materials Article

Rapid Removal of Zinc(II) from Aqueous Solutions Using a Mesoporous Activated Carbon Prepared from Agricultural Waste Xiaotao Zhang 1,2,† 1 2 3

* †

ID

, Yinan Hao 2,† , Ximing Wang 2, *

ID

and Zhangjing Chen 3

College of Science, Inner Mongolia Agricultural University, Hohhot 010018, China; [email protected] College of Material Science and Art Design, Inner Mongolia Agricultural University, Hohhot 010018, China; [email protected] Department of Sustainable Biomaterials Virginia Tech University, Blacksburg, VA 24061, USA; [email protected] Correspondence: [email protected] Contributed equally to this paper.

Received: 5 August 2017; Accepted: 25 August 2017; Published: 28 August 2017

Abstract: A low-cost activated carbon (XSBLAC) prepared from Xanthoceras Sorbifolia Bunge hull via chemical activation was investigated to determine its adsorption and desorption properties for zinc(II) ions from aqueous solutions. XSBLAC was characterized based on its N2 -adsorption/desorption isotherm, EDX, XRD, SEM and FTIR results. An adsorption study was conducted in a series of experiments to optimize the process variables for zinc(II) removal using XSBLAC. Modeling the adsorption kinetics indicated good agreement between the experimental data and the pseudo-second-order kinetic model. The Langmuir equilibrium isotherm fit the experimental data reasonably well. The calculated enthalpy (∆H0 ), entropy (∆S0 ) and Gibbs free energy (∆G0 ) values revealed the endothermic and spontaneous nature of the adsorption process. HNO3 displayed the best desorption performance. The adsorption mechanism was investigated in detail through FTIR and SEM/EDX spectroscopic analyses. The results suggested that XSBLAC is a potential biosorbent for removing zinc(II) from aqueous solutions. Keywords: activated carbon; adsorption; zinc(II); desorption; mechanism

1. Introduction Zinc(II) is frequently found in effluents discharged from industries, such as electroplating, pigments, battery manufacturing units, mining, metallurgy and municipal wastewater treatment plants. Zinc(II) is a well-known toxic metal ion and can threaten human life by bioaccumulating in the food chain. The World Health Organization recommended a maximum acceptable zinc concentration in drinking water of 5.0 mg/L [1]. The removal of zinc(II) ions or decreasing its concentration to the permitted levels before discharge is necessary to prevent deleterious effects on the ecosystem and public health [2]. Numerous methods have been employed to remove zinc(II) ions from wastewater, including precipitation, coagulation, ion exchange, membrane filtration, and electrolysis [3–7]. The cost and the generation of harmful wastes are important parameters relevant to most of these processes, especially for low concentrations of zinc(II). Currently, adsorption is regarded as a powerful technology for removing zinc(II) from aqueous solutions. Commercial activated carbon (AC) has been studied as an adsorbent for wastewater treatment; it has highly developed porosity, a large internal surface area, and relatively high mechanical strength [8]. However, manufacturing AC is very expensive [9]. To overcome this disadvantage, AC can be produced from a wide variety of raw and low-cost

Materials 2017, 10, 1002; doi:10.3390/ma10091002

www.mdpi.com/journal/materials

Materials 2017, 10, 1002

2 of 18

agricultural waste byproducts, including coconut shell, pecan shell, rice husk, oil palm shell, sugarcane bagasse, palm shell, and sawdust [10–17]. The production of AC from agricultural by-products is an increasingly interesting research field because it addresses the problem of the disposal of agro-residues. As the major source of energy tree species, Xanthoceras Sorbifolia Bunge has been widely and extensively cultivated in northern areas, especially the Inner Mongolia autonomous region of China, which has played a very significant role in the agricultural industry. Moreover, the Inner Mongolia region owns lots of processing industrial enterprises, which often produce a number of pollutants containing heavy metals. Therefore, we could efficiently utilize the biomass resources of Xanthoceras sorbifolia Bunge, preparing it as a potential high adsorbent and applying it to industrial wastewater for the purpose of “waste to manage waste”. It is an attractive method to solve issues related to the sustainable development of industrialization in the north of China. From this point of view, the way in which agriculture is combined with industry of the local region to manage the ecological environment is the novelty of this work. In practice, Xanthoceras Sorbifolia Bunge hull (XSBL) is a by-product of production. XSBL is available in large quantities at no cost and could be a good basis for the development of adsorbent materials. Currently, the existing literature contains no information regarding the removal of zinc(II) using Xanthoceras Sorbifolia Bunge hull AC (XSBLAC). The present work aims to prepare AC from XSBL via chemical activation using H3 PO4 and characterize its ability to treat zinc(II)-containing wastewater. This XSBLAC production process is novel because it adopts the National Invention Patent method [18]. The properties of XSBLAC are investigated by analyzing N2 adsorption/desorption, energy dispersive spectrometry (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) results. Furthermore, the adsorption and desorption capacities of zinc(II) are observed in detail. Each factor influencing the adsorption and desorption behavior of XSBLAC, including the initial concentration of zinc(II), pH value, adsorption temperature, adsorption time, HNO3 concentration, desorption temperature and desorption time, is systematically studied using various scenarios. The adsorption kinetics and isotherms on XSBLAC are determined; the enthalpy (∆H0 ), entropy (∆S0 ) and Gibbs free energy (∆G0 ) values are calculated; and the mechanism of zinc(II) adsorption is discussed based on Fourier transform infrared (FTIR) and SEM/energy-dispersive X-ray (EDX) spectroscopic analyses. Finally, exploratory research results regarding the recycling application of XSBLAC offer a reference for zinc(II) removal, and its regeneration ability is evaluated after five adsorption/desorption cycles, showing good recycling ability. 2. Experiments 2.1. Materials The XSBL used in this study as a raw material was collected from Chifeng, China. Prior to use, the sample was washed with hot deionized water to remove dirt particles, dried in the sun for a specific period of time, ground in a high-speed rotary cutting mill, and screened to a particle size of 0.5–0.8 mm for experimental use. A stock solution of zinc(II) (1000 mg/L) was prepared using analytical reagent (A.R.)-grade nitrate salts purchased from Tianjin Beilian Fine Chemicals Co., Ltd. (Tianjin, China). All other chemicals were reagent grade and were used without further purification. All solutions were prepared using deionized water. 2.2. AC Preparation Solid XSBL residue was washed to neutrality using deionized water and dried at 100 ◦ C for 24 h in a hot air oven (Memmert VO400, Schwabach, Germany). Subsequently, the dried mass was soaked in H3 PO4 (85 wt %) and magnetically stirred (500 rpm) for 1 h. The impregnation ratio was calculated as the ratio of the weight of H3 PO4 in solution to the weight of the XSBLBL used. The material was carbonized at 500 ◦ C in a tube furnace (FSX2-12-15N, Tianjin, China) for 4 h. The sample was then

Materials 2017, 10, 1002

3 of 18

cooled to room temperature in a nitrogen atmosphere and washed with deionized water until the pH level stabilized. It was dried in an oven overnight at 120 ◦ C, ground, and finally sieved to 200 mesh [18] using standard sieves (Model Φ200). Then, this carbon powder was stored in an airtight packet for experimental use. 2.3. Adsorption Studies An amount of XSBLAC 0.1000 g (BS210S, Sartorius, Gottingen, Germany) was accurately weighed and added to 50 mL of zinc(II) solution with a known concentration. The suspension was stirred at a uniform speed of 120 rpm in a thermostatic shaker (SHA-C, Zhangjiagang, China), and its pH was adjusted using a certain amount of NaAc-HAc solution with a pH meter (PB-10, Sartorius, Germany). When the adsorption equilibrium was reached, the mixture was centrifuged at 6000 rpm for 10 min (H2050R, Changsha, China). The residual concentration of zinc(II) in the supernatant was determined using an ultraviolet (UV)-Visible spectrophotometer (TU-1901, Beijing, China). The adsorption experiments were conducted with different initial zinc(II) concentrations, pH values, adsorption temperatures and times. Considering the experimental errors, three experiments were run in parallel under the same conditions, and the obtained results were based on the average values. The adsorption capacity of the zinc(II) solution was determined using Equation (1) [19]: qt,1 =

(C0 − Ct )V1 × 65.38 m1

(1)

where qt,1 (mg/g) is the adsorption capacity at time t (min). C0 and Ct,1 (mol/L) refer to the initial zinc(II) concentration and the final concentration at time t (min), respectively. V 1 (mL) refers to the volume of zinc(II), and m1 (g) is the mass of the adsorbent. In the calculation qt,1 , no losses of zinc(II) to any other mechanisms (e.g., volatilization, sorption on glassware, or degradation) were assumed. 2.4. Desorption and Regeneration Studies Samples (0.1000 g) of zinc(II)-loaded XSBLAC were accurately weighed and transferred into 50-mL HNO3 solutions with different concentrations. Each mixture was placed in an ultrasonic cleaning machine (KS-300EI, Ninbo, China). When equilibrium was reached at a certain temperature, the suspension was centrifuged, and the desorbed zinc(II) concentrations in the solution were determined. Considering the experimental errors, three experiments were performed, and the reproducibility of the results was within ±3%. The desorption capacity of the zinc(II)-loaded XSBLAC was calculated according to Equation (2) [20]. Ct,2 V2 × 65.38 qt,2 = (2) m2 where qt,2 (mg/g) refers to the desorption amount at time t (min). Ct,2 (mol/L) is the zinc(II) concentration in the desorbed solution at time t (min). V 2 (mL) refers to the total volume of the desorption solution, and m2 (g) is the mass of the zinc(II)-loaded XSBLAC. Repeated batch experiments were performed to examine the reusability of XSBLAC for zinc(II) removal. The material was washed with deionized water to remove the remaining acid and dried in an oven (DZF-6210, Shanghai, China) at 120 ◦ C for subsequent zinc(II) adsorption. The zinc(II)-adsorption and desorption capacities were determined and analyzed. The adsorption/desorption process was performed consecutively five times. 3. Results and Discussion 3.1. Adsorbent Characterization The specific surface area and porous system of the XSBLAC were characterized based on N2 -adsorption/desorption isotherms (Micromeritics ASAP 2020, Norcross, GA, USA). XRD analysis of the powdered samples was performed using an X-ray power diffractometer with a Cu anode (PAN

Materials 2017, 10, 1002 Materials 2017, 10, 1002

4 of 18 4 of 19

Alytical Co., Co., X’pert X’pert PRO, PRO, Almelo, Almelo, The The Netherlands) Netherlands) at at 40 40 kV kV and and 40 40 mA mA and and scanning scanning from from 44° to 18 18° ◦ to ◦ Alytical at 8°/min. The morphological changes and surface analyses of the samples were conducted using ◦ at 8 /min. The morphological changes and surface analyses of the samples were conducted using SEM-EDX (HITACHI S-4800, Tokyo, Japan). FTIR spectra were recorded in KBr pellets with a FTIR SEM-EDX (HITACHI S-4800, Tokyo, Japan). FTIR spectra were recorded in KBr pellets with a FTIR spectrophotometer (Thermo Nicolet, NEXUS, TM, Waltham, MA, USA). spectrophotometer (Thermo Nicolet, NEXUS, TM, Waltham, MA, USA). 3.2. Properties of XSBLAC XSBLAC 3.2. Properties of N22-adsorption/desorption information regarding thethe porosity of N -adsorption/desorptionisotherms isothermsprovide providequalitative qualitative information regarding porosity carbonaceous adsorbents [21]. The surface physical parameters of XSBLAC obtained from its N 2of carbonaceous adsorbents [21]. The surface physical parameters of XSBLAC obtained from its adsorption/desorption isotherm are summarized in Table 1. From the results, the highest surface area N 2 -adsorption/desorption isotherm are summarized in Table 1. From the results, the highest surface 2/g) and 3/g) of XSBLAC 2 (688.62 m total pore (0.377 cm were calculated using theusing t-plotthe method. area (688.62 m /g) and totalvolume pore volume (0.377 cm3 /g) of XSBLAC were calculated t-plot Additionally, the structure of XSBLAC was found be loose, on its method. Additionally, the structure of XSBLAC wastofound to bewith loose,many with pores many generated pores generated surface. According to the International Union ofUnion Pure and Applied (IUPAC), the mesopore on its surface. According to the International of Pure and Chemistry Applied Chemistry (IUPAC), the structure of XSBLAC can be supported by its average pore diameter (D p = 2.2 nm). mesopore structure of XSBLAC can be supported by its average pore diameter (Dp = 2.2 nm). Table 1. Pore Pore structure structure parameters parameters of of the the sample sample studied studied in in this this work. work. Table 1. Sext Sext/SBET Smic Vtot Vmeso Vmeso/Vtot Dp SBET Sample SBET 2 S(m S Vtot Vmeso V(%) Dp 2/g) 3/g) 3/g) ext 2/g) Sext /S meso /Vtot (nm) BET mic (m (%) (m (cm (cm /g) Sample (m2 /g) (m2 /g) (%) (m2 /g) (cm3 /g) (cm3 /g) (%) (nm) XSBLAC 688.62 477.87 69.4 210.43 0.377 0.252 66.8 2.2 XSBLAC 477.87 69.4 surface210.43 0.377 0.252area, Sext66.8 Number of688.62 analyses: three. SBET—specific area; Sext—mesopore surface /SBET—ratio 2.2 of Number of analyses: three.toSBET —specific surface area; Sext —mesopore surface SextV/S of mesopore BET —ratio mesopore surface area specific surface area, Smic —micropore surfacearea, area, tot—total pore volume, surface area to specific surface area, Smic —micropore surface area, Vtot —total pore volume, Vmeso —mesopore volume, —mesopore volume, Vmeso/Vtot—ratio of mesopore volume to total pore volume, Dp—average Vmeso/V V meso tot —ratio of mesopore volume to total pore volume, Dp —average pore size.

pore size.

Figure 1 shows the pore size size distribution distribution of XSBLAC. XSBLAC has pore sizes between 0.5 nm and 5.5 nm and displays a wide pore size distribution with a low pore volume. From Table 2, it can be seen that the mesopore mesopore surface increased remarkably. remarkably. Thus, a significant amount of micropores became mesopores.

Figure 1. 1. Pore size distribution distribution of of XSBLAC. XSBLAC. Figure Pore size

Elemental analysis was conducted for XSBLACasand XSBL byby EDX. indicated in Table 2, the Table 2. Global Surface Composition Determined EDXAs Analysis. C, O, N, Cl and Na contents for XSBLAC were as follows: 80.26%, 12.92%, 5.62%, 0.75% and 0.45%, respectively. XSBLAC from XSBL using Na the(at activating reagent HN/C 3PO4(%) during Sample Additionally, C (at %) O (at %) prepared N (at %) Cl (at %) %) O/C (%) the treatment process showed a higher C content (80.26%) than XSBL (60.33%). XSBLAC 80.26 12.92 5.62 0.75 0.45 16.1 0.07 XSBL

60.33

34.96

3.41

0.93

0.37

60.0

0.06

Table 2. Global Surface Composition as Determined by EDX Analysis.

Elemental analysis forNXSBLAC and XSBL the Sample C (at %) was conducted O (at %) (at %) Cl (at %) by EDX. Na (at As %) indicated O/C (%)in Table N/C2,(%) C,XSBLAC O, N, Cl and Na contents for XSBLAC were 80.26%, 12.92%, 0.75% and 0.07 0.45%, 80.26 12.92 5.62as follows: 0.75 0.45 5.62%,16.1 XSBL 60.33 34.96 prepared3.41 0.37 60.0 H3 PO4 during 0.06 respectively. Additionally, XSBLAC from XSBL 0.93 using the activating reagent the treatment process showed a higher C content (80.26%) than XSBL (60.33%).

Materials 2017, 10, 1002 Materials 2017, 10, 1002

5 of 18 5 of 19

XRD is is anan effective morphologicalfeatures featuresofofadsorbents. adsorbents. Figure XRD effectivemethod methodfor fordetermining determining the morphological Figure 2 2 shows XRD imagesofofXSBL XSBLand andthe the prepared prepared XSBLAC. inin thethe XRD shows thethe XRD images XSBLAC.Two Twobroad broadpeaks peaksare arepresent present XRD ◦ and ◦ and attributed precursor, observednear near2θ2θ==15 15° and 22 22° ofof XSBLBL; this of of thethe precursor, observed attributedto tothe thealiphatic aliphaticchain chain XSBLBL; this Materials 2017, 10, 1002 5 of 19 indicates that XSBLBLhas hasaaturbostratic turbostraticstructure structure and defined XRD pattern indicates that XSBLBL definedamorphous amorphouscarbons. carbons.The The XRD pattern ◦ , 29° XSBLAC (Figure2)2)shows showsdiffraction diffraction peaks peaks at 2θ 2θ = = 26 26°, 30°, because ofof internal of of XSBLAC (Figure 29◦ and and 30◦of ,likely likely because XRD is an effective method for determiningat the morphological features adsorbents. Figure 2internal etching during H 3PO4 activation and increased numbers of complex pores in XSBLAC and shaped etchingshows during increased of complex pores XSBLAC and shaped 3 PO 4 activation theH XRD images of XSBLand and the preparednumbers XSBLAC. Two broad peaks are in present in the XRD graphite-like microcrystalline layers, suggesting the formation of more carbon-based graphite-like microcrystalline layers, suggesting the formation ofaliphatic moreamorphous amorphous carbon-based of the precursor, observed near 2θ = 15° and 22° and attributed to the chain of XSBLBL; this graphite microcrystallites [22]. Additionally, the dispersion peak (2θ = 15°) persisted, illustrating that ◦ indicates that XSBLBL has a turbostratic structure and defined amorphous carbons. The XRD pattern graphite microcrystallites [22]. Additionally, the dispersion peak (2θ = 15 ) persisted, illustrating that XSBLAC existed(Figure in a type of amorphous state. of existed XSBLAC diffraction peaks at 2θ = 26°, 29° and 30°, likely because of internal XSBLAC in a type2)ofshows amorphous state. etching during H3PO4 activation and increased numbers of complex pores in XSBLAC and shaped graphite-like microcrystalline layers, suggesting the formation of more amorphous carbon-based graphite microcrystallites [22]. Additionally, the dispersion peak (2θ = 15°) persisted, illustrating that XSBLAC existed in a type of amorphous state.

Figure2.2.XRD XRD patterns patterns of of raw Figure raw XSBL XSBLand andXSBLAC. XSBLAC.

The SEM images of XSBL and2.XSBLAC areofpresented inXSBLAC. Figure 3. This figure shows that the Figure XRD patterns raw XSBL and The SEM images of XSBL and XSBLAC are presented in Figure 3. This figure shows that the surface of XSBL contains many thin sheets or layers with large pores in its structure (Figure 3a). surface of XSBL contains many thin withinporous large its structure (Figure The images of XSBL anda sheets XSBLACor arelayers presented Figurepores 3. Thisin figure shows thatpores the and3a). However, theSEM AC appeared to have well-developed coarse surface with irregular However, the AC appeared to have a well-developed coarse porous surface with irregular pores and surface of XSBL contains many thin sheets or layers with large pores in its structure (Figure 3a). breaks (Figure 3b). Mixtures of micropores and mesopores are present in the SEM images. This result However, the AC appeared to have a well-developed coarse porous surface with irregular pores and breaks 3b). Mixtures of micropores andthe mesopores are presentofininorganic the SEMcompounds images. This may (Figure be attributed to the creation of pores and substantial removal in result the breaks (Figure 3b). Mixtures ofof micropores and mesopores are present in the SEM images.compounds This result in the may be attributed to the creation pores and the substantial removal of inorganic raw structure during activation, eventually yielding a porous surface. may be attributed to the creation of pores and the substantial removal of inorganic compounds in the raw structure during activation, eventually yielding a porous surface. raw structure during activation, eventually yielding a porous surface.

Figure SEMimage image of of raw XSBLAC (b).(b). Figure 3.3.SEM raw XSBL XSBL(a) (a)and and XSBLAC Figure 3. SEM image of raw XSBL (a) and XSBLAC (b).

3.3. Effect of Value pH Value Adsorption 3.3. Effect of pH on on Adsorption 3.3. Effect of pH Value on Adsorption The pH value of the zinc(II) solution is highly influential and must be considered during the The pH value of the zinc(II) solution is highly influential and must be considered during the adsorption process. Therefore, experiments were initially performed to optimize the pH value, The pH of the zinc(II)experiments solution is highly influential and must be considered during adsorption value process. Therefore, were initially performed to optimize the pH value,the testing values from 2.0 to 6.0, to investigate the effect of pH on zinc(II) removal, as shown in Figure adsorption process. Therefore, experiments were initially performed to optimize the pH value, testing testing values from 2.0 to 6.0, to effect pH on zinc(II) removal, as shown inofFigure 4. The adsorption of zinc(II) byinvestigate XSBLAC wasthe highly pHofdependent because the superficial charge values from 2.0 to of 6.0,zinc(II) to investigate the effect of pH on zinc(II)because removal, shown in Figure 4. The adsorption highly dependent theas superficial of 4. XSBLAC is affected by theby pHXSBLAC value [23].was Indeed, thepH maximum adsorption capacity for zinc(II) charge was The adsorption of zinc(II) by XSBLAC was highly pH dependent because the superficial charge + XSBLAC is affected theofpH [23].can Indeed, the maximum adsorption capacity for zinc(II) zinc(II) was of achieved at a pH by value 5.2.value This result be explained by the competition between H and + XSBLAC is affected by the pH value [23]. Indeed, the maximum adsorption capacity for zinc(II) was achieved at a pH value of 5.2. This result can be explained by the competition between H and zinc(II) + achieved at a pH value of 5.2. This result can be explained by the competition between H and zinc(II)

Materials 2017, 10, 1002

6 of 18

Materials 2017, 10, 1002 Materials 2017, 10, 1002

6 of 19 6 of 19

ions for activated adsorption sites on the XSBLAC surface at low pH levels; when the pH increases, ions for activated adsorption sites on the XSBLAC surface at low pH levels; when the pH increases, the covered H+activated leaves the AC surface, making more adsorption available zinc(II). According ions for sites on the XSBLAC at low sites pH levels; when for the pH increases, the covered H+ leaves adsorption the AC surface, making moresurface adsorption sites available for zinc(II). According θ + to the solubility product constant of zinc(II) hydrolysis (pK θ sites = 16.92), as the pH value increases, the the covered H leaves the AC surface, making more adsorption available for zinc(II). According to the solubility product constant of zinc(II) hydrolysis (pKsp sp = 16.92), as the pH value increases, the the solubility product of zinc(II) hydrolysis spθ = 16.92), as pH value increases, functional the negativetocharge density alsoconstant increases, resulting in the(pK production of the oxygen-containing negative charge density also increases, resulting in the production productionofof oxygen-containing functional negative charge density also increases, resulting inpH thevalues oxygen-containing functional that no groups and/or ligands in XSBLAC. Therefore, all used were below 6.0, ensuring groups and/or ligands in XSBLAC. Therefore, valuesused usedwere were below ensuring that no groups and/or ligands in XSBLAC. Therefore, all all pH pH values below 6.0,6.0, ensuring that no hydroxide precipitation occurred during the adsorption process. Similar Similar pHeffects effectsonon the adsorption hydroxide precipitation occurred during process. the adsorption hydroxide precipitation occurred duringthe theadsorption adsorption process. Similar pHpH effects on the adsorption of of heavy have been reported in the [24,25].Hence, Hence, the optimal pH value zinc(II) heavy metals havehave been reported inin the literature [24,25]. Hence, the optimal value forfor zinc(II) ofmetals heavy metals been reported theliterature literature [24,25]. the optimal pHpH value for zinc(II) adsorption inin this study was 5.2. adsorption in this study adsorption this study waswas 5.2.5.2.

Figure 4. Effect the values pH values adsorption capacity XSBLAC for zinc(II). (Adsorption Figure 4. Effect of theofpH onon thetheadsorption capacityofof XSBLAC for zinc(II). (Adsorption experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.015 mol/L; pH range: 2.0–6.0; experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.015 mol/L; pH range: 2.0–6.0; Figure 4. Effect of60the pH valuestime: on the adsorption capacity of XSBLAC for zinc(II). (Adsorption temperature: °C; adsorption 40 min). temperature: 60 ◦ C; adsorption time: 40 min). experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.015 mol/L; pH range: 2.0–6.0; 3.4. Effect of Temperature on Adsorption temperature: 60 °C; adsorption time: 40 min).

3.4. Effect of ItTemperature on Adsorption is important to investigate the effect of temperature on zinc(II) adsorption for practical 3.4.ItEffect of Temperature onrelationship Adsorption applications. Here, between and the capacity of XSBLAC is important to the investigate the effecttheoftemperature temperature onadsorption zinc(II) adsorption for practical is discussed under isothermal conditions at different temperatures, as shown in Figure 5. The

applications. Here, the relationship between the temperature andon thezinc(II) adsorption capacityfor of XSBLAC It is important to investigate the effect of temperature adsorption practical is adsorption capacity of XSBLAC increased as the temperature increased from 25 to 60 °C, indicating applications. the relationship between the temperature andas theshown adsorption capacity of XSBLAC discussed underHere, isothermal conditions at different temperatures, in Figure 5. The adsorption that high temperatures facilitate the adsorption of zinc(II) on the surface of XSBLAC. This result can ◦ C, indicating is discussed under isothermal conditions at different temperatures, as shown in Figure 5. The capacity of XSBLAC increased as the temperature increased from 25 to 60 that high be attributed to the fact that high temperature may produce a loosening effect within the structure adsorption capacity of increased as the temperature increased from 25result to 60 °C, indicating temperatures facilitate theXSBLAC adsorption of on the surface of XSBLAC. This can be attributed and activate the functional groups of zinc(II) XSBLAC, leading to further adsorption of zinc(II) ions into the high temperatures facilitate at the adsorption zinc(II) on effect the of the XSBLAC. This result can structure of AC. However, temperatures 60 °C, the surface adsorption capacity decreased, to that the fact that high temperature may produce of aabove loosening within structure and activate accounting for the endothermic behavior of the adsorption process. These results can be attributed to be attributed to the fact that high temperature may produce a loosening effect within the structure the functional groups of XSBLAC, leading to further adsorption of zinc(II) ions into the structure the chemical adsorption groups action between XSBLAC and zinc(II). In theadsorption following experiments, a pHinto of the the at functional of XSBLAC, to further of zinc(II)accounting ions of and AC.activate However, temperatures above 60 ◦ C,leading the adsorption capacity decreased, for 5.2 was selected as the optimum pH value. of AC. However, atadsorption temperatures aboveThese 60 °C, the adsorption capacity decreased, thestructure endothermic behavior of the process. results can be attributed to the chemical accounting for the endothermic behavior of the adsorption process. These results can be attributed to adsorption action between XSBLAC and zinc(II). In the following experiments, a pH of 5.2 was selected the chemical adsorption action between XSBLAC and zinc(II). In the following experiments, a pH of as the optimum pH value. 5.2 was selected as the optimum pH value.

Figure 5. Effect of temperature on the adsorption capacity of XSBLAC for zinc(II). (Adsorption experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.015 mol/L; pH range: 5.16; temperature: 25–75 °C; adsorption time: 40 min).

Figure Effectofoftemperature temperature on on the the adsorption adsorption capacity Figure 5. 5.Effect capacity of of XSBLAC XSBLACfor forzinc(II). zinc(II).(Adsorption (Adsorption experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.015 mol/L; pH range: experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.015 mol/L; pH range:5.16; 5.16; temperature: 25–75◦ C; °C;adsorption adsorptiontime: time: 40 40 min). min). temperature: 25–75

Materials 2017, 10, 1002 Materials 2017, 10, 1002

7 of 18 7 of 19

3.5. Adsorption Kinetics Kinetics 3.5. The effect effect of of adsorption adsorption time time on on the the removal removal of of zinc(II) zinc(II) by by XSBLAC XSBLAC was was investigated, investigated, and and the the The results are shown in Figure greatly increased from 10 10 to to 40 results Figure 6. 6. The Theadsorption adsorptioncapacity capacityofofXSBLAC XSBLAC greatly increased from min until equilibrium and then remained nearly stable as the adsorption time increased further. This 40 min until equilibrium and then remained nearly stable as the adsorption time increased further. behavior maymay be why zinc(II) ions first adsorbed ononthe functional This behavior be why zinc(II) ions first adsorbed thesurface’s surface’sunsaturated unsaturated activated activated functional sites, subsequently subsequently diffused diffused into the XSBLAC’s micropores, and finally adsorbed on functional sites, sites, thereby saturating saturating the the material’s material’s mesopores mesopores and and reaching reaching adsorption adsorption equilibrium. equilibrium. Therefore, Therefore, an an thereby adsorptiontime timeof of40 40min minwas waschosen chosenasasthe the optimal equilibrium time adsorption of zinc(II) adsorption optimal equilibrium time forfor thethe adsorption of zinc(II) on on XSBLAC under experimental conditions. XSBLAC under the the experimental conditions.

Figure 6. 6. Effect Effect of of time time on on the the adsorption adsorption capacity capacity of XSBLAC XSBLAC for zinc(II). (Adsorption experimentsFigure sample dosage: dosage: 0.05 0.05 g; g; initial initial zinc(II) zinc(II)concentration: concentration:0.015 0.015mol/L; mol/L; pH value: 5.16; temperature: 60 ◦°C; sample C; adsorption time: time: 10–80 10–80 min). min). adsorption

To study potential rate-controlling steps of adsorption, pseudo-secondTo studythe the potential rate-controlling steps of pseudo-first-order, adsorption, pseudo-first-order, order, intraparticle diffusion, Elovich and Bangham kinetic models aremodels generally to fitused the pseudo-second-order, intraparticle diffusion, Elovich and Bangham kinetic are used generally experimental data, which are described as Equations (3)–(7) [26–28]. to fit the experimental data, which are described as Equations (3)–(7) [26–28].

kt k t1  qqt,1  lg lg( t ,1)) = lg(qqee − lgqqe e− 1 2.303 2.303

(3) (3)

tt 11 tt = +  2 2 qqt,1 kk22qqee qqe e t ,1

(4) (4)

qt = k i t0.5 0 .5

(5) (5)

qt  ki t

qt =

qt 

1 1 ln(αβ) + ln t β β 1 1



ln( ) 

log qt = log kr +

ln t

1 log t m

(6)

(6) (7)

1

where qe and qt,1 are the amounts of zinc(II) adsorbed (mg/g) log qions log t at equilibrium and at time t (min), (7) t  log k r  m k2 [g·(mg/min)−1 ] is the rate constant of respectively; k1 (min−1 ) is the pseudo-first-order rate constant; 0.5 )−1 ] is an intraparticle diffusion the pseudo-second-order adsorption kinetic equation; ki [mg(mg/g) ·(g·minat where qe and qt,1 are the amounts of zinc(II) ions adsorbed equilibrium and at time t (min), − 1 rate constant; α [mg · (g · min) ] is an initial adsorption rate; and β (g/mg) to the surface −1 −1] related respectively; k1 (min ) is the pseudo-first-order rate constant; k2 [g·(mg/min)is is the rate constant of −1 ·min−1 ] is the rate constant of adsorption; coverage and activation energy for chemisorption; k [mg · g 0.5 −1 r the pseudo-second-order adsorption kinetic equation; ki [mg·(g·min ) ] is an intraparticle diffusion 1/m an indicator of the adsorption −1] is an intensity. rate is constant; α [mg·(g·min) initial adsorption rate; and β (g/mg) is related to the surface The fits of these five models were checked by keach linear plot ln(qrate 7a), −1·min −1] of e −q t ) versusoft (Figure coverage and activation energy for chemisorption; r [mg·g is the constant adsorption; 0.5 (Figure 7c), q versus lnt (Figure 7d), and lnq verse lnt (t/q ) versus t (Figure 7b), q versus t t t t 1/mtis an indicator of the adsorption intensity. 2 and constant values for the five adsorption kinetic models were (Figure 7e), respectively. The R The fits of these five models were checked by each linear plot of ln(qe − qt) versus t (Figure 7a),

(t/qt) versus t (Figure 7b), qt versus t0.5 (Figure 7c), qt versus lnt (Figure 7d), and lnqt verse lnt (Figure 7e),

Materials 2017, 10, 1002 Materials 2017, 10, 1002

8 of 19 8 of 18

respectively. The R2 and constant values for the five adsorption kinetic models were calculated and are given in Table 3. According to According the calculated kinetic model parameters in Table 3 and from3 calculated and are given in Table 3. to the calculated kinetic model parameters in Table comparing the experimental equilibriumequilibrium adsorption capacity, it was clearlyitfound that thefound calculated and from comparing the experimental adsorption capacity, was clearly that qthe e value (103.82 mg/g) agreed with the experimental data (110.48 mg/g), and the pseudo-second-order calculated qe value (103.82 mg/g) agreed with the experimental data (110.48 mg/g), and the 2 (0.9416), indicating that model produced a good fit based on thea value the pseudo-second-order pseudo-second-order model produced good of fit R based on the value of R2 (0.9416), indicating that the kinetics model can reflect the adsorption of XSBLAC. Therefore, it XSBLAC. is obviousTherefore, that chemical pseudo-second-order kinetics model can process reflect the adsorption process of it is adsorption should be the rate limiting step of the adsorption of zinc(II) ions onto the prepared obvious that chemical adsorption should be the rate limiting step of the adsorption of zinc(II) ions activated carbon XSBLAC. onto the prepared activated carbon XSBLAC. 2.0 1.0

(a) pseudo-first-order kinetic model

1.9 1.8 1.7

0.8 t /qt

log (qe- qt)

( b) pseudo-second-order kinetic model

0.9

1.6 1.5

0.6

y = - 0.0084x + 1.8829

1.4

0.7

R2 = 0.6349

y = 0.0083x + 0.2829 R2 = 0.9416

0.5

1.3 1.2

0.4

0

10

20

30

40

50

60

70

80

90

0

10

20

30

40

Time (min)

140 120

50

60

70

80

90

Time (min)

120

(c) Intraparticle diffusion kinetic model

100

(d) Elovich kinetic model

100

60

qt (mg/g)

qt (mg/g)

80

80

y=15.302x-19.502 R2=0.7901

40

60 y=-82.6734x+44.66 R2=0.8724

40 20

20 3

4

5

6

7

8

2.0

9

2.5

3.0

2.2

3.5

4.0

4.5

lnt

t 0.5 (min 0.5)

(e) Bangham kinetic model

2.0

logqt

1.8

y=0.7942x+0.6307 R2=0.7576

1.6 1.4 1.2

1.0

1.2

1.4

1.6

1.8

2.0

logt

Figure pseudo-second-order (b);(b); intraparticle diffusion (c); Elovich (d) and Figure 7. 7. Pseudo-first-order Pseudo-first-order(a); (a); pseudo-second-order intraparticle diffusion (c); Elovich (d) Bangham (e) adsorption kinetic equations fitting and Bangham (e) adsorption kinetic equations fittingcurves curvesofof experimental experimental data. (Adsorption (Adsorption experiments-sample experiments-sample dosage: dosage: 0.05 0.05 g; g; initial initial zinc(II) concentration: 0.015 0.015 mol/L; mol/L; pH pH value: value: 5.16; 5.16; ◦ C). temperature: temperature: 60 60 °C).

Materials 2017, 10, 1002

9 of 18

Table 3. Comparison of the estimated adsorption rate constants, qe , and the correlation coefficients of the five kinetic models. Metal

Zinc(II)

qe (exp) (mg/g)

103.82

Parameters

Pseudo-First-Order

R2

0.6349

Constants

k1 qe (cal)

0.01329

min−1

76.36 mg/g

Pseudo-First-Order

K2 qe (cal)

0.9416 2.764 × 10−4 min−1 110.48 mg/g

Intraparticle Diffusion

ki

0.7901 15.302 mg/(g min0.5 )

Elovich

α β

0.8724 11.2501 mg/(g min) 0.01327 g/mg

Bangham Model 0.7576 Kr (mg·g−1 ·min−1 ) m

0.7942 0.1004

Materials 2017, 10, 1002 Materials 2017, 10, 1002

10 of 18 10 of 19

3.6. 3.6. Adsorption Adsorption Isotherms Isotherms The The effect effect of of the the initial initial zinc(II) zinc(II) concentration concentration on on the the adsorption adsorption capacity capacity of of XSBLAC XSBLAC is is shown shown in in Figure Figure 8. 8. The The initial initial concentration concentration of of zinc(II) zinc(II) substantially influenced the adsorption capacity, capacity, which which increased increased until equilibrium butbut hardly increased as increasedsharply sharplyas asthe theinitial initialzinc(II) zinc(II)concentration concentration increased until equilibrium hardly increased the zinc(II) concentration was increased further.further. Thus, the zinc(II)-removal process probably involves as the zinc(II) concentration was increased Thus, the zinc(II)-removal process probably an interaction between the activated on XSBLAC and zinc(II),and which may result increase involves an interaction between thesites activated sites on XSBLAC zinc(II), whichfrom maythe result from in zinc(II) concentration acceleratingaccelerating the bondingthe of zinc(II) XSBLAC, increasing theinitial increase in initial zinc(II) concentration bondingonto of zinc(II) ontothereby XSBLAC, thereby the driving the forcedriving underlying concentration gradient [29]. gradient Increasing theIncreasing zinc(II) concentration increasing force the underlying the concentration [29]. the zinc(II) further did notfurther increasedid thenot adsorption capacity because the activated sites XSBLACsites were concentration increase the adsorption capacity because theon activated onsaturated. XSBLAC An initial zinc(II)An concentration 0.015 mol/L of was used to ensure thatto the adsorption were saturated. initial zinc(II)ofconcentration 0.015 mol/L was used ensure that theequilibrium adsorption equilibrium would be reached. would be reached.

Figure 8. 8. Effect Effect of of initial initial zinc(II) zinc(II) concentration concentration on on the the adsorption adsorption capacity capacity of of XSBLAC XSBLAC for for zinc(II). zinc(II). Figure (Adsorption experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.005–0.025 mol/L; (Adsorption experiments-sample dosage: 0.05 g; initial zinc(II) concentration: 0.005–0.025 mol/L; pH ◦ pH value: temperature: 60 adsorption °C; adsorption 40 min). value: 5.16;5.16; temperature: 60 C; time:time: 40 min).

Adsorption isotherm models are commonly used to describe the characteristics of an adsorption Adsorption isotherm models are commonly used to describe the characteristics of an adsorption process between solid and liquid phases when adsorption equilibrium is reached. The linear forms process between solid and liquid phases when adsorption equilibrium is reached. The linear forms of of the Langmuir and Freundlich isotherms are represented, respectively, by Equations (8) and (9) [30]: the Langmuir and Freundlich isotherms are represented, respectively, by Equations (8) and (9) [30]:

C

1

C

Ce e  1  Cee = bq + q qqe e bq qmm mm

(8) (8)

1 ln  ln lnkkf f +1 lnlnCCe e ln q qee = nn

(9) (9)

where Ce is the concentration of zinc(II) at equilibrium (mol/L), qe is the amount adsorbed (mg/g), where Ce is the concentration of zinc(II) at equilibrium (mol/L), qe is the amount adsorbed (mg/g), qm qm is the complete monolayer adsorption capacity (mg/g), b is the Langmuir constant related to the is the complete monolayer adsorption capacity (mg/g), b is the Langmuir constant related to the adsorption capacity (L/mg), and n and kf are the Freundlich constants. adsorption capacity (L/mg), and n and kf are the Freundlich constants. A plot of Ce /qt versus Ce is shown in Figure 9a, and the Langmuir constants were calculated using A plot of Ce/qt versus Ce is shown in Figure 9a, and the Langmuir constants were calculated using the slopes and intercepts listed in Table 4. Similarly, a plot of lnqe versus lnCe (Figure 9b) was used to the slopes and intercepts listed in Table 4. Similarly, a plot of lnqe versus lnCe (Figure 9b) was used to evaluate the Freundlich constants, which are also presented in Table 3. From the correlation coefficient evaluate the Freundlich constants, which are also−4presented in Table 3. From the correlation (R2 ) values in 2Table 4, R2 value of 0.9793 (p (1.327 × 10 ) < 0.01) for the Langmuir isotherm adsorption coefficient (R ) values in Table 4, R2 value of 0.9793 (p (1.327 × 10−4) < 0.01) for the Langmuir isotherm 2 model is higher than that for the Freundlich model of R value of 0.7683 and there adsorption model is higher than that for the Freundlich model of R2 value(pof(0.0026) 0.7683 < (p0.01), (0.0026) < 0.01), was no significant in the adsorption of zinc(II) ions. The experimental data for the adsorption process and there was no significant in the adsorption of zinc(II) ions. The experimental data for the had better correlation coefficients values and better fits with the Langmuir isotherm model than with adsorption process had better correlation coefficients values and better fits with the Langmuir the Freundlich model 9). Therefore, the adsorption of zinc(II)the onto XSBLAC was found isotherm model than(Figure with the Freundlich model (Figureprocess 9). Therefore, adsorption process of 2 = 0.9793, p < 0.01), with a maximum monolayer adsorption to follow the Langmuir isotherm model (R zinc(II) onto XSBLAC was found to follow the Langmuir isotherm model (R2 = 0.9793, p < 0.01), with

a maximum monolayer adsorption capacity of 103.82 mg/g. This result revealed the anchoring of

zinc(II) to the abundant functional groups on XSBLAC with the formation of monolayer surface coverage that was homogenous in nature. Moreover, the characteristics of the Langmuir equation can be explained in terms of the equilibrium parameter (RL) according to Equation (10) [31]:

RL 

Materials 2017, 10, 1002

1 1  bC0

11(10) of 18

where b is the Langmuir constant (L/mg), and C0 is the initial concentration (mol/L). The value of RL capacity of 103.82 mg/g. This result revealed the anchoring of zinc(II) to the abundant functional describes the nature of the adsorption: irreversible (RL = 0); favorable (0 < RL < 1); linear (RL = 1); groups on XSBLAC with the formation of monolayer surface coverage that was homogenous in nature. unfavorable (RL > 1) [32]. In this study, the RL value over the given initial zinc(II) concentration range Moreover, the characteristics of the Langmuir equation can be explained in terms of the equilibrium was calculated (0.1362) and is shown in Table 4, confirming that XSBLAC was favorable for zinc(II) parameter (RL ) according to Equation (10) [31]: adsorption under the adsorption conditions employed. 1

Table 4. Adsorption equilibrium constants R obtained from the Langmuir and Freundlich isotherms for (10) L = 1 + bC0 zinc(II) adsorption onto XSBLAC. (Adsorption experiment sample dosage: 0.05 g; pH value: 5.2; temperature: 60 °C; adsorption 40 min). where b is the Langmuir constanttime: (L/mg), and C0 is the initial concentration (mol/L). The value of

RL describes the nature ofLangmuir the adsorption: irreversible (RL = 0); favorable (0 < RL < 1); Equation linear (RL = 1); Isotherm Equation Freundlich Isotherm qm 2 unfavorable (R > 1) [32]. In this study, the R value over the given initial zinc(II) concentration range (mg/g) L b (mg/L) RL qeL(mg/g) R p 1/n kf (mg/L) R2 p 0.0039 0.1362 0.9793 1.327 × 10−4 that 0.4133 0.7683 for0.0026 was103.82 calculated100.76 (0.1362) and is shown in Table 4, confirming XSBLAC4.614 was favorable zinc(II) adsorption under the adsorption conditions employed.

Figure 9. Langmuir Langmuir (a) (a) and and Freundlich Freundlich (b) (b) isotherm isotherm equations equations fitting fitting curves curves of ofthe theexperimental experimentaldata. data. ◦ 5.16; temperature: temperature: 60 60 °C; C;adsorption adsorption time: time: (Adsorption experiments-sample dosage: 0.05 g; pH value: 5.16; 40 min). min). 40 Table 4. Adsorptionparameters equilibrium constants obtained from the Langmuir and Freundlich Thermodynamic were calculated to determine which process isotherms would occur 0 0 0 for zinc(II) adsorption onto XSBLAC. (Adsorption experiment sample dosage: 0.05 g; pH value: 5.2; spontaneously. The variation of the thermodynamic parameters (ΔH , ΔS and ΔG ) should provide ◦ temperature: 60 C; adsorption time: 40 min). insight into the mechanism and adsorption of an isolated system [33]. The enthalpy (ΔH0) and entropy 0 (ΔS ) values can be calculated from the slope and intercept of the plot of lnK vs. 1/T according to Langmuir Isotherm Equation Freundlich Isotherm Equation qm (11) [34]: Equation (mg/g) 103.82

qe (mg/g) 100.76

b (mg/L) 0.0039

RL 0.1362

R2

0.9793

ln K 

0

p

1.327 S × H  R RT 10−4

0

1/n

kf (mg/L)

R2

p

0.4133

4.614

0.7683

0.0026

(11)

Thermodynamic were gas calculated determine which processtemperature would occur where R (8.314 J/mol·K)parameters is the universal constant,toand T (K) is the adsorption in 0 , ∆S0 and ∆G0 ) should provide spontaneously. The variation of the thermodynamic parameters (∆H 0 Kelvin. The Gibb’s free energy change (ΔG ) was calculated from the Langmuir equilibrium constant 0 insight the mechanism and adsorption of an isolated in unitsinto of liters per mole according to Equation (12) [35]:system [33]. The enthalpy (∆H ) and entropy (∆S0 ) values can be calculated from the slope and intercept of the plot of lnK vs. 1/T according to (12) G 0   RT ln K Equation (11) [34]: ∆S0 ∆H 0 where K (L/mol) is from the Langmuir ln equation K = and − has units of liters per mole. The Gibb’s free (11) R RT energy (ΔG0) was −19.91 KJ/mol at 60 °C, and its negative values verified the thermodynamic where R (8.314 J/mol·K) is the universal gas constant, and T (K) is the adsorption temperature in Kelvin. The Gibb’s free energy change (∆G0 ) was calculated from the Langmuir equilibrium constant in units of liters per mole according to Equation (12) [35]: ∆G0 = − RT ln K

(12)

Materials 2017, 10, 1002

12 of 18

where K (L/mol) is from the Langmuir equation and has units of liters per mole. The Gibb’s free energy (∆G0 ) was −19.91 KJ/mol at 60 ◦ C, and its negative values verified the thermodynamic feasibility and spontaneity of the adsorption process under the applied experimental conditions. ∆H0 was 59.68 KJ/mol, which further confirmed the endothermic behavior of XSBLAC. The positive value of ∆S0 (0.239 J/K/mol) may be attributed to the affinity of XSBLAC for zinc(II) and the increasing randomness at the solid-liquid interface. It can be concluded that the adsorption of XSBLAC was an endothermic and spontaneous process based on the thermodynamic data of the adsorption isotherm. Many thermodynamic properties of polymer-solutions such as solubilities, swelling equilibria, and the colligative properties can be expressed in terms of the polymer-solvent interaction parameter. Generally, the Flory–Huggins interaction parameter describes the degree of segregation in polymer blends and block copolymers, which is typically expressed as a function of temperature, T. To establish a relationship between χ and T, we follow the approach of Rana et al. and calculate using Equations (13)–(15) [36–38]. ∆G0 = ∆H 0 − T∆S0 (13) ∆G0 = RT (

φ1 φ2 ln φ1 + ln φ2 + χφ1 φ2 ) N1 N2 α χ= +β T

(14) (15)

where φ1 is the volume fraction of XSBLAC particles, φ2 is the volume fraction of zinc(II), N1 is the molecular volume of XSBLAC particles, N2 is the molecular volume of zinc(II), χ is the F-H interaction parameter, T is the temperature, α and β are parameters for enthalpic and entropic contributions to χ, respectively. χ has been found to decrease with temperature with a dependence that is approximately linear with, but not proportional to 1/T for many systems. In this work, the χ with a value of 0.34 is extrapolated and obtained, which is representative of a miscible system and an exothermic heat of XSBLAC and zinc(II). It should be noted that the χ value is very close to zero, suggesting more favorable mixing at the temperature of 60 ◦ C, which more conducive to the adsorption process between XSBLAC and zinc(II). 3.7. Desorption and Regeneration Performing desorption and regeneration studies of spent adsorbents can produce various benefits, such as lowering the cost associated with the adsorption process, recovering valuable zinc(II), and reducing possible secondary pollution. Thus, a desorbing solution must be inexpensive, effective, and non-polluting. Batches of desorption and regeneration experiments were conducted in the present work using HCl, H2 SO4 , HNO3 , H3 PO4 , C2 H5 OH and NaOH as the desorbing eluent to test their effects on desorption (Figure 10a). Figure 10a clearly shows that NaOH is almost ineffective at releasing bonded zinc(II) ions from XSBLAC. The desorption efficiency of C2 H5 OH was slightly higher than that of NaOH but still showed weak potential for detaching zinc(II) compared with acid solutions. Among the four acidic desorption solutions, HCl, H2 SO4 , HNO3 and H3 PO4 , and HNO3 were found to be good reagents for the regeneration of zinc(II)-loaded XSBLAC. This result was expected because under acidic conditions, electrostatic interactions occurred on the surface of XSBLAC, which was became protonated by H+ ions, thereby allowing the desorption of positively charged zinc(II). The desorption efficiency of HNO3 was better than those of HCl, H2 SO4 and H3 PO4 for zinc(II) removal from XSBLAC. The effect of using HNO3 solution as a desorption reagent on the desorption capacity of zinc(II)-loaded XSBLAC, including the HNO3 concentration, desorption temperature and ultrasonic desorption time, is discussed in detail. The desorption capacity initially increased and then decreased with increasing HNO3 concentration (Figure 10b), possibly because the accumulated H+ concentration increases the concentration gradients of zinc(II) and H+ , which constitute the driving force underlying ion exchange and favoring the desorption process [39]. The desorption capacity first increased and then slightly decreased with increasing desorption temperature (Figure 10c); this behavior is likely

Materials 2017, 10, 1002 Materials 2017, 10, 1002

13 of 18 13 of 19

attributable to the higher temperature, which may enhance the efficiency and activity of the attributable to the higher temperature, which may enhance the efficiency and activity of the adsorption adsorption sites of XSBLAC until equilibrium is reached [40]. The desorption capacity initially sites of XSBLAC until equilibrium is reached [40]. The desorption capacity initially increased and increased and then remained constant with increasing ultrasonic desorption time (Figure 10d), then remained constant with increasing ultrasonic desorption time (Figure 10d), consistent with the consistent with the production of holes, and then reached saturation under ultrasonic conditions [41]. production of holes, and then reached saturation under ultrasonic conditions [41]. Subsequently, the Subsequently, the effect of regenerating and reusing the XSBLAC for five consecutive effect of regenerating and reusing the XSBLAC for five consecutive adsorption/desorption cycles adsorption/desorption cycles of zinc(II) was investigated, and the results are listed in Table 5. The of zinc(II) was investigated, and the results are listed in Table 5. The zinc(II)-adsorption capacity zinc(II)-adsorption capacity decreased only slightly with increasing reuse times, and the desorbed decreased only slightly with increasing reuse times, and the desorbed XSBLAC was effective for XSBLAC was effective for re-adsorption in the fourth cycle under the experimental conditions used. re-adsorption in the fourth cycle under the experimental conditions used. Thus, XSBLAC presents Thus, XSBLAC presents excellent reusability and can be employed as a suitable reagent for the excellent reusability and can be employed as a suitable reagent for the removal of zinc(II) from removal of zinc(II) from wastewater numerous times. wastewater numerous times. 80

Desorption capacity (mg/g)

Desorption capacity (mg/g)

80

(a)

70 60 50 40 30 20

75

70

65

60

10 0

HCl

0.1

H2SO4

0.3

0.4

0.5

HNO3 concentration (mol/L)

80

(c)

75 75

Desorption capacity (mg/g)

Desorption capacity (mg/g)

0.2

H3PO4 C2H5OH NaOH

HNO3

80

70 65 60 55 25

(b)

(d)

70 65 60 55 50 45

30

35

40

45

50

55

60

65

70

75

20

30

Temperature (oC)

40

50

60

70

80

Time (min)

Figure Figure 10. 10. Effects Effects of of various various desorption desorption reagents reagents (a); (a); HNO HNO33 concentration concentration (b); (b); temperature temperature (c) (c) and and time (d) on desorption capacity of XSBLAC for zinc(II). time (d) on desorption capacity of XSBLAC for zinc(II). Table Table 5. 5. Adsorption/desorption Adsorption/desorptioncapacities capacitiesof ofzinc(II) zinc(II)on onXSBLAC XSBLACafter afterfive five consecutive consecutive cycles. cycles. Recycle Times Recycle Times Adsorption capacity (mg/g) Adsorption capacity (mg/g) Desorption capacity (mg/g) Desorption capacity (mg/g)

1st 1st 103.82 103.82 78.52 78.52

2nd 2nd 94.06 94.06 73.31 73.31

3rd 3rd 92.62 92.62 71.05 71.05

4th 4th 80.83

5th 5th 42.44 80.8360.76 42.44 19.17 60.76 19.17

3.8. Adsorption Mechanism 3.8. Adsorption Mechanism The surface functional groups of the adsorbent highly influence its characteristics and The surface functional of the adsorbent highly its characteristics and adsorption adsorption capabilities. Thegroups FTIR spectra of XSBLAC afterinfluence the adsorption and desorption of zinc(II) capabilities. spectra of absorbance XSBLAC after the adsorption desorption of zinc(II) are presentedThe in FTIR Figure 11. The bands of XSBLACand (Figure 11a) were foundare to presented include a in Figure 11. The absorbance bands of XSBLAC (Figure 11a) to include overlapping broad overlapping band at 3200–3500 cm−1, with a peak at were 3422 found cm−1, which can abebroad attributed to the −1 , with a peak at 3422 cm−1 , which can be attributed to the O–H stretching band at 3200–3500 cm O–H stretching of the hydroxyl groups and intermolecular hydrogen bonds. The position and of the hydroxyl groups and intermolecular Thebonds. position asymmetry this asymmetry of this band implied the presencehydrogen of strong bonds. hydrogen Thisand peak shifted to of lower band number implied 3420 the presence strong hydrogen Thisofpeak shifted to lower wave number wave cm−1 andof weakened after the bonds. adsorption zinc(II) (Figure 11b), indicating that 1 and weakened after the adsorption of zinc(II) (Figure 11b), indicating that the O–H and the 3420O–H cm−and the the corresponding hydrogen bonds of XSBLAC were broken to react with zinc(II). The −1 were corresponding hydrogen bonds of XSBLAC broken to react with zinc(II). The bands at 11a) 2452 and and bands at 2452 and 2350 cm related towere the C–H stretching of methyl groups (Figure nearly disappeared after adsorption of zinc(II) ions (Figure 11b). The intensity of the peak at

Materials 2017, 10, 1002

14 of 18

Materials 2017, 10, 1002 2350 cm−1 were related

14 of 19

to the C–H stretching of methyl groups (Figure 11a) and nearly disappeared −1 approximately cm−1 (Figure 11a) 11b). revealed characteristics ofatthe C=O stretching after adsorption1670 of zinc(II) ions (Figure The the intensity of the peak approximately 1670incmthe carboxylic acid which were to lowered number and greatly (Figure 11a)organic revealed thegroups, characteristics of theshifted C=O stretching inwave the carboxylic organic acidreduced groups, after adsorption (Figurewave 11b).number Logically, the peaksreduced appearing at 1184 andadsorption 1127 cm−1 (Figure 11b). 11a) whichzinc(II) were shifted to lowered and greatly after zinc(II) 1 (Figure were assigned to P=O groups at because H3PO 4 was as the activating agent, to leading to the Logically, the peaks appearing 1184 and 1127 cm−used 11a) were assigned P=O groups −1 (Figure formation phosphate acid esters [42]. Furthermore, the band approximately 1043 cmacid because H3ofPO as the activating agent, leading to theat formation of phosphate esters 11a) [42]. 4 was used − 1 was ascribed to C–Oatstretching in saturated aliphatic groups, and ascribed –C–O and Furthermore, thethe band approximately 1043 cm (Figure 11a) was to –O–C=O the C–O stretching of the carboxyl groups, whichand was–C–O broadened and reduced after of adsorption of zinc(II) in saturated aliphatic groups, and –O–C=O stretching the carboxyl groups,(Figure which 11b) was −1 [43]. Adsorption bands that at of approximately 968, 815, andAdsorption 764 cm were related to C–H broadened and reduced afteroccurred adsorption zinc(II) (Figure 11b) [43]. bands that occurred −1 may be assigned to the groups of the benzene ringand and aromatic alcohols, the peak at 560 cmbenzene at approximately 968, 815, 764 cm−1 were relatedand to C–H groups of the ring and aromatic − 1 outer-of-plane bending of560 C–O ofbe alcohols; these peaks shrank afterbending adsorption (Figure 11b) alcohols, and the peak at cmbonds may assigned to the outer-of-plane of C–O bonds of [44]. In addition, the FTIR spectrum of XSBLAC after11b) the [44]. desorption of zinc(II) ions (Figure 11c) alcohols; these peaks shrank after adsorption (Figure In addition, the FTIR spectrum of almost coincided that ofof the original XSBLAC 11a).coincided These changes in the absorption XSBLAC after the with desorption zinc(II) ions (Figure (Figure 11c) almost with that of the original peaks indicated theThese activated sites on the absorption surface of XSBLAC contained hydroxyl and carboxylic XSBLAC (Figurethat 11a). changes in the peaks indicated that the activated sites on functional and contained the free carboxyl groups became carboxylates, which suggested during the the surfacegroups, of XSBLAC hydroxyl and carboxylic functional groups, and the free carboxyl adsorption reaction between zinc(II) ions and XSBLAC. Generally, ion exchange occurred, and groups became carboxylates, which suggested during the adsorption reaction between zinc(II) ions chemical bondsGenerally, were formed between zinc(II) andand the chemical –OH, –C=O, andwere –O–C=O groups of XSBLAC. and XSBLAC. ion exchange occurred, bonds formed between zinc(II) Above FTIR spectra comparison showed that XSBL-AC may bespectra applicable as an effective and theall, –OH, –C=O, and –O–C=O groups of XSBLAC. Above all, FTIR comparison showed renewable adsorber of applicable zinc(II) from wastewater. that XSBL-AC may be as an effective renewable adsorber of zinc(II) from wastewater.

Figure Figure 11. 11. FTIR FTIR spectra spectra of of XSBLAC XSBLAC (a); (a); after after adsorption adsorption (b) (b) and and after after desorption desorption (c) (c) of of zinc(II). zinc(II).

The surface morphologies of XSBLAC before and after zinc(II) adsorption are presented in The surface morphologies of XSBLAC before and after zinc(II) adsorption are presented in Figure 12. From Figure 12a, it can be seen that the surface of XSBLAC contained many thin sheets Figure 12. From Figure 12a, it can be seen that the surface of XSBLAC contained many thin sheets and layers of irregular shapes and sizes with steps and broken edges, which may be an appropriate and layers of irregular shapes and sizes with steps and broken edges, which may be an appropriate structure for zinc(II) adsorption. After the adsorption process, the XSBLAC surface was evenly structure for zinc(II) adsorption. After the adsorption process, the XSBLAC surface was evenly packed with zinc(II) ions and the sheet-stacking structure disappeared (Figure 12b). Distributions of packed with zinc(II) ions and the sheet-stacking structure disappeared (Figure 12b). Distributions the protruding tips are not evenly spread, hinting that zinc(II) was adsorbed on the activated sites of the protruding tips are not evenly spread, hinting that zinc(II) was adsorbed on the activated which mainly existed on the protruding tips of the microporous surface, and only selected functional sites which mainly existed on the protruding tips of the microporous surface, and only selected groups are involved in the adsorption of zinc(II) ions. These results indicated that the adsorption of functional groups are involved in the adsorption of zinc(II) ions. These results indicated that the XSBLAC toward zinc(II) may be a chemical interaction, supporting the previously proposed adsorption of XSBLAC toward zinc(II) may be a chemical interaction, supporting the previously adsorption mechanism. proposed adsorption mechanism.

Materials 2017, 10, 1002

15 of 18

Materials 2017, 10, 1002 Materials 2017, 10, 1002

15 of 19 15 of 19

Figure of zinc(II). zinc(II). Figure 12. 12. SEM SEM images images of of XSBLAC XSBLAC before before (a) (a) and and after after (b) (b) adsorption adsorption of Figure 12. SEM images of XSBLAC before (a) and after (b) adsorption of zinc(II).

The EDX is an analytical technique used for elemental analysis. EDX analysis of XSBLAC was The EDX analyticaltechnique techniqueused usedfor forelemental elementalanalysis. analysis.EDX EDX analysis XSBLAC was The EDX is isanananalytical analysis ofofXSBLAC was performed to confirm existence of zinc(II) on zinc(II)-loaded XSBLAC. The elemental compositions performedtotoconfirm confirmexistence existenceof ofzinc(II) zinc(II) on on zinc(II)-loaded zinc(II)-loaded XSBLAC. The performed The elemental elemental compositions compositionsof of XSBLAC before and after zinc(II) adsorption were analyzed, and the results are presented in Figure ofXSBLAC XSBLACbefore beforeand andafter afterzinc(II) zinc(II)adsorption adsorptionwere wereanalyzed, analyzed,and andthe theresults resultsare arepresented presentedininFigure Figure13. 13. The EDX results of XSBLAC (Figure 13a) revealed the presence of C (80.26%), O (12.92%), N EDX results of XSBLAC (Figure 13a)13a) revealed the presence of C (80.26%), O (12.92%), N (5.62%), 13.The The EDX results of XSBLAC (Figure revealed the presence of C (80.26%), O (12.92%), N (5.62%), Cl (0.75%) and Na (0.45%). After zinc(II) adsorption, two new peaks of zinc(II) were found Cl (0.75%) and Na (0.45%). After zinc(II) adsorption, two new peaks of zinc(II) were found in the EDX (5.62%), Cl (0.75%) and Na (0.45%). After zinc(II) adsorption, two new peaks of zinc(II) were found in the EDX spectrum (Figure 12b); the at % values of the zinc(II)-loaded XSBLAC were C (77.76%), O (Figure 12b); the at % values zinc(II)-loaded XSBLAC were C (77.76%), (10.73%), inspectrum the EDX spectrum (Figure 12b); the at of % the values of the zinc(II)-loaded XSBLAC were CO (77.76%), ON (10.73%), N (5.21%), Zn (5.88%), Cl (0.31%) and Na (0.11%). This result verified the presence of zinc(II) (5.21%),N Zn(5.21%), (5.88%),Zn Cl(5.88%), (0.31%)Cl and Na (0.11%). resultThis verified presence zinc(II)of ions on the (10.73%), (0.31%) and NaThis (0.11%). resultthe verified the of presence zinc(II) ions on the surface of XSBLAC after adsorption [45]. From the SEM-EDX results, it was concluded surface of surface XSBLACofafter adsorption [45]. From the it was concluded XSBLAC ions on the XSBLAC after adsorption [45].SEM-EDX From theresults, SEM-EDX results, it wasthat concluded that XSBLAC could to adsorb zinc(II) from wastewater. could to adsorb zinc(II) fromzinc(II) wastewater. that XSBLAC could to adsorb from wastewater.

Figure 13. EDX analysis of XSBLAC before (a) and after (b) adsorption zinc(II). Figure EDX analysis XSBLAC before and after adsorption zinc(II). Figure 13.13. EDX analysis of of XSBLAC before (a)(a) and after (b)(b) adsorption zinc(II).

3.9. Comparison with Previously Reported Data for Zinc(II) Adsorption 3.9. Comparison with Previously Reported Data Zinc(II) Adsorption 3.9. Comparison with Previously Reported Data forfor Zinc(II) Adsorption Table 6 shows various adsorbents that have been applied in removing zinc(II) from aqueous Table6 6shows showsvarious variousadsorbents adsorbentsthat thathave havebeen beenapplied appliedininremoving removingzinc(II) zinc(II)from fromaqueous aqueous Table solution, as reported in the previous literature, for comparison purposes. Through the comparative solution, reported theprevious previousliterature, literature,for forcomparison comparisonpurposes. purposes.Through Through thecomparative comparative solution, asas reported ininthe the study, we can conclude that XSBLAC is one of the most powerful adsorbents prepared for industrial study,we wecan canconclude concludethat thatXSBLAC XSBLAC one the most powerful adsorbents prepared industrial study, is is one ofof the most powerful adsorbents prepared forfor industrial wastewater treatment. wastewatertreatment. treatment. wastewater

Materials 2017, 10, 1002

16 of 18

Table 6. Comparison of qmax for removal of zinc(II) by various adsorbents. Adsorbent Material

qmax (mg/g)

Reference

XSBLAC SWCNTs MWCNTs PAV Sil/PE1/GA0.5 Frontinalis antipyretica PWS Coir

103.82 43.66 32.68 13.04 32.79 14.7 82 8.6

This study [46] [46] [46] [47] [48] [49] [50]

4. Conclusions In summary, a novel XSBLAC was successfully prepared and employed in solution for the removal of zinc(II) ions. The investigated adsorbent was inexpensive and eco-friendly. The experimental results showed that XSBLAC has excellent adsorption capacity for zinc(II) ions from aqueous solutions, which can be attributed to its high surface area (688.62 m2 /g), total pore volume (0.377 cm3 /g), O-containing functional activated groups, and electrostatic attraction to zinc(II). Batches of adsorption tests were conducted, and the experimental values obtained were found to be in good agreement with those predicted by the models. The adsorption process was well described by the pseudo-second-order model. The adsorption equilibrium data of zinc(II) on the surface of XSBLAC was suitable for the Langmuir model (maximum monolayer adsorption capacity of 103.82 mg/g), and thermodynamic studies revealed the spontaneous and endothermic nature of the adsorption process. XSBLAC was shown to exhibit an effective desorption performance when washed with an HNO3 solution as an eluent, and the maximum desorption capacity obtained was 78.52 mg/g. Additionally, the adsorption capacity after regeneration was even higher until four adsorption/desorption cycles. Furthermore, FTIR and SEM/EDX results demonstrated that zinc(II) ions were mainly adsorbed by selected activated groups in coarse mesopores on the XSBLAC surface; the adsorption mechanism was likely a chemisorption process. Overall, the XSBLAC prepared in this study has potential applications as an alternative and low-cost adsorbent for the removal of zinc(II) ions from aqueous solutions. Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (21467021); Leading Program of Science and Technology of Inner Mongolia (2016-fiance), Leading Program of Science and Technology of Inner Mongolia (20140401), and Leading Program of Science and Technology of Inner Mongolia (20140609); Project of Grasslands Outstanding Person of Inner Mongolia (2013) and Program of R&D Team Building of Inner Mongolia (2014). Author Contributions: Z.X.T. and W.X.M. conceived and designed the experiments; Z.X.T. and H.Y.N. performed the experiments; Z.X.T. and W.X.M. analyzed the data; W.X.M. and C.Z.J. contributed reagents/materials tools; Z.X.T., H.Y.N., W.X.M. and C.Z.J. wrote the paper. Conflicts of Interest: The authors declare that they have no competing interests.

References 1. 2.

3.

4. 5.

Hawari, A.; Rawajfih, Z.; Nsour, N. Equilibrium and thermodynamic analysis of zinc ions adsorption by olive oil mill solid residues. J. Hazard. Mater. 2009, 168, 1284–1289. [CrossRef] [PubMed] Lalhruaitluanga, H.; Jayaram, K.; Prasad, M.N.; Kumar, K.K. Lead(II) adsorption from aqueous solutions by raw and activated charcoals of Melocanna baccifera Roxburgh (bamboo)—A comparative study. J. Hazard. Mater. 2010, 175, 311–318. [CrossRef] [PubMed] Wahby, A.; Abdelouahab-Reddam, Z.; El Mail, R.; Stitou, M.; Silvestre-Albero, J.; Sepúlveda-Escribano, A. Mercury removal from aqueous solution by adsorption on activated carbons prepared from olive stones. Adsorption 2011, 17, 603–609. [CrossRef] Shinde, N.R.; Bankar, A.V.; Kumar, A.R.; Zinjarde, S.S. Removal of Ni(II) ions from aqueous solutions by biosorption onto two strains of Yarrowia lipolytica. J. Environ. Manag. 2012, 102, 115–124. [CrossRef] [PubMed] Minceva, M.; Fajgar, R.; Markovska, L.; Meshko, V. Comparative study of Zn2+ , Cd2+ , Pb2+ removal from water solution using natural clinoptilolitic zeolite and commercial granulated activated carbon equilibrium of adsorption. Sep. Sci. Technol. 2008, 43, 2117–2143. [CrossRef]

Materials 2017, 10, 1002

6.

7. 8.

9. 10. 11. 12.

13. 14.

15. 16.

17.

18. 19. 20. 21. 22. 23.

24. 25. 26. 27. 28.

17 of 18

Puzii, A.M.; Stavitskaya, S.S.; Poddubnaya, O.I.; Vikarchuk, V.M.; Tsyba, N.N. Structural and adsorption properties of active carbon from coconut shells modified with phosphorus heteroatoms. J. Therm. Anal. Calorim. 2016, 124, 1–16. [CrossRef] Lin, J.; Wu, Z.; Tseng, W. Extraction of environmental pollutants using magnetic nanomaterials. Anal. Methods 2010, 2, 1874–1879. [CrossRef] Depci, T.; Kul, A.R.; Önal, Y. Competitive adsorption of lead and zinc from aqueous solution on activated carbon prepared from Van apple pulp: Study in single- and multi-solute systems. Chem. Eng. J. 2012, 200, 224–236. [CrossRef] Linares-Solano, A.; López-González, J.D.D.; Molina-Sabio, M.; Rodriguez-Reinoso, F. Active carbons from almond shells as adsorbents in gas and liquid phases. J. Chem. Technol. Biotechnol. 2010, 30, 65–72. [CrossRef] Kikuchi, Y.; Qian, Q.; Machida, M.; Tatsumoto, H. Effect of ZnO loading to activated carbon on Pb(II) adsorption from aqueous solution. Carbon 2006, 44, 195–202. [CrossRef] Bansode, R.R.; Losso, J.N.; Marshall, W.E.; Rao, R.M.; Portier, R.J. Adsorption of metal ions by pecan shell-based granular activated carbons. Bioresour. Technol. 2003, 89, 115–119. [CrossRef] Nakagawa, Y.; Molina-Sabio, M.; Rodríguez-Reinoso, F. Modification of the porous structure along the preparation of activated carbon monoliths with H3 PO4 and ZnCl2 . Microporous Mesoporous Mater. 2007, 103, 29–34. [CrossRef] Manjula, D.M. Heavy Metals Removal from Industrial Wastewater by Nano Adsorbent Prepared from Cucumis Melopeel Activated Carbon. J. Nanomed. Res. 2017, 5, 00102. [CrossRef] Cronje, K.J.; Chetty, K.; Carsky, M.; Sahu, J.N.; Meikap, B.C. Optimization of chromium (VI) sorption potential using developed activated carbon from sugarcane bagasse with chemical activation by zinc chloride. Desalination 2011, 275, 276–284. [CrossRef] Issabayeva, G.; Aroua, M.K.; Sulaiman, N.M. Removal of lead from aqueous solutions on palm shell activated carbon. Bioresour. Technol. 2006, 97, 2350–2355. [CrossRef] [PubMed] Rahman, M.M.; Awang, M.; Mohosina, B.S.; Kamaruzzaman, B.Y.; Nik, W.B.W.; Adnan, C.M.C. Waste palm shell converted to high efficient activated carbon by chemical activation method and its adsorption capacity tested by water filtration. APCBEE Procedia 2012, 1, 293–298. [CrossRef] Gonçalves, M.; Sánchez-García, L.; Oliveira Jardim, E.D.; Silvestre-Albero, J.; Rodríguez-Reinoso, F. Ammonia removal using activated carbons: Effect of the surface chemistry in dry and moist conditions. Environ. Sci. Technol. 2014, 45, 10605–10610. [CrossRef] Wang, X.; Hao, Y.; Ding, L.; Xue, Z. The Preparation Method and Application of Xanthoceras Sorbifolia Bunge Hull Activated Carbon Loaded with KOH. Chiana Patent ZL2011100844764, 17 January 2013. Romero-Anaya, A.J.; Lillo-Ródenas, M.A.; Lecea, S.M.D.; Linares-Solano, A. Hydrothermal and conventional H3 PO4 activation of two natural bio-fibers. Carbon 2012, 50, 3158–3169. [CrossRef] Zhang, J.; Wang, A. Adsorption of Pb(II) from aqueous solution by Chitosan-g-poly(acrylic acid)/Attapulgite/Sodium humate composite hydrogels. J. Chem. Eng. Data 2010, 55, 2379–2384. [CrossRef] Yang, J.G. Heavy metal removal and crude bio-oil upgrading from sedum plumbizincicola harvest using hydrothermal upgrading process. Bioresour. Technol. 2010, 101, 7653–7657. [CrossRef] [PubMed] Wang, Q.; Li, J.; Chen, C.; Ren, X.; Hu, J.; Wang, X. Removal of cobalt from aqueous solution by magnetic multiwalled carbon nanotube/iron oxide composites. Chem. Eng. J. 2011, 174, 126–133. [CrossRef] Myglovetsa, M.; Poddubnayaa, O.I.; Sevastyanovac, O.; Lindströmc, M.E.; Gawdzikd, B.; Sobiesiakd, M.; Tsybaa, M.M.; Sapsaye, V.I.; Klymchuke, D.O.; Puziy, A.M. Preparation of carbon adsorbents from lignosulfonate by phosphoric acid activation for the adsorption of metal ions. Carbon 2014, 80, 771–783. [CrossRef] Khan, M.A.; Ngabura, M.; Choong, T.S.; Masood, H.; Chuah, L.A. Biosorption and desorption of nickel on oil cake: Batch and column studies. Bioresour. Technol. 2012, 103, 35–42. [CrossRef] [PubMed] Horsfall, M.; Abia, A.A. Sorption of cadmium(II) and zinc(II) ions from aqueous solutions by cassava waste biomass (Manihot sculenta Cranz). Water Res. 2003, 37, 4913–4923. [CrossRef] [PubMed] Shahryari, Z.; Goharrizi, A.S.; Azadi, M. Experimental study of methylene blue adsorption from aqueous solutions onto carbon nano tubes. Int. J. Water Resour. Environ. Eng. 2010, 2, 016–028. Karagoz, S.; Tay, T.; Ucar, S.; Erdem, M. Activated carbon from waste biomass by sulfuric acid activation and their use on methylene blue adsorption. Bioresour. Technol. 2008, 99, 6214–6222. [CrossRef] [PubMed] Rodriguez, A.; Garcia, J.; Ovejero, G.; Mestanza, M. Adsorption of anionic and cationic dyes on activated carbon from aqueous solutions: Equilibrium and kinetics. J. Hazard. Mater. 2009, 172, 1311–1320. [CrossRef]

Materials 2017, 10, 1002

29. 30.

31. 32. 33.

34. 35. 36. 37. 38. 39.

40. 41. 42. 43. 44. 45.

46. 47. 48. 49. 50.

18 of 18

Ramalingam, S.; Parthiban, L.; Rangasamy, P. Biosorption Modeling with Multilayer Perceptron for Removal of Lead and Zinc Ions Using Crab Shell Particles. Arab. J. Sci. Eng. 2014, 39, 8465–8475. [CrossRef] Mishra, V.; Balomajumder, C.; Agarwal, V.K. Biosorption of Zn(II) onto the surface of non-living biomasses: A comparative study of adsorbent particle size and removal capacity of three different biomasses. Water Air Soil Pollut. 2010, 211, 489–500. [CrossRef] Fernandez, E.; Hugi-Cleary, D.; López-Ramón, M.V.; Stoeckli, F. Adsorption of phenol from dilute and concentrated aqueous solutions by activated carbons. Langmuir 2003, 19, 9719–9723. [CrossRef] Rozada, F.; Otero, M.; García, A.I.; Morán, A. Application in fixed-bed systems of adsorbents obtained from sewage sludge and discarded tyres. Dyes Pigment 2007, 72, 47–56. [CrossRef] Sandesh, K.; Kumar, R.S.; Jagadeeshbabu, P.E. Rapid removal of cobalt (II) from aqueous solution using cuttle fish bones; equilibrium, kinetics, and thermodynamic study. Asia-Pac. J. Chem. Eng. 2013, 8, 144–153. [CrossRef] Singha, B.; Das, S.K. Biosorption of Cr(VI) ions from aqueous solutions: Kinetics, equilibrium, thermodynamics and desorption studies. Colloids Surf. B Biointerfaces 2011, 84, 221–232. [CrossRef] [PubMed] Xu, D.; Tan, X.L.; Chen, C.L.; Wang, X.K. Adsorption of Pb(II) from aqueous solution to MX-80 bentonite: Effect of pH, ionic strength, foreign ions and temperature. Appl. Clay Sci. 2008, 41, 37–46. [CrossRef] Chakraborty, S.S.; Maiti, N.; Mandal, B.M.; Bhattacharyya, S.N. Miscibility and phase diagrams for poly(vinyl methyl ether) and polyacrylate blend systems. Polymer 1993, 34, 111–114. [CrossRef] Rana, D.; Mandal, B.M.; Bhattacharyya, S.N. Analogue calorimetry of polymer blends: Poly(styreneco-acrylonitrile) and poly(phenyl acrylate) or poly(vinyl benzoate). Polymer 1996, 37, 2439–2443. [CrossRef] Rana, D.; Mandal, B.M.; Bhattacharyya, S.N. Analogue Calorimetric Studies of Blends of Poly(vinyl ester)s and Polyacrylates. Macromolecules 1996, 29, 1579–1583. [CrossRef] Febrianto, J.; Kosasih, A.N.; Sunarso, J.; Ju, Y.H.; Indraswati, N.; Ismadji, S. Equilibrium and kinetic studies in adsorption of heavy metals using biosorbent: A summary of recent studies. J. Hazard. Mater. 2009, 162, 616–645. [CrossRef] [PubMed] Gill, R.; Mahmood, A.; Nazir, R. Biosorption potential and kinetic studies of vegetable waste mixture for the removal of nickel(II). J. Mater. Cycles Waste Manag. 2013, 15, 115–121. [CrossRef] Morsy, R. Synthesis and physicochemical evaluation of hydroxyapatite gel biosorbent for toxic Pb(II) removal from wastewater. Arab. J. Sci. Eng. 2016, 41, 2185–2191. [CrossRef] Daifullah, A.A.M.; Girgis, B.S. Impact of surface characteristics of activated carbon on adsorption of BTEX. Colloids Surf. A 2003, 214, 181–193. [CrossRef] Deng, L.; Su, Y.; Su, H.; Wang, X.; Zhu, X. Sorption and desorption of lead (II) from wastewater by green algae Cladophora fascicularis. J. Hazard. Mater. 2007, 143, 220–225. [CrossRef] Bulut, Y.; Tez, Z. Adsorption studies on ground shells of hazelnut and almond. J. Hazard. Mater. 2007, 149, 35–41. [CrossRef] [PubMed] Molina-Sabio, M.; Gonçalves, M.; Rodríguez-Reinoso, F. Oxidation of activated carbon with aqueous solution of sodium dichloroisocyanurate: Effect on ammonia adsorption. Microporous Mesoporous Mater. 2011, 142, 577–584. [CrossRef] Lu, C.Y.; Chiu, H.S. Adsorption of zinc(II) from water with purified carbon nanotubes. Chem. Eng. Sci. 2006, 61, 1138–1145. [CrossRef] Kathrine, C.; Hansen, H.C.B. Sorption of zinc and lead on coir. Bioresour. Technol. 2007, 98, 89–97. [CrossRef] Ghoul, M.; Bacquet, M.; Morcellet, M. Uptake of heavy metals from synthetic aqueous solutions using modified PE1-silica gels. Water Res. 2003, 37, 729–734. [CrossRef] Kargi, F.; Cikla, S. Biosorption of zinc(II) ions onto powdered waste sludge (PWS): Kientics and isotherm. Enzym. Microb. Technol. 2006, 38, 705–710. [CrossRef] Coleman, N.J.; Lee, W.E.; Slipper, I.J. Interactions of aqueous Cu2+ , Zn2+ and Pb2+ ions with crushed concrete fines. J. Hazard. Mater. 2005, 121, 203–213. [CrossRef] [PubMed] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).