Journal of Environmental Management 201 (2017) 268e276

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Research article

Synthesis of biochar from residues after biogas production with respect to cadmium and nickel removal from wastewater Aleksandra Bogusz a, Katarzyna Nowak b, Magdalena Stefaniuk a, Ryszard Dobrowolski c, Patryk Oleszczuk a, * a

Department of Environmental Chemistry, Faculty of Chemistry, Maria Skłodowska-Curie University, Maria Curie-Skłodowska Square 3, 20-031, Lublin, Poland Institute of Agrophysics, Polish Academy of Sciences, Doswiadczalna 4, 20-290, Lublin, Poland c Department of Analytical Chemistry, Faculty of Chemistry, Maria Skłodowska-Curie University, Maria Curie-Skłodowska Square 3, 20-031, Lublin, Poland b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 February 2017 Received in revised form 7 June 2017 Accepted 9 June 2017

The objective of the study was to investigate the ability of biochars prepared under different temperatures (400  C and 600  C) from the residue of biogas production (RBP) for the adsorption of cadmium (Cd(II)) and nickel (Ni(II)) ions from aqueous solution. Furthermore, the RBP biochars adsorption capacity was compared with adsorption capacity of biochar produced from wheat straw at 600  C (BCS600). The kinetics of the adsorption, the sorption isotherms, the influence of solution pH and the interfering ions (chlorides and nitrates) were investigated. The desorption of Cd(II) and Ni(II) by hydrochloric and nitric acid from biochars was also investigated. The different types of feedstock used for biochar (BC) preparation (RBP and biomass) determined the physico-chemical properties of biochars and hence their adsorption abilities. Generally, biochars produced from RBPs (regardless of temperature) had the greater capacity to adsorb Cd(II) and Ni(II) than the biochar produced from wheat straw. Of the tested models (Freundlich and Langmuir), the Langmuir model was demonstrated to be the best to describe the sorption of Cd(II) and Ni(II). For the kinetic study, the adsorption process proceeded the fastest for BCU400 than BCU600. Furthermore, BCU600 was the most resistant to the influence of interfering ions on adsorption. For the desorption study, BCU400 was characterized by the highest reproducibility of the surface. The comparison of the results obtained in each adsorption step between RBP biochars and BCS600 suggested that the residue from biogas production could be successfully applied for the removal of Cd(II) and Ni(II) ions from aqueous solutions. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Biochar Biogas residue Adsorption Heavy metals Interfering ions

1. Introduction The global demand for energy is growing, but natural sources of energy are limited. For this reason, new renewable technologies as new energy sources are being investigated. Common renewable sources of energy are organic residues, such as a feedstock for biogas production. Biogas is obtained during anaerobic decomposition, which is fermentation by microorganisms. In 2013, the European energy production from biogas reached 13.4 million tons of oil equivalents (Mtoe) and increased by approximately 10.2% in 2014 (Weiland, 2010).

* Corresponding author. Department of Environmental Chemistry, University of Maria Skłodowska-Curie, pl. M. Curie-Sklodowskiej 3, 20-031, Lublin, Poland. E-mail address: [email protected] (P. Oleszczuk). http://dx.doi.org/10.1016/j.jenvman.2017.06.019 0301-4797/© 2017 Elsevier Ltd. All rights reserved.

The increase of energy production in biogas plants is connected with an increase in the number of residues generated during biogas production (RBP). For example, Pilarski et al. (2008a,b) showed that the production of RBP is equal approximately 67 000 t per year (Pilarski et al., 2008a,b). Solutions are need for safe utilization of this large amount of residue. The most common method of RBP management is its application to the soil, because of high amounts of mineral nitrogen (mainly in the form of NH3), micro- and macroelements and organic matter (OM) which are necessary for plant growth. Recently, the conversion of organic residue (including RBP) to biochar became a very common method of organic residue utilization. It was shown that biochars produced from RBP contained less heavy metals and PAHs compared to raw material (Stefaniuk et al., 2016; Stefaniuk and Oleszczuk, 2015). Moreover, RBPbiochar was more stable, both in terms of toxic compoundsaromatic compounds and positive components-nutrients

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(Lehmann, 2007). In addition to the direct application of biochar produced from RBP to soil, it can also potentially be used as an adsorbent for the removal of heavy metals from water and wastewater (Inyang et al., 2012). To date, the most frequently used material for heavy metals removal from water and wastewater was activated carbon (AC) (Fu and Wang, 2011). Therefore, biochar (especially produced from waste materials) can replace conventional AC and become a cheap contaminant sorbent, combining many functions at the same time (Ahmad et al., 2014). Nickel and cadmium are ubiquitous heavy metals characterized by toxic and carcinogenic properties. In the literature, there were only a few reports concerning the application an et al., 2005; Kılıç et al., of biochar for the removal of nickel (Erdog 2013) and cadmium (Regmi et al., 2012; Xu et al., 2013) from water an et al. and wastewater. For example, Kılıç et al. (2013)and Erdog (2005) reported that the adsorption capacity of biochar produced from almond shells and from waste apricots to nickel were 22.22 mg/g and 101.01 mg/g, respectively. Moreover, there were reports (Regmi et al., 2012; Xu et al., 2013) regarding the adsorption of cadmium by switchgrass biochar (activated) (34 mg/g) and by dairy manure-derived biochar (51.4 mg/g). However, in the literature, there was little research on the application of biochar produced from RBP for the removal of heavy metals from water and wastewater. Inyang et al. (2012) studied the use of this type of biochar for the removal of heavy metal (Pb, Cu, Ni and Cd) mixtures from aqueous solution. The authors showed that biochars were effective in the removal a mixture of heavy metals from aqueous solutions, but a detailed investigation concerning the adsorption process only focused on lead. There is still a lack of data concerning the sorption mechanisms regarding Ni(II) and Cd(II) separately. Moreover, there is no information regarding the influence of pH solution, interfering ions and the desorption process, which can be highly important from a practical point of view. The aim of this study was to investigate the adsorption capacity of Ni(II) and Cd(II) ions to biochars produced from the RBPs. Biochars produced in two different temperatures (400  C and 600  C) were investigated. Additionally, RBPs biochar produced at 600  C was compared with biochar produced from wheat straw (a very common biomass used for biochar production) at the same temperature. The kinetics and the effect of pH and interfering ions (NO 3 and Cl), as well as the desorption of Ni(II) and Cd(II) were investigated in detail. 2. Materials and methods 2.1. Reagents The initial standard stock solutions of the studied heavy metal ions, Ni(II) and Cd(II) (1000 mg/L of each), were prepared by the dissolution of powdered Ni(NO3)2$6H2O and Cd(NO3)2$4H2O (POCH, Gliwice, Poland), respectively, in a redistilled water (Merck Millipore, Poland). The calibration curves of Ni(II) and Cd(II) ions were established using the standard solutions of Ni(II) and Cd(II) prepared in 0.5 mol/L HNO3 by proper dilution from stock solutions (1000 mg/L of each) (Merck, Darmstadt, Germany). Furthermore, nitric acid Suprapure (36%) (POCH, Gliwice, Poland) and sodium hydroxide solutions (Merck, Darmstadt, Germany) were applied for the pH adjustment of solutions. 2.2. Biochars Three different biochars were tested in the experiment. Two biochars, BCU400 and BCU600, were produced from RBP at the pyrolysis temperatures of 400  C and 600  C, respectively. Feedstock used for the BCUs pyrolysis (RBP) was produced during the

269

thermophilic fermentation from maize silage (55%), straw (15%), sugar beet bagasse (15%), pomace of fruit (10%) and manure (5%). The third biochar used in the experiment, BCS600, was produced from wheat straw at 600  C, and it was provided by Mostostal Sp. z.o.o. (Poland). Biochar BCS600 was used as a comparison to the biochars produced from RBP. It was shown (Bogusz et al., 2015) that BCS600 was characterized by a good sorption capacity to different heavy metals and could be used as a reference for the testing of other biochars produced from different feedstock. Before pyrolysis, RBPs were mixed and dried at 30e35  C for 7 days. Next, the samples were ground in a ceramic mortar and passed through a sieve of 2 mm. RBPs (35e75 g) were pyrolized in the furnace of own-construction (Stefaniuk and Oleszczuk, 2015) via slow pyrolysis. The rate of the pyrolysis heating was 25  C/min, and the maximum temperature was kept constant for 5 h. The oxygen-atmosphere was kept constant by a flow of nitrogen of 630 mL N2/min. Detailed information regarding the pyrolysis conditions were provided in previous research (Stefaniuk and Oleszczuk, 2015). Information concerning the methods used for physico-chemical characterization of biochars are presented in Internet Electronic annex. 2.3. Sorption experiments The effect of time on the adsorption of metal ions onto biochars was determined using time intervals, up to 24 h. The initial concentrations of Cd(II) and Ni(II) ions in the single element solutions were both 100 mg/L. The biochars masses were 0.2 g ± 0.03 g. The optimal pH was adjusted to 5.5 for both ions. The kinetics solutions were agitated on a shaker (Elpinþ, Poland) at a constant speed of 120 rpm at room temperature (22 ± 2  C). Next, the mixtures were filtered with 0.45 mm PTFE syringe filters (AlfaChem, Poland). The filtrates were analyzed for the heavy metals concentrations. The influence of pH on the adsorption of Ni(II) and Cd(II) ions onto BCs was investigated in the range of initial pH solutions from 2 to 7 for Cd(II) and from 2 to 8 for Ni(II). The initial concentrations of Cd(II) and Ni(II) ions and masses of BCs were 100 mg/L and 0.2 g (±0.03 g), respectively. The other adsorption conditions were the same as in the kinetic experiment. After 24 h, the adsorption and equilibrium pH were measured to select the optimal pH and to detect the changes in the solutions. The adsorption isotherms of Ni(II) and Cd(II) were performed in the single component systems at an initial pH of 5.5 adjusted by sodium hydroxide or nitric acid. The 0.2 g ± 0.03 g of each biochar was mixed with 50 mL solutions of either Cd(II) or Ni(II), in the range of concentrations from 5 mg/L to 500 mg/L. The solutions were shake on a shaker (120 rpm), with a temperature of 22 ± 2  C, for a specific period of contact time, which was determined on based on the kinetics study. After shaking, the samples were filtered, and the concentration of Cd(II) or Ni(II) was measured in the supernatant. 2.4. Effect of interfering ions The interferences (nitrites and chlorides) were determined at a pH of 5.5. Solutions consisted of either potassium nitrate (KNO3), from 0.001 M to 1 M, or sodium chloride (NaCl), from 0.001 M to 2 M, and Cd(II) or Ni(II). The concentration of Cd(II) or Ni(II) ions was kept constant (100 mg/L) in the KNO3 or NaCl solutions. The pH of the solutions was adjusted with concentrated sodium hydroxide or nitric acid. The other adsorption conditions were the same as in the kinetic experiment. The effect of interfering ions was evaluated by the comparison of the adsorption of Cd(II) or Ni(II) ions obtained for solutions without interfering ions with the results obtained in the experiment with interfering ions.

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2.5. Desorption experiment A desorption study, with increasing concentrations of HCl or HNO3, was also conducted. Biochars BCU400, BCU600 and BCS600, with known amounts of loaded Cd(II) or Ni(II) ions, were applied. BCs loaded with Cd(II) or Ni(II) were placed into Eppendorf probes (volume of 2 mL) and mixed with 2 mL of either hydrochloric acid or nitric acid. The concentration of desorptive agents was increased in each subsequent solution in the series, from 0 M to 9.5 M and from 0 M to 14.5 M, for HCl and HNO3, respectively. The suspensions were agitated in a shaker (120 rpm) at room temperature (22 ± 2  C) for 24 h. The final suspensions after the particular experiments (kinetics, isotherms, the influence of interfering ions, and desorption) were centrifuged and filtered. Then, the supernatant solution was analyzed for Ni(II) and Cd(II) ions using the flame atomic absorption spectrometer VARIAN Spectra AA-880 (Carl Zeiss, Jena, Germany), with a hollow cathode lamp HCL (Varian). Detailed information regarding the metals ions determination is presented in SI (Table S1). The obtained kinetics and isotherm data were analyzed using the models described in SI (Data analysis).

3. Results and discussion 3.1. Biochar properties The properties of the investigated biochars are presented in Table 1. Biochar BCU600, obtained from the same feedstock as BCU400 but at a higher temperature, was characterized by only a 6.5% higher SBET. The values of SBET of both biochars were in the range (1e50 m2/g) for biochars obtained from industrial organic wastes such as eucalyptus sawdust, activated sludge or cow manure (Xu et al., 2013; Shinogi and Kanri, 2003; Martins et al., 2007). However SBET values are smaller (50e500 m2/g) comparing to biochars produced from plant residues such as wood charcoal, sugarcane bagasse or wheat residue (Xu et al., 2013; Shinogi and Kanri, 2003; Chun et al., 2004). However, considering the higher values of the total pore volume (PV) and SBET, it was assumed that BCU600 had a higher microporous structure, which may have had an influence on the greater adsorption capacity of this biochar with respect to the studied metals (Chen et al., 2011). Moreover, BCU600 had higher CEC value than BCU400. The reference biochar, BCS600, exhibited the lowest pH and the highest PV, SBET and mineral ash content compared to the BCU biochars (Table 1). The differences in the physico-chemical properties of RBPs biochars and BCS600 closely correlated with the fact that BCS600 was obtained from different feedstock (wheat straw). The temperature of pyrolysis and type of feedstock used also had an influence on the elemental composition (C, H, N and O) and ratios of O/C, H/C and (OþN)/C of the investigated biochars (Table 1). Lower O/C, (OþN)/C and H/C values of BCU600 compared to BCU400 showed that BCU600 was more hydrophobic (lower O/

C), less polar (lower (OþN)/C) and more aromatic and carbonized (lower H/C) than BCU400. The comparison of properties of BCU biochars to the reference biochar BCS600 showed that BCS600 was the most hydrophobic and aromatic (the lowest O/C and H/C) and the least polar (the lowest (OþN)/C) of all of the investigated biochars. Generally, more aromatic and less polar biochars are produced at higher temperatures comparing to lower ones (Ahmad et al., 2012; Sun et al., 2011; Kim et al., 2013). 3.2. Spectroscopic analyses The FTIR spectra of BCU400 and BCU600 differed significantly (Fig. S1). In the spectrum of BCU400, there were more clear bands compared to BCU600. The FTIR spectra confirmed previous results, where the higher ratio (O þ N)/C was observed for BCU400 than BCU600, indicating the higher polarity of BCU400. The increased pyrolysis temperature decreased the intensity of the bands. The spectra of biochars BCU600 and BCS600 (produced during pyrolysis at the same temperature) were very similar. This result may suggest similar compositions of the biochars, which depended on the temperature of pyrolysis. The spectrum of biochar produced at a lower temperature (400  C) contained bands in the range of 4000 to 2500 cm1. The band at approximately 3300 cm1 corresponded to the stretching vibrations of OH groups (yOH), which was presented in phenols, water or alcohols (Chen et al., 2012). In addition, BCU400 had bands from the aromatic ring at ~1600 cm1 and at ~870 cm1, which proved the presence of carboxylic groups and phenol but also the presence of other aromatic compounds, respectively (Uchimiya et al., 2010). In conclusion, spectra FTIR did not distinguish the biochars due to feedstock. At a higher pyrolysis temperature (600  C), there were changes in the composition of the biochars, which were visible in the spectra. On the basis of Table S2 concerning XRD analysis, it can be assumed that BCU600 before metal adsorption consisted mainly of magnesium calcium carbonate (76 ± 2%), silicon oxide (12.8 ± 0.4%), calcium carbonate (6 ± 1%), copper sulfide (3.0 ± 0.3%) and potassium chloride (1.9 ± 0.2%). Furthermore, XPS results (Table S3) showed that in the composition of BCU600 carbon dominated (62.7At%), especially in form of C-C/C-H (61At%), and oxygen (21.4At%) in form of C]O (55.3At%). Moreover, the other elements were detected in the surface of BCU600, such as nitrogen (1.3At%), sodium (1.4At%), silicon (0.9At%), calcium (2.4At%), phosphorous (0.9At%), potassium (7.5At %) and chlorine (1.4At%). 3.3. Sorption kinetics The time of the equilibrium state reached for each examined biochar was different (Fig. 1). For Cd(II) ions, the equilibrium state was achieved after 4 h for BCU400 and 4.5 h for BCU600. The adsorption of Ni(II) proceeded and followed similar times for

Table 1 Physicochemical properties of BCU400, BCU600 and BCS600 (mean ± SD). Biochar

pH

CEC

Elemental composition C

H

Ash N

H/C

(OþN)/C

O/C

SBET

PV

O

BCU400 10.5 ± 0.03 7.42 ± 0.21 49.7 ± 1.15 2.8 ± 0.08 2.3 ± 0.15 15.0 ± 0.26 30.3 ± 2.31 0.7 ± 0.08 0.3 ± 0.06 0.2 ± 0.08 6.49 ± 0.06 0.011 ± 0.03 BCU600 12.2 ± 0.02 7.78 ± 0.18 51.3 ± 1.68 1.3 ± 0.1 2 ± 0.22 10.1 ± 0.17 35.4 ± 1.89 0.3 ± 0.1 0.18 ± 0.09 0.15 ± 0.11 6.8 ± 0.13 0.022 ± 0.06 BCS600 9.9 ± 0.02 6.12 ± 0.13 54 ± 2.43 1.8 ± 0.09 0.9 ± 0.14 2.3 ± 0.13 41.1 ± 2.41 0.033 ± 0.07 0.06 ± 0.1 0.043 ± 0.09 26.3 ± 0.11 0.026 ± 0.07 pH in KCl, CEC- cation exchange capacity [meq/100 g], CHNO e the contribution [%] of carbon, hydrogen, nitrogen and oxygen, Ash e ash content [%], H/C e ratio of hydrogen to carbon, (OþN)/C - polarity index, O/C e ratio of oxygen to carbon, SBET e specific surface area [m2/g], PV- pore volume [cm3/g].

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Fig. 1. Adsorption kinetics of Cd(II) and Ni(II) ions onto BCU400, BCU600 and BCS600; m ¼ 0.2 g ± 0.03 g, V ¼ 50 mL, Cions ¼ 100 mg/L, T ¼ 22 ± 2  C.

BCU400 and BCU600. In the case of BCS600, the adsorptions of both Cd(II) and Ni(II) by these biochars was faster than BCUs biochars and equal to 4 h and 2 h, respectively. Two primary steps were distinguished in the adsorptions of Cd(II) and Ni(II) by biochars. The first step of adsorption was fast and correlated with the accumulation of metal ions on the biochar surface. The second step of the adsorption process was slower, because most of the active sites were occupied by the molecules of adsorbate. Considering the rate of equilibration in terms of metal ions on the selected sorbents, the equilibrium state was reached faster for Ni(II). This was directly related to the radius of the hydrated Cd(II) and Ni(II) ions (Table S4). Hydrated Ni(II) ions had a smaller radius than Cd(II) ions; therefore, Ni(II) ions penetrated into the pores of biochars faster than Cd(II) ions. The other factor that may have affected the differences in the adsorption kinetic between Cd(II) and Ni(II) ions was the solubility of the metals hydroxide (M(OH)2). The lower value of the solubility of nickel hydroxide (Table S4) determined at the same time had a lower affinity to the solvent (water) and higher affinity to the surface of the biochars compared to cadmium hydroxide. On the contrary, cadmium hydroxide exhibited a higher value of the solubility and a lower affinity to the surface of BCS600, which extended the adsorption equilibrium for the Cd. The differences in kinetics between BCU400 and BCU600 were correlated with different physico-chemical characteristics of these biochars. The differences in the physico-chemical properties were mainly related to the different temperatures of pyrolysis. In the cases of BCU400 and BCU600, the time of reaching the equilibrium state was similar for Cd(II) (all BCU systems) and longer for BCU600 compared to BCU400 for Ni(II). BCU400 was characterized with a higher content of O-containing groups than BCU600, and this property conditioned the faster kinetics of metals (Li et al., 2017). On the contrary, low pore volume made pores less available for adsorbate ions, which reduced the speed of the process on BCU400 (Chen et al., 2008). In the case of BCU600, the situation was completely different. This material was characterized by a lower content of oxygen functional groups, but the access to them was much easier due to the greater volume of pores. Moreover, one hypothesis was that BCU600 obtained at a higher temperature of pyrolysis would have a higher ability to retain water, due to its greater microporosity. Thus, the contact between biochar and water was intensified, and this led to the increased adsorption capacity of the studied metals on BCU600, in comparison to BCU400 (Qian et al., 2015).

The comparison of the kinetics data between the investigated BCU biochars and reference biochar BCS600 (Fig. 1) showed that the equilibrium state was reached faster for BCS600 than both BCU biochars. First, the carbon and oxygen contents should be considered as important factors. BCS600 was characterized by the highest carbon content but also the lowest oxygen content. This result suggested that there was the lowest amount of O-containing groups on the BCS600 surface, which were mainly responsible for the adsorption process. This fact, in combination with the highest pore volume (PV), showed that the fastest adsorption of Cd(II) and Ni(II) ions onto BCS600 was associated with the high availability of a low amount of functional groups, which played a role in active centers. The comparison of these data with the highest value of SBET concluded that O-containing groups on BCS600 were mainly located on the surface, not within pores, but what exactly made them more available for metal ions is unknown. Furthermore, the differences between BCUs and BCS600 were explained by not only higher O-containing groups content but also by the surface pH of biochars. During the study, at pH 5 of the solution, Ni(II) and Cd(II) presented in the cationic forms of Cd2þ and Ni2þ. Considering the pH of biochars (Table 1), it is worth noting that BCU400 and BCU600 were more alkaline than BCS600, which was equal to the presence of a large amount of alkaline active centers on the RBPs biochars' surfaces. When the surfaces of biochars were negatively charged, Ni(II) and Cd(II) ions were more likely attracted to these surfaces. The increase in the negative surface charge density increased the size of the adsorption maximum of Ni(II) and Cd(II) ions on the surface of the carbon material. In turn, the increased time to achieve the equilibrium state reduced the availability of pores. The kinetics data were modeled using a pseudo-first-kinetic equation and a pseudo-second-kinetic-equation. In both metal ions, the best model to describe the kinetics was a PSO equation (R2  0.999) (Table 2, Fig. S2). The comparison of the parameter k2 for the respective systems of metal-biochar concluded that the adsorption process occurred according to the present order, the fastest for BCS600 and BCU400 and the slowest for BCU600. The regression coefficient obtained for the PSO model indicated that the process was based on the chemisorption (Mohan et al., 2011). Moreover, the theoretical adsorption capacities, aeq, were found to be in good agreement with those obtained experimentally, aexp,k (Table 2). Considering the adsorption process from the heavy metals point of view, the highest values of k2 were obtained for

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Table 2 Parameters for kinetics simulated by different equations. Metal

Biochar

Kinetic model Pseudo efirst order

Cd

Ni

BCU400 BCU600 BCS600 BCU400 BCU600 BCS600

aexp,k Pseudo e second order

k1

R2

k2

R2

aeq

0.003 0.0027 0.024 0.0048 0.0034 0.004

0.8117 0.6541 0.1637 0.5028 0.6061 0.7469

0.012 0.0009 0.044 0.0027 0.0017 0.007

1.000 0.999 1.000 1.000 0.999 1.000

36.36 39.22 17.92 27.1 31.35 16.26

36.06 38.72 17.92 27.77 30.88 16.15

aeq - the amount adsorbed at equilibrium time [mg/g], k1 [1/min] and k2 [g/mg$min] - the rate constants of the pseudo e first e order equation and pseudo e second e order equation, respectively, R e regression coefficient, aexp,k e maximal adsorption capacity achieved in the kinetic study [mg/g].

Ni(II), which confirmed that the kinetics of Ni(II) were faster than Cd(II).

3.4. Effect of solution pH The increase in the pH caused the increase in the adsorption capacity of Cd(II) and Ni(II) by all of the biochars (Fig. 2). The observed change resulted due to the modification of the groups on the biochar surface under different pH values. In a low pH (pH 2), the adsorption of Cd(II) and Ni(II) did not appear because of the protonation of surface groups by Hþ ions derived from the solution. In these conditions, the Hþ ions competed with heavy metal ions. The surface groups became deprotonated with the increase of the pH (from 2 to 5), and Cd(II) and Ni(II) ions were bound to the surface of the biochar. The change of pH also affected the ionic forms of the investigated compounds. In the strongly acidic pH ( 6, surface precipitation of the sorbent of the heavy metal ions, in the form of hydroxides (Ni(OH)2 and Cd(OH)2), may have occurred. However, this process was different from the adsorption, and it should be separately studied (Barczak et al., 2015). The optimum pH to the highest adsorption capacity of investigated metals by BCUs (BCU400 and BCU600) ranged from 5 to 6.5

for Cd(II) and from 4 to 6.5 for Ni(II). BCS600 had the same range as the RBPs biochars for Cd(II) and from 5 to 7.5 for Ni(II). These values were used to optimize the process of heavy metal sorption, and the optimum pH for all BCs-metal systems was recognized as pH 5. The curves of the percentage adsorption of Cd(II), as a function of pH, were similar for BCU400 and BCU600 (Fig. 2), which were produced from the same feedstock, but in different temperatures. Slightly different differences were observed between BCUs and BCS600, which confirmed that the mechanisms of Cd(II) adsorption by biochars produced from diverse feedstock were different. However, the curves of Cd(II) adsorption were similar for all the investigated BCs (Fig. 2). In the case of Ni(II), the adsorption by biochars obtained from various raw materials and at different temperatures of pyrolysis were comparable for all BCs-metal systems. There was clearly a rapid increase of adsorption at pH > 2 (except for BCS-Cd, which was at pH > 3). Speciation of studied metals versus pH was performed by using Visual MINTEQ version 3.1. The distribution of the chemical species of Cd and Ni in the various pH was presented in Fig. S3. Regarding to Cd species, the main form of this metal in the solution under different pHs was Cd2þ and above the pH about 3.5 CdOHþ and Cd(OH)2 also appeared. Similar results were obtained by Lee et al. (2012) and Merrikhpour and Jalali (2012), where the dominated form of Cd(II) under different pHs was Cd2þ. In the case of nickel, the predominant form was also positively charged Ni2þ and above pH of 4.5 also NiOHþ and Ni(OH)2 was observed. The presence of hydroxide species of studied metals suggested that at the experimental pH (5.5) adsorption might follow by precipitation.

3.5. Sorption isotherms All of the sorption isotherms were nonlinear (Fig. 3). The Langmuir model (LM) was the best model when compared to the Freundlich model (FM) (Table 3, Fig. S4). The application of LM had the highest coefficient of determination (R2) for both Cd and Ni (Table 3). The good fit to the LM indicated that the adsorption of metal ions could be considered as a monolayer process if the surface consists of a finite number of identical (homogenous) adsorption centers. The calculated values of am and KL (Table 3) exhibited an upward trend in the following order: BCS600 < BCU400 < BCU600. Moreover, am and KL values confirmed that the BCU600 showed the highest adsorption capacity for both Cd(II) and Ni(II) among the investigated biochars. The am values were not equal to aexp,i, which meant that a

Fig. 2. The influence of initial pH on adsorption of Cd(II) and Ni(II) ions onto BCU400, BCU600 and BCS600; m ¼ 0.2 g, V ¼ 50 mL, Cions ¼ 100 mg/L, t ¼ 24 h, T ¼ 22 ± 2  C.

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Fig. 3. Initial runs of adsorption isotherms of Cd(II) and Ni(II) ions onto BCU400, BCU600 and BCS600 fitted to LM; m ¼ 0.2 g ± 0.03 g, V ¼ 50 mL, T ¼ 22 ± 2  C, t ¼ 24 h.

Table 3 Parameters of heavy metal ions sorption isotherms fitted with Langmuir and Freundlich models. Metal

Cd

Ni

Biochar

BCU400 BCU600 BCS600 BCU400 BCU600 BCS600

Langmuir

aexp,i

KL

am

R2

0.04 0.105 0.043 0.112 0.146 0.095

68.63 76.34 32.57 27.86 34.84 17.67

0.9418 0.9334 0.9896 0.9937 0.9953 0.9865

66.49 72.7 30.88 27.19 34.21 16.59

Freundlich K

1/n

R2

0.063 0.209 0.064 0.073 0.174 0.097

1.83 4.05 0.84 1.77 2.62 1.87

0.835 0.266 0.835 0.631 0.782 0.678

am - the maximal theoretical adsorbed amount (sorption capacity) [mg/g], KL - the Langmuir constant - the quasi Gaussian energetic heterogeneity of the adsorption system, R e regression coefficient, aexp,i - the maximal experimental amount adsorbed at equilibrium time [mg/g], K, n e empirical constants indicative of sorption capacity and sorption intensity.

monolayer was not accomplished, because the adsorption capacity obtained experimentally (aexp,i) was lower than the calculated adsorption capacity (am) (Table 3). A possible reason for these differences could be related to the adsorption of interferences on the active sites on the surfaces of the biochars. A good fit with the LM was in good agreement with the studies of other authors (Demirbas, 2002; Hasar, 2003; Kadirvelu, 2001;  ska et al., 2012; Meena et al., 2005). For example, Kołodyn Demirbas (2002) tested a biochar obtained from hazelnut shells with respect to Ni(II) ions, and the best fit of the isotherm data was observed for the LM. Adsorption of Ni(II) was also performed by Hasar (2003) on the almond husk biochar that was activated and non-activated by H2SO4, and the best fitting was determined for the  ska et al. LM. A pig and cow manure were examined by Kołodyn (2012) regarding the adsorption of Cd(II) ions. In this study, the LM better described the obtained data than other models. The adsorption capacities obtained in the current experiment were similar or higher to those presented by other authors (Table S5). The differences between the results reported in other works were probably related with the differences in the structures and chemical compositions between the tested materials. A possible mechanism of adsorption of Cd(II) and Ni(II) is complex. Three primary mechanisms can be considered  ska et al., 2012): (1) the binding to the surface groups (Kołodyn containing oxygen (surface chemistry); (2) the mineral

precipitation of hydroxides and co-precipitation in form of carbonates and phosphates on the biochar surface and pores and (3) ion exchange. However, determining the predominant mechanism depends on the type of biochar. For a biochar with a high content of mineral ash and a lower content of O-containing groups (e.g., BCS600 and BCU600), the primary mechanism of adsorption was likely the surface precipitation. For a biochar that has a significant amount of oxygen groups on the surface (e.g., BCU400), these groups were mainly involved in the process of adsorption. A correlation analysis was performed to better understand the sorption mechanism of the studied metal ions and results are presented in Electronic annex (Fig. S5, Table S6). A few strong relationships were observed when considering data presented. For Cd, the strongest relationship was observed for the O/C ratio versus adsorption capacity and SBET versus adsorption capacity. These results suggested that the adsorption of Cd(II) ions was controlled by surface complexation to O-containing functional groups and physical adsorption. In the case of Ni, a significant relationship was observed between the maximum adsorption capacity and the pH of biochar. It can be assumed that the mechanism for the adsorption of Ni(II) ions included surface precipitation and/or electrostatic repulsion. Moreover, the high pH of biochar and the significant content of mineral ash also showed that the adsorption of the studied metals could proceed as a cation p e interaction. To specify the adsorption mechanism of Cd and Ni post-sorption analyses including FTIR, XPS and XRD were performed. The mechanism of the sorption of Ni(II) and Cd(II) was complex. XRD analysis (Table S2) suggested that adsorption of Ni and Cd on BCU600 proceeded via precipitation with the creation of carbonates, e.g. CdCO3 and NiCO3. It can be also confirmed by FTIR analysis (Fig. S6) and shift of peak at about 1400 cm1 to the higher wavenumbers. In the case of XPS (Table S3), the decrease in the content of carbonates for BCU600-Cd and BCU600-Ni in comparison to native BCU600 was observed, which confirmed that CO2 3 ions participated in the adsorption of Cd(II) and Ni(II). The precipitation with carbonates was also observed by other authors (Trakal et al., 2014; Cui et al., 2016; Zhang et al., 2015). Moreover, taking into account the content of Na, Ca and K before and after the adsorption of Cd and Ni in the XPS data (Table S3) it can be assumed that the amount of the alkali elements decreased (Na, K, Ca in the case of Ni), what suggests the ion exchange mechanism of adsorption (Ahmad et al., 2014). The value of CEC (Table 1) also

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confirm these observations. The comparison between the increase of the metal phosphate content (Table S3) and the decrease of the intensity of band at about 1250 cm1 (associated with vibration of P]O) showed that adsorption can also proceeded with precipitation in form of phosphoric compounds. It was also confirmed for Ni(II) regarding to XRD results (Table S2). It was also noted that for Ni(II) and Cd(II), adsorption process runs via surface precipitation in form of hydroxides (Table S3). Furthermore, in the FTIR spectra (Fig. S6) the changes suggested surface complexation of metal ions with the Ocontaining groups. Complexation by O-containing functional groups was confirmed in the other reports (Sun et al., 2014; Wan et al., 2014). Furthermore, it was previously reported (Ahmad et al., 2014; Inyang et al., 2012) that the type of feedstock also had an influence on the adsorption of heavy metal ions. In the current study, the results indicated that biochars derived from RBP (BCU) that consisted of animal manure (in some part) had a significantly higher adsorption capacity than biomass - derived biochar (BCS600), especially for Cd(II). Xu et al. (2013) stated that the biochar derived from dairy manure exhibited a higher adsorption capacity than biochar obtained from rise husk with respect to Pb, Cu, Zn and Cd removal from aqueous solution. 3.6. Effect of potentially interfering ions The presence of interfering ions caused a decrease in the adsorption of the studied biochars (BCU400, BCU600, and BCS600), and the extent of this effect depended on the type of biochar and metal ions (Fig. 4). The exception was a system with all biochars and Cd(II) ions, where the nitrates did not affect the adsorption process. In other cases, the interfering ions affected the sorption even at low concentrations, and further changes in the interfering ions concentrations maintained the effect at the same level. Generally, all the studied biochars were more resistant to the presence of nitrates   (NO 3 ) than chlorides (Cl ). Cl were easily bound to the surface occupied by aliphatic carbonaceous groups, thus they had a greater effect on the adsorption capacity (Xu et al., 2013). Gupta and Ali (2004) also reported the influence of chlorides on adsorption of Pb and Cr by biochar form wastewater was greater than influence of nitrates. The pH of the investigated biochars ranged from 9.9 to 12.2. In these conditions, Cd(II) and Ni(II) ions may precipitate on the biochar surface as the hydroxides - Ni(OH)2(s) or Cd(OH)2(s), respectively (Fiol et al., 2006), blocking the active sites on the biochars surface (Ahmad et al., 2014; Karthikeyan et al., 1999) and thus reduce the impact of interfering ions. The results for BCU600 confirmed that is was characterized by the highest pH (pH 12.2), which was affected by interfering ions to a lesser extent than the other tested biochars. The competition between the interfering ions (chlorides and nitrates) and Ni(II) and Cd(II) for active sites on the surface of the sorbent caused less efficiency to remove heavy metals from solution (Bogusz et al., 2015). Thus, materials resistant to these ions are better adsorbents for heavy metals. In the case of the sorption of Ni(II), nitrate and chloride ions had the least impacts on the adsorption by BCUs biochar than BCS. This was due to the higher pH and higher polarity of these materials than the reference biochar from biomass (BCS600). 3.7. Desorption study Considering the reuse of biochars as an adsorption agent, the effect of the type and concentration of desorbing reagent must be investigated (Fig. 5). Moreover, relevant information about the

reversibility of the adsorption process of heavy metals ions on the examined materials would be obtained on the basis of the desorption study. Nitric acid was the most effective in the desorption of Ni and Cd from biochar. In the case of Cd(II) ions, a greater differentiation between the used reagents was observed. For BCU400, the type of the desorbing agent was not crucial, because both nitric acid(V) and hydrochloric acid achieved almost complete desorption. Therefore, an almost complete desorption of Cd ions (90e100%) from the BCU400 was achieved using 1 M of HCl, while the concentration of HNO3 needed to obtain total desorption was 10 M. A slightly greater variation in the efficiency of desorption was observed for BCU600 and BCS600. The use of hydrochloric acid gave a maximum desorption efficiency of 80% of Cd(II) ions (already at concentrations of 1 M), while the usage of 14.5 M of HNO3 resulted in the 90% desorption of the ions. The lowest ion desorption efficiencies of Cd(II) ions obtained for the reference biochar (BCS600) were 70% for 9.5 M of HCl and 80% in 4 M of HNO3. Comparing the acids used for the desorption study, the results obtained for Ni(II) ions were characterized by a smaller differentiation in comparison to Cd(II). In most cases, almost complete desorption (90e100%) was achieved for both HNO3 and HCl. No significant differences were particularly observed for BCUs. They were noticeable in the case of the reference biochar (BCS600), especially in the concentration range of 1 Me7 M, but finally 100% efficiency of desorption was obtained. In most cases, the increase in the concentration of the desorbing reagent did not cause a further increase in desorption. Furthermore, for systems for which total desorption (100%) was not obtained, it was necessary to conduct a re-desorption process. Moreover, it can be stated that the BCU400 material weakly bound heavy metal ions (Fig. 5), because the desorption in the studied systems of this material was close to 100%. Other materials, BCS600 and BCU600, bonded metal ions on their surface with similar force. Considering the desorption value obtained for each single biochar, it was assumed that RBP biochars exhibited higher reversibility of the adsorption process for both Cd(II) and Ni(II) ions. Based on the comparison of desorption for both metals, it was concluded that Ni(II) ions had weaker bonds with the surface, because their desorption was close to 100%. The incomplete desorption of Cd(II) may be related to the strong bonding of the metal ions to the mineral fractions of BCU600 and BCS600. These materials were characterized by a significantly higher content of mineral fractions than BCU400. Moreover, another important factor which affected the extent of desorption may be the difference between the ionic radii of Cd(II) and Ni(II). Hydrated ions of Cd(II) had a much larger ionic radius than ions of Ni(II). Thus, Cd(II) ions were more strongly bound to the surface, thereby preventing the total desorption of the tested materials. 4. Conclusions Biochars (BCU) produced from RGP had a higher capacity to adsorb Cd(II) and Ni(II) than biochar produced from biomass. Thus, BCUs may be successfully used for the removal of Cd(II) and Ni(II) ions from aqueous solution. The sorption of the investigated metal ions strongly depended on the initial pH of the solution and physico-chemical properties of the investigated biochars. BCU pyrolyzed at a higher temperature was more effective in the adsorption of Cd(II) and Ni(II) than BCU produced at a lower temperature. These results were likely related to the higher specific surface area, pore volume and mineral fraction content of BCU produced at a higher temperature. Considering the adsorption of particular ions, Cd(II) ions were adsorbed faster, at a higher extent and more strongly than Ni(II) ions, due to the Cd(II) ions smaller

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275

Fig. 4. The influence of chlorides and nitrates on Cd(II) and Ni(II) adsorption onto BCU400, BCU600 and BCS600; m ¼ 0.2 g, V ¼ 50 mL, C ¼ 100 mg/L, t ¼ 24 h, T ¼ 25  C.

Fig. 5. Desorption of Cd(II) and Ni(II) from BCU400, BCU600 and BCS600 in respect to nitric acid and hydrochloric acid concentration; m ¼ 0.008 g, V ¼ 2 mL, ACd(BCU400) ¼ 41.58 mg/ g, ACd(BCU600) ¼ 42.03 mg/g, ACd(BCS600) ¼ 18.26 mg/g, ANi(BCU400) ¼ 30.17 mg/g, ANi(BCU600) ¼ 31.94 mg/g, ANi(BCS600) ¼ 15.63 mg/g, t ¼ 24 h, T ¼ 25  C.

radius and their higher affinity to the biochar surface. Chlorides could be considered as the most interfering ions. The desorption experiment showed that in most cases, a single washing step by

nitric acid would be sufficient for the removal of metal ions from biochars.

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Synthesis of biochar from residues after biogas production with respect to cadmium and nickel removal from wastewater.

The objective of the study was to investigate the ability of biochars prepared under different temperatures (400 °C and 600 °C) from the residue of bi...
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