Journal of Hazardous Materials 273 (2014) 118–123
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Removal of cadmium ions from wastewater using innovative electronic waste-derived material Meng Xu a , Pejman Hadi b , Guohua Chen b , Gordon McKay b,∗ a b
Fok Ying Tung Graduate School, Hong Kong University of Science and Technology, Hong Kong Special Administrative Region Chemical and Biomolecular Engineering Department, Hong Kong University of Science and Technology, Hong Kong Special Administrative Region
h i g h l i g h t s • • • •
A novel developed adsorbent material derived from waste printed circuit boards’ component. The innovative adsorbent material can effectively remove cadmium ions from aqueous solutions. The maximum capacity for cadmium ion removal is 2.1 mmol/g. Cadmium removal capacity is either equivalent or better than commercial resins.
a r t i c l e
i n f o
Article history: Received 19 January 2014 Accepted 16 March 2014 Available online 28 March 2014 Keywords: Cadmium removal Adsorption isotherms Ion exchange Waste printed circuit boards
a b s t r a c t Cadmium is a highly toxic heavy metal even at a trace level. In this study, a novel material derived from waste PCBs has been applied as an adsorbent to remove cadmium ions from aqueous solutions. The effects of various factors including contact time, initial cadmium ion concentration, pH and adsorbent dosage have been evaluated. The maximum uptake capacity of the newly derived material for cadmium ions has reached 2.1 mmol/g at an initial pH 4. This value shows that this material can effectively remove cadmium ions from effluent. The equilibrium isotherm has been analyzed using several isotherm equations and is best described by the Redlich–Peterson model. Furthermore, different commercial adsorbent resins have been studied for comparison purposes. The results further confirm that this activated material is highly competitive with its commercial counterparts. © 2014 Elsevier B.V. All rights reserved.
1. Introduction During the past few decades, heavy metal pollution has become a serious environmental problem. Heavy metals, being controversial in the definition, often refer to elements having atomic weights between 63.5 and 200.6, and a specific gravity greater than 5.0 [1]. Some heavy metals such as cadmium, lead and mercury are not required for human activities; they are non-biodegradable and persistent so that they accumulate in living organisms [2]. Among all the heavy metals, cadmium is known as one of the most toxic ones. It is a probable human carcinogen especially toward the kidney [3]. Exposure to cadmium, even at a trace level,
∗ Corresponding author. Tel.: +852 2358 8412; fax: +852 2358 0054. E-mail addresses: [email protected] (M. Xu), [email protected] (P. Hadi), [email protected] (G. Chen), [email protected] (G. McKay). http://dx.doi.org/10.1016/j.jhazmat.2014.03.037 0304-3894/© 2014 Elsevier B.V. All rights reserved.
is believed to be a severe risk for human beings [4,5]. Besides the direct exposure, cadmium pollutants can also accumulate in plankton and plants and eventually transfer to human beings via the food chain. If adsorbed by the human body, it will affect bone metabolism causing fragility [6], deformation of the reproductive tract and endocrine system and induce premature birth and reduced birth weights [4,7]. A major source of cadmium pollution in the environment comes from industrial wastewater steam, such as effluents from electroplating, battery, metal finishing and printed circuit board industries [8]. Due to the importance of removing cadmium from wastewaters, several methods have been developed to treat wastewatercontaining heavy metal ions. Chemical precipitation, as a widely used method in industries [9], is a relatively simple and inexpensive, but highly pH dependent process. Membrane filtration is another technique to treat wastewater with heavy metals which depends on the permeability of the membranes and operating principles [10]. However, due to the membrane fouling problems,
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2. Materials and methods Nomenclature aF AT aL aLF aRP aT BDR bT bRP Ce E KL KLF KRP Kt nLF qDR qe qm SSE T t
Freundlich isotherm constant Temkin isotherm constant, L/mmol Langmuir isotherm constant, L/mmol Langmuir–Freundlich isotherm constant Redlich–Peterson isotherm constant, (L/mmol)bRP Toth isotherm constant Dubinin–Radushkevinch isotherm constant Temkin isotherm Redlich–Peterson isotherm exponent solution phase mental ion concentration at equilibrium, mmol/L mean free energy of sorption, KJ/mol Langmuir isotherm constant, L/g Langmuir–Freundlich isotherm constant Redlich–Peterson isotherm constant, L/g Toth isotherm constant Langmuir–Freundlich isotherm exponent Dubinin–Radushkevich isotherm constant, mmol/g solid phase mental ion concentration at equilibrium, mmol/g Langmuir monolayer saturation capacity, mmol/g sum of the squares of the error absolute temperature, K Toth isotherm exponent
membrane filtration is still not popular in large scale industrial wastewater treatment. Electrodialysis is also a promising separation process using semi-permeable ion-selective membranes. It has not been widely utilized in the large-scale applications because of the high operation cost to supply the energy requirement [11–13]. Nevertheless, adsorption is an effective and economic process that removes metal ions by contacting adsorbents with wastewater [14–18]. On the other hand, electronic waste (e-waste) has become one of the most important problems of the electronic industry development which has raised public concern in the last few years. With more frequent updating of electronic equipment, the product life has been shortened and more outdated products are becoming obsolete, thus raising a serious environmental problem when disposed due to their toxic nature [19–21]. Printed circuit boards (PCBs) as core components of the majority of the electronic products have been the focus of attention for recycling. One of the most eco-friendly technologies currently used for PCB recycling is to separate the PCBs into metallic and non-metallic powder; the metallic powder is of high value and can be marketed for reuse, however, the non-metallic powder which accounts for 70% of the total PCB weight has always been considered as a low value byproduct and is either dumped into landfill or simply used as a filler [19,22,23]. In this research, the idea is to recycle the electronic waste material to develop a novel adsorbent material to treat heavy metal loaded wastewater. The focus of this work is to activate the nonmetallic powder derived from waste PCBs to remove cadmium ions from effluent. The uptake capacity for cadmium ions were determined by adsorption isotherm experiments using activated non-metallic powder (A-NMP) derived from PCBs. The impacts of some key parameters, namely contact time, initial cadmium ions concentration, pH and adsorbent dosage on the adsorption capacity have been examined. Furthermore, the equilibrium isotherms have been analyzed using several different isotherm model equations.
2.1. Adsorbent preparation 2.1.1. Materials The raw material is the non-metallic powder (NMP) recycled from printed circuit boards. It was micron-sized and was provided by a local company in Hong Kong. Potassium hydroxide (KOH, >85%) for activation was purchased from Sigma–Aldrich Company, UK. 2.1.2. Activation process The precursor, NMP, was impregnated with the activating agent, KOH, under continuous stirring for 3 h at room temperature. The impregnation ratio, defined as the weight ratio of KOH to NMP, was fixed at 2. The resulting slurry was then heated to 250 ◦ C at a rate of 5 ◦ C/min for 3 h in a 18 L muffle furnace (AAF 11/18, Carbolite, UK) under a nitrogen atmosphere (purity 99.99%+). Then, the furnace was cooled down to room temperature with the nitrogen atmosphere constant. The resultant material was then washed with hot and cold water in turn for several times, subsequently dried at 110 ◦ C for 24 h and stored in a desiccator for later experiments. 2.1.3. Characterization The surface morphology of the raw (NMP) and the activated materials (A-NMP) were characterized by scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX). The samples were sprinkled onto an adhesive carbon tape supported on a metallic disk. EDX elementary mapping was performed at randomly selected areas on the surface of the particles. The Fourier transform infrared spectroscopy (FTIR) was used to detect the functional groups present on the materials. By using the KBr disk technique, the FTIR spectra of the raw and activated samples were obtained with a FTS 6000 FTIR spectrometer in the range of 4000–400 cm−1 . The samples were well blended with KBr and then desorbed at room temperature and pressed to obtain IRtransparent pellets. The BET surface areas of the NMP and A-NMP have been determined by N2 adsorption–desorption of porous structure at liquid nitrogen temperature (Auto sorb 1-Quantachrome instrument). 2.2. Adsorbate The metal solutions were prepared using analytical grade Cd (II) nitrate (Sigma–Aldrich, UK) and deionized water. All solutions were adjusted to a desired pH using dilute nitric acid and sodium hydroxide. The pH measurements were made by a pH meter (Orion Hong Kong, model 420A pH meter). 2.3. Experimental methods All the adsorption tests were carried out at 20 ◦ C in a temperature-controlled shaker by mixing accurately weighed amounts of adsorbents with adsorbate solutions of specific concentrations in plastic bottles. The agitation speed of the shaker was set at 120 rpm. Sampling was performed in specific time intervals. The initial and final concentrations of the sample solutions were measured by an inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) (Optima 7300 DV, Perkin-Elmer). The uptake capacities (qe ) were calculated using the following equation: qe =
V (C0 − Ce ) m
(1)
where C0 and Ce are the initial and final concentrations (mmol/L) of the solution respectively, V is volume of the solution in liters, and m is the mass of the adsorbent in grams.
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Fig. 1. SEM-EDX elementary mapping of A-NMP.
The final pH values of the solutions were measured. 2.3.1. Equilibrium contact time studies A contact time experiment was performed initially to determine the time required to reach the quasi-equilibrium state. A fixed mass of 50 mg A-NMP was added to each bottle containing 50 ml of 4 mmol/L cadmium solution with a pH value of 4. Then, the test bottles were placed into the shaker bath. Sampling was conducted at specific times (3, 6, 12, 24, 36, 48, 72, 96, 120 h) and the solution samples were collected in the sample tubes. The removal capacities were calculated using Eq. (1) and plotted against time to provide the equilibrium contact time for the reaction. 2.3.2. pH effect studies 5 mmol/L cadmium (II) nitrate solutions with different pH values were used to study the effect of the solution pH. The adsorbent dosage was fixed at 1 g/L. The bottles were then placed into the shaker for 4 days to reach the equilibrium. 2.3.3. Equilibrium sorption isotherms Various concentrations of cadmium ion solutions were prepared from a 10 mmol/L stock solution. The initial cadmium ion concentrations ranged from 0.2 to 7.5 mmol/L, while the pH values of the metal solutions were adjusted to 4 and the adsorbent dosage were fixed at 1 g/L. The bottles were placed into the shaker bath at 120 rpm and each bottle was removed from the shaker after 4 days for sampling. 2.3.4. Adsorbent dosage effect The dosage effect of the A-NMP for cadmium ion adsorption was carried out by adding various amounts of the activated material ranging from 25 to 200 mg into 100 ml solutions with cadmium concentrations of 5 mmol/L and pH values of 4.
2.4. Comparison with commercial resins Six different commercial resins, namely Lewatit TP 207, Amberlite IRC 7481 chelating resin, Purolite S930Plus ion exchange resin, Diaion CR11, Dowex Marathon C sodium form and Suqing D401 sodium form, were used to adsorb cadmium ions for comparison purpose. The adsorption experiments were carried out in the same experimental conditions outlined in Section 2.3.3. 3. Results and discussion 3.1. Characterization Fig. 1 shows the SEM-EDX elementary mapping results of A-NMP. It illustrates that during the whole activating process, potassium is distributed evenly onto the material. Other characterizations have been mentioned in our previous paper [24]. The FTIR results show that after the activation process, there is a broad band at 3442 cm−1 in A-NMP which is attributed to the O H stretching mode of hydroxyl groups. This peak has much stronger intensity after the activation process. This is due to the cleavage of some siloxane bridges and the formation of silanol groups. Another peak at 1013 cm−1 whose intensity considerably increases after the modification process is assigned to siloxane groups. This peak increases because of the development of new pores by opening the cage which results in more exposure of the siloxane groups to be detected. The N2 adsorption–desorption analysis shows that the raw material, NMP, is a non–porous material while high porosity has been developed after the activation process. The results show that the specific surface area (SBET ) increases from 0.9 m2 /g for the original NMP to 222 m2 /g for A-NMP. The significant change of the
M. Xu et al. / Journal of Hazardous Materials 273 (2014) 118–123
Fig. 2. Removal rate of cadmium ions against time (initial cadmium ion concentration 4 mmol/L at initial pH 4 with adsorbent dosage 1 g/L).
surface area makes the activated material more typical as a highly efficient adsorbent. 3.2. Equilibrium contact time studies Fig. 2 shows the plot of the removal rate of cadmium ions by the adsorbent. It is observed that the adsorption process comes to a quasi-equilibrium state after 72 h. In order to ensure that the equilibrium has been attained, the solutions were agitated for 5 days in all the following experiments.
121
Fig. 4. A-NMP dosage effect on the cadmium ions adsorption (initial cadmium ion concentration 5 mmol/L at initial pH 4).
to the high concentration of hydrogen ions which compete with the diffusion of metal ions resulting in the hydrogen ions to be adsorbed on the surface of the adsorbent instead of the metal ions. The final pHs for all the solutions are below 6 which demonstrates that no precipitation occurs during the experiments.
3.3.1. pH effect The results in Fig. 3 shows that at relatively low initial pH levels, A-NMP is highly pH dependent and will gradually reach the equilibrium state after the pH value is above 3.5. The material adsorbs very small amounts of cadmium ions at pH 2 and with the increase of the pH value, the adsorption capacity gradually increases correspondingly which is consistent with the results found by other researchers [25,26]. The low uptake capacity at low pH levels is due
3.3.2. Adsorbent dosage effect Adsorbent dosage is also an important parameter which determines the capacity of an adsorbent for a given initial concentration of the adsorbate. It has been studied for the removal of Cd2+ ions from aqueous solutions by varying the amount of A-NMP mass while keeping the other parameters constant. As shown in Fig. 4, it is clear that by increasing the adsorbent dosage from 0.25 to 2.00 g/L, the adsorption capacity, the amount of Cd2+ adsorbed per unit mass decreases while the adsorption efficiency, the removal percentage of Cd2+ ions, increases from 10% to 60% for an initial Cd2+ concentration of 5 mmol/L. It is because by increasing the adsorption dosage, the number of available adsorption sites increases so as to increase the removal efficiency [26]. On the other hand, the decrease in adsorption capacity with an increase in the adsorbent dosage is reported to be due to the unsaturation of adsorption sites during
Fig. 3. Adsorption capacity of A-NMP for cadmium ions at various initial pH values (initial cadmium ion concentration 5 mmol/L with adsorbent dosage 1 g/L).
Fig. 5. Equilibrium isotherm of cadmium adsorption with Redlich–Peterson adsorption model fit (initial pH 4 with adsorption dosage 1 g/L).
3.3. Equilibrium studies
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Table 1. This table summarizes the overall SSE values of the seven equilibrium isotherm models. According to the SSE results of the different models in the cadmium ion adsorption system, it was found that all the models except the Dubinin–Radushkevich model describe the adsorption onto the A-NMP well. Based on the calculated parameters for the experimental data, the best-fit model is Redlich–Peterson followed by Langmuir–Freundlich and Toth which also fit the experimental data properly with very small SSEs. It is worthy of note that all of these equations are extensions of Langmuir model. The fit of Redlich–Peterson adsorption model on the experimental isotherm data has been shown in Fig. 5. According to the hypothesis of Langmuir model, the adsorption sites are homogeneous with monolayer coverage. However, the experimental data do not completely fit the Langmuir isotherm which implies that not all sites represented homogeneous ion exchange with identical ions. This is because some of the functional groups carry potassium ions attached to the surface oxygen groups, whereas in some other hydrogen is the ion exchanger. The different functional groups as well as the combination of physisorption and ion exchange might cause the surface to behave in a heterogeneous manner. Although it cannot be described completely by the Langmuir assumption, the uptake seems to follow the monolayer adsorption process since only Cd2+ can bond with the functional groups on the surface.
the adsorption process [27]. Also particle aggregation which leads to smaller total surface area and longer diffusional path length may also cause the decrease of adsorption density. However, the authors believe that the decrease in the number of metal molecules available for each active site with an increase in the amount of adsorbent results in a reduction in the concentration-gradient between the bulk of the solution and the surface of the adsorbent. The effect of the concentration-gradient as a driving force for the adsorption is clearly observed in the change of the uptake capacity with the initial metal concentration (see Fig. 5). 3.3.3. Equilibrium isotherm analysis By increasing the initial Cd2+ ion concentration, the amount of Cd2+ adsorbed per unit mass of A-NMP increases, as shown in Fig. 5. At low initial concentrations, the availability of the adsorption sites is relatively high which means the Cd2+ ions can be adsorbed easily. Correspondingly, at high initial solution concentration, because of the limitation of the total available adsorption sites, a decrease in adsorption efficiency, in terms of percentage cadmium removed can be expected. The maximum adsorption capacity was found to be 2.1 mmol/g. The fit of the experimental equilibrium isotherm data is compared using the Langmuir, Freundlich, Langmuir–Freundlich, Redlich–Peterson, Temkin, Toth and Dubinin–Radushkevich isotherm equations. The variables of each model were determined by a built-in function of Microsoft Office Excel program. The model parameters for each isotherm were calculated using the respective equations by minimizing the difference between the experimental data and the data calculated from the models. The sum of the error squares (SSE) method was used to obtain the best fit isotherm constants. SSE =
(qexp − qcal )2
3.4. Adsorption mechanism Both chemisorption and physisorption are believed to occur during the adsorption process. The chemisorption mechanism of the A-NMP to uptake Cd2+ ions can be explained as an ion-exchange mechanism. The Cd2+ ions are exchanged with potassium or hydrogen ions present in the A-NMP structure. The existence of potassium ions in the solutions after the adsorption verifies this hypothesis. Other properties of the adsorption behavior such as the pH effect of the aqueous media also confirmed this assumption [28]. Besides ion-exchange,
(2)
The theoretical sorption capacity (qcal ) is obtained from the isotherm models. The best fit model was expected to have the lowest SSE value. The isotherm constants and SSE values are shown in Table 1 Isotherm constants of A-NMP for cadmium ions adsorption. KL Ce 1+aL Ce
KL aL SSE
88.8 42.6 0.0125
aF bF SSE
1.99 0.057 0.0034
KLF Ce LF
KLF aLF nLF SSE
8.15 3.06 0.214 0.0028
KRP Ce
KR aR bR SSE
343 170 0.955 0.0026
ln(AT Ce )
AT bT SSE
306,000 21,000 0.0033
Kt Ce
Kt at t SSE
2.49 0.0529 0.239 0.0029
qDR BDR SSE
2.07 6E−10 0.166
Langmuir
qe =
Freundlich
qe = aF CebF
Sips (Langmuir–Freundlich)
qe =
Redlich–Peterson
qe =
n
Temkin
qe =
Toth
qe =
1+(aLF Ce )nLF
b
1+aRP ce RP
RT bT
(at +Cet )
1/t
Dubinin–Radushkevich qe = qDR exp E=
1
2BDR
−BDR RT ln
1+
1 Ce
2
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References
Fig. 6. Comparison of the adsorption capacities of A-NMP with the commercial resins (initial pH 4 with adsorbent dosage 1 g/L).
physisorption is believed to occur because of the mismatch of the mole balance of the adsorbed cadmium ions and desorbed potassium ion. A reduction in the surface area of the adsorbent after metal uptake verifies the involvement of physisorption in addition to chemical ion exchange. Further characterizations of the used material will be conducted to confirm and complete the mechanism. 3.5. Comparison with commercial resins Fig. 6 shows the adsorption capacities of six different commercial resins compared with the A-NMP. It is obvious that the adsorption efficiency of the A-NMP for cadmiums ions is higher than that of Lewatit TP 207, Amberlite IRC7481 Chelating resin, Diaion CR11, Dowex Marathon C sodium form and Suqing D401 sodium form. The best performing adsorbent among all the commercial adsorbents is Purolite S930Plus ion exchange resin, and interestingly the A-NMP has an equivalent performance compared to it. 4. Conclusion The non-metallic powder recycled from PCBs seems to be a suitable precursor for the production of an adsorbent material. The chemical activation process with potassium hydroxide as activating reagent results in the production of a porous material which can effectively adsorb cadmium ions from effluent with a maximum capacity of 2.1 mmol/g. The adsorption equilibrium isotherm results show a better fit with Langmuir-type models. The comparison of the adsorption capacities of several commonly recognized commercial adsorbents with this novel material under similar conditions has proved that the efficiency of this newly developed material is either better or equivalent to the commercial products. Acknowledgement The authors would like to thank the Hong Kong Research Grant Council for their support of this research.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Removal of cadmium ions from wastewater using innovative electronic waste-derived material Meng Xu a , Pejman Hadi b , Guohua Chen b , Gordon McKay b,∗ a b
Fok Ying Tung Graduate School, Hong Kong University of Science and Technology, Hong Kong Special Administrative Region Chemical and Biomolecular Engineering Department, Hong Kong University of Science and Technology, Hong Kong Special Administrative Region
h i g h l i g h t s • • • •
A novel developed adsorbent material derived from waste printed circuit boards’ component. The innovative adsorbent material can effectively remove cadmium ions from aqueous solutions. The maximum capacity for cadmium ion removal is 2.1 mmol/g. Cadmium removal capacity is either equivalent or better than commercial resins.
a r t i c l e
i n f o
Article history: Received 19 January 2014 Accepted 16 March 2014 Available online 28 March 2014 Keywords: Cadmium removal Adsorption isotherms Ion exchange Waste printed circuit boards
a b s t r a c t Cadmium is a highly toxic heavy metal even at a trace level. In this study, a novel material derived from waste PCBs has been applied as an adsorbent to remove cadmium ions from aqueous solutions. The effects of various factors including contact time, initial cadmium ion concentration, pH and adsorbent dosage have been evaluated. The maximum uptake capacity of the newly derived material for cadmium ions has reached 2.1 mmol/g at an initial pH 4. This value shows that this material can effectively remove cadmium ions from effluent. The equilibrium isotherm has been analyzed using several isotherm equations and is best described by the Redlich–Peterson model. Furthermore, different commercial adsorbent resins have been studied for comparison purposes. The results further confirm that this activated material is highly competitive with its commercial counterparts. © 2014 Elsevier B.V. All rights reserved.
1. Introduction During the past few decades, heavy metal pollution has become a serious environmental problem. Heavy metals, being controversial in the definition, often refer to elements having atomic weights between 63.5 and 200.6, and a specific gravity greater than 5.0 [1]. Some heavy metals such as cadmium, lead and mercury are not required for human activities; they are non-biodegradable and persistent so that they accumulate in living organisms [2]. Among all the heavy metals, cadmium is known as one of the most toxic ones. It is a probable human carcinogen especially toward the kidney [3]. Exposure to cadmium, even at a trace level,
∗ Corresponding author. Tel.: +852 2358 8412; fax: +852 2358 0054. E-mail addresses: [email protected] (M. Xu), [email protected] (P. Hadi), [email protected] (G. Chen), [email protected] (G. McKay). http://dx.doi.org/10.1016/j.jhazmat.2014.03.037 0304-3894/© 2014 Elsevier B.V. All rights reserved.
is believed to be a severe risk for human beings [4,5]. Besides the direct exposure, cadmium pollutants can also accumulate in plankton and plants and eventually transfer to human beings via the food chain. If adsorbed by the human body, it will affect bone metabolism causing fragility [6], deformation of the reproductive tract and endocrine system and induce premature birth and reduced birth weights [4,7]. A major source of cadmium pollution in the environment comes from industrial wastewater steam, such as effluents from electroplating, battery, metal finishing and printed circuit board industries [8]. Due to the importance of removing cadmium from wastewaters, several methods have been developed to treat wastewatercontaining heavy metal ions. Chemical precipitation, as a widely used method in industries [9], is a relatively simple and inexpensive, but highly pH dependent process. Membrane filtration is another technique to treat wastewater with heavy metals which depends on the permeability of the membranes and operating principles [10]. However, due to the membrane fouling problems,
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2. Materials and methods Nomenclature aF AT aL aLF aRP aT BDR bT bRP Ce E KL KLF KRP Kt nLF qDR qe qm SSE T t
Freundlich isotherm constant Temkin isotherm constant, L/mmol Langmuir isotherm constant, L/mmol Langmuir–Freundlich isotherm constant Redlich–Peterson isotherm constant, (L/mmol)bRP Toth isotherm constant Dubinin–Radushkevinch isotherm constant Temkin isotherm Redlich–Peterson isotherm exponent solution phase mental ion concentration at equilibrium, mmol/L mean free energy of sorption, KJ/mol Langmuir isotherm constant, L/g Langmuir–Freundlich isotherm constant Redlich–Peterson isotherm constant, L/g Toth isotherm constant Langmuir–Freundlich isotherm exponent Dubinin–Radushkevich isotherm constant, mmol/g solid phase mental ion concentration at equilibrium, mmol/g Langmuir monolayer saturation capacity, mmol/g sum of the squares of the error absolute temperature, K Toth isotherm exponent
membrane filtration is still not popular in large scale industrial wastewater treatment. Electrodialysis is also a promising separation process using semi-permeable ion-selective membranes. It has not been widely utilized in the large-scale applications because of the high operation cost to supply the energy requirement [11–13]. Nevertheless, adsorption is an effective and economic process that removes metal ions by contacting adsorbents with wastewater [14–18]. On the other hand, electronic waste (e-waste) has become one of the most important problems of the electronic industry development which has raised public concern in the last few years. With more frequent updating of electronic equipment, the product life has been shortened and more outdated products are becoming obsolete, thus raising a serious environmental problem when disposed due to their toxic nature [19–21]. Printed circuit boards (PCBs) as core components of the majority of the electronic products have been the focus of attention for recycling. One of the most eco-friendly technologies currently used for PCB recycling is to separate the PCBs into metallic and non-metallic powder; the metallic powder is of high value and can be marketed for reuse, however, the non-metallic powder which accounts for 70% of the total PCB weight has always been considered as a low value byproduct and is either dumped into landfill or simply used as a filler [19,22,23]. In this research, the idea is to recycle the electronic waste material to develop a novel adsorbent material to treat heavy metal loaded wastewater. The focus of this work is to activate the nonmetallic powder derived from waste PCBs to remove cadmium ions from effluent. The uptake capacity for cadmium ions were determined by adsorption isotherm experiments using activated non-metallic powder (A-NMP) derived from PCBs. The impacts of some key parameters, namely contact time, initial cadmium ions concentration, pH and adsorbent dosage on the adsorption capacity have been examined. Furthermore, the equilibrium isotherms have been analyzed using several different isotherm model equations.
2.1. Adsorbent preparation 2.1.1. Materials The raw material is the non-metallic powder (NMP) recycled from printed circuit boards. It was micron-sized and was provided by a local company in Hong Kong. Potassium hydroxide (KOH, >85%) for activation was purchased from Sigma–Aldrich Company, UK. 2.1.2. Activation process The precursor, NMP, was impregnated with the activating agent, KOH, under continuous stirring for 3 h at room temperature. The impregnation ratio, defined as the weight ratio of KOH to NMP, was fixed at 2. The resulting slurry was then heated to 250 ◦ C at a rate of 5 ◦ C/min for 3 h in a 18 L muffle furnace (AAF 11/18, Carbolite, UK) under a nitrogen atmosphere (purity 99.99%+). Then, the furnace was cooled down to room temperature with the nitrogen atmosphere constant. The resultant material was then washed with hot and cold water in turn for several times, subsequently dried at 110 ◦ C for 24 h and stored in a desiccator for later experiments. 2.1.3. Characterization The surface morphology of the raw (NMP) and the activated materials (A-NMP) were characterized by scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX). The samples were sprinkled onto an adhesive carbon tape supported on a metallic disk. EDX elementary mapping was performed at randomly selected areas on the surface of the particles. The Fourier transform infrared spectroscopy (FTIR) was used to detect the functional groups present on the materials. By using the KBr disk technique, the FTIR spectra of the raw and activated samples were obtained with a FTS 6000 FTIR spectrometer in the range of 4000–400 cm−1 . The samples were well blended with KBr and then desorbed at room temperature and pressed to obtain IRtransparent pellets. The BET surface areas of the NMP and A-NMP have been determined by N2 adsorption–desorption of porous structure at liquid nitrogen temperature (Auto sorb 1-Quantachrome instrument). 2.2. Adsorbate The metal solutions were prepared using analytical grade Cd (II) nitrate (Sigma–Aldrich, UK) and deionized water. All solutions were adjusted to a desired pH using dilute nitric acid and sodium hydroxide. The pH measurements were made by a pH meter (Orion Hong Kong, model 420A pH meter). 2.3. Experimental methods All the adsorption tests were carried out at 20 ◦ C in a temperature-controlled shaker by mixing accurately weighed amounts of adsorbents with adsorbate solutions of specific concentrations in plastic bottles. The agitation speed of the shaker was set at 120 rpm. Sampling was performed in specific time intervals. The initial and final concentrations of the sample solutions were measured by an inductively coupled plasma-atomic emission spectrophotometer (ICP-AES) (Optima 7300 DV, Perkin-Elmer). The uptake capacities (qe ) were calculated using the following equation: qe =
V (C0 − Ce ) m
(1)
where C0 and Ce are the initial and final concentrations (mmol/L) of the solution respectively, V is volume of the solution in liters, and m is the mass of the adsorbent in grams.
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Fig. 1. SEM-EDX elementary mapping of A-NMP.
The final pH values of the solutions were measured. 2.3.1. Equilibrium contact time studies A contact time experiment was performed initially to determine the time required to reach the quasi-equilibrium state. A fixed mass of 50 mg A-NMP was added to each bottle containing 50 ml of 4 mmol/L cadmium solution with a pH value of 4. Then, the test bottles were placed into the shaker bath. Sampling was conducted at specific times (3, 6, 12, 24, 36, 48, 72, 96, 120 h) and the solution samples were collected in the sample tubes. The removal capacities were calculated using Eq. (1) and plotted against time to provide the equilibrium contact time for the reaction. 2.3.2. pH effect studies 5 mmol/L cadmium (II) nitrate solutions with different pH values were used to study the effect of the solution pH. The adsorbent dosage was fixed at 1 g/L. The bottles were then placed into the shaker for 4 days to reach the equilibrium. 2.3.3. Equilibrium sorption isotherms Various concentrations of cadmium ion solutions were prepared from a 10 mmol/L stock solution. The initial cadmium ion concentrations ranged from 0.2 to 7.5 mmol/L, while the pH values of the metal solutions were adjusted to 4 and the adsorbent dosage were fixed at 1 g/L. The bottles were placed into the shaker bath at 120 rpm and each bottle was removed from the shaker after 4 days for sampling. 2.3.4. Adsorbent dosage effect The dosage effect of the A-NMP for cadmium ion adsorption was carried out by adding various amounts of the activated material ranging from 25 to 200 mg into 100 ml solutions with cadmium concentrations of 5 mmol/L and pH values of 4.
2.4. Comparison with commercial resins Six different commercial resins, namely Lewatit TP 207, Amberlite IRC 7481 chelating resin, Purolite S930Plus ion exchange resin, Diaion CR11, Dowex Marathon C sodium form and Suqing D401 sodium form, were used to adsorb cadmium ions for comparison purpose. The adsorption experiments were carried out in the same experimental conditions outlined in Section 2.3.3. 3. Results and discussion 3.1. Characterization Fig. 1 shows the SEM-EDX elementary mapping results of A-NMP. It illustrates that during the whole activating process, potassium is distributed evenly onto the material. Other characterizations have been mentioned in our previous paper [24]. The FTIR results show that after the activation process, there is a broad band at 3442 cm−1 in A-NMP which is attributed to the O H stretching mode of hydroxyl groups. This peak has much stronger intensity after the activation process. This is due to the cleavage of some siloxane bridges and the formation of silanol groups. Another peak at 1013 cm−1 whose intensity considerably increases after the modification process is assigned to siloxane groups. This peak increases because of the development of new pores by opening the cage which results in more exposure of the siloxane groups to be detected. The N2 adsorption–desorption analysis shows that the raw material, NMP, is a non–porous material while high porosity has been developed after the activation process. The results show that the specific surface area (SBET ) increases from 0.9 m2 /g for the original NMP to 222 m2 /g for A-NMP. The significant change of the
M. Xu et al. / Journal of Hazardous Materials 273 (2014) 118–123
Fig. 2. Removal rate of cadmium ions against time (initial cadmium ion concentration 4 mmol/L at initial pH 4 with adsorbent dosage 1 g/L).
surface area makes the activated material more typical as a highly efficient adsorbent. 3.2. Equilibrium contact time studies Fig. 2 shows the plot of the removal rate of cadmium ions by the adsorbent. It is observed that the adsorption process comes to a quasi-equilibrium state after 72 h. In order to ensure that the equilibrium has been attained, the solutions were agitated for 5 days in all the following experiments.
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Fig. 4. A-NMP dosage effect on the cadmium ions adsorption (initial cadmium ion concentration 5 mmol/L at initial pH 4).
to the high concentration of hydrogen ions which compete with the diffusion of metal ions resulting in the hydrogen ions to be adsorbed on the surface of the adsorbent instead of the metal ions. The final pHs for all the solutions are below 6 which demonstrates that no precipitation occurs during the experiments.
3.3.1. pH effect The results in Fig. 3 shows that at relatively low initial pH levels, A-NMP is highly pH dependent and will gradually reach the equilibrium state after the pH value is above 3.5. The material adsorbs very small amounts of cadmium ions at pH 2 and with the increase of the pH value, the adsorption capacity gradually increases correspondingly which is consistent with the results found by other researchers [25,26]. The low uptake capacity at low pH levels is due
3.3.2. Adsorbent dosage effect Adsorbent dosage is also an important parameter which determines the capacity of an adsorbent for a given initial concentration of the adsorbate. It has been studied for the removal of Cd2+ ions from aqueous solutions by varying the amount of A-NMP mass while keeping the other parameters constant. As shown in Fig. 4, it is clear that by increasing the adsorbent dosage from 0.25 to 2.00 g/L, the adsorption capacity, the amount of Cd2+ adsorbed per unit mass decreases while the adsorption efficiency, the removal percentage of Cd2+ ions, increases from 10% to 60% for an initial Cd2+ concentration of 5 mmol/L. It is because by increasing the adsorption dosage, the number of available adsorption sites increases so as to increase the removal efficiency [26]. On the other hand, the decrease in adsorption capacity with an increase in the adsorbent dosage is reported to be due to the unsaturation of adsorption sites during
Fig. 3. Adsorption capacity of A-NMP for cadmium ions at various initial pH values (initial cadmium ion concentration 5 mmol/L with adsorbent dosage 1 g/L).
Fig. 5. Equilibrium isotherm of cadmium adsorption with Redlich–Peterson adsorption model fit (initial pH 4 with adsorption dosage 1 g/L).
3.3. Equilibrium studies
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Table 1. This table summarizes the overall SSE values of the seven equilibrium isotherm models. According to the SSE results of the different models in the cadmium ion adsorption system, it was found that all the models except the Dubinin–Radushkevich model describe the adsorption onto the A-NMP well. Based on the calculated parameters for the experimental data, the best-fit model is Redlich–Peterson followed by Langmuir–Freundlich and Toth which also fit the experimental data properly with very small SSEs. It is worthy of note that all of these equations are extensions of Langmuir model. The fit of Redlich–Peterson adsorption model on the experimental isotherm data has been shown in Fig. 5. According to the hypothesis of Langmuir model, the adsorption sites are homogeneous with monolayer coverage. However, the experimental data do not completely fit the Langmuir isotherm which implies that not all sites represented homogeneous ion exchange with identical ions. This is because some of the functional groups carry potassium ions attached to the surface oxygen groups, whereas in some other hydrogen is the ion exchanger. The different functional groups as well as the combination of physisorption and ion exchange might cause the surface to behave in a heterogeneous manner. Although it cannot be described completely by the Langmuir assumption, the uptake seems to follow the monolayer adsorption process since only Cd2+ can bond with the functional groups on the surface.
the adsorption process [27]. Also particle aggregation which leads to smaller total surface area and longer diffusional path length may also cause the decrease of adsorption density. However, the authors believe that the decrease in the number of metal molecules available for each active site with an increase in the amount of adsorbent results in a reduction in the concentration-gradient between the bulk of the solution and the surface of the adsorbent. The effect of the concentration-gradient as a driving force for the adsorption is clearly observed in the change of the uptake capacity with the initial metal concentration (see Fig. 5). 3.3.3. Equilibrium isotherm analysis By increasing the initial Cd2+ ion concentration, the amount of Cd2+ adsorbed per unit mass of A-NMP increases, as shown in Fig. 5. At low initial concentrations, the availability of the adsorption sites is relatively high which means the Cd2+ ions can be adsorbed easily. Correspondingly, at high initial solution concentration, because of the limitation of the total available adsorption sites, a decrease in adsorption efficiency, in terms of percentage cadmium removed can be expected. The maximum adsorption capacity was found to be 2.1 mmol/g. The fit of the experimental equilibrium isotherm data is compared using the Langmuir, Freundlich, Langmuir–Freundlich, Redlich–Peterson, Temkin, Toth and Dubinin–Radushkevich isotherm equations. The variables of each model were determined by a built-in function of Microsoft Office Excel program. The model parameters for each isotherm were calculated using the respective equations by minimizing the difference between the experimental data and the data calculated from the models. The sum of the error squares (SSE) method was used to obtain the best fit isotherm constants. SSE =
(qexp − qcal )2
3.4. Adsorption mechanism Both chemisorption and physisorption are believed to occur during the adsorption process. The chemisorption mechanism of the A-NMP to uptake Cd2+ ions can be explained as an ion-exchange mechanism. The Cd2+ ions are exchanged with potassium or hydrogen ions present in the A-NMP structure. The existence of potassium ions in the solutions after the adsorption verifies this hypothesis. Other properties of the adsorption behavior such as the pH effect of the aqueous media also confirmed this assumption [28]. Besides ion-exchange,
(2)
The theoretical sorption capacity (qcal ) is obtained from the isotherm models. The best fit model was expected to have the lowest SSE value. The isotherm constants and SSE values are shown in Table 1 Isotherm constants of A-NMP for cadmium ions adsorption. KL Ce 1+aL Ce
KL aL SSE
88.8 42.6 0.0125
aF bF SSE
1.99 0.057 0.0034
KLF Ce LF
KLF aLF nLF SSE
8.15 3.06 0.214 0.0028
KRP Ce
KR aR bR SSE
343 170 0.955 0.0026
ln(AT Ce )
AT bT SSE
306,000 21,000 0.0033
Kt Ce
Kt at t SSE
2.49 0.0529 0.239 0.0029
qDR BDR SSE
2.07 6E−10 0.166
Langmuir
qe =
Freundlich
qe = aF CebF
Sips (Langmuir–Freundlich)
qe =
Redlich–Peterson
qe =
n
Temkin
qe =
Toth
qe =
1+(aLF Ce )nLF
b
1+aRP ce RP
RT bT
(at +Cet )
1/t
Dubinin–Radushkevich qe = qDR exp E=
1
2BDR
−BDR RT ln
1+
1 Ce
2
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References
Fig. 6. Comparison of the adsorption capacities of A-NMP with the commercial resins (initial pH 4 with adsorbent dosage 1 g/L).
physisorption is believed to occur because of the mismatch of the mole balance of the adsorbed cadmium ions and desorbed potassium ion. A reduction in the surface area of the adsorbent after metal uptake verifies the involvement of physisorption in addition to chemical ion exchange. Further characterizations of the used material will be conducted to confirm and complete the mechanism. 3.5. Comparison with commercial resins Fig. 6 shows the adsorption capacities of six different commercial resins compared with the A-NMP. It is obvious that the adsorption efficiency of the A-NMP for cadmiums ions is higher than that of Lewatit TP 207, Amberlite IRC7481 Chelating resin, Diaion CR11, Dowex Marathon C sodium form and Suqing D401 sodium form. The best performing adsorbent among all the commercial adsorbents is Purolite S930Plus ion exchange resin, and interestingly the A-NMP has an equivalent performance compared to it. 4. Conclusion The non-metallic powder recycled from PCBs seems to be a suitable precursor for the production of an adsorbent material. The chemical activation process with potassium hydroxide as activating reagent results in the production of a porous material which can effectively adsorb cadmium ions from effluent with a maximum capacity of 2.1 mmol/g. The adsorption equilibrium isotherm results show a better fit with Langmuir-type models. The comparison of the adsorption capacities of several commonly recognized commercial adsorbents with this novel material under similar conditions has proved that the efficiency of this newly developed material is either better or equivalent to the commercial products. Acknowledgement The authors would like to thank the Hong Kong Research Grant Council for their support of this research.
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