Toxic Heavy Metal Capture Using a Novel Electronic Waste-Based

Article pubs.acs.org/est

Toxic Heavy Metal Capture Using a Novel Electronic Waste-Based MaterialMechanism, Modeling and Comparison Pejman Hadi, John Barford, and Gordon McKay* Chemical and Biomolecular Engineering Department, Hong Kong University of Science and Technology, Hong Kong SAR S Supporting Information *

ABSTRACT: In the modern communication era, the disposal of printed circuit boards is ecologically of dire concern on a global scale. The two prevalent methods applied for the disposal of this waste are either incineration or landfilling both of which are viewed with skepticism due to their negative environmental impact. Activation of the nonmetallic fraction of this waste leads to the development of a mesoporous material with highly functional groups which can potentially be applied for heavy metal uptake. The removal of copper, lead, and zinc was studied employing a cost-effective novel adsorbent based on waste printed circuit boards. The results indicate that the modification of the original e-waste material has a considerable effect on its surface area enhancement. Adsorption experiments revealed that the modified novel material had uptake capacities of 2.9 mmol Cu, 3.4 mmol Pb, and 2.0 mmol Zn per each gram of the adsorbent which are significantly higher values than its commercial counterparts used in industry.

1. INTRODUCTION Although water is a source of life and a natural resource that sustains our environment and supports livelihoods, it is also a source of risk and vulnerability which has confronted humanity with some of its greatest challenges throughout history. One of the major water crises is associated with industrial wastewaters containing heavy metal ions hazardous for human physiology and other biological systems when the tolerance levels are exceeded. Consequently, their concentration must be carefully controlled and monitored to levels complying with the relevant environmental regulations for various bodies of water.1,2 A wide range of technologies has been employed for the removal of metal ion pollutants from effluent streams, namely coagulation,3 precipitation,4 reverse osmosis,5 filtration,6 and adsorption.7−10 Among these technologies, adsorption has been shown to be the most promising technique. The process of adsorption implies the presence of an adsorbent solid that binds molecules by physical attractive forces, ion exchange, and/or chemical binding. One prominent advantage of adsorption lies in the fact that the persistent compounds are removed, rather than being broken down to potentially dangerous compounds that may be produced by some other processes.11 Other practical advantages include high separating power12 and ease of process operation and control.13 Nonetheless, the use of the most common adsorbent, that is, activated carbon, has been found to be costly14 which has prompted researchers to search for more economical sources of raw materials to reduce the cost. Development of low cost and highly efficient adsorbents derived from carbonaceous waste has been the focus of great research interest. Materials such as seaweed,15 peanut shell,16 sawdust,17 peat,18 bagasse,19 and bone char20 have been © 2013 American Chemical Society

investigated for this purpose. Although some of these materials have shown good adsorption efficacy, there are two major challenges regarding these carbonaceous materials. The first challenge is concerned with their low yield and the second one is the very high process temperatures required which immensely affects the ultimate adsorbent cost. Hence, it is imperative to look for other inexpensive waste sources which can produce adsorbents with the same adsorption efficiency and which can be manufactured in relatively high yield and at lower process temperatures. Recently, the application of synthetic aluminosilicate materials to trap metal ions in their accessible internal channels has gained momentum.21 However, the drawbacks for these high-tech materials are their tedious production routes and relatively high cost which hinders their widespread application in heavy metal adsorption. Yet, it is common knowledge that printed circuit boards (PCBs) are composed of plastics and aluminosilicates22 and accordingly, it is postulated that the nonmetallic component of waste PCBs can theoretically be visualized as a viable alternative to immobilize metal ions on their surface. Over recent years, there has been an ever-increasing global concern about the management of PCB wastes and the lack of a proper methodology for their disposal.23,24 The two prevalent conventional methodologies applied to treat this waste are either incineration or landfilling. Both of these have Received: Revised: Accepted: Published: 8248

January 11, 2013 May 27, 2013 July 9, 2013 July 10, 2013 dx.doi.org/10.1021/es4001664 | Environ. Sci. Technol. 2013, 47, 8248−8255

Environmental Science & Technology

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6−8 h in vacuo. Adsorption/desorption isotherms, performed under a constant nitrogen flow rate to an adsorption cutoff p/p0 = 0.98, typically required 8−12 h to complete. Surface areas and total micropore volumes were estimated by application of the BET equation and the t-plot method, respectively, using the associated instrument software called AS1Win1.5. Mesopore volumes were calculated by subtracting the micropore volume (obtained from the application of the t-plot method) from the total volume of nitrogen adsorbed at p/p0 = 0.98. 2.4. Elemental Composition. An elemental analyzer (Vario EL III, Varian) was used to determine the carbon, hydrogen, nitrogen, and sulfur (CHNS) contents of the sample. 2.5. FTIR Measurements. Fourier transform infrared spectroscopy has been used to identify the functional groups that exist on the various materials. FTIR spectra of the raw and activated samples were obtained with a FTS 6000 FTIR spectrometer in the range of 4000−400 cm−1 using the KBr disc technique. Well-blended material−KBr (1:100) mix was ground, then desorbed at room temperature and pressed to obtain IR-transparent pellets. 2.6. XRF Studies. The X-ray fluorescence spectroscopy (JEOL JSX-3201Z) technique was employed to determine the concentration of the detectable elements in both the precursor and the activated material. 2.7. XPS Studies. X-ray photoelectron data were acquired with an XPS-PHI5600 system with a monochromatic Al Kα source (excitation energy, hυ = 1486.6 eV) at a voltage of 10 kV and a current of 15 mA. The low resolution spectrum for the whole range was acquired at 70 eV pass energy, whereas the high resolution narrow range spectra for elements of interest were acquired at 20 eV pass energy. The spectra were corrected using C(1s) at 284.6 eV binding energy as reference. 2.8. Aqueous Adsorption. Aqueous adsorption tests were performed using different metal solutions in single-component systems at various concentrations in the range of 0.5−5 mM. The aqueous adsorption efficiencies of untreated NMP and activated material (A-NMP) for the compounds were carried out by mixing 50 mg of the sample with 50 mL of the metal solutions of fixed concentration. Subsequently, full aqueous adsorption isotherms were obtained using the activated material. All adsorption tests were carried out at 20 °C in a temperature-controlled shaker by mixing adsorbate solutions of specific concentrations with accurately weighed masses of adsorbent in plastic bottles. The mixtures were shaken for 5 days and the sampling was performed at specified times followed by filtration. Preliminary tests showed that the contact times employed were sufficient to reach steady-state conditions. To minimize the effect of pH on the adsorption tests, the initial pH values of all stock solutions were adjusted by the addition of dilute nitric acid or sodium hydroxide to a pH value of 4. The initial and final concentrations of the solutions were measured by ICP-AES (Optima 7300 DV, Perkin-Elmer). These data were used to calculate the adsorption capacity, qe, of the adsorbent and figures of adsorption capacity, qe , against equilibrium concentration, Ce, were plotted. The final pH of each isotherm point was recorded and the pH range of each data point was 5.4−5.7. Furthermore, in order to study the effect of pH, the initial concentration of the metal solution was kept constant at 5 mM and the pH value was varied in the range of 2−6.

considerable environmental impact and have been hindered due to the evolution of potent toxic gases such as dioxins and furans in the case of the former technique25,26 and lack of enough landspace and toxic contamination of the land in the case of the latter technique.27−29 Without systems and technologies in place to effectively capture and manage these wastes, the accumulation of the e-wastes has and will continue to be a significant environmental problem at a global level. Although several technologies, such as hydrometallurgy30 and pyrometallurgy,31 have been proposed to recover the valuable components from the waste PCBs, they do not focus on the holistic treatment of PCB wastes in solving the related environmental issues. The mechanical−physical approaches are now attracting more attention and entail a crushing step aimed at stripping metal from the base plates and thereafter separation of metal and nonmetal fractions by various techniques including shape separation,32 conductivity-based separation,33 and density-base separation.34 Although the metal fraction of the waste PCBs can be put on the market, the nonmetallic fraction which comprises 70% of the entire waste PCBs presents a further environmental challenge. Separation processes are unlikely to be economically attractive by taking advantage of only the metallic fraction and disposing of the nonmetallic portion. Therefore, the investigation of potential applications for these nonmetallic fractions of waste printed circuit boards (NMP) is an important area for research and/or practical application in industry. To date, to the best knowledge of the authors, there are no systematic studies on the novel application of NMPs as an adsorbent. Therefore, this breakthrough study focuses on the feasibility of effluent detoxification using this potential innovative adsorbent with the aim of recycling these waste resources in a more profitable way as well as protecting the environment from an ever-increasing ecological contamination.

2. MATERIALS AND METHODS 2.1. Materials. The micrometer-size nonmetallic fraction of waste printed circuit boards (NMP) was employed as the starting material in this work. The NMP powder was provided by a local company in Hong Kong. Potassium hydroxide (assay >85%) was purchased from Sigma-Aldrich and used as the chemical activating agent. 2.2. Activation Process. The “as received” precursor (NMP) was impregnated with 1 M KOH solution for 3 h to give an impregnation ratio of 2:1, where the impregnation ratio is defined as the weight ratio of activating agent to raw material. The slurry was stirred frequently during impregnation in order to achieve good mixing. After the impregnation process was completed, the resultant slurry was heated in a 18 L muffle furnace at 5 °C/min to 250 °C for 3 h under a flowing nitrogen atmosphere to inert the vessel and to remove all the volatile products arising from the thermal decomposition of the precursor. After completion of the activation, the furnace was allowed to cool down to room temperature under a nitrogen atmosphere. The resultant activated material was washed sequentially several times with hot and cold distilled water. Finally the sample was dried in an oven at 110 °C for 24 h and then stored in a desiccator for later use. 2.3. Nitrogen Gas Adsorption. The precursor and the activated material were analyzed by continuous volumetric nitrogen gas adsorption, at liquid nitrogen temperature, using an Autosorb1-Quantachrome automatic adsorption analyzer. Prior to the analysis, the samples were outgassed at 120 °C for 8249

dx.doi.org/10.1021/es4001664 | Environ. Sci. Technol. 2013, 47, 8248−8255


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Furthermore, the bands at 1013 cm−1 and 1036 cm−1 in NMP and A-NMP spectra are ascribed to the siloxane entity. It is envisaged that the activation process results in the evolution of a porous structure causing the availability of more surface siloxanic functional groups and thus the absorption of more light at its corresponding wavelength which accounts for the stronger absorption band in the case of A-NMP. 3.3. XPS Studies. The surface chemistry of the modified and nonmodified materials has been extensively studied by Xray photoelectron spectroscopy (XPS). A wide scan spectrum of treated and untreated material is presented in Figure 2. The

3. RESULTS AND DISCUSSION 3.1. Elemental Analysis. One of the fundamental contributions of this work was to obtain an in-depth understanding of the composition of the original waste material and to measure its compositional change after the activation process. In this regard, the carbon amounts in the original (NMP) and treated (A-NMP) waste materials were 21and 1.6 wt %, respectively (Supporting Information (SI) Table S1). Thus, most of the carbon content of the raw material has been burnt off during the activation process despite the low processing temperature which is due to the catalyzing effect of potassium and calcium.35,36 On the other hand, according to the carbon elemental results, a high fraction of the elements present in NMP is nonCHNS elements which must be identified and measured by other experimental techniques. XRF analysis revealed that the dominant constituents of the original waste material were aluminum, silicon, calcium, and bromine accounting for 11.4, 50.6, 29.7, and 4.1 mol % for NMP and 10.3, 53.5, 26.6, and 0 mol % for A-NMP, respectively (SI Table S2). It is thought that the first three components probably originated from calcium aluminosilicate, one of the major constituents of printed circuit boards. It was noted that the compositions of these components did not vary significantly during the activation process, while the latter component from the flame retardants is completely removed from the adsorbent product during the activation and/or washing steps. 3.2. FTIR Studies. In order to determine the structure of the material before and after the activation practice, analytical characterization of the materials by Fourier transform infrared spectroscopy (FTIR) was used to monitor the changes in the course of modification. Figure 1 gives the IR spectrum of these

Figure 2. X-ray photoelectron spectroscopy (XPS) diagram of the modified and unmodified materials.

most intense photoelectron peaks are found in 102 eV, 153 eV, 285 eV, 293 eV, 347 eV, and 531 eV which are attributed to Si (2p), Si (2s), C (1s), K (2p), Ca (2p), and O (1s). As illustrated in this Figure, the intensities of oxygen, silicon, calcium, and potassium peaks have considerably increased on the surface of the material after the activation process, whereas a smaller carbon peak has been detected on the surface of ANMP compared to NMP (SI Table S3). Each scenario creates different, but consistent trends regarding what is happening in the system; since the material is treated at a relatively high temperature, carbon is burnt off during the treatment process which accounts for the lower carbon amount after activation compared to the original waste material. This is in good agreement with the elemental analysis data showing the same trend for the carbon reduction after the activation process. Moreover, an increase in the intensity of surface oxygen signifies that oxygen atoms in the silicate network come into the surface during the reaction either as Si−OH or Si−O-X groups where X can be either Si or Al. To further clarify this statement, it should be noted that there are already many Si− O-X groups on the surface of the untreated material. Nonetheless, when the original material reacts with KOH, its surface area increases due to the cleavage of the siloxane functional groups resulting in the exposure of more oxygen atoms as either reacted silanol or unreacted siloxane groups in the network. The same justification holds for the increase of the surface calcium, silicon, and potassium elements. 3.4. Textural Properties. The porous structure of the material is one of the essential properties in determining its ability to adsorb the metal ions from the waste effluents. Therefore, the specific surface areas (SBET), and micropore volumes of NMP and A-NMP were calculated by applying the

Figure 1. FTIR spectra of treated and virgin materials.

two materials in a certain range of wavelengths which best characterizes the material. The characteristic peak at 2928 cm−1 implies the stretching C−H bond whose intensity is considerably decreased in the A-NMP spectrum compared to the NMP spectrum. This is due to the carbon burnoff after the activation process verifying the elemental analysis results. The strong absorption band peaked at around 3443 cm−1 in the ANMP spectrum can be assigned to the O−H stretching moiety. On account of the depolymerization of the network in the course of activation, more Si−O−Si and Si−O−Al bonds are broken down into hydroxyl-containing silanol groups and hence the intensity of the peak is increased after activation. 8250



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Brunauer, Emmett and Teller (BET) equation and t-plot method, respectively. It was determined from the results that the specific surface area (SBET) of the material increases from the trivial amount of 0.9 m2/g for the original waste material to 222 m2/g after the activation making it more likely to function as a highly efficient adsorbent. The N2 adsorption−desorption isotherm for the nonactivated material could be categorized as type III which results from the nonporous structure of the original waste material with weak adsorbate−adsorbent interactions. The corresponding isotherm for the activated material is indicative of a type IV isotherm with a distinct H3 hysteresis loop according to the International Union of Pure and Applied Chemistry (IUPAC) classification, which is characteristic of mesoporous materials (see Figure 3 and SI

any micropores. Barrett−Joyner−Halenda calculations for the pore-size distribution, derived from desorption data, reveal a narrow distribution for A-NMP centered at 10 nm. 3.5. Activation Mechanism. The above-characterization methods can give an idea about the activation mechanism of the precursor. It is believed that impregnation of the precursor with the activating reagent causes the hydroxyl groups to diffuse into the tetrahedral network and attack the siloxane bonds resulting in the weakening of the latter bonds. However, this reaction increases the local disorder and entropy and therefore indicating an endothermic reaction. Nevertheless, since impregnation is carried out at room temperature, there is not enough activating energy available to overcome the energy barrier for depolymerization reaction and cleave the bonds to form nonbridging oxygen. In order to provide sufficient energy for the reaction to convert the unreactive siloxane moieties into reactive silanol functional groups, the temperature is increased which then brings about the development of many pores due to the siloxane bond cleavage verified by the BET surface analysis. Furthermore, the development of hydroxyl groups after the activation process was validated by FTIR and XPS confirming the proposed mechanism as depicted in Scheme 1. It is notable that all the parameters are optimized to achieve a higher surface area and enhanced removal efficiency. For instance, when the activation temperature is quite low, the activation energy is not sufficient and no activation occurs whereas at higher temperatures, the pore walls further react bringing about the integration of evolved pores and thus a decrease in the surface area of the material. 3.6. Metal Uptake Studies. The metal removal efficiency of the treated material was investigated in several synthetic wastewater solutions with different concentrations (0.5−5M) of various metal ions at a pH level of 4. These were compared with both the original waste material and three commonly used industrial adsorbents, called Suqing D401, Lewatit TP207 and MCM-41. The isotherms showed the relationship between the

Figure 3. N2 adsorption−desorption isotherm of the activated material.

Figure S1). The latter isotherm exhibited one steep increase at relative pressure p/p0 > 0.8 which was interpreted as capillary condensation in mesopores. Moreover, there is not such a steep rise in the low pressure region which is indicative of the lack of

Scheme 1. Illustrative Activation Mechanism; (a) Aluminosilicate Impregnated in KOH Solution, (b) Reaction of Aluminosilicate with KOH and Development of Pores

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mechanism. Nevertheless, since the mole balance of the adsorbed and desorbed ions mismatch, it is believed that some physisorption also occurs in the adsorption process. 3.8. Comparative Study. Removal of copper, lead, and zinc metal ions by applying the adsorption method has also been studied by other researchers. Peric et al.21 captured 0.4 mmol Cu, 0.45 mmol Pb, and 0.2 mmol Zn metals on a natural zeolite. Rafatullah and co-workers37 have compared the copper and lead adsorption capacities of their adsorbent with eight other researchers and concluded that their removal capability was much higher than these researches. According to their tabulated results, their maximum adsorption was 32 mg/g Cu and 34 mg/g Pb (equivalent to 0.51 mmol/g Cu and 0.16 mmol/g Pb). Qiu et al.38 claimed the maximum exchange level of 2.081 mmol/g Cu, 2.53 mmol/g Pb and 1.53 mmol/g Zn on a synthetic zeolite resin. Comparing their results with many literature values, they concluded a higher adsorption capacity for their material than the commonly used natural zeolites. In a comprehensive review study, the heavy metal removal efficiencies of many low cost adsorbents have been compared by Kurniawan et al.14 Table 1 summarizes the highest reported data in this study for comparison purposes.

amounts of copper, zinc, and lead adsorbed (qe) and their equilibrium concentration (Ce) in solution. NMP cannot adsorb any metal ions, as expected, most probably because of the negligible surface area and no functional moieties involved. However, as shown in Figure 4, A-NMP not only has a better

Figure 4. Copper removal efficiency of A-NMP and three industrial adsorbents and the fit of the Redlich-Peterson adsorption model.

Table 1. Summary of the Highest Reported Adsorption Capacities of Low-Cost Adsorbents and Activated Carbon Extracted from Literature (Kurniawan, Chan, Lo, & Babel, 2006)

removal efficiency compared to the untreated sample, but also functions considerably better than its industrial counterparts for copper ion removal. The trend for the zinc and lead metal ions are similar to the copper metal ion except that the removal efficiency of A-NMP for zinc and lead are 2.0 and 3.4 mmol/g respectively (SI Figure S2). This high efficiency can be attributed to the high porosity development in the modified material and also functionalization of the material during the activation process which not only removes the hydrophobicity problem of the untreated material, but also enhances the interaction between the metal ion and the adsorbent. 3.7. Adsorption Mechanism. The heavy metal (Me2+) adsorption mechanism on the activated material has been illustrated in Scheme 2 which is an ion-exchange mechanism. According to this hypothesis, Me2+ is either exchanged with potassium or hydrogen ions present in the structure of A-NMP resulting in the liberation of potassium ions into the solution. The existence of the potassium ions in the solution after the adsorption process measured by ICP-AES validates this

adsorption capacity (mg/g) source of adsorbent agricultural waste industrial byproducts natural materials miscellaneous

activated carbon this work

adsorption capacity (mmol/g)

type of adsorbent

Cu

Zn

Cu

Zn

soybean hull blast furnace slag

154.9 133.35

NA 103.33

2.46 2.12

NA 1.59

HCl-treated clay cofee residue and clay coal calcined phosphate GAS type C e-waste

83.3 31.5

63.2 NA

1.3 0.5

0.97 NA

NA 29.8

27.2 NA

NA 0.47

0.42 NA

NA 184.5

20 117

NA 2.96

0.31 1.80

Scheme 2. Illustrative Adsorption Mechanism; (a) before Adsorption, (b) after Adsorption

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Table 2. Isotherm Parameters of Copper, Lead and Zinc Adsorption onto the Activated Material model Freundlich

Langmuir

formula

qe =

αF. Ce1/ nF

qe =

KL . Ce 1 + αL . Ce

Sips (Langmuir−Freundlich)

qe =

Redlich-Petereson (RP)

Toth

Temkin

qe =

qe =

αF 1/nF SSE KL αL SSE KS αS 1/nS SSE KR αR βR SSE KTH αTH nTH SSE KT βT SSE

KS. Ce1/ nS 1 + αS. Ce1/ nS KR . Ce 1 + αR . Ce βR

K TH. Ce (αTH + Ce nTH)1/ nTH

qe = βT . ln(K T. Ce)

3.9. pH Effect on the Adsorption Efficiency. Since pH level affects the solubility of the adsorbates, the surface charge of the adsorbent as well as the ionization degree of the adsorbate, the removal of heavy metals from aqueous solutions is also influenced. In order to investigate the role of pH in metal removal efficiency, the initial pHs of the metal solutions were varied over a range of 1.5−6. The adsorption study could not be carried out experimentally at pH values higher than 6 due to the feasibility of metal hydroxide precipitation. The results showed that the adsorbent was not active in the extreme acidic range. At pH levels of less than 6, the percent removal does not change significantly until it reaches pH level of 3, below which the adsorption capacity is drastically affected until it vanishes at pH level of 1.5 (SI Figure S3). The decrease in the adsorption capacity with lowering the pH is on account of the increased number of H+ ions in the bulk which competes with the diffusion of metal co-ions. It is known that hydronium ions compete with metal ions to be adsorbed on the adsorbent which results in lower adsorption capacity. Furthermore, the adsorbed hydrogen ions repel the metal ions due to the repulsive force as a result of charge interaction. 3.10. Adsorption Modeling. The adsorption isotherms have been simulated by applying the well-established fundamental models tabulated in Table 2. It is a common practice to obtain the parameters via the application of the linear regression statistical analysis of the two-component models. Nonetheless, since linearization itself creates significant error, in this study, a nonlinear optimization approach has been adopted to evaluate the fit of the isotherm equation to the experimental data. The suitability of the model equations were evaluated by minimizing the sum of squared error (SSE) across the liquid phase concentration range using the Excel software on a trial-and-error basis. According to the SSE results, all the models except the Temkin model describe the metal adsorption onto the activated material well using the parameters obtained. Interestingly, all the models that best-fit the predicted isotherms based on calculated parameters for the experimental

Cu

Pb

Zn

2.75 0.069 0.13 224.4 76.4 0.007 140 47.3 0.87 0.006 245.8 84.3 0.99 0.004 2.96 0.019 0.85 0.006 3E6 0.18 0.10

3.35 0.04 0.095 454.8 131.1 0.013 425.1 122.5 0.98 0.014 304.8 86.5 1.03 0.0003 3.45 0.002 1.56 0.0003 24011 0.33 2.36

1.64 0.097 0.004 17.0 9.1 0.017 35.0 19.4 1.31 0.006 14.8 7.6 1.04 0.013 1.98 0.099 0.55 0.0007 1.3E6 0.12 0.011

data, that is, Sips, RP, and Toth, are extensions of the Langmuir model and the modifications applied in the models only result in a slight improvement in the SSE. For instance, considering RP model, β is very close to unity in which case it is transformed into the Langmuir equation which is indicative of the reliability of the Langmuir model in this study. The ability of the Redlich-Peterson model to approximate the experimental adsorption data for copper is illustrated in Figure 4 and SI Figure S4. The Langmuir monolayer capacities of the adsorbent for copper, lead, and zinc were found to be 2.92, 3.52, and 1.95 mmol per gram of adsorbent, respectively. In order to find a correlation between the metal ion uptake and metal ion properties, several parameters including electronegativity, stability constant, size of hydrated ion, atomic weight, and mass-to-charge ratio were considered by Dastgheib et al.39 who found that only electronegativities and stability constants correlate with the adsorption levels. The electronegativities of copper, zinc, and lead are 1.90, 2.33, and 1.65, respectively, and the first stability constants of the associated metal hydroxides are 6.3, 4.4, and 6.2, respectively. Considering that one of the dominant mechanisms in this system is ion exchange at the surface level, it is proposed that the more electronegative metal ions will have more attraction to the electrons and thus will result in a higher adsorption affinity. The metal uptake of the adsorbent is in good agreement with the order of electronegativities mentioned. Furthermore, the stability constant is a measure of the stability of the complex formed by a metal ion with a ligand (in this case hydroxide) in aqueous solution. Therefore, higher stability constants result in stronger affinity of the metal ion and hence higher adsorption. According to the above-values, zinc has the lowest stability constant which validates the low metal uptake. However, copper and lead ions have very close stability constants implying that other factors will be more dominant in their adsorption behavior. Overall, these values suggest, as a general trend, that higher electronegativity and higher stability constant correspond to higher adsorption levels. 8253



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(15) Kratochvil, D.; Pimentel, P.; Volesky, B. Removal of trivalent and hexavalent chromium by seaweed biosorbent. Environ. Sci. Technol. 1998, 32, 2693−8. (16) Zhu, C. S.; Wang, L. P.; Chen, W. B. Removal of Cu(II) from aqueous solution by agricultural by-product: peanut hull. J. Hazard. Mater. 2009, 168, 739−46. (17) Shukla, A.; Zhang, Y. H.; Dubey, P.; Margrave, J.; Shukla, S. The role of sawdust in the removal of unwanted materials from water. J. Hazard. Mater. 2002, 95, 137−52. (18) Crist, R.; Martin, J. R.; Chonko, J.; Crist, D. R. Uptake of metals on peat moss: an ion-exchange process. Environ. Sci. Technol. 1996, 30, 2456−61. (19) Valix, M.; Cheung, W. H.; McKay, G. Preparation of activated carbon using low temperature carbonisation and physical activation of high ash raw bagasse for acid dye adsorption. Chemosphere 2004, 56, 493−501. (20) Chen, S. B.; Zhu, Y. G.; Ma, Y. B.; McKay, G. Effect of bone char application on Pb bioavailability in a Pb-contaminated soil. Environ. Pollut. 2006, 139, 433−9. (21) Perić, J.; Trgo, M.; Vukojević Medvidović, N. Removal of zinc, copper and lead by natural zeolite-a comparison of adsorption isotherms. Water Res. 2004, 38, 1893−9. (22) Wallenberger, F. T.; Hicks, R. J.; Bierhals, A. T. Design of environmentally friendly fiberglass compositions: ternary eutectic SiO2−Al2O3−CaO compositions, structures and properties. J. NonCryst. Solids 2004, 349, 377−87. (23) Long, L.; Sun, S.; Zhong, S.; Dai, W.; Liu, J.; Song, W. Using vacuum pyrolysis and mechanical processing for recycling waste printed circuit boards. J. Hazard. Mater. 2010, 177, 626−32. (24) Wong, M. H.; Wu, S. C.; Deng, W. J.; Yu, X. Z.; Luo, Q.; Leung, O. W.; Wong, C. S. C.; Luksemburg, W. J.; Wong, S. Export of toxic chemicalsA review of the case of uncontrolled electronic-waste recycling. Environ. Pollut. 2007, 149, 131−40. (25) Duan, H.; Li, J.; Liu, Y.; Yamazaki, N.; Jiang, W. Characterization and inventory of PCDD/Fs and PBDD/Fs emissions from the incineration of waste printed circuit board. Environ. Sci. Technol. 2011, 45, 6322−8. (26) Söderström, G.; Marklund, S. PBCDD and PBCDF from incineration of waste-containing brominated flame retardants. Environ. Sci. Technol. 2002, 36, 1959−64. (27) Li, Y.; Li, J.; Chen, S.; Diao, W. Establishing indices for groundwater contamination risk assessment in the vicinity of hazardous waste landfills in China. Environ. Pollut. 2012, 165, 77−90. (28) Wang, X.; Gaustad, G. Prioritizing material recovery for end-oflife printed circuit boards. Waste Manage. 2012, 32, 1903−13. (29) Splajt, T.; Ferrier, G.; Frostick, L. E. Monitoring of landfill leachate dispersion using reflectance spectroscopy and groundpenetrating radar. Environ. Sci. Technol. 2003, 37, 4293−8. (30) Oishi, T.; Koyama, K.; Alam, S.; Tanaka, M.; Lee, J. C. Recovery of high purity copper cathode from printed circuit boards using ammoniacal sulfate or chloride solutions. Hydrometallurgy 2007, 89, 82−8. (31) Cui, J.; Zhang, L. Metallurgical recovery of metals from electronic waste: A review. J. Hazard. Mater. 2008, 158, 228−56. (32) Koyanaka, S.; Ohya, H.; Endoh, S.; Iwata, H.; Ditl, P. Recovering copper from electric cable wastes using a particle shape separation technique. Adv. Powder Technol. 1997, 8, 103−11. (33) Luga, A.; Dǎscǎlescu, L.; Morar, R; Csorvassy, I.; Neamiu, V. CoronaElectrostatic separators for recovery of waste non-ferrous metals. J. Electrost. 1989, 23, 235−43. (34) Zhang, S.; Forssberg, E. Mechanical separation-oriented characterization of electronic scrap. Resour., Conserv. Recycl. 1997, 21, 247−69. (35) Yuan, S.; Chen, X. li; Li, J.; Wang, F. CO2 gasification kinetics of biomass char derived from high-temperature rapid pyrolysis. Energy Fuels 2011, 25, 2314−21. (36) Tsai, W.; Chang, C.; Wang, S.; Chang, C.; Chien, S. Preparation of activated carbons from corn cob catalyzed by potassium salts and subsequent gasification with CO2. Bioresour. Technol. 2001, 78, 203−8.

The results obtained in this study demonstrate the high heavy metal removal efficiency for a cost-effective novel adsorbent produced from an environmentally problematic electronic waste. Copper, lead, and zinc uptakes by this innovative material were tested and not only the modified material functions significantly better than the original waste material, but also has a better removal efficiency than three common industrial adsorbents.

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ASSOCIATED CONTENT

S Supporting Information *

Some additional Tables and Figures are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +852 23588412; fax: +852 23580054; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Hong Kong Research Grant Council for their support of this research. REFERENCES

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Toxic Heavy Metal Capture Using a Novel Electronic Waste-Based

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