Mechanistic Insights into Copper Removal by Pyrolytic Tire Char

Apr 8, 2010 - pyrolysis of waste tires for the production of active carbon ... although numerous studies on the use of pyrolytic tire chars have been ...
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Ind. Eng. Chem. Res. 2010, 49, 4528–4534

Mechanistic Insights into Copper Removal by Pyrolytic Tire Char through Equilibrium Studies Augustine Quek,† Xiu-Song Zhao,‡ and Rajashekhar Balasubramanian*,† Department of Chemical and Biomolecular Engineering and DiVision of EnVironmental Science and Engineering, Faculty of Engineering, National UniVersity of Singapore, Singapore 117576, Singapore

This study showed that copper(II) can be removed from aqueous solution by activated pyrolytic tire char in three mechanistically distinct ways. On the basis of equilibrium studies, the mechanisms involved in the adsorptive removal of copper(II), namely, precipitation, surface adsorption, and pore diffusion, were elucidated. Precipitation of copper(II) resulted from changes in the solution pH to neutral levels. This was attributed to amphoteric zinc oxide on the char surface, formed during production of the pyrolytic char. Surface adsorption was revealed by X-ray photoelectron spectroscopy data, which showed a significant increase in copper(II) on the char surface after shaking in the copper solution. This surface adsorption took place despite the relatively low surface area and porosity of the char. However, some cracks and fissures were found to exist in the char that can trap small species such as copper ions. These trapped copper species were partially recovered by microwave-assisted acid digestion of the char. Introduction A recent estimate indicates that about 1.3 billion waste tires are produced annually worldwide.1 It is logical to assume that one tire enters the waste stream for every tire that is produced. Waste tires have been known to harm landfill covers and, in the case of incineration, produce harmful pollutants such as polycyclic aromatic hydrocarbons, benzene, styrene, phenols, and butadiene.2 The problem of scrap tires would also be amplified in the near future as rapid urbanization and improvements in standards of living increase vehicle population. In addition, other waste streams such as wastewaters would also increase. There is thus an urgent need to find effective, economic, and environmentally sound solutions to treating scrap tires and wastewaters. Pyrolysis has been shown to be a technically feasible method for recovering valuable products from wastes, such as carbon sorbents and oil and gas from scrap tires.3-8 Pyrolysis can be thought of as the thermal breakdown of long-chain organic polymers into simpler molecules in the absence of air. Specifically, numerous studies have shown the favorable use of pyrolysis of waste tires for the production of active carbon adsorbents9-21 for wastewater treatment.8,10,16-21 The use of pyrolyzed tire chars to remove hazardous compounds from waste aqueous streams can be a cost-effective solution. However, although numerous studies on the use of pyrolytic tire chars have been carried out over the years,3-21 the mechanisms involved in the sorption process are not completely understood yet. An early work by Lucchesi and Maschio used pyrolyzed tire char for the adsorptive removal of acid black and orange II dyes.8 They showed that tire chars had a higher adsorption capacity for the dyes despite having a much lower surface area than commercial activated carbon.8 This poor correlation between surface areas and adsorption capacities was also corroborated by later works.14,16,17,19 Very little research has been carried out so far to provide mechanistic insights into the adsorption of metal ions onto * To whom correspondence should be addressed. Tel: +65 6516 5135. Fax: +65 6516 5266. E-mail: [email protected]. † Division of Environmental Science and Engineering. ‡ Department of Chemical and Biomolecular Engineering.

pyrolyzed tire chars, as compared to those for organic molecules. CO2-activated tire char was used to remove chromium(VI) by Hamadi et al.,18 who showed that chromium(VI) could be removed within 2 h, with the highest removal at low pHs and high temperatures. This showed that electrical forces are primarily responsible for attracting the chromate anions to the positively charged tire char surface at low pH values. Helleur et al.17 obtained carbonized tire char at 600 °C and reported that the copper(II) adsorption capacity for these chars was three times as much as that for commercial activated carbons, even though the commercial carbons had more than twice the surface area of the carbonized char. There was also an improvement in lead adsorption when 2% oxygen was used during activation. They inferred from their results that more pores become available for the adsorbate after activation but without measurement of the pore volumes.17 In our earlier work, the removal of copper and lead from postpyrolysis oxygenated (PPO) tire char has also been shown to be higher than that of commercial activated carbons,22 although the latter had a higher surface area. A range of physical and chemical processes exists for the removal of heavy metals from wastewater. These include thermochemical precipitation, filtration, ion exchange, sorption via surface complexation/chelation, and pore filling.23 It is therefore likely that more than one mechanism is operative in the removal process of heavy metals from solution in the presence of a solid material such as tire char. This paper reports the results of a fundamental mechanistic study in relation to removal of copper ions from an aqueous solution using pyrolytic char, as well as the implication for the removal of other heavy metals. Copper was selected because it is ubiquitous in the anthropogenic environment, largely as a result of its use in agriculture (e.g., pesticides), the building industry, and electronic products. This has caused subsequent disposal problems for copper from contaminated drinking water to electronic waste disposal. The toxicity of copper arises mainly from its ability to accept and donate single electrons as it changes oxidation state. This redox reaction catalyzes the production of very reactive radical ions such as the hydroxyl radical.24 The catalytic activity of copper is used by the enzymes that it is associated with and is thus only toxic when unsequestered and unmediated. Thus, the mechanisms of the removal of copper from aqueous

10.1021/ie901289e  2010 American Chemical Society Published on Web 04/08/2010

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streams such as wastewaters by pyrolytic tire char should be understood in the interest of human health. This study reveals three distinct mechanisms for copper removal: precipitation, surface adsorption, and pore diffusion. The mechanistic insights are provided through a combination of pH measurements, X-ray photoelectron spectroscopy (XPS), and microwave-assisted acid digestion. The results showed that copper removal in an aqueous medium by pyrolyzed tire chars activated in a low-oxygen atmosphere is not a simple adsorption process. Thus, traditional curve-fitting methods using isotherm models or adsorption energies do not fully express the variety of reactions underlying heavy-metal removal. The need for detailed mechanistic investigations is even more important when using sorbents made from waste solids, which are heterogeneous in nature.

Table 1. Properties of the Pyrolyzed Char and the Raw Tire

Materials and Methods

run, at least three samples were used and one flask of the solution without solids was also tested as a blank experiment. The effect of the pH was studied using eight different initial pH values ranging from 2 to 9 at a room temperature of 24 °C. The solutions were then filtered off using a 0.45 µm paper filter to remove the solids from the solutions. For XPS measurements, all chars were dried in an oven at 50 °C for 24 h before measurement. The copper concentrations before and after adsorption were measured using inductively coupled plasma optical emission spectrometry (Perkin-Elmer ICP Optima 3000DV) and atomic absorption spectroscopy (Perkin-Elmer AAnalyst 300) to check that the results are not machinedependent. At least four different concentrations and a blank (deionized water) were used for instrumental calibration and the generation of a calibration plot.

Pyrolysis and Activation. Pyrolysis of scrap tire samples was carried out in a horizontal tubular reactor with a N2 gas flow rate of 0.6-0.8 L/min, at a heating rate of 20 °C/min, until 550 °C was reached. This temperature was held for 1 h in order to ensure that all pyrolytic reactions were completed. The temperature of 550 °C was chosen for further experiments based on the optimum results obtained, and it is the commonly accepted temperature at which pyrolysis of the tires is believed to be completed.17,21,25-27 The char was further activated in situ by PPO, where 7% oxygen (with the balance 93% nitrogen) was introduced into the pyrolysis furnace to oxygenate the char after pyrolysis was completed and the temperature started to decrease. This oxygenation was carried out between the temperatures of 550 and 250 °C, and the resulting char was named P550250. Tire char pyrolyzed without oxygenation was also produced in a similar manner and referred to as NoPPO. The details of the pyrolysis and activation process have been reported elsewhere.22 Char Characterization. The ASTM D3172-07 method was used in the proximate analysis, using a Carbolite CWF 1100 furnace (Sheffield, U.K.). For bulk metal composition, a composite mixture of three concentrated chemicals (HNO3:HF: H2O2 ) 14:1:4) was used to digest the pyrolyzed chars. The digestion process was assisted by microwave irradiation by placement of the mixtures in a microwave (MLS-1200 mega, Milestone srl, Sorisole, Italy) at a maximum power of 600 W for 12 min. The digested solutions were diluted and analyzed using inductively coupled plasma mass spectrometry (ICP-MS; Perkin-Elmer Elan 6100 ICP-MS, Perkin-Elmer Inc., Waltham, MA). The ζ potential readings were taken with a Zeta-sizer Nano from Malvern Instruments Ltd. (Worcestershire, U.K.). For measurements of the pore-size distributions and BrunauerEmmett-Teller (BET) surface areas, the NOVA 4200 Multistation Anygas Sorption Analyzer (Quantachrome Instruments, Boynton Beach, FL) was used. X-ray Photoelectron Spectroscopy (XPS). The char samples were characterized using XPS (Kratos AXIS Hsi, from Kratos Inc., Manchester, U.K.) for surface elements. It is capable of detecting up to 1/1000 entities, or 0.1% atomic concentration. A curve-fitting software (XPSPEAK, version 4.1) was used to deconvolute the peaks and elucidate the corresponding surface functional groups. Equilibrium Sorption Studies. For equilibrium experiments, each sample of 100 mg of tire char, with 100 mL of a copper solution (Cu(NO3)2) at 25 mg/L (3.93 × 10-4 mol/L), was shaken for 14 h at a speed of 150 rpm, with the initial pH of the solutions adjusted to 5.0 ((0.2) at 24 °C (297 K). For each

volatile matter (wt %) fixed carbon (wt %) ash (wt %) moisture (wt %) Cu (wt %) Fe (wt %) Si (wt %) Zn (wt %) Ca (wt %) Mg (wt %) K (wt %) pH at zero charge BET surface area (m2/g) pore volume (pore size < 50 nm) (cm3/g)

NOPPO

PPO char (P550250)

0.41 ( 0.25 90.5 ( 2.5 8.63 ( 0.25 0.06 ( 0.10 0.008 ( 0.001 0.089 ( 0.009 2.662 ( 0.075 2.525 ( 0.007 0.368 ( 0.068 0.040 ( 0.012 0.142 ( 0.07 7.95 72.4 ( 12.1 0.379 ( 0.022

5.81 ( 1.01 82.9 ( 6.50 10.9 ( 1.81 0.39 ( 0.02 0.017 ( 0.003 0.205 ( 0.078 3.766 ( 0.350 3.518 ( 0.353 0.382 ( 0.038 0.117 ( 0.04 0.157 ( 0.04 4.16 74.5 ( 15.1 0.253 ( 0.012

Results and Discussion Adsorbent Bulk and Surface Characteristics. Table 1 shows several characteristics of the pyrolyzed chars, including the ash content, mineral content, and surfaces areas. Among the 24 elements analyzed by ICP-MS, only some could be detected, and these elements are shown in Table 1. Of these elements, only zinc and alkaline-earth metals are considered to be of significance in the adsorption of heavy metals because these elements can change the pH in the bulk solution. From Table 1, it can be seen that zinc and silicon are the only major inorganic constituents in the tire char, which is consistent with previous findings.4,7,11,28,29 Among these elements, zinc is the more reactive element, with the next-largest component being calcium, which is added as carbonate during the tire manufacturing process. Both the carbonates and amphoteric zinc oxide (ZnO) play a role in moderating the pH of the aqueous environment. The low surface area of 74.5 m2/g agrees with those resulting from previous works.11,14 These are likely to influence the adsorption capacities of the pyrolytic char and adsorption mechanisms involved, as discussed in later sections. Surface Composition. Figure 1 shows the major atomic species on the char surface. The biggest peak is the carbon peak at a binding energy (BE) of 280-295 eV, while the other major species are oxygen (BE ) 525-540 eV) and zinc (BE ) 1021.8-1021.9 eV). No other peaks of any significant signal could be detected by this method, indicating that any surface functional groups must be composed of only Zn, O, C, and possibly H atoms. Figure 2 shows the BE peaks for the Zn2p electrons. Similar peaks were obtained and analyzed for the C1s, O1s, and N1s atoms. Curves were also fitted for each BE according to its

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Figure 1. Wide-scan XPS spectrum of pyrolyzed, (a) unoxygenated char and (b) char oxygenated from 550 to 250 °C for 2.5 h using 7% oxygen (P550250).

Figure 2. Zn2p spectra curve fitting for (a) NoPPO char, showing only the zinc peak, and (b) P550250 char, showing a large oxide peak. Table 2. Quantification of Surface Elements Based on Their Peak Areas by XPS relative atomic concentration (%) element

NoPPO

P550250

Zn O C N

1.57 3.66 94.0 0.76

6.79 8.85 83.7 0.65

corresponding functional group(s), which have been identified by earlier researchers.30,31 Table 2 gives the surface relative mass concentrations of these four elements, as calculated from their peak areas. No strong peaks for nitrogen could be detected (Table 2), indicating a lack of nitrogen groups on the carbon surface, and these groups are thus of no further interest for discussion. The values also strongly indicate that oxygenation of the char does indeed occur in this PPO process. The Zn2p peak of atomic zinc at a BE of 1021.8-1021.9 eV and its oxide peak at 1022.5 eV30 can be used to identify the extent of oxidation that PPO has on the pyrolyzed char. Figure 2 shows the result of this fit. The char that has been oxygenated (P50250) shows a large oxide peak to the left of the main zinc peak, with the oxide peak area larger than the zinc peak area. The unoxygenated char (NoPPO) shows almost no oxide peak. Similarly, the curve fitting for the various peaks to the O1s spectra30-32 for the two char types revealed that the ZnO peak (BE ) 530.4 eV) was much larger for P550250 than for the NoPPO char (530.8 vs 24.7 units). The oxygenated char also had higher proportions of C-O-H and/or C-O-C groups compared to CdO groups, which suggests a higher degree of saturation and higher oxygen concentration on the surface than the unoxygenated char. Small peaks could also be fitted for chemisorbed oxygen.

Table 3. Assigned Groups’ BEs and Respective Relative Areas to the Carbon Peak30,31 assigned group(s) on P550250

BEs (eV)

relative area (m2/g)

π-π* transitions in aromatics COOH, COOR carbonyl CdO C-O, C-O-C main graphitic peak carbidic

291.2-292.1 289.3-290.0 287.5-288.1 286.3-287.0 284.6-285.1 282.6-282.9

7.681 5.755 4.587 22.82 100 0.174

Table 3 shows the assignments and the BEs for the different peaks corresponding to each functional group under the C1s spectra, together with the normalized peak areas, compared to the main graphitic peak for P550250. The peak corresponding to the C-O group was the second largest, in relation to the other oxygen peaks as well as to the main graphitic peak. Effect of the pH on Copper Removal. It was observed that the pH varied significantly from its initial value after the char was shaken in a solution for a few hours. The final pH value was measured to be 7 ((0.4), with the exceptions of initial pH 2 and 3. This variation in the pH can be explained by the fact that ZnO inherent in the tire char has an amphoteric character and is able to adsorb both acidic and alkaline species in water. Precipitation is thus a significant mechanism for heavy-metal removal in this case, and above an initial pH 7, there is total removal of all aqueous copper, with precipitation being the main mechanism. The different amounts of copper removed at various pH values are shown in Figure 3. The removal of copper by adsorption is the amount removed that was measured against a blank, which is another 100 mL of a copper solution of the same concentration and initial pH but without the char sorbent. Subtracting the values from the blank values would measure copper removal due to the mechanisms other than the precipitation mechanism.

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Figure 3. Percentage of copper removed and final pH with varying initial pH.

Figure 4. Equilibrium copper speciation in the aqueous phase.

Because the solution pH is considered to be a major factor influencing the sorption chemistry, a different initial solution pH was used in the adsorption studies. Copper removal through sorption showed a peak at around pH 4 and decreased drastically from pH 7 to 9. This trend is due to the low solubility of the metal at these higher pHs,35 precipitating most of the copper even without the tire char. Several studies have also observed the phenomenon of increased solution pH after the addition of the sorbent.34-36 Machida et al.34 also found significant increases in the pH for charcoal ash in a solution. The pH did not increase when the charcoal was demineralized with inorganic acids but caused a decline in lead removal instead. Namasivayam and co-workers35,36 observed that carbons made from coirpith and peanut shells were able to increase the pH of a solution through an ion-exchange mechanism. The equilibrium copper speciation for copper at each pH is well-known and can be calculated from software. The freeware MINTEQ was used in this case (Visual MINTEQ, version 2.53, by Jon Petter Gustafsson, Stockholm, Sweden). The result of the copper solubility calculation at each pH is shown in Figure 4. On the basis of these calculations, the char-induced precipitaion accounts for a reduction of 1.23 × 10-4 M (7.81 mg/L) in concentration, or approximately one-third of the initial concentration of 3.93 × 10-4 M (25 mg/L), at an initial pH of 5. Because this is less than the total amount of copper removed from solution, other mechanisms were involved in the removal of the remaining copper by PPO tire char. This is shown in the following sections. Surface Adsorption. The surface of the char before and after shaking in a copper solution was characterized using XPS. There is a small, but significant, increase in the intensity of the peaks due to copper at BEs of 928-960 eV30 (Figure 5). Intensity counts increased from 680 cps at a maximum peak height before

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addition into a copper solution to more than 850 cps after shaking in a copper solution. The difference in the relative atomic copper surface concentrations before and after shaking in a copper solution was approximately 3.5%. Assuming a char surface area of 75 m2/g (Table 1) and assuming a monolayer adsorption, only about 2.63 m2/g of the surface was adsorbed with copper. Using an ionic radius of 0.087 nm for Cu2+, it can be estimated that 1.10 × 1020 Cu atoms were adsorbed on the char surface for each 1 g of char. This translates to a 1.83 × 10-4 mol/L (11.6 mg/L) decrease in the concentration. Thus, adsorption accounts for almost half (∼48.5%) of the total amount of copper removed from the solution under these experimental conditions. Thus, the removal of aqueous copper by pyrolytic tire char can be explained by at least two mechanisms, precipitation and surface adsorption. This will be further discussed in a later section. The adsorptive ability of carbon surfaces has traditionally been attributed to various oxygen surface functional groups. Functional groups such as carboxyl, hydroxyl, and carboxylic, presented earlier (Table 3), are essential for the sorption of heavy metals34-39 such as copper. These functional groups and their respective areas are presented in Table 4, as calculated using data from Tables 2 and 3. The total surface area of the char, 74.5 m2/g, from Table 1 was used. Next the surface areas covered by each element are calculated from Table 2. For carbon, the atomic concentration is 83.7%, which covers 62.4 m2/g. Then 62.4 m2/g was used as the basis to calculate the other carbon functional groups by proportion. For example, carbonyl accounts for 3.25% (4.587/141) of 62.4 m2/g, which gives 2.03 m2/g. The result is shown in Table 4, which also shows that the surface areas of these functional groups are more than sufficient for the amount of copper adsorbed, as calculated earlier. In addition, the mineral content of carbon also plays a significant role in heavy-metal removal. Machida et al.34 found drastic decreases in lead removal after demineralization of charcoal, with a decline from 99% to as much as 4% lead removal. Kikuchi et al.38 found that ZnO loading on an activated carbon increased the removal of lead, copper, and cadmium. They concluded that hydroxyl groups, created on ZnO, are responsible for lead adsorption, while carboxyl groups are adsorption sites for the oxidized activated carbon. In this work, the surface area of ZnO on the char is more than 5 m2/g (Table 4), which is significantly more than the area of the adsorbed copper on the char surface (3.5%). The total area of all possible functional groups of 23.2 m2/g (Table 4) is 10 times more than that of the adsorbed copper surface. One reason is that competition with protons (H+) and hydronium ions (H3O+), especially at lower initial pHs, means that a sizable portion of the usable surface area is adsorbed with these ions. Adsorption of H+ ions also explains the increases in the pH values that were observed (Figure 3). Another reason is that copper adsorbed mostly as complexes,40 shown in Figure 4, which actually takes up more space than the assumed uncoordinated copper ion (Cu2+). Last, surface metal oxides have also been shown to adsorb heavy metals through the formation of mixedcation hydroxides,41 which require even more surface area than a simple adsorption of copper complexes. Acid Desorption and Acid Digestion. In addition to analysis of the solution, the chars were drained from the solution and also subjected to further analysis. After drying at room temperature (24 °C) overnight, the chars were shaken in 0.1 M nitric acid for 4 h at 150 rpm, in order to desorb the attached copper. The acid solution was diluted and analyzed. However, only a

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Figure 5. XPS copper peak of (a) P550205 char not used in an aqueous solution with a maximum intensity of 680 cps and of (b) P550205 after shaking in a copper solution for 14 h, maximum intensity 850 cps, both at BEs between 928 and 960 eV. Table 4. Calculated Surface Areas of Functional Groups Known for Heavy-Metal Sorption group 2

surface area (m /g)

COOH, COOR

carbonyl CdO

C-O, C-O-C

aromatics

ZnO

total

2.55

2.03

10.09

3.40

5.1

23.2

portion (∼52%) of the copper could be recovered, as compared to its initial amount. The acid-leached chars were then subjected to microwaveassisted acid digestion, as described earlier, to further break down the chars and recover any copper within the chars themselves. A composite mixture of three concentrated chemicals (HNO3:HF:H2O2 ) 14:1:4) was used to digest the pyrolyzed chars. The digestion process was assisted by microwave irradiation by placing the mixtures in a microwave at a maximum power of 600 W for 12 min. The digested solutions were diluted and analyzed using ICP-MS. The results are shown in Figure 6. Assuming that the total amount of copper removed is due to precipitation, surface adsorption, and pore diffusion only, the theoretical amounts of copper recoverable from surface adsorption by acid leaching and from pore diffusion by acid digestion can be calculated. The amount of copper recovered in the experiments was less than the calculated theoretical amount. One reason is that not all of the char could be removed from the copper solution for acid leaching and digestion. The unrecovered chars were mainly extremely small particle sizes that either coated the walls of the glassware as a thin film or were washed out with the solution. These small-sized chars could have a higher proportion of copper on them compared to the rest of the recovered chars. Although the amount recovered was less than expected, the values are still qualitatively meaningful. This provided evidence

that there is a proportion of copper removed from the solution that is located within the chars. A greater proportion of the expected copper amount was recovered from acid digestion (∼68%) than from acid leaching (∼52%). This indicates that a certain proportion of copper was able to overcome surface boundary barriers, entered, and resided in the few micro- and mesopores in the char. In addition, these copper species were able to resist washing and acid leaching and could be detected only when the chars were well broken up. It is thus evident that copper removal from an aqueous solution by pyrolytic tire char does not take place through a single mechanism. On the basis of the results presented, precipitation accounts for about one-third (32.9%) of the removal because of the pH increase caused by amphoteric ZnO in an acidic solution. Adsorption onto the char surface is the major mechanism and accounts for nearly half (48.5%) of the copper sequestration from solution under the experimental conditions. Less than one-fifth (18.6%) of the total copper removed could be found adsorbed within the char itself because of the added barriers that fluid dynamics present. These findings are summarized in the figure below (Figure 7). The oxygenated, pyrolytic tire char could remove aqueous copper from solution through multiple mechanisms despite having a relatively low surface area (Table 1). The effect of the surface reactivity over the surface area was also observed by other researchers. Aziz et al.33 studied copper removal by

Figure 6. Expected copper recovery (theoretical) from copper-sorbed char and the actual results (experiment) after sorption in a 25 mg/L (3.93 × 10-4 mol/L) copper solution.

Figure 7. Copper removal from a solution through various mechanisms.

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Figure 9. Effect of the temperature on various copper removal techniquess through precipitation and sorption. Figure 8. Final total soluble copper concentration at different temperatures and different pHs, with an initial concentration of 3.93 × 10-4 mol/L (25 mg/L), as calculated by MINTEQ. Table 5. Precipitation and Sorption Copper Removal at Different Temperatures temp total removed (°C) (mol/L) 24 35 50

-4

3.77 × 10 3.75 × 10-4 3.73 × 10-4

precipitation mol/L

% of total -4

1.23 × 10 1.30 × 10-4 1.49 × 10-4

32.6 34.7 39.9

sorption (surface + pore) mol/L

% of total -4

2.70 × 10 2.45 × 10-4 2.24 × 10-4

71.6 65.3 60.1

limestone and activated carbon in both batch and continuous processes. Under their experimental conditions, there was little difference in copper removal regardless of the ratio of limestone to activated carbon used. They concluded that both adsorption and absorption processes were responsible for copper removal, and limestone can be used as a cost-effective medium to replace activated carbon for metal removal. Helleur et al.17 also found higher lead and copper removal for carbonized tire char than activated carbon. Temperature Effects. The same experiment (25 mg/L, initial pH 4.5) was carried out at two other different temperatures of 35 and 50 °C. On the basis of MINTEQ calculations, the pH change was less than 0.3 units for a temperature change from 24 to 50 °C, which is less than the standard deviation, measured for the final pH change here, and was thus not considered to be significant. The total soluble copper concentration at each temperature was also calculated, and the amount of copper precipitated could thus be calculated. This is shown in Figure 8. Overall, a slight decrease in copper removal was found with an increase in the temperature. The quantitative measurements are shown in Table 5, and the relative proportions can be seen in Figure 9. Results from Table 5 and Figure 9 showed that, although more copper was precipitated from a solution at higher temperatures at a final pH of 7, less copper was removed through the other two mechanisms of surface adsorption and pore diffusion. This observation agrees with the fact that adsorption is an exothermic reaction.39 The implication is that, at higher temperatures, copper removal by pyrolytic tire char through sorption processes becomes less important, while precipitation turns out to be more significant. One reason for the increase in the precipitation of copper at increased temperatures is that, at higher temperatures, the solubility of CO2 decreases. This increases the pH as well as the concentration

of other noncarbonate copper species. These two factors together increase the precipitation of copper from a solution. Conclusion Several mechanistic insights are provided for the removal of copper by pyrolyzed oxygenated tire char. The buffering of the pH to neutral levels is believed to be due to the ZnO present on the tire char. The amphoteric nature of ZnO helps to reduce the solubility of the copper ions when the initial pH is acidic and to sequester aqueous copper out of the solution through precipitation. This precipitative removal becomes more significant at higher temperatures, accounting for almost 40% of the total copper removed at 50 °C. Sorption of the copper can occur on the exterior tire char surface as well as diffuse deeper into the cracks within the chars. At a temperature of 24 °C, copper adsorption on the surface accounts for nearly 50% of the total copper removed, while pores remove about 18%. The overall copper removal reaction was independent of temperature changes due to increased precipitation, with lower sorption at higher temperatures and at an equilibrium pH of 7. This work also showed that, in addition to physical properties such as high surface areas, chemical properties such as surface chemistry also play important roles in adsorption processes. In addition, the tire char sorbents also do not require hazardous chemicals or high energy consumption to produce. There is thus potential for both material recovery and reuse and pollution abatement by utilizing waste tires for char production. Although the carbon chars produced would not necessarily be appropriate for critical applications such as potable water treatment, applications related to the treatment of industrial wastewaters are still viable. Literature Cited (1) Beecham, M. Global Market ReView of AutomotiVe TyressForecasts to 2014; Aroq Ltd.: Worcestershire, U.K., 2008. (2) Reisman, J. I. Air Emissions from Scrap Tire Combustion. EPA600/R-97-115, 1997. (3) Mui, E. L. K.; Ko, D. C. K.; McKay, G. Production of Active Carbons from Waste TyressA Review. Carbon 2004, 42, 2789. (4) Ko, D. C. K.; Mui, E. L. K.; Lau, K. S. T.; McKay, G. Production of Activated Carbons from Waste TiresProcess Design and Economical Analysis. Waste Manage. 2004, 24, 875. (5) Unapumnuk, K.; Lu, M.; Keener, T. C. Carbon Distribution from the Pyrolysis of Tire-derived Fuels. Ind. Eng. Chem. Res. 2006, 45, 8757. (6) Arabiourrutia, M.; Olazar, M.; Aguado, R.; Lo´pez, G.; Barona, A.; Bilbao, J. HZSM-5 and HY Zeolite Catalyst Performance in the Pyrolysis of Tires in a Conical Spouted Bed Reactor. Ind. Eng. Chem. Res. 2008, 47, 7600.

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ReceiVed for reView August 16, 2009 ReVised manuscript receiVed March 1, 2010 Accepted March 23, 2010 IE901289E