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Ind. Eng. Chem. Res. 2006, 45, 1557-1562

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Optimum pH for Cr6+ Co-removal with Mixed Cu2+, Zn2+, and Ni2+ Precipitation Jing-Mei Sun,*,†,‡ Feng Li,† and Ju-Chang Huang‡ School of EnVironmental Science and Engineering, Tianjin UniVersity, Tianjin 300072, P. R. China, and Department of CiVil Engineering, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

It is advisable to co-remove Cr(VI) with available Cu(II), Zn(II), and Ni(II) since they coexist in most plating wastewater. Previous studies showed that coprecipitation and adsorption are the main mechanisms contributable to Cr(VI) co-removal with Cu(II) precipitation, and both are highly pH dependent. This study presents the effect of pH on Cr(VI) co-removal with mixed metal precipitation in batch tests and also in a continuous compact system. Batch tests indicate that a maximum of 46.8 mg L-1 Cr(VI) was co-removed with the precipitation of Cu(II), Zn(II), and Ni(II), each 150 mg L-1, at pH of 7.0-7.3. However, co-removal of Cr(VI) decreased significantly with further pH increasing. Therefore in the continuous system, a two-stage nucleated precipitation technology was designed with the first stage being operated at around pH 7.2 to obtain maximum Cr(VI) co-removal and the second stage at around pH 9.2 to achieve further Cr(VI) co-removal with Zn(II) and Ni(II) precipitation. 1. Introduction Many industries, including electroplating, printed circuit, metal finishing, and dye manufacturing, etc., generate wastewater containing variable levels of Cr(VI), Cu(II), Zn(II), and Ni(II). Common methods for removing heavy metals from wastewater include precipitation, oxidation/reduction followed by precipitation, adsorption, complexation and cementation, electrolytic reduction for recovery, ion exchange, reverse osmosis or evaporative separation for polishing effluent or metal recovery, or a combination of these processes. Due to its cost effectiveness and being technically simple to operate, chemical precipitation is the most widely used method for heavy metal removal from industrial wastewater, especially for high-loading (either volume or mass) wastewater streams.1,2 Removal of heavy metal ions as Cu2+, Zn2+, and Ni2+ in aqueous solution typically involves the use of hydroxides, carbonates, or sulfides to induce metal precipitation followed by a solid-liquid separation, solid dewatering, and solidification.2,3 The entire treatment scheme requires a substantial space for installing the treatment facilities. This is not applicable for the small-scale industrial operation or for cities with lack of space, such as Hong Kong, Tokyo, etc. Removal of heavy metals can also be achieved by heterogeneous (surface) precipitation.4-6 That is, some solid media are provided to allow most of the precipitation to take place on the media surface, i.e., heterogeneous deposition. Even though some precipitation takes place in liquid phase, i.e., bulk precipitation or discrete precipitation, some of their precipitates can further adsorb or coat on the surface of solid media (others may form small particles (fines) through secondary nucleation in liquid phase). In these two cases, the solid-liquid separation step can be easily accomplished by capturing these media. Further, the solid media presented in heterogeneous precipitation can also lower the activation energy to enable the precipitation to take place at a lower solubility product of metal and ligand. For example, in a heterogeneous environment, precipitation can * To whom correspondence should be addressed. Tel.: +86 22 27890017. Fax: +86 22 87402072. E-mail: [email protected]. † Tianjin University. ‡ Hong Kong University of Science and Technology.

occur when the solubility product is slightly higher than the theoretical Ksp, while in a homogeneous environment, precipitation can only occur at a product concentration approximately 40 to 50 times higher than the Ksp.1,7 That is, heavy metal removal can be obtained at a lower pH value, and alkaline dosage is less than that in the homogeneous precipitation process. Cr(VI) exists mainly in soluble forms of HCrO4-, Cr2O72-, and CrO42- in aqueous solution, with HCrO4- being predominant at pH below 6.5 while changing to CrO42- being predominant at pH above 6.5. All of HCrO4-, Cr2O72-, and CrO42- cannot react with either carbonate or hydroxide ions to form a precipitate unless they are first reduced to trivalent form (Cr3+ ions). However, the process of reducing the Cr(VI) to Cr(III) followed by hydroxide precipitation of Cr(III) consumes a large amount of reducing agents and produces a large quantity of chemical sludge, both of which increase the cost of treatment. Some have attempted to remove Cr(VI) directly by adsorption on oxide surfaces, such as pure and peroxide-modified titanium dioxide (TiO2, anatase),8,9 amorphous aluminum hydroxide, and iron oxyhydroxide (ferrihydrite).10,11 However, all these require large quantities of adsorbents and will also result in production of a large amount of sludge residues. Besides adsorption, some other methods, such as selective ion-exchange,12 membrane technology,13 and liquid-liquid extraction,14 have also been studied for Cr(VI) removal, but none of these seems suitable for treating industrial plating wastewater with high Cr(VI) concentrations due to high cost and difficulty in operation. In this study, an approach of Cr(VI) co-removal was created through dosing of Na2CO3 into a mixed solution containing Cr(VI), Cu(II), Zn(II), and Ni(II) to induce metal precipitation. On the basis of previous studies,15 co-removal of Cr(VI) would be obtained through coprecipitation with other heavy metals by forming metal-chromate precipitates and adsorption onto the being formed basic metal-carbonate precipitates. It is not the intention of this paper to advocate the use of Cu(II), Zn(II), and Ni(II) as an agent to remove Cr(VI). However, since they coexist in most industrial wastewater, it is advisable to use the available Cu(II), Zn(II), and Ni(II) to maximize the co-removal of Cr(VI) first. Another advantage of the technology is that the continuous single-step compact system using fluidized metal

10.1021/ie050956o CCC: $33.50 © 2006 American Chemical Society Published on Web 01/27/2006

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strippers allows the precipitates produced to coat on the media surface, in which precipitation of heavy metals, co-removal of Cr(VI), and solid-liquid separation occur. Precipitate dewatering is not required, and metals can be recovered by dissolving the metal-coated sands with acids. Therefore, the technology is simple in operation, space-saving, and cost-effective in practical application. It is feasible and advisable for the pretreatment of industrial wastewater with high concentration of metals, especially for the small-scale industries. 2. Materials and Methods 2.1. Synthetic Metal Solutions and Reagents. All synthetic metal-bearing solutions used in this study were prepared by dissolving reagent grade CuCl2‚2H2O (from Riedel-dehae¨n), ZnCl2 (from Nacalai Tesque), NiCl2‚6H2O (from Riedeldehae¨n), and K2Cr2O7 (from Nacalai Tesque) into doubledistilled deionized water for batch tests or into tap water for continuous operations. Unless otherwise specified, the solution contained 60 mg L-1 Cr(VI) and 150 mg L-1 each of Cu(II), Zn(II), and Ni(II). The metals and concentrations were employed because they were typical in the investigated plating wastewaters. Initial pH of the solution was adjusted to around 3 with concentrated HNO3 (70%) to prevent metal precipitation during preparation of synthetic metal-bearing solution. Carbonate solution (0.5 N) was prepared by dissolving industrial grade Na2CO3 into double-distilled deionized water for batch tests or into tap water for continuous operations. 2.2. Experimental Procedures/Operation. In batch tests, operation of Cr(VI) co-removal with mixed metal precipitation is similar to that of Cr(VI) co-removal with Cu(II) precipitation.15 Na2CO3 solution (0.5 N) was added dropwise using a titration buret to a 500 mL metal solution to progressively increase pH and induce the metals’ precipitation. Slight mixing of the solution was conducted using a magnetic stir bar for a well mixture of the alkaline drops with the mother solution immediately. For each pH increment in a range of 4.5 to 10.0, a reaction time of 15 min was allowed for the system to reach equilibrium. Thereafter, a 10 mL aliquot was withdrawn and passed through a 0.45 µm membrane filter before the solution pH was increased to a higher value by further Na2CO3 dosing. The filtrate was acidified with concentrated HNO3 and stored in a polypropylene bottle for subsequent metal analyses. The filter paper contains no studied metals in a blank test. In continuous operation, the metal stripper system employed is shown in Figure 1. It comprised two fluidized sand columns in series, each built with a 36 mm (diameter) Plexiglas tube to a total height of 800 mm. Each column was packed with quartz sand to a static height of 450 mm. 25-72 mesh (150-600 µm) standard quartz sand (BS4550) was used to allow a good fluidizing and also provide a large surface for precipitate coating. The synthetic metal-bearing solution was pumped to the bottom of the first column using a peristaltic pump (Cole Palmer, Masterflex, Model 77200-50) at a flow rate of approximately 350 mL min-1, which was adequate to fluidize the sands to around 600 mm, i.e., 33% expansion. Na2CO3 solution (0.5 N) was injected to each column at a location of 40 mm from the bottom of each column, also using a peristaltic pump (Cole Palmer, Masterflex, Model 77200-50), to increase the operating pH and induce nucleated metal precipitation. pH controller (JENCO, Model 3672) was installed at the top of each column to monitor and control the operating pH within a designed range (pH of 7.2 ( 0.3 or 9.2 ( 0.3). The residence time of waste stream was approximately 1 min in each column. Samples withdrawn at different time intervals were acidified with 1:1

Figure 1. Experimental setup of metal strippers using the nucleated precipitation technology.

Figure 2. Co-removal of Cr(VI) with mixed Cu(II), Zn(II), and Ni(II) precipitation during dosing with Na2CO3.

diluted HNO3 and stored in polypropylene bottles for subsequent metal analysis. 2.3. Analytical Methods. Metal concentrations were determined using an atomic absorption spectrophotometer (HITACHI Z-8200 Polarized Zeeman, Japan; accurate to 0.01 mg L-1) in accordance with Standard Method 3500.16 Soluble metal concentration was determined from filtered liquid sample (passing through a 0.45 µm membrane filter), while total metal concentration was determined from unfiltered samples with their pH first lowered to less than 2 using 1:1 diluted HNO3. pH was measured using an Orion pH meter (Model 420-A). Daily calibration with proper buffer solutions (pH 4.01, 7.00, and 10.01) was performed to ensure its accuracy (to be 0.01). ζ-Potentials of metal precipitates at different pH were determined using a ζ-potential analyzer (zeta plus, Brookhaven instrument corporation, accurate to 0.1 mV). Morphologies of the sand-particle/metal-precipitate combination were examined spectroscopically using a scanning electron microscope (SEM) (PHILIPS, JSM-6300F) under a 10 kV voltage. 3. Results and Discussion 3.1. Co-removal of Cr(VI) in Batch Tests. 3.1.1. Effect of pH on Cr(VI) Co-removal with Mixed Cu(II), Zn(II), and Ni(II) Precipitation. Figure 2 shows the percentages and quantities of Cr(VI), Cu(II), Zn(II), and Ni(II) removal at different pH values during titration of Na2CO3 stepwise to induce metal precipitation. All data are the average of three repeated tests.

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Figure 3. ζ-Potential of the mixture at various pH during dosing Na2CO3 into the mixed Cr-Cu-Zn-Ni solution.

Figure 2 indicates a significant Cr(VI) removal started at pH of around 5.3 and reached to maximum at pH of 7.0-7.3. In a pH range of 5.3 to 6.5, the increase of Cr(VI) removal with increasing pH was very fast, reaching to approximately 72% or 43.2 mg L-1 of Cr(VI) removal under 1.2 pH unit increase. The fast removal appeared to synchronize with Cu(II) removal and was believed to be contributed to the coprecipitation with Cu(II) and adsorption onto freshly formed copper-carbonatehydroxide precipitates based on our previous study on the mechanisms of Cr(VI) co-removal with Cu(II) precipitation.15 In the pH range of 6.5 to 7.3, further Cr(VI) co-removal was observed even though at a relatively slight level, i.e., only approximately 6% or 3.6 mg L-1 (78% - 72% ) 6% or 46.8 - 43.2 ) 3.6 mg L-1) under 0.8 pH unit increase. Since almost all Cu(II) has been solidified in the mentioned pH range, the slight removal was believed to be due to the adsorption of positive Zn(II) and Ni(II) ions onto the negative surface sites of copper-carbonate-hydroxide precipitates. Cation adsorption increases positive surface charges, which makes electrostatic adsorption of anions more favorable.17 However, beyond pH of 7.0-7.3, precipitation of Zn(II) and Ni(II) took place, therefore significant amounts of Zn(II) and Ni(II) removal were observed until they reached almost 100% solidification at pH of around 8.0 and 9.2, respectively. At the same time, the surface charges (detected through ζ-potential) become negative at pH of around 7.5, as shown in Figure 3, which shows variation of ζ-potentials with increasing pH during precipitation of mixed Cr(VI), Cu(II), Zn(II), and Ni(II) solution. The charge reversal of the precipitates led to much higher Cr(VI) desorption from the precipitates compared with electrostatic adsorption of Cr(VI) onto the precipitate surface. Therefore, the co-removal of Cr(VI) decreased with any further raising of pH beyond 7.0-7.3. A total of 46% or 21.6 mg L-1 Cr(VI) which has been solidified from the solution below pH 7.0-7.3 was released into the solution again during pH of 7.3-10.0, i.e., (78% - 42%)/ 78% ) 46% or 46.8 - 25.2 ) 21.6 mg L-1. 3.1.2. Comparison of Cr(VI) Co-removal with Mixed CuZn-Ni Precipitation to the Sum of Cr(VI) Co-removal with Each of Single Cu, Zn, and Ni Precipitation at Different pH. Figure 4 shows the amount of Cr(VI) co-removal with mixed Cu(II), Zn(II), and Ni(II) precipitation described in Figure 2 (solid circular points) and the amount of Cr(VI) co-removal with the precipitation of 150 mg L-1 single Cu(II) (open square points), Zn(II) (open rhombic points), and Ni(II) (open triangular points), respectively. The amounts of Cr(VI) co-removal with each of single Cu(II), Zn(II), and Ni(II) precipitation are simply added. The trend of the sum values versus pH is shown as a solid line (no points) in Figure 4.

Figure 4. Comparison of Cr(VI) co-removal with mixed Cu-Zn-Ni precipitation to the sum of Cr(VI) co-removal with each of single Cu, Zn, and Ni precipitation.

It is interesting to note that the maximum Cr(VI) co-removal with mixed Cu-Zn-Ni precipitation (solid circular points) reached to approximately 46.8 mg L-1 or 78% occurring at a pH of 7.0-7.3. This is apparently higher than the maximum level of the sum of Cr(VI) co-removal with each of single Cu(II), Zn(II), and Ni(II) precipitation (solid line without data points), i.e., approximately 41 mg L-1 or 68% at pH of 7.0-7.2 and 8.2 (at which the maximum level of Cr(VI) co-removal with either single Zn(II) or Ni(II) precipitation was obtained, respectively). In the pH range of 6.2-7.7, the co-removal of Cr(VI) with mixed Cu-Zn-Ni precipitation was higher than the maximum level (i.e., 41 mg L-1) of the sum of single metal precipitation. This is attributed to the adsorption of positive Zn(II) and Ni(II) ions onto the negative surface sites of coppercarbonate-hydroxide precipitates at pH values lower than 7.07.3 increases positive surface charges and then enhances the electrostatic adsorption of anions, as documented before. However, above pH 8.0, co-removal of Cr(VI) with mixed CuZn-Ni precipitation (solid circular points) becomes much lower than the corresponding level of the sum of single metal precipitation (solid line) under the same/corresponding pH conditions. The exact mechanisms were not clear, and further investigation should be continued. However, the observations of Figure 4 suggest that the optimum Cr(VI) co-removal with mixed Cu(II), Zn(II), and Ni(II) precipitation should be realized at two-stage operation. In the first stage, the operating pH is controlled in a range of 7.0-7.5 to obtain the greatest extent of Cr(VI) solidification and removal and also almost 100% Cu(II) removal as shown in Figure 2. The remaining metals in the effluent of the first stage are mainly Zn(II) and Ni(II), and a small amount of Cr(VI). Therefore, an operating pH of 9.09.5 can get a further Cr(VI) co-removal with Zn(II) and Ni(II) precipitation, and it can also complete the Zn(II) and Ni(II) precipitation. 3.1.3. Effect of pH on Co-removal of Cr(VI) with Mixed Zn(II) and Ni(II) Precipitation. Before the investigation of the two-stage operation, another batch test was conducted by dosing Na2CO3 stepwise to a solution containing 150 mg L-1 Zn(II), 150 mg L-1 Ni(II), and 30 mg L-1 Cr(VI) to increase pH progressively to around 9.5. The removal of Cr(VI), Zn(II), and Ni(II) at each pH increment is shown in Figure 5. Figure 5 shows that in a pH range of 7.0-7.8, co-removal of Cr(VI) with mixed Zn(II) and Ni(II) precipitation increased with any increase of pH. A maximum of approximately 31% or 9.36 mg L-1 Cr(VI) removal was obtained at pH of around 7.8. Thereafter, co-removal of Cr(VI) decreases with further pH

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Figure 5. Co-removal of Cr(VI) with mixed Zn(II) and Ni(II) precipitation during dosing with Na2CO3.

increase. On the other hand, removal of Zn(II) and Ni(II) were only approximately 92% and 53%, respectively, at pH of 7.8;

complete Zn(II) and Ni(II) precipitation and/or removal required a pH of above 9.0. At pH of 9.2, co-removal of Cr(VI) was 21% or 6.24 mg L-1, i.e., only around 10% or 3.12 mg L-1 lower than the maximum level. Therefore, the optimum operating pH for all of Cr(VI), Zn(II), and Ni(II) removal should be slightly above 9.0. 3.2. Co-removal of Cr(VI) in a Two-Stage Continuous System under Different Operating pH. After the investigation of batch tests, a continuous-flow nucleated precipitation study using two-stage fluidized sand strippers (Figure 1) was conducted under the control of the first stage at a pH of around 7.2 and the second stage of around 9.2. The metals remaining in the effluents of the first and second columns were detected, and the averages of three repeated tests are shown in Figure 6a. To allow for a comparison of the performance results between the two-stage and single-stage operation, in one special test the operating pH of both stripper columns was controlled at the

Figure 6. Performance of nucleated precipitation of mixed Cr(VI), Cu(II), Zn(II), and Ni(II) solution by dosing with Na2CO3.

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Figure 7. SEM micrographs of various sands under different magnifications.

same value of around 9.2, and the treatment performance is shown in Figure 6b. Figure 6a shows that in the first stage (pH 7.2), only Cr(VI) and Cu(II) were removed while no Zn(II) and Ni(II) removal was observed. The removal of Cr(VI) and Cu(II) reached a steady state after 8 h of operation. For the second stage (pH 9.2), significant removal of Zn(II) and Ni(II) were observed, and it took only approximately 4 h to reach their steady states. Small additional portions of Cr(VI) and Cu(II) were also removed in the second stage, and their removal was quite steady from the beginning of the column operation. Thus, the startup of the system requires 8 h to reach a steady state. During the period, sand surfaces have been completely covered by the metal precipitates. This could be calculated by assuming an even coating/adsorption of the precipitates onto sand surface (no calculation was presented here because of space limitation) and also observed from SEM analyses of the sand-particle/metalprecipitate combinations. Parts a and b of Figure 7 are SEM morphologies of the combinations from the first and second columns after 8 h performance in the two-stage treatment. Three graphs of each series of figure represent morphologies of the same sample but under different magnification. For a comparison, the SEM micrographs of virgin sand with different magnification are shown in Figure 7c. The morphologies of Figure 7a,b under each magnification were distinctly different from the corresponding micrographs of Figure 7c. This indicates the surface of the sands from the first and second columns were completely covered/coated with metal precipitates. After the steady state is reached, further coating/adsorption of the precipitates onto the fluidized particles has good lattice similarity since the particle surface has been precipitate-lattice instead of sand (SiO2)-lattice. Thus, total metals in each effluent kept a relatively steady level in the subsequent stripping operation. Also, the second and third morphologies (with ×2000 and ×20000 magnification, respectively) of Figure 7a were distinctly different from the corresponding micrographs of Figure 7b, because the main components of the former (Figure 7a) were Cr(VI) and Cu(II), while the later (Figure 7b) were Zn(II) and Ni(II).

For the two-stage treatment approach, the amounts of Cr(VI) and Cu(II) remaining in the effluent of the first column were 35 mg L-1 and 48 mg L-1, respectively; i.e., reaching to as high as 42% Cr(VI) removal and 68% Cu(II) removal in the first stage (pH 7.2, Figure 6a). These were significantly higher than the corresponding 21% Cr(VI) removal and 50% Cu(II) removal realized in the first column of the single-stage treatment (pH 9.2, Figure 6b). On the other hand, the removal efficiencies of Zn(II) and Ni(II) in the first stage of the two-stage treatment were only 3% and 7%, respectively (pH 7.2, Figure 6a), but were increased to correspondingly 42% for Zn(II) and 47% for Ni(II) in the same column when the operating pH was controlled at around 9.2 in the single-stage treatment (Figure 6b). After the second stage of the two-stage treatment approach, the remaining Cr(VI), Cu(II), Zn(II), and Ni(II) were 32 mg L-1, 26 mg L-1, 36 mg L-1, and 32 mg L-1, respectively; i.e., a total removal of 47% for Cr(VI), 83% for Cu(II), 76% for Zn(II), and 79% for Ni(II) (Figure 6a). The corresponding removal efficiencies for the single-stage treatment were 22% for Cr(VI), 55% for Cu(II), 50% for Zn(II), and 53% for Ni(II) (Figure 6b). As expected, co-removal of Cr(VI) by pursuing a twostage treatment approach was considerably improved. The higher Cr removal in the two-stage treatment approach contributed to higher Cu(II) removal and higher adsorption capacity of Cr(VI) onto Cu-precipitates at an operating pH of 7.2 based on previous batch tests. The higher Cu(II) removal was attributed to the easier coating of the precipitates on the sand surface since the precipitates carried lower charges (positive) at an operating pH of 7.2 (see Figure 3) and the lower supersaturation or slower rates of nucleation/crystallite growth due to less Na2CO3 dosing. Lower supersaturation or slower nucleation/crystallite growth would lead to less discrete metal precipitation or small particles (fines) in the liquid phase, thus lower total metals in the effluent. Moreover, the effluent metal concentration is still relatively high, and all metal residuals in the continuous flow fluidized stripper technology were somewhat higher than those obtained in the previous batch studies. Higher metal residual was believed to be partly due to some small particles (fines) in the effluent,

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from discrete metal precipitation and the coated/adsorbed precipitates being shelled off sand surface into the effluent, and partly due to short retention time (only approximately 1 min) or insufficient surface area in each column not allowing the complete Cr(VI) adsorption onto metal precipitates or all precipitates effectively plated onto the fluidized sands. To verify above assumptions, soluble metals in the effluents of each column were measured, and the results are shown in triangular points of Figure 6 (solid triangular points for column I and open triangular points for column II). Figure 6a shows that for the two-stage treatment approach, soluble Cu(II) in the effluent of column I is only approximately 3 mg L-1 among 48 mg L-1 total residual (i.e., around 48 - 3 ) 45 mg L-1 in insoluble form), and soluble Cu(II) in the effluent of column II is close to zero (i.e., almost all 26 mg L-1 Cu(II) in insoluble form). This indicates that under the operating pH of around 9.2 in the second stage, all approximately 26 mg L-1 Cu(II) leakage was most probably due to small particle formation in the liquid phase (discrete metal precipitation) or the coated/adsorbed precipitates shelling off sand surfaces, while not due to insufficient metal precipitation, limited surface area, or short retention time. However, under the operating pH of around 7.2 in the first stage, approximately 3 mg L-1 Cu(II) leakage (in soluble form) was due to insufficient metal precipitation, at least 26 mg L-1 Cu(II) leakage (in insoluble form) was due to small particle formation in the liquid phase (discrete metal precipitation) or the coated/adsorbed precipitates shelling off sand surfaces, and the remaining Cu(II) leakage (i.e., less than 48 3 - 26 ) 19 mg L-1 in insoluble form) was possibly due to limited surface area or short retention time. Similarly, soluble concentrations of Zn(II) and Ni(II) are close to their total concentrations in the effluent of column I because of insufficient metal precipitation at pH of around 7.2, while all residuals in the effluent of column II are in their insoluble form due to small particle formation in the liquid phase (discrete metal precipitation) or the coated/adsorbed precipitates shelling off sand surfaces, limited surface area, or short retention time. In the one-stage treatment approach, approximately 67 mg L-1 Cu(II) leakage was in small particles (fines) due to discrete metal precipitation in the liquid phase or the coated/adsorbed precipitates shelling off sand surfaces into the effluent. This is much higher than 26 mg L-1 in the two-stage treatment, because of higher supersaturation or faster nucleation/crystallite growth in the one-stage approach. 4. Conclusion A maximum of approximately 78% or 46.8 mg L-1 Cr(VI) was co-removed at a pH of 7.0-7.3 in the batch test when dosing with Na2CO3 stepwise into a mixed solution containing Cu(II), Zn(II), Ni(II) (each 150 mg L-1), and Cr(VI) (60 mg L-1). However, co-removal of Cr(VI) decreased significantly with pH further increasing to 10.0 even though Zn(II) and Ni(II) precipitation continued. Therefore, in the continuous-flow system, a two-stage nucleated precipitation technology was designed with the first stage being operated at a pH of around 7.2 to obtain maximum Cr(VI) co-removal and the second stage at a pH of around 9.2 to achieve further Cr(VI) co-removal with Zn(II) and Ni(II) precipitation and also Zn(II) and Ni(II) removal. For the synthetic solution containing 60 mg L-1 Cr(VI) and 150 mg L-1 each of Cu(II), Zn(II), and Ni(II), a total of

47% for Cr(VI), 83% for Cu(II), 76% for Zn(II), and 79% for Ni(II) were obtained under the operation, which were much higher than the corresponding removal of 22% for Cr(VI), 55% for Cu(II), 50% for Zn(II), and 53% for Ni(II) using the same experimental setup but controlling the operating pH of both columns at around 9.2. Acknowledgment This study was supported in part by a research funding from Tianjin Municipal Science and Technology Commission (Contract No.: 033113111) and in part by an RGC research grant (Contract No.: HKUST6034/01E). The authors express their appreciation to Dr. Lynn Smith for assistance with improvements in English writing. Literature Cited (1) Richardson, H. W. Handbook of Copper Compounds and Applications; Marcel Dekker: New York, 1997. (2) Patterson, J. W.; Allen, H. E.; Scala, J. J. Carbonate Precipitation for Heavy Metals Pollutants. J. Water Pollut. Control Fed. 1977, 49, 2397. (3) Meanally, S.; Benefield, L.; Reed, R. B. Nickel Removal from a Synthetic Nickel-Plating Wastewater using Sulfide and Carbonate for Precipitation and Coprecipitation. Sep. Sci. Technol. 1984, 19, 191. (4) Aktor, H. Continuous High-Rate Removal of Chromate in a Fluidized Bed without Sludge Generation. Water Sci. Technol. 1994, 30, 31. (5) Benjamin, M. M.; Sletten, R. S.; Bailey, R. P.; Bennett, T. Sorption and Filtration of Metals using Iron-Oxide-Coated Sand. Water Res. 1996, 30, 2609. (6) Nielsen, P. B.; Christensen, T. C.; Vendrup, M. Continuous Removal of Heavy Metal from FGD Wastewater in a Fluidized Bed without Sludge Generation. Water Sci. Technol. 1997, 36, 391. (7) Snoeyink, V. L.; Jenkins, D. Water Chemistry; Wiley-Interscience: New York, 1980. (8) Weng, C. H.; Wang, J. H.; Huang, C. P. Adsorption of Cr(VI) onto TiO2 from Dilute Aqueous Solutions. Water Sci. Technol. 1997, 35, 55. (9) Vasileva, E.; Hadjiivanov, K.; Mandjukov, P. Adsorption of Cr6+ Oxo Anions on Pure and Peroxide-Modified TiO2 (Anatase). Colloid Surfaces A 1994, 90, 9. (10) Zachara, J. M.; Girvin, D. C.; Schmidt, R. C.; Resch, C. T. Chromate Adsorption on Amorphous Iron Oxyhydroxide in the Presence of Major Groundwater Ions. EnViron. Sci. Technol. 1987, 21, 589. (11) Mesuere, K.; Fish, W. Chromate and Oxalate Adsorption on Goethite: 1. Calibration of Surface Complexation Models. EnViron. Sci. Technol. 1992, 26, 2357. (12) Segupta, A. K. Modifying Ion - Exchange Resin Composition for Selective Removal of Potentially Toxic Hexavalent Chromium Species. 18th Mid-Atlantic Industrial Waste Conference; Blacksburg, VA, 1986. (13) Ho, W. S.; Poddar, T. K. New Membrane Technology for Removal and Recovery of Chromium from Waste Waters. EnViron. Prog. 2001, 20, 44. (14) Painter, R. D.; Yarbrough, D. W. Use of Liquid-Liquid Extraction for the RemoVal of Chromium from Industrial Wastewater; American Institute of Chemical Engineers 1991 Summer National Meeting, Pittsburgh, PA, 1991. (15) Sun, J. M.; Shang, C.; Huang, J. C. Co-removal of Hexavalent Chromium with Copper Precipitation in Synthetic Plating Wastewater. EnViron. Sci. Technol. 2003, 37, 4281. (16) APHA. Standard Methods for the Examination of Water and Wastewater, 20th ed.; American Public Health Association/American Water Works Association/Water Pollution Control Federation: Washington D.C., 1998. (17) Richard, F. C.; Bourg, A. C. M. Aqueous Geochemistry of Cr: A review. Water Res. 1991, 25, 807.

ReceiVed for reView August 21, 2005 ReVised manuscript receiVed December 29, 2005 Accepted December 30, 2005 IE050956O