Reduction of Hexavalent Chromium with the Brown Seaweed Ecklonia

A consistent 2 mm spot size was analyzed on all surfaces using a MgKα (hλ = 1253.6 ... at 48 mg/L, indicating that 52 mg/L of total Cr was removed f...
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Environ. Sci. Technol. 2004, 38, 4860-4864

Reduction of Hexavalent Chromium with the Brown Seaweed Ecklonia Biomass DONGHEE PARK,† YEOUNG-SANG YUN,‡ AND J O N G M O O N P A R K * ,† Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, Korea, and Division of Environmental and Chemical Engineering and Research Institute of Industrial Technology, Chonbuk National University, Chonbuk 561-756, Korea

A new type of biomass, protonated brown seaweed Ecklonia sp., was used for the removal of Cr(VI). When synthetic wastewater containing Cr(VI) was placed in contact with the biomass, the Cr(VI) was completely reduced to Cr(III). The converted Cr(III) appeared in the solution phase or was partly bound to the biomass. The Cr(VI) removal efficiency was always 100% in the pH range of this study (pH 1 ∼ 5). Furthermore, the Cr(VI) reduction was independent of the Cr(III) concentration, the reaction product, suggesting that the reaction was an irreversible process under our conditions. Proton ions were consumed in the ratio of 1.15 ( 0.02 mol of protons/mol of Cr(VI), and the rate of Cr(VI) reduction increased with decreasing the pH. An optimum pH existed for the removal efficiency of total chromium (Cr(VI) plus Cr(III)), but this increased with contact time, eventually reaching approximately pH 4 when the reaction was complete. The electrons required for the Cr(VI) reduction also caused the oxidation of the organic compounds in the biomass. One gram of the biomass could reduce 4.49 ( 0.12 mmol of Cr(VI). From a practical viewpoint, the abundant and inexpensive Ecklonia biomass could be used for the conversion of toxic Cr(VI) into less toxic or nontoxic Cr(III).

Introduction Chromium and its compounds are widely used in industry, with the most usual and important sources coming from the electroplating, tanning, water cooling, pulp producing, and ore and petroleum refining processes (1). The effluents from these industries contain both Cr(VI) and Cr(III) in concentrations ranging from tens to hundreds of mg/L. Cr(VI) is known to be toxic to both plants and animals, as a strong oxidizing agent and potential carcinogen (2). In contrast, Cr(III) is generally only toxic to plants in very high concentrations and is less toxic, or nontoxic, to animal (3). Because of these differences, the discharge of Cr(VI) to surface water is regulated to below 0.05 mg/L by the U.S. EPA, while total Cr, including Cr(III), Cr(VI), and its other forms, is regulated to below 2 mg/L. Therefore, the reduction of Cr(VI) is * To whom correspondence should be addressed. Phone: +8254-279-2275. Fax: +82-54-279-2699. E-mail: [email protected]. † Pohang University of Science and Technology. ‡ Chonbuk National University. 4860

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imperative to meet the discharge levels, and recycling and reuse are promoted. Consequently, the removal of Cr(VI) from industrial wastewater has attracted much research interest. Existing chemical treatment processes for the lowering of Cr(VI) concentrations generally involve the aqueous reduction of Cr(VI) to Cr(III) using various chemical reagents, with the subsequent adjustment of the solution pH to near-neutral conditions, for the precipitation of the Cr(III) ions produced. However, these methods have been considered undesirable due to the use of expensive and toxic chemicals, poor removal efficiency for meeting regulatory standards, and the production of large amounts of chemical sludge (4, 5). An alternative for dealing with the problem of Cr(VI) wastewater may be to remove Cr(VI) via sorption-based processes, wherein synthetic resin (6), activated carbon (7), fly ash-wollastonite (8), carbon slurry (9), inorganic sorbent materials (10), or the so-called biosorbents derived from dead biomass. Of these, biosorbents are considered the cheapest, most abundant, and environmentally friendly option (4, 5). Because of these advantages, there has been extensive research exploring appropriate biosorbents able to effectively remove Cr(VI), such as sawdust (11-14), moss peat (15), agricultural byproduct (16-18), food industrial waste (19), plants (2022), fungi (23-26), bacteria (27-29), microalgae (30-33), and seaweed (4, 5, 34). Most reports claim that the Cr(VI) was removed from aqueous systems by anionic adsorption (12-14, 17-19, 2134). Some researchers have reported that the removal of Cr(VI) was partly through reduction, as well as anionic adsorption, and the partial reduction could take place only under strongly acidic conditions (pH < 2.5) (4, 5, 11, 15, 16, 20). This study presents a new type of biomaterial, protonated biomass of brown seaweed Ecklonia, capable of completely removing Cr(VI) even at pH 5.0, and the underlying Cr(VI) removal mechanism has been elucidated. The factors affecting the Cr(VI) removal efficiency have been investigated.

Materials and Methods Preparation of the Biomass. The brown seaweed, Ecklonia sp., was collected along the seashore of Pohang, Korea. In our previous study (35), this seaweed biomass was shown to be a good biosorbent of Cr(III). After swelling and rinsing with deionized-distilled water, the sun-dried biomass was cut into approximately 0.5 cm sized pieces. The cut biomass was treated with a 1 M H2SO4 solution for 24 h, which replaced the natural mix of ionic species with protons and sulfates. The acid-treated biomass, designated as protonated biomass in this article, was washed with several times with deionizeddistilled water and then oven dried at 80 °C for 24 h. The resulting dried biomass was later stored in a desiccator and used for the following experiments. Cr(VI) Removal Experiments. The Cr(VI) removal experiments were carried out in flasks. The following set of factors was chosen as the standard conditions: 5 g/L biomass, 100 mg/L initial Cr(VI) concentration, 0 mg/L initial Cr(III) concentration, 200 mL working volume, and pH 2 at room temperature (20 ∼ 25 °C). Sufficient solution/biomass contact time was allowed until the Cr(VI) concentration remained unchanged, ranging from hours to weeks. The stock solutions of Cr(VI) and Cr(III) used in all the experiments were made by dissolving analytical grade K2CrO4 (Kanto) and CrCl3‚6H2O (Sigma), respectively, in deionized-distilled water and were freshly prepared every time. Each trial was performed by bringing into contact the desired amount of biomass with 200 mL of a Cr(VI) solution of known 10.1021/es035329+ CCC: $27.50

 2004 American Chemical Society Published on Web 08/04/2004

concentration in a 500 mL Erlenmeyer flask. The flasks were agitated on a shaker at 200 rpm. In the pH-stat experiments, the solution pH was maintained at the desired value using 0.5 M H2SO4 or 1 M NaOH solutions. The changes in the working volume due to NaOH or H2SO4 addition were negligible. Meanwhile, in the pH-shift experiments, the solution pH was not adjusted after bubbling of N2 gas to remove O2 and CO2 from the system, but the final pH was measured for comparison with the initial pH. With the exception of the experiments conducted under biomasslimited conditions, the experiments were continued until the Cr(VI) had been completely removed. Samples for Cr(VI) and total chrome concentration analyses were intermittently removed from the flasks and appropriately diluted. The total volume of withdrawn samples never exceeded 2% of the working volume. XPS (X-ray Photoelectron Spectroscopy) Analysis. XPS was employed to determine the valence state of the Cr bound on the biomass. The Cr-laden biomass was obtained through contact with 200 mg/L Cr(VI) at pH 2.0 for 2 days, while the Cr(III)-laden biomass was obtained through contact with 200 mg/L Cr(III) at pH 4.0 for 2 days. Prior to mounting for XPS, the biomass was washed with deionized-distilled water several times and then freeze-dried. The resulting biomass was transported to the spectrometer in a portable, gastight chamber. CrCl3‚6H2O (Sigma) and K2CrO4 (Kanto) were used as Cr(III) and Cr(VI) reference compounds, respectively. Spectra were collected on a VG Scientific model ESCALAB 220iXL. A consistent 2 mm spot size was analyzed on all surfaces using a MgKR (hλ ) 1253.6 eV) X-ray source at 100 W and pass energy of 0.1 eV for 10 high-resolution scans. The system was operated at a base pressure of 2 × 10-8 mbar. The calibration of the binding energy of the spectra was performed with the C 1s peak of the aliphatic carbons, which is at 284.6 eV. Chromium Analysis. A colorimetric method, as described in the standard methods (36), was used to measure the concentrations of the different Cr species. The pink colored complex, formed from 1,5-diphenycarbazide and Cr(VI) in acidic solution, was able to be spectrophotometrically analyzed at 540 nm (Spectronic 21, Milton Roy Co.). To estimate for total Cr, the Cr(III) was first converted to Cr(VI) at high temperature (130 ∼ 140 °C) by the addition of excess potassium permanganate prior to the 1,5-diphenycarbazide reaction. The Cr(III) concentration was then calculated from the difference between the total Cr and Cr(VI) concentrations. The detection limit of this method was 0.03 mg/L.

Results and Discussion Reduction of Cr(VI) into Cr(III). To examine the Cr(VI) removal characteristic of the Ecklonia biomass, the Cr concentrations and pH profiles were investigated, with no pH adjustment (Figure 1). The Cr(VI) concentration was found to sharply decrease and was removed to be below the lower limit of detection for analytical method employed. Meantime, the Cr(III), which was not initially present, appeared in solution and increased in proportion to the Cr(VI) depletion. These results indicated that the Cr(VI) was reduced into Cr(III) when brought into contact with the biomass. The solution pH, within the initial few minutes, decreased abruptly from 2.00 to 1.94, then increased, and finally equalized to 2.07 after 6 h of contact time. In a control experiment, with no Cr(VI) present, the pH also decreased abruptly, from 2.00 to 1.92, in the first few minutes, but subsequently remained unaltered (data not shown). Thus, the initial sharp decrease in the solution pH was considered as a result of the efflux of protons from the protonated biomass. While, the increase of the solution pH then likely to be related to the removal of Cr(VI). In the region where the solution pH increased, the amount of protons that

FIGURE 1. Dynamics of Cr(VI) removal by protonated Ecklonia biomass during pH-shifting experiments. Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L biomass concentration, initial pH 2.0. Symbols: (O) Cr(VI); (2) Cr(III); and (3) pH.

FIGURE 2. Cr 2p spectra of the Cr-laden biomass and Cr(III)-laden biomass; the former was obtained after Cr(VI) biosorption at pH 2.0, and the latter was obtained after Cr(III) biosorption at pH 4.0. disappeared was proportional to the amount of Cr(VI) removed, reflecting the proton ion participation in the removal of the Cr(VI). Thus, it could be expected for the removal rate of Cr(VI) to increase as the solution pH decreased. After the complete Cr(VI) removal, the final Cr(III) concentration in the solution remained at 48 mg/L, indicating that 52 mg/L of total Cr was removed from the solution. It could be assumed that the total Cr removed was bound to the biomass. To characterize the valence state of the Cr bound on the biomass, X-ray photoelectron spectroscopy (XPS) was employed. Low-resolution XPS spectra of the Cr-unloaded biomass indicated that other than C, N, and O, no significant contributions were present from other elements associated with biomass surfaces. High-resolution spectra collected from the Cr 2p core region indicated that indeed there was no Cr associated with the biomass surfaces (data not shown). However, high-resolution spectra of the Cr-laden biomass and the Cr(III)-laden biomass indicated that there were significant contributions of the Cr bound on the biomass (Figure 2). Significant bands appeared at binding energies of 577.0 ∼ 578.0 and 586.0 ∼ 588.0 eV; the former corresponds to Cr 2p3/2 orbitals, the latter to Cr 2p1/2 orbitals. The Cr 2p3/2 orbitals are assigned at 577.2 eV (CrCl3) and 576.2 ∼ 576.5 eV (Cr2O3) for Cr(III) compounds, while Cr(VI) forms are characterized by higher binding energies such as 578.1 eV (CrO3) or 579.2 eV (K2Cr2O7) (37). The spectra of the Cr-laden biomass was same to that of the Cr(III)-laden biomass. There VOL. 38, NO. 18, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effect of the initial Cr(III) concentration on the Cr(VI) reduction. Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L biomass concentration. Symbols: (O, b) initial Cr(III) ) 0 mg/L; (4, 2) initial Cr(III) ) 50 mg/L; (3, 1) initial Cr(III) ) 100 mg/L; open symbols are Cr(VI); and closed symbols are Cr(III). have been previous studies (16, 38) on the Cr species bound to the biomass. Lytle et al. (38) reported that Cr(VI) taken from the fine lateral roots of wetland plants was rapidly reduced to Cr(III). Cardea-Torresdey et al. (16) reported that Cr(VI) could be bound to an oat byproduct, but easily reduced to Cr(III) by positively charged functional groups, and subsequently adsorbed by available carboxyl groups. These studies reached the same conclusions as this study, that is, the bound Cr was in the Cr(III) state. Therefore, it can be concluded that the Cr(VI) was removed from aqueous phase and finally reduced to Cr(III). Irreversibility of Cr(VI) Reduction. To investigate the effect of Cr(III) on the Cr(VI) reduction, the initial Cr(III) concentration was varied from 0 to 100 mg/L (Figure 3). The presence of Cr(III) did not affect the Cr(VI) reduction. In addition, no reoxidation of the reduced Cr(III) occurred in these experiments. Therefore, it can be concluded that the reduction of Cr(VI) by the Ecklonia biomass was an irreversible process under our experimental condition. From a practical viewpoint, this result might encourage the application of the Ecklonia biomass as a Cr(VI) remover because there was no interference of the final reaction product (Cr(III)) in the reduction of Cr(VI). Effect of pH. As shown in the pH-shifting experiments (Figure 1), protons were consumed during the Cr(VI) removal; therefore, the Cr(VI) removal was studied at various solution pHs (Figure 4). As seen in Figure 4, the Cr(VI) removal rate increased with decreasing pH. The contact time required for complete Cr(VI) removal varied from 12 to 480 h and was pH dependent. In all the experiments conducted in this study, the Cr(VI) was completely removed from aqueous phase, even at pH 5. However, the removal efficiency of the total Cr was 16.7% at pH 1, 73.2% at pH 3, and 57.2% at pH 5, respectively. The existence of optimum pH for total Cr removal will be discussed later. Although many previous studies (10, 11, 13-17, 20-23) have shown the Cr(VI) removal rate to increase with decreasing pH, no nonliving biomass capable of completely removing Cr(VI) at pH 5 has been reported. Proton Consumption with Cr(VI) Reduction. To evaluate the amount of protons required for Cr(VI) reduction, pHshift batch experiments were conducted under uniform initial solution pH and biomass concentration conditions, while the initial Cr(VI) concentration was varied from 0 to 200 mg/ L. To avoid the pH variation by carbon dioxide from air, the experiments were carried out in a N2 environment. Figure 4862

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FIGURE 4. Dynamics of Cr(VI) removal by protonated Ecklonia biomass at various pHs. Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L biomass concentration. Symbols: (O) pH 1.0; (2) pH 3.0; and (3) pH 5.0.

FIGURE 5. Ratio of proton consumption to reduced Cr(VI). Conditions: 5 g/L biomass concentration, 0 ∼ 200 mg/L of initial Cr(VI) concentration. Symbols: (O) final pH and (9) amount of proton consumed vs amount of Cr(VI) reduced. 5 shows that final solution pH increased as increasing the amount of reduced Cr(VI). The proton consumption ratio required for the reduction of Cr(VI) can be calculated by subtracting the proton concentration of each trial from that of the control. As shown in Figure 5, the ratio was almost constant when reduced Cr(VI) was less than 2 mmol/L. However, as reduced Cr(VI) increased to more than 2 mmol/ L, the ratio deviated from the constant. This likely resulted from the complex participation of the protons in following reactions: (a) protons can be released, or bound, from, or to, various functional groups of the biomass depending on the solution pH (35). (b) Protons are consumed during the reduction of Cr(VI) as follows:

Cr2O72- + 14H+ + 6e- T 2Cr3+ + 7H2O CrO42- + 8H+ + 3e- T Cr3+ + 4H2O

E° ) +1.33V (1) E° ) +1.48V

(2)

HCrO4- + 7H+ + 3e- T Cr3+ + 4H2O

E° ) +1.35V (3)

H2CrO4 + 6H+ + 3e- T Cr3+ + 4H2O

E° ) +1.33V (4)

(c) Protons are released from the biomass during Cr(III) binding via proton-Cr(III) ion exchange (35). In the case of protonated Ecklonia biomass, the functional group related to the interaction between proton and Cr(III) is a carboxyl

TABLE 1. Cr(VI) Removal under Biomass Limiting Conditiona biomass concentration [g/L] initial Cr(VI) concentration [mg/L] pH [-] final Cr(VI) concentration [mg/L] Cr(VI) reduced/biomass [mmol/g]

0.0515 100 2.00 88.3 4.37

0.1685 100 2.00 59.6 4.61

a Experiments were conducted until the Cr(VI) concentration did not nearly change (30 days).

group having a pKa of 4.6 ( 0.1 (35). Thus, the effects of reactions a and c might have been relatively small below pH 3.0 as compared to reaction b. However, these effects cannot be ignored at pH > 3, where Cr(III) can easily occupy the functional groups by replacing protons. The decrease of the proton/Cr(VI) ratio could be explained with the preferential binding of the Cr(III) to the biomass at the high final pH resulted from the reduction of high concentration of Cr(VI). Since acidic conditions are practically favorable for the fast removal of Cr(VI), the constant ratio (1.15 ( 0.02 mol of proton consumption per mol of reduced Cr(VI)) at a low final pH range (pH < 3) could be used for a rough estimation of the proton consumption per Cr(VI) reduced in practical operations. Oxidation of the Biomass and Available Electrons. For the reduction of Cr(VI) to Cr(III), not only protons, but also electrons are required. The electrons required for the reduction of Cr(VI) were possibly supplied from the biomass, resulting in the oxidation of the organic compounds of the biomass, resulting in the partial release of soluble organics. When the biomass was long-term contacted with Cr(VI), there was distinct increases in concentrations of dissolved organic compounds and inorganic carbons (e.g., dissolved CO2, HCO3-) in the solution phase, as compared with the Cr(VI) free control (data not shown). The appearance of inorganic carbons in the effluent implied that parts of organic carbons of the biomass were completely oxidized into CO2. Furthermore, it was observed that the surface of the biomass was rougher after contact with Cr(VI) (data not shown). To evaluate the available electrons able to be supplied from the biomass, Cr(VI) was brought into contact with a small amount of the biomass (Table 1). In these biomasslimited experiments, some of the Cr(VI) could possibly remain after the biomass was completely oxidized. As a result, 1 g of the biomass could reduce 4.49 ( 0.12 mmol of Cr(VI) at pH 2. In other words, since 3 mol of electrons are required for the reduction of 1 mol of Cr(VI) to Cr(III), it can be suggested that 1 g of the biomass possessed 13.47 ( 0.36 mmol of available electrons. From a practical viewpoint, only 223 g of Ecklonia biomass is required for the reduction of 1 mol of Cr(VI), while 834 g of FeSO4‚7H2O, which is a common Cr(VI) reductant, is required for the same level of reduction. Effect of Temperature. Temperature may play an important role in the reduction of Cr(VI) to Cr(III). Therefore, batch experiments were performed at pH 2.0 to examine the temperature dependency of Cr(VI) reduction by the biomass in the range of 5 ∼ 45 °C (Figure 6). The increase of temperature greatly increased the Cr(VI) removal rate. Some previous studies (11, 15, 29) supported our results. In general, the increase of temperature induces the rate of a redox reaction (39). Discussion on Existence of Optimum pH for the Removal of Total Cr. The removal efficiency of total Cr in aqueous phase was investigated at various pHs and with different contact times (Figure 7). Removal efficiency was calculated from the mass balance for total Cr in aqueous phase. As a matter of course, the removal efficiency of total Cr increased with increasing contact time. An optimum pH existed for the total Cr removal efficiency at a fixed contact time. However,

FIGURE 6. Effect of the temperature on the Cr(VI) reduction. Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L biomass concentration, pH 2.0. Symbols: (O) 5 °C; (9) 15 °C; (4) 25 °C; (1) 35 °C; and (0) 45 °C.

FIGURE 7. Optimum pH for the removal efficiency of total Cr. Conditions: 100 mg/L initial Cr(VI) concentration, 5 g/L biomass concentration. Symbols: (O) 1 h; (9) 12 h; (4) 48 h; and (1) 480 h. Cr(VI) in all the experiments was completely removed within 480 h. the optimum pH increased with increasing contact time, eventually reaching approximately 4 on completion of the reaction. The reason the optimum pH increased can be explained as follows: at a high pH, Cr(VI) is very slowly reduced to Cr(III), and Cr(III) is easily bound to the biomass. Thus, as the pH increases, the reduction rate of Cr(VI) is rate-limited: as soon as Cr(III) is formed from Cr(VI), it is bound to the biomass. However, since Cr(VI) reduction is irreversible, although slow at a high pH, as the contact time increases, Cr(VI) can be completely reduced to Cr(III), which is then removed by biosorption. The question as to why the optimum pH was 4 was likely due to the release, or destruction, of Cr(III)-binding sites during the oxidation of the biomass. The release of organic compounds from the seaweed biomass is known to increase at elevated pHs (40). The soluble form of Cr(III) binding sites, such as carboxyl groups, can form complexes with Cr(III), which may still exist in the aqueous phase. The existence of an optimum pH for the efficient removal of total Cr by the seaweed, Sargassum, biomass has previously been observed (4, 5). However, these reports showed that the optimum pH for the efficient removal of total Cr was in the vicinity of 2 ∼ 2.5. To accurately evaluate the optimum pH conditions, sufficient contact time should also be considered. In our experiments, the complete reaction required 480 h, while only 6 h was given in the previous VOL. 38, NO. 18, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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reports (4, 5). It was interesting to note that the optimum pH after 6 h of contact time was also between 1.5 and 2.5 in our experiments.

Acknowledgments This work was financially supported by the KOSEF through the AEBRC at POSTECH and partially by Grant R08-2003000-10987-0 from the Basic Research Program of the KOSEF.

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Received for review November 30, 2003. Revised manuscript received May 30, 2004. Accepted June 28, 2004. ES035329+