Ind. Eng. Chem. Res. 2009, 48, 2113–2117
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Removal of Cr(VI) Ions by Spent Tea and Coffee Dusts: Reduction to Cr(III) and Biosorption Syam K. Prabhakaran,† K. Vijayaraghavan,‡ and R. Balasubramanian*,† DiVision of EnVironmental Science and Engineering, National UniVersity of Singapore, Singapore 117576, and Singapore-Delft Water Alliance, National UniVersity of Singapore, 1 Engineering DriVe 2, Singapore 117576, Singapore
The potential use of spent tea and coffee dusts was investigated for the removal of Cr(VI) from aqueous solution. The removal mechanism was identified as the reduction reaction of Cr(VI) to Cr(III), followed by Cr(III) sorption to the biomass. The phenolic compounds in tea and coffee dusts serve as electron-donor groups for rapid reduction of Cr(VI). The pH edge experiments revealed that Cr(VI) reduction by both tea and coffee dusts was independent of pH whereas reduced Cr(III) adsorption onto biomass was strongly dependent on pH. Isotherm experiments revealed that tea and coffee dusts possess maximum chromium uptakes of 44.9 and 39.0 mg/g, respectively, at pH 4. Among the two isotherm models (Langmuir and Toth), the Toth model better described the chromium biosorption isotherms with high correlation coefficients and low percent error values. A kinetic model based on the redox reaction between Cr(VI) and biomass successfully described the kinetic data. A comparison of these kinetic data with those from Sargassum and UlVa sp., revealed that Cr(VI) reduction rate of coffee dust was 40 times faster than that of UlVa biomass and 144 times faster than that of Sargassum biomass. 1. Introduction The presence of heavy metals in aqueous streams is of great concern because of their toxic and carcinogenic effects on human health and aquatic organisms. Chromium is one such heavy metal found in natural deposits as ores and also found in several other natural materials in its compound form. It can also enter aquatic systems through discharge of concentrated industrial effluents. Chromium can occur at several different oxidation states ranging from -4 to +6.1 However, only hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) are the stable forms in the natural environments. While Cr(VI) is predominant in natural aquifers, Cr(III) prevails in the municipal wastewater rich in organics.2 Cr(VI) is known to have 100fold more toxicity than Cr(III) because of its high water solubility and mobility.3 Thus, the USEPA has set the maximum contaminant level (MCL) for Cr(VI) in domestic water supplies to be 0.05 mg/L, while total Cr containing Cr(III), Cr(VI), and other species of chromium is regulated to be discharged below 2 mg/L.4 Hence various technologies were employed to treat the chromium-bearing wastewaters at the point of source. Some physiochemical methods such as adsorption, chemical reduction, precipitation, solvent extraction, and ion exchange have proven to be effective for removal of Cr(VI) from aqueous solutions and industrial effluents. However, they suffer from huge capital investment or energy requirement.5 There has been considerable attention on the search for alternatives of the otherwise expensive or energy intensive processes. In recent years, research attention has been focused on biological methods for the treatment of metal-bearing effluents,6 some of which are in the process of commercialization. Among these methods, biosorption has been demonstrated to possess good potential to replace conventional methods for the removal of metal ions. Bacteria, fungi, algae, and industrial and * Corresponding author: E-mail:
[email protected]. Tel.: +6565165135. Fax: +65-67744202. † Division of Environmental Science and Engineering. ‡ Singapore-Delft Water Alliance.
agricultural wastes are examples of biomass-derived metal sorbents.7,8 Several biosorbents were identified to effectively remove Cr(VI) from aqueous solutions, and the most important include Apsergillus niger,9 Rhizopus arrhizus,10 Ecklonia sp.11 and Sargassum sp.12 However, most of the early studies claimed that Cr(VI) was removed from the aqueous phase through sorption mechanism, whereby anionic Cr(VI) ion species bind to the positively charged groups of biomaterials.13 It has been recently recognized that these findings were misinterpreted due to errors in measuring concentrations of different chromium species in the aqueous phase, insufficient contact time required for equilibrium, and the lack of information about the oxidation state of the chromium bound to biomaterials.14,15 Due to extensive consumption throughout the world, coffee and tea preparation produces large amount of wastes, which are known antioxidants. These wastes are considered to be good metal scavengers from solutions and wastewaters as they contain certain functional groups.16 This work investigated the potential of spent coffee and tea wastes in removal of Cr(VI) from aqueous solution for the first time. The removal ability of these waste materials was compared to that of two algal biosorbents (Sargassum sp. and UlVa sp.). These seaweeds are well-known Cr(VI) biosorbents and have been used in many studies.12,17-19 Mechanistic insights into physicochemical interactions involved in the removal of Cr(VI) by the biomaterials used in the study are provided. Kinetics of the removal process including kinetic modeling is discussed. 2. Experimental Methods 2.1. Biosorbents and Chromium Solutions. Spent tea and coffee dusts were collected locally and washed several times with deionized water. They were then dried under the sun, and this dried powder was directly used as an adsorbent. Tea and coffee dusts were designated as TD and CD, respectively, in the present article. Sargassum and UlVa biomasses were collected from the beaches of Labrador Park in Singapore. The biomass was extensively washed with deionized water and sun-
10.1021/ie801380h CCC: $40.75 2009 American Chemical Society Published on Web 01/15/2009
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Figure 1. Effect of solution pH on the chromium reduction and biosorption potential of TD and CD (biosorbent dose ) 3 g/L; temperature ) 23 ( 2 °C; initial Cr(VI) concentration ) 30 mg/L).
Figure 2. Effect of biosorbent dosage on the Cr(VI) reduction efficiency of TD and CD (pH ) 4; temperature ) 23 ( 2 °C; initial Cr(VI) concentration ) 23 mg/L).
dried. The dried biomass was then grounded in a blender to obtain particles in the size range of 0.5 to 1 mm. All reagents and metals used were of analytical-reagent grade. Ultrapure (Millipore) water was used to prepare all the solutions. The hexavalent chromium solutions were prepared by dissolving appropriate amounts of potassium dichromate (K2Cr2O7) in Ultrapure water. 2.2. Batch Experiments for Cr(VI) Reduction and Biosorption. Cr(VI) reduction and removal potential of TD and CD were examined under batch mode of operation. Each batch experimental trail was performed by adding 0.3 g of each biomaterial with 100 mL of desired Cr(VI) concentration in a 250 mL Erlenmeyer flask at different pH conditions. The mixture in the conical flasks was agitated on a shaker at 150 rpm at 23 ( 2 °C. The initial solution pH was adjusted using 0.1 M HCl or 0.1 M NaOH and the same chemicals were used to control the pH during the experiments. After 10 h, samples were filtered by passing through a 0.45 µm PTFE membrane filter and analyzed for total chromium and Cr(VI) concentration. Kinetic experiments were conducted using the same procedure as above, except that the samples were collected at different time intervals to determine the time point at which the equilibrium was attained. The total concentration of chromium i.e. the sum of the concentration of Cr(VI) and Cr(III) was determined using an inductively coupled plasma-optical emission spectrometer (ICPOES, Perkin-Elmer model 3100) at a wavelength of 357.9 nm. The concentration of Cr(VI) was determined by a calorimetric method. The complex formed due to 1,5-diphenylcarbazide and Cr(VI) in acidic solution was analyzed in a UV-spectrophotometer at 540 nm. The trivalent chromium concentration was determined from the difference between total chromium and Cr(VI) concentrations.
mium, which subsequently shifts to other forms such as CrO42and Cr2O72- as the pH increases. These low acidic conditions would also cause the surface of the sorbent to be protonated to a higher extent, which results in a strong attraction between the negatively charged Cr(VI) complex ions and positively charged biomass surface. However, there is also a possibility that Cr(VI) can be reduced to Cr(III) due to electron donor groups of biomaterial and also because of high redox potential value (>+1.3 V at standard condition) of Cr(VI). 12,15 To confirm this phenomenon, efforts were made to quantify the concentration of Cr(III) in the solution. The results clearly indicated that Cr(VI) was completely reduced to Cr(III) in the examined pH range of 1-6. The acidic pH accelerates the redox reaction in aqueous and solid phases, since the protons participate in this reaction.11 Phenolic and tannin groups have been reported as electron-donor groups of biomaterials.20 Polyphenols are the most important ingredient in tea and coffee. More than 30 polyphenolic compounds were present in a complex form, which includes catechin, flavones, anthocyan, and phenolic acid.21,22 Thus the reactions leading to reduction of Cr(VI) to Cr(III) can be summarized as follows:
3. Results and Discussion 3.1. Influence of pH. Preliminary experiments on the effect of equilibrium pH on the Cr(VI) biosorption revealed that strong acidic conditions are required to obtain maximum removal efficiencies. Experiments were conducted in the pH range of 1-6 at an initial Cr(VI) concentration of 30 mg/L (Figure 1). Results revealed that pH 4 was optimum for Cr(VI) removal by both TD and CD. Hexavalent chromium exists as CrO42-, HCrO4-, H2CrO4, HCr2O7-, or Cr2O72-, depending on the pH of the medium and the total Cr(VI) concentration.11 At low pH solutions, HCrO4- is the prevalent form of hexavalent chro-
HCrO4- + 7H+ + 3e- f Cr3+ + 4H2O
(1)
Cr2O72- + 14H+ + 6e- f 2Cr3+ + 7H2O
(2)
Eventually, the formed Cr(III) cations can bind to the negatively charged groups such as carboxyl of the biomaterial. The results indicated that Cr(VI) was completely reduced to Cr(III) by both TP and CP at all pH conditions examined in this study. However, only less than 60 and 56% of total chromium were adsorbed onto TD and CD, respectively, in the pH range of 1-6 (Figure 1). 3.2. Influence of Biosorbent Dosage. The influence of biosorbent dosage on the Cr(VI) reduction and biosorption was examined by varying dosages from 0.5 to 5 g/L. Figure 2 presents typical sets of results obtained by varying dosages of tea and coffee dusts during Cr(VI) removal. As the biosorbent dose increases, the extent of Cr(VI) reduction also increases. An increase in biosorbent concentration generally increases the number of binding sites, which in turn increases the Cr(VI) reduction. It is also worth noting that biosorbent concentration of 3 g/L was able to reduce Cr(VI) completely and further increase in biosorbent dose resulted in unaltered reduction efficiency. In contrast, Cr(VI) uptake decreased with the increase in biomass dose. Since the uptake is a measure of the amount
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Figure 3. Isotherms during chromium biosorption onto TD and CD (pH ) 4; temperature ) 23 ( 2 °C; biosorbent dose ) 3 g/L). Curves were predicted by the Toth model.
of Cr(VI) ions bound by unit weight of the biomass, its magnitude decreased with the increase in biomass dose. This decline may also be due to the role of several other factors. The important factors include: (a) at high sorbent dosages, the available metal ions are insufficient to cover all the exchangeable sites on the biosorbent, usually resulting in low metal uptake; and (b) at high sorbent dosages, interference between binding sites also results in low uptake.8 In the present study, the lowest biosorbent dosage (0.5 g/L) resulted in the lowest Cr(VI) reduction and highest Cr(VI) uptake for both tea and coffee dusts. Considering the importance of both reduction and biosorption for our study, 3 g/L of tea or coffee dusts were selected as optimum biosorbent dosage for further studies, as it showed better reduction and biosorption potential comparatively. 3.3. Isotherm. Since maximum reductions of Cr(VI) followed by Cr(III) adsorption were observed at pH 4, the isotherm experiments were conducted at this pH to evaluate the full saturation potential of TD and CD. The initial Cr(VI) concentrations were varied from 0 to 500 mg/L at a fixed biosorbent dose of 3 g/L. Isotherms were determined by plotting total chromium bound to the biomass (mg/g) to the residual Cr(III) concentration (mg/L). The experimental isotherms (Figure 3) followed a general trend, i.e., the Cr(III) uptake increased with increasing Cr(VI) concentration and reached saturation at higher equilibrium concentrations. The isotherms obtained for both TD and CD can be classified as “L” isotherms without a strict plateau i.e., the ratio between the concentration of solute remaining in solution and that sorbed on the solid decreases when the solute concentration increases, providing a concave curve.23 The Langmuir model, with the assumption that the reduced Cr(III) remains in an equilibrium state between the biomass and aqueous solution, can be expressed as QCr(III) )
max QCr(III) bCCr(III) 1 + bCCr(III)
(3)
where QCr(III) is the amount of the reduced Cr(III) bound to per max is the maximum Cr(III) uptake unit of biomass (mg/g), QCr(III) capacity of the biosorbent (mg/g), b is the Langmuir affinity constant (L/mg), and CCr(III) is the equilibrium Cr(III) concentration (mg/L). The model parameters were evaluated using nonlinear regression in Sigma Plot (version 4.0, SPSS, USA) software. The Langmuir model served to estimate the maximum uptake values, which could not be reached in the experiments. The constant b represents the affinity between the sorbent and sorbate. Maximum Cr(III) uptakes of 44.9 and 39.0 mg/g were
Figure 4. Reduction and biosorption kinetics during Cr(VI) removal by TD and CD (pH ) 4; temperature ) 23 ( 2 °C; biosorbent dose ) 3 g/L).
Figure 5. Reduction and biosorption kinetics during Cr(VI) removal by Sargassum sp. and UlVa sp. (pH ) 4; temperature ) 23 ( 2 °C; biosorbent dose ) 3 g/L).
observed for TD and CD, respectively. Also, TD exhibited higher affinity constant of 0.019 L/mg, compared to 0.013 L/mg for CD. Even though the model was able to predict the Cr(III) biosorption isotherm curves with high correlation coefficients (>0.997), the percent error associated with the prediction was very high in the order of greater than 30%. Therefore, a three parameter model, viz. the Toth model, was applied for the present isotherm data. The Toth model,24 derived from potential theory, has proven useful in describing sorption in heterogeneous systems and can be represented as QCr(III) )
max QCr(III) bT CCr(III)
[1 + (bTCCr(III))1 ⁄ nT]nT
(4)
where bT is the Toth model constant (L/mg) and nT the Toth model exponent. The model was able to describe the chromium biosorption isotherms of both TD and CD with very high correlation coefficients (>0.999) and low percent error values max , bT, and nT values were (
