Study Of Chromium(?) Reduction Using EGCG And EGCG-Fe Complex In Aqueous Solution

Epigallocatechin gallate (EGCG) is the most abundant catechin in tea polyphenol and known for its excellent antioxidative and coordinative property. This study focus on hexavalent chromium reduction by EGCG and EGCG-Fe complex and its reaction mechanism, to exploit the potential of Cr(?) remediation utilizing tea polyphenol.As one of the most important natural occurring antioxidant in green tea, EGCG possesses potent reducing ability. Experiments showed that EGCG can effectively reduce Cr(?) in aqueoues solution at pH<4. In terms of 100?M Cr(?) wastewater sample,90.86% initial Cr(?) were reduced within 2 hours by equalmole EGCG at pH=3,25?. With a prolonged reaction time, one mole EGCG could reduce more than 3 moles of Cr(?) at acidic condition. Experiment results indicated that reacthion between Cr(?) and EGCG appears an overall second-order reaction, which follows first-order to both Cr(VI) and EGCG concentrations. The reaction kinetics influenced by solution pH consists of two stages:(?) first-order with respect to [H+] at pH 3.07? 4.29 and (?) zero-order in [H+] over the pH range from 4.86 to 7.05. Parallel reaction pathways are probably responsible for the different pH dependence of the reaction order. Based on this theory, an emprical equation was proposed to predict reaction kinetics in the pH range from 3.07-7.05. In addition to the amount of [H+] participated in reaction, solution pH also affected the Cr(?) speciation and stability of EGCG. Hence at neutral and alkaline condition (pH>7.35), the variation of Cr(?) species and enhanced EGCG autoxidation would result in significant decline of Cr(?) reduction rate. Moreover, HPLC-MS/MS and UV-VIS analyses were performed to illustrate more details of the redox reaction. EGCG quinone was identified as the preliminary oxidation product of EGCG and the trihydroxyphenyl B ring of EGCG was determined as the most reactive moiety to Cr(?). Accordingly, a mechanism of Cr(?) reduction involving EGCG-Cr ester formation was proposed.Apart from reducing ability, EGCG is also a prominent chelator of transition metal ion. The complex formed by EGCG and Fe(?) (or Fe(?)) could markedly accelerate Cr(?) reduction rate. The reaction rate constant of Cr(?) reduction by EGCG-Fe(?) increased with the decrease of solution pH, achieving values of 3356.3, 2049.5,1453.7 and 1022.0 M-1s-1 at pH 3.75,4.02,4.32 and 4.61 respectively. The formation of EGCG-Fe(?) complex, however, was favored at higher pH, thus the apparent reaction rates were enhanced as the solution pH rise. At pH<5, EGCG-Fe(?) could reduce Fe(?) to Fe(?), then forming high reactive intermediates such as EGCG semiqueione and EGCG semiqueione-Fe(?) complex, which are probably responsible for the great improved Cr(?) reduction rate. Therefore, EGCG-Fe(?)/Fe(?) would be formed at acidic condition and achieve a high Cr(?) reduction efficiency. Besides, the presence of dissolved oxygen would compete with Cr(?) to oxidize EGCG-Fe(?) and depress the reduction rate, whereas the effect of dissolved oxygen was negligible at pH<4. Hence, lower solution pH and dissolved oxygen concentration are in favor of Cr(?) remediation by EGCG-Fe complex.