Adsorption characteristics of metal-capture-agent immobilized PAC for low concentration Cr(VI) in water at low temperature

As a kind of metal-capture-agent, food grade tannic-acid(TA) was immobilized on commercial powdered activated carbon(PAC). Adsorption of low concentration Cr(VI)(0.500 mg·L^-1) on the TA-PAC was investigated at various dosage and pH, and adsorption capacities and adsorption isotherms were measured at 275 K. Compared with HNO₃- and NaOH-modified PAC(N-PAC and OH-PAC), the results obtained from the BET, Boehm's titration and FTIR indicated that TA-PAC is of larger specific surface area, total pore volume and more abundant acid functional groups. The performance of TA-PAC, expressed by removal efficiency of Cr(VI)and total chromium, is much more excellent than N-PAC and OH-PAC. Its adsorption capacity for Cr(VI) was found to be pH-dependent, the removal efficiency both Cr(VI) and total chromium were notably higher in the acidic medium(pH=3.0-6.0)than in alkaline medium(pH=7.0-10.0). The adsorption data for total chromium were fitted using two-parameter isotherm models and the related parameters were estimated. Freundlich model is the best for data of C_e from 0.010 to 0.404 mg·L^-1, R²=0.9941, indicating a heterogeneous coverage of chromium on TA-PAC. Langmuir(I), Langmuir(II)and Temkin models didn't fit the data well, because of occurring piecewise node at 0.149 mg·L^-1, 0.151 mg·L^-1 and 0.163 mg·L^-1 respectively. The adsorption mechanism of TA-PAC for Cr(VI) was assumed to be chemical process, such as surface reduction of Cr(VI), esterification between chromate and catechol, and ion exchange.