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 Ce 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.