Removal of Cr(VI) Ions by Spent Tea and Coffee Dusts: Reduction to Cr(III) and Biosorption

Abstract

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.