Composting improves biosorption of Pb2+ and Ni2+ by renewable lignocellulosic materials. Characteristics and mechanisms involved

Composting may alter the sorption properties of organic materials deriving from renewable sources. The widely available olive tree pruning waste (OTPW) and its composted form (COTPW) were comparatively tested for Pb2+ and Ni2+ removal in single- and double-metal systems. Pb2+ biosorption was higher than that of Ni2+ and increased metal biosorption was observed up to pH 5.0. The process followed pseudo-second order kinetics and described by the Langmuir isotherm. Surface area, total pore volume and zeta potential values were increased following composting, while cation exchange capacity was over-doubled (from 37.6 to 87.4 cmol, kg(-1)) leading to higher Pb2+ and Ni2+ biosorption and improved biosorption at elevated temperatures. There was 144%, 78%, and 148% increase in the maximum sorption capacity for Pb2+ and 29%, 59%, and 108% for Ni2+ at 10, 25 and 60 degrees C respectively. FTIR analysis indicated significant shifts in the chemical structure of OTPW as a result of composting, in line with oxidative decomposition processes. Among the desorption solutions tested, HNO3 and EDTA showed maximum recovery of both metals. Physisorption of both metals was greatly reduced by composting, leading to a biosorbent that retained Pb2+ and Ni2+ more efficiently. The suppressive effect of Ni2+ on Pb2+ sorption on OTPW throughout the whole range of Pb+2 concentrations, and the suppressive effect of Pb+2 on Ni+2 sorption at low Nr(2+) concentrations were both alleviated when composted OTPW was used. Overall, OTPW proved to be a highly efficient biosorbent, especially for Pb2+, and composting resulted in multifunctional improvement of sorption characteristics. (C) 2013 Elsevier B.V. All rights reserved.