Teff straw characterization and utilization for chromium removal from wastewater: Kinetics, isotherm and thermodynamic modelling

Abstract

Significant numbers of studies have been reported on use of various types of agricultural wastes as adsorbent. Teff straw originates from Ethiopia where chromium pollution is one of the major pollution problem. Therefore, in the present study, teff straw was tested as an adsorbent for hexavalent chromium removal from aqueous solution. Characterization of teff straw was done using various instruments such as X-ray diffraction, scanning electron micrograph (SEM) and Fourier transform infra-red (FTIR) spectroscopy. Batch experiments were carried out to study the effects like pH, initial chromium concentration, contact time and dose of teff straw on the efficiency of the adsorption process. Adsorption of Cr(VI) on teff straw adsorbent was by a complex process. Better realistic description of adsorption kinetics was seen from pseudo-second order rate equation. The equilibrium sorption characteristics of Cr(VI) from aqueous solutions with concentrations in the range of 100–900mg/L were studied at different temperatures in the range of 288–318K, and temperature increment was found to have a positive effect on the sorption process. Langmuir isotherm gave better representation over wide range of temperature. At adsorbent dose=10g/L, pH 2 and contact time=6h, maximum Cr(VI) uptake per unit mass of teff straw (qmax) was found as 58, 70, 79, and 86mg/g at 288, 298, 308, and 318K, respectively. The negative value of change in Gibbs free energy (ΔGo) indicated spontaneous adsorption of Cr(VI) onto teff straw adsorbent. Isosteric heats of adsorption (ΔHst,a) were low at low levels of surface coverage and increased progressively.