Chromium removal from electroplating wastewater by coir pith

Coir pith is a by-product from padding used in mattress factories. It contains a high amount of lignin. Therefore, this study investigated the use of coir pith in the removal of hexavalent chromium from electroplating wastewater by varying the parameters, such as the system pH, contact time, adsorbent dosage, and temperature. The maximum removal (99.99%) was obtained at 2% (w/v) dosage, particle size <75microm, at initial Cr(VI) 1647mgl(-1), system pH 2, and an equilibrium time of 18h. The adsorption isotherm of coir pith fitted reasonably well with the Langmuir model. The maximum Cr(VI) adsorption capacity of coir pith at 15, 30, 45 and 60 degrees C was 138.04, 197.23, 262.89 and 317.65mgCr(VI)g(-1) coir pith, respectively. Thermodynamic parameters indicated an endothermic process and the adsorption process was favored at high temperature. Desorption studies of Cr(VI) on coir pith and X-ray absorption near edge structure (XANES) suggested that most of the chromium bound on the coir pith was in Cr(III) form due to the fact that the toxic Cr(VI) adsorbed on the coir pith by electrostatic attraction was easily reduced to less toxic Cr(III). Fourier transform infrared (FT-IR) spectrometry analysis indicated that the carbonyl (CO) groups and methoxy (O-CH(3)) groups from the lignin structure in coir pith may be involved in the mechanism of chromium adsorption. The reduced Cr(III) on the coir pith surface may be bound with CO groups and O-CH(3) groups through coordinate covalent bonding in which a lone pair of electrons in the oxygen atoms of the methoxy and carbonyl groups can be donated to form a shared bond with Cr(III).