Sorption and Reductive Transformation of Hexavalent Chromium On Coconut-Shell-Derived Black Carbon.

Coconut shell (CS) is a waste by-product from agriculture. In this study, CS was converted to carbonized materials by pyrolysis at different temperatures. The extent and rate of Cr(VI) sorption of CS and CS-derived carbons were studied as a function of pH, contact time, and initial Cr(VI) concentration. The structures of sorbed Cr species on the sorbents were determined using X-ray absorption spectroscopy. XPS and FT-IR spectra were obtained for the sorbents before and after reaction to identify the functional groups responsible for sorption and reduction of Cr(VI) on the surfaces of the CS-derived carbon. The Cr(VI) sorption of CS-derived carbons was primarily dependent on the surface area and porosity of the sorbents. Upon decreasing solution pH, the Cr(VI) sorption on CS-derived carbon increased due to increasing positive charges on the surfaces of the sorbents. In addition, the reduction of Cr(VI) to Cr(III) became more significant at lower pH, which also contributed to the Cr(VI) removal from solutions. The removal of Cr(VI) by CS-derived carbons are dependent on their pyrolysis temperature. CS and CS-250 were better than other carbon materials in terms of the Cr(VI) removal rate. As the pyrolysis temperature was increased to 350¢XC and above, the removal of Cr(VI) decreased. The surface properties of CS-derived carbon, in particular the surface functional groups, may play a a significant role in Cr(VI) sorption and reduction. The functional groups involving in Cr(VI) reduction would be the phenolic, carboxylic and oxygen substituted aromatic functional groups(-OCH and ¡VOH).