Organic carbon and inorganic silicon speciation in rice-bran-derived biochars affect its capacity to adsorb cadmium in solution

Biochar enriched in oxygenated functional groups and minerals has a high adsorption capacity for heavy metals. The roles and contributions of the organic and mineral fractions in biochars to heavy metal adsorption are critical to understanding the long-term effect of biochar application in contaminated soil. Batch adsorption experiments were performed on differently treated rice-bran-derived biochars to determine the contributions of the organic and mineral fractions to cadmium (Cd) adsorption in this study. Rice-bran-derived biochars were produced at 300 and 700 A degrees C (named R300 and R700). The Cd adsorption onto the original biochars and biochars treated with water, hydrochloric acid (HCl), and HCl + HF was compared on the basis of their adsorption isotherm curves. Biochar properties were characterized by CHN elemental analysis, XRD, zeta potential, Fourier transform infrared spectroscopy (FTIR), and SEM-EDX before and after Cd adsorption. The Cd adsorption capacity gradually decreased as the water-washing times increased, and ten- and hundred-fold decreases were observed on acid-treated biochars. The relative contribution of water-soluble matter to the decrease of Cd in solution was 87.4 % and 71.4 % for R300 and R700, respectively. The contribution of HCl-soluble minerals was 11.5 % (R300) and 28.3 % (R700), while the role of insoluble silicon oxide could be ignored. The adsorption mechanism between Cd and the organic fractions or minerals in biochars was complexation, as illustrated by the FTIR spectra. Both the organic and mineral fractions in biochars play an important role in Cd adsorption. The relative contribution of water-soluble matter was much higher than that of HCl-soluble ashes and insoluble silicon oxide minerals. The results indicate that the adsorption capacity of biochars for Cd may greatly decrease in leached and acidic soils with the loss of soluble organic and mineral fractions.