Biochars derived from various crop straws: characterization and Cd(II) removal potential.

Five types of biochars prepared from four crop straws and one wood shaving at 600 °C were characterized, and their sorption to Cd(II) were determined to investigate the differences in capacity to function as sorbents to heavy metals. Surface areas and pore volumes of the biochars were inversely correlated to the lignin content of raw biomass. The biochars derived from crop straws displayed more developed pore structure than wood char due to the higher lignin content of wood. Sorption capacity of the biochars to Cd(II) followed the order of corn straw>cotton straw>wheat straw>rice straw>poplar shaving, which was not strictly consistent with the surface area of the chars. The surface characteristics of chars before and after Cd(II) sorption were investigated with scanning electron microscopy equipped with an energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy, which suggested that the higher sorption of Cd(II) on corn straw chars was mainly attributed to cation exchange, surface precipitation of carbonate, and surface complexation with oxygen-containing groups. This study indicated that crop straw biochars exhibit distinct sorption capacities to heavy metals due to various surface characteristics, and thus the sorption efficiency should be carefully evaluated specific to target contaminant.