Abstract:The adsorption characteristics of Pb2+ in water by rice-straw biochar were elucidated by adsorption isotherm and kinetics, and the adsorption mechanism of biochar was qualitatively and quantitatively revealed by means of elemental analysis, BET-N2, Zeta potential, SEM-EDS, FT-IR, XRD and XPS, along with the calculation for relative distribution of each adsorption mechanism. The results indicated that the maximum adsorption capacity of Pb2+ by biochar were 233.48 mg.g-1、236.00 mg.g-1 and 237.50 mg.g-1 under the temperatures of 20℃、30℃ and 40℃, respectively. The experimental data fitted the Langmuir (R2 > 0.96) better than the Freundlich isotherm, which suggested monolayer adsorption and a homogeneous distribution of active surface sites. Under the initial concentrations of 50,100and 300 mg.L-1, the equilibrium time of biochar were 30 min, 90 min and 360 min, respectively, which followed the pseudo-second-order mechanism and corresponded to a chemisorption process. The main mechanisms responsible for Pb2+ adsorption by rice-straw biochar included chemical precipitation, cation exchange, Cπ-electron coordination, and functional group complexation, in which the chemical precipitates could be mainly Pb4(CO3)2(SO4)(OH)2 and PbCO3, accounting for 47.15%-50.81% of total adsorption. Cation exchange also played an important role in the adsorption mechanisms due to high contribution proportions to total adsorption (32.82%-37.77%). Among the adsorption mechanisms, both the Cπ-electron coordination and functional group complexation could contribute 15.03% to 16.37% to the total adsorption, but these were overshadowed by the chemical precipitation and cation exchange, accounting for 83.63% to 84.92% in the total adsorption. These suggested rice-straw biochar could be an efficient adsorbent for the removal of Pb2+ from aqueous solution.
