Biochar (BC) derived from waste products is a cost-effective sorbent for remediation of metal-contaminated soils. We studied the kinetics and adsorption mechanisms for removal of metal ions, such as lead (Pb 2+) and cadmium (Cd 2+) with biochar. The adsorption capacities of BC for Pb 2+ and Cd 2+ increased after alkaline treatment. The highest sorption capacities were 175.53 and 68.08 mg g-1 , for Pb and Cd, respectively. The Langmuir adsorption isotherm and pseudo second kinetic equation could well fit the adsorption processes, revealing that the sorption mechanisms of Pb 2+ and Cd 2+ by BC are complex and predominantly controlled by chemisorption. BC has a higher affinity for Pb than Cd, due to easy hydrolysis of Pb at low pH. Furthermore, precipitation as carbonate minerals (2PbCO 3 •Pb(OH) 2 and CdCO 3) and complexation with functional groups (carboxyl and hydroxyl) were also important for adsorption of Pb and Cd by BC.
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