Unraveling sorption of lead in aqueous solutions by chemically modified biochar derived from coconut fiber: A microscopic and spectroscopic investigation.

In this study, we examined the efficacy of nine different types of coconut-fiber derived biochars (CFBs), prepared at different temperatures and chemically modified with ammonia, hydrogen peroxide and nitric acid, to remove lead (Pb2+) from aqueous solutions. Langmuir-qm values of the biochars pyrolyzed at 300°C and modified with ammonia and nitric acid increased from 49.5 to 105.5 and 85.2mgg-1, respectively, compared to control (unmodified), whereas hydrogen peroxide treatment had no effect. The maximum amount of Pb adsorbed on biochars was in the order of CFB-700>MCFB-300-NH3·H2O>CFB-500>MCFB-300-HNO3>CFB-300. X-ray absorption fine structure (XAFS) spectroscopy results revealed that Pb-montmorillonite, Pb(C2H3O2)2, PbSO4, Pb-Al2O3 and Pb3(PO4)2 were the five most important Pb species observed in Pb-loaded biochars, and as such, favoring Pb immobilization in aqueous solutions. Overall, the sorption capacity of CFBs pyrolyzed at 300°C substantially increased for Pb2+ with ammonia and nitric acid modification. However, these chemical modifications did not improve the sorption of Pb on CFBs pyrolyzed at temperatures ≥500°C, thereby highlighting a temperature dependent response of chemically modified biochars to Pb sorption in this study.