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

•Chemical modification increased Pb adsorption of the biochars pyrolyzed at 300 °C.•Modification enhanced free carboxyl and hydroxyl groups on biochars' surfaces.•Pb-montmorillonite and Pb(C2H3O2)2 were the dominant species of the Pb-loaded biochars.•The chemical modification decreased the exchangeable Pb on the Pb-loaded biochars.•Chemical modification reduced Pb adsorption on the biochars pyrolyzed at 700 °C.

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 85.2 mg g− 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.

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