Spectroscopic investigations and density functional theory calculations reveal differences in retention mechanisms of lead and copper on chemically-modified phytolith-rich biochars

A better understanding of different retention mechanisms of potentially toxic elements (PTEs) by biochars during the remediation of contaminated sites is critically needed. In this study, different spectroscopic techniques including synchrotron-based micro-X-ray fluorescence (mu-XRF), X-ray absorption fine structure (XAFS), and near edge XAFS spectroscopy (NEXAFS), were used to investigate the spatial distributions and retention mechanisms of lead (Pb) and copper (Cu) on phytolith-rich coconut-fiber biochar (CFB), and ammonia, nitric acid and hydrogen peroxide modified CFB (MCFB) (i.e., ACFB, NCFB and HCFB). The mu-XRF analyses indicated that sorption sites on ACFB and NCFB were more efficient compared to those on CFB and HCFB to bind Pb/Cu. XAFS analyses revealed that the percentage of Pb species as Pb(C2H3O2)(2) increased from 22.2% (Pb-loaded CFBs) to 47.4% and 41.9% on Pb-loaded NCFBs and HCFBs, while the percentage of Cu(OH)(2) and Cu(C2H3O2)(2) increased from 5.8% to 32.8% (Cu-loaded CFBs) to 41.5% and 43.4% (Cu-loaded NCFBs), and 27.1% and 35.1% (Cu loaded HCFBs), respectively. Due to their similar atomic structures of Pb/Cu, Pb(C2H3O2)(2)/Pb-loaded montmorillonite and Cu(C2H3O2)(2)/Cu(OH)(2) were identified as the predominant Pb/Cu species observed in Pb-and Cu-loaded MCFBs. The NEXAFS analyses of carbon confirmed that increasing amounts of carboxylic groups were formed on HCFB and NCFB by oxidizing carbon-containing functional groups, which could provide additional active binding sites for Pb/Cu retention. Results from the X-ray photoelectron spectroscopy analyses of nitrogen showed that azido-groups of ACFB played major roles in Pb/Cu retention, while amide-groups and pyridine groups of NCFB primarily participated in Pb/Cu retention. Overall, density functional theory calculations suggested that silicate and the synergistic effect of hydroxyl and carboxylic-groups on MCFBs were highly efficient in Pb retention, while azido-groups and/or carboxylic-groups played major roles in Cu retention. These results provide novel insights into the PTE retention mechanisms of MCFBs.

Automated summary

This study investigates the retention mechanisms of potentially toxic elements (PTEs) by different biochars, revealing that ammonia, nitric acid, and hydrogen peroxide modified biochars show more efficient sorption sites for Pb and Cu compared to unmodified biochar. The results provide novel insights into the PTE retention mechanisms of modified biochars.