Effective removal of Hg2+ and Cd2+ in aqueous systems by Fe–Mn oxide modified biochar: A combined experimental and DFT calculation

The Fe–Mn oxide modified biochar (FM–BC) was synthesized using straw biochar, and the adsorption mechanism of Hg2+ and Cd2+ was explained in–depth through adsorption experience data, characterization, and density functional theory (DFT) calculation. Adsorption kinetics and isotherm fitting suggested that FM–BC contained more adsorption sites than pristine biochar, and chemisorption was the rate–limiting step in Hg2+ and Cd2+ adsorption. The maximum adsorption capacities of Hg2+ and Cd2+ reached 86.82 and 131.03 mg·g−1, respectively. Combining the characterization results (SEM–EDS, XRD, FTIR and XPS), it can be found that chemical complexation reaction (monodentate or multidentate inner–sphere complexes) was the main adsorption mechanisms for Hg2+ and Cd2+ adsorption. Moreover, the coprecipitation reaction was another significant mechanism of removing Cd2+ by FM–BC. The DFT calculation results illustrated that oxygen–containing functional groups and metal–π interaction were crucial for the excellent adsorption performance of Hg2+ and Cd2+ onto FM–BC. The carbon structures doped with Fe/Mn atoms have significant adsorption energies for Hg2+ and Cd2+, and Fe/Mn atoms provided more electrons to promote the electron transfer. These innovative findings provide theoretical basis for the application of FM–BC as a high–performance adsorbent in the purification of Hg and Cd contaminated wastewater.