Interpret the elimination behaviors of lead and vanadium from the water by employing functionalized biochars in diverse environmental conditions

Wide-ranging researches have been executed to treat groundwater from different mining areas, although complex behaviors of diverse metal ion species in the groundwater have not been illustrated clearly. This research study explored the mechanisms through which Pb(II) and V(V) are eliminated in single and binary-metal removal processes by oxygen, nitrogen, and sulfur-doped biochars also considering the kinetic and characterization techniques. The adsorption efficiency of V (V) was enhanced by oxygen-doped biochar at pH 4 with an adsorption capacity of ~70 mg/g. However, Pb (II) was rapidly removed at pH 6 with a higher adsorption capacity of ~180 mg/g by the nitrogen and sulfur-doped biochar forming PbCO₃ and V(CO)₆ crystals along the single-metal removal process. These results could be explained by the Hard Soft Acid Base theory. The hard Lewis acid vanadium was attracted by the hard Lewis base oxygen, and the intermediate Lewis acid lead was attracted by the intermediate and soft Lewis base nitrogen and sulfur. Besides, the removal ability of Pb(II) and V(V) in the binary-metal removal process showed a similar phenomenon for all types of biochars at pH 4 with the adsorption capacity of ~400 mg/g for Pb(II) and 175 mg/g for V(V), but the composition of vanadium species remains unclear on the surface of the biochars. Initially, H₃V₂O₇⁻, H₂VO₄⁻, and HVO₄²⁻ species were electrostatically attracted by the oxygen-based functionalities, then V(V) species was partially reduced to VO²⁺ by the oxygen, nitrogen, and sulfur functionalities in different ratios. Finally, H₃V₂O₇⁻, H₂VO₄⁻, and HVO₄²⁻ species produced Pb₅(VO₄)₃Cl and Pb₂V₂O₇ which co-precipitate with Pb(II), but VO²⁺ does not generate any form of precipitates. The above-explained technique supports the treatment of vanadium mining groundwater with valuable vanadinite (Pb₅(VO₄)₃Cl) mineral.