Microwave-assisted hydrothermal treatment of soybean residue and chitosan: Characterization of hydrochars and role of N and P transformation for Pb(II) removal

Abstract

The beneficial utilization of biomass wastes and residues for environmental remediation is the focus of many research efforts towards environmental sustainability. In this study, agricultural residue (soybean, (SB)) and a biopolymer (chitosan, (CS)) were examined for enhanced heavy metals (HMs) removal. Specifically, SB, CS, and mixtures of SB and CS at different mass ratios (SB:CS) were treated to obtain hydrochars (HCs) via microwave-assisted hydrothermal treatment (Mw-HTT). Synthesized HCs were subsequently characterized by elemental analysis (EA), SEM/EDX, XRD, FT-IR, XPS, and sequential extraction to estimate nitrogen (N) and phosphorus (P) transformation during Mw-HTT. The N remained same in CS HCs and decreased in SB HCs based on EA results, and sequential extraction indicated that P decreased after Mw-HTT in SB and CS HCs. Lead (Pb(II)) adsorption was carried out using as-synthesized HCs, and experimental results were examined using standard adsorption isotherm and kinetic models. The R2 value (≥ 0.960) with acceptable n and Kf confirmed that Freundlich isotherm was suitable. For adsorption kinetics, pseudo-2nd-order model showed R2 ≥0.94 and good agreement between calculated and actual adsorption capacities, providing insights into adsorption mechanisms for CS, SB, and SB:CS HCs. It was found that N and P transformed during Mw-HTT, which played a role in Pb(II) removal from solutions.