Insight into the adsorption mechanisms of Cr(vi) from aqueous solution using biochar prepared <i>via</i> conventional pyrolysis and microwave-assisted pyrolysis.
Abstract
The management of hexavalent chromium (Cr(vi)) in aqueous environments remains a critical challenge in environmental engineering due to its high toxicity, mobility, and industrial prevalence. This study investigates the synthesis and performance of a structurally robust, highly porous rice husk-derived biochar prepared via a novel integrated approach of microwave-assisted pyrolysis followed by hydrothermal functionalization (RHB-MW), comparatively evaluating it against biochar prepared via conventional thermal carbonization coupled with the same hydrothermal post-treatment (RHB-Calc). Optimal preparation conditions utilizing internal volumetric microwave heating (800 W, 10 minutes), prior to hydrothermal conditioning, successfully preserved the cellular architecture of the biomass, yielding an advanced biochar with a superior specific surface area of 296.56 m2 g-1 and a mesoporous architecture (average pore size of 5.41 nm). The Langmuir maximum adsorption was 39.52 and 33.38 mg g-1 for RHB-MW and RHB-Calc, respectively. The macroscopic adsorption process for both materials closely aligned with the Krishnamurti isotherm, suggesting complex, cooperative sorption behavior. Kinetic modeling indicated that RHB-MW behavior is consistent with pseudo-second-order chemisorption pathways, while RHB-Calc displayed kinetic limitations typical of an energetically heterogeneous surface described by the Elovich model. Cyclic regeneration trials utilizing 0.5 M NaOH demonstrated highly effective desorption via alkaline deprotonation and electrostatic repulsion; RHB-MW retained an adsorption capacity of 26.32 mg g-1 by the third cycle, whereas RHB-Calc capacity fell sharply to 15.98 mg g-1. These findings highlight that integrating microwave-assisted pyrolysis with hydrothermal treatment is a highly effective, physically transformative upcycling strategy for converting abundant agricultural residues into high-performance, structurally resilient mesoporous matrices for complex heavy metal remediation.
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