Effect of Low-Molecular-Weight Organic Acids and Silicon on Arsenic Adsorption and Desorption in a Paddy Soil of Bengal Delta Plain: Insights from Thermodynamics and Equilibrium Modeling

The transfer of arsenic (As) from contaminated soil to the human body through rice represents a health catastrophe in the Bengal Delta Plain (BDP), emphasizing the need for a deeper understanding of the indispensable factors influencing the mobility and retention of As in the paddy soils of BDP. This study examined the effects of temperature and rice root-secreted low-molecular-weight organic acids (LMWOAs), at field-relevant levels, on As adsorption and desorption, while evaluating the influence of silicon (Si) on As desorption in BDP paddy soil, using thermodynamic analysis and equilibrium modeling. Arsenic adsorption and desorption experiments were conducted at two temperatures (20 and 35 °C) with and without LMWOAs, and the effect of Si (0–100 mg L⁻¹) on As desorption was evaluated. The adsorption data were analyzed using a newly developed R package, “AdIsMF” for linear and nonlinear Freundlich and Langmuir isotherm models. Results revealed a considerable adsorption capacity (q_max: 698–813 mg kg⁻¹) driven by endothermic, entropy-driven adsorption processes. The q_max and adsorption affinity (K_l) increased at higher temperatures to 12.4–13.3% and 19.5–26.6%, respectively. LMWOAs had minimal impact on adsorption isotherm, slightly reducing K_l (1.14 to 0.98 L mg⁻¹) alone, but significantly impacting the energetics of As adsorption. The linear Langmuir model outperformed others in model selection criteria and accuracy measures, indicating monolayer adsorption on homogeneous surfaces. The adsorption mechanism was inferred to be predominantly chemisorption, supported by physisorption. Desorption studies revealed irreversible As binding to soil, with greater desorption at lower temperatures. Silicon concentrations above 1 mg L⁻¹ significantly enhanced As mobility, with cumulative desorption reaching 31.5% under 100 mg L⁻¹ Si. These findings underscore the importance of soil temperature, i.e. rice cultivation season, on As availability, and dose-optimization of Si amendments to mitigate As risks in paddy systems.