Corncob and rice husk biochar mitigate potentially toxic elemental mobility in flooded-paddy-mine-contaminated soils

Abstract Redox-induced changes in soils via flooding influence the solubility and mobility of potentially toxic elements (PTEs) and, by extension, trigger an upsurge of their transport into surrounding ecosystems. We simulated an anaerobic paddy field environment through flooding of amended artisanal-gold mine-contaminated soils in an 85-day pot incubation experiment. We studied the redox-induced changes that occurred in these amended soils treated with 5% rates of compost, corn-cob biochar, rice husk biochar (RB), and poultry manure (PM) whilst under rice cultivation. The biogeochemical drivers that may influence PTE mobilisation under dynamically changing redox conditions, such as those in paddy soils, were then investigated. Slurry samples were collected from each flooded pot with a syringe after the 85-day incubation period for laboratory analysis of seven PTE concentrations, including As, Cd, Cu, Fe, Mn, Pb, and Zn. We employed linear regression models to identify the variables controlling the mobilisation and release of these PTEs from the multi-element mine-contaminated sites. The study identified Eh as the dominant control on PTE mobilisation, with temperature acting as a secondary but important driver. The influence of temperature was particularly evident in regulating the behaviour of As, Fe, and Mn, suggesting that temperature-dependent redox and biogeochemical processes contribute substantially to PTE mobilisation. In contrast, pH exerted a comparatively limited influence, indicating that PTE mobilisation was governed more strongly by redox conditions and temperature than by pH. These findings highlight the potential of corncob and rice husk biochar to mitigate elemental mobility in flooded mine-contaminated soils.