Effect of the combined treatment of red mud and gypsum on plant-available arsenic in acidic and alkaline soils

Abstract

Red mud (RM) is noted for its ability to immobilize heavy metals by absorbing and precipitating them. However, RM's high alkalinity increases soil dissolved organic carbon (DOC), which hinders the absorption of anionic metals like arsenic (As), thereby reducing its immobilization effectiveness. Conversely, phos phogypsum (G) lowers soil pH and DOC levels. Thus, combining RM and G is proposed to enhance immobilization efficiency. This study examined their combined impact on heavy metal(loid) immobilization in 39 As-contaminated soils (19 acidic, 20 alkaline). Each soil was divided into ‘untreated’ and ‘RM+G treated’ groups in 5 L pots. After incubation, rice was cultivated for five months and harvested, with subsequent analysis of soil pH, DOC, and rice As concentrations. Results indicate that in most cases, the RM+G treatment maintained or lowered soil pH compared to untreated soils, with exceptions in 3 acidic and 3 alkaline soils. Despite RM’s alkalizing effect, G effectively countered this. Additionally, RM+G treatment reduced soil DOC levels in the majority of soils, except for 1 acidic and 2 alkaline soils. In acidic soils, the RM+G treatment significantly reduced rice As concentrations exceeding the Korean threshold (0.5 mg kg-1), from 7 instances in untreated soils to 1 instance. However, in alkaline soils, the number of instances exceeding the threshold increased from 10 in untreated soils to 15 in RM+G treated soils, indicating varied effectiveness. Overall, while RM and G combined effectively reduce plant-available As in acidic soils, their efficacy in alkaline soils shows mixed results, suggesting the need for tailored approaches based on soil characteristics.