Arsenic contamination and rare earth element composition of acid mine drainage impacted soils from South Africa

Acid mine drainage (AMD) impacted soils occur as soils near weathered pyritic mine tailings, directly receiving AMD or in proximity to AMD polluted waters. They pose environmental and human health risks and present challenges to restoration strategies for better land use management. This study investigated the chemical composition and geochemical evidence of AMD impact in four top soils from gold and coal mining regions near Johannesburg, South Africa with a focus on characterising metal contamination and rare earth element (REE) content. Soil physico-chemical properties (pH, electrical conductivity, texture, cation exchange capacity, carbon content, and sulfate) were measured and analysed and pseudo-total elemental contents were determined from a microwave digestion analysed by ICP-MS. The results showed evidence of soil acidification and exceedingly high sulfate (>7000 mg kg−1) and calcium (>200 g kg−1) contents. High arsenic contents (>300 mg kg−1) were found for some of the samples, exceeding the national soil screening value of 23 mg kg−1. Total REE content ranged from 51.7 to 110 mg kg−1 and favoured the light over heavy rare earth elements. The results suggest that there are complex dynamics between AMD and soil element fractions. Arsenic was unexpected as it had not previously been well documented in AMD polluted groundwater, surface waters, or mine seepage from the region, and REEs, with which AMD is often enriched, were not retained in the topsoil samples. These novel findings indicate major challenges to post-mining land rehabilitation.