Effects of organic matter and ageing on the bioaccessibility of arsenic

► Adding organic matter to contaminated soils may increase arsenic bioaccessibility. ► Ageing soils with >25% organic matter can lead to increased arsenic bioaccessibility. ► No changes in arsenic bioaccessibility for soils poor in organic matter (mean 7.7%). ► No changes in arsenic bioaccessibility for samples containing arsenopyrite. ► Organic matter in soil may favour oxidation of trivalent arsenic to pentavalent form.