Kinetics of plasticiser release and degradation in soils

Despite the increasing use of emerging phthalate and non-phthalate plasticisers as replacements for restricted phthalates, few studies have investigated their rates of entry and persistence in soils. We investigated release of the emerging plasticiser diethyl hexyl terephthalate (DEHTP) from polyvinyl chloride microplastics (PVC; 4 mm diameter; 21% DEHTP w/w) in soils in a 3-month laboratory study. DEHTP was released rapidly, with 6.6-12.1 ng DEHTP released per mg PVC within <2 h, although this was a small proportion of the amount in the pellets (<0.006%). Degradation rates of 8 phthalate plasticisers and 4 non-phthalate emerging plasticisers in the soils were measured in a separate 3-month laboratory study. For 7 of the 12 plasticisers, pseudo-first order half-lives were <30 days, suggesting relatively low persistence. 5 higher molecular weight plasticisers, including the emerging trioctyl trimellitate and DEHTP, were more persistent, with half-lives >100 days. Plasticiser half-lives in soils were significantly positively correlated with logK_OW. Degradation was typically slower in acidic heathland (pH 3.8; organic matter 3.7%), than in alkaline grassland (pH 7.3; OM 16%) or sandy loam agricultural (pH 5.3; OM 5%) soils. Rapid release and potential persistence of some emerging plasticisers in soils indicates that presence of these contaminants may increase in the future.