Impact of geochemical iron and sulfur redox cycling on arsenic release along a salinity gradient: Sea level rise implications.

Climate change is accelerating sea-level rise (SLR), increasing the intensity and frequency of saltwater intrusion, and coastal storm flooding. Areas that are heavily contaminated with pollutants such as arsenic (As) in coastal areas may be influenced by tidal cycles. The effects of seawater intrusion and prolonged flooding will drive chemical and mineralogical changes which may threaten water quality and coastal ecosystem health. We investigated the re-mobilization of As from heavily contaminated urban sediments (13.3 g kg-1 As), over a seawater salinity gradient for 35 d combining anoxic incubation and X-ray absorption spectroscopy. We observed that As mobility is closely related to the reductive dissolution of Fe oxyhydroxides because the variation of ∼1.3 % of the total Fe dissolution was associated with ∼9 % of total As release in solution. Sulfidation boosted by salinity increased FeIII-mineral reductive dissolution, which increased As release at short term incubation (before 14 d), however at long term incubation (at 35 d), sulfidation also re-immobilized the soluble As to stable new solid phases such as Asx-Sy minerals and coprecipitation/adsorption with FeS minerals. Our results demonstrate the threat that SLR has on As release and immobilization from contaminated coastal sediments or soils due to biogeochemical redox cycling of Fe and S.