Hydrochemical, mineralogical and isotopic investigation of arsenic distribution and mobilization in the Guandu wetland of Taiwan

This study explores the distribution and mobilization of As in the ecosystem of the Guandu wetlands, Taiwan. The chemical parameters, chemical sequential extraction, mineralogical compositions, and sulfur isotopic compositions (i.e., δ34S[SO4] and δ18S[SO4]) of porewater and two sediment core samples (S2 and S5, locate in the inner and outer sites of the Guandu wetland) were analyzed to characterize As spatial distribution. The crucial mechanisms of the biogeochemical processes that control As mobility in wetland ecosystems were inferred. Based on factor analysis and cluster analysis, the vertical distributions of the redox zones in S2 and S5 were classified as oxidizing, transitional, and reducing zones, respectively. The mineralogical characteristics showed that adsorption and desorption are the major processes which control As retention in the surface sediment under cyclic aerobic/anaerobic conditions. Aqueous As and Fe were restrained because of oxidation, whereas aqueous Fe precipitated as amorphous metal oxides (i.e., FeO, FeOOH, and Fe2O3). Subsequently, aqueous As was adsorbed onto the surfaces of Fe(hydr)oxides, resulting in a high solid As content in the oxidizing zone. The high aqueous As content in the boundary of the transitional and reducing zones was caused by the reductive dissolution of highly dissolved Fe compounds through the microbial respiration of organic matter (OM). In the reducing zone, As3+ can be constrained by the formation of FeS2 in sediment during bacterial sulfate reduction that is governed by the relative enrichment of the δ34S[SO4] and δ18S[SO4] values. Sulfur disproportionation and the redox of elemental sulfur (S0) are additional reaction paths that cause As cycling. Arsenic mobility in the Guandu wetland is primarily caused by the reductive dissolution of As-containing Fe-oxyhydroxides and the redox cycling of sulfate/sulfide, accompanied by the respiration of OM.