Redox Dynamics of Arsenic Species in the Root-Near Environment of Juncus effusus Investigated in a Macro-Gradient-Free Rooted Gravel Bed Reactor

In the framework of investigating the dynamics of As species within the planted soil beds of treatment wetlands, the redox dynamics of As species particularly in the root‐near environment of the rhizosphere were investigated. For this purpose, long‐term experiments were carried out using a specially designed macro‐gradient‐free rooted gravel bed reactor, planted with Juncus effusus to treat an artificial wastewater containing As (200 μg As/L). The exceptional quality of the biofilm processes at the helophyte root‐surfaces in treatment wetlands were of special importance in this investigation. The results showed that under C‐deficient conditions, a highly efficient As immobilization (> 85 %), obviously due to adsorption and/or co‐precipitation, was attained. The addition of organic carbon immediately caused an elevated As concentration and enrichment of As(III) (nearly 80 % of total As) in the reactor. Increasing the SO42– concentration in the artificial wastewater inflow facilitated a high As immobilization (> 82 %) under sulfate reducing condition. In principle, a highly efficient microbial dissimilatory sulfate reduction contributed to S2– formation and a greater As immobilization (most likely as As2S3) under C surplus and reducing conditions. Significant differences in As immobilization were observed by varying the inflow of the SO42– concentration (0.2, 5, 10, 25 S/L) under C surplus conditions. More As(III) precipitates (15 % less in the outflow) when the inflow of the SO42– concentration was decreased from 25 mg S/L to 10 mg S/L. Immobilized As showed greater instability by releasing As(V) (up to 85 % of total As) due to changes in the dynamic redox conditions inside the reactor. Re‐oxidation of reduced sulfur into other S species (e.g. S0, SO42–) due to plant‐root mediated O2 release probably caused an oxidative dissolution of already precipitated insoluble As (e.g. As2S3) and as a consequent As remobilization. The findings of this study highlighted the significance of SO42– in relation to organic C supply in planted soil beds treating As‐contaminated wastewater under constructed wetland conditions. To characterize the redox dynamics of As species and the transformation processes particularly in the root‐near environment of the helophytes in treatment wetlands were the prime focus of this study. A specially designed macro‐gradient‐free rooted gravel bed reactor, planted with Juncus effusus, was used to treat an artificial wastewater containing As.