Redox processes of sulfur and manganese in a constructed wetland

Constructed wetlands are very popular in terms of wastewater treatment today. Formation of redox potential gradients inside such a system strongly influences the wastewater-treatment efficiency. Individual oxidation forms of sulfur, and dissolved and precipitated manganese forms were determined in the vegetation bed of the constructed wetland. The aim of the speciation analysis was to contribute to the characterization of its redox properties. Sulfur was mostly oxidized at the inflow. The concentration of sulfates at the inflow varied from 25 to 55 mg/l, while concentration of sulfides was always lower than 6.0 mg/l and mostly even lower than 1.0 mg/l. However, sulfates were reduced during the pretreatment and the wastewater flow through the vegetation bed. The concentration of total manganese varied from 0.2 to 0.8 mg/l. Approximately 60% of Mn was precipitated at the inflow. The content of precipitated Mn forms declined to ca. 40-50% at the inflow zone. this content was almost constant across the vegetation bed to the outflow when water was sampled from 60-cm depth. However, the content of precipitated Mn forms increased to ca. 74% for samples from 20-cm depth. With respect to the aeration of the system, manganese can be precipitated as MnO(2) in these samples.