“Mn(II) Curtain” in the riparian sediment at the lower reaches of the Hanjiang River, China

Abstract

Manganese (Mn) is an abundant valent-variable element whose transformation plays an essential role in pollutant fates. This study investigated the distributions of total Mn (T-Mn) and its existing forms, such as exchangeable Mn (EX-Mn), those bound to carbonate (Carb-Mn), iron (manganese) oxides (Ox-Mn), organic matter or sulfide (Om-Mn), and residual lattice combined state Mn (Res-Mn) in three two-dimension sediment profiles of the riparian zone at the lower reaches of Hanjiang River, China, with different water flow directions caused by the dam. The mechanism for the distribution of the Mn forms was discussed through correlation analysis with other sediment indices and modelling studies using the hydrogeochemical simulation software PHREEQC. Results showed that Res-Mn dominated the whole profiles, and the Carb-Mn and Ex-Mn were accumulated and formed an “Mn(II) curtain” at the interface of groundwater and river water, most likely derived from reducing Ox-Mn and Om-Mn at low Eh of this area. The correlation analysis results demonstrated that T-Mn positively correlated with Fe, Ca, Mg, and P in the sediments, inferring the deposition of the primary geological environment or coprecipitation in the riparian zone. Both EX-Mn and Carb-Mn were positively correlated with the average depth of the sampling site and the moisture content of sediments, indicating that the reductive environment below the water level was more favourable for the accumulation of EX-Mn and Carb-Mn. Ox-Mn tended to accumulate above the groundwater level, and Om-Mn were mainly enriched in the subsurface of farmland. The modelling results suggested that the formation of “Mn(II) curtain” was related to the release of CO2 pressure, high HCO3– concentration from the regional groundwater or the aerobic decomposition of TOC from river water, and the low Eh caused by the depth or exhaustion of the DO from river water. This study helps us understand the hydrogeochemical process and environmental fate of Mn and related pollutants in the riparian zone.