A laboratory study of the presence and transformation of dissolved Mn(III) across the sediment–water interface of an anoxic freshwater body

Dissolved Mn(III) has been verified to be an important component of Mn cycling in a natural water system. However, the existence and stability of dissolved Mn(III) and its interactions with environmental factors, such as pH, dissolved oxygen (DO), oxidation–reduction potential (ORP) and dissolved organic carbon (DOC) in freshwater body are still not clear. In this paper, the dynamic characteristics of dissolved Mn (Mn diss ), Mn(III), pH, DO, ORP and DOC across the sediment–water interface of anoxic freshwater environment (Wangjuan Reservoir of Qingdao City) were studied through laboratory experiments. The correlations between Mn diss and these key environmental factors, as well as between Mn(III) and environmental factors were quantitatively analyzed. The results indicated that concentration gradient was the main driving force of Mn diss and Mn(III) of each layer overlying water, and there were obvious differences between the concentrations of Mn diss and Mn(III) of overlying and pore water. In sediment pore water, dissolved Mn(III) concentrations were as high as 9.35 mg/L and constituted up to 23% of the total dissolved Mn pool, indicating that dissolved Mn(III) could stably exist in anoxic freshwater body and mainly came from the reduction process of Mn oxides in sediment. According to data analysis results, both the concentrations of Mn diss and Mn(III) of pore water had significant correlations with the values of pH, ORP, and DOC of sediment pore water. The correlation coefficients of Mn diss with the three factors were − 0.861, − 0.882 and − 0.934, respectively, while, the correlation coefficients of Mn(III) with them were − 0.836, − 0.671 and − 0.851, respectively.