GEOCHEMICAL DRIVERS OF MN REMOVAL IN DRINKING WATER RESERVOIRS UNDER HYPOLIMNETIC OXYGENATION

Manganese (Mn) is a drinking water contaminant with possible neurotoxic effects, which threatens drinking water quality worldwide in regions of Mn-rich bedrock. Hypolimnetic oxygenation (HOx) is a novel water treatment method deployed in lakes and reservoirs to control bottom water (hypolimnion) concentrations of metals and nutrients, including Mn. HOx systems restore dissolved oxygen to the hypolimnion, promoting oxidation of metals into insoluble forms that settle from the water column. Previous work in two Southwestern Virginia drinking water reservoirs documented large disparities in HOx system efficacy for Mn removal across sites. The contrasting geochemistry of both reservoirs suggests abiotic drivers influence Mn removal rates in lakes and reservoirs with HOx systems.

Our batch experiments simulated the effects of pH and alkalinity on Mn removal rates in oxygenated lakes and reservoirs. In the absence of light, we observed substantial Mn removal within 14 days under high pH conditions (pH 10-11) and negligible removal in solutions under pH 8. In experiments with pH 10-11 and alkalinity over 80 mg/L CaCO₃, near-total Mn removal occurred within 24 hours. Mn removal co-occurred with precipitation of visible particles (~300 μm diameter) in these experiments. We analyzed particulates formed in batch experiments using electron diffraction spectroscopy and scanning electron microscopy. Most are MnO_x with sheet-like and amorphous textures, consistent with Mn removal driven by oxidation in our experimental solutions. Our observations of a positive correlation between pH and Mn oxidation rates are consistent with Morgan (1969) and Diem and Stumm (1984). Elevated alkalinity in high pH solutions promotes Mn oxidation by maintaining initial high pH conditions through buffering and may promote Mn carbonate formation, thus catalyzing Mn oxidation. Our research elucidates impacts of individual and multiple geochemical variables on Mn oxidation and removal in freshwaters, which may inform water resource managers’ decisions on installing HOx systems for Mn removal. In addition, our results can improve understanding of Mn cycling in natural waters.