Impact of Organic Carbon on Manganese Release, Colloid Formation, and Aggregation in Surface and Groundwater

The interaction of manganese (Mn) oxides with natural organic matter (NOM) can mobilize Mn, impacting groundwater quality. However, the formation of Mn colloids, critical determinants of Mn transport and aggregation, is often overlooked. To investigate Mn behavior and colloid formation upon C amendment, humic acid was reacted with Mn oxide suspensions at different C:Mn molar ratios (C:Mn = 0-15) over 200 h. The addition of organic carbon promoted the formation of highly stabilized and mixed-valence Mn colloids through reductive dissolution and complexation, with "aqueous" Mn (<450 nm, C:Mn = 15) release increasing by 56.3% from 0 to 200 h and colloidal Mn increasing by up to 31.9% (3-450 nm, C:Mn = 15, t = 200 h) compared to without carbon addition. Analysis of Mn oxidation state revealed that C-Mn colloids contained Mn in multiple oxidation states, and the percentage of Mn(II,III) relative to total Mn increased with increasing C concentrations. In surface water, the hydrodynamic diameter of both Mn and C-Mn colloids remained stable. In groundwater, C-Mn colloids (C:Mn = 3) remained stable (150 nm) over 30 days, while Mn colloids aggregated into larger particles. Analysis of natural surface and groundwaters identified a substantial fraction of Mn (up to 19.2 and 27.2%, respectively) existing in colloidal phases. These findings shed light on the intricate cycling of Mn among particulate, colloidal, and dissolved phases, which governs Mn fate and transport in the environment.