Impact of organic carbon-Mn oxide interactions on colloid stability and contaminant metals in aquatic environments

Interactions between organic carbon and Mn oxides can lead to the formation of C-Mn colloids, which play a crucial role in regulating Mn mobility in the environment. Despite the significance of these interactions, the impact of C-Mn oxide interactions on the mobility of these colloids, particularly in the presence of contaminant metals, remains poorly understood. This study investigated the aggregation kinetics of C-Mn colloids formed through the reaction between humic acid and Mn oxides at three C:Mn molar ratios in the presence of divalent cations (Ca²⁺ and Mg²⁺). The introduction of organic carbon increased the stability (i.e., ability to resist aggregation) of C-Mn colloids compared to pure Mn(IV) colloids, as reflected in the higher critical coagulation concentration (CCC). As C:Mn molar ratios rose from 0.5 to 3 during colloid formation, the CCCs for the resulting C-Mn colloids increased from 3.6 mM to 7.2 mM Ca²⁺. However, at the highest C:Mn ratio (C:Mn=15), the CCCs decreased slightly to 7.0 mM Ca²⁺, with a similar trend observed for Mg²⁺. The stability of C-Mn colloids was affected by characteristics including electrostatic repulsion, surface functional groups, and Mn(II) content, resulting from reaction with dissolved organic carbon. Based on CCCs, C-Mn colloids were most stable in the presence of Mn²⁺ (6.9 mM), followed by Co²⁺ (5.9 mM), Zn²⁺ (2.7 mM), and Cd²⁺ (1.9 mM). The capacity of contaminant metals to destabilize C-Mn colloids followed the reverse order, with Cd²⁺ having the greatest destabilizing effect. Variations among the metals were influenced by factors such as atomic radius, hydration shell, electronegativity, and electrostatic repulsion. These results provide new insight into the aggregation behavior of C-Mn colloids and the mechanisms controlling the fate and mobility of associated contaminant metals. This knowledge has important implications for understanding contaminant transport in natural waters and optimizing water treatment processes.