Characteristics of Humic Acid-Iron Colloid Stability and Its Mechanism of Binding with As(III)

The water-soil interface contains substantial amounts of dissolved organic matter (DOM) and iron minerals. However, the stability and aggregation behavior of DOM-Fe colloids under varying concentrations and mineral compositions remain unclear. Therefore, the binding behavior of these composite colloids with As(III) requires further investigation. This study analyzes the surface charge and particle size variations of the composite colloids to understand their stability and the binding process with As. Results indicate that the C/Fe molar ratio affects the dispersion of the three types of humic acid-iron (HA-Fe) composite colloids. As the C/Fe ratio increases, the Zeta potential of the composite colloids increases, and their particle size decreases, with a minimum size of 135, 232, and 188 nm. When the C/Fe molar ratio is below 4.7, sedimentation occurs, with the maximum sedimentation value (C/C₀) reaching 0.7. As pH increases, the Zeta potential of all three HA-Fe composite colloids increases by 6.3, 8.1, and 6.2 mV respectively, and their particle sizes decrease, with average reductions of 336, 483, and 256 nm. The binding capacity with As(III) increases as the C/Fe ratio rises from 0 to 23.3. At a C/Fe ratio of 23.3, the binding coefficients (logK_D) with As(III) are 2.42, 2.86, and 2.96 for the three composite colloids, respectively. Among them, the HA-FeO(OH) composite colloid shows the highest binding rate with arsenic at 92% when the C/Fe ratio is 23.3. The binding mechanisms include complexation, redox reactions, and cation bridge formation. The findings provide new insights into the environmental behavior of DOM-Fe at water-soil interfaces and their impact on pollutant migration.