Sedimentation kinetics and stability mechanisms of iron and manganese colloids in simulated groundwater

Abstract

The occurrence of different forms of iron and manganese colloids in the subsurface environment has been widely reported. However, their sedimentation kinetics and stability mechanisms have been scarcely addressed. In this study, the effect of pH, electrolytes, and fulvic acid on the 24 h-scale sedimentation kinetics of iron and manganese colloids was investigated, and their mechanisms were described by the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) and extended DLVO (XDLVO) theories. The results indicated that the colloidal sedimentation behavior was not only affected by their structures, but also by their hydrochemical environments. The sedimentation rate of these colloids increased with increasing pH. The electric double layer of colloids was compressed at a higher electrolyte concentration and valence state, further inducing colloidal sedimentation. However, fulvic acid could retard colloidal sedimentation as a result of electrostatic repulsion and a steric hindrance effect. The DLVO and XDLVO theory results suggest that both the total potential energy and the energy barrier decreased with increasing electrolyte concentration or valence state, whereas they increased with increasing fulvic acid concentration. Moreover, the repulsive potential was dominant for iron colloid sedimentation in Na⁺ and Ca²⁺, whereas the sedimentation of manganese colloids was mainly controlled by the repulsive potential in Na⁺ and the attractive potential in Ca²⁺. These findings provide a basic reference for understanding the fate and transport of iron and manganese colloids and associated contaminants in various subsurface environments.