Enhanced cadmium immobilization in saturated media by gradual stabilization of goethite in the presence of humic acid with increasing pH.

Goethite (Gt) and humic acid (HA) are important components of soil that significantly affect Cd mobility. In this study, the co-transport of Cd2+ and Gt with/without HA in saturated sand columns was investigated by monitoring the breakthrough curves at different pH values. A solute transport model was used to study Cd2+ transport and retention in the saturated sand in the presence of Gt and HA, and a colloid transport model was used to describe the Gt colloid (GtC) transport in the columns. Our results showed that the transport behaviors of Cd2+ and Gt colloids/aggregates were regulated by pH. Cadmium transport was significantly inhibited at high pH due to its adsorption on the sand and Gt. Moreover, Gt retention was gradually stabilized with increasing pH regardless of its forms, i.e., individual colloids (GtC) or larger assemblages of particles due to aggregation (GtA). This retention was obviously enhanced in the presence of HA. Thus, the superposition of increased Cd2+ adsorption on Gt and Gt retention (stabilization) enhanced the immobilization of Cd2+ at high pH. In addition to stabilizing Gt, HA further enhance Cd2+ adsorption on Gt, thus promoting Cd2+ immobilization. However, only a small amount of organic-matter-bound Cd2+ was observed in the columns with injected HA. The major fractions of retained Cd2+ were exchangeable Cd2+ and Fe-oxide-bound Cd2+. Our results provide new insights into the roles of Gt and HA in the transport and mobilization of Cd2+ in soil-groundwater systems.