Enhanced role of humic acid on the transport of iron oxide colloids in saturated porous media under various solution chemistry conditions

Abstract

Humic acid (HA), a naturally occurring and widely distributed natural organic matter in the environment, is one of the most important factors controlling the transport and fate of mineral colloids. This study examined the influence of HA (0–10 mg/L) on the transport of goethite and hematite colloids in saturated quartz sand-packed columns under various solution chemistry conditions (including ionic strength and pH). We found that the transport-enhancement effect of HA on goethite colloids was greater than that on hematite colloids because the steric hindrance and electrostatic repulsion between HA-bound goethite colloids and sand stronger than those between HA-bound hematite colloids and sand. Besides, Ca2+ could be a bridging agent between HA and sand; and consequently, enhanced the steric hindrance between iron oxide colloids and sand, which was an important mechanism for the transport-enhancement effect of HA in the presence of Ca2+. Interestingly, with increasing HA concentration from 3 to 10 mg/L, the transport-enhancement effect of HA at higher ionic strength (i.e., 20 mM NaCl) was to a larger extent than that at lower ionic strength (i.e., 5 mM NaCl); meanwhile, the enhancement effect of HA at lower pH (i.e., 5.0) was to a larger extent than that at higher pH (i.e., 9.0). The observations were attributable to the surface charge and aggregation properties of HA-bound iron oxide colloids affected by solution chemistry coupled with HA concentration. Furthermore, DLVO theory extended by steric repulsion (XDLVO) could explain the observed trend of colloid transport under the test conditions.