Abstract
Soil colloids significantly facilitate the transport of contaminants; however, little is known about the effects of highly reactive iron oxide and the most representative organic matter on the transport of soil colloids with different physicochemical properties. This study investigated the effects of goethite (GT) and humic acid (HA) on the sedimentation and transport of soil colloids using settling and column experiments. The stability of soil colloids was found to be related to their properties and decreased in the following order: black soil colloids (BSc) > yellow soil colloids (YSc) > fluvo-aquic soil colloids (FSc). Organic matter increased the stability of BSc, and ionic strength (Ca2+) promoted the deposition of FSc. Colloids in individual and GT colloids (GTc) coexistence systems tended to stabilize at high pH and showed a pH-dependence whereby the stability decreased with decreasing pH. The interaction of GTc and kaolinite led to a dramatic sedimentation of YSc at pH 4.0. HA enhanced the stability of soil colloids, especially at pH 4.0, and obscured the pH-dependent sedimentation of soil colloids. The transport ability of soil colloids was the same as their stability. The addition of GT retarded the transport of soil colloids, which was quite obvious at pH 7.0. This retardation effect was attributed to the transformation of the surface charge of sand from negative to positive, which increased the electrical double-layer attraction. Although sand coated with GT–HA provided more favorable conditions for the transport of soil colloids in comparison to pure sand, the corresponding transport was relatively slow. This suggests that the filtration effect, heterogeneity, and increased surface roughness may still influence the transport of soil colloids.
Highlights
Heterogeneous natural environmental systems are characterized by an abundance of natural colloids and nanoparticles
Provided more favorable conditions for the transport of soil colloids in comparison to pure sand, the corresponding transport was relatively slow. This suggests that the filtration effect, heterogeneity, and increased surface roughness may still influence the transport of soil colloids
All of the soils used in this experiment were silty loam, with the silt fraction (79.0–88.9%) dominating the particle size distribution, which was followed by sand (10.8–16.4%) and clay (0.17–0.38%)
Summary
Heterogeneous natural environmental systems are characterized by an abundance of natural colloids and nanoparticles. Colloidal particles are usually smaller than ~ 1 μm in at least one dimension and have high specific surface areas (10–800 m2 g−1 ) [1]. These factors determine the fate and transport of pollutants to a great extent and contribute to environmental pollution in adjacent surface waterbodies and groundwater. Soil type determines the physicochemical properties of soil colloids; differences in the chemical and mineralogical properties of soil colloids influence their stability [8], pollutant load [9], and mobility [10], which further affects their ability to transport pollutants.
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