Natural colloid mobilization and leaching in wettable and water repellent soil under saturated condition

Abstract

The coupled transport of pollutants that are adsorbed to colloidal particles has always been a major topic for environmental sciences due to many unfavorable effects on soils and groundwater. This laboratory column study was conducted under saturated moisture conditions to compare the hydrophobic character of the suspended and mobilized colloids in the percolates released from a wettable subsoil and a water repellent topsoil. Both soils with different organic matter content were analyzed for wettability changes before and after leaching using sessile drop contact angles as well as water and ethanol sorptivity curves, summarized as repellency index. Hydrophobicity of the effluent suspensions was assessed using the C18 adsorption method. Water repellency level of the repellent soil decreased after leaching but remained on a lower level of water repellency, while, the wettable soil remained wettable. The leached colloids from the repellent soil were predominantly hydrophilic and the percentage of the hydrophobic colloid fraction in the effluent did not systematically changed with time. Total colloid release depended on soil carbon stock but not on soil wettability. Our results suggest that due to the respective character of transported colloids a similar co-transport mechanism for pollutants may occur which does not depend explicitly on soil wettability of the releasing horizon, but could be more affected by total SOM content. Further studies with a wider range of soils are necessary to determine if the dominant hydrophilic character of leached colloids is typical. Due to the mostly hydrophilic colloid character we conclude also that changes in wettability status, i.e. of wettable subsoil horizons due to the leachate, may not necessarily occur very fast, even when the overlaying topsoil is a repellent soil horizon with a high organic matter content.

Source, Distribution, and Risk Estimation of Hazardous Elements in Farmland Soils in a Typical Alluvial–Lacustrine Transition Basin, Hunan Province

“…An increase in pH will increase the negative charge on the soil colloid surface, resulting in the precipitation of carbonates and the formation of hydroxides by bioavailable heavy metals, thereby reducing the mobility of heavy metals [ 46 ]. Organic matter in soil can form stable substances that are different from ions containing metals, which, in turn, affects the mobility and morphological composition of soil heavy metals and metalloids [ 47 ].…”

“…This implies that PC1 represents natural sources of As and Pb. PC2 showed a strong positive load on Cd, Pb, Zn, Sb, mineral element SiO2, and organic matter, indicating that the heavy metal contents were affected by soil clay and organic matter [ 47 ]. Soil organic matter is highly capable of retaining or immobilizing metals, thereby affecting their migration and distribution in the soil [ 48 ].…”

Colloid transport through soil and other porous media under transient flow conditions—A review

“…However, solution chemistry, grain size, pore flow velocity, and colloid concentration all influence the shape of the DLVO interaction energy curve, the magnitude of the energy barrier, and the depth of the primary and secondary minimum (Carstens et al, 2017; Liu et al, 2018; Rasmuson et al, 2019; Rastghalam et al, 2019; Xu et al, 2016; Yang et al, 2019) and with that the deposition efficiency of colloids from bulk solution. These parameters have also been found to control colloid detachment, the release of deposited colloids from the soil grain, which is dependent on the balance of repulsive forces between colloid and grain surfaces (Chequer, Bedrikovetsky, Carageorgos, Badalyan, & Gitis, 2019; Ma et al, 2016; Ma, Guo, Lei, et al, 2018; Mohanty, Saiers, & Ryan, 2016; Sepehrnia, Fishkis, Huwe, & Bachmann, 2018; VanNess et al, 2019). In ideal porous media and saturated conditions, colloid retention is mainly controlled by the net rate of colloid deposition and release.…”

“…The dynamic nature of nonsteady state flow can cause shifts in chemical factors (e.g., ionic strength, pH, surface charge, and chemical composition of the pore water) and physical factors (e.g., pore size distribution, shrinking, and swelling of the soil) in the unsaturated zone, which might fundamentally influence the importance and magnitude of some of the processes assumed to be primary controls on colloid transport in steady‐state flow systems (Saiers, Hornberger, Gower, & Herman, 2003; Saiers & Lenhart, 2003a, 2003b; Torkzaban, Hassanizadeh, Schijven, Bruijn, & Husman, 2006; Torkzaban, Hassanizadeh, Schijven, & van den Berg, 2006; Wang et al, 2019). Despite the fact that colloid transport in transient flow has received the least attention in colloid research to date (Baumann, 2007; Lazouskaya & Jin, 2008; Lazouskaya, Jin, & Or, 2006; Sang et al, 2013; Wan & Wilson, 1994a), transient flow experiments are often considered suitable for making inferences on the mobilization of colloids, in addition to the transport and retention mechanisms (Cheng & Saiers, 2010; Chequer et al, 2019; Ma et al, 2016; Mohanty et al, 2016; Sepehrnia et al, 2018). This is particularly important for nonpoint source pollution mitigation since transient flow more closely mimics the phenomenon of colloid transport and colloid‐facilitated transport of contaminants as it occurs in real vadose zone environments (Gao et al, 2006; McCarthy & McKay, 2004; Saiers & Lenhart, 2003a; Zhuang et al, 2007).…”