Influence of surfactants on unsaturated water flow and solute transport

Abstract

Surfactants can reduce soil water retention by changing the surface tension of water and the contact angle between the liquid and solid phases. As a result, water flow and solute transport in unsaturated soil may be altered in the presence of surfactants. In this study, the effects of a representative nonionic surfactant, Triton X-100, on coupled water flow and nonreactive solute transport during unsaturated flow conditions were evaluated. Batch reactor experiments were conducted to measure the surfactant sorption characteristics, while unsaturated transport experiments were performed in columns packed with 40-270 mesh Ottawa sand at five initial water contents. Following the introduction of surfactant solution, the rate of water percolation through the sand increased; however, this period of rapid water drainage was followed by decreased water percolation due to the reduction in soil water content and the corresponding decrease in unsaturated hydraulic conductivity behind the surfactant front. The observed changes in water percolation occurred sequentially, and resulted in faster nonreactive solute transport than was observed in the absence of surfactant. A one-dimensional mathematical model accurately described coupled water flow, surfactant, and solute transport under most experimental conditions. Differences between model predictions and experimental data were observed in the column study performed at the lowest water content (0.115 cm 3 /cm 3 ), which was attributed to surfactant adsorption at the air-water interface. These findings demonstrate the potential influence of surfactants additives on unsaturated water flow and solute transport in soils, and demonstrate a methodology to couple these processes in a predictive modeling tool.

Surfactant-induced flow compromises determination of air-water interfacial areas by surfactant miscible-displacement

“…2B shows that as surfactant solution displaces water in the column, a rapid decrease in S W is induced that is proportional in magnitude to the influent SDBS concentration, followed by a modest rewetting. This surfactantinduced drainage is consistent with the enhanced SDBS transport through the column observed and also with both SMD experiments for A I determination that utilized hanging water columns (Brusseau et al 2015, Costanza-Robinson et al 2012 and other simulations and experiments unrelated to A I determination (Bashir et al 2011, Chen and Kibbey 2006, Karagunduz et al 2015, Smith and Gillham 1999, Zartman and Barsch 1990.…”

“…that hysteresis exerts minimal impact on surfactant R T (e.g., Fig. 3C and Table in Karagunduz et al 2015), and consequently, on A I determination. In fact, R T derived from simulations that excluded hysteresis matched experimentally measured values slightly better than those that included hysteresis.…”

“…This drainage was associated with transient effluent flowrates of up to 27% above the steady-state conditions that existed prior to the surfactant introduction and 300% variation in estimated A I , depending on how the drainage was accommodated in the data analysis. Studies utilizing surfactants for A I determination by methods other than SMD, as well as unsaturated SMD experiments unrelated to A I determination report similar surfactant-induced drainage (Bashir et al 2011, Chen and Kibbey 2006, Karagunduz et al 2015, Smith and Gillham 1999, Zartman and Barsch 1990. While use of lower surfactant concentrations reduces the magnitude of surfactant-induced flow Kibbey 2006, Zartman andBarsch 1990), even low concentrations (e.g., 0.05 mM) can induce substantial surfactant-induced flow and drainage (Costanza-Robinson et al 2012).…”

“…Considerable variation in surfactant concentration can occur over short distances (i.e., the length of a solute front), resulting in pressure head gradients that can induce flow. The potential for surfactant-induced flow to affect conventional SMD experiments, and thereby A I measurement, is recognized (Brusseau et al 2015, Karagunduz et al 2015, Kibbey and Chen 2012. Costanza-Robinson et al (2012) found that as the surfactant (SDBS) pulse displaced resident water within the column, 24-51%, depending on influent SDBS concentration, of the water drained from the column.…”

“…Brusseau et al (Brusseau et al 2007 only observed surfactant-induced drainage when using a hanging water column and not when using a vacuum chamber. They suggested that the strong vacuum control of the vacuum chamber prevents surfactant-induced drainage from occurring, even as others have observed surfactant-induced drainage when utilizing a vacuum chamber (Karagunduz et al 2015). No explanation was provided regarding why a hanging water column should offer any less experimental control, nor for why a vacuum chamber should render the system immune to the uncontested physical basis for surfactant-induced drainage.…”