Characterizing Time‐Dependent Contact Angles for Sands Hydrophobized with Oleic and Stearic Acids

Abstract

Capillary barriers (CBs) represent useful, low‐cost systems for limiting water infiltration and controlling seepage at solid waste landfills in semiarid and arid regions. The application of CBs in wet regions can be problematic due to loss of water‐impermeable properties under high‐frequency precipitation. A potential solution is to alter soil grain surfaces to become water repellent by mixing or coating the soil cover material with hydrophobic agents (HAs). In this study, hydrophobic CBs comprised of sands mixed with environmentally friendly HAs (oleic acid [OA] and stearic acid [SA]) were studied. Water repellency (WR) characteristics for hydrophobized sand samples with different HA contents and representing different coating methods (mixing in and solvent aided) were measured. Initial contact angles (αi) for OA‐coated samples sharply increased with increasing HA content and reached peak values of 97 to 101° at 0.75 to 1.0 g HA kg−1 sand, whereafter αi gradually decreased. Measured αi values for SA‐coated samples increased sharply to 90° and then gradually reached a maximum of 108° at 6.0 g kg−1 HA content. Each test sample exhibited a decrease in contact angle (α) with time (t) and reached an apparent equilibrium after around 1200 s. The time dependence of α was expressed by an exponential function, α = αi exp (−At), where A is the coefficient of temporal change in WR (s−1). While the A values for the solvent‐aided OA‐coated samples were relatively constant (between 3 × 10−4 and 6 × 10−4 s−1), A values for the mixing‐in OA‐coated samples fluctuated. Generally, the solvent‐aided coating method yielded less time dependency of α and higher WR persistence.

Characterization of water repellency for hydrophobized grains with different geometries and sizes

“…First, all six different materials were washed with a lowfoaming neutral cleansing agent, and then rinsed thoroughly with distilled water several times following the procedure of Subedi et al (2012). The washed grains were air dried under ambient temperature and stored under climate controlled condition (20°C and 50 % RH) for 2 weeks.…”

“…Moreover, a landfill capping system is necessary for preventing biointrusion and controlling landfill gas emissions (Bozkurt et al 2001). Modern engineering capping systems such as & N. Senani Wijewardana senaniwijewardana@gmail.com 1 geo-membranes, geo-textiles, and geo-synthetic clay liners are technically effective (Koerner and Daniel 1997), but, they are not acceptable for waste landfills in developing countries due to economic and technical constraints (Sharma and Reddy 2004;Simon and Müller 2004;Subedi et al 2012). Therefore, earthen cover systems (Benson and Khire 1995) including evapotranspirative covers (Scanlon et al 2005) and capillary barrier cover systems (CBCSs; Tidwell et al 2003; Morris and Stormont 1998) have been studied in recent years as acceptable and low-cost techniques for waste landfills in developing countries.…”

“…Then the water retained in the fine-grained layer can be removed by evapotranspiration (Khire et al 2000) and/or diverted to drainage along the inclined interface of the layers (Ross 1990;Steenhuis et al 1991;Stormont 1995). As examined by Dell'Avanzi et al (2010) and Subedi et al (2012), a possible technique to enhance the abilities of a capillary barrier and control seepage is to make the coarse-grained materials water repellent by mixing or coating them with hydrophobic agents (HAs). Various HAs have been tested to alter the hydrophobicity of grain surfaces: silicone resins, amine acetates, fluorochemicals (Fink 1970), polyoxyalkylated diethylenetriamine (Alexandrova et al 2011), dichlorodimethylsilane (DCMS) (Liu et al 2012), polytetrafluorethylene (Dell' Avanzi et al 2010), stearic acid (Leelamanie et al 2008;Subedi et al 2012;González-Peñaloza et al 2013), and oleic acid (OA; Subedi et al 2012).…”

“…As examined by Dell'Avanzi et al (2010) and Subedi et al (2012), a possible technique to enhance the abilities of a capillary barrier and control seepage is to make the coarse-grained materials water repellent by mixing or coating them with hydrophobic agents (HAs). Various HAs have been tested to alter the hydrophobicity of grain surfaces: silicone resins, amine acetates, fluorochemicals (Fink 1970), polyoxyalkylated diethylenetriamine (Alexandrova et al 2011), dichlorodimethylsilane (DCMS) (Liu et al 2012), polytetrafluorethylene (Dell' Avanzi et al 2010), stearic acid (Leelamanie et al 2008;Subedi et al 2012;González-Peñaloza et al 2013), and oleic acid (OA; Subedi et al 2012). They reported that the hydrophobized grains were fully effective to repel the water, based on the results of tests of water repellency (WR) such as water drop penetration time (King 1981), molarity of an ethanol droplet test (Roy and McGill 2002) and sessile drop method (SDM; Bachmann et al 2000a, b).…”

“…In this study, OA (molar mass of 282.46 g mol -1 , density of 0.895 g cm -3 , Kanto Chemical Corp., Tokyo, Japan) was used as a HA to make hydrophobized materials by a mixing-in method (Subedi et al 2012). OA is a monounsaturated omega-9 fatty acid consisting of CH 3 and CH 2 groups, a carboxyl group (COOH) and a vinyl CH group (double bond CH=CH).…”