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.

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BackgroundFluoride contamination of groundwater, both anthropogenic and natural, is a major problem worldwide and hence its removal attracted much attention to have clean aquatic systems. In the present work, removal of fluoride ions from drinking water tested using synthesized γ-Fe2O3 nanoparticles.MethodsNanoparticles were synthesized in co-precipitation method. The prepared particles were first characterized by X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). Density functional theory (DFT) calculations on molecular cluster were used to model infrared (IR) vibrational frequencies and inter atomic distances.ResultsThe average size of the particles was around 5 nm initially and showed a aggregation upon exposure to the atmosphere for several hours giving average particle size of around 5–20 nm. Batch adsorption studies were performed for the adsorption of fluoride and the results revealed that γ-Fe2O3 nanoparticles posses high efficiency towards adsorption. A rapid adsorption occurred during the initial 15 min by removing about 95 ± 3 % and reached equilibrium thereafter. Fluoride adsorption was found to be dependent on the aqueous phase pH and the uptake was observed to be greater at lower pH. Fourier transform infrared spectroscopy (FT-IR) was used for the identification of functional groups responsible for the adsorption and revealed that the direct interaction between fluoride and the γ-Fe2O3 particles.ConclusionsThe mechanism for fluoride removal was explained using the dehydoxylation pathway of the hydroxyl groups by the incoming fluoride ion. FT-IR data and other results from the ionic strength dependence strongly indicated that formation of inner-spherically bonded complexes. Molecular clusters were found to be good agreement with experimental observations. These results show direct chemical interaction with fluoride ions.