Thermodynamic quantities of surface formation of aqueous electrolyte solutions X. Aqueous solution of 2:1 valence-type salts

How does calcium and sodium in saline water affect the potential of different rates and size of sugarcane biochar to soil water repellency remediation?

Soil water repellency (SWR) is a growing problem in arid and semiarid regions with non-saline water limitations. The objective of this study was to investigate the potential of different rates and sizes of sugarcane biochar on reducing soil water hydrophobicity when using saline and non-saline water. Eleven sugarcane biochar application rates from 0 to 10% with two different sizes including <0.25 and 0.25-1 mm were studied. The experiment was conducted in two strongly and extremely water-repellent soil. In addition, to investigate the effect of electrolyte concentration on biochar potential for SWR reduction, calcium chloride and sodium chloride electrolyte solutions with 5 concentrations including 0, 0.15, 0.3, 0.45 and 0.6 mol L-1 were considered. The results showed that both sizes of biochar reduced the soil water repellency. While in strongly repellent soil the 4% biochar was enough to change the soil from strongly repellent to hydrophilic, in extremely water-repellent soil, the use of 8% of fine biochar and 6% of coarse biochar changed the extremely water-repellent soil to slightly hydrophobic and strongly hydrophobic respectively. Increasing the electrolyte concentration caused the expansion of soil hydrophobicity and reduced the positive effect of biochar to water repellency management. Increasing the electrolyte concentration in sodium chloride solution has a greater effect on increasing hydrophobicity than in calcium chloride solution. In conclusion, biochar could suggest as a soil-wetting agent in these two hydrophobic soils. However, the salinity of water and the its dominant ion could increase the amount of biochar for soil repellency reduction.

Surface tension and surface Δχ-potential of concentrated Z+:Z- electrolyte solutions

Schmutzer's model for the surface of aqueous electrolyte solutions is generalized to Z+:Z- salts. The thickness of the ion-free layer is calculated from the thickness of the "hydrophobic gap" at the water surface (1.38Å) and the radii of the ionic hydration shells. The overlap between the adsorption and the diffuse double layers is accounted for. The proposed model predicts the dependence of the surface tension σ and the surface Δχ-potential on the electrolyte concentration c(el) in agreement with the available data, without adjustable parameters. The Hofmeister effect on σ for salts of the same valence type is explained with their ion-specific activity coefficients. The negative value (toward air) of the Δχ-potential of most 1:1 electrolytes originates from the dipole moment of the water molecules at the surface. The negative χ-potential due to water dipoles is inversely proportional to the dielectric permittivity ε of the solution. Since ε diminishes as c(el) increases, most 1:1 electrolyte solutions exhibit a more negative χ-potential than pure water (Δχ<0). The Hofmeister series of Δχ of 1:1 salts (Δχ(LiCl) ≈ Δχ(NaCl)<Δχ(KCl)<Δχ(KF)) follows the corresponding series of ε (ε(LiCl) ≈ ε(NaCl)<ε(KCl)<ε(KF)). The theory allows the estimation of the surface potential χ0 of pure water from the experimental data for electrolyte solutions; the result, χ0 ≈ -100 mV, confirms the value currently accepted in the literature.