Microcalorimetric studies of the effects of MgCl2 concentrations and pH on the adsorption of DNA on montmorillonite, kaolinite and goethite

This study attempted to comprehend the interaction mechanisms between DNA and the common minerals in soil such as montmorillonite, kaolinite and goethite at various environmental conditions. The effects of MgCl ₂ concentrations (0, 1, 10 and 60 mM) and pH (3.0, 5.0, 7.0 and 9.0) on the adsorption of DNA on the examined minerals were investigated by the equilibrium adsorption and direct measurement of adsorption enthalpies. The adsorption isotherms fitted by the Langmuirian model revealed that the maximum capacities and affinities of DNA adsorption on minerals increased with MgCl ₂ concentrations and decrease of pH. No DNA was adsorbed on montmorillonite at pH 9.0. The values of DNA adsorption enthalpies (Δ H _ads) ranged from −0.3 to 4.9 kJ g ^{− 1} at different systems. The Δ H _ads values decreased with increasing MgCl ₂ concentrations and decreasing pH. The adsorption of DNA on montmorillonite, kaolinite and goethite was an exothermic reaction (−0.3 < Δ H _ads < − 0.1 kJ g ^{− 1}) at 60 mM MgCl ₂ and pH 3.0, suggesting a more significant electrostatic attraction in the adsorption process. In contrast, DNA adsorption on minerals became endothermic (0.1 < Δ H _ads < 4.9 kJ g ^{− 1}) at 0–10 mM MgCl ₂ and pH 5.0–9.0 and dehydration effects were considered as the dominant driving forces for DNA adsorption on minerals. The thermodynamic parameters presented in this study have important implications for clarifying the binding mechanisms between DNA and mineral particles in soil and associated environments.