Surface charge properties of variable charge soils influenced by environmental factors

The variable charge soils that have been developed in tropical and subtropical zones are unique soils compared with the constant charge soils that have been developed in Europe and North America, especially the sensitivity of the particle surface charges to environmental factors in the variable charge soils. In this study, to avoid the interference of side reactions, experiments were performed using a fast potentiometric acid-base titration under non-equilibrium condition, and simulation by the surface complexation model to investigate the effects of environmental factors, including the impact of CO2 (in nitrogen/in air), the impact of different ionic strengths, the impact of dry-wet rotation (different soil-to-liquid (S/L) ratios), and the impact of seasonal temperature variation (different reaction temperatures), on the surface acidity constants and on the protonation/deprotonation processes in a typical variable charge soil collected in southern China. The titration results demonstrate that the observed measured apparent pH when surface charge equal to zero (pHQ=0) were significantly negative shifted in air, and negative correlations were proven between the S/L ratios, the reaction temperatures and the observed pHQ=0 values. The simulation results from the fast titration prove that changing the S/L ratios modifies the surface species concentration (≡SOH, ≡SOH2+, and ≡SO−), further influencing the modification of the surface charge properties of the soil. Meanwhile, an increase in temperature not only increases the amount of the surface species concentration but also causes an enlargement of the protonation/deprotonation processing range, which may increase the acid-base buffer capacity of soils. Taken together, these results can help us to further understand the natural soil acidification processes occurring in variable charge soils, especially in the southern areas of China.