Phosphate adsorption at variable charge soil/water interfaces as influenced by ionic strength

The effect of phosphate adsorption on zeta potential of the colloids of variable charge soils and the effect of ionic strength on phosphate adsorption by the soils were investigated using batch experimental method. The presence of phosphate resulted in the decrease in zeta potential and isoelectric point (IEP) of the colloids of the soils, which further suggested that the phosphate was adsorbed specifically by these soils. The effect of phosphate adsorption on zeta potential was correlated with the content of free Fe/Al oxides in the soils; the higher the content of Fe/Al oxides in a soil the greater was the decrease in zeta potential and IEP of the soil colloids. The intersection of phosphate adsorption-pH curves at different ionic strengths (a characteristic pH) was obtained for 2 Oxisols. Above this pH, the adsorption of phosphate increased with increasing ionic strength, whereas below it the reverse trend occurred. The intersect pH was 4.60 for the Oxisol from Guangdong and 4.55 for the Oxisol from Yunnan, which was lower than the values of PZSE (point of zero salt effect) of these soils, but near the PZNC (point of zero net charge) of the soils. The effects of ionic strength and pH on phosphate adsorption by these soils were interpreted with the help of an adsorption model developed previously by Bowden et al. The results of zeta potential suggested that the potential in an adsorption plane became less negative with increasing ionic strength above the soil PZNC and decreased with increasing ionic strength below the soil PZNC. These results support the hypothesis of the adsorption model that the potential in the adsorption plane changed with ionic strength with an opposite trend to the surface charge of these soils. The phosphate adsorption by these soils was related not only to the ionic strength but also to the types of electrolytes present. K⁺ induced a greater increase in phosphate adsorption than Na⁺ due to the greater affinity of the soils to K⁺ than Na⁺.