Adsorption of Sulfate and Phosphate on Andisols

This research studied the factors influencing the adsorption of phosphate and sulfate added alone or as a mixture on Andisols with different chemical, physico-chemical, and mineralogical properties and the desorption of sulfate (and phosphate) previously fixed on selected soil samples by increasing concentrations of phosphate (and sulfate). All the samples adsorbed amounts of phosphate 2–5 times greater than those of sulfate. By increasing pH, phosphate adsorption only slightly decreased, whereas sulfate retention decreased dramatically. At pH>5.5, sulfate sorption was usually very low or negligible. Allophanic materials increased the adsorption of the nutrients, whereas organic carbon and silicate prevented the adsorption of phosphate and especially of sulfate. When the anions were added as a mixture, sulfate adsorption was drastically decreased even in the presence of low amounts of phosphate (initial phosphate/sulfate molar ratio (R)≪1). A possible explanation of these findings is that phosphate adsorption on soil samples decreased their surface charge and point of zero charge (PZC), preventing the fixation of sulfate ions on more negative surfaces. Finally, pH strongly influenced the competition in adsorption between phosphate and sulfate; at pH values <4.5 sulfate competed with phosphate, probably forming inner-sphere complexes, and was able to replace some phosphate anions previously adsorbed on the surfaces of soil samples.