Effect Of pH, Phosphate and/or Malate on Sulfate Sorption on Andisols

The sulfate sorption was studied on Andisols with variable mineralogical composition and low organic matter content in presence and absence of phosphate or malate ligands at different pH. Crystalline clay mineralogy was similar for all the studied samples, except for the 2C horizon of the pedon 2, which did not show crystalline minerals. The soil samples showed content of allophane ranging from 16% to 42%. The sulfate adsorption decreased when descending the first 3 horizons (A1, 2A2 and 3Bw1) of the pedon 1. This behavior was attributed to the decreasing organic carbon content. At different pH sulfate sorption was drastically decreased by increasing the pH from 4.0 to 7.0, precisely by 80% for sample 5 to 100% for sample 1. Experiments on the competitive sorption of phosphate and sulfate on volcanic soils were carried out. Sulfate sorption was reduced even in the presence of low concentrations of phosphate ions (phosphate/sulfate molar ratio << 1). Sulfate sorption was influenced by many factors as the organic carbon content, pH, the mineralogical composition of soil samples and the order of sulfate and phosphate addition into the soils. Sulfate competes with phosphate particularly when added before phosphate and at low pH values (pH < 4.5). Phosphate more than malate affected the kinetics of sulfate sorption onto a volcanic soil containing a large amount of allophanic materials