Sorption of ionisable organic compounds by field soils. Part 2: cations, bases and zwitterions

Sorption of a range of permanent cations, bases and zwitterionic compounds was measured as a function of pH in two soil types. Pyridinium cations were more strongly sorbed (K(d) 10- > 1000) than aliphatic cations (K(d) < 5). At very low pH, sorption of the aliphatic cations sharply decreased, probably because they are displaced by protons. Most weak bases, including carbendazim and pyridines, were strongly sorbed (K(d) 9-35) at low pH, where they would be appreciably protonated, sorption becoming much weaker at soil pH values > 6. However, an additional mechanism of sorption was observed for those zwitterions capable of chelation (e.g. picolinic acid and alanine) which gave rise to high K(d) values at pH values near neutral. Inorganic phosphate was strongly sorbed (K(d) > 140) save at very low pH. Glyphosate and inorganic phosphate were sorbed very strongly at pH values near to 4 (K(d) > 200). The very strong sorption was attributed to ligand exchange interaction. Sorption of picolinic acid was similar when measured in water or calcium chloride solution (0.01 M). However, sorption decreased with increasing concentration of calcium chloride up to 1 M, probably because the protonated form of picolinic acid was displaced by calcium ions.