Soil Sorption of Nickel in presence of Citrate or Arginine

The influence from 0.5 and 5 mmol L^-1 citrate andarginine on nickel sorption in sandy loamy topsoil ofa Mollic Hapludalf has been investigated in order toevaluate the effect of organic complexing agents onnickel retention in soils. Both ligands, which formstrong complexes with nickel, represent common soilsolutes but also serve as models for oxygen andnitrogen donor ligands in fulvic acids and otherorganic soil solutes. In the pH range 4.5–8,sorption edges showed a dramatic decrease in nickelsorption in presence of citrate, but not in thepresence of arginine. These differences were furtherquantified at pH 6.0 through sorption isothermscovering initial nickel concentrations in the range3.4–42.6 μmol L^-1. Again, the amounts ofnickel sorbed without and in the presence of 0.5 and5 mmol L^-1 arginine were not significantlydifferent, whereas 0.5 and 5 mmol L^-1 citratereduced sorption by 50 to 90%. The ligand effectscould be expressed through variation of the Freundlichaffinity constant K. The results indicate thattrivalent positively charged nickel-arginine complexes(NiH₃arg₂ ³⁺) are sorbed to cationexchange sites, while in the presence of citrate sitesfor the bonding of monovalent negatively chargednickel-citrate complexes (Nicit^-) are sparse.