THERMODYNAMIC ASPECTS OF NICKEL HUMIC-ACID INTERACTIONS

NiHA stability constants (beta values) have been derived at different temperatures from the amounts of nickel (Ni2+) and nickel humate (NiHA ), determined to be present, in equilibrated nickel humic acid mixture s at 4 degrees C, 20 degrees C, 40 degrees C, 60 degrees C and 80 degr ees C. The constants were deduced using (i) a simple Scatchard plot, ( ii) a metal-humic acid charge neutralisation model and (iii) an electr ostatic/discrete site complexation model as implemented in the CHESS g eochemical speciation code. The derived values depended on whether bul k solution (Scatchard and Charge Neutralisation Approaches) or humic s urface metal concentrations (Electrostatic Approach) were employed in the calculations. The Scatchard and Charge Neutralisation approaches w ere found to be mathematically equivalent. The differently derived con stants and their temperature dependencies have been used to calculate corresponding thermodynamic Delta H, Delta G and Delta S values. The v alues obtained are compared and features of thermodynamic significance discussed.