Stability and Thermodynamics Characteristics of Humic Acid -Nickel Complexes

The effects of pH, ionic strength and temperature on the stability constants of humic acid-nickel were studied using cation-exchange equilibrium method, in order to detect the stability characteristics of humic acid-nickel complexes under different pH, ionic strength and temperature conditions, which will be great significant to the study on the environmental behavior of nickel ions. The results showed that the values of logK increased successively as pH increased from 3.5 to 7.0, since when the amount of H+ increased in the system, the weak acidic reactant spots of the humic acid were saturated, so the reaction between humic acid and nickel ions were restrained. With the decrease of the amount of H+ in the system, the humic acid changed from reunite state at lower pH into extend state at higher pH, so the reaction between the inner reactant spots of humic acid with the nickel ions became more easily, and the humic acid-nickel complexes more steady. The impact of ionic strength on logK was complicated, when it increased from 0.00 to 0.05 mol·L-1, logK decreased from 2.733 to 2.463, while further increase of ionic strength from 0.05 to 0.10 mol·L-1 resulted in an increase of logK from 2.463 to 3.897. The interaction between humic acid and nickel ions maybe involved a two-phase system and was the result of colligation reaction between nickel ions and the inside and outside reaction spot of humic acid. The increase of temperature from 298～308 K caused an decrease of logK from 3.897 to 2.574, showing that higher temperature was unfavorable to the reaction between humic acid and nickel ions. The thermodynamic function variations in complexation reaction were all negative, △rGmθ 0、△rHmθ 0、△rSmθ0, indicating a spontaneous reaction. The coordination numbers x, were all lower than 1, suggesting that the complexation of humic acid with nickel involved several coordination groups.