On the dissolution kinetics of humic acid particles: Effects of pH, temperature and Ca2+ concentration

The dissolution kinetics of humic acid particles has been studied in batch experiments by UV–vis spectrophotometry, and the effects of pH, temperature, stirring rate and Ca2+ concentrations are reported. The dissolution rate increases by more than two orders of magnitude by increasing the pH from 4 to 11 in 10−3 M KCl solutions. There is also an important increase in the dissolution rate by increasing temperature and stirring rate. The presence of Ca2+ in the electrolyte solution, on the contrary, strongly decreases the dissolution rate at constant pH. The results suggest that the increase in the dissolution rate by increasing pH is strongly related to the breaking of hydrogen bonds that hold HA molecules in the solid phase and to the increased electrostatic repulsion among negatively charged humic acid molecules. This electrostatic repulsion overcomes attractive intermolecular forces among individual molecules constituting HA particles and thus a progressive dissolution takes place. Calcium ions, on the contrary, seem to block negatively charged groups of the surface molecules reducing electrostatic repulsion. These ions may also act as bridges between functional groups of two adjacent molecules, increasing the attractive intermolecular forces.