THE FORMATION OF HUMIC COATINGS ON MINERAL PARTICLES UNDER SIMULATED ESTUARINE CONDITIONS - A MECHANISTIC STUDY

The adsorption of three humic substances isolated from the River Dodde r (Eire) and one commercial humic acid (Aldrich) by six minerals was s tudied and found to be influenced by mineral type, the nature of the h umics and by the solution characteristics. Adsorption was heavily pH-d ependent, with lower uptake at higher pH values due to increased ioniz ation of the humic substances. The amount of humic substances adsorbed was in the order: Aldrich humic acid > Dodder humic acid > Dodder ful vic acid > Dodder hydrophilic macromolecular acid corresponding to a d ecreasing aromatic carbon content and increasing O/C ratio. Desorption experiments showed Aldrich humic acid was the most difficult to be de sorbed, followed by Dodder humic acid, Dodder fulvic acid then Dodder hydrophilic macromolecular acid. Humic uptake was found to increase wi th ionic strength due to the neutralization of charges on both mineral surfaces and humic substances, and more importantly, the compression of diffuse double layers; this also showed that van der Waals attracti on is an important part of the interactive forces between humic substa nces and mineral particles. The amount of humic substances adsorbed va ried with mineral type, particle size and surface area, with aluminium oxide having the highest capacity. The effect of temperature on humic adsorption was shown to vary with humic type and two types of reactio n mechanism have been proposed responsible for the humic-clay interact ions.