The study of the strontium and calcium binding by humic acid

The solutions of humic acid and its salts with metals have colloidal character. The complexation properties of humic acid in different conditions (pH, concentration, ionic strength,...) were estimated by ultrafiltration technique with polysulphonate membrane (10000 Da) under 400 kPa pressure. The binding of strontium by humic acid was studied by using of the solutions of strontium and humic acid in wide concentration range of phone salts and pH values as a model solutions of soil leaching. It has been found, that recovery of humic acid was slowly decreased with increasing pH-value what agrees to the full repulsion state of humic acid in solutions with neutral and basic pH-value. The polyelectrolyte nature of humic acid leads to the strontium binding which increases in the strength (strontium recovery (60% at pH =7) with the increasing degree of ionization of the carboxylate groups. The strontium complexation in the presence of calcium was studied in different series of metal concentrations at pH 5. The explanation of interaction of a humic acid with strontium at presence of calcium is based on the Kim-model of metal charge neutralization upon complexation to humic acid functional groups. The proton exchange capacity of acid depends on the origin of humic acid and can be determined by pH titration of humic acid. Values of free concentrations of strontium and calcium was obtained from speciation ultrafiltration experiment with parallel samples as metal concentrations in filtrate. The ultrafiltration recovery of humic acid at pH 5 was quantitative and that means all formed humic acid complexes (Sr or Ca) were separated from bulk solution and seized on the membrane filter. Metal concentrations values in filtrate were corrected with respect to permeation factor for strontium and calcium. (authors)