URANIUM ADSORPTION ON FERRIHYDRITE - EFFECTS OF PHOSPHATE AND HUMIC-ACID

Uranium adsorption on ferrihydrite was studied as a function of pH in systems equilibrated with air, in the presence and absence of added ph osphate and humic acid (HA). The objective was to determine the influe nce of PO43- and HA on uranium uptake. Below pH 7, the sorption of UO2 2+ typically increases with increasing pH (the 'low pH sorption edge') , with a sharp decrease in sorption above this pH value (the 'high pH edge'). The presence of Sigma PO43- of 10(-4) mol/L moved the low pH e dge to the left by approximately 0.8 pH units. The PO43- was strongly bound by the ferrihydrite surface, and the increased uptake of U was a ttributed to the formation of ternary surface complexes involving both UO22+ and PO43-. The addition of HA (9 mg/L) increased U uptake at pH values below 7, with little effect at higher pH values. The positions of the pH edges were also affected by the ionic strength and total U content. These experiments show that sorption interactions involving P O43- and HA must be considered in order to model the behavior of U in natural systems, in which these components are often present.