The Effect of Organic Ligands on the Sorption of Neodymium, Gadolinium and Uranium onto Nontronite and Goethite

The sorption of the rare earth elements (REE) Nd(III) and Gd(III) onto goethite in the presence of Suwannee River fulvic acid in 0.1 m NaCl solutions at 25 ºC was investigated quantitatively. The experiments involved batch titrations whereby the concentrations of REE and/or fulvic acid remaining in solution were determined as a function of pH. In the absence of fulvic acid, removal of REE from solution is enhanced in the presence of goethite over the pH range from 6 to 8, compared to the unary system (REE only) in which precipitation of an amorphous hydroxide occurred at pH greater than or equal to 8. In the absence of REE, removal of fulvic acid from solution is enhanced in the presence of goethite in the pH range from 2 to 8 at least 9, compared to a unary (fulvic acid only) system. The presence of fulvic acid at concentrations from 10 to 50 ppm enhanced REE sorption onto goethite slightly at pH less than 7, but had no discernable effect at higher pH values. Fulvic acid at a concentration of 100 ppm exhibited a greater enhancement of REE sorption at pH < 7, but inhibited REE sorption slightly at pH > 7. Experiments investigating the effect of sorption of REE onto goethite by citrate were also performed. However, these studies were not completed owing to experimental difficulties. The results obtained in this study represent an important contribution to the ultimate goal of predicting the mobility of trivalent REE (and analogous trivalent actinides) in the presence of natural organic matter and goethite