An Experimental Study On The Mechanism Controlling Trace Metal Distribution In Surface Water During Water/Colloid Interactoins

Fractionations between LREE and HREE in water, especially between those with the same electric charge and approximately identical ionic radii, such as Y3+/Ho3+, Zr4+/Hf4+ and Nb5+/Ta5+, Ce anomalies and REE tetrad effect, as well as the causes and controlling mechanisms of different factors have become the essential objects of study in REE geochemistry of water and are also the important theoretical bases for reasonably interpreting variations in REE and trace metal composition of waters in nature. Up to now, much work has been done on interactions at the water/particulate interface, but little experimental work has been done on the capability of colloids or other particulates to adsorb the REE and other high-valent cations (e.g. Zr, Hf, Nb and Ta). In previous experiments, little attention was paid to the controlling factors (e.g. pH, aqueous medium type, inorganic and organic ligands, etc.) and their controlling mechanism. Therefore, little experimental evidence has been developed for the diversity in the distribution and compositional variation of REE and other trace metals in water.By series of batch experiments, the sorption/scavenging of di-(Cu2+), tri-(Ln3+), tetra-(Zr4+, Hf4+) and quinque-(Nb5+, Ta5+) valent ions upon hydrous ferric oxide (HFO) in different aqueous medium conditions was quantitatively investigated in this paper. The influence of interaction at the water/colloid interface affected by dissolved ligands and pH on the mechanism offractionations between light and heavy rare earth elements (REE) and other elemental pairs (e.g. Y-Ho, Zr-Hf and Nb-Ta) was mainly discussed. The speciation of rare earth elements in solution was also simulated. The results indicated that the process that HFO was produced controlled the kinetic characteristic of trace metal element sorption upon HFO, and aqueous medium conditions, including pH, electrolytes, dissolved ligands, the concentration of sorbent and sorbate and the speciation of sorbate, were the main factors that affected trace metal interaction at the water/colloid interface. The partition coefficients of REE during the process of Interaction at the water/colloid interface displayed different distribution patterns affected by different factors, but they all showed the negative Y anomalies, and the positive Ce anomalies and the REE tetrad effects under lower pH conditions, and they displayed LREE-riched, HREE-riched, HREE-riched, and LREE-riched distribution patterns respectively on the influences of CO32-, SCO42-, PO43- and humic acid, and the mechanism of REE interaction at the interface of water and HFO was combinations of ion exchange and specific sorption process. In the range of natural water pH values, zirconium and hafnium, niobium and tantalum all fractionated, but displayed relative inertia behaviors and preferably coherent behavior which exerted their behavior of geochemical twin pairs during the process of sorption or co-precipitation.The above results provided the experimental bases for reasonably interpreting variations in REE and trace metal composition of surface waters in nature.