Controls of interactions between iron hydroxide colloid and water on REE fractionations in surface waters: Experimental study on pH-controlling mechanism

The influence of pH on the partitioning behavior of REE at the water/particulate interface has been studied experimentally. At the beginning of colloid formation the adsorption of REE on iron hydroxide colloids is dominant, followed by REE desorption. Finally adsorption and desorption tend to reach equilibration. The capability of iron hydroxide colloids to adsorb the HREE is greater than that to adsorb the LREE. With increasing pH, LREE/HREE fractionations will take place between iron hydroxide colloids and water, leading to the reduction of their partition coefficient ratio (DLREE/DHREE). The DREE distribution patterns show Y anomalies (DY/DHo <1), with obvious REE tetrad effects appearing under low pH conditions. Experimental results have shown that there do exist REE tetrad effects in nature. In addition to pH, the chemical type of surface water and ion intensity are also the important factors controlling REE tetrad effects and leading to fractionations between particulate-adsorbed REE and dissolved REE.