THE ADSORPTION OF EUROPIUM TO COLLOIDAL IRON OXYHYDROXIDES AND QUARTZ- THE IMPACT OF PH AND AN AQUATIC FULVIC-ACID

The adsorption of europium to colloidal Fe2O3, FeOOH, Fe(OH)(3) and Si O2 was investigated as a function of pH and the presence of an aquatic fulvic acid (FA). The colloids were present at concentrations represe ntative of the upper levels of natural waters (50-500 mg/l). Measureme nts with Photon Correlation Spectroscopy showed size distribution typi cally within the ranges 200-700 nm, 40-400 nm and 50-300 nm for Fe2O3, FeOOH and SiO2, respectively, while the amorphous Fe(OH)(3) formed re latively large particles (> 1000 nm; outside the range of the method f or measurements of size distribution). Total concentrations of Eu and the fulvic acid were 10(-8) mol l(-1) and 2 mg l(-1), respectively. Th e adsorption was affected by pH, the composition of the colloidal phas e as well as the presence of fulvic acid. The uptake was almost quanti tative in the Fe-systems at pH > 5.5-6, with 50% uptake at pH of 4.5-5 .5 (4-5 units below pH(zpc)). For quartz, the maximum adsorption was a round 90% (at pH 9), with 50% adsorption at pH 7 (4 pH units above pH( zpc)). The adsorption can not be related to coulombic attraction betwe en the surfaces and the metal species alone, but rather to a specific chemical adsorption, where the pH-dependence would be related to the d istribution of Eu between different species (on the surface and in sol ution). The presence of FA slightly increased the absorption in the lo wer pH range, corresponding to an adsorption of Eu-fulvate complexes. The adsorption was reaching 100% at higher pH values in the Fe(OH)(3) system, while it was decreased in the other three systems compared to the experiments without FA. The decrease could be attributed to compet ition between inorganic Eu species and the FA-molecules about sites on the surfaces, an effect not obvious in the Fe(OH)(3)-systems due to t he much larger surface area available. The results illustrate the impo rtance of inorganic colloids (of Fe and Si) as mobile adsorbents and c arriers of trivalent elements at trace levels in aquatic systems. They also indicate that the presence of FA, at environmental concentration levels, would effect the distribution between species in solution and a suspended particulate phase in the colloidal size range, with conse quences for the transport properties of the trace element.