THE INFLUENCE OF HUMIC-ACID ON THE ADSORPTION OF EUROPIUM ONTO INORGANIC COLLOIDS AS A FUNCTION OF PH

The effect of pH (2-10) and humic acid (HA) concentration on the adsorption of HA onto a range of pure-phase inorganic colloids (delta-alumina, haematite, manganese dioxide, amorphous silica and titanium dioxide) has been studied, and the influence of HA on the colloidal zeta-potential has been investigated. The results showed a decrease in the adsorption of HA with increasing pH, due to both electrostatic effects and specific site interactions. Adsorption was influenced by colloid surface area, HA concentration and the nature of the mineral surface, with acidic surfaces exhibiting a lower affinity for adsorption than more basic surfaces. The HA was shown to confer a more negative surface charge on the colloids with the exception of silica. Natural organic matter (NOM) is ubiquitous in the aquatic environment, which would indicate that natural inorganic colloids are likely to possess NOM coatings and be predominantly negatively charged. The presence of HA was shown to strongly influence the adsorption of (152)EU (1 x 10(-9) mol 1(-1)) onto the inorganic colloids over the pH range investigated, except in the manganese dioxide system. (152)EU adsorption onto the bare colloids was shown to be highly pH dependent, adsorption increasing with pH. The mechanism appeared not to be purely electrostatic, but due to metal speciation and the nature of the surface sites, i.e. specific site binding. In the presence of HA, Eu adsorption generally showed an increase at low pH due to adsorption of EuHA complexes onto the colloid surface. At high and intermediate pH, adsorption was generally reduced owing to desorption of EuHA complexes from the colloid surface, and because inorganic Eu species were being prevented from adsorbing onto the colloid surface, a possible cause being a blocking mechanism by the HA. It can be concluded that if inorganic colloids are to be included in metal speciation and transport codes, a thorough understanding of the ternary interactions of NOM with toxic metals and inorganic colloids must be gained.