Adsorption of uranium on clay and the effect of humic substances

Adsorption of hexavalent uranium (4.2x10(-5) mol/l) from aqueous solutions on "cypris" clay was studied as a function of pH (1-12), concentration of coal humate (0, 0.3 and 3 g/l) and clay concentration (5 and 50 g/l). A batch method was used for the experiments. Uranium remaining in the solution after the adsorption was determined by spectrophotometry of its arsenazo-III complex. Simultaneously, the distribution of coal humate between the solid and liquid phases was measured by UV spectrophotometry. Both the adsorption of uranium and the distribution of coal humate were strongly pH-dependent. The adsorption of uranium passed through two maxima whose form and position on the pH-scale depended on the concentration of clay and on the humate to clay ratio. The precipitation/adsorption of humate on clay was high at low pH values and decreased with increasing pH. The slope of the decrease and its position on the pH-scale depended on the humate to clay ratio. Presence of humate enhanced the adsorption of uranium at pH < 4-7.5 (depending on the concentrations of clay and humate) and suppressed it at pH > 4-8. The results were compared with literature data and qualitatively interpreted. Conclusions were drawn on probable mechanisms of uranium adsorption in the absence and presence of humate, and on possible applications of cypris clay as barrier or filling material and of the clay-humate system for wastewater treatment.