Rate and extent of cobalt sorption to representative aquifer minerals in the presence of a moderately strong organic ligand

There is an increasing awareness that rate-limited sorption reactions can play an important role in the transport of solutes in groundwater. The rate and extent of reactions between aqueous metals and mineral surfaces are affected by many factors, including the temperature, the presence of organic chelating agents, and adsorbent mineralogy. Cobalt sorption was investigated in terms of temperature, citrate concentration, and silica sand surface coating. The kinetic sorption data were described well by two simultaneous second-order reactions. The results suggested that decreasing temperature or the presence of citrate resulted in a slower approach to equilibrium for Co sorption to the uncoated silica sand that contained small amounts of secondary minerals. Using the same sand coated with an amorphous Fe(III) oxide, increasing temperature or the presence of citrate resulted in a faster approach to equilibrium for Co sorption. The equilibrium adsorption isotherms were described well by a generalized two-layer surface complexation model. Citrate decreased the extent of Co sorption to the uncoated silica; the effect was most pronounced at low temperature. Conversely, citrate increased the extent of Co sorption to the Fe-coated silica. These results suggest that citrate decreased the rate and extent of adsorption to the uncoated silica through the formation of a stable anionic aqueous complex that has a lower affinity for the surface than Co2+. Conversely, the higher anion sorption capacity of the Fe-coated silica increased the rate and extent of Co sorption with citrate present, presumably through the formation of an organo-metallic ternary surface complex. (C) 1998 Elsevier Science Ltd. All rights reserved.