RATE AND EXTENT OF COBALT SORPTION TO REPRESENTATIVE AQUIFER MINERALSIN 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 s urfaces are affected by many factors, including the temperature, the p resence of organic chelating agents, and adsorbent mineralogy. Cobalt sorption was investigated in terms of temperature, citrate concentrati on, and silica sand surface coating. The kinetic sorption data were de scribed well by two simultaneous second-order reactions. The results s uggested that decreasing temperature or the presence of citrate result ed 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 temp erature or the presence of citrate resulted in a faster approach to eq uilibrium for Co sorption. The equilibrium adsorption isotherms were d escribed well by a generalized two-layer surface complexation model. C itrate decreased the extent of Co sorption to the uncoated silica; the effect was most pronounced at low temperature. Conversely, citrate in creased the extent of Co sorption to the Fe-coated silica. These resul ts 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+. Converse ly, the higher anion sorption capacity of the Fe-coated silica increas ed 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.