The influence of surfactants on the adsorption of heavy metal ions by clays

The effect of a range of commercially available, water soluble surfactants on the uptake of Cu, Zn, Cd or Pb ions by three types of clay (kaolinite, illite and a montmorillonite) over the pH range 3–10 has been investigated. Charged surfactants influenced the phase distribution process when present at low concentrations (<0.005% w/v) but much higher levels of non-ionic species (> 0.1% w/v) were required to produce observable effects. The cationic surfactants reduced metal ion uptake by montmorillonite suspensions by competing for surface sites, but with illite and kaolinite the added organic cations had much smaller effects on the equilibrium position. The addition of anionic surfactants led to increased metal loss from solution, particularly in the presence of an alkyl benzene sulphonate. It is considered that the controlling process in this case was precipitation of sparingly soluble metal-surfactant species. The effect of the non-ionic organics (all ethoxylates) varied with the type of clay involved and the metal ion studied. The concentrations used were of the order required for micelle formation, and it is considered that the micelles became involved in the metal ion distribution process. With montmorillonite suspensions, the presence of surfactant reduced the amount of metal ion sorbed by the solid, but with kaolinite and illite evidence of a reduction in adsorption site availability was apparent in only a few of the systems examined. Of the remainder, many had no effect but twice this number caused some enhanced loss of metal ion from solution. Few of the added surface active agents significantly influenced the pH at which the metal ions precipitated as hydroxy species. The implication of the results on metal ion levels in natural waters has been considered.