pH effects in the adsorption of heavy metal ions by clays

The amount of hydroxonium or hydroxyl ion sorbed by the sodium form of clays (kaolin, illite or montmorillonite) has been found to increase in proportion with the amount added (i.e., data fits a Boedeker type equation). The adsorptive capacities, and bonding strengths (as evaluated from Langmuir isotherm plots) varied with clay type and nature of the adsorbing species (values for OH^? being greater than for H⁺).The presence of clay suspensions reduced the pH required for total precipitation of heavy metal ions (e.g., Cu, Pb, Cd, Zn), as hydroxy species, and this behaviour has been re-examined in terms of observed pM, pOH relationships and the effect of ligand additions.