Interactions of Cd and Pb with Humate-Palygorskite and Humate-Sepiolite Complexes

Various geochemical studies have yielded conflicting data on whether humic coatings decrease or increase adsorption of heavy metals by soil minerals. The objective of the present study is to determine how humate pre-adsorption affects subsequent retention of Cd and Pb by palygorskite and sepiolite, as special silicate clay minerals of soil in many arid regions. For this purpose, a series of equilibrium batch experiments were conducted on the interactions of Pb and Cd with Ca-palygorskite and Ca-sepiolite before and after humate adsorption. The results showed that the Langmuir (L), Freundlich (F), Langmuir-Freundlich (LF), and Toth (T) equations satisfactorily described metal sorption data on the minerals. In the presence of humate as the pre-adsorbate, the values for sorption capacities of palygorskite and sepiolite for Cd and Pb slightly decreased. This can be attributed to the competition between humates and metal ions for mineral active sites and steric hindrance of the adsorbed humates, which reduces the access of metal ions to the mineral surface and internal channels. Humate coatings decreased the adsorption equilibrium constants of Cd, suggesting that the affinity of the organo-clays for Cd sorption is lower than those of Ca-clays. The values for the heterogeneity factor (beta) generally showed an increasing trend with increasing humate coverage on palygorskite and sepiolite, which can be explained by the increased diversity of adsorption centers on humate-clay complexes. It may be concluded that the presence of humate bound on fibrous clay surfaces can influence the sorption, and hence the bioavailability and mobility of heavy metals in fibrous clay-containing arid and semiarid soils.