Utilization of fertilizers and pesticides in agriculture,industrialization and car traffic in urban areas release heavy metals into soils and environment. High concentrations of heavy metals in soils cause long-term risks to ecosystems and health humans. Among the heavy metals, Cd and Pb are of the most hazardous to the environment. Clay minerals, having great surface area and cation exchange capacity, usually show a considerable ability to sorb heavy metals. Palygorskite and sepiolite, mainly found in arid and semi-arid regions, are clay minerals with high sorption potentials. Humic substances (HS) play a major role in the fate of metals in soils and waters. Many important processes in the environment are influenced through interaction with humic substances. In most natural environments, mineral surfaces are coated to a large extent with adsorbed natural organic matter such as humic substances. In this research, sorption of humate anions by palygorskite and sepiolite was studied. Sorption of lead ions by the clays and the humate-clay complexes was also investigated. FTIR studies were carried on humate adsortion by sepiolite and palygorskite. The results showed that sepiolite had significantly higher potential for humate sorption compared to palygorskite that can be due to the higher surface area and wider structural channels of sepiolite. FTIR results showed that characteristic bands of CH 3 groups of humate were observed on the organo-mineral spectra confirming the adsorption of humate by sepiolite and palygorskite minerals. Both Langmuir and Freundlich equations satisfactorily described Cd and Pb sorption by the fibrous clays and the humate-clay complexes. Sorption isotherms appeared to be L-type reflecting a relatively high affinity of the metals to the sorbents. Maximum sorption capacity of sepiolite for Cd and Pb was higher than that of palygorskite and the Ca-clays displayed higher sorption capacity compare to humate-clay complexes for both clays that can be due to steric effects and competition between humate and heavy metals cations to sorb on the mineral surfaces and site blockage if similar sites are occupied. With increasing of metal ions sorbed, pH decrease due to inner-sphere complexation of metals on the clay surface functional groups releasing H + from the surface sites. The equilibrium pH decrease was more pronounced in case of sepiolite suggesting that inner-sphere complex had greater share to sorption of Pb by the minerals compared to Cd. Key words : Fibrous clay minerals, Isotherm,Organominerals, Heavy metals.
