Bacteria are the most abundant microorganisms in the soil. The roles of these microorganisms in soil fertility, nutrient cycling and formation and stability of soil structure are well known. Pseudomonas putida is a unique soil bacterium regarding its various functions in the soil. About 80-90% of the soil bacterial cells can be adsorbed on the surface of the soil colloids. Clay minerals with large specific surface area and high cation exchange capacity act as strong adsorbents and preferred habitat for bacteria. The interactions between bacteria and clay minerals can significantly alter the activity and ecological distribution of the bacteria, as well as the biomineralization, mineral weathering and biodegradation of pollutants in the soil. Heavy metals contamination due to natural and anthropogenic sources is a global environmental concern. Heavy metals, like lead, can be adsorbed by the surfaces of both clay minerals and bacteria. High concentration of lead, as one of the most toxic heavy metals in the environment, can cause severe toxic effects to exposed plants, animals and humans. This study examined the interactions between P. putida with palygorskite and sepiolite clay minerals, as the clays frequently found in arid and semiarid region soils. Also, the abilities of the P. putida and its composites with the clays to accumulate lead were investigated . Result of the equilibrium studies showed that Freundlich and Langmuir models were good models describing P. putida sorption on palygorskite and sepiolite. A higher affinity as well as a larger amount of adsorption of P. putida was found on sepiolite as compared to the palygorskite. Pseudomonas putida showed a considerable capacity to sorb lead ions. Sorption of lead decreased with the increase of bacteria in composite bacteria-clay. Composites of bacteria-sepiolite were more effective than bacteria-palygorskite in lead absorption.
