Adsorption kinetics of Pb and Cd by two plant growth promoting rhizobacteria

A bench study was carried out to characterize the kinetics of two plant growth promoting rhizobacteria (PGPR) Azotobacter chroococcum and Bacillus megaterium to adsorb heavy metals from solution. Adsorption of Pb2+ and Cd2+ by bacterial cells was processed quickly with an equilibration achieved within 5 min. The adsorptions were fitted well with Freundlich and Langmuir isotherm models. The comparison of isotherm parameters indicated that A. chroococcum had a stronger capacity to bind metal ions than B. megaterium, with an average increase of 59.8% for Pb2+ and 75.6% for Cd2+, respectively. Both bacteria had a stronger affinity to Pb2+ than Cd2+ since Pb2+ was more easily bound with the phosphoryl groups on the cell surface than Cd2+. This demonstrated that the presence of bacteria in the rhizosphere may result in the reduction of mobile ions in soil solution.