A decade of groundbreaking research into the bio-sorption properties of extracellular polymers (EPS) from sludge and purely cultured bacteria has revealed that the composition and structure of these heterogeneous macromolecules favor sequestration of heavy metals. Comprised chiefly of proteins, polysaccharides, lipids, and nucleic acids, EPS has been used extensively in heavy metal remediation schemes. The use of EPS in heavy metal remediation is a cost-effective and environmentally friendly approach. Rhizobacteria such as Pseudomonas and Bacillus have been identified as plant growth promoting bacteria (PGPB) with the ability to enhance nutrient acquisition and reduce the adverse effects of plant toxins (including heavy metals). The growth of these bacteria is associated with secretion of EPS which control their mobility in the soil as well as influence other soil chemical processes. Therefore, our research was designed to investigate the changes in the surface charge properties of variable charge soils after interaction with EPS, and how this interaction affected heavy metal mobility and nutrient retention.
The ability of EPS to influence the surface charge properties of variable charge soils is dependent on its concentration and solution pH, with the effect being small at lower pH. Interaction of the soil colloids with EPS resulted in the zeta potential of the soil colloids to become more negative than the colloids without EPS. EPS increased the overall surface negative charge of the soils through a charge transfer mechanism, thereby, enhancing the adsorption of heavy metals (Cu, Cd, Pd). Also, the formation of EPS-metal-soil complexes resulted in enhanced metal sorption and accounted for the metal adsorbed via the non-electrostatic mechanism.
In a combined system, organic acids can inhibit the ability of EPS to enhanced heavy metal sorption by reducing the system pH. In a single system containing just single ligand, it was observed that the ability of oxalate to enhanced Pb sorption by an acidic Ultisol was superior to the abilities of citrate and EPS (117 mg L-1 of ligand), but when the concentrations were increased to 800 mg L-1, EPS was more effective. The findings of these studies demonstrate that the presence of EPS in variable charge soils control heavy metal bioavailability and nutrient retention. For remediation of polluted soils, it is recommended that wild legumes, which are rich in different rhizobacteria species be encouraged to colonize such lands so as to convert the heavy metals and organic pollutants to non-available forms. Furthermore, wheat and soybeans cultivation should also be encouraged so as to increase the presence of Pseudomonas in soils and enhance their CEC.
It was challenging to design a unique work given that my supervisor Profesor Xu Ren-Kou has published extensively on the physicochemical properties of variable charge soils. Nevertheless, the diverse and conducive nature of our research group gave room for out of the box thinking. Making use of the online resources provided by both the Institute of Soil Science (ISSCAS) and UCAS, drafting the article was not much of a challenge. We consulted the different journals, mastered the requirements, and with the guidance of my supervisor, we have two publications from this work in Environmental Pollution and Ecotoxicology and Environmental Safety.
By Nkoh Jackson Nkoh, CAS-TWAS President’s Fellow (2016)
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science Chinese Academy of Science (ISSCAS), University of Chinese Academy of Sciences