The role of extracellular polymeric substances on the sorption of natural organic matter

In this study, the influence of extracellular polymeric substances (EPS) composition and quantity was explored for biosorption of natural organic matter (NOM), using variants of Pseudomonas aeruginosa and Pseudomonas putida. Model EPS (sodium alginate beads) were tested and sorption capacity for NOM was also elucidated. In the absence of divalent ions, minimal NOM biosorption was observed and differences among strains were negligible. Under presence of divalent ions, biosorption of NOM was proportional to the amount of EPS secreted by P. aeruginosa variants. For sorption tests with model EPS, divalent ions also promoted biosorption of tested NOM, and total biosorption was also proportional to alginate quantity. Carboxyl group content in both alginate EPS and NOM appeared to be linked to increased biosorption via bridging with divalent ions. The alginate overproducing strain possessed more potential NOM biosorption sites, while the wild-type and alginate deficient strains possessed fewer potential binding sites. In comparison, P. putida, secreting protein-based EPS, behaved differently for NOM biosorption, due to its hydrophobicity and the structural characteristics of proteins. Hydrophobic interactions appeared to enhance the biosorption of more hydrophobic Suwannee River humic acid by P. putida, whose biosorption of more hydrophilic NOM variants was similar to the alginate deficient strain. Mechanistically, the presence of a diffuse electrical double layer will present potential energy barriers limiting biosorption; however, divalent ion concentrations in the aquatic environment will promote biosorption processes, permitting functional group interactions between EPS and NOM. Bridging between hydrophilic carboxyl groups on alginate EPS and NOM appeared to be the dominant form of biosorption, while hydrophobic interactions enhanced biosorption for protein-based EPS.