Spectra metrology for interaction of heavy metals with extracellular polymeric substances (EPS) of Pseudomonas aeruginosa OMCS-1 reveals static quenching and complexation dynamics of EPS with heavy metals

The adsorption behavior and interaction mechanisms of extracellular polymeric substances (EPS) of Pseudomonas aeruginosa OMCS-1 towards chromium (Cr), lead (Pb), and cadmium (Cd) were investigated. EPS-covered (EPS-C) cells exhibited significantly higher (p<0.0001; two-way ANOVA) removal of Cr (85.58±0.39%), Pb (81.98±1.02%), and Cd (73.88±1%) than EPS-removed (EPS-R) cells. Interactions between EPS-heavy metals were spontaneous (ΔG<0). EPS-Cr(VI) and EPS-Pb(II) binding were exothermic (ΔH<0), while EPS-Cd(II) binding was endothermic (ΔH˃0) process. EPS bonded to Pb(II) via inner-sphere complexation by displacement of surrounding water molecules, while EPS-Cr(VI) and EPS-Pb(II) binding occurred through outer-sphere complexation via electrostatic interactions. Increased zeta potential of Cr (29.75%), Pb (41.46%), and Cd (46.83%) treated EPS and unchanged crystallinity (CIXRD=0.13), inferred EPS-metal binding via both electrostatic interactions and complexation mechanism. EPS-metal interaction was predominantly promoted through hydroxyl, amide, carboxyl, and phosphate groups. Metal adsorption deviated EPS protein secondary structures. Strong static quenching mechanism between tryptophan protein-like substances in EPS and heavy metals was evidenced. EPS sequestered heavy metals via complexation with C-O, C-OH, C=O/O-C-O, and NH/NH2 groups and ion exchange with –COOH group. This study unveils the fate of Cr, Pb, and Cd on EPS surface and provides insight into the interactions among EPS and metal ions for metal sequestration. Heavy metal pollution is a serious and persistent environmental issue worldwide; when untreated, get accumulated in natural ecosystems and adversely affects living organisms. Heavy metals including chromium, lead and cadmium are listed among the priority pollutants and regarded as carcinogens by the US Environmental Protection Agency (US EPA) and International Agency for Research on Cancer (IARC). The inhabitant biofilm-forming multi-metal resistant bacterium, Pseudomonas aeruginosa OMCS-1, showed resistance to high concentrations of Cr, Pb, and Cd. The EPS of the bacterium exhibited higher binding affinity towards heavy metals and facilitated the removal of Cr (85.58%), Pb (81.98%), and Cd (73.88%).