Thermodynamics of binding interactions between extracellular polymeric substances and heavy metals by isothermal titration microcalorimetry

- The thermodynamics of binding between EPS and heavy metals was investigated.
- Extracellular proteins were major participants in EPS/heavy metal binding.
- EPS/heavy metal binding was spontaneous and driven mainly by an enthalpy change.
- Divalent cations impeded EPS/heavy metal binding by electrostatic interaction.
- Trivalent cations competed with heavy metal ions for EPS binding sites.

Extracellular polymeric substances (EPS) play a crucial role in heavy metal bio-adsorption using activated sludge, but the interaction mechanism between heavy metals and EPS remains unclear. Isothermal titration calorimetry was employed to illuminate the mechanism in this study. The results indicate that binding between heavy metals and EPS is spontaneous and driven mainly by enthalpy change. Extracellular proteins in EPS are major participants in the binding process. Environmental conditions have significant impact on the adsorption performance. Divalent and trivalent cations severely impeded the binding of heavy metal ions to EPS. Electrostatic interaction mainly attributed to competition between divalent cations and heavy metal ions; trivalent cations directly competed with heavy metal ions for EPS binding sites. Trivalent cations were more competitive than divalent cations for heavy metal ion binding because they formed complexing bonds. This study facilitates a better understanding about the interaction between heavy metals and EPS in wastewater treatment.

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