Adsorption of Pb(II), Cd(II) and Zn(II) by extracellular polymeric substances extracted from aerobic granular sludge: Efficiency of protein

• EPS extracted from AGS exhibited large adsorption capacity for heavy metals.
• High protein content full of functional groups was closely related to adsorption.
• Abundant carboxyl and hydroxyl groups were the main adsorption sites.
• Complexation through COO− and OH was the dominate adsorption mechanism.
• Metal ions could be efficiently desorbed from EPS under acidic conditions.

Extracellular polymeric substances (EPS) in aerobic granular sludge (AGS) play an important role in the removal of heavy metals. In this study, EPS with high protein content (86.8%) were extracted from AGS using a cation exchange resin extraction method. EPS exhibited large adsorption capacities for Pb(II), Cd(II) and Zn(II) of 1587.3, 1470.6 and 1123.6 mg/g respectively, as calculated by the Langmuir isotherm model. The excellent biosorption performance of EPS was closely related to the large concentrations of carboxyl (0.615 mmol/g) and hydroxyl groups (0.525 mmol/g) located on protein surfaces. FTIR and XPS analysis demonstrated that the dominant adsorption mechanism was complexation by carboxyl and hydroxyl groups. Moreover, extracted EPS, both with hydrophobic and hydrophilic groups, were found to comprise an amphiphilic material, which could also capture metal ions through electrostatic attraction and ion-exchange. Furthermore, the heavy metal ions could be efficiently desorbed from EPS under acidic conditions and reused. The study indicates that protein in EPS filled with carboxyl and hydroxyl groups is key to the heavy-metal adsorption process. The results provide basic guidance for the full utilization of EPS in AGS for bioremediation in wastewater treatment.

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