Nitrogen aeration alters the spatial distribution and metal adsorption of extracellular polymeric substances in waste-activated sludge

Abstract

Extracellular polymeric substances (EPS) extracted from waste-activated sludge (WAS) have the potential to remove heavy metal ions from wastewater; both the spatial distribution and metal adsorption of EPS from WAS after nitrogen aeration were systematically investigated in this study. Compared with air aeration, nitrogen aeration significantly improved the contents of proteins (PN) and polysaccharides (PS) in the Slime-EPS (S-EPS) and loosely-bound EPS (LB-EPS), significantly increased the PS content, and slightly increased the PN content in the tightly-bound EPS (TB-EPS). The variations in the fluorescence intensities (FI) of the peaks I and II for the S-EPS, LB-EPS and TB-EPS were basically consistent with the abovementioned variations in the concentrations of these EPS. Notably, nitrogen aeration dramatically improved the content of protein-like substances in the LB-EPS. For the same aeration time, the Pb²⁺ reclamation rates obtained by the LB-EPS extracted from the nitrogen-aerated WAS were much higher than those achieved by the LB-EPS extracted from the air-aerated WAS. The FTIR analyses further indicated that nitrogen aeration improved the contents of the functional groups, especially -OH, -COOH and -NH₂, responsible for binding heavy metals, in both the LB-EPS and TB-EPS. The SEM analyses verified that the nitrogen scours contributed to the EPS release, and Pb²⁺ reclamation was achieved via the attachment of Pb²⁺ onto the edge of the EPS. The influences of the nitrogen aeration on the spatial distribution and metal adsorption of the EPS in WAS were revealed for the first time in this study. Thus, this study lays the foundation for the application of nitrogen aeration in the resource utilization of WAS.

Fig. 1. Effects of aeration on the (a) PN and (b) PS contents of the S-EPS, LB-EPS and TB-EPS extracted from WAS. A and N represent the aeration by air or nitrogen, respectively. NA represents the additional aeration after aeration by nitrogen for 4 h.

Fig. 2. Variations in the 3D-EEM spectra of the S-EPS during aeration. N1–N4: nitrogen aeration for 1–4 h at the intervals of one hour. The same is shown below. NA1–NA4: air aeration after nitrogen aeration for 1–4 h. A1–A4: air aeration for 1–4 h.

Fig. 3. Variations in the 3D-EEM spectra of the LB-EPS during aeration. N1–N4: nitrogen aeration for 1–4 h at the intervals of one hour. The same is shown below. NA1–NA4: air aeration after nitrogen aeration for 1–4 h. A1–A4: air aeration for 1–4 h.

Fig. 4. Variations in the 3D-EEM spectra of the TB-EPS during aeration. N1–N4: nitrogen aeration for 1–4 h at the intervals of one hour. The same is shown below. NA1–NA4: air aeration after nitrogen aeration for 1–4 h. A1–A4: air aeration for 1–4 h.

Fig. 5. FTIR spectra of the extracted LB-EPS (a), TB-EPS (b) LB-EPS after the adsorption of Pb²⁺ (c) and TB-EPS after the adsorption of Pb²⁺ (d) from WAS by nitrogen or air aeration.

Fig. 6. SEM images of WAS (a) before and (b) after being aerated by nitrogen for 4 h. The EPS extracted from WAS aerated by nitrogen (c) before and (d) after the adsorption of Pb²⁺.