Effect of hexavalent chromium on extracellular polymeric substances of granular sludge from an aerobic granular sequencing batch reactor

The effect of hexavalent chromium (Cr(VI)) on extracellular polymeric substances (EPS) of granular sludge was investigated in an aerobic granular sequencing batch reactor (GSBR). With the increase of Cr(VI) concentration from 0 to 30 mg L−1, the polysaccharide (PS) in loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) increased from 1.68 to 5.05 mg g−1 VSS and 1.89 to 7.57 mg g−1 VSS, respectively, and the protein (PN) in LB-EPS and TB-EPS increased from 11.57 to 38.86 mg g−1 VSS and 12.76 to 62.98 mg g−1 VSS, respectively. The PN/PS ratios in LB-EPS and TB-EPS increased from 6.90 to 7.69 and 6.75 to 8.32, respectively, with the increase of Cr(VI) concentration from 0 to 30 mg L−1. The five peaks identified by three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy in LB-EPS and TB-EPS were attributed to protein-like and humic acid-like substances. Fourier transform infrared (FTIR) spectra indicated that the increase of Cr(VI) concentration had distinct effects on the functional groups of PN and PS in LB-EPS and TB-EPS. The X-ray photoelectron spectroscopy (XPS) results showed that a portion of Cr(VI) was reduced to Cr(III). The relative hydrophobicity (RH) of the granular sludge exhibited a better linear correlation with the PN/PS ratio in TB-EPS (R2 = 0.9782) than that in LB-EPS (R2 = 0.9172).