| Algal biofilm technology has advantages in removal of nitrogen and phosphorus in wastewater treatment. However, toxicants, e.g., heavy metals, usually coexist with nitrogen and phosphorus in sewage treatment, which have toxic effects on algal cells and their extracellular polymeric substances(EPS). As is a kind of the macromolecular substance generated by organisms, e.g., algae, EPS has a variety of biological roles on algal cells. However, the role of EPS on the removal of nitrogen and phosphorus under the stress of heavy metals are not very clear.In this paper, Chlorella vulgaris(C.vulgaris) was employed to investigate how Cd2+(as a representative heavy metal) affects the secretion of EPS and in turn affects the removal efficiency of NH4+-N and PO43--P. Moreover, the distribution of NH4+-N,PO43--P and Cd2+in the EPS and algae cells were studied. The composition differences of EPS under the stress of Cd2+ were determined by the three-dimensional excitation-emission matrix(3D-EEM) spectrum and Fourier transform infrared(FTIR)spectrum. Additionally, relationship between EPS contents with the removal efficiency of nutrients was investigated. The following results were obtained:(1) Heavy metal Cd2+ wad high toxicity to the growth of C. vulgaris with the EC50 value 1.17 mg/L. The removal of NH4+-N and PO43--P, growth rate and esterase activity of C. vulgaris cells were inhibited under the stress of high concentration of Cd2+(more than 0.5 mg/L). When the concentration of Cd2+ was applied in 1.0 mg/L,the average growth rate and the removal efficiencies of NH4+-N and PO43--P by algae was 3.29×105 cells/(m L*d), 42.3% and 32.7%, however yet remained 59.4%, 62.6%and 64.9% of the control(0 mg/L Cd2+) in the 6-d cultivation time, respectively.Meanwhile, the removal quantities of NH4+-N and PO43--P by a single cell also could remain 69.3% and 62.5% of the control. Additionally, research results of the cell esterase activity indicated that EPS had a function of resisting toxicants as a protective barrier in the removal of NH4+-N and PO43--P under the stress of Cd2+.(2) Compared to the control, the contents of TN and TP in S-EPS increased by56.8% and 30.7% while augmented by 26.8% and 27.5% in B-EPS under the stress of1.0 mg/L of Cd2+, respectively. Moreover, the TN and TP contents in a single cell were lower than the control, however still remained by 82.5% and 979% the control.Furthermore, the content of Cd2+ in B-EPS was about 55.4% of the initial Cd2+concentration(1.0 mg/L) and outclassed that in S-EPS(about 19.5%) in the 6-d cultivation time, indicating that B-EPS played a more crucial role in the adsorption of Cd2+. Additionally, compared to the control, both the diffusion and absorption rates nutrients decreased in the treatment(1.0 mg/L of Cd2+). However, the diffusion rates of NH4+-N and PO43--P from S-EPS to B-EPS yet remained approximately 65.8% and64.1%, and the cell and absorption rates of NH4+-N and PO43--P also remained about62.0% and 56.2% of the control in the 6-d cultivation, respectively.(3) In the 6-d cultivation time, the contents of S-EPS and B-EPS were 15.56×10-7μg/cell and 4.38×10-7 μg/cell in the treatment, enhancing by 20.2% and 30.4% in the6-d cultivation time, respectively, compared to the control. Meanwhile, the contents of polysaccharide(PS) in the S-EPS and B-EPS were 9.74×10-7μg/cell and 5.45×10-7μg/cell, increasing by 17.5% and 20.3%, while that of the protein(PN) were 7.53×10-7μg/cell and 3.17×10-7 μg/cell, increasing by 19.3% and 33.2% under the stress of 1.0mg/L of Cd2+, compared to the control, respectively. 3D-EEM spectrum verified that S-EPS mainly was composed of tryptophan-like and humic acid-like substances while B-EPS mainly contained tryptophan-like and tyrosine-like substances. Additionally,the tryptophan-like and tyrosine-like components of PN and humic acid-like substances in the S-EPS or B-EPS were enhanced under the stress of 1.0 mg/L of Cd2+.Moreover, the red shifts of the peak tryptophan-like in S-EPS and peak tyrosine-like in B-EPS were occured in the presence of Cd2+. FTIR spectrum indicated that 1.0mg/L of Cd2+ significantly decreased the stretching vibration intensities for-OH or-NH(3446-3460 cm-1) and C=O(-1642 cm-1, attributed to proteins) in both S-EPS and B-EPS, C-O-C and C-O(-1081 cm-1, attributed to polysaccharides) in S-EPS,which confirmed that EPS, especially for polysaccharides and proteins, could easily bind with Cd2+. Additionally, EPS contents had a satisfactory correlation with the removal efficiencies of NH4+-N(R2=0.9905) and PO43--P(R2=0.9414) under the stress of 1.0 mg/L of Cd2+. |
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