Experimental and Theoretical Approaches for the Surface Interaction between Copper and Activated Sludge Microorganisms at Molecular Scale

Abstract

Interactions between metals and activated sludge microorganisms substantially affect the speciation, immobilization, transport, and bioavailability of trace heavy metals in biological wastewater treatment plants. In this study, the interaction of Cu(II), a typical heavy metal, onto activated sludge microorganisms was studied in-depth using a multi-technique approach. The complexing structure of Cu(II) on microbial surface was revealed by X-ray absorption fine structure (XAFS) and electron paramagnetic resonance (EPR) analysis. EPR spectra indicated that Cu(II) was held in inner-sphere surface complexes of octahedral coordination with tetragonal distortion of axial elongation. XAFS analysis further suggested that the surface complexation between Cu(II) and microbial cells was the distorted inner-sphere coordinated octahedra containing four short equatorial bonds and two elongated axial bonds. To further validate the results obtained from the XAFS and EPR analysis, density functional theory calculations were carried out to explore the structural geometry of the Cu complexes. These results are useful to better understand the speciation, immobilization, transport, and bioavailability of metals in biological wastewater treatment plants. Interactions between metals and microorganisms play an important role in the geochemical cycling of trace heavy metals. In biological wastewater treatment processes, microorganisms in activated sludge have a high metal complexation capacity and then substantially affect the speciation, immobilization, transport, and bioavailability of metals in biological wastewater treatment plants. Previous study showed that there are many heavy metals (e.g., copper) in municipal and industrial wastewaters 1 . It is of great significance to study the surface complexation of metals on microorganisms and to understand their fates in biological wastewater treatment plants. In addition, extracellular polymeric substances (EPS), high-molecular weight compounds secreted by microorganisms, also have a significant impact on the fates of heavy metals 2,3 . The relevant functional groups involved in the interaction between metals and microorganisms are reported to be -COOH, -OH, -NH 2 , and -PO 4 , etc 4 . However, there are contradictory reports about the interaction between heavy metals and microbial cells in the presence and absence of EPS. In some studies it was found that the presence of EPS did not significantly affect the interaction 5 ; while others reported that the presence of EPS had a substantial effect on the interaction 6 . Such a contradiction may be attributed to the different experimental techniques employed in these works. Furthermore, the complicated structures of activated sludge and high water content make it difficult to explore the interaction mechanisms between heavy metals and microbial cells at a microscale. Previous studies showed that activated sludge microorganisms had high complexing capability to Cu [7][8][9] , which was particle-diffusion-controlled, and followed the pseudo-sec...

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“…41 The g values suggested that the distorted square pyramidal octahedral coordination of copper within the siliceous framework demonstrating the copper (II) complex (Figure S2). 58 The fluorescence spectra explored the absorption characteristics of Dox along with its coordination to the mesoporous framework (Figure 4D). The sample specimens were excited at an absorption maximum of Dox (λ ex : 440 nm) and resulted in maximum emission at around 580 nm (λ em ) 40 (Figure 4D-a).…”

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“…The FT-IR spectra of extracellular proteins also revealed that some carbohydrates and nucleic acids could bind with proteins. Many previous studies showed the high binding activity of EPS toward metal ions, dyes and pharmaceutical 23 24 25 26 . Figure 5 shows the protein amounts related to binding activity (organic molecules and ions) in anaerobic sludge and aerobic sludge were similar, whereas they were more abundant than those in anoxic sludge.…”

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“…The adsorption process of Cu(II) and Pb(II) from aqueous solution onto the raw and the treated MK was described by the Langmuir isotherm equation, which has been widely used in previous studies to explore the adsorption isotherms and to determine the maximum adsorption capacity of sorbent materials toward pollutants [8,17,18]. Following is a linearized form of the Langmuir isotherm: ); e q is the amount of Cu(II) or Pb(II) adsorbed at equilibrium (mg g -1 ); max q is the maximum adsorption capacity of MK at equilibrium (mg g -1 ); and b (L mg -1 ) is the Langmuir constant [19,20]. were involved in the hydrothermal reactions of raw MK.…”