Differential ultraviolet-visible absorbance spectra for characterizing metal ions binding onto extracellular polymeric substances in different mixed microbial cultures

Ultraviolet-visible (UV-vis) absorbance spectra was adopted to quantify the binding of major metal ions (e.g., Na(I), Ca((II)), Fe(III), Cu(II), and Pb(II)) on extracellular polymeric substances (EPSs) extracted from different mixed cultures. The results showed that the differential absorbance spectra (DAS) provided discernible features for revealing the changes in optical properties of EPSs induced by metals, i.e., the intensity of DAS increased largely with incrementally increased metal concentrations (Fe(III), Cu(II), and Pb(II)). It can be assumed attributable to the changes in the conformations and inter-chromophores of the EPS biomolecules. In addition, the changes in spectral parameters of DSlope_325-375 (spectral slope in the range of wavelengths 325-375 nm) and DA₃₀₀ (differential absorbance at 300 nm) were found to be closely related to the amounts of metals bound onto all extracted EPSs, particularly for Fe(III) and Cu(II). The decreased S_R (the ratio of slope_275-295 to slope_350-400) of the EPS solutions after dosage of metals suggested increased molecular weight or size of the EPS biomolecules. Deconvolution of the DAS yielded six Gaussian bands, which were present in all of the EPS samples with various metals. Moreover, the relative contributions of different Gaussian bands in the DAS were determined by the nature of EPS-metal ions interactions good correlated with the covalent-bonding index. This study concluded that DAS and selected spectral parameters (DA₃₀₀, DSlope_325-375 and S_R) can be used to successfully characterize the binding of metals onto EPS at environmentally relevant concentrations.