Fluorescence approach for investigating binding properties between metals and soluble microbial products from a biological wastewater treatment plant

The interactions between three metals (Cu²⁺, Fe³⁺ and Al³⁺) and soluble microbial products (SMP) from the effluent of wastewater treatment plant (WWTP) were investigated using three dimensional excitation-emission matrix (EEM) fluorescence spectroscopy coupled with self-weighted alternating trilinear decomposition (SWATLD) analysis. Two main components in the SMP were identified as protein-like substances at Ex/Em (230-235/345-350) nm and humic-like substances at Ex/Em (340-350/425-430) nm. The SWATLD approach could discriminate SMP into different meaningful components and estimate their relative concentrations. The conditional stability constants (Log K_c) were calculated to evaluate the metal complexing properties of the SMP by Ryan-Weber Model. The two components in SMP was found to be strong ligands for the three metals based on their conditional stability constants, in an order of Cu²⁺ > Al³⁺ > Fe³⁺. The interactions between SMP and metals also can be affected by solution conditions, such as pH, ionic strength and temperature. The results of this study would provide an approach to characterize the SMP complexing properties and are useful to understand the transformation of metals in aqueous environments.