As one of the major soluble organic compounds in membrane bioreactors, soluble microbial product (SMP) can combine the surface sites on hydrous ferric oxides (HFO), which can compete with HFO for the uptake of phosphate. In this study, SMP was extracted from an MBR plant with long-term stable operation, and the effects of SMP on phosphate adsorption onto fresh HFO (FHFO) were investigated at pH values of 6.0 and 7.8 in a two-phase system composed of FHFO (solid phase) and SMP/phosphate (liquid phase). The phosphate adsorption capacity of FHFO was found to decrease significantly with increase in the pH value and SMP concentration. A diffuse layer model (DLM) and a charge distribution multisite complexation (CD-MUSIC) model, which take these factors into account, were developed to describe the speciation of SMP and phosphate on FHFO surface. Both of the models provided good description of phosphate adsorption behavior in the presence of SMP at different pH. The equilibrium constants of surface complexation reactions between SMP and HFO were obtained by inversion calculation as log10K = 15.1 ± 0.11 in DLM model and log10K = 12.1 ± 0.5 in CD-MUSIC model, which can improve the equilibrium constants database of surface complexation models.

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