The new fate of MCLR revealed by dialysis equilibrium and theoretical calculations: Influence from DOM and Fe(II)/Mn(II)

Abstract

Microcystin-LR (MCLR) is a highly toxic pollutant widely distributed in eutrophic waters. The interplay between dissolved organic matter (DOM) and metal ions has been extensively reported; however, the impact of DOM-metal complexes on MCLR remains to be thoroughly studied. In this research, equilibrium dialysis was employed to study the kinetics and isotherms of MCLR adsorption by DOM, meanwhile the Density functional theory (DFT) calculations provided insights into the theoretical mechanisms of Fe(II)/Mn(II) involvement in the adsorption process. The adsorption kinetics revealed that the process is physicochemical, with chemisorption being the critical factor controlling the adsorption rate. The adsorption isotherms confirmed that Fe(II) alters the spatial arrangement of adsorption sites on the superficies of DOM, with the complexes of Fe(II) and DOM from the macrophyte-dominated regions of Lake Taihu (MDR-Fe(II)) showing the uppermost adsorption capacity of 6193.965 µg/g among all combinations. The influence of different cations (K+, Ca2+, Na+, Mg2+) on the adsorption of MCLR to DOM is divergent, with metals either reducing MCLR adsorption through competition for adsorption sites or increasing it by forming ternary complexes. The pH decrease reduced the affinity of DOM to metal ions while simultaneously enhancing MCLR adsorption through changes in its ionization state. DFT calculations demonstrated that Fe(II)/Mn(II) reduces the resistance to interaction between functional groups on DOM and MCLR, with carboxyl-Fe(II)-carboxyl interactions showing the highest binding energy (ΔG = −7.253 eV). Overall, Fe(II)/Mn(II) enhances the adsorption of MCLR via altering the structure and surface electrostatic potential distribution of DOM. This research indicates that the ultimate fate of MCLR in aquatic ecosystems is affected by DOM and metal ions.