Study On The Ternary Fulvic Acid-iron Oxides-heavy Metal Coupled Reaction System Based On Coprecipitation Versus Adsorption

The binding properties of dissolved organic matter(DOM)with iron oxides and metal ions are very important for the biogeochemical dynamic cycling in the aquatic environment,and their coupled interactions mainly occur via adsorption and coprecipitation.In order to deepen the understanding of the complex interactions among fulvic acid(FA),Fe(III)and Cu(II)in the environment,the present study compared the sorption and removal capacities of Fe-FA-Cu ternary by conducting adsorption and coprecipitation batch experiments.Based on the organic acid extraction experiment,the availability of Cu in the solid products of different systems was compared,and the impact of solution pH and Fe/OC ratios on the removal effect of Cu was analyzed.The structural changes of FA in ternary systems were successfully characterized by the combination of spectroscopy and chemometric analysis,the binding rules and mechanisms of FA in such complex interactions were discussed,providing helpful theoretical basis for the further discussion of the environmental behaviors of DOM in water environment and for heavy metal pollution remediation.The conclusions of this paper are as follows:(1)FA significantly changed the structural properties of precipitates and affected the sorption effect of the system on heavy metals.With the decrease of Fe/OC ratios(i.e.,the increase of coexistence FA),the particle size of ferrihydrites reduced significantly.The sorption and removal efficiency of Cu in CPT system was higher than that in SOR system.The residual soluble Cu concentration in the solution decreased with the increase of pH value,and most of Cu were removed when the pH value at 6 or above.Different Fe/OC ratios would inhibit or promote the removal of Cu,the addition of FA led to the further reduction of Cu concentration in CPT system with Fe/OC of 1:3 while the Cu removal was inhibited in SOR system.(2)the binding mode of different products with Cu make its availability different.In the extraction experiment,the level of extracted Cu in CPT solid products was obviously lower than that in SOR products(up to 40%-60%).This difference may be attributed to the more tightly bound Cu in CPT product,while Cu in SOR product was adsorbed on the outer layers so that more Cu could release to the solutions.SOR samples treated with oxalic acid had stronger sorption capacity towards Cu than those treated with malic acid and citric acid.Citric acid showed the strongest ability to activate iron ions,and there was a significant linear correlation between the extraction rate of Cu and Fe in CPT precipitates,proving that the dissolution of iron oxides is one of the important mechanisms for citric acid to activate Fe.(3)the ZPC values of CPT and SOR suspensions were 5.6 and 7.3,respectively.The surface charges in SOR system exhibited a faster descend trend than that in CPT,indicating the different affinity for Cu.pH values,reaction modes and surface charges of suspensions had great influence on the formation of functional groups in FA and their subsequent participation in Fe-Cu binding process.EEM-PARAFAC results showed that there were two hum-like components C1 and C3 in Fe-FA-Cu ternary complexes,as well as one tryptophan-like component C2.Component C2 played a more crucial role in SOR system.Each component was highly sensitive to pH interference,and C2 and C3 showed significant negative correlation in both two systems,while the correlations between C1 with C2 and C3 were much weaker.(4)2D-FTIR-COS results demonstrated that the polysaccharide C-O and carboxyl O=C-O in FA molecules played an important role in Fe-Cu binding process.Aromatic C=C had the highest sensitivity to pH perturbation in SOR system,while the lowest in CPT system,which was due to the presence of more FA with high molecular weight in SOR system.In both two systems,the structural changes of FA with increased pH follow the order: polysaccharide group C-O > carbonyl C=O > carboxyl O=C-O > aromatic C=C,indicating that the binding affinity of hydrophilic groups is stronger than that of hydrophobic groups.These promising results are helpful for understanding the coupled interaction and its mechanisms in the multi-phase system.An enhanced picture regarding the structural variations of FA in ternary systems was constructed,and the impact of pH and SOR/CPT was successfully compared and discussed,highlighting the potential of combined spectroscopic and chemometric technology in studying the complicated process in aquatic environment.