The impact of small organic molecules on Fe(II) coagulation: Facilitating vs. shielding mechanisms on charge transfer

Abstract

During the Fe(II) coagulation process, organics can significantly alter the structure of formed flocs, thereby influencing their efficacy in water treatment. However, the underlying mechanisms are not fully understood. This study investigates the impact of small organic molecules (SOM) on Fe(II) coagulation using serine, cysteine, histidine, and citric acid as examples. Results demonstrate that different SOM can significantly change the coagulation behavior by forming particles with distinct nanostructures, including flake-shaped γ-FeOOH, spherical γ-FeOOH, and ferrihydrite globules. The detection of Fe2+ in solution partially explains these phenomena, as Fe2+ can catalyze lattice rearrangement through charge transfer. By controlling the oxidation rate of Fe2+, SOM can influence the structure of flocs: cysteine and serine prolong the existence time of Fe2+ and promote the formation of highly crystalline γ-FeOOH, while citric acid accelerates Fe2+ oxidation, resulting in the opposite effect. However, histidine, despite delaying the oxidation of Fe2+, inhibits the formation of crystalline minerals, leading to the presence of flocs containing spherical γ-FeOOH. Mediated electrochemical analyses indicate that this is due to the adsorption of SOM on flocs, which hinders the effective entry of Fe2+ into the solid phase and disrupts the charge transfer. This study demonstrates that SOM can affect the interaction between Fe2+ and the nanostructure of flocs in two ways: directly influencing the oxidation rate of Fe2+ and indirectly interfering with the charge transfer between flocs and free Fe2+, which highlights the critical role of Fe(II)-Fe(III) charge transfer in coagulation and provides new possibilities for analyzing more complex organics-coagulation systems.