Insight into in-situ chemical oxidation by Fe(II)-containing minerals: The role of inherent Fe(II)-OH in Fe(II)-Al LDHs

Abstract

Fe–OH–H–O–O–SO3 bond formation is an important mechanism for improving electron transfer and Fe(II)/Fe(III) cycling in iron-based materials for persulfate activation. However, Fe–OH has been mostly reported to be derived from surface-adsorbed water or Fe(III)–OH. The mechanism of the inherent Fe(II)–OH in the structure has been rarely reported. This study found that Fe(II)-Al layered double hydroxides (LDHs) with an inherent Fe(II)–OH structure in the environment can rapidly catalyze peroxymonosulfate (PMS) and degrade Bisphenol A (BPA) within a short time, even highly concentrated BPA. Through various characterizations and theoretical calculations, the Fe–OH–Fe and Fe–OH–Al in the metal layer of LDHs are suggested to have good electron conduction properties due to the Fe–OH in the structure, which contributes to the redox between Fe(II) and Fe(III). Meanwhile, LDHs have an excellent adsorption energy for PMS due to the special positive electric property of metal layers. The electrons can be effectively transferred to the adsorbed PMS through the Fe–OH bond, enabling quick PMS decomposition. Moreover, unlike the green rust with a similar structure, the presence of Al contributes to the structural stability of LDHs during the reaction process. Furthermore, the part of the in-situ structural changes (from LDHs to green rust) caused by the transformation of Fe(II) to Fe(III) also contributes to the improvement of the activation efficiency because oxygen vacancies are generated in this process. Considering that Fe(II)-Al LDHs exist in groundwater and soil, the inherent Fe-OH in their structure plays a key role in in-situ chemical oxidation. Therefore, this work provides a new perspective for the in-situ chemical oxidation of persulfate by natural Fe(II) containing minerals.