The effect of gallic acid interactions with iron-coated clay on surface redox reactivity

Adsorption and redox reactions between organic matter and natural reactive surfaces have a significant impact on pollutant sequestration and transformation, and on the effectivity of water and soil remediation practices. Herein, the interactions between an organic phenolic acid, gallic acid (GA), and clay coated with iron oxides (FeOx-MMT), were explored. Adsorption and desorption experiments revealed that GA has a higher affinity for FeOx-MMT than for native clay. The adsorption to FeOx-MMT was irreversible and only slightly affected by salinity, suggesting strong inner-sphere complexation. The GA-FeOx-MMT complex was characterized using cyclic-voltammetry, UV-Vis spectroscopy, FTIR, and XPS measurements. The results showed oxidation and transformation of GA on the surface and a considerable reduction of the surface iron. The resulting GA-FeOx-MMT complex had increased catalytic properties, enhancing hydrogen peroxide decomposition, and creating significantly more radicals than FeOx-MMT and raw clay. This led to the destruction of GA on the surface of the clay-iron complex, resulting in loss of activity over time. Our findings suggest a correlation between overall GA adsorption, consequent iron reduction and oxidant decomposition. This means that in systems where such constituents are present, these types of interactions need to be taken into consideration in terms of predicting the fate of pollutants in the environment, and for properly evaluating soil and water chemical treatment processes.