Spectral induced polarization of clay-oxide hybrid particles

Pore-Scale Mechanisms for Spectral Induced Polarization of Calcite Precipitation Inferred from Geo-Electrical Millifluidics

Spectral induced polarization (SIP) has the potential for monitoring reactive processes in the subsurface. While strong SIP responses have been measured in response to calcite precipitation, their origin and mechanism remain debated. Here we present a novel geo-electrical millifluidic setup designed to observe microscale reactive transport processes while performing SIP measurements. We induced calcite precipitation by injecting two reactive solutions into a porous medium, which led to highly localized precipitates at the mixing interface. Strikingly, the amplitude of the SIP response increased by 340% during the last 7% increase in precipitate volume. Furthermore, while the peak frequency in SIP response varied spatially over 1 order of magnitude, the crystal size range was similar along the front, contradicting assumptions in the classical grain polarization model. We argue that the SIP response of calcite precipitation in such mixing fronts is governed by Maxwell-Wagner polarization due to the establishment of a precipitate wall. Numerical simulations of the electric field suggested that spatial variation in peak frequency was related to the macroscopic shape of the front. These findings provide new insights into the SIP response of calcite precipitation and highlight the potential of geoelectrical millifluidics for understanding and modeling electrical signatures of reactive transport processes.

Iron-Montmorillonite-Cyclodextrin Composites as Recyclable Sorbent Catalysts for the Adsorption and Surface Oxidation of Organic Pollutants

Iron-clay-cyclodextrin composites were designed as sorbent catalysts to adsorb and oxidize pollutants from water. The clay-iron backbone served as a mechanical support and as a heterogeneous Fenton catalyst, and the cyclodextrin monomers or polymers cross-linked with polyfluorinated aromatic molecules were used to accommodate adsorption of the pollutants. The composite based on iron-clay-cyclodextrin-polymers (Fe-MMT-βCD-DFB) exhibited superior adsorption and degradation of the model pollutants, bisphenol A (BPA), carbamazepine (CBZ), and perfluorooctanoic acid (PFOA), compared to the monomer-based composite and the native iron clay. The variety of adsorption sites, such as the polyfluorinated aromatic cross-linker, cyclodextrin toroid, and iron-clay surface, resulted in high adsorption affinity toward all pollutants; BPA was primarily adsorbed to the cyclodextrin functional groups, CBZ showed high affinity toward the Fe-MMT surface and the Fe-MMT-βCD-DFB composite, whereas PFOA was adsorbed mainly to the βCD-DFB polymer. Degradation, using H₂O₂, was highly efficient, reaching over 90% degradation in 1 h for BPA and CBZ and ∼80% for PFOA. The composite also showed excellent degradation efficiency in a multicomponent system with all three model pollutants. Furthermore, the composite's activity remained steady for five consecutive cycles of adsorption and degradation. The ability to remediate a broad range of pollutants, and the high overall removal exhibited by this novel material, demonstrates the potential for future application in water remediation technologies.

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.