Influence of metal substitution on goethite redox properties and contaminant reduction

The growing demand for critical metals such as nickel (Ni), cobalt (Co), copper (Cu), aluminum (Al), and zinc (Zn) has intensified interest in understanding their environmental cycling and sustainable recovery. Iron (oxyhydr)oxides, particularly goethite and ferrihydrite, are abundant in soils and sediments and play a central role in controlling metal mobility and redox-driven contaminant transformations. This study investigates how substitution of these critical metals into goethite and ferrihydrite structure influences their redox properties and reactivity, with implications for both natural biogeochemical cycling and remediation strategies. I systematically synthesized a suite of metal-substituted goethites and ferrihydrites and characterized their structural and redox behavior. Electrochemical measurements revealed that metal substitution alters the Fe(III)-oxyhydroxide||Fe(II) redox potential, with Ni causing the most significant shift more oxidizing as compared to unsubstituted goethite. This shift is important because it aligns with behavior more like ferrihydrite, another iron oxide that is more amorphous and reactive in the soil. Furthermore, I investigated trends in ionic radius of the substituting cation to observe Ni again being the outlier which highlights the complexity of substitution effect on mineral structure.To evaluate environmental relevance, nitrobenzene was used as a model contaminant in batch reduction experiment. I found that unsubstituted goethite chemically reduced nitrobenzene the fastest while Ni-substituted goethite slowed the rate of nitrobenzene reduction the most. Additionally, redox potential shifts were found to strongly correlate with reduction kinetics, establishing a linear free energy relationship that provides predictive framework for contaminant transformation in metal-substituted systems. These findings offer new insight into the role of metal substitution in critical metal cycling and support the design of more sustainable, targeted remediation as well as metal extraction methods.