Biogeochemical redox processes are essential for understanding and managing environmental contamination. These processes involve electron transfer reactions that influence the chemical speciation, bioavailability, toxicity, and mobility of elements such as Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg, Tc, and U. Redox reactions can be driven by microorganisms and natural organic matter, and they play a key role in the formation and dissolution of mineral phases. Redox-active humic substances and mineral surfaces can catalyze the transformation and degradation of organic contaminants. Understanding these processes is crucial for predicting and protecting water quality and ecosystem health, as well as for developing engineered remediation strategies. Redox processes significantly impact the fate and transport of trace elements and organic contaminants. For example, the reduction of trace metals can either decrease or increase their mobility, depending on the specific metal and reaction conditions. Similarly, the redox state of arsenic influences its toxicity and mobility, with arsenite generally being more mobile and toxic than arsenate. Microbial activity can influence the speciation of trace metals through changes in sorption and precipitation equilibria, as well as solid-solution partitioning of metal-binding organic matter. Redox processes also affect the behavior of metalloids such as arsenic and antimony. These elements can exist in multiple oxidation states, and their redox transformations are influenced by microbial activity and environmental conditions. For instance, the reduction of arsenic can lead to its sequestration by secondary minerals, while the oxidation of arsenic can increase its mobility. Similarly, the redox state of antimony can influence its mobility and toxicity, with antimony in different oxidation states having different environmental impacts. The impact of redox processes on organic contaminants is also significant. Microbial activity can reduce electron shuttles, such as humic substances, which can then transfer electrons to organic contaminants, facilitating their degradation. Additionally, redox processes can influence the fate of organic contaminants through the formation of reactive metal oxides and the transformation of organic compounds. Research needs include improving instrumentation for studying redox transformations at the molecular scale, understanding electron transfer mechanisms between microbes and minerals, and elucidating the redox activity of humic substances. Additionally, there is a need to develop models that can predict the fate and transport of contaminants in redox-dynamic environments, as well as to address the impacts of climate change on biogeochemical cycling. Overall, a deeper understanding of biogeochemical redox processes is essential for managing environmental contamination and protecting ecosystem health.Biogeochemical redox processes are essential for understanding and managing environmental contamination. These processes involve electron transfer reactions that influence the chemical speciation, bioavailability, toxicity, and mobility of elements such as Fe, Mn, C, P, N, S, Cr, Cu, Co, As, Sb, Se, Hg, Tc, and U. Redox reactions can be driven by microorganisms and natural organic matter, and they play a key role in the formation and dissolution of mineral phases. Redox-active humic substances and mineral surfaces can catalyze the transformation and degradation of organic contaminants. Understanding these processes is crucial for predicting and protecting water quality and ecosystem health, as well as for developing engineered remediation strategies. Redox processes significantly impact the fate and transport of trace elements and organic contaminants. For example, the reduction of trace metals can either decrease or increase their mobility, depending on the specific metal and reaction conditions. Similarly, the redox state of arsenic influences its toxicity and mobility, with arsenite generally being more mobile and toxic than arsenate. Microbial activity can influence the speciation of trace metals through changes in sorption and precipitation equilibria, as well as solid-solution partitioning of metal-binding organic matter. Redox processes also affect the behavior of metalloids such as arsenic and antimony. These elements can exist in multiple oxidation states, and their redox transformations are influenced by microbial activity and environmental conditions. For instance, the reduction of arsenic can lead to its sequestration by secondary minerals, while the oxidation of arsenic can increase its mobility. Similarly, the redox state of antimony can influence its mobility and toxicity, with antimony in different oxidation states having different environmental impacts. The impact of redox processes on organic contaminants is also significant. Microbial activity can reduce electron shuttles, such as humic substances, which can then transfer electrons to organic contaminants, facilitating their degradation. Additionally, redox processes can influence the fate of organic contaminants through the formation of reactive metal oxides and the transformation of organic compounds. Research needs include improving instrumentation for studying redox transformations at the molecular scale, understanding electron transfer mechanisms between microbes and minerals, and elucidating the redox activity of humic substances. Additionally, there is a need to develop models that can predict the fate and transport of contaminants in redox-dynamic environments, as well as to address the impacts of climate change on biogeochemical cycling. Overall, a deeper understanding of biogeochemical redox processes is essential for managing environmental contamination and protecting ecosystem health.