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Polycrystalline ice formation concentrates mineral nanoparticles into liquid boundaries between growing ice crystals. Here we show that minutes of freezing dictate iron mineral fate over subsequent months of aqueous aging. A single freeze–thaw cycle irreversibly aggregates ferrihydrite through converging physical and chemical mechanisms. Freeze concentration collapses electrostatic barriers while cryosuction strips hydration layers and compresses nanoparticles into micrometer-scale planar aggregates. Chemical evidence points to interfacial (hydr)oxo bridging, alongside hydrogen bonding, that resists disaggregation. These mechanisms lock nanoparticles into mesocrystal-like assemblages that retain their nanoscale identity but inhibit dissolution–reprecipitation to goethite, instead favoring solid-state transformation to hematite. Ice formation thus acts as a geochemical reactor, driving aggregation and interfacial bonding that redirect iron speciation, with broad implications for nutrient cycling and carbon preservation across the cryosphere.

Conspectus: Iron oxide (nano)minerals are abundant and reactive components of natural systems, exerting a profound influence on the environmental fate of contaminants. Among these, pharmaceutical pollutants have gained increasing attention due to their ubiquitous presence in aquatic and terrestrial environments, their persistence, and their potential to harm ecosystems and human health, particularly through contributions to antimicrobial resistance.Iron oxides play a dual role in mitigating pharmaceutical pollution: they serve as adsorptive surfaces and as redox-active materials capable of transforming pharmaceutical compounds. Adsorption is governed by the affinity between pharmaceutical functional groups and specific iron oxide surface sites, while redox-active pharmaceuticals may undergo abiotic transformations upon electron transfer with Fe(III) or mixed-valent (Fe(II)–Fe(III)) minerals. These interactions can yield breakdown products with altered bioactivity and toxicity, adding complexity to environmental risk assessments. However, these processes remain poorly understood due to experimental limitations and the absence of reliable predictive models. Predicting pharmaceutical behavior under environmentally relevant conditions requires bridging multiple spatial and temporal scales.In this Account, we provide a multiscale evaluation of how iron oxides influence the fate and remediation potential of pharmaceutical pollutants, integrating findings from experimental and modeling studies at molecular, interface, pore, column, and field levels. We also explore how Machine Learning can link across scales and uncover emergent patterns in complex data sets. Environmental factors, including solution chemistry, mineral properties, and co-occurring components, significantly influence these interactions.This approach allows us to identify key knowledge gaps, draw connections between molecular-scale reactivity and macroscale environmental processes, and highlight opportunities for developing predictive tools for environmental risk assessment. The insights gained extend beyond pharmaceuticals to a wide range of emerging organic contaminants. Advancing this field requires interdisciplinary collaboration at the intersection of molecular geochemistry, environmental materials science, mineralogy, hydrology, and microbiology.

Cold-region ecosystems are highly sensitive to climate change, yet the geochemical processes shaping their future remain poorly understood. Here, we show that ice systematically enhances mineral dissolution through freeze concentration into microscale reactive hot spots. Using goethite nanoparticles as a model iron oxide and environmentally relevant inorganic anions common in soils, waters, and aerosols (chloride, fluoride, sulfate), we demonstrate that ligand-promoted dissolution rates under mildly acidic conditions scale with binding affinity in both ice and liquid water, with ice enhancing rates across all reactive ligands. Fluoride, the strongest complexing agent, increased dissolution more than fourfold in ice, while weakly binding perchlorate produced no measurable dissolution in either phase. Reactions persisted well below the eutectic temperature, mediated by minute volumes of liquid-like water stabilized within networks of micron-sized mineral aggregates. Our findings highlight ice as a dynamic medium driving iron release, with implications for nutrient availability, carbon cycling, and biogeochemical feedbacks in rapidly warming polar and alpine regions.

Dissolved Mn(III) plays a critical role in controlling the fate of environmental contaminants. Using ciprofloxacin (CIP) as a model organic pollutant, this study examines how goethite and gibbsite, common soil minerals, affect the oxidative reactivity of Mn(III) stabilized by ligands of varying complexation strength. Among the four ligands tested, pyrophosphate (PP), a redox-inert but strongly complexing ligand, exhibited contrasting behavior depending on whether minerals were absent or present. In mineral-free systems, Mn(III)-PP complexes were generally unreactive toward CIP oxidation. In contrast, mineral surfaces enhanced CIP oxidation by promoting pH-dependent dissociation of Mn(III)-PP complexes. At pH 4, Mn(III)-PP preferentially adsorbed onto minerals, destabilizing Mn(III) complexes in solution and favoring its conversion to hydrolyzed MnOH²⁺ species. These intermediates underwent disproportionation, generating MnO2 colloids that subsequently promoted Mn(III) dissociation/disproportionation. Spectroscopy (UV-Vis, vibrational, and X-ray photoelectron) revealed a progressive decline in aqueous Mn(III)-PP complexes, and a corresponding accumulation of Mn(III)/Mn(IV) in solid phases. These results highlight Mn(III)-ligand coordination at mineral surfaces as a key regulator of Mn(III) reactivity. By elucidating the oxidation mechanisms of organic pollutants mediated by dissolved Mn(III) on minerals, this study enhances our understanding of Mn(III) dynamics in aquatic environments.

Schwertmannite and jarosite are naturally occurring iron (Fe) oxyhydroxysulfates with strong sorption capacities for hexavalent uranium [U(VI)] in various acidic sulfate-rich environments. These metastable minerals commonly undergo recrystallization, particularly in the presence of dissolved Fe²⁺ [Fe(II)_aq], which may influence the fate of associated U(VI). Here, we quantified molecular-level changes in U repartitioning and speciation when U(VI)-sorbed schwertmannite and jarosite reacted with Fe(II)_aq under near-neutral and anaerobic conditions over 2 weeks. The results show that Fe(II)_aq additions promoted rapid mineral transformation to goethite via a dissolution–reprecipitation pathway, proceeding (near-completely) for schwertmannite but slowly and incompletely for jarosite. Importantly, even at early transformation stages when goethite likely only started forming on the surface of the transforming minerals, the recrystallization process led to near-complete retention of U, predominantly as U(VI), within the structure of the neo-formed goethite. Subsequent U reduction to U(V) increased with time but remained incomplete, even after extensive mineral transformation in the presence of 1–50 mM Fe(II)_aq for 2 weeks. The results demonstrate that Fe(II)-promoted recrystallization of Fe-oxyhydroxysulfates can rapidly and persistently lock both U(VI) and U(V) into chemically stable goethite, with important implications for predicting U behavior and designing remediation strategies in various acidic and U-contaminated environments.

Pyrite, a ubiquitous sulfide mineral, exerts a strong influence on the fate of coexisting As(III) and As(V) species in natural settings, such as gold deposits, sedimentary basins, and hydrothermal systems. However, the As(III) adsorption and oxidation mechanisms on pyrite at neutral pH remain contested. Through oxic and anoxic kinetic experiments using pyrite with varying oxidation degrees, we demonstrate that As(III) adsorbs preferentially to the Fe(III) (oxy)(hydr)oxide coatings rather than to pyrite sites. Contrary to prevailing assumptions, the HO•radicals contribute minimally to As(III) oxidation at circumneutral pH. Spectroscopy and molecular simulations revealed that pyrite-generated H2O2oxidizes As(III) via an inner-sphere electron transfer process. This heterogeneous oxidation likely proceeds through a ternary surface complexation involving arsenite and Fe sites. These findings challenge both the conventional radical-dominated pathway and the assumed mechanism of natural arsenopyrite formation. By elucidating the dominant As(III) sorption/oxidation pathway on pyrite surfaces, our findings reshape our current understanding of arsenic geochemistry. They also inform risk assessment and remediation strategies for arsenic-impacted environments.

Ice often mediates unexpected reactions in the Cryosphere, acting as a fascinating geochemical reactor. Mineral–organic interactions in frozen environments, such as soils and permafrost, are crucial for explaining the flux of soluble iron during melting events, yet the mechanisms remain misunderstood. This study elucidates the unique roles of freezing in the dissolution of iron oxyhydroxide nanoparticles (α–FeOOH) by oxalate, a low molecular weight dicarboxylate, under acidic conditions. From time-resolved experiments conducted over 4 d, we demonstrate that soluble iron was released through reactions in minute volumes of liquid water trapped between ice micrograins. Freeze concentration of nanoparticles, oxalate, and protons into this liquid water drove oxalate- and proton-promoted dissolution reactions at temperatures as low as −30 °C. Remarkably, ice at −10 °C dissolved more iron than liquid water at 4 °C under high oxalate loadings, and even more than at 25 °C under low oxalate loadings. In contrast, high salinity subdued dissolution. Also, sequential freeze-thaw cycles enhanced dissolution by releasing unreacted oxalate that was previously locked in ice. By resolving the chemical controls on mineral dissolution in ice, this work can help explain how freeze-thaw events are supplying new fluxes of soluble iron to nature.

Mn(III) species play critical roles in determining the environmental fate of antibiotics released into natural systems. However, their reactivity is strongly influenced by complexation reactions with (in)organic ligands. This study investigates the impact of Mn(III) complexation with pyrophosphate (PP), a model environmental ligand, on the redox-driven degradation of ciprofloxacin (CIP), a widely used antibiotic and environmental contaminant. Spectroscopic analysis and thermodynamic modeling revealed that Mn(III)-PP complexes initially dissociate into MnOH2+ species, which can then disproportionate and form MnO2 colloids. Both dissociation and disproportionation reactions had comparable trends at pH 4 and 7, with reactivities that were strongly dependent on Mn(III):PP ratios. The progress of CIP oxidation following direct coordination with Mn compounds over time was sigmoidal, with an initial lag phase attributed to Mn(III)-PP complex dissociation and disproportionation. CIP degradation was predominantly governed by pH, with maximal rate constants decreasing from k = 0.390 h−1 at pH 3 to k = 0.065 h−1 at pH 5, and no CIP removal under circumneutral to alkaline conditions. Cyclic voltammetry also confirmed that the strongly pH-dependent redox potential of the Mn(III)/Mn(II) couple aligned with facile CIP oxidation under acidic conditions. These collective findings indicated that ligand complexation, such as with PP, enhanced Mn(III) stability and mitigated dissociation and disproportionation reactions. The new insight provided by this work on the speciation and redox activity of Mn(III) should thereby be considered for understanding ciprofloxacin degradation in contaminated water systems.

Cross-linked melamine imprinted monoliths targeting glyphosate were synthesized using 4-phosphonobutanoic acid (PBA) and N-(phosphonomethyl)iminodiacetic acid (PMIDA) as templates. The binding capacities, evaluated in an aqueous medium, showed that both PMIDA and PBA promoted selective binding sites with imprinting factors of 2.5 and 1.7, respectively. Despite a relatively low imprinting factor, the polymer imprinted with PMIDA showed a noticeably higher binding efficiency in the presence of sodium chloride compared to the nonimprinted reference, demonstrating an ability to selectively target the desired analytes in real sample matrices. Spectroscopic investigations using Fourier transform infrared and 1H nuclear magnetic resonance spectroscopy revealed the formation of “memory pockets” for glyphosate molecules in the imprinted melamine-formaldehyde scaffold promoted by simultaneous contributions from (i) hydrogen bonding with N-H/O-H moieties and (ii) electrostatic interaction toward the triazine ring.