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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.

The fate of the antibiotic ciprofloxacin (CIP) in natural waters can be strongly affected by interactions with nanominerals, such as goethite (GT; α-FeOOH). Using classical molecular dynamics, this study resolved the early stages of CIP adsorption on the four main crystallographic faces of GT nanoparticles, which is otherwise difficult to study experimentally. These early stages are driven by (i) electrostatic attraction, and (ii) the establishment of non-specific bonds between CIP (carboxyl, keto, amine) and GT (surface OH) functional groups. Simulations revealed that the medium-range (<1.5 nm) approach was not influenced by crystallographic orientation, but primarily by local positive charges generated by the interfacial orientation of GT surface OH groups. As a result, the deprotonated CIP− species reached the highest densities and residence times near GT surfaces, followed by the zwitterionic CIP−/+ species, and finally the protonated CIP+ species. Hydrogen bond numbers follow the same trend, and result from interactions between CIP carboxyl and GT surface O(H) groups. However, the protonated CIP+ species formed more hydrogen bonds and the most stable hydrogen bonds with the reactive OH groups of the (100) and (110) faces of GT. Additionally, protonation facilitated access to the (010) face by allowing the carboxyl group to fit into a tightly-bound water layer. By resolving the very first stages of CIP-GT interactions, this study established a foundational understanding of the precursor species which ultimately lead to redox-active Fe-bonded CIP species that can alter antimicrobial resistance in nature. These findings should thus contribute to a deeper understanding of mineral-organic interactions and to antibiotic transport in nature.

Ciprofloxacin (CIP), a commonly used antibiotic, is today found in natural waterways and terrestrial environments alongside trace heavy metal contaminants. CIP however has a weak affinity for iron (oxy)hydroxide minerals, which often control contaminant transport in nature. This weak affinity is caused by the electrostatic repulsion between positively charged mineral surfaces and the CIP piperazine ring. Using goethite (α-FeOOH), a representative iron (oxy)hydroxide nanomineral, we show that the presence of Cu(II) greatly enhances CIP adsorption while at the same time catalyzes CIP oxidation to byproducts, which are new to nature. The CIP uptake was greatest at circumneutral pH and in saline conditions, where Cu(II), CIP, and mineral surface charges were the least repulsive. Vibrational spectroscopy and molecular simulations revealed that the enhanced uptake of CIP was caused by the the coordination of metal-bonded Cu(II)-CIP surface complexes on goethite. The inner-sphere Cu(II)-CIP complex also facilitated CIP oxidation into a series of new products, which we identified by mass spectrometry. Finally, to predict Cu(II) and quinolone loadings prior to redox-driven reactions, we propose a multisite surface complexation model using Cu(II)-CIP ternary surface complexes, alongside an ion pair to account for the ionic strength dependence on loadings. The information developed in this work will help tracking the fate of CIP in contaminated aquatic environments.

Per- and polyfluoroalkyl substances (PFAS) are ubiquitous environmental contaminants that often co-occur with heavy metals. Despite their prevalence, the mobility of PFAS in complex, multicomponent systems, particularly at the molecular scale, remains poorly understood. The vast diversity of PFAS and their low concentrations alongside anthropogenic and natural substances underscore the need for integrating mechanistic insights into the sorption models. This study explores the influence of metal cations (Cu(II), Cd(II), and Fe(II)) on the adsorption of four common PFAS (PFOA, PFOS, PFDA, and GenX) onto goethite (α-FeOOH), a common iron (oxyhydr)oxide in both aquatic and terrestrial environments. PFAS adsorption was highly dependent on the PFAS type, pH, and metal ion concentration, with a surface complexation model effectively predicting these interactions. Cu(II) and Cd(II) enhanced PFOS and PFDA adsorption via ternary complexation while slightly reducing PFOA and GenX adsorption. Under anoxic conditions, Fe(II) significantly increased the adsorption of all PFAS, showing reactivity greater than those of Cu(II) and Cd(II). Additionally, natural organic matter increased PFAS mobility, although metal cations in groundwater may counteract this by enhancing PFAS retention. These findings highlight the key role of metal cations in PFAS transport and offer critical insights for predicting PFAS behavior at oxic-anoxic environmental interfaces.

Understanding contaminant transport through unsaturated porous media is a considerable challenge, given the complex interplay of nonlinear physical and biogeochemical processes driven by variations in water saturation. In this study, we tackled this challenge through a series of column experiments involving fine (100-300 μm) and coarse (1.0-1.4 mm) sand particles coated with birnessite (MnO₂) under variable saturation degrees. Dynamic flow experiments in sand columns revealed that desaturation altered the ability of MnO₂ in removing tetracycline (TTC), a redox-active antibiotic, yet the effect depends on the sand type and then on the saturation degree. Moderate saturation degrees in fine-grained sand columns promoted fractional and preferential water flow which favored a more acidic pH and increased dissolved oxygen levels. These conditions enhanced TTC removal, despite the reduced physical accessibility of reactive phases. In contrast, lower saturation degrees in coarse-grained sand columns induced stronger flow heterogeneity with a very small fraction of the water content participating in flow. The mobility behavior of all the columns was predicted using transport models that consider TTC adsorption and transformation, as well as dual porosity under variable water saturation degrees. This research offers valuable insights into predicting the fate and transport of redox-active contaminants in unsaturated soils and subsurface environments.

As critical transition zones between the land and the sea, estuaries are not only hotspots of hydrogeochemical and microbial processes/reactions, but also play a vital role in processing and transferring terrestrial fluxes of metals and nutrients to the sea. This study focused on three estuaries in the Gulf of Bothnia. All of them experience frequent inputs of acidic and Mn/metal-rich creek waters due to flushing of acid sulfate soils that are widespread in the creekś catchments. Analyzing existing long-termwaterchemistrydata revealed a strong seasonal variation of Mn loads, with the highest values in spring (after snow melt) and autumn (after heavy rains). We sampled surface waters, suspended particulate matter (SPM), and sediments from the estuarine mixing zones and determined the loads and solid-phase speciation of Mn as well as the composition and metabolic potentials of microbial communities. The results showed that the removal, cycling, and lateral transport of Mn were governed by similar phases and processes in the three estuaries. Manganese X-ray absorption spectroscopy data of the SPM suggested that the removal of Mn was regulated by silicates (e.g., biotite), organically complexed Mn(II), and MnOx (dominated by groutite and phyllomanganates). While the fractional amounts of silicate-bound Mn(II) were overall low and constant throughout the estuaries, MnOx was strongly correlated with the Mn loadings of the SPM and thus the main vector for the removal of Mn in the central and outer parts of the estuaries, along with organically complexed Mn(II). Down estuary, both the fractional amounts and average Mn oxidation state of the MnOx phases increased with (i) the total Mn loads on the SPM samples and (ii) the relative abundances of several potential Mn-oxidizing bacteria (Flavobacterium, Caulobacter, Mycobacterium, and Pedobacter) in the surface waters. These features collectively suggested that the oxidation of Mn, probably mediated by the potential Mn-oxidizing microorganisms, became more extensive and complete towards the central and outer parts of the estuaries. At two sites in the central parts of one estuary, abundant phyllomanganates occurred in the surface sediments, but were converted to surface-sorbed Mn(II) phases at deeper layers (>3–4 cm). The occurrence of phyllomanganates may have suppressed the reduction of sulfate in the surface sediments, pushing down the methane sulfate transition zone that is typically shallow in estuarine sediments. At the outermost site in the estuary, deposited MnOx were reduced immediately at the water–sediment interface and converted most likely to Mn carbonate. The mobile Mn species produced by the Mn reduction processes (e.g., aqueous Mn(II) and ligand complexed Mn(III)) could partly diffuse into the overlying waters and, together with the estuarine Mn loads carried by the surface waters, transfer large amounts of reactive Mn into open coastal areas and subsequently contribute to Mn shuttling and inter-linked biogeochemical processes over the seafloor. Given the widespread occurrence of acid sulfate soils and other sulfidic geological materials on many coastal plains worldwide, the identified Mn attenuation and transport mechanisms are relevant for many estuaries globally.

Pharmaceutical compounds often coexist in mixtures rather than as individual entities. However, little is known about their co-adsorption and co-mobility in soil and groundwater. In this study, we investigated the adsorption of a quinolone antibiotic (nalidixic acid, NA) and an anti-inflammatory agent (niflumic acid, NFA) onto two soils from France and Sweden in water-saturated soil columns. Despite its lower hydrophobicity, adsorption of NA is much greater than NFA, which can be ascribed to the presence of both carbonyl and carboxylic groups in NA molecule. The data suggest that adsorption to soil components can mainly take place through hydrogen bonding and surface complexation mechanisms, prevailing over hydrophobic interactions. Accordingly, more sorption of NA and NFA was observed in the Swedish soil because it contains more clay content, and much higher Al and Fe contents than the French soil. Injection of NA/NFA mixture in the column did not modify the breakthrough behavior compared to single systems, although cooperative adsorption was observed under static batch conditions. Ca²⁺ inhibited NA adsorption by forming a soluble NA-Ca²⁺ complex but promoted NFA adsorption both in single and binary systems. The mobility in soil columns was well predicted using a new transport model that accounts for both kinetics and binding reactions of NA and NFA to soil constituents. This work will help in accurately predicting the mobility of coexisting pharmaceutical compounds in soils.

Redox-driven reactions involving Mn(II) species adsorbed at Mn(IV) oxide surfaces can release Mn(III) in the form of dissolved Mn(III)-ligand species in natural waters. Using pyrophosphate (PP) as a model ligand, we show that freezing accelerates and enhances Mn(III) formation in the form of Mn(III)-PP complexes. This freeze-promoted reaction is explained by the concentration of Mn(IV) oxides and solutes (Mn(II), Na⁺, and Cl^-) into the minute fractions of liquid water locked between ice (micro)crystals - the Liquid Intergrain Boundary (LIB). Time-resolved freezing experiments at −20 °C showed that Mn(III) yields were greatest at low salt (NaCl) content. In contrast, high salt content promoted Mn(III) formation through chloride complexation, although yields became lower as the cryosalt mineral hydrohalite (NaCl·2H₂O) dehydrated the LIB by drawing water into its structure. Consecutive freeze-thaw cycles also showed that dissolved Mn(III) concentrations increased within the very first few minutes of each freezing event. Because each thaw event released unreacted PP previously locked in ice, each sequential freeze-thaw cycle increased Mn(III) yields, until ∼80% of the Mn was converted to Mn(III). This was achieved after only seven cycles. Finally, temperature-resolved freezing experiments down to −50 °C showed that the LIB produced the greatest quantities of Mn(III) at −10 °C, where the volumes were greater. Reactivity was however sustained in ice formed below the eutectic (−21.3 °C), down to −50 °C. We suspect that this sustained reactivity was driven by persistent forms of supercooled water, such as Mn(IV) oxide-bound thin water films. By demonstrating the freeze-driven production of Mn(III) by comproportionation of dissolved Mn(II) and Mn(IV) oxide, this study highlights the potentially important roles these reactions could play in the production of pools of Mn(III) in natural water and sediments of mid- and high-latitudes environments exposed to freeze-thaw episodes.

In a boreal acidic sulfate-rich subsoil (pH 3-4) developing on sulfidic and organic-rich sediments over the past 70 years, extensive brownish-to-yellowish layers have formed on macropores. Our data reveal that these layers ("macropore surfaces") are strongly enriched in 1 M HCl-extractable reactive iron (2-7% dry weight), largely bound to schwertmannite and 2-line ferrihydrite. These reactive iron phases trap large pools of labile organic matter (OM) and HCl-extractable phosphorus, possibly derived from the cultivated layer. Within soil aggregates, the OM is of a different nature from that on the macropore surfaces but similar to that in the underlying sulfidic sediments (C-horizon). This provides evidence that the sedimentary OM in the bulk subsoil has been largely preserved without significant decomposition and/or fractionation, likely due to physiochemical stabilization by the reactive iron phases that also existed abundantly within the aggregates. These findings not only highlight the important yet underappreciated roles of iron oxyhydroxysulfates in OM/nutrient storage and distribution in acidic sulfate-rich and other similar environments but also suggest that boreal acidic sulfate-rich subsoils and other similar soil systems (existing widely on coastal plains worldwide and being increasingly formed in thawing permafrost) may act as global sinks for OM and nutrients in the short run.