Mineral dust can indirectly impact climate by nucleation of atmospheric solids, for example, by heterogeneously nucleating ice in mixed-phase clouds or by impacting the phase of aerosols and clouds through contact nucleation. The effectiveness toward nucleation of individual components of mineral dust requires further study. Here, the nucleation behavior of metal oxide nanoparticle components of atmospheric mineral dust is investigated. A long-working-distance optical trap is used to study contact and immersion nucleation of ammonium sulfate by transition-metal oxides, and an environmental chamber is used to probe depositional ice nucleation on metal oxide particles. Previous theory dictates that ice nucleation and heterogeneous nucleation of atmospheric salts can be impacted by several factors including morphology, lattice match, and surface area. Here, we observe a correlation between the cationic oxidation states of the metal oxide heterogeneous nuclei and their effectiveness in causing nucleation in both contact efflorescence mode and depositional freezing mode. In contrast to the activity of contact efflorescence, the same metal oxide particles did not cause a significant increase in efflorescence relative humidity when immersed in the droplet. These experiments suggest that metal speciation, possibly as a result of cationic charge sites, may play a role in the effectiveness of nucleation that is initiated at particle surfaces.

Bare rock surfaces in dry to semiarid places of the world often host a black-brown accretion rich in Mn and Fe known as rock varnish. The varnish surface presents an ideal environment for microbial development. A burgeoning interdisciplinary arena of scholarship focuses on the biogeochemical fingerprints of life in severe settings. Given that a large number of researchers hypothesize that varnish formation is a key process by microorganisms, the high altitude Ladakh remains a largely unexplored research setting. Thus, as one of the world’s harshest dry deserts, we selected Ladakh as the focus for this investigation into the nature of organic biomarkers found in subaerial rock varnish in this severe climate. Microbial fingerprinting using organic biomarkers and isotopic analyses in conjunction with electron microscopy reveals the presence of organic metabolites such as fatty acids, alkyl benzenes, oxime, amide, and fatty acids that we interpret as resulting from mineral–microbial interactions. We hypothesize that a newly discovered change in surface wettability characteristics from hydrophilic (in host rock) to hydrophobic (in varnish) might be important in facilitating the development of microbial processes that could be related to varnish formation.

This study aims to explore the damage and permeability enhancement effect of acidification on rock. First, an X-ray diffraction qualitative analysis was conducted on the roof rock in the coal seam of Jiulishan Coal Mine in Jiaozuo City, Henan Province, China, by an X-ray diffractometer. The optimum acid solution ratio (12% HCl+1% HF) was obtained based on the principle of acidification acting on the mineral composition. Based on previous tests, tests for mechanical properties and gas permeability of rock samples before and after acidification were carried out. Under the condition of different acidification times, the rock damage degree was quantitatively characterized by the changes in the mass loss and mechanical properties of rock, and the relationships of acidification time with the rock damage degree and permeability were explored. The results reveal the following. (1) Macroscopically, acidification damages rock by deteriorating its mechanical properties, and the damage mode transitions from brittle damage to ductile damage. The damage degree is positively correlated with the acidification time; however, when the reaction reaches a certain extent, acidification barely causes mechanical damage to the rock. (2) Microscopically, minerals in the rock mass are dissolved, hence promoting the connectivity between pores and enhancing the permeability. In addition, there is a chain causality for rock permeability enhancement; that is, the acidification time determines the amount of minerals dissolved inside the rock, and the mineral dissolution intensifies rock damage, which then enhances the permeability. The evolution of rock permeability with acidification time can be concluded as follows: the permeability increases rapidly first, then rises slowly, and finally levels off; the optimum acidification time is in the range of 8–24 h with permeability enhanced by 220–260%.

Organic acids are demonstrating outstanding performance in adsorption of heavy metals as a result of their biocompatible and green characteristics, which have attracted widespread attention. In this paper, the extraction rate of citric acid extracts for heavy metals of Cu and Cr in the sludge were investigated. The effects of time, pH, concentration, temperature, and other factors were studied. The results demonstrated that the leaching rate of Cu by citric acid was 82.35% with a reaction time of 18 h, pH 3, solvent concentration of 0.4 mol/L, and reaction temperature of 30 °C. In addition, the leaching rate of Cr was 93.86% at a reaction time of 15 h, pH of 3, solvent concentration of 0.4 mol/L, and reaction temperature of 30 °C. Furthermore, the distribution of environmental heavy metals was evaluated by the BCR three-state continuous extraction method for ionic patterns, which showed that the sludge was leached by citric acid and the heavy metals mainly existed in the exchangeable state and subsequently in the residual state. Eventually, the complexation of citric acid with heavy metal ions was further confirmed by molecular dynamics simulations, which provided an alternative approach and a new perspective for the purification of environmental heavy metals by organic acids.

The non-ideal solid solutions between calcite and smithsonite, [(Ca1–xZnx)CO3], were synthesized, and their interaction with different aqueous solutions at 25 °C was experimentally investigated. The X-ray diffraction spectra indicated that all synthesized minerals exhibited the calcite structure exclusively. After 180–240 days of dissolution in N2-degassed water (NDW) and air-saturated water (ASW), the aqueous Zn concentrations reached a constant value ranging from 0.002565 to 0.006133 and 0.002710 to 0.006374 mmol/L for the solid solutions with low Zn/(Zn + Ca) mole ratios (XZn < 0.075) or from 0.005416 to 0.076400 and 0.005128 to 0.067222 mmol/L for the solid solutions with high XZn (>0.864), respectively. After 180–240 days of dissolution in CO2-saturated water (CSW), the aqueous Zn concentrations reached a constant value ranging from 0.005938 to 0.081753 mmol/L for all solid solutions. The aqueous Zn/(Ca + Zn) mole ratios were considerably lower than the solid XZn. The aqueous Zn and Ca concentrations generally increased with increasing XZn for solid solutions with XZn < 0.075, while they decreased with increasing XZn for solid solutions with XZn > 0.864. The average solubility products (Ksp) (≈ ion activity products at the constant state) were determined to be 10–8.36±0.10, 10–8.33±0.03, and 10–8.28±0.06 for calcite [CaCO3] in NDW, ASW, and CSW, respectively. Similarly, the average solubility products were determined to be 10–10.65±0.12, 10–10.60±0.08, and 10–10.47±0.06 for smithsonite [ZnCO3] in NDW, ASW, and CSW, respectively. The logarithm of Ksp showed a slight increase with increasing XZn for solid solutions with XZn < 0.075, whereas it decreased with increasing XZn for the solids with XZn > 0.864. In the Lippmann diagram constructed with the Guggenheim coefficients a0 = 2.72 and a1 = −0.266 for the [(Ca1–xZnx)CO3] solid solutions, it was observed that the solid solutions dissolved non-stoichiometrically and moved progressively up to the minimum stoichiometric saturation curve for pure smithsonite and the solutus curve and then along them from right to left, finally reaching the saturation curve for calcite. The coexistence of Zn-poor aqueous solutions with ZnCO3-rich solids highlights the findings of the [(Ca1–xZnx)CO3] mineral–water reaction and its significance in the zinc geochemical cycle in Earth’s surface environments, contributing to a comprehensive understanding of these processes.

Clathrate hydrates of methane as well as its ambient pressure analogues of tetrahydrofuran (THF) and cyclopentane have been reported to undergo partial dissociation at temperatures as low as ∼200 K in the presence of ammonia (NH3). This behavior has significant implications for the hydrocarbon cycle on Titan because the destabilization of subsurface clathrate deposits could contribute to the replenishment of atmospheric methane. In addition, the ternary H2O–THF–NH3 system exhibits great chemical complexity, with a previous report of an unknown THF–NH3-rich phase. In this study, we have investigated the H2O–THF–NH3 system at 1 bar with X-ray diffraction and Raman spectroscopy to further characterize the inhibiting effect of ammonia on clathrates and the various phases formed between 90 and 280 K. Our data are in agreement with previous Raman spectroscopy and calorimetry studies and confirm the partial dissociation of THF clathrates in aqueous solutions of ammonia between 200 and 270 K. Transposing this effect to methane clathrates implies that the presence of small amounts of ammonia in Titan’s icy shell could trigger partial melting of these subsurface methane reservoirs. Furthermore, these new data enabled the characterization of a previously unknown THF–NH3-rich phase observed below 220 K. Our analysis suggests a trigonal crystal system for this novel compound, with a unit cell volume of 1617 Å3 at 90 K.

Selenium (Se) is both a micronutrient required for most life and an element of environmental concern due to its toxicity at high concentrations, and both bioavailability and toxicity are largely influenced by the Se oxidation state. Environmentally relevant fungi have been shown to aerobically reduce Se(IV) and Se(VI), the generally more toxic and bioavailable Se forms. The goal of this study was to shed light on fungal Se(IV) reduction pathways and biotransformation products over time and fungal growth stages. Two Ascomycete fungi were grown with moderate (0.1 mM) and high (0.5 mM) Se(IV) concentrations in batch culture over 1 month. Fungal growth was measured throughout the experiments, and aqueous and biomass-associated Se was quantified and speciated using analytical geochemistry, transmission electron microscopy (TEM), and synchrotron-based X-ray absorption spectroscopy (XAS) approaches. The results show that Se transformation products were largely Se(0) nanoparticles, with a smaller proportion of volatile, methylated Se compounds and Se-containing amino acids. Interestingly, the relative proportions of these products were consistent throughout all fungal growth stages, and the products appeared stable over time even as growth and Se(IV) concentration declined. This time-series experiment showing different biotransformation products throughout the different growth phases suggests that multiple mechanisms are responsible for Se detoxification, but some of these mechanisms might be independent of Se presence and serve other cellular functions. Knowing and predicting fungal Se transformation products has important implications for environmental and biological health as well as for biotechnology applications such as bioremediation, nanobiosensors, and chemotherapeutic agents.

The grim status of India’s air quality is commonly attributed to alarming levels of particulate matter (PM). This has led to the government formulating several strategies to control PM in the country. For example, the National Clean Air Programme (NCAP) was launched in 2019 to reduce 30% PM concentrations by 2024 in nonattainment cities (NACs). However, studies across the world found these PM-centric action plans to be detrimental to ozone-related air quality. The present study is among the first few investigations that employ the Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model to comprehensively understand the effect of the NCAP on PM and ozone in India. In the 2024 business-as-usual scenario, while 50% of NACs have deteriorating PM2.5 levels, all but 32% of other NACs meet the Indian National Ambient Air Quality Standards (INAAQS) with the NCAP. The NACs that violated the INNAQS are mainly situated in the Indo-Gangetic Plain. For ozone, with the implementation of the NCAP, 80% of NACs meet the INAAQS. This reduction in ozone is attributed to the indicator ratio of formaldehyde to oxides of nitrogen (HCHO/NOy), indicating that the Indian region is volatile organic compound-limited (VOC-limited), and thereby, reduction in nitrogen oxides and VOC emissions results in a decrease in ozone. Further, another reason can be the increase in forest cover as a part of the NCAP, which has increased ozone deposition velocity by ∼10.5%. This study indicates that the current action plans of India, if implemented successfully, can also alleviate the problem of secondary pollutants like ozone, saving around 12% of lives.

Pollen grains significantly contribute to the aerosol population, and levels are predicted to increase in the future. Under humid atmospheric conditions, pollen grains can rupture creating pollen grain fragments referred to as subpollen particles (SPPs) which are dispersed into the atmosphere with wind. In this laboratory study, SPP emission factors were determined for ryegrass, Lolium sp., and giant ragweed,Ambrosia trifida, in terms of the number of SPPs produced per pollen grain and the number of SPPs produced per m2, which were compared to previously measured live oak,Quercus virginiana, emission factors. The SPP emission factors were 4.9 × 1013 ± 4.3 × 1013 SPPs per m2 for ryegrass, 1.3 × 1015 ± 1.1 × 1015 SPPs per m2 for giant ragweed, and 1.1 × 1015 ± 1.6 × 1015 SPPs per m2 for live oak. SPPs and whole pollen grains from these species were evaluated for their ice nucleation efficiency in immersion and contact mode freezing. Measurements of the ice nucleation efficiency indicate that SPPs are weakly effective INPs in immersion mode, but that pollen grains represent a source of moderately efficient INPs in immersion and contact modes.

In the interstellar medium, six molecules have been conclusively detected in the solid state in interstellar ices, and a few dozen have been hypothesized and modeled to be present in the solid state as well. The icy mantles covering micrometer-sized dust grains are, in fact, thought to be at the core of complex molecule formation as a consequence of the local high density of molecules that are simultaneously adsorbed. From a structural perspective, the icy mantle is considered to be layered, with an amorphous water-rich inner layer surrounding the dust grain, covered by an amorphous CO-rich outer layer. Moreover, recent studies have suggested that the CO-rich layer might be crystalline and possibly even be segregated as a single crystal atop the ice mantle. If so, there are far-reaching consequences for the formation of more complex organic molecules, such as methanol and sugars, that use CO as a backbone. Validation of these claims requires further investigation, in particular on acquiring atomistic insight into surface processes, such as adsorption, diffusion, and reactivity on CO ices. Here, we present the first detailed computational study toward treating the weak interaction of (pure) CO ices. We provide a benchmark of the performance of various density functional theory methods in treating the binding of pure CO ices. Furthermore, we perform an atomistic and in-depth study of the binding energy of CO on amorphous and crystalline CO ices using a pair-potential-based force field. We find that CO adsorption is represented by a large distribution of binding energies (200–1600 K) on amorphous CO, including a significant amount of weak binding sites (<350 K). Increasing both the cluster size and the number of neighbors increases the mean of the observed binding energy distribution. Finally, we find that CO binding energies are dominated by dispersion and, as such, exchange-correlation functionals need to include a treatment of dispersion to accurately simulate surface processes on CO ices. In particular, we find the ωB97M-V functional to be a strong candidate for such simulations.

Terrestrial enhanced weathering of alkaline silicate minerals is a promising climate change mitigation strategy with the potential to limit the global temperature rise. The formation and accumulation of pedogenic carbonate and bicarbonate in soils/subsoils and groundwater offers a large sink for C storage; the amount of soil inorganic carbon (SIC) presently held within soils has been estimated to be 720–950 Gt of C. These values can be augmented by the addition of a variety of calcium and magnesium silicates via enhanced weathering. While the concept of the application of finely milled silicate rocks for faster weathering rates is well established, there has been limited discussion on the role of local climate, natural SIC content (i.e., the SIC innately present in the soil), and soil pH (among other important agronomic factors) on silicate weathering when applied to croplands, especially in view that the aim is to establish terrestrial enhanced weathering as a carbon dioxide removal (CDR) strategy on a global scale. In this work, we emphasized the importance of soil pH and soil temperature on silicate weathering and looked to estimate an upper limit of (i.e., constrain) the global capacity until the year 2100 for enhanced rock weathering (ERW) to draw down CO2 in the form of accumulated pedogenic carbonate or soluble bicarbonate. We assessed the global spatial distribution of cropland soil pH, which serves as a proxy for local innate SIC; annual rate of pluvial (rainfall) precipitation; and soil temperature, and found that the potential CO2 drawdown difference between faster and slower weathering silicates is narrower in Asia, Africa, and South America, while the gap is larger for Europe, North America, and Oceania.

A detailed quantum chemical investigation of a new reaction mechanism possibly leading to the formation of cyanoketene (NC–CH═C═O) in the interstellar medium (ISM) was carried out. Different reaction channels have been found by the AutoMeKin program, and the structures and harmonic force fields of the key stationary points have been characterized at the density functional theory level employing last-generation double-hybrid functionals. Finally, single-point computations at those geometries by state-of-the-art composite wave function methods provided accurate energies for the evaluation of thermochemical and kinetic parameters in the framework of an Ab Initio Transition State Theory based Master Equation (AITSTME) strategy. Our results indicate that the barrier-less association reaction of the formyl radical (HCO•) to the cyanocarbene radical (HCCN) can lead to the formation of cyanoketene under the harsh conditions of the ISM. Canonical rate constants computed for temperatures up to 600 K show that the most abundant product is indeed cyanoketene. The formation of other, even more stable, species involves higher activation energies and/or less favorable multi-step processes. Furthermore, to aid the search of cyanoketene, still undetected in the ISM, its rotational spectrum was recorded up to 530 GHz. The refined set of spectroscopic constants obtained in this way allows for spectral predictions from the microwave to the terahertz region, particularly for the bright b-type transitions, which can be targeted for the identification of cyanoketene in spectral line surveys. Despite cyanoketene was already sought without success in a variety of astronomical sources, we suggest to look for it in those sources where HCO or HCCN have already been detected, namely, W3, NGC2024, W51, K3-50, IRC + 2016, and TMC-1.

Various geochemical proxies have been developed to determine if ancient sedimentary strata were deposited in marine or nonmarine environments. A critical parameter for proxy reliability is the residence time of aqueous species in seawater, which is rarely considered for proxies relying on stable isotopes and elemental abundance ratios. Differences in residence time may affect our ability to track geologically short-lived alternations between marine and nonmarine conditions. To test this effect for sulfur and nitrogen isotopes and sulfur/carbon ratios, we investigated a stratigraphic section in the Miocene Oberpullendorf Basin in Austria. Here, previous work revealed typical seawater-like rare earth element and yttrium (REY) systematics transitioning to nonmarine-like systematics. This shift was interpreted as a brief transition from an open marine depositional setting to a restricted embayment with a reduced level of exchange with the open ocean and possibly freshwater influence. Our isotopic results show no discernible response in carbonate-associated sulfate sulfur isotopes and carbon/sulfur abundance ratios during the interval of marine restriction inferred from the REY data, but nitrogen isotopes show a decrease by several permil. This observation is consistent with the much longer residence time of sulfate in seawater compared with REY and nitrate. Hence, this case study illustrates that the residence time is a key factor for the utility of seawater proxies. In some cases, it may make geochemical parameters more sensitive to marine water influx than paleontological observations, as in the Oberpullendorf Basin. Particular care is warranted in deep time, when marine residence times likely differ markedly from the modern.

Phosphorus (III) oxide (P4O6) has been suggested to be a major component of the gas phase phosphorus chemistry in the atmospheres of gas giant planets and of Venus. However, P4O6’s proposed role is based on thermodynamic modeling, itself based on values for the free energy of formation of P4O6 estimated from limited experimental data. Values of the standard Gibbs free energy of formation (ΔGo(g)) of P4O6 in the literature differ by up to ∼656 kJ/mol, a huge range. Depending on which value is assumed, P4O6 may either be the majority phosphorus species present or be completely absent from modeled atmospheres. Here, we critically review the literature thermodynamic values and compare their predictions to observed constraints on P4O6 geochemistry. We conclude that the widely used values from the NIST/JANAF database are almost certainly too low (predicting that P4O6 is more stable than is plausible). We show that, regardless of the value of ΔGo(g) for P4O6 assumed, the formation of phosphine from P4O6 in the Venusian atmosphere is thermodynamically unfavorable. We conclude that there is a need for more robust data on both the thermodynamics of phosphorus chemistry for astronomical and geological modeling in general and for understanding the atmosphere of Venus and the gas giant planets in particular.

Amorphous magnesium carbonate (AMC: MgCO3·nH2O) is a metastable phase with respect to crystalline magnesium carbonate. AMC has been suggested to be an important mineral, a major process of CO2 fixation in alkaline saline lakes. Understanding the role of AMC for carbon fixation in alkaline lakes requires knowledge of the solubility of AMC under the full range of relevant environmental conditions, but its solubility is poorly known for low water temperatures. In this study, the solubility of AMC at 7 °C was measured in laboratory experiments and compared with chemical analyses of water samples from Olgoy Lake, a natural alkaline saline lake in Mongolia. Laboratory measurements using both supersaturation and undersaturation methods provided very similar solubility products of AMC (log Ksp = −5.20 ± 0.02) at 7 °C, which was markedly higher than the solubility product at 25 °C (log Ksp = −5.59). The temperature dependence of the solubility of AMC is much greater than that of monohydrocalcite, another important authigenic calcium carbonate in alkaline lakes. Analyses of water from Olgoy Lake consistently showed a significant temperature dependency of ion activity products with respect to magnesium carbonate. The relatively high solubility of AMC at low temperatures implies that the formation of AMC requires relatively high concentrations of dissolved components in lakes. Our results suggest that the formation of AMC is favored more in summer than in winter.

The modification of clay minerals by exopolysaccharides (EPS) may significantly increase their adsorption capacity for heavy metals. This study focused on the adsorption of EPS (produced by Rhizobium tropici)-modified montmorillonite (MMT) and kaolinite (KLT) for Cs and Sr and the influence of external factors (pH, sulfate, and phosphate). The characterization of the composites was carried out using X-ray diffraction (XRD), Fourier transform infrared (FTIR), atomic force microscopy (AFM), and scanning electron microscopy/energy-dispersive X-ray analysis. With EPS modification, the adsorption capacity of MMT for Cs and Sr reached 256 and 90.9 mg/g, respectively, which were significantly improved by 53.8 and 54.5% compared to MMT alone, respectively. The adsorption capacity of KLT for Sr improved by 10.7%. KLT did not adsorb Cs either before or after EPS modification. The adsorption isotherms for Sr on MMT, EPS-MMT, KLT, and EPS-KLT as well as Cs on MMT and EPS-MMT were better described with the Freundlich adsorption models, indicating a heterogeneous layered adsorption process. XRD, FTIR, and AFM analysis confirmed the interlayer reaction of Sr/Cs with EPS-MMT. The Sr amounts adsorbed on EPS-MMT composites increased significantly with increasing pH, while the pH influence was not obvious on Cs adsorption but still slightly increased at pH 7 and then dropped at pH 9. In the presence of 50 and 500 mg/L sulfate, the Sr amount absorbed decreased by 12.5, and 29.3%, respectively. On the contrary, there was a significant increase in Cs adsorption by 12.2 and 33.9%, respectively. In the presence of phosphate, a significant increase (64.5%) was observed for Cs adsorption under 50 mg/L phosphate loading, but 500 mg/L phosphate inhibited (65.8%) the adsorption. In contrast, there was no significant change of Sr adsorption under different phosphate concentrations. The current study would provide a new insight for the application of biopolymers in remediation of Sr- and Cs-contaminated areas.

The search for life on other bodies in our solar system is currently focused on Ocean Worlds, those bodies known to contain liquid water, as water is one of the requirements for life as we understand it. In the search for organic biosignatures that would indicate the presence of past or current life, liquid samples from these bodies would utilize an initial sample preparation step in which interfering substances such as salts can be removed before analysis. Previous work on potential sample preparation techniques for these samples evaluated solid phase extraction (SPE) on cation exchange media, but only explored amino acid analytes at low salt to analyte ratios and used high concentration eluents. This work utilized mixed-mode ion exchange solid phases, developing methods for elutions in low concentration solutions and evaluating polar analytes with a range of functionalities. Method development revealed the need for a pre-elution step in the process to decrease the elution volume required. Both anion and cation exchange media were evaluated for the capture of analytes from solutions that simulated Earth’s oceans. The Oasis MCX and Strata X-C cation exchange media provided the best results, with >90% retention of all analytes including amino acids, organic amines, nucleotides, peptides, and an oligonucleotide. These cation exchange media retained even anionic components, including glutamic acid and organic acids, with >90% efficiency. These analytes were released in the wash step, but salt ion removal was completed before release, allowing this technique to be used for desalting of these analytes. Extraction of a 14 component mixed analyte solution also showed retention of all analytes, with testing of analyte concentrations down to 100 nM in 35 g/L simulated ocean solution. Oasis MAX and Strata X-A anion exchange media did not retain glutamic acid, fumaric acid, or dipicolinic acid when the salt concentration was high; these anionic analytes were easily extracted from low salt solutions. The anion exchange media showed a range of functionality for extracting other analytes from the simulated ocean water, capturing >90% of tryptophan and phenylalanine, but retaining <50% of the valine and >20% of the glycine. Irradiation exposure, as a model for the solar irradiation expected during deployment to extraterrestrial locations, did not affect the performance of any ion exchange media tested. A reversed-phase SPE column was directly coupled to a cation exchange column, to investigate removal of nonpolar compounds that might bind to and block the mixed-mode ion exchange media.

Manganese oxide modified activated carbon (MOMAC) was synthesized as a novel in situ sediment and soil amendment for treatment of redox-sensitive contaminants, such as mercury (Hg), through buffering of reduction–oxidation (redox) potential and sorption. This study characterized MOMAC synthesis products at three different Mn concentrations on activated carbon (AC) surfaces and compared them with homogeneously precipitated Mn oxide (MnOx) and unmodified AC for properties influencing redox buffering and sorption capacity. Bulk spectroscopic analyses (XAS and XPS), XRD, and electron microscopy showed that homogeneous MnOx matched the local structure of vernadite (δ-Mn(IV)O2), while MOMAC formed aggregates on the AC surface composed mostly of vernadite with fractions of manganite (γ-Mn(III)OOH) (17–46%) and sorbed Mn(II) (11–21%). Higher bulk surface area and lower Mn average oxidation state were associated with MOMAC and are attributed to the reduction of Mn(IV) by Mn(II) adsorbed on AC or diffused into AC pores. Cation exchange reactions of Na+ and Ca2+ also contributed to Mn oxidation state changes by driving disproportionation of Mn(III) to Mn(II) and Mn(IV). In batch slurry experiments with and without Hg-contaminated sediment from Oak Ridge National Laboratory (TN, U.S.A.), addition of MOMAC and MnOx resulted in higher solution redox potential and lower pH compared to AC and no-amendment controls. MnOx poised solution redox at higher potential than MOMAC, but MOMAC was more effective at sorbing Hg released by the oxidation of sediment HgS(s), Hg0, and/or organic-associated Hg. By combining redox buffering with sorption, MOMAC is a promising in situ amendment that may efficiently target redox-sensitive contaminants in aquatic sediments.

Atmospheric brown carbon (BrC) can impact the radiative balance of the earth and form photooxidants. However, the light absorption and photochemical properties of BrC from different sources remain poorly understood. To address this gap, dilute water extracts of particulate matter (PM) samples collected at Davis, CA over one year were analyzed using high resolution aerosol mass spectrometry (HR-AMS) and UV–vis spectroscopy. Positive matrix factorization (PMF) on combined AMS and UV–vis data resolved five water-soluble organic aerosol (WSOA) factors with distinct mass spectra and UV–vis spectra: a fresh and an aged water-soluble biomass burning OA (WSBBOAfresh and WSBBOAaged) and three oxygenated OA (WSOOAs). WSBBOAfresh is the most light-absorbing, with a mass absorption coefficient (MAC365 nm) of 1.1 m2 g–1, while the WSOOAs are the least (MAC365 nm = 0.01–0.1 m2 g–1). These results, together with the high abundance of WSBBOAs (∼52% of the WSOA mass), indicate that biomass burning activities such as residential wood burning and wildfires are an important source of BrC in northern California. The concentrations of aqueous-phase photooxidants, i.e., hydroxyl radical (·OH), singlet molecular oxygen (1O2*), and oxidizing triplet excited states of organic carbon (3C*), were also measured in the PM extracts during illumination. Oxidant production potentials (PPOX) of the five WSOA factors were explored. The photoexcitation of BrC chromophores from BB emissions and in OOAs is a significant source of 1O2* and 3C*. By applying our PPOX values to archived AMS data at dozens of sites, we found that oxygenated organic species play an important role in photooxidant formation in atmospheric waters.

This study establishes a theoretical foundation for the oxidation pathway of monovalent thallium (Tl(I)) to trivalent thallium (Tl(III)) on birnessite, which is responsible for the over million-times enrichment of Tl in marine ferromanganese deposits over seawater concentration. Tl(I) oxidation occurs on vacant Mn(IV) sites located on the basal planes of the birnessite layers and on the edge sites, in agreement with experiment. Two Mn(IV) atoms are reduced to Mn(III) when Tl(I) gives up two electrons in two one-electron steps with formation of an intermediate Tl(II) inner-sphere complex. Tl(I) oxidation is facilitated at pH > 4–5 by the partial hydrolysis of the Tl(III) inner-sphere product on the reactive basal and edge sites. Oxidation by O2 is thermodynamically unfavorable. Although density functional theory has predictive power for an intermediate Tl(II) complex, it would be difficult to characterize it as Tl(II) is highly reactive and therefore probably short-lived. These findings provide the first atomic-scale description of the oxidation of Tl(I) by a manganese oxide and fill gaps in our understanding of global thallium sequestration in natural systems.