Frozen Clay Minerals as a Potential Source of Bioavailable Iron and Magnetite

Iron (Fe) is an essential micronutrient that affects biological production. Iron-containing clay minerals are an important source of bioavailable iron. However, the dissolution of iron-containing clay minerals at temperatures below the freezing point has not been investigated. Here, we demonstrate the enhanced reductive dissolution of iron from a clay mineral in ice in the presence of iodide (I-) as the electron donor. The accelerated production of dissolved iron in the frozen state was irreversible, and the freeze concentration effect was considered the main driving force. Furthermore, the formation of magnetite (Fe3O4) after the freezing process was observed using transmission electron microscopy analysis. Our results suggest a new mechanism of accelerated abiotic reduction of Fe(III) in clay minerals, which may release bioavailable iron, Fe(II), and reactive iodine species into the natural environment. We also propose a novel process for magnetite formation in ice. The freezing process can serve as a source of bioavailable iron or act as a sink, leading to the formation of magnetite.

Geochemical Characterization and Heavy Metal Sources in PM10 in Arequipa, Peru

Particulate matter smaller than 10 μm (PM10) is an important air pollutant that adversely affects human health by increasing the risk of respiratory and cardiovascular diseases. Recent studies reported multiple extreme PM10 levels at high altitude Peruvian cities, which resulted from a combination of high emissions and limited atmospheric circulation at high altitude. However, the emission sources of the PM10 still remain unclear. In this study, we collected PM10 samples from four sites (one industrial site, one urban site, and two rural sites) at the city of Arequipa, Peru, during the period of February 2018 to December 2018. To identify the origins of PM10 at each site and the spatial distribution of PM10 emission sources, we analyzed major and trace element concentrations of the PM10. Of the observed daily PM10 concentrations at Arequipa during our sampling period, 91% exceeded the World Health Organization (WHO) 24-h mean PM10 guideline value, suggesting the elevated PM10 strongly affected the air quality at Arequipa. The concentrations of major elements, Na, K, Mg, Ca, Fe, and Al, were high and showed little variation, suggesting that mineral dust was a major component of the PM10 at all the sites. Some trace elements, such as Mn and Mo, originated from the mineral dust, while other trace elements, including Pb, Sr, Cu, Ba, Ni, As and V, were from additional anthropogenic sources. The industrial activities at Rio Seco, the industrial site, contributed to significant Pb, Cu, and possibly Sr emissions. At two rural sites, Tingo Grande and Yarabamba, strong Cu emissions were observed, which were likely associated with mining activities. Ni, V, and As were attributed to fossil fuel combustion emissions, which were strongest at the Avenida Independencia urban site. Elevated Ba and Cu concentrations were also observed at the urban site, which were likely caused by heavy traffic in the city and vehicle brake wear emissions.

Influence of chemical aging on physico-chemical properties of mineral dust particles: A case study of 2016 dust storms over Delhi

The physico-chemical properties of dust particles collected During Dust Storm (DDS) and After Dust Storm (ADS) events were studied using Scanning Electron Microscope coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS), X-ray Fluorescence Spectroscopy (XRF) and X-ray Photoelectron Spectroscopy (XPS). Morphological and compositional change in dust particles were observed as they react with the anthropogenic pollutants present in the urban environment. The calcite rich particles were observed to transform into calcium chloride, calcium nitrate, and calcium sulfate on reacting with the chlorides, nitrates, and sulfates present in the urban atmosphere. The frequency distributions of Aspect Ratio (AR) for the DDS and ADS particles were observed to be bimodal (mode peaks at 1.2 and 1.5) and monomodal (mode peak at 1.1), respectively. The highly irregular shaped solid dust particles were observed to transform into nearly spherical semisolid particles in the urban environment. XPS analysis confirms the high concentration of oxides, nitrates, and chlorides at the surface of ADS samples which show the signatures of mineral dust particles aging. Species with a high value of imaginary part of refractive index (like Cr metal, Fe metal, Cr₂O_3, FeO, Fe₂O₃) were observed at the surface of dust particles. At 550 nm wavelength, the light-absorbing potential of the observed species along with black carbon (BC) was found to vary in the order; Cr metal > Fe metal > Cr₂O₃> FeO > BC > Fe₂O₃> FeOOH. The presence of the aforementioned species on the surface of ADS particles will tremendously affect the particle optical and radiative properties compared to that of DDS particles. The present work could reduce the uncertainty in the radiation budget estimations of mineral dust and assessment of their climatic impacts over Delhi.

Phenomenology and geographical gradients of atmospheric deposition in southwestern Europe: Results from a multi-site monitoring network

This article presents the results of atmospheric deposition from a 15-sites network which cover remote, agricultural, urban and industrial areas in the Iberian Peninsula and the Balearic Islands, with the aim of exploring geographical, climatic and natural vs anthropogenic gradients. Annual average fluxes of global deposition, discriminating insoluble (3,5-20,7 g m^-2 year^-1) and soluble-inorganic (7,1-45,5 g m^-2 year^-1) aerosols are discussed, seasonal patterns are regarded, and an attempt to estimate the impact of the main sources is presented. The wide range of atmospheric deposition fluxes (DF) regarding soluble (DF_SOL) and insoluble (DF_INS) has been investigated taking into consideration the contribution from nearby to long-distance sources, such as African dust, or regional-to-nearby ones, which include agricultural dust in the Ebro Valley, industrial emissions at different parts, urban dust at all cities, or saline dust resuspension from a dissicated lake bed. DF_SOL is made up of marine aerosols, prevailing in coastal areas, with few exceptions in the Ebro Valley; nitrogen-species, homogeneously distributed across the network, with few exceptions due to agricultural sources; mineral dust, enhanced in the Ebro Valley owing to regional and agricultural emissions; and phospathe, displaying comparable values to other studies in general, but three hotspots at regional background environments have been identified. DF_INS particles followed the aridity pattern, especially where anthropogenic emissions take place. Our estimates indicate that the regional dust to DF_INS in the Ebro Valley represented 23-30%, overpassing 50% at intensive agricultural areas. Similarly, urban-metropolitan contributions accounted for 37-45% at the four cities, and 55% at the industrial one. African dust deposition was enhanced in the Central Pyrenees (75-80%) as a result of the magnification of atmospheric washout processes, and in south-eastern Iberia (69%) owing to the higher frequency of dust outbreaks.

Photo-oxidation of particle phase iron species dominates the generation of reactive oxygen species in secondary aerosol

This study presents an experimental investigation on the photochemical transformation of iron species in aerosol including dissolution of insoluble iron species into soluble fraction, and soluble ferric oxidation to ferrous form. This process has significantly contributed to the aerosol oxidative potential in generation of reactive oxygen species (ROS). We conducted both laboratory experiment of UV irradiation and real world solar irradiation on large variation of aerosol samples for the characterization of iron speciation in insoluble and soluble fractions to investigate their transformation under photooxidation process. The results showed that the real world solar irradiation significantly increased the soluble Fe(II) fraction, and this is corroborated by laboratory simulation of UV irradiation showing increasing soluble Fe(II) fraction with elongating aging time. The results further exhibited that the dissolution of iron component into soluble fraction was a dominant process, followed by the conversion of soluble ferric to ferrous ions. Further, the study confirmed that the oxidative potential of particulate matter (PM) is attributed dominantly to the abundance of transition metals, i.e. Fe, and the incremental ROS generation after photochemical process is attributed largely to the transformation of solid phase iron species to soluble Fe(II). The results suggest that transition metals, for example by iron in this study, play an important role in secondary aerosol process.

Influence of Atmospheric Processes on the Solubility and Composition of Iron in Saharan Dust

Aerosol iron was examined in Saharan dust plumes using a combination of iron near-edge X-ray absorption spectroscopy and wet-chemical techniques. Aerosol samples were collected at three sites located in the Mediterranean, the Atlantic, and Bermuda to characterize iron at different atmospheric transport lengths and time scales. Iron(III) oxides were a component of aerosols at all sampling sites and dominated the aerosol iron in Mediterranean samples. In Atlantic samples, iron(II and III) sulfate, iron(III) phosphate, and iron(II) silicates were also contributors to aerosol composition. With increased atmospheric transport time, iron(II) sulfates are found to become more abundant, aerosol iron oxidation state became more reduced, and aerosol acidity increased. Atmospheric processing including acidic reactions and photoreduction likely influence the form of iron minerals and oxidation state in Saharan dust aerosols and contribute to increases in aerosol-iron solubility.

Increase in dust storm related PM10 concentrations: A time series analysis of 2001-2015

Over the last decades, changes in dust storms characteristics have been observed in different parts of the world. The changing frequency of dust storms in the southeastern Mediterranean has led to growing concern regarding atmospheric PM10 levels. A classic time series additive model was used in order to describe and evaluate the changes in PM10 concentrations during dust storm days in different cities in Israel, which is located at the margins of the global dust belt. The analysis revealed variations in the number of dust events and PM10 concentrations during 2001-2015. A significant increase in PM10 concentrations was identified since 2009 in the arid city of Beer Sheva, southern Israel. Average PM10 concentrations during dust days before 2009 were 406, 312, and 364 μg m(-3) (median 337, 269,302) for Beer Sheva, Rehovot (central Israel) and Modi'in (eastern Israel), respectively. After 2009 the average concentrations in these cities during dust storms were 536, 466, and 428 μg m(-3) (median 382, 335, 338), respectively. Regression analysis revealed associations between PM10 variations and seasonality, wind speed, as well as relative humidity. The trends and periodicity are stronger in the southern part of Israel, where higher PM10 concentrations are found. Since 2009 dust events became more extreme with much higher daily and hourly levels. The findings demonstrate that in the arid area variations of dust storms can be quantified easier through PM10 levels over a relatively short time scale of several years.

Rapid and gradual modes of aerosol trace metal dissolution in seawater

Atmospheric deposition is a major source of trace metals in marine surface waters and supplies vital micronutrients to phytoplankton, yet measured aerosol trace metal solubility values are operationally defined, and there are relatively few multi-element studies on aerosol-metal solubility in seawater. Here we measure the solubility of aluminum (Al), cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) from natural aerosol samples in seawater over a 7 days period to (1) evaluate the role of extraction time in trace metal dissolution behavior and (2) explore how the individual dissolution patterns could influence biota. Dissolution behavior occurs over a continuum ranging from rapid dissolution, in which the majority of soluble metal dissolved immediately upon seawater exposure (Cd and Co in our samples), to gradual dissolution, where metals dissolved slowly over time (Zn, Mn, Cu, and Al in our samples). Additionally, dissolution affected by interactions with particles was observed in which a decline in soluble metal concentration over time occurred (Fe and Pb in our samples). Natural variability in aerosol chemistry between samples can cause metals to display different dissolution kinetics in different samples, and this was particularly evident for Ni, for which samples showed a broad range of dissolution rates. The elemental molar ratio of metals in the bulk aerosols was 23,189Fe: 22,651Al: 445Mn: 348Zn: 71Cu: 48Ni: 23Pb: 9Co: 1Cd, whereas the seawater soluble molar ratio after 7 days of leaching was 11Fe: 620Al: 205Mn: 240Zn: 20Cu: 14Ni: 9Pb: 2Co: 1Cd. The different kinetics and ratios of aerosol metal dissolution have implications for phytoplankton nutrition, and highlight the need for unified extraction protocols that simulate aerosol metal dissolution in the surface ocean.

Atmospheric transport and deposition of mineral dust to the ocean: implications for research needs

This paper reviews our knowledge of the measurement and modeling of mineral dust emissions to the atmosphere, its transport and deposition to the ocean, the release of iron from the dust into seawater, and the possible impact of that nutrient on marine biogeochemistry and climate. Of particular concern is our poor understanding of the mechanisms and quantities of dust deposition as well as the extent of iron solubilization from the dust once it enters the ocean. Model estimates of dust deposition in remote oceanic regions vary by more than a factor of 10. The fraction of the iron in dust that is available for use by marine phytoplankton is still highly uncertain. There is an urgent need for a long-term marine atmospheric surface measurement network, spread across all oceans. Because the southern ocean is characterized by large areas with high nitrate but low chlorophyll surface concentrations, that region is particularly sensitive to the input of dust and iron. Data from this region would be valuable, particularly at sites downwind from known dust source areas in South America, Australia, and South Africa. Coordinated field experiments involving both atmospheric and marine measurements are recommended to address the complex and interlinked processes and role of dust/Fe fertilization on marine biogeochemistry and climate.

Coal fly ash as a source of iron in atmospheric dust

Anthropogenic coal fly ash (FA) aerosol may represent a significant source of bioavailable iron in the open ocean. Few measurements have been made that compare the solubility of atmospheric iron from anthropogenic aerosols and other sources. We report here an investigation of iron dissolution for three FA samples in acidic aqueous solutions and compare the solubilities with that of Arizona test dust (AZTD), a reference material for mineral dust. The effects of pH, simulated cloud processing, and solar radiation on iron solubility have been explored. Similar to previously reported results on mineral dust, iron in aluminosilicate phases provides the predominant component of dissolved iron. Iron solubility of FA is substantially higher than of the crystalline minerals comprising AZTD. Simulated atmospheric processing elevates iron solubility due to significant changes in the morphology of aluminosilicate glass, a dominant material in FA particles. Iron is continuously released into the aqueous solution as FA particles break up into smaller fragments. These results suggest that the assessment of dissolved atmospheric iron deposition fluxes and their effect on the biogeochemistry at the ocean surface should be constrained by the source, environmental pH, iron speciation, and solar radiation.

Formation of iron nanoparticles and increase in iron reactivity in mineral dust during simulated cloud processing

The formation of iron (Fe) nanoperticles and increase in Fe reactivity in mineral dust during simulated cloud processing was investigated using high-resolution microscopy and chemical extraction methods. Cloud processing of dust was experimentally simulated via an alternation of acidic (pH 2) and circumneutral conditions (pH 5-6) over periods of 24 h each on presieved (<20 microm) Saharan soil and goethite suspensions. Microscopic analyses of the processed soil and goethite samples reveal the neo-formation of Fe-rich nanoparticle aggregates, which were not found initially. Similar Fe-rich nanoparticles were also observed in wet-deposited Saharen dusts from the western Mediterranean but not in dry-deposited dust from the eastern Mediterranean. Sequential Fe extraction of the soil samples indicated an increase in the proportion of chemically reactive Fe extractable by an ascorbate solution after simulated cloud processing. In addition, the sequential extractions on the Mediterranean dust samples revealed a higher content of reactive Fe in the wet-deposited dust compared to that of the dry-deposited dust These results suggestthat large variations of pH commonly reported in aerosol and cloud waters can trigger neo-formation of nanosize Fe particles and an increase in Fe reactivity in the dust

Atmospheric iron deposition: global distribution, variability, and human perturbations

Atmospheric inputs of iron to the open ocean are hypothesized to modulate ocean biogeochemistry. This review presents an integration of available observations of atmospheric iron and iron deposition, and also covers bioavailable iron distributions. Methods for estimating temporal variability in ocean deposition over the recent past are reviewed. Desert dust iron is estimated to represent 95% of the global atmospheric iron cycle, and combustion sources of iron are responsible for the remaining 5%. Humans may be significantly perturbing desert dust (up to 50%). The sources of bioavailable iron are less well understood than those of iron, partly because we do not know what speciation of the iron is bioavailable. Bioavailable iron can derive from atmospheric processing of relatively insoluble desert dust iron or from direct emissions of soluble iron from combustion sources. These results imply that humans could be substantially impacting iron and bioavailable iron deposition to ocean regions, but there are large uncertainties in our understanding.

FTIR Spectroscopy Combined with Isotope Labeling and Quantum Chemical Calculations to Investigate Adsorbed Bicarbonate Formation Following Reaction of Carbon Dioxide with Surface Hydroxyl Groups on Fe2O3 and Al2O3

FTIR spectroscopy combined with isotope labeling experiments and quantum chemical calculations is used to investigate the adsorption of carbon dioxide on hydroxylated metal oxide surfaces. In particular, transmission FTIR spectra following CO2 adsorption on hydroxylated nanoparticulate Fe2O3, alpha-Al2O3, and gamma-Al2O3 particles at 296 K are reported. As expected, reaction of CO2 with these surfaces results in the formation of adsorbed bicarbonate and carbonate. In this study, the vibrational spectrum of the bicarbonate product is analyzed in detail through the use of isotope labeling experiments and quantum chemical calculations. The experimental and calculated vibrational frequencies of adsorbed HC16O3-, DC16O3-, HC18O3-, HC16O18O2-, and HC18O16O2- indicate that bicarbonate bonds to the surface in a bridged structure. There is some evidence from the mixed isotope experiments that following initial nucleophilic attack of OH, the formation of the final bicarbonate structure involves a proton transfer. On the basis of energetic considerations, the proton transfer mechanism most likely occurs through an intermolecular process involving either coadsorbed hydroxyl groups and/or carbonate.

Surface Reactions of Carbon Dioxide at the Adsorbed Water−Iron Oxide Interface

Despite the fact that carbon dioxide is an abundant atmospheric gas with profound environmental implications, there is little information on the reaction of carbon dioxide at the adsorbed water-oxide interface. In this study, the chemistry of carbon dioxide at the adsorbed water-iron oxide interface is investigated with FTIR spectroscopy. As shown here, the thin water layer on the iron oxide surface plays an important role in the surface chemistry of carbon dioxide. In particular, adsorbed water enhances CO(2) uptake, undergoes isotope exchange with CO(2) in O(18)-labeled experiments, and influences the chemical nature of the predominant adsorbed product on the surface from bicarbonate to carbonate. The resultant thin water film is acidic in nature from the reaction of CO(2). The IR spectrum recorded of adsorbed carbonate at the adsorbed water-iron oxide interface is remarkably similar to that at the bulk liquid water-iron oxide interface. Since reactions in thin water films estimated to be approximately 2 layers will play a role in a number of environmental processes, it is essential to understand the chemistry of these "wet" interfaces with atmospheric gases.

Laboratory studies of potential mechanisms of renoxification of tropospheric nitric acid

Laboratory studies of the heterogeneous reactions between HNO3 in thin water films on silica surfaces and gaseous NO, CO, CH4, and SO2, proposed as potential "renoxification" mechanisms in the atmosphere, are reported. Transmission FTIR was used to monitor reactants and products on the silica surface and in the gas phase as a function of time. No reaction of CO, CH4, or SO2 was observed; upper limits to the reaction probabilities (gamma(rxn)) are < or = 10(-10) for CO and SO2 and < or = 10(-12) for CH4. However, the reaction of HNO3 with NO does occur with a lower limit for the reaction probability of gammaNO > or = (6 +/- 2) x 10(-9) (2s). The experimental evidence shows that the chemistry is insensitive to whether the substrate is pure silica or borosilicate glass. Nitric acid in its molecular form, and not the nitrate anion form, was shown to be the reactive species, and NH4NO3 was shown not to react with NO. The HNO3-NO reaction could be a significant means of renoxification of nitric acid on the surfaces of buildings and soils in the boundary layer of polluted urban atmospheres. This chemistry may help to resolve some discrepancies between model-predicted ozone and field observations in polluted urban atmospheres.

Long-range transport of mineral dust in the global atmosphere: impact of African dust on the environment of the southeastern United States

Soil dust is a major constituent of airborne particles in the global atmosphere. Dust plumes frequently cover huge areas of the earth; they are one of the most prominent and commonly visible features in satellite imagery. Dust is believed to play a role in many biogeochemical processes, but the importance of dust in these processes is not well understood because of the dearth of information about the global distribution of dust and its physical, chemical, and mineralogical properties. This paper describes some features of the large-scale distribution of dust and identifies some of the geological characteristics of important source areas. The transport of dust from North Africa is presented as an example of possible long-range dust effects, and the impact of African dust on environmental processes in the western North Atlantic and the southeastern United States is assessed. Dust transported over long distances usually has a mass median diameter <10 microm. Small wind-borne soil particles show signs of extensive weathering; consequently, the physical and chemical properties of the particles will greatly depend on the weathering history in the source region and on the subsequent modifications that occur during transit in the atmosphere (typically a period of a week or more). To fully understand the role of dust in the environment and in human health, mineralogists will have to work closely with scientists in other disciplines to characterize the properties of mineral particles as an ensemble and as individual particles especially with regard to surface characteristics.