Atmospheric Processing of Volcanic Glass: Effects ... - ACS Publications

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Atmospheric Processing of Volcanic Glass: Effects on Iron Solubility and Redox Speciation Elena C. Maters,*,† Pierre Delmelle,† and Steeve Bonneville‡ †

Earth and Life Institute, Environmental Sciences, Université catholique de Louvain, Croix du Sud 2, bte L7.05.10, B-1348 Louvain-la-Neuve, Belgium ‡ Biogéochimie et Modélisation du Système Terre, Département Géosciences, Environnement et Société, Université Libre de Bruxelles, Avenue Franklin Roosevelt 50, CP160/02, B-1050 Brussels, Belgium S Supporting Information *

ABSTRACT: Volcanic ash from explosive eruptions can provide iron (Fe) to oceanic regions where this micronutrient limits primary production. Controls on the soluble Fe fraction in ash remain poorly understood but Fe solubility is likely influenced during atmospheric transport by condensationevaporation cycles which induce large pH fluctuations. Using glass powder as surrogate for ash, we experimentally simulate its atmospheric processing via cycles of pH 2 and 5 exposure. Glass fractional Fe solubility (maximum 0.4%) is governed by the pH 2 exposure duration rather than by the pH fluctuations, however; pH 5 exposure induces precipitation of Fe-bearing nanoparticles which (re)dissolve at pH 2. Glass leaching/ dissolution release Fe(II) and Fe(III) which are differentially affected by changes in pH; the average dissolved Fe(II)/Fetot ratio is ∼0.09 at pH 2 versus ∼0.18 at pH 5. Iron release at pH 2 from glass with a relatively high bulk Fe(II)/Fetot ratio (0.5), limited aqueous Fe(II) oxidation at pH 5, and possibly glassmediated aqueous Fe(III) reduction may render atmospherically processed ash a significant source of Fe(II) for phytoplankton. By providing new insight into the form(s) of Fe associated with ash as wet aerosol versus cloud droplet, we improve knowledge of atmospheric controls on volcanogenic Fe delivery to the ocean.

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INTRODUCTION In vast areas of the world’s oceans (>30%), marine primary production (MPP) is limited by an insufficient supply of bioavailable iron (Fe),1−3 an essential micronutrient for processes including photosynthesis, respiration and nitrogen fixation.4 Continental dusts such as soil particles, glacial flour, and fly ash play a key role in alleviating the Fe deficiency in these waters upon atmospheric deposition.5,6 Since MPP represents a control on carbon dioxide exchange between the atmosphere and the ocean, and thereby contributes to climate regulation over millennial time scales,7,8 considerable effort has been dedicated to quantifying Fe input to seawater by continental dust deposition.5,9,10 Volcanic ash is increasingly recognized as an Fe source to the surface ocean.11 The millenial Fe flux to the Pacific Ocean from these aluminosilicate particles produced by explosive eruptions is estimated to be comparable to that from mineral dust from arid and semiarid regions,12 with millimeter to meter scale ash layers in deep ocean sediment evidencing ash input to the ocean throughout Earth’s history.13 Further, geochemical analyses of ocean sediment and ice sheet drill cores point to a relationship between periods of intense volcanism and global cooling at several points in time possibly driven by increases in MPP induced by ash fallout.14,15 Importantly, volcanic impacts © 2016 American Chemical Society

on climate via perturbations to the carbon cycle, including by ocean Fe fertilization by ash, are increasingly evoked14−16 alongside the more established effect on climate of radiative forcing by volcanogenic sulfate aerosols.17 Recent field and laboratory results confirm that ash deposition can modify seawater biogeochemistry by releasing Fe.11,18−22 A wide range of Fe release values from volcanic ash in (sea)water (18−37 000 nmol Fe g−1 ash in 1 h) has previously been reported,12,18,23 and controls on the fraction of Fe in ash that can be supplied to the ocean remain poorly understood.11 Olgun et al.12 found no correlation between ash Fe content (∼1 to 8 at. %) and Fe solubility for over 40 samples from different eruptions. Ayris and Delmelle24 highlighted the possibility of various volcanic and atmospheric controls on ash Fe solubility but these have yet to be fully elucidated; the former have only recently been explored by thermodynamic modeling of ash-gas interactions at high temperature25 while the latter still await investigation. Ash is likely subjected to physicochemical processes during long-range transport similar Received: Revised: Accepted: Published: 5033

December 22, 2015 April 25, 2016 April 29, 2016 April 29, 2016 DOI: 10.1021/acs.est.5b06281 Environ. Sci. Technol. 2016, 50, 5033−5040

Article

Environmental Science & Technology

oxidation of volcanic SO2 and typically dominates the acid aerosol load in an atmospheric ash cloud.17 Twelve hours of exposure at each pH value, mimicking the wet aerosol (pH 2) and cloud droplet (pH 5) phases, was chosen based on the estimated lifetime of aerosols and clouds in the atmosphere.46 Here the terms “wet aerosol” and “cloud droplet” are used to designate two potential pH conditions and not to encompass all physicochemical conditions corresponding to these phases in the atmosphere. The solution pH was raised by addition of 6 and 1 M (dropwise > pH 3) NH4OH and lowered by addition of 3.6 M H2SO4. A control experiment exposing the glass powder to a H2SO4 solution at constant pH 2 for 36 h was also performed as a point of comparison. Details of the experimental protocol are given in the SI. Briefly, triplicate experiments were conducted in polypropylene batch reactors covered with parafilm at 25 °C in the dark under constant gentle stirring at a solid-to-solution ratio of 1 g L−1. This value is intermediate within the very wide range of particle loadings estimated in atmospheric aerosols and clouds.47 During the experiments, subsamples of the batch solution were collected at various time intervals (minute to hour scale), filtered through 0.2 μm cellulose acetate membrane filters, and stored in capped plastic tubes in the dark at ∼4 °C until dissolved Fe analyses (within 2 days). Subsamples at pH 5 were acidified (to ∼ pH 2) with 1.8 M H2SO4 immediately after filtration to preserve the dissolved Fe and Fe(II)/Fe(III) ratio. At the end of the experiments, the glass remaining in solution was recovered by vacuum filtration, rinsed with ultrapure water and dried in air for subsequent spectroscopic and/or microscopic analyses. Dissolved Iron Analyses. Total Fe (Fetot = Fe(II) + Fe(III)) and Fe(II) concentrations in solution subsamples were determined colorimetrically by the Ferrozine method48 using a Genesys 10S UV−vis spectrophotometer and a 1 cm cell path length. The Ferrozine, buffer and reducing reagents were prepared as described by Viollier et al.49 Standard solutions ranging from 0 to 5000 ppb of Fe(II) were prepared from (NH4)2Fe(SO4)2·6H2O dissolved in pH 2 H2SO4. The detection limit was 2.5 ppb (∼0.05 μM). Measurements were performed within minutes of Ferrozine and buffer reagent addition and under inactinic illumination to minimize the potential for (photo)oxidation of Fe(II).50 The Fe(III) concentration was calculated from the difference between measured Fetot and Fe(II) concentrations.

to those known to enhance Fe solubility in airborne mineral dust.26−28 In particular, exposure to water condensationevaporation cycles can significantly modify Fe partitioning between dissolved and particulate phases, with large pH fluctuations in the solution surrounding solid particles (i.e., a highly acidic film in the “wet aerosol” phase outside of clouds versus a less acidic droplet in the “cloud droplet” phase within clouds29) suggested to be a key aspect of atmospheric processing.27,29−31 Condensation-evaporation cycles probably affect volcanic ash which is coemitted with acidic gases and condensates (e.g., H2SO4, HCl, possibly HF) during eruption and whose hygroscopic nature promotes water adsorption.32,33 Atmospheric processing of ash has not previously been studied, yet knowledge of what governs Fe solubility in ash during its lifecycle from magma source to ocean sink is essential for assessing its capacity to deliver bioavailable Fe to the surface ocean.11 Consideration of Fe redox speciation is also important given that ferrous (II) Fe is much more soluble than ferric (III) Fe and thus may be regarded as the form most readily bioavailable in seawater.2,34−36 Dissolved Fe redox speciation has seldom been reported in previous atmospheric processing studies on continental dusts30,37,38 and has never been measured in volcanic ash leachates. Here we investigate experimentally for the first time the influence of pH variations on Fe(II) and Fe(III) mobilization from a powdered glass as a proxy for volcanic ash transported long distances to the ocean. Specifically, a time series of dissolved Fe(II) and Fe(III) concentrations are measured in batch reactors containing glass suspensions in H2SO4 solution either cycled between pH 2 and pH 5 or kept constant at pH 2 as a point of comparison. In addition, we apply bulk and surface analytical and geochemical modeling techniques to elucidate changes in Fe speciation within the solid and aqueous phases induced by the simulated atmospheric processing.

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MATERIALS AND METHODS Volcanic Glass Sample. A glass powder of andesitic composition (SiAl0.34Fe0.09Mg0.13Ca0.13Na0.13K0.03Ti0.01O3) was used in the present study as a proxy for the primary constituent of volcanic ash.39 Moreover, ash delivered to the open ocean following long-range transport is likely to be enriched in glassy fragments due to earlier gravitational settling of crystalline particles.40 In addition, the largest and most explosive eruptions correspond to violent caldera-forming ignimbrite events which generate ash clouds dominated by the glassy component and which are most susceptible to lead to ash deposition far from source.15,41 Details on synthesis and characterization of the glass are provided in the Supporting Information (SI). The particle size distribution is broadly comparable to that reported for natural ash from various explosive eruptions,42 with particles spanning 100 μm in diameter capable of being transported 100s to 1000s of km from the volcano before gravitational settling.42−44 Batch Dissolution Experiments. The atmospheric processing experiment involved exposing the glass powder to a H2SO4 solution subjected to changing acidity via three cycles of pH 2 and pH 5 over a 72 h period. This approach has previously been applied to simulate atmospheric processing of various Fe-bearing dusts.27,29−31,45 The preference for using H2SO4 over other atmospheric acidic compounds (e.g., HNO3, organic acids) that may also interact with ash particles is justified on the basis that H2SO4 is readily produced by

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RESULTS Total Fe Trends. Dissolved Fetot concentrations (in μmol g−1 of glass) in H2SO4 solution cycled between pH 2 and pH 5 over 72 h, and the corresponding measured pH values (±0.1 pH unit), are shown in Figure 1. During the first pH 2 phase, dissolved Fetot concentrations increased rapidly within 1 h and attained a maximum value of 5.1 ± 0.2 μmol g−1 after 12 h. During the first pH 5 phase, dissolved Fetot concentrations declined steeply within 1 min and attained a minimum value of 1.1 ± 0.2 μmol g−1 after 12 h (at 24 h total). This overall pattern was reproduced during the subsequent two cycles of pH change. Lowering the solution pH for the second and third pH 2 phases induced a steep increase in dissolved Fe tot concentrations to 3.2 ± 0.1 and 3.4 ± 0.1 μmol g−1 after 1 min, respectively, exceeding that measured after 1 min of exposure for the first pH 2 phase (1.0 ± 0.1 μmol g−1). In addition, dissolved Fetot concentrations increased overall during the second and third pH 2 phases, reaching values of 5.8 ± 0.2 and 6.4 ± 0.1 μmol g−1 after each 12 h period (at 36 and 60 h 5034

DOI: 10.1021/acs.est.5b06281 Environ. Sci. Technol. 2016, 50, 5033−5040

Article

Environmental Science & Technology

Figure 1. (A) Solution pH values and (B) corresponding fractional Fetot solubility and dissolved Fetot, Fe(II) and Fe(III) concentrations (C inset) measured over 72 h in H2SO4 solution cycled between pH 2 and pH 5 containing powdered glass at a 1 g L−1 solid-to-solution ratio. Values represent triplicate mean with error bars equivalent to the standard deviation of the mean.

total), respectively. In contrast, raising the solution pH for the second and third pH 5 phases induced an almost immediate decrease in dissolved Fetot concentrations, reaching values of 0.8 ± 0.1 and 0.7 ± 0.1 μmol g−1 after each 12 h period (at 48 and 72 h total), respectively. Dissolved Fetot concentrations (in μmol g−1 of glass) in H2SO4 solution at pH 2 over 36 h, representing the control experiment, are shown in Figure 2a. Also incorporated in this plot for comparison are the dissolved Fetot concentrations from the pH 2 phases of the cycling experiment merged together, that is, with the pH 5 phases removed. The pattern of dissolved Fetot concentrations during the control and cycling experiments are remarkably similar, with a rapid initial increase within the first hour and a more gradual increase thereafter to final values of 6.2 ± 0.1 and 6.4 ± 0.1 μmol g−1 after 36 h, respectively. Fe(II) and Fe(III) Trends. Dissolved Fetot, Fe(II) and Fe(III) concentrations (in μmol g−1 of glass) in H2SO4 solution cycled between pH 2 and pH 5 over 72 h are shown in Figure 1. Dissolved Fe(III) concentrations followed a cyclic pattern as noted above for dissolved Fetot concentrations. Dissolved Fe(II) concentrations increased to 1.5 ± 0.2 μmol g−1 after 12 h during the first pH 2 phase, dropping to