The biogeochemical cycling of arsenic in a changing climate

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University of Delaware
This study focuses on the impact of a changing climate on the biogeochemical cycling of Arsenic (As) in wetlands and high manganese (Mn) soils. The aims were to assess the stability of As in wetlands impacted by sea level rise and to determine the physical and chemical properties and reaction kinetics of As oxidation under different temperature and pH regimes in high manganese soils from Graskop, South Africa. ☐ Coastal wetlands sequester large amounts of environmental contaminants, in their sediments and biomass, including carcinogenic As. However, sea level rise and saltwater inundation will potentially destabilize marsh ecosystems and can cause previously sorbed contaminants to be released. Marsh plants play a critical role in the fate and transport of As in wetlands through direct absorption into root tissue, sorption to iron plaques surrounding the roots, and incorporation and secretion of As in aboveground biomass. These relationships were tested on common reed, Phragmites australis, which was raised in a controlled growth chamber and exposed to saline conditions ranging from freshwater to seawater. The experiments were first conducted hydroponically and then in natural marsh soil. Porewater samplers were used to determine the redox state, pH, and As concentration in the soil experiments. Arsenic in the root plaques was analyzed using a modified dithionite citrate bicarbonate (DCB) method and the total As in plant biomass was measured with inductively coupled plasma optical emission spectrometry (ICP-OES). The distribution/association of As in the roots collected from these experiments was also investigated using synchrotron-based micro-X-ray absorption near edge structure (micro-XANES) and micro-X-ray fluorescence (micro-XRF) spectroscopy, respectively, at the National Synchrotron Light Source (NSLS), Argonne National Laboratory, and SLAC National Accelerator Laboratory. Roots and outer plaques were analyzed whole and as cryosectioned 30 μm cross-sections to spatially differentiate the oxidation state of As sorbed on the plaques and absorbed in different regions of the roots. Synchrotron analysis showed changes in As speciation with increased salinity, with more As-sulfide compounds found in a saline environments. The XRF maps of common reed roots shows the distribution and uptake of As(III) and As(V) in plant roots. These maps indicate As(III) moves in through the root tip and then is complexed by the plant to become a more stable and less toxic As compound. The saltwater intrusion potted study has shown how wet and dry cycles can release Fe, Mn, and As into the porewater of wetland sediments. This study provides key insight to understand the impact of sea level rise on the cycling, mobility, and speciation of redox sensitive As and other contaminants in wetlands. ☐ Similarly, Mn-oxides play a critical role in regulating As oxidation and reduction reactions in the environment. Manganese-oxides govern many geochemical reactions due to their abundance and high reactivity. Despite their importance in cycling redox sensitive compounds in natural systems, much remains unknown about the reactivity of Mn-oxides formed under environmental conditions and they may be altered by changes in climate. To study how these Mn-oxides react, soils were collected from Graskop, South Africa. Three soil profiles were excavated with a range of Mn concentrations. Each profile was further separated based on horizons, some containing over 20% Mn. Manganese nodules are ubiquitous in these soils. The soil in each horizon was thoroughly characterized to determine chemical and physical properties, including cation exchange capacity (CEC), point of zero charge (PZC), and Brunauer–Emmett–Teller (BET) surface area. X-ray powder diffraction (XRD) was used to characterize the mineralogy of the crystalline material found in the clay fraction. Scanning electron microscopy (SEM) images were taken of each soil to characterize the particle distribution and mineralogy. A series of batch reactions were used to determine the capacity of these soils and nodules to oxidize As(III) into As(V) under a variety of factors that may be impacted by an altered climate. The conditions of the reaction were varied from pH 4.5, 7.2, and 9.0 and temperatures ranged from 4.5 ˚C, 23 ˚C, and 40 ˚C to elucidate how differing pH and temperature influenced the oxidation reaction. Aliquots collected from these expePriments were analyzed by liquid chromatography-inductively coupled plasma-mass spectrometry (LC-ICP-MS). Solid samples from the reactions were taken to NSLS II and analyzed on beamline 4-BM (XFM) to determine the changes in manganese oxidation state after reacting with arsenite under varying conditions. X-ray fluorescence maps were collected on thin sections of these soils to analyze the distribution and speciation of Mn and Fe in natural soils and nodules. The As(III) batch reactions have demonstrated the oxidative potential of natural and synthetic Mn-oxides. The kinetics changed dramatically under differing pH and temperatures, which altered the oxides’ reactivity and potential to sorb As(V). Increasing the temperature also increased the reaction rates, releasing excess As(V) which was not initially sorbed. The Mn soils were least reactive at a circumneutral pH and were able to oxidize As(III) more rapidly at pH 4.5 and 9.0. The naturally formed Mn soils reacted similarly to synthetic Mn-oxides, indicating studies conducted with laboratory-made Mn-oxides are applicable to Mn reactivity in the environment. These experiments add to understanding the role of Mn-oxides in controlling redox sensitive reactions in the present and future environment.