The impact of redox potential and salinity on arsenic cycling and mobility in iron oxide systems
Date
2021
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Sea level rise (SLR) as a result of climate change is affecting the world’s coastlines, especially in the mid-Atlantic region of the US. Impending SLR will change current coastal hydrologic regimes and cause an increase in salinity and flooding, which can lead to major variations in metal/metalloid cycling. Arsenic (As) is a ubiquitous element that can accumulate in dangerous concentrations in coastal soils due to anthropogenic or natural processes. Arsenic is of particular interest due to its carcinogenic nature as well as its sensitivity to changes in redox potential, which generally cause it to become more mobile as redox conditions reduce. Currently, there are limited data on the geochemical controls governing As cycling in variable saline and fresh water environments. The aim of this study was to simulate SLR in a controlled laboratory setting and investigate its effects on arsenic sorption and desorption to iron oxides, which are commonly found all throughout the world. Additionally, this study investigated the impact of seawater and variable redox conditions on As geochemistry of an arsenic-ferrihydrite coprecipitate. ☐ To help address this knowledge gap, arsenate was sorbed to goethite and ferrihydrite in artificial river water (ARW) and artificial seawater (ASW) systems. Arsenate sorption isotherms revealed that slightly more As sorbed to both goethite and ferrihydrite in ASW compared to ARW. Arsenate pH envelopes in ARW revealed that as the pH increases, As sorption to goethite and ferrihydrite decreases. Since goethite and ferrihydrite have a variable surface charge, as the pH increases, the mineral surfaces become increasingly negative and repel the negatively charged arsenate oxyanion. In ASW, however, it was shown that at pH > 8, there was more As sorption than in ARW in both goethite and ferrihydrite. The high concentration of divalent cations, such as Mg2+ and Ca2+, in seawater sorb more to the mineral surface as the pH increases. Sorbed cations will promote the adsorption of the negatively charged arsenate oxyanion through reduction in the electrostatic potential. Probing of the local bonding environment of arsenate adsorbed to goethite and ferrihydrite at high and low pH and in both ARW and ASW was investigated by synchrotron-based bulk extended X-ray absorption fine structure (EXAFS) spectroscopy at the National Synchrotron Light Source-II. Arsenic EXAFS analysis revealed that at high pH, the As-Fe interatomic distance is slightly increased compared to low pH. Although the main binding complex is bidentate binuclear to the goethite surface, this stretch in the As-Fe interatomic distance may represent the presence of some monodentate binding complexes as well. The presence of ternary-like complexes at high pH between divalent cations and arsenate cannot be ruled out. Desorption experiments revealed that high ionic strength in ASW may play a small role in the desorption of arsenate from goethite and ferrihydrite, but the greatest impact is from ligand exchange. ☐ When investigating the impact of seawater and variable redox conditions on As and Fe geochemistry of an arsenic-ferrihydrite coprecipitate, it was revealed that as the redox potential decreased, Fe reductive dissolution increased, but the coprecipitate reprecipitated as magnetite. Through Fe EXAFS, it was found that there was more magnetite formation in ARW compared to ASW, 66% vs 32% formed, respectively. The high concentration of SO42- in ASW mostly likely was adsorbed onto the mineral surface and inhibited reductive dissolution. Additionally, As sequential extractions revealed that there was overall more As associated with well-crystallized/less reactive iron phases, such as magnetite, in ARW than ASW. Arsenic EXAFS also showed that at low redox potential, As became more associated with structural phases, most likely the newly formed magnetite. ☐ Additional experiments are needed to further elucidate the role of seawater in arsenate adsorption at a high pH and its effect on the arsenate-iron oxide binding complex. Also, further investigation into the role of SO42- on As and Fe cycling throughout a shifting redox potential is needed.
Description
Keywords
Arsenic, Sea level rise, Seawater in arsenate adsorption