Root, and 5 co-authors, reported in 2009 that the mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels down gradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface).
Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases.
The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwater.
This study evaluated whether both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.
Mechanistic processes controlling arsenic speciation, sequestration, and mobilization in shallow subsurface sediments that span a range of redox conditions were examined
over short vertical and horizontal spatial scales (centimeters to meters) using a combination of spectroscopic and chemical analyses and geochemical modeling. Chemical and spectroscopic characterization of vertical and lateral changes in sediment speciation and pore water chemistry showed that two different mechanisms for arsenic attenuation were active: first, sulfate reduction in the top zone, resulting in formation of arsenic and iron sulfide in sediments, provided attenuation during vertical infiltration of surface water; second, oxidation and sorption in the lower oxidized zone by the influx of oxygenated aquifer water promoted lateral attenuation. Distinct differences in the local atomic structure around arsenic in realgar-type compared to orpiment-type phases, in particular the presence of As–As dimeric cations [As-As]2+ with realgar, are diagnostic of the presence of these phases (or their polymorphs).
The authors concluded that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient conditions, with realgar predicted as the stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. The presence of both types of these phases at arsenic concentrations elevated above background is consistent with thermodynamic relationships (Nordstrom and Archer, 2003; Helz and Tossell, 2008), although there remain uncertainties in the rates at which realgar-type phases may form at ambient temperatures and in the thermodynamic data for arsenic sulfide species and phases.
Geochemical simulation using the reaction progress approach to provide thermodynamic constraints (solubility and sorption) during changes from oxidized to reduced
conditions, with model parameters generalized from field observations, lend insight into dynamic processes and factors that may control dissolved concentrations. Arsenic
mobilization may occur in the zone affected by seasonal fluctuations of groundwater level potentially during two conditions: when groundwater level rises, flooding reduced sediments and promoting oxidative dissolution of sulfide; and when water level falls and oxidized sediments become progressively reduced as microbial action consumes O2 and other electron acceptors, reducing arsenic, iron, and manganese (the latter being particularly important in low-Fe systems, e.g., Amirbahman et al., 2006; Choi et al., 2009).
During an oxidative cycle, oxidation of iron and arsenic, leading to (co-)precipitation of Fe(III)-(hydr)oxides and arsenate and/or pH-controlled arsenate sorption, will be an effective attenuation mechanism as long as oxidative capacity is maintained and the ratio of arsenic to labile iron (and potentially manganese) is low. During a reduction cycle, arsenite (and labile ferrous iron) will be partitioned to groundwater until sulfate reduction produces sufficient dissolved sulfide concentrations to exceed the solubility of solid sulfide phases. At intermediate redox states, the concentration of labile iron in the system will determine whether green-rust-type phases (which may age to magnetite) form that may provide new sorption sites for arsenic. In freshwater aquifers with low dissolved sulfate and thus limited potential for sulfate reduction, pH-controlled sorption and competition with other dissolved species will control dissolved arsenic concentrations.
See a previous paper on the same site: https://www.pnas.org/content/pnas/101/38/13703.full.pdf
See a paper on similar subject at a site in Inner Mongolia: http://i2mportal.com/resource_detail.php?resource_id=9411
For supplementary data cited in this paper, see:
http://www.i2mconsultants.com/downloads/corp/1-s2.0-S0016703709004219-mmc1.pdf
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