The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions

Abstract

The chemical speciation of arsenic in sediments and porewaters of aquifers is the critical factor that determines whether dissolved arsenic accumulates to potentially toxic levels. Sequestration of arsenic in solid phases, which may occur by adsorption or precipitation processes, controls dissolved concentrations. We present synchrotron x-ray absorption spectra of arsenic in shallow aquifer sediments that indicate the local structure of realgar (AsS) as the primary arsenic-bearing phase in sulfate-reducing conditions at concentrations of 1-3 mmol⅐kg ؊1 , which has not previously been verified in sediments at low temperature. Spectroscopic evidence shows that arsenic does not substitute for iron or sulfur in iron sulfide minerals at the molecular scale. A general geochemical model derived from our field and spectroscopic observations show that the ratio of reactive iron to sulfur in the system controls the distribution of solid phases capable of removing arsenic from solution when conditions change from oxidized to reduced, the rate of which is influenced by microbial processes. Because of the difference in solubility of iron versus arsenic sulfides, precipitation of iron sulfide may remove sulfide from solution but not arsenic if precipitation rates are fast. The lack of incorporation of arsenic into iron sulfides may result in the accumulation of dissolved As(III) if adsorption is weak or inhibited. Aquifers particularly at risk for such geochemical conditions are those in which oxidized and reduced waters mix, and where the amount of sulfate available for microbial reduction is limited. Despite intensive study in recent years, quantitative biogeochemical models that explain and predict conditions controlling the uptake or release of dissolved arsenic in groundwaters remain incomplete, due partly to a lack of knowledge of arsenic speciation in subsurface sediments. Worldwide, elevated concentrations of arsenic (Ͼ10 g⅐liter Ϫ1 , the World Health Organization and new U.S. Environmental Protection Agency drinking-water standard) in groundwater have the potential to have an adverse impact on Ϸ90 million people (1, 2), including 13 million in the United States (3, 4). Arsenic is a known carcinogen and mutagen that can lead to both acute and chronic adverse health effects (5, 6). Arsenic in ground-and surface waters results from both natural and anthropogenic sources, but its mobility and attenuation, and thus impact on humans and other organisms, is directly tied to its chemical speciation (3,7,8).Recent studies using direct spectroscopic characterizations have begun to verify differing chemical states of arsenic in natural samples and their influence on arsenic mobility (9-12). Under oxic conditions, it is widely accepted that arsenic in sediments is removed from solution by adsorption to or coprecipitation with ferric oxyhydroxide and possibly by adsorption to manganese oxide and phyllosilicate minerals (7, 13). As subsurface conditions change from oxidized to progressively more reduced, As(V) is reduced to As(III...

Arsenic bioavailability to rice plant in paddy soil: influence of microbial sulfate reduction

“…Microbial sulfate reduction often resulted in subsequent precipitation of As by a mixed population of SRB (Jong and Parry 2003;Rittle et al 1995). Conversely, under sulfate-reducing conditions, As may react with excess sulfide to first form soluble thioarsenite complexes, which may then precipitate as As-S solid phases such as amorphous As 2 S 3 or realgar (AsS) as dissolved sulfide concentrations increase (O'Day et al 2004;Bostick et al 2005). The thioarsenite complexes are more easily formed in a low Fe 2+ and high organic matter environment (Couture et al 2013;Burton et al 2014), while not commonly reported in paddy soil.…”

“…5), which may be through the coprecipitation of As(III) with reducing S 2− and iron sulfides under anaerobic conditions. It has been suggested that low As mobility in soil solution is related to the conditions of high sulfur in anaerobic paddy soils and also in sediments of aquifers, peat, and floodplains (O'Day et al 2004;Langner et al 2012;Burton et al 2014). The release of As to soil solution and its accumulation in rice plants in the transition from oxidizing to reducing may depend on the amount of available iron and sulfur in the paddy soil, rate of reductive dissolution of Fe(III) phases, SO 4 2− reducing rate, and the rate of As precipitation with S 2− .…”

Naturally occurring arsenic in terrestrial geothermal systems of western Anatolia, Turkey: Potential role in contamination of freshwater resources

“…In geothermal environments, mobility of As can be partly controlled by its chemical speciation. The dominant As species in a given environmental setting is controlled by many parameters, such as pH, temperature, adsorption and dissolution reactions and redox conditions which are driven by the presence of organic matter and inorganic electron donors such as sulfides [8][9][10][11][12][13]. Arsenic is generally released from the host rock by dissolution under reducing conditions, where high residence time, and high temperature and pressure of the fluids favor its mobilization [2,14].…”

Mobilization and speciation of arsenic from hydrothermally altered rock containing calcite and pyrite under anoxic conditions

“…Arsenic is a toxic trace element that is ubiquitous in nature, and is usually concentrated in mineral sulfide ore bodies (Fleet et al, 1989;Huston et al, 1995;Smedley and Kinniburgh, 2002). Serious health problems like arsenicosis, keratosis and certain types of cancers (e.g., skin, lungs, liver, bladder and kidneys) have been linked to the chronic intake of this element through contaminated drinking water sources (Cebrian et al, 1983;Chakraborty and Saha, 1987;Chen et al, 1985;Chen et al, 1992;O'Day et al, 2004;Sengupta, 2002;Smith et al, 1992;Tseng et al, 1968;Zaldivar, 1974).…”