Arsenic In Rocks Research Articles

The effect of Na–Ca–Cl brines on the dissolution of arsenic from arsenopyrite under geologic carbon dioxide storage conditions

Arsenopyrite (FeAsS) is one of the most common mineral sources of arsenic in rocks, sediments, and ores. The dissolution of arsenopyrite releases arsenic, which has potential implications for human and environmental health. With carbon dioxide storage in saline, sedimentary geologic formations being an important component of mitigation strategies for global warming, the effect of injected CO2 mixed with anoxic brines on dissolution of arsenic from arsenopyrite merits examination. Arsenopyrite dissolution was studied using anoxic Na–Ca–Cl brines in the presence and absence of CO2, at both ambient and high temperature and pressure conditions. Results revealed that the dissolution rate of arsenic ranged from 10−10 to 10−12mol/m2s, with an average dissolution rate of 10–10.7±0.2mol/m2s in the absence of CO2. Understanding effects of particular brine constituents on arsenopyrite dissolution requires further investigation, however. The observed arsenopyrite dissolution rates under anoxic conditions are about two orders of magnitude lower than those observed under oxidizing conditions. The presence of dissolved CO2 in brines resulted in dissolution rates marginally lower than those observed in the absence of CO2, with an average dissolution rate of 10–11.2±0.5mol/m2s, even though a reduction in pH to approximately 3.5 was observed. Increase in temperature and pressure from 25°C to 60°C, and 1bar to 100bars respectively, increased As dissolution rate from arsenopyrite by 1logunit.

The occurrence and dominant controls on arsenic in the Newark and Gettysburg Basins

Elevated arsenic (As) concentrations in groundwater and rocks have been found in crystalline and sedimentary aquifers from New England to Pennsylvania, USA. The arsenic geochemistry and water–rock interactions of the Northern Appalachian Mountains and the Newark Basin have been researched at length, however, little is known about arsenic in the Gettysburg Basin. Both the Newark and Gettysburg Basins were formed during the breakup of Pangea, sediment deposition occurred during the Triassic and lithologies are of similar depositional environment. We compile and review the work done in the Newark Basin and collect new samples in the Gettysburg Basin for comparison. The Gettysburg Basin has 18%–39% of rock samples with arsenic concentrations greater than the crustal average of 2mg/kg, while the Newark Basin has 73% to 95% of rock samples above the crustal average. The strongest controls on arsenic in rocks of the Gettysburg Basin are the relationship between arsenic and iron and silicon concentrations while the strongest controls in the Newark Basin are the relationship between arsenic and iron and organic carbon concentrations. The groundwater arsenic concentrations follow similarly with 8–39% of water samples from the Gettysburg Basin above 10μg/L and 24–54% of water samples from the Newark Basin above 10μg/L. The strongest controls on arsenic in water of the Gettysburg Basin are pH, alkalinity and silicon, while the strongest controls in the Newark Basin are pH and alkalinity.

Arsenic in volcanic geothermal fluids of Latin America

Numerous volcanoes, hot springs, fumaroles, and geothermal wells occur in the Pacific region of Latin America. These systems are characterized by high As concentrations and other typical geothermal elements such as Li and B. This paper presents a review of the available data on As concentrations in geothermal systems and their surficial discharges and As data on volcanic gases of Latin America. Data for geothermal systems in Mexico, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, Ecuador, Bolivia, and Chile are presented. Two sources of As can be recognized in the investigated sites: Arsenic partitioned into volcanic gases and emitted in plumes and fumaroles, and arsenic in rocks of volcanic edifices that are leached by groundwaters enriched in volcanic gases. Water containing the most elevated concentrations of As are mature Na–Cl fluids with relatively low sulfate content and As concentrations reaching up to 73.6mgL−1 (Los Humeros geothermal field in Mexico), but more commonly ranging from a few mgL−1 to tens of mgL−1. Fluids derived from Na–Cl enriched waters formed through evaporation and condensation at shallower depths have As levels of only a few μgL−1. Mixing of Na–Cl waters with shallower meteoric waters results in low to intermediate As concentrations (up to a few mgL−1). After the waters are discharged at the ground surface, As(III) oxidizes to As(V) and attenuation of As concentration can occur due to sorption and co-precipitation processes with iron minerals and organic matter present in sediments. Understanding the mechanisms of As enrichment in geothermal waters and their fate upon mixing with shallower groundwater and surface waters is important for the protection of water resources in Latin America.

The interstitial location of selenium and arsenic in rocks associated with coal mining using ultrasound extractions and principal component analysis (PCA)

The release of selenium and arsenic from coal mine wastes into main waterways is an environmental cause for concern in the mining industry due to a myriad of subsequent ecotoxicological problems associated with the two metalloids. In a 2002 USEPA study undertaken in a mountaintop removal/valley fill (MTR/VF) mining area in southern West Virginia, measured Se concentrations were higher than the stipulated 5 ng/mL in 66 out of the 213 water samples collected. We studied the chemical composition of forty seven randomly selected pulverized core rock samples collected from depths of 25 ft to 881 ft from MTR/VF sites to determine the amounts of bioaccessible (ultrasound leachable) As and Se concentrations and their tentative locations within the rock matrix. The application of principal component analysis (PCA) to the chemical data, suggested that ultrasound leachable selenium concentrations were associated with 14 Å d-spacing phyllosilicate clays (chlorite, montmorillonite and vermiculite all 2:1 layered clays) whilst ultrasound leachable arsenic concentrations were closely related to the concentration of illite, another 2:1 phyllosilicate clay. Negative correlations between leachable arsenic and selenium with kaolinite a 1:1 layered clay, were also observed. We used the observed negative correlations to rule out the presence of selenium or arsenic in 1:1 kaolinite. Hence mining waste from MTR/VF sites containing substantial amounts of illite and 14 Å d-spacing clays may require to be placed in priority landfills or valley fills.

Case studies – Arsenic

Arsenic is found naturally in the environment. People may be exposed to arsenic by eating food, drinking water, breathing air, or by skin contact with soil or water that contains arsenic. In the U.S., the diet is a predominant source of exposure for the general population with smaller amounts coming from drinking water and air. Children may also be exposed to arsenic because of hand to mouth contact or eating dirt. In addition to the normal levels of arsenic in air, water, soil, and food, people could by exposed to higher levels in several ways such as in areas containing unusually high natural levels of arsenic in rocks which can lead to unusually high levels of arsenic in soil or water. People living in an area like this could take in elevated amounts of arsenic in drinking water. Workers in an occupation that involves arsenic production or use (for example, copper or lead smelting, wood treatment, pesticide application) could be exposed to elevated levels of arsenic at work. People who saw or sand arsenic-treated wood could inhale/ingest some of the sawdust which contains high levels of arsenic. Similarly, when pressure-treated wood is burned, high levels of arsenic could be released in the smoke. In agricultural areas where arsenic pesticides were used on crops the soil could contain high levels of arsenic. Some hazardous waste sites contain large quantities of arsenic. Arsenic ranks #1 on the ATSDR/EPA priority list of hazardous substances. Arsenic has been found in at least 1,014 current or former NPL sites. At the hazardous waster sites evaluated by ATSDR, exposure to arsenic in soil predominated over exposure to water, and no exposure to air had been recorded. However, there is no information on morbidity or mortality from exposure to arsenic in soil at hazardous waste sites. Exposure assessment, community and tribal involvement, and evaluation and surveillance of health effects are among the ATSDR future Superfund research program priority focus areas. Examples of exposures to arsenic in drinking water, diet and pesticide are given.

Determination of arsenic in rocks and ores with a sequential X‐ray spectrometer

AbstractThe determination of As making use of the Kα net intensity requires subtraction of the overlapping Pb Lα peak. As the interfering intensity cannot be measured directly, a calculated value has to be obtained from measurement at the Pb Lβ line. A mathematical approach was developed which is based on a typical mean value for the ratio between the Pb Lα and Lβ net intensities. Three measurements per sample are required and the routine calibration procedure is set up with three standard samples. Measurements are fed into a computer programmed to calculate both As and Pb concentrations. The programme corrects for matrix effects by means of one of the measurements that is taken at a chosen background point.The method was devised for fast routine analysis and can be applied to samples of varied composition with the limitation that the As and Pb emission lines are not subjected to significant enhancement. Samples are prepared by grinding the material to a suitable fineness and then pressing it into a tablet. The attainable accuracy and limit of detection for As depend on the Pb level present in each sample.

Neutron activation analysis of arsenic in rocks and sediments by direct evolution with chloride- and bromide-condensed phosphoric acid reagents

A simple method is presented for the determination of arsenic in rocks and in sediments by neutron activation. After irradiating a sample it is, without any other treatment, directly heated in condensed phosphoric acid containing sodium chloride or sodium bromide to evolve arsenic as arsenic(III) chloride or bromide. The distillate is absorbed in distilled water, in which arsenic is later precipitated in elementary form by adding hypophosphite to the solution. From arsenite, arsenic(III) oxide, arsenate and arsenic(III) sulphide, arsenic chloride can be evolved with NaCl-CPA reagent, but elementary arsenic and arsenic(V) oxide do not react with it. However, metallic arsenic is found to react with KIO3-NaCl-CPA and arsenic(V) oxide with the NaBr-CPA, both both evolving arsenic(III) chloride or bromide. Therefore, successive distillations, the first with NaCl-CPA and the second with NaBr-CPA, give a satisfactory means of differential determination of arsenic(III) and arsenate as well as arsenic(V) oxide. For the elementary arsenic a problem still now remains. The chemical recovery of carrier goes well beyond 95%.

The determination of arsenic in rocks, sediments and minerals by arsine generation and atomic absorption spectrometry

An arsine generation-atomic absorption method for the rapid and precise determination of 0.04–4000 p.p.m. arsenic in geological materials is described. The siliceous sample is decomposed with perchloric, nitric and hydrofluoric acids and potassium permanganate solution, and the residue is dissolved in dilute hydrochloric acid. Arsine is generated with potassium iodide, tin(II) chloride and zinc powder, and introduced to an argon—hydrogen flame. The method is applied to various standard rocks, NBS mineral standards, and geochemical exploration samples. The relative standard deviation is 4–14 %.