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Distribution of Arsenic in the Environment NATURAL SOURCES Earth's Crust Arsenic ranks twentieth among the elements in abundance in the earth's crust. The abundance of arsenic in the continental crust of the earth is generally given as 1.5-2 ppm. Thus, it is relatively scarce. Nevertheless, it is a major constituent of no fewer than 245 mineral species. Arsenic is found in high concentration in sulfide deposits, where it is present as the native element or alloys (four minerals), arsenides (27 minerals), sulfides (13 minerals), sulfosalts (sulfides of arsenic with metals, such as lead, copper, silver, and thallium, 65 minerals), and the oxidation products of the foregoing (two oxides, 11 arsenites, 116 arsenates, and seven silicates). Of these minerals, arse- nopyrite is by far the most common. In addition, many sulfides contain appreciable amounts of arsenic in solid solution; the most important of these is pyrite, which has a maximal arsenic content of about 5% (common range, 0.02-0.5%~. The arsenic-bearing sulfides and sul- fosalts oxidize readily; under surface conditions, oxidation proceeds to arsenic trioxide and to the arsenate stage. 16
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Distribution of Arsenic in the Environment TABLE 3-1 Arsenic in Igneous Rocksa 17 Arsenic Concentration, ppm No. Range Usually Rocks Analyses Reported Average Ultrabasic 37 0.3-16 3.0 Basalts, gabbros 146 0.06-113 2.0 Andesites, dacites 41 0.5-5.8 2.0 Granitic 73 0.2-13.8 1.5 Silicic volcanic 52 0.2-12.2 3.0 aEstimated on the basis of data of On~shi602 and Boyle and Jonasson.94 Igneous and Sedimentary Rock Concentrations of arsenic in igneous rocks are listed in Table 3-1. No trend of concentration is apparent with respect to content of silica or other major elements. The limited data available indicate rather uni- form distribution of arsenic among the major constituent minerals, except for slight enrichment in the sulfide minerals of igneous rocks. Data on the concentration of arsenic in sedimentary rocks are summarized in Table 3-2. Shales, clays, phosphate rocks, and sedimen TABLE 3-2 Arsenic in Sedimentary Rocksa Arsenic Concentration, ppm No. Range Usually Rocks Analyses Reported Average Limestones 37 0.1-20 1.7 Sandstones 1 1 0.6-120 2.0 Shales and clays 324 0.3-490 14.5b Phosphorites 282 0.4-188 22.6 Sedimentary iron ores 110 1-2,900 400? Sedimentary manganese ores (up to 1.5%) Coal 1,150 0-2,000 13c aEstimated on the basis of data of Onishi602 and Boyle and Jonasson.94 Excluding one sample with arsenic at 490 ppm. CBoyle and Jonasson94 gave 4 ppm.
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18 ARSENIC tary iron and manganese oxides are notably enriched in arsenic. The data of Tourtelot794 indicate that most of the arsenic in nonmarine clays and shales is associated with the clay minerals, whereas a considerable proportion of the arsenic in offshore marine samples is present as pyrite. Tourtelot, Schultz, and Gill795 found a correlation between the arsenic and organic carbon concentrations. A similar correlation was observed by Ruch, Kennedy, and Shimp689 for unconsolidated sedi- ments of Lake Michigan; they attributed this arsenic to manes activities- the arsenic content in surface sediments (0-6 cm) averaged more than twice that at depths greater than 20 cm (12.4 vs. 5.3 ppm). It should be noted that a higher than average content of arsenic is commonly found in sandstones, shales, and coals associated with uranium mineralization in Utah, Colorado, Wyoming, and South Dakota; this suggests considerable mobility of arsenic. High concentrations of arsenic (maximum, 2,100 ppm; average, 1 15 ppm; median, 60 ppm) have also been noted in sediments from the area of hot brines in the Red Sea.345 408 Most of the analyses for phosphorites3~6 797 are related to samples from the United States (Table 3-31. There is considerable variation in arsenic content, even from a single area, and no correlations with concentrations of phosphorus pentoxide, organic matter, or other major constituents are proved. Gulbrandsen3~6 suggested a correlation of arsenic with organic matter for the phosphorites of the Phosphoria Formation (Montana, Wyoming, and Idaho); Stow764 found no such correlation for Florida land-pebble phosphate, but found a positive correlation with iron content. The available analyses have been made on whole rock; consequently, correlations of arsenic with other con- stituents can be made with confidence only if the purified phosphate mineral and associated clay material are determined. It would be especially desirable to conduct such studies on samples of high arsenic content. Soil Arsenic is present in all soils, and the geologic history of a particular soil determines its arsenic content.308 The natural arsenic content in virgin soils varies from 0.1 to 40 ppm. The average is about 5-6 ppm, but it varies considerably among geographic regions. 159 Soils overlying sulfide ore desposits commonly contain arsenic at several hundred parts per million; the reported maximum is 8,000 ppm. This arsenic may be present in unweathered sulfide minerals or in an inorganic anion state. The most common sulfide is arsenopyrite, although arse !
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Distribution of Arsenic in the Environment TABLE 3-3 Arsenic in Phosphoritesa 19 Arsenic Concentration, ppm No. Locality and Type of Rock Analyses Range Average Median Soup Carolina, river rock 4 56.8-88.1 68.4 64.3 South Carolina, land rock 4 9.2-27.5 17.4 15.9 Florida, hard rock 8 1.4-9.6 5.4 5.7 Florida, land pebble 31 3.6-21.2 11.9 11.6 Florida, soft rock 6 0.4-18.6 7.5 5.7 Tennessee, blue rock 7 8.4-37.7 20.4 19.8 Tennessee, brown rock 25 5.1-56.1 14.6 12.5 Tennessee, white rock 3 4.8-21.7 10.6 5.2 Kentucky 3 6.7-12.7 9.9 10.3 Arkansas 8 14.6-188.2 61.0 43.8 Oklahoma 3 15.6-19.3 17.6 17.9 Montana 25 < 10-106 40.0 30 Idaho 27 8.4-60 18.5 15 Wyoming 17 < 10-150 26.4 17 Utah 14 8.4-43.2 16.0 14 British Columbia 1 28.3 28.3 Europe 10 7.6-54.8 25.1 20.8 North AfIica 13 7.0-36.7 17.4 16.3 Israel ? 20-40 ? ? Insular (West Indies, Pacific) 21 5.1-76.2 16.3 12.0 Southern Australia 2 20.3-24.3 22.3 22.3 Summarized mainly from Tremearne and Jacob797 and Gulbrandsen.3l6 nosulf~des of almost any metal cation can be found. Inorganic arsenate may be bound to iron and aluminum cations or oxides or to any other cation present (such as calcium, magnesium, lead, and zinc). Arsenic may also be bound to the organic matter in soils, in which case it is released into the soil solution as the organic matter is oxidized and is then available for plant uptake or fixation by soil cations.675 Some arsenic from other inorganic forms is also available for plant uptake, inasmuch as the slightly soluble iron and aluminum arsenates and the soil solution are in equilibrium. The amount released for plant uptake is a function of the particular chemical and physical forms of individual arsenic compounds. The amount of available arsenic (ex- tracted with 0.05 N hydrochloric acid and 0.025 N sulfuric acid) is small in virgin soils and averages about one-tenth of the total arsenic present in most cultivated soils. t59330833~i
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20 Water ARSENIC The cycle of arsenic in natural waters has recently been reviewed by Ferguson and Gavis.249 Data on the arsenic content of waters and sediments are summarized in Tables 3-4 and 3-5. Sugawara and Kanamori768 showed that the ratio of As(V) to total arsenic was close to 0.8: 1 in ocean water. Braman97 reported ratios of 0.56 :1 and 0.81:1 for a tidal flat and saline bay water, respectively. He also found that As(III), methanearsonic acid, and cacodylic acid were present. The ratio of As(V) to As(III), based on thermodynamic calculations, should be 1026: 1 for oxygenated seawater at a pH of 8.1. In reality, it is 0.1: 1 to 10 :1. This unexpectedly high As(III) content is caused, at least in part, by biologic reduction in seawater.393 The content of arsenic in seawater is a small fraction (perhaps 0.1%) of the amount calculated to have been carried into the sea. Nearly all the arsenic has been precipi- tated or adsorbed on marine clays (probably most important), phos- phorite, and hydrous oxides of iron and manganese. The scavenging of arsenic from solution by coprecipitation with hydrous oxides of iron and manganese in laboratory experiments is well known, but its occurrence in natural waters has not been studied in detail. Moenke55 noted that spring waters (pH, 5.1) of high arsenic content precipitated about 80~o of their arsenic in iron-rich sediments within 160 m of the source of entry. The high content of arsenic in hot springs is notable; fumarolic gases have been reported to contain arsenic at up to 0.7 ppm. Extremely high arsenic concentrations have been reported in some groundwaters from areas of thermal activity,3~2448 from areas of rocks with high arsenic content,86 294~83 and in some waters of high dissolved-salt content.478 85~ Most of the other high values reported in rivers and lakes and in sediments (Tables 3-4 and 3-5) are probably due to industrial contami- nation. Angino and others have shown that household detergents (mostly of the high-phosphate type) widely used in the United States contained arsenic at 1-73 ppm; their use probably contributes signifi- cant amounts of arsenic to surface waters. Sollins,75~ however, felt that, after dilution during use, the concentration would be well below the recommended maximum and constitute no particular hazard. It has been generally assumed that surface waters, like the ocean, are "self- purifying" with respect to arsenic-i.e., that the arsenic is removed from solution by deposition with sediments; but quantitative studies are lacking. Sediments are always higher in arsenic than the waters with which they are associated. The data on ground waters are inadequate. About 3% of the analyses show arsenic at more than 50 ppb, the 1962 maximal permissible
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Distribution of Arsenic in the Environment TABLE 3-4 Arsenic in Fresh Waters 21 Arsenic Concentration, Water ,ug/liter (ppb) Reference United States, lakes: New York, Chautauqua 3.5-35.6 474 Michigan 0.5-2.4 720 Superior 0.1-1.6 720 Wisconsin 4.0-117 141 California, Searles 198,000-243,000 851 California 0.0-100a 478 0.0-2,000b 478 Florida, Echols 3.58 99 Florida, Magdelene 1.75 99 United States, rivers: Hillsborough 0.25 99 Withlacoochee 0.42 99 Fox (polluted watershed) 100-6,000 107 Yellowstone 4.5 231 Narrow 0.90 659 Providence 0.75-0.90 659 Seekonk 2.48-3.45 659 Sugar Creek (contaminated) <10-1,100 224, 859 Columbia 1.6 602 Schuylkill 30-180 436 United States, canals: Florida <10-20 305 United States, well water: California ~10-<2,000 296 Florida 0.68 99 Minnesota (contaminated) 11,800-21,000 244 Washington 5.0-6.0 241 Oregon 0.00-1,700 294 United States, Puget Sound 1.5-1,200 186, 187 United States, rainwater: Rhode Island 0.82 659 Washington, Seattle 17 186 Argentina, Cordoba, drinking water 480-1,490 traces-300 315 42 Bosnia, Shebrenica, spring 4,607 385 Canada, well water 0.5-15 302 <2.3-7.500 883 Chile 800 86
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22 TABLE 3-4 (Continued) ARSENIC Arsenic Concentration, Water ,ug/liter (ppb) Reference 1 Italy, Modena Province: Groundwater 3.0-5.0 824 Subsurface <0.4-2.1 155 Japan: Rain 0.01-13.9 405 Rivers (40) 0.25-7.7 405 Aomori Prefecture 30-3,950 588 Lakes 0.16-1.9 602 Germany: Elbe River 20-25 602 Rhine River 3.1 432 Greece, lakes 1.1-54.5 602 Formosa, well water 800 242 New Zealand, rivers: Waikato Rived 5-100 448 Waiotapu Valley trace-276,000 312 Yagnob, Daiyee River, suspended 100 300 445 Sweden: Rivers 0.2-0.4 602 Glacial ice 2.0-3.8 847 Antarctica 0.60-0.75 405 Spring waters, California, Kamchatka, U.S.S.R., New Zealand 130-1,000 851 Oil- and gas-field waters, California, Louisiana, Hungary 0.0-5,800 851 Thermal waters, Wyoming, Nevada, California, Alaska, Iceland 20-3,800 851 Spring waters, e U.S.S.R., Wyoming, Algeria, Iceland 30-500 851 Dissolved solids, <2,000 ppm. Dissolved solids, >2,000 ppm. CHigh in bicarbonate; of geothermal origin. dHigh in bicarbonate and boron. eDeposit traverUne.
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Distribution of Arsenic in the Environment TABLE 3-5 Arsenic in Sediments 23 Arsenic Concentration, Locality ppm Reference United States: New York, Chautauqua 0.5-306.0 694 Texas 3.C' 3 0.8-8.0 654 Winyah Bay 8.0-12.0 394 Lake Michigan 5.0-30.0 689 7.2-28.8 720 Lake Superior 2.8-5.4 720 Lalces, Wisconsin 0.1-45.0 727 Sugar Creek (conta'Tunated) 4,470-66,700 859 Puget Sound 2.9-10,000 186 Washington, rivers Skagit 15-34 186,187 Stillaguamish 17-48 186,187 Snohom~sh 22-74 186,187 Duwam~sh 15-40 186,187 Puyallup 2.6-7.5 186,187 Nisqually 4.5-12 186,187 Dosewallips 7.4 186,187 Duckabush 6.8 186,187 Japan 0.0-93.4 405 Minamata area 4.7-60 319 Netherlands, Rhine Delta ND-3 10 197 New Zealand: Wa~otapu Valley muds 51-14,250 312 Manne 6.6 652 Pelagic 40 819 England <2-5,000 38,456,789 ND = Not detected. concentration in drinking water.8088~3 In view of recent reports of chronic arsenic poisoning attributed to the use of such waters in Chile86 and in Oregon,294 further study is imperative. The volcanic rocks from which the arsenic-rich waters come in Oregon are of a type that is common in the western United States.262
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24 Plants ARSENIC Arsenic is ubiquitous in the plant kingdom. Its concentration varies from less than 0.01 to about 5 ppm (dry-weight basis). Appendix A lists the arsenic concentrations of some plants and plant products. Differ- ences in arsenic content probably reflect species differences in plants and, in a larger sense, environmental and edaphic factors in a particular geographic region. Plants growing in arsenic-contaminated soils gen- erally have higher residues than plants grown in normal soils. Arsenic concentrations are less than S.O ppm (dry wt) or O.S ppm (fresh wt) for untreated vegetation, whereas treated plants may have much higher concentrations. However, values for some nontreated plants are as high as or higher than those for plants that were treated with arsenic or grown in arsenic-contaminated soil. Natural variations among plants, plant species, available soil arsenic, and growing conditions are all responsible in part for these discrepancies. There appears to be little chance that animals would be poisoned by consuming plants that contain arsenic residues from contaminated soils, because plant injury occurs before toxic concentrations could appear. Marine plants, particularly algae and seaweed, may have extremely high arsenic contents. In 11 varieties of British seaweed examined, a range of 5.2 ppm (in Chondrus crispus) to 94 ppm (in Laminaria digitata) was recorded.398 In green algae, the amount of arsenic varied inversely with the apparent chlorophyll content, from O.OS to S.O ppm on a dry-weight basis.5~9 For brown algae, values of around 30 ppm have been reported. Animals and Humans Arsenic is present in all living organisms (Appendix B). Marine fish may contain up to 10 ppm; coelenterates, some mollusks, and crusta- ceans may contain higher arsenic concentrations. Freshwater fish contain up to about 3 ppm, although most values are less than 1 ppm. Domestic animals and man generally contain less than 0.3 ppm on a wet-weight basis; The total human body content varies between 3 and 4 mg and tends to increase with age. With the exception of hair, nails, and teeth, analyses have revealed that most body tissues contain less than 0.3 ppm. The median arsenic content in 1,000 samples of human hair was O.S I ppm, as determined by neutron-activation analysis.743 The median concentrations for males and females were 0.62 and 0.37 ppm, respec
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Distribution of Arsenic in the Environment 25 lively. Arsenic content of hair has served as an indicator in incidents of suspected poisoning. Values greater than about 2-3 ppm indicate possible poisoning, although higher concentrations have been recorded in occupational surveys. For example, a survey of workers in a copper processing plant in Czechoslovakia showed mean arsenic contents of 178 ppm in 21 persons exposed to air containing arsenic tr~ox~ae at 1 01-5.07 m~/m3 and 56.6 prim in 18 persons exposed to air containing . · . . , 7 , 0.08-0.18 mg/m3; a control (nonexposeu) group naa mu. Pam.- such occupational surveys, it is important to distinguish between exogenous arsenic from atmospheric pollution and cosmetics and that from ingestion. Nail clippings from a patient with acute polyneuritis from arsenic poisoning contained arsenic at 20-130 Pam. The normal ~r~.nic content of nails is 0.43-1.08 ppm.380 w_ The arsenic content ot urine can vary normally from 0.1 to 1.0 ppm. Great daily variations exist and depend on the amount of arsenic in various foodstuffs. It is generally high after consumption of seafood. When arsenic is ingested, the amount excreted increases over several days to a maximum and then declines to normal. Some of the highest concentrations of arsenic in biota are encoun- tered in marine organisms. The average arsenic content of freshwater fish including shad, gar, carp, bullhead, pickerel, bluegill, black bass, white bass, buffalo, and horned dace varied up to 2.1 ppm.233 The average oil content of these fish was only 2.49%, but the oil carried 22.8% of the total arsenic present. The arsenic in the liver oil of the large-mouthed black bass averaged 30 ppm. These values are generally lower than those reported for marine fish, which range up to 32.4 ppm for cod. Shrimp contain arsenic at 3.8-128 ppm on a dry-weight basis.~72 A survey of canned seafood showed the following arsenic concentrations: clams, 15.9 ppm; oysters, 16.0 ppm; smoked oysters, 45.8 ppm; lobsters, 22.1 ppm; and shrimp, 19.9 ppm.203 Air Trace amounts of arsenic may be present in air. Although no 24-h maximal atmospheric concentration has been set in the United States, 3 ,ug/m3 has been recommended in the U.S.S.R. and Czechoslo- vakia.676 The threshold limit recommended for industrial workers is 500 ,ug/m3 for arsenic and its compounds and 200 ,ug/m3 for arsine.425 Exposure standards for inorganic arsenic have recently been proposed by the Occupational Safety and Health Administration.809 They limit
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26 ARSENIC air concentration to ''4 ,ug As/m3 of air averaged over an eight hour period." A ceiling limit of 10 ,ug/m3 is proposed for any 15-min period during a work shift. Data on emission of arsenic to the atmosphere have been sum- marized by Sullivan770 and by Davis and Associates and are dis- cussed at the end of this chapter. Arsenic content in air and dust is summarized in Table 3-6. In areas remote from industrial contamina- tion, air concentrations of arsenic generally are less than 0.02 ,ug/m3, whereas in urban areas they vary from less than 0.01 to 0.16 ,ug/m3. Two of the air values reported as ''United States, Miscellaneous" were 2.50 ,ug/m3 in Anaconda, Montana, in 1961-1962 (the maximum) and 1.40 ,ug/m3 in E1 Paso, Texas, in 1964.77° MAN-MADE SOURCES Production Data on domestic and world production, imports, and domestic con- sumption of arsenic from 1964 to 1973, as shown in Table 3-7, were obtained from the Bureau of Mines, Minerals in the U.S. Economy. Much of the arsenic processed in the United States is imported in copper ore and concentrates. An equal amount is imported as arsenic compounds. Agriculture is the largest user of arsenic, accounting for about 80~o of the demand. Figure 3- 1 indicates sources of arsenicals by country and type of material in 1973. Tables 3-8, 3-9, and 3-10 show U.S. imports for consumption of white arsenic (arsenic trioxides, U.S. imports of arsenicals by class, and world production of white arse- nic.812 In the United States, arsenic is produced entirely as a by-product of the smelting of nonferrous-metal ores. Domestic production of arsenic has been adversely affected since the 1920's, when very large quan- tities of imported by-product arsenic became available from a copper mine in Sweden whose ore contained a high proportion of arsenic. The demand for arsenic was reduced after World War II by the advent of organic substances developed during and after the war that were used as pesticides and for other purposes for which arsenic had previously been used. The resulting surplus of by-product arsenic kept the price of white arsenic (77% arsenic metal) at 6.25-6.75 cents/Ib (13.8-14.9 cents/kg) from July 1968 through 1973. However, in early 1974, the price increased to 13 cents/lb (28.7 cents/kg). Arsenic is a troublesome contaminant in ores. Some arsenic com
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Distribution of Arsenic in the Environment 69 transfers to and from a field for the organoarsenical herbicides. They concluded that transfers involving reduction to methylarsines, soil erosion, and crop uptake were the primary redistribution mechanisms in this model. Treatment with cacodylic acid resulted in a theoretical buildup of arsenic of 2.6-3.3 ppm/ha pet year, whereas MSMA accumu- lated at only 1.5-1.9 ppm/ha per year. They concluded that ''arsenic is mobile and nonaccumulative in the air, plant and water phases of the agronomic ecosystem. Arsenicals do accumulate in soil, but redistribu- tion mechanisms preclude hazardous accumulations at a given site.''701 This model does not include the application of arsenic trioxide to desiccate cotton before harvest. Inputs into the environment and a redistribution of arsenic in the terrestrial ecosystem are presented in Figure ~-4. Natural inputs are from volcanic action, decay of plant matter, and weathering of minerals within the soil, whereas man-made sources of arsenic are combustion of coal and oil, smelting of ores, and use of fertilizers and pesticides. The largest sink for man-made arsenic in the environment is the soil. Onishi and Sandell calculated a balance between igneous rocks (arsenic content, 2 ppm) and sedimentary deposits (shale and sedi- ments, 10 ppm; sandstone and limestone, 1.5 ppm).603 They observed that, if the amounts of sediments equaled that of weathered rocks, then much of the arsenic in sediments must come from volcanism. At present, this input is small, and weathering of continental rocks is in approximate balance with oceanic sediment deposition. Using esti- mates of arsenic weathering (45,000 tons/year, or 41,000 tonnes/year) and deposition rates, Ferguson and Gavis concluded that "there is no substantial imbalance between natural weathering and deposition of arsenic at present."249 The amount of arsenic from weathering trans- ported to the oceans as part of the dissolved load of the rivers is 33,000 tons/year (30,000 tonnes/year). Arsenic from man-made sources is redistributed either through industrial processes, such as the burning of coal, or by the refining of oil for gasoline and fuel oil. Man's activity does cause high environmental concentrations at some locations. Estimates are available for an arsenic balance at a coal-fired steam plant in Memphis, Tennessee.85 The balance for most trace elements is satisfactory. Elements that can be present in a gaseous form (e.g., arsenic and mercury) are not completely recovered. Most arsenic recovered was in the precipitation inlet fly ash, but 52-64% of the arsenic in coal could not be found. It may have been lost in the gas stream. Coutant et al.~72a found that "only a small percentage of arsenic is emitted from the stacks" and that it did not pose an important problem from an air-pollution standpoint. "Arsenic tended
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Distribution of Arsenic in the Environment 71 to be distributed continuously through the system as a function of temperature," and ''there is a definite tendency for concentration of arsenic in the lower temperature deposits in the combustion system." As coal utilization increases, the amount of arsenic escaping to the environment will increase, unless proper control measures are used. Smelter activities have traditionally introduced large amounts of arsenic into the environment. The copper smelter at Tacoma, Washing- ton, has been examined for arsenic emission into the environment. Crecelius et al. reported that input amounts to 200,000 kg of arsenic trioxide per year into the air via stack dust, 20,000-70,000 kg of arsenic per year into Puget Sound through dissolved arsenicals in its liquid effluent discharge, and 1,500,000 kg of arsenic per year in crystalline slag dumped into the Sound. The installation of more pollution- control equipment at this smelter is planned, so the amount of arsenic released into the air and water will decrease significantly. Information has been collected, to the extent available, to develop a pattern of arsenic emission into the environment. It included informa- tion on the arsenic associated with mineral raw materials and fuels, on the arsenic content of salable mineral products, on solid waste dis- carded by mineral processors, and on effluent from mineral plants. Complete material-balance reports were obtainable for only a few plants. However, considerable incomplete evidence was accumulated. These data were used to trace the disposition of arsenic through mineral processing steps and consumption in commodities containing significant quantities of arsenic. They were also used to determine the distribution of arsenic throughout commercial production and the disposition of arsenic used in agriculture and industry. Arsenic emis- sion to the atmosphere was calculated with the factors listed in Table 3-19. TABLE 3-19 Arsenic Emission Factorsa Arsenic Source Arsenic Concentration Mining and milling 0.45 tonne/million tonnes of copper, lead. zinc. silver. gold. Or uranium ore Smelting and refining 955 tonnes/million tonnes of copper produced 591 tonnes/million tonnes of zinc produced 364 tonnes/million tonnes of lead produced Coal 1.4 tonnes/million tonnes of coal burned Petroleum 5.2 kg/million barrels of petroleum aCalculated on the basis of Davis and Associates' and Minerals Yearbook 1968.~2
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72 ARSENIC The principal source of atmospheric arsenic from manufacturing is the processing of nonferrous metals. Gualtieri classified loo of copper and copper-lead-zinc ores as being arsenical and stated that they have an average arsenic: copper ratio of 1: 50.3~4 Analyses of nonferrous ores considered nonarsenical are not available. However, reference to mineralogic descriptions of other principal nonferrous mining districts indicates that arsenic minerals usually occur in trace quantities, are seldom visible in ore specimens picked at random, and have not caused serious pollution. It is apparent that the arsenic content of nonarsenical ores is less than one-tenth that of arsenical ores. Arsenic concentra- tions would be equivalent to 160 ppm in arsenical ore containing 0.8% copper and 12 ppm in nonarsenical ore containing 0.6% copper. The arsenic concentrations of rocks in the earth's crust are shown in Tables 3-1 and 3-2 as: granite, 1.5 ppm; other igneous, 2.0-3.0 ppm; lime- stone, 1.7 ppm; sandstone, 2.0 ppm; and shales and clays, 14.5 ppm We may assume rock distribution in nonferrous-metal deposits as: granite, 25%; other igneous, 25%; limestone, 25%; sandstone, 15%; and shale, logo. The average arsenic content of unmineralized rock in mining districts would then be over 3 ppm. The average arsenic con- tent of waste moved in mining is estimated as the average of the values for ore and unmineralized material, or 81 ppm for arsenical districts and 7 ppm for nonarsenical districts. An estimated 40~o of the arsenic in copper or copper-lead-zinc ore is left in the concentrator tailings. Much of the arsenic can be allowed to enter the tailing or can be depressed into an iron sulfide tailing, provided that the arsenic mineral does not contain valuable metals. The tailing is deposited on the surface, and some will be blown away by the wind; however, this quantity should not exceed 1% of the annual output. Arsenic in gold and uranium mill tailings is subject to similar wind losses. Arsenic minerals in tailing dunes may eventually weather to water-soluble compounds that will probably be transported over short distances before reacting with iron, aluminum, calcium, and magnesium in the soil to form largely insoluble substances. Most of the arsenic emitted to the atmosphere during nonferrous- metal production results from smelting. At the primary smelter, arsenic contained in the ores and concentrates becomes distributed among the metal product, slag, speiss (a heavy metallic mixture of iron and nonferrous arsenides), flue dust, and atmospheric emission. Arsenic in metal is removed by pyrometallurgic or electrolytic refining methods; the arsenic-containing residues are recirculated to the smelting fur- naces. After recovery of by-products, primary furnace slag is discarded. Speiss is sent to smelters with facilities for processing high-arsenic ma
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Distribution of Arsenic in the Environment 73 serial. Flue dust contains much of the volatile arsenic that is expelled from the furnace melt and collected in the stack-gas cleaning system. Some finely divided arsenic escapes ordinary dust-precipitating units, but additional cooling and cleaning of the furnace gases, as is done be- fore sulfuric acid recovery, should capture most of the finely divided material. Flue dust is ordinarily recirculated to the furnaces, some of it being removed, if necessary, to keep excessive arsenic from ac- cumulating in the system. The high-arsenic flue dust usually contains considerable metal value and, like the speiss, is shipped to the smelter equipped for processing it. At this smelter, the flue dust and speiss are roasted with fluxes to remove as much arsenic as possible. The arsenic is refined to commercial-grade material, and the calcine is smelted for its metal content. Atmospheric arsenic emission during smelting was estimated for 1968 conditions by Davis and Associates on the basis of material balances and sampling data obtained from industrial sources. i96 Aver- age arsenic emission was estimated at 4.9 lb/ton (2.5 kg/tonne) of copper produced, 1.3 lb/ton (0.65 kg/tonne) of zinc, and 0.8 lb/ton (0.4 kg/tonne) of lead. Information obtained in February 1974 showed that arsenic emission at smelters processing arsenical copper ores was much reduced from the 1968 emission and that the average arsenic emission from copper smelting was 2.1 lb/ton (1.05 kg/tonne) of metal. No new data on emission from zinc and lead smelters are available. However, some information was obtained on the arsenic content of ores and concen- trates. On the basis of the indicated smelter inputs of arsenical and nonarsenical concentrates and estimated percentage stack losses for smelters treating various types of ore, the recovery factors were estimated. These estimates are similar to those determined by the Davis study for lead and zinc smelters.~96 Arsenic is in all coal and may be associated with metal sulfides, clay minerals, or organic material in the coalbed. Using data developed by Abernethy,2 Davison estimated that U.S. coal contains arsenic at an average of 10 ppm in eastern fields, 5 ppm in midwestern fields, and l ppm in western fields. A small fraction of the arsenic in coal escapes dust-collecting equipment and reaches the atmosphere. Cuffe and Gerstle estimated the average arsenic discharge to the atmosphere from power plants at 0.000064 grains (0.004 ma) per standard cubic foot, with l lb (0.4536 kg) of coal being burned for each 160 scf of flue gas.~90 This is equivalent to 1.4 ppm of the coal burned. This factor should be applicable to industry-wide coal use, inasmuch as nearly all coal consumed is burned in plants with fly-ash control equipment.
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Distribution of Arsenic in the Environment 75 Assuming 600 million tons (544 million tonnes) of coal burned per year in the United States' this would correspond to the emission of 840 tons (762 tonnes) of arsenic. The arsenic content of petroleum was investigated by Davis and Associates' who obtained analyses of 110 oils.~96 The average content was 0.042 ppm' or about 5.2 kg/million barrels. A future problem may arise from producing oil from shale. Oil from Colorado shale contained arsenic at 82 ppm. This arsenic, however' could be removed by contact with a mixture of nickel sulfide and molybdenum sulfide on alumina under reducing conditions.572 All arsenic present was removed until there was 7.2% arsenic on the alumina; thereafter, arsenic was found in the effluent gases. Inconsequential arsenic emission results from mining and processing of phosphate rock. The average arsenic content of mine run rock is estimated at 5.7 ppm; of washed rock' 12.0 ppm; and of discarded material, 2.6 ppm, on the basis of an analysis by Tremearne and Jacob797 and production data shown in Bureau of Mines Mineral Yearbc)oks.~2 Total arsenic placed in waste impoundments would be about 200 tons (181 tonnes) annually. of which perhaps 1 ton (0.9 tonne) would be expected to enter the atmosphere through weathering. About 17~ of phosphate rock is used for electric-furnace manufacture of elemental phosphorus. The total arsenic in the furnace feed is about 60 tons (54 tonnes)' of which only a small proportion would reach the atmosphere. Iron ore contains arsenic, but only insignificant quantities of it are emitted during iron and steel production. Boyle and Jonasson showed arsenic contents of hematite up to 160 ppm and of magnetite up to 3 ppm.94 Arsenic occurs in part in the form of scorodite, a very stable arsenate of iron. In the blast furnace, the arsenic compounds are reduced to elemental arsenic, which combines with iron to form iron arsenide and dissolves in the metal; very little of the contained arsenic reaches the atmosphere. Table 3-20 shows an industrial balance for arsenic emission into the environment based on the estimated emission factors, the rate of consumption of mineral fuels, and the rate of production of nonferrous metals, including arsenic. Cartons surveyed arsenic input and movement in the United States. He estimated a total movement of about 119,000 tons (108,000 tonnes) of arsenic per year (Table 3-211. He distinguished between arsenic that is found in end products and arsenic that is dissipated onto land' emitted in air and water, or destined for landfills. Of the 108~000 tonnes, most is fixed in products in which the arsenic is immobile or is
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76 TABLE 3-21 Summary of U.S. Arsenic Flow, Dissipation, and Emission, 1974a ARSENIC Location of Arsenic Arsenic Flow, Ready Environ tonsb mental Transport End products: Steel Cast iron Other Dissipation to land: Steel slag Pesticides Copper leach liquor Other Airborne emission: Waterborne effluent: Phosphate detergents Other Landfill wastes: 26,438 17,089 3,638 5,711 63,030 39,690 11,565 9,702 2,073 9,757 Losses from copper-smelting 5,292 Pesticides 2,536 Coal 717 Other 1,212 165 121 44 19,691 10,584 3,748 1,984 3,375 Copper flue dusts Copper-smelting slag Coal fly ash Other No No No Unknown Yes Yes Yes Yes Yes Yes Yes Yes Yes No No No No Derived from Carton. bTo convert to tonnes, multiply values in table by 0.9072. deposited in landfills as waste material. The remainder is in a form that can move readily within the environment. About half the mobile arsenic comes from the use of pesticides. That which is applied to land becomes predominantly fixed in insoluble compounds and is only minimally available for transport. Arsenic that is emitted into air or water is most mobile and of greatest concern to the general population surrounding the points of emission. It is the air- borne arsenic trioxide residues that have been implicated in the arsenic-cancer question. This topic is discussed in Chapter 6. Arsenic from man-made sources eventually reaches the soil. Pro- cessed arsenic is applied by way of pesticides and through natural contamination of fertilizer materials. Arsenic that is gaseous or is adsorbed onto particulate matter is removed from the atmosphere through fallout or in rain. It is-deposited on vegetation, on soil, or in
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Distribution of Arsenic in the Environment 77 water. Once in the water, arsenic can be accumulated to some extent by various forms of aquatic life. Arsenate in solution is adsorbed or incorporated into phytoplankton and algae, and an organic compound is synthesized. Fish, when they consume the algae, incorporate this organic arsenic compound. In some cases, the arsenical is further metabolized to yield high-molecular-weight lipid materials, proteins, or easily soluble low-molecular-weight compounds. The arsenical from aquatic life, when consumed, is generally eliminated with very little accumulation. i7~ Pesticidal arsenic that is deposited on the land may have several fates. A portion of methanearsonic acid and cacodylic acid may be reduced to volatile arsines (under both aerobic and anaerobic condi- tions), but the predominant degradation product is arsenate.879 Under anaerobic conditions, these two compounds are reduced to volatile arsines. Arsenate and arsenite are also reduced or methylated to volatile arsines under some conditions. ~75 499 Braman detected di- methylarsine and trimethylarsine or their oxidation products above grass that had been treated with sodium arsenite, methanearsonic acid, cacodylic acid, and phenylarsonic acid. 97 Volatile arsenicals were detected from soils treated with sodium arsenate, MSMA, and cacodylic acid. Volatilization occurred under both aerobic and anaerobic condi- tions. Amounts volatilized were 0.64, 8.22, and 14.10% of the applied arsenate, MSMA, and cacodylic acid, respectively, in 150 days under aerobic conditions. Under anaerobic conditions, the amounts produced from arsenate, MSMA, and cacodylic acid were 1.60, 0.84, and 4.485S, respectively. Regardless of initial form or oxidative condition, only dimethylarsine was detected.875 Arsenate, from pesticide or from fallout and runoff, is fixed in the soil as slightly soluble salts of iron, aluminum, calcium, and mag- nesium. These may be true compounds or surface-adsorbed reaction products. In addition, some arsenic is bound in organic forms in the soil. The arsenic that is not in an insoluble form is available for leaching into ground water, is available for uptake by plants and trees, and appears in spring water. As indicated earlier, all vegetation contains arsenic. Burning of agricultural wastes and forest and grass fires redistribute arsenic into the atmosphere, from which it is redeposited on the earth through particulate fallout or rain. Fungi and bacteria in the soil metabolize arsenic and the methylated derivatives to methylar- sines. The methylarsines are unstable and are oxidized to As(V). Some of the reactions are shown in Table 3-22. These processes are mediated by microorganisms, as well as by chemical action. The faster reactions are the more environmentally important ones. The stable forms of
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Distribution of Arsenic in the Environment 79 man-made arsenic in the environment are o-arsenic acid and its salts. All other forms of methylated arsenic compounds yield o-arsenic acid in soil as a major sink. This form, however, can be methyl- ated and put back into the cycle in nature. Braman detected arsenic in the III, V, methanearsonic, and cacodylic forms in Florida water. 99 His samples could not have been contaminated by pesticide application, so these forms appear to be part of the natural cycle. The most important concept with respect to arsenic cycling in the environment is constant change. Arsenic appears everywhere in every living tissue and is constantly being oxidized, reduced, or otherwise metabolized. In the soil environment, insoluble or slightly soluble compounds are constantly being resolubilized and the arsenic pre- sented for plant uptake or reduction by organisms and chemical pro- cesses. Man has modified the arsenic cycle only by causing localized high concentrations.
Representative terms from entire chapter: