P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 et aL, 1992), though significant seasonal variations in As(Ill/AsT ratios varying between 0. 1-0.9 but are typ speciation as well as absolute concentration have been cally around 0.5-0.6(DPHE/BGS/MML, 1999: Smedley found Concentrations and relative proportions of As(V et al, 2001b). Ratios in reducing groundwaters from Inner and As(lIn vary according to changes in input sources, Mongolia are typically 0.6-0.9(Smedley et al., 2001a) redox conditions and biological activity. The presence of Concentrations of organic forms are generally low or As(lID may be maintained in oxic waters by biological negligible in groundwaters(e.g. Chen et al., 1995) reduction of As(V). particularly during summer month Higher relative proportions of As(lln have been found 2. 4. Impact of redox kinetics on arsenic speciation in river stretches close to inputs of As(lI-dominated industrial effluent(Andreae and Andreae, 1989)and Redox reactions are important for controlling the waters with a component of geothermal water. behaviour of many major and minor species in natural Proportions of As(lln) and As(v) are particularly waters, including that of As. However, in practice. variable in stratified lakes where redox gradients can be redox equilibrium is often achieved only slowly and the with estuarine waters, distinct changes in As speciation ments (O, C, N,S and Fe). Redox-sensitive minor and occur in lake profiles as a result of redox changes. For trace elements such asAs respond to these change example, in the stratified, hypersaline and hyperalkaline rather than control them. The slow rate of many het Mono Lake( California, USA), there is a predominance erogeneous redox reactions is supported by the studies of As(V) in the upper oxic layer and of As(lln) in the of Wersin et al. 1991) who estimated that the complete 2000). Rapid oxidation of As(im occurs as a result of Swiss lake sediment would take more than I ka. Equili- microbial activity during the early stages of lake turn- brium thermodynamic calculations predict that As(V) over(Oremland et al., 2000). The As oxidation occurs concentrations should be greater than As(lincon- before Fe(ln) oxidation. Unlike Mono Lake, speciation centrations in all but strongly reducing conditions, i.e. of As in lakes does not necessarily follow that expected where SO4 reduction is occurring. While this is indeed from thermodynamic considerations. Recent studies often found to be the case, such theoretical behaviour is have shown that arsenite predominates in the oxidised not necessarily followed quantitatively in natural waters epilimnion of some stratified lakes whilst arsenate may where different redox couples can point to different persist in the anoxic hypolimnion(Kuhn and Sigg, 1993; implied redox potentials(Eh values), reflecting thermo- Newman et aL., 1998). Proportions of As species may also dynamic disequilibrium( Seyler and Martin, 1989; Eary vary according to the availability of particulate Fe and and Schramke, 1990: Kuhn and Sigg, 1993). In Oslo- Mn oxides(Pettine et al. 1992; Kuhn and Sigg, 1993) fjord, Norway, As(lIn) was found under oxidising con- Organic forms of As are usually minor in surface ditions(Abdullah et aL, 1995). Also, in oxygenated waters. In lake waters from Ontario. Azcue and nriagu seawater, the As(V)/As(III) ratios should be of the order 1995)found As(lll) concentrations of 7-75 ug I of 105-1026(Andreae, 1979)whereas measured ratic As(V)of 19-58 ug I-I and only 0.01-1.5 ug I-I of of 0. 1-250 have been found, largely supported by bio- onganic As. Nonetheless, proportions of organic forms logical transformations (Johnson and Pilson, 1975 As can increase as a result of methylation tions Cullen and Reimer, 1989). Oxidation of As(ln by dis- catalysed by microbial activity(bacteria, yeasts, algae). solved O2, so-called oxygenation, is a particularly slow The dominant organic forms found are dimethylarsinic eaction. For example, Johnson and Pilson(1975)gave acid (DMAA: (CH3)2Aso(OH)) and mono- half-lives for the oxygenation of As(In) in seawater methylarsonic acid (MMAA; CH3AsO(OH)), where As ranging from several months to a year is present in both cases in the pentavalent oxidation Other studies have demonstrated the stability of state Proportions of these two species have been noted As(V)/As(lID) ratios over periods of days or weeks dur- to increase in summer as a result of increased microbial ing water sampling when no particular care was taken to activity(e.g. Hasegawa, 1997). The organic species may prevent oxidation, again suggesting relatively slow oxi- so be more prevalent close to the sediment-water dation rates. Andreae(1979)found stable ratios in sea- interface(Hasegawa et al, 1999) water for up to 10 days(4 C). Cherry et al. (1979)found In groundwaters, the ratio of As(lll) to As(V) can from experimental studies that the As(v)/As(lll) ratios vary greatly as a result of variations in the abundance of were stable in anoxic solutions for up to 3 weeks but redox-active solids, especially organic C, the activity of that gradual changes occurred over longer timescales microorganisms and the extent of convection and diffu Cherry et al. (1979)suggested that the measured As(V/ on of O2 from the atmosphere. In strongly reducing As(lIn ratios in natural waters, especially groundwaters, aquifers(Fe(lll) and SOa reducing aquifers), As(lll might be used as an indicator of the ambient redox(eh) typically dominates, as expected from the redox sequenc conditions as the redox changes are sufficiently rapid to Reducing As-rich groundwaters from Bangladesh have occur over periods of years. Yan et al. (2000)have also
et al., 1992), though significant seasonal variations in speciation as well as absolute concentration have been found. Concentrations and relative proportions of As(V) and As(III) vary according to changes in input sources, redox conditions and biological activity. The presence of As(III) may be maintained in oxic waters by biological reduction of As(V), particularly during summer months. Higher relative proportions of As(III) have been found in river stretches close to inputs of As(III)-dominated industrial effluent (Andreae and Andreae, 1989) and in waters with a component of geothermal water. Proportions of As(III) and As(V) are particularly variable in stratified lakes where redox gradients can be large and seasonally variable (Kuhn and Sigg, 1993). As with estuarine waters, distinct changes in As speciation occur in lake profiles as a result of redox changes. For example, in the stratified, hypersaline and hyperalkaline Mono Lake (California, USA), there is a predominance of As(V) in the upper oxic layer and of As(III) in the reducing layer (Maest et al., 1992; Oremland et al., 2000). Rapid oxidation of As(III) occurs as a result of microbial activity during the early stages of lake turnover (Oremland et al., 2000). The As oxidation occurs before Fe(II) oxidation. Unlike Mono Lake, speciation of As in lakes does not necessarily follow that expected from thermodynamic considerations. Recent studies have shown that arsenite predominates in the oxidised epilimnion of some stratified lakes whilst arsenate may persist in the anoxic hypolimnion (Kuhn and Sigg, 1993; Newman et al., 1998). Proportions of As species may also vary according to the availability of particulate Fe and Mn oxides (Pettine et al., 1992; Kuhn and Sigg, 1993). Organic forms of As are usually minor in surface waters. In lake waters from Ontario, Azcue and Nriagu (1995) found As(III) concentrations of 7–75 mg l1 , As(V) of 19–58 mg l1 and only 0.01–1.5 mg l1 of organic As. Nonetheless, proportions of organic forms of As can increase as a result of methylation reactions catalysed by microbial activity (bacteria, yeasts, algae). The dominant organic forms found are dimethylarsinic acid (DMAA; (CH3)2AsO(OH)) and monomethylarsonic acid (MMAA; CH3AsO(OH)2), where As is present in both cases in the pentavalent oxidation state. Proportions of these two species have been noted to increase in summer as a result of increased microbial activity (e.g. Hasegawa, 1997). The organic species may also be more prevalent close to the sediment-water interface (Hasegawa et al., 1999). In groundwaters, the ratio of As(III) to As(V) can vary greatly as a result of variations in the abundance of redox-active solids, especially organic C, the activity of microorganisms and the extent of convection and diffusion of O2 from the atmosphere. In strongly reducing aquifers (Fe(III)- and SO4-reducing aquifers), As(III) typically dominates, as expected from the redox sequence. Reducing As-rich groundwaters from Bangladesh have As(III)/AsT ratios varying between 0.1–0.9 but are typically around 0.5–0.6 (DPHE/BGS/MML, 1999; Smedley et al., 2001b). Ratios in reducing groundwaters from Inner Mongolia are typically 0.6–0.9 (Smedley et al., 2001a). Concentrations of organic forms are generally low or negligible in groundwaters (e.g. Chen et al., 1995). 2.4. Impact of redox kinetics on arsenic speciation Redox reactions are important for controlling the behaviour of many major and minor species in natural waters, including that of As. However, in practice, redox equilibrium is often achieved only slowly and the redox potential tends to be controlled by the major elements (O, C, N, S and Fe). Redox-sensitive minor and trace elements such as As respond to these changes rather than control them. The slow rate of many heterogeneous redox reactions is supported by the studies of Wersin et al. (1991) who estimated that the complete reductive dissolution of Fe(III) oxides in an anoxic Swiss lake sediment would take more than 1ka. Equilibrium thermodynamic calculations predict that As(V) concentrations should be greater than As(III) concentrations in all but strongly reducing conditions, i.e. where SO4 reduction is occurring. While this is indeed often found to be the case, such theoretical behaviour is not necessarily followed quantitatively in natural waters where different redox couples can point to different implied redox potentials (Eh values), reflecting thermodynamic disequilibrium (Seyler and Martin, 1989; Eary and Schramke, 1990; Kuhn and Sigg, 1993). In Oslofjord, Norway, As(III) was found under oxidising conditions (Abdullah et al., 1995). Also, in oxygenated seawater, the As(V)/As(III) ratios should be of the order of 1015–1026 (Andreae, 1979) whereas measured ratios of 0.1–250 have been found, largely supported by biological transformations (Johnson and Pilson, 1975; Cullen and Reimer, 1989). Oxidation of As(III) by dissolved O2, so-called oxygenation, is a particularly slow reaction. For example, Johnson and Pilson (1975) gave half-lives for the oxygenation of As(III) in seawater ranging from several months to a year. Other studies have demonstrated the stability of As(V)/As(III) ratios over periods of days or weeks during water sampling when no particular care was taken to prevent oxidation, again suggesting relatively slow oxidation rates. Andreae (1979) found stable ratios in seawater for up to 10 days (4 C). Cherry et al. (1979) found from experimental studies that the As(V)/As(III) ratios were stable in anoxic solutions for up to 3 weeks but that gradual changes occurred over longer timescales. Cherry et al. (1979) suggested that the measured As(V)/ As(III) ratios in natural waters, especially groundwaters, might be used as an indicator of the ambient redox (Eh) conditions as the redox changes are sufficiently rapid to occur over periods of years. Yan et al. (2000) have also P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 527
P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 concluded that the As(v)/As(Ill) ratio may be used as a rapidly being catalysed by bacteria with rate constant reliable redox indicator for groundwater syste anging from 0.02 to 0.3 day-(Oremland et aL., 2000). (1988)found that the Eh calculated from the As(V)- cally and biologically(Abdullah et al, 19%ed chemi ever, this optimism may be unfounded since Welch et al Methylated As species are also readily oxidise As(ln couple neither agreed with that from the Fe(ll) Less is known about the rate of solid-phase reductio Fe(lll and other redox couples nor with the measured of As(V) to As(lIn but there have been some studies Eh. Measurements of Eh in natural waters using Pt with soils and sediments. The evidence from soils is th electrodes are known to be problematic(Lindberg and under moderately reducing conditions(Eh< 100 mV) Runnells, 1984). The reliability of the As redox couple induced by flooding, As(V) is reduced to As(lm) in a as a redox indicator therefore remains to be seen It matter of days or weeks and adsorbed As(V) is released clearly important that where such comparisons are as As(Ill(Masscheleyn et aL., 1991; Reynolds et al made, the Eh measurements are carried out without 1999). Masscheleyn et al.(1991) found from laboratory disturbing the natural redox environment (Yan et al. experiments that some of the as was released before Fe, 2000). In cases where the aquifer is strongly stratified implying reductive desorption from Fe oxides rather groundwater flow induced by pumping during sampling than reductive dissolution. Up to 10%o of the total As in or use may also lead to the mixing of waters with very the soil eventually became soluble. Smith and Jaffe different redox potentials. Perhaps the most that can b (1998)modelled As(V) reduction in benthic sediments as said at present is that the existence of As(lIn) implies a first order reaction with respect to arsenate, with a rate reducing conditions somewhere in the system coefficient of 125 a-I Laboratory studies show that the kinetics of oxyge- nation of As(ln are slowest in the slightly acid range, around pH 5 (Eary and Schramke, 1990)which is why 3. Sources of arsenic water samples are often acidified to about this ph to preserve their in situ speciation. Eary and Schramke 3.1. Minerals ( 1990) also gave an empirical rate equation for the reaction over the pH range 8-12.5. This was based on 3.1.1. Major arsenic minerals the concentration (activity) of the H,AsOs species in Arsenic occurs as a major constituent in more than solution. They suggested that the half-life for As(ln) in 200 minerals, including elemental As, arsenides, sul natural waters is 1-3 a, although the rate may be greater hides, oxides, arsenates and arsenites. A list of some of because of the presence of unknown aqueous species or the most common As minerals is given in Table 2. Most oxide particles, especially Mn oxides. Certainly there is are ore minerals or their alteration products. However, considerable evidence that mn oxides can increase the these minerals clatively rare in the natural environ- rate of As(ln oxidation with half-lives being reduced to nent. The greatest concentrations of these minerals as little as 10-20 min in the presence of Mn-oxide par- occur in mineralised areas and are found in close asso- ticles(Oscarson et al., 1981; Scott and morgan, 1995) ciation with the transition metals as well as Cd, Pb, ag, This is used to advantage in the removal of As(lln) from Au, Sb, P, w and Mo. The most abundant As ore drinking water(Driehaus et aL., 1995). The rate of oxida- mineral is arsenopyrite, FeAss. It is generally believed ion is independent of the concentration of dissolved O2 that arsenopyrite, together with the other dominant As (Scott and Morgan, 1995), the rate being controlled by the sulphide minerals realgar and orpiment, are only formed te of a surface reaction. Less is known about the role of under high temperature conditions in the earth's crust Fe oxides in altering the oxygenation kinetics. Phote However, authigenic arsenopyrite has been reported in chemical oxidation and reduction may be additional sediments by Rittle et al.(1995)and orpiment has factors in surface waters. Titanium-containing particles recently been reported to have been formed by may aid the photo-oxidation(Foster et al., 1998) bial precipitation(Newman et al., 1998). Although often In the natural environment, the rates of both As(ll) present in ore deposits, arsenopyrite is much less abun- xidation and As(V) reduction reactions are controlled dant than arsenian (As-rich") pyrite(Fe(s, As)) which isms and can be orders of magnitude is probably the most important source of As in or greater than under abiotic conditions. For example, zones(Nordstrom, 2000) sterile water samples have been observed to be less sus- Where arsenopyrite is present in sulphide ores asso- eptible to speciation changes than non-sterile ciated with sediment-hosted Au deposits, it tends to be (Cullen and reimer, 1989). Wilkie and Hering (1998) the earliest-formed mineral, derived from hydrothermal found that As(m in geothermal waters input to streams solutions and formed at temperatures typically of 100C in Sw USA oxidised rapidly downstream(pseudo first- or more. This is followed by the formation of rarer order half-life calculated at as little as 0.3 h) and they native As and thereafter arsenian pyrite. Realgar an attributed the fast rate to bacterial mediation. The orpiment generally form later still. This paragenetic duction of As(v) to As(Im in Mono Lake was also sequence is often refected by zonation within sulphid
concluded that the As(V)/As(III) ratio may be used as a reliable redox indicator for groundwater systems. However, this optimism may be unfounded since Welch et al. (1988) found that the Eh calculated from the As(V)– As(III) couple neither agreed with that from the Fe(II)– Fe(III) and other redox couples nor with the measured Eh. Measurements of Eh in natural waters using Pt electrodes are known to be problematic (Lindberg and Runnells, 1984). The reliability of the As redox couple as a redox indicator therefore remains to be seen. It is clearly important that where such comparisons are made, the Eh measurements are carried out without disturbing the natural redox environment (Yan et al., 2000). In cases where the aquifer is strongly stratified, groundwater flow induced by pumping during sampling or use may also lead to the mixing of waters with very different redox potentials. Perhaps the most that can be said at present is that the existence of As(III) implies reducing conditions somewhere in the system. Laboratory studies show that the kinetics of oxygenation of As(III) are slowest in the slightly acid range, around pH 5 (Eary and Schramke, 1990) which is why water samples are often acidified to about this pH to preserve their in situ speciation. Eary and Schramke (1990) also gave an empirical rate equation for the reaction over the pH range 8–12.5. This was based on the concentration (activity) of the H2AsO3 - species in solution. They suggested that the half-life for As(III) in natural waters is 1–3 a, although the rate may be greater because of the presence of ‘unknown aqueous species’ or oxide particles, especially Mn oxides. Certainly there is considerable evidence that Mn oxides can increase the rate of As(III) oxidation with half-lives being reduced to as little as 10–20 min in the presence of Mn-oxide particles (Oscarson et al., 1981; Scott and Morgan, 1995). This is used to advantage in the removal of As(III) from drinking water (Driehaus et al., 1995). The rate of oxidation is independent of the concentration of dissolved O2 (Scott and Morgan, 1995), the rate being controlled by the rate of a surface reaction. Less is known about the role of Fe oxides in altering the oxygenation kinetics. Photochemical oxidation and reduction may be additional factors in surface waters. Titanium-containing particles may aid the photo-oxidation (Foster et al., 1998). In the natural environment, the rates of both As(III) oxidation and As(V) reduction reactions are controlled by micro-organisms and can be orders of magnitude greater than under abiotic conditions. For example, sterile water samples have been observed to be less susceptible to speciation changes than non-sterile samples (Cullen and Reimer, 1989). Wilkie and Hering (1998) found that As(III) in geothermal waters input to streams in SW USA oxidised rapidly downstream (pseudo firstorder half-life calculated at as little as 0.3 h) and they attributed the fast rate to bacterial mediation. The reduction of As(V) to As(III) in Mono Lake was also rapidly being catalysed by bacteria with rate constants ranging from 0.02 to 0.3 day1 (Oremland et al., 2000). Methylated As species are also readily oxidised chemically and biologically (Abdullah et al., 1995). Less is known about the rate of solid-phase reduction of As(V) to As(III) but there have been some studies with soils and sediments. The evidence from soils is that under moderately reducing conditions (Eh<100 mV) induced by flooding, As(V) is reduced to As(III) in a matter of days or weeks and adsorbed As(V) is released as As(III) (Masscheleyn et al., 1991; Reynolds et al., 1999). Masscheleyn et al. (1991) found from laboratory experiments that some of the As was released before Fe, implying reductive desorption from Fe oxides rather than reductive dissolution. Up to 10% of the total As in the soil eventually became soluble. Smith and Jaffe´ (1998) modelled As(V) reduction in benthic sediments as a first order reaction with respect to arsenate, with a rate coefficient of 125 a1 . 3. Sources of arsenic 3.1. Minerals 3.1.1. Major arsenic minerals Arsenic occurs as a major constituent in more than 200 minerals, including elemental As, arsenides, sulphides, oxides, arsenates and arsenites. A list of some of the most common As minerals is given in Table 2. Most are ore minerals or their alteration products. However, these minerals are relatively rare in the natural environment. The greatest concentrations of these minerals occur in mineralised areas and are found in close association with the transition metals as well as Cd, Pb, Ag, Au, Sb, P, W and Mo. The most abundant As ore mineral is arsenopyrite, FeAsS. It is generally believed that arsenopyrite, together with the other dominant Assulphide minerals realgar and orpiment, are only formed under high temperature conditions in the earth’s crust. However, authigenic arsenopyrite has been reported in sediments by Rittle et al. (1995) and orpiment has recently been reported to have been formed by microbial precipitation (Newman et al., 1998). Although often present in ore deposits, arsenopyrite is much less abundant than arsenian (‘As-rich’) pyrite (Fe(S,As)2) which is probably the most important source of As in ore zones (Nordstrom, 2000). Where arsenopyrite is present in sulphide ores associated with sediment-hosted Au deposits, it tends to be the earliest-formed mineral, derived from hydrothermal solutions and formed at temperatures typically of 100 C or more. This is followed by the formation of rarer native As and thereafter arsenian pyrite. Realgar and orpiment generally form later still. This paragenetic sequence is often reflected by zonation within sulphide 528 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568
P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002 )517-568 Table 2 Major As minerals occurring in nature Mineral Occurrence Native arsenic Hydrothermal veins ein deposits and norites Realgar Ass ein deposits, often associated with orpiment, clays and limestones, also deposits n hot Orpiment Hydrothermal veins, hot springs Cobaltite High-temperature deposits, metar The most abundant As mineral ly in mineral veins Cu, Fe),S13 hydrothermal veins Hydrothermal veins Secondary mineral formed by oxidation of realgar, arsenopyrite and other As minerals FeAsO42H,0 Ni, Co)3(AsO4)2.8H20 Secondary mineral Mg:(AsO4h.8H20 Secondary mineral, smelter wastes Pharmacosiderite Fe3(AsO4)2(OH)3. 5H2o Oxidation product of arsenopyrite and other As minerals minerals, with arsenopyrite cores zoning out to arsenian acid mine drainage, and for the presence of As problems pyrite and realgar-orpiment rims. Oxides and sulphates around coal mines and areas of intensive coal burning. are formed at the latest stages of ore mineralisation High As concentrations are also found in many oxide (Arehart et al., 1993) minerals and hydrous metal oxides, either as part of the mineral structure or as sorbed species. Concentrations 3. 1.2. Rock-forming minerals in Fe oxides can also reach weight percent values Though not a major component, As is also often (Table 3), particularly where they form as the oxidation present in varying concentrations in other common products of primary Fe sulphide minerals, which have rock-forming minerals. As the chemistry of As follo an abundant supply of As. Adsorption of arsenate to closely that of s, the greatest concentrations of the ele- hydrous Fe oxides is particularly strong and sorbed ment tend to occur in sulphide minerals, of which pyrite loadings can be appreciable even at very low As con is the most abundant Concentrations in pyrite, chalco- centrations in solution(Goldberg, 1986: Manning and pyrite, galena and marcasite can be very variable, even Goldberg, 1996: Hiemstra and van Riemsdijk, 1996) within a given grain, but in some cases exceed 10 wt. Adsorption to hydrous Al and Mn oxides may also be (Table 3). Arsenic is present in the crystal structure of important if these oxides are present in quantity (e.g. many sulphide minerals as a substitute for s. Besides Peterson and Carpenter, 1983; Brannon and Patrick, being an important component of ore bodies, pyrite is 1987). Arsenic may also be sorbed to the edges of clays also formed in low-temperature sedimentary environ and on the surface of calcite ( Goldberg and Glaubig ments under reducing conditions. Such authigenic pyrite 1988), a common mineral in many sediments. However, plays a very important role in present-day geochemical these loadings are much smaller on a weight basis than cycles. It is present in the sediments of many rivers, for the Fe oxides. Adsorption reactions are responsible commonly forms preferentially in zones of intense As found in most natural wat oxic) concentrations of lakes and the oceans as well as of many aquifers. Pyrite for the relatively low(and non reduction such as around buried plant roots or other Arsenic concentrations in phosphate minerals are nuclei of decomposing organic matter. It is sometimes variable but can also reach high values, for example up to present in a characteristic form as framboidal pyrite. 1000 mg kg-I in apatite(Table 3). However, phosphate During the formation of this pyrite, it is likely that some minerals are much less abundant than oxide minerals of the soluble As will also be incorporated. Pyrite is not and so make a correspondingly small contribution to stable in aerobic systems and oxidises to Fe oxides with the As concentration in most sediments. Arsenic can the release of large amounts of So4, acidity and asso- also substitute for Si+, Al+, Fe+ and Ti ciated trace constituents, including As. The presence of mineral structures and is therefore present in many pyrite as a minor constituent in sulfide-rich coals is ulti other rock-forming minerals, albeit at much lower con ately responsible for the production of 'acid rain and centrations. Most common silicate minerals contain
minerals, with arsenopyrite cores zoning out to arsenian pyrite and realgar-orpiment rims. Oxides and sulphates are formed at the latest stages of ore mineralisation (Arehart et al., 1993). 3.1.2. Rock-forming minerals Though not a major component, As is also often present in varying concentrations in other common rock-forming minerals. As the chemistry of As follows closely that of S, the greatest concentrations of the element tend to occur in sulphide minerals, of which pyrite is the most abundant. Concentrations in pyrite, chalcopyrite, galena and marcasite can be very variable, even within a given grain, but in some cases exceed 10 wt.% (Table 3). Arsenic is present in the crystal structure of many sulphide minerals as a substitute for S. Besides being an important component of ore bodies, pyrite is also formed in low-temperature sedimentary environments under reducing conditions. Such authigenic pyrite plays a very important role in present-day geochemical cycles. It is present in the sediments of many rivers, lakes and the oceans as well as of many aquifers. Pyrite commonly forms preferentially in zones of intense reduction such as around buried plant roots or other nuclei of decomposing organic matter. It is sometimes present in a characteristic form as framboidal pyrite. During the formation of this pyrite, it is likely that some of the soluble As will also be incorporated. Pyrite is not stable in aerobic systems and oxidises to Fe oxides with the release of large amounts of SO4, acidity and associated trace constituents, including As. The presence of pyrite as a minor constituent in sulfide-rich coals is ultimately responsible for the production of ‘acid rain’ and acid mine drainage, and for the presence of As problems around coal mines and areas of intensive coal burning. High As concentrations are also found in many oxide minerals and hydrous metal oxides, either as part of the mineral structure or as sorbed species. Concentrations in Fe oxides can also reach weight percent values (Table 3), particularly where they form as the oxidation products of primary Fe sulphide minerals, which have an abundant supply of As. Adsorption of arsenate to hydrous Fe oxides is particularly strong and sorbed loadings can be appreciable even at very low As concentrations in solution (Goldberg, 1986; Manning and Goldberg, 1996; Hiemstra and van Riemsdijk, 1996). Adsorption to hydrous Al and Mn oxides may also be important if these oxides are present in quantity (e.g. Peterson and Carpenter, 1983; Brannon and Patrick, 1987). Arsenic may also be sorbed to the edges of clays and on the surface of calcite (Goldberg and Glaubig, 1988), a common mineral in many sediments. However, these loadings are much smaller on a weight basis than for the Fe oxides. Adsorption reactions are responsible for the relatively low (and non-toxic) concentrations of As found in most natural waters. Arsenic concentrations in phosphate minerals are variable but can also reach high values, for example up to 1000 mg kg1 in apatite (Table 3). However, phosphate minerals are much less abundant than oxide minerals and so make a correspondingly small contribution to the As concentration in most sediments. Arsenic can also substitute for Si4+, Al3+, Fe3+ and Ti4+ in many mineral structures and is therefore present in many other rock-forming minerals, albeit at much lower concentrations. Most common silicate minerals contain Table 2 Major As minerals occurring in nature Mineral Composition Occurrence Native arsenic As Hydrothermal veins Niccolite NiAs Vein deposits and norites Realgar AsS Vein deposits, often associated with orpiment, clays and limestones, also deposits from hot springs Orpiment As2S3 Hydrothermal veins, hot springs, volcanic sublimation products Cobaltite CoAsS High-temperature deposits, metamorphic rocks Arsenopyrite FeAsS The most abundant As mineral, dominantly in mineral veins Tennantite (Cu,Fe)12As4S13 Hydrothermal veins Enargite Cu3AsS4 Hydrothermal veins Arsenolite As2O3 Secondary mineral formed by oxidation of arsenopyrite, native arsenic and other As minerals Claudetite As2O3 Secondary mineral formed by oxidation of realgar, arsenopyrite and other As minerals Scorodite FeAsO4.2H2O Secondary mineral Annabergite (Ni,Co)3(AsO4)2.8H2O Secondary mineral Hoernesite Mg3(AsO4)2.8H2O Secondary mineral, smelter wastes Haematolite (Mn,Mg)4Al(AsO4)(OH)8 Conichalcite CaCu(AsO4)(OH) Secondary mineral Pharmacosiderite Fe3(AsO4)2(OH)3.5H2O Oxidation product of arsenopyrite and other As minerals P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 529
P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002 )517-568 Table 3 Typical As concentrations in common rock-forming minerals As concentration range(mg kg") References 0-77,000 Baur and Onishi(1969); Boyle and Jonasson(1973 Marcasite Dudas(1984): Fleet and Mumin(1997) Baur and Onishi(1969) 10-5000 Baur and Onishi (1969) Oxide up to 160 Baur and Onishi(19 Fe oxide(undifferentiated) up to 2000 Fe(ll) oxyhydroxide Pichler et al. (1999) Magnetite Baur and Onishi(1969) bartz 0.4-1.3 Baur and Onishi (1969) Feldspar Biotite amphibole BBBB uuuu dddd 0.05-0.8 Baur and Onishi(1969) Boyle ar n(1973) Boyle ar n(1973) on(1973) Sulphate minerals Gypsum/ anhydrite 1-6 Boyle and Barite Boyle and Jarosite 34-1000 Boyle and Other minerals Apatite 1-1000 Baur and Onishi(1969) Boyle and Jonasson(1973) Halite Stewart(1963 Fluorite Boyle and Jonasson(1973) round 1 mg kg-I or less. Carbonate minerals usually of high-As waters(Nicolli et al., 1989; Smedley et al contain less than 10 mg kg-I As(Table 3). 2002). This may relate to the reactive nature of recent acidic volcanic material, especially fine-grained ash and 3. 2. Rocks sediments and so its tendency to give rise to Na-rich high-pH ground waters(Section 5) 3.2.1. Igneous rocks Arsenic concentrations in igneous rocks are 3. 2.2. Metamorphic rocks low. Ure and Berrow (1982)quoted an average value of Arsenic concentrations in metamorphic rocks tend to 1.5 mg kg- for all igneous rock types(undisting reflect the concentrations in their igneous and sedimen- Averages for different types distinguished by silica con- tary precursors. Most contain around 5 mg kg- or less tent (Table 4) are slightly higher than this value but Pelitic rocks(slates, phyllites) typically have the highest generally less than 5 mg kg. volcanic glasses are only concentrations with on average ca 18 mg kg(Table 4) an averag (Table 4). Overall, there is relatively little difference 3.2.3. Sedimentary rocks between the different igneous rock types. Despite not The concentration of As in sedimentary rocks is typi- especially ashes, are often implicated in the generation slightly above average terrestrial abundance. Average
around 1mg kg1 or less. Carbonate minerals usually contain less than 10 mg kg1 As (Table 3). 3.2. Rocks, sediments and soils 3.2.1. Igneous rocks Arsenic concentrations in igneous rocks are generally low. Ure and Berrow (1982) quoted an average value of 1.5 mg kg1 for all igneous rock types (undistinguished). Averages for different types distinguished by silica content (Table 4) are slightly higher than this value but generally less than 5 mg kg1 . Volcanic glasses are only slightly higher with an average of around 5.9 mg kg1 (Table 4). Overall, there is relatively little difference between the different igneous rock types. Despite not having exceptional concentrations of As, volcanic rocks, especially ashes, are often implicated in the generation of high-As waters (Nicolli et al., 1989; Smedley et al., 2002). This may relate to the reactive nature of recent acidic volcanic material, especially fine-grained ash and its tendency to give rise to Na-rich high-pH groundwaters (Section 5). 3.2.2. Metamorphic rocks Arsenic concentrations in metamorphic rocks tend to reflect the concentrations in their igneous and sedimentary precursors. Most contain around 5 mg kg1 or less. Pelitic rocks (slates, phyllites) typically have the highest concentrations with on average ca. 18 mg kg1 (Table 4). 3.2.3. Sedimentary rocks The concentration of As in sedimentary rocks is typically in the range 5–10 mg kg1 (Webster, 1999), i.e. slightly above average terrestrial abundance. Average Table 3 Typical As concentrations in common rock-forming minerals Mineral As concentration range (mg kg1 ) References Sulphide minerals: Pyrite 100–77,000 Baur and Onishi (1969); Arehart et al. (1993); Fleet and Mumin (1997) Pyrrhotite 5–100 Boyle and Jonasson (1973); Marcasite 20–126,000 Dudas (1984); Fleet and Mumin (1997) Galena 5–10,000 Baur and Onishi (1969) Sphalerite 5–17,000 Baur and Onishi (1969) Chalcopyrite 10–5000 Baur and Onishi (1969) Oxide minerals Haematite up to 160 Baur and Onishi (1969) Fe oxide (undifferentiated) up to 2000 Boyle and Jonasson (1973) Fe(III) oxyhydroxide up to 76,000 Pichler et al. (1999) Magnetite 2.7–41Baur and Onishi (1969) Ilmenite <1Baur and Onishi (1969) Silicate minerals Quartz 0.4–1.3 Baur and Onishi (1969) Feldspar <0.1–2.1 Baur and Onishi (1969) Biotite 1.4 Baur and Onishi (1969) Amphibole 1.1–2.3 Baur and Onishi (1969) Olivine 0.08–0.17 Baur and Onishi (1969) Pyroxene 0.05–0.8 Baur and Onishi (1969) Carbonate minerals Calcite 1–8 Boyle and Jonasson (1973) Dolomite <3 Boyle and Jonasson (1973) Siderite <3 Boyle and Jonasson (1973) Sulphate minerals Gypsum/anhydrite <1–6 Boyle and Jonasson (1973) Barite <1–12 Boyle and Jonasson (1973) Jarosite 34–1000 Boyle and Jonasson (1973) Other minerals Apatite <1–1000 Baur and Onishi (1969), Boyle and Jonasson (1973) Halite <3–30 Stewart (1963) Fluorite <2 Boyle and Jonasson (1973) 530 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568
P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002 )517-568 Table 4 Typical As concentrations in rocks, sediments, soils and other surficial deposits Rock/sediment type As concentration average and/ No of Reference (mg kg") Ultrabasic rocks(peridotite, dunite, 1.5(0.03-15.8) Basic rocks(basalt) 23(0.18-113 Basic rocks(gabbro, dolerite) 1.5(0.06-28) Onishi and Sandell(1955); termediate(andesite, trachyte, latite) 2.7(0.55.8) Baur and Onishi(1969) Intermediate(diorite, granodiorite, syenite) 1.0(0.09-13.4) Boyle and Jonasson(1973) Acidic rocks(rhyolite) 4.3(3. re and Berrow(1982): Acidic rocks(granite, aplite 1.3(0.2-15) Riedel and Eikmann(1986 Acidic rocks(pitchstone (2.2-122) 5.5(2.2-7.6) 5.5(0.7-11) Phyllite/slate 1.1(<0.1-18.5) 63(0.445) 3-15(upto490 Non-marine shale/mudstone 0-12 4.1(0.6-120) 15 Onishi and Sandell (1955); 2.6(0.1-20.1 Baur and Onishi(1969); 21(0.4-188) 5 Boyle and Jonasson(1973); Iron formations and Fe-rich sediment 1-2900 Cronan(1972): Riedel and Evaporites(gypsum/anhydrite 3.5(0.1-10) Eikmann(1986): Welch et al. Coals 0.3-35,000 (1988): Belkin et al. (2000) Bituminous shale(Kupferschiefer 100-900 Unconsolidated sediments arIous 3(0.6-50) Azcue and Nriagu(1995) Alluvial sand (bangladesh) 9(1.06.2) BGS and DPHE (2001) Alluvial mud/clay(Bangladesh) 6.5(2.7-14.7) BGS and DPHE (2001) River bed sediments( Bangladesh) Datta and Subramanian(1997 Lake sediments, Lake Superi 2.0(0.58.0) Allan and Ball(1990) Lake sediments. British Colombia .5(0.944) Cook et al. (1995) Glacial till. British Colom bia 92(1.9170) Cook et al. (1995) World average river sediments Martin and Whitfield (1983) Stream and lake silt(Canada) Boyle and Jonasson(1973) Loess silts, Argentina 5.4-18 Smedley et al. (2002) Continental margin sediments Legeleux et al. (1994 (argillaceous, some anoxic variou 72(0.1-55) Boyle and Jonasson(1973) Peaty and bog 13(2-36 Ure and Berrow (1982) Acid sulphate cid sulphate Soils near sulph e Soils (Canada) Gustafsson and Tin(1994) 15-45 Dudas(1984): Dudas et al. (1988) 126(2-8000 Boyle and Jonasson(1973) Contaminated surficial deposits Mining.contaminated lake sediment 342(80-1104) Azcue et al.(1994, 1995 ritish Colombia (continued on next page)
Table 4 Typical As concentrations in rocks, sediments, soils and other surficial deposits Rock/sediment type As concentration average and/ or range (mg kg1 ) No of analyses Reference Igneous rocks Ultrabasic rocks (peridotite, dunite, kimberlite etc) 1.5 (0.03–15.8) 40 Basic rocks (basalt) 2.3 (0.18–113) 78 Basic rocks (gabbro, dolerite) 1.5 (0.06–28) 112 Onishi and Sandell (1955); Intermediate (andesite, trachyte, latite) 2.7 (0.5–5.8) 30 Baur and Onishi (1969); Intermediate (diorite, granodiorite, syenite) 1.0 (0.09–13.4) 39 Boyle and Jonasson (1973); Acidic rocks (rhyolite) 4.3 (3.2–5.4) 2 Ure and Berrow (1982); Acidic rocks (granite, aplite) 1.3 (0.2–15) 116 Riedel and Eikmann (1986) Acidic rocks (pitchstone) 1.7 (0.5–3.3) Volcanic glasses 5.9 (2.2–12.2) 12 Metamorphic rocks Quartzite 5.5 (2.2–7.6) 4 Hornfels 5.5 (0.7–11) 2 Phyllite/slate 18 (0.5–143) 75 Boyle and Jonasson (1973) Schist/gneiss 1.1 (<0.1–18.5) 16 Amphibolite and greenstone 6.3 (0.4–45) 45 Sedimentary rocks Marine shale/mudstone 3–15 (up to 490) Shale (Mid-Atlantic Ridge) 174 (48–361) Non-marine shale/mudstone 3.0–12 Sandstone 4.1 (0.6–120) 15 Onishi and Sandell (1955); Limestone/dolomite 2.6 (0.1–20.1) 40 Baur and Onishi (1969); Phosphorite 21 (0.4–188) 205 Boyle and Jonasson (1973); Iron formations and Fe-rich sediment 1–2900 45 Cronan (1972); Riedel and Evaporites (gypsum/anhydrite) 3.5 (0.1–10) 5 Eikmann (1986); Welch et al. Coals 0.3–35,000 (1988); Belkin et al. (2000) Bituminous shale (Kupferschiefer, Germany) 100–900 Unconsolidated sediments Various 3 (0.6–50) Azcue and Nriagu (1995) Alluvial sand (Bangladesh) 2.9 (1.0–6.2) 13 BGS and DPHE (2001) Alluvial mud/clay (Bangladesh) 6.5 (2.7–14.7) 23 BGS and DPHE (2001) River bed sediments (Bangladesh) 1.2–5.9 Datta and Subramanian (1997) Lake sediments, Lake Superior 2.0 (0.5–8.0) Allan and Ball (1990) Lake sediments, British Colombia 5.5 (0.9–44) 119 Cook et al. (1995) Glacial till, British Colombia 9.2 (1.9–170) Cook et al. (1995) World average river sediments 5 Martin and Whitfield (1983) Stream and lake silt (Canada) 6 (<1–72) 310 Boyle and Jonasson (1973) Loess silts, Argentina 5.4–18 Arribe´re et al. (1997); Smedley et al. (2002) Continental margin sediments (argillaceous, some anoxic) 2.3–8.2 Legeleux et al. (1994) Soils Various 7.2 (0.1–55) 327 Boyle and Jonasson (1973) Peaty and bog soils 13 (2–36) 14 Ure and Berrow (1982) Acid sulphate soils (Vietnam) 6–4125 Gustafsson and Tin (1994) Acid sulphate soils (Canada) 1.5–45 18 Dudas (1984); Dudas et al. (1988) Soils near sulphide deposits 126 (2–8000) 193 Boyle and Jonasson (1973) Contaminated surficial deposits Mining-contaminated lake sediment, British Colombia 342 (80–1104) Azcue et al. (1994, 1995) (continued on next page) P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 531