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Environment International 35 (2009) 1243–1255

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Environment International

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Review article Secondary arsenic minerals in the environment: A review

Petr Drahota a,b,⁎, Michal Filippi a a Institute of Geology, Academy of Sciences of the Czech Republic, v.v.i., Rozvojová 269, 165 00 Prague 6 — Lysolaje, Czech Republic b Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic article info abstract

Article history: Information on arsenic (As) speciation in solid materials is critical for many environmental studies concerned Received 10 March 2009 with As stability and/or mobility in natural As-impacted soils and mining or industrial sites contaminated by Accepted 10 July 2009 As. The investigation of these systems has provided evidence for a number of secondary As minerals that Available online 7 August 2009 have often played a signiﬁcant role in As mobility in the solid phase–water system. This paper presents a list of environmentally important secondary As minerals in contaminated soil and waste systems, summarizes Keywords: the information about their origin, occurrence, environmental stability and thermodynamics, and proposes Arsenic Secondary arsenic mineral several important avenues for further investigation. Environmental sample © 2009 Elsevier Ltd. All rights reserved. Solubility Environmental stability

Contents

1. Introduction ...... 1243 2. Environmentally important secondary as minerals ...... 1244 2.1. As oxides ...... 1244 2.2. Fe arsenates ...... 1244 2.2.1. Well-crystallized Fe arsenates ...... 1244 2.2.2. Poorly crystalline and amorphous Fe(III) arsenates ...... 1248 2.2.3. Pharmacosiderite group ...... 1250 2.2.4. Ca–Fe(III) arsenates ...... 1250 2.3. Fe sulphoarsenates and sulphoarsenites ...... 1251 2.4. Ca, Mg and Ca–Mg arsenates ...... 1251 2.5. Other metal arsenates ...... 1252 3. Summary and tasks for future research ...... 1253 Acknowledgements ...... 1253 References ...... 1253

1. Introduction sulphide minerals realgar and orpiment are also found. These primary As-bearing minerals are listed in Table 1. While As does not readily More than 300 arsenic (As) minerals are known to occur in nature. substitute into the structures of the major rock-forming minerals, it Of these approx. 60% are arsenates, approx. 20% are sulphides and can easily occur as a minor component in the abundant Fe sulphide sulphosalts, 10% are oxides and the rest are arsenites, arsenides, native mineral pyrite (e.g., Fleet et al., 1989, 1993; Savage et al., 2000; elements and metal alloys (Bowell and Parshley, 2001). The most Zachariáš et al., 2004; Blanchard et al., 2007). When these primary important primary As-bearing minerals are those where the As occurs minerals are exposed to the atmosphere and surface or ground waters, as the anion (arsenide) or dianion (diarsenide), or as the sulfarsenide alteration reactions cause formation of secondary As minerals, such as anion(s); these anions are bonded to metals such as Fe (löllingite, simple As oxides or more complex phases with As, oxygen and various arsenopyrite), Co (cobaltite) and Ni (gersdorfﬁte). The simple As metals. The latter group of minerals comprises arsenite and arsenate minerals that are formed by linking As(III)-oxo-anion groups or As (V)-oxo-anion groups, respectively, to a variety of mono-, di- and ⁎ Corresponding author. Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic. trivalent metal cations. Secondary arsenite minerals are rare in natural E-mail address: [email protected] (P. Drahota). environments, usually occurring as the products of hydrothermal

1244 P. Drahota, M. Filippi / Environment International 35 (2009) 1243–1255

Table 1 arsenolite, is a relatively common secondary mineral formed from Ideal formulas of the most common primary As-bearing minerals referred to in the text. oxidation of native As, arsenopyrite, löllingite, and from weathering of Mineral Formula System scorodite. Arsenolite has occurred as efﬂorescencents on the walls of Arsenopyrite FeAsS Monoclinic former underground mine workings, such as in the Jáchymov ore district, Cobaltite CoAsS Orthorombic Czech Republic (e.g., Ondruš et al.,1997). This mineral has been also found Enargite Cu3AsS4 Orthorombic in association with scorodite and native sulphur in arsenopyrite/ ﬁ Gersdorf te NiAsS Cubic löllingite-rich processing waste (Filippi, 2004)(Figs. 2 and 3a, b, c), Löllingite FeAs Orthorombic 2 weathered saprolite (Bowell,1994), and in an old tailings pile (e.g., Ashley Orpiment As2S3 Monoclinic

Pyrite FeS2 Cubic and Lottermoser, 1999; Juillot et al., 1999; Borba et al., 2003; Haffert and Realgar AsS Monoclinic Craw, 2008). Tennantite (Cu, Ag, Fe, Zn)12As4S13 Cubic The monoclinic dimorph of As trioxide, claudetite, is often intimately associated with arsenolite. It has occurred as the oxidation product of native As in dump wastes (Matsubara et al., 2001), as the alterations under mildly reducing conditions. Conversely, secondary speleothem related to the alteration of enargite, tennantite, or arsenate compounds comprise a large class of minerals that have been arsenopyrite in the breccia-pipe mineralization at Corkscrew Cave, found in many oxidized environments. Similar to phosphate minerals, Arizona (Onac et al., 2007), and as the efflorescents at the furnace and the arsenate tetrahedra is bonded to octahedrally coordinated condensed flue sites (Ashley and Lottermoser, 1999). It is assumed transition metal ions (e.g., Fe, Mn, Ni) or to large, divalent cations that claudetite could represent an important secondary reservoir of As (e.g., Ca, Ba, Pb). Because of variations and multiples of the bonding in the metasedimentary rocks of the Maine watershed, USA, patterns, the relatively open structures of some arsenate minerals originating from the orpiment and arsenopyrite oxidation (Foley cause extensive substitution of cations, anions and water, and solid and Ayuso, 2008). In situ formation of claudetite in the absence of Fe solutions (e.g., Mutter et al., 1984; Behrens et al., 1998). All the oxyhydroxides and aqueous Fe complexes from oxidation of orpiment secondary As minerals mentioned in the text are listed in Table 2. is assumed in the reaction (Eq. (1)) (Foley and Ayuso, 2008): Recently, the investigation of both the natural and anthropogenic ð Þþ ð Þþ ð Þ→ ð Þþ þð Þþ 2−ð ÞðÞ As geochemical anomalies, using modern analytical tools (micro- As2S3 s 3H2O l 6O2 g As2O3 s 6H aq 3SO4 aq 1 Raman, XAS techniques), has provided a better picture of As bonding to a The stabilities of arsenolite and claudetite are quite similar, since particular substrate. Wang and Mulligan (2008) compiled the research they have nearly similar free energies of formation: arsenolite on the surface structure and surface complexing of As species in the −576.34 kJ/mol and claudetite −576.53 kJ/mol (Nordstrom and inorganic solid phase that play an important role in governing As Archer, 2003). Nevertheless, claudetite is slightly more stable than mobility in most natural systems. As another result of this approach, a arsenolite under ambient conditions (the difference −0.19 kJ/mol). large number of secondary As minerals have been found in highly Arsenolite and claudetite dissolve at pHb8 and temperature to 90 °C contaminated soils, stream sediments, former industrial sites and mine according to the Eq. (2) (Pokrovski et al., 1996); these are stable in tailings (e.g., Utsunomiya et al., 2003; Paktunc et al., 2004; Cancès et al., equilibrium with waters of high pH (Nordstrom and Archer, 2003). 2008; Filippi et al., 2009). The ability of secondary As minerals to immobilize As and control its dissolved concentrations depends on the ð Þþ ð Þ→ 0ð ÞðÞ As2O3 s 3H2O l 2H3AsO3 aq 2 solubility of these phases, which is highly variable. Precipitation of secondary As minerals has always been reported in very rich-As Water in equilibrium with arsenolite can contain up to 10–16 g/L environments, where the amount of As usually exceeded the availability As (Pokrovski et al., 1996). Since it is highly soluble in water and acids, of the surface ligand-bonding sites, and the concentrations of dissolved As its presence in nature may strongly influence the As concentration in and metal cations exceeded the solubility product of the newly-formed acid mine/waste waters (Haffert and Craw, 2008). secondary As mineral. These environments are usually represented by severalchemicalsystemssuchasAs(III)–O (e.g., former processing 2.2. Fe arsenates plants), Fe(III)–As(V)±S(VI)–H2O(e.g.,lowpH-sulfidic mining sites), Ca, Mg±(K, Ba, Na)–Fe(III)–As(V)–H2O (e.g., high pH-mining sites and An examination of the accredited Fe arsenates by the International contaminated alkaline soils) and Pb±Fe(III)–As(V)±S(VI)–H2O(e.g., Mineralogical Association, as found in the Mindat.org mineral polymetallic mining sites and former industrial sites). database (Ralph and Chau, 2008), resulted in 16 minerals. Of these, The aim of this paper is to present a detailed summary of the current only scorodite, kaňkite, symplesite, pharmacosiderite, arseniosiderite knowledge of the important secondary As minerals documented in and yukonite are relatively common in highly contaminated sites and contaminated environment (Figs. 1 and 2). Fairly current extensive have been studied in the context of environmental issues. The reviews on As geochemistry (e.g., Riveros et al., 2001;Smedleyand majority of these minerals are members of Fe(II, III)–As(V) and Ca– Kinniburgh, 2002; O'Day, 2006) have already listed major primary and Fe(III)–As(V) systems that often prevail in contaminated natural secondary As minerals, which has helped to explain diverse properties of environments. As in the natural environments. This review focuses on secondary As minerals from prevalent chemical systems in nature and describes origin, 2.2.1. Well-crystallized Fe arsenates occurrence, environmental stability and thermodynamics of the second- Under acidic conditions of the Fe(III)–As(V) system, scorodite is by ary As minerals from these systems. Minerals are sorted according to the far the most common secondary As mineral, usually originating from chemical composition based on the anionic constituents, and divided into arsenopyrite or As-bearing pyrite oxidation. Scorodite was found as groups which are commonly used in the mineralogical and environmental the main secondary As mineral in many types of environments (Figs. 1 literature. and 2), ranging from natural weathered mineralized rocks (e.g., Utsunomiya et al., 2003), various types of naturally contaminated soils 2. Environmentally important secondary as minerals in the different climatic areas (e.g, Bowell, 1994; Morin et al., 2002; Pfeifer et al., 2004; Filippi et al., 2007) to various mine and industrial 2.1. As oxides wastes and tailings contaminated by As (e.g., Davis et al., 1996; Foster et al., 1998; Juillot et al., 1999; Craw et al., 2002; Néel et al., 2003; Arsenic trioxide is the primary product of As smelters. In nature, it Filippi, 2004; Frau and Ardau, 2004; Paktunc et al., 2004; Flemming exists in two allotropic modifications. The most common cubic form, et al., 2005; Mahoney et al., 2005; Mains and Craw, 2005; Salzsauler Author's personal copy

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Table 2 Secondary As minerals referred to in the text and their solubility data for standard state conditions, 25 °C and 1 atm.

0 Mineral group Mineral Formula System log Ksp ΔGf Arsenic solubility Reference (kJ/mol) conditionsa b As oxides Arsenolite As2O3 Cubic −1.37 −576.34 – Nordstrom and Archer (2003) –– 11.1 g/L; 22 °C Pokrovski et al. (1996) b Claudetite As2O3 Monoclinic −1.40 −576.53 – Nordstrom and Archer (2003) –– 10.1 g/L; 22 °C Pokrovski et al. (1996) cd Fe arsenates AFA/pitticite Fex(AsO4)y(SO4)z·nH2O Amorphous −23.0 −1268.72 Tozawa et al. (1978) Langmuir et al. (2006) −22.67 −1267.1 75–15370 mg/L at Robins (1987) pH 0.5–2.4e –– 25–130 mg/L at Tozawa et al. (1978) pH 1.82–3.10; 25 °C

Arseniosiderite Ca2Fe3O2(AsO4)3·3H2O Monoclinic –– 3.1–27 mg/L at Krause and Ettel (1989) pH 6.85–8.15; 25 °C

Kaatialaite Fe(H2AsO4)3·5(H2O) Monoclinic –– 5.4 g/L; TCLP test Taboada et al. (2008)

Kaňkite Fe3(AsO4)·3.5H2O Monoclinic –– –

Kolfanite Ca2Fe3O2(AsO4)3·2H2O Monoclinic –– 1.36 mg/L at pH 8; 20 °C Becze and Demopoulos (2007)

Parasymplesite Fe3(AsO4)2·8H2O Monoclinic –– –

Pharmacosiderite K[Fe4(OH)4(AsO4)3]·6.5H2O Cubic –– – c Scorodite FeAsO4·2H2O Orthorombic −25.4 −1282.42 0.33–5.89 mg/L at Bluteau and Demopoulos pH 5.01–6.99; 22 °C (2007) −25.86 −1285.05cdKrause and Ettel (1988) Langmuir et al. (2006) −24.41 −1279.2 0.11–463 mg/L at Krause and Ettel (1988) pH 0.97–7.92; 23 °C −21.7 −1263.52 1.8–10.3 mg/L at Dove and Rimstidt (1985) pH 5.53–6.36; 25 °C f Symplesite Fe3(AsO4)2·8H2O Triclinic −33.25 −3751.02 0.024–7 mg/L at Johnston and Singer (2007) pH 6.0–9.1; 27 °C† −40.43 −3792.01f – Khoe et al. (1991)

Yukonite Ca7Fe12(AsO4)10(OH)20· Amorphous –– 1.16–5.11 at pH 7.56–8.82; Becze and Demopoulos (2007)

15H2O (?) 20 °C –– 6.3–51 mg/L at Krause and Ettel (1989) pH 5.5–6.15; 25 °C g Fe sulphoarsenates/ Beudantite PbFe3(AsO4)(SO4)(OH)6 Hexagonal −10.53 −3055.6 – Gaboreau and Vieillard (2004) sulphoarsenites −15 −3081.12g – Roussel et al. (2000) b0.02 mg/L at pH 4.3–4.65; Krause and Ettel (1989) 25 °C h Bukovskýite Fe2(AsO4)(SO4)(OH)·7H2O Triclinic −9.00 −3480 – Gas'kova et al. (2008)

Sarmientite Fe2(AsO4)(SO4)(OH)·5H2O Monoclinic –– –

Tooeleite Fe6(AsO3)4(SO4)(OH)4·4H2O Monoclinic –– –

Zýkaite Fe4(AsO4)3(SO4)(OH)·15H2O Orthorombic –– –

Ca, Mg arsenates Haidingerite Ca(AsO3OH)·H2O Orthorombic −4.79 −1533 2050 mg/L at pH 6.22; 23 °C Bothe and Brown (1999b) –– 3120–4360 mg/L at pH 4.93; Swash and Monhemius (1996) TCLP test

Hörnesite Mg3(AsO4)2·8H2O Monoclinic –– 300–1100 mg/L at pH 6.5–7.4 Chukhlantsev (1956) i Pharmacolite Ca(HAsO4)·2H2O Monoclinic −4.68 −1808.21 5919 mg/L at pH 6.7; 25 °C Rodríguez-Blanco et al. (2007) –– 3120–4360 mg/L at pH 4.93; Swash and Monhemius (1996) TCLP test

Picropharmacolite Ca4Mg(AsO4)2 Triclinic –– –

(HAsO3OH)2·11H2O d Weilite CaHAsO4 Triclinic 2.36 −1292.48 Mahapatra et al. (1986) Gas'kova et al. (1999) –– 2170–3610 mg/L at pH 4.93; Swash and Monhemius (1996) TCLP test –– 540–764 mg/L at pH 3–8; Mahapatra et al. (1986) 35 °C d Other metal Adamite Zn2(AsO4)(OH) Orthorombic 5.71 −1252.29 Magalhães et al. (1988) Gas'kova et al. (1999)

arsenates Annabergite Ni3(AsO4)2·8H2O Monoclinic –– 47.8–1449 mg/L at pH 3–9; Yuan and Demopoulos (2005) 22 °C −28.38 −3488.57jdNishimura et al. (1990) Langmuir et al. (1999) −27.29j −3482.34 – Essington (1988) d Austinite CaZn(AsO4)(OH) Orthorombic 6.88 −1651.13 Magalhães et al. (1988) Gas'kova et al. (1999) k d Bayldonite PbCu3(AsO4)2(OH)2 Triclinic −0.19 −1810.6 Magalhães et al. (1988) Magalhães and Silva (2003) d Clinoclase Cu3(AsO4)(OH)3 Monoclinic 10.1 −1209.48 Magalhães et al. (1988) Gas'kova et al. (1999) d Conichalcite CaCu(AsO4)(OH) Orthorombic 1.29 −1470.17 Magalhães et al. (1988) Gas'kova et al. (1999)

Cornubite Cu5(AsO4)2(OH)4 Triclinic 12.4 −2057.9 – Magalhães et al. (1988) d Duftite PbCu(AsO4)(OH) Orthorombic −1.98 −959.92 Magalhães et al. (1988) Gas'kova et al. (1999)

Erythrite Co3(AsO4)2·8H2O Monoclinic –– –

Euchroite Cu2(AsO4)(OH)·3(H2O) Orthorombic 3.28 −1552.7 – Magalhães et al. (1988) l Fornacite Pb2Cu(AsO4)(CrO4)(OH) Monoclinic −44.66 −1956.86 0.03 mg/L at pH 6.96 Lee and Nriagu (2008) m Köttigite Zn3(AsO4)2·8H2O Monoclinic −32.40 −4030.48 16 mg/L at pH 4.87 Lee and Nriagu (2008)

Legrandite Zn2(AsO4)(OH)·H2O Monoclinic 5.97 −1488.6 – Magalhães et al. (1988) d Mansﬁeldite AlAsO4·2H2O Orthorombic −2.74 −1730.78 Whiting (1992) Gas'kova et al. (1999) −1.38n −1720.8 – Essington (1988) o d Mimetite Pb5(AsO4)3Cl Hexagonal −28.24 −2675.5 Inegbenebor et al. (1989) Magalhães and Silva (2003) −17.95o −2616.8 dComba et al. (1988) Twidwell et al. (1992) d Olivenite Cu2(AsO4)(OH) Monoclinic 2.39 −845.52 Magalhães et al. (1988) Gas'kova et al. (1999) (continued on next page) Author's personal copy

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Table 2 (continued)

0 Mineral group Mineral Formula System log Ksp ΔGf Arsenic solubility Reference (kJ/mol) conditionsa

Schultenite Pb(AsO3OH) Monoclinic −11.76 −805.66 8.8 mg/L at pH 4.68; 25 °C Lee and Nriagu (2008) −12.52p −809.62 dMagalhães et al. (1988) Magalhães and Silva (2003) d Sterlinghillite Mn3(AsO4)2·4H2O Monoclinic 7.42 −4045.17 Whiting (1992) Gas'kova et al. (1999)

0 0 0 The log Ksp and ΔGf values in italics were calculated from corresponding ΔGf and log Ksp values, respectively, and from the reactions marked with letters in superscripts. The ΔGf values for the aqueous species were taken from Wagman et al. (1982), Bard et al. (1985) and Nordstrom and Archer (2003). a pH and total As values are the highest and lowest levels presented in the reference. There is no relation between the pH and the solubility values in the column. b As2O3(s)+3H2O(l)=2H3AsO3(aq). c 3+ 3− FeAsO4·2H2O(s)=Fe (aq)+AsO4 (aq) +2H2O(l). d Reference that gives the original solubility data which has been recomputed in the reference in the next column. e The data was read from a graph. f + 2+ 2− Fe3(AsO4)2·8H2O(s)+4H (aq)=3Fe (aq) +2HAsO4 (aq)+8H2O(l). g + 2+ 3+ − 2− PbFe3(AsO4)(SO4)(OH)6(s) +8H (aq)=Pb (aq)+3Fe (aq)+H2AsO4 (aq)+SO4 (aq)+6H2O(l). h + 3+ − 2− Fe2(AsO4)(SO4)(OH)·7H2O(s)+3H (aq)=2Fe (aq)+H2AsO4 (aq) +SO4 (aq)+8H2O(l). i + 2+ 2− Ca(HAsO4)·2H2O(s)+H (aq)=Ca (aq) +H2AsO4 (aq)+2H2O(l). j 2+ 3− Ni3(AsO4)3·8H2O(s)=3Ni (aq)+2AsO4 (aq)+8H2O(l). k + 2+ 2+ − PbCu3(AsO4)2(OH)2(s)+6H (aq)=Pb (aq)+3Cu (aq)+2H2AsO4 (aq)+2H2O(l). l + 2+ 2+ − 2− Pb2Cu(AsO4)(CrO4)(OH)(s)+3H (aq) =2Pb (aq)+Cu (aq)+H2AsO4 (aq) +CrO4 (aq) +H2O(l). m + 2+ − Zn3(AsO4)2·8H2O(s) +4H (aq) =3Zn (aq)+2H2AsO4 (aq)+8H2O(l). n + 3+ − AlAsO4·2H2O(s)+2H (aq)=Al (aq)+H2AsO4 (aq)+2H2O(l). o + 2+ − − Pb5(AsO4)3Cl(s)+6H (aq)=5Pb (aq)+3H2AsO4 (aq)+Cl (aq)). p 2+ 2− PbHAsO4(s)=Pb (aq)+HAsO4 (aq).

et al., 2005). Scorodite persistence in acidic environments suggests calculated value of the solubility product (log Ksp =−25.4) from the that its solubility may control the concentration of dissolved As in long-term experiments at pH 5 and 7 was in general agreement with waters with pH less than approximately 3 (e.g., Frau and Ardau, 2004; (but slightly lower than) the solubility products of other authors as Moldovan and Hendry, 2005). Congruent dissolution of scorodite cited in Langmuir et al. (2006). The free energy of formation for occurs at low pH through the following reaction (Eq. (3)) (Dove and crystalline scorodite was given as −1279.2 kJ/mol (Krause and Ettel, Rimstidt, 1985): 1988). Subsequent works by Langmuir et al. (2006) and Bluteau and Demopoulos (2007) on crystalline scorodite, corresponded at slightly ð Þþ þð Þ→ −ð Þþ ð Þ2þð Þþ ð ÞðÞ more negative free energy of formation (Table 2). The highly different FeAsO4·2H2O s H aq H2AsO4 aq Fe OH aq H2O l 3 solubility values (Fig. 4) and up to four orders of magnitude difference At higher pH (pH 2.43, 23 °C, Krause and Ettel, 1989), scorodite for the published solubility products of Fe(III) arsenates (Table 2)were dissolves incongruently, forming Fe hydroxide and arsenate oxyanions probably a result of the different crystallinities of the studied materials − 2− and perhaps of the presence of a greater number of compounds in the (H2AsO4 or HAsO4 ;pKa2 =6.99, Nordstrom and Archer, 2003): tested materials (Krause and Ettel, 1988, 1989; Langmuir et al., 2006). − þ They found that both natural and synthetic crystalline scorodite is FeAsO ·2H OðsÞþH OðlÞ→H AsO ðaqÞþFeðOHÞ ðsÞþH ðaqÞð4Þ 4 2 2 2 4 3 approximately 100 times less soluble than the apparently amorphous Fe (III) arsenates (Table 2 and Fig. 4). The more negative free energy of ð Þþ ð Þ→ 2−ð Þþ ð Þ ð Þþ þð ÞðÞ FeAsO4·2H2O s H2O l HAsO4 aq Fe OH 3 s 2H aq 5 crystalline scorodite indicates that it is the more stable phase than amorphous Fe(III) arsenates (Table 2) and could be expected to form Scorodite has relatively low solubility under oxidizing conditions from less crystalline compounds with time. (Table 2) but, under reducing conditions (below Eh~100 mV), dissolved The precipitation and synthesis of scorodite have attracted sub- As concentrations can increase dramatically due to reductive dissolution stantial interest because of the potential of this compound to be a good of both Fe and As (Rochette et al., 1998). The reductive dissolution of carrier for the ﬁxation of As from As-rich but Fe-deﬁcient waste scorodite could be catalyzed by dissimilatory Fe(III)-reducing bacteria, solutions or solids (Riveros et al., 2001; Filippou and Demopoulos,1997). e.g., Shewanella alga (Cummings et al., 1999)orDesulfuromonas Precipitation of scorodite has been observed under hydrothermal palmitatis (Papassiopi et al., 2003), which release dissolved arsenate as conditions (e.g., Dutrizac and Jambor, 1988) and also at temperatures a result of dissimilatory reduction of Fe(III) to Fe(II). below the boiling point using a combination of seeding and super- The solubility products of Fe(III) arsenates, including scorodite, have saturation control applied to chloride (Demopoulos et al., 1995)and been studied by many authors (e.g., Chukhlantsev, 1956; Tozawa et al., sulphate solutions (Singhania et al., 2005). 1978; Dove and Rimstidt, 1985; Krause and Ettel, 1988, 1989; Zhu and Kaňkite is a 1:1 crystalline ferric mineral like scorodite. In contrast to Merkel, 2001; Harvey et al., 2006; Langmuir et al., 2006; Bluteau and scorodite, it is generally a rare mineral in nature; however, in the past few Demopoulos, 2007) and the consequent solubility data for studied years, it has been detected in soil (e.g., Bowell, 1994), mine tailing compounds exhibits large variations. More recently, Langmuir et al. (DeSisto, 2008) and some mine wastes as a locally common mineral. Rich (2006) re-evaluated the published solubility data for scorodite from occurrence of kaňkite has been documented in strongly weathered

Krause and Ettel (1988) (log Ksp =−25.86±0.03) and obtained values of historical mine wastes in several sites in central Europe (e.g., Čech et al., ~90 mg/L As at pH 1 and pH 5, and ~0.4 mg/L As at pH 2.5. Harvey et al. 1976; Ondruš et al.,1999; Hloušek, 2000; Siuda, 2004)(Fig.1a, b) and New (2006) reported minimum solubility values of ~19 mg/L As at pH 1; in the Zealand (Haffert and Craw, 2007). It is often associated with scorodite, presence of goethite, the solubility value is ~7 mg/L As at pH 2.5 and with amorphous Fe(III) arsenates (Fig. 3e, f, g) and usually with Fe(III) ~1200 mg/L As at pH 5; in the presence of ferrihydrite, the minimum sulphoarsenates, such as bukovskýite and zýkaite. In these mineral solubility value of As is ~0.1 mg/L at pH 2.5 and ~4 mg/L at pH 5. Bluteau associations, kaňkite is usually the youngest phase (Filippi et al., 2009). In and Demopoulos (2007) reported much lower solubility values of addition it is less stable than scorodite (Čech et al., 1976) so its ability to 0.35 mg/L As at pH 5. They found that the dissolution of scorodite over release As and inﬂuence its aqueous concentration under speciﬁc the environmentally important range of pH 5 to 9 was very low. The geochemical environments should be very high. Only two studies have Author's personal copy

P. Drahota, M. Filippi / Environment International 35 (2009) 1243–1255 1247

Fig. 1. Type old mining sites for secondary As minerals (a, b) and a natural As-rich geochemical anomaly (c) with occurrence of secondary As minerals (all sitesarefromtheCzechRepublic). a — Forested mediaeval waste dump close to the Giftkies arsenopyrite mine (Jáchymov ore district): rich occurrence of Fe(III) arsenates; b — large mediaeval waste dump near the Kuntery complex ore mine in Kaňk (Kutná Hora ore district): rich occurrence of Fe(III) arsenates and sulphoarsenates; c — agriculturally exploited ﬁeld with arsenopyrite-gold mineralization of the Mokrsko gold deposit close to the village of Mokrsko: common occurrence of Ca–Fe arsenates. The secondary As minerals from these sites are given in Fig. 3.

concerned in the identiﬁcation of this mineral in complex environmental The mineral kaatialaite is very rare secondary Fe(III) arsenate that samples (Frost and Kloprogge, 2003; Filippi et al., 2009). No solubility and has been reported in very acidic environments. Raade et al. (1984) thermodynamic data have been reported for this phase. described this mineral in the waste dump of the löllingite-containing Author's personal copy

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Fig. 2. Example of the derelict processing plant site in the Přebuz abandoned mine area (Czech Republic) with ﬁnely milled arsenopyrite/löllingite concentrate from the 1950s before (2003) and after (2009) partial remediation. The surﬁcial part of the concentrate body was cemented and partly overcrusted by scorodite, arsenolite and rare kaatialaite. Examples of the secondary As minerals are given in Fig. 3.

Kaatiala pegmatite mine in Finland. Filippi (2004) described rare scorodite) are frequently reported as important As-carriers at highly kaatialaite with scorodite and arsenolite in arsenopyrite/löllingite- contaminated sites (Figs.1a,band3e,f,g). These compounds are rich processing waste (Figs. 2 and 3a) with pH of the leachate close to characterized by two broad peaks in the powder XRD patterns (e.g., 1. According to Zhu and Merkel (2001) kaatialaite is more stable under Paktunc et al., 2008); therefore, they are usually referred to as amorphous strong acid conditions (pHb2) than scorodite. This means that during ferric arsenate (AFA) (Leblanc et al., 1996; Langmuir et al., 1999; Pichler the dissolution of scorodite under strong acid conditions when pH et al., 2001; Gieré et al., 2003; Paktunc et al., 2004; Salzsauler et al., 2005) continues to decrease, scorodite can also change into kaatialaite or pitticite (Filippi et al., 2004; Siuda, 2004; Salzsauler et al., 2005)which (Eq. (6)). comprises amorphous compounds with no apparent stoichiometry (Dunn, 1982). The local structure of AFA has no resemblance to that of ð Þþ þð Þþ ð Þ→ ð Þ ð Þ 3FeAsO4·2H2O s 6H aq 3H2O l Fe H2AsO4 3·5H2O s scorodite or other crystalline Fe(III) arsenates (Paktunc et al., 2008). It is þ þ 2Fe3 ð6Þ postulated that AFA is a small polymer of single chains of corner-sharing FeO octahedra with bridging arsenate tetrahedra alternating along the The toxicity leaching procedure test performed on synthetic 6 chains. Natural occurrences of AFA accommodate wide variations in their kaatialaite indicated very high solubility values in weak 0.004 M composition. High amounts of S, similar to less P and Ca, are apparently acetic acid (5.1 g/L As) and water (5.4 g/L As) (Taboada et al., 2008). not essential for its formation (Dunn, 1982). High-sulphur AFA has often Consequently, it cannot be proposed as a suitable compound for As been referred to as amorphous iron sulphoarsenates (AISA) (e.g., Gieré disposal. et al., 2003; Salzsauler et al., 2005). AFA is usually closely associated with On the contrary, ferrous arsenate is of particular interest as a low scorodite, kaňkite (Fig. 3f, g) and crystalline sulphoarsenates at low pH, solubility mineral (Table 2)(Khoe et al., 1991; Johnston and Singer, 2007). such as in sulphide-rich mine dumps, tailings and acid mine drainage It is represented by two minerals, symplesite and parasymplesite, (e.g., Courtin-Nomade et al., 2002; Gieré et al., 2003; Filippi et al., 2004; crystallizing in the triclinic and monoclinic systems, respectively. Siuda, 2004; Salzsauler et al., 2005). It commonly forms homogenous gel- Secondary precipitation of these compounds has not yet been widely like materials. The surface of the AFA is usually botryoidal and it exhibits a determined in natural systems, although it is supported by several indices perfect conchoidal fracture. Natural occurrences of AFA have also been (Lin and Puls, 2003). The chemical composition of a secondary alteration reported from a sea-floor hydrothermal vent (Rancourt et al., 2001)andat product in the former mine tailing at La Petite Faye, France closely geothermal springs (Inskeep et al., 2004). corresponds to symplesite (Courtin-Nomade et al., 2002). Symplesite was Langmuir et al. (2006) studied the behavior of As in buried mine identified as a minor secondary product of the microbial reduction by tailings using re-evaluated solubility products of AFA and scorodite. They dissimilatory Fe(III) reducing bacteria in microcosm experiments concluded that the conversion of AFA to scorodite is possible (at least in (Papassiopi et al., 2003), and geochemical modeling suggested its the region of pHb3). Two recent studies investigated this transformation oversaturation in many natural systems (Johnston and Singer, 2007). reaction and found strong pH and temperature control of the kinetics of Several studies reported thermodynamic data for ferrous arsenate the conversion (Le Berre et al., 2008; Paktunc et al., 2008). It is fast at pH 1, compounds (Khoe et al., 1991; Sadiq, 1997; Johnston and Singer, 2007). moderate at pH 2 and 3, and slow at pH 4.5 (Paktunc et al., 2008). Several On the basis of controlled laboratory precipitation experiments, Johnston studies have evaluated the solubility of Fe(III) arsenate compounds, and Singer (2007) recently reported a several orders of magnitude larger including AFA, and reported decreasing solubilities with increasing Fe/As solubility product for symplesite (log K =−33.25) than that in Khoe sp molar ratio of the compounds (e.g., Krause and Ettel,1989; Virčíková et al., et al. (1991) (log K =−40.43). Using their solubility constant, Johnston sp 1995; Courtin-Nomade et al., 2002). For example, when the molar Fe/As and Singer (2007) found oversaturation with respect to symplesite in ratio of solid was equal to 2, the total concentration of dissolved As was several reduced groundwaters of Bangladesh and alkaline drinking water 7mg/LatpH5and25°C(Krause and Ettel, 1989). At an Fe/As molar ratio in USA. The results suggested that symplesite could be a significant sink of 16, the total dissolved As values were b0.5 mg/L. However, compounds for As(V) in reduced waters and in some alkaline waters. with different Fe/As molar ratios are not composed of a single phase. Lower solubility values indicate ferrihydrite formation, which takes place 2.2.2. Poorly crystalline and amorphous Fe(III) arsenates at pH≥2. The results of Paktunc et al. (2008) suggest that AFA is absent in In addition to well-crystallized Fe(III) arsenates, poorly crystalline precipitates with As/Fe molar ratio N5 and negligible amounts of AFA were Fe(III) arsenates (usually considered to be amorphous equivalents of detected in the precipitates with an Fe/As molar ratio of 4. On the basis of Author's personal copy

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Fig. 3. Secondary As minerals from mining, industrial and natural rich-As environments documented in Figs.1 and 2:a— wall of the arsenopyrite/löllingite (ars/lö) concentrate body covered by powdery arsenolite and crusty scorodite; b — slightly weathered ars/lö fragment covered by a powdery mixture of arsenolite and scorodite (8 cm in diameter); c — transparent octahedral crystal of arsenolite (1 mm in diameter) on scorodite in a cavity of the ars/lö concentrate; d — highly weathered ars/lö fragment (5 cm in longer axis) almost transformed into scorodite; (photos a, b, c, d weretakeninthePřebuz abandoned mine); e — secondary Fe(III) arsenates and Fe(III) oxyhydroxides in situ (50 cm in length); f — rock fragments cemented by amorphous Fe(III) arsenate (AFA/pitticite) with rare green scorodite and kaňkite; g — weathered rock fragment coated by scorodite (grey-green), kaňkite (pistachio-green) and AFA/pitticite (brown) (photos e, f, g are from the Giftkies mine dump in the Jáchymov district); h — detail of the exposed mine dump slope with secondary Fe(III) arsenates and sulphoarsenates; i — disintegrating aggregate of scorodite and kaňkite in situ in the dump wall; j — nodular aggregate of bukovskýite 5 cm in diameter; j — (photos h, i, j are from the Kuntery mine dump in the Kaňk); k — sampling tube ﬁlled by weathered granodiorite with As mineralization; l — up to 4 mm large complex grains composed of pharmacosiderite, arseniosideriteandFeoxyhydroxides(photosk,lweretakenintheMokrskogold deposit). the solubility data of Tozawa et al. (1978) (Fig. 4), Langmuir et al. (2006) Langmuir et al. (2006) from the less reliable solubility data of estimated the solubility product of AFA as log Ksp =−23.0±0.3. This Chukhlantsev (1956) and Robins (1987),equaltologKsp =−22.06± value is close to the solubility product values for AFA computed by 0.10 and −22.67±0.31, respectively. Author's personal copy

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exhibited high selectivity for Sr and Cs (Behrens et al., 1998), which might make them useful for the clean-up of radioactive waste. Morin et al. (2002) and Drahota et al. (2009) documented Ba and K, and Ca and Ba replacement in the pharmacosiderite structure, respectively, as a function of equilibration with different groundwater composition in the weathering proﬁles. To our knowledge, no solubility and thermodynamic data for the pharmacosiderite minerals have been determined. This is may be partly due to the relative lack of the pure natural phase, since attempts to synthesize pharmacosiderite by precipitation, gel growth and hydro- thermally have been unsuccessful (Mutter et al., 1984; Ela et al., 2006).

2.2.4. Ca–Fe(III) arsenates Three secondary minerals occur in the Ca–Fe(III)–As(V) system. Among them, mainly arseniosiderite and less yukonite have been recognized in near neutral natural environments, such as waste mine dumps, cyanidation tailings (Paktunc et al., 2003, 2004) and naturally contaminated soils (Figs.1c and 3k, l), stream bed sediments and altered rocks from arsenopyrite weathering (Pieczka et al., 1998; Borba and Figueiredo, 2004; Frau and Ardau, 2004; Filippi et al., 2007; Cancès et al., 2008). Both Ca–Fe arsenates are very often intimately associated with pharmacosiderite and less with other Fe(III) arsenates. Paktunc et al. (2004), Filippi et al. (2007) and Drahota et al. (2009) documented frequent arseniosiderite replacements of pharmacosiderite and scorodite, and its coprecipitation with goethite. These results indicated that Ca–Fe arsenates are commonly formed during maturing of the parental Fig. 4. Solubility of amorphous ferric arsenate (AFA) and crystalline scorodite (FeAsO4·2H2O) environment when sulphides are consumed and an increase in the pH at 22–25 °C. makes the antecedents of arseniosiderite (scorodite or pharmacosiderite) unstable. This hypothesis is supported by the change in the relative proportion of arseniosiderite and pharmacosiderite in undisturbed soil profiles, with the mutual disappearance of pharmacosiderite and 2.2.3. Pharmacosiderite group increment of arseniosiderite in some topsoil horizons (Drahota et al., Minerals of the pharmacosiderite group (A [M (OH) (AsO ) ]·nH O) x 4 4 4 3 2 2009). A hypothetical reaction (Eq. (7)) can be written for Ba- appear to be a relatively common and stable As-carriers in some highly pharmacosiderite incongruent dissolution to arseniosiderite (Drahota contaminated soils (Morin et al., 2002; Borba and Figueiredo, 2004; Filippi et al., 2009): et al., 2007; Cancès et al., 2008)(Figs. 1c and 3k, l)andminewastes (Brown et al., 1990; Paktunc et al., 2004). The structure of pharmacosi- ½ ð Þ ð Þ ð Þð Þ − Ba Fe4 OH 5 AsO4 3 ·5 H2O s derite minerals consists of an open zeolitic framework [Fe (OH) (AsO ) ] þ 4 4 4 3 þ 2Ca2 ðaqÞ→Ca Fe ðAsO Þ O ·3ðH OÞðsÞþFeðOHÞ0ðaqÞ with large cations in channels (K, Ba, Na and sometimes Ca) and Fe and Al 2 3 4 3 2 2 3 þ 2þð Þþ ð Þþ þð ÞðÞ in octahedral sites (Table 3). Minerals of the pharmacosiderite group are Ba aq 2H2O l 2H aq 7 often associated with Ca–Fe arsenates (arseniosiderite, yukonite) and less The solubility of arseniosiderite and yukonite are reported by with unstable scorodite in soils (Filippi et al., 2007; Cancès et al., 2008), Krause and Ettel (1989) as follows: 6.7 mg/L at pH 6.8 and 25 °C for stream bed sediments (Frau and Ardau 2004) and cyanidation tailings arseniosiderite, and 6.3 mg/L at pH 6.15 and 25 °C for yukonite. Cancès (e.g., Paktunc et al., 2004). The occurrence of pharmacosiderite and Ca–Fe et al. (2008) reported dissolution of arseniosiderite in the arsenopyr- arsenates in different environments points at pharmacosiderite precipita- ite-contaminated topsoil and the subsequent scavenging of released tion from high As(V) and low Fe(III) solutions close to neutral pH values; As by poorly ordered Fe oxyhydroxides at the bottom of the soil otherwise, Fe(III) arsenates (e.g., AFA, scorodite) or Fe oxyhydroxides profile. Nevertheless, the persistence of arseniosiderite in mature soil would be preferentially formed at low pH or a higher Fe/As molar ratio, above the As-rich bedrock at the Mokrsko–West gold deposit, Czech respectively (Krause and Ettel, 1989; Swash and Monhemius, 1995). Republic, could correspond to the high environmental stability of Several studies examined the properties of the minerals of the arseniosiderite under specific conditions, in which the high activity of pharmacosiderite group. Frost and Kloprogge (2003) and Filippi et al. the Ca(II) ion (e.g., from the dissolved calcite) reduces the solubility of (2007) investigated the Raman spectra, while Mutter et al. (1984) and arseniosiderite (Drahota et al., 2009). Brown et al. (1990) investigated the Mössbauer effect and thermogravi- Several hydrothermal studies have investigated the Ca–Fe(III)–As(V) metric properties. Like zeolites proper, pharmacosiderite minerals are system, involving synthesis, characterization and preliminary solubility extremely tolerant of cation exchanges in their channels (Mutter et al., testing of Ca–Fe(III)–As(V) compounds (Swash and Monhemius, 1995; 1984). Similarly, synthetic structural analogues of pharmacosiderite Becze and Demopoulos, 2007). In the latter study, direct precipitation and aging from a gypsum saturated matrix solution at pH 8 and 95 °C led to the formation of crystalline yukonite and kolfanite. Similarly, Jia and Demopoulos (2008) found that Fe(III) arsenate coprecipitates to a Table 3 poorly crystalline Ca–Fe(III)–As(V) phase in the presence of calcite at pH Minerals from the pharmacosiderite group. 8 and 22 °C which, upon heating to 75 °C, converted to crystalline Composition Name Reference yukonite. In the hydrometallurgical industry, As is generally removed

K[Fe4(OH)4(AsO4)3]·6.5H2O Pharmacosiderite Zemann (1948) from acidic solutions by coprecipitation with Fe(III) upon neutralization K[Al4(OH)4(AsO4)3]·6.5H2O Alumopharmacosiderite Schmetzer et al. (1981) by addition of lime (CaO). Since some type of Ca–Fe(III)–As(V) Ba[Fe4(OH)5(AsO4)3]·5H2O Barium-pharmacosiderite Walenta (1994) association in coprecipitates may convert to some crystalline Ca–Fe – (Na,K)[Fe4(OH)4(AsO4)3]·6 7H2O Sodium-pharmacosiderite Peacor and Dunn (1985) arsenates upon aging, further work is needed to clarify the formation Author's personal copy

P. Drahota, M. Filippi / Environment International 35 (2009) 1243–1255 1251 conditions and stability of these minerals, in particular in order to Berikul Au mine, Russia (Gieré et al., 2003) and in gossan ores from Rio evaluate their suitability for As sequestering under natural conditions. Tinto, Spain (Roca et al., 1999; Nieto et al., 2003). Romero et al. (2006, 2007) observed that precipitation of beudantite has efﬁciently removed As and Pb from mildly acidic leachets (pH=2.9–4.0) in the 2.3. Fe sulphoarsenates and sulphoarsenites oxidation zone of Pb–Zn mine tailings in Taxco and Zimapán, Mexico. Similarly, beudantite and scorodite have been found as the main As- The group of hydrous Fe(III) basic sulphoarsenates and sulphoarse- carriers in a mine tailing at La Petite Faye (Roussel et al., 2000; nites includes 12 minerals accredited by the International Mineralogical Courtin-Nomade et al., 2002; Néel et al., 2003). Association, as found in the The Mindat.org mineral database (Ralph and Two studies reported thermodynamic data for beudantite. Roussel

Chau, 2008). In comparison with Fe arsenates, Fe(III) sulphoarsenates and et al. (2000) approximated the solubility product at log Ksp =−15 sulphoarsenites are usually less common in nature and consequently using the Gibbs free energy of formation for Pb-jarosite, assuming an attract less interest by investigators. However, if present, these equilibrium state between beudantite and Pb-jarosite. The approx- fi compounds could play a signi cant role in dissolved As control, since imation is lower than solubility product log Ksp =−10.53 inferred they are generally highly soluble in comparison with Fe arsenates. We from the empirically predicted Gibbs free energy of formation of chose three environmentally important representatives from this group, beudantite (Gaboreau and Vieillard, 2004). Assuming preceding although other minerals have also been reported in the literature (e.g., thermodynamic data and congruent dissolution of beudantite in the sarmientite, zýkaite, see e.g., Gieré et al., 2003; Siuda, 2004; Márquez et al., pH range 2–4.5, dissolved Pb concentrations controlled by the 2006). beudantite solubility would not exceed the drinking water standards From this group, namely bukovskýite (Fig. 3j) has been found in As- (Roussel et al., 2000). Moreover, Romero et al. (2007) suggested that rich tropical soils in the Ashanti region in Ghana (Bowell, 1994), in the incorporation of Zn and Cu into the structure of beaudantite may bacterial oxidation processing wastes of the Sao Bento mine in Brazil further lower its solubility. (Márquez et al., 2006) and particularly in strongly weathered, sulphide-rich mine wastes (Čech et al., 1976; Gieré et al., 2003; 2.4. Ca, Mg and Ca–Mg arsenates Mains and Craw, 2005; Gas'kova et al., 2008)(Figs.1b and 3h). In other natural environments, bukovskýite has been identified in precipitates An examination of the accredited minerals from this group (Ralph of acidic mine waters at Carnoulès creek, France (Leblanc et al., 1996) and Chau, 2008) resulted in 11 Ca arsenates, 3 Mg arsenates and 4 Ca– and at mine pit lakes (Bowell and Parshley, 2005; Triantafyllidis and Mg arsenates. Of Ca arsenate compounds, weilite and pharmacolite Skarpelis, 2006). In natural assemblages, it is usually associated with have been reported in a pit wall of the Getchell mine in Nevada (Bowell scorodite, kaňkite and rarely also with zýkaite. Based on the and Parshley, 2005). These minerals occurred in association with thermodynamic calculations (Gas'kova et al., 2008) and field observa- haidingerite and minor picropharmacolite in the oxidation zone of the tions (Mains and Craw, 2005), bukovskýite seems to be more soluble Sainte Marie Pb–Zn mines in France (Pierrot, 1964) and in the buried than scorodite (at least in acid environments). The free energy of wastes at an industrial site close to Marseille in southern France (Juillot formation of bukovskýite was calculated on the basis of the ideal et al., 1999). At the industrial site, Ca arsenate minerals formed as a Δ 0 − mixing model, which yielded the value of Gf = 3480±20 kJ/mol result of the interaction of run-off waters with As-bearing sulphidic (Gas'kova et al., 2008). The formation of bukovskýite takes place from wastes and underlying limestone (Juillot et al., 1999). The anhydrous oversaturated acid solutions (according to composition of pore water arsenate weilite was the first phase formed during the neutralization of in tailings (Gieré et al., 2003), they are tens of grams of As(V) and acidic As-bearing solutions, followed by the precipitation of pharma- 2− hundred grams of SO4 ) according to the Eq. (8) (Gas'kova et al., colite (pH 5.9 and 6.8). These observations are in good agreement with 2008): the laboratory experiments of Ca arsenate precipitation at various pH values (Pierrot, 1964); weilite was the first Ca arsenate precipitated ð Þþ ð Þþ ð Þþ : ð Þ FeS2 s FeSO4·7H2O s H3AsO3 aq 4 5O2 g between pH 3 and 5, followed by haidingerite for pH up to 6, and þ ð Þ→ ð Þð Þð Þ ð Þþ 2−ð Þ H2O l Fe2 AsO4 SO4 OH ·7H2O s 2SO4 aq fi þ followed nally by pharmacolite at pH above 6. Calcium arsenates have þ 4H ðaqÞð8Þ often occurred as efflorescences on the walls of abandoned underground mine workings in the Jáchymov ore district, northern Czech Tooeleite is the only arsenite–sulphate mineral that has recently Republic (Ondruš et al., 1997); pharmacolite has been identified as the been identified in the stromatolite-like deposits in the acid mine main efflorescent at the furnace and condensed flue Mole River site, waters at Carnoulès creek (Morin et al., 2003, 2007)intheU.S.Mine, New South Wales (Ashley and Lottermoser, 1999). Depending on the Utah, USA (Cesbron and Williams, 1992), and in the bacterial pH (H AsO− or HAsO2−;pK =6.99, Nordstrom and Archer, 2003), oxidation processing wastes (Márquez et al., 2006). Laboratory 2 4 4 a2 precipitation of Ca arsenates could occur through Eqs. (10) and (11) experiments showed that specific strains, having an Acidithiobacillus (Juillot et al., 1999): ferrooxidans genotype, induced formation of tooeleite by oxidation of Fe(II) to Fe(III) in the presence of high dissolved As(III) (Morin et al., −ð Þþ 2þð Þþn ð Þ→ n ð Þþ þð ÞðÞ 2003). It can also be easily synthesized in the laboratory by pre- H2AsO4 aq Ca aq H2O l CaHAsO4· H2O s H aq 10 cipitation from the pure components at temperatures up to 90 °C (Nishimura and Robins, 2008). 2−ð Þþ 2þð Þþn ð Þ→ n ð ÞðÞ HAsO4 aq Ca aq H2O l CaHAsO4· H2O s 11 Beudantite is a representative mineral of the beudantite group that is structurally related to the alunite–jarosite family of minerals The presence Ca and Mg arsenates in the environment indicates (Szymański, 1988). The formation of beudantite (Eq. (9)) is usually a high concentration of dissolved As(V), taking into account that related to the oxidation of galena and arsenopyrite with release of As, aqueous Ca and Mg concentrations are usually controlled by Fe and Pb (Roussel et al., 2000): equilibrium with less-soluble Ca or Mg compounds (Magalhães, 2þð Þþ 3þð Þþ 2−ð Þþ −ð Þ 2002). The solubility of Ca arsenates has been studied by many Pb aq 3Fe aq SO4 aq H2AsO4 aq þ authors since they usually control As immobilization in rich-As þ 6H OðlÞ→PbFe ðAsO ÞðSO ÞðOHÞ ðsÞþ8H ðaqÞð9Þ 2 3 4 4 6 environments, which have been treated with cement, lime, gypsum Beudantite has been identified in the precipitates of acidic mine and pozzolanic materials (e.g., Bothe and Brown,1999a; Moon et al., waters at Carnoulès creek, France (Leblanc et al., 1996). Accumulation 2004; Rodríguez et al., 2008). For example, Rodríguez-Blanco et al. of As in a jarosite–beudantite solid solution has been reported at the (2007) have shown that, at about pH 7, the interaction of dissolved Author's personal copy

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As(V) and gypsum resulted in concomitant precipitation of pharmaco- 1988; Gas'kova et al., 1999; Lee and Nriagu, 2007), but these have been lite crystals (Eq. (12)): studied only occasionally since they are relatively rare and have not been seriously considered for disposal practices. Their occurrence in 2−ð Þþ ð Þ→ ð Þþ 2−ð ÞðÞ HAsO4 aq CaSO4·2H2O s CaHAsO4·2H2O s SO4 aq 12 nature is usually restricted to the specific chemical system in natural environments such as oxidized polymetal hydrothermal ores, mill- Swash and Monhemius (1996) reported high solubilities for weilite, tailings of “five-element-type” (Ag–Bi–Co–Ni–As) deposits and ranging from 2170 to 3610 mg/L, and for haidingerite and pharmacolite, environments polluted by arsenic pesticides. The crystallization of ranging from 3120 to 4360 mg/L. Bothe and Brown (1999b) reported a these minerals in similar environments can, however, impose a value of 2050 mg/L for the solubility of haidingerite at pH 6.22. Other Ca significant control on the dissolved concentration of both As and the arsenates, synthesized from aqueous lime-As slurries with varying Ca/As specific metal. Among the large number of secondary Zn, Cu, Pb, Ni molar ratios (from 1:0.8 to 4:1), also have high solubilities (Bothe and and Co arsenate minerals, there are several minerals that could be Brown,1999a,1999b; Moon et al., 2004; Zhu et al., 2006) with a maximum abundant and have significantly controlled dissolved concentrations of 47,000 mg/L for Ca3(AsO4)2·3H2OatpH5.85(Zhu et al., 2006). of As in a specific natural environment. However, the solubility of Ca arsenate compounds in excess free lime Leadarsenateswereemployedfordecadesaspesticidesinagriculture, yielded an apparent solubility of b0.1 mg/L (Nishimura et al., 1985). and residues of an original contaminant, often schultenite, remain in soils Similar to this experimental observation, the high stability of Ca arsenate (Cancès et al., 2005; Arai et al., 2006). Synthetic schultenite in minerals (weilite, haidingerite and pharmacolite) in the system of natural contaminated soils has undergone site-specific dissolution reactions and waters out of equilibrium with respect to aqueous As(V) concentrations resulted in heterogeneous As sequestration mechanisms ranging from As may be indicated by a common ion effect, in which high Ca activity in coprecipitation with Fe(III) oxyhydroxides to precipitation of amorphous solution inhibits the dissolution of Ca arsenates (Juillot et al., 1999). orpiment and As–Ca coprecipitates (Cancès et al., 2005; Arai et al., 2006). Although the solubility of Ca arsenates has been widely investigated, Natural schultenite occurs in the oxidized zone of Pb–As hydrothermal determination of their solubility products has received less attention with deposits, such as in oxidized arsenopyrite-bearing quartz vein at the King some significant exceptions. Gas'kova et al. (1999) calculated the thermo- County, Washington (Falls et al., 1985). Schultenite is stable under dynamic solubility product of weilite at log Ksp =−2.36 from the data of conditions of low pH (b2.5), low chloride concentrations (b10−3 mol/L) Mahapatra et al. (1986). The solubility product of haidingerite (log Ksp = and relatively high activity of Pb and As(V) ions (Magalhães and Silva, −4.79) was estimated by Bothe and Brown (1999b). The solubility 2003). The recent determinations of the free Gibbs energy of formation of product of pharmacolite is close to those estimated for both weilite and 0 schultenite yielded values of ΔGf =−809.62 (Magalhães and Silva, 2003) haidingerite, and corresponds to log Ksp =−4.68±0.04 (Rodríguez- and ΔG0=−805.66 kJ/mol as reported by Lee and Nriagu (2007). Blanco et al., 2007). In addition to the high solubilities of Ca arsenate f Probably the most common secondary Pb arsenate mineral, mimetite, compounds, there is strong evidence that Ca arsenates are not stable in the is much less soluble than schultenite and is even formed from weakly presence of carbon dioxide in the air at a pHN8.3 (Nishimura et al., 1983) saline solutions. Magalhães and Silva (2003) demonstrated that mimetite and decompose slowly to secondary calcite and soluble arsenateoxyanions 2− 3− is a very stable mineral at low dissolved Pb and As(V); the stability field (HAsO4 or AsO4 ;pKa3 =11.80, Nordstrom and Archer, 2003): covers the pH range of most natural waters, even when they contain − − dissolved carbonate and sulphate. Moreover, the crystallization of a Ca ðAsO Þ ðsÞþ3HCO ðaqÞþOH ðaqÞ→3CaCO ðsÞ 3 4 2 3 3 phosphate/arsenate solid solution (phosphomimetite) reduced the þ 2−ð Þþ ð ÞðÞ 2HAsO4 aq H2O l 13 dissolved As concentrations from several grams per liter to less than 0.2 ppb (Twidwell et al., 1994). Magalhães and Silva (2003) recently ð Þ ð Þþ 2−ð Þ→ ð Þþ 3−ð ÞðÞ reviewed the solubility product constants and free energies of formation Ca3 AsO4 2 s 3CO3 aq 3CaCO3 s 2AsO4 aq 14 of mimetite and other Pb arsenates, and established stability diagrams for the environmentally relevant Pb minerals including schultenite, mimetite Donahue and Hendry (2003) calculated the possible equilibrium and other Pb arsenate minerals. composition of the pore waters in the Rabbit Lake U-tailing with Some deposits, such as hydrothermal five-element associations and various Ca arsenate minerals using PHREEQC, which yielded a wide many uranium deposits, contain Ni–Co–Fe arsenides and sulfarsenides. range of dissolved As concentrations from 2.9 to 1248 mg/L. Based on The oxidation of these primary As minerals typically leads to the the most probable Ca arsenate compound Ca (OH) (AsO ) ·4H O 4 2 4 2 2 formation of greenish and pinkish secondary compounds, annabergite present in the uranium mine mill tailing, they concluded that the and erythrite, respectively. The precipitation of erythrite has been equilibrium dissolved As concentrations in the range between 13 and expected to exert significant, if not dominant, control over the dissolved 126 mg/L are very similar to those measured in the water samples. concentrations of Co and As, and probably also over the concentrations Voigt et al. (1996) reported the precipitation of Mg arsenate hörnesite of Ni, which substitutes for Co (Jambor and Dutrizac,1995). Annabergite as a product of the reaction of Mg-rich ground waters with soils has been identified in association with scorodite and Fe oxyhydroxides contaminated by As-bearing smelter wastes from California (Eq. (15)): in many uranium mine tailings from the McClean Lake, Midwest and 3− 2þ Cigar Lake ore deposits (Langmuir et al.,1999; Mahoney et al., 2005)and 2AsO ðaqÞþ3Mg ðaqÞþ8H OðlÞ→Mg ðAsO Þ ·8H OðsÞð15Þ 4 2 3 4 2 2 from the Rabbit Lake deposit (Donahue et al., 2000), all in northern The presence of hörnesite in contaminated soil was supported and Saskatchewan in Canada. Geochemical modeling and mineralogical quantified by combination of XANES and EXAFS techniques (Foster et al., analyses indicated that Mg-rich annabergite may precipitate above pH 1997). Hörnesite has also been found in association with claudetite, 5–6fromraffinate solutions that are high in Ni and low in Fe (Langmuir pharmacolite and Ca–Mg arsenates in the speleothems at Corkscrew et al., 1999). Similarly, Mahoney et al. (2007) documented the Cave (Onac et al., 2007). Little information is available on the solubility of precipitation of annabergite in the vicinity of unreacted bases where this mineral and it probably differs from the values of approx. 300 to local pH values reached distinctively higher values than bulk solution 1100 mg/L at pH from 6.5 to 7.4 determined by Chukhlantsev (1956). pHs of uranium tailings neutralization circuit. In accordance with these natural observations, experimental study has shown an increase in 2.5. Other metal arsenates annabergite solubility with decreasing pH (Yuan et al., 2005), reaching a minimum solubility value of 48 mg/L As and 0.2 mg/L Ni at pH 9. There are other metal arsenates, such as those of Zn, Cu, Pb, Ni and Langmuir et al. (1999) corrected annabergite solubility products from Co, which are usually less soluble and more stable in the pH range of Nishimura et al. (1990) for the presence of Ni complexes and reported natural waters than the Ca arsenates or Fe arsenates (Magalhães et al., the value of log Ksp =−28.38. Author's personal copy

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