Arsenate sulfate

Arsenate sulfate refers to a class of inorganic compounds containing both arsenate (AsO₄³⁻) and sulfate (SO₄²⁻) anions, often found in mineral forms or as secondary products in geochemical and industrial processes. These compounds are typically associated with arsenic-rich environments, such as mine tailings, hydrothermal systems, and hypersaline lakes, where arsenic and sulfur coexist. Arsenate sulfates are of interest in mineralogy, environmental chemistry, and biogeochemistry due to their role in arsenic mobility, toxicity, and microbial interactions. A notable example is beudantite.^[1][2]

Chemistry and structure

Arsenate sulfates are complex compounds that incorporate arsenate (AsO₄³⁻) and sulfate (SO₄²⁻) anions, often coordinated with metal cations such as iron (Fe), copper (Cu), or manganese (Mn). The arsenate ion consists of a central arsenic atom (oxidation state +5) tetrahedrally coordinated with four oxygen atoms, carrying a -3 charge. The sulfate ion, similarly tetrahedral, has a sulfur atom (oxidation state +6) bonded to four oxygen atoms, with a -2 charge. In arsenate sulfates, these anions may form distinct structural units or substitute for each other due to their similar tetrahedral geometry and charge. For example, beudantite (PbFe₃(OH)₆(SO₄)(AsO₄)) exhibits a layered alunite-type structure. Solubility is low (Ksp ~10⁻²⁰ to 10⁻³⁰), but acidic conditions (pH < 3) or microbial activity can mobilize arsenic. The compounds are stable under oxidizing conditions but may reduce to arsenite (As(III)) phases in anoxic environments.^[1]

Natural occurrence

Nishanbaevite, an arsenate sulfate mineral found in fumaroles

Arsenate sulfates occur naturally as minerals in oxidized zones of arsenic-rich ore deposits and anthropogenically in acid mine drainage (AMD) and smelting residues and hypersaline lakes. Notable examples include:

• Beudantite (PbFe₃(AsO₄)(SO₄)(OH)₆): Found in Tsumeb, Namibia, and AMD sites, with a yellow-green hue and alunite-type structure.
• Jarosite-arsenojarosite series (KFe₃(AsO₄,SO₄)₂(OH)₆): Common in acidic sulfate-rich environments, with arsenate (AsO₄^3-, As(V)) substituting for sulfate, distinct from arsenite (AsO₃^3-, As(III)) or elemental arsenic (As(0)) phases.

These minerals form in acidic (pH 2–5), oxidizing environments where sulfate and arsenate ions co-precipitate with Fe³⁺ or Pb²⁺. Global occurrences include the Carnoulès mine, France, and gold mines in Nevada, USA. In hypersaline lakes like Mono Lake and Searles Lake, California, arsenate and sulfate ions contribute to arsenic cycling, influenced by microbial reduction processes. Anthropogenic sources dominate due to mining and smelting activities.^[1][2][3]

Formation processes

Arsenate sulfates form through abiotic and biotic processes in arsenic- and sulfur-rich environments. Abiotically, they precipitate during the oxidation of sulfide minerals (e.g., arsenopyrite, FeAsS) in AMD or hydrothermal systems, where arsenate and sulfate ions co-precipitate with metal cations. For example, pressure oxidation (POX) of refractory gold ores produces sulfoarsenates as residues, with compositions like phase 3 (Fe_0.9-1.3As_0.3-0.6S_0.4-0.7O₄(OH)_0.3-3.3).Biotically, bacteria like Thiobacillus mediate arsenate sulfate formation by oxidizing sulfide or reducing arsenate, as seen in Mono Lake sediments. Microbial sulfate reduction under anoxic conditions can couple with arsenate reduction, forming arsenate sulfates in the presence of iron. pH and redox potential (pe) control speciation: arsenate dominates in oxygenated, high-pe waters, while arsenite prevails in anoxic, low-pe conditions. Pourbaix diagrams illustrate these transitions, with arsenate sulfates stable at neutral to acidic pH.^[3][4]

Applications

Arsenate sulfates have limited direct applications but are significant in industrial and environmental contexts. In gold mining, sulfoarsenates in POX residues stabilize arsenic, reducing its release into tailings leachate. These compounds are studied to optimize arsenic immobilization during ore processing. Historically, arsenate-based compounds, including those with sulfate components, were used in pesticides (e.g., lead hydrogen arsenate), but their toxicity led to bans in many countries. Arsenate sulfates like copper arsenate were minor components in pigments, such as Egyptian blue, before being replaced by safer alternatives. In analytical chemistry, arsenate sulfates serve as reference materials for studying arsenic speciation and mobility in contaminated sites. Their stability in mineral form informs strategies for remediating arsenic-polluted soils and waters.^[5]

Environmental impact

Arsenate sulfates play a dual role in environmental arsenic dynamics. On one hand, they immobilize arsenic in stable mineral phases (e.g., sulfoarsenates), reducing its bioavailability in mine tailings and sediments. Jarosite's ability to incorporate arsenate limits arsenic release in AMD. On the other hand, elevated sulfate concentrations in water can trigger arsenic desorption from sediments, as sulfate competes with arsenate for adsorption sites on iron (hydr)oxides. A 2021 study showed that 10% of desorbed arsenic results from competitive adsorption, 21% from iron oxide reduction, and 69% from microbial activity. In hypersaline lakes, microbial arsenate and sulfate reduction cycles mobilize arsenic, increasing its environmental risk. Arsenate sulfates contribute to arsenic contamination in groundwater when dissolved, posing health risks like arsenic poisoning upon chronic exposure. Their toxicity stems from arsenate's similarity to phosphate, disrupting biochemical reactions.^{[4][3] [2][6]}

Analytical methods

Analyzing arsenate sulfates requires techniques to distinguish arsenate, sulfate, and coexisting ions. Titration methods, such as barium chloride with magnesium EDTA, separate sulfate from arsenate and phosphate in solution, avoiding interference from alkaline earth metals. Radioisotope assays using [⁷³As]arsenate and [³⁵S]sulfate measure microbial reduction rates in sediments, as applied in Mono Lake studies. Denaturing gradient gel electrophoresis (DGGE) and PCR amplification of 16S rRNA and arrA genes characterize microbial communities driving arsenate sulfate formation.^[3][7]

Examples

	Chemical formula	Crystal system	Space group	Unit cell	Volume	Density	Comments	References
Nishanbaevite	KAl2O(AsO4)(SO4)	orthorhombic	Pbcm	a = 15.487 b = 7.258 c = 6.601 Z = 4	742.1	3.012	biaxial (–), α = 1.552, β ≈ γ = 1.567	[8]
Vasilseverginite	Cu9O4(AsO4)2(SO4)2	monoclinic	P21/n	a = 8.1131 b = 9.9182 c = 11.0225 β = 110.855° Z = 2	828.84		biaxial (–), α 1.816, β 1.870, γ 1.897, 2V ~ 30(15)°	[9]
Beudantite	PbFe3(OH)6SO4AsO4	trigonal	R3_m	a = 7.32, c = 17.02 Z=3	789.79	4.48	Uniaxial (-) nω = 1.957 nε = 1.943 Birefringence: 0.014	[10]
Gallobeudantite	PbGa3(AsO4)(SO4)(OH)6	trigonal	R3m	a = 7.225, c = 17.03 Z=3	769.88	4.87	uniaxial (–) nω = 1.763 nε = 1.750	[11]
	K4(SO4)(HSO4)2(H3AsO4)	triclinic	P1_	a=8.9076 b=10.1258 c=10.6785 α=72.525°, β=66.399°, γ=65.516°, Z=2	792.74			[12]
	Rb4(SO4)(HSO4)2(H3AsO4)	monoclinic	P21	a = 5.868 b = 13.579, c = 11.809 β = 94,737°				[13]
	Rb3(HSO4)2.5(H2AsO4)0.5	triclinic	P	a = 7.471 b = 7.636 c = 12.193 α = 71.91° β = 73.04° γ = 88.77°			superprotonic phase over 404 K; decompose 473K	[14]
	Cs2(HSO4)(H2AsO4)	monoclinic	P21/n					[15]