Thermodynamic mixing properties of alunite supergroup minerals: Quantum-mechanical modeling and thermodynamic analysis of sulfate, chromate, selenate, phosphate, and arsenate solid solutions, as well as uranyl incorporation

Abstract In various geological settings, alunite supergroup minerals, including the end members alunite [KAl3(SO4)2(OH)6] and jarosite [KFe3(SO4)2(OH)6], occur in extensive solid solution on the cation and anion sites, enabling them to accommodate a variety of elements. Jarosite is known for scavenging heavy metals on the K site (e.g., Pb in solid solution between jarosite and plumbojarosite) in acid mine drainage settings, while both jarosite and alunite can be important in controlling toxic anions such as selenate, chromate, or arsenate. However, there is no thermodynamic information on the solubility of these important cations and anions as a function of temperature, in part because this information is difficult to obtain experimentally. In this study, thermodynamic mixing properties of sulfate-chromate (S-Cr), sulfate-selenate (S-Se), and sulfate-phosphate-arsenate (S-P-As) solid solutions in alunite supergroup minerals are investigated based on quantum-mechanical modeling and statistical thermodynamic analysis. S-Cr and S-Se solid solutions in alunite and jarosite are due to the mixing of anions with equivalent charges on the sulfate site. The enthalpy of mixing ( Δ H mix ) is lowest at 0 K (−273 °C) and increases with increasing temperature; it also depends on the arrangement of atoms on the sulfate site (i.e., atomic ordering). Δ H mix is almost constant at −70 °C and higher temperatures. These findings imply that S-Cr and S-Se solid solutions tend to be complete at room temperature and no ordering is acquired at or above ambient conditions. The Gibbs free energy of mixing ( Δ G mix ) indicates that jarosite is more flexible to accommodate chromate and selenate at the sulfate site than the alunite structure and that complete solid solution can be formed between jarosite and both Se- and Cr-analogues at temperatures above −80 °C. Our modeling results of solid solutions in jarosite and alunite demonstrate the critical role of alunite supergroup minerals in controlling toxic elements for long-term immobilization including the relatively favorable incorporation of uranyl (UO22+) into plumbojarosite. The S-P-As solid solution is also explored between alunite family minerals [DAl3(TO4)2(OH, H2O)6] (D is K+, Na+ or Ca2+; TO4 is SO42−, PO43− or AsO43−). The energetic barrier for mixing (reflected by the peak enthalpy of mixing) between ions with different charges (i.e., S-P and As-S solid solution; ∼2–3 kJ/mol of exchangeable atoms) is higher by a factor of two or three compared to anions with the equal charge (P-As solid solution; 1–1.5 kJ/mol of exchangeable atoms). Below 700 °C, ternary S-P-As mixing over a range of compositions in ternary space having alunite, crandallite [CaAl3(PO4)6(OH)5(H2O)], and arsenocrandallite [CaAl3(AsO4)6(OH)5(H2O)] as end members shows large miscibility gaps at compositions close to the ratio of S:P:As = 4:1:1 and 1:1:1 at the TO4 site. Ternary S-P-As mixing between woodhouseite, arsenowoodhouseite, and either alunite or natroalunite shows that arsenate is more compatible with sulfate in natroalunite than sulfate in alunite, whereas substitution of sulfate with phosphate is energetically more favorable in alunite than natroalunite. Our computed phase diagrams of S-P-As mixing suggest that binary solid solutions between pairs of sulfate, phosphate and arsenate in alunite family minerals scarcely occur below 100 °C, is limited at temperatures from 100 to 300 °C and become extensive or complete above 300 °C.