Synthesis and Characterization of the Synthetic Minerals Villyaellenite and Sarkinite, Mn5(AsO4)2(HAsO4)2 · 4H2O and Mn2(AsO4)(OH)

Dimorphism of MnHAsO4(H2O): natural monoclinic krautite and its synthetic triclinic modification

The crystal structure of natural krautite, MnHAsO4(H2O), was re-evaluated from a cotype specimen, confirming the previously reported monoclinic symmetry for this mineral (space group P21, Z = 8, a = 8.0093(5), b = 15.9372(10), c = 6.8065(4) Å, β = 96.534(2)° at room temperature, 5662 structure factors, 302 parameters, R1 = 0.0295, wR2 = 0.0770). Although hydrogen atoms could not be located from the single crystal X-ray diffraction study, the higher accuracy and precision of the results allowed to derive the hydrogen-bonding scheme (O⋯O = 2.55–2.90 Å) in the crystal structure of krautite. Crystals of synthetic MnHAsO4(H2O) were grown by mixing aqueous solutions of NH4H2AsO4 and MnSO4 and keeping the formed gel at 105 °C for several days. The obtained triclinic crystals were systematically and polysynthetically twinned by contact on (010). Separation of reflections from two individual domains made it possible to determine and refine the crystal structure (space group P 1 ‾ $P‾{1}$ , Z = 8, a = 8.0105(16), b = 15.991(4), c = 6.8029(12) Å, α = 92.635(8), β = 96.534(2), γ = 90.151(8)° at room temperature, 7226 structure factors, 255 parameters, R1 = 0.0445, wR2 = 0.1381). The triclinic polymorph of MnHAsO4(H2O) does not show a direct group-subgroup relation with monoclinic krautite. Triclinic MnHAsO4(H2O) is closely related with other triclinic M IIHAsO4(H2O) (M = Co, Cu, Zn, Mg) mineral phases. Quantitative structural comparisons between the five M IIHAsO4(H2O) compounds revealed a high similarity between the Mn and Co members, and between the Zn and Mg members, respectively. Subtle distinctions between the two pairs are ascribed to a different hydrogen-bonding scheme. Although the Cu member has a similar hydrogen-bonding scheme as the Mn and Co pair, its structural similarity with triclinic MnHAsO4(H2O) is low due to the strain of the crystal structure caused by the Jahn-Teller distortions of the [CuO6] octahedra.

A combined abiotic oxidation-precipitation process for rapid As removal from high-As(III)-Mn(II) acid mine drainage and low As-leaching solid products

A combination process of Fenton-like and catalytic Mn(II) oxidation via molecular oxygen-induced abio-oxidation of As(III)-Mn(II)-rich acid mine drainage (AMD) is developed to rapidly and efficiently remove As and obtain low As-leaching solids in this study. The effect of pH, temperature, oxygen flow rate and neutralization reagent on As removal was investigated. The results showed that pH was important to As removal efficiency, which achieved maximum in 0.25-2 h, but decreased from ∼100 % to ∼92.6 % with the increase of pH 5-9. pH, temperature and oxygen flow rate played key roles in As(III) oxidation. The increase of As(III) oxidized from 16.8 to 67.1% to 98.6-99.0 % occurred as increasing the pH 5-9, 25-95 °C and oxygen flow rate of 0-2.4 L min^-1. NaOH or Ca(OH)₂ as base was less important to As removal. The mechanism involved Fenton-like reaction between Fe(II) and O₂ for produced Fe(III) (oxy)hydroxide association with As(III + V) and Mn(II), catalytic Mn(II) oxidation for the formation of Mn(III, IV) oxides, and further As(III) oxidation by Mn(III, IV) oxides. As-bearing six-line ferrihydrite was the main solid product for low As-leaching fixation. pH 8, 95 °C and oxygen flow rate of 1.6 L min^-1 were optimal for As removal.

A new form of NaMnAsO4

The new form of NaMnAsO₄ (denoted as β) crystallizes in the monoclinic crystal system and is isotypic with one form of NaCoPO₄ and with NaCuAsO₄.

A new form of NaMnAsO₄, sodium manganese(II) orthoarsenate, has been obtained under hydro­thermal conditions, and is referred to as the β-polymorph. In contrast to the previously reported ortho­rhom­bic α-polymorph that crystallizes in the olivine-type of structure and has one manganese(II) cation in a distorted octa­hedral coordination, the current β-polymorph contains two manganese(II) cations in [5]-coordination, inter­mediate between a square-pyramid and a trigonal bipyramid. In the crystal structure of β-NaMnAsO₄, four [MnO₅] polyhedra are linked through vertex- and edge-sharing into finite {Mn₄O₁₆} units strung into rows parallel to [100]. These units are linked through two distinct orthoarsenate groups into a framework structure with channels propagating parallel to the manganese oxide rows. Both unique sodium cations are situated inside the channels and exhibit coordination numbers of six and seven. β-NaMnAsO₄ is isotypic with one form of NaCoPO₄ and with NaCuAsO₄.