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The Raman spectra of synthetic compounds equivalent to the variscite group: FeAsO 4 ·2H 2 O AlAsO 4 ·2H 2 O, GaAsO 4 ·2H 2 O, and InAsO 4 ·2H 2 O are reported. In particular, upon comparison of FeAsO 4 ·2H 2 O to AlAsO 4 ·2H 2 O, it is observed that the Type II (weak) H-bond lengths in the latter are slightly longer, which is postulated to affect the stability (As release) in water at pH 5 and 7. Arsenate stretching and bending vibrations were found to be distinct in terms of spectral structure and therefore well suited for fingerprinting. The calculated As-O bond strengths from existing crystallographic data showed no significant variations. The strongest ν 1 (AsO 4 3− ) stretch was used to monitor the As-O bonding interactions in the four As-O-M units, where a shift of 114 cm −1 was observed in the order FeAsO 4 ·2H 2 O (lowest) < InAsO 4 ·2H 2 O < GaAsO 4 ·2H 2 O < AlAsO 4 ·2H 2 O (highest); this order also followed exactly the measured arsenic release of these phases. This shift in ν 1 (AsO 4 3− ) position was rationalized to stem from the differences in the electronegativities of the M 3+ cations. The trends mentioned above were verified and found to also hold for the isostructural phosphate analogues strengite (FePO 4 ·2H 2 O) and variscite (AlPO 4 ·2H 2 O) using published data. Therefore, it is postulated that, as observed with the stability of solution complexes, there may be a correlation between the electronegativity of the M 3+ cation in these isostructural phases and their stability (As or P release) in water.

This paper reports on the hydrothermal synthesis, structural characterization, and chemical stability-leachability of three metal arsenates, namely gallium arsenate dihydrate (GaAsO 4 ‚2H 2 O), indium arsenate dihydrate (InAsO 4 ‚2H 2 O), and lanthanum arsenate (LaAsO 4 ). The new standard synthesis method involves hydrothermal precipitation at 433 K (160 °C) from equimolar (0.3 M) M(III)-As(V) nitrate solutions over a period of 24 h. The produced materials were found to be essentially stoichiometric and to exhibit very good crystallinity. The two dihydrates were found further to be made up of uniformly grown crystallites either aggregated (GaAsO 4 ‚2H 2 O) or nonaggregated (InAsO 4 ‚2H 2 O), reflecting their common orthorhombic crystal habit, while LaAsO 4 consisted of large aggregated particles with monoclinic habit features. In terms of stability, InAsO 4 ‚ 2H 2 O and LaAsO 4 were found to release less than 1 mg/L arsenic when subjected to a TCLP-like leachability test (24 h contact at pH 5) while GaAsO 4 ‚2H 2 O released 2.4 mg/L arsenic. An extended leachability study over a period of several weeks resulted in higher concentrations of arsenic released via an incongruent dissolution mechanism. Of the three compounds, LaAsO 4 was determined to be the most stable with arsenic equilibrium solubility equal to 4 and 13 mg/L, respectively, at pH 5 and 7 at 22 °C.

The valence states of Mg-Al alloys are compared to those of reference materials (pure Mg and Al metals, and intermetallics). Two methods based on x-ray emission spectroscopy are proposed to determine the phases and their proportion: first, by analyzing the Al valence spectra of the Mg-rich alloys and the Mg valence spectra of the Al-rich alloys; second by fitting with a linear combination of the reference spectra the Al spectra of the Al-rich alloys and the Mg spectra of the Mg-rich alloys. This enables us to determine that Al and Al 3 Mg 2 are present in the 0 -43.9 wt% Al composition range and Mg and Al 12 Mg 17 are present in the 62.5 -100 wt% Al composition range. In the 43.9 -62.5%Al range, the alloy is single phase and an underestimation of the Al content of the alloy can be estimated from the comparison of the bandwidth of the alloy spectrum to the bandwidths of the reference spectra.