Arsenic speciation in synthetic jarosite

Abstract

Iron sulfate minerals may incorporate arsenic released from oxidizing arsenian pyrite and other sulfide minerals. For example, in the southern Mother Lode gold district of California, As is concentrated in jarosite, ideally KFe 3(SO 4) 2(OH) 6, that formed by the oxidation of sulfides in mineralized outcrops and mine tailings containing ∼100 to ∼2000 ppm As. These weathering crusts, which are vulnerable to storm erosion, are of concern as a source for the transport of As into watersheds. Both arsenatian jarosite and scorodite [FeAsO 4·2H 2O] provide only temporary storage mechanisms for As in the environment because of the limited pH and redox conditions over which they are stable. To evaluate the extent of incorporation of As into jarosite, a series of jarosites in the system K 2O–Fe 2O 3–As 2O 5–H 2SO 4–H 2O was synthesized at 95 °C and 1 bar. Potassium arsenate was included at relative molar proportions of 1–50% As T,aq (defined as 100*As/(As+S)) in the starting solutions. Arsenic was incorporated into all samples at higher proportions relative to S than in the starting solutions. Synchrotron X-ray diffraction results indicated that jarosite was formed from starting solutions that contained up to 20% As T,aq. An X-ray amorphous phase also formed in samples containing As and, at 25% As T,aq in the starting fluid, only an amorphous phase was obtained. Starting fluids with 33% and 50% As T,aq produced poorly crystalline scorodite. Scanning electron microscope (SEM) images show textural differences among the jarosite samples, from relatively uniform anhedral to subhedral particles 0.5–1 μm across in low-As jarosite samples, to smaller grains (<0.2 μm) dispersed in a groundmass in the samples richer in As. Results from SEM, SXRD and Raman spectroscopy suggest that most As in the synthetic samples enters the amorphous phase, but quantitative analytical electron microprobe (AEM) analyses showed that the grains of jarosite contain up to ∼30% As T,s. Iron K-edge and As K-edge extended X-ray absorption fine-structure (EXAFS) spectra of the arsenatian jarosite showed As and Fe environments consistent with substitution of As for S. SXRD indicated unit-cell expansion with increased substitution of As in jarosite. The extent to which As is accommodated in the jarosite structure may be limited by the charge-balance mechanism and deficiency of Fe in octahedral sites. The X-ray amorphous material produced in the syntheses exhibits short-range order and is similar to scorodite over small domains, on the order of 10–14 Å. Raman spectroscopy showed several broad peaks in the 750–950-cm −1 Raman shift spectral region, associated with As–O vibrations. The relative intensities of bands related to As–OFe and As–O vibrations are also consistent with substitution of As for S in jarosite, and with small, scorodite-like domains in the amorphous material. Collectively, these data show that the strong association between Fe and arsenate during nucleation of the solid materials leads to As retention in the oxidized Fe minerals produced during weathering of arsenian pyrite and other As-rich sulfide minerals.