Identification of diagenetic calcium arsenates using synchrotron-based micro X-ray diffraction

Resumen

In this work, we
identify the type of calcium arsenates found in sediment samples
from an aquifer located in Matehuala, San Luis Potosí, México.
Sediments in contact with levels up to 158 mg/L of arsenic in
neutral pH water were studied by X-ray diffraction, scanning
electron microscopy coupled to energy dispersive X-ray analyses
(SEM-EDS), and synchrotron based X-ray diffraction. Identification
of these calcium arsenates by X-ray analysis has proved to be very
difficult to achieve because the precipitates of interest are on the
microscale and immerse in a matrix of calcite, gypsum, and quartz
comprising nearly 100 % of the samples. Needle-like specimens
composed of calcium, arsenic, and oxygen were, however, commonly
observed in sediment samples during SEM-EDS analyses in
backscattered mode. Synchrotron based X-ray analyses revealed some
peaks that were compared with published data for guerinite,
haindingerite, and pharmacolite suggesting that these were the
calcium arsenates present in sediments, the calcium arsenates that
control the solubility of arsenic in the contaminated aquifer in
Matehuala, and the calcium arsenates that prevail in the long-term
in the environment after cycles of dissolution and precipitation.
The identification of these calcium arsenates is consistent with the
environmental conditions prevailing at the study area and the
SEM-EDS observations. However, its identification is not unequivocal
as the comparison of experimental data collected in single crystal
specimens against X-ray diffraction references collected in powders
prevents a strictly proper identification of the specimens analyzed.
In this way, scorodite was also identified by synchrotron based Xray
analyses however its presence is inconsistent with the environmental
conditions and the calcium arsenate associations found in this
study. Scorodite identification was therefore considered tentative.
A thorough examination, with additional and/or improved analytical
techniques, should be undertaken to find an environmentally sound
explanation for the diffraction peaks assigned to scorodite, which
might be from a clay a mineral, probably with no arsenic.