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

Abstract

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.

Distribution of Arsenic and Risk Assessment of Activities on Soccer Pitches Irrigated with Arsenic-Contaminated Water

“…Matehuala geology is represented by sedimentary marine rocks, which is part of the eastern physiographic province of the Sierra Madre Oriental [ 63 ]. Therefore, soils at the study area are largely dominated by either calcite or gypsum [ 16 , 44 , 64 ]. The stratigraphic sequence of Matehuala consists mainly of PreOxfordian, Upper Jurassic, and Cretaceous carbonated series, a little portion of Tertiary volcanic rocks that cover the city, and Quaternary alluvial material deposited on the Matehuala valley [ 63 ].…”

“…Cycles of dissolution and precipitation of secondary arsenate minerals formed in soil after irrigation may also account for the accumulation of As in the topsoil and the vertical migration of As. Highly soluble arsenate minerals, such as haidingerite (CaHAsO 4 ·H 2 O; pk sp = 10 −4.79 ) and pharmacolite (CaHAsO 4 ·2H 2 O; pk sp = 10 −4.68 ), in addition to guerinite (Ca 5 H 2 (AsO 4 ) 4 ·9H 2 O; pk sp = 10 −30.69 ) have been found in the old abandoned smelter in Matehuala, as a result of the precipitation of As in a calcium-rich environment [ 64 , 66 , 67 ]. Previous studies have shown that As is highly soluble in water, and the more gypsum the soil contains, the lower the solubility of As, while higher calcite results in higher dissolution [ 16 ].…”

Síntesis De Arseniatos De Calcio (Guerinita, Haidingerita Y Farmacolita) Morfológicamente Similares a Los Encontrados en Suelos Contaminados

“…Para Ca 5 H 2 (AsO 4 ) 4 .nH 2 O con n = 9 para guerinita. Las fases identificadas en este trabajo concuerdan con las fases identificadas por Castillo et al (2015), quien identificó guerinita, haidingerita y farmacolita en muestras de suelos contaminados con arseniatos de calcio recolectados en Matehuala (Castillo et al 2015). La obtención de distintas fases de arseniatos de calcio en la síntesis 1 y 2 podría deberse a las diferencias de pH al comienzo de los experimentos (Fig.…”

“…En ambientes naturales e impactados, la identificación inequívoca de arseniatos de calcio ha demostrado, sin embargo, ser muy difícil de llevar a cabo (Ondrus et al 1997, Donahue y Hendry 2003, Onac et al 2007, Pantuzzo y Ciminelli 2010, Martínez-Villegas et al 2013, Castillo et al 2015) debido a la presencia de variados y diferentes arseniatos de calcio en muestras compuestas principalmente por otros minerales primarios (Ondrus et al 1997, Donahue y Hendry 2003, Onac et al 2007, Pantuzzo y Ciminelli 2010, Martínez-Villegas et al 2013, Castillo et al 2015. Aunado a la falta de reproducibilidad y ambigüedad de patrones de difracción de fases puras de arseniatos de calcio que no empatan con alguna fase conocida (Myneni et al 1997, Bothe y Brown 1999a, 1999b.…”

Chemical speciation of lead adsorbed onto volcanic ashes by ICP-OES and XANES

“…The ICP-OES measurements were performed by dissolving with concentrated HNO 3 and HCl the VA powder in a proportion of 0.1 g of VA in 10 mL (1:100 w/v) [15]. The equipment used was an ICP-OES brand Varian model 738 ES with the torch placed at the axial position and nebulizer flow of 0.75 L min −1 .…”