Arsenic mobility controlled by solid calcium arsenates: A case study in Mexico showcasing a potentially widespread environmental problem
Highlights
► We showcase the extent of arsenic release from calcium arsenate dissolution. ► We show this process releases several tons of dissolved arsenic per year. ► We found arsenic mobility is controlled by calcium and calcium-binding anions. ► We alert to a potential widespread concern in calcium rich environments.
Introduction
Natural and human activities that have negative impacts on environmental systems have led us to consequences of large dimensions, such as the As crisis that affects nearly 50 million people only in south and east Asia due to ingestion of arsenic contaminated water (Ravenscroft et al., 2009). In order to deal with As contamination efficiently, it is necessary to identify the origin and the mechanisms of mobilization in a given environment. Reductive dissolution, alkali desorption, sulfide oxidation, and geothermal processes are source mechanisms recognized to release dissolved As species to natural waters (Ravenscroft et al., 2009). Among these, sulfide oxidation is one of the mechanism occurring at most mining sites, which could explain the As mobilization associated to acid mine drainage from As-rich sulfidic minerals (e.g. 1000 mg/L at pH < 1) (Bowell, 2002). However, in the presence of neutralizing materials (e.g., calcite, oxides and hydroxides), As released from sulfide (and As) oxidation would immobilize in situ through strong As(V) sorption processes to newly-formed iron oxides or coprecipitation processes (Violante and Pigna, 2002; Cheng et al., 2009; Fendorf and Kocar, 2009; Ravenscroft et al., 2009; Johnston et al., 2011) if a high Fe/As ratio prevails (USEPA, 2000; Paktunc et al., 2003; Ravenscroft et al., 2009). Therefore, in order to explain As water pollution at near neutral pH in environments that are not iron-rich (such as is the case in the present investigation), other mechanisms need to be elucidated. Understanding of these mechanisms will aid in their detection and in proposing appropriate mitigation schemes.
In cases where the Fe/As ratio is low, iron arsenate minerals may dominate (Paktunc et al., 2003), but if other metal cations (e.g. Ca2+; Mg2+, Pb2+) are present in sufficiently high concentrations, the corresponding arsenate salt will form depending on pH (Walker et al., 2009; Villalobos et al., 2010). Lead, mixed Pb–Cu, and Ca arsenates have been reported to form in soils contaminated with residues from metallurgical processes, rich in these metals (Villalobos et al., 2010). The present investigation pertains to a contaminated environment where the contents of both iron and other metals are very low, and calcium is the dominant cation. Although calcium arsenates are not as highly insoluble as those of other divalent metals, they are known to remove large amounts of arsenate from aqueous solutions (Bothe and Brown, 1999a; Rodríguez-Blanco et al., 2007; Villalobos et al., 2010) and therefore are believed to control the mobility of As in calcium-dominated environments.
In the present work we identify an important contamination source of mobile As in a mining complex adjoining an urban center in Mexico, investigate the mechanism of As release involved, and determine the geochemical controls of As. We monitored the concentration of As in the Matehuala-Cerrito Blanco artesian hydraulic complex on a monthly basis for a year to determine the spatial and temporal behavior of As and the mechanism that controls its mobility, using geochemical speciation modeling and electron scanning microscopy observations of sediments.
Santa Maria de la Paz is a mining district located in the north-central part of San Luis Potosí, Mexico, in Villa de la Paz municipality and on the Sierra del Fraile eastern foothill, 8 km west of an urban center (Matehuala City) Matehuala, S.L.P. (Fig. 1a). Mining activity here dates 240 years back and currently comprises exploitation of skarn deposits of copper, silver, zinc, lead, and gold from El Fraile hill (Castillo-Nieto and Carranza-Alvarado, 1996; Castro-Larragoitia et al., 1997). Millions of tons of active and historical tailings are estimated to have been accumulated on the surrounding terrains of the mine on a not managed private property, where access to the public is not restricted (Castro-Larragoitia et al., 1997; Manz and Castro, 1997; Razo et al., 2004). Additional polluting activities on the site include accumulation of slags and construction debris from an inactive metal ore smelter that operated within Matehuala City until the 1960's (Manz and Castro, 1997).
Tailings from Santa Maria de la Paz are composed mainly of calcite, and to a lower extent of quartz (Castro-Larragoitia et al., 1997). Arsenopyrite is the most important As-bearing mineral (Castro-Larragoitia et al., 1997), yielding 4.0 g of As per kg tailing in average, which is concentrated in the particle size fractions below 63 μm (up to 50 g/kg). The composition of the slags from the demolished smelter is dominated by SiO2, FeO, CaO and, to a lower extent by Al2O3, MgO, MnO, K2O and heavy metals (Manz and Castro, 1997). Total As concentrations in slags (38 g/kg average) are higher than the average As in tailings and are known to partially dissolve in acidic water, carbonated water, and humic-rich water (2.8–5.5 mg/kg) (Castro-Larragoitia et al., 1997; Manz and Castro, 1997). No information has been published on the characterization of the demolition debris and the soils from the smelter.
Previous studies have suggested that dispersion of tailings and slags has contaminated soils, sediments, water, birds, crops, and affected children in an area surrounding the mining district of over 100 km2 (Castro-Larragoitia et al., 1997; Yanez et al., 2003; Razo et al., 2004; Chapa-Vargas et al., 2010). A remarkable case of As contamination in the area is the Matehuala-Cerrito Blanco artesian hydraulic complex, where aqueous As concentrations between 5.9 and 7.2 mg/L have been reported (Fig. 1a) (Razo et al., 2004) (compare with the Mexican limit of 0.4 mg/L of waters used for non-drinking purposes). This artesian hydraulic complex is about 6 km long and comprises a gallery with 24 dug wells, a channel, and a pond where water is stored and used for recreational activities, watering animals, and irrigation of agricultural fields (Fig. 1a). The hydraulic complex is found within a perched aquifer that runs in a W to E direction and is believed not to mix with a low-As shallow aquifer (<21 μgAs/L) (Razo et al., 2004) that runs NW to SE between 15 and 50 m in depth (INEGI, 2007; Gónzalez-Grijalva, 2009) (Fig. 1a). The perched aquifer is recharged naturally by rainfall and artificially by water that is continuously discharged from the mine located in Santa Maria de la Paz (Fig. 1a). Although the source of As contamination in the Matehuala-Cerrito Blanco artesian hydraulic complex could be related to some mining activities at Santa Maria de la Paz, the specific source and mechanism of As mobilization remains unclear and is not associated to water from the current operation of the mine because the aqueous As concentration in this potential source (<0.4 mg/L) is less than 10% of that in the hydraulic complex (Fig. 1a) (Razo et al., 2004).
Section snippets
Establishing the spatial and temporal dynamics of As mobility
Water samples were collected for over a year on a monthly basis at eight different sampling points. Sampling point A is a spring located within a private Gun Club, 200 m south of the inactive smelter and a pile of slags (Fig. 1). Sampling points B–H are located in dug wells and a channel of the Matehuala-Cerrito Blanco hydraulic complex. Location, geographical positions, and dimensions of the different sampling points are given in Table S1 and Fig. S1 (Supporting information) and Fig. 1b. Water
On the spatial and temporal dynamics of As mobility
Arsenic showed a spatial behavior in a piece-wise linear fashion with three different levels of concentration over time (Fig. 2a). In sampling point A (the water resurgence where As pollution originates), As concentrations were 91 mg/L average between September 2008 and January 2009, which decreased to a mean value of 44 mg/L between February and May 2009, and increased again but up to 155 mg/L average between June and August 2009 (Fig. 2a). In the Cerrito Blanco hydraulic system (sampling
Acknowledgments
This study was partially funded by DGA-IPICyT and grants SEP-CONACYT-U46506-Z, and FMSLP-2005-C01-32. Special thanks are due to Janete Morán, Andrés Arredondo, Rosalina Tovar, and David Gaytan who helped with many of the analyses, to Mr. Tomás Ferrendiz who showed us the area, to land owners who kindly permitted access to their properties, and Jessica Martínez who formatted the paper. We also thank Gladis Labrada, Edgar Peralta, Ana Iris Peña, and Bety Rivera of the LINAN and Mary Carmen Rocha,
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