Natural Attenuation of Arsenic in Semiarid Soils Contaminated by

Chapter 17

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch017

Natural Attenuation of Arsenic in Semiarid Soils Contaminated by Oxidized Arsenic Wastes MargaritaGutiérrez-Ruíz,MarioVillalobos*,Francisco Romero, and Pilar Fernández-Lomelín Environmental Bio-Geochemistry Group, LAFQA, Instituto de Geografía, National Autonomous University of Mexico (UNAM), Circuito Exterior, Ciudad Universitaria, México, Coyoacán, 04510, D.F., México

This work presents experimental evidence for the natural attenuation of arsenic in semi-arid soils contaminated by wastes containing oxidized arsenic species. This evidence was obtained through measurements of water-soluble As in aqueous soil extracts of extensive soil sampling sets, and by comparing them with the solubility of the original As species in the wastes. Additionally, wet chemical analyses of total As, Pb, Zn, Cd, Cu, Mn, Fe, Ca, and pH, were conducted on these samples. Selected fine soil fractions of high total As content but low water-soluble As content were analyzed by Scanning Electron Microscopy coupled with Energy Dispersive X-ray Fluorescence Spectroscopy, and revealed predominant associations between As, Pb, and Zn, and little to none of As with Fe. This evidence suggests attenuation is due to formation of very low-solubility heavy metal-like arsenates as arsenic-containing wastes equilibrate with the soil media. Thermodynamic solubility calculations, using total elemental contents and data from pure heavy metal arsenates, further support this hypothesis.

© 2005 American Chemical Society

In Advances in Arsenic Research; O'Day, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Arsenic is a toxic element to plants and animals because of its affinity for proteins, lipids and other cell components (1,2). It has been associated with a number of different cancers, as well as to cardiovascular and neurological effects, depending on the mode and level of exposure (1,3). Arsenic is commonly found in parent rock and mineral environments in low oxidation states [usually 0, -1 and - 2 (4,5)], appearing associated to sulfidic minerals, typically as arsenopyrite [FeAsS] or arsenical pyrite [Fe(As,S) ], and to analogous sulfides and arsenides of other transition metals (e.g., of Pb, Zn and Cu) (3,4). In oxidized environments a number of different arsenite [of As(III)] and arsenate [of As(V)] minerals of transition metals exist (3-5). Consequently, As is a natural component of Pb, Cu, Zn and A u ores (6) and inevitably enters metallurgical processing (3). Arsenic and other trace metals accompanying the original parent mineral ores appear in the mining wastes. Thus, As is ubiquitous in mineral wastes and the soil environments in which they are disposed of. The bioavailability of heavy metals in general depends on their reactivity and solubility. The mobility of As in soils is inextricably linked to its speciation because a considerable range of mobility behaviors is observed for the stable chemical species under ambient conditions (4,5). The present work is concerned with aerated semi-arid contaminated soil environments where the stable As species are the higher oxidation states, namely As(III) and As(V), which are the most mobile chemical forms in aqueous environments (6,7). These two chemical forms are readily interchangeable under variable redox conditions in soils, such as those that exist during alternating saturated and unsaturated regimes (5,7,8). Iron oxides have been identified to reduce the mobility of oxidized As species through a range of mechanisms from adsorption to coprecipitation (916). However, some evidence suggests that under certain conditions, this control may be exerted by the lower solubility of mixed Fe-metal arsenates (10,14,17,18). For example, Beudantite (Pb-Fe arsenate-sulfate-hydroxide mineral) has been detected by X-ray diffraction (XRD) in acid sulfide-rich tailings from gold mining (10,14). Morin et al. (18) detected a secondary iron arsenate containing barium (pharmacosiderite) in a natural soil developed from weathering of arsenopyrite and other reduced As minerals that originate from hydrothermal processes. In contrast, Lumsdon et al. (19) inferred that adsorption to hydrous ferric oxides, and not arsenate mineral formation, was controlling As availability in similar As-contaminated soil environments, by performing thermodynamic calculations and modeling using concentration values of species extracted from the soils. However, they did not consider thermodynamic solubility constants of metal arsenates other than those of Ca and Fe(III), which


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237 show much higher As solubilities at circumneutral pH values (7), despite the high concentrations of first-row transition metals in the soils studied. The role of iron oxides in As retention has been amply demonstrated in the literature, however, few studies have been conducted on industrial residues rich in heavy metals, which may provide under specific conditions, available fractions of these elements. Davis et al. (17) identified, using electron microprobe analysis and wavelength dispersive spectroscopy, as major Asbearing liberated particles in smelter-impacted soils and house dusts, a complex solid substitution series described as heavy metal - As oxides, with very low contributions from Fe - As oxides. The present work is part of an investigation to seek remediation strategies of soils contaminated with As and other heavy metals. Here, we show evidence of natural As attenuation in the soil environments studied as due to insolubilization processes through formation of heavy metal arsenate compounds. Two different semi-arid sites in Mexico where soils have been contaminated with As(III) and As(V) wastes were studied. Wet chemical analyses of total and water-soluble As contents, total metal contents and other soil parameters, as well as electron microscopy coupled with energy dispersive spectroscopy were performed.

Materials and Methods Study Area Sampling sites are located in semi-arid regions in north and central Mexico (Fig. 1) on terrain surrounding smelting complexes located in Monterrey (MONT) and San Luis Potosi (SLP) cities. The M O N T plant operated 107 years producing Pb, Ag, Au, Sb, B i alloys and Se. Mainly two wastes were produced: "paspurrias" (a white mixture of Ca-carbonates with Ca-arsenates, at high pH) and slag (a black vitrified solid with As and heavy metals). The wastes are stored in a superficial disposal site (Fig. 1), but traces of paspurrias are still visible in the surrounding soils. The study area (= 60 Ha) in M O N T is underlain by brown, clayey, alkaline soils with calcareous nodules (caliche). The annual average temperature (AAT) is 22 °C, average rainfall (AAR) is 638.5 mm, and average evaporation rate (AAER) is 1941 mm. In the SLP site a smelter has operated since 1890, producing Cu (Cu- Plant), A s 0 (As- Plant) and "calcinas" (a subproduct rich in Pb and As). This site is located in the N E of the city. The wastes (melt slag) are deposited in two places (Fig. 1). A residential area is located East of the As- Plant (Morales Distric 8,800 inhabitants = 100 Ha,). The A A T is 24.5 °C, the A A R is 351 mm, and the A A E R is 2072 mm. Two different sites were sampled (Fig. 1): the Morales 2

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239 district, composed of alluvial soils rich in calcareous nodules; and an uninhabited field composed of rhyolite soils (= 28 Ha).


Soil and waste sampling and storage Forty two test pits were dug to a depth of 2.50 m in the M O N T site, collecting samples approximately every 15 to 25 cm to obtain 162 soil samples. In the SLP site, 100 soil samples were collected (0-15 cm). Each sample was a composite of 5-10 sub samples taken from 1 Ha grid squares. Thirty subsurface soil samples were collected from 6 cores of total depths of 80 to 120 cm, by taking samples every 15 - 30 cm depth intervals. Product and sub-product samples were collected: Black As (high content of A s 0 ) , Furnace Dusts (rich in Pb and As), Dust of Converters (rich in Cu), and "Calcinas". 2

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Chemical and physical analysis Air-dried samples were disaggregated, sieved (

Natural Attenuation of Arsenic in Semiarid Soils Contaminated by

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