Chromium occurrence in a nickel laterite profile and its implications to surrounding surface waters

Highlights

• In the Ni laterites, the bulk of Cr is fixed in recalcitrant minerals while only <1% is bioavailable and easily mobilizable.

• Water-extractable Cr(VI) content from the Ni laterite profile surpassed drinking water and freshwater standards.

• First attempt to estimate Cr(VI) infiltration flux (62 to 3446 t/km²/yr) in Ni laterites.

• Area-normalized Cr(VI) fluxes in surrounding streams are mostly ≤50 kg/km²/yr except during stormflow conditions.

Abstract

Ultramafic rocks are considered as the most important geogenic sources of Cr whose hexavalent species is of environmental concern. In tropical to subtropical areas (e.g. New Caledonia, Brazil, Philippines), prolonged and pervasive weathering of ultramafic rocks produces Ni laterite ores which are further enriched in Cr. While elevated levels of Cr have been identified in Ni laterites, Cr availability and export from these deposits and its contribution to natural waters remain poorly understood. Therefore, this study examined the speciation and flux of Cr in a Ni laterite profile through sequential extraction and leaching experiments coupled with physical, mineralogical, and geochemical characterization. These were correlated to the behavior and export of Cr in surrounding surface waters studied through hydrochemical investigation. This work demonstrates, through the example of the Rio Tuba Ni laterite deposit, that pervasive weathering of ultramafic rocks in tropical areas generate very important reservoirs of Cr(VI). Although Cr is primarily fixed in insoluble fractions (>89%) and least in bioavailable and easily mobilizable phases (<1%), due to the inherent elevated Cr content (up to 2.9 wt%) of the Ni laterites, the latter still provided water-extractable Cr(VI) contents surpassing drinking water and freshwater standards. By comparison with the silicate-rich saprolite unit, the overlying Fe-(oxyhydr)oxide dominated laterite layer yielded lower Cr(VI) contents due to its acidic nature and higher amounts of organic matter and Fe-(oxyhydr)oxides which promote reduction and readsorption of Cr(VI). During water infiltration, Cr(VI) migrates downward along the profile at a rate of 62 to 3446 t/km²/yr. While the alkaline and oxidizing conditions of the surrounding surface waters favor Cr(VI) mobilization, only a fraction of Cr(VI) from the Ni laterites reaches these water bodies. Cr(VI) concentrations (≤213 μg/L) and fluxes (mostly ≤50 kg/km²/yr) in the surface waters being significantly lower than that of the Ni laterites reflect the action of processes (e.g. dilution, reduction, adsorption) that attenuate the release of this species along water flow paths.

Introduction

Chromium (Cr) is a transition metal widely used in various industrial processes (metallurgical, refractory, and chemical) for its resistance to impact and corrosion (Kotaś and Stasicka, 2000). It occurs in two common oxidation states (III and VI), where Cr(III) is an essential nutrient (at a tolerable daily intake) (EU, 2011; EFSA, 2014) involved in the metabolism of glucose, fat, and protein (Anderson, 1989) while Cr(VI) is a known toxic pollutant and carcinogen via oral and inhalation routes (WHO, 2003; ATSDR, 2012). Because of this, much interest has been drawn to the sources and evaluation of Cr(VI) occurrence (Economou-Eliopoulos et al., 2016). Significant levels of Cr(VI) in the environment are typically associated with the extensive industrial use of chromium. However, weathering of ultramafic rocks has been identified as an important Cr(VI) contributor to ground and surface waters (Robles-Camacho and Armienta, 2000; McClain and Maher, 2016; McClain et al., 2017).

Ultramafic rocks contain the most Cr among the general rock types, with an average concentration of 2980 mg/kg (Alloway and Ayres, 1997) nearly a hundred times more than the global average Cr content of the upper crust (35 mg/kg) (Shaw et al., 1967; Shaw et al., 1976; Wedepohl, 1995). Soils derived from these rocks and its metamorphic equivalents (i.e. serpentinites), generally called serpentine soils, could be further enriched in Cr with concentrations as high as 80,000 mg/kg (Oze et al., 2004; Chrysochoou et al., 2016). In these geogenic sources, Cr occurs mainly in the trivalent state in Cr-bearing minerals (e.g. chromite, olivine, serpentine), as insoluble (oxyhydr)oxide complexes, or strongly adsorbed onto soil component surfaces (Schroeder and Lee, 1975; Fendorf, 1995; Kotaś and Stasicka, 2000; Garnier et al., 2013; Morrison et al., 2015; Hseu et al., 2018). During weathering, Cr(III) may be released and oxidized to Cr(VI) by hydrogen peroxide, dissolved oxygen, and primarily by Mn(III/IV)-oxides (Schroeder and Lee, 1975; Eary and Rai, 1987; Richard and Bourg, 1991). Conversely, Cr(VI) may be reduced back to Cr(III) by Fe(II), organic matter, sulfide, and microbes (Schroeder and Lee, 1975; Bartlett, 1991; Fendorf et al., 2000) or attenuated by adsorption onto positively charged surfaces (e.g. by Fe-oxides) (Rai et al., 1989).

While ultramafic rocks and derived products cover only a small portion of the Earth's surface, they are distributed in populated areas within the Circum-Pacific margin and the Mediterranean (Oze et al., 2004), possibly exposing a great number of people to significant levels of Cr. Cycling of Cr from these rocks and soils have provided natural waters with significant levels of Cr(VI) (Robles-Camacho and Armienta, 2000; McClain and Maher, 2016; Kazakis et al., 2017; McClain et al., 2017), occasionally exceeding regulatory limits for drinking water (50–100 μg/L) (USEPA, 2006; WHO, 2017). Extreme outliers (up to 1.62 mg/L Cr(VI)) were measured in surface waters and groundwaters of subtropical to tropical areas (e.g. New Caledonia, Greece) (Gunkel-Grillon et al., 2014; Economou-Eliopoulos et al., 2016) where ultramafic rocks are weathered into stratified ore deposits known as Ni laterites. Nickel laterites account for 60% of the identified land-based Ni resources of the world (USGS, 2020) and 60% of the annual global nickel production (Butt and Cluzel, 2013). These deposits have been exploited for Ni and Co, and recently, they have been sought for rare earth elements (REE), platinum group elements (PGE), and Sc (e.g. in Cuba, Dominican Republic, New Caledonia) (Elias, 2002; Butt and Cluzel, 2013; Aiglsperger et al., 2016; Teitler et al., 2019). Nickel laterite ores are extracted through surface mining methods developing large opencast areas. This could enhance the release of Cr in surrounding aqueous environments. Despite this, the availability and transport of Cr in Ni laterites and affected natural waters remain poorly understood.

This research examines a Ni laterite catchment in the Philippines, the world's second-largest producer of Ni ore (USGS, 2020). The Ni ores are primarily mined from Ni laterite deposits associated with ophiolites and ophiolitic terranes widely distributed throughout the archipelago (Encarnación, 2004; Yumul, 2007). This work focuses on the Rio Tuba Ni laterite deposit which represents one of the Philippines' largest Ni ore reserves. The objectives of this study were to determine the speciation and flux of Cr in the Rio Tuba Ni laterite profile and to compare these to the occurrence and export of Cr in the surrounding mining-influenced surface waters. Here, sequential extraction was conducted to determine the solid phase association or fractionation of Cr which provides important information about its potential availability and toxicity (Arnason and Fletcher, 2003; Filgueiras et al., 2002; Ahlf et al., 2009). In addition, a water-extractable leaching experiment was performed to estimate the Cr(VI) content of the labile, and thus, more toxic and bioavailable fractions. From these data, Cr(VI) fluxes through Ni laterites were estimated and correlated to that of the surrounding water bodies. The results of this study contribute to the understanding of the potential of Cr(VI) release in intensely weathered ultramafic rocks in tropical environments, crucial in the evaluation of potential environmental risks and possible remediation measures in these Cr-rich geologic backgrounds.