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and health risks of toxic geogenic, and (3) highlight mitigation measures and future research needs. Figure 1.1 summarizes the key focus on the chapter, including the nature, human exposure pathways, and health risks of toxic geogenic contaminants in serpentinitic geological systems and their ultramafic protoliths.
Figure 1.1 Summary of the nature, environmental sources, and human exposure and health risks of toxic geogenic contaminants in serpentinitic geological systems and their ultramafic protoliths. Adapted and redrawn based on Gwenzi [4].
1.2 Serpentinitic Geological Systems
1.2.1 Nature, Occurrence, and Geochemistry
The serpentine group consists of three minerals: (1) chrysotile, which is a type of asbestos; (2) antigorite; and (3) lizardite (www.minerals.net; www.geology.com). The hydrothermal transformation of ultramafic rocks or protoliths such pyroxenites and peridotites results in the formation of serpentines [9, 11]. In this chapter, the term serpentinitic geological systems is broadly used to refer to both serpentines sensu stricto, and their ultramafic protoliths or precursor rock assemblages. This is because both serpentines and their ultramafic protoliths contain anomalously high concentrations of toxic contaminants [4].
Serpentinitic geological systems are sparsely distributed across all continents, particularly along plate convergent boundaries such as Circum-Pacific margin [4]. Countries hosting significant serpentinitic geological systems include, the USA (California, Oregon), Spain, Australia (Western Australia), Sri Lanka and Zimbabwe (Great Dyke) [4, 12–15]. In Africa, the Great Dyke in Zimbabwe is linear ultramafic geological formation hosting serpentinitic geological systems and other associated petrological assemblages [4, 16, 17]. Serpentinitic geological systems have a unique geochemistry, defined by anomalously high concentrations of toxic contaminants [1]. Of human health concerns are high concentrations of chrysotile asbestos, toxic metals specifically Cr, Ni, Fe, Mn, Co, and Cd [1, 9], and potential enrichment of rare earth elements.
Toxic contaminants in serpentinitic geological systems are released via natural and anthropogenic activities. Natural processes include biotic and abiotic weathering processes, and subsequent erosion and mass movement [18]. Anthropogenic activities include agriculture (i.e., tillage practices) and mining operations including metallurgical processing, construction, and sculpturing, carving, and engraving [4]. Once released from the host rock toxic contaminants are widely disseminated via wind and hydrologically-mediated processes. These processes include surface and sub-surface runoff, water and wind erosion, infiltration and groundwater recharge, and surface water-groundwater exchanges [4]. Moreover, due to the low moisture retention and infiltration of serpentine soils, they are highly susceptible to high runoff, erosion and mass movement (e.g., landslides) [19]. Therefore, the toxic contaminants from serpentinitic geological systems tend to be widely disseminated into the surrounding environment, resulting in off-site environmental pollution and human health risks [4, 19].
1.2.2 Occurrence and Behavior of Toxic Contaminants
1.2.2.1 Chrysotile Asbestos
Chrysotile is a fibrous asbestos mineral, which is a human carcinogen [4, 20]. Due to its toxicity, chrysotile and other types of asbestos have been long-banned in most developing countries. The physico-chemical behavior of chrysotile is defined by its crystal habit, fiber size, chemical composition, biopersistence, and surface reactivity, including capacity to generate reactive oxygen radicals [4, 21, 22]. Its fibrous nature, low density, and long residence time, coupled with unique aerodynamic properties make it a unique contaminant. For example, due to its biopersistence and aerodynamic properties, chrysotile can be transported over long distances from the sources to other environmental compartments [4]. Chrysotile undergoes limited biochemical degradation and is not taken up by plants due to its fibrous nature. Evidence shows that acid rain may promote the corrosion of chrysotile, and its release into the environment. In serpentinitic geological systems, chrysotile co-occurs with other contaminants such as toxic metals [21–23], thus may act as a carrier in this regard. The co-occurrence of chrysotile and other toxic contaminants points to potential synergistic interactions between the two, a process reported to have adverse human health effects [21, 22].
A few studies exist on the environmental and biogeochemical behavior of chrysotile, particularly in aqueous systems [24, 25]. The following summary findings are evident from existing data: (1) coagulation and filtration significantly remove chrysotile in aqueous systems, including drinking water supplies [26], (2) strong organic acids promote the loss of crystallinity, and release of Mg, which alters the surface charge from an initial positive to a negative one [24, 25]. Therefore, some studies suggest that the release of organic acids from organic amendments (sawdust, peat, compost, manure, and biochar) can be used as basis to reclaim serpentinitic geological systems such as mine tailings and waste dumps [4, 24].
1.2.2.2 Toxic Metals
Serpentinitic geological systems contain extremely high concentrations of toxic metals, specifically Fe, Ni, Cr, Co, Mn, and Zn [4, 27, 28]. The total concentrations of toxic metals have been reported to be in the order of hundreds and thousands mg per kg of dry soil. For example, the following total concentration of toxic metals was reported in serpentinitic geological systems in Albania: 3,865 mg Cr/kg, 3,579 mg Ni/kg, 2,495 mg Zn/kg, 1,107 mg Cu/kg, and 476 mg Co/kg [26]. High concentrations of toxic metals were also reported elsewhere in India and Zimbabwe [27]. Two points are noteworthy literature on toxic metals: (1) high total concentrations of Cr and Ni are considered universal in serpentines, but they co-occur with other toxic metals (e.g., Zn, Fe, and Mn) in equally high concentrations, and (2) the bulk of the data on toxic metals report total concentrations, which indicate the total pool in the soil, but provide no indication on the bioavailable and bioaccessible fractions [4]. The lack of data on bioavailable and bioaccessible concentrations makes it difficult to infer human health risks from such data. In some instances (e.g., Cr), some metal species (e.g., Cr (VI)) are more toxic than others (e.g., Cr(III)), but data on speciation are rarely provided in most studies on toxic metals.
The literature on metal geochemistry in serpentinitic geological environments is dominated by Cr, particularly Cr(VI), probably due to its high toxicity [4, 29]. Specifically, several studies exist on the role of Mn minerals (e.g., MnO2, birnessite,) in accelerating the dissolution of Cr-bearing spinels such as chromite (FeCr2O4) and magnesiochromite (MgCr2O4), and the oxidation of Cr(III) to Cr(VI) [1, 30]. The accelerated dissolution and oxidation account for extremely high concentrations of Cr(VI) detected in aquatic ecosystem in serpentinitic geological systems [9, 29]. For instance, a recent review showed that high Cr(VI) concentrations above the World Health Organization (WHO) limit for drinking water (i.e., 50 μg L−1 equivalent to 960 nM Cr(VI)) have been detected in both surface water and groundwater in several countries [4]. Such high concentrations may pose significant human health risks in cases where communities rely on untreated drinking water.
Unlike chrysotile, toxic metals may undergo uptake and bioaccumulation by wild plants and food crops. Rhizospheric processes including, root-soil interactions and the release of root exudates are critical in the biogeochemical behavior of toxic metals [4, 31]. For instance, root-soil interactions and root exudates may increase the bioavailability and bioaccesibility of toxic metals, and their subsequent uptake and bioaccumulation by plants and crops [31]. Such processes may transfer toxic metals from the soil system into the human food chain, thereby posing human health risks.
1.2.2.3 Rare Earth Elements