Metal(loid)s remobilization and mineralogical transformations in smelter-polluted savanna soils under simulated wildfire conditions

Highlights

• Wildfires affect smelter-polluted biomass-rich savanna topsoils.

• Laboratory wildfire simulations using combustion experiments at 250–850 °C.

• Cd, Cu, Pb, and Zn released at >550–600 °C, but As remobilized already at >275 °C.

• Temperature-dependent remobilization related to solid speciation of contaminants.

• Local protection of mining and smelting sites against wildfires needed.

Abstract

The surroundings of mines and smelters may be exposed to wildfires, especially in semi-arid areas. The temperature-dependent releases of metal(loid)s (As, Cd, Cu, Pb, Zn) from biomass-rich savanna soils collected near a Cu smelter in Namibia have been studied under simulated wildfire conditions. Laboratory single-step combustion experiments (250–850 °C) and experiments with a continuous temperature increase (25–750 °C) were coupled with mineralogical investigations of the soils, ashes, and aerosols. Metals (Cd, Cu, Pb, Zn) were released at >550–600 °C, mostly at the highest temperatures, where complex aerosol particles, predominantly composed of slag-like aggregates, formed. In contrast, As exhibited several emission peaks at ~275 °C, ~370–410 °C, and ~580 °C, reflecting its complex speciation in the solid phase and indicating its remobilization, even during wildfires with moderate soil heating. At <500 °C, As was successively released via the transformation of As-bearing hydrous ferric oxides, arsenolite (As₂O₃) grains attached to the organic matter fragments, metal arsenates, and/or As-bearing apatite, followed by the thermal decomposition of enargite (Cu₃AsS₄) at >500 °C. The results indicate that the active and abandoned mining and smelting sites, especially those highly enriched in As, should be protected against wildfires, which can be responsible for substantial As re-emissions.

Introduction

Wildfire activity has increased in numerous places on Earth partially as a result of climate change among other factors (Westerling et al., 2006; Gross et al., 2020; Kganyago and Shikwambana, 2020; Weber and Yadav, 2020; Witze, 2020). Wildfires cause atmospheric pollution, contribute to the long-range transport of aerosols (Creamean et al., 2016) and have direct effects on human health (Weinhold, 2011), soil properties (Campos et al., 2016; Pereira et al., 2019), land erosion and the subsequent contamination of watersheds (Bladon et al., 2014; Cerrato et al., 2016; Abraham et al., 2017; Rahman et al., 2018). Since the last decade, the environmental effects of wildfires have attracted the attention of many scientists and resulted in the publication of focused special issues of scientific journals (Muñoz-Rojas and Pereira, 2020 and references therein) and books (Pereira et al., 2019).

Compared to the above-mentioned environmental impacts, the processes related to wildfire-driven emissions of trace elements have been studied less, although Nriagu (1989) estimated that the global emissions of metal(loid) contaminants from wildfires and biomass burning average 1900 t Pb, 7600 t Zn, 3800 t Cu, 190 t As and 110 t Cd per year. Laboratory and field experiments demonstrated that Hg is especially easily mobilized even by moderate fires, when the soil and biomass are heated to temperatures around 350 °C (Wiedinmeyer and Friedli, 2007; Friedli et al., 2009; Campos et al., 2015; Abraham et al., 2018a; Howard at al., 2019; Tuhý et al., 2020a). Aerosols from biomass burning have been found to contain many trace metallic elements, including Cu, Zn, Pb, and Cd, as demonstrated by studies from the African savanna (Gaudichet et al., 1995), Australia (Isley and Taylor, 2020), the USA (Jahn et al., 2021) and Asia (Kayee et al., 2020), where wildfires are quite frequent. Many studies confirmed that the emitted metals could be transported hundreds of kilometers from the fire (Young and Jan 1977; Creamean et al., 2016).

Soils contaminated by human activities appear to contribute to trace metal remobilization by wildfires. For example, Pb isotopic investigations of the ash from wildfires in the USA and Australia showed that the ash contained industrial Pb from gasoline combustion (Odigie and Flegal, 2011, 2014; Kristensen and Taylor, 2014; Wu et al., 2017). Other studies from Australia compared the Pb isotopic compositions of aerosols before, during, and after the wildfire periods; the authors found that the interpretation of the Pb isotopic patterns is not straightforward, but the contribution of remobilized anthropogenic Pb seems to occur (Kristensen et al., 2017; Isley and Taylor, 2020).

There is a particularly high risk of contaminant remobilization back into the atmosphere in highly polluted semi-arid areas in the vicinity of abandoned or active mines and smelters (Abraham et al., 2018a,b). Particulates emitted by mining and smelting industries or windblown from waste disposal sites have very complex chemical and mineralogical compositions; they adhere to the surfaces of vegetation, are deposited onto the soil surface as leaf litter, and are incorporated into the topsoil organic matter (Kříbek et al., 2016; Ettler et al., 2016; Tuhý et al., 2020b), where they become vulnerable to transformations during the wildfire-induced heating. It is important to stress that wildfires underneath the tree canopies cause greater soil heating due to the wood fuel presence (Pereira et al., 2019). Several studies from Cu mining areas in Zambia hypothesized that this phenomenon could be responsible for the greater re-emission of metals from contaminated wooded savanna than from grassland sites (Mihaljevič et al., 2011; Ettler et al., 2014).

However, the mechanisms and processes controlling the remobilization of metal(loid) contaminants from the burned soil and biomass remain poorly understood. To help to fill this knowledge gap, this paper's main objective is to investigate the release of metal(loid)s from highly polluted biomass-rich savanna topsoils collected near mines and smelters located in semi-arid Namibia under simulated wildfire conditions. Many previous laboratory wildfire-simulation studies focusing on soil heating have been conducted in muffle furnaces (e.g., Johnston et al., 2019), but this technique has many limitations, such as the lack of oxygen circulation and the impossibility of aerosol sampling (Pereira et al., 2019 and references therein). For this reason, we used a novel setup of laboratory burning experiments and combined them with comprehensive chemical and multi-method mineralogical investigations of the initial materials, ashes, and aerosols. This approach enabled one to estimate the fluxes of the metals (Cd, Cu, Pb, Zn) and the metalloid (As), which were the main potentially toxic elements in the studied topsoils, and to describe the related mineralogical transformations over a broad temperature range corresponding to conditions extending from low to high-intensity wildfires (250–850 °C).