Morphologies and size distribution of metal-rich particles
Metal-rich particles emitted from smelters are largely composed of angular, spherical particles, and agglomerates of small spherical particles. Angular particles are typically associated with fugitive emissions of dust particles from concentrate ores and other raw materials during handling operations (e.g., transportation, storage, or use) and expelled materials not heated to the melting temperature (Ettler et al., 2016; Gregurek et al., 1999, Shukurov et al., 2014). On the other hand, spherical and agglomerate-bearing particles are formed by quenched melt droplets during smelting of Pb and Zn concentrate ores, and flue gas cleaning processes emitted by the smelter smokestack (Henderson et al., 1998, Gregurek et al., 1998, 1999, Knight & Henderson, 2006, Lanteigne et al., 2012, 2014, Shukurov et al., 2014; Ettler et al., 2014a, 2016).
Regarding the morphologies and size distribution of metal-rich particles emitted from Met-Mex, our analysis revealed the presence of two primary categories: spherically shaped particles and irregularly shaped particles, often forming agglomerates. These metal-rich particles encompass molten materials released during the smelting process, materials that did not attain the necessary melting temperatures, as well as unmelted or raw ores.
Sobanska et al. (1999) conducted a study on the morphology and size distributions of “primary” dust emitted by typical pyrometallurgical lead smelters with dust filters. They found that 90% of the particle distribution was around 5 µm and had no predominant morphology, while the remaining 10% was between 10–20 µm in size. In smelter operation without dust filters, the presence of metal-rich particles and metal-rich slag agglomerates larger than 20 µm is common (Cusano et al., 2017). Large particles (> 10 µm), including the agglomerates, can be released from fugitive concentrate ore during handling operations and by incomplete smelter operations, while fine and ultrafine particles are primarily released during smelting and refining operations.
Met-Mex emitted during decades huge amounts of dust related to out control processes, historically low efficiency of emission controls, large volumes of Pb production, and non-strict and little-enforced environmental regulations in México. A gradual decrease in the emission factors was achieved in the past five decades, with a significant progress in the dust control efficiency achieved two decades ago (1999–2001). We estimated that 0.68 (0.4 to 0.97) ton of PM d− 1 (148 to 355 ton of PM y− 1), equivalent to 118–320 ton of PM10 and 89–284 ton of PM2.5 were emitted before 1999. In 1999, PROFEPA measurements of the emission rate of PM via chimneys of Met-Mex accounted a daily rate of 0.5 ton (183 tons of PM per year), equivalents to 0.42 ton of PM10 d− 1 (152 ton y− 1) and 0.33 ton of PM2.5 d− 1 (122 ton y− 1) (PROFEPA, 1999). SEMARNAT (2010) estimated that the metallurgical industry emitted 0.70 ton of PM10 d− 1 (257 ton y− 1) and 0.21 ton of PM2.5 d− 1 (76 ton y− 1). Our calculations reveled maximum emission rates of 0.26 ton of PM10 d− 1 (95.4 ton y− 1) and 0.23 ton of PM2.5 d− 1 (84.8 ton y− 1) in 2010.
Current daily emission rates vary from 54 to 130 kg of PM10 or 44 to 114 kg of PM2.5 from the metallurgical complex. Regarding to the significant progress in the PM control efficiency achieved two decades ago to the present, the emissions of PM10 and PM2.5 are still significative. In our study, metal-rich particles mixed in the urban dust accounts < 10% for PM2.5 and close to 60% for PM10, while ultrafine (< 0.1 µm) and fine (0.1 to 2.5 µm) close 10%, and > 30% for inhalable particles > 10 µm.
These fine metal-rich particles represent an elevated health risk for the population regarding to the tiny particles but mainly because the elevated contain significant levels of toxic metals, including Pb, Cd, and Zn. For example, our analysis by SEM-XRD techniques of collected PM samples revealed that the content of Pb-rich particles ranging from 60–70% in PM10 and PM2.5.
The particle size distribution is an important parameter for determining particle dynamics, displacement, deposition, and the duration of metal-rich particle suspended in the atmosphere. The size of the metallic particles is also prime for human incorporation. The finest size metallic particles are more easily inhaled, ingested, and absorbed through dermal contact. For example, fine particles (< 2.5 µm) can penetrate the smaller airways in the peripheral lung region close to alveolar interstitium, while ultrafine particles (< 0.1 µm) can penetrate until the alveolar interstitium (Fig. 2). In the alveolar region of the lungs, the rate of absorption of Pb particles is about 32% of the deposition in the lungs (USEPA, 1994). Additionally, the finest particles adsorb more heavy metals compared to coarse-grained ones, which is attributed to the larger surface areas of their components (Witt et al., 2014).
We did not study the size distribution of metal-rich particles as a function of the distance from the smelter and refining factory. However, we observed a higher number of larger particles in closer samples to Met-Mex. It is expected that coarser particles are deposited in the vicinity of the factory and the surrounding urban environments, while fine and ultrafine particles can travel longer distances away from the complex (Csavina et al., 2011, 2012, 2014; Ettler, 2016).
Mineral composition of weathered Pb-rich particles
No previous studies have described the mineralogy of metal-rich particles emitted by the Ag-Cd-Pb-Zn smelter and refining complex in Torreón. Pb rich-particles emitted from non-ferrous smelter and refining complexes contain a variety of compounds, including sulfides (PbS, ZnS) and their oxidation products such as sulfates (PbSO4), oxides (PbO, ZnO, CdO, As2O3), oxide-sulfates (PbO⋅PbSO4), carbonates (PbCO3), Cl-bearing phases (PbClOH, Pb4O3Cl2), metallic elements (Pbº, Znº) and Pb silicates (Sobanska et al., 1999; Manceau et al., 2000; Ettler et al., 2020).
All these minerals were observed in our collected dust samples. Secondary minerals were mostly observed around or in incrustations within cavities of primary minerals, such as galena, and in molten agglomerates. A notable absence of Cl-bearing phases was observed in our urban dust. This can be explained by the difficulty in identifying non-crystalline phases by XRD and/or their high solubility, which causes rapid dissolution and mobilization into the environmental matrices (from aerosols to urban dust soils, to soils and water) (Ettler et al., 2005).
Although the formation of secondary Pb and Zn minerals during the pyrometallurgical process, as transformation products, is highly probable, the chemical composition of Pb-particles may change over time during weathering (Spear et al., 1998; Piatak et al., 2004; Piatak & Seal, 2010; Root et al., 2015). The presence of secondary minerals should be indicative of the degree of oxidation before the subsequent processing (smelting, refinery) and alteration during and after emission to the atmosphere, deposition, and accumulation in the urban area. In this study, the presence of cerussite, litharge, and anglesite around pieces of galena may indicates that PbS has undergone partial oxidation (Hettiarachchi & Pierzynski, 2004). Numerous particles of galena, either liberated or incorporated, have been converted into secondary minerals of Pb, providing clear evidence of weathering processes affecting these sulfide mineral particles. Nevertheless, the presence of particles of galena as the primary phase in urban dust also suggest a slow kinetic weathering process (Rucker, 2000; Witt et al., 2013, 2014), giving that emissions began over 120 years ago and that most Pb-particles were emitted prior to the 1960s (Soto-Jiménez and Flegal, 2021). Changes in the mineralogy of the Pb-rich particles historically emitted by Pb-Zn smelting and refining activities evidence the progress in weathering.
The presence of secondary Pb minerals can also be associated to the ore concentrate coming from the mines to feed the Met-Mex smelter. In Mexico, the production of silver, cadmium, lead, and zinc primarily relies on the exploitation sphalerite (ZnS) and galena (PbS). Galena and sphalerite may also be accompanied by a minor presence of other secondary minerals, including cerussite (PbCO3), smithsonite (ZnCO3), smithsonite (ZnCO3), maghemite (FeZnCO3), hydrozincite (Zn[(OH)3CO3]2), and hemimorphite (Zn2SiO4). Notably, these secondary minerals form through the weathering of the lead and zinc sulfides and are primarily found in shallow deposits. We assumed that the presence of secondary minerals in the concentrates processed by Met-Mex is highly improbable because Peñoles and its subsidiaries operate both underground and open pit mines, typically extracting ores from depths of up to 1,000 meters below surface outcrops. Secondary minerals formed because of the weathering of Pb and Zn sulfides are primarily found in shallow deposits. Furthermore, the flotation and lixiviation processes employed during the concentration of galena and sphalerite effectively remove impurities, including probably these secondary minerals. Even, if some secondary Pb minerals were to arrive in the smelter feed concentrate, the carbonate minerals easily break down during Pb oxidation at high temperatures. Thus, the presence of first-generation secondary Pb minerals is negligible, such as was observed in the analyzed samples.
Environmental impacts of lead
Upon emission, galena (PbS) comes into contact with environmental elements such as air and water, which accelerate its weathering and facilitate the subsequent development of lead phases, such as PbSO4 and PbCO3 (Benvenuti et al., 2000; Razo et al., 2004). This weathering process transforms galena into lead phases characterized by higher lead (Pb) contents and enhanced bioaccessibility compared to the original galena. For instance, cerussite and anglesite exhibit Pb contents of up to 77.5% and 74%, respectively, surpassing the 50–60% Pb content of galena. Importantly, these lead phases demonstrate increased bioaccessibility compared to galena (Ruby et al., 1999). Under gastric conditions, lead carbonates and lead sulfates are notably more soluble than galena (IPCS, 1989; Gasser et al., 1996; Ruby et al., 1999; Casteel et al., 2006; Argyraki, 2014; Bosso & Enzweiler, 2008; Ettler et al., 2020).
Thus, the weathering alterations of Pb-rich particles to finer particles, more concentrated in Pb, and forming secondary minerals exacerbates the solubility, availability, and toxicity of Pb in urban environments (Ettler et al., 2020; Spear et al., 1998). The Pb-rich particles undergoing weathering are susceptible to be more accessible and available when they are incorporated into the human body through different routes, including inhalation in aerosols, ingestion in soils and dust, and absorption by dermal contact (Birmili et al., 2006).
This study presents compelling evidence of the physicochemical transformation of lead (Pb)-rich particles, which is manifested through alterations in their particle size, elemental composition, and mineral content. Specifically, secondary minerals become incorporated into significantly smaller particles, mostly ranging from < 0.1 to 4 µm in size, in contrast to the larger original sulfide particles. Given the enduring presence of Pb and its ongoing physical and chemical changes, it is imperative that Met-Mex takes proactive measures by implementing an extensive remediation plan, covering a wide radius around the Torreón complex. This plan should aim to control, mitigate, and prevent the emission and concentration of Pb within urban environments.
Furthermore, there is a pressing need for Met-Mex to expedite the transition from strict control to advanced abatement strategies, incorporating state-of-the-art filtration technology at each stage of their processes. Failure to do so may necessitate the consideration of closing the Pb-smelting operations within the Met-Mex complex, which is situated in the heart of Torreón city. In addition to these measures, it is crucial that Met-Mex allocates resources to support independent scientific studies focused on assessing the risks posed to the population of Torreón, which numbers over 725,000 individuals. These studies should address the potential hazards associated with Pb-rich particles emitted from the smelter complex.