Post-deposition Weathering of Pb-rich Particles From a Pb-Zn Smelting and Refining Factory Under Semiarid Conditions


 In this study, urban dust samples were collected at 1 km radius surrounding one of the largest Ag-Cd-Pb-Zn smelting and refining complex in the world (Met-Mex Peñoles), which is in operation in Torreón (North México) since 1901. Metal-rich particles in urban dust were analyzed for elemental composition, and Pb-rich particles were identified, characterized, and analyzed for mineral identification by using conventional techniques such as scanning electron microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and X-ray powder diffraction (XRD). Pb-rich particles showed a variety of sizes and morphologies and different contents of Pb and other elements. Pb-rich particles were related to the fugitive and non-controlled emissions from Met-Mex Peñoles. Galena occurs in individual and metal-rich agglomerate particles. The presence of secondary Pb minerals (e.g., Pb carbonates, Pb sulfate, and Pb oxides) evidenced the weathering in Pb-rich particles and metal-rich agglomerates. Secondary Pb minerals are incorporated in finer particles than original sulfide minerals, and they are also more concentrated in Pb and chemically more available than galena for the environment and humans. Physical-chemical transformations on the weathered Pb-rich particles are increasing the availability and toxicity of lead in the urban dust and the potential impacts on the environment and human health.


Introduction
Fugitive and non-controlled metal-rich particles emissions from mining and non-ferrous metallurgical are the main source of metals to the atmosphere (Bollhöfer & Rosman, 2001), and then to the urban environments, biota, and humans . The physical and chemical properties of the metal-rich particles in urban impacted by mining-metallurgical activities depend on the processed mineral, industrial stage, and the emission controls implemented. In the case of Ag-Cd-Pb-Zn smelter and re neries complexes, metal-rich particles are emitted to the atmosphere in a variety of sizes, morphologies, and elemental and mineral composition (Adamo et al., 1996;Gregurek et al., 1998Gregurek et al., , 1999Ohmsen, 2001; Knight & Henderson, 2005. These metal-rich particles contain high contents of potentially toxic elements (PTE) including As, Cd, Cu, Hg, Mn, Pb, and Zn.
Metal-rich particles emitted from smelter and re neries complexes are not inert, but they are subject to weathering processes that involve a set of transformations of physical and chemical properties during their transport, atmospheric precipitation, and post-deposition (Chopin & Alloway, 2007;Ettler et al., 2014a,b;Sobanska et al., 2016). The progress of the weathering processes on the metal-rich particles depends on the prevailing conditions in deposition environments (e.g., oxidizing condition, water content, temperature, pH, microorganisms, and presence of other inorganic and organic compounds).
In Torreón, located on an arid to semiarid climate region in northern México, is installed one of the worldwide largest Ag-Cd-Pb-Zn smelters and re ning complexes (Mex-Mex Peñoles), which is in operation since 1901. Mex-Mex metallurgical complex has been a primary source of metal-rich particles and PTE to rural and urban areas in Torreón since decades ago The alterations of the physicochemical properties of Pb-rich smelting particles post-deposition, emitted for decades, related to weathering has not yet been investigated in Torreon. The weathering process of Pb-rich particles in uence the metal mobility and availability and exacerbate the potentially deleterious effects of Pb on human health and the environment, but little knowledge exists about these mechanisms in arid or semiarid climate regions (Root et al., 2015), such as Torreón city.
In the present research, metal-rich particles present in urban environmental dust collected in Torreón were observed for physical characterization and weathering by scanning electron microscopy (SEM). Then, the metal-rich particles were bombardment by an electron beam to characterize the elemental composition by Energy Dispersive X-Ray Spectroscopy (EDS). Finally, we focused on the weathered Pb-rich particles for particle size distribution, morphology, and elemental and mineral composition by using SEM-EDS and X-ray powder diffraction (XRD) techniques. Mineral analyses were compared by the mineralogical database on the International Centre for Diffraction Data (https://www.icdd.com/).

Background Met-Mex
Details about the historical Ag-Cd-Pb-Zn smelter and re neries complex, Met-Mex Peñoles, are provided by Soto-Jiménez & Flegal (2021). Brie y, the smelter in Torreón was built in 1901 to treat the ores concentrates of Peñoles mines. A few years later, become one of Mexico's largest industrial complexes, producing Pb, Ag, and As. Torreón smelter was competing with The ASARCO El Paso lead smelter, constructed in the late 19th century, to process ore from the rich mining districts of Chihuahua (Marcosson, 1949). Beginning 1960s, the facility was expanded and diversi ed to process other metals and chemical by-products over time. The expansion included a Zn smelter-electrolytic re nery plant in 1961; three H 2 SO 4 plants in 1961,1970,1976; improvements in those plants in 1988 and 2000, an Ag re nery (1975), and NH 4 SO 4 fertilizer and SO 2 liquid plants in 1990. Presently, Met-Mex is divided into two circuits: the Pb-Ag and Zn circuits. The Pb-Ag circuit has the Pb smelter, where Ag-Cd-Pb-Zn concentrates are processed to produce Pb bullion, and the Ag-Pb re nery that receives Pb bullion and other byproducts to produce Au, Ag, Pb, and Bi. The installed capacity in the Pb smelter is 180,000 tons of Pb bullion, and that of the Ag-Pb re nery is 180,000 tons of Pb, 118 million oz of Ag, 1.9 million oz of Au, and 1,440 tons of Bi. The zinc circuit, where an electrolytic process to produce re ned Zn treats, Zn concentrates has an installed capacity of 360,000 tons of Zn ingots (Peñoles Report, 2017). In 2018, the production volumes were Au 1.13 million oz, Ag 64.9 million oz, Pb 112,840 ton, Zn 228,842 ton, Bi 333 ton, and 639 ton of Cd (Peñoles Report, 2018 Background Torreón city Torreón city, located in northern México, has a population of > 725,000 (INEGI, 2021). It is the largest of four sister cities of the metropolitan "La Laguna" zone (Torreón and Matamoros in Coahuila, and Gómez-Palacios and Lerdo in Durango), with a population > 1.6 million inhabitants (INEGI, 2021). The "La Laguna" region is characterized by an arid to semiarid climate, with very low precipitation (mean 225 mm y − 1 ), elevated evaporation (mean 2,000 mm y − 1 ), and average summer and winter temperatures of 31 and 16ºC, respectively. Relative humid average 38%.
Based on a wind vectorial analysis of 10 years dataset, the prevalent wind in the region goes from northwest to southeast during winter, and from northeast to southwest for the rest of the year. Thermic inversions are common from November to March, these events exacerbate the ambient pollution in the city because the pollutants are retained in the atmospheric upper layers. Besides, powerful convection ows are generated as the result of contact between the compressed cold air and warm air, which results in resuspension and mobilization of particles to the atmosphere. The resuspension may result in dust storms (in the region called tolvanera) that emit large amounts of dust into the atmosphere with the potential to be transported long distances.
The topography is characterized by a mountain chain of the Sierra de las Noas (> 400 m high; 1500 m) and the at basin of Agua Naval River. This river divides the cities of Gomez and Lerdo, and then ows southward to Torreón City. The North-South-oriented Sierra de las Noas hill is another natural limit of the city.

Sample collection
Atmospheric particulate matter sinking by dry deposition was collected in the urban area in Torreón in situated cross-sectionally sites, depending on the measured wind direction and at speci c distances from the Met-Mex complex (Fig. 1). Eight acrylic frame plates (1 m 2 ) recovered with low-density polyethylene (LDPE) were rotativity placed in different sites covering most of the city area and the prevailing weather conditions in the region. The deposition devices were collected above the home ceiling (> 3 m height) for 12 to 48 hours under stable weather conditions (no rain was observed and samples for dust storm events were excluded). A total of 18 aerosol samples were collected in the urban area of Torreón in a 2-km radius from Met-Mex, including 3 samples representing the urban background obtained in three sites far away from the complex emissions or another known source of lead.

SEM, EDS, and XRD analysis
Aerosols samples were oven-dried (65-70ºC) for 72 h and then equilibrated in a desiccator for 48 h. Aliquots of urban dust samples were examined under the scanning electron microscope (SEM) to determine the size and morphology. Few milligrams of urban dust samples were prepared by sprinkling aliquots onto a carbon-impregnated tape and mounted in aluminum holders. Measurements were made using a Zeiss model EVO MA10 SEM at high vacuum operating with a beam scan accessory V (20 kV). High de nition and high magni cation SEM images for each sample (triplicated of 150x150 mm area per sample) were taken at four magni cations, namely 500x, 1500x, 3000x, and 6000x obtained by backscattered electron detector.
The elements with a large atomic number (e.g., Pb, atomic number Z = 82) produce an intense bright white color because more backscattered electrons are emitted when the primary electron beam interacts with the nuclei of the element. The high-resolution SEM images were analyzed by a softwareassisted method (ImageJ software developed by the U.S. National Institutes of Health) to determine the particles diameter. Particle counting was visually done in the magni ed SEM images. Particles were classi ed in ve different size fractions based on their deposition e ciency along the respiratory tract ( Fig. 2). Brie y, (1) > 10 µm: inhalable since they can easily enter the nose and mouth, (2) < 10 µm (PM 10 ): thoracic since they can penetrate deep in the respiratory system and generally deposited by impaction in the extra thoracic airway region, (3) < 4 µm: respirable because they are small enough to pass completely through the respiratory system and enter the bloodstream and are deposited by impaction in the tracheobronchial airway in the deep lungs, (4) < 2.5 µm (PM 2.5 ): ne because penetrate the smaller airways and are deposited by sedimentation in the peripheral lung region in the tracheobronchial airway close to alveolar interstitium, and (5) < 0.1 µm (PM 0.1 ): ultra ne because penetrate the smaller airways and is deposited by diffusion often in the alveolar interstitium. Also, inhalable particles were divided in large (> 10 to 25 µm), and giants (> 25 µm).
The deposited aerosol samples were analyzed by Energy Dispersive X-Ray Spectroscopy (EDS) detector (Bruker XFlash 4010) coupled to the Zeiss EVO MA10 SEM (Full Scale: 20KeV(20eV/ch,1Kch), Acc. Volt: 20.0 KV, Probe Current: 1.781E-08 A). The sample preparation for SEM-EDS analysis agreed to Bern et al. (2009) procedure. Brie y, aliquot of sample was weighted (0.1-0.2 g), splitted to obtain a sample mass between 10 and 20 mg, placed in suspension with isopropanol (10 mg dust mL − 1 ), transferred to polycarbonate substrate (0.4 µm pore size lters a xed to 13 mm aluminum stubs with conductive carbon adhesive tabs), air-dried, and coated with a thin (∼100 Å) conductive carbon lm using a carbon evaporator with a rotating stage. Metal-rich particles with different sizes and morphologies were analyzed by EDS-SEM for quanti cation of elemental composition. Then, we focused on the identi cation of minerals in the metallic particles rich in Pb based on the chemical composition analyses (in weight %) and morphology of the SEM-EDS analysis.
Because of the complexity for the identi cation of phases within the urban dust, X-ray powder diffraction analysis was done on selected Pb-rich particles and metal-rich agglomerates slag as an independent method verifying the mineralogy of SEM-EDS. XRD measurements were done with a Philips PW 1729 X-ray diffractometer at a voltage of 40 kV and current of 30 mA using Co Kα (1.79 Å) and Cu Kα radiation (1.54 Å), respectively. Diffraction patterns from Pb-rich particles were collected over a scan range of 5-75° 2θ with a step size of 0.02° 2θ and dwell times of 4-10 s. The identi cation of the probable minerals was supported by consulting the mineralogical database on the International Centre for Diffraction Data (https://www.icdd.com/ ).
Elemental composition of rich-metal particles An elemental composition analysis was conducted by SEM-EDS in the most common metallic particles observed in each airborne dust sample collected in the urban environmental area around Met-Mex. Results of the quanti cation of elemental abundances (in weight %) are summarized in Table 1 and displayed in Figs. 4 and 5. Lead peaks in EDS spectra observed in urban dust collected in the Met-Mex vicinity were differentiated from other metals (e.g., Al, Ag, Cd, Cu, Fe, Hg, and Zn), metalloids (As, Bi, and Si), and not metals (Ca, Cl, Mg, P, and S), which appear together in the low part of the spectra. Fe, 6 % Cu, 1.5% Cd, 0.7% Al, 0.3% Ag, and 0.2% of Ti and Hg. Metalloids in Pb-rich particles included Bi (median 2.1%), Si (2%), and As (1%). Non-metal contents showed median values of 3.4% for S, 2.7% for Ca, 0.6% for P, and 0.9% for Mg.
The chemical analysis among different Pb-rich particles observed in urban dust collected in surrounding Met-Mex sites, distinguished in size and morphological, showed signi cant differences among them. However, Pb-rich particles in the same or different sample, with similar morphologies characteristics, showed similarity in the elemental composition. Overall, a relationship was observed between the particle size and the elemental composition, regarding Pb content (Table 1; Fig. 4-5). Most of the ultra ne and ne particles (< 2.5 µm) showed higher contents of Pb (70-90% of Pb) and less proportion of other elements than large (> 10 to 25 µm) with xx-yy%, and giant particles (> 25 µm) with < 25% of all Pb mass.
In Table 1, we also included the elemental composition of composite samples collected far away from Met-Mex (at 4, 5.3, and 7.2 km to northeastward), representing the urban background in Torreón. Signi cant differences were observed between metal-rich particles collected far away versus those collected in the vicinity of Met-Mex. In background urban samples, the median contents of elements were Ag 0.2%, Al 12.7%, As 1.5%, Ca 10.4%, Cl 1.4%, Cu 5.6%, Fe 13.9%, Mg 0.6%, P 0.7%, Pb 1.7%, S 1%, Si 48.6%, Ti 1%, while Bi, Cd and Zn were undetected. The composition of the background samples is representative of natural soils in the region. Overall, Pb contents in metal-rich particles collected far away were signi cantly lower than those collected around 1 km radios Met-Mex, while levels of Si and Al were signi cantly higher.

Identi cation of most probable minerals in metal-rich particles
Because of the complexity of the chemical composition (metals Al, Ag, Cd, Cu, Fe, Hg, and Zn; metalloids As, Bi, and Si, and non-metals Ca, Mg, P, and S), X-ray diffraction analyses, in a combination of SEM-EDS, were practiced in representative samples of urban dust with most common Pb-rich particles for identi cation of minerals ( Table 2). The presence of sulfur in the particle with Pb and Zn peaks indicates the presence of basic Pb and Zn minerals, galena (PbS) and sphalerite (ZnS), in the urban dust samples collected in the vicinity of Met-Mex. Individual particles of galena ore and relicts of this mineral were observed into agglomerated pieces of slags. In many Ag-Cd-Pb-Zn mining districts in México, the predominant forms of Pb and Zn are galena and sphalerite, respectively. Thus, sul de mineral concentrates are mostly processed to produce lead re ned. The presence of galena and sphalerite is indicative of fugitive emissions of sul de mineral concentrates, while metal-rich agglomerates slag containing galena, evidencing an incomplete smelting of those concentrates processed.

Morphologies and size distribution of metal-rich particles
Non-data about the morphologies and size distribution of metal-rich particles emitted from the Met-Mex are available. However, metal rich-particles emitted from smelters consist largely of angular, spherical particles and agglomerates of small spherical particles. Angular particles are related to the fugitive emission of dust particles of concentrate ores and other raw material from the 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. Spherical and agglomeratebearing particles represent quenched melt droplets during smelting of Pb and Zn concentrate ores, and ue 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., 2014Ettler et al., 2014aEttler et al., , 2016. Sobanska et al. (1999) studied the morphology and size distributions of 'primary' dust emitted by typical pyrometallurgical lead smelters with dust lters, they found that 90% of the particle distribution was around 5 µm and had no 'predominant morphology', while the remaining 10% was 10-20 µm. In smelter operation without dust lters, the presence of metal-rich particles and metal-rich agglomerates slag is common > 20 µm (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 the ne and ultra ne particles are mainly releasing from smelting and re ning operations.
We did not study the metal-rich particle size distribution in function of distance to the smelter and re ning factory, however, a higher number of larger particles were more present in closer samples to Met-Mex. It is expected that coarser particles are deposited in the vicinity of the factory and the . In this study, changes in the mineralogy of the Pb-rich particles historically emitted by Pb-Zn smelting and re ning activities evidence the progress in the weathering. The presence of other secondary minerals around pieces of galena (e.g., cerussite, litharge, and anglesite) is indicative that PbS has undergone partial oxidation in the urban environment (Hettiarachchi & Pierzynski, 2004). The notable absence of Cl-bearing phases can be explained by the di culty to identify non-crystalline phases by XRD and/or because they have high solubility, rapidly dissolved and mobilized into the environmental matrices (from aerosols to urban dust soils, to soils and water) (Ettler et al., 2005).
A lot of particles of galena or with galena incorporated are being converted into secondary minerals of Pb, which is clear evidence of weathering processes occurring on these sul de mineral particles. However, the presence of particles of galena in urban dust, as the primary phase, also denotes a slow kinetic weathering process (Rucker, 2000;Witt et al., 2013Witt et al., , 2014, considering that emissions started 120 years ago and more of the Pb-particles were emitted before the 1960s (Soto-Jiménez and Flegal, 2021).

Implications for impacts on the environment and health of lead
Lead contents in cerussite increase up to 77.5% and anglesite up to 74% compared to galena that contains 50-60% by mass. Pb carbonates and Pb sulfates are more soluble under gastric conditions than galena (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 ner particles, more concentrated in Pb, and forming secondary minerals, exacerbates the solubility, availability, and toxicity of the Pb in urban environments (Ettler et al., 2020;Spear et al., 1998). The Pbrich particles in weathering process are susceptible to be more accessible and available when are incorporated into the human body by different routes, including inhalation in aerosols, ingestion in soils and dust, and absorption by dermal contact (Birmili et al., 2006).
In this study, the alteration of the physical of Pb-rich particles is evidenced by changes in the particle size, elemental contents, and mineral composition.
For example, secondary minerals are incorporated in much smaller particles (mostly to < 0.1 to 4 µm in size) than the original sul de. The distribution of particle size is an important parameter for determining particle dynamics, displacement, deposition, and the aerosol stay duration in the atmosphere.
The size of the metallic particles is prime for human incorporation. The nest size metallic particles are more easily to be inhaled, ingested, and absorbed by dermal contact. For example, ne (< 2.5 mm) particles can penetrate the smaller airways in the peripheral lung region close to alveolar interstitium, while ultra ne (< 0.1 mm) particles 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). Also, the nest particles absorb more heavy metals in comparison to coarse-grained ones, which is attributed to the larger surface areas of their components (Witt et al., 2014).

Conclusions
In Torreón, the presence of Pb-rich particles historically emitted continue in the modern urban dust and because Pb is a persistent pollutant with limited mobility, the pollution will continue for centuries unless major abatement operations are employed. Knowledge of the long-term weathering alterations of metal-rich particles emitted from non-ferrous smelting and re ning operations is useful to understand the mechanisms by which PTEs are released into the environment.
In the present, signi cant amounts of metal-rich particles occur in the urban dust surrounding Met-Mex in Torreón, including a lot of Pb-rich particles, which show a variety of size, morphology, and elemental and mineral composition. Analysis of particle size distribution, morphology, and elemental and mineral composition by using SEM-EDS and X-ray powder diffraction (XRD) techniques evidenced alterations by weathering processes on Pb-rich particles. The weathering of Pb-rich particles is reducing the particle sizes (Pb-rich particles to ner particles), increasing the presence of secondary minerals such as Pb carbonates, Pb sulfate, and Pb oxides containing highest Pb contents. All physical-chemical transformations on the weathered Pb- Availability of data and material: Included in this work.
Code availability: Not applicable.  Micromorphology of metal-rich particles in urban dust observed by SEM: a) small Pb-rich particles adhered to coarse smelter slag, b) small Pb-rich particles incorporated to unaltered sul des from concentrate ores, c) metal-rich agglomerates particle, d) galena incorporated into metal-rich agglomerates particle, e) small spherical metal-rich particle, f). spherical metal-particles into metal-rich agglomerates slag.

Figure 4
Energy Dispersive X-Ray Spectroscopy spectrum in conjunction with scanning electron microscopy and elemental composition (% weight) of Pb-rich particles in urban dust collected in adjacent site to the smelter (TP1-3).

Figure 5
Energy Dispersive X-Ray Spectroscopy spectrum in conjunction with scanning electron microscopy and elemental composition (% weight) of Pb-rich particles in urban dust collected in adjacent site to the smelter (TP2-3).

Figure 6
Surface morphology and secondary phases onto weathered Pb-rich particles in urban dust collected in the vicinity of Met-Mex as observed by SEM. The presence of Pb carbonate minerals (cerussite, hydrocerussite), Pb-sulfate (anglesite), and Pb-oxides (lithargo) were identi ed in weathered Pb-rich particles.