The environmental hazard caused by smelter slags from the Sta. Maria de la Paz mining district in Mexico

PII: S0269-7491

(97)001

07-3

Environmental Pollution, Vol. 98, No. I, pp. 7-13, 1997 © 1998 Elsevier Science Ltd. All fights reserved Printed in Great Britain 0269-7491/97 $17.00 + 0.00

ELSEVIER

THE ENVIRONMENTAL H A Z A R D CAUSED BY SMELTER SLAGS FROM THE STA. MARIA DE LA PAZ MINING DISTRICT IN MEXICO Michael M a n z a* and L. Javier Castro b aUFZ Centrefor Environmental Research Leipzig-Halle, Department of Chemical Ecotoxicology, PO Box 2, D-04301 Leipzig, Germany bFacultad de Ciencias de la Tierra de la UANL. Ap. Postal. 104, C.P. 67700 Linares, N.L. Mexico (Received 13 December 1996; accepted 24 July 1997) Abstract The noxious potential of metallurgical tailings fiom the Ag/Pb/Zn/Cu mining district of Sta. Maria de la Paz (Mexico) is ascertained via the chemical characterization of slag material. Batch experiments using various extraction solutions (e.g. natural rainwater, water rich in humic substances, ammonium nitrate solution) revealed information on the dissolution behaviour of the elements Fe, Mn, Ni, Cu, Zn, As, Ba and Pb from slag with respect of grain size (slag material screened under 2 mm and analytically fine fraction) and elution time (2, 7.5, 24 and 240h). An initial ecotoxicological assessment of the results is made and the environmental danger caused by large volumes of slag tailings is discussed. © 1998 Elsevier Science Ltd. All rights reserved

Keywords: Mining, arsenic, tailings, environmental pollution, slag.

Kerzfassung Das Schadstoffpotential metallurgischer Rfickstdnde der Verhfittung von Erzen im Ag-Pb-Zn-Cu- Minendistrikt yon Sta. Maria de la Paz, Mexiko wird durch chemische Charakterisierung des Schlackenmateriales ermittelt. Batch-Experimente, die mit verschiedenen Extraktionsmitteln (z.B. natfirliches Regenwasser, huminstoffreiches Wasser, Ammoniumnitratlb'sung ) durchgeffihrt wurden, geben Aufschlufl fiber das L6"slichkeitsverhalten der Elemente Fe, Mn, Ni, Cu, Zn, As, Ba und Pb aus Schlacken in Abdhngigkeit yon der Korngrdfle (Schlackenmaterial < 2 mm und analysenfeine Fraktion ) und der Elutionsdauer (2, 7.5, 24 und 240 Stunden). In einer ersten Einschdtzung werden die Ergebnisse 6kotoxikologisch bewertet und das yon den Schlackenhalden ausgehende Umweltgefdhrdungspotential diskutiert.

soils caused by mining activities. The majority of this pollution (which is often located in the vicinity of smelters), is the result of the deposition of metal oxides or tailings from the atmosphere (Little and Martin, 1972; Koning, 1974; Lobersli and Steinnes, 1988; Puchelt, 1992). Contaminated sites can also be attributed to the considerable accumulation of slag and flotation material in soils (Simon, 1978; Gryschko et al., 1995), as has been found to be the case in the Black Forest region (Germany) and in La Paz (Mexico). The aforementioned research projects focused on investigations into heavy metal pollution in different compartments. In a second phase, several investigations were carried out with the aim of characterizing the mobility and availability of these substances from soils and slags (Castro, 1995; Manz, 1995; Castro et al., 1997). In 1991-1995, extensive geochemical investigations were conducted in the mining district of Sta. Maria de La Paz in the Central Mexican Uplands in order to establish the potential risk to the environment created by mining activities (Castro, 1995). We decided to augment these findings by investigating the massive quantities of slag tailings originally stemming from the smelting of metal ore and still being present. In addition to geochemical characterization, a number of batch tests were also performed using different solvents. Various elution agents designed to emulate near-natural conditions enabled the mobilization behaviour of inorganic elements (arsenic and heavy metals) to be studied in relation to the extraction time and grain size of a variety of slag material (Manz, 1995). When considering the findings obtained, it must be stressed that slag is merely one of a number of contamination sources affecting the environment. The mining arca's soil and vegetation contain high levels of pollutants introduced by atmospheric input and wastewater from past and present mining activities (due in no small part to the mine owner's lack of environmental awareness).

INTRODUCTION

Over the past few years, numerous publications have reported on arsenic and heavy metal contamination in *To whom correspondence should be addressed.

GEOGRAPHIC DESCRIPTION

The mining area lies on the eastern edge of the Central Mexican Upland between the coordinates 100o45, and

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Fig. 1. Map of the investigation area (after Castro, 1995). 100°35'W and 23o37' and 23°44'N, about 500 km NNW to Mexico City (Fig. 1). It comprises an asymmetric north-south depression, whose deepest point is located about 2 km east of the town of Matehuala at an altitude of 1550m above sea level. The small town of La Paz in the west lies at an altitude of 1800 m above sea level. The upland has a semiarid climate, with seasonal waterbearing brooks resulting from summer rain. Using the FAO classification, the soil types in the area range from calcic to gypsic xerosoles (FAO-Unesco, 1988). The mines and the dressing plant lie on the western edge of the area, near La Paz. It was here in the mid-18th century that Ag-Pb-Zn ore was discovered in hydrothermal veins, along with Cu-Au ore in seams. The ore has been worked from that time, virtually continuously. In 1992 the Ag-Pb-Zn mines were shut down as a reaction to the low prices on the international market; but the Cu-Au mines are still operating. The ore was dressed by flotation and smelted in a plant on the northern edge of the mining district outside the town Matehuala (near H4 in Fig. 1) until the mid1960s, when the plant was closed down. The ore was subsequently smelted and processed at other facilities elsewhere in Mexico or abroad. This immense activity has resulted in many contaminated sites, the most significant of which are shown in Fig. 1, namely the flotation tailings (H1-H3) to the east of La Paz and a slag tailings heap measuring 400 mx250 m with a height of up to 8 m on the outskirts

of Matehuala (H4). Nowadays, the former smelting area is undergoing redevelopment and some of the slag material has been used for road-metal, footpaths and railroad routes. The presence of huge quantities of tailings begs the question as to the precise environmental risk they pose.

MATERIAL AND METHODS The slag tailings were sampled at different locations, the material subsequently being mixed to produce a composite sample weighing about 2kg. Preparation involved the following stages: 1. Cleaning, screening and crushing to a grain size less than 2 mm; 2. Removal of secondary weathered fragments; 3. Homogenization of the whole material less than 2 mm; 4. Grinding of about half of the slag material in a planetary mill; 5. Secondary homogenization of the ground material. In order to determine the total concentration of pollutants, the ground material was dissolved with H F HNO3-HC104 after Heinrichs and Herrmann (1990). The elements in the solution were determined using Inductively Coupled Plasma Source Mass Spectrometry

The environmental hazard caused by smelter slags from the Sta. Maria de la Paz mining district in Mexico (ICP-MS) and various atomic absorption speetrophotometric methods. Additional measurements were also made (X-ray fluorescence and neutron activation analysis) in order to evaluate the results. Batch experiments with a slag/solution ratio of 1:10 were performed in polyethylene bottles on a horizontal shake machine using both slag material under 2 mm and analytically fine material. As natural processes are not subject to constant pH, the batch tests were not carried out at a constant pH either. Several different eluents were used in the 24-h batch experiments: 1. Acidified demineralized H20 (pH 1, standardized with HNO3), to estimate the extractability of heavy metals under extremely aggressive conditions; 2. Natural rainwater (RW) with a pH of 5.4, to simulate near-natural leaching conditions; 3. Water rich in humic substances (HW) from an upland moor lake near Kaltenbronn in the Black Forest (Germany) with a pH of about 4, to simulate the concentration of humic substances in a soil solution; 4. A 1 M ammonium nitrate solution (NH4NO3) with a pH of 4.8--a method which has been used in Germany since 1995 for the extraction of available trace elements from soils (DIN 19730); 5. A solution of 0.25 g CaCO3 in 1 litre bidistilled water with a pH of about 7 at the beginning of the batch test, to simulate the effect of a calcareous soil solution. In addition, extensive time-dependent batch experiments (2, 7.5, 24 and 240 h) with rainwater, humic water and calcareous water were conducted on the screened material. Some batch tests were repeated to facilitate the evaluation of the results. For some metals (Cd, Cr, Sb and Bi) the concentrations in the eluates were often under the detection limit of the analytically method used.

RESULTS AND DISCUSSION The area under investigation is characterized by high arsenic, lead, zinc, copper and cadmium pollution. The flotation residue and the slag are deposited in huge, unsecured tailings, and are consequently exposed to direct rainfall and wind erosion. Arsenic and heavy metals have accumulated in the mine water and the tailings seepage. This water reaches the preliminary clarifier virtually unaltered and may enter the ground water. Moreover, the contaminated water is also used to irrigate vegetables and cereals (chiefly maize). This environmental risk has been highlighted by several investigations in the surrounding area (Castro, 1995).

Chemical characterization and poUution potential In macroscopic terms, the basic glass slag has a colour ranging from black to greyish black and appears glassy,

9

similar to an obsidian. It has a homogeneous dense structure and fractures sharply. The metallurgical residue of the tailings mainly has a particle size range with a diameter ranging from below 5 em to an analytically fine fraction. Unsmelted ore material is sometimes encountered. Investigations using the microsonde on comparable slag invariably show that heavy metals can often be found drop-shaped inclusions in an oxide silicate ground mass. In some cases the heavy metals (e.g. zinc and lead) occur as silicates, which are formed under extremely high smelting temperatures (Faber, 1954; Bachmann, 1982; Manz, 1995). In addition to high fractions of SiO2, Fe203 and CaO, the main components also include high concentrations of a number of heavy metals, especially zinc (2.86%; Fig. 2). Various iron ores and limestone were added as flux during the smelting process. The unusually high levels of arsenic for slag of this type (0.38%) are remarkable, as most of the arsenic present usually escapes during the smelting process, as arsenic oxide. The slag's high arsenic concentration may be due to the type and efficiency of the smelting method used; alternatively it could be the result of the repeated addition of arsenic and heavy metal oxides accumulating in the condensing chamber during the roasting and smelting process. It can be seen in Fig. 2 that apart from arsenic, high concentrations of the environmentally significant elements Zn, Pb, Cu and Sb are also present in the metallurgical residue. Resalts of the batch experiments In order to determine the metal fractions which are soluble under extremely aggressive conditions, the slag (grain size < 2 mm) was eluted for 24 h with an eluate comprising a solution adjusted to a pH of 1 using HNO3 (Table 1). These batch experiments were not conducted under constant pH conditions, and the values at the end of the experiment were approximately 4.4. The relatively low mobilization rates (concentrations in mg kg -1 in Table 1, calculated on the basis of the original slag) of Cu, As, Sb and Pb may be attributed to the low solubilities of these compounds, under these pH conditions. Furthermore, metals already dissolved during the batch experiment were readsorbed on negatively charged slag surfaces, or specifically adsorbed, for example, to dissolved iron oxides. The high percentage concentration of Ni is remarkable, although the absolute concentrations dissolved are very low. It will be fixed to a cation exchange complex, because the main amounts can be released by ammonium nitrate. One aim of the chemical analyses was to distinguish the quantities of As and heavy metals mobilized from different grain sizes. The screened (< 2 mm) and analytically fine ground slag material were shaken for 24h with various elution agents (Figs 3 and 4). During the experiments, the concentrations in the eluates were frequently below the analytical detection limit and therefore some elements are ignored. Figure 3 shows the results of the batch experiments carried out on the screened material (grain size <2ram). The lowest

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amounts were mobilized by the watery calcareous solution (Fe: 3.9 m g k g -1, Zn: 2 . 3 m g k g -1, As: 1 . 3 m g k g - l ) ; the largest quantities were mobilized by the 1 M a m m o n i u m nitrate solution (Cu: 1 4 2 m g k g - l , Zn: 59.1 m g k g - l , Pb: 18.8 mg kg-1). C o m p a r e d to total content in the slag, the m a x i m u m amount o f any element dissolved by CaCO3 was just 0.05% (Ba), in the same order of magnitude as the figures for H W (0.4%, Cu), and R W (0.07%, Pb and Ba). In contrast, NH4NO3 mobilized as much as 19% (Ni), although this

hardly presents a risk owing to the low total content o f Ni in the metallurgical residues. In the case of R W and H W , the concentrations in the eluates were actually below the detection limit. However, the situation is quite different with regard to the environmentally significant heavy metals and As, which are present in much higher fractions in the slag and are also mobilized in greater quantities. Nearly all the findings in Fig. 4 for analytically fine slag material reveal even larger quantities mobilized, for example 2 0 0 m g k g -1 of Cu in the

The environmental hazard caused by smelter slags from the Sta. Maria de la Paz mining district in Mexico

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case of NH4NO3. By way of comparison, the mobilization levels for the other eluates and elements are as follows:

CaCO3: RW: HW: NH4NO3:

Fe 4.9, Ba 4.0 and As 2.4mgkg-n; Fe 17.4, Zn 2.6 and As 2.3mgkg-Z; Fe 106, Zn 14.3, Cu 9.3 and As 3.8mgkg-1; Cu 190, Zn 117 and Ba 30.4mgkg -I.

Despite the changing pH values, it was possible to determine the solubility sequences for some elements in terms of their relative solubility fractions (Table 2). The use o f different extraction agents and fractions (crushed < 2 mm, analytically fine material) show different solubility sequences at the end of the batch test. The reason for this is that arsenic for example has a significantly higher solubility from its compounds in the face of a higher pH value. In Table 2 ratios of amounts extracted between the two different grain sizes are shown. With the exception of Pb and Cu, more heavy metals are mobilized from analytically fine material, as a result of its larger surface area, leading to a better attack by chemical action. In contrast the dissolved Pb can be trapped by newly formed hydrous iron oxides (Gadde and Laitinen, 1974). The experiments with natural water rich in humic substances showed that very large quantities of certain heavy metals in the slag material can sometimes be mobilized (Figs 4 and 5). This extraction agent was used as it has a chemical composition similar (but more diluted) to that of moisture in the upper mineral soil horizon, and thus enables estimation o f the order of magnitude in which pollutants may be available to Table 2. Ratios between amounts of heavy metal extracted from different grain sizes (finely ground < 2 ram) by various extraction agents (batch-time 24 h)

Extraction agent

Mn Fe

H20 with C a C O 3 Rainwater(RW) Water rich on humic substances (HW) 1M NH4NO3

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plants for a short time before sorption onto various soil components in the 'soil solution'. Slag with a grain size < 2 mm was used for the timedependent batch experiments. Elution took place for 2, 7.5, 24 and 240h using three different elution agents (RW, H W and NH4NO3). Once again, a constant solidsolution ratio of l:10 (5 g slag to 50 ml extraction solution) was selected for each batch experiment. Despite the homogenization of the material, considerable fluctuations in concentration sometimes occurred, owing to the non-homogeneities in the slag (mainly when using the screened material). These could not be completely excluded, as the metallic inclusions in particular occurred with differing density, size and distribution. The metals exhibit very different elution behaviour according to the use of extraction agents and with increasing elution time, which is chiefly due to the changing pH during the course of the experiment (Fig. 5). The rise in pH over time is most clearly apparent with the rainwater because of its low mineralization. In many cases, an equilibrium between the solid and dissolved phases was reached for the heavy metals after being shaken for just 24 h. In contrast, some elements failed to achieve equilibrium even after an extraction period lasting 240h. In some batch experiments, the heavy metal content in the filtrate was actually observed to decrease with increasing elution time. This could be due to a number of reasons: • •

The rising pH during the experiment; Exceedance of the solubility product of certain compounds; • Increased adsorption of positively charged cations onto newly formed Fe hydroxides and oxyhydroxides, and thus removal from the eluate; • Readsorption on the slag surface of the partic!es.

Ba Pb

0.83 1.34 1.98 0.48 2.07 0.55

CRITICAL REVIEW AND P R O S P E C T S The residues accumulating during ore processing, with their high levels of As, Pb, Zn and Cu, were dumped onopen, unsecured slag-heaps, and were thus exposed to

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order to avoid erosion) or by sealing them with plastic sheeting. This would prevent the smallest particles from drifting over the surrounding district. In order to better assess the danger to the ecosystem emanating from the residues, supplementary ecotoxicological investigations are essential. These could be carried out indirectly in the form of further possible extraction methods already conducted on similar material (Kramer et al., 1988; F6rstner et al., 1991; Gupta, 1991) or directly via toxicological tests on salt extracts (L~itseh et al., 1991).

REFERENCES

Bachmann, H.-G. (1982) The identification of slags from archaeometallurgical sites. Institute of Archeology Occasional Publication, 6. Bowen, H. J. M. (1979) Environmental Chemistry of the Elements. Academic Press, New York. Castro, J. (1995) Umweltauswirkungen des Bergbaus im semiariden Gebiet yon Santa Maria de la Paz, Mexiko. Karlsruher Geochemische Hefte, 10 (English and extended Spanish abstract included). Castro, J., Kramar, U. and Puehelt, H. (1997)200-year mining activities at La Paz, San Luis Potosi/Mexico---consequences to environment and geochemical exploration. Journal of Geochemical Exploration, 58, 81-91. Faber, W. (1954) Mikroskopie der Metallhfitten-Schlacken.In Handbuch der Mikroskopie in der Technik, Band 2, Teil II, Hrsg. H. Freund, pp. 521-526. Umschau-Verlag, Frankfurt am Main. FAO-Unesco (1988) Soil Map of the World. FAO Rom. F6rstner, U., Calmano, W. and Kienz, W. (1991) Assessment of long-term metal mobility in heat-processing wastes. Water, Air, and Soil Pollution 57-58, 319-328. Gadde, R. R. and Laitinen, H. A. (1974) Studies of heavy metal adsorption by hydrous iron and manganese oxides. Analytical Chemistry 46, 2022-2026. Glooschenko, W. A. and Arafat, N. (1988) Atmospheric deposition of arsenic and selenium across Canada using Sphagnum moss as a biomonitor. Science Total Environment 73, 269-275.

The environmental hazard caused by smelter slags from the Sta. Maria de la Paz mining district in Mexico Goldenberg, G. (1990) Die Schlacken und ihre Analysen. Freiburger Univ.-Bl. 109, 147-172. Goldenberg, G. (1994) Arch~iometaUurgische Untersuchungen zum MetallhQttenwesen im Siidschwarzwald (Friihgeschichte bis 19 Jahrhundert). Diss. Univ. Freiburg: 300 S.; Freiburg i. Br. Gryschko, R., Horlacher, D., Buchleither, Y. and Berg, T. (1995) Schwermetallaufnahme von Gemiisepflanzen auf 'Schlackebrden'. Mitteilungen der Deutschen Bodenkundlichen Gesellschaft 76, 277-280. Gupta, S. K. (1991) Assessment of ecotoxicological risk of accumulated metals in soils with the help of chemical methods standardized through biological tests. In Heavy Metals in the Environment, ed. J. P. Vernet, pp. 55-65. Elsevier, Amsterdam, London, New York, Tokyo. Heinrichs, H. and Herrmann, A. G. (1990) Praktikum der analytischen Geochemie. Springer, Heidelberg. Koning, H. W. (1974) Lead and cadmium contamination in the area immediately surrounding a lead smelter. Water, Air and Soil Pollution 3, 63-70. Kramer, J. R., Gleed, J., Brassard, P. and Collins, P. V. (1988) Comparison of various leachate extraction procedures for the characterization of inorganics in wastes. Proceedings of the Ontario Ministry of the Environment Technology Transfer Conference, MOE, Toronto. L~itsch, A., Reinecke, H. and Schwedt, G. (1991) Leuchtbakterientest zur Beurteilung der Toxizit/it yon Extrakten aus

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Erzabraumhalden. Erzmetal144, 188-191. Lronard, A. (1991) Arsenic. In Metals and Their Compounds in the Environment. Occurrence, Analysis and Biological Relevance, ed. E. Merian, pp. 751-774. VCH, Weinheim, New York. Little, P. and Martin, M. H. (1972) A survey of zinc, lead and cadmium in soil and natural vegetation around a smelting complex. Environmental Pollution 3, 241-254. Lebersli, E. M. and Steinnes, E. (1988) Metal uptake in plants from a birch forest area near a copper smelter in Norway. Water, Air and Soil Pollution 37, 25-39. Manz, M. (1995) Umweltbelastungen durch Arsen und Schwermetalle in Bfden, Halden, Pflanzen und Schlacken ehemaliger Bergbaugebiete des Mittleren und SQdlichen Schwarzwaldes. Karlsruher Geochemische Hefte, 7 (English abstract included). Piver, W. T. (1983) Mobilization of arsenic by natural and industrial processes. In Biological and Environmental Effects of Arsenic, ed. B. A. Fowler, pp. 1-50. Elsevier, Amsterdam. Puchelt, H. (1992) Environmental inorganic geochemistry of the continental crust. In The Handbook of Environmental Chemistry, Vol. 1, Part F, ed. O. Hutzinger, pp. 29--63. Springer, Berlin. Simon, E. (1978) Heavy metals in soils, vegetation development and heavy metal tolerance in plant populations from metalliferous areas. New Phytologist 81, 175-188.