___________________________________________________________________________
__________________________________________________________________________ 1) Institute of Geotechnics, Slovak Academy of Sciences, Watsonova 45, 043 53 Košice, Slovak Republic
2) Institute of Geotourism, BERG Faculty, Technical University of Košice, Němcovej 32, 040 01 Košice, Slovak Republic
Tomáš Schütz
1), Sergej Straj
ň
ák
2)Uptake of dangerous elements from industrial
wastewaters
Abstract
The contamination of water by heavy metals through the industrial wastewater is a worldwide environmental problem. Heavy metals like cadmium may come from various industrial sources such as electroplating, metal finishing, metallurgy, chemical manufacturing. In nature, cadmium is often associated with sphalerite (ZnS). This association is caused by the geochemical similarity between zinc and cadmium which makes geological separation unlikely. As a consequence, cadmium is produced mainly as a byproduct from mining, smelting, and refining sulfidic ores of zinc, and, to a lesser degree, lead and copper. The cadmium is soluble in aquatic conditions, reacts with surrounding elements and transforms into different compounds. It accumulates in the bodies of fish and other aquatic organisms and plants. Human health is at risk through the consumption of contaminated food and water. Various heavy metals might be present in wastewater from mining production and ores treatment. In order to reduce of risk for human body from the mining activity, adsorption process provides an attractive alternative treatment to other removal techniques because it is more economical and readily available. Some natural materials such as clays are being considered as alternative low-cost adsorbents [1]. Bentonite is one of the most popular clay rock with exceptional adsorption properties. It is possible to use bentonite as a sorbent either without any treatment or in its modified form. The chemical modifications is feasible due to the presence of water molecules at the surface and exchangeable cations in the interlayer space of montmorillonite structure. [5]. These qualities explicitly show the sorption ability of harmful pollutants from the potential liquid leakage.
Key words: industrial wastewaters, water contamination, adsorbents
Absorpcja (wychwytywanie) niebezpiecznych pierwiastków
z wód przemysłowych
Streszczenie
Zanieczyszczenie wody metalami ciężkimi przez odpadowe wody przemysłowe jest problemem środowiskowym na całym świecie. Metale ciężkie , na przykład kadm mogą pochodzić z różnych źródeł takich jak galwanotechnika, obróbka powierzchniowa metali, metalurgia, przemysł chemiczny. W przyrodzie kadm jest często powiązany ze sfalerytem (ZnS). To powiązanie jest spowodowane geochemicznym podobieństwem pomiędzy cynkiem i kadmem, które sprawia, że geologiczne rozdzielenie jest nieprawdopodobne. W rezultacie kadm jest głównie produktem ubocznym pochodzącym z przemysłu górniczego, hutniczego oraz rafinacji siarczkowych rud cynku a w mniejszym stopniu ołowiu oraz miedzi. Kadm jest rozpuszczalny w wodzie, reaguje z otaczającymi go pierwiastkami i przekształca się w różne związki chemiczne. Odkłada się w organizmach ryb oraz innych organizmach
wodnych i roślinach. Zdrowie człowieka jest narażone na ryzyko spowodowane spożyciem zanieczyszczonej żywności i wody. Różne metale ciężkie mogą być obecne w ściekach z produkcji górniczej i przeróbki rud. Aby zmniejszyć negatywny wpływ działalności górniczej na ludzki organizm, procesy wychwytywania są korzystną alternatywną metodą oczyszczania w stosunku do innych metod, ponieważ są bardziej ekonomiczne i łatwo dostępne. Jako możliwe tanie adsorbenty bierze się tu pod uwagę niektóre materiały naturalne [1]. Bentonit jest jedną z najpopularniejszych skał ilastych o wyjątkowych właściwościach adsorpcyjnych. Możliwe jest zastosowanie bentonitu jako sorbentu bez żadnego przetworzenia lub w postaci zmodyfikowanej. Chemiczne modyfikacje są możliwie dzięki obecności cząstek wody na powierzchni i wymiennych kationów w przestrzeni międzywarstwowej montmorylonitu [5]. Te właściwości wyraźnie wskazują na zdolność sorpcji szkodliwych substancji zanieczyszczających z ewentualnego wycieku.
Słowa kluczowe: ścieki przemysłowe, zanieczyszczenie wód, adsorbenty
1. Introduction
Almost all societies depend on the availability and use of mined products such as minerals and metals. They are the basis of our wealth and ensure economic development all over the world. But the expansion of mining operations into environmentally sensitive and fragile areas has increased the level of environmental destruction and the impact on basic ecosystem services and biodiversity. Furthermore, inadequate provision for closure and post-closure is leaving a growing number of abandoned and/or orphan mining sites around the world [4].
1.1. Land use and biodiversity
Habitat alteration is one of the most significant potential threats to biodiversity associated with mining. It may occur at any stage in the mine cycle with the greatest potential for temporary or permanent alteration of terrestrial and operation. Additionally, exploration often requires the construction of access routes, transportation corridors and temporary camps to house workers, all of which may result in land-clearing and population influx to a varying extent [4].
1.2. Water use and quality
Management of water use and quality in and around mine sites can be a significant issue. Potential contamination of water sources may occur early in the mine cycle during the exploration stage and many factors including indirect impacts (e.g. population migration) can result in negative impacts to water quality. Though the extraction and subsequent processing of minerals, metals and metal compounds tend to become chemically more available, which can result in a acid or alkaline drainage. Reduction of surface and groundwater availability is also a concern at the local level and for communities in the vicinity of mining sites, particularly, in arid regions, or in regions of high agricultural potential [4].
1.3. AMD
Acid Mine Drainage (AMD), can be a consequence of mining coal or mineral deposits. A large amount of scientific research has been conducted to determine the chemical reactions that create acidity and lead to the precipitation of dissolved metals, but despite improvements in prediction and prevention methods, acid mine drainage problems persist. The acidity of mine drainage is caused primarily by the oxidation of pyrite, a mineral containing Iron and sulphide, commonly found in tailings, overburden and other mine waste piles. The rate of oxidation depends on the following: reactive surface area of the pyrite, the oxygen concentration and pH of the water, and the presence of Iron-oxidizing bacteria (e.g. Thiobacillus ferroxidans). The potential toxicity of mine water and its adverse affects on the environment can be ascribed to its four main characteristics that are acidity, iron and its precipitates, trace metals (e.g. cadmium, zinc, copper, lead etc.) and turbidity. Sulphate is another regular component in mine water as it is formed during pyrite oxidation. Not all of these components have to be present in mine water in order to cause harm but in most cases they are found in combination with each other. More distinct are the terms Acid Mine Drainage and Alkaline Mine Drainage. The former is acidic water (pH <5.0), laden with iron, sulphate and other metals, which forms under natural conditions when geologic strata containing pyrite are exposed to the atmosphere or oxidizing environments. AMD can form from mining, both in surface and in underground mines. Alkaline mine drainage is water that has a pH of 6.0 or above, but may still have dissolved metals that can create acid by oxidation and hydrolysis. The drainage quality (acid or alkaline) depends on the acid and alkaline minerals contained in the geologic material [4].
1.4. Hazardous materials from mineral processing
Hazardous materials may be used at various stages of mineral extraction, for example cyanide for gold leaching. Such materials should be handled, stored and transported in such a way as to avoid leaks, spills into soils, surface water and groundwater resources. [4].
1.5. Air quality
Managing ambient air quality at mine sites is important at all stages of the mine cycle. Airborne emissions may occur during each stage of the mine cycle, but particularly during exploration, development, construction and operation. The main sources include dust escaping from blasting, exposed surfaces such as tailings facilities, stockpiles, waste dumps, haul roads and infrastructure, and to a lesser extent, gases from combustion of fuels in equipment and vehicles [4].
1.6. Waste
Mines generate large volumes of waste, involving materials that must be removed to gain access to the mineral resource, such as topsoil, overburden and
waste rock, as well as tailings remaining after minerals have been largely extracted from the ore. Some of this waste is inert and consequently unlikely to be a significant environmental hazard apart from smothering river beds and the risk of collapse if stored in large quantities. However other fractions, in particular those generated by the non-ferrous metal mining, industry, may contain large quantities of dangerous substances, such as heavy metals. Structures such as waste dumps, tailings impoundments and/or dams, and containment facilities should be planned, designed, and operated in such a way that geotechnical risks and environmental impacts are appropriately assessed and managed all the way through the mine cycle [4].
1.7. Bentonite
Surface under the contaminated water can be protected by bentonite. The solution can be reached by the use of seal material which might eliminate the above mentioned risks. The demands on the seal system are quick and simple installation, no harm to the environment and cost-effectiveness [13]. In order to meet these requirements, it has been suggested to use a seal layer consisting of natural materials such as clay, e.g. bentonite. Bentonite belongs to chemically stabile rocks while its usage does not pose threat to the environment. In general, this clay is composed of dominant smectite group of minerals, mainly montmorillonite that has features typical for the crystal structure of this group [11].
Montmorillonite is characteristic for its high swelling ability and low hydraulic conductivity, good sorption properties in comparison with other minerals [5]. Moreover, the unique property of smec tite is the expandability of its structure and the presence of exchangeable cations. The distance between the structural layers can vary depending on the diameter of exchange cations. It means that the interlayer space is either reduced or spread. Thus montmorillonite belongs to highly universal natural adsorbents able to accept the substances with quite a wide range of element size. Besides that, it has high affinity to water, organic compounds, etc. [14]. Adsorption capacity of bentonite can be enhanced by various modifications of its structure. Example of such modification is natrification. Different natrification agents may be used for this purpose such as NaCl, NaF, Na2CO3, NaOH.
Natrification salt Na2CO3 is used almost exclusively thanks to its financial
accessibility [5]. Another kind of bentonite modification is the magnetic modification,when, for example, the bentonite is coated with iron oxides [9].
2. Materials and methods
Bentonite is fine-grain material with the content of the particles that by their size belong to natural micro and nanomaterials [7]. The bentonite used in this study originated from the Slovak deposit Stará Kremnička – Jelšový potok. This natural bentonite (B) contains almost monomineral fraction of montmorillonite (> 90%) with the particles size below 20 µm [6]. The particular amount of natural bentonite was converted to its monoionic sodium form. The natrified bentonite (NaB) was prepared from the slurry, which contained the activating agent (Na2CO3) and distilled water to
which the bentonite was added. The stabilization took 24 hours at ambient temperature. The final product was dried at 60°C an d then it was mashed manually. Subsequently, the manganese oxide – natural bentonite composite (Mn-B), manganese oxide – natrified bentonite composite (Mn-NaB) and reference sample of „pure“ manganese oxides (Ref-Mn) were prepared according to the method represented by the reaction (1):
2KMnO4 + 8HCl → 2MnO2 + 2KCl + 3Cl2 + 4H2O (1)
The Mn-B and Mn-NaB were prepared in a weight ratio 1:1 (bentonite: manganese oxides). Ref-Mn was prepared without the addition of bentonites. Process of manganese oxides precipitation included these few steps: Potassium permanganate was dissolved in distilled water in a beaker and kept in a water bath at 90°C for 15 min. Bentonite was added into the pu rple solution and this suspension was mixed gently for 10 min. After that 2M HCl was slow added dropwise to the suspension and heated in a water bath at 90°C. After the titration, the mixture was stirred further 30 min. The final product was cooled at the air and washed several times using double distilled water, dried in oven at 100°C for 24 hours and stored. X-Ray diffraction data were obtained by the diffractometer Bruker D8 Advance (40 kV, 40 mA), working with the CuKα radiation. JCPDS (Joint Committee for Powder Diffraction Data – International Centre for Diffraction Data) database was used to analyze the diffraction peaks. Thermal analysis (TG and DTA) were carried out at temperature up to 800°C in air on a Derivatograph – C (MOM) under the conditions: sample weight 200 mg, heating rate 10°C/min, and Al2O3
crucible. The electrokinetic properties of the particles in the aqueous solutions play a significant role in understanding of the adsorption properties at the solid – solution interface [3]. Zeta potential measurements were carried out by Malvern Zetasizer Nano Z, which works with the technique of the laser Doppler electrophoresis. The changes in the surface charge were determined as a function of pH. The initial suspensions having different pH values were prepared by the addition of NaOH and HNO3. The infrared spectra were obtained using the KBr disc technique using
Bruker Tensor 27 FTIR spectrometer. For each sample 64 scans were measured in the 4000-400 cm-1 spectral range in the abs mode with a resolution of 4 cm-1. The
sorption properties of selected samples were examined by sorption of cadmium cations from the synthetic solution Cd(NO3)2.4H2O at the ambient temperature. The
initial concentrations of Cd2+ were in the range from 10 to 700 mg/L. The
concentration of adsorbent was 1 g/L. The sorption was done in polyethylene tubes on the rotary shaker, pH of the solution with the help of NaOH and HNO3 was set to
5. Shaking took 24 hours. The concentration of the metals was determined by the method of atomic adsorption spectroscopy (Varian 240 RS/2400).
3. Results and discussion
The X-ray diffraction analysis of natural bentonite (B) confirmed montmorillonite as dominant mineral phase (Fig. 1a). The activation of bentonite by sodium cations caused structural changes of natural bentonite (Fig. 1b). It affected mainly (001) reflection of montmorillonite. Shift of NaB peak to the right on x axis points to the cation exchange from interlayer space for cations with the smaller atomic radius. Moreover, after the natrification, (003) reflection of montmorillonite disappeared and
the intensities of the (100) and (110) reflections were reduced. Reference sample of manganese oxides with the strongest (001) reflection with d001 value of 0.717 nm
(Fig. 1c) corresponds to birnessite – type manganese oxide. [15,2]. Precipitation of manganese oxides on bentonite caused structural changes of both composites Mn-B and Mn-NaMn-B. The main (001) reflection of montmorillonite was reduced and (005) reflection disappeared (Fig. 1d,e). On the other side, (001), (002), (200) and (310) reflections of birnessite – type manganese oxide appeared in diffraction patterns of Mn-B and Mn-NaB (Fig. 1d,e). However, d001 value of manganese oxides was
shifted from 0.717 nm to 0.705 nm in diffraction patterns of Mn-B and 0.701 nm in diffraction patterns of Mn-NaB and d002 value from 0.366 nm (Ref-Mn) to 0.352 nm
(Mn-B) and 0.354 nm (Mn-NaB). These changes were probably associated with the ability of the montmorillonite to the exchange of cations from water solutions.
Fig. 1. X-ray diffraction patterns of the B (a), NaB (b), Ref-Mn (c), Mn-B (d) and Mn-NaB (e), [12]
Thermal studies of investigated samples are shown in Figures 2-6. The first dominant mass loss of natural bentonite (Fig. 2) is due to the dehydration of interparticle water, adsorbed water and interlayer water. The DTA doubled endothermic peak that corresponded to this change was observed around 137 and 200°C. The exothermic peak of DTA curve resulting f rom the started recrystallization was detected at 790°C (Fig. 2) [10]. The first step in the thermal decomposition of reference sample of birnessite type – manganese oxide (Ref-Mn) related with the loss of adsorbed water. The exothermic peak around 540°C on DTA curve may be attributed to the phase transformation caused by the formation of Mn2O3 and
cryptomelane (Fig. 4) [11]. The main weight loss of Mn-NaB (Fig. 6) related to the removing of adsorbed water below 150°C. TG curve of Mn-NaB showed lower weight loss in the temperature range from 200 to 800°C than in the case of Ref-Mn.
The zeta potential values (ζ) measured for each final pH value of suspensions are shown in Figure 7. There was not reached an isoelectric point for natural bentonite. The reason is in nonstoichiometric substitutions of Al for Si in the tetrahedral sheets and Mg for Al in the octahedral sheets of the 2:1 layered structure of montmorillonite. The saturation of bentonite with the sodium cations within the NaB sample did not have a significant effect on the zeta potential values of bentonite, which remained negative during the whole measurement. With the increase in pH, absolute values of zeta potential of Ref-Mn, Mn-B and Mn-NaB were overall higher than in the case of natural and natrified bentonite (Fig. 7). Although the Ref-Mn changed zeta potential from negative to positive values at pH ≈1.5. In addition, the composites (Mn-B and Mn-NaB) did not achieve zero values of zeta potential and showed higher stability than Ref-Mn in the whole pH range (Fig. 7).
Fig. 2. TG and DTA curves of B [12]
Fig. 4. TG and DTA curves of Ref-Mn [12]
Fig. 6. TG and DTA curves of Mn-NaB [12]
In the Figure 8 are shown absorption spectra of all measured samples. Absorption band in the infrared spectrum of natural bentonite (B) at 3639 cm-1
corresponded to stretching vibrations of OH groups of bentonite. Broad absorption band at 3430 cm-1 presented stretching vibrations of OH groups. The absorption
bands at 1642 cm-1 corresponded to the bending OH vibrations of water molecules.
The bending vibrations of AlAlOH and AlMgOH were observed at wavenumbers 912, 841 cm-1. Absorption band at 1037 cm-1 related to the stretching vibration of
Si-O groups, while bands at 520 and 454 cm-1 belonged to bending vibrations of
Al-O-Si and Si-OSi. Additionally, in the FTIR spectra of Mn-B and Mn-NaB were observed another absorption bands at wavenumbers 1625 and 1386, 1384 cm-1
attributed to bending vibrations of nitrogen molecules and water molecules, respectively.
Fig. 8. Infrared spectra of B (a), NaB (b), Mn-B (c) and Mn- NaB (d), [12]
The last but very important step in this study was the comparison of sorption properties of bentonite before and after its modifications. The Figure 9 shows the isotherms of Cd2+ sorption on the natural bentonite (B) and modified samples: NaB,
Mn-B and Mn-NaB. The sorption data were fitted with the help of linear shape of Langmuir isotherm (2), [8]: m e m e e
Q
C
b
Q
q
C
+
=
1
(2) where
Ce is the equilibrium concentration of the metal ions in the solution,
qe is the amount of the adsorbed metal related to the weight unit of the adsorbent (mg/g),
Qm represents the maximal adsorption capacity (mg/g) and b is the sorption equilibrium constant (L/mg).
Fig. 9. Sorption of Cd2+ on B, NaB, Mn-B and Mn-NaB [12]
The high correlation coefficients for all evaluated samples (R2 >0.98) confirm that
Langmuir model is appropriate chosen for describing the sorption data. The maximum adsorption capacity of natural bentonite (B) was 63.29 mg .g-1. Mn-NaB
reached the highest maximum adsorption capacity of all samples (108.69 mg .g-1).
Conclusion
Abandoned mines can be used for exposure and for development of geotourism (for example one of the mines are those in the Štiavnica area or those in the Gemer region). With the development of geotourism is also connected more number of visitors and overnight stays.
In this case, ensuring the safety of abandoned mines and stability of wastes after maining activity must be top priority A non-toxic environment is the necessary requirement to use abandoned mines for tourism and to for development of new hotel in near area. The results of sorption experiments showed, that bentonite especially after its modification by manganese oxides is appropriate alternative to remove the dangerous pollutants like heavy metals from aqueous solutions. This fact opens ability to use of the natural modified materials for relatively cheap and fast resolution of problems with mining wastewaters.
Acknowledgement
This work was supported by the Slovak Grant Agency for Science VEGA No. 2/0115/12 and 2/0114/13 and the projects NanoCEXmat I (26220120019) and NanoCEXmat II (26220120035).
References
[1] Babel, S., Kurniawan, T., 2003, Low-cost adsorbents for heavy metals uptake from contaminated water: a review. Journal of Hazardous Materials, 97 (1-3), 219–243. [2] Cheney, M., Bhowmik, P., Qian, S., Joo, S., Hou, W., Okoh, J., 2008, A New Method
of Synthesizing Black Birnessite Nanoparticles: From Brown to Black Birnessite with Nanostructures. Journal of Nanomaterials 2008, 1-8.
[3] Duman, O., Tunç, S., 2009, Electrokinetic and rheological properties of Na-bentonite in some electrolyte solutions. Microporous and Mesoporous Materials, 117 (1-2), 331–338.
[4] Egerer, H., Mining and environment in the western Balkans, 2010, This study was prepared by Zoi Environment Network on behalf of UNEP Vienna in the framework of the Environment and Security Initiative - South Eastern Europe with support of the Austrian Development Agency (ADA) and the Ministry of Foreign Affairs of Finland. [5] Galamboš, M., Kufčáková, J., Rosskopfová, O., Rajec, P., 2010, Adsorption of
cesium and strontium on natrified bentonites. Journal of Radioanalytical and Nuclear Chemistry, 283 (3), 803-813.
[6] Jesenák, J., Hlavatý, V., 2000, Laboratory device for sedimentation of fine bentonite fractions. Scripta Facultatis Scientiarum Naturalium Universitatis Masarykianae Brunensis Geology, 28-29, 33-36.
[7] Kraus, I., 2011, Nerastné bohatstvo a človek na príklade environmentálnych surovín Slovenska. Geovedy pre každého, projekt LPP-0130-09.
[8] Langmuir, I., 1918, The adsorption of gases on plane surfaces of glas, mica and platinum. Journal of the American Society, 40 (9), 1361-1403.
[9] Mockovčiaková, A., Orolínová Z., Škvarla J., 2010, Enhancement of the bentonite sorption properties. Journal of Hazardous Materials, 180 (1-3), 274-281.
[10] Önal, M., Sarikaya, Y., 2007, Thermal behaviour of a bentonite. Journal of Thermal Analysis and Calorimetry, 90 (1), 167-172.
[11] Ramalingam, K., Kamatchi, T., Sumod, P., 2006, Synthesis, spectral, thermal and CO2 absorption studies on birnessites type layered MnO6 oxide. Transition Metal Chemistry, 31 (4), 429-433.
[12] Schütz, S., Dolinská, S., Danková, Z., Briančin, J., Mockovčiaková, A., Strajňák, S., 2013, Removal of heavy metals by manganese – modified natural material 2013. In: Proceedings of 15. Balkan Mineral Processing Congress : Volume 2 : June 12. - 16, 2013, Sozopol, Bulgaria. – Sofia, Bulgaria: Publishing House St. Ivan Rilski, p. 1005-1008. ISBN 978-954-353-218-6.
[13] Strajňák, S., Schütz T., 2012, Bentonite – subgrade for a wind park in Sobrance district. In: The Asian conference on arts and cultures 2012: Bangkok, Thailand, August 9-10, 2012. – Bangkok: Srinakharinwirot University, p. 273-280. ISBN 980-7-83102-4.
[14] Šucha, V., 2001, Íly v geologických procesoch. Acta Geologica Universitas Comenianae, Monografická séria, Univerzita Komenského v Bratislave, Bratislava. p.159.
[15] Tu, S., Racz, G., Goh, T., 1994, Transformations of synthetic birnessite as affected by pH and manganese concentration. Clays and Clay Minerals, 42 (3), 321-330.
