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desorption of Zn, Cd, Pb, Ni, Cu, and Cr by clay-bearing mining wastes
Applied Clay Science 65–66 (2012) 6–13
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Competitive sorption/desorption of Zn, Cd, Pb, Ni, Cu, and Cr by clay-bearing mining wastes Edeltrauda Helios-Rybicka ⁎, Rafał Wójcik AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, 30‐059 Krakow, A. Mickiewicza 30, Poland
a r t i c l e
i n f o
Article history: Received 26 September 2011 Received in revised form 31 May 2012 Accepted 4 June 2012 Available online 20 July 2012 Keywords: Kaolinite-bearing mining waste Smectite-basalt weathering waste Competitive sorption Desorption Retention
a b s t r a c t The competitive sorption–desorption of trace metal ions by kaolinite-bearing coal mining wastes in Upper Silesia and by smectite-bearing waste from a basalt quarry in Lower Silesia, both in Poland, were studied. Generally, at higher clay contents, the sorption on both kaolinite wastes followed the same sequence of efficiency: Cr~ Zn > Pb> Cu > Ni≥ Cd, while the sequence for the smectite waste was Zn> Pb> Cd> Cu ≥ Ni > Cr. The retention of the metal ions followed the same sequence for all clay-bearing wastes: Cr> Pb > Cu > Zn > Ni≥ Cd. The sorption of the clay-bearing coal mining wastes was considerable, though the kaolinite and illite were the main components. There were no significant differences between both kaolinite-bearing wastes in the sorption/desorption capacity. In spite of this, the highest pH (approx. 8) of the KaW-1 kaolinite-bearing waste and the presence of iron (hydr)oxides may be crucial in explaining the observed differences. The clay waste samples showed similar retention for all metal ions except Pb. The highest retention was observed for Cr. Lead showed a lower retention, and the difference among clay wastes was noticeable. The amounts of metal ions sorbed and desorbed by the clay wastes depended on the type of clay waste, its composition and grain size, trace metal type and concentration, solid/solution (s/l) ratio, pH and the presence of other trace metal ions. It is concluded that in addition to adsorption, precipitation controlled the removal of trace metal ions from multimetal solutions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Mining activities generate huge volumes of waste. For example, each extracted ton of hard coal generates 250–300 kg of waste. In 2007, Silesian coal mines produced approximately 30.5 million tons of waste. Coal mining waste is not highly utilised, but it can be used for purposes such as land reclamation. At the end of 2008, almost 600 million tons of coal mining waste had accumulated in the Upper Silesian Coal Basin (USCB) of Poland. Approximately 46% of this waste remained underground, and 54% was dumped on the surface. Coal mining activities impose harmful, often irreversible, impacts on terrestrial and aquatic environments, of which the most important occur during are waste generation, e.g., changes in landscape and hydrological and hydrochemical transformation of surface water flows (Helios-Rybicka and Rybicki, 2003). Coal mining waste in the USCB consists mostly of carboniferous claystones, mudstones and sandstones. All waste samples contain variable amounts of clay minerals, mainly kaolinite and illite, herein referred to as kaolinite waste. Among different mining wastes, clays are very common. Because of their sorption properties, they could be used as the mineral
⁎ Corresponding author. Tel.: + 48 12 617 39 68; fax: + 48 12 633 29 36. E-mail addresses: [email protected] (E. Helios-Rybicka), [email protected] (R. Wójcik). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.06.006
sorbents reducing the migration of toxic substances, e.g., trace metal ions (Sikora and Budek, 1996). There are many publications concerning the sorption behaviour of individual trace metal ions on clays, clay minerals and soils (Abollino et al., 2008; Bhattacharyya and Sen Gupta, 2008; Churchman et al., 2006; Helios-Rybicka and Kyzioł, 1990; Sikora and Budek, 1992, 1996). However, only a few articles reported on the competitive sorption of multiple trace metals (Antoniadis and Tsadilas, 2007; Helios Rybicka and Jędrzejczyk, 1995; Sipos, 2009; Srivastava et al., 2005; Vega et al., 2006; Zhang and Zheng, 2007). Trace elements can be difficult to remove from aqueous solution. Sorption is one class of mechanism that reduces the mobility of trace metals and their chemical species in geochemical cycles. One of the methods that has been very widely developed is called geochemical engineering (Förstner, 1996; Schuiling, 1990). Sorption has become a preferred technique for removal, recovery and recycling of hazardous trace metal ions from polluted water and wastewater (Bhattacharyya and Sen Gupta, 2008; Du and Hayashi, 2006). Many types of sorbents are used for removing trace metal ions from aqueous solution, but clays and clay minerals are the most important natural scavengers for these pollutants in soil-water systems (Bhattacharyya and Sen Gupta, 2008; Helios Rybicka and Jędrzejczyk, 1995; Wang et al., 2007). The aim of this study is to understand the processes of competitive sorption/desorption from multimetal solutions of Zn, Pb, Cr, Cd, Cu, and Ni and their retention on kaolinite-bearing coal mining wastes and on smectite products of basalt weathering.
E. Helios-Rybicka, Rł. Wójcik / Applied Clay Science 65–66 (2012) 6–13
of approximately 0.5 kg each were taken from this batch of material and used for further analyses.
Table 1 Physic-chemical parameters of the clay wastes. Parameter
KaW-1
KaW-2
fraction b 63 μm Mineralogical composition
Fraction b 63 μm a (%) pH-H2O CEC b (meq/100 g) SSA c (m2/g) TOC d (%) Main components f (%)
SiO2 Al2O3 Fe2O3
f
Trace metals (mg/kg)
Cd Co Ni Cr Cu Pb Zn Mn
quartz kaolinite illite dolomite calcite hematite 28 8.1 8.0 (−) 14.0 45.2 35–60 20.1 17–28 2.7 1.5–5.5 2.7 50 83 89 116 74 173 249
7
SmW raw
quartz kaolinite illite chlorite calcite
smectite kaolinite hematite goethite
12 7.0 8.9 (−) 7.8 e 47.8
60 6.6 56.8 102 (−) 32.8 (−) 20.6 (−) 15.5 (−) 3.9 87 143 407 39 22 108 1138
23.8 1.8 1.7 51 75 102 46 58 80 180
a wet sieving, b 1 M MnCl2, c N2 BET, d Rock-Eval, e after Kotarba et al. (2002), (−) not determined, italics after Skarżyńska (1997), f HF + HClO4 digestion, AAS.
2. Materials and methods Competitive sorption and desorption experiments were performed on two kaolinite-bearing wastes from the coal mines of the USCB (The Borynia mine sample is labelled KaW-1, and the Brzeszcze mine sample is labelled KaW-2) and a smectite product of basalt weathering waste from the Męcinka basalt quarry in Lower Silesia (sample SmW). All types of waste contained variable amounts of clay minerals with a predominance of kaolinite or smectite, hereafter called kaolinite wastes or smectite waste. The samples of carbonaceous kaolinite bearing wastes from the Borynia and Brzeszcze mines, each 5 kg, were taken from the fresh heaps. The samples after homogenisation were disintegrated, and then one-quarter of this material was mechanically crumbled. Samples
2.1. Characterisation of clay-bearing wastes The kaolinite wastes showed a shaly structure with the interbedding laminations of the organic substances and weak to medium compaction. Both kaolinite wastes are hardly water-slaking, sand-gravel mixtures with a low silt-clay size fraction. After wet sieving, the size fraction b63 μm was 28 mass% of the Borynia kaolinite waste (sample KaW-1) and 12 mass% of the Brzeszcze kaolinite waste (sample KaW-2). The size fraction b63 μm of both kaolinite wastes was used for the sorption/ desorption experiments. Electron microscopy studies of the clay-bearing waste from Brzeszcze revealed weak compaction but high porosity; the size of inter-aggregate pores ranged between 10 and 15 μm and that of inner-aggregate pores ranged between 1 and 3 μm. Finely dispersed organic matter was also observed (Kozielska-Sroka, 2004). The mineralogical composition of the fraction b63 μm in both kaolinite waste samples (KaW-1 and KaW-2) and in the smectite waste row sample (SmW) was determined by powder X-ray diffraction (PXRD). Quartz, kaolinite and illite were the primary minerals in the kaolinite waste samples, while the predominant component in the smectite waste was smectite (Żyła and Sikora, 1993), and kaolinite was also detected (Table 1). Trace amounts of chlorite, carbonates (calcite, dolomite), goethite and hematite were also found. The concentration of carbonates was measured using the Scheibler apparatus. In all clay waste samples, the contents were below the detection limit (b2 %). The low content of carbonates was accompanied by the low contents of CaO (0.3–1.8 %) and MgO (0.3–2.1 %) determined for the kaolinite-bearing coal mining wastes from the USCB (Skarżyńska, 1997). The unfractionated sample (SmW) of smectite waste described in earlier works (Sikora and Budek, 1996; Żyła and Sikora, 1993) was used for a previous experiment (Helios Rybicka and Jędrzejczyk, 1995) and in the present sorption/desorption experiments. Strong aggregation is a characteristic property of this material. Thus, the composition of the b2 μm fraction was 25% and was approximately 35% for the 2–63 μm fraction (Żyła and Sikora, 1993, sample VI/1/7). The specific surface area (SSA) of the smectite waste sample was 102 m2/g (Helios Rybicka and Jędrzejczyk, 1995). Generally, the content of the main components in both kaolinite waste samples was similar (Table 1). For the smectite waste, the content of SiO2 was lower than that of Fe2O3 compared with the kaolinite wastes. The composition of the kaolinite samples agreed with the concentration ranges of carbonaceous kaolinite shales at the USCB (Skarżyńska, 1997). The total organic carbon (TOC) content of the two samples of kaolinite waste (KaW-1) was measured with the Rock-Eval 6 Turbo apparatus. The TOC content was rather high in both and was very similar between the two (14.0%). Kotarba et al. (2002) measured the TOC content in 30 samples of carbonaceous clay shales from the USCB using the Rock-Eval method. The data showed the TOC to be between 0.92% and 11.8%. For the kaolinite waste from the Brzeszcze coal mining region, the mean content of TOC was 7.8%. 2.2. Sorption/desorption
Fig. 1. pH of clay dispersion after sorption as related to the s/l ratio.
Multimetallic solutions of Zn, Cd, Ni, Cu, Pb and Cr nitrate with each metal concentration equal to 5 mg/dm 3 (at pH 5.5) were mixed with each clay waste sample to obtain clay/water dispersions with solid concentrations of 5, 10, 20 and 40 g/dm 3 corresponding to solid/solution (s/l) ratios of 0.005, 0.01, 0.02, 0.04. The concentrations of the trace metal ions per unit mass of the adsorbents were 1000, 500, 250 and 125 mg/kg. The dispersions were shaken for 12 hours and separated from the solid materials by centrifugation; the pH was then measured in the supernatants. The adsorbed metal
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Fig. 2. Relative sorption of Pb, Cd, Ni, Zn, Cu and Cr on clay waste at different s/l ratios.
Fig. 3. Sorption of trace metal ions at different initial concentrations in a multi-metal solution.
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samples were determined using the sequential extraction procedure (step V—modified by the current authors) proposed by Förstner et al. (1981). This procedure, previously proposed for sediment, is outlined below. The three clay samples with the highest clay content (40 g/dm 3) were selected after the sorption experiments. The extraction steps, fractions, (extractants) and extracted components were as follows: I Exchangeable fraction, (1 M NH4OAc, pH 7), exchangeable metal ions; II Carbonate, (1 M NaOAc, pH 5, HOAc), metal ions as carbonates; III Easily reducible, (0.1 M NH2OH HCl 0.01 M HNO3), metal ions bound by Mn-oxides; IV Moderately reducible, (0.1 M oxalate buffer, pH 3), metal ions bound by iron (hydr)oxides; V Residual, (conc. HNO3), metal ions in lithogenic material, organic matter and sulphides. Fig. 4. Total amount of the trace metal ions sorbed on clay wastes as a function of the total amount of metal ions in the equilibrium solution.
ions were desorbed in 1 M ammonium acetate solution (shaking time 2 hours). These solutions were filtered (0.45 μm), and the trace metal contents were determined by atomic absorption spectroscopy (AAS). The same desorption procedure was used to determine the extractable amounts of trace metals in the actual kaolinite coal mining and the smectite wastes. 2.3. Sequential extraction The chemical forms of the sorbed trace metal ions and of the original metal amount (see Table 1) in the kaolinite and smectite waste
The reproducibility of the data from duplicate measurements was estimated as b5%. 3. Results 3.1. Sorption results The pH of the equilibrated kaolinite waste sample dispersions varied from 6.1 to 7.2 and from 5.9 to 6.4 for the smectite waste samples. The pH increased with an increasing s/l ratio, sharply for s/l b0.02 (Fig. 1). At the highest s/l ratio, 100% of Zn and almost 90% of Pb were sorbed on kaolinite waste. The smectite waste, independent of s/l, sorbed approximately 90%, almost the same relative amount of Pb. The kaolinite waste samples removed 100% of Cr, and the smectite
Fig. 5. Desorption of trace metal ions as related to their amounts sorbed on clay wastes.
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sample removed only 64% of Cr, both independent of the s/l ratio. All samples sorbed 70% to 80% of Cu. The sorbed amounts of Cd, Ni and Zn varied between 12–95%, 15–82% and 28–100%, respectively, and increased with increasing s/l ratios (Fig. 2). However, the amounts of metal ions sorbed in mg per kg sorbent decreased with increasing s/l ratios. The relation between sorbed amounts (mmol/kg) of trace metal ions and the initial concentration in the multimetal solution (mmol/ kg) (Fig. 3) was nearly linear for Pb, Cu and Cr over the entire range of initial metal concentrations. Lead, because of its high molar mass, showed the lowest initial molar concentrations. Sorption curves of Zn, Ni and Cd were similar. The competitive sorption curves (Fig. 4) of metal ions on the kaolinite waste samples were very similar.
3.2. Desorption experiments The raw kaolinite waste sample desorbed Ni, Zn and Cu in amounts of 14 mg/kg, 8.9 mg/kg, and 1.4 mg/kg, respectively; the smectite waste desorbed only 2.8 mg/kg of Ni. The concentrations of the remaining metal ions in all cases were b0.5 mg/kg. After adsorption, the clay waste samples desorbed 60-90% of Cd adsorbed and 50-70% of Ni adsorbed (Fig. 5). Lesser amounts of desorption were observed for Zn (16–65%), Cu (15–50%) and Pb (5–58%). The desorption of some metal ions depended on the presence or the absence of other metal ions; for example, sorbed Cu ions on the kaolinite particles impeded the release of Zn ions; therefore, the highest desorption was observed in the absence of Cu ions (Wang et al., 2007). Conversely, the desorption of Pb2+ increased considerably with increasing concentrations of Cu and Zn ions (Yang et al., 2006).
The desorption of Cr occurred much less than the other five metal ions and reached 13% in the case of KaW-2. The degree of desorption generally followed the order Cd ≥ Ni > Zn > Cu > Pb ≥ Cr, independent of the type of clay waste. The clay waste samples showed similar retention for all metal ions except Pb 2+. The highest retention was observed for Cr 3+, while the retention of Pb ions was smaller and varied noticeably with the clay waste sample (Fig. 6).
3.3. Sequential extraction The sequential extraction procedure was used for the assessment of the main sorbents of trace metal ions in different types of samples, e.g., sediments, soils, and wastes (Verner et al., 1996, Helios Rybicka and Wilson, 2000). This method has several limitations (Förstner et al., 1981). This method allows the determination of the total amounts of metal ions present in the various mineral phases of the investigated samples; in our case, we determined the original and sorbed amounts of metal ions. In all waste samples, the relative amounts of exchanged trace metal ions varied in wide ranges (Fig. 7): Cd 60–84%, Ni 27–33%, Zn 8–27%, Cu 9–20%, Pb 4–19% and Cr 1–2%. In the kaolinite waste samples, large amounts of up to 40% of Cu and Cr, 35% of Pb, 30% of Zn, 28% of Ni and 15% of Cd were found as carbonates. In the smectite waste sample, the relative amounts of metal ions as carbonates were higher for Cd (27%) and lower for Zn (15%), Cu and Pb (each 13%), Ni (10%) and Cr (7%). Trace metal ions released as exchangeable and as carbonates were defined as the potentially mobile fraction.
Fig. 6. Retention of trace metal ions as related to their amounts sorbed on clay wastes.
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In the kaolinite samples (KaW-1, KaW-2), 26% of the total Pb and up to 19% of Cu and Zn were associated with Mn-oxides (easily reducible fraction). The Fe-(hydr)oxides (moderately reducible fraction) in the kaolinite wastes bound mainly Cu (up to 23%), but in the smectite waste, the Fe-(hydr)oxides bound mainly Pb (65%). The smectite waste sample (SmW) exhibited the lowest mobility of the trace metals (step V); 75% of the total Cr, approximately 40% of Ni and Zn, and 30% of Cu remained immobile (Fig. 7). The kaolinite waste samples also showed considerable relative amounts of immobile trace metals, i.e., Cr, Ni, Zn and Pb up to 45%, 23%, 26% and 16%, respectively.
4. Discussion The efficiency of clay wastes to remove trace metal ions from aqueous solutions was studied in competitive sorption/desorption experiments. In competitive sorption experiments, Cr was totally removed from the multimetal solution by both kaolinite wastes, which showed similar sorption capacities. Generally, at an s/l >0.01, the sorption on both kaolinite wastes followed the same sequence of efficiency: Cr~ Zn >Pb > Cu > Ni ≥ Cd. For the smectite waste, the sequence was different: Zn > Pb > Cd >Cu≥ Ni > Cr. At a high s/l, the studied clay wastes sorbed similar amounts of Pb, Zn and Cu. One could expect relatively low sorption for the kaolinite wastes due to a considerably lower CEC than the CEC of smectite (Table 1). However, the kaolinite waste samples showed large amounts of sorbed metal ions. The smectite waste showed considerably higher sorption over
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the entire range of s/l ratios for Cd and Ni, while only at the lowest s/l ratio for Zn and Pb (Fig. 2). The presence of Fe-Mn-oxides and organic matter in the clay wastes are also effective sorbents and can significantly contribute to the sorption of trace metal ions, especially of Pb, Zn and Cu (Abollino et al., 2008; Benjamin and Leckie, 1981; Brümmer et al., 1988; Helios-Rybicka and Förstner, 1986; Mckenzie, 1980). The impact of Fe-coatings on the sorption and binding energy of trace metal ions by clay minerals (also under competitive conditions) have been previously investigated (Helios Rybicka et al., 1995; Helios-Rybicka and Förstner, 1986; Helios-Rybicka and Schoer, 1982). Fe-(hydr)oxides associated with kaolinite and smectite are only effective sorbents in that they increase the effective surface area. All of the clay wastes studied showed higher sorption of Pb, Cu and Cr, whereas Cd, Ni and Zn remain mostly in solution, especially at the lowest s/l. With increasing s/l ratios, the Pb, Cu and Cr contribution to the total amount of sorbed metal ions decreased but increased for Zn, Cd and Ni (Fig. 8, Table 2). The results obtained for Pb, Cu and Zn are consistent with the findings of Sipos (2009) who studied the competitive sorption of these metal ions in a soil profile. Cu and Pb ions displaced Cd, Ni and Zn at an s/l b0.02 from the sorption sites in the clay wastes studied. Given a certain amount of sorbed metal ions, the fraction of sorbed Cd, Ni and especially Zn started to decrease while that of Pb and Cu increased. This is consistent with recent observations of sorption/desorption processes of trace metal ions on different soils and mineral sorbents (Brigatti et al., 2000; Fontes and Gomes, 2003; Sipos, 2009; Zhang and Zheng, 2007).
Fig. 7. Sequential extraction results as relative portions of trace metals released from the clay waste samples (with s/l = 0.04) after competitive sorption.
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Fig. 8. Contribution of each metal to the total amounts of metal ions sorbed on clay wastes at different s/l ratios.
The sum of the sorbed trace metal ions increased with increasing s/l ratios (Fig. 9). The smectite waste was the most effective mineral sorbent, but the kaolinite wastes also showed a high sorption for the metal ions, especially at higher s/l ratios. Considerable differences in the total amounts of metal ions sorbed by the smectite and kaolinite waste samples were not observed. Thus, the pH, which is higher for the kaolinite waste dispersions (Fig. 1) that also have a high TOC, can be crucial in enhancing the sorption of trace metal ions. For all clay wastes, the retention of trace metal ions decreased as Cr ≫ Pb > Cu > Zn > Ni ≥ Cd. The high sorption and retention of Cr and Pb on the clay wastes was caused by the associated Fe-Mn-oxides. In previous studies (Helios Rybicka and Wilson, 2000), it was confirmed by SEM EDX that Cr was mainly associated with Fe-Mn-oxides. The highest immobilisation was shown in the smectite waste sample, in particular for Cr, Pb and Cu; above 95% of sorbed Cr, 80% of Pb and 50% of Cu were immobile (Fig. 10). High immobilisation of these metal
ions (Cr 95%, Pb 65% and Cu 60%) was also observed in the kaolinite wastes.
5. Conclusion Both kaolinite waste samples immobilised relatively large amounts of metal ions. In addition to quartz, the kaolinite and illite coal mining wastes also contained Fe-(hydr)oxides, traces of carbonates and finely dispersed carbonaceous inclusions. The presence of these associated minerals may also cause considerable sorption. The kaolinite waste
Table 2 Variation of the metal sorption between the smallest and highest s/l values (calculated after Zhang and Zheng, 2007). Waste sample
Metal (%)* Pb
Cd
Ni
Zn
Cu
Cr
KaW-1 KaW-2 SmW
40 44 42
− 75 − 74 − 35
− 64 − 62 − 26
− 54 − 46 − 23
55 53 24
66 62 43
* y = (a/b − 1) × 100%, where y is the variation (in %) of the sorption of trace metal ions. a—sorption at the lowest s/l values. b—sorption at the highest s/l values.
Fig. 9. Relative total amount of metal ions sorbed on clay waste samples at different s/l ratios.
E. Helios-Rybicka, Rł. Wójcik / Applied Clay Science 65–66 (2012) 6–13
Fig. 10. Immobilisation of metal ions (%) in relation to their amounts sorbed on clay wastes (samples KaW-1 and SW).
samples revealed small differences in sorption and desorption that may also be due to different pH values. When considering the total amounts of sorbed metal ions, substantial differences between the two types of wastes were not observed. Although the smectite waste was the more effective sorbent, kaolinite wastes also sorbed high amounts of metal ions. There were significant differences among the behaviours of trace metal ions in the sequential extraction procedure. The kaolinite wastes contained high percentages of potentially mobile Cd, Ni and Zn ions, i.e., 95%, 40%, and 50%, respectively, of their total amounts were exchangeable and dissolved as carbonates. Relatively large amounts of metal ions bound on Mn–Fe oxides revealed the substantial role played by these oxides in the sorption process. In addition to the trace metal sorption, the precipitation as hydroxides on Fe-(hydr) oxide coatings always present in the clays may play a significant role in the removal of metal ions from solutions (see 3.3.). Finally, the amounts of metal ions sorbed and desorbed by the clay wastes depended on the type of clay waste, its composition and grain size, trace metal type and concentration, s/l ratio, pH and the presence of other trace metal ions. A very important point is that the kaolinite waste sorbents are important clay-bearing coal mining wastes that are mined in huge amounts and need to be utilised. Both kaolinite coal mining wastes and smectite waste can be used as geochemical barriers and can also be utilised for the treatment of water or sewage that is usually contaminated with many toxic metal ions. The results obtained for the kaolinite wastes clearly revealed that (i) some portion of the trace metal ions can be immobilised, but (ii) the remaining metal ions are transferred from the clay wastes to the infiltrating waters and may be subsequently sorbed on the waste that resides in deeper layers of the barrier. Acknowledgements This study was supported by the AGH University of Science and Technology as Project No. 11.11.140. References Abollino, O., Giacomino, A., Malandrino, M., Mentasti, E., 2008. Interaction of metal ions with montmorillonite and vermiculite. Applied Clay Science 38, 227–236. Antoniadis, V., Tsadilas, C.D., 2007. Sorption of cadmium, nickel, and zinc in mono- and multimetal systems. Applied Geochemistry 22, 2375–2380. Benjamin, M.M., Leckie, J.M., 1981. Multiple site adsorption of Cd, Cu, Zn and Pb on amorphous iron oxyhydroxide. Journal of Colloid and Interface Science 79, 209–221.
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Wang, S., Nan, Z., Zeng, J., Hu, T., 2007. Desorption of zinc by the kaolin from Suzhou, China. Applied Clay Science 37, 221–225. Yang, J.Y., Yang, X.E., Loch, J.P.G., Schentu, J.L., Stoffella, P.J., 2006. Effects of pH, organic acids, and inorganic ions on lead desorption from soils. Environmental Pollution 143, 9–15. Zhang, M., Zheng, S., 2007. Competitive adsorption of Cd, Cu, Hg and Pb by agricultural soils of the Changjiang and Zhujiang deltas in China. Journal of Zhejiang University SCIENCE A. http://dx.doi.org/10.1631/jzus.2007.A1808. 1808-1815. Żyła, M., Sikora, W.S., 1993. Sorption properties of smectite weathering products of Lower Silesia basaltic rocks. Mineralogia Polonica 24 (1–2), 63–71.
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Competitive sorption/desorption of Zn, Cd, Pb, Ni, Cu, and Cr by clay-bearing mining wastes Edeltrauda Helios-Rybicka ⁎, Rafał Wójcik AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, 30‐059 Krakow, A. Mickiewicza 30, Poland
a r t i c l e
i n f o
Article history: Received 26 September 2011 Received in revised form 31 May 2012 Accepted 4 June 2012 Available online 20 July 2012 Keywords: Kaolinite-bearing mining waste Smectite-basalt weathering waste Competitive sorption Desorption Retention
a b s t r a c t The competitive sorption–desorption of trace metal ions by kaolinite-bearing coal mining wastes in Upper Silesia and by smectite-bearing waste from a basalt quarry in Lower Silesia, both in Poland, were studied. Generally, at higher clay contents, the sorption on both kaolinite wastes followed the same sequence of efficiency: Cr~ Zn > Pb> Cu > Ni≥ Cd, while the sequence for the smectite waste was Zn> Pb> Cd> Cu ≥ Ni > Cr. The retention of the metal ions followed the same sequence for all clay-bearing wastes: Cr> Pb > Cu > Zn > Ni≥ Cd. The sorption of the clay-bearing coal mining wastes was considerable, though the kaolinite and illite were the main components. There were no significant differences between both kaolinite-bearing wastes in the sorption/desorption capacity. In spite of this, the highest pH (approx. 8) of the KaW-1 kaolinite-bearing waste and the presence of iron (hydr)oxides may be crucial in explaining the observed differences. The clay waste samples showed similar retention for all metal ions except Pb. The highest retention was observed for Cr. Lead showed a lower retention, and the difference among clay wastes was noticeable. The amounts of metal ions sorbed and desorbed by the clay wastes depended on the type of clay waste, its composition and grain size, trace metal type and concentration, solid/solution (s/l) ratio, pH and the presence of other trace metal ions. It is concluded that in addition to adsorption, precipitation controlled the removal of trace metal ions from multimetal solutions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Mining activities generate huge volumes of waste. For example, each extracted ton of hard coal generates 250–300 kg of waste. In 2007, Silesian coal mines produced approximately 30.5 million tons of waste. Coal mining waste is not highly utilised, but it can be used for purposes such as land reclamation. At the end of 2008, almost 600 million tons of coal mining waste had accumulated in the Upper Silesian Coal Basin (USCB) of Poland. Approximately 46% of this waste remained underground, and 54% was dumped on the surface. Coal mining activities impose harmful, often irreversible, impacts on terrestrial and aquatic environments, of which the most important occur during are waste generation, e.g., changes in landscape and hydrological and hydrochemical transformation of surface water flows (Helios-Rybicka and Rybicki, 2003). Coal mining waste in the USCB consists mostly of carboniferous claystones, mudstones and sandstones. All waste samples contain variable amounts of clay minerals, mainly kaolinite and illite, herein referred to as kaolinite waste. Among different mining wastes, clays are very common. Because of their sorption properties, they could be used as the mineral
⁎ Corresponding author. Tel.: + 48 12 617 39 68; fax: + 48 12 633 29 36. E-mail addresses: [email protected] (E. Helios-Rybicka), [email protected] (R. Wójcik). 0169-1317/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2012.06.006
sorbents reducing the migration of toxic substances, e.g., trace metal ions (Sikora and Budek, 1996). There are many publications concerning the sorption behaviour of individual trace metal ions on clays, clay minerals and soils (Abollino et al., 2008; Bhattacharyya and Sen Gupta, 2008; Churchman et al., 2006; Helios-Rybicka and Kyzioł, 1990; Sikora and Budek, 1992, 1996). However, only a few articles reported on the competitive sorption of multiple trace metals (Antoniadis and Tsadilas, 2007; Helios Rybicka and Jędrzejczyk, 1995; Sipos, 2009; Srivastava et al., 2005; Vega et al., 2006; Zhang and Zheng, 2007). Trace elements can be difficult to remove from aqueous solution. Sorption is one class of mechanism that reduces the mobility of trace metals and their chemical species in geochemical cycles. One of the methods that has been very widely developed is called geochemical engineering (Förstner, 1996; Schuiling, 1990). Sorption has become a preferred technique for removal, recovery and recycling of hazardous trace metal ions from polluted water and wastewater (Bhattacharyya and Sen Gupta, 2008; Du and Hayashi, 2006). Many types of sorbents are used for removing trace metal ions from aqueous solution, but clays and clay minerals are the most important natural scavengers for these pollutants in soil-water systems (Bhattacharyya and Sen Gupta, 2008; Helios Rybicka and Jędrzejczyk, 1995; Wang et al., 2007). The aim of this study is to understand the processes of competitive sorption/desorption from multimetal solutions of Zn, Pb, Cr, Cd, Cu, and Ni and their retention on kaolinite-bearing coal mining wastes and on smectite products of basalt weathering.
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of approximately 0.5 kg each were taken from this batch of material and used for further analyses.
Table 1 Physic-chemical parameters of the clay wastes. Parameter
KaW-1
KaW-2
fraction b 63 μm Mineralogical composition
Fraction b 63 μm a (%) pH-H2O CEC b (meq/100 g) SSA c (m2/g) TOC d (%) Main components f (%)
SiO2 Al2O3 Fe2O3
f
Trace metals (mg/kg)
Cd Co Ni Cr Cu Pb Zn Mn
quartz kaolinite illite dolomite calcite hematite 28 8.1 8.0 (−) 14.0 45.2 35–60 20.1 17–28 2.7 1.5–5.5 2.7 50 83 89 116 74 173 249
7
SmW raw
quartz kaolinite illite chlorite calcite
smectite kaolinite hematite goethite
12 7.0 8.9 (−) 7.8 e 47.8
60 6.6 56.8 102 (−) 32.8 (−) 20.6 (−) 15.5 (−) 3.9 87 143 407 39 22 108 1138
23.8 1.8 1.7 51 75 102 46 58 80 180
a wet sieving, b 1 M MnCl2, c N2 BET, d Rock-Eval, e after Kotarba et al. (2002), (−) not determined, italics after Skarżyńska (1997), f HF + HClO4 digestion, AAS.
2. Materials and methods Competitive sorption and desorption experiments were performed on two kaolinite-bearing wastes from the coal mines of the USCB (The Borynia mine sample is labelled KaW-1, and the Brzeszcze mine sample is labelled KaW-2) and a smectite product of basalt weathering waste from the Męcinka basalt quarry in Lower Silesia (sample SmW). All types of waste contained variable amounts of clay minerals with a predominance of kaolinite or smectite, hereafter called kaolinite wastes or smectite waste. The samples of carbonaceous kaolinite bearing wastes from the Borynia and Brzeszcze mines, each 5 kg, were taken from the fresh heaps. The samples after homogenisation were disintegrated, and then one-quarter of this material was mechanically crumbled. Samples
2.1. Characterisation of clay-bearing wastes The kaolinite wastes showed a shaly structure with the interbedding laminations of the organic substances and weak to medium compaction. Both kaolinite wastes are hardly water-slaking, sand-gravel mixtures with a low silt-clay size fraction. After wet sieving, the size fraction b63 μm was 28 mass% of the Borynia kaolinite waste (sample KaW-1) and 12 mass% of the Brzeszcze kaolinite waste (sample KaW-2). The size fraction b63 μm of both kaolinite wastes was used for the sorption/ desorption experiments. Electron microscopy studies of the clay-bearing waste from Brzeszcze revealed weak compaction but high porosity; the size of inter-aggregate pores ranged between 10 and 15 μm and that of inner-aggregate pores ranged between 1 and 3 μm. Finely dispersed organic matter was also observed (Kozielska-Sroka, 2004). The mineralogical composition of the fraction b63 μm in both kaolinite waste samples (KaW-1 and KaW-2) and in the smectite waste row sample (SmW) was determined by powder X-ray diffraction (PXRD). Quartz, kaolinite and illite were the primary minerals in the kaolinite waste samples, while the predominant component in the smectite waste was smectite (Żyła and Sikora, 1993), and kaolinite was also detected (Table 1). Trace amounts of chlorite, carbonates (calcite, dolomite), goethite and hematite were also found. The concentration of carbonates was measured using the Scheibler apparatus. In all clay waste samples, the contents were below the detection limit (b2 %). The low content of carbonates was accompanied by the low contents of CaO (0.3–1.8 %) and MgO (0.3–2.1 %) determined for the kaolinite-bearing coal mining wastes from the USCB (Skarżyńska, 1997). The unfractionated sample (SmW) of smectite waste described in earlier works (Sikora and Budek, 1996; Żyła and Sikora, 1993) was used for a previous experiment (Helios Rybicka and Jędrzejczyk, 1995) and in the present sorption/desorption experiments. Strong aggregation is a characteristic property of this material. Thus, the composition of the b2 μm fraction was 25% and was approximately 35% for the 2–63 μm fraction (Żyła and Sikora, 1993, sample VI/1/7). The specific surface area (SSA) of the smectite waste sample was 102 m2/g (Helios Rybicka and Jędrzejczyk, 1995). Generally, the content of the main components in both kaolinite waste samples was similar (Table 1). For the smectite waste, the content of SiO2 was lower than that of Fe2O3 compared with the kaolinite wastes. The composition of the kaolinite samples agreed with the concentration ranges of carbonaceous kaolinite shales at the USCB (Skarżyńska, 1997). The total organic carbon (TOC) content of the two samples of kaolinite waste (KaW-1) was measured with the Rock-Eval 6 Turbo apparatus. The TOC content was rather high in both and was very similar between the two (14.0%). Kotarba et al. (2002) measured the TOC content in 30 samples of carbonaceous clay shales from the USCB using the Rock-Eval method. The data showed the TOC to be between 0.92% and 11.8%. For the kaolinite waste from the Brzeszcze coal mining region, the mean content of TOC was 7.8%. 2.2. Sorption/desorption
Fig. 1. pH of clay dispersion after sorption as related to the s/l ratio.
Multimetallic solutions of Zn, Cd, Ni, Cu, Pb and Cr nitrate with each metal concentration equal to 5 mg/dm 3 (at pH 5.5) were mixed with each clay waste sample to obtain clay/water dispersions with solid concentrations of 5, 10, 20 and 40 g/dm 3 corresponding to solid/solution (s/l) ratios of 0.005, 0.01, 0.02, 0.04. The concentrations of the trace metal ions per unit mass of the adsorbents were 1000, 500, 250 and 125 mg/kg. The dispersions were shaken for 12 hours and separated from the solid materials by centrifugation; the pH was then measured in the supernatants. The adsorbed metal
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Fig. 2. Relative sorption of Pb, Cd, Ni, Zn, Cu and Cr on clay waste at different s/l ratios.
Fig. 3. Sorption of trace metal ions at different initial concentrations in a multi-metal solution.
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samples were determined using the sequential extraction procedure (step V—modified by the current authors) proposed by Förstner et al. (1981). This procedure, previously proposed for sediment, is outlined below. The three clay samples with the highest clay content (40 g/dm 3) were selected after the sorption experiments. The extraction steps, fractions, (extractants) and extracted components were as follows: I Exchangeable fraction, (1 M NH4OAc, pH 7), exchangeable metal ions; II Carbonate, (1 M NaOAc, pH 5, HOAc), metal ions as carbonates; III Easily reducible, (0.1 M NH2OH HCl 0.01 M HNO3), metal ions bound by Mn-oxides; IV Moderately reducible, (0.1 M oxalate buffer, pH 3), metal ions bound by iron (hydr)oxides; V Residual, (conc. HNO3), metal ions in lithogenic material, organic matter and sulphides. Fig. 4. Total amount of the trace metal ions sorbed on clay wastes as a function of the total amount of metal ions in the equilibrium solution.
ions were desorbed in 1 M ammonium acetate solution (shaking time 2 hours). These solutions were filtered (0.45 μm), and the trace metal contents were determined by atomic absorption spectroscopy (AAS). The same desorption procedure was used to determine the extractable amounts of trace metals in the actual kaolinite coal mining and the smectite wastes. 2.3. Sequential extraction The chemical forms of the sorbed trace metal ions and of the original metal amount (see Table 1) in the kaolinite and smectite waste
The reproducibility of the data from duplicate measurements was estimated as b5%. 3. Results 3.1. Sorption results The pH of the equilibrated kaolinite waste sample dispersions varied from 6.1 to 7.2 and from 5.9 to 6.4 for the smectite waste samples. The pH increased with an increasing s/l ratio, sharply for s/l b0.02 (Fig. 1). At the highest s/l ratio, 100% of Zn and almost 90% of Pb were sorbed on kaolinite waste. The smectite waste, independent of s/l, sorbed approximately 90%, almost the same relative amount of Pb. The kaolinite waste samples removed 100% of Cr, and the smectite
Fig. 5. Desorption of trace metal ions as related to their amounts sorbed on clay wastes.
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sample removed only 64% of Cr, both independent of the s/l ratio. All samples sorbed 70% to 80% of Cu. The sorbed amounts of Cd, Ni and Zn varied between 12–95%, 15–82% and 28–100%, respectively, and increased with increasing s/l ratios (Fig. 2). However, the amounts of metal ions sorbed in mg per kg sorbent decreased with increasing s/l ratios. The relation between sorbed amounts (mmol/kg) of trace metal ions and the initial concentration in the multimetal solution (mmol/ kg) (Fig. 3) was nearly linear for Pb, Cu and Cr over the entire range of initial metal concentrations. Lead, because of its high molar mass, showed the lowest initial molar concentrations. Sorption curves of Zn, Ni and Cd were similar. The competitive sorption curves (Fig. 4) of metal ions on the kaolinite waste samples were very similar.
3.2. Desorption experiments The raw kaolinite waste sample desorbed Ni, Zn and Cu in amounts of 14 mg/kg, 8.9 mg/kg, and 1.4 mg/kg, respectively; the smectite waste desorbed only 2.8 mg/kg of Ni. The concentrations of the remaining metal ions in all cases were b0.5 mg/kg. After adsorption, the clay waste samples desorbed 60-90% of Cd adsorbed and 50-70% of Ni adsorbed (Fig. 5). Lesser amounts of desorption were observed for Zn (16–65%), Cu (15–50%) and Pb (5–58%). The desorption of some metal ions depended on the presence or the absence of other metal ions; for example, sorbed Cu ions on the kaolinite particles impeded the release of Zn ions; therefore, the highest desorption was observed in the absence of Cu ions (Wang et al., 2007). Conversely, the desorption of Pb2+ increased considerably with increasing concentrations of Cu and Zn ions (Yang et al., 2006).
The desorption of Cr occurred much less than the other five metal ions and reached 13% in the case of KaW-2. The degree of desorption generally followed the order Cd ≥ Ni > Zn > Cu > Pb ≥ Cr, independent of the type of clay waste. The clay waste samples showed similar retention for all metal ions except Pb 2+. The highest retention was observed for Cr 3+, while the retention of Pb ions was smaller and varied noticeably with the clay waste sample (Fig. 6).
3.3. Sequential extraction The sequential extraction procedure was used for the assessment of the main sorbents of trace metal ions in different types of samples, e.g., sediments, soils, and wastes (Verner et al., 1996, Helios Rybicka and Wilson, 2000). This method has several limitations (Förstner et al., 1981). This method allows the determination of the total amounts of metal ions present in the various mineral phases of the investigated samples; in our case, we determined the original and sorbed amounts of metal ions. In all waste samples, the relative amounts of exchanged trace metal ions varied in wide ranges (Fig. 7): Cd 60–84%, Ni 27–33%, Zn 8–27%, Cu 9–20%, Pb 4–19% and Cr 1–2%. In the kaolinite waste samples, large amounts of up to 40% of Cu and Cr, 35% of Pb, 30% of Zn, 28% of Ni and 15% of Cd were found as carbonates. In the smectite waste sample, the relative amounts of metal ions as carbonates were higher for Cd (27%) and lower for Zn (15%), Cu and Pb (each 13%), Ni (10%) and Cr (7%). Trace metal ions released as exchangeable and as carbonates were defined as the potentially mobile fraction.
Fig. 6. Retention of trace metal ions as related to their amounts sorbed on clay wastes.
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In the kaolinite samples (KaW-1, KaW-2), 26% of the total Pb and up to 19% of Cu and Zn were associated with Mn-oxides (easily reducible fraction). The Fe-(hydr)oxides (moderately reducible fraction) in the kaolinite wastes bound mainly Cu (up to 23%), but in the smectite waste, the Fe-(hydr)oxides bound mainly Pb (65%). The smectite waste sample (SmW) exhibited the lowest mobility of the trace metals (step V); 75% of the total Cr, approximately 40% of Ni and Zn, and 30% of Cu remained immobile (Fig. 7). The kaolinite waste samples also showed considerable relative amounts of immobile trace metals, i.e., Cr, Ni, Zn and Pb up to 45%, 23%, 26% and 16%, respectively.
4. Discussion The efficiency of clay wastes to remove trace metal ions from aqueous solutions was studied in competitive sorption/desorption experiments. In competitive sorption experiments, Cr was totally removed from the multimetal solution by both kaolinite wastes, which showed similar sorption capacities. Generally, at an s/l >0.01, the sorption on both kaolinite wastes followed the same sequence of efficiency: Cr~ Zn >Pb > Cu > Ni ≥ Cd. For the smectite waste, the sequence was different: Zn > Pb > Cd >Cu≥ Ni > Cr. At a high s/l, the studied clay wastes sorbed similar amounts of Pb, Zn and Cu. One could expect relatively low sorption for the kaolinite wastes due to a considerably lower CEC than the CEC of smectite (Table 1). However, the kaolinite waste samples showed large amounts of sorbed metal ions. The smectite waste showed considerably higher sorption over
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the entire range of s/l ratios for Cd and Ni, while only at the lowest s/l ratio for Zn and Pb (Fig. 2). The presence of Fe-Mn-oxides and organic matter in the clay wastes are also effective sorbents and can significantly contribute to the sorption of trace metal ions, especially of Pb, Zn and Cu (Abollino et al., 2008; Benjamin and Leckie, 1981; Brümmer et al., 1988; Helios-Rybicka and Förstner, 1986; Mckenzie, 1980). The impact of Fe-coatings on the sorption and binding energy of trace metal ions by clay minerals (also under competitive conditions) have been previously investigated (Helios Rybicka et al., 1995; Helios-Rybicka and Förstner, 1986; Helios-Rybicka and Schoer, 1982). Fe-(hydr)oxides associated with kaolinite and smectite are only effective sorbents in that they increase the effective surface area. All of the clay wastes studied showed higher sorption of Pb, Cu and Cr, whereas Cd, Ni and Zn remain mostly in solution, especially at the lowest s/l. With increasing s/l ratios, the Pb, Cu and Cr contribution to the total amount of sorbed metal ions decreased but increased for Zn, Cd and Ni (Fig. 8, Table 2). The results obtained for Pb, Cu and Zn are consistent with the findings of Sipos (2009) who studied the competitive sorption of these metal ions in a soil profile. Cu and Pb ions displaced Cd, Ni and Zn at an s/l b0.02 from the sorption sites in the clay wastes studied. Given a certain amount of sorbed metal ions, the fraction of sorbed Cd, Ni and especially Zn started to decrease while that of Pb and Cu increased. This is consistent with recent observations of sorption/desorption processes of trace metal ions on different soils and mineral sorbents (Brigatti et al., 2000; Fontes and Gomes, 2003; Sipos, 2009; Zhang and Zheng, 2007).
Fig. 7. Sequential extraction results as relative portions of trace metals released from the clay waste samples (with s/l = 0.04) after competitive sorption.
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Fig. 8. Contribution of each metal to the total amounts of metal ions sorbed on clay wastes at different s/l ratios.
The sum of the sorbed trace metal ions increased with increasing s/l ratios (Fig. 9). The smectite waste was the most effective mineral sorbent, but the kaolinite wastes also showed a high sorption for the metal ions, especially at higher s/l ratios. Considerable differences in the total amounts of metal ions sorbed by the smectite and kaolinite waste samples were not observed. Thus, the pH, which is higher for the kaolinite waste dispersions (Fig. 1) that also have a high TOC, can be crucial in enhancing the sorption of trace metal ions. For all clay wastes, the retention of trace metal ions decreased as Cr ≫ Pb > Cu > Zn > Ni ≥ Cd. The high sorption and retention of Cr and Pb on the clay wastes was caused by the associated Fe-Mn-oxides. In previous studies (Helios Rybicka and Wilson, 2000), it was confirmed by SEM EDX that Cr was mainly associated with Fe-Mn-oxides. The highest immobilisation was shown in the smectite waste sample, in particular for Cr, Pb and Cu; above 95% of sorbed Cr, 80% of Pb and 50% of Cu were immobile (Fig. 10). High immobilisation of these metal
ions (Cr 95%, Pb 65% and Cu 60%) was also observed in the kaolinite wastes.
5. Conclusion Both kaolinite waste samples immobilised relatively large amounts of metal ions. In addition to quartz, the kaolinite and illite coal mining wastes also contained Fe-(hydr)oxides, traces of carbonates and finely dispersed carbonaceous inclusions. The presence of these associated minerals may also cause considerable sorption. The kaolinite waste
Table 2 Variation of the metal sorption between the smallest and highest s/l values (calculated after Zhang and Zheng, 2007). Waste sample
Metal (%)* Pb
Cd
Ni
Zn
Cu
Cr
KaW-1 KaW-2 SmW
40 44 42
− 75 − 74 − 35
− 64 − 62 − 26
− 54 − 46 − 23
55 53 24
66 62 43
* y = (a/b − 1) × 100%, where y is the variation (in %) of the sorption of trace metal ions. a—sorption at the lowest s/l values. b—sorption at the highest s/l values.
Fig. 9. Relative total amount of metal ions sorbed on clay waste samples at different s/l ratios.
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Fig. 10. Immobilisation of metal ions (%) in relation to their amounts sorbed on clay wastes (samples KaW-1 and SW).
samples revealed small differences in sorption and desorption that may also be due to different pH values. When considering the total amounts of sorbed metal ions, substantial differences between the two types of wastes were not observed. Although the smectite waste was the more effective sorbent, kaolinite wastes also sorbed high amounts of metal ions. There were significant differences among the behaviours of trace metal ions in the sequential extraction procedure. The kaolinite wastes contained high percentages of potentially mobile Cd, Ni and Zn ions, i.e., 95%, 40%, and 50%, respectively, of their total amounts were exchangeable and dissolved as carbonates. Relatively large amounts of metal ions bound on Mn–Fe oxides revealed the substantial role played by these oxides in the sorption process. In addition to the trace metal sorption, the precipitation as hydroxides on Fe-(hydr) oxide coatings always present in the clays may play a significant role in the removal of metal ions from solutions (see 3.3.). Finally, the amounts of metal ions sorbed and desorbed by the clay wastes depended on the type of clay waste, its composition and grain size, trace metal type and concentration, s/l ratio, pH and the presence of other trace metal ions. A very important point is that the kaolinite waste sorbents are important clay-bearing coal mining wastes that are mined in huge amounts and need to be utilised. Both kaolinite coal mining wastes and smectite waste can be used as geochemical barriers and can also be utilised for the treatment of water or sewage that is usually contaminated with many toxic metal ions. The results obtained for the kaolinite wastes clearly revealed that (i) some portion of the trace metal ions can be immobilised, but (ii) the remaining metal ions are transferred from the clay wastes to the infiltrating waters and may be subsequently sorbed on the waste that resides in deeper layers of the barrier. Acknowledgements This study was supported by the AGH University of Science and Technology as Project No. 11.11.140. References Abollino, O., Giacomino, A., Malandrino, M., Mentasti, E., 2008. Interaction of metal ions with montmorillonite and vermiculite. Applied Clay Science 38, 227–236. Antoniadis, V., Tsadilas, C.D., 2007. Sorption of cadmium, nickel, and zinc in mono- and multimetal systems. Applied Geochemistry 22, 2375–2380. Benjamin, M.M., Leckie, J.M., 1981. Multiple site adsorption of Cd, Cu, Zn and Pb on amorphous iron oxyhydroxide. Journal of Colloid and Interface Science 79, 209–221.
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