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Cadmium decontamination through ball milling using an expandable clay mineral
Applied Clay Science 182 (2019) 105256
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Research Paper
Cadmium decontamination through ball milling using an expandable clay mineral
T
⁎
P. Di Leoa,d, , M.D.R. Pizzigallob, N. Ditarantoc, R. Terzanob a
CNR-IMAA, Tito Scalo, (Potenza), Italy Dipartimento Scienze del Suolo della Pianta e degli Alimenti, Università degli Studi di Bari “Aldo Moro”, Bari, Italy c Dipartimento di Chimica, Università degli Studi di Bari “Aldo Moro”, Bari, Italy d Dipartimento delle Culture Europee e del Mediterraneo, Basilicata University, Matera, Italy b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mechanochemistry Cadmium Clay minerals Dioctahedral smectite
Mechanochemical treatments have been widely used for the remediation of soils and sediments polluted by organic and inorganic pollutants. However, there has been still limited knowledge about the molecular mechanisms underlying the mechanochemical transformations responsible of heavy metals immobilization on clay minerals. In the present study, the ability of a dioctahedral smectite to retain cadmium (Cd) as induced by mechanochemical treatments was investigated. The smectite was ground with different amounts of Cd (from 0.3 to 5.2% w/w). Solid-state characterizations (XRF, XRD, XPS, FT-IR, NMR) as well as desorption isotherm experiments were carried out to understand the critical metal sorption mechanisms onto dioctahedral smectite occurring in mechanochemical interactions. The “entrapping efficiency” and the type of interaction between clay surfaces and cadmium were evaluated. Cd immobilization degree was assessed by extract analyses and expressed as leachable fraction of cations. Leaching was both performed with deionized water and 1 M MgCl2 solution. Spectroscopic analyses evidenced that long time grinding (24 h) allowed Cd sorption on two sites: Type I, exhibiting characteristic of an outer-sphere complex likely localized in the montmorillonite interlayers and coordinated by two water molecule shells, i.e. in an exchangeable position; Type II sites, where Cd was more tightly bound to the TOT layers, either onto broken edges via the new OHs formed during the mechanochemical treatments or onto montmorillonite surfaces likely bridged via a water molecule. Desorption isotherms also confirmed a two-adsorption site model. This information could be useful to understand, develop and manage new remediation technologies based on mechanochemical treatments.
1. Introduction The contamination of soils and sediments by heavy metals has received an ever-increasing consideration in the last decade, due to their potential risk for the environment and the ecosystem. Remediation of contaminated soils and sediments is a very difficult challenge to be figured out. Therefore, the implementation of simple, fast and economic remediation technologies represents a very complex scientific issue. Generally speaking, immobilization processes for the treatment of heavy metals (here after HM) contaminated soils are often preferred, owing to the limitations and costs of extractive techniques. Actually, immobilization techniques prevent HM spreading in the environment as consequence of leaching by enhancing the physical characteristics of soils as well as the solubility/toxicity of HM compounds. One of the most applied immobilization techniques is adsorption, due to its high ⁎
efficiency to remove metal ions, its easy application, large availability of adsorbents, and low cost (Huang and Blankenship, 1984; Ovadyahu et al., 1998; Đukić et al., 2015). In this respect, mechanochemical treatments furnish a better and more efficient alternative to this technique. Clay minerals exhibit remarkable properties as metal adsorbents in soil remediation due to their large presence in nature and low toxicity. Indeed, these materials are considered very efficient due to their high adsorption, ion exchange, and swelling properties as well as their large surface area, layered structure, abundance and low cost (Ghorbel-Abid et al., 2010; Zhao et al., 2011). Polluted soils can be mechanochemically treated with clay minerals and oxides to promote degradation of both organic and inorganic pollutants (Lapides et al., 2002; Napola et al., 2006; Concas et al., 2007; Montinaro et al., 2007, 2008; Pizzigallo et al., 2011; Di Leo et al., 2012;
Corresponding author at: CNR-IMAA, Tito Scalo, (Potenza), Italy. E-mail address: [email protected] (P. Di Leo).
https://doi.org/10.1016/j.clay.2019.105256 Received 5 April 2019; Received in revised form 6 August 2019; Accepted 7 August 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
Applied Clay Science 182 (2019) 105256
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RAW CLAY MINERAL Bentolite L (BL)
dried sample 37°C
not - milled
homoionized Ca-Bentolite with 0.5 M CaCl2 (Ca-BL)
Cd-exchanged Bentolite with CdCl2 0.1 M (Cd-BL)
milled (6 h and 24 h) dried sample 37°C
spectroscopic analyses: XRD, XPS, FTIR, and solid-state NMR
grinding from 1 to 24 hours at 700 rpm with different CdCl 2 amounts (CdCl2-BL) sample ground from 1 to 24 hours at 700 rpm
leaching with deionized water
leaching with 1 M MgCl2
Centrifuging at 1500 rpm + drying at 37°C
Solid-state analyses: XRD, XPS, FTIR, solid- state NMR
solid-state analyses: XRD, XPS, FTIR, solid- state NMR
liquid-phase analyses: ICP/OES
Fig. 1. Experimental procedure.
Rickwood Spec. This is a montmorillonite. Its chemical composition (wt %), determined by XRF, is as follows: SiO2 72.7, Al2O3 13.7, MgO 2.7, Fe2O3 0.7, CaO 2.0, Na2O, 0.2, K2O 0.1, TiO2 0.2, Loss On Ignition 7.7. Its CEC is 100 meq/100 g (Borden and Giese, 2001) and exhibits the following crystallochemical formula corresponding to half unit cell: (Ca,Na)0.3(Al,Mg)2Si4O10(OH)2(H2O)10.
Zhang et al., 2012; Di Leo et al., 2013; Ancona et al., 2016; Wang et al., 2017 among others). This technique consists in dry-grinding different substrates (i.e. toxic substances and adsorbing surfaces) in a high-energy ball mill to gather, tacking the advantages of mechanical forces, the transformation of a contaminant in a less toxic form. These forces are able to perturb the crystal structure of solids (both sorbents or contaminated soils) and provide an easier chemical transformation through grinding treatments. In fact, the catalytic capacity of the material is improved upon grinding by the breaking of bonds in the solid medium, which make possible the interaction between the new active surfaces and the contaminants (see Cagnetta et al., 2016 for a critical review). Several studies described the influence of mechanochemical activation on the morphological and microstructural changes of clay mineral substrates (Ramadan et al., 2010; Vdovic et al., 2010; Ancona et al., 2016), but only a limited number of studies investigated the use of mechanical milling on their adsorption properties (Nenadovic et al., 2009; Vdovic et al., 2010; Kumrić et al., 2013). Besides, Concas et al. (2007), Montinaro et al. (2007), Montinaro et al. (2008), and Nomura et al. (2008) reported that mechanochemical dry milling allows activation of chemical reactions between clay minerals (both natural and synthesized) surfaces and HM cations by inducing different types of mechanical stresses and without any other energy supply. The present study investigates the effect of mechanochemical treatments (dry grinding by means of a zirconia planetary ball mill) on the ability of smectites to “entrap” Cd. To this purpose, a dioctahedral smectite, i.e. Bentolite L, was ground with different amounts of Cd and for different times. The understanding of the critical metal sorption mechanisms onto smectite in mechanochemical interactions was obtained using techniques for solid-state analysis (XRF, XRD, XPS, FT-IR, NMR) and desorption experiments. The “entrapping efficiency” and the type of interactions between the clay surfaces and the HM were evaluated by leaching with both deionized water and 1 M MgCl2 solutions both Cd-exchanged smectie as well as the Ca-smectite milled with different amounts of CdCl2 as solid salt.
2.2. Mechanochemical treatment Before mechanochemical treatments BL has been homoionized with 0.5 M CaCl2 solutions for 24 h (hereafter Ca-BL). A cadmium homoionized BL (hereafter Cd-BL) was obtained using 0.2 N CdCl2 solution, starting from the homoionized Ca-BL (Di Leo and O'Brien, 1999). Cadmium chloride used in the experiments was purchased by Sigma Aldrich and exhibits 99.0% of purity. Enriched 113Cd CdCl2, from Cambridge Isotope Laboratories Inc., was also used to prepare the samples for NMR analysis. The mechanochemical treatment were carried out using a planetary ball mill (Pulverisette 7), by Frischt, Oberstain, Germany. The reactor consisted of two zirconia jars with 7 zirconia balls (10 mm diameter). Each pot was operated at high energy (700 rpm) for different milling times, from 1 to 24 h, both for the Ca-BL and the Cd-BL samples. Specifically, each step of 15 min of milling was followed by a 15 min break. Samples after 24 h milling has been leached with both deionized water and 1 M MgCl2 solutions. Besides, Ca-BL have been milled with different amounts of CdCl2 as solid salt (here after CdCl2-BL) and, after milling, was leached with both deionized water and 1 M MgCl2 solutions. All the experiments were conducted in triplicate. The complete experimental procedure is described in Fig. 1. 2.3. Analytical procedures Solid state analyses were carried out on Ca-BL and Cd-BL samples ground for 6 h and 24 h as follows: X-Ray fluorescence (XRF) analysis was performed with an energy dispersive portable spectrometer (Thermo NITON XL3T 900S, USA), operating at 50 kV and 40 μA. BL have been added with elvacite® solution (acetone 16% -w/v) and subsequently pressed into a pellet using a pressure of 10 tons. The Loss On Ignition (L.O.I.) was determined after heating the sample at 950 °C overnight. X-Ray Diffraction (XRD) patterns was collected on a Rigaku Miniflex
2. Materials and methods 2.1. Materials The dioctahedral smectite used in the present paper for mechanochemical treatments is Bentolite L (hereafter BL), purchased from 2
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indicates that the stacking of the layers gets disrupted and lost, proving evidences for increasing amorphization of the montmorillonite structure with grinding, i.e. delamination (Frost et al., 2001; Hrachová et al., 2007). Ca-BL samples before and after grinding for 6 h and 24 h were also analyzed by FTIR (Fig. 3A). The intense bands at 1040 and 1090 cm−1 arise from Si-O-Si stretching vibrations (Farmer and Russel, 1964; Bala et al., 2000). Changes in these bands are due to the destruction of the clay crystals with prolonged grinding. The unification of these bands into a single broad one at about 1082 cm−1 after 6 h grinding (see Fig. 3Ab) suggests the rupture along sheets parallel to the ab planes: indeed, modifications of SieO band arising from vibrations perpendicular to the 001 plane indicates that delamination occurs mainly in the direction of the c-axis, in accordance with XRD results. Besides, the decrease in the intensity of SieO bands after 24 h-grinding (see Fig. 3Ac) is likely due to “prototropy” (Yariv and Lapides, 2000), i.e. some of the atoms from the octahedral layer may approach the oxygens common to the tetrahedral and octahedral layers, perturbing SieO vibrations. Since protons are the principal diffusing atoms, changes in the spectrum can be attributed to surface protonation. Finally, layer fractures along ac and bc planes rise the number of exposed functional groups -AlOH, -SiOH, -Al-O−, SieOe, and Al-O-Si and therefore increase the ion-exchange capacity (Yariv e Lapides, 2000 and reference therein). The bands due to water-water hydrogen bonds (e.g., Mn+ –O–H–O–H–), free water molecules (hydration water or water weakly hydrogen-bound to the surface oxygens of the tetrahedral sheet), and water bending modes are observed near 3627, 3438 and 1640 cm−1, respectively. On the other hand, the bands corresponding to the vibrational frequencies of Al2OH and MgAlOH occur near 919 and 844 cm−1, respectively (Farmer and Russel, 1964). Prolonged milling (from 6 h to 24 h; compare curves b and c from Fig. 3A) causes indeed the diminishing and/or disappearance of 919 and 844 / 786 cm−1 bands as well as of the one at 3627 cm−1. This is related to changes in the OH groups in the montmorillonite structure. It is likely that the OHs either remain bound to the degraded matrix by unsaturated broken bonds or adsorbed as water molecules (Cicel and Kranz, 1981), since these bands do not completely disappear. The absorption band at 521 cm−1 is ascribed to Si-O-R3+ and R3+OH vibrations (where R3+ = Al, Mg o Li). This indicates that mechanochemical treatment causes the rupture between the octahedral and tetrahedral sheets in montmorillonite, and the perpendicular-to-thelayer plane periodicity is totally damaged (Cicel and Kranz, 1981). Xray patterns recorded after 6 h and 24 h grinding have indeed confirmed the absence of periodicity in c direction (see Fig. 2Ab). The absorption band at 468 cm−1 has been assigned either to SieO or Si-O-Si vibrations (Stubican and Roy, 1961; Farmer and. Russel, 1964). This band although preserved, became gradually more diffuse during grinding. Mechanochemical grinding affects chemical bonds in the OeH, Si–O–Al, and Si–O–Si groups from montmorillonite. After 6 h milling, the modification of the Ca-BL structure (Fig. 3Ab) mainly regards the OH stretching band at 3627 cm−1 and the bending bands at 916 cm−1 (Al2OH) and 844 cm−1 (AlMgOH). These decrease in intensity during the first 6 h - grinding, whereas only a small OH band is recognized in the spectrum of Ca-BL after 24 h - grinding. A broad band, attributable to water adsorbed on the amorphous product, dominates the OH stretching region. The changes in the SieO stretching (1040 cm−1), Si–O–Al (521 cm−1), and Si–O–Si (468 cm−1) bending vibrations are to be more complex. The intensity of the band at 521 cm−1 decrease markedly (Hrachová et al., 2007) with grinding time and is absent in the 6 h ground sample spectrum, thus indicating the breakdown of the Si–O–Al bonds, i.e. destruction of octahedral-to-tetrahedral sheet connections. The Si–O–Si band become broader and its intensity partly decreases after 6 h grinding. However, the Si–O–Si band in the spectrum of the 24 h ground sample remain broad and occur at 472 cm−1. The SieO stretching bands have a peculiar behavior with progressive
II diffractometer with a CuKα radiation source and sample spinner. Operational conditions were 30 kV, 15 mA, scan interval from 2.0° – 10.0° Δ2θ, with a 0.02° step scan and an 0.5° Δ2θ/min acquisition rate. X-Ray Photoelectron Spectroscopy (XPS) spectra were acquired by a Theta Probe Thermo Scientific spectrometer equipped with a monochromatized AlKα source (spot = 400 μm). Survey and high-resolution spectra were acquired in constant analyzer energy (CAE) mode with a pass energy of 150 eV and 100 eV respectively. Charge neutralization was achieved using a flood-gun in low energy mode (− 1 eV). Checking of Binding Energy (BE) scale linearity was tested using gold, silver and copper reference materials according to ISO 15472: 2001. The calibration of the BE scale was performed by fixing the BE of the alkyl carbon component at 284.8 ± 0.1 eV. Surface atomic percentages were averaged out of three replicates. Fourier-Transform Infra-Red Spectroscopy (FT-IR) spectra were acquired with a Nicolet 5 PC FT infrared spectrometer, using pressed KBr pellets on a spectral range of 4000–400 cm−1. Nominal resolution was 2 cm−1 and final spectra are the average of 64 scans. 113 Cd solid-state Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker MSL - 300 spectrometer using Single Pulse Excitation (SPE), Cross Polarization (CP), Magic-Angle Spinning (MAS), and dipolar DECoupling (DEC). The relevant experimental parameters are: spinning speeds 4–5 kHz; contact time 10–12 ms; proton decoupler power level 55 kHz; and recycle delays of 1 s and 4 s. In an attempt to minimize interferences from spinning side bands for some spectra, a Total Sideband Suppression (TOSS) was used. The CP sequence incorporated a 90° proton flip-back pulse immediately after the acquisition period in order to reduce the recycle delay necessary between scans. All MAS and CPMAS NMR spectra were recorded on samples equilibrated at room humidity and are referred to a 0.1 M Cd(ClO4)2 solution. 2.4. Desorption experiments To choose the most suitable milling time to perform desorption experiments - i.e. the time at which the maximum Cd adsorption was presumable observed - the mixture containing the highest Cd percentage (5.2%) was preliminarily milled at different times ranging from 1 to 24 h (see Fig. 6). An aliquot of 5 g solid CdCl2-BL at different metal ion concentrations (from 0.3 to 5.2% w/w) was therefore mechanically milled for 24 h. After milling, 100 mg of each solid mixture was extracted with 10 mL of deionized water or 1 M MgCl2 solution, by stirring for 24 h. Afterwards, the suspensions were centrifuged at 4 °C for 10 min. at 2288 g and the supernatants filtered through 0.2 μm regenerated cellulose acetate filters. The Cd concentration in the extracts was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, iCAP 6000 SERIES, THERMO Scientific). 3. Results and discussion 3.1. Mechanochemical treatment of Ca-BL Ca-BL were analyzed using XRD, XPS and FTIR before and after 6 h and 24 h grinding. Diffraction patterns for the Ca-BL not milled and milled for two different times are reported in Fig. 2A. The XRD investigations provided information about the periodicity of the structure in various grinding stages. The XRD pattern of Ca-BL is osservable a diffraction peak at d001 = 15.8 Å (Fig.2Aa). Grinding of Ca-BL have produced a marked structural alteration, with a progressive broadening of the (001) peak and a drastically decreasing intensity of the non-basal diffraction peak (d100 = 4.51 Å). The width at half-height of the (001) diffraction peak increases enormously during grinding and the peak almost disappears after 24 h grinding (see Fig. 2Ac). A reduction in the number of layers in individual particles and the non-uniformity of d00z values caused by the milling process are probable reasons for this behavior (Cicel and Kranz, 1981). The loss of intensity of the (001) peak 3
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Fig. 2. XRD patterns (A) a) not-milled Ca-BL, b) 6 h and c) 24 h milling; (B) a) not-milled Cd-BL, b) 6 h and c) 24 h milling; (C) Ca-BL/CdCL2 (5%) milled at increasing times.
Fig. 3. FT-IR spectra (A) a) not-milled Ca-BL, b) 6 h and c) 24 h milling; (B) a) not-milled Cd-BL, b) 6 h and c) 24 h milling.
octahedral sheets damaged. Consequently, the montmorillonite layers are modified and the formation of a three-dimensional framework constituted of linked [SiO4] and [AlO4] tetrahedra occurred. Two factors may affect the position of the 1040 cm−1 band: layered vs threedimensional organization of the [SiO4] tetrahedra and Si/Al ratio in the aluminosilicate network. At the beginning of the grinding process (up to 6 h), the [SiO4] units prevail; the SieO band thus appears at higher positions in the Si–O(Si) bridges of amorphous SiO2 adsorption region.
treatments. In the 6 h-ground Ca-BL spectrum, the SieO bands at 1090 and 1040 cm−1 merge to a broad band at 1082 cm−1. Additional grinding up to 24 h cause a significant broadening and intensity reduction of this band. The very broad SieO band exhibits a pronounced shoulder near 950 cm−1 (Fig. 3Ac). These changes are likely due to alteration of the montmorillonite structure during grinding. The disappearance of the bands at 3627, 916, and 845 cm−1 suggests that the octahedral OHs coordinating central atoms are released and the 4
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Fig. 4. XPS wide scan Atomic concentrations (%) from XPS quantitative analysis for Ca-BL (A) a) before and b) after 24 h milling; for Cd-BL XPS wide scan (B) a) before and after b) 24 h milling. High resolution Cd 3d XPS spectra Cd-BL C) a) not-milled, b) 24 h milling, c) leached with deionized water after 24 h milling, and d) leached with MgCl2 1 M solutions after 24 h milling. High resolution Cd 3d XPS spectra of Ca-BL + CdCl2 (5% w/w) (D) a) 6 h and b) 24 h milling.
3.2. Mechanochemical treatment of Cd-BL
An increase of the number of [AlO4] makes the SieO (Al IV) bridge contribution (i.e. absorption near 1000 cm−1) more important, and the complex band merge to one. The broad absorption bands at 786 and 750 cm−1 might correspond to the SieO bending and AleO stretching vibrations, respectively, similarly to glassy forms of feldspars spectra (i.e. amorphous aluminosilicates). The nearly total absence of bands at 786 and 750 cm−1 in the spectrum relative to the 24 h ground sample probably indicates that the three-dimensional aluminosilicate network is partially damaged (Hrachová et al., 2007). XPS wide scan spectrum of Ca-BL (Fig. 4A) exhibits C (C1s), Mg (MgKLL), Ca (Ca2p), O (O1s and OKLL), Al (Al2p and Al2s), and Si (Si2p and Al2s) peaks. Data about surface atomic percentage are reported in Table 1. The content of surface Mg content undergoes changes upon prolonged grinding (expressed as atomic ratio with respect to Ca and Al content), likely due to migration of Mg from octahedral surface layers into the bulk, which makes it not anymore detectable by XPS. This is in accordance with FTIR results, indicating that OHs coordinating central atoms (mainly Al and secondarily Mg) from octahedral layers are released and octahedral sheets are damaged.
The total Cd amount in the Cd-BL, as determined by XRF analysis, is 2.51% w/w (43.23 meq/100 g). After grinding Cd-BL for 24 h and extracting with 1 M MgCl2 solution, the total Cd content was reduced to 1.42% w/w (25.19 meq/100 g). Cd-BL samples have been characterized by XRD, FT-IR, XPS and NMR before and after 6 h and 24 h milling (Figs. 2B, 3B, 4B, C respectively). After 6 h milling, the basal d001 and d003 reflections (at 15.50 and 5.01 Å) disappeared. Similarly to what observed for Ca-BL, this is a consequence of delamination: the complete reduction of 1st order reflection is indicative of a periodicity break along c-direction. Cd-BL IR spectra do not exhibit any specific difference with respect to the Ca-BL spectra previously reported and discussed (compare Fig. 3B and Fig. 3A). XPS wide scan spectrum of Cd-BL (Fig. 4B) exhibits C (C1s), Cd (Cd3d), Mg (MgKLL), Ca (Ca2p), O (O1s and OKLL), Al (Al2p and Al2s), and Si (Si2p and Si2s) peaks. Surface atomic percentages results are reported in Table 2. The last column reports Cd/Al ratio, showing that Cd content is reduced by 70% of its initial amount after 1 M MgCl2
Table 1 Atomic concentration (%) from XPS quantitative analysis for Ca-BL. Ca Bentolite L
not milled 24 h milling
C%
O%
Si%
Al%
Mg%
Ca%
6.0 ± 0.9 17.0 ± 6.0
60.0 ± 0.6 55.0 ± 4.0
22.1 ± 0.5 19.8 ± 1.1
6.7 ± 0.5 5.5 ± 0.5
3.1 ± 0.5 1.7 ± 0.5
1.2 ± 0.5 1.2 ± 0.5
5
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Table 2 Atomic concentration (%) from XPS quantitative analysis for Cd-BL. Cd Bentolite L
not-milled 24 h milling H2O leaching MgCl2 leaching
C%
O%
5.9 ± 1.4 8.5 ± 0.5 11.3 ± 1.5 17.5 ± 1.4
62.9 63.0 57.8 48.5
Si% ± ± ± ±
1.1 0.5 1.4 0.6
20.1 19.3 19.9 16.5
± ± ± ±
0.5 0.5 0.5 0.5
Al%
Mg%
6.7 ± 0.5 6.9 ± 0.5 7.10 ± 0.5 6.5 ± 0.5
4.7 1.5 3.2 7.9
± ± ± ±
0.7 0.5 0.5 0.5
Cl%
Cd%
– – – 3.0 ± 0.5
0.7 0.7 0.7 0.2
± ± ± ±
0.2 0.2 0.2 0.2
shielding, as the lattice-oxygen ions contribute at lesser extent to the coordination of such cations. Hence, in a two-layer hydrate, as in BL (where the first coordination shell of the exchangeable cation contains H2O molecules only), the shielding is at a maximum. Therefore, the interlayer cation is totally dissociated from the clay surfaces and is relatively free of motion. Thus, the narrow component at −0.5 ppm occurring in the Cd-BL (Fig. 5a) could arise from Cd2+ in the interlayers. This Cd is surrounded by H2O molecules and forms outer-sphere complexes (Di Leo and O'Brien, 1999; Di Leo and Cuadros, 2003), therefore it is in Type I configuration. MAS NMR spectrum of Cd-BL grinded for 24 h evidence that mechanochemical treatment is responsible for changes in the Cd chemical environment. In the spectrum are present two new components with chemical negative shift, at −20 ppm, and a positive chemical shift, at 20 ppm (Fig. 5b). The latter is quite broad: the observed 20 ppm peak broadening may have been caused by chemical shift dispersion in the MAS experiments (Di Leo and O'Brien, 1999). The component with more negative chemical shift (− 20 ppm) is likely ascribable to Cd in adsorption sites bridged with OHs to either external surfaces or broken edges via a water molecule (i.e. in the Type IIb), thus in a relatively less strong interaction (Di Leo and Cuadros, 2003). On the other hand, the component with more positive chemical shift (20 ppm) could indicate a Cd forming inner-sphere complexes (Type IIa), i.e. relatively more strongly interacting with the OHs migrated on the external surfaces or on the broken edges owing to prolonged grinding.
extraction. Due to XPS surface sensitivity, Cd reduction after treatment must refer only to Cd on the surface, in an exchangeable position, whereas the remaining Cd is non-exchangeable. To better characterize Cd absorption sites, and to define how mechanochemical treatment affects them, high resolution XPS spectra have been acquired both for not-milled and 24 h - ground Cd-BL before and after extractions with distilled water or MgCl2 solution. Grinding affects the Cd3d XPS BE (eV) and the possible sites for Cd (Fig. 4Ca-d). Specifically, in the XPS spectrum of the not-milled sample (Fig. 4Ca), only the Cd component at BE = 407.4 ± 0.3 eV is observed. After 24 h grinding (Fig. 4Cb), a second component centered at lower energy is visible, i.e. at BE = 406.3 ± 0.1 eV. The two components suggest the presence of two Cd adsorption sites: the high energy component (407.4 ± 0.3 eV) is compatible with an adsorption site exhibiting the characteristics of an outer-sphere complex (i.e. exchangeable Cd), likely localized in the montmorillonite interlayers and coordinated by two water molecules shells (hereafter defined as Type I). The lower energy component (406.3 ± 0.1 eV) is compatible with sites where Cd is more tightly bound to the TOT layers. This may correspond either to Cd on external surfaces/broken edges (hereafter defined as Type IIa) or bound by the new OHs formed by the mechanochemical treatment or likely bridged via a water molecule (hereafter defined as Type IIb). Indeed, FTIR analysis exhibits a progressive change in the status of the hydroxyl groups in the Cd-BL structure as a consequence of the prolonged milling. No evidence of the original arrangement of OHs in the partly destroyed structure is observed and they either remain in the system bound in the form of water molecules adsorbed on the degraded matrix or in the form of OHs bound to unsaturated broken bonds (see Fig. 3B). Besides, 24 h milling is a suitable time to produce fractures in the layers along ab and bc planes, thus increasing the number of exposed functional groups AlOH, SiOH, AleOe, SieOe, and Al-O-Si (Yariv and Lapides, 2000). On the other hand, the lower energy spectral component (see Fig. 4Ca) could also arise from Cd adsorbed onto montmorillonite external surfaces - but still bridged via a water molecule therefore still in an exchangeable position. XPS spectra recorded after extractions (Fig. 4Cc and d) has allowed a better assignment of the Cd3d signals. Extracting with MgCl2 solution has completely removed the 407.4 ± 0.3 eV component, being this relative to Cd in an exchangeable site, whereas the lower energy component (406.3 ± 0.1 eV) was only reduced. This indicates that the latter component is arising from Cd in a more stable adsorption sites, i.e. more tightly bound to the TOT layers (i.e. in an inner sphere complex). This implies that the prolonged milling is quite efficient in reducing Cd mobility into the Cd-BL. Solid-state NMR spectroscopy carried out on 113Cd-BL samples supports the hypothesis of a two Cd adsorption site model. In Fig. 5 are reported the NMR spectra for the not-milled and 24 h milled samples. In the former, only one well defined symmetric peak is observable (chemical shift = − 0.5 ppm). According to Di Leo and O'Brien (1999) and Di Leo and Cuadros (2003), this peak is attributable to Cd coordinated with oxy species. The nuclei shielding (no strong negative chemicalshift) of exchangeable cations by electron shells is affected by electron transfer with tetrahedral sheet oxygen ions. As a consequence of this, the exchangeable cations strongly interacting with the basal oxygen ions have therefore a negative chemical shift. Hydrated phases exhibiting one or two H2O layers in the interlayer have a less effective
3.3. Mechanochemical treatment of Ca-BL - CdCl2 solid mixtures Ca-BL was milled with 5% w/w (44.48 meq Cd/100 g) solid CdCl2 salt for increasing time from 1 up to 24 h (CdCl2-BL). Mixtures were analyzed by XRF after extractions with 1 M MgCl2 solution. Results indicate that the Cd content retained in the solid phase increase at increasing milling time, from 5.37 to 32.91 meq/100 g. Starting from 5 h, the concentration reaches a plateau, thus suggesting that the capacity of montmorillonite to adsorb Cd by solid-solid mechanochemical treatment has reach almost its maximum after 5 h milling (Fig. 6). XRD diffraction patterns at increasing milling times (Fig. 2C) evidenced that a 6 h milling treatment of CdCl2-BL is necessary to get the attenuation of the d001 reflection, i.e. delamination of the mineral, similarly to what observed for the Ca-BL (see. Fig. 2A). Milling Ca-BL with different CdCl2 amounts for 6 h and 24 h does not produce any spectroscopic (FTIR) difference from what observed for the milling of the untreated clay mineral. In order to better define the characteristics of Cd adsorption sites, and how these are affected by prolonged milling, high resolution XPS spectra were recorded for CdCl2-BL (5% w/w). In Fig. 4D (a-b) the spectra for different grinding times (6 h and 24 h) are reported. Prolonged milling induces changes on cadmium BE. Specifically, in the 6 h XPS spectrum the two Cd components, relative to Type I and Type II configurations, are visible (Fig. 4 Da; see also Fig. 4C). A 24 h grinding has caused the disappearance of the low BE component relative to the Cd in Type IIa configuration (Fig. 4Db). Considering that after grinding only 0.1% of Cd – expressed as atomic concentration (data not shown) – is left in the mixture, it can be assumed that ~ 20% of Cd expressed as atomic concentration has been entrapped in more stable adsorption 6
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Fig. 5. Solid-state NMR spectra for
113
Cd-BL a) not-milled and b) 24 h milling.
Csol = 4.924 ∗ (Ceq)0.926
sites.
Best fitting of experimental data with Langmuir equation implies that only one ion can be hold by each site (monolayer sorption; sites with equal energy). On the other hand, the Freundlich equation is suitable for a highly heterogeneous surface and the isotherm shape with a lack of plateau indicates multilayer adsorption (Singh et al., 2006). The value n is connected to the distribution of bound ions on the adsorbent surface. The Kf and 1/n values are related to the sorption capacity and intensity, respectively. A significant adsorption takes place at low concentration if the values of 1/n are lower than unity. On the other hand, for higher concentrations, the increase of the entrapped amount becomes less noteworthy and viceversa (Juang et al., 1996). In accordance with desorption isotherms - as experimental data give the best fitting with a Langmuir equation when using aqueous extractant – Cd is adsorbed only in a monolayer (Cd in Type IIa configuration). On the other hand, more than a monolayer covering is possible, as experimental data are better interpolated with a Freudlich equation when using saline extractant (Cd in Type IIb configuration). Therefore, desorption data confirm the presence of two types of absorption sites for Cd after grinding.
3.4. Cd desorption isotherms On the basis of the results presented in the previous paragraphs, the 24 h milling time - at which the maximum adsorption was observed (see Fig. 6) - was chosen for desorption isotherm experiments. The isotherms were achieved by using initial Cd concentrations in the solid phase ranging from 6.68 to 92.09 meq/100 g. Cd concentrations determined in deionized water or 1 M MgCl2 solution extracts were used to construct isotherms according to Langmuir and Freundlich equations. Isotherm parameters for desorption and correlation coefficients for both equations are reported in Table 3. The cadmium content per unit mass of solid phase according to the extractant used, i.e. aqueous or MgCl2, followed different trends. For the aqueous extracts, the Laugmuir eq. (L Eq. [1]) gives the best experimental data interpolation (R2 = 0.9954; Fig. 7A), as follows:
Csol =
263.49 Ceq (1 + 3.61 Ceq)
[2]
[1]
where Csol is the Cd concentration in solid phase (meq/100 g) and Ceq the concentration in the liquid phase (meq/l), 3.61 is Kl, and 263.49 corresponds to Kl * b (for parameter details see Table 3). On the other hand, the experimental Cd desorption data for the MgCl2 extracts are better fit with the Freundlich eq. (F Eq. [2], R2 = 0.9373; Fig. 7B):
4. Conclusions The effect of mechanochemical treatments (dry grinding by means of a zirconia planetary ball mill) on the ability of smectites to “entrap”
30
20
10
0
6
12
Fig. 6. Kinetics of Cd adsorption onto Ca-BL. 7
18
24
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Table 3 Desorption mean constants and correlation coefficients for Langmuir and Freundlich equations. Freundlich
Langmuir
Kf (meq/100)*(L/meq) BentoliteL acqueous extracts BentoliteL MgCl2 extracts
1/n
42.49 4.92
2
1/n
R
0.4 0.92
0.971 0.937
Kl (L/meq)
b (meq/100 g)
R2
3.6 0.19
73 53.7
0.995 0.626
octahedra were therefore released and octahedral sheets were damaged. Long time experiments (24 h) suggested that milling facilitates Cd sorption onto two adsorption sites. The first one, i.e. Type I, exhibited outer-sphere complex characteristics and Cd in an exchangeable position, coordinated by two water molecules shells. The second one, Type II, was compatible with adsorption sites where Cd was more tightly bound to the TOT layers. This corresponded either to Cd on broken edges (Type IIa) bound via the new OHs formed by means of mechanochemical treatment or to Cd onto montmorillonite surfaces (Type IIb) bridged via a water molecule. The difference between desorption trends for Cd with the two extractants (deionized water and MgCl2 solution) were attributed to the presence of the different adsorption sites Type IIa and b. In accordance with desorption isotherms, Cd in Type IIa configuration formed only a monolayer, as experimental data gave the best fitting with a Langmuir equation. On the other hand, for Cd in Type IIb configuration, more than a monolayer covering was
Cd were investigated in the present study. To this purpose, the dioctahedral smectite BL was ground with different amounts of Cd and for different times. The understanding of the critical metal sorption mechanisms onto smectite in mechanochemical interactions was obtained using techniques for solid-state analysis (XRF, XRD, XPS, FT-IR, NMR) and desorption experiments. The “entrapping efficiency” and the type of interactions between the clay surfaces and Cd were evaluated by leaching with both deionized water and 1 M MgCl2 solutions both Cdexchanged smectie as well as the Ca-smectite milled with different amounts of CdCl2 as solid salt. Spectroscopic analyses evidenced changes in Ca-BL structure during grinding and the breaking of periodicity along c-direction: prolonged grinding induced delamination. Grinding for 24 h caused a partial destruction of the three-dimensional aluminosilicate network, likely due to migration of Mg from octahedral surface layers into the bulk. The OHs coordinating central atoms (mainly Al and secondarily Mg) in the
80
A) Cd adsorbed (meq/100g)
70 60 50 40 30 20 10 0
0
1
2
3
4
5
Ce (meq/l)
Cd adsorbed (meq/100g)
B)
40
30
20
10
0
0
1
2
3
4
5
6
7
Ce (meq/l) Fig. 7. s. Desortpion isotherm for Cd onto Ca-BL (aqueous extract) after 24 h milling (A). Desortpion isotherm for Cd onto Ca-BL (MgCl2 extract) after 24 h milling (B). 8
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possible, as experimental data gave the best fitting with a Freudlich equation. The basic knowledge achieved in the present study on the interaction of Cd with Ca-BL induced by mechanical forces can be used to develop new remediation strategies based on the application of clay minerals to mechanochemically decontaminate soils and sediments polluted by heavy metals.
Huang, C.P., Blankenship, D.W., 1984. The removal of Hg(II) from dilute aqueous solution by activated carbon. Water Res. 18, 37–46. Juang, R.S., Wu, F.C., Tseng, R.L., 1996. Adsorption isotherms of phenolic compounds from aqueose sotutions onto activated carbon fibers. J. Chem. Eng. Data 41, 487–492. Kumrić, K.R., Đukić, A.B., Trtić, T.M., Vukelić, N.S., Stojanović, Z., Grbović Novaković, J.D., Matović, Lj. Lj, 2013. Simultaneous removal of divalent heavy metals from aqueous solutions using raw and mechanochemically treated interstratified montmorillonite/kaolinite clay. Ind. Eng. Chem. Res. 52, 7930–7939. Lapides, I., Yariv, S., Dolodnitsky, G., 2002. Simultaneous DTA-TG study of montmorillonite mechanochemically treated with crystal-viole. J. Therm. Anal. Calorim. 67, 99–112. Montinaro, S., Concas, A., Pisu, M., Cao, G., 2007. Remediation of heavy metals contaminated soils by ball milling. Chemosphere. 67, 631–639. Montinaro, S., Concas, A., Pisu, M., Cao, G., 2008. Immobilization of heavy metals in contaminated soils through ball milling with and without additives. Chem. Eng. J. 142, 271–284. Napola, A., Pizzigallo, M.D.R., Di Leo, P., Spagnolo, M., Ruggiero, P., 2006. Mechanochemical approach to remove phenantrene from a contaminated soil. Chemosphere. 65, 1583–1590. Nenadovic, S., Nenadovic, M., Kovacevic, R., Matovic, Lj, Matovic, B., Jovanovic, Z., Grbovic Novakovic, J., 2009. Influence of diatomite microstructure on its adsorption capacity for Pb(II). Sci. Sinter. 41, 309–317. Nomura, Y., Fujiwara, K., Takada, M., 2008. Lead immobilization in mechanochemical fly ash recycling. J. Mater. Cycles Waste. 10, 14–18. Ovadyahu, D., Yariv, S., Lapides, I., Deutsch, Y., 1998. Mechanochemical adsorption of phenol by TOT swelling clay minerals II. Simultaneous DTA and TG study. J. Therm. Anal. 51, 431–447. Pizzigallo, M.D.R., Di Leo, P., Ancona, V., Spagnuolo, M., Schingaro, E., 2011. Effect of aging on catalytic properties in mechanochemical degradation of pentachlorophenol by birnessite. Chemosphere. 82, 627–634. Ramadan, A.R., Esawi, A.M.K., Gawad, A.A.A., 2010. Effect of ball milling on the structure of Na+-montmorillonite and organo-montmorillonite (Cloisite 30B). Appl. Clay Sci. 47, 196–202. Singh, S.P., Ma, L.Q., Hendry, M.J., 2006. Characterization of aqueous lead removal by phosphatic clay: equilibrium and kinetic studies. J. Hazard. Mater. 136, 654–662. Stubican, V., Roy, R., 1961. Isomorphus substitution and infrared spectra of the layerlattice silicates. Am. Mineral. 46, 32–51. Vdovic, N., Jurina, I., Scapin, S.D., Sondi, I., 2010. The surface properties of clay minerals modified by intensive dry milling. Appl. Clay Sci. 48, 575–580. Wang, H., Hwang, J., Huang, J., Xu, Y., Yu, G., Li, W., Zhang, K., Liu, K., Cao, Z., Ma, X., Wei, Z., Wang, Q., 2017. Mechanochemical remediation of PCB contaminated soil. Chemosphere. 168, 333–340. Yariv, S., Lapides, I., 2000. The effect of mechanochemical treatments on clay minerals and the mechanochemical adsorption of organic material onto clay minerals. J. Mater. Synth. Process. 8, 223–233. Zhang, K., Huang, J., Zhang, W., Yu, Y., Deng, S., Yu, G., 2012. Mechanochemical degradation of tetrabromobisphenol A: performance, products and pathway. J. Hazard. Mater. 243, 278–285. Zhao, J.Z., Xiao, W.L., Li, M., Xiang, Z.-P., Li, L.J., Wang, J., 2011. The effective pressure law for permeability of clay-rich sandstones. Pet. Sci. 8, 194–199.
References Ancona, V., Di Leo, P., Pizzigallo, M.D.R., 2016. Sequestration of catechol and pentachlorophenol by mechanochemically treated kaolinite. Clay Clay Miner. 64, 488–497. Bala, P., Samantaray, B.K., Srivastava, S.K., 2000. Dehydration transformation in Camontmorillonite. Bull. Mater. Sci. 23, 61–67. Borden, D., Giese, R.F., 2001. Baseline studies of the clay minerals society sorce clays: cation exchange capacity measurements by the ammonia-eletrode method. Clay Clay Miner. 49 (5), 444–445. Cagnetta, G., Robertson, J., Huang, J., Zhang, K., Yu, G., 2016. Mechanochemical destruction of halogenated organic pollutants: a critical review. J. Hazard. Mater. 313, 85–102. Cicel, B., Kranz, G., 1981. Mechanism of montmorillonite structure degradation by percussive grinding. Clay Miner. 16, 151–162. Concas, A., Montinaro, S., Pisu, M., Cao, G., 2007. Mechanochemical remediation of heavy metals contaminated soils: modelling and experiments. Chem. Eng. Sci. 62, 5186–5192. Di Leo, P., Cuadros, J., 2003. 113Cd, 1H MAS NMR and FTIR Analysis of Cd2+ adsorption on Dioctahedral and Trioctahedral smectite. Clay Clay Miner. 51, 403–414. Di Leo, P., O'Brien, P., 1999. Nuclear magnetic resonance (NMR) study of Cd2+ sorption on montmorillonite. Clay Clay Miner. 47, 761–768. Di Leo, P., Pizzigallo, M.D.R., Ancona, V., Di Benedetto, F., Mesto, E., Schingaro, E., Ventruti, G., 2012. Mechanochemical transformation of an organic ligand on mineral surfaces: the efficiency of birnessite in catechol degradation. J. Hazard. Mater. 201202, 148–154. Di Leo, P., Pizzigallo, M.D.R., Ancona, V., Di Benedetto, F., Mesto, E., Schingaro, E., Ventruti, G., 2013. Mechanochemical degradation of pentachlorophenol onto birnessite. J. Hazard. Mater. 244-245, 303–310. Đukić, A.B., Kumrić, K.R., Vukelić, N.S., Dimitrijević, M.S., Baščarević, Z.D., Kurko, S.V., Matović, L.Lj, 2015. Simultaneous removal of Pb2 +, Cu2 +, Zn2 + and Cd2 + from highly acidic solutions using mechanochemically synthesized montmorillonite–kaolinite/TiO2 composite. Appl. Clay Sci. 103, 20–27. Farmer, V.C., Russel, J.D., 1964. The infrared Spectra of layer silicates. Spectrochim. Acta 20, 1149–1173. Frost, R.L., Makó, É., Kristóf, J., Horváth, E., Kloprogge, J.T., 2001. Mechanochemical treatment of kaolinite. J. Colloid Interface Sci. 239, 458–466. Ghorbel-Abid, I., Galai, K., Trabelsi-Ayadi, M., 2010. Retention of chromium(III) and cadmium(II) from aqueous solution by illitic clay as a low-cost adsorbent. Desalination. 256, 190–195. Hrachová, J., Madejová, J., Billik, P., Komadel, P., ˇStefan Fajnor, V., 2007. Dry grinding of Ca and octadecyltrimethylammonium montmorillonite. J. Colloid Interface Sci. 316, 589–595.
9
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research Paper
Cadmium decontamination through ball milling using an expandable clay mineral
T
⁎
P. Di Leoa,d, , M.D.R. Pizzigallob, N. Ditarantoc, R. Terzanob a
CNR-IMAA, Tito Scalo, (Potenza), Italy Dipartimento Scienze del Suolo della Pianta e degli Alimenti, Università degli Studi di Bari “Aldo Moro”, Bari, Italy c Dipartimento di Chimica, Università degli Studi di Bari “Aldo Moro”, Bari, Italy d Dipartimento delle Culture Europee e del Mediterraneo, Basilicata University, Matera, Italy b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mechanochemistry Cadmium Clay minerals Dioctahedral smectite
Mechanochemical treatments have been widely used for the remediation of soils and sediments polluted by organic and inorganic pollutants. However, there has been still limited knowledge about the molecular mechanisms underlying the mechanochemical transformations responsible of heavy metals immobilization on clay minerals. In the present study, the ability of a dioctahedral smectite to retain cadmium (Cd) as induced by mechanochemical treatments was investigated. The smectite was ground with different amounts of Cd (from 0.3 to 5.2% w/w). Solid-state characterizations (XRF, XRD, XPS, FT-IR, NMR) as well as desorption isotherm experiments were carried out to understand the critical metal sorption mechanisms onto dioctahedral smectite occurring in mechanochemical interactions. The “entrapping efficiency” and the type of interaction between clay surfaces and cadmium were evaluated. Cd immobilization degree was assessed by extract analyses and expressed as leachable fraction of cations. Leaching was both performed with deionized water and 1 M MgCl2 solution. Spectroscopic analyses evidenced that long time grinding (24 h) allowed Cd sorption on two sites: Type I, exhibiting characteristic of an outer-sphere complex likely localized in the montmorillonite interlayers and coordinated by two water molecule shells, i.e. in an exchangeable position; Type II sites, where Cd was more tightly bound to the TOT layers, either onto broken edges via the new OHs formed during the mechanochemical treatments or onto montmorillonite surfaces likely bridged via a water molecule. Desorption isotherms also confirmed a two-adsorption site model. This information could be useful to understand, develop and manage new remediation technologies based on mechanochemical treatments.
1. Introduction The contamination of soils and sediments by heavy metals has received an ever-increasing consideration in the last decade, due to their potential risk for the environment and the ecosystem. Remediation of contaminated soils and sediments is a very difficult challenge to be figured out. Therefore, the implementation of simple, fast and economic remediation technologies represents a very complex scientific issue. Generally speaking, immobilization processes for the treatment of heavy metals (here after HM) contaminated soils are often preferred, owing to the limitations and costs of extractive techniques. Actually, immobilization techniques prevent HM spreading in the environment as consequence of leaching by enhancing the physical characteristics of soils as well as the solubility/toxicity of HM compounds. One of the most applied immobilization techniques is adsorption, due to its high ⁎
efficiency to remove metal ions, its easy application, large availability of adsorbents, and low cost (Huang and Blankenship, 1984; Ovadyahu et al., 1998; Đukić et al., 2015). In this respect, mechanochemical treatments furnish a better and more efficient alternative to this technique. Clay minerals exhibit remarkable properties as metal adsorbents in soil remediation due to their large presence in nature and low toxicity. Indeed, these materials are considered very efficient due to their high adsorption, ion exchange, and swelling properties as well as their large surface area, layered structure, abundance and low cost (Ghorbel-Abid et al., 2010; Zhao et al., 2011). Polluted soils can be mechanochemically treated with clay minerals and oxides to promote degradation of both organic and inorganic pollutants (Lapides et al., 2002; Napola et al., 2006; Concas et al., 2007; Montinaro et al., 2007, 2008; Pizzigallo et al., 2011; Di Leo et al., 2012;
Corresponding author at: CNR-IMAA, Tito Scalo, (Potenza), Italy. E-mail address: [email protected] (P. Di Leo).
https://doi.org/10.1016/j.clay.2019.105256 Received 5 April 2019; Received in revised form 6 August 2019; Accepted 7 August 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.
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RAW CLAY MINERAL Bentolite L (BL)
dried sample 37°C
not - milled
homoionized Ca-Bentolite with 0.5 M CaCl2 (Ca-BL)
Cd-exchanged Bentolite with CdCl2 0.1 M (Cd-BL)
milled (6 h and 24 h) dried sample 37°C
spectroscopic analyses: XRD, XPS, FTIR, and solid-state NMR
grinding from 1 to 24 hours at 700 rpm with different CdCl 2 amounts (CdCl2-BL) sample ground from 1 to 24 hours at 700 rpm
leaching with deionized water
leaching with 1 M MgCl2
Centrifuging at 1500 rpm + drying at 37°C
Solid-state analyses: XRD, XPS, FTIR, solid- state NMR
solid-state analyses: XRD, XPS, FTIR, solid- state NMR
liquid-phase analyses: ICP/OES
Fig. 1. Experimental procedure.
Rickwood Spec. This is a montmorillonite. Its chemical composition (wt %), determined by XRF, is as follows: SiO2 72.7, Al2O3 13.7, MgO 2.7, Fe2O3 0.7, CaO 2.0, Na2O, 0.2, K2O 0.1, TiO2 0.2, Loss On Ignition 7.7. Its CEC is 100 meq/100 g (Borden and Giese, 2001) and exhibits the following crystallochemical formula corresponding to half unit cell: (Ca,Na)0.3(Al,Mg)2Si4O10(OH)2(H2O)10.
Zhang et al., 2012; Di Leo et al., 2013; Ancona et al., 2016; Wang et al., 2017 among others). This technique consists in dry-grinding different substrates (i.e. toxic substances and adsorbing surfaces) in a high-energy ball mill to gather, tacking the advantages of mechanical forces, the transformation of a contaminant in a less toxic form. These forces are able to perturb the crystal structure of solids (both sorbents or contaminated soils) and provide an easier chemical transformation through grinding treatments. In fact, the catalytic capacity of the material is improved upon grinding by the breaking of bonds in the solid medium, which make possible the interaction between the new active surfaces and the contaminants (see Cagnetta et al., 2016 for a critical review). Several studies described the influence of mechanochemical activation on the morphological and microstructural changes of clay mineral substrates (Ramadan et al., 2010; Vdovic et al., 2010; Ancona et al., 2016), but only a limited number of studies investigated the use of mechanical milling on their adsorption properties (Nenadovic et al., 2009; Vdovic et al., 2010; Kumrić et al., 2013). Besides, Concas et al. (2007), Montinaro et al. (2007), Montinaro et al. (2008), and Nomura et al. (2008) reported that mechanochemical dry milling allows activation of chemical reactions between clay minerals (both natural and synthesized) surfaces and HM cations by inducing different types of mechanical stresses and without any other energy supply. The present study investigates the effect of mechanochemical treatments (dry grinding by means of a zirconia planetary ball mill) on the ability of smectites to “entrap” Cd. To this purpose, a dioctahedral smectite, i.e. Bentolite L, was ground with different amounts of Cd and for different times. The understanding of the critical metal sorption mechanisms onto smectite in mechanochemical interactions was obtained using techniques for solid-state analysis (XRF, XRD, XPS, FT-IR, NMR) and desorption experiments. The “entrapping efficiency” and the type of interactions between the clay surfaces and the HM were evaluated by leaching with both deionized water and 1 M MgCl2 solutions both Cd-exchanged smectie as well as the Ca-smectite milled with different amounts of CdCl2 as solid salt.
2.2. Mechanochemical treatment Before mechanochemical treatments BL has been homoionized with 0.5 M CaCl2 solutions for 24 h (hereafter Ca-BL). A cadmium homoionized BL (hereafter Cd-BL) was obtained using 0.2 N CdCl2 solution, starting from the homoionized Ca-BL (Di Leo and O'Brien, 1999). Cadmium chloride used in the experiments was purchased by Sigma Aldrich and exhibits 99.0% of purity. Enriched 113Cd CdCl2, from Cambridge Isotope Laboratories Inc., was also used to prepare the samples for NMR analysis. The mechanochemical treatment were carried out using a planetary ball mill (Pulverisette 7), by Frischt, Oberstain, Germany. The reactor consisted of two zirconia jars with 7 zirconia balls (10 mm diameter). Each pot was operated at high energy (700 rpm) for different milling times, from 1 to 24 h, both for the Ca-BL and the Cd-BL samples. Specifically, each step of 15 min of milling was followed by a 15 min break. Samples after 24 h milling has been leached with both deionized water and 1 M MgCl2 solutions. Besides, Ca-BL have been milled with different amounts of CdCl2 as solid salt (here after CdCl2-BL) and, after milling, was leached with both deionized water and 1 M MgCl2 solutions. All the experiments were conducted in triplicate. The complete experimental procedure is described in Fig. 1. 2.3. Analytical procedures Solid state analyses were carried out on Ca-BL and Cd-BL samples ground for 6 h and 24 h as follows: X-Ray fluorescence (XRF) analysis was performed with an energy dispersive portable spectrometer (Thermo NITON XL3T 900S, USA), operating at 50 kV and 40 μA. BL have been added with elvacite® solution (acetone 16% -w/v) and subsequently pressed into a pellet using a pressure of 10 tons. The Loss On Ignition (L.O.I.) was determined after heating the sample at 950 °C overnight. X-Ray Diffraction (XRD) patterns was collected on a Rigaku Miniflex
2. Materials and methods 2.1. Materials The dioctahedral smectite used in the present paper for mechanochemical treatments is Bentolite L (hereafter BL), purchased from 2
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indicates that the stacking of the layers gets disrupted and lost, proving evidences for increasing amorphization of the montmorillonite structure with grinding, i.e. delamination (Frost et al., 2001; Hrachová et al., 2007). Ca-BL samples before and after grinding for 6 h and 24 h were also analyzed by FTIR (Fig. 3A). The intense bands at 1040 and 1090 cm−1 arise from Si-O-Si stretching vibrations (Farmer and Russel, 1964; Bala et al., 2000). Changes in these bands are due to the destruction of the clay crystals with prolonged grinding. The unification of these bands into a single broad one at about 1082 cm−1 after 6 h grinding (see Fig. 3Ab) suggests the rupture along sheets parallel to the ab planes: indeed, modifications of SieO band arising from vibrations perpendicular to the 001 plane indicates that delamination occurs mainly in the direction of the c-axis, in accordance with XRD results. Besides, the decrease in the intensity of SieO bands after 24 h-grinding (see Fig. 3Ac) is likely due to “prototropy” (Yariv and Lapides, 2000), i.e. some of the atoms from the octahedral layer may approach the oxygens common to the tetrahedral and octahedral layers, perturbing SieO vibrations. Since protons are the principal diffusing atoms, changes in the spectrum can be attributed to surface protonation. Finally, layer fractures along ac and bc planes rise the number of exposed functional groups -AlOH, -SiOH, -Al-O−, SieOe, and Al-O-Si and therefore increase the ion-exchange capacity (Yariv e Lapides, 2000 and reference therein). The bands due to water-water hydrogen bonds (e.g., Mn+ –O–H–O–H–), free water molecules (hydration water or water weakly hydrogen-bound to the surface oxygens of the tetrahedral sheet), and water bending modes are observed near 3627, 3438 and 1640 cm−1, respectively. On the other hand, the bands corresponding to the vibrational frequencies of Al2OH and MgAlOH occur near 919 and 844 cm−1, respectively (Farmer and Russel, 1964). Prolonged milling (from 6 h to 24 h; compare curves b and c from Fig. 3A) causes indeed the diminishing and/or disappearance of 919 and 844 / 786 cm−1 bands as well as of the one at 3627 cm−1. This is related to changes in the OH groups in the montmorillonite structure. It is likely that the OHs either remain bound to the degraded matrix by unsaturated broken bonds or adsorbed as water molecules (Cicel and Kranz, 1981), since these bands do not completely disappear. The absorption band at 521 cm−1 is ascribed to Si-O-R3+ and R3+OH vibrations (where R3+ = Al, Mg o Li). This indicates that mechanochemical treatment causes the rupture between the octahedral and tetrahedral sheets in montmorillonite, and the perpendicular-to-thelayer plane periodicity is totally damaged (Cicel and Kranz, 1981). Xray patterns recorded after 6 h and 24 h grinding have indeed confirmed the absence of periodicity in c direction (see Fig. 2Ab). The absorption band at 468 cm−1 has been assigned either to SieO or Si-O-Si vibrations (Stubican and Roy, 1961; Farmer and. Russel, 1964). This band although preserved, became gradually more diffuse during grinding. Mechanochemical grinding affects chemical bonds in the OeH, Si–O–Al, and Si–O–Si groups from montmorillonite. After 6 h milling, the modification of the Ca-BL structure (Fig. 3Ab) mainly regards the OH stretching band at 3627 cm−1 and the bending bands at 916 cm−1 (Al2OH) and 844 cm−1 (AlMgOH). These decrease in intensity during the first 6 h - grinding, whereas only a small OH band is recognized in the spectrum of Ca-BL after 24 h - grinding. A broad band, attributable to water adsorbed on the amorphous product, dominates the OH stretching region. The changes in the SieO stretching (1040 cm−1), Si–O–Al (521 cm−1), and Si–O–Si (468 cm−1) bending vibrations are to be more complex. The intensity of the band at 521 cm−1 decrease markedly (Hrachová et al., 2007) with grinding time and is absent in the 6 h ground sample spectrum, thus indicating the breakdown of the Si–O–Al bonds, i.e. destruction of octahedral-to-tetrahedral sheet connections. The Si–O–Si band become broader and its intensity partly decreases after 6 h grinding. However, the Si–O–Si band in the spectrum of the 24 h ground sample remain broad and occur at 472 cm−1. The SieO stretching bands have a peculiar behavior with progressive
II diffractometer with a CuKα radiation source and sample spinner. Operational conditions were 30 kV, 15 mA, scan interval from 2.0° – 10.0° Δ2θ, with a 0.02° step scan and an 0.5° Δ2θ/min acquisition rate. X-Ray Photoelectron Spectroscopy (XPS) spectra were acquired by a Theta Probe Thermo Scientific spectrometer equipped with a monochromatized AlKα source (spot = 400 μm). Survey and high-resolution spectra were acquired in constant analyzer energy (CAE) mode with a pass energy of 150 eV and 100 eV respectively. Charge neutralization was achieved using a flood-gun in low energy mode (− 1 eV). Checking of Binding Energy (BE) scale linearity was tested using gold, silver and copper reference materials according to ISO 15472: 2001. The calibration of the BE scale was performed by fixing the BE of the alkyl carbon component at 284.8 ± 0.1 eV. Surface atomic percentages were averaged out of three replicates. Fourier-Transform Infra-Red Spectroscopy (FT-IR) spectra were acquired with a Nicolet 5 PC FT infrared spectrometer, using pressed KBr pellets on a spectral range of 4000–400 cm−1. Nominal resolution was 2 cm−1 and final spectra are the average of 64 scans. 113 Cd solid-state Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker MSL - 300 spectrometer using Single Pulse Excitation (SPE), Cross Polarization (CP), Magic-Angle Spinning (MAS), and dipolar DECoupling (DEC). The relevant experimental parameters are: spinning speeds 4–5 kHz; contact time 10–12 ms; proton decoupler power level 55 kHz; and recycle delays of 1 s and 4 s. In an attempt to minimize interferences from spinning side bands for some spectra, a Total Sideband Suppression (TOSS) was used. The CP sequence incorporated a 90° proton flip-back pulse immediately after the acquisition period in order to reduce the recycle delay necessary between scans. All MAS and CPMAS NMR spectra were recorded on samples equilibrated at room humidity and are referred to a 0.1 M Cd(ClO4)2 solution. 2.4. Desorption experiments To choose the most suitable milling time to perform desorption experiments - i.e. the time at which the maximum Cd adsorption was presumable observed - the mixture containing the highest Cd percentage (5.2%) was preliminarily milled at different times ranging from 1 to 24 h (see Fig. 6). An aliquot of 5 g solid CdCl2-BL at different metal ion concentrations (from 0.3 to 5.2% w/w) was therefore mechanically milled for 24 h. After milling, 100 mg of each solid mixture was extracted with 10 mL of deionized water or 1 M MgCl2 solution, by stirring for 24 h. Afterwards, the suspensions were centrifuged at 4 °C for 10 min. at 2288 g and the supernatants filtered through 0.2 μm regenerated cellulose acetate filters. The Cd concentration in the extracts was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, iCAP 6000 SERIES, THERMO Scientific). 3. Results and discussion 3.1. Mechanochemical treatment of Ca-BL Ca-BL were analyzed using XRD, XPS and FTIR before and after 6 h and 24 h grinding. Diffraction patterns for the Ca-BL not milled and milled for two different times are reported in Fig. 2A. The XRD investigations provided information about the periodicity of the structure in various grinding stages. The XRD pattern of Ca-BL is osservable a diffraction peak at d001 = 15.8 Å (Fig.2Aa). Grinding of Ca-BL have produced a marked structural alteration, with a progressive broadening of the (001) peak and a drastically decreasing intensity of the non-basal diffraction peak (d100 = 4.51 Å). The width at half-height of the (001) diffraction peak increases enormously during grinding and the peak almost disappears after 24 h grinding (see Fig. 2Ac). A reduction in the number of layers in individual particles and the non-uniformity of d00z values caused by the milling process are probable reasons for this behavior (Cicel and Kranz, 1981). The loss of intensity of the (001) peak 3
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Fig. 2. XRD patterns (A) a) not-milled Ca-BL, b) 6 h and c) 24 h milling; (B) a) not-milled Cd-BL, b) 6 h and c) 24 h milling; (C) Ca-BL/CdCL2 (5%) milled at increasing times.
Fig. 3. FT-IR spectra (A) a) not-milled Ca-BL, b) 6 h and c) 24 h milling; (B) a) not-milled Cd-BL, b) 6 h and c) 24 h milling.
octahedral sheets damaged. Consequently, the montmorillonite layers are modified and the formation of a three-dimensional framework constituted of linked [SiO4] and [AlO4] tetrahedra occurred. Two factors may affect the position of the 1040 cm−1 band: layered vs threedimensional organization of the [SiO4] tetrahedra and Si/Al ratio in the aluminosilicate network. At the beginning of the grinding process (up to 6 h), the [SiO4] units prevail; the SieO band thus appears at higher positions in the Si–O(Si) bridges of amorphous SiO2 adsorption region.
treatments. In the 6 h-ground Ca-BL spectrum, the SieO bands at 1090 and 1040 cm−1 merge to a broad band at 1082 cm−1. Additional grinding up to 24 h cause a significant broadening and intensity reduction of this band. The very broad SieO band exhibits a pronounced shoulder near 950 cm−1 (Fig. 3Ac). These changes are likely due to alteration of the montmorillonite structure during grinding. The disappearance of the bands at 3627, 916, and 845 cm−1 suggests that the octahedral OHs coordinating central atoms are released and the 4
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Fig. 4. XPS wide scan Atomic concentrations (%) from XPS quantitative analysis for Ca-BL (A) a) before and b) after 24 h milling; for Cd-BL XPS wide scan (B) a) before and after b) 24 h milling. High resolution Cd 3d XPS spectra Cd-BL C) a) not-milled, b) 24 h milling, c) leached with deionized water after 24 h milling, and d) leached with MgCl2 1 M solutions after 24 h milling. High resolution Cd 3d XPS spectra of Ca-BL + CdCl2 (5% w/w) (D) a) 6 h and b) 24 h milling.
3.2. Mechanochemical treatment of Cd-BL
An increase of the number of [AlO4] makes the SieO (Al IV) bridge contribution (i.e. absorption near 1000 cm−1) more important, and the complex band merge to one. The broad absorption bands at 786 and 750 cm−1 might correspond to the SieO bending and AleO stretching vibrations, respectively, similarly to glassy forms of feldspars spectra (i.e. amorphous aluminosilicates). The nearly total absence of bands at 786 and 750 cm−1 in the spectrum relative to the 24 h ground sample probably indicates that the three-dimensional aluminosilicate network is partially damaged (Hrachová et al., 2007). XPS wide scan spectrum of Ca-BL (Fig. 4A) exhibits C (C1s), Mg (MgKLL), Ca (Ca2p), O (O1s and OKLL), Al (Al2p and Al2s), and Si (Si2p and Al2s) peaks. Data about surface atomic percentage are reported in Table 1. The content of surface Mg content undergoes changes upon prolonged grinding (expressed as atomic ratio with respect to Ca and Al content), likely due to migration of Mg from octahedral surface layers into the bulk, which makes it not anymore detectable by XPS. This is in accordance with FTIR results, indicating that OHs coordinating central atoms (mainly Al and secondarily Mg) from octahedral layers are released and octahedral sheets are damaged.
The total Cd amount in the Cd-BL, as determined by XRF analysis, is 2.51% w/w (43.23 meq/100 g). After grinding Cd-BL for 24 h and extracting with 1 M MgCl2 solution, the total Cd content was reduced to 1.42% w/w (25.19 meq/100 g). Cd-BL samples have been characterized by XRD, FT-IR, XPS and NMR before and after 6 h and 24 h milling (Figs. 2B, 3B, 4B, C respectively). After 6 h milling, the basal d001 and d003 reflections (at 15.50 and 5.01 Å) disappeared. Similarly to what observed for Ca-BL, this is a consequence of delamination: the complete reduction of 1st order reflection is indicative of a periodicity break along c-direction. Cd-BL IR spectra do not exhibit any specific difference with respect to the Ca-BL spectra previously reported and discussed (compare Fig. 3B and Fig. 3A). XPS wide scan spectrum of Cd-BL (Fig. 4B) exhibits C (C1s), Cd (Cd3d), Mg (MgKLL), Ca (Ca2p), O (O1s and OKLL), Al (Al2p and Al2s), and Si (Si2p and Si2s) peaks. Surface atomic percentages results are reported in Table 2. The last column reports Cd/Al ratio, showing that Cd content is reduced by 70% of its initial amount after 1 M MgCl2
Table 1 Atomic concentration (%) from XPS quantitative analysis for Ca-BL. Ca Bentolite L
not milled 24 h milling
C%
O%
Si%
Al%
Mg%
Ca%
6.0 ± 0.9 17.0 ± 6.0
60.0 ± 0.6 55.0 ± 4.0
22.1 ± 0.5 19.8 ± 1.1
6.7 ± 0.5 5.5 ± 0.5
3.1 ± 0.5 1.7 ± 0.5
1.2 ± 0.5 1.2 ± 0.5
5
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Table 2 Atomic concentration (%) from XPS quantitative analysis for Cd-BL. Cd Bentolite L
not-milled 24 h milling H2O leaching MgCl2 leaching
C%
O%
5.9 ± 1.4 8.5 ± 0.5 11.3 ± 1.5 17.5 ± 1.4
62.9 63.0 57.8 48.5
Si% ± ± ± ±
1.1 0.5 1.4 0.6
20.1 19.3 19.9 16.5
± ± ± ±
0.5 0.5 0.5 0.5
Al%
Mg%
6.7 ± 0.5 6.9 ± 0.5 7.10 ± 0.5 6.5 ± 0.5
4.7 1.5 3.2 7.9
± ± ± ±
0.7 0.5 0.5 0.5
Cl%
Cd%
– – – 3.0 ± 0.5
0.7 0.7 0.7 0.2
± ± ± ±
0.2 0.2 0.2 0.2
shielding, as the lattice-oxygen ions contribute at lesser extent to the coordination of such cations. Hence, in a two-layer hydrate, as in BL (where the first coordination shell of the exchangeable cation contains H2O molecules only), the shielding is at a maximum. Therefore, the interlayer cation is totally dissociated from the clay surfaces and is relatively free of motion. Thus, the narrow component at −0.5 ppm occurring in the Cd-BL (Fig. 5a) could arise from Cd2+ in the interlayers. This Cd is surrounded by H2O molecules and forms outer-sphere complexes (Di Leo and O'Brien, 1999; Di Leo and Cuadros, 2003), therefore it is in Type I configuration. MAS NMR spectrum of Cd-BL grinded for 24 h evidence that mechanochemical treatment is responsible for changes in the Cd chemical environment. In the spectrum are present two new components with chemical negative shift, at −20 ppm, and a positive chemical shift, at 20 ppm (Fig. 5b). The latter is quite broad: the observed 20 ppm peak broadening may have been caused by chemical shift dispersion in the MAS experiments (Di Leo and O'Brien, 1999). The component with more negative chemical shift (− 20 ppm) is likely ascribable to Cd in adsorption sites bridged with OHs to either external surfaces or broken edges via a water molecule (i.e. in the Type IIb), thus in a relatively less strong interaction (Di Leo and Cuadros, 2003). On the other hand, the component with more positive chemical shift (20 ppm) could indicate a Cd forming inner-sphere complexes (Type IIa), i.e. relatively more strongly interacting with the OHs migrated on the external surfaces or on the broken edges owing to prolonged grinding.
extraction. Due to XPS surface sensitivity, Cd reduction after treatment must refer only to Cd on the surface, in an exchangeable position, whereas the remaining Cd is non-exchangeable. To better characterize Cd absorption sites, and to define how mechanochemical treatment affects them, high resolution XPS spectra have been acquired both for not-milled and 24 h - ground Cd-BL before and after extractions with distilled water or MgCl2 solution. Grinding affects the Cd3d XPS BE (eV) and the possible sites for Cd (Fig. 4Ca-d). Specifically, in the XPS spectrum of the not-milled sample (Fig. 4Ca), only the Cd component at BE = 407.4 ± 0.3 eV is observed. After 24 h grinding (Fig. 4Cb), a second component centered at lower energy is visible, i.e. at BE = 406.3 ± 0.1 eV. The two components suggest the presence of two Cd adsorption sites: the high energy component (407.4 ± 0.3 eV) is compatible with an adsorption site exhibiting the characteristics of an outer-sphere complex (i.e. exchangeable Cd), likely localized in the montmorillonite interlayers and coordinated by two water molecules shells (hereafter defined as Type I). The lower energy component (406.3 ± 0.1 eV) is compatible with sites where Cd is more tightly bound to the TOT layers. This may correspond either to Cd on external surfaces/broken edges (hereafter defined as Type IIa) or bound by the new OHs formed by the mechanochemical treatment or likely bridged via a water molecule (hereafter defined as Type IIb). Indeed, FTIR analysis exhibits a progressive change in the status of the hydroxyl groups in the Cd-BL structure as a consequence of the prolonged milling. No evidence of the original arrangement of OHs in the partly destroyed structure is observed and they either remain in the system bound in the form of water molecules adsorbed on the degraded matrix or in the form of OHs bound to unsaturated broken bonds (see Fig. 3B). Besides, 24 h milling is a suitable time to produce fractures in the layers along ab and bc planes, thus increasing the number of exposed functional groups AlOH, SiOH, AleOe, SieOe, and Al-O-Si (Yariv and Lapides, 2000). On the other hand, the lower energy spectral component (see Fig. 4Ca) could also arise from Cd adsorbed onto montmorillonite external surfaces - but still bridged via a water molecule therefore still in an exchangeable position. XPS spectra recorded after extractions (Fig. 4Cc and d) has allowed a better assignment of the Cd3d signals. Extracting with MgCl2 solution has completely removed the 407.4 ± 0.3 eV component, being this relative to Cd in an exchangeable site, whereas the lower energy component (406.3 ± 0.1 eV) was only reduced. This indicates that the latter component is arising from Cd in a more stable adsorption sites, i.e. more tightly bound to the TOT layers (i.e. in an inner sphere complex). This implies that the prolonged milling is quite efficient in reducing Cd mobility into the Cd-BL. Solid-state NMR spectroscopy carried out on 113Cd-BL samples supports the hypothesis of a two Cd adsorption site model. In Fig. 5 are reported the NMR spectra for the not-milled and 24 h milled samples. In the former, only one well defined symmetric peak is observable (chemical shift = − 0.5 ppm). According to Di Leo and O'Brien (1999) and Di Leo and Cuadros (2003), this peak is attributable to Cd coordinated with oxy species. The nuclei shielding (no strong negative chemicalshift) of exchangeable cations by electron shells is affected by electron transfer with tetrahedral sheet oxygen ions. As a consequence of this, the exchangeable cations strongly interacting with the basal oxygen ions have therefore a negative chemical shift. Hydrated phases exhibiting one or two H2O layers in the interlayer have a less effective
3.3. Mechanochemical treatment of Ca-BL - CdCl2 solid mixtures Ca-BL was milled with 5% w/w (44.48 meq Cd/100 g) solid CdCl2 salt for increasing time from 1 up to 24 h (CdCl2-BL). Mixtures were analyzed by XRF after extractions with 1 M MgCl2 solution. Results indicate that the Cd content retained in the solid phase increase at increasing milling time, from 5.37 to 32.91 meq/100 g. Starting from 5 h, the concentration reaches a plateau, thus suggesting that the capacity of montmorillonite to adsorb Cd by solid-solid mechanochemical treatment has reach almost its maximum after 5 h milling (Fig. 6). XRD diffraction patterns at increasing milling times (Fig. 2C) evidenced that a 6 h milling treatment of CdCl2-BL is necessary to get the attenuation of the d001 reflection, i.e. delamination of the mineral, similarly to what observed for the Ca-BL (see. Fig. 2A). Milling Ca-BL with different CdCl2 amounts for 6 h and 24 h does not produce any spectroscopic (FTIR) difference from what observed for the milling of the untreated clay mineral. In order to better define the characteristics of Cd adsorption sites, and how these are affected by prolonged milling, high resolution XPS spectra were recorded for CdCl2-BL (5% w/w). In Fig. 4D (a-b) the spectra for different grinding times (6 h and 24 h) are reported. Prolonged milling induces changes on cadmium BE. Specifically, in the 6 h XPS spectrum the two Cd components, relative to Type I and Type II configurations, are visible (Fig. 4 Da; see also Fig. 4C). A 24 h grinding has caused the disappearance of the low BE component relative to the Cd in Type IIa configuration (Fig. 4Db). Considering that after grinding only 0.1% of Cd – expressed as atomic concentration (data not shown) – is left in the mixture, it can be assumed that ~ 20% of Cd expressed as atomic concentration has been entrapped in more stable adsorption 6
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Fig. 5. Solid-state NMR spectra for
113
Cd-BL a) not-milled and b) 24 h milling.
Csol = 4.924 ∗ (Ceq)0.926
sites.
Best fitting of experimental data with Langmuir equation implies that only one ion can be hold by each site (monolayer sorption; sites with equal energy). On the other hand, the Freundlich equation is suitable for a highly heterogeneous surface and the isotherm shape with a lack of plateau indicates multilayer adsorption (Singh et al., 2006). The value n is connected to the distribution of bound ions on the adsorbent surface. The Kf and 1/n values are related to the sorption capacity and intensity, respectively. A significant adsorption takes place at low concentration if the values of 1/n are lower than unity. On the other hand, for higher concentrations, the increase of the entrapped amount becomes less noteworthy and viceversa (Juang et al., 1996). In accordance with desorption isotherms - as experimental data give the best fitting with a Langmuir equation when using aqueous extractant – Cd is adsorbed only in a monolayer (Cd in Type IIa configuration). On the other hand, more than a monolayer covering is possible, as experimental data are better interpolated with a Freudlich equation when using saline extractant (Cd in Type IIb configuration). Therefore, desorption data confirm the presence of two types of absorption sites for Cd after grinding.
3.4. Cd desorption isotherms On the basis of the results presented in the previous paragraphs, the 24 h milling time - at which the maximum adsorption was observed (see Fig. 6) - was chosen for desorption isotherm experiments. The isotherms were achieved by using initial Cd concentrations in the solid phase ranging from 6.68 to 92.09 meq/100 g. Cd concentrations determined in deionized water or 1 M MgCl2 solution extracts were used to construct isotherms according to Langmuir and Freundlich equations. Isotherm parameters for desorption and correlation coefficients for both equations are reported in Table 3. The cadmium content per unit mass of solid phase according to the extractant used, i.e. aqueous or MgCl2, followed different trends. For the aqueous extracts, the Laugmuir eq. (L Eq. [1]) gives the best experimental data interpolation (R2 = 0.9954; Fig. 7A), as follows:
Csol =
263.49 Ceq (1 + 3.61 Ceq)
[2]
[1]
where Csol is the Cd concentration in solid phase (meq/100 g) and Ceq the concentration in the liquid phase (meq/l), 3.61 is Kl, and 263.49 corresponds to Kl * b (for parameter details see Table 3). On the other hand, the experimental Cd desorption data for the MgCl2 extracts are better fit with the Freundlich eq. (F Eq. [2], R2 = 0.9373; Fig. 7B):
4. Conclusions The effect of mechanochemical treatments (dry grinding by means of a zirconia planetary ball mill) on the ability of smectites to “entrap”
30
20
10
0
6
12
Fig. 6. Kinetics of Cd adsorption onto Ca-BL. 7
18
24
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Table 3 Desorption mean constants and correlation coefficients for Langmuir and Freundlich equations. Freundlich
Langmuir
Kf (meq/100)*(L/meq) BentoliteL acqueous extracts BentoliteL MgCl2 extracts
1/n
42.49 4.92
2
1/n
R
0.4 0.92
0.971 0.937
Kl (L/meq)
b (meq/100 g)
R2
3.6 0.19
73 53.7
0.995 0.626
octahedra were therefore released and octahedral sheets were damaged. Long time experiments (24 h) suggested that milling facilitates Cd sorption onto two adsorption sites. The first one, i.e. Type I, exhibited outer-sphere complex characteristics and Cd in an exchangeable position, coordinated by two water molecules shells. The second one, Type II, was compatible with adsorption sites where Cd was more tightly bound to the TOT layers. This corresponded either to Cd on broken edges (Type IIa) bound via the new OHs formed by means of mechanochemical treatment or to Cd onto montmorillonite surfaces (Type IIb) bridged via a water molecule. The difference between desorption trends for Cd with the two extractants (deionized water and MgCl2 solution) were attributed to the presence of the different adsorption sites Type IIa and b. In accordance with desorption isotherms, Cd in Type IIa configuration formed only a monolayer, as experimental data gave the best fitting with a Langmuir equation. On the other hand, for Cd in Type IIb configuration, more than a monolayer covering was
Cd were investigated in the present study. To this purpose, the dioctahedral smectite BL was ground with different amounts of Cd and for different times. The understanding of the critical metal sorption mechanisms onto smectite in mechanochemical interactions was obtained using techniques for solid-state analysis (XRF, XRD, XPS, FT-IR, NMR) and desorption experiments. The “entrapping efficiency” and the type of interactions between the clay surfaces and Cd were evaluated by leaching with both deionized water and 1 M MgCl2 solutions both Cdexchanged smectie as well as the Ca-smectite milled with different amounts of CdCl2 as solid salt. Spectroscopic analyses evidenced changes in Ca-BL structure during grinding and the breaking of periodicity along c-direction: prolonged grinding induced delamination. Grinding for 24 h caused a partial destruction of the three-dimensional aluminosilicate network, likely due to migration of Mg from octahedral surface layers into the bulk. The OHs coordinating central atoms (mainly Al and secondarily Mg) in the
80
A) Cd adsorbed (meq/100g)
70 60 50 40 30 20 10 0
0
1
2
3
4
5
Ce (meq/l)
Cd adsorbed (meq/100g)
B)
40
30
20
10
0
0
1
2
3
4
5
6
7
Ce (meq/l) Fig. 7. s. Desortpion isotherm for Cd onto Ca-BL (aqueous extract) after 24 h milling (A). Desortpion isotherm for Cd onto Ca-BL (MgCl2 extract) after 24 h milling (B). 8
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possible, as experimental data gave the best fitting with a Freudlich equation. The basic knowledge achieved in the present study on the interaction of Cd with Ca-BL induced by mechanical forces can be used to develop new remediation strategies based on the application of clay minerals to mechanochemically decontaminate soils and sediments polluted by heavy metals.
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