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Mechanochemistry: A review of surface reactions and environmental applications
Applied Clay Science 67–68 (2012) 141–150
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Research paper
Mechanochemistry: A review of surface reactions and environmental applications Ahmed Nasser, Uri Mingelgrin ⁎ Institute of Soil, Water and Environmental Sciences, Volcani Center, Bet Dagan 50250, Israel
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 21 November 2011 Accepted 21 November 2011 Available online 22 December 2011 Keywords: Mechanochemstry Minerals Organic pollutants Polycyclic aromatic hydrocarbons Remediation Soils
a b s t r a c t Mechanochemistry is the study of physico-chemical transformations generated by mechanical force. This force may break down crystals, thus exposing fresh, active surfaces and enhance the mass transfer required for reaction partners in the solid state to make the contact required for initiating a chemical reaction. Application of mechanical force may facilitate sorption-induced steric enhancement of transformations catalyzed by ions of transition metals (e.g., cations of such metals serving as counter ions in exchange complexes of clays). The formation of mixed Cu–Na–montmorillonite from Na–montmorillonite ground with CuCl2 in the presence of imazaquin and the resultant breakdown of imazaquin is one example of this phenomenon. Reported mechanochemical reactions which involve minerals, include redox reactions, polymerization and polymer rearrangements, recrystallization and dehydration. The more widespread methods for removing organic contaminants from soils are based on biological degradation. Biological processes, however, have certain shortcomings such as the substantial time they require, or the significant concentrations of harmful residues that are left in the soil. Alternative soil remediation schemes are, therefore, needed and mechanochemical procedures may be part of such innovative cleanup protocols. Mechanochemistry can be applied at the field scale and mechanochemical techniques were already suggested as a clean and cheap approach to hazardous waste management and destruction. Furthermore, soil tillage is actually a process in which mechanical force is applied to the soil and hence, it may induce a variety of mechanochemical reactions. Clays and metal (especially iron and manganese) oxides catalyze numerous transformations of organic compounds when such compounds are sorbed on the minerals' surfaces. The catalytic efficiency of these minerals is, at least in part, due to their strong acidity, both Bronsted and Lewis, and to the large specific surface area they may possess. Manganese and iron are redox active and their minerals are thus capable of supporting both electron and proton transfer reactions. Since iron oxides are abundant in soils, transformations augmented by these minerals are of considerable environmental importance. In the present review, case studies of mechanochemically induced transformations of pollutants are presented. The breakdown of pesticides (2,4-D, imazaquin and atrazine) and of other organic pollutants such as DCP and PAHs achieved by light grinding for short durations (a few minutes) in a mixture with minerals which are soil components or the derivatives of such components is described. Among the tested minerals, Al– and Cu–montmorillonite, iron oxides and Mn oxides were the most effective heterogeneous catalysts. The effectiveness of the minerals depended strongly on the degraded pollutant and on the conditions (e.g., the moisture content) under which the mechanochemical reaction took place. Although the presence of water often hinders the degradation, in some cases the reverse is true. Na–montmorillonite in the presence of CuCl2 was effective in degrading imazaquin when the mixture was ground wet. This demonstrates a detoxification process applicable to real-world systems which are often wet. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Mechanochemistry is a branch of science that deals with physicochemical and chemical transformations induced by mechanical force. Such a force (e.g., as applied by grinding) may perturb the crystal structure of solids (thus exposing or creating fresh surfaces rich in active catalytic sites), and provide the driving force for the mass
⁎ Corresponding author. Tel.: + 972 3 9683641; fax: + 972 3 9683301. E-mail address: [email protected] (U. Mingelgrin). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.11.018
transfer required for bringing into contact reactants of solid state chemical reactions. Pressure or shear stress applied to impacted particles may facilitate both unique phase transitions and chemical transformations. The enhancement of catalytic capacity upon grinding may be in many cases due to the supply by mechanical means of the energy required to break bonds in the solid medium, thus exposing fresh surfaces, rich in unstable sites (the partner atoms in the broken bonds) to the substrate molecules. The mechanical agitation may also help orient the sorbed molecules relative to the newly formed, catalytically efficient sites, at the optimal conformation for the surface reaction to occur (Beyer and Clausen-Schaumann, 2005;
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Hall et al., 1996; Loiselle et al., 1997; Mingelgrin et al., 1978; Nasser et al., 2000; Pizzigallo et al., 2004; Shin et al., 2000; Tanaka et al., 2005). Reported mechanochemical solid state reactions include a wide range of processes, such as oxidation-reduction, polymerization and polymer rearrangement, recrystallization and dehydration (e.g., Balaz et al., 2005; Filipovic-Petrovic et al., 2002; Harchova et al., 2007; Kalinkin et al., 2006; Mendelvici, 2001). Mechanochemical techniques were used to manipulate the mode of adsorption of organic substances on clay minerals (Landau et al., 2002) and were suggested as an easy, clean and cheap novel approach to polluted soils and hazardous waste management and destruction (Brike et al., 2004; Caschili et al., 2006; Hall et al., 1996; Loiselle et al., 1997; Montinaro et al., 2007; Napola et al., 2006; Nasser et al., 2000; Pizzigallo et al., 2004; Shin et al., 2000; Tanaka et al., 2005). Mechanochemistry can be applied relatively easily at the field scale for remediation of polluted soils (e.g. http://www. tribochem.de, 2002). Furthermore, soil tillage practices such as plowing, or disking are actually processes in which a mechanical force is applied to soil particles and hence, soil tillage may initiate a variety of mechanochemically induced reactions. Clay minerals and certain metal (e.g., iron and manganese) oxides are major contributors to the catalysis of abiotic transformations of organic pollutants in natural environments (e.g., Nzengung et al., 2001; Wolfe et al., 1990). Clays may constitute half or more of the solid fraction of soils and are known to catalyze numerous transformations of organic compounds when those are sorbed on their surfaces (e. g., Mingelgrin et al., 1977; Nasser et al., 1997; Wang and Huang, 2003; Weiss et al., 2002; Wolfe et al., 1990). The catalytic efficiency of clay minerals and metal oxides is attributed in many cases to their capacity to function as strong acids, both Bronsted and Lewis, and to the large specific surface area they often possess (especially expandable clays). Manganese dioxides, for example, have highly acidic surfaces due to the interaction upon hydration of terminal hydroxyl groups and of bridging oxygen atoms with water molecules. Like manganese, iron is redox active and is capable of supporting both electron and proton transfer reactions. Since iron oxides are abundant in soils, transformations enhanced by these minerals are environmentally important. It is reasonable to assume that application of mechanical force will facilitate sorption-induced steric enhancement of many transformations catalyzed by transition metals, for example by cations of such metals serving as counter ions in exchange complexes of clays. The potential role of exchangeable copper in enhancing mechanochemical degradation is demonstrated below for imazaquin. Mechanochemistry does not only deal with enhanced catalysis. Applying mechanical force may induce, for example, the formation of solid phases which are of environmental importance or otherwise of interest. Yariv et al. (1991, 1994) reported the formation of kaolinite intercalated with alkali halides (CsCl and CsBr) by mechanical force (grinding). Yariv and Lapides (2000) described mechanochemical phenomena occurring during grinding of kaolinite. Kaolinite was ground in the absence as well as in the presence of alkali halides and using IR spectroscopy, those authors demonstrated the occurrence of four types of mechanochemical effects: diffusion of atoms (mainly protons, “prototropy”), delamination, layer breakdown, and adsorption of atmospheric water by the amorphous grinding product. When the smectites montmorillonite, beidellite, saponite, and Laponite were ground with excess phenol (ibid.), in most cases the first 5 min of grinding led to an increase in the adsorption of the phenol while additional grinding resulted in a decrease in phenol adsorption. Given below (imazaquin section) is an example of the formation of Cu–montmorillonite from Na–montmorillonite ground with CuCl2 and the enhancement of the degradation of imazaquin by this transformation. The overall objective of this review is to present case studies of mechanochemically induced transformations of pollutants, including pesticides (e.g., atrazine, 2,4-D and imazaquin) and other organic substances such as 2,4-dichlorophenol (DCP) or polycyclic aromatic
hydrocarbons (PAHs), using light grinding for short durations (on the order of minutes) in a mixture with minerals. The investigated pollutants and their structural formulae are presented in Appendix 1. The minerals used were soil components or derivatives of such components, specifically clays and metal oxides. 2. 2,4-D and DCP The role of manganese oxides in promoting oxidation of organic compounds is well documented (e.g., Cheney et al., 1996) and accordingly, a number of the mechanochemical reactions described in the present review involve such oxides. Birnessite was ground manually with wide range of pollutants: 2,4-D (Nasser et al., 2000), atrazine (Shin et al., 2000) organo-clorinated compounds (Pizzigallo et al., 2004) and imazaquin (Nasser et al., 2009). Excluding imazaquin, the other mentioned compounds degraded in a matter of hours at ambient temperature on the surface of birnessite. The mechanochemical degradation of the widely used herbicide 2,4-D (2,4-dichlorophenoxyacetic acid; the molecular structure is presented in Appendix 1) on synthetic birnessite (δ-MnO2) was investigated by Nasser et al. (2000). The authors used light grinding (5 min in an agate mortar -O.D 100 mm- and pestle). The apparent activation energy, evaluated by heat conduction calorimetry, was estimated to be 37 kJ mol − 1. Two degradation products were identified, 2,4-dichlorophenol (DCP), and CO2. No consumption of gaseous oxygen was detected, but some Mn(IV) of the birnessite was reduced Mn(II). A positive correlation was observed between the amount of Mn(II) produced and CO2 evolved, suggesting that the decomposition of 2,4-D involved the Mn oxide, birnessite, as a source of oxygen and Mn(IV) as an electron acceptor. The mechanochemical degradation of 2,4-D on birnessite is summarized in Fig. 1. Nasser et al. (2000) reported that, DCP, the primary degradation product of 2,4-D, further degrades on the surface of birnessite. Hence, the present authors undertook to study the effect of applying mechanical force on the fate of DCP in the presence of birnessite as well as other solid phases. Aside from birnessite, hematite (α-Fe2O3) and Na– and Cu–montmorillonite were tested as solid phases. DCP (its molecular structure is presented in Appendix 1) is one of the most important chlorophenols used in the manufacture of pesticides. It is the key-intermediate in the synthesis of 2,4-D. DCP is also a major feedstock in the production of certain compounds used in mothproofing, antiseptics and seed disinfectants (Bauer et al., 1999; Kroschwitz, 1993; USEPA, 1980). Thus, chlorophenol is considered as an EPA priority pollutant and it was detected in hazardous waste sites, wastewater and drinking water (ATSDR, 1995; Laurenti
Fig. 1. Kinetics of 2,4-D disappearance after grinding for 5 min in the presence of birnessite (adapted from Nasser et al., 2000).
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Table 1 LCMS—detected degradation products of DCP in ground birnessite–DCP mixture vs. DCP standard. RT (min)
Measured molecular mass
Calculated atomic composition
8.7a 10.4 11.6 12.4 12.6 14.1 15.0 16.0 16.3 16.7 17.7
160.96 252.98 286.94 320.91 378.97 412.93 446.89 446.89 538.92 538.92 572.88
C6H3OCl2 C12H7O2Cl2 C12H6O2Cl3 C12H5O2Cl4 C18H10O3Cl3 C18H9O3Cl4 C18H8O3Cl5 C18H8O3Cl5 C24H12O4Cl5 C24H12O4Cl5 C24H11O4Cl6
a
Standard deprotonated DCP (not observed in treated mixtures).
Table 2 Degradation of DCP in contact with selected minerals: 24 hours after mixing, immediately after 5 min of grinding and 24 hours after grinding. Mineral
Birnessite Na–montmorillonite Cu–montmorillonite Hematite
DCP degradation (%) After contact for 24 hours
Immediately after grinding
24 hours after grinding
20 0 5 0
90 0 0 8
93 27 16 15
et al., 2003; Majumder and Gupta, 2007; Wentz, 1989). It can be found in soils as a result of partial degradation of herbicides such as 2,4-D, (Bhandari and Xu, 2001). The mechanical force was applied by grinding a mixture of each mineral and DCP (25:1 ratio) manually, using mortar and pestle for 5 min. Samples of the ground mixtures were then extracted with methanol and the DCP in the extract determined by HPLC. The identification of the degradation products was performed by LCMS and Table 1 lists the components found after application of the mechanical force on the birnessite–DCP mixture. The results summarized in Table 1 demonstrate that two main reactions took place during the grinding process: dimerization (or a higher degree of polymerization) and dechlorination. The extent of degradation of DCP when ground in the presence of a number of minerals is summarized in Table 2. Again, birnessite exhibited a high catalytic efficiency and hematite, Na– and Cu–montmorillonite displayed a considerably lower catalytic capacity. Based on the finding of Nasser et al. (2000), that during the degradation of 2,4-D on birnessite some Mn(IV) was reduced to Mn(II), it was suggested that the decomposition of DCP might involve Mn as an electron acceptor resulting in the formation Mn(II). The formation of Mn(II) should increase the solubility of the manganese species present, and indeed, it was demonstrated that DCP degradation on birnessite was associated with a considerable increase in the extractability of manganese from the mineral into an acidic aqueous solution (Fig. 2). The above discussion demonstrates that application of a mild mechanical force for a short duration can enhance the degradation of 2,4-D and, even to a greater extent, of its major degradation product DCP. The manganese oxide, birnessite, was shown to be very effective in inducing the degradation.
Fig. 2. The amount of Mn extracted from birnessite, before and after grinding, and from a DCP/birnessite mixture after grinding, to an acidified aqueous solution.
in Appendix 1), brought about by light grinding with a mortar and pestle in the presence of various minerals, including soil-derived clays. Grinding of carbendazim (25:1 ratio) manually, using mortar and pestle for 5 min with Al–montmorillonite resulted in the degradation of 85% of the pesticide's content within 5 min. Other minerals (Fig. 3) were not effective in degrading carbendazim, demonstrating the often encountered high specificity of heterogeneous catalylists. The degradation mechanism in the present case is possibly related to the high acidity of the surface of Al–montmorillonite and the basic nature of the amino group of carbendazim (Appendix 1).
4. Imazaquin The effectiveness of mechanochemical processes in enhancing degradation of the herbicide imazaquin (2-[4,5-dihydro-4-methyl-4-(1methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid) by soil components was investigated in some detail (Nasser et al., 2009). The molecular structure of imazaquin is presented in Appendix 1. Imazaquin is a widely used herbicide with a broad-spectrum of weed control. The molecule has two ionizable functional groups: a carboxyl group
3. Carbendazim Another interesting example of mechanochemical degradation is the breakdown of carbendazim (methyl 1H-benzimidazol-2-ylcarbamate; a benzimidazole fungicide, the molecular structure of which is presented
Fig. 3. Kinetics of carbendazim degradation following 5 min of grinding with various minerals (m/m0 is the fraction of carbendazim remaining).
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Fig. 4. Imazaquin remaining after 5 min of grinding with different minerals (m/m0 is the fraction of imazaquin remaining).
Fig. 7. Fraction of added imazaquin that remained after 5 min of grinding (M/Mo) in the presence of solutions of chlorides of selected transition metals.
Fig. 5. Fraction of added imazaquin that remained after 5 min of dry grinding (M/Mo) as a function of the imazaquin/mineral ratio (Nasser et al., 2009).
(acid, pKa =3.8) and a quinoline group (basic, pKb =2.0; Stougaard et al., 1990). Its persistence in soil may be relatively high. Grinding of imazaquin for 5 min with Cu–montmorillonite, Al– montmorillonite and hematite resulted in 90%, 56% and 71% degradation of the herbicide, respectively (Fig. 4). Grinding was less efficient in inducing degradation of imazaquin by birnessite and Ca–montmorillonite. Agsorb (a commercial trade name for a natural mixture of attapulgite and montmorillonite) and Na–montmorillonite were the least effective in degrading imazaquin. Grinding with Na–montmorillonite to which CuCl2 was added was, however, as efficient as grind-
Fig. 6. Fraction of added imazaquin (M/Mo) that remained after 5 min of wet grinding with water, HCl or NaOH solution.
ing with Cu–montmorillonite in degrading the herbicide. The mechanical force applied was, as in the previous cases, manual grinding of the mixtures of imazaquin and the minerals, using mortar and pestle. Mixing the minerals with imazaquin for 5 min without grinding, or grinding imazaquin with no mineral present, did not induce degradation of the herbicide. The degradation rate of imazaquin was examined as a function of herbicide load (3.9, 8.9, 16.7 and 26.6 mg imazaquin/g mineral) and of time of grinding. Na–montmorillonite was not effective in degrading imazaquin over the whole range of ratios (Fig. 5). Hematite was very effective in degrading imazaquin in the lower range of loadings (3.9 and 8.9 mg/g), but the efficiency of degradation declined with the increase in the imazaquin/hematite ratio. This is expected due to the limited number of active sites on the surface of hematite, the particles of which lack, unlike montmorillonite, any internal surfaces. The more complicated dependence of the efficiency of Al–montmorillonite in degrading imazaquin on the reactant-mineral ratio (Fig. 5) may reflect the complex effect of the imazaquin–montmorillonite ratio on the extent of delamination of the layered mineral as a result of grinding and on the penetration of the herbicide into the interlayer spaces. In the framework of this study, the pressure applied by the pestle was estimated at 1500 ± 75 g cm − 2 applied for a contact time of 7.5 seconds during the 5 min of grinding. Prolonged grinding of imazaquin, up to 30 min, was performed using a ball mill. The prolonged grinding of imazaquin that was attempted with Na–montmorillonite,
Fig. 8. X-Ray diffractograms of: A. Na–montmorillonite, B. Cu–montmorillonite and C. Na–montmorillonite ground with CuCl2 (Nasser et al., 2009).
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Al–montmorillonite and hematite yielded a similar extent of degradation to that achieved after 5 min of manual grinding. This finding suggests that a plateau in the degradation process was reached after grinding for a relatively short duration of a few minutes. Mineral-herbicide mixtures that were wetted to 30% moisture content with an aqueous solution (0.1 N HCl, 0.1 N NaOH, 1 N chloride of Ni 2 +, Cu 2 + or Fe 3 + or distilled water) during the grinding process were also investigated. The catalytic efficiency of hematite
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was reduced in the presence of water (Fig. 6). The presence of free water diminishes the polarity and hence the dissociability of the minerals' outermost shell of water of hydration, and hence the BrÖnsted and Lewis acidity as well as basicity of the potentially catalytic sites at the minerals' surface. Wetting, therefore, lowers the capacity of minerals to enhance many transformations of organic compounds. On the other hand, wetting birnessite, which in the dry state was less efficient than both Al–montmorillonite and hematite in
Fig. 9. Possible transformation products of imazaquin as detected by LC-MS analysis (Nasser et al., 2009).
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degrading imazaquin, did improve somewhat the mineral's catalytic efficiency. The addition of either HCl or NaOH did not seem to strongly affect the minerals' degradation efficiency (Fig. 6). When a solution of Ni 2 +, Cu 2 +or Fe 3 + chloride was added during the grinding of imazaquin with the minerals, the most effective in degrading imazaquin was the addition of Cu 2 + to the imazaquin/ Na–montmorillonite mixture. Addition of iron solution to the minerals also improved their catalytic efficiency in degrading imazaquin during wet grinding (Fig. 7), but to a lesser extent than the addition of Cu 2 +. The addition of Cu 2 + to the imazaquin/Na–montmorillonite mixture enabled degradation of more than 90% of the initial imazaquin during the 5 min of grinding (Fig. 7). When imazaquin was ground after being wetted with the solution of CuCl2 in the absence of the mineral, no degradation of imazaquin was detected. Yet, when Cu–montmorillonite was exposed to either dry or wet grinding together with imazaquin, the extent of imazaquin degradation was similar to that achieved by grinding with Na–montmorillonite to which CuCl2 was added (Figs. 4 and 7). This finding suggests that Cu–montmorillonite might have been formed during the wet grinding of Na–montmorillonite to which a Cu 2 + solution was added, and that the formed Cu- montmorillonite is the species responsible for the degradation of imazaquin. Support for this hypothesis is lent by XRD (Fig. 8) of Na– and Cu–montmorillonite and of Na–montmorillonite ground with Cu 2 + (wet grinding). All three patterns indicate the presence of montmorillonite and of some quartz. The patterns of Na–montmorillonite and Cu–montmorillonite differ by a slight displacement of the peak maximum from 28.52° (Na-mont.) to 28.81° (Cu-mont.) and by Cu–montmorillonite having a sharper peak than Na–montmorillonite at the vicinity of 7.06°. In addition, Cu–montmorillonite contains traces of copper chloride as indicated by the low peaks at 16.19° and 15.5°. Na–montmorillonite ground with Cu 2 + shows an enlargement and a broadening of the peak at 7.06°. Moreover, Na–montmorillonite ground with Cu 2 + contained halite (NaCl), identified by the two peaks at 27.45° and 31.82°, which was
Fig. 10. (a) Concentrations of atrazine (AT), DDA, DIA, and DEA in methanol extracts obtained during the reaction of atrazine on birnessite (BIRN) and (b) cryptomelane type II (CR-II) (from Shin et al., 2000).
not present in Na–montmorillonite (Fig. 8). Formation of halite in this sample is further evidence for the partial exchange of Na by Cu as the counter ion of the montmorillonite during the grinding of Namontmorillonite with CuCl2. LC-MS analysis revealed that the mechanochemical transformation of imazaquin resulted in the formation of a dimer and several breakdown products (Fig. 9). The variety of products detected indicates that several pathways for transformation of imazaquin occurred simultaneously, including dehydroxylation, demethylation, decarboxylation and condensation. Similarly, a host of metabolites was identified as photodegradation products of imazaquin in water (Barkani et al., 2005).
5. Atrazine Shin et al. (2000) reported the mechanochemical degradation of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine; the molecular structure is presented in Appendix 1) adsorbed on four synthetic manganese oxides. Atrazine, a herbicide, is an important pollutant, detected in water systems worldwide. Its reactivity with soil minerals is thus of obvious importance. Synthetic pyrolusite (βMnO2), birnessite (δ-MnO2), and cryptomelane types I and II (αMnO2), were used as the degrading solid media. The authors followed the decomposition products that were formed after deposition of atrazine onto the Mn oxide surfaces and grinding the atrazine/oxide mixtures manually for 5 min using an agate mortar and pestle. The interaction of atrazine with birnessite produced (Fig. 10) a considerable amount of the metabolites DDA (didealkylatrazine), DIA (deisopropylatrazine), and DEA (deethylatrazine). However, the concentrations of the metabolites observed did not correspond to the amount of the parent atrazine that disappeared, apparently because a substantial amount of CO2 was produced over time as well. The degradation of atrazine also produced Mn(II), but a positive correlation between the amount of Mn(II) produced and atrazine degraded was found only for cryptomelane II (Fig. 11), suggesting that the dominant pathway for the decomposition of atrazine at the surface of this mineral involved the oxide serving as an electron acceptor in a reaction that yields Mn(II).
Fig. 11. Disappearance of atrazine (ATZ) and release of Mn(II) during the interaction of atrazine with cryptomelane type II (from Shin et al., 2000).
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1.0
0.8
0.8
Pyrene (M/Mo)
Pyrene (M/Mo)
a 1.0
0.6
0.4
0.2
b
0.6
0.4
0.2
0.0
0.0 no grinding
manual grinding
no grinding
1.0
0.8
0.8
Pyrene (M/Mo)
Pyrene (M/Mo)
c 1.0
0.6
0.4
manual grinding
d
0.6
0.4
0.2
0.2
0.0
147
0.0 no grinding
manual grinding
no grinding
manual grinding
Fig. 12. Fraction (M/M0) of pyrene remaining with or without 5 min of grinding. (a) Na–montmorillonite (b) birnessite (c) magnetite (d) Cu–montmorillonite. T0 refers to the instance immediately after grinding and T24 to 24 hours afterward (Joseph-Ezra, 2011).
6. PAH Polycyclic aromatic hydrocarbons (PAH) are important environmental pollutants with a relatively high resistance to microbial degradation. Due to their hydrophobic (i.e. low aqueous solubility) nature,
1.0
they are adsorbed strongly to the organic fraction of the soil's solid phase, thus further reducing their vulnerability to microbial degradation (Alexander, 2000; Mulder et al., 2001). Joseph-Ezra (2011) performed a detailed study on the mechanochemical degradation of certain polycyclic aromatic hydrocarbons, in particular pyrene and phenanthrene, under light grinding with soil minerals for short durations. Below is a summary of some of the results. Pyrene (C16H10) and phenanthrene (C14H10), are polycyclic aromatic hydrocarbons (their molecular structure is presented in
PHE (M/Mo)
0.8 0.6 no grinding
10
0.4
0.0 no grinding
manual grinding
Fig. 13. Fraction (M/M0) of the added phenanthrene (PHE) remaining on magnetite loaded with phenanthrene, with and without manual grinding for 5 min. T0 refers to the instance immediately after grinding and T24 to 24 hours afterward (Joseph-Ezra, 2011).
Frequency (%)
manual grinding
0.2
5
0 0.10
1.00
10.00
100.00
1000.00
Particle size (µm) Fig. 14. Particle size distribution of magnetite before and after grinding for 5 min.
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Appendix 1), that often contaminate soils (e.g., Harayama, 1997). They are resistant to most natural degradation processes and hence take a long time to dissipate (ATSDR, 1995). Napola et al. (2006) tested the efficacy of grinding by a ball mill in the remediation of a soil contaminated by phenanthrene and also investigated the influence of birnessite addition to the soil before grinding on phenanthrene removal. These authors obtained 20-50% removal and addition of birnessite to the soil did not affect significantly the extent of phenanthrene degradation. Joseph-Ezra (2011) tested samples of magnetite (Fe3O4), manganese oxide (birnessite, δ-MnO2), Na–montmorillonite and Cu–montmorillonite and a sample from the surface horizon of a sandy loam soil. The samples were loaded with pyrene and/or phenanthrene at various concentrations (up to 1,000 μg PAH/g mineral) and ground manually. Manual grinding was performed using a mortar and pestle for 5 min. The rate of degradation and the nature of the degradation products were determined by HPLC and LCMS. Na–montmorillonite and birnessite did not bring about the degradation of pyrene, with or without grinding, while grinding with Cu– montmorillonite did actually retard the degradation of pyrene in comparison to the rate of degradation of this PAH when incubated in the presence of Cu–montmorillonite without applying a mechanical force (the non-ground samples displaying considerable degradation of pyrene, ~ 50% in 24 h). Grinding pyrene with magnetite resulted in significant degradation (~50%), while in the non-ground samples no degradation was detected. The longer the grinding time, the higher was the rate of degradation. Magnetite has been found to be the most effective mineral in degrading pyrene by grinding (Fig. 12). Grinding with magnetite caused immediate degradation of both pyrene and phenanthrene. The degradation of phenanathrene when ground with magnetite is summarized in Fig. 13. The results presented in Figs. 12 and 13 (Joseph-Ezra, 2011) were obtained using a magnetite sample, the apparent diameter of 90% of the particles of which was 298 μm or less before grinding. After manual grinding, the size of 90% of the particles was reduced to 21 μm or less (Fig. 14). Thus, the enhanced degradation of the PAHs brought about by grinding was possibly due to the exposure of new, still active surface areas as the mineral's crystals were broken down. Accordingly, the extent of mechanochemical degradation of the PAHs on magnetite increased with the duration of grinding, while the particle size decreased and more active sites were exposed (Joseph-Ezra, 2011). Addition of 5% magnetite to the soil loaded with an initial concentration of 500 μg/g of each pyrene and phenanthrene and grinding did not induce significant degradation. However, adding water to the PAH-spiked soil/magnetite mixtures and then drying before grinding, resulted in a 10% degradation of phenanthrene and 16% degradation of pyrene. When magnetite was added to the contaminated soil, the PAH molecules were already adsorbed to the soil's solid phase, and therefore their contact with the mineral was hindered. The presence of water after wetting allowed the diffusive movement of the PAH molecules toward the magnetite particles, thus enabling the degradation process to occur.
HPLC runs displayed new peaks after grinding with magnetite. These peaks and the decline in the intensity of the peaks of the PAHs, are evidence of the degradation of the PAHs. LCMS analysis revealed that the major degradation mechanisms of pyrene and phenanthrene when ground with magnetite were redox processes. In the case of both pyrene and phenanthrene, oxidized products were detected (Table 3).
7. Summary and conclusions The most common methods for removing organic pollutants from the soil are based on biological degradation (e.g., Khan et al., 2004). However, biological procedures have certain shortcomings, including the fact that they may require considerable time or leave high concentrations of harmful residues in the soil (Mulder et al., 2001). Therefore, it may be advantageous to develop complementary soil remediation methods based on mechanochemical procedures. The data presented in this review demonstrated that light grinding of a variety of organic contaminants together with certain minerals is an effective way to bring about the contaminants' degradation. The mechanochemical approach can serve as a basis for remediation schemes for soils contaminated with numerous organic pollutants. And indeed, the interest in the discipline of mechanochemistry is fast expanding, as indicated by the volume of publications in that field in recent years. Mechanochemical procedures, such as light grinding for short periods of time (in the order of minutes) enabled easy and fast degradation of an assortment of organic pollutants ranging from 2,4-D and DCP, through imazaquin and atrazine to various PAHs. Among the minerals tested, Al– and Cu–montmorillonite, iron oxides and Mn oxides were shown to be the most effective heterogeneous catalysts (or more precisely reaction partners), but the relative efficiency of each of these minerals depends strongly on the target pollutant. The degradation rate of the pollutants is also strongly affected by the conditions under which the mechanochemical process takes place (e.g., moisture content). Although the presence of water often hinders the degradation process, in some cases the reverse is true. Thus, Na–montmorillonite in the presence of CuCl2 (the joint grinding of which resulted in partial replacement of Na by Cu as the montmorillonite's counter cation) efficiently enhanced the degradation of imazaquin when wet. This observation suggests a process of detoxification applicable to realworld polluted systems which are often wet, in which the naturally occurring Na–montmorillonite (or bentonite) serves, together with CuCl2, as the remediating agent.
Acknowledgements The authors are indebted to Julius Ben Ari for his performance and analysis of the LCMS runs and to Hadas Josef-Ezra for her invaluable work on the mechanochemical degradation of PAH.
Table 3 LCMS—detected degradation products of pyrene and phenanthrene after grinding with magnetite (Joseph-Ezra, 2011).
Pyrene degradation products
Phenanthrene degradation products
Retention time (min)
Molecular mass-detected compound, positive mode (APCI)
Molecular mass-detected compound, negative mode
Atomic composition of neutral molecule
9.11 7.15
219.08 235.07
217.07 (ESI) 232.05 (APCI) 233.06 (ESI)
C16H10O C16H10O2
6.37
233.06 235.07 195.08 211.07
5.5 6.5
C16H9O2 C16H11O2 C14H10O C14H10O2
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Appendix 1. List of compounds discussed in the review
Compound
Molecular formula
2,4-Dichlorophenoxyacetic acid (2,4-D)
C8H6Cl2O3
2,4-Dichlorophenol (DCP)
C6H4Cl2O
Atrazine
C8H14ClN5
Carbendazim
C9H9N3O2
Imazaquin
C17H17N3O3
Phenanthrene
C14H10
Pyrene
C16H10
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Research paper
Mechanochemistry: A review of surface reactions and environmental applications Ahmed Nasser, Uri Mingelgrin ⁎ Institute of Soil, Water and Environmental Sciences, Volcani Center, Bet Dagan 50250, Israel
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 21 November 2011 Accepted 21 November 2011 Available online 22 December 2011 Keywords: Mechanochemstry Minerals Organic pollutants Polycyclic aromatic hydrocarbons Remediation Soils
a b s t r a c t Mechanochemistry is the study of physico-chemical transformations generated by mechanical force. This force may break down crystals, thus exposing fresh, active surfaces and enhance the mass transfer required for reaction partners in the solid state to make the contact required for initiating a chemical reaction. Application of mechanical force may facilitate sorption-induced steric enhancement of transformations catalyzed by ions of transition metals (e.g., cations of such metals serving as counter ions in exchange complexes of clays). The formation of mixed Cu–Na–montmorillonite from Na–montmorillonite ground with CuCl2 in the presence of imazaquin and the resultant breakdown of imazaquin is one example of this phenomenon. Reported mechanochemical reactions which involve minerals, include redox reactions, polymerization and polymer rearrangements, recrystallization and dehydration. The more widespread methods for removing organic contaminants from soils are based on biological degradation. Biological processes, however, have certain shortcomings such as the substantial time they require, or the significant concentrations of harmful residues that are left in the soil. Alternative soil remediation schemes are, therefore, needed and mechanochemical procedures may be part of such innovative cleanup protocols. Mechanochemistry can be applied at the field scale and mechanochemical techniques were already suggested as a clean and cheap approach to hazardous waste management and destruction. Furthermore, soil tillage is actually a process in which mechanical force is applied to the soil and hence, it may induce a variety of mechanochemical reactions. Clays and metal (especially iron and manganese) oxides catalyze numerous transformations of organic compounds when such compounds are sorbed on the minerals' surfaces. The catalytic efficiency of these minerals is, at least in part, due to their strong acidity, both Bronsted and Lewis, and to the large specific surface area they may possess. Manganese and iron are redox active and their minerals are thus capable of supporting both electron and proton transfer reactions. Since iron oxides are abundant in soils, transformations augmented by these minerals are of considerable environmental importance. In the present review, case studies of mechanochemically induced transformations of pollutants are presented. The breakdown of pesticides (2,4-D, imazaquin and atrazine) and of other organic pollutants such as DCP and PAHs achieved by light grinding for short durations (a few minutes) in a mixture with minerals which are soil components or the derivatives of such components is described. Among the tested minerals, Al– and Cu–montmorillonite, iron oxides and Mn oxides were the most effective heterogeneous catalysts. The effectiveness of the minerals depended strongly on the degraded pollutant and on the conditions (e.g., the moisture content) under which the mechanochemical reaction took place. Although the presence of water often hinders the degradation, in some cases the reverse is true. Na–montmorillonite in the presence of CuCl2 was effective in degrading imazaquin when the mixture was ground wet. This demonstrates a detoxification process applicable to real-world systems which are often wet. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Mechanochemistry is a branch of science that deals with physicochemical and chemical transformations induced by mechanical force. Such a force (e.g., as applied by grinding) may perturb the crystal structure of solids (thus exposing or creating fresh surfaces rich in active catalytic sites), and provide the driving force for the mass
⁎ Corresponding author. Tel.: + 972 3 9683641; fax: + 972 3 9683301. E-mail address: [email protected] (U. Mingelgrin). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.11.018
transfer required for bringing into contact reactants of solid state chemical reactions. Pressure or shear stress applied to impacted particles may facilitate both unique phase transitions and chemical transformations. The enhancement of catalytic capacity upon grinding may be in many cases due to the supply by mechanical means of the energy required to break bonds in the solid medium, thus exposing fresh surfaces, rich in unstable sites (the partner atoms in the broken bonds) to the substrate molecules. The mechanical agitation may also help orient the sorbed molecules relative to the newly formed, catalytically efficient sites, at the optimal conformation for the surface reaction to occur (Beyer and Clausen-Schaumann, 2005;
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Hall et al., 1996; Loiselle et al., 1997; Mingelgrin et al., 1978; Nasser et al., 2000; Pizzigallo et al., 2004; Shin et al., 2000; Tanaka et al., 2005). Reported mechanochemical solid state reactions include a wide range of processes, such as oxidation-reduction, polymerization and polymer rearrangement, recrystallization and dehydration (e.g., Balaz et al., 2005; Filipovic-Petrovic et al., 2002; Harchova et al., 2007; Kalinkin et al., 2006; Mendelvici, 2001). Mechanochemical techniques were used to manipulate the mode of adsorption of organic substances on clay minerals (Landau et al., 2002) and were suggested as an easy, clean and cheap novel approach to polluted soils and hazardous waste management and destruction (Brike et al., 2004; Caschili et al., 2006; Hall et al., 1996; Loiselle et al., 1997; Montinaro et al., 2007; Napola et al., 2006; Nasser et al., 2000; Pizzigallo et al., 2004; Shin et al., 2000; Tanaka et al., 2005). Mechanochemistry can be applied relatively easily at the field scale for remediation of polluted soils (e.g. http://www. tribochem.de, 2002). Furthermore, soil tillage practices such as plowing, or disking are actually processes in which a mechanical force is applied to soil particles and hence, soil tillage may initiate a variety of mechanochemically induced reactions. Clay minerals and certain metal (e.g., iron and manganese) oxides are major contributors to the catalysis of abiotic transformations of organic pollutants in natural environments (e.g., Nzengung et al., 2001; Wolfe et al., 1990). Clays may constitute half or more of the solid fraction of soils and are known to catalyze numerous transformations of organic compounds when those are sorbed on their surfaces (e. g., Mingelgrin et al., 1977; Nasser et al., 1997; Wang and Huang, 2003; Weiss et al., 2002; Wolfe et al., 1990). The catalytic efficiency of clay minerals and metal oxides is attributed in many cases to their capacity to function as strong acids, both Bronsted and Lewis, and to the large specific surface area they often possess (especially expandable clays). Manganese dioxides, for example, have highly acidic surfaces due to the interaction upon hydration of terminal hydroxyl groups and of bridging oxygen atoms with water molecules. Like manganese, iron is redox active and is capable of supporting both electron and proton transfer reactions. Since iron oxides are abundant in soils, transformations enhanced by these minerals are environmentally important. It is reasonable to assume that application of mechanical force will facilitate sorption-induced steric enhancement of many transformations catalyzed by transition metals, for example by cations of such metals serving as counter ions in exchange complexes of clays. The potential role of exchangeable copper in enhancing mechanochemical degradation is demonstrated below for imazaquin. Mechanochemistry does not only deal with enhanced catalysis. Applying mechanical force may induce, for example, the formation of solid phases which are of environmental importance or otherwise of interest. Yariv et al. (1991, 1994) reported the formation of kaolinite intercalated with alkali halides (CsCl and CsBr) by mechanical force (grinding). Yariv and Lapides (2000) described mechanochemical phenomena occurring during grinding of kaolinite. Kaolinite was ground in the absence as well as in the presence of alkali halides and using IR spectroscopy, those authors demonstrated the occurrence of four types of mechanochemical effects: diffusion of atoms (mainly protons, “prototropy”), delamination, layer breakdown, and adsorption of atmospheric water by the amorphous grinding product. When the smectites montmorillonite, beidellite, saponite, and Laponite were ground with excess phenol (ibid.), in most cases the first 5 min of grinding led to an increase in the adsorption of the phenol while additional grinding resulted in a decrease in phenol adsorption. Given below (imazaquin section) is an example of the formation of Cu–montmorillonite from Na–montmorillonite ground with CuCl2 and the enhancement of the degradation of imazaquin by this transformation. The overall objective of this review is to present case studies of mechanochemically induced transformations of pollutants, including pesticides (e.g., atrazine, 2,4-D and imazaquin) and other organic substances such as 2,4-dichlorophenol (DCP) or polycyclic aromatic
hydrocarbons (PAHs), using light grinding for short durations (on the order of minutes) in a mixture with minerals. The investigated pollutants and their structural formulae are presented in Appendix 1. The minerals used were soil components or derivatives of such components, specifically clays and metal oxides. 2. 2,4-D and DCP The role of manganese oxides in promoting oxidation of organic compounds is well documented (e.g., Cheney et al., 1996) and accordingly, a number of the mechanochemical reactions described in the present review involve such oxides. Birnessite was ground manually with wide range of pollutants: 2,4-D (Nasser et al., 2000), atrazine (Shin et al., 2000) organo-clorinated compounds (Pizzigallo et al., 2004) and imazaquin (Nasser et al., 2009). Excluding imazaquin, the other mentioned compounds degraded in a matter of hours at ambient temperature on the surface of birnessite. The mechanochemical degradation of the widely used herbicide 2,4-D (2,4-dichlorophenoxyacetic acid; the molecular structure is presented in Appendix 1) on synthetic birnessite (δ-MnO2) was investigated by Nasser et al. (2000). The authors used light grinding (5 min in an agate mortar -O.D 100 mm- and pestle). The apparent activation energy, evaluated by heat conduction calorimetry, was estimated to be 37 kJ mol − 1. Two degradation products were identified, 2,4-dichlorophenol (DCP), and CO2. No consumption of gaseous oxygen was detected, but some Mn(IV) of the birnessite was reduced Mn(II). A positive correlation was observed between the amount of Mn(II) produced and CO2 evolved, suggesting that the decomposition of 2,4-D involved the Mn oxide, birnessite, as a source of oxygen and Mn(IV) as an electron acceptor. The mechanochemical degradation of 2,4-D on birnessite is summarized in Fig. 1. Nasser et al. (2000) reported that, DCP, the primary degradation product of 2,4-D, further degrades on the surface of birnessite. Hence, the present authors undertook to study the effect of applying mechanical force on the fate of DCP in the presence of birnessite as well as other solid phases. Aside from birnessite, hematite (α-Fe2O3) and Na– and Cu–montmorillonite were tested as solid phases. DCP (its molecular structure is presented in Appendix 1) is one of the most important chlorophenols used in the manufacture of pesticides. It is the key-intermediate in the synthesis of 2,4-D. DCP is also a major feedstock in the production of certain compounds used in mothproofing, antiseptics and seed disinfectants (Bauer et al., 1999; Kroschwitz, 1993; USEPA, 1980). Thus, chlorophenol is considered as an EPA priority pollutant and it was detected in hazardous waste sites, wastewater and drinking water (ATSDR, 1995; Laurenti
Fig. 1. Kinetics of 2,4-D disappearance after grinding for 5 min in the presence of birnessite (adapted from Nasser et al., 2000).
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Table 1 LCMS—detected degradation products of DCP in ground birnessite–DCP mixture vs. DCP standard. RT (min)
Measured molecular mass
Calculated atomic composition
8.7a 10.4 11.6 12.4 12.6 14.1 15.0 16.0 16.3 16.7 17.7
160.96 252.98 286.94 320.91 378.97 412.93 446.89 446.89 538.92 538.92 572.88
C6H3OCl2 C12H7O2Cl2 C12H6O2Cl3 C12H5O2Cl4 C18H10O3Cl3 C18H9O3Cl4 C18H8O3Cl5 C18H8O3Cl5 C24H12O4Cl5 C24H12O4Cl5 C24H11O4Cl6
a
Standard deprotonated DCP (not observed in treated mixtures).
Table 2 Degradation of DCP in contact with selected minerals: 24 hours after mixing, immediately after 5 min of grinding and 24 hours after grinding. Mineral
Birnessite Na–montmorillonite Cu–montmorillonite Hematite
DCP degradation (%) After contact for 24 hours
Immediately after grinding
24 hours after grinding
20 0 5 0
90 0 0 8
93 27 16 15
et al., 2003; Majumder and Gupta, 2007; Wentz, 1989). It can be found in soils as a result of partial degradation of herbicides such as 2,4-D, (Bhandari and Xu, 2001). The mechanical force was applied by grinding a mixture of each mineral and DCP (25:1 ratio) manually, using mortar and pestle for 5 min. Samples of the ground mixtures were then extracted with methanol and the DCP in the extract determined by HPLC. The identification of the degradation products was performed by LCMS and Table 1 lists the components found after application of the mechanical force on the birnessite–DCP mixture. The results summarized in Table 1 demonstrate that two main reactions took place during the grinding process: dimerization (or a higher degree of polymerization) and dechlorination. The extent of degradation of DCP when ground in the presence of a number of minerals is summarized in Table 2. Again, birnessite exhibited a high catalytic efficiency and hematite, Na– and Cu–montmorillonite displayed a considerably lower catalytic capacity. Based on the finding of Nasser et al. (2000), that during the degradation of 2,4-D on birnessite some Mn(IV) was reduced to Mn(II), it was suggested that the decomposition of DCP might involve Mn as an electron acceptor resulting in the formation Mn(II). The formation of Mn(II) should increase the solubility of the manganese species present, and indeed, it was demonstrated that DCP degradation on birnessite was associated with a considerable increase in the extractability of manganese from the mineral into an acidic aqueous solution (Fig. 2). The above discussion demonstrates that application of a mild mechanical force for a short duration can enhance the degradation of 2,4-D and, even to a greater extent, of its major degradation product DCP. The manganese oxide, birnessite, was shown to be very effective in inducing the degradation.
Fig. 2. The amount of Mn extracted from birnessite, before and after grinding, and from a DCP/birnessite mixture after grinding, to an acidified aqueous solution.
in Appendix 1), brought about by light grinding with a mortar and pestle in the presence of various minerals, including soil-derived clays. Grinding of carbendazim (25:1 ratio) manually, using mortar and pestle for 5 min with Al–montmorillonite resulted in the degradation of 85% of the pesticide's content within 5 min. Other minerals (Fig. 3) were not effective in degrading carbendazim, demonstrating the often encountered high specificity of heterogeneous catalylists. The degradation mechanism in the present case is possibly related to the high acidity of the surface of Al–montmorillonite and the basic nature of the amino group of carbendazim (Appendix 1).
4. Imazaquin The effectiveness of mechanochemical processes in enhancing degradation of the herbicide imazaquin (2-[4,5-dihydro-4-methyl-4-(1methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid) by soil components was investigated in some detail (Nasser et al., 2009). The molecular structure of imazaquin is presented in Appendix 1. Imazaquin is a widely used herbicide with a broad-spectrum of weed control. The molecule has two ionizable functional groups: a carboxyl group
3. Carbendazim Another interesting example of mechanochemical degradation is the breakdown of carbendazim (methyl 1H-benzimidazol-2-ylcarbamate; a benzimidazole fungicide, the molecular structure of which is presented
Fig. 3. Kinetics of carbendazim degradation following 5 min of grinding with various minerals (m/m0 is the fraction of carbendazim remaining).
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Fig. 4. Imazaquin remaining after 5 min of grinding with different minerals (m/m0 is the fraction of imazaquin remaining).
Fig. 7. Fraction of added imazaquin that remained after 5 min of grinding (M/Mo) in the presence of solutions of chlorides of selected transition metals.
Fig. 5. Fraction of added imazaquin that remained after 5 min of dry grinding (M/Mo) as a function of the imazaquin/mineral ratio (Nasser et al., 2009).
(acid, pKa =3.8) and a quinoline group (basic, pKb =2.0; Stougaard et al., 1990). Its persistence in soil may be relatively high. Grinding of imazaquin for 5 min with Cu–montmorillonite, Al– montmorillonite and hematite resulted in 90%, 56% and 71% degradation of the herbicide, respectively (Fig. 4). Grinding was less efficient in inducing degradation of imazaquin by birnessite and Ca–montmorillonite. Agsorb (a commercial trade name for a natural mixture of attapulgite and montmorillonite) and Na–montmorillonite were the least effective in degrading imazaquin. Grinding with Na–montmorillonite to which CuCl2 was added was, however, as efficient as grind-
Fig. 6. Fraction of added imazaquin (M/Mo) that remained after 5 min of wet grinding with water, HCl or NaOH solution.
ing with Cu–montmorillonite in degrading the herbicide. The mechanical force applied was, as in the previous cases, manual grinding of the mixtures of imazaquin and the minerals, using mortar and pestle. Mixing the minerals with imazaquin for 5 min without grinding, or grinding imazaquin with no mineral present, did not induce degradation of the herbicide. The degradation rate of imazaquin was examined as a function of herbicide load (3.9, 8.9, 16.7 and 26.6 mg imazaquin/g mineral) and of time of grinding. Na–montmorillonite was not effective in degrading imazaquin over the whole range of ratios (Fig. 5). Hematite was very effective in degrading imazaquin in the lower range of loadings (3.9 and 8.9 mg/g), but the efficiency of degradation declined with the increase in the imazaquin/hematite ratio. This is expected due to the limited number of active sites on the surface of hematite, the particles of which lack, unlike montmorillonite, any internal surfaces. The more complicated dependence of the efficiency of Al–montmorillonite in degrading imazaquin on the reactant-mineral ratio (Fig. 5) may reflect the complex effect of the imazaquin–montmorillonite ratio on the extent of delamination of the layered mineral as a result of grinding and on the penetration of the herbicide into the interlayer spaces. In the framework of this study, the pressure applied by the pestle was estimated at 1500 ± 75 g cm − 2 applied for a contact time of 7.5 seconds during the 5 min of grinding. Prolonged grinding of imazaquin, up to 30 min, was performed using a ball mill. The prolonged grinding of imazaquin that was attempted with Na–montmorillonite,
Fig. 8. X-Ray diffractograms of: A. Na–montmorillonite, B. Cu–montmorillonite and C. Na–montmorillonite ground with CuCl2 (Nasser et al., 2009).
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Al–montmorillonite and hematite yielded a similar extent of degradation to that achieved after 5 min of manual grinding. This finding suggests that a plateau in the degradation process was reached after grinding for a relatively short duration of a few minutes. Mineral-herbicide mixtures that were wetted to 30% moisture content with an aqueous solution (0.1 N HCl, 0.1 N NaOH, 1 N chloride of Ni 2 +, Cu 2 + or Fe 3 + or distilled water) during the grinding process were also investigated. The catalytic efficiency of hematite
145
was reduced in the presence of water (Fig. 6). The presence of free water diminishes the polarity and hence the dissociability of the minerals' outermost shell of water of hydration, and hence the BrÖnsted and Lewis acidity as well as basicity of the potentially catalytic sites at the minerals' surface. Wetting, therefore, lowers the capacity of minerals to enhance many transformations of organic compounds. On the other hand, wetting birnessite, which in the dry state was less efficient than both Al–montmorillonite and hematite in
Fig. 9. Possible transformation products of imazaquin as detected by LC-MS analysis (Nasser et al., 2009).
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degrading imazaquin, did improve somewhat the mineral's catalytic efficiency. The addition of either HCl or NaOH did not seem to strongly affect the minerals' degradation efficiency (Fig. 6). When a solution of Ni 2 +, Cu 2 +or Fe 3 + chloride was added during the grinding of imazaquin with the minerals, the most effective in degrading imazaquin was the addition of Cu 2 + to the imazaquin/ Na–montmorillonite mixture. Addition of iron solution to the minerals also improved their catalytic efficiency in degrading imazaquin during wet grinding (Fig. 7), but to a lesser extent than the addition of Cu 2 +. The addition of Cu 2 + to the imazaquin/Na–montmorillonite mixture enabled degradation of more than 90% of the initial imazaquin during the 5 min of grinding (Fig. 7). When imazaquin was ground after being wetted with the solution of CuCl2 in the absence of the mineral, no degradation of imazaquin was detected. Yet, when Cu–montmorillonite was exposed to either dry or wet grinding together with imazaquin, the extent of imazaquin degradation was similar to that achieved by grinding with Na–montmorillonite to which CuCl2 was added (Figs. 4 and 7). This finding suggests that Cu–montmorillonite might have been formed during the wet grinding of Na–montmorillonite to which a Cu 2 + solution was added, and that the formed Cu- montmorillonite is the species responsible for the degradation of imazaquin. Support for this hypothesis is lent by XRD (Fig. 8) of Na– and Cu–montmorillonite and of Na–montmorillonite ground with Cu 2 + (wet grinding). All three patterns indicate the presence of montmorillonite and of some quartz. The patterns of Na–montmorillonite and Cu–montmorillonite differ by a slight displacement of the peak maximum from 28.52° (Na-mont.) to 28.81° (Cu-mont.) and by Cu–montmorillonite having a sharper peak than Na–montmorillonite at the vicinity of 7.06°. In addition, Cu–montmorillonite contains traces of copper chloride as indicated by the low peaks at 16.19° and 15.5°. Na–montmorillonite ground with Cu 2 + shows an enlargement and a broadening of the peak at 7.06°. Moreover, Na–montmorillonite ground with Cu 2 + contained halite (NaCl), identified by the two peaks at 27.45° and 31.82°, which was
Fig. 10. (a) Concentrations of atrazine (AT), DDA, DIA, and DEA in methanol extracts obtained during the reaction of atrazine on birnessite (BIRN) and (b) cryptomelane type II (CR-II) (from Shin et al., 2000).
not present in Na–montmorillonite (Fig. 8). Formation of halite in this sample is further evidence for the partial exchange of Na by Cu as the counter ion of the montmorillonite during the grinding of Namontmorillonite with CuCl2. LC-MS analysis revealed that the mechanochemical transformation of imazaquin resulted in the formation of a dimer and several breakdown products (Fig. 9). The variety of products detected indicates that several pathways for transformation of imazaquin occurred simultaneously, including dehydroxylation, demethylation, decarboxylation and condensation. Similarly, a host of metabolites was identified as photodegradation products of imazaquin in water (Barkani et al., 2005).
5. Atrazine Shin et al. (2000) reported the mechanochemical degradation of atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine; the molecular structure is presented in Appendix 1) adsorbed on four synthetic manganese oxides. Atrazine, a herbicide, is an important pollutant, detected in water systems worldwide. Its reactivity with soil minerals is thus of obvious importance. Synthetic pyrolusite (βMnO2), birnessite (δ-MnO2), and cryptomelane types I and II (αMnO2), were used as the degrading solid media. The authors followed the decomposition products that were formed after deposition of atrazine onto the Mn oxide surfaces and grinding the atrazine/oxide mixtures manually for 5 min using an agate mortar and pestle. The interaction of atrazine with birnessite produced (Fig. 10) a considerable amount of the metabolites DDA (didealkylatrazine), DIA (deisopropylatrazine), and DEA (deethylatrazine). However, the concentrations of the metabolites observed did not correspond to the amount of the parent atrazine that disappeared, apparently because a substantial amount of CO2 was produced over time as well. The degradation of atrazine also produced Mn(II), but a positive correlation between the amount of Mn(II) produced and atrazine degraded was found only for cryptomelane II (Fig. 11), suggesting that the dominant pathway for the decomposition of atrazine at the surface of this mineral involved the oxide serving as an electron acceptor in a reaction that yields Mn(II).
Fig. 11. Disappearance of atrazine (ATZ) and release of Mn(II) during the interaction of atrazine with cryptomelane type II (from Shin et al., 2000).
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1.0
0.8
0.8
Pyrene (M/Mo)
Pyrene (M/Mo)
a 1.0
0.6
0.4
0.2
b
0.6
0.4
0.2
0.0
0.0 no grinding
manual grinding
no grinding
1.0
0.8
0.8
Pyrene (M/Mo)
Pyrene (M/Mo)
c 1.0
0.6
0.4
manual grinding
d
0.6
0.4
0.2
0.2
0.0
147
0.0 no grinding
manual grinding
no grinding
manual grinding
Fig. 12. Fraction (M/M0) of pyrene remaining with or without 5 min of grinding. (a) Na–montmorillonite (b) birnessite (c) magnetite (d) Cu–montmorillonite. T0 refers to the instance immediately after grinding and T24 to 24 hours afterward (Joseph-Ezra, 2011).
6. PAH Polycyclic aromatic hydrocarbons (PAH) are important environmental pollutants with a relatively high resistance to microbial degradation. Due to their hydrophobic (i.e. low aqueous solubility) nature,
1.0
they are adsorbed strongly to the organic fraction of the soil's solid phase, thus further reducing their vulnerability to microbial degradation (Alexander, 2000; Mulder et al., 2001). Joseph-Ezra (2011) performed a detailed study on the mechanochemical degradation of certain polycyclic aromatic hydrocarbons, in particular pyrene and phenanthrene, under light grinding with soil minerals for short durations. Below is a summary of some of the results. Pyrene (C16H10) and phenanthrene (C14H10), are polycyclic aromatic hydrocarbons (their molecular structure is presented in
PHE (M/Mo)
0.8 0.6 no grinding
10
0.4
0.0 no grinding
manual grinding
Fig. 13. Fraction (M/M0) of the added phenanthrene (PHE) remaining on magnetite loaded with phenanthrene, with and without manual grinding for 5 min. T0 refers to the instance immediately after grinding and T24 to 24 hours afterward (Joseph-Ezra, 2011).
Frequency (%)
manual grinding
0.2
5
0 0.10
1.00
10.00
100.00
1000.00
Particle size (µm) Fig. 14. Particle size distribution of magnetite before and after grinding for 5 min.
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Appendix 1), that often contaminate soils (e.g., Harayama, 1997). They are resistant to most natural degradation processes and hence take a long time to dissipate (ATSDR, 1995). Napola et al. (2006) tested the efficacy of grinding by a ball mill in the remediation of a soil contaminated by phenanthrene and also investigated the influence of birnessite addition to the soil before grinding on phenanthrene removal. These authors obtained 20-50% removal and addition of birnessite to the soil did not affect significantly the extent of phenanthrene degradation. Joseph-Ezra (2011) tested samples of magnetite (Fe3O4), manganese oxide (birnessite, δ-MnO2), Na–montmorillonite and Cu–montmorillonite and a sample from the surface horizon of a sandy loam soil. The samples were loaded with pyrene and/or phenanthrene at various concentrations (up to 1,000 μg PAH/g mineral) and ground manually. Manual grinding was performed using a mortar and pestle for 5 min. The rate of degradation and the nature of the degradation products were determined by HPLC and LCMS. Na–montmorillonite and birnessite did not bring about the degradation of pyrene, with or without grinding, while grinding with Cu– montmorillonite did actually retard the degradation of pyrene in comparison to the rate of degradation of this PAH when incubated in the presence of Cu–montmorillonite without applying a mechanical force (the non-ground samples displaying considerable degradation of pyrene, ~ 50% in 24 h). Grinding pyrene with magnetite resulted in significant degradation (~50%), while in the non-ground samples no degradation was detected. The longer the grinding time, the higher was the rate of degradation. Magnetite has been found to be the most effective mineral in degrading pyrene by grinding (Fig. 12). Grinding with magnetite caused immediate degradation of both pyrene and phenanthrene. The degradation of phenanathrene when ground with magnetite is summarized in Fig. 13. The results presented in Figs. 12 and 13 (Joseph-Ezra, 2011) were obtained using a magnetite sample, the apparent diameter of 90% of the particles of which was 298 μm or less before grinding. After manual grinding, the size of 90% of the particles was reduced to 21 μm or less (Fig. 14). Thus, the enhanced degradation of the PAHs brought about by grinding was possibly due to the exposure of new, still active surface areas as the mineral's crystals were broken down. Accordingly, the extent of mechanochemical degradation of the PAHs on magnetite increased with the duration of grinding, while the particle size decreased and more active sites were exposed (Joseph-Ezra, 2011). Addition of 5% magnetite to the soil loaded with an initial concentration of 500 μg/g of each pyrene and phenanthrene and grinding did not induce significant degradation. However, adding water to the PAH-spiked soil/magnetite mixtures and then drying before grinding, resulted in a 10% degradation of phenanthrene and 16% degradation of pyrene. When magnetite was added to the contaminated soil, the PAH molecules were already adsorbed to the soil's solid phase, and therefore their contact with the mineral was hindered. The presence of water after wetting allowed the diffusive movement of the PAH molecules toward the magnetite particles, thus enabling the degradation process to occur.
HPLC runs displayed new peaks after grinding with magnetite. These peaks and the decline in the intensity of the peaks of the PAHs, are evidence of the degradation of the PAHs. LCMS analysis revealed that the major degradation mechanisms of pyrene and phenanthrene when ground with magnetite were redox processes. In the case of both pyrene and phenanthrene, oxidized products were detected (Table 3).
7. Summary and conclusions The most common methods for removing organic pollutants from the soil are based on biological degradation (e.g., Khan et al., 2004). However, biological procedures have certain shortcomings, including the fact that they may require considerable time or leave high concentrations of harmful residues in the soil (Mulder et al., 2001). Therefore, it may be advantageous to develop complementary soil remediation methods based on mechanochemical procedures. The data presented in this review demonstrated that light grinding of a variety of organic contaminants together with certain minerals is an effective way to bring about the contaminants' degradation. The mechanochemical approach can serve as a basis for remediation schemes for soils contaminated with numerous organic pollutants. And indeed, the interest in the discipline of mechanochemistry is fast expanding, as indicated by the volume of publications in that field in recent years. Mechanochemical procedures, such as light grinding for short periods of time (in the order of minutes) enabled easy and fast degradation of an assortment of organic pollutants ranging from 2,4-D and DCP, through imazaquin and atrazine to various PAHs. Among the minerals tested, Al– and Cu–montmorillonite, iron oxides and Mn oxides were shown to be the most effective heterogeneous catalysts (or more precisely reaction partners), but the relative efficiency of each of these minerals depends strongly on the target pollutant. The degradation rate of the pollutants is also strongly affected by the conditions under which the mechanochemical process takes place (e.g., moisture content). Although the presence of water often hinders the degradation process, in some cases the reverse is true. Thus, Na–montmorillonite in the presence of CuCl2 (the joint grinding of which resulted in partial replacement of Na by Cu as the montmorillonite's counter cation) efficiently enhanced the degradation of imazaquin when wet. This observation suggests a process of detoxification applicable to realworld polluted systems which are often wet, in which the naturally occurring Na–montmorillonite (or bentonite) serves, together with CuCl2, as the remediating agent.
Acknowledgements The authors are indebted to Julius Ben Ari for his performance and analysis of the LCMS runs and to Hadas Josef-Ezra for her invaluable work on the mechanochemical degradation of PAH.
Table 3 LCMS—detected degradation products of pyrene and phenanthrene after grinding with magnetite (Joseph-Ezra, 2011).
Pyrene degradation products
Phenanthrene degradation products
Retention time (min)
Molecular mass-detected compound, positive mode (APCI)
Molecular mass-detected compound, negative mode
Atomic composition of neutral molecule
9.11 7.15
219.08 235.07
217.07 (ESI) 232.05 (APCI) 233.06 (ESI)
C16H10O C16H10O2
6.37
233.06 235.07 195.08 211.07
5.5 6.5
C16H9O2 C16H11O2 C14H10O C14H10O2
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Appendix 1. List of compounds discussed in the review
Compound
Molecular formula
2,4-Dichlorophenoxyacetic acid (2,4-D)
C8H6Cl2O3
2,4-Dichlorophenol (DCP)
C6H4Cl2O
Atrazine
C8H14ClN5
Carbendazim
C9H9N3O2
Imazaquin
C17H17N3O3
Phenanthrene
C14H10
Pyrene
C16H10
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