Molybdenum (Mo) and tungsten (W) are proven to be important environmental contaminants. The understanding of the sorption mechanism of molybdenum and tungsten on mineral sorbents can enable the development of methods for their effective removal from aqueous solutions and their subsequent management. Mo and W play an important role in the chemical industry with the following applications:
Molybdenum is used as an alloying agent; as electrodes for electrically heated glass furnaces and fire hearths; in nuclear energy applications; as a catalyst in the refining of petroleum; in radio and light bulbs as a coupling element; and as flame- and corrosion-resistant coatings for other metals.
Tungsten is used as cemented carbide; as alloys; in electronics and electrical industries; in chemical applications; and as glass-to-metal seals. Tungsten oxides have two unique properties: intercalation and polycondensation. Thus, there is much opportunity for tungsten to find application in a fuel cell or energy-saving technologies in the future.
Mo and W are widely distributed in nature and occur in the form of various minerals, with the most common being molybdenite (MoS2
), wulfenite (PbMoO4
), powellite (CaMoO4
), wolframite ((Fe,Mn)WO4
), pinalite (Pb3
), and ferberite (FeWO4
). Weathering of rocks and sediments rich in Mo and W has resulted in their high concentration in groundwater [1
]. Moreover, the concentrations of Mo and W in the environment are significantly enhanced by anthropogenic inputs from coal-resource development, fly ash, sewage sludge, and hard rock mining activity [2
]. The environmental behavior of aqueous solutions containing molybdenum and tungsten is very complex because Mo(VI) and W(VI) anions occur as a monomer only in alkaline or neutral solutions. Under even slightly acidic conditions, they tend to polymerize in the form of isopoly molybdates and tungstates with possible biotic toxicity implications [3
]. Molybdates and tungstates usually coexist in the contaminated water, therefore, it is essential to understand Mo(VI) and W(VI) interactions in wastewater and to find an approach to efficiently remove these contaminations.
Smectites are a diverse group of clay minerals with a 2:1 layer silicate structure. Montmorillonite is the most common member of the smectite group, which has excellent sorption capacity for many heavy metals [5
]. A distinctive feature of the smectite group that determines its sorption potential is that water and other polar molecules can—by entering between the unit layers—cause the structure to expand in the direction normal to the basal plane. The smectite group is characterized by an overall negative charge. Therefore, all cations are easily absorbed onto their surface [8
]. These properties are determined by substitutions located in the tetrahedral and octahedral smectite layers, as well as by incomplete negative charges located on the edges of crystallites—adjacent to oxygen atoms and OH−
groups. However, natural smectites exhibit a weak affinity for the anionic forms of metals, which limits their use [9
]. Modification of smectite with organic compounds such as quaternary ammonium salts yields a material with improved sorption properties in terms of anionic form [6
To date, only the adsorption of molybdates on goethite and gibbsite has been extensively studied [13
]. However, in recent years, new, functionalized materials such as magnetic macroporous cross-linked copolymers of glycidyl methacrylate [14
] or thiol-containing organic molecule of pyrite [15
] have become increasingly popular in the immobilization of molybdates from aqueous solutions. In addition, an evaluation of the adsorption of mono- and polytungstates onto selected soil minerals (gibbsite, birnessite, kaolinite, illite, and montmorillonite) has been described by Sen Tuna [4
] and Iwai and Hashimoto [7
]. Tungstate has also been shown to adsorb strongly onto iron oxyhydroxide mineral surfaces [16
]. Other studies on the sorption of molybdenum and tungsten on a variety of materials have also been reported [20
]. However, despite many studies on the immobilization of molybdenum and tungsten on different sorbents, there is still a lack of information about the sorption of mono- and polymolybdates and tungstates onto modified clay, particularly in terms of sorption mechanisms.
The aim of this paper is to investigate the structural and textural features of organically-modified montmorillonites with adsorbed anions. In our earlier studies, we analyzed the influence of surfactant amount, kinetics, pH, and initial concentration of Mo, W on the sorption of molybdates and tungstates by organo-smectites [12
]. However, the proper understanding of the changes in structural and textural properties of used sorbents are crucial to finding an effective method for the disposal of both compounds. An additional challenge is to identify conditions in which the stable and nontoxic form of Mo(VI) and W(VI) will be achieved after the sorption process. This paper presents the results obtained after studying the features of organically-modified montmorillonite with absorbed molybdate and tungstate anions.
The sorption experiments revealed that Na-M has no sorption capacity for anions due to the negative charge of the surface; this finding is consistent with the observation of Murray [40
]. However, surface modification with surfactants caused a change in the surface charge and an increase in the sorption capacity of Na-M relative to anions. The negative surface of Na-M can be easily transformed into a positively charged one by replacing the metal ions with large organic cations, such as DDTMA and DDDDMA. The modification process includes the introduction of organic cations into ion-exchangeable positions. Thus, the negative electrostatic charge of structural layers are naturally compensated by cations adsorbed in the interlayer space [41
]. DDTMA-M is proven to have a better sorption capacity than DDTMA-M. It can be explained by the differences in the structure of these molecules. A previous study indicated that with an increasing amount of carbon chain and its length, the sorption efficiency decreases [42
], which is also proven by the results obtained in this study.
The sorption of Mo(VI) and W(VI) resulted in an increase in pH values. The reaction of molybdates and tungstates with OH−
groups in the structure of unmodified and modified montmorillonite resulted in alkalinization of the solution. Some researchers [43
] indicate that low-molecular-weight substances containing diol-groupings can form complexes with metal-oxyanions. For Mo and W, these hydroxy compounds can form bi- or poly-nuclear complexes. OH−
groups might form polyol-complexes with Mo and W anions [44
]. However, it is also possible that molybdates and tungstates may be present in the solution in the form of polyions developed from metal, water, and OH−
]. During sorption, a simple form of monoion is sorbed, and OH−
groups remain in the solution, thus alkalizing it. The immobilization of both Mo(VI) and W(VI) in the mixed solution is difficult due to the interference of these elements. Mo(VI) and W(VI) anions occur as a monomer only in alkaline or neutral solutions. Under acidic conditions, they polymerize to the form of isopoly molybdates and tungstates causing limited sorption of these anions from mixed solutions [46
]. The sorption of Mo(VI) and W(VI) is limited at pH > 5 and pH > 6, respectively [12
]. Mo-oxyanions are less stable than W-oxyanions and are more sensitive to pH change; further, with the increasing pH (from 3.5 to 5.5), the sorption capacity of molybdates decreased more rapidly than that of tungstate [44
]. This finding can explain the higher sorption capacity of W(VI) ions.
The present findings demonstrate that montmorillonites modified with DDTMA and DDDDMA are effective sorbents of Mo(VI) and W(VI). The most effective sorbent for the removal of Mo(VI) and W(VI) was DDTMA-M, which adsorbed 388 mmol/kg Mo(VI) and 537 mmol/kg W(VI). Literature data reveal, that removal of tungstate using popular sorbents such as gibbsite, goethite or birnessite was significantly lower than the results obtained in this study (201, 174 and 24 mmol/kg, respectively) [4
]. Previous studies also show, that for some sorbents, such as ferrihydrite, the sorption capacities obtained by different researchers differ significantly, form 120 mmol/kg to 420 mmol/kg [7
]. It was observed that popular sorbents are also less effective than organically modified montmorillonite. Goethite was able to immobilize 160 mg/kg of Mo(VI); gibbsite—200 mmol/kg; aluminum oxide—70 mmol/kg; kaolinite—1.5–3 mmol/kg; illite—3.75 mmol/kg [13
]. Resembling sorption capacities can be observed for drinking water treatment residues—324 mmol/kg; or iron oxide-coated sand—305 mmol/kg [49
The possibility of regeneration and reuse of different organically modified clay minerals have already been discussed in the literature. Desorption isotherms show apparent variability in desorption behavior among the different anions and also, among different concentrations of the same anion. In general, desorption did not appear to be completely reversible for any of the organically modified clay minerals; however, in many cases it was possible to desorb up to 70% of absorbed anion [51
The impact of surfactants on the structure of montmorillonite is confirmed by XRD results. The modification of montmorillonite with DDTMA increased the distance to 14.4 Å, while the addition of DDDDMA caused an increase up to 16.8 Å. This difference is probably caused by the size of the surfactant molecule—DDTMA has a shorter carbon chain, while DDDDMA has two long carbon chains attached to pentavalent nitrogen. According to the literature data, the thickness of the montmorillonite unit is 9.70 Å and the molecular dimension of DDTMA is approximately 3.8 Å in height and 18.0 Å in length when the alkyl chain of the DDTMA is parallel to the plane of montmorillonite [54
]. Thus, it is assumed that the obtained d-value indicates a horizontal arrangement of surfactant molecules in the interlayer spaces [56
Poor sorption capacity of Na-M regarding anions are confirmed—there are no new phases that could generate peaks on the diffractogram. For DDTMA-M with Mo and W adsorbed, there are visible changes in the diffractograms; some additional peaks are seen, which may suggest the presence of new crystalline phases in the sample. Analysis of the DDTMA-M-MoW pattern also indicates the presence of additional peaks, which may suggest the emergence of new crystalline phases. The speculation is based on the occurrence of two distinct peaks at 28.05 Å and 10.4 Å. The peak corresponding to the d001
value in the montmorillonite is 14.4 Å. The shift to higher d value suggests that the sorption processes resulted in a slight reduction in the interlayer distance. We assume that the reduction of the montmorillonite interlayer distance may be caused by the reaction of Mo(VI) and W(VI) with the organic cations. The emerging phase may generate a specific arrangement in the space between the packets, which occupies less space than the organic cation or the precipitation of the phase on the surface of the organo-montmorillonite, which was also suggested by Bajda and Kłapyta [6
]. Diffractograms of DDDDMA-M in which Mo and W was adsorbed contain additional peaks, which are comparable for all the diffractograms of DDDDMA-M with absorbed ions. This observation suggests the occurrence of similar phases in the structure of DDDDMA-M after sorption processes. Peaks corresponding to the characteristic plane (001) of montmorillonite (19 Å and 18.6 Å) indicate an increase in the distance between the organo-montmorillonite packets. It is probably caused by the precipitation of the phase in the interlayer space [6
]. However, it must be taken into account, that precipitation of anion salts on the surface is also possible. Previous studies concerning the immobilization of chromates, vanadates, molybdates and tungstates onto HDTMA-modified clay minerals suggest that precipitation is probable [58
]. Thus, we believe that similar phenomena may occur during the immobilization of molybdates and tungstates onto DDTMA-M and DDDDMA-M. However, there is no unambiguous proof of this. Mechanisms of immobilization of Mo(VI) and W(VI) are similar. The proposed mechanisms for molybdates are presented in Figure 7
The FTIR spectra of Na-M-Mo, Na-M-W, and Na-M-MoW prove that sorption of anions did not occur. There are no changes that could be related to the presence of new bonds and functional groups containing Mo or W. However, the occurrence of new bands on the spectra of DDTMA-M and DDDDMA-M with adsorbed Mo(VI) and W(VI) allowed for the conclusion that polymerized forms of molybdenum and tungsten ions were present in the sample in the form of the Mo7
ion or W7
]. A decrease in the intensity of the band connected with OH−
groups’ vibrations (the bands’ intensity for DDTMA-M and DDDDMA-M decreases compared with Na-M) indicates some reduction of interlayer water content caused mainly by surface hydrophobisation and by displacement of water molecules in the interlayer space by DDTMA+ and DDDDMA+ cations. A more visible effect in the total reduction of interlayer water content is observed for DDTMA-M and DDTMA-W. It is possible to assume that Mo(VI) and W(VI) precipitated with DDTMA in interlayer spaces of montmorillonite and completely removed the OH−
A significant difference in specific surface values of the modified montmorillonite is caused by blocking the access of the N2
probe to the pores in DDTMA-M and DDDDMA-M by large organic cations. The shape of all low-temperature sorption/desorption isotherms for all sorbents corresponds to type II [36
] which suggest that sorption is multilayered and unlimited in the range of high relative pressures. The initial shape of the isotherm for Na-M indicates monolayer adsorption, and at the inflection site of the isotherm, multilayer adsorption began. The inflection points of the isotherms for DDTMA-M and DDDDMA-M are not clear. This indicates that in the beginning, the monolayers and the formation of multilayers on the surface of the adsorbent occurred [60
]. In the range of relative pressures below p/p0 ~0.45, micropores are filled, and above this value, meso- and macropores are filled. For all sorbents, hysteresis loops are visible, which are directly related to the capillary action of liquid nitrogen occurring in the mesopores [36
]. Type H3 of the hysteresis shape indicate that Na-M, DDTMA-M, and DDDDMA-M are materials characterized by slit-shaped pores [61
A nitrogen sorption/desorption isotherm constructed for DDTMA-M also corresponds to type II isotherms [36
]. In the range of low relative pressures, multilayer adsorption on the materials was found because of the lack of pronounced inflection of the sorption curves. Monolayers consisting of N2
molecules formed a multilayer [60
]. An interesting phenomenon was observed for DDTMA-M-MoW. The sorption curve has reached values below zero. This may indicate the evolution of nitrogen from the sample during analysis. This was possible because nitrogen is one of the components of the DDTMA molecule.
Analysis of high-resolution XP spectra was performed in correlation with the Na-M surface state. The C 1s spectrum changes are the effect of surface treatment of all the analyzed samples. Two spectrum features were then observed, namely the component assigned to carbonates disappeared and an additional component appeared at approximately 283.3 eV. This additional component was ascribed to carbon–metal bonding (possibly through oxygen or nitrogen) that indicated the presence of the DDTMA-metal system at the Na-M surface. Additionally, the B peak intensity increased and the peak maximum shifted to approximately 286.5 eV, which indicated the presence of C-O and C-N bonding after the treatment.
An analysis of O 1s core excitation revealed that the performed treatment resulted in a pronounced increase in the A peak assigned to hydroxyl groups at the cost of a decrease in the B peak (O-Si bonds). Such behavior could indicate preferential adsorption of DDTMA compounds on O-Si-bonds. The adsorption process resulted in the appearance of an additional peak at approximately 529.0 eV. This peak could indicate the presence of defected metal-oxide systems at the treated surfaces. The change in intensities of O 1s spectrum components was correlated with the metal ions used in the treatment. It was found that Mo(VI) ion adsorption resulted in a larger concentration of hydroxyl groups (lower O-Si amount) at the surface than in the case of the W(VI) ion system. This behavior indicates a different mechanism of Mo(VI) and W(VI) ion adsorption at the Na-M surface. The electronic composition of Al 2p core excitation was found to be stable and comparable to the one at the Na-M surface for all the treated surfaces. In the case of Si 2p spectra, the change caused by Mo(VI) ions was strong and resulted in an increase (from 4.4% to 22.8%) in the intensity of the component at 102.1 eV. This component was ascribed to Si-O–metal bonding and indicated adsorption of Mo(VI) ions on Si-O groups. Application of W(VI) ions in the treatment resulted in increased hydroxylation of silicate ions. When the system of both ions was used for the treatment, the Si 2p line showed mainly (95.1%) the basic silica state at the treated surface.
The SEM method enabled the observation of new phases on the surface of modified montmorillonites with absorbed ions. In the SEM image of DDTMA-M-Mo, incrustations on the lamellar edges of montmorillonite are visible. To summarize the results so far, it is possible to state that it is an inorganic–organic salt. The source of organic salt components is DDTMA cation and inorganic molybdate ion [6
]. In the SEM image, there are visible inlays of these salts on the surface of DDTMA-M. Their presence affected the morphology of the montmorillonite surface, and the disappearance of the tissue structure can be observed. A similar image was obtained for a DDTMA-M-W, however, the inorganic–organic salt consisting of the tungstate anion and the DDTMA cation takes the form of a “coating” on the surface of the montmorillonite. A different morphology of DDTMA-M-Mo and DDTMA-M-W is caused by the effect of different amounts of these ions being absorbed on DDTMA-M. DDTMA-M shows a higher sorption capacity for W(VI) than for Mo(VI). A large amount of adsorbed W(VI) ions resulted in the formation of inorganic–organic salt covers on the montmorillonite surface. For DDTMA-M-MoW, a similar morphology in the form of covers on the surface of DDTMA-M can be observed. The form of salt precipitations on organo-montmorillonite depends on the availability of sites for crystallization. The salt crystallized in areas where organic cations are found. On the less accessible surface, ions formed single concentrations [6
In the SEM image for DDDDMA-M-Mo, larger forms of salt precipitations were visible than those present on DDTMA-M-Mo. The form of the precipitations indicates a lower availability of the montmorillonite surface to crystallize the salt; this was the case for DDTMA-M. The lower effectiveness of DDDDMA to modification of Na-M is related to the smaller amount of organic cations present in/on the organo-montmorillonite, and thus fewer available places are accessible for the crystallization of organic–inorganic salt. For DDDDMA-M-W, the precipitation of salts is visible. W(VI) sorption on DDDDMA-M was three times higher than Mo(VI) sorption. Thus, the precipitation of an inorganic–organic salt consisting of a tungstate anion occupied a larger area than the precipitation of salt with Mo(VI). The SEM image for DDDDMA-M-MoW, shows a crystallized inorganic–organic salt; however, despite the crystallization of salt, it does not has an impact the montmorillonite tissue structure.