The concentration of molybdenum in unpolluted fresh surface water is usually low. Indeed, it is about 0.5 μg/dm3 in river water, whereas it is no more than 10.0 μg/dm3 in marine and oceanic water [1–3].

Nevertheless, the problem of natural water pollution with molybdenum is important. The maximum permissible concentrations (MPCf = 1 μg/dm3) for molybdenum are significantly high in water bodies located near the mining, mineral processing, petrochemical, and metallurgical industries. The concentration of molybdenum in the wastewater of copper smelters, nonferrous metal processing plants, mines, and enrichment plants is 40–50, 50–60, 100–8000, and 2500–4000 μg/dm3, respectively.

The mining enterprises located in Kirovsk (Murmansk oblast), for example, have a significant anthropogenic impact on surface water. The main sources of pollution are discharges of mine and quarry water from mines, as well as wastewater from enrichment plants. MPC in wastewater is significantly high for a number of components (Mo, F, and Al); however, one of the main pollutants of natural waters is molybdenum found near the extraction and processing of apatite-nepheline ores in an amount CMo = 80–200 μg/dm3, which is significantly higher than that both for fishery ponds and the background values.

This deposit of apatite-nepheline ores is assigned to the Khibiny massif, in which there is a molybdenite (MoS2) having no industrial importance. Natural geological conditions are violated and the redox conditions are changed during its development, which leads to the oxidation of molybdenite and leaching of molybdenum. Sodium water and pH values shifted to the alkaline region facilitate the transition of molybdenum from an insoluble form to a solution.

The main methods for the purification of wastewater from molybdenum ions are reverse osmosis, ultrafiltration, electrodialysis, sorption, and ion exchange [4–6].

The electrodialysis and purification processes with osmotic membranes have some disadvantages despite their high efficiency. Firstly, electrodialyzers and reverse osmosis plants have a high cost and require high-energy consumption. Secondly, they require thorough water treatment, so that they are inappropriate for the purification of large tonnage mine waters with a multicomponent composition.

Reagent purification methods of wastewater from molybdenum implies the conversion of molybdenum to an insoluble form and its subsequent removal from the solution. Their disadvantages include the low degree of extraction of molybdenum from dilute solutions, which does not allow reaching the MPCf values for water to be purified. In addition, the significant consumption and high cost of the reagents together with the bulkiness of equipment make it impossible to use them to purify mine water from molybdenum to the standard values.

The published data showed that sorption and ion-exchange purification methods from molybdenum are currently most widely used, because they possess environmental and economic efficiency [7, 8]. Nevertheless, the current purification technological methods based on sorption and ion exchange require substantial refinement, because wastewater from mining enterprises has specific properties (multicomponent, multitonnage, and irregularity of inflow). Currently, the sorbents based on production waste are considered to be promising, because this leads not only to a solution of environmental problem but also to a significant decrease in the cost of the final product [9].

EXPERIMENTAL

The hydrochemistry of Mo depends on its forms in solutions at certain E and pH values; Fig. 1 shows the diagram of various molybdenum forms depending on E and pH. This diagram allows predicting the occurrence of molybdenum in general terms [10].

Fig. 1.
figure 1

Diagram of various molybdenum forms depending on E and pH.

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Molybdenum migrates in groundwater in a hexavalent form. Molybdenum exists as its acid compounds (H2\({\text{Mo}}_{4}^{0}\), \({\text{HMoO}}_{4}^{ - }\), and \({\text{MoO}}_{4}^{ - }\)) in oxygen-containing groundwater at pH > 2. Most part of molybdenum in natural waters is in a dissolved state, because its oxoanions (\({\text{HMoO}}_{4}^{ - }\) and \({\text{MoO}}_{4}^{{2 - }}\)) have a weak affinity to the solid surface [2, 11].

The chemical properties of molybdenum (solubility of its oxides, molybdenum acid, and molybdenum compounds with sodium) determine its significant migration ability and accumulation in natural waters within large E and pH values in various chemical compositions of groundwater.

A study of the hydrochemistry of molybdenum showed that molybdenum is coadsorbed onto many hypergene neoplasms and especially with iron hydroxides [2, 11].

The wastes containing reactive iron are assigned to those from the chemical industry, metallurgical production, and metal processing waste. Metal wastes are considered to be promising as a sorbent, because they are cheap and available iron-containing material, which is obtained at most industrial enterprises.

The St3sp1 unalloyed steel chip sample was used in the laboratory studies. The chips were moistened with distilled water and then kept in air for 24 h to obtain oxidized iron forms with a developed specific surface area and increased reactivity.

The sorption ability of modified sorbent relative to molybdenum was assessed on model solutions under static conditions. Chemically pure ammonium molybdenum acid (NH4)6Mo7O24 · 4H2O, which gives the molybdate ion \({\text{MoO}}_{4}^{{2 - }}\) during dissociation, was used to prepare model molybdenum solutions. The model solutions were prepared with a concentration of СMo = 100 μg/dm3, corresponding to the average amount of molybdenum in the mine waters of the production facilities.

Steel chips were placed into a model solution in a mass ratio S : L = 1 : 250 while stirring on a KS 260 basic IKA orbital shaker (at a stirring rate of 300 rpm) for 20 h. The studies were performed at 18°C and a pH of 5.2. The concentration of molybdenum was found before and after purification on an AAS-7000 atomic absorption spectrometer with electrothermal atomization. As a result, we found that the efficiency of purification of model solutions from molybdenum on a metal waste sorbent is more than 99%.

RESULTS AND DISCUSSION

An St3sp1 unalloyed steel sample was used to identify the mechanism of purification of model solutions from molybdenum with iron-containing metalworking waste. Iron corrosion products have a different composition depending on the corrosion conditions. Fe(OH)3, Fe(OH)2, FeOOH, FeO, Fe3O4, and Fe2O3 appear frequently [12]. Iron corrosion products are evenly distributed over the entire metal surface, have a loose structure, and occupy a volume that is 2.5–3 times larger than that of corroded steel.

A steel chip sample (1 g) was placed in 250 cm3 of deionized water with a pH of 3, 5, 7, and 9 while being stirred for 72 h to obtain corrosion products. The precipitate obtained was separated from the solution, dried, and analyzed on an MS1104Em Mössbauer spectrometer with a 57Co(Cr) source, which makes it possible to study nuclear gamma resonance spectra on iron ions. The composition of the corrosion products at different pH values is almost the same but their mass varies quite strongly, because the rate of the corrosion process is different. The Mössbauer spectroscopy data indicate (Fig. 2) that the doublet is assigned to iron(III) oxide, whereas the sextets are attributable to iron(II) and iron(III) oxides; and in addition, the Fe2O3 and Fe3O4 is 32.2 and 67.8%, respectively.

Fig. 2.
figure 2

Mössbauer absorption spectrum of corrosion products of St3sp1 steel chips obtained at pH = 5.

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Pure iron(II) and chemically pure iron(III) oxides were used as reagents in the experiments to find the efficiency of the molybdenum purification of the model solutions with each of the corrosion products obtained. Samples of 1 g of FeO, Fe2O3, and Fe3O4 were placed in the model solutions with a molybdenum concentration of 1 mg/dm3 and a volume of 250 cm3 under stirring on a KS 260 basic IKA orbital shaker (the stirring rate is 300 rpm) for 20 h. The experiments were performed at 18°C and a pH of 5.2. Moreover, the model solutions were degassed with a water-jet vacuum pump and closed with a plug throughout the entire contact time to avoid the oxidation of FeO and Fe3O4 to Fe2O3. Then the solutions were sedimented for 30 min and centrifuged.

The concentration of molybdenum was found before and after purification on an AAS-7000 atomic absorption spectrometer. Table 1 shows the results of the analysis obtained after the calculation of the mean arithmetic purification efficiency from two parallel experiments. It indicates that the extraction of molybdenum ions proceeds in the presence of all iron oxides but a higher purification efficiency from molybdenum is achieved with iron(III) oxide.

Table 1.   Efficiency of molybdenum purification of model solutions with FeO, Fe3O4, and Fe2O3
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The model molybdenum solution (СMo = 1000 mg/dm3) was mixed with 1 g of a St3sp1 steel chip sample under stirring for 72 h to find the composition of the products after purification. The chips were preliminarily wetted with distilled water and then kept in air for 24 h for the oxidation processes to proceed.

The resulting precipitate was separated from the solution, dried, and analyzed on a Shimadzu XRD-6000 X-ray diffractometer.

The X-ray phase analysis showed that the product obtained after purification of the model solution from molybdenum is partially presented as an X-ray amorphous phase. The identification of a crystalline phase, however, indicates that it corresponds to the reference X-ray spectrum of Fe2(MoO4)3 (no. 31-0642 of the PDF ICDD file standard) according to the interplanar distances and line intensities (Fig. 3).

Fig. 3.
figure 3

Diffractogram of product obtained after purification of model solution from molybdenum and the library reference spectrum of Fe2(MoO4)3.

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The data obtained make it possible to assume that the mechanism of purification of model solutions from molybdenum on iron-containing metalworking waste is based on the chemisorption process, which proceeds with the formation of insoluble iron molybdates and their subsequent sorption on the developed corroded steel chip surface.

The gross analysis of the products obtained after the interaction between the molybdenum-containing solution and the corrosion products of steel chips, however, does not allow the identification of the structural features of the precipitate formed. As a result, electron microscopy was used to confirm the hypothesis and to describe the structure of the compounds formed. For this purpose, an St3sp1 unalloyed steel plate, previously treated with distilled water in air, was placed in the model solution with a molybdenum concentration of 1000 mg/dm3. The solution was stirred by shaking to intensify the formation of iron and molybdenum compounds on the surface of the plate. The experiment was performed for 7 days, and the plate was removed from the solution and dried in air. Visual inspection of the product showed that the upper layer of the sediment was presented as a loose ocher X‑ray amorphous phase, whereas the layer adjacent to the metal had a denser structure and darker color. Both layers were studied with scanning electron microscopy and energy dispersive analysis on a Tescan Vega TS electron microscope equipped with an Oxford Instruments INCAx-ACT energy dispersive microanalysis system (Fig. 4): region A corresponds to the lower sediment layer, whereas region B is assigned to the upper one.

Fig. 4.
figure 4

Electron microscopic image of the product obtained after interaction between iron and molybdenum-containing solution.

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We found that the mass ratio of Mo and Fe in the upper layer is 6.5 : 1.0, whereas it is 1.5 : 1.0 in the lower layer, which corresponds to iron(III) molybdate. The data indicate that iron(III) molybdate is formed only in the immediate vicinity of the surface of the steel plate, whereas the upper layers are enriched in molybdenum and contain iron only as a minor impurity.

The chemisorption isotherms were constructed to study the kinetics of the purification process of model solutions from molybdenum. St3sp1 steel chip samples with various specific surface areas (30, 25, 15, and 8 cm2/g) were used in the chemisorption. The specific surface area of the steel chip samples prior to the corrosion processes was found with a BET method on a Quantachrome NOVA 1000e high-speed specific surface and pore size analyzer.

Model solutions with the molybdenum concentration ranging from 0.1 to 1000 mg/dm3 were prepared to construct chemisorption isotherms. The steel chip samples with various specific surface area values (30, 25, 15, and 8 cm2/g) with a weight of 0.4 g were added to the model solutions (V = 100 cm3) under stirring on a KS 260 basic IKA orbital shaker at a stirring rate of 300 rpm for the equilibrium contact time to ensure the ratio S : L = 1 : 250.

The static exchange capacity (SEC) of the chemisorbent was calculated according to the standard method developed at the Scientific Research Institute of Mineral Raw Materials (STO RosGeo 08-002-98. Technological methods for studying mineral raw materials” [13]). The following balance equation can be used if a single injection of a sorbent g (g) and a certain amount of the treated water are known:

$${\text{SEC}}\,g + V\,{{C}_{{{\text{eq}}}}} = V\,{{C}_{{{\text{init}}}}},$$
(1)

where SEC is the static exchange capacity (mg/g), g is the mass of a dry sorbent (g), V is the volume of the solution added to a sorbent (dm3), Cinit is the concentration of the extracted component in the initial solution (mg/dm3), and Ceq is the residual (equilibrium) concentration of the extracted component after interaction with the sorbent for equilibrium time (mg/dm3).

The SEC under the given operation conditions of the experiment was found from the following balance equation:

$$\left[ {{\text{SEC}} = ({{C}_{{{\text{init}}}}} - {{C}_{{{\text{eq}}}}})V} \right]{\text{/}}g.$$
(2)

The chemisorption isotherms were constructed from the data obtained (Fig. 5).

Fig. 5.
figure 5

Isotherms of chemisorption process of molybdenum from model solutions on steel chips (T = 18°C) at a specific surface area: (1) 30, (2) 25, (3) 15, and (4) 8 cm2/g.

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Figure 5 indicates that the chemisorption is much higher for chips with a larger specific surface area than those with a lower specific surface area at the same equilibrium concentration. In addition, the isotherm curves of molybdenum chemisorption from a solution on steel chips with various specific surface areas are different. When the specific surface area is 25 and 30 cm2/g, the SEC increases sharply with an increase in the concentration of molybdenum in the initial solution. There is only a slight increase in the SEC for steel chips with a specific surface area of 15 cm2/g with an increase in the concentration of molybdenum in the initial solution (SЕC = 18 mg/g). When the specific surface area is lower (8 cm2/g), the SEC takes the form of a saturation curve with the maximum value (SЕC = 8.8 mg/g). This is due to the fact that a thin layer of iron molybdates is formed on the surface of steel chips with a small specific area at high concentrations of molybdenum in the initial solution, and the passivation proceeds, which prevents the formation of new oxidized iron forms and, as a consequence, the further purification process. In the case of preliminarily treated chips, the surface is initially more developed and the passivation process is slower, so that the SEC is higher. When the specific surface area of the chips is larger the corrosion process is faster than the formation of iron molybdates.

The studies showed that the rate of chemisorption of molybdenum on steel chips from solutions depends on the intensity of the formation of iron oxides to form insoluble molybdenum compounds. The total exchange capacity of the modified steel chips with a specific surface of more than 15 cm2/g is therefore limited only by its corrosion rate and can theoretically be achieved after its complete dissolution.

In addition, the desorption of molybdenum from chemisorption products was studied. For this purpose, the treated steel chips together with the precipitate obtained after the purification of the model solution from molybdenum were placed into an ultrasonic bath for 120 min at 45°C and 1 dm3 of deionized water. The resulting solution was removed from the ultrasonic bath and centrifuged; then the concentration of molybdenum was measured on an AAS-7000 atomic absorption spectrometer with electrothermal atomization, which appeared to be lower than the detection limit (less than 0.1 μg/dm3). These results indicate that there is no desorption of molybdenum from the chemisorption products.

As noted above, the precipitate obtained during the purification process is presented as a mixture of insoluble iron molybdates, oxides, and hydroxides, which can be considered as a commercial intermediate for the production of ferromolybdenum according to the known technologies [14–16]. Such use of the spent chemisorbent is environmentally effective, cost-effective, and allows us to get a valuable industrial product from production waste.

CONCLUSIONS

(1) We found that the mechanism of purification of model solutions from molybdenum with iron-containing metalworking waste implies the formation of iron(III) molybdate as a surface layer with strong adhesion to the steel surface in the first stages and the formation of amorphous layers enriched in molybdenum oxide on the surface of this molybdate(VI) in the following chemisorption stages.

(2) The study of chemisorption kinetics showed that the SEC of steel chips relative to molybdenum is limited by the passivation process of iron, which proceeds, when iron molybdates are formed on the surface of steel chips with a specific surface of less than 15 cm2/g. In addition, the total exchange capacity of the modified steel chips with a specific surface of more than 15 cm2/g is limited only by the corrosion rate and can theoretically be achieved after its complete dissolution.

(3) The precipitate formed during the purification process, existing as insoluble iron molybdates, oxides, and hydroxides, can be used as a raw material to produce ferromolybdenum.

(4) The laboratory studies indicate that chemisorbent based on iron-containing metalworking waste may be used for the purification of wastewater from molybdenum(VI) ions to the standard values, which will allows us to solve the problem in an environmentally efficient and economical way.