Molybdenum is located in group VI, period V, of the transition series as a metallic element. The atomic number for this element is 42, and it has a relative atomic mass of 95.94 g/mol, with melting and boiling points of 2623 and 4639 °C, respectively. Because molybdenum metal does not freely exist in nature, it typically presents either as molybdenite (molybdenum disulfide: MoS
2), wulfenite (lead molybdate), or powellite (calcium molybdate). The direct mining of molybdenite, which is commonly undertaken, enables the metal to be obtained. Furthermore, the metal can also be recovered as a byproduct of copper mining
[23][24][25]. Molybdenum is commonly used in the industrial manufacturing of non-ferrous alloys, special steels, electrical contacts, X-ray tubes, spark plugs, tungsten production, glass-to-metal seals, and pigments. Due to several unique properties, molybdenum disulfide can be used as a lubricant additive, and molybdenum compounds are used in fertilizers or directly on seeds to mitigate molybdenum deficiency in crops
[26][27].
Pure molybdenum naturally exists as a silvery-white metal with variable oxidation states between 2 and 6, the most stable of which are Mo
4+ and Mo
6+ [28]. Because molybdenum primarily exists as molybdate anions (MoO
42−) in nature, it can be combined to form an assortment of polymolybdate compounds
[26][29][30]. Examples of these compounds that are soluble in an aqueous medium at room temperature are sodium molybdate (Na
2MoO
4), ammonium molybdate ((NH
4)
2MoO
4), and ammonium paramolybdate ((NH
4)
6Mo
7O
24.4H
2O). By comparison, molybdenum trioxide (MoO
3) is sparingly soluble, and other compounds such as molybdenite (MoS
2), calcium molybdate (CaMoO
4), molybdenum chloride (MoCl
5), and metallic molybdenum (Mo) are completely insoluble in an aqueous medium
[26][28]. Molybdenum is one of the important trace elements needed by most living organisms in daily life processes
[7]. Molybdenum is commonly present at the active site and plays a role as a cofactor to more than 50 enzymes involved in sulfur, nitrogen. and carbon cycles
[18][19]; these enzymes include aldehyde oxidase, nitrogenase, sulfite oxidase, and xanthine oxidase. In addition, molybdenum acts as an agent in electron transport
[19][28].
2. Identification of the Mo-Reducing Enzyme
The optimization of Mo-blue reduction for the purpose of bioremediation offers a method for the removal of molybdenum pollutants in addition to physical and chemical methods. Presently, molybdenum pollutants are chemically treated as a remediation technique
[31]. Microbial-based molybdenum bioremediation offers several advantages over chemical-based remediation. First, Mo-blue is colloidal and forms a precipitate with bacterial biomasses
[32]. This is suitable for bioremediation via entrapment with membranes, in which cells are either enclosed or immobilized. The main challenge in the elucidation of the mechanism of reduction is the identification of the enzyme. Though the enzyme has been purified and characterized in one bacterium
[33], the sequencing of the enzyme was not successful.
3. Conclusions
This review aimed to provide an update on the current knowledge regarding the molybdenum-reducing bacteria that have been isolated to date. This review also provided an update on the ability of some molybdenum-reducers to degrade other xenobiotics, which is a feature that needs to be studied and optimized in the future. A number of these xenobiotics, such as amides, can be used as electron donors for molybdenum reduction, and the ability of other xenobiotics to act as electron donors should be studied in the future. The inhibitory effect on the molybdenum reduction of cationic heavy metals, such as mercury, copper, and silver, is also seen in other anionic reductions, such as chromate and arsenate microbial reductions. Thus, means to combat this issue need to be studied for real remediation applications in the future. The true identity of the molybdenum-reducing enzyme is currently unknown, and the future sequencing of the purified enzyme could reveal the underlying mechanism behind the reduction process. Inhibition kinetics studies have shown that molybdate is toxic at high concentrations. Thus, the limits of the capability of bacteria to remediate sites highly contaminated by molybdenum, in addition to molybdenum-rich effluent from mine tailings, should be studied. The activation energy for molybdenum reduction also needs to be explored in future studies to distinguish molybdenum from other anionic metal reductions and other xenobiotic degradation processes.