Molybdenum

Typical wastewaters

• mine drainage (molybdenum and copper porphyry mines) molybdate MoO₄²⁻ leached from molybdenite bearing waste and tailings; protonated HMoO₄⁻ and molybdic acid in acid mine water the US ore mining subpart limits Cd, Cu, Zn, Pb and As in molybdenum mine drainage but not molybdenum; finished water near US mines up to 200 µg/L (WHO)
• cooling tower blowdown and closed loop drains (molybdate corrosion inhibitor) molybdate MoO₄²⁻ dosed as sodium molybdate, leaving with blowdown Abu Dhabi limits molybdenum to 10 mg/L in trade effluent to sewer
• steel, hydrodesulfurisation catalyst and pigment plants molybdate MoO₄²⁻ in oxic effluent above pH 6 named by the element entry; no effluent figure read

1 · Identity

Symbol, number

Mo, 42

Oxidation states in water

+6 as molybdate, MoO₄²⁻, the stable form in oxic water at neutral and alkaline pH, with HMoO₄⁻ and H₂MoO₄ (molybdic acid) in acid water and polymolybdates only at high concentration and low pH; +4 as molybdenite (MoS₂) and MoO₂ under reducing conditions, where molybdenum is fixed as the sulfide.

Note

The element entry covers molybdenite, roasting and the alloy uses; this chapter is the anion in supplies, mine water and cooling systems.

2 · Occurrence in water

Natural sources

Oxidative weathering of molybdenite: molybdenum disulfide is sparingly soluble but readily oxidised to the more soluble molybdates, which are stable in water in the absence of a reducing agent (WHO). Molybdenum is the most abundant dissolved transition metal in seawater at about 0.01 mg/L.

Anthropogenic sources

Molybdenum mining and copper porphyry mines (drinking water up to 200 µg/L near mining operations, tap water up to 580 µg/L in Colorado, WHO); molybdate corrosion inhibitors in cooling and closed loop systems, which leave with blowdown; molybdenum in fertilisers and seed treatment; steel, catalyst (cobalt molybdenum hydrodesulfurisation) and pigment plants (element entry); sewage sludge from all of these.

3 · Speciation

Above pH 6 molybdenum is the molybdate dianion, unhydrolysed, unsorbed by most surfaces and as mobile as sulfate; in acid water it protonates to HMoO₄⁻ and molybdic acid and sorbs on iron and aluminium oxides in the manner of the other oxyanions. It has no redox chemistry in oxic water; under sulfidic, reducing conditions it is removed as molybdenum sulfide, which is why molybdenum is enriched in black shales and why sulfide reduction is the only chemical sink. Molybdate is a non oxidising anodic corrosion inhibitor: it adsorbs on the passive film of steel as Mo(VI) and competes with chloride without being reduced.

Solubility

Molybdenum disulfide is sparingly soluble; sodium and ammonium molybdates are freely soluble; calcium molybdate precipitates on steel in calcium hydroxide solution (JES 2016). No solubility products are printed in the sources read.

Hydrolysis

Molybdic acid dissociates stepwise to HMoO₄⁻ and MoO₄²⁻ in acid water; the constants are not printed in the sources read, so none are quoted.

Complexation

Molybdate competes with selenate for zero valent iron surfaces (WHO selenium document citing Zhang 2005) and by extension for iron oxide sorption sites. No constants quoted.

Precipitates

Molybdenum sulfide under reducing conditions; calcium molybdate in hard alkaline water; ferric molybdate is not documented in the sources read.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Molybdate is stable and needs only acidification for total molybdenum; filter 0.45 µm for dissolved. In cooling water samples the inhibitor dose (mg/L) dwarfs natural levels (µg/L), so dilution blanks matter.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Essential, estimated daily requirement 0.1 to 0.3 mg for adults; intakes 100 to 240 µg/day in the US; the human NOAEL of 0.2 mg/L in drinking water over 2 years underlies the WHO health based value, with concerns about that study's quality; EPA RfD 0.005 mg/kg per day (WHO, EPA).

Bioaccumulation

Not addressed in the sources read.

Ecotoxicity

Not addressed in the sources read; no US EPA aquatic criterion for molybdenum appears in the table.

Flags

• The natural water concentrations are 1940s to 1960s US surveys; WHO says more modern studies show much lower levels.
• The seawater figure (0.01 mg/L) is a single abundance figure from Jefferson Lab via PubChem.
• The molybdenite oxidation and calcium molybdate equations are mass balances written here; WHO and the JES abstract describe them in words.
• No removal process was read for molybdenum; the removal rows are analogies to selenate and arsenate and say so.
• The brief's former WHO guideline value of 70 µg/L is consistent with the 2011 revision noted in the fact sheet, but the WHO history document was not read.
• The corrosion inhibitor mechanism rests on one 2016 abstract in pH 12.5 calcium hydroxide, not on cooling water conditions.
• Abu Dhabi lists molybdenum only for sewer discharge (10 mg/L); other GCC states not read.

Gaps

• No treatment performance data for molybdenum were read; WHO's documents have no treatment section.
• Dissociation constants of molybdic acid, molybdate sorption constants and molybdenum sulfide solubility are not printed in the sources read.
• No industrial or municipal wastewater concentrations were read; the mine water figures are drinking water near mines.
• No ecotoxicity data or aquatic criterion was read.
• Molybdenum in sewage sludge land application limits and in EU or GCC discharge standards beyond Abu Dhabi were not read.
• The Standard Methods phosphate and silica methods are cited by number only.
• The thiomolybdate stoichiometry is the sum of the four stepwise sulfidations reported in the Erickson and Helz abstract; the full paper was not read and no stability constants are quoted.
• The heptamolybdate condensation is cited to Baes and Mesmer by chapter, from the chapter, not re-read.
• The 12:1 phosphomolybdate stoichiometry is standard analytical chemistry; Standard Methods 4500-P was cited by number and not re-read.

Sources

Identity

Name and symbol

Molybdenum, Mo

Atomic number

42 protons

Position

group 6 · period 5 · d-block · transition metal

CAS number

7439-98-7

Atomic structure

Atomic mass

95.95 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d⁵
[Kr] 5s¹⁴d⁵

Electrons per shell

2, 8, 18, 13, 1

Valence electrons

6 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

2,896 K (2,622.85 °C)

Boiling point

4,912 K (4,638.85 °C)

Density

10.2 g/cm3

Appearance

gray metallic

Thermal conductivity

138 W/(m·K)

Electrical resistivity

53.4 nΩ·m at 20 °C

Electrical conductivity

18.73 MS/m

Crystal structure

body-centered cubic

Molar heat capacity

24.06 J/(mol·K)

Chemical properties

Oxidation states

+6

Electronegativity

2.16 (Pauling Scale)

Ionisation energy

7.092 eV
1st 684.3, 2nd 1,560, 3rd 2,618 kJ/mol

Electron affinity

0.746 eV

Atomic radius

empirical 154, covalent 154, van der Waals 209 pm

Ionic radius

Mo³⁺ 69; Mo⁴⁺ 65; Mo⁵⁺ 61; Mo⁶⁺ 59 pm

Reactivity

A hard, refractory group 6 metal that shows no visible reaction with air or water at room temperature and has little tendency to form a simple cation in solution; it is attacked by the halogens, hydrogen peroxide and oxidising acids.

with water

Does not react with water at room temperature.

with oxygen, air

No visible reaction at room temperature; oxidises weakly from 300 C and in bulk above 600 C to the volatile trioxide: $\ce{2 Mo + 3 O2 -> 2 MoO3}$.

with acids

Resists non-oxidising acids; it is attacked by oxidising agents such as hydrogen peroxide and by nitric acid or aqua regia, which oxidise it towards molybdic acid.

with halogens

Attacked by the halogens; fluorine gives the stable hexafluoride: $\ce{Mo + 3 F2 -> MoF6}$, and chlorine gives the pentachloride: $\ce{2 Mo + 5 Cl2 -> 2 MoCl5}$.

Typical compounds

• MoS₂ molybdenum disulfide molybdenite, the ore; layered solid lubricant
• MoO₃ molybdenum trioxide from roasting the ore; reduced by hydrogen to the metal
• H₂MoO₄ molybdic acid parent of the molybdates
• (NH₄)₆Mo₇O₂₄ ammonium heptamolybdate soluble molybdate for catalysts and pigments
• MoF₆ molybdenum hexafluoride stable, volatile hexafluoride
• MoCl₅ molybdenum pentachloride dark green dimeric solid

Occurrence, production and use

Crustal abundance

1.2 milligrams per kilogram

Oceanic abundance

1×10-2 milligrams per liter

Occurrence and sources

Molybdenum is also recovered as a by-product of copper and tungsten mining operations. The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate.

• molybdenite (MoS2) in porphyry molybdenum deposits Climax and Henderson (Colorado); primary mines exist only in China and the United States
• by-product molybdenite in porphyry copper deposits Chile, Peru, Mexico, Arizona, Montana, Nevada, Utah
• crustal and oceanic abundance about 0.8 ppm (BGS figure via RSC); 1.2 mg/kg crust and 0.01 mg/L seawater (PubChem)

Extraction, production

Oxidative roasting of molybdenite concentrate to molybdic oxide

$\ce{2 MoS2 (s) + 7 O2 (g) -> 2 MoO3 (s) + 4 SO2 (g)}$

RSC states the ore is roasted to molybdenum oxide; USGS names the product as molybdic oxide (MoO3, 57 percent Mo) made at three US roasting plants; sulfur dioxide as the sulfur product is implied by roasting in air, not named. Rhenium is recovered from the roaster gases

Reduction of molybdic oxide to metal powder, ferromolybdenum and chemicals

$\ce{MoO3 (s) + 3 H2 (g) -> Mo (s) + 3 H2O (g)}$

RSC states the oxide is then reduced to the metal but names no reductant; USGS lists ferromolybdenum, metal powder and chemicals (ammonium molybdates, pigments) as the intermediate products

Uses

Molybdenum has a high melting point and is used to make the electrodes of electrically heated glass furnaces. Some electrical filaments are also made from molybdenum. The metal is used to make some missile and aircraft parts and is used in the nuclear power industry. Molybdenum is also used as a catalyst in the refining of petroleum.

Molybdenum is primarily used as an alloying agent in steel. When added to steel in concentrations between 0.25% and 8%, molybdenum forms ultra-high strength steels that can withstand pressures up to 300,000 pounds per square inch. Molybdenum also improves the strength of steel at high temperatures. When alloyed with nickel, molybdenum forms heat and corrosion resistant materials used in the chemical industry.

Molybdenum disulfide (MoS2), one of molybdenum's compounds, is used as a high temperature lubricant. Molybdenum trioxide (MoO3), another molybdenum compound, is used to adhere enamels to metals. Other molybdenum compounds include: molybdic acid (H2MoO4), molybdenum hexafluoride (MoF6) and molybdenum phosphide (MoP2).

It is used in certain nickel-based alloys, such as the "Hastelloys(R)" which are heat-resistant and corrosion-resistant to chemical solutions. Molybdenum oxidizes at elevated temperatures. The metal has found recent application as electrodes for electrically heated glass furnaces and forehearths. The metal is also used in nuclear energy applications and for missile and aircraft parts. Molybdenum is valuable as a catalyst in the refining of petroleum. It has found applications as a filament material in electronic and electrical applications. Molybdenum is an essential trace element in plant nutrition; some lands are barren for lack of this element in the soil. Molybdenum sulfide is useful as a lubricant, especially at high temperatures where oils would decompose. Almost all ultra-high strength steels with minimum yield points up to 300,000 psi (lb/in.2) contain molybdenum in amounts from 0.25 to 8%. Biologically, molybdenum as a trace element is necessary for nitrogen fixation and other metabolic processes.

• Steel and alloys: alloying agent in cast iron, alloy and stainless steel and superalloys for strength, hardness, wear and corrosion resistance; heating elements, drills, saw blades, engine parts; little substitution possible principal use per USGS; no percentage given
• Chemicals (catalysts, pigments, lubricants): hydrodesulfurisation and other petroleum catalysts; molybdenum orange pigment; ammonium and other molybdates; molybdenum disulfide lubricant additive; circuit-board inks, electrodes
• Mining: primary porphyry molybdenum mines and by-product recovery at porphyry copper mines; molybdate in extractive-waste water
• Energy: strong demand expected from power generation and infrastructure projects
• Agriculture: essential micronutrient in nitrogenase and about 50 other enzymes

Safety, toxicity

GHS classification, signal word Danger

• H225 Highly Flammable liquid and vapor Flammable liquids
• H228 Flammable solid Flammable solids
• H361 Suspected of damaging fertility or the unborn child Reproductive toxicity
• H315 Causes skin irritation Skin corrosion/irritation
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation

Discovery and name

Discovered by

Carl Wilhelm Scheele

Discovered

1778

First isolated

Peter Jacob Hjelm

Named by

not in sources

Origin of the name

from Greek Μόλυβδος, 'lead', since its ores were confused with lead ores

The metal is silvery white, very hard, but is softer and more ductile than tungsten. It has a high elastic modulus, and only tungsten and tantalum, of the more readily available metals, have higher melting points. It is a valuable alloying agent, as it contributes to the hardenability and toughness of quenched and tempered steels. It also improves the strength of steel at high temperatures.