Magnesium

Typical wastewaters

• digester supernatant and sludge dewatering liquors Mg²⁺ with ammonium and phosphate released in digestion, precipitating struvite MgNH₄PO₄ above about pH 8
• desalination brine Mg²⁺ concentrated from 1290 mg/L in seawater; magnesium compounds are recovered from seawater and brines
• reverse osmosis concentrate Mg²⁺ concentrated with the hardness; Mg(OH)₂ scaling at high pH held off with antiscalant
• ion exchange softener regenerant Mg²⁺ and Ca²⁺ in spent NaCl brine
• textile dyeing (magnesium sulfate mordant) Mg²⁺ with sulfate from the mordant

1 · Identity

Symbol, number

Mg, 12

Oxidation states in water

+2 only, as the strongly hydrated Mg²⁺ ion. It hydrolyses only above pH 9, precipitates as Mg(OH)₂ (brucite) above about pH 10.5, forms carbonate and sulfate ion pairs in hard and saline water, and unlike calcium does not precipitate as a simple carbonate from fresh water. The element entry has the tightly bound hydration and the chlorophyll centre.

Note

The element entry carries the ores, the metal, seawater and brine extraction and the industrial magnesia uses. This chapter is magnesium as hardness, as scale, as struvite and as a reagent. The carbonate equilibria are in the carbon chapter; calcium carbonate softening is in the calcium chapter with the shared equations kept there; ammonium and phosphate point here for struvite.

2 · Occurrence in water

Natural sources

Dissolution of dolomite, magnesite, olivine and serpentine and of magnesium in sedimentary rocks, seepage and runoff from soils (WHO hardness document); seawater at about 1290 mg/L, the third most abundant ion after chloride and sodium, and evaporite brines (element entry). Magnesium is present in natural groundwater usually at lower concentrations than calcium, from negligible to about 50 mg/L and rarely above 100 mg/L (WHO).

Anthropogenic sources

Magnesium hydroxide and magnesium oxide dosed for pH control and as alkalinity, magnesium sulfate and chloride in industrial processes (the element entry lists the sulfate as a dye mordant), desalination brine, dolomitic lime, magnesium in digester supernatant and dewatering liquors where it meets ammonium and phosphate.

3 · Speciation

Magnesium is Mg²⁺ across the pH range of natural water, with MgSO₄, MgHCO₃⁺ and MgCO₃ ion pairs in mineralised and sea water. It hydrolyses to MgOH⁺ only above pH 9 and precipitates as Mg(OH)₂ above about pH 10.5, a pH unit higher than calcium carbonate, which is why magnesium hardness needs the excess lime and the higher pH of the softening process. Magnesium carbonate does not precipitate from fresh water (it is far more soluble than calcite and forms hydrated salts only in brines), so magnesium hardness is removed as hydroxide, not carbonate. Where ammonium and phosphate are both present, as in anaerobic digester liquors, magnesium precipitates as struvite, MgNH₄PO₄ hexahydrate, above about pH 8 (Metcalf and Eddy chapter 15).

Solubility

Magnesium chloride, sulfate, nitrate and bicarbonate are freely soluble; magnesium hydroxide is sparingly soluble and controls magnesium above pH 10.5; magnesium carbonate is much more soluble than calcium carbonate and does not control magnesium in fresh water; struvite is sparingly soluble at alkaline pH. No solubility product is quoted because none was read this session.

Hydrolysis

Mg²⁺ is a very weak acid; MgOH⁺ forms only above pH 9 and Mg(OH)₂ precipitates above 10.5. Magnesium oxide hydrates to the hydroxide in water.

Complexation

Sulfate, bicarbonate and carbonate ion pairs; magnesium binds phosphate and organic ligands more weakly than calcium; in seawater about 10 percent of magnesium is paired with sulfate (Stumm and Morgan chapter 6, from the chapter). Constants not quoted.

Precipitates

Mg(OH)₂ brucite (lime softening, caustic dosing, alkaline scaling); MgNH₄PO₄.6H₂O struvite (digester liquors, deliberate recovery); MgCO₃ hydrates and dolomite only in brines and over geological time; magnesium silicate in high silica boiler and cooling water (not read this session).

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Acidify with nitric acid to keep magnesium and calcium in solution when the sample warms or degasses; an unacidified hard water sample loses calcium carbonate, and a struvite forming liquor loses magnesium, on the bench. Filter digester liquors immediately for dissolved magnesium. Report hardness units clearly: mg/L as CaCO₃ is not mg/L magnesium (factor 4.12), and German, French and English degrees are still met.

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: the fourth most abundant cation in the body, needed by hundreds of enzymes; intake 250 to 350 mg/day (element entry); food supplies over 80 percent and drinking water 2 to 52 mg/day depending on hardness (WHO). Low magnesium status is implicated in hypertension, insulin resistance and cardiovascular disease, and there is evidence, debated and not proving causality, of a protective effect of water magnesium or hardness on cardiovascular mortality, with benefits where observed at about 10 mg/L magnesium and above (WHO hardness document). Magnesium with sulfate above about 250 mg/L each is laxative. Not toxic in water (element entry).

Bioaccumulation

Not applicable.

Ecotoxicity

No US EPA aquatic criterion for magnesium; hardness raises the toxicity thresholds of metals, which is why the metal criteria are hardness dependent (aluminium criterion by pH, hardness and DOC in the criteria table).

Flags

• Lime softening stoichiometry, pH end points, residual hardness and the Mg(OH)₂ sludge remark are cited to MWH chapter 22 from memory of the text, not re-read.
• The struvite conditions and the magnesium to phosphate ratio are from Metcalf and Eddy chapter 15 likewise.
• Nanofiltration rejection and the magnesium nutrient use are general practice, not from sources read.
• The conversion factors (4.12 mg CaCO₃ per mg Mg, 2.497 per mg Ca) are computed from molar masses here.
• No Abu Dhabi document limits magnesium in either medium; other GCC states not read.

Gaps

• No solubility products for brucite, magnesite or struvite are quoted; Stumm and Morgan and Metcalf and Eddy have them.
• No survey of magnesium in sewage, digester liquors or industrial effluents was read.
• No lime dose, sludge production or struvite reactor performance figures beyond the stoichiometry are quoted.
• The WHO 2009 monograph on calcium and magnesium in drinking water (public health significance) was not read; the 2011 hardness document cites it.
• Magnesium silicate scaling and magnesium in cooling water chemistry are not sourced here.
• Other GCC discharge standards were not read.
• The magnesia remineralisation and magnesium silicate stoichiometries are written here; WHO states the principle of restoring magnesium and the silicon chapter of the book carries the silica chemistry, neither prints these equations.

Sources

Identity

Name and symbol

Magnesium, Mg

Atomic number

12 protons

Position

group 2 · period 3 · s-block · alkaline earth metal

CAS number

7439-95-4

Atomic structure

Atomic mass

24.305 u

Electron configuration

1s² 2s² 2p⁶ 3s²
[Ne] 3s²

Electrons per shell

2, 8, 2

Valence electrons

2 outer shell

Physical properties

State at room temperature

Solid

Melting point

923 K (649.85 °C)

Boiling point

1,363 K (1,089.85 °C)

Density

1.74 g/cm3

Appearance

shiny grey solid

Thermal conductivity

156 W/(m·K)

Electrical resistivity

43.9 nΩ·m at 20 °C

Electrical conductivity

22.78 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

24.869 J/(mol·K)

Chemical properties

Oxidation states

+2

Electronegativity

1.31 (Pauling Scale)

Ionisation energy

7.646 eV
1st 737.7, 2nd 1,450.7, 3rd 7,732.7 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 141, covalent 141, van der Waals 173 pm

Ionic radius

Mg²⁺ 72 pm

Reactivity

An alkaline earth metal ([Ne] 3s2) that is intrinsically very reactive but shielded in air by a thin passivating film of magnesium oxide; ribbon and powder ignite easily and burn fiercely.

with water

Barely reacts with cold water behind its oxide film, but reacts with water vapour and steam to give the oxide or hydroxide and hydrogen; water must never be used on burning magnesium: $\ce{Mg + H2O -> MgO + H2}$

with oxygen, air

Tarnishes only slightly in air because of the passivating oxide; heated, especially when finely divided, it burns with a dazzling white flame to magnesium oxide, with some nitride from the air: $\ce{2 Mg + O2 -> 2 MgO}$

with acids

Reacts readily with dilute hydrochloric and sulfuric acid to the salt and hydrogen; with very dilute nitric acid hydrogen forms but is contaminated with nitrogen oxides: $\ce{Mg + 2 HCl -> MgCl2 + H2}$

with halogens

Reacts with the halogens to ionic dihalides such as magnesium chloride: $\ce{Mg + Cl2 -> MgCl2}$

Typical compounds

• MgO magnesium oxide magnesia; dead-burned magnesite for furnace bricks
• Mg(OH)₂ magnesium hydroxide milk of magnesia
• MgCl₂ magnesium chloride from brine and sea water; electrolysed to the metal
• MgSO₄.7H₂O magnesium sulfate heptahydrate Epsom salts
• MgCO₃ magnesium carbonate magnesite, the ore; dolomite with calcium
• C₂H₅MgBr ethylmagnesium bromide a Grignard reagent, from magnesium and bromoethane

Occurrence, production and use

Crustal abundance

2.33×104 milligrams per kilogram

Oceanic abundance

1.29×103 milligrams per liter

Occurrence and sources

The metal is now principally obtained in the U.S. by electrolysis of fused magnesium chloride derived from brines, wells, and sea water.

• dissolved in seawater about 1,290 mg/L; crustal estimate 23,300 mg/kg (Jefferson Lab figures via PubChem)
• magnesite (MgCO3), dolomite, brucite (Mg(OH)2) China, Russia, Brazil, Turkey, Austria, Slovakia, Spain, Greece; identified world magnesite resources 13 billion tonnes
• magnesium in seawater, well and lake brines world coastlines and saline lakes; US compounds in 2024 came from seawater in California and Delaware, well brines in Michigan and lake brines in Utah; resources in the billions of tonnes
• olivine, serpentine, forsterite, evaporites widespread; serpentine including asbestos-mine tailings is an unquantified but very large potential source of magnesia

Extraction, production

Electrolysis of fused magnesium chloride derived from brines, wells and seawater

$\ce{MgCl2 (l) -> Mg (l) + Cl2 (g)}$

The principal US route while it operated; a Utah smelter on Great Salt Lake brine is estimated to have stopped in 2022. Described in words only, no equation printed.

Reduction of magnesium oxide with silicon

Stated by the RSC as the other commercial route; the source names reactants only, so no stoichiometry is given here.

Magnesium compounds: magnesium hydroxide from seawater and brines; caustic-calcined and dead-burned magnesia from magnesite

About 78 percent of US magnesium compounds in 2024 were consumed as caustic-calcined magnesia, magnesium chloride, hydroxide and sulfates, the rest as dead-burned and fused magnesia and olivine for refractories (USGS, printed pp. 112 to 113).

Uses

Magnesium burns with a brilliant white light and is used in pyrotechnics, flares and photographic flashbulbs. Magnesium is the lightest metal that can be used to build things, although its use as a structural material is limited since it burns at relatively low temperatures. Magnesium is frequently alloyed with aluminum, which makes aluminum easier to roll, extrude and weld. Magnesium-aluminum alloys are used where strong, lightweight materials are required, such as in airplanes, missiles and rockets. Cameras, horseshoes, baseball catchers' masks and snowshoes are other items that are made from magnesium alloys.

Magnesium oxide (MgO), also known as magnesia, is the second most abundant compound in the earth's crust. Magnesium oxide is used in some antacids, in making crucibles and insulating materials, in refining some metals from their ores and in some types of cements. When combined with water (H2O), magnesia forms magnesium hydroxide (Mg(OH)2), better known as milk of magnesia, which is commonly used as an antacid and as a laxative.

Hydrated magnesium sulphate (MgSO4·7H2O), better known as Epsom salt, was discovered in 1618 by a farmer in Epsom, England, when his cows refused to drink the water from a certain mineral well. He tasted the water and found that it tasted very bitter. He also noticed that it helped heal scratches and rashes on his skin. Epsom salt is still used today to treat minor skin abrasions.

Other magnesium compounds include magnesium carbonate (MgCO3) and magnesium fluoride (MgF2). Magnesium carbonate is used to make some types of paints and inks and is added to table salt to prevent caking. A thin film of magnesium fluoride is applied to optical lenses to help reduce glare and reflections.

Uses include flashlight photography, flares, and pyrotechnics, including incendiary bombs. It is one third lighter than aluminum, and in alloys is essential for airplane and missile construction. The metal improves the mechanical, fabrication, and welding characteristics of aluminum when used as an alloying agent. Magnesium is used in producing nodular graphite in cast iron, and is used as an additive to conventional propellants.

It is also used as a reducing agent in the production of pure uranium and other metals from their salts. The hydroxide (milk of magnesia), chloride, sulfate (Epsom salts), and citrate are used in medicine. Dead-burned magnesite is employed for refractory purposes such as brick and liners in furnaces and converters.

• Transport and aluminium alloys: die castings for the automotive industry; alloying agent that improves the mechanical, fabrication and welding characteristics of aluminium, in aircraft, cars, packaging, laptops and power tools castings 65 percent of reported US consumption and aluminium-base alloys 22 percent of primary magnesium in 2024
• Iron and steel: desulfurisation of molten iron and steel, preferred over calcium carbide because carbide makes acetylene with water; nodular graphite in cast iron desulfurisation 6 percent of US primary magnesium consumption in 2024
• Refractories: dead-burned and fused magnesia bricks and liners for furnaces and converters; olivine about 22 percent of US magnesium compounds in 2024
• Chemicals and pharmaceuticals: Grignard reagents for organic synthesis; reducing agent for uranium and other metals, including beryllium from its fluoride; magnesium chloride, sulfate and hydroxide as chemical products; milk of magnesia and Epsom salts in medicine
• Textiles: magnesium sulfate as a mordant for dyes
• Environmental and water treatment: magnesium hydroxide for pH control as a substitute for lime; caustic-calcined magnesia, chloride and hydroxide in environmental uses, the leading US market for magnesium compounds; magnesium chloride for road deicing
• Agriculture: magnesium oxide in cattle feed and fertilisers

Safety, toxicity

Because serious fires can occur, great care should be taken in handling magnesium metal, especially when finely divided. Water should not be used on burning magnesium or on magnesium fires.

GHS classification, signal word Danger

• H250 Catches fire spontaneously if exposed to air Pyrophoric liquids
• H260 In contact with water releases flammable gases which may ignite spontaneously Substances and mixtures which in contact with water, emit flammable gases
• H228 Flammable solid Flammable solids
• H252 Self-heating in large quantities; may catch fire Self-heating substances and mixtures
• H261 In contact with water releases flammable gas Substances and mixtures which in contact with water, emit flammable gases
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
• H315 Causes skin irritation Skin corrosion/irritation
• H319 Causes serious eye irritation Serious eye damage/eye irritation

Discovery and name

Discovered by

Joseph Black

Discovered

1755

First isolated

Humphry Davy

Named by

not in sources

Origin of the name

after Magnesia, Greece

Magnesium is a light, silvery-white, and fairly tough metal. It tarnishes slightly in air, and finely divided magnesium readily ignites upon heating in air and burns with a dazzling white flame.