Mercury

Typical wastewaters

• chlor-alkali (legacy mercury cells, decommissioning) Hg(II) complexes and dissolved elemental mercury, treated to 3 to 15 µg/L at the outlet of the mercury treatment unit during decommissioning or conversion the mercury cell process was stopped in the EU on 11 December 2017 and cannot be BAT under any circumstances; contaminated sewers and sediments release mercury for years
• coal fired power (flue gas desulfurisation wastewater) Hg(II) in the scrubber purge, treated to 34 ng/L monthly and 103 ng/L daily under the 2020 US BAT limits raw FGD mercury implied in the microgram per litre range; older or smaller units 356 and 788 ng/L
• ore mining and dressing (copper, lead, zinc, gold, silver, molybdenum mines and mills) Hg(II) in mine drainage and mill water, limited at 1 µg/L monthly limit, not a measured concentration
• refineries and factories; landfill and cropland runoff inorganic Hg(II), the US EPA source list for mercury in drinking water no concentration read
• textile and leather wet processing Hg(II) as a ZDHC restricted metal, limited at 0.01 mg/L foundational, 0.005 progressive and 0.001 aspirational, with sludge thresholds of 1 mg/kg textile and 0.2 mg/kg leather a limit, not a measured occurrence

1 · Identity

Symbol, number

Hg, 80

Oxidation states in water

+2 as Hg²⁺, present in water almost entirely as neutral hydroxide and chloride complexes (Hg(OH)₂, HgClOH, HgCl₂) and, in sulfidic water, as HgS and bisulfide complexes; 0 as dissolved elemental mercury, volatile and lost to air; +1 as Hg₂²⁺ only in unusual chemistry; and organic mercury, above all monomethylmercury CH₃Hg⁺, formed by bacteria in sediments. Almost all mercury in uncontaminated drinking water is thought to be inorganic Hg²⁺ (WHO).

Note

The element entry gives the metal's reactions and the calomel, cinnabar and chloride compounds. In water the questions are which complex, whether it has been methylated, and how far below 1 µg/L a treatment can go.

2 · Occurrence in water

Natural sources

Weathering of cinnabar and mercury bearing sulfide deposits; local mineral deposits produce higher groundwater levels (WHO); rainwater carries 5 to 100 ng/L, with means as low as 1 ng/L (WHO background document, IPCS 1990). Methylation of inorganic mercury occurs in both fresh water and seawater (WHO).

Anthropogenic sources

Chlor-alkali mercury cells, the process the EU stopped on 11 December 2017 and the BAT conclusions declare not BAT under any circumstances; electrical appliances, dental amalgam and mercury compounds (WHO); refineries, factories, landfill and cropland runoff (US EPA source list); flue gas desulfurisation wastewater at coal fired power plants, regulated in the US at 34 ng/L monthly; ore mining and dressing at 1 µg/L monthly; artisanal gold amalgamation in the ledger.

3 · Speciation

Dissolved inorganic mercury in oxic fresh water is not the free ion but neutral Hg(OH)₂, HgClOH and HgCl₂, with the chloride complexes winning as chloride rises (HgCl₃⁻ and HgCl₄²⁻ in seawater); natural organic matter binds Hg(II) strongly at reduced sulfur sites, so most river mercury is organically bound or on particles. In anoxic sulfidic water mercury is HgS, one of the least soluble solids known, unless excess sulfide or organic thiols redissolve it. Bacteria in anoxic sediments methylate Hg(II) to methylmercury, the form that bioaccumulates; the balance between methylation and demethylation sets the methylmercury level (WHO).

Solubility

Mercury(II) chloride and nitrate are soluble; mercury sulfide has a very low solubility (WHO background document). No solubility product is printed in the sources read; the Stumm and Morgan table gives HgS among the least soluble sulfides (from the chapter, not re-read).

Hydrolysis

Hg²⁺ hydrolyses at low pH: Hg(OH)₂ is the dominant species above about pH 4 in chloride free water, which is why mercury behaves as a neutral, poorly sorbed molecule rather than a cation (Stumm and Morgan chapter 6, from the chapter).

Complexation

Chloride (HgCl⁺, HgCl₂, HgCl₃⁻, HgCl₄²⁻), hydroxide, sulfide and bisulfide, and the reduced sulfur groups of natural organic matter; constants not printed in the sources read.

Precipitates

HgS (cinnabar or metacinnabar) from sulfide precipitation and in anoxic sediments; Hg(OH)₂ and HgO only at high pH and high concentration; mercury co-precipitated on Fe(OH)₃ and Al(OH)₃ flocs in coagulation.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Preserve with bromine monochloride (EPA 1631E) or acid; mercury adsorbs to and volatilises from ordinary bottles, so use acid cleaned glass or fluoropolymer and follow Method 1669 clean hands technique. Discrete autosamplers contaminate at the ng/L level (EPA 1631E). Samples above 100 ng/L must be diluted; iodide above tens of µg/L interferes with the purge. Methylmercury needs a separate, unoxidised aliquot.

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

Inorganic mercury targets the kidney in humans and animals (tubular necrosis, proteinuria); acute oral poisoning causes haemorrhagic gastritis and colitis; mercury(II) chloride can raise benign tumours where tissue is damaged and is weakly genotoxic without point mutations (WHO). Mean dietary intake 2 to 20 µg/day; JECFA PTWI for total mercury 5 µg/kg body weight with no more than 3.3 µg/kg as methylmercury (1972, reassessed 1988 and 2003; WHO background document). Methylmercury is the neurotoxin of Minamata and of fish advisories.

Bioaccumulation

Methylmercury bioaccumulates and biomagnifies in aquatic food chains; the EU EQS in biota is 20 µg/kg wet weight, set because the water column value cannot protect predators and humans eating fish (Directive 2013/39/EU). Inorganic mercury in drinking water is not the exposure route of concern; food is (WHO).

Ecotoxicity

US EPA aquatic life criteria: freshwater 1.4 µg/L acute and 0.77 µg/L chronic, saltwater 1.8 and 0.94 µg/L (1995), with the caution that they may not protect where methylmercury is a substantial part of total mercury. EU EQS: 0.07 µg/L annual average and maximum in inland and other surface waters, biota 20 µg/kg.

Flags

• The occurrence figures are 1990s IPCS compilations in the WHO 2005 background document; no modern survey was read.
• The FGD and chlor-alkali numbers are limits, not measured effluent concentrations.
• The hydrolysis and chloride speciation equations are cited to Stumm and Morgan chapter 6 from memory of the text; the mechanism of methylation (sulfate reducers, hgcAB genes) is not sourced and is left as 'bacteria in anoxic sediments' as WHO puts it.
• The HgS redissolution equation is written here from the EPA statement about soluble mercury sulfide species at excess sulfide.
• The US steam electric mercury limits carry compliance dates and were revised in 2024; check applicability before quoting.
• Abu Dhabi values cover two media (marine 0.001 mg/L, sewer 0.5 mg/L); other GCC states were not read.
• The Minamata Convention text was not reachable this session; the chlor-alkali ban date is cited to the EU mercury regulation.

Gaps

• No source read gives mercury in seawater, municipal wastewater influent or specific industrial effluents as measured ranges; only limits and 1990s compilations are quoted.
• Solubility products, hydrolysis and chloride complexation constants and methylmercury formation rates are not printed in the sources read.
• The Minamata Convention (adoption, entry into force, Annex B dates) was not read; the EU regulation stands in.
• Mercury removal by chemical reduction (borohydride, stannous chloride) and by membrane extraction, described in the EPA capsule report chapters 5 and 6, was not read in detail.
• EPA 245.1 and 245.7 detection limits were not read; the WHO 0.05 µg/L figure stands for cold vapour AAS.
• Other GCC discharge standards (Saudi, Oman, Qatar) were not read.
• Bacterial methylation of Hg(II) to methylmercury is described by WHO as a pathway and no source read prints a stoichiometry or a rate for it, so no methylation equation is written.
• The thiol resin equation is written in the resin convention from the mechanism the EPA capsule report describes.

Sources

Identity

Name and symbol

Mercury, Hg

Atomic number

80 protons

Position

group 12 · period 6 · d-block · transition metal

CAS number

7439-97-6

Atomic structure

Atomic mass

200.592 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰
[Xe] 6s²⁴f¹⁴⁵d¹⁰

Electrons per shell

2, 8, 18, 32, 18, 2

Valence electrons

12 ns and (n-1)d

Physical properties

State at room temperature

Liquid

Melting point

234.32 K (-38.83 °C)

Boiling point

629.88 K (356.73 °C)

Density

13.5336 g/cm3

Appearance

shiny, silvery liquid

Thermal conductivity

8.30 W/(m·K)

Electrical resistivity

961 nΩ·m

Electrical conductivity

1.04 MS/m

Crystal structure

rhombohedral

Molar heat capacity

27.983 J/(mol·K)

Chemical properties

Oxidation states

+2, +1

Electronegativity

2 (Pauling Scale)

Ionisation energy

10.438 eV
1st 1,007.1, 2nd 1,810, 3rd 3,300 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 132, covalent 132, van der Waals 209 pm

Ionic radius

Hg⁺ 119; Hg²⁺ 102 pm

Reactivity

The only metal liquid at room temperature, mercury sits between the noble metals and its group 12 relatives zinc and cadmium in reactivity: it does not tarnish in dry air or react with water or non-oxidising acids, but it dissolves in oxidising acids, combines directly with sulfur and the halogens, and forms Hg(I) dimers as well as Hg(II) compounds.

with water

Does not react with water.

with oxygen, air

Does not tarnish in dry air at room temperature; heated near its boiling point it slowly oxidises to the red oxide: $\ce{2 Hg + O2 -> 2 HgO}$, which decomposes back to the elements near 400 C.

with acids

Unaffected by hydrochloric acid or dilute sulfuric acid; hot concentrated sulfuric acid dissolves it: $\ce{Hg + 2 H2SO4 -> HgSO4 + SO2 + 2 H2O}$, and nitric acid gives the nitrate: $\ce{Hg + 4 HNO3 -> Hg(NO3)2 + 2 NO2 + 2 H2O}$, with excess mercury giving the Hg(I) salt instead.

with halogens

Combines directly with the halogens on gentle heating; chlorine gives the sublimable mercury(II) chloride: $\ce{Hg + Cl2 -> HgCl2}$, while excess mercury gives calomel Hg2Cl2.

Typical compounds

• HgCl₂ mercury(II) chloride corrosive sublimate, violently poisonous white solid
• Hg₂Cl₂ mercury(I) chloride calomel, Cl-Hg-Hg-Cl with a metal-metal bond
• HgS mercury(II) sulfide cinnabar, the ore, and the pigment vermilion
• HgO mercury(II) oxide red oxide of mercury batteries, decomposes near 400 C
• Hg(CNO)₂ mercury fulminate primary explosive once used in detonators
• Hg(NO₃)₂ mercury(II) nitrate soluble salt from the metal and nitric acid

Occurrence, production and use

Crustal abundance

8.5×10-2 milligrams per kilogram

Oceanic abundance

3×10-5 milligrams per liter

Occurrence and sources

• cinnabar (HgS), with droplets of native mercury China, Kyrgyzstan, Tajikistan, Spain (Almaden, closed 2003), Slovenia, Mexico, Peru, Ukraine; Alaska, Arkansas, California, Nevada, Texas
• by-product of gold-silver ore processing and of colonial silver-mining waste Nevada mines; Mexico reclaims mercury from Spanish colonial residues
• crustal and oceanic abundance about 0.03 ppm crust (BGS via RSC); 0.085 mg/kg crust and 0.00003 mg/L seawater (PubChem)

Extraction, production

Roasting cinnabar in a current of air and condensing the vapour

$\ce{HgS (s) + O2 (g) -> Hg (g) + SO2 (g)}$

RSC states cinnabar is heated in air and the mercury vapour condensed; sulfur dioxide as the sulfur product is implied by roasting, not named

Secondary mercury by retorting of lamps, batteries, amalgam, switches, thermostats and contaminated soils

eight US facilities of six companies in 2024; LED replacement of fluorescent lamps is increasing the recycled flow

Uses

Mercury can be used to make thermometers, barometers and other scientific instruments. Mercury conducts electricity and is used to make silent, position dependent switches. Mercury vapor is used in streetlights, fluorescent lamps and advertising signs.

Mercury easily forms alloys with other metals, such as gold, silver, zinc and cadmium. These alloys are called amalgams. Amalgams are used to help extract gold from its ores, create dental fillings (in the case of silver) and help extend the life of dry cell batteries (in the case of zinc and cadmium).

Mercury forms useful compounds with other elements. Mercuric chloride (HgCl2) is a very poisonous salt and was once used to disinfect wounds. Mercurous chloride (Hg2Cl2), also called calomel, is an antiseptic used to kill bacteria. Mercuric sulfide (HgS) is used to make a red paint pigment called vermilion. Mercuric oxide (HgO) is used to make mercury batteries.

Mercury is poisonous and can enter the body through the respiratory tract, the digestive tract or directly through the skin. It accumulates in the body, eventually causing severe illness or death.

The metal is widely used in laboratory work for making thermometers, barometers, diffusion pumps, and many other instruments. It is used in making mercury-vapor lamps and advertising signs, etc. and is used in mercury switches and other electronic apparatus. Other uses are in making pesticides, Mercury cells for caustic soda and chlorine production, dental preparations, anti-fouling paint, batteries, and catalysts.

• Chemicals (chlor-alkali mercury cell, VCM catalysts): flowing mercury cathode of the mercury-cell chlor-alkali process; anode: $\ce{2 Cl- -> Cl2 + 2 e-}$; cathode: Na+ + e- + Hgx -> NaHgx; decomposer: 2 NaHgx $\ce{+ 2 H2O -> 2 NaOH + H2 + 2}$ Hgx; overall: $\ce{2 NaCl + 2 H2O -> 2 NaOH + H2 + Cl2}$ (cak-bref-2014, PDF p40); one US mercury-cell plant still ran in 2024; not BAT in the EU since 2017; mercury(II) catalysts for coal-based (acetylene) vinyl chloride monomer in China; mercury carried in mercury-cell caustic into steam-cracker scrubbing liquor about 163 t of elemental mercury circulates in US manufacturing processes (catalysts, cell cathodes), almost all reused (usgs-mcs2025-mercury)
• Electrical devices: relays, sensors, switches, valves, rectifiers relays, sensors, switches and valves 65 percent of US end use (usgs-mcs2025-mercury)
• Dentistry and medicine: dental amalgam (mercury and silver as biocides against recurrent decay); formulated products: buffers, fixatives, vaccine preservatives dental amalgam 27 percent, formulated products 7 percent of US end use (usgs-mcs2025-mercury)
• Pharmaceuticals (fine-chemical synthesis): mercury reagents in reductions and other unit processes of organic fine-chemical synthesis
• Lighting: fluorescent and compact fluorescent lamps, being replaced by LEDs bulbs, lamps and lighting 1 percent of US end use (usgs-mcs2025-mercury)
• Mining (gold amalgamation): mercury amalgamation of gold in artisanal and small-scale mining, the largest anthropogenic emission source; mercury reported in extractive-waste water at four precious-metal sites; by-product recovery at large gold-silver mines
• Pigments and consumer products (legacy): vermilion (HgS) pigment, now rarely used; button batteries, skin-lightening creams, folk medicines, fireworks, pesticides in some countries

Safety, toxicity

GHS classification, signal word Danger

• H330 Fatal if inhaled Acute toxicity, inhalation
• H400 Very toxic to aquatic life Hazardous to the aquatic environment, acute hazard
• H410 Very toxic to aquatic life with long lasting effects Hazardous to the aquatic environment, long-term hazard
• H290 May be corrosive to metals Corrosive to Metals
• H300 Fatal if swallowed Acute toxicity, oral
• H360 May damage fertility or the unborn child Reproductive toxicity
• H360D May damage the unborn child Reproductive toxicity
• H372 Causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
• H317 May cause an allergic skin reaction Sensitization, Skin
• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H370 Causes damage to organs Specific target organ toxicity, single exposure
• H341 Suspected of causing genetic defects Germ cell mutagenicity
• H360FD May damage fertility; May damage the unborn child Reproductive toxicity

Discovery and name

Discovered by

Ancient Egyptians

Discovered

before 1500 BCE

First isolated

not in sources

Named by

not in sources

Origin of the name

from the planet Mercury, with which it was associated in medieval alchemy

It is a heavy, silvery-white metal; a rather poor conductor of heat, as compared with other metals, and a fair conductor of electricity. It easily forms alloys with many metals, such as gold, silver, and tin, which are called amalgams. Its ease in amalgamating with gold is made use of in the recovery of gold from its ores. The most important salts are mercury chloride (corrosive sublimate - a violent poison), mercurous chloride (calomel, occasionally still used in medicine), mercury fulminate, a detonator widely used in explosives, and mercuric sulfide (vermilion, a high-grade paint pigment). Organic mercury compounds are important. It has been found that an electrical discharge causes mercury vapor to combine with neon, argon, krypton, and xenon. These products, held together with van der Waals' forces, correspond to HgNe, HgAr, HgKr, and HgXe. Mercury is a virulent poison and is readily absorbed through the respiratory tract, the gastrointestinal tract, or through unbroken skin. It acts as a cumulative poison and dangerous levels are readily attained in air. Air saturated with mercury vapor at 20°C contains a concentration that exceeds the toxicity limits. The danger increases at higher temperatures. It is important therefore that mercury be handled with care. Containers of mercury should be securely covered and spillage should be avoided. If it is necessary to heat mercury or mercury compounds, it should be done in a well-ventilated hood. Methyl mercury is a dangerous pollutant and is now widely found in water and streams. The triple point of mercury, -38.8344C, is a fixed point on the International Temperature Scale (ITS-90).