Zinc

Typical wastewaters

• chemical sector effluent (raw materials, plant corrosion, cooling water inhibitors) Zn²⁺ in metal bearing streams 5.6 to 250 mg/L before treatment, final effluent generally at or below 300 µg/L
• viscose rayon and acrylic fibre spinning Zn²⁺ from zinc sulfate spin baths and zinc chloride solvent; 6,796 (viscose) and 3,325 (acrylic) µg/L daily maximum
• electroplating and metal finishing total zinc 2.61 daily maximum and 1.48 mg/L monthly average
• mine drainage (copper, lead, zinc, gold, silver ores) Zn²⁺ in mine drainage, 1.5 daily maximum and 0.75 mg/L monthly average
• cooling tower blowdown (zinc corrosion inhibitors) Zn²⁺ from zinc based cooling water treatment, 1.0 mg/L total zinc
• municipal sewage (galvanised pipe corrosion, zinc orthophosphate) Zn²⁺ in sewage; urban wastewater treatment plants were the largest single source of zinc to water in the 2010 E-PRTR the WHO zinc document names corroding galvanised pipe and brass, and the EPA 2016 corrosion control guidance names zinc orthophosphate, as the loads behind it
• textile dyeing (zinc containing cationic dyes, viscose) total zinc 0.04 to 0.5 mg/L BAT-AEL, up to 0.8 mg/L with viscose fibres or zinc containing cationic dyes

1 · Identity

Symbol, number

Zn, 30

Oxidation states in water

+2 only, Zn²⁺ and its hydrolysis products, carbonate and sulfide solids; 0 as the sacrificial metal of galvanised steel and zinc anodes, which corrode to Zn²⁺.

Note

The element entry covers the ores, smelting and the amphoteric chemistry in outline; this chapter is the pipe, the effluent and the sludge.

2 · Occurrence in water

Natural sources

Weathering of sphalerite and other zinc sulfides; natural soil zinc 1 to 300 mg/kg; surface water usually below 10 µg/L and groundwater 10 to 40 µg/L (WHO background document).

Anthropogenic sources

Corrosion of galvanised iron pipe, brass and zinc alloy fittings, most in soft, acidic, carbon dioxide rich water (WHO); corrosion of plant pipework, tank insulation and roofs, raw materials, viscose fibre production and zinc based cooling water corrosion inhibitors in the chemical sector (CWW BREF section 2.4.3.9); zinc orthophosphate dosed for lead and copper control, which raises zinc loads to the sewage works (EPA OCCT 2016); electroplating and metal finishing, zinc and lead mine drainage, battery manufacture and cooling tower blowdown, each with its own US effluent limit. In the E-PRTR for 2010, 115 chemical installations emitted 150 t of zinc to water, 6 percent of a total of 2,513 t, and urban waste water treatment plants were the largest single source (CWW BREF section 1.2.3.11).

3 · Speciation

Zinc is Zn²⁺ over the whole natural pH range; hydrolysis to ZnOH⁺ begins only around pH 9, so free and ion paired zinc dominates in most waters. Its solids are Zn(OH)₂ and hydrozincite type basic carbonates in alkaline water, ZnCO₃ (smithsonite) in carbonate rich water, and ZnS in sulfidic sediments and anaerobic sludge, which holds zinc at very low solubility. The hydroxide is amphoteric and redissolves as zincate above about pH 11, so precipitation plants have a pH window on both sides.

Solubility

Zinc salts of the common anions (chloride, sulfate, nitrate) are freely soluble; Zn(OH)₂, ZnCO₃ and ZnS are the controlling solids. Chemical precipitation can reach about 1 to 10 mg/L for a single metal with lime and lower values with sulfide (CWW BREF citing VITO 2010); the solubility products themselves are not printed in the sources read.

Hydrolysis

Stepwise to ZnOH⁺, Zn(OH)₂, Zn(OH)₃⁻ and Zn(OH)₄²⁻; the first hydrolysis constant is near pK 9 (Stumm and Morgan chapter 6, from the chapter, not re-read), which is why zinc stays as the free ion in neutral water and why hydroxide precipitation needs pH 9 or more.

Complexation

Sulfate and bicarbonate ion pairs; natural organic matter; chelating agents such as EDTA keep zinc dissolved and make hydroxide precipitation incomplete (CWW BREF). Constants are not quoted because the sources read print none.

Precipitates

Zn(OH)₂, basic zinc carbonate (hydrozincite), ZnCO₃, ZnS; zinc phosphate on pipe walls where zinc orthophosphate is dosed.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Galvanised fittings, brass taps and rubber tubing contaminate samples; distinguish first draw from flushed samples when the question is the pipe. Acidify to pH below 2 for total zinc; filter 0.45 µm in the field for dissolved zinc. Zinc is ubiquitous in laboratory dust and gloves, so blanks matter at the µg/L level.

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: nearly 200 zinc enzymes; requirement 15 to 20 mg/day for adult men; JECFA PMTDI 1 mg/kg body weight. Acute: vomiting above about 500 mg zinc sulfate, mass poisonings from acidic drinks kept in galvanised containers; chronic excess causes copper deficiency (zinc therapy at 150 to 405 mg/day); no health based guideline value (WHO).

Bioaccumulation

Homeostatically regulated in animals, absorption 10 to 90 percent and biological half time about one year in humans (WHO); not a bioaccumulating contaminant in the mercury sense. Zinc toxicosis in livestock is copper deficiency.

Ecotoxicity

US EPA aquatic life criteria, dissolved zinc: freshwater 120 µg/L acute and 120 µg/L chronic at hardness 100 mg/L as CaCO₃ (hardness dependent, Appendix B equations), saltwater 90 and 81 µg/L (1995). Activated sludge inhibition threshold 5 to 20 mg/L (CWW BREF).

Flags

• The WHO concentration ranges are 1980s and 1990s compilations (Finnish surveys, Nriagu 1980); no recent survey was read.
• The seawater figure (4.9 µg/L) is a single abundance figure from Jefferson Lab via PubChem.
• The hydrolysis pK near 9 and the Zn(OH)₂ solubility minimum near pH 9.5 are cited to textbook chapters from memory, not re-read this session.
• The zincate, carbonate, sulfide and galvanised corrosion equations are electron and mass balances written here; the sources name the reactions but do not print them.
• CWW BREF Table 3.36 is one plant's pretreatment data and Tables 3.40 and 3.112 are single reference plants, not sector statistics.
• The activated sludge inhibition range 0.08 to 0.5 mg/L (UK Environment Agency 1997 as cited by the BREF) carries no rate reduction figure and conflicts with the 5 to 20 mg/L threshold of Table 3.115.
• Abu Dhabi values: marine outfall 0.5 mg/L, sewer 1 mg/L; other GCC states not read.
• EPA 2018 health advisory row for zinc read from a PDF table whose column alignment was verified against the cadmium row.

Gaps

• No source read gives zinc in municipal wastewater as a concentration; the CWW BREF only says urban WWTPs are the largest E-PRTR source.
• No solubility products or hydrolysis constants are printed in the sources read; Stumm and Morgan has them but was not re-read.
• No detection limit was read for the zincon method or for ISO 8288 beyond the WHO figure.
• Zinc in mine drainage and smelter effluent as concentrations is in the ledger's mining chapter, not here.
• Other GCC discharge standards (Saudi, Oman, Qatar) were not read.
• The reagent chemistry of zinc orthophosphate film formation is described by the EPA only in outline; the equation added here is the simple zinc phosphate solid, not a measured film composition.
• The hydrozincite, zinc anodic half reaction and EDTA chelation equations are from Stumm and Morgan chapters 6 and 8, from the chapter, not re-read; no stability constants, solubility products or electrode potentials beyond the standard zinc value were read.

Sources

Identity

Name and symbol

Zinc, Zn

Atomic number

30 protons

Position

group 12 · period 4 · d-block · transition metal

CAS number

7440-66-6

Atomic structure

Atomic mass

65.38 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰
[Ar] 4s²³d¹⁰

Electrons per shell

2, 8, 18, 2

Valence electrons

12 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

692.68 K (419.53 °C)

Boiling point

1,180 K (906.85 °C)

Density

7.134 g/cm3

Appearance

silver-gray

Thermal conductivity

116 W/(m·K)

Electrical resistivity

59.0 nΩ·m at 20 °C

Electrical conductivity

16.95 MS/m

Crystal structure

hexagonal close packed

Molar heat capacity

25.47 J/(mol·K)

Chemical properties

Oxidation states

+2

Electronegativity

1.65 (Pauling Scale)

Ionisation energy

9.394 eV
1st 906.4, 2nd 1,733.3, 3rd 3,833 kJ/mol

Electron affinity

0 eV

Atomic radius

empirical 122, covalent 122, van der Waals 139 pm

Ionic radius

Zn²⁺ 74 pm

Reactivity

A group 12 metal ([Ar] 3d10 4s2) with a single +2 oxidation state and chemistry much like magnesium; a moderately reactive metal and strong reducing agent (E = -0.76 V), comparable to manganese, whose fresh surface tarnishes to a protective basic carbonate film.

with water

Does not react with cold water because the surface film of oxide and basic carbonate protects it.

with oxygen, air

Tarnishes quickly in air to a passivating layer of basic zinc carbonate; at high red heat it burns with a bright bluish-green flame to white clouds of zinc oxide: $\ce{2 Zn + O2 -> 2 ZnO}$

with acids

Reacts readily with acids (and with alkalis to zincates); hydrochloric or sulfuric acid strips the passivating layer and then dissolves the metal with evolution of hydrogen, very pure zinc reacting only slowly: $\ce{Zn + 2 HCl -> ZnCl2 + H2}$

with halogens

Reacts with all the halogens, violently with fluorine and with decreasing vigour down the group so that iodine gives only a little heat: $\ce{Zn + Cl2 -> ZnCl2}$

Typical compounds

• ZnO zinc oxide white pigment, rubber, cosmetics; amphoteric
• ZnS zinc sulfide sphalerite, the main ore; luminous screens, lithopone
• ZnCO₃ zinc carbonate smithsonite ore; basic carbonate is the tarnish film
• ZnCl₂ zinc chloride important soluble salt; flux
• ZnSO₄ zinc sulfate soluble salt of the colourless Zn2+ ion
• Zn(C₂H₅)₂ diethylzinc pyrophoric organozinc reagent

Occurrence, production and use

Crustal abundance

7.0×101 milligrams per kilogram

Oceanic abundance

4.9×10-3 milligrams per liter

Occurrence and sources

The principal ores of zinc are sphalerite (sulfide), smithsonite (carbonate), calamine (silicate), and franklinite (zinc, manganese, iron oxide). One method of zinc extraction involves roasting its ores to form the oxide and reducing the oxide with coal or carbon, with subsequent distillation of the metal.

• sphalerite or zinc blende (ZnS), smithsonite (ZnCO3), calamine (silicate), franklinite sulphide and oxidised zinc lead deposits; principal mining areas China, Australia and Peru, also India, Mexico, the United States, Bolivia, Kazakhstan, Russia and Sweden
• zinc concentrates carrying germanium and gallium Alaska and Tennessee zinc mines; germanium and gallium are recovered from zinc processing residues
• electric arc furnace dust and galvanising residues secondary zinc recovered as crude zinc oxide at smelters

Extraction, production

Roasting of sphalerite concentrate to zinc oxide

$\ce{2 ZnS + 3 O2 -> 2 ZnO + 2 SO2}$

Balanced from the reactants and products stated by the sources: concentrate is roasted to the oxide (RSC, PubChem) and roasting metal sulphides releases the sulphur dioxide that non ferrous smelters turn into sulphuric acid (LVIC-AAF BREF).

Reduction of zinc oxide with carbon, or leaching and electrowinning

$\ce{ZnO (s) + CO (g) -> Zn (g) + CO2 (g)}$

The equation given is the retort and imperial smelting reduction step, taken from the added secondary source. Refined production about 13.7 million tonnes in 2024 (estimate).

Zinc sulphide pigment by precipitation

$\ce{ZnSO4 + Na2S -> ZnS + Na2SO4}$

The SIC BREF states that zinc sulphide and lithopone pigments are made from zinc sulphate and sodium sulphide; balanced from those reactants and the sulphide product, with sodium sulphate as the only possible co product. The ammonia plant desulphurisation step $\ce{H2S + ZnO -> ZnS + H2O}$ is stated verbatim by the LVIC-AAF BREF.

Uses

Roughly one third of all metallic zinc produced today is used in a process known as galvanization. During galvanization, an object that is subject to corrosion, such as an iron nail, is given a protective coating of zinc. The zinc can be applied to an object by dipping it in a pool of molten zinc, but it is most often applied through an electroplating process. Sacrificial zinc anodes are used in cathodic protection systems to protect exposed iron from corrosion. Metallic zinc is also used to make dry cell batteries, roof cladding and die castings.

Zinc is used to make many useful alloys. Brass, an alloy of zinc that contains between 55% and 95% copper, is probably the best known zinc alloy. Brass was first used about 2,500 years ago and was widely used by the ancient Romans, who used it to make such things as coins, kettles and decorative items. Brass is still used today, particularly in musical instruments, screws and other hardware that must resist corrosion. Zinc is alloyed with lead and tin to make solder, a metal with a relatively low melting point used to join electrical components, pipes and other metallic items. Prestal®, an alloy containing 78% zinc and 22% aluminum, is a strange material that is nearly as strong as steel but is molded as easily as plastic. Nickel silver, typewriter metal, spring brass and German silver are other common zinc alloys.

Zinc oxide (ZnO), a common zinc compound, forms when metallic zinc is exposed to the air and forms a protective coating that protects the rest of the metal. Zinc oxide is used in paints, some rubber products, cosmetics, pharmaceuticals, plastics, printing inks, soap and batteries, among other things. Zinc sulfide (ZnS), another zinc compound, glows when it is exposed to ultraviolet light, X-rays or electrons and is used to make luminous watch dials, television screens and fluorescent light bulbs. Zinc chloride (ZnCl2) is another zinc compound that is used to protect wood from decay and insects.

The metal is employed to form numerous alloys with other metals. Brass, nickel silver, typewriter metal, commercial bronze, spring bronze, German silver, soft solder, and aluminum solder are some of the more important alloys.

Large quantities of zinc are used to produce die castings, which are used extensively by the automotive, electrical, and hardware industries. An alloy called Prestal(R), consisting of 78 percent zinc and 22 percent aluminum, is reported to be almost as strong as steel and as easy to mold as plastic. The alloy said to be so moldable that it can be molded into form using inexpensive ceramics or cement die casts.

Zinc is also used extensively to galvanize other metals such as iron to prevent corrosion. Zinc oxide is a unique and very useful material for modern civilization. It is widely used in the manufacture of paints, rubber products, cosmetics, pharmaceuticals, floor coverings, plastics, printing inks, soap, storage batteries, textiles, electrical equipment, and other products. Lithopone, a mixture of zinc sulfide and barium sulfate, is an important pigment.

Zinc sulfide is used in making luminous dials, X-ray and TV screens, and fluorescent lights.

The chloride and chromate are also important compounds. Zinc is an essential element in the growth of human beings and animals. Tests show that zinc-deficient animals require 50 percent more food to gain the same weight as an animal supplied with sufficient zinc.

• Galvanising and alloys: hot dip and electro galvanised steel for car bodies, lamp posts, barriers and bridges, the leading use of refined zinc; die casting alloys; brass, nickel silver and aluminium solder galvanised steel was the leading use of refined zinc in the United States in 2024, followed by brass and bronze and zinc base alloys (USGS, ranking only)
• Chemicals: zinc sulphate in the sulphuric acid spin bath that regenerates viscose fibre, leaving zinc hydroxide and zinc sulphide sludge; zinc sulphide and lithopone white pigments and zinc oxide; pelletised zinc oxide absorbing hydrogen sulphide from ammonia plant feed; copper oxide and zinc oxide shift catalyst; zinc stearate as the main mould release agent in polystyrene; zinc micronutrient in NPK fertilisers; zinc discharged from phosphoric acid, superphosphate, viscose, pigment and silicone plants
• Rubber, paint and consumer products: zinc oxide in paints, rubber, cosmetics, pharmaceuticals, plastics, inks, soaps, batteries and textiles; zinc sulphide in luminous paints and fluorescent lights
• Textiles: zinc borate as a flame retardant synergist; zinc is a compound in the textile chapter's discharge list
• Pharmaceuticals and fine chemicals: zinc among the heavy metals discharged from processes involving metals (OFC BREF)
• Mining: zinc ore is a base metal ore of the MWEI BREF; zinc is listed among substances discharged from base metal, precious metal, iron ore and bauxite extraction

Safety, toxicity

Zinc is not considered to be toxic, but when freshly formed ZnO is inhaled a disorder known as oxide shakes or zinc chills sometimes occurs. Where zinc oxide is encountered, recommendations include providing good ventilation to avoid concentration exceeding 5 mg/m3, (time-weighted over an 8-hour exposure, 40-hour work week).

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
• 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
• H320 Causes eye irritation Serious eye damage/eye irritation

Discovery and name

Discovered by

Indian metallurgists

Discovered

before 1000 BC

First isolated

Andreas Sigismund Marggraf

Named by

not in sources

Origin of the name

probably from the German zinke, "tooth-like", since its crystals are needle-like

Zinc is a bluish-white, lustrous metal. It is brittle at ordinary temperatures but malleable at 100 to 150°C. It is a fair conductor of electricity, and burns in air at high red heat with evolution of white clouds of the oxide.

It exhibits superplasticity. Neither zinc nor zirconium is ferromagnetic; but ZrZn2 exhibits ferromagnetism at temperatures below 35°K. It has unusual electrical, thermal, optical, and solid-state properties that have not been fully investigated.