Cadmium

group 12 · period 5 · d-block · transition metal

fullCadmium is regulated everywhere at a few µg/L (WHO 3, EU 5, US 5) because it accumulates in the kidney for decades, it is an EU priority hazardous substance with hardness banded quality standards, and it reaches water from phosphate fertiliser, galvanised pipe, plating, pigments and zinc smelting; in water it is one soluble cation that has to be precipitated at high pH or as sulfide.

Typical wastewaters

electroplating and metal finishing (cadmium plating) Cd²⁺ in rinse water; cadmium cyanocomplexes in cyanide bath rinses until the cyanide is oxidised US limit 0.69 mg/L daily, 0.26 mg/L monthly; hydroxide precipitation needs about pH 11, sulfide works lower
mine drainage and zinc and lead smelting Cd²⁺ dissolved in acid water, remobilised from sediment as acidity rises; chloride complexes in brines US ore mining limit 0.10 mg/L daily, 0.05 mg/L monthly for copper, lead and zinc mines
phosphate fertiliser (diffuse, from phosphate rock cadmium) Cd²⁺ leached from fertilised soil to groundwater WHO: a major source of diffuse pollution
pigment and nickel-cadmium battery manufacture Cd²⁺; species not given by the source US EPA lists metal refineries, waste batteries and paints as sources of the MCL
textile and leather wet processing (cadmium pigments and stabilisers) total cadmium as a ZDHC Table 2 metal (0.1 mg/L foundational); sludge thresholds 1 mg/kg textile and 2 mg/kg leather

In the ledger's plant and process records, discharged by: NPK and CN fertilisers (Chemicals) · Phosphoric acid (wet process) (Chemicals) · Speciality inorganic pigments (iron oxide, chromium oxide, CIC, zinc sulphide, lithopone) (Chemicals) · Superphosphates (Chemicals) · Base metal ores (Cu, Ni, Pb, Sn, Zn) (Mining) · Bauxite, alumina, magnesite and ilmenite (Mining) · Industrial minerals (potash, salt, kaolin, magnesite and others) (Mining) · Precious metal ores (Au, Ag, Pt): gold and silver extraction (Mining)

1 · Identity

Symbol, number: Cd, 48
Oxidation states in water: +2 only, as Cd²⁺ and its chloride complexes; the element entry notes it forms only Cd₂+. It hydrolyses late, so it stays dissolved to high pH, and it substitutes for zinc in everything from sphalerite to galvanised coatings, which is why it travels with zinc into water.
Note: Cadmium in natural water is found mainly in bottom sediments and suspended particles, and its solubility is governed largely by acidity: sediment bound cadmium dissolves when acidity rises (WHO background document).

2 · Occurrence in water

Natural sources: Weathering of zinc ores (sphalerite carries about 0.03 percent cadmium, element entry); remobilisation from sediment by acidification (WHO). Unpolluted natural water is usually below 1 µg/L.
Anthropogenic sources: Fertilisers produced from phosphate ores, a major source of diffuse pollution (WHO); impurities in the zinc of galvanised pipe and in cadmium containing solders in fittings, water heaters, coolers and taps (WHO); cadmium plating (US metal finishing limit 0.69 mg/L daily), cadmium sulfide and selenide pigments, nickel-cadmium batteries, zinc and lead smelting and mine drainage (0.10 mg/L limit for copper, lead and zinc mines); local air pollution deposits. The ledger's mining and chemical chapters carry the plant level figures.

matrix	typical range	note
unpolluted natural water	below 1 µg/L	median dissolved cadmium at 110 stations worldwide below 1 µg/L, maximum 100 µg/L in the Rio Rimac, Peru
surface water, Rhine and Danube (1988)	0.1 (0.02 to 0.3); 0.025 µg/L1980s	averages
drinking water, general	usually below 1 µg/L	Netherlands 1982: 0.1 to 0.2 µg/L detected in only 1 percent of samples from 256 plants
drinking water, corrosive or acidified supplies	approaching 5 (Sweden, shallow wells in acidified soil); 1 to 26 mean (Saudi Arabia, private wells and corroded pipes) µg/L region-dependent; the Saudi figures are 1988 samples partly from corroded pipes	soft water of low pH is more corrosive to cadmium bearing plumbing (WHO)

3 · Speciation

Cadmium(II) is a soluble, weakly hydrolysing cation across the pH of natural water. It adsorbs on iron and manganese oxides, clays and organic matter more strongly as pH rises, and it is chloride complexed in seawater and brines, which keeps it mobile there. Cd(OH)₂ needs a higher pH than the zinc, copper or nickel hydroxides (about 11 in the textbook solubility curves), so hydroxide precipitation of cadmium runs at the top of the range and often falls short; carbonate precipitation as CdCO₃ works lower, and sulfide precipitation lowest of all. Acidification is the release mechanism: it dissolves sediment bound cadmium and corrodes cadmium out of galvanised pipe.

condition	dominant species	note
fresh water, pH 6 to 8, oxic	Cd²⁺; minor CdCl⁺, CdSO₄, CdHCO₃⁺; cadmium adsorbed on particles	mostly particle bound in natural water (WHO)
seawater and brines	CdCl⁺, CdCl₂ (aq), CdCl₃⁻	chloride complexation; textbook, not in the WHO text
acid water, mine drainage, acidified wells	Cd²⁺ dissolved; sediment cadmium remobilised	WHO: suspended or sediment bound cadmium may dissolve with an increase in acidity
lime or caustic above pH 10.5 to 11	Cd(OH)₂ (s)	hydroxide precipitation window; higher than for other plating metals
carbonate or sulfide dosing	CdCO₃ (s), CdS (s)	lower residuals

Solubility: Cd(OH)₂ controls only in strong alkali; CdCO₃ in carbonate rich alkaline water; CdS wherever sulfide exists. No solubility products quoted; the sources read print none.
Hydrolysis: Weak; Cd(OH)⁺ becomes significant only above about pH 9, so cadmium salts are nearly neutral in water.
Complexation: Chloride (seawater), organic matter, and in plating baths cyanide and ammonia, which hold cadmium against hydroxide precipitation; constants not quoted.
Precipitates: Cd(OH)₂, CdCO₃, CdS (treatment and sediments); cadmium co-precipitated in zinc hydroxide, iron hydroxide and calcium carbonate softening sludge; cadmium phosphate in soils treated with phosphate fertiliser.

\ce{Cd^2+ + H2O <=> CdOH^+ + H+}

the first hydrolysis step, significant only above about pH 9, which is why cadmium salts are nearly neutral in water and why the hydroxide precipitation window sits so high; no constant is printed in the sources read

\ce{Cd^2+ + 3 Cl^- -> CdCl3^-}

seawater and brines; the chloro complexes hold cadmium in solution, cut its sorption on particles and raise its mobility in saline water; Stumm and Morgan chapter 6, from the chapter, not re-read, and the WHO text does not give it

\ce{Cd^2+ + 2 OH- -> Cd(OH)2 (s)}

lime or sodium hydroxide at high pH, about 11 for the solubility minimum (Metcalf and Eddy chapter 6 curves, from the chapter, not re-read); the CWW BREF lists hydroxide precipitation

\ce{Cd^2+ + CO3^2- -> CdCO3 (s)}

soda ash or lime softening at pH 9 to 10; WHO: 0.002 mg/L achievable by coagulation or precipitation softening

\ce{Cd^2+ + HS^- -> CdS (s) + H+}

sulfide precipitation at pH 8 to 9; lowest residuals; BREF chemical precipitation variant

\ce{Cd^2+ + 4 CN^- -> Cd(CN)4^2-}

cadmium cyanide plating baths and their rinses; the complex passes straight through lime dosing, so the cyanide has to be destroyed before any precipitation step; Stumm and Morgan chapter 6, from the chapter, not re-read

\ce{2 RNa + Cd^2+ -> R2Cd + 2 Na^+}

cation or chelating resin, R the resin in the sodium form; hardness competes on a non selective resin and the regenerant carries the cadmium out as a concentrate

4 · Role in treatment

as a problem

leaching from galvanised pipe and solders

cadmium impurity in zinc coatings and solders corrodes out in soft, low pH water

WHO background document; product control is the remedy

hydroxide precipitation falls short

Cd(OH)₂ solubility minimum sits near pH 11, above the window used for zinc, copper and nickel

the reason cadmium plating rinses are treated with sulfide or at a separate pH

complexed cadmium in plating rinses

cyanide baths hold cadmium as cyanocomplexes until the cyanide is oxidised

cyanide destruction precedes precipitation

\ce{CN^- + OCl^- -> CNO^- + Cl^-}

first stage of alkaline chlorination at pH 10 or above, which frees the cadmium for precipitation; the second stage takes cyanate on to nitrogen and bicarbonate at pH 8 to 9; Metcalf and Eddy chapter 6, from the chapter, not re-read

diffuse load from fertiliser

phosphate rock cadmium applied to soil reaches groundwater and crops

WHO: a major source of diffuse pollution; food is the main exposure

sludge and biosolids

all removed cadmium ends in hydroxide, carbonate or sulfide sludge

ZDHC sludge thresholds 1 mg/kg (textile) and 2 mg/kg (leather) dry weight

as a reagent

none

cadmium is not a treatment reagent; it is a contaminant of zinc reagents and galvanised hardware

5 · Removal and control

coagulation and precipitation softening (drinking water)

adsorption on and co-precipitation with iron or aluminium hydroxide flocs; lime softening precipitates CdCO₃ and Cd(OH)₂ with the calcium carbonate

\ce{Cd^2+ + CO3^2- -> CdCO3 (s)}

WHO fact sheet: 0.002 mg/L should be achievable using coagulation or precipitation softening

Efficiency: to 2 µg/L (WHO)
Interferences: low pH, complexing agents

hydroxide precipitation (industrial effluent)

lime or caustic to about pH 11, flocculation, settling, filtration, then neutralisation

\ce{Cd^2+ + 2 OH- -> Cd(OH)2 (s)}

the standard metal finishing train; cyanide destroyed first in cadmium cyanide plating

Efficiency: to the US metal finishing limit of 0.26 mg/L monthly; lower needs sulfide
Interferences: complexing agents; pH overshoot redissolves zinc and chromium co-precipitated with it

sulfide precipitation

CdS precipitated with sodium sulfide or organosulfide, insoluble enough to work in the presence of complexing agents

\ce{Cd^2+ + HS^- -> CdS (s) + H+}

pH 8 to 9; sulfide excess controlled

Efficiency: lower residuals than hydroxide; no figure printed in the sources read
Interferences: excess sulfide; colloidal CdS needs a coagulant

ion exchange

cation or chelating resin takes Cd²⁺ from rinse water and groundwater; the regenerant carries the cadmium

\ce{2 RNa + Cd^2+ -> R2Cd + 2 Na^+}

BREF lists ion exchange for metals

Efficiency: not quoted
Interferences: hardness competition on non selective resin; complexed cadmium

source control

cadmium free solders and galvanising, corrosion control in soft acidic supplies

WHO: contamination from galvanised pipe and solders

6 · Analytics

method	standard	detection limit	note
ICP-MS	EPA 200.8 (mass 111); ISO 17294-2; Standard Methods 3125	0.01 µg/L (WHO fact sheet); EPA 200.8 instrument detection limit 0.1 µg/L scanning, 0.02 µg/L selected ion monitoring	molybdenum oxide ions overlap the cadmium masses in molybdenum rich samples; the method's correction equations apply
graphite furnace AAS	ISO 15586; Standard Methods 3113	0.1 µg/L (WHO background document)
flame AAS	Standard Methods 3111; ISO 8288 (the ISO 1985 and 1986 flame methods cited by WHO)	2 µg/L (fact sheet); 5 µg/L (background document)	adequate for effluent, not for the 3 to 5 µg/L drinking water values
ICP-OES	EPA 200.7; ISO 11885	not read

Sampling pitfalls: Cadmium adsorbs on container walls and on suspended particles, and most cadmium in natural water is particle bound (WHO), so total cadmium needs acid digestion of the unfiltered sample and dissolved cadmium needs field filtration before acidification. A first draw sample from galvanised or soldered plumbing after stagnation gives the leaching value; a flushed sample gives the supply.

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.

drinking water

body	limit	note
WHO GDWQ 4th ed. with addenda (2022)	0.003 mg/L	3 µg/L from the JECFA PTMI of 25 µg/kg body weight per month with a 10 percent allocation to water (high intake from food), 60 kg, 2 L/day; kidney tubular dysfunction the critical effect; assessment 2011
EU DWD 2020/2184	5.0 µg/L	Annex I Part B
US EPA NPDWR	0.005 mg/L	MCL and MCLG 0.005 mg/L; health effect listed as kidney damage; sources listed as corrosion of galvanised pipes, natural deposits, metal refineries, waste batteries and paints

discharge

body	limit	note
EU EQS (Directive 2013/39/EU), cadmium and its compounds, priority hazardous substance 6	annual average 0.08 or less (class 1), 0.08 (2), 0.09 (3), 0.15 (4), 0.25 (5); maximum allowable 0.45 or less (1), 0.45 (2), 0.6 (3), 0.9 (4), 1.5 (5) µg/L	inland surface waters by hardness class: below 40, 40 to 50, 50 to 100, 100 to 200, 200 mg CaCO₃/L and above; other surface waters annual average 0.2 µg/L with the same maximum allowable values
EU CWW BREF BAT-AEL (Decision 2016/902)	not set	cadmium is not among the metals with a BAT-AEL (Cr, Cu, Ni, Zn)
US EPA 40 CFR 433.14, metal finishing (BAT), cadmium (total)	0.69 daily maximum; 0.26 monthly average mg/L	with copper 3.38 and 2.07, nickel 3.98 and 2.38, silver 0.43 and 0.24 mg/L
US EPA 40 CFR 440.103, copper, lead, zinc, gold, silver and molybdenum ore mines and mills (BAT)	0.10 daily maximum; 0.05 30-day average mg/L	taken from the book's lead water chapter; section not re-read this session
Abu Dhabi ADS 23/2017, discharge to the marine environment (EAD)	0.05 mg/L region-dependent; marine discharge only	Table 1 maximum allowable concentration at the point of discharge
Abu Dhabi DoE Trade Effluent Control Regulations 2022, discharge to sewer	1 mg/L region-dependent; sewer discharge, not receiving water	Table A₄ Metals

industry thresholds

sector	body	limit	note
textile and leather	ZDHC Wastewater Guidelines v₂.1 (2022), Table 2	0.1 foundational; 0.05 progressive; 0.01 aspirational mg/L	textile and leather alike; sludge total cadmium threshold 1 mg/kg (textile) and 2 mg/kg (leather) dry weight (Table 4A); cadmium pigments and stabilisers are the textile sources

8 · Health and environmental effects

Toxicity: The kidney is the target: cadmium accumulates there with a biological half life of 10 to 35 years, and the JECFA PTMI of 25 µg/kg body weight per month is the dietary exposure that keeps urinary cadmium below the 5.24 µg per gram creatinine breakpoint for beta-2-microglobulin excretion (WHO fact sheet). Daily oral intake is 10 to 35 µg, smoking adds to it, and the margin between the PTMI and actual intake is small. Carcinogenic by inhalation (IARC Group 2A), no evidence by the oral route.
Bioaccumulation: Accumulates in the human kidney over decades (WHO); in water it partitions to sediment and suspended particles. Aquatic bioaccumulation is not addressed in the sources read.
Ecotoxicity: US EPA aquatic life criteria (2016): freshwater acute 1.8 µg/L at 100 mg/L hardness (hardness dependent, dissolved); the 2016 freshwater chronic criterion was vacated by a court order of 18 August 2023 and the 2001 update applies; saltwater 33 µg/L acute and 7.9 µg/L chronic. The EU annual average EQS of 0.08 to 0.25 µg/L by hardness class is the tighter working ceiling.

Flags

The Cd(OH)₂ solubility minimum pH and the chloride complexes are cited to Metcalf and Eddy chapter 6 and Stumm and Morgan chapter 6 from memory of the text, not re-read this session.
The precipitation equations are written here; the CWW BREF names the techniques without printing them.
The occurrence figures are 1980s surveys compiled in the 2011 WHO background document; the Saudi values come partly from corroded private plumbing.
The US freshwater chronic cadmium criterion is vacated; only the 2001 value applies and it was not read.
The mine drainage limit is reused from the book's lead chapter; the CFR section was not re-read.
EU law was read on legislation.gov.uk mirrors because eur-lex did not respond; eur-lex urls kept for consistency.
Abu Dhabi values cover two media (marine 0.05 mg/L, sewer 1 mg/L); other GCC states not read.
Standard Methods and ISO method numbers other than those in the sources read (EPA 200.8, ISO 17294-2 and the methods the WHO documents cite) are quoted from memory and were not confirmed this session.

Gaps

No seawater, municipal wastewater or raw plating effluent cadmium concentration was read.
The EU fertiliser cadmium limit (Regulation 2019/1009) was not read; the fertiliser source is quoted from WHO without a number.
No solubility products or complex constants quoted; the textbooks were not re-read.
Reverse osmosis and adsorbent removal of cadmium are not covered because no read source gives figures.
EU law was read on legislation.gov.uk mirrors; other GCC discharge standards were not read.
The chloride and cyanide complexation and the alkaline chlorination stoichiometries are cited to the textbooks by chapter, from the chapters, not re-read; no constants are quoted.

Sources

WHO Guidelines for Drinking-water Quality, 4th ed. incorporating the first and second addenda (2022), chapter 12 chemical fact sheet, Cadmium (pp. 354 to 355)
WHO, Cadmium in Drinking-water, background document for development of WHO Guidelines for Drinking-water Quality, WHO/SDE/WSH/03.04/80/Rev/1 (2011), sections 1.3, 1.4, 2 and 3.2
40 CFR 440.103, Effluent limitations (BAT), copper, lead, zinc, gold, silver and molybdenum ores subcategory
Directive (EU) 2020/2184 on the quality of water intended for human consumption (recast), Annex I Part B (read on the legislation.gov.uk mirror of the directive)
US EPA, National Primary Drinking Water Regulations (table of MCLs and treatment techniques)
Directive 2013/39/EU amending Directives 2000/60/EC and 2008/105/EC as regards priority substances, Annex I Part A (read as the legislation.gov.uk PDF of the adopted directive)
Commission Implementing Decision (EU) 2016/902 establishing BAT conclusions for common waste water and waste gas treatment/management systems in the chemical sector (CWW), BAT 12 Table 3 and footnotes c, d and g (read on the legislation.gov.uk mirror)
Best Available Techniques Reference Document for Common Waste Water and Waste Gas Treatment/Management Systems in the Chemical Sector (CWW BREF 2016), chapter 3 (chemical precipitation with hydroxide and sulfide, ion exchange)
40 CFR 433.14, Effluent limitations (BAT), metal finishing point source category
Abu Dhabi Specification ADS 23/2017, Environmental Specifications for Land-Based Liquid Discharges to the Marine Environment (Environment Agency Abu Dhabi), Table 1
Abu Dhabi Department of Energy, Trade Effluent Control Regulations 2022 (DoE/PD/R01/005, effective 1 January 2022), Appendix Table A4 Metals
ZDHC Wastewater Guidelines Version 2.1 (November 2022), Table 2 heavy metals and Table 4A sludge
US EPA Method 200.8, Revision 5.4 (1994), Determination of trace elements in waters and wastes by ICP-MS, sections 1.7 and 7.1, Table 1 (instrument detection limits)
US EPA, National Recommended Water Quality Criteria, Aquatic Life Criteria Table
Standard Methods for the Examination of Water and Wastewater (online edition), 3111 (flame AAS), 3120 (ICP-OES), 3125 (ICP-MS)
Stumm, W. and Morgan, J. J., Aquatic Chemistry, 3rd ed. (Wiley, 1996), chapter 6 (metal ions in aqueous solution: hydrolysis and complexation) and chapter 7 (precipitation and dissolution)
Metcalf and Eddy, Wastewater Engineering: Treatment and Resource Recovery, 5th ed. (McGraw-Hill, 2014), chapter 6 (chemical precipitation of heavy metals; hydroxide and sulfide solubility versus pH)

Identity

Name and symbol: Cadmium, Cd
Atomic number: 48 protons
Position: group 12 · period 5 · d-block · transition metal
CAS number: 7440-43-9

Atomic structure

Atomic mass: 112.414 u
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰
[Kr] 5s²⁴d¹⁰
Electrons per shell: 2, 8, 18, 18, 2
Valence electrons: 12 ns and (n-1)d

isotope	mass (u)	abundance
¹⁰⁶Cd	105.906 460(8)	1.2 %
¹⁰⁸Cd	107.904 184(8)	0.8 %
¹¹⁰Cd	109.903 008(3)	12.4 %
¹¹¹Cd	110.904 184(3)	12.7 %
¹¹²Cd	111.902 764(2)	24.1 %
¹¹³Cd	112.904 408(2)	12.2 %
¹¹⁴Cd	113.903 365(2)	28.7 %
¹¹⁶Cd	115.904 763(1)	7.5 %

natural isotopic composition, mole fraction

Physical properties

State at room temperature: Solid
Melting point: 594.22 K (321.07 °C)
Boiling point: 1,040 K (766.85 °C)
Density: 8.69 g/cm3
Appearance: silvery bluish-gray metallic
Thermal conductivity: 96.6 W/(m·K)
Electrical resistivity: 72.7 nΩ·m
Electrical conductivity: 13.76 MS/m
Crystal structure: hexagonal close packed
Molar heat capacity: 26.02 J/(mol·K)

Chemical properties

Oxidation states: +2
Electronegativity: 1.69 (Pauling Scale)
Ionisation energy: 8.994 eV
1st 867.8, 2nd 1,631.4, 3rd 3,616 kJ/mol
Electron affinity: 0 eV
Atomic radius: empirical 144, covalent 144, van der Waals 158 pm
Ionic radius: Cd²⁺ 95 pm

Reactivity: A soft group 12 metal that sits below zinc and behaves much like it, though less reactive: it tarnishes in moist air, burns when heated, dissolves in dilute acids to Cd2+ salts but, unlike zinc, not in alkalis; the metal and its compounds are toxic.
with water: Does not react with water at ordinary temperatures.
with oxygen, air: Tarnishes slowly in moist air and burns when heated to brown cadmium oxide: $\ce{2 Cd + O2 -> 2 CdO}$ .
with acids: Dissolves in dilute acids with evolution of hydrogen, giving the chloride, sulfate or nitrate: $\ce{Cd + 2 HCl -> CdCl2 + H2}$ ; it does not dissolve in alkalis.
with halogens: Combines with the halogens on heating to the dihalides: $\ce{Cd + Cl2 -> CdCl2}$ .

Typical compounds

CdO cadmium oxide brown oxide from burning the metal
CdS cadmium sulfide greenockite; brilliant yellow pigment, photocopier drums
CdSe cadmium selenide red pigment and quantum-dot semiconductor
CdTe cadmium telluride thin-film solar cells, infrared detectors
CdCl₂ cadmium chloride from the metal in hydrochloric acid
Cd(OH)₂ cadmium hydroxide electrode material in nickel-cadmium cells

Occurrence, production and use

Crustal abundance: 1.5×10-1 milligrams per kilogram
Oceanic abundance: 1.1×10-4 milligrams per liter

Occurrence and sources

substituting for zinc in sphalerite (ZnS), about 0.03 percent Cd in typical zinc ores all zinc deposits; recovered at zinc smelters
greenockite (CdS) weathered sphalerite and wurtzite
trace impurity in sedimentary phosphate rock carried into wet-process phosphoric acid and fertilisers
crustal and oceanic abundance about 0.08 ppm crust (BGS via RSC); 0.15 mg/kg crust and 0.00011 mg/L seawater (PubChem)

Extraction, production: By-product of zinc leaching of roasted sulfide concentrates
cadmium follows zinc through roasting and sulfuric acid leaching and is separated from the zinc electrolyte; the sources state the route but no equation. Secondary cadmium comes from spent nickel-cadmium batteries, copper-cadmium alloy scrap, electric-arc-furnace dust and CdTe panels
Carbon reduction of cadmium oxide (Stromeyer, 1817)
RSC states the brown oxide was heated with lampblack to give the metal; whether the carbon leaves as CO or CO2 is not stated, so no equation is written

Uses

Cadmium is a poisonous metal and its use is somewhat limited for this reason. Like zinc, cadmium can be electroplated to other materials to protect them from corrosion. Cadmium easily absorbs neutrons and is used to make control rods for nuclear reactors. Cadmium is also used in rechargeable nickel-cadmium batteries.

Cadmium is alloyed with silver to form solder, a metal with a relatively low melting point used to join electrical components, pipes and other metallic items. Cadmium based solders must be handled with care to prevent cadmium poisoning. Cadmium alloys are also used to make low friction bearings that are highly resistant to fatigue.

Hydrated cadmium sulfate (3CdSO4·5H2O), one of cadmium's compounds, is used in a device called a Weston cell, a type of battery that produces a precise voltage used to calibrate medical and laboratory equipment. Cadmium sulfide (CdS), another cadmium compound, is a yellow powder that is used as a pigment. Other cadmium compounds are used in the phosphors of black and white television sets and in the blue and green phosphors in color television sets.

Cadmium is a component of some of the lowest melting alloys; it is used in bearing alloys with low coefficients of friction and great resistance to fatigue; it is used extensively in electroplating, which accounts for about 60% of its use. It is also used in many types of solder, for standard E.M.F. cells, for Ni-Cd batteries, and as a barrier to control nuclear fission. Cadmium compounds are used in black and white television phosphors and in blue and green phosphors for color TV tubes. It forms a number of salts, of which the sulfate is most common; the sulfide is used as a yellow pigment. Cadmium and solutions of its compounds are toxic.

Batteries: nickel-cadmium rechargeable batteries; being displaced by lithium-ion and nickel-metal-hydride RSC states 80 percent of cadmium produced (undated); USGS names NiCd batteries the main use
Solar and electronics: cadmium telluride thin-film solar panels (about 21 GW per year of world capacity in 2024); cadmium-zinc-telluride substrates for radiation detectors; phosphors formerly in colour television tubes
Chemicals (pigments, stabilisers, fertilisers): cadmium sulfide and sulfoselenide yellow, orange and red pigments; barium-cadmium stabilisers in flexible PVC, now replaced by barium stabilisers; unwanted cadmium carried from phosphate rock into wet-process phosphoric acid and fertilisers
Coatings: cadmium electroplating of steel for aircraft fasteners and oil platforms; zinc-nickel substitutes elsewhere
Nuclear: neutron-absorbing control rods; cadmium-silver-indium alloys
Mining: cadmium in zinc-lead ore, tailings and extractive-waste water; 13 base-metal sites reported it in discharges

Safety, toxicity

Failure to appreciate the toxic properties of cadmium may cause workers to be unwittingly exposed to dangerous fumes. Silver solder, for example, which contains cadmium, should be handled with care. Serious toxicity problems have been found from long-term exposure and work with cadmium plating baths. Exposure to cadmium dust should not exceed 0.01 mg/m3 (8-hour time-weighted average, 40-hour week). The ceiling concentration (maximum), for a period of 15 min, should not exceed 0.14 mg/m3. Cadmium oxide fume exposure (8-hour, 40-hour week) should not exceed 0.05 mg/m3, and the maximum concentration should not exceed 0.05 mg/m3. These values are presently being restudied and recommendations have been made to reduce the exposure.

GHS classification, signal word Danger

H250 Catches fire spontaneously if exposed to air Pyrophoric liquids
H330 Fatal if inhaled Acute toxicity, inhalation
H341 Suspected of causing genetic defects Germ cell mutagenicity
H350 May cause cancer Carcinogenicity
H361fd Suspected of damaging fertility; Suspected of damaging the unborn child Reproductive toxicity
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
H302 Harmful if swallowed Acute toxicity, oral
H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation
H361 Suspected of damaging fertility or the unborn child Reproductive toxicity
H372 Causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
H370 Causes damage to organs Specific target organ toxicity, single exposure

Discovery and name

Discovered by: Karl Samuel Leberecht Hermann and Friedrich Stromeyer
Discovered: 1817
First isolated: not in sources
Named by: Friedrich Stromeyer
Origin of the name: after calamine, in which it was found, itself named after Cadmea

Conventions

Valence electrons are counted by the usual convention: the outer shell for s- and p-block elements, ns and (n-1)d for the d-block, ns, (n-1)d and (n-2)f for the f-block. Lanthanides and actinides are placed in the f-block with no group number. Electrical conductivity is the reciprocal of the printed resistivity. Ionic radii are Shannon effective radii, six-coordinate unless noted. Where a field reads “not in sources” the value was not found; it is a gap, not a zero. Regulatory limits are the published values and change often, so check the standard in force at your site and the numbers written into your own permit before you design to them.

Data

Element records, isotopes, radii and the descriptive text come from PubChem (NCBI), the Los Alamos National Laboratory periodic table, IUPAC CIAAW and the IAEA Atomic Mass Data Center. Appearance, thermal conductivity, electrical resistivity, crystal structure, discovery and the origin of each name come from Wikipedia and Periodic-Table-JSON, used under CC BY-SA 4.0. Hazard classifications come from the ECHA C&L inventory via PubChem. Ionic radii follow R. D. Shannon (1976). The water chapters cite their own sources at the foot of each entry, and are written to the level of Snoeyink and Jenkins, Stumm and Morgan, MWH's Water Treatment and Metcalf and Eddy.