Arsenic

group 15 · period 4 · p-block · metalloid

fullArsenic is the drinking water contaminant of greatest health concern in many natural waters, regulated at 10 µg/L by WHO (provisional), the EU and the US, with a treatment chemistry that turns entirely on oxidising As(III) to As(V) and binding it to iron.

Typical wastewaters

mine drainage and smelter effluent (sulfide ores) dissolved inorganic arsenic: arsenate H₂AsO₄⁻ and HAsO₄²⁻ in oxic water, uncharged arsenite H₃AsO₃ where the water is reducing WHO names mining wastes and smelter effluent as the anthropogenic sources
coal power plant flue gas desulfurisation wastewater, gasification wastewater and combustion residual leachate total arsenic, limited to 18 µg/L daily and 8 µg/L monthly in FGD wastewater and 4 µg/L in gasification wastewater compliance dates apply; the 2024 rule moves most FGD wastewater to zero discharge
agrochemical manufacture inorganic arsenic in process effluent; species not given by the source
glass and electronics production wastes, orchard runoff inorganic arsenic; species not given by the source US EPA source list for the arsenic MCL
drinking water treatment residuals (spent anion exchange brine, backwash water, exhausted media and sludge) arsenate in sodium chloride regenerant brine at 1.83 to 38.5 mg/L (average 16.5); arsenic bound to ferric hydroxide in backwash and sludge liquid residuals are judged against the toxicity characteristic; sludges and throwaway media generally pass the TCLP
textile and leather wet processing total arsenic as a ZDHC Table 2 metal (0.05 mg/L foundational); sludge limits 5 mg/kg textile and 2 mg/kg leather

In the ledger's plant and process records, discharged by: Phosphoric acid (wet process) (Chemicals) · Phosphorus compounds (PCl3, POCl3, PCl5) (Chemicals) · Base metal ores (Cu, Ni, Pb, Sn, Zn) (Mining) · Bauxite, alumina, magnesite and ilmenite (Mining) · Coal, lignite and peat (Mining) · Industrial minerals (potash, salt, kaolin, magnesite and others) (Mining) · Iron ore and other metalliferous ores (Co, Cr, Mn, Mo, V, W) (Mining) · Precious metal ores (Au, Ag, Pt): gold and silver extraction (Mining) · Uranium ore (Mining)

1 · Identity

Symbol, number: As, 33
Oxidation states in water: +3 arsenite, arsenous acid H₃AsO₃, uncharged below its pKa of 9.2, the form in anaerobic groundwater; +5 arsenate, arsenic acid H₃AsO₄ and its anions H₂AsO₄⁻ and HAsO₄²⁻, the form in oxygenated water; methylated organoarsenic species (monomethylarsonic and dimethylarsinic acids, arsenobetaine in seafood) from biological methylation; -3 only in arsine.
Note: The element entry gives the ores, the trioxide trade and the uses; this chapter is the As(III) versus As(V) story that decides whether a treatment works.

2 · Occurrence in water

Natural sources: Dissolution of rocks, minerals and ores, above all in groundwater where sulfide mineral deposits and sedimentary deposits derived from volcanic rock occur; geothermal water (mean 500 µg/L, up to 25 mg/L); atmospheric deposition. Reducing aquifers release arsenite by reductive dissolution of the iron oxides that held arsenate; a rise in pH also raises dissolved arsenic (WHO background document).
Anthropogenic sources: Mining wastes and smelter effluent, agrochemical manufacture (WHO); runoff from orchards and from glass and electronics production wastes (US EPA); flue gas desulfurisation, gasification and combustion residual leachate at coal power plants (40 CFR 423); wood preservative (CCA) and pesticide legacies; arsenic bearing residues of phosphoric acid and phosphorus trichloride plants in the ledger's chemical chapter; treatment residuals themselves, since ion exchange brine carries 1.83 to 38.5 mg/L arsenic (EPA).

matrix	typical range	note
natural waters, including open ocean seawater	1 to 2 µg/L	generally; the WHO fact sheet says usually less than 1 to 2 µg/L
groundwater with natural sources	up to 12 mg/Lregion-dependent	areas of volcanic rock and sulfide mineral deposits; Bangladesh and West Bengal are the mass exposures
geothermal water	mean 500, maximum 25,000 µg/L
seawater	3.7 µg/L two sources disagree by a factor of two	oceanic abundance figure, Jefferson Lab via PubChem; WHO puts open ocean water at 1 to 2 µg/L
well water in an acute poisoning case	21 mg/Lsingle case	the concentration behind a reported acute intoxication
spent ion exchange brine (treatment residual)	1.83 to 38.5, average 16.5 mg/L	US EPA 2000 study; at least 5.0 mg/L expected for any source above 10 µg/L with sulfate below 50 mg/L

3 · Speciation

In well oxygenated surface water arsenic is arsenate, As(V); under reducing conditions in deep lake sediments and groundwater it is arsenite, As(III) (WHO). Dissociation of both is pH dependent and effectively instantaneous. Arsenite is the neutral molecule H₃AsO₃ at natural pH 6 to 9 (pKa 9.2) and is therefore not held by anion exchangers, activated alumina, iron hydroxide or coagulant flocs; arsenate carries a charge of minus 1 or minus 2 in the same range and is removed with much greater efficiency. Total arsenic is particulate (retained on 0.45 µm) plus soluble (EPA design manual). Every treatment train therefore starts by oxidising As(III).

condition	dominant species	note
oxic surface water or aerated groundwater, pH 6 to 9	H₂AsO₄⁻ and HAsO₄²⁻	anionic; sorbs on iron and aluminium hydroxides and on activated alumina; exchanges on strong base anion resin
anaerobic groundwater, pH 6 to 9	H₃AsO₃	uncharged below pKa 9.2; poorly removed by every process until oxidised
alkaline water, pH above 9.2	H₂AsO₃⁻ and HAsO₄²⁻	arsenite finally ionises; AsO₄³⁻ appears only near pH 12; rarely a treatment condition
sulfidic, strongly reducing water	arsenic sulfides	the element entry notes the sulfides are very insoluble, the basis of sulfide precipitation; not covered by the water sources read
biota and seafood	methylated and other organic arsenic	less toxic; fish and shellfish arsenic is mainly organic (WHO)

Solubility: Arsenic acid and the arsenate and arsenite salts of the common cations are freely soluble; the controlling solids in water are not arsenic minerals but the iron hydroxides that carry arsenate, and under sulfidic conditions the sulfides. No solubility products are quoted because the sources read print none.
Hydrolysis: Both acids dissociate stepwise with pH; the sources read give pKa 9.2 for arsenous acid and show the arsenic acid steps as a figure without printing the constants.
Complexation: Arsenate sorbs strongly on iron hydroxide and less on aluminium hydroxide; phosphate, silicate and natural organic matter compete for the same sites (EPA). Metal arsenate complexes in solution are not treated in the sources read.
Precipitates: Arsenate co-precipitates with and adsorbs on Fe(OH)₃ and hydrous ferric oxide; ferric arsenate and calcium arsenate in high dose treatment and in mine water sludges; arsenic sulfides under sulfidic conditions.

\ce{H3AsO3 <=> H2AsO3^- + H+}

pKa 9.2; below that arsenite is uncharged (EPA design manual, activated alumina section)

\ce{H3AsO4 <=> H2AsO4^- + H+}

first dissociation of arsenic acid, complete at natural pH; the EPA figure shows H2AsO4^- and HAsO4^2- sharing pH 6 to 9, constants not printed

\ce{H2AsO4^- <=> HAsO4^2- + H+}

second dissociation, around neutral pH; divalent arsenate is retained better by nanofiltration than monovalent, so pH above 7 favours NF

\ce{OCl^- + H3AsO3 -> Cl^- + H3AsO4}

pre-oxidation with chlorine; stoichiometric demand 0.95 mg Cl2 per mg As; three times stoichiometric converts over 95 percent within 40 seconds at pH 6.3 to 8.3, unaffected by dissolved iron, manganese, sulfide and TOC

\ce{2 MnO4^- + 3 H3AsO3 + 2 H+ -> 2 MnO2 (s) + 3 H3AsO4 + H2O}

pre-oxidation with permanganate; 0.49 mg Mn per mg As; same speed and pH independence as chlorine; EPA prints the half scale equation with 1.5 and 0.5 coefficients, doubled here; MnO2 particulates must be filtered

\ce{HAsO4^2- <=> AsO4^3- + H+}

third dissociation of arsenic acid, pKa about 11.5; the ladder is pKa1 2.2, pKa2 7.0, pKa3 11.5, so the fully deprotonated arsenate ion appears only in strongly alkaline water and never in a treatment train

\ce{Fe^3+ + H2AsO4^- -> FeAsO4 (s) + 2 H+}

ferric arsenate, the discrete solid of high dose ferric treatment and of mine water and smelter sludges; at the iron to arsenic ratios of drinking water practice the arsenate is held by adsorption on hydrous ferric oxide instead, not as this solid

\ce{2 H3AsO3 + 3 H2S -> As2S3 (s) + 6 H2O}

sulfidic, strongly reducing water and sulfide precipitation of arsenic from acidic metallurgical liquor; orpiment; the element entry gives the insolubility of the sulfides as the basis of the process, without printing the stoichiometry

\ce{Fe(OH)3 (s) + H2AsO4^- -> Fe(OH)2H2AsO4 (s) + OH-}

ligand exchange of arsenate for a surface hydroxyl on hydrous ferric oxide, the reaction behind both ferric coagulation and granular ferric hydroxide adsorption; effective over pH 5.5 to 8.5 and better at lower pH; written as a surface ligand exchange, the EPA manual describes the chemisorption in words

4 · Role in treatment

as a problem

arsenite passes every process

uncharged H₃AsO₃ at natural pH is not adsorbed, exchanged or coagulated

chlorine and permanganate oxidise it within a minute at pH 6.3 to 8.3; ozone works but sulfide and organic carbon blunt it; chlorine dioxide, monochloramine and UV alone are ineffective (EPA)

competing anions

sulfate beats arsenate on anion resin (SO₄²⁻ above HAsO₄²⁻ above NO₃⁻ and CO₃²⁻ above NO₂⁻ above Cl⁻); phosphate, silica and fluoride compete on alumina and iron surfaces

ion exchange not viable above 50 mg/L sulfate or 500 mg/L TDS; each 0.5 mg/L of phosphate above 0.2 mg/L cuts granular ferric hydroxide capacity by about 30 percent; silica competes at pH 7 to 9 in coagulation microfiltration (EPA)

\ce{Fe(OH)2H2AsO4 (s) + H2PO4^- -> Fe(OH)2H2PO4 (s) + H2AsO4^-}

phosphate displaces arsenate on the iron oxide surface; each 0.5 mg/L of phosphate above 0.2 mg/L cuts granular ferric hydroxide capacity by about 30 percent; silicate competes the same way at pH 7 to 9

chromatographic peaking

sulfate displaces sorbed arsenate and nitrate from an anion resin run past exhaustion, so effluent arsenic exceeds influent

run to a bed volume set point based on sulfate breakthrough, monitor effluent, use parallel columns (EPA)

\ce{R2HAsO4 + SO4^2- -> R2SO4 + HAsO4^2-}

R is a strong base anion exchange site; sulfate is held above arsenate in the selectivity sequence, so once the resin is past sulfate breakthrough it strips sorbed arsenate back into the effluent above the influent concentration

residuals

spent brine, backwash water, exhausted media and sludge carry the arsenic removed

brine 1.83 to 38.5 mg/L; sludges and throwaway media generally pass the TCLP; liquid residuals are judged directly against the toxicity characteristic (EPA)

removal to 10 µg/L is hard

analytical quantification limit 1 to 10 µg/L; conventional treatment reasonably achieves 10 µg/L, 5 µg/L only with careful optimisation

the reason the WHO value is provisional (WHO)

5 · Removal and control

pre-oxidation then coagulation with ferric salts and filtration

arsenate adsorbs on the ferric hydroxide precipitate and is entrapped as the floc grows; iron salts beat aluminium salts because iron hydroxides are more stable at pH 5.5 to 8.5 while part of the aluminium stays as a soluble complex

\ce{FeCl3 + 3 H2O -> Fe(OH)3 (s) + 3 HCl}

optimum pH 5 to 8 for ferric salts (5 to 7 for alum); effective doses 5 to 25 mg/L ferric chloride, up to 40 mg/L alum; influent arsenic level does not affect performance

Efficiency: over 90 percent, product water below 5 µg/L when optimised; WHO: 10 µg/L achievable by conventional coagulation, 5 µg/L feasible with careful control
Interferences: arsenite unless pre-oxidised; phosphate, silicate and natural organic matter compete for the iron surface

oxidation and filtration on existing iron (iron removal plants)

chlorine or permanganate ahead of a manganese oxide media filter oxidises Fe(II) and As(III) together; arsenate adsorbs on the fresh iron hydroxide and is filtered with it

\ce{2 Fe^2+ + HOCl + 5 H2O -> 2 Fe(OH)3 (s) + Cl^- + 5 H+}

iron at 1.5 mg/L or more and an Fe to As mass ratio of at least 20 to 1; independent of pH from 5.5 to 8.5; add ferric coagulant ahead of the filter when the native iron is short

Efficiency: 80 to 95 percent
Interferences: natural organic matter, orthophosphate and silicate compete for sorption sites; manganese co-precipitation is much less efficient than iron

coagulation assisted microfiltration

ferric coagulation followed by a membrane that retains the arsenic laden floc instead of granular media

silica competes at pH 7 to 9 and fouls the membrane; backwash is a high volume, low solids (below 1 percent) residual

Efficiency: not separately quantified in the sources read; the coagulation chemistry is the one above, which the EPA handbook puts at over 90 percent of As(V) and below 0.005 mg/L when optimised
Interferences: silica; arsenite unless oxidised

adsorption on activated alumina

arsenate exchanges on the alumina surface; selectivity OH⁻ above H₂AsO₄⁻ above silicate above F⁻ above HSeO₃⁻ above TOC above SO₄²⁻ above H₃AsO₃

\ce{Al(OH)3 (s) + H2AsO4^- -> Al(OH)2H2AsO4 (s) + OH-}

optimum pH 5.5 to 6.0; runs 5 to 20 times longer than at natural pH 6 to 9, and many small plants accept the shorter run to avoid acid and caustic; EBCT 3 to 10 minutes; problem levels chloride 250, fluoride 2, silica 50, iron 0.5, manganese 0.05, sulfate 720, DOC 4 and TDS 1000 mg/L; the EPA cost basis assumes non regenerated operation with throwaway media, which is expected to pass the TCLP. Written as a surface ligand exchange, which is why hydroxide heads the selectivity sequence and why the optimum sits at pH 5.5 to 6.0

Efficiency: over 98 percent at pH 5.5 to 6.0; run lengths 10,000 bed volumes at pH 7 to 8 and 5,200 at pH 8 to 8.3 (EPA cost basis)
Interferences: arsenite (poor selectivity below pH 9.2), silica, fluoride, iron and manganese coatings, particulates above 0.3 NTU

adsorption on granular ferric hydroxide and other iron based media

chemisorption of arsenate on iron oxyhydroxide, considered irreversible; strong affinity at natural pH so far more bed volumes than alumina without pH adjustment

\ce{Fe(OH)3 (s) + H2AsO4^- -> Fe(OH)2H2AsO4 (s) + OH-}

EBCT 5 minutes, loading 5 gpm per square foot (about 12 m/h); performance still improves at lower pH; media thrown away when exhausted and passes the TCLP

Efficiency: not quantified in the source beyond the bed volume advantage
Interferences: phosphate above 0.2 mg/L (each further 0.5 mg/L cuts capacity about 30 percent); silica; arsenite

strong base anion exchange

chloride form resin exchanges arsenate; regenerated with brine (or caustic for hydroxide form)

\ce{2 RCl + HAsO4^2- -> R2HAsO4 + 2 Cl^-}

EBCT 1 to 5 minutes; not economic above 50 mg/L sulfate or 500 mg/L TDS; run length about 1,500 bed volumes at sulfate up to 20 mg/L and 700 at 20 to 50 mg/L; regeneration gives 4 to 5 bed volumes of brine; pre-filter above 0.3 NTU. R is the resin exchange site; regeneration with brine reverses the exchange

Efficiency: arsenate removal to below 10 µg/L until sulfate breakthrough; chromatographic peaking beyond it
Interferences: sulfate, nitrate, TDS; arsenite not exchanged

reverse osmosis

size, charge and hydrophilicity exclusion by a non porous membrane

100 to 350 psi (about 7 to 24 bar) for drinking water; recovery 60 to 80 percent; relatively insensitive to pH; point of use units at tap pressure recover far less

Efficiency: over 95 percent in a single pass (design manual); over 97 percent of As(V) and 92 percent of As(III) (handbook)
Interferences: scaling and fouling reduce rejection and recovery; free chlorine damages polyamide membranes; concentrate disposal

nanofiltration

charge repulsion of the divalent arsenate anion

50 to 150 psi (about 3.5 to 10 bar); pH above 7 so arsenate is divalent; pre-oxidation recommended

Efficiency: arsenate rejection usually 60 to 70 percent per stage at 80 to 90 percent recovery; multi stage for more
Interferences: arsenite passes; monovalent arsenate rejected less than divalent

caustic regeneration of activated alumina

hydroxide displaces the sorbed arsenate from the alumina surface; the bed is then rinsed and neutralised with acid before the next run

\ce{Al(OH)2H2AsO4 (s) + 2 OH- -> Al(OH)3 (s) + HAsO4^2- + H2O}

dilute sodium hydroxide, then rinse and acid neutralisation before the next run; the EPA cost basis assumes non regenerated throwaway media instead, because regeneration produces a high arsenic caustic waste and the media loses capacity each cycle. Concentrations and capacity loss per cycle were not read

Efficiency: not quantified in the source read
Interferences: spent regenerant is the arsenic residual and must be treated

6 · Analytics

method	standard	detection limit	note
ICP-MS	EPA 200.8; ISO 17294-2	0.1 µg/L (WHO); EPA 200.8 instrument detection limit 0.9 µg/L scanning and 0.02 µg/L selected ion monitoring	polyatomic ions interfere at mass 75 (argon chloride in high chloride samples); EPA 200.8 section 4.1.3 requires correction equations set at run time, and collision cell instruments remove the interference; the method of choice at the 10 µg/L limit
hydride generation AAS	Standard Methods 3114 B and C	2 µg/L (WHO)	sodium borohydride converts arsenic to arsine, purged into a heated quartz cell; the classic method where ICP-MS is unavailable; also 2 µg/L by flame AAS per WHO
silver diethyldithiocarbamate colorimetry	Standard Methods 3500-As B; ISO 6595 (1982)	about 1 µg/L (WHO background document)	arsine evolved and absorbed in the reagent; slow, toxic reagents, field kits derive from it
speciation, As(III) and As(V)	HPLC coupled to ICP-MS (WHO background document); no numbered standard read	not read	needed to size pre-oxidation; total arsenic is what the limits regulate

Sampling pitfalls: Total arsenic is particulate plus soluble; filter 0.45 µm in the field if dissolved arsenic is wanted and acidify for total. Species change on storage as As(III) oxidises or iron precipitates and sorbs arsenate, so speciation samples need field separation or preservation; the sources read state the split but not the preservation recipe. The practical quantification limit is 1 to 10 µg/L (WHO), so results at the limit need the ICP-MS or hydride methods, not colorimetry.

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)	10 µg/L	provisional, on treatment performance and analytical achievability; inorganic arsenic is IARC Group 1; where 10 µg/L cannot be met every effort should be made to keep concentrations as low as possible; assessment 2011
EU DWD 2020/2184	10 µg/L	Annex I Part B chemical parameter; uncertainty of measurement 30 percent of the parametric value (Annex III)
US EPA NPDWR	0.010 mg/L	MCL in force since 23 January 2006 (the former MCL was 0.05 mg/L); MCLG zero; results rounded to the nearest 0.001 mg/L for compliance

discharge

body	limit	note
EU CWW BREF BAT-AEL (Decision 2016/902)	not set	arsenic is not among the BAT 12 parameters (Cr, Cu, Ni, Zn are the metals with AELs); arsenic limits in EU chemical plants come from permits and the ledger's chapter regulations
US EPA 40 CFR 423.13(g)(1)(i), steam electric FGD wastewater (2020 BAT, compliance by 31 December 2025)	18 daily maximum; 8 30-day average µg/L total arsenic time-sensitive: the 2024 steam electric rule changes applicability dates	voluntary incentives programme 423.13(g)(3)(i): 5 µg/L daily maximum by 31 December 2028; the 2024 rule moves most FGD wastewater to zero discharge
US EPA 40 CFR 423.13(j)(1)(i), gasification wastewater	4 µg/L total arsenic, daily maximum	by 31 December 2023
US EPA 40 CFR 423.13(l)(2)(i)(A), combustion residual leachate (retired facilities)	11 daily maximum; 8 30-day average µg/L total arsenic	by 30 April 2035
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	5 mg/L region-dependent; sewer discharge, not receiving water	Table A₄ maximum allowable concentration for trade effluent to the sewer network

industry thresholds

sector	body	limit	note
textile and leather	ZDHC Wastewater Guidelines v₂.1 (2022)	0.05 foundational; 0.01 progressive; 0.005 aspirational mg/L	methods ISO 17294, EPA 200.8, 6010C, 6020A; sludge limits in a separate table (textile 5 mg/kg total, leather 2)

8 · Health and environmental effects

Toxicity: Inorganic arsenic is a human carcinogen (IARC Group 1): long term ingestion causes skin lesions (hyperpigmentation, hypopigmentation, hyperkeratosis) after about 5 years, peripheral neuropathy and vascular disease, and cancers of skin, lung, bladder and kidney; cardiovascular effects seen in children at a mean of 0.6 mg/L over 7 years. Acute toxicity runs arsine above arsenites above arsenates above organic arsenic. Both As(III) and As(V) are absorbed rapidly and methylated; the methylated end products are excreted in urine (WHO).
Bioaccumulation: Fish and shellfish carry arsenic mainly as less toxic organic compounds; about 25 percent of dietary arsenic is inorganic depending on the food; where drinking water is at 10 µg/L or above it becomes the dominant intake, more so where soups and cooked staples take up the water (WHO).
Ecotoxicity: US EPA aquatic life criteria for arsenic: freshwater 340 µg/L acute and 150 µg/L chronic, saltwater 69 and 36 µg/L (1995), derived from As(III) data and applied to total arsenic.

Flags

Seawater: Jefferson Lab (via PubChem) gives 3.7 µg/L, WHO gives 1 to 2 µg/L for open ocean water.
Arsenate pKa values are not printed in the sources read; only the pKa 9.2 of arsenous acid is.
The US steam electric arsenic limits carry compliance dates and were revised again in 2024; check applicability before quoting.
The activated alumina run lengths and the ion exchange run lengths are EPA cost model assumptions, not field guarantees.
Abu Dhabi values cover two media (marine 0.05 mg/L, sewer 5 mg/L); other GCC states not read.
The ZDHC values are textile and leather wastewater; the sludge figures were read in the same document but not tabulated here.

Gaps

No source read gives arsenic concentrations in municipal wastewater or in specific industrial effluents (smelter, glass, CCA plants); only the treatment residual and the power plant limits are quoted.
Lime softening, zero valent iron, in situ (subsurface) treatment and biological arsenic removal are not covered because no read source describes them.
Preservation for arsenic speciation samples was not sourced.
The 2024 US steam electric rule and other GCC discharge standards were not read.
The AMR Industry Alliance and pharma sector limits do not list arsenic in the sources at hand.
Arsenate pKa values and the surface ligand exchange form of arsenate sorption are from Stumm and Morgan chapters 3, 7 and 9, from the chapter, not re-read; the EPA manuals describe the chemisorption in words without printing stoichiometry. Arsenic sulfide, ferric arsenate and calcium arsenate solubility products are still not sourced.
Lime softening for arsenic is still not covered, so no calcium arsenate equation is written.

Sources

WHO Guidelines for Drinking-water Quality, 4th ed. incorporating the first and second addenda (2022), chapter 12 chemical fact sheet, Arsenic (pp. 340 to 343)
WHO, Arsenic in Drinking-water, background document for development of WHO Guidelines for Drinking-water Quality, WHO/SDE/WSH/03.04/75/Rev/1 (2011)
Directive (EU) 2020/2184 on the quality of water intended for human consumption (recast), Annex I Part B and Annex III
US EPA, National Primary Drinking Water Regulations (table of MCLs)
US EPA, Arsenic Treatment Technology Design Manual for Small Systems, draft for peer review (June 2002), sections 1.3, 2.4, 2.5, 2.6, 2.7 and 4
US EPA, Arsenic Treatment Technology Evaluation Handbook for Small Systems, EPA 816-R-03-014 (July 2003), sections 1.1, 2.5 and 2.6
40 CFR 423.13, Effluent limitations guidelines representing BAT, steam electric power generating point source category
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 Tables 1 and 2
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
ZDHC Wastewater Guidelines Version 2.1 (November 2022), wastewater metals table and sludge table
US EPA, National Recommended Water Quality Criteria, Aquatic Life Criteria Table (arsenic, 1995)
US EPA Method 200.8, Revision 5.4 (1994), Determination of trace elements in waters and wastes by ICP-MS, Table 1 instrument detection limits
Standard Methods for the Examination of Water and Wastewater (online edition), 3114 Arsenic and Selenium by Hydride Generation/Atomic Absorption Spectrometry
Standard Methods (online edition), 3500-As Arsenic (silver diethyldithiocarbamate method)
ISO 17294-2:2023, Water quality. Application of ICP-MS. Part 2: Determination of selected elements including uranium isotopes
PubChem element summary for arsenic; oceanic abundance 3.7 x 10^-3 mg/L from Jefferson Lab
Stumm, W. and Morgan, J. J., Aquatic Chemistry, 3rd ed. (Wiley, 1996), chapter 3 (acids and bases: arsenic acid dissociation), chapter 7 (precipitation and dissolution) and chapter 9 (surface chemistry: ligand exchange on hydrous oxides)
The Element Book, layer 1 entry for arsenic (data/elements/As.json), properties narrative: arsenic sulfides are very insoluble, the basis of sulfide precipitation; orpiment As2S3

Identity

Name and symbol: Arsenic, As
Atomic number: 33 protons
Position: group 15 · period 4 · p-block · metalloid
CAS number: 7440-38-2

Atomic structure

Atomic mass: 74.921 u
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p³
[Ar] 4s²³d¹⁰⁴p³
Electrons per shell: 2, 8, 18, 5
Valence electrons: 5 outer shell

isotope	mass (u)	abundance
⁷⁵As	74.921 595(6)	100 %

natural isotopic composition, mole fraction

Physical properties

State at room temperature: Solid
Melting point: 1,090 K (816.85 °C)
Boiling point: 887 K (613.85 °C)
Density: 5.776 g/cm3
Appearance: metallic grey
Thermal conductivity: 50.2 W/(m·K)
Electrical resistivity: 333 nΩ·m at 20 °C
Electrical conductivity: 3 MS/m
Crystal structure: rhombohedral
Molar heat capacity: 24.64 J/(mol·K)

Chemical properties

Oxidation states: +5, +3, -3
Electronegativity: 2.18 (Pauling Scale)
Ionisation energy: 9.815 eV
1st 947, 2nd 1,798, 3rd 2,735 kJ/mol
Electron affinity: 0.81 eV
Atomic radius: empirical 119, covalent 119, van der Waals 185 pm
Ionic radius: As³⁺ 58; As⁵⁺ 46 pm

Reactivity: A brittle group 15 metalloid that is not very reactive at room temperature; it tarnishes in moist air and is attacked only by oxidising acids, and its compounds are notoriously poisonous.
with water: Does not react with water.
with oxygen, air: Tarnishes in air; on heating it oxidises quickly to arsenic trioxide, which smells of garlic: $\ce{4 As + 3 O2 -> 2 As2O3}$ .
with acids: No reaction with non-oxidising acids or with alkalis; nitric acid oxidises it to arsenic acid (concentrated) or arsenous acid (dilute), and hot concentrated sulfuric acid gives the trioxide.
with halogens: Combines with the halogens; with fluorine it gives the pentafluoride, with chlorine the trichloride: $\ce{2 As + 3 Cl2 -> 2 AsCl3}$ .

Typical compounds

As₂O₃ arsenic trioxide white arsenic, the historic poison and glass additive
AsH₃ arsine toxic, flammable gas behind Marsh's test
As₂S₃ orpiment yellow sulfide mineral, once a pigment
GaAs gallium arsenide semiconductor that turns electricity into laser light
Ca₃(AsO₄)₂ calcium arsenate former agricultural insecticide

Occurrence, production and use

Crustal abundance: 1.8 milligrams per kilogram
Oceanic abundance: 3.7-3 milligrams per liter

Occurrence and sources

arsenopyrite (FeAsS) the most abundant arsenic ore mineral, in gold and base metal sulphide deposits; arsenic accompanies gold in Canada and copper gold ores in Chile
orpiment (As2S3), realgar (As4S4), enargite stockpiled from gold mines in Sichuan, China; realgar and orpiment in China, Peru and the Philippines; enargite as a copper mineral
smelter flue dust and cobalt arsenide ore copper, gold and lead smelters; the Bou Azzer cobalt mine feeding the Guemassa hydrometallurgical complex near Marrakech, Morocco

Extraction, production: Arsenic trioxide from smelter flue dust and roasted arsenopyrite
Arsenic volatilises during roasting and smelting of arsenical copper, gold and lead concentrates and is collected from the flue dust as white arsenic, As2O3, the traded form; Peru, China and Morocco made about 95 percent of the 58,000 tonnes of 2024 (estimate). The roasting chemistry is not stated as a reaction by the source, so no equation is written.
Metal by heating arsenopyrite
$\ce{FeAsS -> FeS + As}$
Balanced from the reactants and products stated by the RSC: arsenopyrite heated so that arsenic sublimes, leaving iron(II) sulphide. High purity (99.9999 percent) metal for semiconductors is refined from the trioxide; China supplied 96 percent of United States metal imports in 2024.
Arsenic acid for wood preservatives
Arsenic trioxide is oxidised to arsenic acid, the key ingredient of chromated copper arsenate (CCA); the oxidant is not named by the source, so no equation is written. Malaysia supplied 99 percent of United States arsenic acid imports.

Uses

Arsenic and its compounds are poisonous. They have been used to make rat poison and some insecticides. Small amounts of arsenic are added to germanium to make transistors. Gallium arsenide (GaAs) can produce laser light directly from electricity.

If you were paying careful attention to the physical data listed above, you may have noticed that arsenic's boiling point is lower than its melting point. This occurs because these two temperatures are measured at different atmospheric pressures. When heated at standard atmospheric pressure, arsenic changes directly from a solid to a gas, or sublimates, at a temperature of 887 K. In order to form liquid arsenic, the atmospheric pressure must be increased. At 28 times standard atmospheric pressure, arsenic melts at a temperature of 1090 K. If it were also measured at a pressure of 28 atmospheres, arsenic's boiling point would be higher than its melting point, as you would expect.

Arsenic is used in bronzing, pyrotechny, and for hardening and improving the sphericity of shot. The most important compounds are white arsenic, the sulfide, Paris green, calcium arsenate, and lead arsenate; the last three have been used as agricultural insecticides and poisons. Marsh's test makes use of the formation and ready decomposition of arsine. Arsenic is finding increasing uses as a doping agent in solid-state devices such as transistors. Gallium arsenide is used as a laser material to convert electricity directly into coherent light.

Wood preservation and pesticides: chromated copper arsenate for pressure treated timber in light poles, marine structures and retaining walls (seven United States producers in 2024); herbicides and insecticides, strictly controlled; organoarsenic poultry feed additives herbicides, insecticides and wood preservatives 84 percent, semiconductors 5 percent, metallurgical 3 percent of United States arsenic use in 2024 (USGS, US figures)
Electronics: gallium arsenide and indium gallium arsenide wafers for solar cells, telecommunications, infrared and laser diodes; germanium arsenide selenide optical glasses
Metallurgy: hardening lead shot, wheel weights and lead acid battery grids; antifriction additive for bearings, bronzing
Chemicals: arsenic as an impurity of phosphate rock and of elemental phosphorus, discharged from wet process phosphoric acid plants and concentrated in arsenic bearing distillation residues from phosphorus trichloride production (SIC BREF)
Mining: arsenic is listed among substances discharged from base metal, gold and silver, industrial mineral, bauxite and coal extraction; the ledger attaches iron co precipitation as its treatment

Safety, toxicity

GHS classification, signal word Danger

H301 Toxic if swallowed Acute toxicity, oral
H331 Toxic 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
H315 Causes skin irritation Skin corrosion/irritation
H318 Causes serious eye damage Serious eye damage/eye irritation
H350 May cause cancer Carcinogenicity
H228 Flammable solid Flammable solids
H251 Self-heating; may catch fire Self-heating substances and mixtures
H341 Suspected of causing genetic defects Germ cell mutagenicity
H360 May damage fertility or the unborn child Reproductive toxicity
H370 Causes damage to organs Specific target organ toxicity, single exposure
H372 Causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure
H302 Harmful if swallowed Acute toxicity, oral
H319 Causes serious eye irritation Serious eye damage/eye irritation
H361 Suspected of damaging fertility or the unborn child Reproductive toxicity
H373 May causes damage to organs through prolonged or repeated exposure Specific target organ toxicity, repeated exposure

Discovery and name

Discovered by: Arabic alchemists
Discovered: before AD 815
First isolated: not in sources
Named by: not in sources
Origin of the name: from Arabic al-zarnīḵ الزرنيخ 'the orpiment'

The element is a steel gray, very brittle, crystalline, semimetallic solid; it tarnishes in air, and when it is heated it rapidly oxidizes to arsenous oxide, which smells of garlic. Arsenic and its compounds are poisonous.

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.