Oxygen

Typical wastewaters

• municipal sewage and other biodegradable discharges dissolved oxygen depleted by the oxygen demand of organic carbon and ammonia; Abu Dhabi marine discharge must carry at least 3.0 mg/L at the point of discharge
• textile dyeing and finishing and leather tanning dissolved oxygen measured on site and reported, no limit value
• urban waste water treatment plant discharges (EU) oxygen demand of the effluent: BOD₅ 25 mg/L O₂ and COD 125 mg/L O₂, or 70 to 90 and 75 percent reduction Annex I Table 1
• dairy processing effluent BOD₅, the oxygen consumed in five days, as the regulated parameter receiving stations BPT 0.475 kg BOD₅ per 1,000 kg BOD₅ input daily maximum, 0.190 30-day average
• palm oil mill effluent dissolved oxygen 2.6 to 4.1 mg/L with BOD 254 to 1,541 mg/L and COD 1,231 to 2,422 mg/L eight smallholder mills, Nigeria
• intensive aquaculture effluent carbonaceous BOD₅ consumed across fluidised sand biofilters; outlet cBOD₅ 1.7 mg/L, 66 to 82 percent removal
• dam spillway tailwater (total dissolved gas supersaturation) supersaturated dissolved oxygen and nitrogen from spill; EPA criterion 110 percent total dissolved gas, waivers to 120 percent Columbia and Snake River dams
• regulated river below a hydropower station total gas pressure 96 to 133 percent, a chronic pollutant toxic to aquatic respiration Otra River, Norway; fish deaths with gas bubble trauma

1 · Identity

Symbol, number

O, 8

Oxidation states in water

-2 in water itself and in every oxyanion and oxide (sulfate, nitrate, phosphate, carbonate, the hydroxide precipitates); 0 as dissolved O₂ and as ozone O₃ (a neutral molecule that acts as a two electron oxidant to O₂); -1 in hydrogen peroxide and the hydroxyl radical, the intermediates of advanced oxidation. Superoxide and singlet oxygen appear in mechanisms but not in plant design.

Note

The element entry carries the atmosphere, air separation and the biological importance of O₂. This chapter is dissolved oxygen, ozone, peroxide and the oxidation and corrosion they drive. The Fe(II) oxygenation rate law and the oxidant doses for iron and manganese are in the iron chapter; nitrification oxygen demand is in the nitrogen chapter; aerobic BOD oxidation is in the carbon chapter; bromate is in the bromine chapter.

2 · Occurrence in water

Natural sources

Dissolved oxygen enters from the atmosphere and from photosynthesis and is consumed by respiration and by the oxidation of reduced species; saturation falls with temperature and salinity, so warm, saline water holds least. Groundwater below the water table and stratified lake bottoms run out of oxygen and turn over to Fe(II), Mn(II), ammonia, sulfide and methane; that anoxic boundary is where most groundwater treatment problems begin (iron chapter). Ozone and peroxide occur naturally only at trace levels from photochemistry.

Anthropogenic sources

Oxygen depletion by sewage and other biodegradable discharges, the oldest water pollution problem and the reason for BOD limits; supersaturation below dams and in algal blooms; ozone generated on site for disinfection and oxidation; hydrogen peroxide dosed for advanced oxidation, sulfide control and dechlorination; pure oxygen and enriched air in activated sludge; oxygen scavengers (sulfite) in boiler feedwater.

3 · Speciation

Dissolved oxygen is a neutral gas whose equilibrium concentration follows Henry's law and whose kinetics are everything: thermodynamically it should oxidise every reduced species in water, but it reacts fast only with Fe(II) above pH 7, sulfide and sulfite, and needs bacteria for ammonia, organic carbon and methane. Ozone is a stronger oxidant, second only to the hydroxyl radical among chemicals used in water treatment, sparingly soluble, and it decomposes spontaneously by a radical chain that yields about 1.5 mol of hydroxyl radicals per mol of ozone in pure water; under acid conditions direct molecular ozone oxidation dominates, at high pH or with UV or hydrogen peroxide the hydroxyl radical route takes over, which is the basis of advanced oxidation (EPA chapter 3). Hydroxyl radicals react at 10^10 to 10^13 per mol per second, near diffusion control, live microseconds and never exceed about 10⁻12 mol/L; bicarbonate and carbonate scavenge them.

Solubility

Oxygen: about 9.1 mg/L at 20 C in fresh water at sea level, falling with temperature and salinity (Metcalf and Eddy Appendix D). Ozone: 570 mg/L at 20 C for pure ozone gas, more soluble than oxygen but chlorine is 12 times more soluble than ozone; with feed gas below 14 percent ozone the driving force limits residuals to about 1 mg/L (EPA). Hydrogen peroxide is miscible.

Hydrolysis

Not applicable to O₂ and O₃. Hydrogen peroxide is a very weak acid.

Complexation

Molecular oxygen binds to Fe(II) and Mn(II) centres in the first step of their oxidation and to haemoglobin; no aqueous complexes of consequence in treatment.

Precipitates

Oxygen is the oxidant that makes the precipitates: Fe(OH)₃, MnO₂, elemental sulfur from sulfide, and calcium carbonate where CO₂ is stripped by aeration. Oxygen itself does not precipitate.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Dissolved oxygen must be measured in situ or fixed on site: a sample gains oxygen from air on the way to the laboratory if it is anoxic and loses it if it is supersaturated, and biological activity in the bottle changes it in minutes. Fill BOD bottles by displacement without bubbles. Ozone and peroxide residuals decay within minutes and are measured at the sampling point. Temperature, salinity and barometric pressure are needed to convert dissolved oxygen to percent saturation.

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

Dissolved oxygen is not a health parameter in drinking water. Ozone gas is toxic, above 0.2 mg/m₃ for an 8-hour day (element entry), and highly corrosive; ambient monitoring at 0.1 ppm by volume protects plant staff (EPA). Hydrogen peroxide is an irritant and oxidiser in the concentrated solutions dosed. Oxygen enriched atmospheres raise fire risk (element entry: above 25 percent many organic materials are highly flammable).

Bioaccumulation

Not applicable.

Ecotoxicity

Dissolved oxygen is the aquatic life criterion: the US EPA 1986 freshwater criteria and the saltwater criteria for Cape Cod to Cape Hatteras set minimum concentrations by life stage and averaging period (cited in the criteria table, the numbers not read this session); the Abu Dhabi marine specification requires at least 3.0 mg/L in the discharge itself. Ozone and peroxide residuals are acutely toxic to aquatic life and are destroyed before discharge.

Flags

• The oxygen solubility figures and the aeration set point are Metcalf and Eddy Appendix D and chapter 8 from memory, not re-read this session.
• The peroxone and Fenton stoichiometries are the accepted net reactions written here; the EPA manual and Metcalf and Eddy describe the processes without printing them.
• The EPA dissolved oxygen criteria document is a scanned PDF whose text could not be extracted; numeric criteria are therefore not quoted.
• The WHO position on dissolved oxygen is inferred from the absence of a fact sheet; chapter 10 of the GDWQ was not read.
• Deaeration and peroxide quenching performance are general practice, not from sources read.
• Abu Dhabi values cover two media (marine outfall DO at least 3.0 mg/L; sewer no row); other GCC states not read.

Gaps

• No dissolved oxygen survey of rivers, groundwater or effluents was read; saturation values and set points stand in.
• The numeric US dissolved oxygen criteria (1986) and the WHO chapter 10 text were not read.
• No CT values for ozone disinfection are quoted; the EPA manual has the tables.
• No oxygen transfer efficiencies, alpha factors or energy figures are quoted.
• Hydrogen peroxide dosing rates for sulfide, dechlorination and Fenton are not sourced; hydrogen sulfide chemistry is in the sulfur chapter.
• Other GCC discharge standards were not read.

Sources

Identity

Name and symbol

Oxygen, O

Atomic number

8 protons

Position

group 16 · period 2 · p-block · diatomic nonmetal

CAS number

7782-44-7

Atomic structure

Atomic mass

15.9994 u

Electron configuration

1s² 2s² 2p⁴
[He] 2s²²p⁴

Electrons per shell

2, 6

Valence electrons

6 outer shell

Physical properties

State at room temperature

Gas

Melting point

54.36 K (-218.79 °C)

Boiling point

90.2 K (-182.95 °C)

Density

0.0014 g/cm3 (gas at STP, so 1.429 g/L)

Appearance

gas: colorlessliquid and solid: pale blue

Thermal conductivity

26.58×10-3 W/(m·K)

Electrical resistivity

not in sources

Electrical conductivity

not in sources

Crystal structure

cubic

Molar heat capacity

not in sources

Chemical properties

Oxidation states

-2

Electronegativity

3.44 (Pauling Scale)

Ionisation energy

13.618 eV
1st 1,313.9, 2nd 3,388.3, 3rd 5,300.5 kJ/mol

Electron affinity

1.461 eV

Atomic radius

empirical 66, covalent 66, van der Waals 152 pm

Ionic radius

O²⁻ 140 pm

Reactivity

A highly electronegative nonmetal and potent oxidiser; O2 combines with nearly every element to give oxides, slowly at room temperature (rusting, autoxidation of organics) and rapidly in combustion.

with water

Does not react; it dissolves sparingly, more in cold water, which is what sustains aquatic life.

with oxygen, air

O2 does not react with itself, but an electric discharge or ultraviolet light converts it to the reactive allotrope ozone: $\ce{3 O2 -> 2 O3}$

with acids

Does not react with acids.

with halogens

Does not combine with fluorine under ambient conditions; oxygen difluoride and dioxygen difluoride form only in an electric discharge at low temperature and decompose on warming, and there is no direct reaction with chlorine: $\ce{O2 + 2 F2 -> 2 OF2}$

Typical compounds

• H₂O water the most familiar oxide; nine tenths of it by mass is oxygen
• SiO₂ silicon dioxide quartz and sand; the crust is largely silicates
• Fe₂O₃ iron(III) oxide hematite and rust
• Al₂O₃ aluminium oxide corundum, bauxite, ruby and sapphire
• CO₂ carbon dioxide trace gas of the air, product of respiration and burning
• H₂O₂ hydrogen peroxide peroxide, oxygen in the minus one state

Occurrence, production and use

Crustal abundance

4.61×105 milligrams per kilogram

Oceanic abundance

8.57×105 milligrams per liter

Occurrence and sources

Oxygen is the third most abundant element found in the sun, and it plays a part in the carbon-nitrogen cycle, the process once thought to give the sun and stars their energy. Oxygen under excited conditions is responsible for the bright red and yellow-green colors of the Aurora Borealis.

A gaseous element, oxygen forms 21% of the atmosphere by volume and is obtained by liquefaction and fractional distillation. The atmosphere of Mars contains about 0.15% oxygen. The element and its compounds make up 49.2%, by weight, of the earth's crust. About two thirds of the human body and nine tenths of water is oxygen.

In the laboratory it can be prepared by the electrolysis of water or by heating potassium chlorate with manganese dioxide as a catalyst.

• dissolved in seawater about 857,000 mg/L; crustal estimate 461,000 mg/kg (Jefferson Lab figures via PubChem)
• O2 in air 21 percent of the atmosphere by volume; the industrial source
• water and silicate, oxide and carbonate rocks 49.2 percent of the crust by mass and nine tenths of water by weight
• dissolved oxygen surface waters, sustaining aerobic life

Extraction, production

Cryogenic distillation of liquid air, pressure-swing adsorption over zeolite, or ceramic membrane separation

Physical separation, no chemical reaction; about 99 percent of supply. Zeolite beds adsorb nitrogen and let oxygen pass; ceramic membranes give higher purity.

Electrolysis of water

$\ce{2 H2O (l) -> 2 H2 (g) + O2 (g)}$

About 1 percent of supply; described in words only, no equation printed in the sources.

Uses

Oxygen is a highly reactive element and is capable of combining with most other elements. It is required by most living organisms and for most forms of combustion. Impurities in molten pig iron are burned away with streams of high pressure oxygen to produce steel. Oxygen can also be combined with acetylene (C2H2) to produce an extremely hot flame used for welding. Liquid oxygen, when combined with liquid hydrogen, makes an excellent rocket fuel. Ozone (O3) forms a thin, protective layer around the earth that shields the surface from the sun's ultraviolet radiation. Oxygen is also a component of hundreds of thousands of organic compounds.

Plants and animals rely on oxygen for respiration. Hospitals frequently prescribe oxygen for patients with respiratory ailments.

• Iron and steel: oxygen enrichment of blast furnaces and steelmaking; the greatest commercial use of the gas
• Chemicals: ammonia oxidation for nitric acid, $\ce{4 NH3 + 5 O2 <=> 4 NO + 6 H2O (AAF BREF, PDF}$ p124); ethylene oxide by direct oxidation over silver at 15 to 25 bar, C2H4 + 1/2 $\ce{O2 -> C2H4O}$, releasing 105 kJ/mol, with the side reaction $\ce{C2H4 + 3 O2 -> 2 CO2 + 2 H2O}$ releasing 1,323 kJ/mol (LVOC BREF, PDF p400); oxychlorination of ethylene, C2H4 + 1/2 $\ce{O2 + 2 HCl -> C2H4Cl2 + H2O}$, releasing 239 kJ/mol, which recycles the HCl from VCM cracking (PDF p528); sulfur dioxide conversion, SO2 + 1/2 $\ce{O2 <=> SO3}$, delta H0 = -99 kJ/mol, the heart of the sulfuric acid contact process (AAF BREF, PDF p179); formaldehyde, CH3OH + 1/2 $\ce{O2 -> CH2O + H2O}$, delta H = -159 kJ/mol over a metal oxide catalyst (LVOC BREF, PDF p375); synthesis gas for ammonia and methanol; hydrogen peroxide
• Water and waste water treatment: pure oxygen or oxygen-enriched air in activated sludge aeration (CWW BREF, PDF p316 and p321); ozone, with or without UV, hydrogen peroxide and Fenton's reagent as chemical oxidants (PDF p247); a growing use in the treatment of sewage and industrial effluent
• Metal fabrication: oxy-acetylene welding and cutting
• Medicine: oxygen therapy for respiratory ailments

Safety, toxicity

not in sources

Discovery and name

Discovered by

Michael SendivogiusCarl Wilhelm Scheele

Discovered

1604, 1771

First isolated

not in sources

Named by

Antoine Lavoisier

Origin of the name

from the Greek ὀξύς (acid, literally 'sharp', from the taste of acids) and -γενής (producer)

The gas is colorless, odorless, and tasteless. The liquid and solid forms are a pale blue color and are strongly paramagnetic.