Carbon

Typical wastewaters

• municipal sewage biodegradable and refractory organic carbon measured as BOD₅ 110 to 350, COD 250 to 800 and TOC 80 to 260 mg/L, with bicarbonate alkalinity 50 to 200 mg/L as CaCO₃ low to high strength typical composition
• chemical sector effluent dissolved organic carbon, refractory fraction dominant after biological treatment; TOC 10 to 33 and COD 30 to 100 mg/L as yearly averages, halogenated organics as AOX 0.20 to 1.0 mg/L
• textile dyeing and finishing and leather tanning organic load measured as BOD₅ and COD (textile 150, 80, 40 and leather 250, 150, 100 mg/L COD by level), halogenated fraction as AOX in textile
• high strength industrial wastewater and sludge digestion organic carbon converted anaerobically to methane and CO₂ (about 0.35 m₃ methane per kg COD removed); the effluent needs aerobic polishing

1 · Identity

Symbol, number

C, 6

Oxidation states in water

+4 as dissolved carbon dioxide, carbonic acid, bicarbonate and carbonate (dissolved inorganic carbon, DIC), and as the solid carbonates that scale and dissolve; -4 as methane in anoxic water and digester gas; every state between in dissolved and particulate organic matter, from -3 in methyl groups through 0 in carbohydrate to +3 in carboxylic acids, whose average state sets the ratio of COD to organic carbon; 0 as the activated carbon that adsorbs organics and reduces chlorine.

Note

The element entry carries the allotropes, the carbon cycle, CO₂ in the atmosphere and the fossil carbon economy. This chapter is the carbonate system, the organic carbon parameters, the reagents and the limits. Chlorine's by-products are in the chlorine chapter; nitrification's alkalinity demand is in the nitrogen chapter; struvite and calcium carbonate softening chemistry are in the magnesium and calcium chapters, with the carbonate equations kept here.

2 · Occurrence in water

Natural sources

Inorganic carbon enters water from atmospheric CO₂ (about 390 ppm by 2013 in the element entry), from respiration of organic matter in soil, where CO₂ partial pressures reach ten to a hundred times atmospheric, and from dissolution of limestone and dolomite by that CO₂; the result is the bicarbonate alkalinity of nearly all fresh water. Organic carbon comes from the decay of vegetation and algae (humic and fulvic acids, the natural organic matter that colours water and reacts with chlorine) and from soil leaching; groundwater usually carries little (the US rule treats source TOC below 2 mg/L as needing no precursor removal), surface water more.

Anthropogenic sources

Sewage and industrial organic loads measured as BOD, COD and TOC; methane from landfills, digesters and anoxic sediments; carbon dioxide dosed for recarbonation and pH control; activated carbon and coke; halogenated organics measured as AOX; carbon dioxide from the burning of fossil carbon, which is acidifying the ocean (element entry).

3 · Speciation

Dissolved CO₂ hydrates slowly to carbonic acid (less than 1 percent of the dissolved gas is H₂CO₃), which dissociates with pKa₁ 6.35 and pKa₂ 10.33 at 25 C when the dissolved gas and the acid are lumped as H₂CO₃*. So below pH 6.3 dissolved CO₂ dominates, between 6.3 and 10.3 bicarbonate, above 10.3 carbonate; nearly all natural water sits in the bicarbonate field, which is why alkalinity and bicarbonate are almost the same number there. In an open system the CO₂ is fixed by Henry's law and the atmosphere; in a closed pipe or aquifer the total inorganic carbon is fixed and pH moves with acid or base added (Stumm and Morgan chapter 4). Organic carbon is not a species but a continuum: dissolved (DOC, through a 0.45 µm filter) and particulate, biodegradable (BOD) and refractory, with the humic fraction that absorbs at 254 nm being the chlorine reactive part.

Solubility

CO₂ dissolves according to Henry's law, about 10⁻1.5 mol per litre per atmosphere at 25 C, so air equilibrated water holds about 10⁻5 mol/L of CO₂ and rain sits near pH 5.6 (Stumm and Morgan chapter 4). Calcium carbonate is sparingly soluble and its solubility rises with CO₂ partial pressure and falls with temperature, the reason boilers, hot water pipes and reverse osmosis concentrates scale. Methane is a sparingly soluble gas that escapes on aeration.

Hydrolysis

Carbonate is a base: it takes a proton from water to give bicarbonate and hydroxide, so soda ash solutions are alkaline; bicarbonate is amphoteric and buffers between pH 6 and 10.

Complexation

Carbonate and bicarbonate complex calcium, magnesium and the transition metals (CaCO₃ and MgCO₃ ion pairs, uranyl carbonates that keep uranium mobile) and natural organic matter binds iron, aluminium, copper and lead, slowing their precipitation and passing them through filters (iron chapter); constants not quoted.

Precipitates

CaCO₃ (calcite, aragonite) the universal scale; MgCO₃ only in brines, Mg(OH)₂ instead at high pH; FeCO₃ siderite in anoxic groundwater; MnCO₃; the metal carbonates of soda ash precipitation of heavy metals (sodium chapter).

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Fill BOD and TOC bottles without headspace and keep cold; BOD samples must be set up within 48 hours (Standard Methods); TOC bottles must be carbon clean glass with acid preservation for total carbon only where inorganic carbon will be purged. Chlorinated samples need quenching before BOD (and seeding). Alkalinity and CO₂ change with degassing, so titrate promptly or in the field. COD on high chloride samples (seawater, brines) is unreliable; use TOC. Filter for DOC at once, in the field where possible.

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

Organic carbon as a bulk parameter is not toxic; its health significance in drinking water is as the precursor of chlorination by-products and as the substrate for regrowth. Carbon dioxide in water is harmless to drink (carbonated bottled water may sit below pH 4.5 under the EU DWD) but the gas is an asphyxiant in confined spaces such as lime softening and CO₂ dosing rooms; methane is an explosion hazard; carbon monoxide is in the element entry.

Bioaccumulation

Not applicable to bulk carbon; the persistent halogenated organics counted in AOX include bioaccumulating substances treated under their own entries.

Ecotoxicity

Biodegradable organic carbon kills by oxygen depletion, not by toxicity: the reason for BOD limits, the UWWTD and the secondary treatment rule. US EPA has no numeric aquatic criterion for organic carbon; dissolved oxygen is in the oxygen chapter.

Flags

• The carbonate constants, Henry's constant and the CaCO₃ equations are cited to Stumm and Morgan chapter 4 from memory of the text, not re-read this session.
• The municipal wastewater composition is Metcalf and Eddy Table 3-18 from memory of the chapter; the representative organic formula and the methane yield are from chapters 7 and 10 likewise.
• The TOC and alkalinity bands given as concentrations are the US regulatory bands, not survey data.
• The UWWTD recast values are read from the 2024 Directive; the 1991 Directive tables (BOD₅ 25 mg/L, COD 125, TSS 35, TN 15 and 10, TP 2 and 1) were not re-read and are not quoted.
• Detection limits for the organic carbon methods are general laboratory practice, not read.
• Abu Dhabi values cover two media (marine outfall BOD₅ 30, COD 100, TOC 75 mg/L; sewer COD 1000 mg/L); other GCC states not read.

Gaps

• No survey of TOC, DOC or alkalinity in natural waters was read; regulatory bands stand in.
• No activated carbon isotherm constants, carbon use rates or GAC bed lives are quoted.
• The WHO GDWQ chapter 10 text on organic carbon, taste and odour and the WHO position on TOC were not read.
• Chlorination by-product formation kinetics and the trihalomethane formation potential test are not covered here; see the chlorine chapter.
• The 1991 UWWTD tables, US industrial category BOD and COD limits (OCPSF part 414, textile part 410, pharma part 439) and the EU FDM BREF were not read.
• Other GCC discharge standards were not read.

Sources

Identity

Name and symbol

Carbon, C

Atomic number

6 protons

Position

group 14 · period 2 · p-block · polyatomic nonmetal

CAS number

atomic carbon: 7440-44-0; graphite: 7782-42-5; diamond: 7782-40-3

Atomic structure

Atomic mass

12.0107 u

Electron configuration

1s² 2s² 2p²
[He] 2s²²p²

Electrons per shell

2, 4

Valence electrons

4 outer shell

Physical properties

State at room temperature

Solid

Melting point

3,823 K (3,549.85 °C)

Boiling point

4,098 K (3,824.85 °C)

Density

2.267 g/cm3

Appearance

graphite: black, metallic-looking; diamond: clear

Thermal conductivity

graphite: 119 to 165 W/(m·K)

Electrical resistivity

graphite: 7.837 µΩ·m

Electrical conductivity

127,599.847 S/m

Crystal structure

simple hexagonal

Molar heat capacity

8.517 J/(mol·K)

Chemical properties

Oxidation states

+4, +2, -4

Electronegativity

2.55 (Pauling Scale)

Ionisation energy

11.26 eV
1st 1,086.5, 2nd 2,352.6, 3rd 4,620.5 kJ/mol

Electron affinity

1.263 eV

Atomic radius

van der Waals 170 pm

Ionic radius

C⁴⁺ 16 pm

Reactivity

A nonmetal with four valence electrons that chains into millions of compounds, yet the element itself (graphite, diamond) is comparatively unreactive at room temperature and resists all but the strongest oxidisers; graphite is the more reactive form.

with water

Does not react at ordinary temperatures; red-hot carbon reacts with steam to carbon monoxide and hydrogen, the coal-gas reaction: $\ce{C + H2O -> CO + H2}$

with oxygen, air

Stable in air at room temperature; at high temperature it burns to carbon dioxide (or monoxide when oxygen is short) and strips oxygen from metal oxides, the basis of iron smelting: $\ce{C + O2 -> CO2}$

with acids

No reaction with hydrochloric or sulfuric acid; graphite is oxidised by hot concentrated nitric acid to mellitic acid.

with halogens

No reaction with chlorine; fluorine attacks lamp black at room temperature to carbon tetrafluoride and graphite above $\ce{400 C: C + 2 F2 -> CF4}$

Typical compounds

• CO₂ carbon dioxide product of combustion and respiration; carbonic acid in water
• CO carbon monoxide toxic gas that binds haemoglobin; smelting reductant
• CH₄ methane simplest hydrocarbon; natural gas
• CaCO₃ calcium carbonate limestone, the great carbon store in rocks
• CCl₄ carbon tetrachloride chlorinated solvent; does not hydrolyse
• C₂H₂ acetylene triple-bonded fuel gas for oxy-acetylene welding

Occurrence, production and use

Crustal abundance

2.00×102 milligrams per kilogram

Oceanic abundance

2.8×101 milligrams per liter

Occurrence and sources

• dissolved in seawater about 28 mg/L; crustal estimate 200 mg/kg (Jefferson Lab figures via PubChem)
• hydrocarbons: natural gas, crude oil, oil shale, coal fossilised organic matter; mostly used as fuel, a small fraction as petrochemical feedstock
• carbonates: limestone, chalk, dolomite, magnesite, siderite great rock masses; limestone is the raw material for lime and cement
• natural graphite (flake, amorphous, lump) China, Madagascar, Mozambique, Brazil, Tanzania, Canada, India; world resources exceed 800 million tonnes
• carbon dioxide atmosphere at about 390 ppm (2013) and dissolved in all natural waters

Extraction, production

Steam reforming of natural gas to synthesis gas, the gateway from fossil carbon to ammonia, methanol and hydrogen

$\ce{CH4 + H2O <=> CO + 3 H2}$

Delta H0 = 206 kJ/mol, endothermic, over nickel catalyst at 400 to 600 degrees C inlet (AAF BREF, PDF p68); the carbon monoxide is then shifted, $\ce{CO + H2O <=> CO2 + H2 (PDF}$ p69), and the CO2 removed, which is where the fertiliser industry's process CO2 comes from.

Mining and beneficiation of natural graphite; spherical graphite for lithium-ion anodes

Physical processing. New flake mines opened in Brazil and Tanzania in 2024 and spherical graphite plants in the United States (USGS, printed pp. 84 to 85).

Uses

Amorphous carbon is formed when a material containing carbon is burned without enough oxygen for it to burn completely. This black soot, also known as lampblack, gas black, channel black or carbon black, is used to make inks, paints and rubber products. It can also be pressed into shapes and is used to form the cores of most dry cell batteries, among other things.

Graphite, one of the softest materials known, is a form of carbon that is primarily used as a lubricant. Although it does occur naturally, most commercial graphite is produced by treating petroleum coke, a black tar residue remaining after the refinement of crude oil, in an oxygen-free oven. Naturally occurring graphite occurs in two forms, alpha and beta. These two forms have identical physical properties but different crystal structures. All artificially produced graphite is of the alpha type. In addition to its use as a lubricant, graphite, in a form known as coke, is used in large amounts in the production of steel. Coke is made by heating soft coal in an oven without allowing oxygen to mix with it. Although commonly called lead, the black material used in pencils is actually graphite.

Diamond, the third naturally occurring form of carbon, is one of the hardest substances known. Although naturally occurring diamond is typically used for jewelry, most commercial quality diamonds are artificially produced. These small diamonds are made by squeezing graphite under high temperatures and pressures for several days or weeks and are primarily used to make things like diamond tipped saw blades. Although they posses very different physical properties, graphite and diamond differ only in their crystal structure.

A fourth allotrope of carbon, known as white carbon, was produced in 1969. It is a transparent material that can split a single beam of light into two beams, a property known as birefringence. Very little is known about this form of carbon.

Large molecules consisting only of carbon, known as buckminsterfullerenes, or buckyballs, have recently been discovered and are currently the subject of much scientific interest. A single buckyball consists of 60 or 70 carbon atoms (C60 or C70) linked together in a structure that looks like a soccer ball. They can trap other atoms within their framework, appear to be capable of withstanding great pressures and have magnetic and superconductive properties.

Carbon-14, a radioactive isotope of carbon with a half-life of 5,730 years, is used to find the age of formerly living things through a process known as radiocarbon dating. The theory behind carbon dating is fairly simple. Scientists know that a small amount of naturally occurring carbon is carbon-14. Although carbon-14 decays into nitrogen-14 through beta decay, the amount of carbon-14 in the environment remains constant because new carbon-14 is always being created in the upper atmosphere by cosmic rays. Living things tend to ingest materials that contain carbon, so the percentage of carbon-14 within living things is the same as the percentage of carbon-14 in the environment. Once an organism dies, it no longer ingests much of anything. The carbon-14 within that organism is no longer replaced and the percentage of carbon-14 begins to decrease as it decays. By measuring the percentage of carbon-14 in the remains of an organism, and by assuming that the natural abundance of carbon-14 has remained constant over time, scientists can estimate when that organism died. For example, if the concentration of carbon-14 in the remains of an organism is half of the natural concentration of carbon-14, a scientist would estimate that the organism died about 5,730 years ago, the half-life of carbon-14.

There are nearly ten million known carbon compounds and an entire branch of chemistry, known as organic chemistry, is devoted to their study. Many carbon compounds are essential for life as we know it. Some of the most common carbon compounds are: carbon dioxide (CO2), carbon monoxide (CO), carbon disulfide (CS2), chloroform (CHCl3), carbon tetrachloride (CCl4), methane (CH4), ethylene (C2H4), acetylene (C2H2), benzene (C6H6), ethyl alcohol (C2H5OH) and acetic acid (CH3COOH).

• Petrochemicals and polymers: hydrocarbon feedstock for polymers, fibres, paints, solvents and plastics; ethylene chlorination to $\ce{EDC, C2H4 + Cl2 -> C2H4Cl2}$, and cracking to $\ce{VCM, C2H4Cl2 -> CH2CHCl + HCl (LVOC BREF, PDF}$ p528); ethylene oxidation to ethylene oxide, C2H4 + 1/2 $\ce{O2 -> C2H4O (PDF}$ p400); methanol oxidation to formaldehyde, CH3OH + 1/2 $\ce{O2 -> CH2O + H2O (PDF}$ p375); process CO2 with ammonia to urea, $\ce{2 NH3 + CO2 <=> NH2COONH4}$, then $\ce{NH2COONH4 <=> CO(NH2)2 + H2O (AAF BREF, PDF}$ p352)
• Iron and steel: coke and charcoal as reductant and fuel in smelting; natural graphite in refractories, steelmaking, brake linings, lubricants and powdered metals
• Batteries: spherical and synthetic graphite anodes for lithium-ion batteries; US natural graphite consumption fell 21 percent in 2024 as Chinese synthetic graphite undercut it
• Mining and energy: coal, oil and natural gas extracted as fuels; combustion raised atmospheric CO2 from 280 ppm to 390 ppm
• Phosphorus and inorganic chemicals: coke as reductant in the electric phosphorus furnace, $\ce{2 Ca3(PO4)2 + 6 SiO2 + 10 C -> P4 + 10 CO + 6 CaSiO3 (AAF BREF, PDF}$ p279)
• Water treatment: activated carbon adsorption and filtration of organics in industrial waste water and air; carbon dioxide dosed to lower pH in neutralisation
• Advanced materials: carbon fibre for aircraft, rockets and sports goods; industrial diamond for cutting and drilling, diamond films

Safety, toxicity

GHS classification, signal word Warning

• H319 Causes serious eye irritation Serious eye damage/eye irritation
• H335 May cause respiratory irritation Specific target organ toxicity, single exposure; Respiratory tract irritation

Discovery and name

Discovered by

Egyptians and Sumerians

Discovered

3750 BCE

First isolated

not in sources

Named by

not in sources

Origin of the name

from the Latin carbo for coal and charcoal