Halogenated organic compounds (as AOX), HALOGENATED ORGANIC COMPOUNDS
AOX stands for 'Adsorbable organically bound halogens' expressed as chloride, and determined according to the relevant European Standard method.
The AOX value is a global parameter resulting from the application of this standard method which intends to assess the total amount of halogenated organic substances in water. This parameter covers a large group of substances which can be adsorbed from water onto activated carbon. They may be simple volatile substances as e.g. trichloromethane (chloroform), or complex organic molecules as dioxins/furans with a large variety of toxic properties. Some of them are formed in small amounts in nature. Most AOXs are chlorine-containing molecules, but bromo- and iodo-AOXs also occur.
The parameter AOX only exists as a combined analytical measurement of organically bound halogens for the monitoring and control of pollutants in effluents treated using chlorine (or more rarely) bromine and in effluents which still contain chlorine-containing products or intermediates.
One of the main sources of AOX discharges world-wide is the pulp and paper industry. When using chlorine and chlorine chemicals to bleach fibres, some of the chlorine reacts with organic matter to form chlorinated organic substances which may then be discharged with the treated effluent and detected as AOXs.
In the EU, chlorine gas bleaching has been abandoned in favour of more environmentally sound bleaching techniques that use chlorine dioxide and ozone, reducing the amount of AOX in the discharges and reducing the amount of highly chlorinated compounds as well. Smaller quantities of AOX are also formed during the routine chlorination of drinking water, swimming pools, cooling waters and process waters in laundries.
Another main source of AOX discharges are the effluents of all those industrial facilities which contain chlorine-containing substances from manufacturing or use. The chemical industry is mentioned especially, though other sectors such as metal surface treatment, textiles, waste treatment, printing and dry cleaning are also concerned.
There is no direct relationship between AOX value and toxicity. Excessive exposure to chemicals in this diverse grouping of chemicals may affect health with the possible effects depending on the particular chemical. Many of the chemicals detected under the parameter AOX are also individually listed as well. Some of the chlorinated chemicals detected by AOX are toxic to fish and other aquatic organisms at low concentrations. Many are persistent and have a tendency to bioaccumulate also.