Adsorbable Organic Halides (AOX)

General information on AOX analysis

The term "AOX" stands for Adsorbable Organically Bound Halogens. The X represents the elements chlorine, bromine and iodine. AOX is a convention parameter, mainly used for water and waste water quality assessments. It used for compliance with DIN ISO 9652, EPA 1650 C, EPA 9020 B and measures the concentration of organically bound halogens in water samples.

In addition to the determination of AOX in drinking water and municipal and industrial waste waters, the parameter AOX is among others also determined in sewage sludges, soils and sediments. It is regulated in the norms EN16166 and DIN 38409-S18. The AOX of solids, such as paper or cardboard (ISO 11480) used in the paper processing industry can also be determined in addition to the AOX of the waste water. Sewage sludge that is to be applied to agricultural land as fertilizer in particular or which is used for thermal recycling, is subject to strict controls on its AOX content. 

These tests are necessary because many organic halogen compounds are toxic and persistent. They dismantle slowly and accumulate through the food chain. Therefore, the sum parameter AOX is ecotoxicologically highly relevant. Compared to other chemical methods, AOX determination is relatively young. It was developed in the mid-1970s and established itself as a standard parameter in the list of German standard methods for water, wastewater and sludge analysis in 1985.

What is a sum parameter?

The term "sum parameter", or also “summarized impact and substance parameter” is frequently used in environmental analysis to describe characteristics of certain substance groups or substance classes in sum. Each sum parameter is determined according to a defined procedure, an analytical convention. In this process, the majority of individual substances of a substance class are recorded via a common characteristic. This holds the advantage of sum parameters: they can be determined with considerably less effort than a large number of individual substances. The environmental hazard potential of relevant samples of waters, solids, exhaust air, etc. can be estimated very quickly using a sum parameter. Therefore, it is an ideal screening method.

Procedure of an AOX determination

Mainly two methods are used for sample preparation in AOX analysis: the column method and the batch method. In both methods the compounds will adsorb onto activated charcoal. The sample is brought into intensive contact with a special, low-blank value carbon. For the batch method, the activated carbon and sample are shaken for a specific period and then filtered using a polycarbonate filter or the AFU3 sample preparation device. For the column method, the sample is pressed over at least two interconnected columns filled with active carbon.

Even before the adsorption, some parameters have to be inspected closely:

• pH value of the sample
• Inorganic chloride content
• DOC, dissolved organic carbon content
• Presence of oxidizing agents
• Active chlorine content
• Microorganisms
• Amount of particles

The necessary pH value for the adsorption of the sample should be ≤ two. In addition, in the sample the inorganic chloride content should not exceed 1g/l, as regulated in ISO 9562. Otherwise interferences or false-high readings may occur. Recommendations are listed in the according norms, e.g. ISO 9562.

In contrast to chloride, too high a concentration of DOC can lead to underestimation. Oxidizing agents in the sample must be rendered ineffective by the addition of sodium sulfite. Samples are often diluted prior to enrichment on activated carbon to meet all boundary conditions.

Preparing the samples is crucial to the success of the AOX analysis. There are many factors that can distort the results. The organic chloride content of algae and microorganisms for example or high salt contents in the samples can result in false-high readings or scattering findings. A standard-compliant preparation of the samples is a decisive factor for reliable analyses.

After sample preparation, the AOX adsorbed carbon is burned in a high oxygen atmosphere at temperatures of at least 950°C. The organic halogen compounds will be converted into halogen hydracid. Further products of the combustion are carbon dioxide, water as well as other gaseous oxides (SO₂, NO_x, ...). Concentrated sulfuric acid removes the produced water from the gas streanm. The remaining gases that are released during the combustion are absorbed in acetic electrolytes. The content of halides (Cl, Br, I, but not F) is determined by microcoulometric titration. The result of the AOX analysis is expressed in µg/l Cl.