Adsorbable organically bound halogens

from Wikipedia, the free encyclopedia

The AOX ( A dsorbierbare O rganisch bound halogens ; which X is generally an abbreviation for any chemicals in the halogen used) is a group of parameters of the chemical analysis , which primarily to assess water and sewage sludge is used. The sum of the organic halogens that can be adsorbed on activated carbon is determined. These include chlorine , bromine and iodine compounds . Organic fluorine compounds are not recorded by this analysis method. Since the chlorine-containing compounds usually clearly predominate in the organically bound halogens, mg Cl / L was used as the unit for AOX .

history

The determination method for AOX was developed in the mid-1970s by Wolfgang Kühn at the University of Karlsruhe as part of his doctoral thesis. AOX has been a standard parameter in the list of German standard methods for water, waste water and sludge analysis since 1985 . In 1990 the AOX was added to the list of pollutant parameters according to Section 3 of the Waste Water Tax Act (AbwAG). In order to be able to better control the entry of organically bound halogens into the soil and thus into the groundwater and surface water , the AOX content in sewage sludge has to be examined since 1992 in accordance with Section 3 (5) of the Sewage Sludge Ordinance (AbfKlärV) , if it is agricultural or horticultural soil is to be applied; according to her, the sum of the AOX could not be higher than 500 mg per kg of sewage sludge dry matter.

Analytical method

The method of determination for AOX is specified in DIN EN ISO 9562 (previously DIN EN 1485 or DIN 38409-H14). The organically bound halogens in the sample are bound (adsorbed) to the activated carbon either by shaking out in the presence of activated carbon (shaking method ) or by flushing the sample through a glass tube filled with activated carbon (column method).

There are several parameters to consider when preparing the sample: pH , inorganic chloride , DOC and oxidizing agent . The pH value required for enrichment should be less than or equal to 2. The limit for the chloride content in the sample to be measured (not to be confused with the original sample) is 1 g / l . In addition, there may be malfunctions, in particular overdiagnosis . The chloride content can be determined beforehand using various methods . A high DOC (dissolved organic carbon, dissolved organic carbon ) content of more than 10 mg / l, however, can interfere with the adsorption, and generally leads to lower findings . Oxidizing agents must be reduced with sodium sulphite (salt of sulphurous acid) (proof with potassium iodide / starch ). The sample is mixed with a sodium nitrate solution and enriched in the respective process. For the shaking method, the activated carbon is added to the sample to be examined and mixed for one hour while shaking - hence the name of the method. In contrast, with the column method, the sample is passed over the activated carbon with a defined flow rate (3.3 ml / min ). The loaded activated carbon is washed with a dilute sodium nitrate washing solution in order to remove any inorganic halide residues ( salts ) present . The activated charcoal is burned in a stream of oxygen at temperatures of around 1000 ° C , usually 960-980 ° C. The reaction mainly produces water , carbon dioxide , nitrogen oxide , sulfur dioxide and the desired hydrogen halides . The water produced in the combustion process is bound with sulfuric acid in a dry trap. The remaining gases are caught in an acetic acid solution and measured by means of microcoulometric titration , silver ions are generated by electricity . The result is given in µg / l Cl.

The accuracy of the analysis method depends on the substances present in the sample ( sample matrix ). In particular, high concentrations of inorganic chlorine-containing and non-halogen-containing organic compounds seem to disturb the analytical method significantly. The high-chloride process (SPE-AOX) (formerly DIN 38409-H22) is available as an alternative .

swell

Organohalogen compounds, i.e. organic substances with at least one halogen atom ( chlorine , bromine , iodine , fluorine ), can come from both natural and anthropogenic sources. There is no general answer to which of the two sources dominates, as there is no reliable knowledge about the natural synthesis quantities of organohalogen compounds. It is estimated that more than 5 million tons of methyl chloride are produced annually from algae production. Organic halogen compounds from natural sources contribute to an AOX base load ( background concentration ) of soils, groundwater and surface waters.

Anthropogenic sources of organohalogen compounds are mainly industrial (and municipal) wastewater, the fertilization of agricultural areas with sewage sludge and the use of halogenated pesticides . In Germany, for example, sewage sludge is not allowed to be applied to or in soils used for forestry, agriculture or horticulture, or given by sewage sludge producers if the AOX content therein exceeds the so-called sewage sludge- related limit of (until 2017 500, since then) 400 mg per kilogram of sewage sludge - Exceeds dry matter (or limit values ​​for other substances).

Expressiveness

The informative value of the AOX is controversial in specialist circles, as this parameter records both practically harmless compounds and highly toxic dioxins and furans in equal measure. In addition, some connections that cannot be identified are also recorded. AOX is a cumulative parameter that is of limited significance with regard to the contamination of a sample with organohalogen compounds, but does not allow any ecotoxicological statements.

Individual evidence

  1. ↑ Section 4, Paragraph 11 of the AbfKlärV in the version valid until October 1, 2017
  2. ↑ until 2017: 500 mg / kg TS according to § 4 Paragraph 11 of the Sewage Sludge Ordinance (AbfKlärV) of April 15, 1992, since 2017 400 mg / kg TS according to § 8 Sewage Sludge Ordinance of September 27, 2017 with No. 2 of its Appendix 1 ; a violation constitutes an administrative offense according to § 36 para. 1 no. 6 AbfKlärV, § 69 KrWG .
  3. ↑ Rainer Schulze-Rettmer: Does the AOX still make sense? KA- Wasserwirtschaft, Abwasser, Abfall 48 (11), pp. 1602-1614 (2001), ISSN  1616-430X