⚗️ AOX — adsorbable organically bound halogens in water and wastewater: adsorption on activated carbon, displacement of inorganic halides, combustion at 950 °C and argentometric titration

Physicochemistry PN-EN ISO 9562

Overview

AOX is not the name of a substance but an analytical convention — the standard says so expressly in its introduction. The result represents the sum of organically bound chlorine, bromine and iodine (but not fluorine) which under defined conditions can be adsorbed on activated carbon, and if the sample is not filtered it also covers the halogens bound to the suspended matter. The scope of the standard is the direct determination in water of usually 10 µg/l of organically bound chlorine, bromine and iodine, expressed as chlorides, adsorbable on activated carbon. The method applies to samples with an inorganic chloride concentration below 1 g/l; samples with higher concentrations are diluted before analysis. It also applies to samples with suspended matter on which halogens are adsorbed (e.g. insoluble halides) — filtering the sample before analysis makes it possible to determine dissolved AOX and AOX in the suspended matter separately. For filtered samples with a high content of inorganic chlorides the standard provides a modified method, with solid-phase extraction (SPE-AOX, Annex A, informative), with an explicit proviso: results obtained by the modified method may differ significantly from the results of the required method.

What it is for. The parameter serves above all for monitoring water quality, but it also plays a part in assessing solid waste (e.g. in aqueous eluates of waste) and sewage sludge. The recipients are municipal wastewater treatment plants, environmental authorities and industrial establishments discharging wastewater directly and indirectly. An industrial plant has to show that its wastewater does not exceed the AOX limit value proper to its branch; a treatment plant checks not only compliance with limit values but also the efficiency of its own process — by comparing AOX before and after treatment. That is the practical value of this indicator: a single number says how much halogen-bound organic matter there is in the wastewater, without deciding which compounds these are.

Three routes of adsorption and the choice between them. The standard describes three procedures of adsorption on activated carbon: shaking, stirring and column adsorption; the choice is imposed by laboratory or administrative rules, and in many countries the column method is obligatory for wastewater. The stirring method (also called carbodisc, with a carbon fibre fleece) requires very experienced staff and a great deal of manual work, which is why it is rare in routine laboratories, and it is poorly suited to samples with solid particles — typical of wastewater; the standard itself warns that with samples containing suspended matter, stirring may not cover the particles sufficiently and then recommends shaking or a column. The shaking (batch) method is obligatory for sewage sludge and other solid samples and is sometimes used for waters, but it has limited automation, a greater risk of contamination from the laboratory atmosphere and offers no simple check on the completeness of adsorption; it is not recommended for waters in which chlorides cannot be brought down by dilution to a value ≤ 500 mg/l Cl⁻. The column method covers samples with and without particles, permits a simple check on the completeness of adsorption (a second column in series), is more resistant to high chloride concentrations thanks to more efficient washing of the carbon with the nitrate solution, and lends itself to automation. The standard itself adds that at high chloride concentrations (about 1 g/l) shaking may give greater interferences than the column procedure.

What to expect from interferences. High AOX values may arise from the presence of active chlorine and of certain inorganic bromine and iodine compounds, which bind to the carbon irreversibly — the reactions of these oxidants with the organic compounds of the sample and with the carbon are prevented by adding sodium sulfite immediately after the sample is taken. Organic bromine and iodine compounds may decompose during combustion into elemental bromine or iodine and pass into higher oxidation states, so that they are sometimes determined incompletely (negative error). Samples containing living cells (microorganisms, algae) may give overestimated (falsely elevated) results because of their chloride content — such samples are not analysed earlier than 8 h after acidification. Alcohols, aromatic compounds and carboxylic acids may lower the result (e.g. at DOC > 100 mg/l). The recovery of some polar and hydrophilic compounds, such as monochloroacetic acid, is incomplete.

Status of the standard. The current Polish edition is PN-EN ISO 9562:2007 — the Polish-language version, published on 28 June 2007, 26 pages, price group M, managed by KT 122 for Water Quality — Chemical Tests — Organic Substances, ICS 13.060.50. It implements EN ISO 9562:2004 [IDT] and ISO 9562:2004 [IDT] and supersedes PN-EN ISO 9562:2005 — the English-language version. The PKN catalogue card for this edition has neither a "Date of withdrawal" field, nor a "Superseded by" field, nor a corrigendum (Ap) — the edition is in force. In the PKN list for number 9562 there are only two items: the withdrawn English version of 2005 and the binding Polish version of 2007, so the trap of "the corrigendum hangs only on one language version" does not occur here — only one version has a current card. On the international side ISO 9562:2004 is the third edition which — as its own foreword states — cancelled and replaced the technically revised second edition ISO 9562:1998; the first edition ISO 9562:1989, which we use below in a few expressly marked places, is thereby doubly out of date. On the European side EN ISO 9562:2004 superseded EN 1485:1996.

What this description does not contain. We have not seen the full text of the standard — PKN and ISO supply it for a fee. The scope, interferences, principle, list of reagents and apparatus (clauses 1–7; clause 8 "Sampling and pretreatment of samples" is also outside the preview) are given from the preview of ISO 9562:2004, and the Polish title and the publication data from the PKN catalogue card. Clause 9 (procedure), 10 (calculation), 11 (precision) and 12 (test report) and the annexes are not in the preview. That is why we give the working parameters of the column method from the Analytik Jena application notes for the multi X 2500 analyser, declared as compliant with DIN EN ISO 9562 (this is a manufacturer's source, not the text of the standard), and a few figures — expressly marked — from the preview of the withdrawn edition ISO 9562:1989, which in those places agrees with the manufacturer's notes. We give no data at all on the precision of the method from the current edition, because we have not seen them.

Method principle

The water sample is acidified with nitric acid. The organic compounds contained in the sample are adsorbed on activated carbon — by shaking, by stirring or by column adsorption. The adsorbed inorganic halides are displaced by washing the carbon with a sodium nitrate solution acidified with nitric acid. The loaded carbon is burned in a stream of oxygen, the hydrogen halides formed are absorbed in an absorption solution, and the halide ions are determined by argentometric titration, for example microcoulometrically. The result is expressed as the mass concentration of chlorides. The distinction between dissolved AOX and AOX bound to suspended matter does not follow from the chemistry of the determination but from whether the sample was filtered before analysis.

Applications

• Control of industrial wastewater discharged directly and indirectly — demonstrating compliance with the branch AOX limit value
• Assessment of the efficiency of a treatment plant: comparison of AOX in raw and treated wastewater
• Monitoring of surface water and groundwater for organic halogen compounds
• Control of waters after chlorine disinfection — chlorinated by-products as a summary indicator (with attention to the addition of sodium sulfite binding active chlorine)
• Testing of aqueous eluates of solid waste and sewage sludge (for solids the shaking method is obligatory)
• Pulp-and-paper and chemical wastewater — the edition ISO 9562:1989 names wastewater from pulp mills as requiring longer adsorption because of high-molecular constituents, and the manufacturer’s note examines raw wastewater from a chemical plant

Key parameters

Standard

Standard number

PN-EN ISO 9562:2007

Title (PL)

Jakość wody — Oznaczanie adsorbowalnych, organicznie związanych chlorowców (AOX)

Title (EN)

Water quality — Determination of adsorbable organically bound halogens (AOX)

Step-by-step procedure

1. Taking and preserving the sample

   Take the sample into a flat-bottomed 1000 ml bottle, preferably of brown glass. Immediately after sampling add the sodium sulfite solution — without it active chlorine and oxidising bromine and iodine compounds will raise the result, binding irreversibly to the carbon. Samples containing living cells (microorganisms, algae) are analysed not earlier than 8 h after acidification.

   🌡 Temperature: 0–4 °C for storage of the sample

2. Deciding: total or dissolved AOX

   If the result is to cover the halogens bound to the suspended matter — do not filter the sample. If dissolved and suspended AOX are to be separated — filter the sample before analysis and state this in the test report. This decision changes the meaning of the result, not merely its value.

3. Checking the chloride concentration and diluting

   The method applies to samples with an inorganic chloride concentration below 1 g/l; richer samples are to be diluted before analysis. For saline and filtered waters the standard provides the modified SPE-AOX method (Annex A, informative), with the proviso that its results may differ significantly.

4. Choosing the adsorption procedure

   Choose shaking, stirring or the column — laboratory or administrative rules decide. For wastewater the column method is obligatory in many countries; for sewage sludge and other solid samples — the shaking method. For samples with suspended matter the standard advises against stirring (the particles are sometimes insufficiently covered), and at chlorides of the order of 1 g/l — against shaking (greater interferences than on a column).

5. Acidification and addition of nitrate (column method)

   According to the manufacturer’s note compliant with DIN EN ISO 9562: bring the sample to pH 2 with concentrated nitric acid and then add 5 ml of the nitrate stock solution to 100 ml of the prepared sample. Samples with a high particle content are to be homogenised before adsorption with a high-speed disperser.

6. Adsorption on the columns

   Pass the sample through two adsorption tubes set in series, each filled with about 50 mg of activated carbon, at a flow rate of 3 ml/min. With samples containing particles a filtration column may be placed before the carbon columns — the particles are retained and the carbon columns do not block.

   ⏱ Time: about 45 min for six samples on a parallel unit according to the manufacturer’s note

7. Washing the carbon with the nitrate solution

   Wash the columns with the nitrate washing solution at the same rate of 3 ml/min — according to the manufacturer’s note 25 ml per determination. This step displaces the inorganic halides from the carbon; without it the inorganic chlorides would be counted as AOX.

8. Combustion

   Burn the loaded carbon in a stream of pure oxygen at a temperature of at least 950 °C. The columns are introduced into a vertical quartz tube (in the arrangement described in the manufacturer’s note this is done by an automatic feeder). The organohalogen compounds are converted into hydrogen halides.

   🌡 Temperature: ≥ 950 °C

9. Drying the gas and detection

   Dry the combustion gas with concentrated sulfuric acid in an absorber and then introduce it into the microcoulometric measuring cell. The chlorides are determined by argentometric titration. Microcoulometry is an absolute method, so according to the manufacturer’s note the analyser does not require calibration — it does require confirmation of correct operation with an AOX standard solution (4-chlorophenol in water).

10. Checking the completeness of adsorption and the agreement of dilutions

    Read the result from the first and from the second column separately. According to the withdrawn edition ISO 9562:1989 the AOX value from the second column should not exceed 10 % of the value from the first — otherwise the sample should be diluted. According to the manufacturer’s note to DIN EN ISO 9562 the results of two different dilutions of the same sample should not differ by more than 10 %; that is the simplest test of whether the matrix is overloading the adsorption.

11. Blanks and checking the procedure

    Determine the total blank — it covers the water, the reagents and the gases. According to the edition ISO 9562:1989 the total blank should not exceed 3 µg (or 30 µg/l respectively), and if it does, the adsorption, the combustion and the microcoulometric determination are checked separately. The same edition requires, when checking the whole procedure, five standard solutions and a blank, and regards the result as acceptable when the correlation coefficient is greater than 0.999; the manufacturer’s note adds the criterion of a slope of 0.95–1.05. Note: these figures come from outside the current edition — before use check the wording of clauses 9 and 10 in the copy of PN-EN ISO 9562:2007 you hold.

12. Calculation and test report

    Express the result as the mass concentration of chlorides (µg/l). In the test report state whether the sample was filtered (dissolved or total AOX), which adsorption procedure was used, what dilution was applied and what the blank was. Without this information the result of an analytical convention is not comparable with any other.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Concentrated nitric acid (65 %) and concentrated sulfuric acid are strongly corrosive and oxidising. Acidify samples and fill the absorber in a fume cupboard, wearing acid-resistant gloves, goggles and a laboratory coat.
• For safety reasons the standard itself recommends buying ready-made 4-chlorophenol standard solutions instead of preparing them from the solid substance — the simplest way of avoiding contact with the reagent during weighing.
• The combustion furnace works at a temperature of at least 950 °C with a flow of pure oxygen — the combination of high temperature and oxygen requires keeping combustible substances away and checking the tightness of the oxygen installation.
• The sulfuric acid from the absorber must not flow back into the heated quartz tube when the temperature or the pressure falls — this leads to violent evaporation and cracking of the tube.
• Laboratory air is sometimes appreciably contaminated with organic halogen compounds (reagents, solvents, correction fluid) — this is not only a problem of the result but also of exposure; the carbon on the filter must not be sucked dry, which reduces both the error and the contact with the atmosphere.
• Industrial wastewater tested for AOX is by assumption sometimes loaded with organohalogen compounds of carcinogenic and mutagenic action — treat the sample as hazardous material and send the waste from adsorption and combustion for chemical disposal.

Frequently asked questions