Measuring AOX with Combustion with Ion Chromatography

AOX has been a vital part of wastewater monitoring since the early 1990s. It was developed as a sum parameter to define the quantity of (non-polar) chlorinated hydrocarbons. This type of substance can be easily enhanced on activated charcoal. Following the combustion of this charcoal in an oxygen atmosphere, the resulting chloride is captured in an absorption solution. The content of chloride can then be established by various methods.

Combustion Module

As a detection technique, coulometric titration managed to succeed primarily due to its sensitivity. The AOX value thereby attained is a value established by the procedure. The steps to be followed are prescribed in EN ISO 9562: 2004 (DEV-H14)^[1]. Since, in the case of samples having high chloride concentrations, the inorganic chloride is not fully removed from the charcoal even after frequent rinsing, the high chloride procedure according to EN ISO 9562: Annex A was formulated.

Ion Chromatography

As sum parameter for organically bound chlorine, organically bound bromine and iodine are also distinguished using coulometric AOX, without, however, any sign of their respective proportions. In the mid-1990s, the question of separating halogens and, specifically, measuring the content of organically bound iodine in different wastewaters arose. As such, OleksyFrenzel et al.^[2] combined preconcentration in activated charcoal followed by combustion and ion chromatography as a technique for establishing formerly organically bound halogens (AOBr, AOCl, AOI).

With the proof of perfluorinated compounds in the environment, the application field of the AOX technique was finally extended for AOF established by the very sensitive ion chromatography.^[3] In principle, it would also be conceivable to establish the proportion of organically bound sulfur. Unfortunately, this is not possible because of residual sulfur levels in activated charcoal found in the market.

Besides establishing organically bound halogens from aqueous matrices, this combustion technique integrated with ion chromatography as determination technique has been progressively used over the last few years for the examination of halogens and sulfur in numerous raw materials and industrial products.

Devices, Chemicals, Methods

To define the levels of organically bound halogens in groundwater, samples of surface water and wastewater are either pumped over activated charcoal columns (column technique) or shaken with activated charcoal (shaking technique).

For the enrichment using the column technique, Hessenwasser uses a low-fluoride system (a1 Envirotech: Mitsubishi MCI TOX Sample Preparator and AOX Enrichment 30).

The combustion and determination are performed with a device combination (Metrohm),^[4] comprising of a combustion module, an autosampler, an absorber module, and an ion chromatograph with chemical and CO₂ suppression as well as UV detection and conductivity.

The combustion in the oxygen stream is enhanced by the automatic incorporation of water (hydropyrolysis). In the case of AOCl, AOF, AOBr, and AOI determination from aqueous matrices, ultrapure water is used as the absorption solution. For the examination of a majority of solids, 100 mg/L H₂O₂ is used as the absorption solution^[5] and a pre-concentration column, Metrosep A PCC 2HC/4.0, is used for the removal of the matrix peak around fluoride.

The ion chromatographic separation is performed on a high capacity separation column (Metrohm A Supp 5-150/4.0, alternatively A Supp 5-250/4.0, guard column Metrosep A Supp 5 Guard/4.0). The eluent used is a solution containing 3.2 mmol/L Na₂CO₃ and 1.0 mmol/L NaHCO₃. The flow rate is fixed to 0.7 mL/minute, while the injected volume of sample is 200 μL.

Table 1 shows the measured retention times for the nitrogen, halogen, and sulfur species. The oxyhalide interferences stated in the table could not be detected in practice, seemingly because of the sulfur content of the activated charcoal.

With the aforementioned experimental conditions, detection limits of approximately 5 μg/L for AOCl, approximately 2 μg/L for AOF, and approximately 1 μg/L for AOBr and AOI could be realized in regular operation using commercial AOX activated coals with an IC injection volume of 200 μL and a sample volume of 100 mL. By blank value optimizations (for example, use of low-chlorine and low-fluorine charcoals), lower detection limits could be realized.

Table 1. Retention times of the tested halogen, nitrogen, and sulfur species on the Metrohm A Supp 5 - 150/4.0 column using an eluent consisting of a solution of 3.2 mmol/L Na₂CO₃ and 1.0 mmol/L NaHCO₃ at a flow rate of 0.7 mL/minute.

Automated Work

When using an autosampler, the activated charcoal in the sample boat will dry out upon extended storage in the sample rack.

If the boat is then moved into the combustion module, care has to be taken to guarantee that no charcoal from the sample boat is lost in the process. To skip this step altogether, wetting of the charcoal extruded from the preconcentration columns with two drops of glycerol, applied with the help of a Pasteur pipette, has proven to be very effective.

The glycerol encapsulates the activated charcoal, so that it stays in the boat. Figure 1 illustrates that the coal does not fall out even if the boat is tilted upside down. As control measurements have revealed, the p.a. glycerol used does not result in a blank value increase for the halogens. Repetitive measurements exhibited no change in the measured concentrations. Furthermore, no drift was observed as a factor of the time the samples were waiting on the autosampler before analysis.

Figure 1. Top left: Extruded activated charcoal in the sample boats, Top right: Wetting with two drops of glycerol. Bottom left: wet activated charcoal. Bottom right: after wetting the activated charcoal with glycerol, it does not fall out of the sample boat even when turned upside down. This effect persists for about 24 hours.

Overview

Using glycerol to fix the charcoal may help prevent possible interference from dropping or flying activated charcoal of automated Combustion IC (CIC) determinations. On the whole, Combustion IC is a compatible technique for markedly increasing the information content from an AOX assay.

The inadequacies of the AOX may continue to be challenging (pretreatment of the sample, preconcentration behavior on charcoal because of varying polarity, and occurrence of high contents of inorganic halogens). But, ion chromatography contrary to coulometric titration offers a lot more information relating to AOX analysis. Therefore, establishing a sum parameter for organically bound fluoride (AOF) apart from the determination of AOCl, AOBr, and AOI is achievable.

By conversion and addition, the AOX of the sample can also be calculated from the AOBr, the AOCl, and the AOI (Table 2) content found. An extension of the measuring approaches to the legally controlled range would be required, since with the determination of the newly standardized sum parameter AOF, a concurrent determination of the AOX would also be possible.

Table 2. Comparison of the measured AOX content in a sewage treatment plant discharge and a sewage sludge with the calculated content (addition of Cl equivalents) from the CIC determination.

Figure 2. Concentration curve for 10 consecutive measurements of a fluoride blank (green) and a solution of 50 μg/L fluoride in the form of 4-fluoro-benzoic acid (blue).

References

Adrian Kaiser², Karl-Heinz Bauer¹, Karin Koch¹, and Konstantin Goettgens¹

Affiliations

¹Hessenwasser GmbH & Co.KG, Darmstadt, Deutschland

²Hochschule Fresenius, Idstein, Deutschland

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.