Transport processes of short chained alkylphenols (SCAP) and other soluble organics (chemical analysis and consulting for research and industry)

Our group is involved in various cooperation projects with industry and international universities concerning the chemical analysis and interpretation of the short chained alkylphenol (SCAP) distribution in surface, ground, process and waste water as well as in soils. The presence and behaviour of individual short chained alkylphenols (SCAP) in the subsoil has extensively been studied. It could be shown on 14 sites, which differ in terms of their geology and hydrogeology, that SCAP are present although the operation at these sites which caused the contamination ceased long ago. SCAP are toxic and widespread contaminants which demand a detailed investigation in terms of their persistence in the environment and also in terms of their individual transport behaviour.

The often used phenolindex is not suitable for the investigation of SCAP in environmental samples. Most SCAP react with a decreased sensitivity and are underrepresented by the index. Para-alkyl SCAP are not detectable by the phenolindex. Simultaneously, other contaminants such as anilines and heterocyclic compounds which almost always appear together with SCAP attribute to the phenolindex. Therefore, it must be concluded that the sum parameter is not precise enough as an input parameter for modelling and on its basis it is rather impossible to predict the development of contamination plumes.

The group participates in the DIN/ISO standardisation of phenol analysis from water and soil and has further developed ultrasensitive methods for their trace detection. This analytical method takes advantage of the latest and commonly established developments in sample preparation and gas chromatography column technique. It could be shown that the separation of underivatised SCAP on medium polarity columns with permethyl-cyclodextrin is possible. Together with headspace SPME, for the selective, water free extraction of SCAP and their selective transfer to the GC injector, the method is economical and is operated fully automated. It is a sensitive and selective analytical procedure which can be applied to very complex samples.

The adsorption mechanism of the highly soluble SCAP is somewhat different from commonly investigated insoluble contaminants such as PAK and BTEX. Their adsorption behaviour can not accurately be described by existing partitioning models and thus their adsorption is commonly overestimated. This requires a thorough investigation on the SCAP’s adsorption mechanism which has been done by numerous batch experiments in this study. The adsorption mechanism of SCAP is that they adsorb in multi-layers onto subbituminous coal which results in a steplike isotherm. The adsorbed layers stabilise themselves by aggregating in hemimicells. SCAP do not show a great tendency to diffuse into the adsorbent and thus intraparticle sorption processes are not predominant. The adsorption capacity is mainly assigned to natural organic matter (NOM). No adsorption was determined for coarse sand and only little adsorption was found for carbonates and dolomites. Overall, the partitioning coefficients of SCAP are very small with the consequence that they are only little retarded in aquifers. Thus, SCAP are very mobile compounds. Under groundwater relevant pH conditions SCAP adsorption onto NOM is not pH dependent. In contrast, the adsorption onto carbonate sediments is pH depended due to the nature of interaction. Generally, the adsorption capacity of carbonates increases between the point of zero charge (ZPC) of the material and the pKa of the SCAP compound.

Results from 1-D transport modelling show that the steplike isotherm effectively decreases the retardation factor. Thus, for total SCAP concentrations in the lower mg/L range the mean retardation factor of SCAP is around 5. However, the easier soluble C0 - C1 SCAP which have a lower distribution coefficient will have retardation factors below 5. Commonly investigated organic contaminants such as BTEX or PAH show retardation factors several orders of magnitude higher than SCAP.

SCAP were found to be very mobile compounds which implies that adsorption as a natural attenuation process does not work effectively for these compounds. Commonly applied remediation systems such as “pump and treat” and also water treatment plants which are both based on activated carbon adsorber columns do not retain SCAP for long. Their breakthrough as toxic organic contaminants happens early with the consequence that whole treatment concept for these soluble contaminants must be put into question.

The investigation of complex contaminations is always very difficult and is best done in combination of several different techniques. For sites which are contaminated by SCAP additional evidence can be gained be using the contaminants themselves as tracers. In order to do so, two parameters are suggested. The PCF (phenol cresols fraction) enables the use of SCAP as partitioning tracers and the MPR (meta- paracresol ratio) as a reactive tracer indicating the presence of oxygen in the aquifer. Thus, the SCAP distribution across contaminated sites can provide valuable information. SCAP distribution pattern are physico-chemically evolved and contain therefore some averaged information about the flow path and aquifer conditions. This in turn supports the long term prediction of the site development. SCAP are little retarded organic compounds describing the maximum extent of the organic plume and thus site investigation should therefore be carried out well beyond the source.

For more information please contact tlicha@gwdg.de