PCB and PCDD partition constants in mixed dissolved phases and the impact of the individual mixture constituents

The impact of mixtures of dissolved phases such as surfactants and humic acids (HA) on highly hydrophobic contaminants (hHOCs, log KOW > 5.5), many of which can be highly toxic e.g. dibenzo-p-dioxins (PCDDs), is unknown. However, it is recognized that due to partitioning processes dissolved organic phases can alter the aqueous solubility of hHOCs and potentially lead to groundwater contamination and offside transport of contamiants. Therefore, methodologies to quantify the mixture impact on the partitioning behaviour of hHOCs is important. 

Mixture behaviour of a binary anionic (C12-2-LAS)-nonionic (C12EO4) surfactant mixtures at two mass ratios (33:67 or 83:17 C12EO4: C12-2-LAS) as well as in mixtures with HA was assessed via polymer-water sorption isotherm, where the critical micelle concentration (CMC) is denoted by the inflection point of the sorption isotherm. Tensiometer measurements were carried out for validation purposes.

A previously reported passive dosing method was applied to determine PCDD and PCB (log KOW 5.8-8.3) partition constants in the different mixture solutions at total surfactant concentrations above the CMC.

The CMCs of both co-surfactants in the mixtures were lower than their CMC in single surfactant solution and were dependent on the mixture ratio. The CMCs of the co-surfactants in either the binary or the ternary mixture with HA increased with increasing HA concentration.

PCDD and PCB partition constants to the C12EO4: C12-2-LAS mixtures were quantified (KMImix) and found to increase with increasing hHOC hydrophobicity as reported in single surfactant solutions. Furthermore, partition constants to micelles (KMImix) for hHOCs in the surfactant mixtures increased with increasing C12EO4 fraction in the mixture, but, stayed below the values for single C12EO4.

A dependency on HA concentration for hHOC partition constants in C12EO4-HA mixtures (KMI-DOC) was observed, where KMI-DOC decreased with increasing HA. In C12-2-LAS-HA mixtures as well as in the ternary mixture log KMI-DOC and log KMImix-DOC were similar for the different HA concentrations as well as in comparison to reported partition constants to single surfactants or HA.

Partition constants of hHOCs to the model mixtures were quantified and the mixture effect of individual dissolved phase constituents was investigated which may be promising for a wide applicability of the techniques in fields such as toxicity testing or bioavailability determination of hHOCs in the environment.