The influence of surfactants on the sorption of phenanthrene in natural waters
Polycyclic aromatic hydrocarbons (PAHs) are widely recognised as harmful, persistent organic pollutants, whilst surfactants are more easily degraded but their ubiquitous use both domestically and industrially ensures their presence in natural waters. Owing to both their hydrophobic and hydrophilic properties, surfactants are capable of both adsolubilisation and solubilisation of co-contaminants, hence, their presence in natural waters is considered extremely significant. The purpose of this research was to determine how surfactants and PAHs influence each other in estuaries where both are co-disposed. This study is the first to employ environmentally realistic concentrations of surfactants and to use natural particles in order to determine how changes in surfactant behaviour impinge on the sorption of a representative PAH to sediment. The experimental method employed involved tracing the solubility and sorption of a '''C-labelled PAH (phenanthrene) in the presence of representative surfactants from the three main groups, i.e., anionic, nonionic and cationic. Initially the four compounds were examined in isolation at two temperatures (8°C and 20°C), salinities and particle concentrations. The next stage involved incorporating each surfactant with phenanthrene in the same experiment and repeating the range of variables. Solubility and adsorption of phenanthrene in the presence of Triton X-100, sodium dodecyl sulphate (SDS) or hexadecyltrimethylammonium iodide (HDTMA) proved to be highly complex. The adsorption, expressed as a Freundlich coefficient, KF was generally nonlinear and varied according to surfactant type, salinity, temperature, and contact sequence. The overriding influence, however, was sediment particulate matter (SPM) concentration. On some occasions the surfactants in isolation displayed unusual particle concentration effects (PCE), such as an increase of surfactant in the aqueous phase as the SPM concentration increased. Despite this, they all generally exaggerate the more typical PCE displayed by phenanthrene, where the particle-water partition coefficient is reduced as particle concentration increases. This effect is then modified by temperature and contact sequence, which has important environmental implications with regard to removal or release of compounds stored within sediments. However, overall, at environmentally relevant concentrations, the surfactants appear to enhance sorption, rather than solubility, with HDTMA the most effective surfactant at removing phenanthrene from the aqueous phase.