Changes to cadmium associations in canal sediments on drying and oxidation

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed in the marine environment. Many PAHs are known to be mutagenic and carcinogenic, so their presence in the environment may constitute a serious threat to the health of humans and ecosystems. Bioremediation has shown promise as a potentially effective and low-cost treatment to eliminate organic pollutants in the environment. However, biodegradation of PAHs in soil or sediment is usually very slow because the bioavailability is limited by a poor mass transfer, since PAHs generally are well known for their low aqueous solubility, high hydrophobicity and tendency to sorb tightly to soil or sediment constituents. Applying surfactants at contaminated site might be one way to facilitate the release of sorbed PAHs from solid matrices and increase the aqueous concentrations of hydrophobic compounds resulting in higher mass transfer rate. Though using chemically produced surfactants has shown promise for significantly reducing time and cost of remediation, the use of these surfactants in environmental treatment processes has been rather limited due to the concerns regarding their toxicity and biodegradation fact. Being biologically synthesized surface-active agents, biosurfactants have the advantages of being readily biodegradable and non-toxic comparable to chemically produced surfactants. This distinct perspective makes them valuable in PAHs bioremediation. In this paper, the effectiveness of anionic rhamnolipid biosurfactant on the desorption and biodegradation of PAHs in sediment slurry was compared with that of synthesized surfactant sodium dodecyl sulfonate (SDS). The artificially fresh-contaminated sediment and aged-contaminated one were used in experiments to determine the effect of aging process on bioremediation. Both biological and chemically synthetic surfactants at high dosages (above effective critical micelle concentration) greatly enhanced desorption of sediment-sorbed phenanthrene, whereas rhamnolipid mixture (Rh-mix) was more efficient than SDS in desorption enhancement. Moreover, Rha-mix showed lower toxicity to the PAH-degrading bacteria. The results of biodegradation experiments showed that the extents of phenanthrene biodegradation were significantly enhanced by both Rha-mix and SDS at the optimal concentrations. When the biodegradation systems amended with rhanmolipid dosages above 400 mg/L and SDS dosages above 5000 mg/L, phenanthrene biodegradation was inhibited because of the mass transfer of phenanthrene from micelle into water. However, Rh-mix at the high concentrations showed lower inhibitory effect on phenanthrene biodegradation than SDS. On the other hand, the lower desorption and biodegradation extent of aged phenanthrene indicated that the aging process could significantly decrease bioavailability of phenanthrene. These results suggested that rhamnolipid may be more effective than SDS in PAH-contaminated sediment remediation processes.