Pollution of soils by persistent organic pollutants and potentially toxic elements (PTEs) is currently a significant concern worldwide. In many contaminated soils not only a mixture of organic contaminants exist, but also PTEs are present. The effect of any remediation treatment used for hydrophobic contaminants removal from co-contaminated soils will have effects on extraction and availability of PTEs present in the soil, and this fact have to be considered in order to avoid a secondary contamination of the surrounding environment or an increased toxicity in the treated soil which can affect a subsequent bioremediation process.
In these cases, it is important to develop appropriate remediation techniques to treat both pollutants, or only to degrade the organics but without altering the inorganic pollutants. These techniques used to be more complex and costly due to the differences in the physicochemical properties of organic and inorganic contaminants.
This is the case of those soils contaminated by creosote, which has been used as a wood preserving agent for many years to treat, for example, railroad ties, power line poles or ship’s hull to prevent rot. Creosote is a very complex mixture of organic compounds, but its main components are polycyclic aromatic hydrocarbons (PAHs), which can make up to 85% of creosote composition. But also some Potentially Toxic Elements (PTEs) such as Cr, Cu or As, are used as wood preserving agents. Wood used to be first treated with inorganic preservatives and then with creosote. The waste sludge produced by these treatments has generally been landfilled, and many ancient facilities of wood treatment with creosote are nowadays brownfield of contaminated soil by PAHs and PTEs. In these cases, the use of some cyclodextrin (CD) derivatives could be a good solution to remediate such co-contaminated soils.
Our research group have selected several CDs (β-cyclodextrin, BCD; hydroxypropyl-β-cyclodextrin, HPBCD; hydroxypropyl-γ-cyclodextrin, HPGCD; and randomly methylated-β-cyclodextrin, RAMEB) as environmentally friendly agents to enhance the apparent solubility and removal of 16 polycyclic aromatic hydrocarbons (PAHs), from a creosote-contaminated soil which had received historically both creosote and inorganic wood preservatives for almost 100 years. But we studied also the effect of such CDs on the removal of the potentially toxic elements (PTEs) present in this soil (Madrid et al., 2019a). For comparison purposes, we have used also a rhamnolipid biosurfactant (RL). Biosurfactants are natural surface-active biomolecules produced by bacteria, fungi and yeast, with lower toxicity and higher biocompatibility and biodegradability than chemical surfactants.
Up to 15.4% of the Σ16 PAHs were extracted using RL as washing solution, but only up to 2% of the Σ16 PAHs was extracted with CDs (4-ring PAHs in higher concentrations). In relation to PTEs, the CDs derivatives that we had selected (BCD, HPBCD and RAMEB) proved to be inefficient for their extraction. The percentages of PTEs extracted with these CDs were in general similar to those extracted by Ca(NO3)2 solution, which simulates the soil solution ionic strength. Therefore, the use of BCD, HPBCD, HPGCD or RAMEB to extract PAHs could be an advantage in those soils where the release of PTEs is not convenient to prevent an increase of the soil and/or groundwater toxicity. However, metal ions are able to form complexes with certain CD derivatives, such as methyl-β-CD, carboxymethyl-β-CD, or glycine-β-CD, which have demonstrated their significant effect on inorganic pollutant removal from soil (Skold et al., 2009; Prochowicz et al., 2016). Therefore, these CDs derivatives should not be used for bioremediation of soils co-contaminated by organic contaminants and inorganic PTEs.
On the contrary, PTEs extraction by the biosurfactant RL used increased dramatically. Up to 21% of Cd, or 14% Pb, Zn, or Ni, or 7% for Cu or As contents could be extracted, indicating the advantage of using the CDs selected in relation to this biosurfactant as extractants in co-contaminated soils.
To check the effect of the application of CDs in the biodegradation of PAHs in the same co-contaminated soil, bioremediation experiments were carried out using HPBCD and the biosurfactant RL for comparison (Madrid et al., 2019b). Both act as bioavailability enhancer agents, but also as biostimulation agents. HPBCD became completely biodegraded in soils, likely because several plant- and fungi-associated bacteria metabolised HPBCD as a carbon source (Fenyvesi et al. 2005). Biosurfactants also stimulate microbial biomass growth to decompose the contaminants (Bezza and Chirwa, 2017). The addition of HPBCD or the rhamnolipid (extraction and biostimulation) combined with a potentially PAHs degrader consortium (bioaugmentation) was selected as the strategy for an enhanced bioremediation that could increase the chance for a successful decontamination of this long-term polluted soil.
HPBCD could reduce the content of some PAHs, especially those with 4 rings. Thus, for example, up to 48% degradation for pyrene and 43% for fluoranthene was achieved. On the contrary, dissipation for 5-ring PAHs was very low and for 6-ring was negligible. 2- and 3-ring PAHs were already absent because this polluted soil had been subjected to natural attenuation for almost 100 years before our experiments, and it is likely that the volatile, water soluble and easily available PAHs were already removed. Substantial loss of PAHs during our experiment due to volatility, photolysis or chemical degradation was, therefore, unlikely.
By contrast, the percentage of PAHs degraded by the biosurfactant was null, and even inhibited the scarce degradation observed in the control treatment. The reason was the high increase in availability of PTEs and, therefore, the high toxicity of the soil after treatment with rhamnolipid, which inhibited the PAH bioremediation process.
The toxicity of the soil during HPBCD and RL treatments at different incubation periods of the biodegradation experiments was checked through Microtox Acute Toxicity test using Aliivibrio fischeri, and also through the enumeration of viable bacteria, achieved by counting the colony forming units (CFUs) per gram of soil. The results demonstrated that there was not an increase in the toxicity in the soil treated with HPBCD in relation to the control soil without any amendment added. But just the opposite happened when using the biosurfactant, where the number of CFUs decayed completely after rhamnolipid application, eliminating any microbial activity in the soil, demonstrating that the HPBCD was a much better choice than RL as extractant agent in such co-contaminated soils.
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