Soil Bioremediation by Cyclodextrins

Comunicación oral presentada en la 6th European Conference on Cyclodextrins. Abstracts book pág. 14 (2019)Pollution of soils is currently a significant concern worldwide. Thousands of sites are contaminated by organic compounds being a hazard to human health and the environment. Soils are major repositories for organic contaminants, which can reach it through their direct application (pesticides or organic amendments in agriculture), accidental or continuous spillages from the industry, or the disposal of industrial and urban waste water. The vast majority of the organic pollutants in soil are poorly soluble in water, and the addition of solubilizing agents (cosolvents, surfactants) is necessary to remove them from soil during the decontamination processes. Many of these extractants are toxic to soil microbial population, or form high-viscosity emulsions difficult to remove and their adsorption onto soils is high. Due to these environmental risks, an alternative is the use of Cyclodextrins (CDs) for the removal of hydrophobic contaminants from soils due to their ability to enhance the aqueous solubility of hydrophobic organic compounds through the formation of water-soluble inclusion complexes. There are other advantages in the use of CDs in soil remediation: (1) they present much lower toxicity for soil microbial populations; (2) CDs are considered biodegradable, which minimizes reagent residuals left in soil; (3) CDs show a minimal tendency to remain adsorbed on the soil surface; (4) CDs are not sensible to changes in pH or ionic strength. Technologies to eliminate organic contaminants from soils using CDs fall into two categories: separation technologies (such as soil washing/flushing), or destruction technologies, which may be either by chemical remediation (chemical oxidation/reduction, photodegradation, electrokinetic remediation) or by biological remediation (Bioremediation). Bioremediation of polluted soils is an emerging technology which uses soil living organisms (microorganisms, plants, earthworms) to remediate sites polluted by organic contaminants, and is considered a cost-effective and environmentally friendly remediation technique. Bioavailability of organic contaminants for their assimilation by soil organisms is a major limitation in the bioremediation of contaminated soils, affecting the clean-up time, cost and end-point of the process. CDs are then used to increase the bioavailability of such pollutants to be degraded by soil organisms. The majority of studies which use CDs for bioremediation of contaminated soils are based on soil microorganisms (bacteria and fungi) which use the contaminants as nutrient or energy source. In contaminated soils, the microbiota reacts immediately and the species capable of degrading the specific contaminant will proliferate, increasing their percentage within the microbial community. However, sometimes it is necessary to inoculate the soil with exogenous specific degrader strains (bioaugmentation). A wide variety of organic contaminants are currently found in soils, but, as the size of the CD molecules is fixed, only those pollutants with a molecular size similar or smaller and an appropriate structure have options to be included inside of CDs internal cavities. References about the variety of contaminants treated and the different CDs used in soil bioremediation will be exposed, as well as the appropriate soil conditions. AcknowledgmentsThis work was supported by the Spanish Ministry of Science, Innovation and Universities (co-funded by FEDER), under the research project CTM2017-82472-C2-1-R