Effects of environmental factors on bioremediation of co-contaminated soils using a multiresistant bacterium

Abstract

Increasing soil pollution problems resulting from industrialization and urbanization has caused worldwide concerns. This problem is extremely acute in areas where the presence of different families of organic pollutants is accompanied by heavy metals, inconcentrations exceeding permissible levels. Both organic and inorganic compoundswhen entering the soil pose a huge threat to human health and natural ecosystem. For instance, according to data from the Environmental Protection Agency (EPA), 40% of hazardous waste sites included in the National Priority List arecontaminated with organic compounds and heavy metals. The treatment of this simultaneous contamination, known as co-contamination, represents a real current challenge.In particular, chromiumcontaminationinsoil and waterhasbeendetectedin association with industrialareas. Cr(VI) is a dangerous pollutant, classified as carcinogenic by EPA. Similarly, residues of the gamma isomer of hexachlorocyclohexane (γ-HCH), commercially known as lindane, have been detected in soils, water, air, plants and animals, because of itsunselective use, mainly in agriculture practices. Lindane is extremely recalcitrant, withnumerous health effects, including neurological problems and cancer. Mixed pollution by chromium and lindane has been detected around the world in sediment and soil, at concentrations up to 140 mg kg-1 and 400 mg kg-1, for chromium and lindane, respectively.The treatment of soils with mixed pollution is a complex issue, because the remediation technologies are different for each class of pollutants. In this context, bioremediation is a low cost technology, which simultaneously allows the degradation of organic compounds and the removal or stabilization of metals into non-toxic or less toxic forms.Actinobacteria are a group of bacteria with a cosmopolitan distribution in differentecosystems. They have demonstratedthe abilityto remove several organic and inorganic pollutants. In this context, bioaugmentation with actinobacteria to degrade organic compounds and remove or stabilize heavy metals represents a promising approach to bioremediate co-polluted environments.In fact, Polti et al. (2014) previously demonstrated the potential of Streptomyces sp. M7 to clean up artificially contaminated non-sterile soils with hexavalent chromium and lindane.However, the bioremediation efficacy depends on several factors and their interactions, including the temperature, the humidity and the initial contaminant concentrations. For proper study of the influence of these factors,it is necessary to apply experimental design methods, which determine the effective factors and their interactions, as well as to model and optimize the whole system. The full factorial design generate maximum information regarding the factors and identify the interactions between separate experimental factors and predict the effect that such interactions could have on the experimental response. As consequence, bioremediation effectiveness could be improved.On the other hand, to assess whether bioremediation processes are acceptable, it is mandatory to investigate toxic effects of microbial metabolites produced during the pollutant removal. Bioindicators change their response in front of changes in environmental pollution. Lactuca sativa is one of therecommended species for this purpose, since it allows evaluating lethal and sublethal effects and it can be usedin samples with high turbidity, reducing pretreatment interference. Furthermore, it has high sensitivity, so it requires a reduced exposure time, it has low cost and does not require sophisticated equipment.This chapter will show a full factorial design like a useful tool to study the effect of environmental factors onabioremediation process. Further, will be demonstrated how the efficiency of this bioprocess could be proved by using L. sativa as bioindicator.