Laccase-based technologies to remove organic pollutants from soils and wastewaters

Abstract

Pollution of soil and water is an environmental issue worldwide. Thereby, the development and implementation of low-cost and eco-friendly treatments for the decontamination of polluted sites and wastewater is a priority. In this regard, the use of biological agents, as white rot fungi, to degrade and detoxify environmental contaminants has emerged as a potential alternative. These microorganisms have been reported to remove a wide range of xenobiotics by the action of the extracellular lignin-modifying enzymes, such as peroxidases and laccases. Laccases (copper-containing oxidases, EC 1.10.3.2) are promising biocatalysts due to their oxidative versatility and low catalytic requirements. Laccases, as well as laccase-mediator systems, have been successfully used to oxidatively detoxify and remove a great number of contaminants. The potential of laccase is explored in the current research aiming to develop continuous or semicontinuous technologies to remove pollutants of concern, such as Polycyclic Aromatic Hydrocarbons (PAHs) and Emerging Organic Contaminants (EOCs), from real environmental matrices. In the first section of the Thesis, the use of laccase is investigated to degrade PAHs. The natural biodegradation of these toxic pollutants is restricted by two major factors: low water solubility and high hydrophobicity, which make PAHs persistent pollutants in soil. Extraction with solvents or surfactants is an alternative to remove PAHs from soils. However, this technique by itself does not degrade the pollutants, but only attains their transfer to another phase. Enzymatic remediation with laccase may be a feasible alternative, but requires the addition of surfactants or solvents to increase the pollutant bioavailability for the enzymatic action. Initially, different reactor configurations (micellar, biphasic and the combination of both) were evaluated to perform the laccase-catalyzed degradation of anthracene, the model PAH. The use of a Two Phase Partitioning Bioreactor (TPPB) with silicone oil and the addition of surfactant in the aqueous phase allowed the enzymatic treatment of high loads of anthracene at fast conversion rate. Subsequently, the use of vegetable oils (sunflower or pomace olive oil) as organic phase in the TPPB was proposed to dissolve high concentrations of the target pollutant. Pomace olive oil led to the best results and after optimizing the operational conditions in the TPPB, a novel remediation process to remove anthracene from polluted soils was proposed. The process consisted of the extraction of anthracene from the soil by the oil followed by its degradation in a surfactant-assisted TPPB, operated with a laccase-mediator system. The main outcomes of this study showed high extraction efficiency for pomace olive oil and high removal rate of anthracene in the TPPB. Moreover, the feasibility of reusing both the aqueous and organic phases of the TPPB in successive batches of anthracene degradation in the reactor was also demonstrated. The second section of the Thesis focuses on the enzymatic removal of EOCs from aqueous systems. The industrial chemical bisphenol A and pharmaceuticals active compounds are examples of EOCs frequently detected in wastewaters, considered as the main source of EOCs entering the environment. The application of laccases in tertiary wastewater treatment requires the retention of the biocatalyst in the reactor. In the current research, two main approaches were considered to fulfill such requirement. On one hand, the use of an enzymatic membrane reactor with a ceramic membrane was considered to perform the continuous removal of BPA from secondary effluent by free laccase. The second approach consisted of the immobilization of laccase onto nanoparticles or magnetic microparticles, prior to its application to remove micropollutants from secondary effluents. Immobilization not only facilitated the retention of the biocatalyst in the system by a high pore size membrane or by applying an external magnetic field, but also increased laccase stability.