Bioremediation of Heavy Metals by Immobilized Microbial Cells and Metabolites

CASTRO, MF; BONILLA, JO; DELFINI, CD; VILLEGAS, LB

Strategies for Bioremediation of Organic and Inorganic Pollutants

CRC Press Taylor & Francis Group

Lugar: Boca Ratón, Florida; Año: 2018; p. 243 - 262

Heavy metals exist in nature as dilute components of the geochemical cycles. In general, soils have heavy metals as a result of geological and edaphic processes. In the last few decades, concentration of these metals increased due to certain human activities. Soil heavy metal pollution derives from an increase in the heavy metal concentration regarding the background level (Nakic et al. 2007, Villegas et al. 2013). All ecosystems, mainly soil, are important habitats for millions of microorganisms including a large variety of fungi, algae, protozoa and different types of bacteria. Microorganisms are an essential part of the living soil and of paramount importance for soil health. In soil, microorganisms are key players in the cycle of nitrogen, sulfur and phosphorus, and in the organic waste decomposition. Also, microbial communities from these ecosystems can interact and utilize heavy metals. Often these metals are present in very high concentrations, and there also exist mixed contamination by the presence of numerous heavy metals at the same time. Unlike organic contaminants, heavy metals cannot be degraded by physicochemical and biological processes, so when they are released into the environment, they remain there indefinitely. For decades, these persistent pollutants have accumulated in the environment and are still being released. This situation is the reason to apply a suitable solution to effectively detoxify and reduce their exposure to the environment. Additionally, metals can leach into groundwater aquifers and contaminate drinking water, and are unreachable by many methods actually available to decontaminate surface soils. Many developing countries are faced with the challenge of reducing human exposure to heavy metals, mainly due to their limited economic capacities to use advanced technologies for their removal (Chowdhury et al. 2016). For this reason, heavy metals provide several unique challenges for remediation.Heavy metals profoundly affect biological systems, either positively because they are essential or negatively because they are toxic, when they are present in excessive amounts. Living organisms incorporate and use these metals; for example, a significant number of proteins require metals for their correct catalytic activity and/or to maintain their structure (Waldron et al. 2009). Clearly, heavy metals are directly or indirectly involved in all aspects of cell growth, metabolism, and cellular differentiation.The importance of heavy metals to human health as well as their impact on the environment has stimulated research on the action of these on biological systems with the aim of understanding their response to the presence of heavy metals and the tolerance mechanisms in many prokaryotic and eukaryotic microorganisms. Moreover, numerous reports have shown the microorganism?s abilities to tolerate and remove heavy metals from pure culture or microbial consortium (Villegas et al. 2008, 2009, Piñón-Castillo et al. 2010, Villegas et al. 2013, Bonilla et al. 2016).Several techniques are used in order to minimize the impact that high concentrations of heavy metals cause to the ecosystems, the most used being the physicochemical methods. These conventional methods applied to the removal of heavy metals are limited to the waste confinement or to the modification of its solubility and toxicity through changes in oxidation state. Heavy metals removal using these methods requires high levels of energy and chemical reagents and the cleanup process is generally incomplete. Moreover, they also generate a toxic waste or scrap that may become a secondary environmental pollutant. In this context, microbial bioremediation is an attractive eco-friendly technology due to a moderate-cost and greater efficiency than traditional techniques (Wagner-Dobler 2003).Bioremediation is a well-described technique which involves the utilization of organisms or part of them in order to transform some toxic pollutants in inert or less-toxic compounds (Dua et al. 2002, Okoh and Trejo-Hernandez 2006, Luqueño et al. 2011). Bioremediation is the most effective management tool to recover polluted environment (Kurmar et al. 2011). Currently, a variety of strategies utilize microbial biomass for the treatment of contaminated sites with different toxins. Although the use of microorganisms to clean up heavy metals contaminated environments or effluents is a proposal that overcomes the use of physico-chemical methods, the application of free microorganisms? presents many limitations.Immobilized cells have been extensively used as an important tool in biotechnological processes, especially for the production of useful chemicals because the immobilization extends the useful life and increases cellular stability and survival (Saifuddin and Raziah 2007). However, little is known about the application of immobilized cells or microbial metabolites in the treatment of wastewaters or bioremediation of contaminated sites with heavy metals. Therefore, the use of immobilized systems as a promising tool to clean up heavy metals-contaminated sites is considered in this chapter.