Usage of microorganisms for bioabsorption of metals

Lectures 20-21

Biosorption is a physiochemical process that occurs naturally in certain biomass which allows it to passively concentrate and bind contaminants onto its cellular structure. Though using biomass in environmental cleanup has been in practice for a while, scientists and engineers are hoping this phenomenon will provide an economical alternative for removing toxic heavy metals from industrial wastewater and aid in environmental remediation.

Pollution interacts naturally with biological systems. It is currently uncontrolled, seeping into any biological entity within the range of exposure. The most problematic contaminants include heavy metals, pesticides and other organic compounds which can be toxic to wildlife and humans in small concentration. There are existing methods for remediation, but they are expensive or ineffective. However, an extensive body of research has found that a wide variety of commonly discarded waste including eggshells, bones, peat, fungi, seaweed, yeast and carrot peels can efficiently remove toxic heavy metal ions from contaminated water. Ions from metals like mercury can react in the environment to form harmful compounds like methylmercury, a compound known to be toxic in humans. In addition, adsorbing biomass, or biosorbents, can also remove other harmful metals like: arsenic, lead, cadmium, cobalt, chromium and uranium. Biosorption may be used as an environmentally friendly filtering technique. There is no doubt that the world could benefit from more rigorous filtering of harmful pollutants created by industrial processes and all-around human activity.

The idea of using biomass as a tool in environmental cleanup has been around since the early 1900’s when Arden and Lockett discovered certain types of living bacteria cultures were capable of recovering nitrogen and phosphorus from raw sewage when it was mixed in an aeration tank. This discovery became known as the activated sludge process which is structured around the concept of bioaccumulation and is still widely used in wastewater treatment plants today. It wasn’t until the late 1970’s when scientists noticed the sequestering characteristic in dead biomass which resulted in a shift in research from bioaccumulation to biosorption.

Any study of biofilms must accept that biofilms may develop in an enormous number of environments, and that the structural intricacies of any single biofilm formed under any specific set of parameters may well be unique to that single environment and microflora. The enormous number of microbial species capable of forming biofilms or interacting with others to do so, together with the very great range of polysaccharides produced, gives rise to an infinite number of permutations. In natural conditions, monospecies biofilms are relatively rare; thus most biofilms are composed of mixtures of micro-organisms. This adds to the interspecies and intraspecies interactions and to the general complexity of the macromolecular mixture present.

The exopolysaccharides (EPS) synthesized by microbial cells vary greatly in their composition and hence in their chemical and physical properties. Some are neutral macromolecules, but the majority are polyanionic due to the presence of either uronic acids (d-glucuronic acid being the commonest, although d-galacturonic and d-mannuronic acids are also found) or ketal-linked pyruvate. Inorganic residues, such as phosphate or rarely sulphate, may also confer polyanionic status. The polysaccharides are essentially very long, thin molecular chains with molecular mass of the order of 0·5–2·0×106 Da, but they can associate in a number of different ways. In several preparations, the polysaccharides have been visualized as fine strands attached to the bacterial cell surface and forming a complex network surrounding the cell. Mayer et al. (1999)⇓ suggested that electrostatic and hydrogen bonds are the dominant forces involved. Ionic interactions may be involved, but more subtle chain–chain complex formation in which one macromolecule ‘fits’ into the other may result in either floc formation or networks which are very poorly soluble in aqueous solvents. Another result may be the formation of strong or weak gels. The polysaccharides can thus form various types of structures within a biofilm. However, in biofilms the polysaccharides do not exist alone but may interact with a wide range of other molecular species, including lectins, proteins, lipids etc., as well as with other polysaccharides. The resultant tertiary structure comprises a network of polysaccharide and other macromolecules, in which cells and cell products are also trapped.

Control questions:

1. Are there specific biofilm polysaccharides?
2. What do we know of the structure and properties of the biofilm polysaccharides?
3. Bioiabsorption process description?
4. What is the relationship of structure to function?

Дата добавления: 2014-10-15