Bioremediation of Industrial Wastes
A variety of pollutants are discharged in the environment from a large number of industries/mills. For example, textile industry alone contributes a significant amount of pollutants to water bodies such as enzymes, acids, alkali, alcohols, phenols, dyes, heavy metals, radionucliods, etc. Traces of zinc, cadmium, mercury, copper, chromium, lead are found in dyes, Upadhyay and Maurya (1996) have discussed about bioremediation of toxic textile effluents by free and immobilized microbial cells and enzymes.
It has been reported that actinomycetes show a higher capacity to bind metal ions as compared to fungi and bacteria. In addition, uptake mechanism of living and dead cells differ. Due to these differences they have potential application in industries. The living microbial cells accumulate metals intracellularly at a higher concentration, whereas dead cells precipitate metals in and around cell walls by several metabolic processes. Dead biomass immobilized on polymeric membrane absorbs uranium well, and immobilized Aspergillus oryzae
cells on reticulated foam particles have been used for Cd removal. Aspergillus niger
biomass contains up to 30 per cent of chitin and glucan. Chitin phosphate and chitosan phosphate of fungi absorb greater amount of U than Cu, Cd, Mn, Co, Mg and Ca (Upadhyay and Maurya, 1996).
Bioremediation of Dyes
There is limited study on microbial degradation of azo and reactive dyes. Maximum number of dyes undergo degradation through reduction. Kulla (1981) discussed azo-reductase of Pseudomonas
strains in the chemostat culture. This enzyme catalyses azo-linkage of the dye. During degradation process of azo, NAD(P) acts as electron donor. Srivastava et ah
(1995) observed degradation of black liquor pulp mill effluents by the strains of Pseudomonas putida.
Some anaerobic bacteria, Streptomyces
and fungi (e.g. Phaenerochaete chrysosporium)
have been characterized for decoloration of chromogenic dyes. The enzymes involved in dye degradation are lignases (lignin peroxidase), Mn (II) dependent peroxidase and glyoxal oxidase. These enzymes are well associated with lignin degrading system.
Bioremediation of Heavy Metals
Bacteria, algae, fungi, actinomycetes and higher plants accumulate high amount of heavy metals in their cells.
Algae. The species of Chlorella, Anabaena inaequalis, Westiellopsis prolifica, Stigeoclonium tenue, Synechococcus
sp. tolerate heavy metals. However, several species of Chlorella, Anabaena,
marine algae have been used for the removal of heavy metals. But the operational condition! limit the practical application of these organisms. Rai et. al.
(1998) studied biosorption i.e.
both adsorption and absorption of Cd++
by a capsulated nuisance cyanobacterium, Microcystis
both from field and laboratory. The naturally occurring cells showed higher efficiency for Cd++
as compared to laboratory cells.
Fungi. Fungi also are capable of accumulating heavy metals in their cells. However, several mechanisms operate in them for removal of heavy metals from the solution, a few of these have been discussed below :
Bioremediation of Coal Wastes through VAM Fungi
- Metabolism-independent accumulation. The positively charged ions in the solution are attracted to negatively charged ligands in cell materials. Biosorption of metal ion occurs on microbial cell surface. But composition of biomass and other factors affect biosorption. For example, in Rhizopus arrhizus adsorption depends on ionic radius of Li3+, Mn2+, Cu2+, Zn2+, Cd2+, Ba2+, Hg2+ and Pb2+. However, binding of Hg2+, Ag2+, Cd2+, A13+, Ni2+, Cu2+ and Pb2+ strongly depends on concentration of yeast cells.
- Metabolism-dependent accumulation. In fungi and yeast, heavy metal ions are transported into the cells through cell membrane. However, as a result of metabolic processes ions are precipitated around the cells, and synthesized intracellularly as metal-binding proteins. Energy-dependent uptake of Cu2+, Cd2+, Co2+, Ni2+, Zn2+ by fungi has been demonstrated. Moreover, intracellular uptake is influenced by certain external factors such as pH, anions, cations and organic materials, growth phase, etc. Metal uptake by growing batch culture was found maximum during lag phase and early log phase in Aspergillus niger, Penicillium spinulosum and Trichoderma viride (Townsley and Ross, 1985).
- Extracellular precipitation and complexation. Fungi produce several extracellular products which can complex or precipitate heavy metals. For example, many fungi and yeast release high affinity Fe-binding compounds that chelate iron. It is called siderophores. The Fe3+ chelates which are formed outside the cell wall are taken up into the cell. In Saccharomyces cerevisiae removal of metals is done by their precipitation as sulphides e.g. Cu2+ is precipitated as CuS (Beveridge, 1989).
Bioremediation of coal waste land through VAM fungi is gaining importance in recent years. Selected VAM fungi are introduced through plants in coal mine areas. Extensive infection of most plant species colonizing coal waste has been observed in India and other countries. It has been found that VAM fungi improved the growth and survival of desirable revegetation species. Increased growth of red maple, maize, alfalfa and several other plants inoculated with VAM fungi growing in coal mine soil has been recorded (Daft and Hacskaylo, 1977).