Urbanization and industrialization have led to the release of numerous toxic pollutants into the environment, including heavy metals from industrial processes and wastewater. These pollutants include antimony, chromium, mercury, aluminium, copper, zinc, nickel, lead, and arsenic. Untreated pollutants from wastewater, from agri-food industries and crude oil also contribute to environmental pollution. Mining processes release toxic chemicals like lead, arsenic, cadmium, and copper. Industrial wastes like cement-making release zinc, copper, and cadmium, which can pollute water bodies. Heavy metals can cause health issues in humans, including altered central nervous systems, liver and kidney dysfunction, cardiovascular diseases, reproductive and immune system malfunctions, and cancers. Improper disposal of these pollutants into soil and water environments can lead to fish death and chronic diseases.
Bioremediation, a more efficient, eco-friendly, and cost-effective technology, is needed to remediate these pollutants. Bioremediation can be carried out using plants and microorganisms, but plants take longer to grow and cannot be easily manipulated. Microbes can mitigate heavy metals, improve soil fertility, and improve plant development. This review discusses the types, mechanisms of bioremediation.
Different pollutants and their toxicity
Air pollutants, including nitrogen oxides, sulfur dioxide, volatile organic compounds, and water pollutants, can cause developmental disorders, respiratory problems, cancers, cardiovascular diseases, and reproductive issues in humans, increasing premature death rates. Pollution can cause respiratory diseases like COPD and asthma, reproductive disorders, liver damage, and cancer in animals. Nitrogen dioxide reduces plant growth and yield, while sulfur dioxide causes acid rain. Ozone exposure impairs photosynthetic rhythm and metabolism. Eutrophication in aquatic environments leads to algal blooms, fish death, and disequilibration in fish diversity.
Types of remediation
Different types of bioremediation exists mainly categorized as physical, chemical and biological bioremediation. In physical bioremediation, booms, skimmers and sorbent materials are used. Different challenges are associated with this type of remediation. For example, the challenge with bloom remediation is its reliance on buoyancy and roll response, which affects the remediation process.
Chemical remediation involves adding chemicals to stabilize and remove heavy metals from the environment. However, these chemicals can also contribute to environmental pollution. Bioremediation is a sustainable, affordable, and safe method using organics like plants and microbes. Growing plants takes longer time. Therefore microbes are preferred due to their rapid growth, ease of use and easy manipulation. Improving microbes’ use in bioremediation is crucial for promoting a sustainable environment.
Microbes as bioremediation agents
Microorganisms convert toxic elements into less toxic compounds, water, carbon dioxide. These less toxic compounds are further degraded by other microbes through mineralization. Bioremediation uses bacteria, fungi, and algae. Microbes are ubiquitous and can absorb various pollutants in unusual environments. Their survival skills, such as acidophiles in acidic, psychrophiles in cold, and halophiles in saline regions, enhance their efficiency.
Mechanisms of microbial bioremediation
Microbes can remove pollutants from the environment through two mechanisms: immobilization and mobilization. Mobilization involves enzymatic oxidation, bioleaching, biostimulation, bioaugmentation, and enzymatic reduction. Immobilization involves bioaccumulation, complexation, biosorption, and precipitation. Microbes help transform pollutants into end products like carbon dioxide and water. Immobilization is used for bioremediation of heavy metals, especially in highly contaminated environments. It can be done using in-situ or ex-situ methods. Microbes like E. asburiae and B. cereus are involved in immobilizing heavy metals. Microbes shield themselves from toxic and harmful compounds by different mechanisms like forming hydrophobic or solvent efflux pump which helps them in protecting cell’s outer membrane.
Enzymatic oxidation
In enzymatic oxidation, pollutant compounds are oxidized from higher to lower oxidation state. During this process, an electron is lost from heavy metals, making them less toxic. This involves an enzyme, produced by microbes.
Enzymatic reduction
Obligate and facultative anaerobes perform enzymatic oxidation, converting pollutants to reduced oxidized states. This method is effective for bioremediation of compounds like polychlorinated dibenzo-p-dioxins and dibenzofurans. Further research is needed to discover other organisms capable of bioremediating environmental pollutants.
Bioaugmentation
Bioaugmentation is a method of bioremediation where microorganisms are added to polluted sites to feed on toxic pollutants. This process is effective, rapid, and cost-effective. External microbes are added to augment resident microbes, while non-resident microbes are added to promote degradation. The effectiveness of these strains depends on factors like competition and adaptation to the new environment. Burkholderia sp. FDS-1 has been reported to degrade nitrophenolic compounds in polluted soil.
Biostimulation
Biostimulation is a cost-effective, environmentally friendly, and efficient method of bioremediation that adds nutrients and metabolites to soil to enhance microbe activity and improve remediation. It is preferable due to indigenous microbes’ competitiveness and helps maintain environmental microbial diversity. However, biostimulation can lead to environmental complications like eutrophication and pollution if nutrients are synthetic. Various bacteria and fungi have shown effectiveness in bioremediation of heavy metals.
Bioleaching
Bioleaching is a method using acidophilic microbes to solubilize solid heavy metals from sediments, especially iron or sulfur pollutants. Iron- or sulfur-oxidizing bacteria, like A. thiooxidans, Aspergillus, Mucor, Penicillium, Cladosporium, and Rhizopus, create an acid environment and solubilize the heavy metals.
Biosorption
Biosorption is the process of adsorbing heavy metals from pollutants using proton and ion displacement, complexation, chelation, and electrostatic forces. Bacteria, fungi, and algae, such as Rhodococcus erythropolis, Streptomyces sp. K11, and Bacillus anthracis, are bioremediation agents. Heavy metal pollutants, like gold, zinc, and copper, are economically valuable and can be recovered through desorption, a reversible step in biosorption.
Bioaccumulation
Bioaccumulation occurs when a compound’s absorption rate exceeds its loss rate, leading to toxic compound accumulation in microbes’ intracellular membranes. Heavy metals move across microbe’s cell membrane through carrier-mediated transport, protein channel, and ion pumps. Many microorganisms are actively bioaccumulating heavy metals such as Rhizopus arrhizus, aspergillus niger, and pseudomonas putida.
Precipitation
Heavy metals or pollutants can be converted into precipitates or crystals during biogeochemical cycling, reducing toxicity. Enzymes and secondary metabolites play a role in this process. Sulfate-reducing bacteria, Bacillus subtilis, and Oceanobacillus indicireducens are associated with heavy metal precipitation in the environment.
Conclusion
So, bioremediation of wastes is an easy, cost effective and environmentally friendly approach. It helps in minimizing the environmental pollution. Using microbes as bioremediation agent is fast and easy approach and beneficial approach as the microorganism are easy to manipulate and they reproduce rapidly.