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Bioremediation: Current Status, Prospects and Challenges

Introduction

The growing industrialization also brings environmental pollution which has become a big concern in today’s world. The pollution not only contaminates the environment but also makes the living organisms seriously ill. There are several types of pollutants such as solid waste, liquid waste, toxic gases, radioactive waste and even heavy metals that are highly persistent in soil. A novel technology should be constructed in order to assure the safety of the living organisms and to maintain the sustainability of the environment from the harmful reactions of environmental pollution, and bioremediation is one of those methods. Bioremediation is the utilization of microorganisms to clean up contaminated soils, aquifers, sludge, residues, and is considered an ecofriendly, economical, efficient and sustainable technology to improve the polluted sites.

Human health and natural ecosystem have gone under threat because of the huge load of contaminants such as polycyclic aromatic hydrocarbons (PAHs), petroleum and related products, pesticides, chlorophenols, and heavy metals that enter the soil and water. Chemical and physical technologies for soil remediation are either inefficient or too costly and bioremediation is believed to be one of the most cost-effective methods for soil remediation. Metals and metalloids like Cr, Pb, Hg, U, Se, Zn, As, Cd, Ag and Ni contaminate the water as they are hazardous to human health and environment. Conventional physicochemical methods such as electrochemical treatment, ion exchange, precipitation, osmosis, evaporation and sorption are too costly and not environmentally friendly. However, bioremediation processes show efficient removal of metals, while present in very low concentrations where physicochemical removal methods fail to operate (Maui and Kumar 2014).

Bioremediation technologies have been developed in the last few decades and are used to alleviate environmental accidents and systematic contaminations. Bioremediation works through degradation, eradication, immobilization, or detoxification of different chemical wastes and hazardous materials through the microbial activities with remediating power from the surrounding sites. The main principle of biodegradation is degrading and transforming pollutants such as hydrocarbons, oil, heavy metal, pesticides and dyes, etc. into a less toxic form. Therefore, bioremediation is referred often as an ecofriendly biological mechanism of recycling wastes into another form that can be less toxic to the enviromrrent.

Bioremediation of solid waste

Numerous studies suggest that exposure to chemicals and other substances or gases like suspended particulate matter, carbon dioxide (C02), carbon monoxide (CO), nitrogen dioxide (N20), sulphur dioxide (S02) and hydrogen sulfide (H2S) emitted from burning of solid waste in the dumping sites is highly dangerous, causing a significant risk to human health (Rim-Rukeh 2014). Compostable organic matter (fruit and vegetable peels, food waste), recyclables (paper, plastic, glass, metals, etc.), toxic substances (paints, pesticides, used batteries, medicines) and soiled waste (blood-stained cotton, sanitary napkins and disposable syringes) are few examples of municipal solid waste (Kausal et al. 2012). Depending on factors such as water content, and temperature, harmful odorous gases are often produced due to the decomposition of piled up solid wastes and particulate solid wastes can be dispersed into the atmosphere and contaminate the surrounding atmospheric environment (Tian et al. 2013). Municipal solid waste (MSW) open dump site operation is an important component of waste management throughout the country. Drurrp site fires are common practice. Both direct and indirect greenhouse gases have been emitted from the MSW management process such as landfill, composting and incineration. Where land is scarce, incineration has been the most successfirl method but the costs of landfills are too high and environmental benefits of incineration are still in question (Kitmanran 2017). In the future, better technology with regard to municipal solid waste open dump site operation and emission control should be developed.

Biorenrediation involving bacteria and fungi or yeast is a comparatively far less expensive and more ecofriendly technology to remediate solid waste than other techniques. Awasthi et al. (2017) reported that some fungal strains of Trichodenna sp., Aspergillus niger and Aspergillus floras are potential strains which are capable to exclude metal from the leachate via biosorption process, among which the Trichodenna sp. was found to be an excellent bioremediating agent for Cd2“ absorption. Bacterial isolates such as Serratiaproteaniaculons S1BD1, Alcaligenes sp. OPKDS2 and Rhodococcus erythropolis OSDS 1 strains with high bioremediating efficiency could potentially not only be used for the bioaugmentation of the solid waste management site but also possess a high tolerance level to a wide range of salinity and pH (Xia et al. 2017).

Apart from microbial bioremediation, another form of bioremediation, phytoremediation uses green plants with higher biomass to uptake pollutants in their tissues and reduce the availability of the pollutants especially heavy metals in the soil. There are two different approaches such as contaminant extraction/degradation and stabilization that can be used for phytoremediation of landfill sites. In the first approach, plants grown in heavy metal-contaminated sites can accumulate heavy metals through then uptake mechanism into different plant tissues like leaves, stem, grain, and roots, which can be harvested and disposed from the contaminated sites. The second approach involves establishing the plant vegetation near landfill site of solid waste that can help in the stabilization and prevention of runoff to surrounding areas from the contaminated sites through acting as a barrier. Moreover, plants can prevent inorganic contaminants by stabilizing them near the root zone from leaching to groundwater. Trees such as Hibiscus tiliaceus, Acacia mangium, Dendrocalanius maroochy, Casuarina species and Eucalyptus grandis growing on landfill caps can also reduce methane emissions from landfills by providing favorable environments for methane- oxidizing bacteria (Venkatraman and Ashwath 2007).

Phytostabilization is a useful technique to rectify toxicity of municipal solid waste containing heavy metals such as Cd, Cu, As, Zn and Cr. Tree species such as Popiihts nigra L. or Agrostis capillaris L. is often used for phytostabilization of Cu-polluted soil and municipal solid waste compost. A few endophytes like Enterobacter sp. and Pseudomonas putida isolated from poplar (Populus deltoides) were found to be able to degrade volatile organic compounds such as trichloroethylene (TCE) (Touceda-Gonzalez et al. 2017).

 
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