Table of Contents:
Challenges of bioremediation
Limitations of bioremediation
Bioremediation also has its own limitations like other methods. Only biodegradable compounds are remediated by bioremediation, and not all compounds are susceptible to rapid and complete degradation such as chlorinated organic or high aromatic hydrocarbons that are resistant to microbial attack. Sometimes, the products of biodegradation may be more persistent or toxic than the parent compound. Bioremediation may take more time than other treatment options, such as excavation and removal of soil or incineration to remediate the contaminants (Zeyaullah et al. 2009).
The field release of genetically engineered microorganisms (GEMs) has some drawbacks such as decreased levels of fitness and the extra energy demands required by the presence of foreign genetic material in the cells (Singh et al. 2011). There are numerous limitations with electro-bioremediation technology that need to be overcome such as solubility of the pollutant and its desorption from the soil matrix, the availability of potential microorganisms at the site of contamination, the ratio between target and non-target ion concentrations, and toxic electrode effects on microbial metabolism (Virkutyte et al. 2002).
However, some of the more recalcitrant and toxic xenobiotic compounds involving highly nitrated and halogenated aromatic compounds and explosives are generally stable, chemically inert under natural conditions, and not common to remediate efficiently by many microorganisms. Some compounds are co-metabolized by microorganisms only in the presence of an alternative carbon source; these compounds cause problems for biodegradation and bioremediation for the reasons of the toxicity of these organic pollutants to the existing microbial populations. Pollutants, such as petroleum hydrocarbons, polycyclic aromatic hydrocarbons and some chlorinated pesticides among others, are not easily available to the microorganisms because of their hydrophobicity and persistence in soil.
The composting efficiency depends on the type of contaminants, temperature, and soil/waste amendment ratio for bioremediation (Antizar-Ladislao et al. 2005). The spent mushroom waste from Pleurotus ostreatus was found to degrade and mineralize DDT in soil (Pumomo et al. 2010). But, Alvey and Crowley (1995) observed that additions of compost suppressed soil mineralization of atrazine were relative to rates in un-amended soils.
The use of windrow treatment may not be the best option to adopt hi remediating soil polluted with toxic volatiles due to periodic turning and production of CH4, i.e., greenhouse gas because of development of anaerobic zone within piled polluted soil.
Bioreactor-based bioremediation is not a popular frill-scale practice as it requires more manpower and is cost ineffective (Pliilp and Atlas 2005). Limitations of land fanning bioremediation techniques are large operating space, reduction in microbial activities due to unfavorable environmental conditions, additional cost due to excavation, and reduced efficacy in inorganic pollutant removal (Khan et al. 2004, Maila and Colete 2004). In hot (tropical) climate regions, it is not suitable for treating soil polluted with toxic volatiles due to its design and mechanism of pollutant removal (volatilization).
In bioshirping technique, excessive soil moisture limits air permeability and decreases oxygen transfer rate, in turn reducing microbial activities and these are the major concerns of this particular in situ technique. The major limitation of biosparing is predicting the direction of airflow. Longer remediation time, pollutant concentration, toxicity, and bioavailability to plant and slow plant growth rate are limiting factors which check the application of phytoremediation (Kuiper et al. 2004, Vangronsveld et al. 2009, Ah et al. 2013). There is a possibility that bioaccumulated toxic contaminants may be transferred along the food chain.
Toxic intermediates produced during the bioremediation process
HCH isomers show resistance towards biodegradation due to then different chemical configuration and complex namre. The complete degradation of small concentrations of HCH was successful, while partial/incomplete reductions resulted when the concentration of contaminant was increased in many of the bioremediation experiments. Incomplete degradation also results in formation of toxic intermediates such as pentachlorocyclohexanols. pentachlorocyclohexeues, and tetrachlorocyclohexane-diols as some of the metabolites of a and у HCH degradation has a higher solubility in water than their parental counterpart (Rama et al. 2008).
The growth of the inoculated microorganisms may be affected by the high concentration of pollutant and the presence of the autochthonous species (Cycon et al. 2017, Chen et al. 2014). The degradative abilities of the inoculated microorganisms may also be attributed to the production of toxic intermediates that are formed during the degradation of lindane and HCH which might inhibit their further growth (Megharaj et al. 2011). The associated challenges of the bioaugmentation technology include both abiotic and biotic factors and environmental conditions like temperature, pH, moisture and organic content of the soil, initial pesticide concentration and additional carbon sources. Besides this, there are many factors that affect the bioaugmentation of the polluted soils and have been discussed in detail by Cycon et al. (2017).
Microorganisms produce biosurfactants under specific growth conditions, which may be anionic or nonionic (Zhang and Miller 1995). Biosurfactant (rhanmolipid) was found to enhance the solubility and the subsequent degradation of phenantlirene by Spliiiigonioiias sp. (Pei et al. 2010). Biosurfactants can be toxic or even utilized preferentially by the pollutant-degrading microorganisms. In a recent report, Hua et al. (2010) demonstrated that a salt-tolerant Enterobacter cloacae mutant could be used as an agent for bioaugmentation of petroleum- and salt contaminated soil due to increased K+ accumulation inside and exopolysaccharide level outside the cell membrane.
Won et al. (1974) used Pseudomonos sp. strain to degrade TNT, which resulted into 2-Amino-4, 6-dinitrotoluene (2-ADNT), 4-Amino-2,6-dinitrotoluene (4-ADNT), 2,6-dinitro- 4-hydroxylaminotoluene (2,6-DHAT), and the corresponding nitrodiaminotoluenes, 2,2’,6,6’- tetranitro-4,4’-azoxytoluene and 2,2’,4.4'-tetranitro-6,6’-azoxytoluene. Lachance et al. (2004) reported that TNT metabolites (4-amino-2,6-dinitrotolueue, 2-amino-4,6-dinitrotoluene) are as toxic as TNT itself. In addition, 2,4-DANT and 2,4-DNT were also observed to be more toxic than TNT to Hyalella azteca and Rana catesbeiana, respectively (Suns and Steevens 2008, Paden et al. 2011). The intermediate metabolites of TNT such as 2-Amino-4, 6-dinitrotoluene (2-ADNT) and 4-Amino-2,6-dinitrotolueue (4-ADNT) are more toxic than parental TNT on the basis of their reproductive toxicities (Kamjanapiboomvong et al. 2009).
Aromatic amines are released after cleavage of azo bonds by microflora containing azoreductases (Prasad and Aikat 2014). Aromatic amines are more persistent in the environment than dyes (Chen et al. 2009). Nitroanilines are produced during the biodegradation of azo dyes under anaerobic conditions, which are toxic and affect the ability and efficiency of the dye decolorizing bacteria (Klialid et al. 2009). According to Tsuboy et al. (2007), the acetoxy group (COCH3) located on the benzene ring can be metabolized by the P450 enzyme and other hepatic enzymes generating radical mutagenic intermediates. Biochemical activation through N-hydroxylation, followed by sulfation, esterification or acetylation reactions, generates reactive intermediates that are able to bind to DNA and largely account for the carcinogenicity of arylamiues (Pinheiro et al. 2004).
Failure of bioremediation process
Due to lesser efficiency, competitiveness, and adaptability, relative to the indigenous members of natural communities, bioaugmentation efforts have failed to remediate the contaminated soils. For example, the bacteria capable of degrading polychlorinated biphenyls (PCBs) in laboratory culture media survived poorly in natural soils, and when these strains were inoculated to remediate PCB-contaminated soils, the result was the failure of bioaugmentation (Blasco et al. 1995). Further investigations revealed that formation of an antibiotic compound, protoanemonin. from 4-chlorocatechol via the classical 3-oxoadipate pathway by the native microorganisms w'as the reason for poor survival of the introduced specialist PCB-degrading strains (Blasco et al. 1997).