Environmental pollution is a major concern in Pakistan and this problem is constantly on the rise. The situation is worsening due to unchecked disposal of industrial and municipal effluents into local environments, spills during transportation, leakage from waste disposal and storage sites, hospital wastes, urban solid waste and excessive agrochemicals application in agriculture. These are the factors mainly contributing to the deterioration of agricultural lands and water quality. In Pakistan, inorganic pollutants (heavy metals etc.) were the focus of many studies but remediation of organic pollutants is rarely addressed. Organic pollutants like pesticides, hydrocarbons, pharmaceuticals, azodyes etc. once released into environment pose serious threats to environment and public health. Pollution by these contaminants is deteriorating the delicate balance of soil and environmental ecosystem, and needs to be remediated. Different remediation technologies are being used e.g. bioremediation, sorption, chemical oxidation, vapor extraction, thermal desorption, and incineration. But application of a sole technique does not provide satisfactory results because of poor remediation efficiency, cost, time and environmental impacts. Therefore, innovative remediation technologies must be developed because the current available technologies are often inadequate to meet economic and ecological demands.
In order to provide relatively inexpensive and highly effective strategies for the remediation of contaminated sites, research has focused on in situ remediation technologies. Coupling of different remediation technologies to improve removal from contaminated sites has achieved very fast, more effective and efficient results. The coupling offers an opportunity to apply various techniques at the same target zone.
Bioremediation is an appealing strategy for cleaning up of contaminated matrix, primarily because of its environment friendly and cost-effective nature. Biological techniques are more appropriate for the bioavailable plume section of a contaminated site (Singh et al., 2009). Most of the petrol hydrocarbons are biodegradable but bioremediation becomes inefficient in lowering long term soil toxicity below the stringent environmental cleanup standards and always leaves more or less complex residues (Plaza et al. 2009). Moreover, bioremediation is limited by longer treatment time and its inability to perform at highly contaminated sites as microbes dont survive there (Atlas, 1995). These limitations of bioremediation could be addressed by its coupling with advanced chemical oxidation treatments which involve injection of strong oxidants into the soil matrix to react with pollutants.
The recent investigation has shown that coupling two common treatments, in situ chemical oxidation and in situ bioremediation is not only feasible but also in many cases provides more efficient and extensive cleanup of contaminated sub-surfaces (Sutton et al. 2011 and references therein). Through coupling chemical oxidation to rapidly remove high concentrations of contaminants and long-term biological breakdown of remaining pollutants, an improved in situ remediation strategy is created that allows rapid redevelopment and extensive contaminant removal. In situ chemical oxidation, chemical oxidants are applied to sub-surface to rapidly degrade pollutants. Alternatively, naturally occurring micro-organisms are also able to break down contaminants relatively slow, but some organic pollutants, chemical pollutants, chlorinated compounds and radionuclides are not remediated because of limited abilities of bioremediation. Also bioremediation can produce toxic metabolites. To enhance bioremediation appropriate condition are maintained by the injecting nutrients or adjusting redox conditions. Through combining these two techniques, redevelopment can occur following chemical oxidation while long term bioremediation ensures removal of residual contaminants.
It has been concluded that pre-treatments of in situ chemical oxidation seems to improve overall remediation by (1) decreasing the pollutant concentration to less toxic levels for the soil biota, (2) improving parent compound bioavailability, (3) producing biodegradable and bioavailable oxidized daughter compounds and by providing oxygen for aerobic biological transformation of contaminants (Sutton et al., 2010)
But oxidants used in Situ Chemical Oxidation can impede the biological activity (Sutton et al., 2010). Due to addition of oxidants, changes in subsurface conditions such as pH which are necessary for microbial growth can kill soil biota. Thus, by optimizing conditions microbial population can not only survive, but thrive. This can be done by adjusting the type and quantity of chemical oxidant used. We may consider following points for oxidants use (1) compatibility of oxidant with microbes rather on the basis of contaminant, (2) through minimizing direct oxidant contact with soil biota and (3) by using low doses of oxidants.
Similarly, parameters such as pH, nutrient availability and temperature can be manipulated to create optimized conditions for bioremediation in chemically
pretreated sites. Coupled bioremediation and In Situ Chemical Oxidation is not only feasible but, when properly applied, can provide rapid,
more extensive and cost-effective remediation than either biological or chemical
techniques alone.
Coupling technologies has been successfully used for the remediation of polycyclic aromatic hydrocarbons, petroleum diesel contaminated sites, water remediation and heavy metals on laboratory level (Rijnaarts, 2010; Silva-Castro et al., 2013; Valderrama et al., 2009; Venny et al., 2012) . By the concept coupling, research-based, cost-effective and efficient, biphasic treatment strategies can be planned for different pollutants under different conditions to improve remediation efficiencies. In view of the facts, the time and research are needed for taking this technology from the laboratory level to the field condition. The optimization of conditions and a full understanding of the processes essentials are required to ensure success of coupling in field applications.
Atlas, R. M. (1995). Petroleum biodegradation and oil spill bioremediation. Marine Pollution Bulletin 31, 178-182.
Rijnaarts, N. B. S. J. T. C. G. H. H. M. (2010). Efforts to improve coupled in situ chemical oxidation with bioremediation: a review of optimization strategies. SOIL AND LANDSCAPE ECOLOGY.
Silva-Castro, G. A., Rodelas, B. n., Perucha, C., Laguna, J., González-López, J. s., and Calvo, C. n. (2013). Bioremediation of diesel-polluted soil using biostimulation as post-treatment after oxidation with Fenton-like reagents: Assays in a pilot plant. Science of the Total Environment 445—446, 347-355.
Singh, A., Kuhad, R. C., and Ward, O. P. (2009). Biological remediation of soil: an overview of global market and available technologies. In “Advances in applied bioremediation”, pp. 1-19. Springer.
Sutton, N. B., Grotenhuis, J. T. C., Langenhoff, A. A. M., and Rijnaarts, H. H. M. (2010). Efforts to improve coupled in situ chemical oxidation with bioremediation: a review of optimization strategies. Journal of Soils and Sediments 11, 129-140.
Valderrama, C., Alessandri, R., Aunola, T., Cortina, J. L., Gamisans, X., and Tuhkanen, T. (2009). Oxidation by Fentons reagent combined with biological treatment applied to a creosote-comtaminated soil. Journal of Hazardous Materials 166, 594-602.
Venny, Gan, S., and Ng, H. K. (2012). Modified Fenton oxidation of polycyclic aromatic hydrocarbon (PAH)-contaminated soils and the potential of bioremediation as post-treatment. Science of The Total Environment 419, 240-249.
The writers are associated with the Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad. They can reached at <muhammad.usman@uaf.edu.pk> and <musmanch@yahoo.com>