African Journal of Microbiology Research Vol. 6(10) pp. 2233-2238, 16 March, 2012 Available online at http://www.academicjournals.org/AJMR DOI: 10.5897/AJMR11.967 ISSN 1996-0808 ©2012 Academic Journals
Review
Soil electerokinetic remediation and its effects on soil microbial activity- A review Iman Tahmasbian1* and Azadeh Nasrazadani2 1
Member of Young Researchers Club, Islamic Azad University, Khorasgan Branch, Isfahan, Iran. Department of Soil Science, College of Agriculture, Islamic Azad University, Khorasgan Branch, Isfahan, Iran.
2
Accepted 9 February, 2012
Over the years, the persistence of heavy metals in the nature associated with their intensive use by modern society has caused metal accumulation in the biosphere and adverse effects on food quality, soil health and the environment. In response to these negative effects, there has been an ongoing development of variety of technologies to treat the contaminated soils with heavy metals. Electrokinetics provides a physical method for the extraction of chemicals from contaminated sites and can be used for in-situ treatment of heavy metals and organic contaminated soil and its operation principle is applying direct electric field in soil to drive pollutant within the soil pores towards the electrode through electromigration, electroosmosis as well as electrophoresis. An important requirement, before using any method for the remediation of contaminated soil, is specifying their efficiency and assurance of having few destructive impacts on soil health. Chemical transformation occurring during the treatment could greatly modify the bioavailability of these metals and possibly make them more hazardous to the living organisms. The aim of this review is to assess the consequences of soil electrokinetic decontamination on biological activities. Key words: Electrokinetic, decontamination, soil, microbial activity, heavy metals.
INTRODUCTION Nowadays, due to the lack of environmental awareness and improving living quality, soil contamination problem is raised all over the world. Heavy metal pollution is released into the environment by various anthropogenic activities, such as industrial manufacturing processes, domestic refuse and waste materials. Excess concentrations of heavy metals in soils have caused the disruption of natural terrestrial ecosystems (Wei et al., 2007; Yadav et al., 2009). A large variety of remediation methods have been developed to reform the damaged environment and ecology so as to protect the public health of this future generation. The varying degree of
*Corresponding author. E-mail:
[email protected]. Tel: 00989360046449.
success of soil remediation depends heavily on the nature of contaminated soil, contaminant type and its concentration on the subsurface and also environmental conditions since due to low hydraulic conductivity in finegrained soils; remediation is so difficult (Yeung, 2011). Electrokinetics is one of the recently developed remediation technologies. It can provide the degradation of soil contaminants and the removal of metals by direct movement of pollutants, in response to the direct electric current (Maini et al., 2000). This process is a hybrid technology for the treatment of contaminated subsurface with organic contaminants and has been demonstrated to be successful with wide ranges of pollutants such as organic chemicals and heavy metals and has emerged as one practical engineering technique for remediation of contaminated soils (Kim et al., 2009; Maini et al., 2000). Several recent studies revealed harmful effects of
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electrokinetic decontamination on the biodegradation and culturability of soil bacteria in direct current (DC) electric fields as they are typically used for electro-bioremediation measures (Shi et al., 2008c; Tiehm et al., 2009; Wick et al., 2004). It couples bioremediation to electrokinetic transport phenomena of nutrients, terminal electron acceptors and contaminant-degrading microbes. During electrobioremediation DC electric fields are normal applied to induce electrokinetic transport phenomena which are particularly effective in fine-grained soils of low hydraulic conductivity (Wick et al., 2010). Electrokinetic remediation methods seem to be the most effective insitu or ex-situ soil decontamination procedure, with high efficiency and time effectiveness in low permeability soils (Shen et al., 2007). Briefly, when DC electric fields are applied to contaminated soil via electrodes placed into the ground, the migration of charged ions occurs. Positive ions such as heavy metals (HM) are attracted to the negatively charged cathode, and negative ions migrate to the positively charged anode (Figure 1). Non-ionic species have been experimentally proved to be transported with the electroosmosis-induced water flow (Virkutyte et al., 2002). Electromigration, electroosmosis and electrophoresis are removal mechanisms in electrokinetic remediation. + The H and the OH ions are generated from the anode and cathode, moving across the pore fluid within soil particles towards the cathode and anode, respectively. + The H cations created at the anode, enhance desorption of the adsorbed metals on the soil surface and at the same time promote the dissolution of precipitated contaminants. The production of OH ions at the cathode increases pH which causes the precipitation of the metals and consequently prevents their movement and reduces the treatment efficiency (Virkutyte et al., 2002). Using a direct electric current in soil results several changes in many aspects, such as soil pH, redox potential and electrolyte concentration. These changes may impact the nature of the clay surface, chemically and the success of the electrochemical remediation (Lear et al., 2004; AlHamdan and Reddy, 2008). As we know, microbial communities play significant roles in recycling of plant nutrients, maintenance of soil structure, detoxification of noxious chemicals and controlling the plant pests and growth (Elsgaard et al., 2001; Filip, 2002). The changes in soil condition such as soil pH, temperature (increased by 1 to 3°C as a result of EK process) and the application of electric current have direct effects on soil microbial activity (Kim et al., 2010). Although little is currently known about the effect of electrokinetics on exposed soil microbial communities, recent studies have suggested that no serious impact on microbial health occurs when it is applied to pristine, noncontaminated soils (Lear et al., 2004). However, there are
some researches about the effects of electrokinetic on soil health in the presence of pollutant, illustrating the adverse effects on soil microbial communication (Wang et al., 2009a, b; Cang et al., 2009; Kim et al., 2010). As the organisms play important roles in soil, investigation of the influences of the electrical field on soil organisms is crucial to assess the efficiency of this method for soil remediation. Therefore, in this review we attempt to scrutinize the effects of electrical field on biological characteristic of soil.
EFFECTS ON MICROBIAL ENUMERATION Electro-osmosis seems to be a feasible way to disseminate bacteria in different soil types even when hydraulic pumping is ineffective. It is well suited even for dense soil types, where other forms of electrokinetics are inefficient. The micro-scale tests show the principal feasibility of dissemination of bacteria in soils in which electro-osmosis functions. In most researches, the electrokinetic remediation systems were applied in small reactors in the laboratory scale. It is undeniable that the referred system can reduce the microbial population where the electrodes are placed in the soil. Kim et al. (2010) showed that the larger number of bacteria in all parts of the soil treated with EK, declined compared with controlled soil. Bacterial abundance near the electrodes was significantly lower than controlled and was counted at the middle distance between the electrodes. In the same experiment, the results of real-time polymerase chain reaction (PCR) indicated that 16S ribosomal ribonucleic acid (rRNA) gene copy in the EKdecontaminated soil significantly decreased in proportion to the controlled system. Kim and his colleagues also demonstrated that bacteria can be transported by Electroosmosis phenomenon as they found a positive correlation between the 16S rRNA gene copy and residual diesel concentration (Kim et al., 2010). The larger gene copy numbers were observed at the middle distance between the electrodes and the soil near the electrodes showed low gene copy numbers. Many experiments prove that electrical field can decrease and increase soil pH near the anode and cathode, respectively (Zhou et al., 2005; Cang et al., 2009; Zhang et al., 2010). On the other hand, the effect of soil pH on specified microorganisms, soil microbial biomass, microbial activity and more recently, microbial community structure, have been investigated previously (Pietri and Brookes, 2008). Wardle (1992) concluded that soil pH is probably at least as important as soil C and N concentrations in influencing the size of the microbial biomass. Soil pH also affects organic C solubility (Andersson et al., 2000) and increases the availability of biologically
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Figure 1. Electromigration and electroosmosis flow in the soil, HM shows heavy metals and other cations.
toxic with the decreasing of pH (Flis et al., 1993). This, in turn, affects microbial community structure (Marstorp et al., 2000) and microbial activity (Bååth and Anderson, 2003). Soil microbial activities are affected by the accumulation of contaminants in soil. Low concentrations of certain transition metals such as cobalt, copper, nickel and zinc are essential for many cellular processes of bacteria. However, higher concentrations of these metals often are cytotoxic. Other heavy metals, including lead, cadmium, mercury, silver and chromium have no known beneficial effects to bacterial cells and are toxic even at low concentrations (Nies, 2004). Lear et al. (2007) found that bacterial counts decreased within the electrokinetic cell as compared to the control treatment In pentachlorophenol (PCP) polluted soil. They reported that application of electrokinetic led to reduce the number of both bacteria and fungi close to the anode. Both the application of PCP and electrokinetics were previously shown to induce changes in the soil microbial community (Martins et al., 1997; Lear et al., 2004, respectively), the latter reducing bacterial numbers close to the anode. Lear et al. (2007) concluded that the movement of PCP towards the anode end of the electrokinetic cell, (similarly observed in Harbottle et al., 2001) may have caused further disruption to the microbial community. Soil management can also change its physical, chemical and biological characteristics and as a result, different responses by biological activities to heavy metal toxicity can be observed. Also, the activities of microorganisms that promote plant growth can be altered by high concentrations of heavy metals (Wang et al.,
2007).
EFFECTS ON ENZYME ACTIVITY However, few studies have been focused on the effect of EK remediation on soil enzyme activities. While, soil enzymes, as the most active soil components, drive the metabolic process of soil and play important roles in soil nutrient cycling and the purification of pollutants. Soil enzyme activities reflect the soil physicochemical properties and heavy metal concentration (Hinojosa et al., 2004). On the other hand, soil enzyme activities are more stable than microbial activity and are easily measured. So, the study on the effect of EK on soil enzymes and its mechanisms will enable us to better assess the influence of EK on soil characteristics and provide the practical application basis (Cang et al., 2009). Soil enzymes are acting as biological catalysts to facilitate different reactions and metabolic processes occurring in biogeochemical cycles of nutrients, maintenance of soil structure, detoxification of pollutants and produce essential compounds for both microorganisms and plants (Moreno et al., 2003; Pan and Yu, 2011). Among the different enzymes in soils, urease, acid phosphatase and dehydrogenase are important in the transformation of different plant nutrients (Pan and Yu, 2011). The inhibition of heavy metal pollution on urease or phosphatases activity was reported by many scientists (Khan et al., 2010). Generally, it has been proved that
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heavy metals can combine with sulf-hydryl groups of proteins, restraining the activity of enzymes. Investigation of enzyme activity after the electrokinetic remediation opened another window to illuminate the influence of the direct current on the soil microbiology. Urease is an extra-cellular enzyme which has been studied since the last few years. Wang et al. (2009) found that the urease activity in the electrokinetic treated soil increased compared with untreated soil (polluted soil). Clean soil is reported to have most urease activity. They observed that the anodic soil had the least urease enzyme activity. Whereas the metal concentration in this part of the soil is low, it was concluded that urease is influenced by a factor except pollution. Cang et al. (2009) showed that the soil urease activity changed very little and was slightly lower than the original value. They concluded that electrokinetic treatments had little negative effect on soil urease activity. With this evidence, the first thing that comes to mind is the effect of pH on urease activity. To eliminate the effects of the pH on enzyme activity, in the same experiment, soil pH level was kept on 3.5, but the observation showed no differences between the pHcontrolled and uncontrolled experiment (Cang et al., 2009). In the other electrokinetic remediation of diesel polluted soil, dehydrogenase activity in the soil was used for investigation of soil microbial activity. In this experiment, middle of the electrokinetic reactor showed the highest dehydrogenase activity, as well. The enzyme activity near the cathode decreased compared to that of the controlled. Interestingly, soil dehydrogenase activity near the anode was enhanced after electrokinetic remediation; despite decreased the number of bacteria (kim et al., 2010). In this experiment Cang et al. (2009) revealed that soil invertase and catalase activities increased and the highest invertase activity was170 times as much as the initial one. The activities of soil acidic phosphatase were lower than the initial ones. Bivariate correlation analyses indicated that the soil invertase and acidic phosphatase activities were significantly correlated with soil pH, EC and dissolved organic carbon but the soil urease activities had no correlation with soil properties. Furthermore, the effects of direct electric current on solution invertase and catalase enzyme protein indicated that it had negative effect on solution catalase activity and little impact on solution invertase activities. From the change of invertase and catalase activities in soil and solution, Cang et al. (2009) concluded that the dominant effective mechanism is changing soil properties by electrokinetic treatments. Moreover, it seems that the response of the microbial population and enzymes to the electrical field depends on their kind and their optimal habitat. In this study, the soil urease activity changed very little and was slightly lower than the original value. It meant that EK treatments had little negative effect on soil
urease activity. By paired sample t-test, the soil urease activities of the two experiments were not significantly different. By comparison with urease, the soil invertase and catalase activities in most column sections were larger than the original value (Cang et al., 2009).
EFFECTS ON MICROBAIL RESPIRATION Although there are not many researches on the effects of electrokinetic on microbial respiration, it is thought that the electrokinetic can reduce microbial respiration. The application of electric current significantly reduced soil microbial respiration below that of the control in PCP polluted soil. Furthermore, respiration was lowest close to the anode end of the electrokinetic chambers (Lear et al., 2007). Altering the soil pH in electrokinetic treated soils can impress the microbial respiration. This was also true for microbial population and probably enzyme activity. In an electrokinetic experiment, the lowest soil microbial respiration was observed in the soil close to anode (Wang et al., 2009a). There was not any correlation with metal concentration and microbial respiration in soil, indicating that soil microbial respiration was reduced by reducing the pH in the anodic soil. On the other hand, Wang et al. (2009b) found that soil respiration decreased in the soil near the cathode. It is illuminating that there were other factors affecting the microbial processes.
EFFECTS ON SOIL MICROBIAL BIOMASS CARBON Soil microbial biomass carbon has already been found to be sensitive to the changes of heavy metals concentrations in soil medium (Giller et al., 1998). Wang et al. (2009a) reported that the anodic soil had higher levels of biomass carbon compared with the polluted soil. There were no significant differences between the polluted soil and cathodic soil. They concluded that this was probably due to the high soil metal concentration in soil near the cathode. In the current study, the obtained results showed that soil microbial biomass carbon was negatively related to soil pH. It is known that the electrokinetics alters soil pH, which is a crucial parameter for soil microbial growth, influencing a complex range of interacting factors, such as membrane integrity and function, and also the bioavailability of nutrients and contaminants (Lear et al., 2004). Hence, the change of the soil microbial biomass carbon may have been enhanced by the combination of low pH and soil metal toxicity that interacted synergistically to make conditions even more harmful (Wang et al., 2009a). The direct effect of the applied electric field on microbial biomass carbon could not be proved (Wang et al., 2009a). Brookes has provided evidence that heavy metals decrease the
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proportion of microbial biomass C in total soil organic matter and soil microbial C has been proposed as a useful measure of soil pollution by heavy metals (Brooke, 1995).
CONCLUSION When an electric current is applied across the electrodes, water moves through the soil medium towards the cathode. By using electro-osmosis, fluid can transfer through the soil particularly in fine sands and low permeability clays. During the electro-osmosis, pH values change at the electrodes and a gradient of moisture is formed across the soil, but these changes can be amended with suitable buffers. Having discussed about the effect of the electrokinetic decontamination on soil, we can conclude that the main factors influencing the microbial community are reducing pH as well as increasing the toxicity of metals in soil. In most cases, microbial population or activities decreased after the electrokinetic remediation. Soil acidification and the accumulation of pollutants near the anode were found to be disadvantageous to soil microbial diversity. Although in certain cases, different results were reported, more detailed studies showed that this reduction is more near the electrodes especially near the anode. Electrokinetics can affect a complex range of interacting factors such as membrane integrity and function via alteration of soil pH. Moreover, low pH increases the toxicity of pollutants. Indeed, combination of low pH and contaminant toxicity make the living conditions more difficult. In general, efficient electrobioremediation of contaminated soil requires that the application of electric fields has no negative effect on microbial ecosystem functioning. From a microbial community's or a microbe's perspective this means that neither DC-related physical, nor concomitant transport, nor release phenomena of nutrients, terminal electron acceptors or contaminants are harmful to its physiology and activity. Although electro-bioremediation appears to be successful, effective application of this technology still requires a better mechanistic understanding of the processes affecting the interactions of contaminantdegrading microorganisms with electro-kinetically mobilized contaminants at the micro-scale and detailed information on both its short and long side-effects on microbial activities. REFERENCES Al-Hamdan AZ, Reddy KR (2008). Transient behavior of heavy metals in soils during electrokinetic remediation. Chemos., 71: 860–871. Andersson S, Nilsson I, Saetre P (2000). Leaching of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in mor humus
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