In this kind of remediation process, the organic matter is electrochemically converted to generate useful by-products, produce bioelectricity, and fuel by the action of microbial metabolic processes Logan and Rabaey, As soil contains the majority of heavy metal in insoluble form, their removal rate was minimum, so the solubility can be achieved by coupling electrokinetic with other techniques. Based on these implications, electro-kinetic EK technique was introduced around s and was widely employed to manage heavy metal contaminated fine-grain soils of low hydraulic conductivity Maini et al.
Here a direct electric current was used to remove fine and low permeability heavy metal particles from the soil with minimum disturbance to the surface.
As voltage was applied between two sides of the electrolytic tank containing contaminated soil, an electric field gradient was created. This low-level electric current aid as a cleaning agent by stimulating the pollutants to transport toward recovery wells involving mechanisms such as electro-osmotic flow, electromigration, and electrophoresis thereby inducing electrochemical reactions Acar and Alshawabkeh, The main advantage of this method is its simple operation, cost-effective, and no subsequent pollution Zhou et al.
But, EK method has also certain restrictions like, bioavailability of the heavy metal and mass transfer between the electrode and pollutants Simoni et al. In order to increase its overcome these restrictions and to achieve high efficiency, an interesting idea of integrating EK remediation with biological method was used and got succeeded by many researchers.
As a biological counterpart, both acidophilic and alkalophilic microbes were employed.
If the acidic bacterium is involved, it will favor EK, whereas the alkalophilic will aid in metal precipitation. In a few instances, few microbes may require additional nutrients as an energy source glucose, starch etc. Some of the interesting works on Bio-EK integrated system, implied by the scientific community were discussed in detail here.
One such study by Rosestolato et al. In a study of EK assisted bioremediation carried out by Azhar et al. Electrokinetic study was conducted using electric current of voltage 50V for a period of 7 days, which was followed by microbial remediation using Lysinibacillus fusiformis bacteria. With a future perspective of symbiotic combination strategies using electrochemically active bacterial cells and electrified interfaces, Varia et al.
Their demonstration concluded on microbial influenced electronation thermodynamics of the metal ion, with an outcome of prospective energy savings. A similar study by Kim et al. They have employed Acidithiobacillus ferrooxidans species to carry out bioleaching process as it was capable to oxidize the reduced sulfur and ferrous ions. This creates an acidic environment in the soil, which was reported to as a suitable condition for removal of heavy metals Nareshkumar et al. Peng et al. In this Bio-EK integrated remediation, bioleaching process was carried out initially to convert the metal to soluble form which favors a faster and higher rate of remediation in electrokinetic method a follow up process in bioelectrokinetics.
From the obtained results they have concluded that the maximum removal of heavy metal was achieved with minimal power consumption, than used for conducting individual EK remediation. A similar bioelectrokinetics remediation work was reported by Huang et al. In this experiment, soil samples were collected and oxidized using iron containing bacterial species and the soil was further treated by electrokinetic method, by which, the metals will start to eliminate with change in the pH of soil.
The corresponding elimination of metal ions of copper, zinc, chromium, and lead was monitored and reported with maximum removal rate.
Dong et al. A pilot study was conducted for a period of 30 days in which surfactant Tween 80 and chelating agents EDTA was added to enhance EK operating conditions. The addition of EDTA was found to play a role in eliminating the heavy metal toxicity in soil and this coupled technique reported This is an emerging method of remediation that has proved to be more effective in terms of metal recovery and being more economical than the other integrated approaches discussed previously.
This combination was initiated with the outstanding results of EK remediation and its compatible operation with phytoremediation Figure 4. When phytoremediation is employed as an individual process, they may offer an economical solution, but, its in situ application is limited by climatic conditions, metal bioavailability, and shallow depths Barber, The recovery yield and process rate also require a significant improvement. However, this can be enhanced by combining phytoremediation with different strategies like transgenic technology, bioaugmentation, remediation with electrokinetics, permeable reactive barrier Cameselle et al.
Laboratory studies on EK and phytoremediation approach has exhibited a respectable vision in heavy metal remediation of Zn, Pb, Cu, Cd, and As. Electrokinetics was also found to play an important role in phytoremediation. A direct current passed between electrodes which placed vertically in soil separates organic and inorganic molecules Cao et al.
Depending on the plant's uptake mechanisms, different strategies like, phytoextraction, phytoevaporation, phytostabilization, rhizodegradation, and rhizofiltration were employed for phytoremediation Halim et al. Figure 4. Bhargavi and Sudha used an electrokinetic assisted phytoremediation process to reduce the levels of chromium and cadmium. In their study, the samples were taken from Bharathi Nagar and Tandalam village of the Ranipet Industrial area.
The collected samples were first remediated using EK method, followed by phytoremediation by extruding the remediated soil samples from the electrokinetic cell. The EK remediated soil was potted to grow the plant Brassica Juncea. For electrokinetic treatment, 50 V of electric current was applied and the removal rate was monitored at a regular interval of time from 5 to 25 days. They reported on This EK remediated soil, employed for phytoremediation showed a promising accumulation of cadmium and chromium in single harvest, which was further increased in subsequent harvests.
A similar study was conducted by Lim et al. Compared to controls, the electric field assisted phytoremediation showed 2—4 times effective removal of lead in the soil. Cang et al. They concluded that the property of the soil was directly influenced by the voltage applied and the growth of plant increased the enzymatic activity of soil to achieve a maximum heavy metal remediation. Electrokinetic coupled phytoremediation using species Lemna minor was tested by Kubiak et al.
In an another study of lead removal from soil was reported by Hodko et al. Phyto-electrokinetic remediation under laboratory scale was studied by O'Connor et al. One of the soil samples was contaminated by copper and the other by cadmium with arsenic. The test soils were filled in the reactor in two separate chambers. An electric current of 30 V was applied simultaneously by seeding with rye grass. A significant removal rate was reported over a period of 98 and 80 days for Cu and Cd-As soil, respectively.
EK enhanced phytoextraction demonstrated by Mao et al. It was then followed by phytoextraction using plants that enhanced the effectiveness of metal removal from the soil. Phytobial remediation is an efficient and eco-friendly solution to remove heavy metals from soil and water.
Phytobial remediation utilizes plants as well as microbes to remove heavy metals from soil and water. As mentioned in literature, phytobial based remediation utilizes the plants to uptake the heavy metals and the microbes will help in degradation of those metallic substances Lynch and Moffat, Figure 5 portrays different mechanism viz. These mechanisms can be enhanced by integrating a suitable bacterium that can secrete multiple plant growth promoting substances PGPS Martin and Ruby, These substances include organic acids, ACC deaminase, siderophores, and biosurfactants that will transform the metals into a bioavailable form Roy et al.
Figure 5. Various microbial interaction with heavy metals Source: Ahemad, Copyright , Springer Nature. Table 7. Plant growth promoting substances realeased by phosphate solubilizing bacteria Source: Ahemad, , Copyright Springer Nature. Phytobial remediation is recognized as cleanest and cheapest approach unlike other invasive technologies. It also has an advantage of being applied to vast areas of contaminated groundwater, soil and sediment.
In addition, its in situ application option was found to decrease the heavy metal distribution in the soil and aids in preserving the top soil. Despite these advantages, this method is restricted to shallow aquifer and soil due to plant root length restriction, potential fear of transfer of heavy metals to the food chain, long duration may require several seasons , regular monitoring due to litter fall , lack of safe proper disposal method, tough metal recovery procedures, and high recycle economy.
Roy et al. Different types of microbes involved in phytobial remediation are discussed here in detail. Free living microbes assist phytoremediation by mobilization, immobilization, and volatilization. Mobilization of metals occurs by different reactions such as volatilization, redox transformation, leaching, and chelation. The microbes like Sulfurospirillum barnesii, Geobacter , and Bacillus selenatarsenatis are used for arsenic removal.
Lee et al. The plant used for phytoremediation accumulates heavy metals as harvested tissue, which can be disposed off. Introducing mobilizing microbes into contaminated water speed up the process of heavy metal accumulation Wang et al. During the immobilization process, the mobility of the contaminant is prohibited by altering the physical and chemical properties Leist et al.
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The oxidase enzymes present in the microbes oxidize the metals and make them immobilize and less toxic. The microbes such as Sporosarcina ginsengisoli, Candida glabrata, Bacillus cereus , and Aspergillus niger were used in immobilization technique to remove heavy metals Littera et al.
In a biotransformation process, a large number of bacteria, fungi, and algae were employed in heavy metal removal using biomethylation process Frankenberger and Arshad, Certain bacteria and fungi that live within the plants are called endophytes. They live within the plant for at least a part of their life cycle without damaging the host.
They are ubiquitously associated with most of the plant, of which some can promote plant growth Ryan et al. Few fungal endophytes will produce secondary metabolites too. Methylobacterium strains from Pteris vittata herb was reported to exhibit heavy metal tolerance Dourado et al.
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