**5.9 Permeable reactive barrier (PRB)**

*Trace Metals in the Environment - New Approaches and Recent Advances*

ity among other in-situ bioremediation techniques [25].

**5.6 Bioslurping**

**5.7 Biosparging**

**5.8 Phytoremediation**

nutrients and moisture to increase bioremediation. That will achieve microbial transformation of pollutants to a harmless state. This technique has gained popular-

This technique combines vacuum-enhanced pumping, soil vapor extraction and bioventing to achieve soil and ground water remediation by indirect providing of oxygen and stimulation of contaminant biodegradation [26]. This technique is planned for products recovery from remediating capillary, light non-aqueous phase liquids (LNAPLs), unsaturated and saturated zones. This technique used to remediate soils which are contaminated with volatile and semi-volatile organic compounds. The method uses a "slurp" that spreads into the free product layer, which pulls up liquids from this layer. The pumping machine transports LNAPLs to the surface by upward movement, where it becomes separated from air and water. In this technique, soil moisture bounds air permeability and declines oxygen transfer rate, which reducing microbial activities. Although this technique is not suitable for low permeable soil remediation, it is cost effective operation procedure due to less

amount of ground water, minimizes storage, treatment and disposal costs.

generally used in treating aquifers contaminated with diesel and kerosene.

possible when some plants such as willow and alfalfa are used [27, 28].

This technique is similar to bioventing in this air is injected into soil subsurface to improve microbial activities which stimulate pollutant removal from polluted sites. However, in bioventing, air is injected in saturated zone, which can help in upward movement of volatile organic compounds to the unsaturated zone to stimulate biodegradation process. The efficiency of biosparging depends on two major factors specifically soil permeability and pollutant biodegradability. In bioventing and soil vapor extraction (SVE), biosparing operation is closely correlated technique known as in-situ air sparging (IAS), which depend on high air-flow rates for volatilization of pollutant, whereas biosparging stimulates biodegradation. Biosparging has been

Phytoremediation is depolluting the contaminated soils. This technique based on plant interactions like physical, chemical, biological, microbiological and biochemical in contaminated sites to diminish the toxic properties of pollutants. Which is depending on pollutant amount and nature, there are several mechanisms such as extraction, degradation, filtration, accumulation, stabilization and volatilization involved in phytoremediation. Pollutants like heavy metals and radionuclides are commonly removed by extraction, transformation and sequestration. Organic pollutants hydrocarbons and chlorinated compounds are mostly removed by degradation, rhizoremediation, stabilization and volatilization, with mineralization being

Some important factors of plant as a phytoremediator include: root system, which may be fibrous or tap depending on the depth of pollutant, above ground biomass, toxicity of pollutant to plant, plant existence and its adaptability to predominant environmental conditions, plant growth rate, site monitoring and above all, time mandatory to achieve the preferred level of cleanliness. In addition, the plant must be resistant to diseases and pests [29]. In phytoremediation removal of pollutant includes uptake, translocation from roots to shoots. Further, translocation and accumulation depends on transpiration and partitioning [30]. However, the process

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This technique is commonly observed as a physical method for remediating contaminated groundwater. However, biological mechanisms are precipitation degradation and sorption of pollutant removal used in PRB method. The substitute terms such as biological PRB, bio-enhanced PRB, passive bioreactive barrier, have been suggested to accommodate the biotechnology and bioremediation aspect of the technique. In general, PRB is an in-situ technique used for remediating heavy metals and chlorinated compounds in groundwater pollution [31, 32].
