Bioslurping

This technique connects vacuum-enhanced pumping, soil vapor extraction, and bioventing to achieve soil groundwater remediation by the indirect provision of oxygen and stimulation of contaminant biodegradation (Gidarakos and Aivalioti 2007). The method is intended for free product recovery, such as light non-aqueous phase liquids (LNAPLs), thus remediating capillary, unsaturated, and saturated zones. It can also be applied to remediate soils contaminated with volatile and semivolatile organic compounds. The system uses a “slurp” that continues into the free product layer, which draws up liquids fr om this layer in a way comparable to that of how a straw draws fluid from any vessel. The pumping mechanism produces the upward movement of LNAPLs to the surface, where it becomes isolated from water and air.

In this technique, excessive soil moisture restricts air permeability and reduces oxygen transfer rate, in turn reducing microbial activities. Although the method is not fit for soil bioremediation with low permeability, it saves costs due to less volume of groundwater resulting from the operation. Thus, it minimizes storage, treatment, and waste disposal costs (Philp and Atlas 2005). Building a

46 Bioremediation Science: From Theory to Practice vacuum on a deep high permeable site and fluctuating water table, which could produce saturated soil lenses that are hard to aerate, is amongst the major concerns of this in situ technique (Azubuike et al. 2016).

Biosparging

This technique is very comparable to bio venting in that ah' is injected into soil subsurface to excite microbial activities to promote pollutant removal from polluted sites. Unlike bioventing, the air is introduced at the saturated zone, which can cause upward mobility of VOCs to the unsaturated zone to improve biodegradation. The effectiveness of biosparging depends on two significant factors, namely, soil permeability, which describes pollutant bioavailability to microbes, and pollutant degradability (Pliilp and Atlas 2005).

As with bioventing and soil vapor extraction (SYE), biosparing is comparable in operation with an almost similar technique known as in situ air sparging (IAS), which depends on high airflow rates to realize pollutant volatilization, whereas biosparging increases biodegradation. Moreover, both mechanisms of pollutant removal are not mutually exclusive for both techniques. Biosparging has been extensively applied in managing aquifers contaminated with petroleum products, especially diesel and kerosene. Kao et al. (2008) reported that biospargiug of benzene, toluene, ethylbenzene, and xylene (BTEX) contaminated aquifer plume resulted in a transfer from anaerobic to aerobic conditions (Azubuike et al. 2016).

A case study conducted in the Damodar Valley in Eastern India revealed that biosparging is effective at removing 75% of contaminants present within a year (Kruger et al. 1997). The initial results achieved in the field trials were also explained using computer modeling programs. The results from the study were used to set the optimum conditions for bioremediation, including proper moisnue content, pH, temperature, nutrients, and carbon sources. The field tests used six separate test sites. Different parameters were tested in each location to investigate the optimum condition.

Phytoremediation

This technique relies on the application of plant interactions (i.e., physical, biochemical, biological, chemical, and microbiological interactions) in contaminated sites to mitigate the toxic effects of pollutants. Depending on the pollutant type (elemental or organic), there are several mechanisms (accumulation or extraction, degradation, filtration, stabilization, and volatilization) involved in phytoremediation (Kruger et al. 1997, McCutcheou et al. 2004, Subramaniam et al. 2006, Yadav et al. 2018). Toxic heavy metals and radionuclides pollutants are mostly removed by extraction, transformation, and sequestration (Ojuederie et al. 2017). On the other hand, organic pollutants like hydrocarbons and chlorinated compounds are dominantly reduced by degradation, rliizoremediation, stabilization, and volatilization (Table 4) by several types of plant species (Meagher 2000, Kuiper et al. 2004).

Some critical factors must be considered when picking a plant species for phytoremediation. This includes-plant root system, i.e., fibrous or tap depending on the depth of pollutant, aboveground biomass, which should not be accessible for annual eating, toxicity of pollutant to plant, plant survival and its adaptability to existing environmental conditions, plant growth rate, site monitoring and above all, time required to achieve the desired level of cleanliness. Besides, the plant should be resistant to diseases and pests (Lee 2013). It has been reported that in some contaminated environments, the process of contaminant removal by plant involves: uptake, which is mostly by passive means, translocation from roots to shoots, which is carried out by xylem flow, and accumulation in the shoot (San et al. 2013).

Purakayastha et al. (2009) conducted a phytoremediation study on crop species of Brassica juncea, Brassica canipestris, Brassica carinata, and Brassica napus. They concluded Brassica carinata cv. DLSC17 to be a suitable species for heavy metals remediation from the soil, which was able to reduce the metals load by 15% forZn. 12% Pb, and 11% forNi from a naturally contaminated soil from peri-urban Delhi. While Unterbrunner et al. (2007) conducted a phytoremediation study

Table 4. Process of phytoremediation with a suitable example of pollutants and then potential plants.

Process

Function

Pollutant

Medium

Plant

References

Phytoextraction

Remove metals pollutants that accumulate in plants. Remove organics from the soil by concentrating them m plants

Cd, Pb, Zn,

As,

Petroleum,

and

Radionuclides

Soil and groundwater

Viola baoshanensis, Helianthus annus, Alfalfa, Poplar, Indian mustard cabbage

(Macek et al. 2000, Zhuang et al. 2007)

Phyto-transformation

Plant uptake and degradation of organic compounds

Xenobiotic

substances

Soil

Cannes

(Subramanian et al. 2006)

Phyto-degradation

Plant and associated microorganism degrade organic pollutants

DDT,

Explosives, waste, and Nitrates

Groundwater

Elodea Canadensis, Duckweed Hybrid poplar

(Garrison et al. 2000, Newman and Reynolds 2004)

Rhizofiltration

Roots absorb and adsorb pollutants, prunarily metals, from water and aqueous waste streams

Zn, Pb, Cd,

As

Also,

Radionuclei

Groundwater

Brassica jimcea, Helianthus annus

(Dushenkov et al. 1995, Veima et al. 2006)

Phyto-stabihzation

Use of plant species to reduce the bioavailability of pollutants in the environment

Cu, Cd, Cr, Ni, Pb, Zn

Soil

Anthyllisvallesiana, Lupmusalbus Hybrid poplar grasses

(Vazquez et aL 2006)

Phyto-volatilization

Use of plants to volatilize pollutants

Se, CC14, EDB, TCE

Zea mays, Brassica sp.

(Ayotamuuo and Cogbara 2007)

on tree species and observed a considerable amount of Cd and Zn accumulation in various willow, poplar, and birch tree species with up to 116 mg Cd kg'1 and 4680 mg Zn kg"1 in leaves of Salix caprea tree, metal concentrations in leaves were not related to total (aqua regia), or labile (1 M NH4N03 extract), concentrations in soil but the accumulation factors (leaf concentration: soil concentration) for Cd and Zn, which followed an inverse log type function.

That the application of plant growth-promoting rhizobacteria (PGPR) might play an essential role in phytoremediation is also reported by many researchers, as PGPR promotes biomass production and tolerance of plants to heavy metals and other unfavorable soil (edaphic) conditions (Barea et al. 2002, Yancheshmeh et al. 2011, De-Bashan et al. 2012, Rajkumar et al. 2012). Brachiaria viatica and maize (com) have also been listed as potent phytoremediators of heavy metal-polluted soil (Tiecher et al. 2016).

 
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