Bioaeration is a technique by which aerobic biodegradation in situ is stimulated by additional oxygen intake to soil bacteria. Unlike the vapor extraction technique in the soil, bioaeration uses low airflows, to support microbiological activity. Usually oxygen is added to the soil by direct injection of air into the polluted land. Injection of air can be done in vertical wells or in horizontal channels. In addition to accelerating degradation, bioaeration also has a side effect, namely to move volatile contaminants through activated soil. The technique usually applies to the unsaturated soil area and is suitable for all compounds that can be aerobically biodegradable (Preston et al., 2011).
Under the generic name of phytoremediation are those methods that usage plants to remove, transfer, stabilize, and destroy contaminants/pollutants from soil, water, sediments. Phytoremediation techniques offer significant potential for certain implementations and allow lands that are much larger than would be possible if traditional remediation technologies were used. A large number of plant species (over 400), starting with pteridophyte ferns and ending with angiosperms such as sunflower or poplar, can be used to remove contaminants from agricultural waste through several mechanisms. Phytoremediation mechanisms include intensified biodegradation in the rhizosphere (rhizodegradation), phytoextraction (phytoaccumulation), phytodegradation, and phytostabilization (Singh and Singh, 2017).
Rhizodegradation takes place in the soil part that surrounds the roots of the plants. Natural substances propagated from plant roots serve as a substrate for MO present in the rhizosphere, thus accelerating the degradation of contaminants/pollutants. Plant roots loosen the soil, leaving room for water transport and aeration. This technique tends to push the water to the surface area and dehydrate the lower saturated areas (Wang et al., 2017).
Phytoextraction is the technique by which plant roots absorb water and nutrients together with soil contaminants/pollutants (such as especially metals). Contaminants/pollutants are not destroyed, but accumulate in the roots, stems, and leaves of plants that can be harvested to remove and destroy contaminants/pollutants. The extraction technique depends on the capacity of plants to grow in soils with high concentrations of metals and their capacity to extract the soil contaminants under specific climatic conditions.
For phytoextraction may be used either plants with exceptional natural capacity to accumulate metals, so-called hyperaccumulators, or plants that produce high amounts of biomass (com, barley, peas, oats, rice, Indian mustard) chemically assisted with additives that improve the capacity to extract contaminants (Tauqeer et al., 2019).
Additives of citric acid, oxalic acid, gallic acid, vanilic acid, classical chelating agents such as ethylenediaminetetraacetate (EDTA) and diethy- lenetriaminepentaacetate (DTPA) or biodegradable chelating agents such as ethylenediamine disuccinate (EDDS), methylglycine diacetate (MGDA) substantially improve soil extraction of Zn, Cd, Cu, and Ni.
However, care must be taken, as these additions have the risk of mobilizing metals in groundwater. The number of hyperaccumulators in the plant kingdom is reduced: about 400 species of vascular plants, the vast majority having a particular affinity for Ni. By definition, hyperaccumulators should accumulate at least 100 mg/g of Cd or As, 1000 mg/g of Co, Cu, Cr, Ni or Pb, 10,000 mg/g Mn or Ni. Certain species of ferns have a special accumulation capacity for As-up to 23,000 mg/kg in the Pteris vitata species. Common buckwheat (Fagopyrum esculentumMoencti) can accumulate up to 4200 mg/kg Pb, being the first Pb hyperaccumulator species that also has high productivity in biomass.
Other plants with the potential for phytoextraction are those of Brassica genus: Brassica jtmcea (Indian mustard) for Cr, Cd (VI), 137Cs, Cu, Ni, Pb, Brassica napus for Pb, Se, Zn, Brassica oleracea (ornamental cabbage) for As, 137Cs, Ni, Tl. Hg bioavailable from the soil can be extracted with barley, wheat, yellow lupine (Lupinus luteus), dog grass (Cynodon dactylon) (Ashraf et al., 2019).
Phytodegradation is the technique of metabolizing contaminants/pollutants in plant tissues. Plants produce enzymes (such as oxygenases and dehaloge- nases) that promote catalytic degradation of contaminants/pollutants hi plant tissue. It is studied the possibility of simultaneous degradation of aromatic compounds and chlorinated aliphatic compounds by this method (Dolphen and Thiravetyan, 2015).