Heavy metals and their compounds are naturally ubiquitous throughout the soil environment. They are introduced naturally through the weathering of the parent materials, and also resulted from a variety of human activities such as mining, smelting, electroplating, and other industrial processes that have metal residues in their waste streams. When heavy metals in soils have been transformed from solid form into either ionic moieties or through biomethylation to organometallic moieties, they could have caused threat to the health of animals and human beings. For instance, the chronic effects of Cd dust or aerosol particulate matter through soil ingestion consist of lung cancer, pulmonary adenocarcinomas, prostatic proliferative lesions, bone fractures, kidney dysfunction, and hypertension.Another example, the excessive intake of Pb (PM2.5, Pb2+ ions or organolead) can damage the nervous, skeletal, circulatory, enzymatic, endocrine, and immune systems. Due to their potentialtoxic, persistent and irreversible characteristic, the heavy metals such as Cd, Cr, As, Hg, Pb, Cu, Zn and Ni have been listed as priory control pollutants by the United States Environmental Protection Agency (USEPA) and caused more and more attention in many part of the world.
According to the report by the Environmental Protection Ministry, 16.1% of China’s soil and 19.4% of its arable land are highly contaminated with cadmium, nickel and arsenic. High levels of toxins and heavy metals are a major concern since rapid industrialization in China has caused irreversible damages to the environment. The study was performed on soil samples from two-thirds of China’s land which is about 6.3 square kilometers. According to researchers, 82.8% of the polluted land was contaminated with inorganic materials including arsenic, copper, mercury, lead and cadmium.
What techniques are used to remediate contaminated soils?
In the past, soil remediation was primarily done by physical removal of soils from contaminated sites for landfilling, incineration or chemical stripping of contaminants from soil resulting soils sufficiently clean to leave on site. In many cases, significant additional damage would result. Evaluation of alternative methods, that were less invasive, providing the similar cleanup results with less damage, were not only allowed, but required.
Given the high costs of traditional remediation techniques, increasing consideration is being given to phytoremediation because of the potential of remediating a site at a relatively low cost compared to conventional remediation methods. Phytoremediation is significantly cheaper than conventional remediation technologies for shallow contamination, and in the case of phytoextraction, target cleanup concentrations are close to the concentrations at the contaminated site. In our project, we have selected hyperaccumulator- Sedum alfredii Hance to uptake heavy metals in the metalliferous soil and increased ability of plants expressing the bacterial enzyme ACC deaminase to accumulate heavy metals.
However, phytoremediation in practice has several constraints at the level of sites as these are with a variety of different contaminants. Further, the success of phytoremediation of metals depends upon a plant's ability to tolerate to accumulate high concentrations of the metals, while yielding a large plant biomass. Due to their importance for practical applications, metal-tolerant plant–microbe associations have been the objective of particular attention due to the potential of microorganisms for bioaccumulating metals from polluted environment or its effects on metal mobilization/immobilization and consequently enhancing metal uptake and plant growth.
What method can be used to alleviate heavy metal stress and accelerate phytoremediation?
Bacillus megaterium is a type of phosphate-solubilizing bacteria that play a significant role in phosphorus nutrition by enhancing its availability to plants through release from organic soil P pools by mineralization and is also a type of rhizosphere bacteria are known to play an essential role in the management of heavy metal stresses in plants. As our chassis, we have intended to highlight Bacillus megaterium as a metal-trap by using genetically engineering method.