Phytoremediation mechanisms for polycyclic aromatic hydrocarbons removing from contaminated soils
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Phytoremediation of polycyclic aromatic hydrocarbons (PAHs) from soil aims to degrade them into less toxic/non toxic compounds and limit their further movement by sequestration and accumulation into the vacuoles. Lipophilic organic compounds such as PAHs are bound strongly to the epidermis of the root tissue and are rarely translocated within plant. There are no reports in the literature data of PAHs being completely mineralized by plants. There is little evidence to suggest that PAHs accumulate to significant degree in plants, but there still is a lot of evidences on the ability of various plant species (most often grasses and legumes), to degrade and dissipate these dangerous contaminants. The primary mechanism controlling the dissipation of PAHs is rhizosphere microbial degradation where microbes use PAHs molecules as carbon substrates for growth, which in final, leads to the breakdown or total mineralization of the contaminants. The process is usually augmented by the excretion of root exudates (e.g., sugars, alcohols, acids, enzymes), and the build-up of organic carbon in the soil, so the proper selection of particular plant species represents a critical management decision for PAHs phytoremediation. These facts favor the rhyzoremediation as the best solution for sites contaminated with PAHs.Abstract This study investigated xenobiotic biodegradation in rhizosphere soil collected from field‐grown plants, grouped for analysis as monocots or dicots. Microbial activity was highest in monocot rhizosphere soils, followed by dicot rhizosphere soils and, finally, nonrhizosphere soils. No differences were seen between these soils in the mineralization of phenol or 2,4‐dichloropbenol (2,4‐DCP), but there were differences in 2,4‐dichlorophenoxyacetic acid (2,4‐D) and 2,4,5‐trichlorophenoxyacetic acid (2,4,5‐T) mineralization. The rate constants for 2,4‐D or 2,4,5‐T mineralization in nonrhizosphere soil were lower than those for either rhizosphere soil. Monocot rhizosphere soil mineralized these compounds faster than dicot rhizosphere soil. Thus, soils that had a prior association with a plant showed significantly increased rates of mineralization for the more recalcitrant compounds tested. In addition, this enhanced mineralization in the rhizosphere appeared to be dependent on the type of plant involved.
Bulk soil
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The existing status on phytoremediation mechanism of organic contaminated soil was reviewed, including direct phytoremediation and enhanced degradation by plant in rhizosphere. The issues for further research on phytoremediation of organic contaminated soil were also discussed
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Phytoextraction process
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Phytoremediation has been regarded as a suitable technique for the pollution control of Ni contaminated soils. In this paper two methods of efficient improvement of phytoextraction are discussed: first, to improve the uptake of Ni by plants by increasing Ni concentration in soil solution; second, to adopt some means based on physiological mechanism of heavy metal hyperaccumulators.
Phytoextraction process
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A radiotracer glucose mineralization assay was used with streptomycin and actidione to monitor the relative seasonal contributions of bacteria and fungi to mineralization processes in soils derived from the rhizosphere-rhizoplane zone of plants from a shortgrass prairie ecosystem. Bacteria played a major role in glucose mineralization in both the rhizosphere and rhizoplane. These results indicate that the bacteria may play a greater role in glucose mineralization processes in the rhizosphere and rhizoplane zones of a semiarid grassland than would be assumed, based on available biomass estimates. This technique appears to be valuable for determining bacterial versus fungal contributions to glucose mineralization in the rhizosphere and rhizoplane and may be useful for measuring the decomposition of other more complex substances in this zone of intense microbial activity.
Soil microbiology
Bulk soil
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Based on a 20-year-long fertilization experiment in paddy field,effects of farming systems on soil organic carbon(SOC) mineralization of rhizosphere and non-rhizosphere in paddy were studied.This experiment included four cultivation treatments: plain-cultivation(rice),ridge-cultivation without tillage(rice),ridge-cultivation without tillage(rice-rape) and plain-cultivation(rice-rape).The results showed that the daily mineralization of SOC declined rapidly in the first seven days and then gradually became more and more stable under different farming systems.At the end of incubation,the daily mineralization of SOC was only 3.1%～6.7% of the first day.Plus,the mineralization rates of SOC was decreased effectively when practiced the rotation of rice and rape.The daily and cumulative mineralization of SOC and mineralization intensity under the treatment of the rhizosphere of ridge-cultivation without tillage(rice-rape) were higher than those of non-rhizosphere,but other treatments were opposite.The daily mineralization of SOC of rhizosphere and non-rhizosphere fitted with a power-function with incubation time.The daily mineralization of SOC of non-rhizosphere was higher than rhizosphere in the whole incubation period of 62 days except the first day in all treatment.And discrepancy rate was between 67.0% and 98.7%.The daily and cumulative mineralization of SOC and mineralization intensity of rhizosphere and non-rhizosphere were significant correlation with pH.The discrepancy between rhizosphere and non-rhizosphere of daily and cumulative mineralization of SOC were evident correlation with microbial biomass carbon and nitrogen.
Nitrogen Cycle
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Mercury (Hg) is considered as a global pollutant, and the remediation of Hg contaminated soil always poses worldwide concern. Phytoremediation is a new technology developed recently for the remediation of Hg contaminated soil, in which, phytoextraction is a kind of most favorable phytoremediation techniques. This paper introduced the phytoremediation techniques of Hg contaminated soil at home and abroad, including phytovolatilization, phytostabilization and phytoextraction, with their application prospects discussed. It was considered that the study of Hg-hyperaccumulation plants and their tolerance mechanisms is of significance for the phytoremediation of Hg contaminated soil.
Mercury
Mercury contamination
Phytoextraction process
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The soil overweight and currently used soil heavy metal pollution prevention measures were introduced,the basic principles of the plant repair for heavy-metal contamination of soil were explored,the comparison of common phytoremediation and chemical improvers for plantrepair was focused on and the phytoremediation research directions were discussed.The result showed that the phytoremediation technology had advantages of low cost,and less chance of secondary pollution and destruction of soil ecological environment,etc.
Phytoextraction process
Environmental Pollution
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