Na Ding

Microbial community structure changes during Aroclor 1242 degradation in the rhizosphere of ryegrass (Lolium multiflorum L.)

Polychlorinated biphenyls in a commercial mixture (Aroclor 1242) were added to soil at 8.0 mg kg(-1) with and without ryegrass (Lolium multiflorum L.) planted in a specially designed rhizobox. At the end of 90 days, the presence of plants significantly increased Aroclor 1242 degradation compared with soils without ryegrass. Phospholipid fatty acids (PLFAs) profiles were affected by the distance from the rhizosphere, indicating a distance-dependent selective enrichment of competent species that may be responsible for efficient Aroclor 1242 degradation. The highest concentration of total PLFAs also occurred at 3 mm from the root zone in planted soils. The numbers of bacteria (cy17:0, 16:0), gram-positive bacteria (a15:0, i16:1, a17:0) and actinomycete (18:2 omega 6,9c) were significantly higher in planted soils than in unplanted soils. Furthermore, individual PLFAs [i16:0, 16:0 N alcohol, 18:0(10Me), i16:1, a15:0, i14:1, 14:0 2OH, 18:1 omega 9c, a17:0, 14:0 3OH, i14:0, a16:0, 16:1 omega 5c] were strongly correlated with the Aroclor 1242 degradation rates (%) (P<0.05) in planted treatments, whereas individual PLFAs of i16:1, 12:0 3OH, 15:0, a15:0 had significant correlations with the Aroclor 1242 degradation rates (%) (P<0.05) in unplanted soils. In particular, individual PLFAs i16:1 had strong correlations with Aroclor 1242 degradation in treatments both with and without ryegrass.

Dissipation of pentachlorophenol in the aerobic-anaerobic interfaces established by the rhizosphere of rice ( Oryza sativa L.) root.

Phytoremediation is an emerging technology for the detoxification and remediation of organic pollutants such as pentachlorophenol (PCP). To investigate the dissipation behavior of PCP in the aerobic-anaerobic interfaces established by the rhizosphere of rice ( L.) root, a glasshouse experiment was conducted using a specially designed rhizobox. The possible biogeochemical mechanisms were also studied through illustration of the dynamic behavior of important electron acceptors and donors that are potentially involved in the reductive dechlorination and aerobic catabolism processes of PCP. The soil was spiked with 20 ± 0.25 and 45 ± 0.25 mg of PCP kg soil. Soil in the rhizobox was divided into five different compartments at various distances from the root surface. Maximum dissipation of PCP in planted soil was observed at 3-mm distance from the root zone as well as rapid changes in concentrations of sulfate, chloride, nitrate, and ammonium at the same distance from the root. In contrast, in the unplanted soil, no difference was observed in the PCP concentration with increasing distance. After 45 d, a significantly higher concentration of PCP was degraded in planted soil compared with unplanted soil. In the unplanted microcosms, about 45% of the initial PCP was lost at both low and high added rates, respectively. This was, proportionately, a significantly smaller percentage compared with the planted rhizosphere (an average of 66 and 64.5%, respectively). Moreover, the correlations of PCP dissipation with SO, NO, and Fe were significantly negative, while the correlations of PCP dissipation with NH, Fe, and Cl were significantly positive. This suggested the oxidization of soil constituents can inhibit aerobic catabolism of PCP by consuming O, and the reduction of soil constituents can inhibit anaerobic reductive dechlorination of PCP. Therefore, the significance of the rhizosphere in phytoremediation of chlorinated compounds such as PCP differs significantly between wetland and rainfed systems.

Dynamic Behavior of Persistent Organic Pollutants in Soil and Their Interaction with Organic Matter

Persistent organic pollutants (POPs) pose threat to environment because of their potential for longrange atmospheric transport, bioaccumulation and toxicity. The POPs behave dynamically in the environment according to their nature of action like volatilization, sorption, desorptin and transportation from their source of production to some where. These POPs migrate on air currents from warmer regions of the globe towards the colder Polar Regions. Once they reach colder temperatures they condense, precipitate out, and are deposited again on the earth’s surface. Environmental variables like temperature, soil pH, moisture have serious effects on the POPs behavior in the soil. Inorganic minerals also have good interaction with the xenobiotics and play an important role in the transformation of xenobiotics. The manganese and iron oxides and clay minerals (e.g. smectites containing Fe(III)) have well-documented properties to promote the oxidation of a number of organic pollutants. Organic matter is considered the most important factor limiting availability and mobility of POPs in soil and a substantial percentage of the total amount of an organic contaminant applied to a soil may become associated with the humic fraction of that soil. Organic pollutants strongly adsorb to carbonaceous sorbents such as black carbon. In particular, activated charcoal (AC) is known for a strong adsorptive capacity due to its high specific surface area. Adsorption to activated charcoal can render hazardous organic pollutants in soils and sediments less available to organisms and hinder their dispersal into unaffected environments. Some studies also show that some sorbents from natural organic materials, such as peat, soybean stalk and pine needle under superheated temperature/ pressure conditions, for sorption of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils. Natural materials significantly decrease the extractability and bioavailability of PAHs from contaminated soils. Main objective of this review article is to compile some valuable information regarding the existence, dynamic behavior, effect of environmental variables on POPs and their interactions with organic matter.

Dynamics of Dissolved Organic Carbon in the Rhizosphere of Ryegrass (Lolium multiflorum L.) Induced by PCBs Pollution

The objective of this study was to examine ryegrass for phytoremediation of polychlorinated biphenyls (PCBs)-contaminated soils. A rhizobox was designed to allow the harvest of an intact layer of rhizosphere soil from plant root without the removal of the root material itself. Ryegrass was grown under controlled conditions in rhizobox for 45, 90 and 135 days. Results indicated that dissipation of PCBs in the rhizosphere were much higher at various layers compared to the no-plant control soils. The DOC concentrations suggested a trend over time. Dissolved organic carbon (DOC) concentrations at 45 days occurred as high as 11.63 mg·L−1 in the rhizosphere, then decrease rapidly to 7.86 mg·L−1 at 90 days after sowing. Finally, there was no significant change from 90 days to 135 days in DOC concentration. Moreover, dissipation rates of PCBs correlated positively with DOC concentrations at 45 days and 90 days after sowing. In this study, an average 6.8% loss of PCBs occurred due to the biotic activity without ryegrass planted.

The Sorption Behavior and Bioavailability of Persistent Organic Pollutants (POPs) in Soils

Persistent organic pollutants (POPs) pose a threat to the environment because of their potential for long-range atmospheric transport, bioaccumulation and toxicity. The POPs behave dynamically in the environment according to their different processes e.g. volatilization, sorption, desorption and transportation. Environmental variables like temperature, soil pH and moisture have serious effects on POPs behavior in soil. Organic as well as inorganic compounds may react with the xenobiotics and play an important role in their transformation in soil. Manganese and iron oxides and clay minerals (e.g. smectites containing Fe(III)) had well-documented properties to promote the oxidation of a number of organic pollutants. Organic matter is considered the most important factor limiting the availability and mobility of POPs in soil and a substantial percentage of the organic contaminants applied to a soil might become associated with soil humic fraction. Organic pollutants strongly adsorbed to carbonaceous sorbents such as black carbon. In particular, activated charcoal (AC) had a strong adsorptive capacity due to its high specific surface area. Adsorption to activated charcoal could render hazardous organic pollutants in soils and sediments less available to organisms and hindered their dispersal into unaffected environment. Some studies also showed that some sorbents from natural organic materials, such as peat, soybean stalk and pine needle under superheated temperature/pressure conditions, significantly affected the sorption of polycyclic aromatic hydrocarbons (PAHs) in contaminated soils. Natural materials significantly decreased the extractability and bioavailability of POPs from contaminated soils. Organic matter also plays a vital role in controlling the contamination of POPs in soil. The rhizosphere effect is showing promising to control POPs contamination in the soil environment. This review evaluated the work on the environmental behavior and bioavailability of POPs in soils as affected by various environmental variables.