Phytoremediation of 137Cs: factors and consequences in the environment

Radionuclide contamination is a concerning threat due to unexpected nuclear disasters and authorized discharge of radioactive elements, both in the past and in present times. Use of atomic power for energy generation is associated with unresolved issues concerning storage of residues and contaminants. For example, the nuclear accidents in Chernobyl 1986 and Fukushima 2011 resulted in considerable deposition of cesium (Cs) in soil, along with other radionuclides. Among Cs radioactive variants, the anthropogenic radioisotope ¹³⁷Cs (t_½?=?30.16 years) is of serious environmental concern, owing to its rapid incorporation into biological systems and emission of β and γ radiation during the decaying process. To remediate contaminated areas, mostly conventional techniques are applied that are not eco-friendly. Hence, an alternative green technology, i.e., phytoremediation, should in future be considered and implemented. This sustainable technology generates limited secondary waste and its objectives are to utilize hyper-accumulating plants to extract, stabilize, degrade, and filter the radionuclides. The review highlights plant mechanisms for up-taking radionuclides and influences of different environmental factors involved in the process, while considering its long-term effects.

     

Potential for phytoextraction of137 Cs from a contaminated soil

Potential for phytoremediation of a soil contaminated with radiocesium was investigated in three phases: (1) hydroponic screening for plant species capable of accumulating elevated levels of cesium in shoots, (2) investigation of several amendments for their potential to increase the bioavailability of ¹³⁷Cs in the contaminated soil, and (3) bioaccumulation of radiocesium in shoots of plants grown in¹³⁷ Cs-contaminated soil.The bioaccumulation ratio for Cs in shoots of hydroponically grown plants ranged between 38 and 165. From solution, dicot species accumulated 2- to 4-fold more cesium in shoots than grasses. In studies investigating the bioavailability of ¹³⁷Cs in aged contaminated soil, ammonium salts were found to be the most effective desorbing agents, releasing approximately 25% of the¹³⁷ Cs. The extent of ¹³⁷Cs desorption from the soil increased with ammonium concentration up to 0.2 M. In a pot study conducted in a greenhouse, there was significant species-dependent variability in the ability to accumulate ¹³⁷Cs in the shoot from contaminated soil. The ability to accumulate ¹³⁷Cs from the soil increased in the order: reed canarygrass (Phalaris arundinacea) < Indian mustard (Brassica juncea) < tepary bean (Phaseolus acutifolius)< cabbage (B. oleracea var. capitata). It was also found that addition of NH₄NO₃ solution to the soil elicited a two- to twelve-fold increase in ¹³⁷Cs accumulation in the shoot. The greatest amount of ¹³⁷Cs (40 Bq g^-1 dw) was removed in shoots of cabbage grown in contaminated soil amended with 80 mmols NH₄NO₃ kg^-1 soil. Bioaccumulation ratios of 2–3 were obtained with the best performing plant species. These values are significantly greater than those previously reported in the literature (usually <0.1) for plants grown on aged contaminated soil. These results indicate that careful species selection along with amendments that increase the bioavailability of¹³⁷ Cs in the soil could greatly enhance the prospects for the use of plants to remediate ¹³⁷Cs-contaminated soils.     

污染土壤的植物修复

污染土壤的植物修复沈德中（中国农业大学资源与环境学院,北京１０００９４）ＰｈｙｔｏｒｅｍｅｄｉａｔｉｏｎｏｆＣｏｎｔａｍｉｎａｔｅｄＳｏｉｌ．ＳｈｅｎＤｅｚｈｏｎｇ（ＣｏｌｅｇｅｏｆＲｅｓｏｕｒｃｅｓａｎｄＥｎｖｉｒｏｎｍｅｎｔ,ＣｈｉｎａＡｇｒｉｃ...     

137Cs uptake in spring wheat (Triticum aestivum L.cv. Tonic) at varying K supply

Spring wheat (Triticum aestivum L. cv. Tonic) was grown for 16 days in a sandy loam soil which was contaminated with ¹³⁷Cs. The soil was fertilised with K at three rates (0,1 and 2 mmol K per 950 g dry soil) and with NO₃ ^--N at two rates (0 and 2 mmol per 950 g dry soil) in a factorial design. The ¹³⁷Cs Activity Concentration (AC) in the shoot tissue significantly reduced 8.2-fold (nil N treatment, p<0.001) and 9.3-fold (highest N dose, p<0.001) with increasing K supply. In contrast, the K application increased the ¹³⁷Cs AC in soil solution 1.7 fold (nil N treatment) or had no significant effect (highest N dose). At similar K application, the application of N increased the ¹³⁷Cs AC in the shoot compared to the control. This effect is most probably due to the increased NH₄ ⁺ concentration in soil solution which increased the ¹³⁷Cs AC in soil solution. The soil solution composition (¹³⁷Cs and K concentration) in the rhizosphere was estimated from the average soil solution composition at day 16 and solute transport calculations. The ¹³⁷Cs AC in the shoot tissue was predicted from the estimated soil solution composition in the rhizosphere and the relationship between K concentration and ¹³⁷Cs uptake derived from a nutrient solution experiment. The predictions of ¹³⁷Cs AC's in the shoot are qualitatively correct for the fertiliser effects but underestimate the observations between 1.4 and 9.9 fold.     

Risk-Based Corrective Action of Hydrocarbon Contamination at a Former Major Urban Petroleum Storage Site in the U.K.

The former site of a major petroleum storage facility adjacent to a major urban watercourse was found to have potentially significant concentrations of hydrocarbons in soil and groundwater that needed to be addressed prior to redevelopment. A series of intrusive investigations were undertaken to collect physical and chemical data for a Quantitative Risk Assessment (QRA) of potential impacts on human health and the wider environment, in order to derive a remedial strategy for redevelopment of the site for light industrial use. A site-specific QRA methodology was devel oped using both U.K. and U.S. guidance to produce Risk-Based Clean-up Levels (RBCLs) for benzene, and other petroleum hydrocarbons. The U.K. has no nationally based guidance on risk assessment and studies are designed by the consultant for submission to the U.K. Environment Agency (EA) for their approval. It is the EA's role to determine whether the work has been undertaken satisfactorily. To achieve these RBCLs, ex situ bioremediation was identified as the best practicable remedial option. This was carried out in windrows using mechanical aeration (to achieve oxygenation with ammonia nitrate granule and woodchip addition) for a total of approximately 5291?m³ of soil. The bioremediation process was successful in achieving the site specific RBCLs for benzene and for other hydrocarbons within an average of 5 to 6 weeks. This article describes the successful implementation of Risk-Based Corrective Action (RBCA) at this petroleum release site as a demonstration of how risk-based remedial standards for contaminated sites can be achieved with regulatory approval.     

Assessing the Potential Threat Landscape of a Proposed Reintroduction Site for Carnivores

This study provides a framework to assess the feasibility of reintroducing carnivores into an area, using African wild dogs (Lycaon pictus) as an example. The Great Fish River Nature Reserve in the Eastern Cape Province, South Africa, has been identified as a potential reserve to reintroduce wild dogs, and we applied this framework to provide a threat assessment of the surrounding area to determine potential levels of human-wildlife conflict. Although 56% of neighbouring landowners and local communities were positive about a wild dog reintroduction, data collected from questionnaire surveys revealed that human-wild dog conflict is a potential threat to wild dog survival in the area. Additional potential threats include diseases, snaring, poaching and hunting wild dogs for the use of traditional medicine. A threat index was developed to establish which properties harboured the greatest threats to wild dogs. This index was significantly influenced by the respondent’s first language (isiXhosa had more positive indices), education level (poorer education was synonymous with more positive threat indices), land use (wildlife ranching being the most negative) and land tenure (community respondents had more positive indices than private landowners). Although threats are present, they can be effectively mitigated through strategies such as carnivore education programs, vaccination campaigns and anti-snare patrols to promote a successful reintroduction of this endangered canid.     

Utilization of Grasses for Potential Biofuel Production and Phytoremediation of Heavy Metal Contaminated Soils

This research focuses on investigating the use of common biofuel grasses to assess their potential as agents of long-term remediation of contaminated soils using lead as a model heavy metal ion. We present evidence demonstrating that switch grass and Timothy grass may be potentially useful for long-term phytoremediation of heavy metal contaminated soils and describe novel techniques to track and remove contaminants from inception to useful product. Enzymatic digestion and thermochemical approaches are being used to convert this lignocellulosic feedstock into useful product (sugars, ethanol, biocrude oil + biochar). Preliminary studies on enzymatic hydrolysis and fast pyrolysis of the Switchgrass materials that were grown in heavy metal contaminated soil and non-contaminated soils show that the presence of lead in the Switchgrass material feedstock does not adversely affect the outcomes of the conversion processes. These results indicate that the modest levels of contaminant uptake allow these grass species to serve as phytoremediation agents as well as feedstocks for biofuel production in areas degraded by industrial pollution.     

Distribution of Na,K, Rb,Cs, and137Cs in some Austrian higher fungi

Biological Trace Element Research - The level of137Cs in 28 species of mushroom collected and analyzed between 1986 and 1992, after the Chernobyl accident, are reported.Xerocomus badius was found...     

Potential of Indigenous Plant Species for the Phytoremediation of Arsenic Contaminated Land in Kurdistan (Iran)

To evaluate the potential of the indigenous plant species for Arsenic (As) phytoremediation, a total of 138 plant samples and 138 soil samples from rooting zones were collected from two As-contaminated areas in Kurdistan, western Iran. The areas were the Sari Gunay Gold Mine (SG) and Ali Abad Village (AA). The soil of both areas naturally contains As, with mining activities at SG. Soil and plant samples were collected at five sites in the SG (SG1 to SG5) and at two sites in the AA (AA1 and AA2). Soil samples were analyzed for total and water-soluble As concentration, as well as for the main soil physical and chemical properties such as electrical conductivity (Ec), pH, organic carbon (C_org.), available phosphorus (P_ava.), and soil texture. Plant samples were analyzed for As concentration in their shoots and roots.

The average total and water–soluble As concentrations in soil were 751.6 and 6.2 ppm at SG and 920.8 and 8.0 ppm at AA, respectively. The highest root and shoot As concentration was found in Juncus inflexus (751.5 ppm) at AA2 and in Astragalus gossypinus (158.7 ppm) at AA1, respectively. With regard to phytoremediation strategies, Acantholimon brachystachyum, Astragalus gossypinus, Stipa barbata, and Ephedra major with a high translocation factor (TF) can be potentially used for As phytoextraction. However, Juncus inflexus, Phragmites australis, Bromus tomentellus, and Elymus sp., which show high bioconcentration factor (BCF) and low TF, are suggested as good candidates for As phytostabilization. In general, the TF values of terrestrial plants were higher than those of amphibious plants; meanwhile, BCF values showed the opposite behavior.     

Phytoremediation Potential of Weeds in Heavy Metal Contaminated Soils of the Bassa Industrial Zone of Douala,Cameroon

Phytoremediation is a promising option for reclaiming soils contaminated with toxic metals, using plants with high potentials for extraction, stabilization and hyperaccumulation. This study was conducted in Cameroon, at the Bassa Industrial Zone of Douala in 2011, to assess the total content of 19 heavy metals and 5 other elements in soils and phytoremediation potential of 12 weeds. Partial extraction was carried out in soil, plant root and shoot samples. Phytoremediation potential was evaluated in terms of the Biological Concentration Factor, Translocation Factor and Biological Accumulation Coefficient. The detectable content of the heavy metals in soils was Cu:70–179, Pb:8–130, Zn:200–971, Ni:74–296, Co:31–90, Mn:1983–4139, V:165–383, Cr:42–1054, Ba:26–239, Sc:21–56, Al:6.11–9.84, Th:7–22, Sr:30–190, La:52–115, Zr:111–341, Y:10–49, Nb:90–172 in mg kg^?1, and Ti:2.73–4.09 and Fe:12–16.24 in wt%. The contamination index revealed that the soils were slightly to heavily contaminated while the geoaccumulation index showed that the soils ranged from unpolluted to highly polluted. The concentration of heavy metals was ranked as Zn > Ni > Cu > V > Mn > Sc > Co > Pb and Cr in the roots and Mn > Zn > Ni > Cu > Sc > Co > V > Pb > Cr > Fe in the shoots. Dissotis rotundifolia and Kyllinga erecta had phytoextraction potentials for Pb and Paspalum orbiculare for Fe. Eleusine indica and K. erecta had phytostabilisation potential for soils contaminated with Cu and Pb, respectively.     

Phytoremediation Potential of Phragmites australis in Hokersar Wetland - A Ramsar Site of Kashmir Himalaya

Heavy metals are an important class of pollutants with both lethal and sublethal effects on organisms. Wetlands are cheap natural alternatives for removal of heavy metals from soils; however, wetland plants vary greatly in their degree of metal uptake. Hokersar wetland, a Ramsar site of Kashmir Himalaya, India is a game reserve of international importance that provides suitable habitat for resident birds and an excellent stopover point for migratory birds visiting from Palaearctic breeding grounds in Central Asia, China, N-Europe and Siberia. The toxicity of chronic dietary metal exposure in birds may have adverse reproductive effects which include decreased egg production, decreased hatchability, and increased hatchling mortality. Thus, the present study aimed to assess the heavy metal sequestration capability of one of the most common wetland plant species Phragmites australis in Hokersar wetland. The accumulation of the different elements was in order of Al > Mn > Ba > Zn > Cu > Pb > Mo > Co > Cr > Cd > Ni. Translocation factor, i.e. ratio of shoot to root metal concentration revealed that metals were largely retained in the roots of P. australis, thus reducing the supply of metals to avifauna and preventing their bio-accumulation.     

Predicting the Effect of Deep-Rooted Hybrid Poplars on the Groundwater Flow System at a Large-Scale Phytoremediation Site

Estimating the effect of phreatophytes on the groundwater flow field is critical in the design or evaluation of a phytoremediation system. Complex hydrogeological conditions and the transient water use rates of trees require the application of numerical modeling to address such issues as hydraulic containment, seasonality, and system design.

In 1999, 809 hybrid poplars and willows were planted to phytoremediate the 317 and 319 Areas of Argonne National Laboratory near Chicago, Illinois. Contaminants of concern are volatile organic compounds and tritium. The site hydrogeology is a complex framework of glacial tills interlaced with sands, gravels, and silts of varying character, thickness, and lateral extent. A total of 420 poplars were installed using a technology to direct the roots through a 25--ft (8--m)-thick till to a contaminated aquifer.

Numerical modeling was used to simulate the effect of the deep-rooted poplars on this aquifer of concern. Initially, the best estimates of input parameters and boundary conditions were determined to provide a suitable match to historical transient ground-water flow conditions. The model was applied to calculate the future effect of the developing deep-rooted poplars over a 6 year period. The first 3 years represent the development period of the trees. In the fourth year, canopy closure is expected to occur; modeling continues through the first 3 years of the mature plantation. Monthly estimates of water use by the trees are incorporated. The modeling suggested that the mature trees in the plantation design will provide a large degree of containment of groundwater from the upgradient source areas, despite the seasonal nature of the trees' water consumption. The results indicate the likely areas where seasonal dewatering of the aquifer may limit the availability of water for the trees. The modeling also provided estimates of the residence time of groundwater in the geochemically altered rhizosphere of the plantation.     

Potential of Phytoremediation for Treatment of PAHs in Soil at MGP Sites

Phytoremediation is a natural, aesthetically pleasing, low-cost technology that employs plant-influenced microbial, chemical, and physical processes to remediate contaminated soils and waters. The Institute of Gas Technology (IGT) conducted a laboratory study to determine the potential of phytoremediation to remediate soils contaminated with polynuclear aromatic hydrocarbons (PAHs). The soils used for the study were collected from a former manufactured gas plant (MGP) site in Newark, NJ. Phytoremediation was assessed both as a primary remediation technology and as a final polishing step for soil treatment. The following three plant species were used for the 6-month laboratory study: alfalfa (Medicago sativa), switch grass (Panicum virgatum), and little bluestem grass (Schizachyrium scoparium). Using both alfalfa and switch grass for primary treatment of PAH-contaminated soil, a 57% reduction in total PAH concentration was observed after 6-months of treatment. Final polishing of that soil using alfalfa further reduced the total PAH concentration in that soil by 15%. Research is in progress with the objective of improving both the efficiency and the economics of phytoremediation for the cleanup of contaminated soils to environmentally acceptable endpoints at MGP sites.     

Evaluation of the Phytoextraction Potential at an Acid-Mine-Drainage-Impacted Site

Four plant species were found naturally growing at an acid mine drainage (AMD)-impacted site contaminated with 9430 mg kg^?1Al, 76,000 Fe mg kg^?1, ～150 mg kg^?1Mn, and 420 mg kg^?1 Mg: soybeans (Glycine max), cattails (Typha latifolia), goldenrods (Solidago sp.), and reed grass (Phragmites australis). The metal uptake selectivity was Fe?Mg～Mn>Al for cattails, Mg>Mn>Fe>Al for goldenrods, and Fe?Al>Mg>Mn for reeds. When metal translocation factors, shoot concentrations, and toxicity of the contaminants were correlated, cattails and reeds were more effective at the site than the soybeans or goldenrods. Cattails had a translocation factor of 3.71 for Al, 3.3 for Mg, 1.98 for Mn, and only 0.2 for Fe. The translocation factors for reeds were much higher for Fe (8.64) and Al (7.3). Cattails (1.11 mg Al g^?1 shoot) and reeds (3.4 mg g^?1 g shoot) were both able to hyperaccumulate Al. Additional research is warranted to ascertain if the uptake efficiencies can be enhanced by the use of chelators.     

Retranslocation of Rb86, Cs137, and K to new leaf growth in bush beans

Summary Bush bean plants were exposed to either Rb⁸⁶ or Cs¹³⁷ with and without carrier rubidium or cesium for 48 hours in a complete nutrient solution. These solutions were then discarded. The plants were then allowed to grow for 11 additional days in complete nutrient solution except that potassium was omitted. Measurement of Rb⁸⁶, Cs¹³⁷, and potassium in new leaves indicated that Cs¹³⁷ was retranslocated to the new growth much slower than was potassium but that proportionately more Rb⁸⁶ was in new leaves than potassium and these results were independent of the presence or absence of the respective carriers. Considerably more Rb⁸⁶ than Cs¹³⁷ was absorbed by the plants either with or without the carriers.     

137Cs as a tracer of recent sedimentary processes in Lake Michigan

To determine recent sediment movement, we measured the levels of ¹³⁷Cs (an artificial radionuclide produced during nuclear weapons testing) of 118 southern Lake Michigan samples and 27 in Green Bay. These samples, taken from 286 grab samples of the upper 3 cm of sediment, were collected in 1975 as part of a systematic study of Lake Michigan sediment. ¹³⁷Cs levels correlated well with concentrations of organic carbon, lead, and other anthropogenic trace metals in the sediment. ¹³⁷Cs had a higher correlation with silt-sized than with clay-sized sediment (0.55 and 0.46, respectively). Atmospherically derived ¹³⁷Cs and trace metals are being redistributed by sedimentary processes in Lake Michigan after being incorporated in suspended sediment. We determined a distribution pattern of ¹³⁷Cs that represents areas of southern Lake Michigan where sediment deposition is occurring.     

低放核素污染土-水介质的植物修复研究进展

植物修复技术是利用植物根系吸收水分和养分的过程来吸收和转化土壤和水体中的污染物,以期达到清除,修复和治理的目的,是用于对土壤-水体中重金属和放射性核素污染清除的生态技术.本文就放射性核素的来源、污染现状、植物对放射性核素的积累筛选以及对污染土壤的修复研究进行综述,以明确植物修复技术在改善环境中的作用,为进一步筛选超积累植物并探讨植物对放射性核素污染的修复机理提供参考.