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▪ Abstract  Contaminated soils and waters pose a major environmental and human health problem, which may be partially solved by the emerging phytoremediation technology. This cost-effective plant-based approach to remediation takes advantage of the remarkable ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. Toxic heavy metals and organic pollutants are the major targets for phytoremediation. In recent years, knowledge of the physiological and molecular mechanisms of phytoremediation began to emerge together with biological and engineering strategies designed to optimize and improve phytoremediation. In addition, several field trials confirmed the feasibility of using plants for environmental cleanup. This review concentrates on the most developed subsets of phytoremediation technology and on the biological mechanisms that make phytoremediation work.

Four sequential extraction procedures (Sposito, Tessier, Silveira and Bureau Communautaire de Reference (BCR)) were used to evaluate the distribution of some metals (Fe, Cu, Cd and Zn) in a contaminated soil around a mining area. The results showed that Fe and Zn were mainly recovered in the recalcitrant soil fractions, while Cd was primarily localized in the exchangeable fraction. Soil Cu was highly associated with organic matter fraction. The amorphous Fe fraction in Silveira could be recognized as part of the Fe-Mn oxide fraction in Tessier and BCR procedures, while the crystalline Fe oxide fraction was classified into the residual fraction in Sposito, BCR and Tessier schemes. Although the same reagent was used to extract target fraction, less carbonate-bound Cu, Cu and Zn were extracted in Tessier procedure as compared to Silveira method, while Tessier scheme yielded a higher proportion of Fe, Cu and Zn in the Fe-Mn oxide fraction than BCR method. Due to the lack of uniformity of experimental conditions and the differences in extraction reagents, the extraction efficiency of metal species varied with the sequential extraction schemes. Therefore, care should be taken when comparing the results obtained by different sequential extraction procedures.

Soil samples were collected at alluvial sites of the Litavka River, which flows through the Beroun and Příbram cities in Central Bohemia Region of the Czech Republic in 2005 and 2006. Higher heavy metal content in soils (Cd, Pb, Zn, Cu) is due to composition of the parent rock, emissions from lead processing industry and the leak of toxic material from the steel works sludge ponds in the 1970s and 1980s. The samples were collected from six sites located at different distances from the contamination source (the former sludge ponds) and chemical and biological properties were determined. The ratio of the microbial biomass carbon to oxidisable carbon content dropped down significantly on more heavily contaminated sites. Basal respiration activity did not correlate with the content of heavy metals in soil, but there was certain declining tendency with increasing intensity of soil contamination. Respiration activities significantly correlated with the total carbon, oxidisable carbon and the total nitrogen content. The metabolic quotient showed higher values with increasing contamination. Dehydrogenases and arylsulphatase activities decreased with increasing contamination. Urease activity has also a declining tendency but its relation to different intensity of contamination was not unambiguous. Urease activity has shown a relationship with the content of total nitrogen in soil. No relationship was found between the total sulphur content and arylsulphatase activity. Dehydrogenases, arylsulfatase and urease activities significantly correlated with the microbial biomass carbon.

The aim of this study was to evaluate and compare the competitive and individual sorption of Cd, Cu, Pb and Zn on three natural soils: a Gleyic Fluvisol (content of Cd 30, Cu 25, Pb 2297 and Zn 3718 mg/kg), a Gleyic Cambisol (content of Cd 5, Cu 29, Pb 1158 and Zn 180 mg/kg) and a Chernozem (content of Cd 0.4, Cu 36, Pb 75 and Zn 67 mg/kg). For evaluation of the sorption and desorption, the Freundlich isotherms were used. The results of the model experiment confirmed that the sorption from single-metal solution was more effective than sorption under multi-metal conditions, due to competitive effects. In all tested soils sorption of Cd, Cu and Zn decreased with the rate of other competitive metals; the Pb sorption was not affected by other competitive metals in solutions. Moreover, during multi-metal sorption, Zn was significantly desorbed in Cambisol. In general, sorptions of Cu, Pb and Zn were greater in uncontaminated soil compared to contaminated soils. Clear trend of impact of the contamination on Cd sorption was not observed. The results showed the sorption decreasing in order Chernozem > Fluvisol > Cambisol. The sorption was the greatest in uncontaminated soil with low mobility of studied metals.

To identify populations with the ability to accumulate heavy metals, approximately 300 accessions pertaining to 30 plant species were grown for 4 wk in a hydroponic media that approximated the nutrient and heavy metal composition of a soil contaminated with moderate levels of cadmium (Cd), copper (Cur), and zinc (Zn). The results indicated that several Brassica spp. exhibited moderately enhanced Zn and Cd accumulation. Selected accessions of Brassica juncea (L.) Czern, B. napus L., and B. rapa L. were then grown in pots with heavy metal-contaminated soil to compare the Zn and Cd phytoextraction by these species to that of Thlaspi caerulescens J. and C. Presl, a known Zn and Cd hyperaccumulator, and two grass species, Agrostis capillaris L. and Festuca rubra L. The three Brassica spp. were the most effective in removing Zn from the contaminated soil, primarily because they produced more than 10 times the shoot biomass produced by T. caerulescens. When the soil was amended with Gro-Power, a commercial soil amendment that improves soil structure and fertility, removal of Zn by plant shoots doubled to more than 30 000 mg Zn pot-1 (4.5 kg). The results suggest that for phytoremediation of metal-polluted soils to be successful, a strategy should be considered that combines rapid screening of plant species possessing the ability to tolerate and accumulate heavy metals with agronomic practices that enhance shoot biomass production and/or increase metal bioavailability in the rhizosphere

Organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) were determined in agricultural soils from the northern states of India. The average concentration of OCPs was 37.67 ± 0.33 ng/g (dry weight − DW) while HCHs alone accounted for 93% followed by DDT (4.27%) and endosulphan (2.51%). The α/γ ratio of HCH (< 0.01–8.64) reflects the use of technical as well as lindane formulations. The ratio of p,p’-DDT/p,p’-DDE (0.16) and o,p’-DDT/p,p’-DDT (< 0.01) indicates the contamination of soils with the past use of technical DDT. The mean concentrations of endosulphan and dieldrin were 0.95 ± 0.53 ng/g (DW)and 0.16 ± 0.07 ng/g (DW), respectively. The average concentration of PCBs was 13.44 ± 0.06 ng/g (DW). The toxic equivalency (TEQ) calculated using WHO 2005-TEFs ranged from 0.01 to 105.40 pg WHO 2005-TEQ/g (DW) with the mean of 13.78 ± 0.11 pg WHO 2005-TEQ/g (DW). PCB-105 (25%), PCB-114 (18%), and PCB-118 (18%) were the dominant congeners and accounted for 61% while a non ortho PCBs contributed only 18% to total DL-PCBs. The contamination of soils is a matter of concern but is not alarming because the observed levels were lower than those given by the Canadian soil quality guidelines.

Previous studies have shown that EDTA is necessary to solubilize soil Pb and facilitate its transport from the soil to the above ground plant tissues. These studies have also suggested that Pb is accumulated in the plant tissue with transpiration as the driving force. We conducted further studies to evaluate the relationship between EDTA soil treatment, plant transpiration, and plant accumulation of Pb and EDTA. Indian mustard (Brassica juncea) plants were grown in soils containing Pb at three different concentrations (1.5, 3.0 and 4.8 mmol/kg) for 5 weeks before being treated with EDTA concentrations ranging from 0 to 10 mmol/kg. Plant shoots and xylem sap were collected and analyzed for Pb and EDTA content using ICP and HPLC, respectively. Water loss was measured for 7 days following EDTA application. Transpiration was not affected at <5 mmol/kg EDTA but, at 10 mmol/kg EDTA transpiration decreased by 80%, whereas accumulation of Pb and EDTA increased. In the Sassafras soil, Pb and EDTA accumulation in the plant shoots continued to increase as the applied EDTA concentration increased, except at the highest level (10 mmol/kg). In soil amended with 4.8 mmol/kg Pb and 10 mmol/kg EDTA, the concentrations of EDTA and Pb in shoots decreased and visible signs of phytotoxicity were observed. The results presented herein support recent studies in hydroponic systems showing that EDTA and Pb are taken up by the plant and suggest that Pb is translocated in the plant as the Pb-EDTA complex. The results also show that the maximum Pb accumulation by plants occurs by maximizing the concentration of the Pb-EDTA complex based on the EDTA extractable soil Pb.

Heavy metal (Cu, Zn, Cd, Pb and Fe) concentrations in salt marsh sediments of the Karnafully River coast were investigated in this study. Sediment samples of four different sites were analysed for selected heavy metals by AAS (atomic absorption spectrophotometry). The mean values of heavy metals were 45.79 µg/g for Cu, 105.0 µg/g for Zn, 0.43 µg/g for Cd, 26.70 µg/g for Pb and 3297.38 µg/g for Fe in this study. It is observed that Pb and Cu have positive correlations (r = 0.370, P < 0.05) and Fe and Zn have negative correlations (r = -0.729, P < 0.05) in the correlation matrix. Except Fe, all the metals were found at contamination levels which may indicate a recent and continuous contamination from domestic and industrial discharges. The results of sediment samples showed that the Karnafully River coast is facing heavy metal pollution.

The hyperaccumulator Thlaspi caerulescens J & C Presl, was grown in seven different soils collected from around Europe that had been contaminated with heavy metals by industrial activity or the disposal of sewage sludge to land. Zinc accumulation factors (shoot concentration/initial soil solution concentration) ranged from 3500-85 000 with a mean value of around 36 000. This compares with mean accumulation factors of 636,66 and 122 for Cd, Ca and Mg, respectively. The concentration of Zn in the shoots was much greater than in the roots. The total removal of Zn and Cd ranged from 8 to 30 and from 0.02 to 0.5 mg kg⁻¹ soil, respectively. The Zn concentration in shoots of T. caerulescens correlated, using a curvilinear relationship, with the initial Zn concentration in soil solution (R² = total Zn 0.78; Zn²⁺ 0.80). There was no relationship between the uptake of Zn and the total Zn concentration in the soil. In most soils, solution pH increased only slightly after growth of T. caerulescens, indicating that acidification was not the mechanism used to mobilise Zn in the soil. Dissolved organic carbon concentrations generally increased but characterisation of the component organic compounds was not attempted. The concentrations of Zn and Cd in soil solution decreased considerably after growth of T. caerulescens. The percentages of Zn and Cd in soil solution present as free ions also decreased. However, the decrease of Zn in soil solution after growth accounted for only about 1% of the total Zn uptake by T. caerulescens. This was much lower than for Cd, Ca and Mg. The results suggest that either T. caerulescens was highly efficient at mobilising Zn which was not soluble initially, or the soils used had large buffering capacities to replenish soil solution Zn within a short time. This work highlights the need to investigate the role of root exudates on the mobilisation of Zn and Cd in soils by the hyperaccumulator T. caerulescens.

Microbially catalyzed reactions, which occur in the natural sulfur cycle, have been integrated in a microbiological process to remove toxic metals from contaminated soils. Bioleaching using sulfuric acid produced by sulfur-oxidizing bacteria was followed by precipitation of the leachate metals as insoluble sulfides by sulfate-reducing bacteria. Metal contaminants including Cd, Co, Cr, Cu, Mn, Ni, and Zn were efficiently leached from an artificially contaminated soil. Mn, Ni, and Zn were the only target elements that were significantly leached from soil minerals. Pb leaching was slow and remained incomplete over a period of 180 days. Mineral components such as Fe, Ca and Mg were also leached but the eventual reduction in soil mass was only approximately 10%. An industrially contaminated soil was also efficiently leached and approximately 69% of the main toxic metals present, Cu, Ni, and Mn, were removed after 175 days. The leachate that resulted from the action of sulfur-oxidizing bacteria on contaminated soil was stripped of metals using an anaerobic bioreactor containing a mixed culture of sulfate-reducing bacteria which precipitated soluble metal species as solid metal sulfides. More than 98% of the metals were removed from solution with the exception of Mn, Ni, and Pb, where 80–90% were removed. The metal content of the resultant effluent liquor was low enough to meet European criteria for discharge into the environment.