Changing contaminant mobility in a dredged canal sediment during a three-year phytoremediation trial

Ecological risk from potentially toxic element legacy contamination in sediment from the Forth and Clyde Canal, Scotland, UK

Industrial activities on the banks of waterways can degrade both the waterbody and the surrounding area and continue to exert pressure on the environment even after the closure of the industries involved. An assessment was undertaken to determine concentration, distribution, mobility and ecological risk of potentially toxic elements (PTE) from legacy contamination in sediments of the Forth and Clyde Canal, UK. Concentrations of PTE, determined by ICP-MS following aqua regia digestion, were 5.54-219 mg kg-1 for As, < 0.025-11.0 mg kg-1 for Cd, 44.8-883 mg kg-1 for Cr, 39.3-618 mg kg-1 for Cu, 35.8-72.1 g kg-1 for Fe, 720-4460 mg kg-1 for Mn, 42.0-154 mg kg-1 for Ni, 93.9-2740 mg kg-1 for Pb, 5.36-122 mg kg-1 for Sn and 288-3640 mg kg-1 for Zn. With the exception of Fe and Mn, higher levels were observed at urban locations than at rural. Enhanced Cr, Pb and Sn content at suburban locations could be attributed to historical industrial activities on the canal bank, while widespread distribution of As and Pb was consistent with atmospheric deposition. In the inner-city area, sediment quality was severely deteriorated, and the potential ecological risk was very high. Fractionation patterns, determined using the modified BCR sequential extraction, indicated a particularly high risk of mobilization for Cd, Mn and Zn, and the highest exchangeable fraction risk from Zn. The research highlights the need to assess and, where necessary, manage legacy contaminated sites in line with the UN 2030 Agenda for Sustainable Development.

Safe utilization and remediation potential of the mulberry-silkworm system in heavy metal-contaminated lands: A review

Mulberry cultivation and silkworm rearing hold a prominent position in the agricultural industries of many Asian countries, contributing to economic growth, sustainable development, and cultural heritage preservation. Applying the soil-mulberry-silkworm system (SMSS) to heavy metal (HM)-contaminated areas is significant economically, environmentally, and socially. The ultimate goal of this paper is to review the main research progress of SMSS under HM stress, examining factors affecting its safe utilization and remediation potential for HM-contaminated soils. HM tolerance of mulberry and silkworms relates to their growth stages. Based on the standards for HM contaminants in various mulberry and silkworm products and the bioconcentration factor of HMs at different parts of SMSS, we calculated maximum safe Cd and Pb levels for SMSS application on contaminated lands. Several remediation practices demonstrated mulberry's ability to grow on barren lands, absorb various HMs, while silkworm excreta can adsorb HMs and improve soil fertility. Considering multiple factors influencing HM tolerance and accumulation, we propose a decision model to guide SMSS application in polluted areas. Finally, we discussed the potential of using molecular breeding techniques to screen or develop varieties better suited for HM-contaminated regions. However, actual pollution scenarios are often complex, requiring consideration of multiple factors. More large-scale applications are crucial to enhance the theoretical foundation for applying SMSS in HM pollution risk areas.

Combined Chemical and Ecotoxicological Measurements for River Sediment Management in an On-Land Deposit Scenario

Sediment management along engineered river systems includes dredging operations and sediment deposition in the sea (capping) or on land. Thus, determining the ecotoxicological risk gradient associated with river sediments is critical. In this study, we investigated sediment samples along the Rhône River (France) and conducted environmental risk assessment tests with the idea to evaluate them in the future for deposit on soil. Based on an on-land deposit scenario, the capacity of the sediment samples from four sites (LDB, BER, GEC, and TRS) to support vegetation was evaluated by characterising the physical and chemical parameters (pH, conductivity, total organic carbon, grain size, C/N, potassium, nitrogen, and selected pollutants), including polychlorinated biphenyls (PCBs) and metal trace elements. All tested sediments were contaminated by metallic elements and PCBs as follows: LDB > GEC > TRS > BER, but only LDB had levels higher than the French regulatory threshold S1. Sediment ecotoxicity was then assessed using acute (plant germination and earthworm avoidance) and chronic (ostracod test and earthworm reproduction) bioassays. Two of the tested plant species, Lolium perenne (ray grass) and Cucurbita pepo (zucchini), were highly sensitive to sediment phytotoxicity. Acute tests also showed significant inhibition of germination and root growth, with avoidance by Eisenia fetida at the least contaminated sites (TRS and BER). Chronic bioassays revealed that LDB and TRS sediment were significantly toxic to E. fetida and Heterocypris incongruens (Ostracoda), and GEC sediment was toxic for the latter organism. In this on-land and spatialised deposit scenario, river sediment from the LDB site (Lake Bourget marina) presented the highest potential toxicity and required the greatest attention. However, low contamination levels can also lead to potential toxicity (as demonstrated for GEC and TRS site), underlining the importance of a multiple test approach for this scenario.

Willow and Herbaceous Species' Phytoremediation Potential in Zn-Contaminated Farm Field Soil in Eastern Québec, Canada: A Greenhouse Feasibility Study

Phytoremediation shows great promise as a plant-based alternative to conventional clean-up methods that are prohibitively expensive. As part of an integrated strategy, the selection of well-adapted plant species as well as planting and management techniques could determine the success of a long-term program. Herein, we conducted an experiment under semi-controlled conditions to screen different plants species with respect to their ability to phytoremediate Zn-contaminated soil excavated from a contaminated site following a train derailment and spillage. The effect of nitrilotriacetic acid (NTA) application on the plants and soil was also comprehensively evaluated, albeit we did not find its use relevant for field application. In less than 100 days, substantial Zn removal occurred in the soil zone proximal to the roots of all the tested plant species. Three perennial herbaceous species were tested, namely, Festuca arundinacea, Medicago sativa, and a commercial mix purposely designed for revegetation; they all showed strong capacity for phytostabilization at the root level but not for phytoextraction. The Zn content in the aboveground biomass of willows was much higher. Furthermore, the degree of growth, physiological measurements, and the Zn extraction yield indicated Salix purpurea ‘Fish Creek’ could perform better than Salix miyabeana, ‘SX67’, in situ. Therefore, we suggest implementing an S. purpurea—perennial herbaceous co-cropping strategy at this decade-long-abandoned contaminated site or at similar disrupted landscapes.

Intercropping improves heavy metal phytoremediation efficiency through changing properties of rhizosphere soil in bamboo plantation

Moso bamboo is considered a potential species for heavy metal (HM) phytoremediation; however, the effect of intercropping on rhizosphere and phytoextraction remains to be elucidated. We comparatively investigated rhizobacteria, soil properties, and phytoextraction efficiency of monoculture and intercropping of Moso bamboo and Sedum plumbizincicola in Cu/Zn/Cd-contaminated soil. Compared with monocultures, intercropping increased the bacterial α-diversity indices (Shannon, Chao1) and the number of biomarkers. Intercropping reduced the contents of soil organic matter (SOM), available nutrients, and Cd and Cu in rhizosphere soils, and reduced the Cd and Zn contents in tissues of sedum. By contrast, Cd and Zn contents in tissues of bamboo increased, and the increase of organic acid in root exudates from intercropping could facilitate the HM absorption. The total amount of Cu, Zn, and Cd removed from the soil in intercropping system was 1.2, 1.9, and 1.8 times than those in monoculture bamboo, respectively. The abundances of Proteobacteria, Acidobacteria, Verrucomicrobia and Actinobacteria were higher in intercropping, playing an important role in soil nutrient cycles and HM remediation. These bacterial communities were closely correlated (P < 0.01) with SOM, available nitrogen, available phosphorus, and HMs. The results suggested this intercropping pattern can increase HM removal efficiency from polluted soils.

Influence of Planting Density on the Phytoremediation Efficiency of Festuca arundinacea in cd-Polluted Soil

Planting density can influence the biomass generation and element uptake capacity of various plants, which are two critical factors that determine the phytoremediation efficiency of plants. A series of 70 d experiments was performed to evaluate the influence of the planting density (10, 15, 20, 25, and 30 g seeds·m^- 2, namely D10, D15, D20, D25, and D30, respectively) of Festuca arundinacea on the decontamination of Cd-polluted soils. The variations in the biomass yield, falling tissue (senescent and dead leaf tissues) proportion, and Cd extraction capacity of the species under different cultivation strategies were determined. The results showed that the biomass generation of the species per square meter increased as the planting density increased, reached a peak at D20, and then decreased significantly. In addition, planting density can change the proportions of different leaf types, and the highest amount of senescent and dead leaves which accumulated significantly more Cd compared with the emerging and mature leaf tissues was observed at D20. A suitable planting density can also drive the species to secrete more dissolved organic matter (DOM), especially hydrophilic fractionations in to the soil, activating more Cd. Therefore, the phytoremediation efficiency of the species was determined by the dry weight of the falling tissues, which contained more than 75% of the leaf Cd. A suitable planting density can enhance the Cd decontamination capacity of F. arundinacea, and the adjustment of the planting density is a practicable and economical method that can be performed in real fields.

Soil microbial community succession and interactions during combined plant/white-rot fungus remediation of polycyclic aromatic hydrocarbons

Despite combined plant/white-rot fungus remediation being effective for remediating polycyclic aromatic hydrocarbon (PAH)-contaminated soil, the complex organismal interactions and their effects on soil PAH degradation remain unclear. Here, we used quantitative PCR, analysis of soil enzyme activities, and sequencing of representative genes to characterize the ecological dynamics of natural attenuation, mycoremediation (MR, using Crucibulum laeve), phytoremediation (PR, using Salix viminalis), and plant-microbial remediation (PMR, using both species) for PAHs in soil for 60 days. On day 60, PMR achieved the highest removal efficiency of all three representative PAHs (65.5%, 47.5%, and 62.4% for phenanthrene, pyrene, and benzo(a)pyrene, respectively) when compared with the other treatments. MR significantly increased the relative abundance of Rhizobium and Bacillus but antagonized the other putative indigenous PAH-degrading bacteria, which were enriched by PR. PR significantly reduced soil nutrients, such as NO₃^- and NH₄⁺, and available potassium (AK), thereby changing the microbial community composition as reflected by redundancy analysis, significantly reducing the soil bacterial biomass relative to that in other treatments. These disadvantages hampered phenanthrene and pyrene removal. MR provided additional nutrients, which counteracted the nutrient consumption associated with PR, thereby maintaining the microbial community diversity and bacterial biomass of PMR at a level achieved in the NA treatment. Combination remediation therefore overcame the disadvantages of using PR alone. These results indicated that inoculation with the combination of S. viminalis and C. laeve synergistically stimulated the growth of indigenous PAH-degrading microorganisms and maintained bacterial biomass, thus accelerating the dissipation of soil PAHs.

Enhancing Salix viminalis L.-mediated phytoremediation of polycyclic aromatic hydrocarbon-contaminated soil by inoculation with Crucibulum laeve (white-rot fungus)

Although plant-white-rot fungi (WRF) remediation is considered efficient in improving polycyclic aromatic hydrocarbon (PAH)-contaminated soil, the prospects for using it remain poorly known. Therefore, we evaluated whether the WRF Crucibulum laeve could improve the phytoremediation of PAH-contaminated soil by Salix viminalis L. A 60-day pot experiment was conducted to investigate the effects of C. laeve inoculation (using two inoculation treatments and a non-inoculated control) on the phytoremediation potential, growth, and antioxidant metabolism of S. viminalis cultivated in PAH-contaminated soil. The S. viminalis-C. laeve association synergistically caused the highest PAH removal rate. Under the S. viminalis-C. laeve treatment, 80% of the biological concentration and translocation factors for all tissues of S. viminalis were > 1, whereas only 20% of these factors were > 1 when S. viminalis was used alone. C. laeve inoculation remarkably enhanced phytoremediation by promoting S. viminalis-based phytoextraction of PAHs from soils. Furthermore, although C. laeve inoculation altered the antioxidant metabolism of S. viminalis by inducing oxidative stress, thereby inhibiting plant growth, the plant's hardiness enabled it to survive and grow normally for 60 days after treatment. Therefore, phytoremediation using S. viminalis inoculated with C. laeve can be considered a feasible approach for the phytoremediation of PAH-contaminated soil.

Bamboo - An untapped plant resource for the phytoremediation of heavy metal contaminated soils

Phytoremediation is a green technology used for the remediation of heavy metal soils. However, up to now, very few plants are known to be both hyperaccumulators and fast-growers. In contrast, some non-hyperaccumulators, which possess lower extraction capacities than hyperaccumulators, are fast-growing species with much higher total biomass yields and are potential alternative phytoremediators. Bamboo is a taxonomic group comprised of 1439 species that are mostly distributed in the tropics and subtropics. Although limited studies on bamboo for phytoremediation, recent studies have shown that some bamboo species have high ability to adapt to metalliferous environments and a high capacity to absorb heavy metals. Bamboo tissues in the rhizome and culm can accumulate a large amount of heavy metals that mainly accumulate in the cell wall, vacuole, and cytoplasm. Certain bamboo species such as moso bamboo, Phyllostachys praecox, have been shown to have a high endurance in metal contaminated soils, enabling a considerable uptake and accumulation of heavy metals. However, excessive concentrations of heavy metals may cause oxidative stress and damage bamboo plants. Therefore, several management strategies have been developed to improve the phytoremediation ability of bamboo species, including the selection of tolerant bamboo species, intercropping with hyperaccumulators, fertilization applications, and employment of chelate in soil. This review demonstrates that bamboo species, which have high biomass productivity, short rotation, and high economic value, can be used for phytoremediation. However, the mechanisms of heavy metal uptake, transport, sequestration, and detoxification of different bamboo species require urgent investigation.

Growth, physiology, and phytoextraction potential of poplar and willow established in soils amended with heavy-metal contaminated, dredged river sediments

Phytotechnologies have been used worldwide to remediate and restore damaged ecosystems, especially those caused by industrial byproducts leaching into rivers and other waterways. The objective of this study was to test the growth, physiology, and phytoextraction potential of poplar and willow established in soils amended with heavy-metal contaminated, dredged river sediments from the Great Bačka Canal near Vrbas City, Serbia. The sediments were applied to greenhouse-grown trees of Populus deltoides Bartr. ex Marsh. clone 'Bora' and Salix viminalis L. clone 'SV068'. Individual pots with trees previously grown for two months were amended with 0, 0.5 and 1.0 kg of sediment containing 400 mg Cr kg^-1, 295 mg Cu kg^-1, 465 mg Zn kg^-1, 124 mg Ni kg^-1, 1.87 mg Cd kg^-1, and 61 mg Pb kg^-1. Following amendment, trees were grown for two seasons (i.e., 2014, 2015), with coppicing after the first season. In addition to growth parameters, physiological traits related to the photosynthesis and nitrogen metabolism were assessed during both growing seasons. At the end of the study, trees were harvested for biomass analysis and accumulation of heavy metals in tree tissues and soils. Application of sediment decreased aboveground biomass by 37.3% in 2014, but increased height (16.4%) and leaf area (19.2%) in 2015. Sediment application negatively impacted the content of pigments and nitrate reductase activity, causing them to decrease over time. Generally, the effect of treatments on growth was more pronounced in poplars, while willows had more pronounced physiological activity. Accumulation patterns were similar to previously-published results. In particular, Zn and Cd were mostly accumulated in leaves of both poplar and willow, which indicated successful phytoextraction. In contrast, other metals (e.g., Cr, Ni, Pb, Cu) were mostly phytostabilized in the roots. Differences in metal allocation between poplar and willow were recorded only for Cu, while other metals followed similar distribution patterns in both genera. Results of this study indicated that the composition of heavy metals in the sediments determined the mechanisms of the applied phytoremediation technique.

Trace element phytoextraction from contaminated soil: a case study under Mediterranean climate

The current field study aims to assess the suitability of four different plant species (i.e. poplar, willow, hemp and alfalfa) to be used for trace element (TE) (i.e. Cd, Cu, Ni, Pb and Zn) phytoextraction under hot-arid Mediterranean climate conditions. Plants were grown for two consecutive years on a moderate TE contaminated soil, supplied with water and mineral nutrients. The growth and physiological parameters were assessed throughout the trial to compare the response of plants to the environmental pollution, and TE uptake rates were measured for aboveground plant tissues. The phytoextraction rate for each species was expressed as a function of aboveground biomass yield and the TE uptake and translocation within the plant. Alfalfa played a significant role in reducing extractable Ni (60.6%) and Zn (46%) in the soil, whereas hemp reduced 32% of extractable Cd and 46% of extractable Pb; poplar decreased extractable Cd (37%), Ni (49%), Pb (46%) and Zn (63%); and willow reduced the extractable Zn (73%) compared to the beginning of the trial. No change in total TE content was observed; however, poplar and willow were able to extract and accumulate the highest amount of Zn (3200 and 5200 g ha^-1 year^-1 respectively) and Cu (182 and 116 g ha^-1 year^-1), whereas hemp, with 36 g ha^-1 year^-1, showed the best phytoextraction potential for Pb. Overall, we found a positive correlation between the phytoextraction rate and biomass yield, extractable TE concentration and translocation factor (TF) and a negative relationship with Ca concentration in the soil.

Soil trace element changes during a phytoremediation trial with willows in southern Québec, Canada

This study determined the changes in trace elements (TE) (As, Cd, Cu, Ni, Pb, Zn) chemistry in the soils of a willow ("Fish Creek" - Salix purpurea, SV1 - Salix x dasyclados and SX67 - Salix miyabeana) plantation growing under a cold climate during a three-year trial. The soil HNO₃-extractable and H₂O-soluble TE concentrations and pools significantly decreased under most cultivars (Fish, SX67). Yet, TE changes showed inconsistent patterns and localized soil TE increases (Ni, Pb) were measured. Temporal changes in soil TE were also detected in control plots and sometimes exceeded changes in planted plots. Discrepancies existed between the amount of soil TE change and the amount of TE uptake by willows, except for Cd and Zn. Phytoremediation with willows could reduce soil Cd and Zn within a decadal timeframe indicating that they can be remediated by willows in moderately contaminated soils. However, the time needed to reduce soil As, Cu, Ni and Pb was too long to be efficient. We submit that soil leaching contributed to the TE decrease in controls and the TE discrepancies, and that the plantation could have secondary effects such as the accelerated leaching of soil TE.

Heavy metal stress in alders: Tolerance and vulnerability of the actinorhizal symbiosis

Alders have already demonstrated their potential for the revegetation of both mining and industrial sites. These actinorhizal trees and shrubs and the actinobacteria Frankia associate in a nitrogen-fixing symbiosis which could however be negatively affected by the presence of heavy metals, and accumulate them. In our hydroponic assay with black alders, quantification of the roots and shoots metal concentrations showed that, in the absence of stress, symbiosis increases Mo and Ni root content and simultaneously decreases Mo shoot content. Interestingly, the Mo shoot content also decreases in the presence of Ni, Cu, Pb, Zn and Cd for symbiotic alders. In symbiotic alders, Pb shoot translocation was promoted in presence of Pb. On the other hand, Cd exclusion in symbiotic root tissues was observed with Pb and Cd. In the presence of symbiosis, only Cd and Pb showed translocation into aerial tissues when present in the nutrient solution. Moreover, the translocation of Ni to shoot was prevented by symbiosis in the presence of Cd, Ni and Pb. The hydroponic experiment demonstrated that alders benefit from the symbiosis, producing more biomass (total, root and shoot) than non nodulated alders in control condition, and in the presence of metals (Cu, Ni, Zn, Pb and Cd). Heavy metals did not reduce the nodule numbers (SNN), but the presence of Zn or Cd did reduce nodule allocation. Our study suggests that the Frankia-alder symbiosis is a promising (and a compatible) plant-microorganism association for the revegetation of contaminated sites, with minimal risk of metal dispersion.

Agronomic Practices for Improving Gentle Remediation of Trace Element-Contaminated Soils

The last few decades have seen the rise of Gentle soil Remediation Options (GRO), which notably include in situ contaminant stabilization ("inactivation") and plant-based (generally termed "phytoremediation") options. For trace element (TE)-contaminated sites, GRO aim to either decrease their labile pool and/or total content in the soil, thereby reducing related pollutant linkages. Much research has been dedicated to the screening and selection of TE-tolerant plant species and genotypes for application in GRO. However, the number of field trials demonstrating successful GRO remains well below the number of studies carried out at a greenhouse level. The move from greenhouse to field conditions requires incorporating agronomical knowledge into the remediation process and the ecological restoration of ecosystem services. This review summarizes agronomic practices against their demonstrated or potential positive effect on GRO performance, including plant selection, soil management practices, crop rotation, short rotation coppice, intercropping/row cropping, planting methods and plant densities, harvest and fertilization management, pest and weed control and irrigation management. Potentially negative effects of GRO, e.g., the introduction of potentially invasive species, are also discussed. Lessons learnt from long-term European field case sites are given for aiding the choice of appropriate management practices and plant species.

Phytoremediation of heavy metal contaminated soil by Jatropha curcas

This study employed Jatropha curcas (bioenergy crop plant) to assist in the removal of heavy metals from contaminated field soils. Analyses were conducted on the concentrations of the individual metals in the soil and in the plants, and their differences over the growth periods of the plants were determined. The calculation of plant biomass after 2 years yielded the total amount of each metal that was removed from the soil. In terms of the absorption of heavy metal contaminants by the roots and their transfer to aerial plant parts, Cd, Ni, and Zn exhibited the greatest ease of absorption, whereas Cu, Cr, and Pb interacted strongly with the root cells and remained in the roots of the plants. J. curcas showed the best absorption capability for Cd, Cr, Ni, and Zn. This study pioneered the concept of combining both bioremediation and afforestation by J. curcas, demonstrated at a field scale.

A field study on phytoremediation of dredged sediment contaminated by heavy metals and nutrients: the impacts of sediment aeration

Compared to traditional chemical or physical treatments, phytoremediation has proved to be a cost-effective and environmentally sound alternative for remediation of contaminated dredged sediment. A field study was conducted in a sediment disposal site predominantly colonized by Typha angustifolia under different sediment moisture conditions to estimate the phytoremediation effects of dredged sediment. The moisture content was 37.30 % and 48.27 % in aerated and waterlogged sediment, respectively. Total nitrogen (TN) content was higher in the waterlogged sediment than in the aerated sediment. The total Cd contents were lower in aerated sediment, which was mainly resulted from the lower exchangeable fraction of Cd. The bioaccumulation of P, Cu and Pb in T. angustifolia was promoted by waterlogging, and the belowground tissue concentrations and accumulation factors (AFs) of Cu were higher than that of other metals, which can be explained by that Cu is an essential micronutrient for plants. Consistent with many previous studies, T. angustifolia showed higher metal levels in roots than in above-ground tissues at both the sediment conditions. Due to the improved biomass produced in the aerated sediment, the removals of nutrients and the metals by plant harvest were higher from aerated sediment than from waterlogged sediment. It was indicated that maintaining the dredged sediment aerated can avoid release risk and plant uptake of metals, while the opposite management option can promote phytoextraction of these contaminants.

Forage and rangeland plants from uranium mine soils: long-term hazard to herbivores and livestock?

Metalliferous uranium mine overburden soils integrated into arable land or stabilized by perennial rangeland plants evoke concern about the quality of crops and the exposure of grazing and thereby soil-ingesting (wildlife) herbivores to heavy metals (HM) and radionuclides. In a 2-year trial, thirteen annual and perennial forage and rangeland plants were thus potted on, or taken from, cultivated field soil of a metalliferous hot spot near Ronneburg (Germany). The content of soil and shoot tissues in 20 minerals was determined by ICP-MS to estimate HM (and uranium) toxicities to grazing animals and the plants themselves, and to calculate the long-term persistence of the metal toxicants (soil clean-up times) from the annual uptake rates of the plants. On Ronneburg soil elevated in As, Cd, Cu, Mn, Pb, U, and Zn, the shoot mineral content of all test plants remained preferentially in the range of "normal plant concentrations" but reached up to the fourfold to sixfold in Mn, Ni, and Zn, the 1.45- to 21.5-fold of the forage legislative limit in Cd, and the 10- to 180-fold of common herb concentrations in U. Shoot and the calculated root concentrations in Cd, Cu, Ni, and Zn accounted for phytotoxic effects at least to grasses and cereals. Based on WHO PTWI values for the tolerable weekly human Cd and Pb intake, the expanded Cd and Pb limits for forage, and reported rates of hay, roots, and adhering-soil ingestion, the tolerable daily intake rates of 0.65/11.6 mg in Cd/Pb by a 65 kg herbivore would be surpassed by the 11- to 27/0.7- to 4.7-fold across the year, with drastic consequences for winter-grazing and thereby high rates of roots and soil-ingesting animals. The daily intake of 5.3-31.5 mg of the alpha radiation emitter, U, may be less disastrous to short-lived herbivores. The annual phytoextraction rates of critical HM by the tested excluder crops indicate that hundreds to thousands of years are necessary to halve the HM and (long-lived) radionuclide load of Ronneburg soil, provided the herbage is harvested at all. It is concluded that the content in Cd/As, Cd, and Cu exclude herbage/Ronneburg soil from the commercial use as forage or pasture land soil for incalculable time spans. Caution is required, too, with the consumption of game.

Phytoremediation of dredged marine sediment: monitoring of chemical and biochemical processes contributing to sediment reclamation

In this study, a pilot phytoremediation experiment was performed to treat about 80 m(3) of silty saline sediments contaminated by heavy metals and organic compounds. After preliminary mixing with a sandy soil and green compost application, three different plant treatments [Paspalum vaginatum (P); P. vaginatum + Spartium junceum (P + S); P. vaginatum + Tamarix gallica (P + T)] were compared to each other and to an unplanted control (C) in order to evaluate the plant efficiency in remediating and ameliorating agronomical and functional sediment properties. The experiment was monitored for one year after planting by taking sediment samples at two depths and performing several chemical and biochemical analyses. After one year, the increase in hydrolytic enzyme and dehydrogenase activities indicated the stimulation of sediment functionality. Additionally, the availability of energy sources derived from organic matter application and plant-root activity promoted the formation of a stable organic matter fraction. Finally, P + S and P + T were also effective in decontaminating polluted marine sediments from both organic (total petroleum hydrocarbons, TPH) and inorganic (heavy metal) pollutants.

Planting woody crops on dredged contaminated sediment provides both positive and negative effects in terms of remediation

There is currently a requirement for studies focusing on the long-term sustainability of phytoremediation technologies. Trace element uptake by Salix, Populus and Alnus species planted in dredged contaminated canal sediment and concentrations in sediment and pore waters were investigated, eight years after a phytoremediation trial was initiated in NW England. Soil biological activity was also measured using invertebrate and microbial assays to determine soil quality improvements. Zinc was the dominant trace metal in foliage and woody stems, and the most mobile trace element in sediment pore water (~14 mg l(-1)). Biological activity had improved; earthworm numbers had increased from 5 to 24, and the QBS index (an index of microarthropod groups in soil) had increased from 70 to 88. It is concluded that biological conditions had improved and natural processes appear to be enhancing soil quality, but there remains a potential risk of trace element transfer to the wider environment.

Intra- and inter-annual variation of Cd, Zn, Mn and Cu in foliage of poplars on contaminated soil

The uptake of trace metals in the leaves of fast-growing woody species is a crucial factor in ecological risk assessment and in the evaluation of phytoextraction potentials. In this study, we present a long-term data series of foliar Cd, Zn, Mn and Cu concentrations in poplar (Populus trichocarpa x P. deltoides). Leaves were collected every three weeks from 2001 until 2007 on three sites, (i) a new plantation on an alluvial soil polluted by river sediments, (ii) a new plantation on an unpolluted soil and (iii) a 10-year old plantation on a polluted dredged sediment soil. In addition, tree rings were measured on the alluvial soil in order to better assess growth over the past seven years. Foliar concentrations of Cd, Zn and Mn decreased considerably with time in the new plantation on polluted soil. Concentrations of Zn and Mn decreased in the new plantation on unpolluted soil as well. The older plantation on polluted soil did not show changes in foliar concentrations for Cd, Zn or Mn. Foliar Cu concentrations slightly increased for all sites. Within one growing season, foliar concentrations of Cd, Zn, Cu and Mn increased towards the end of the season. The tree ring data of the poplars on the alluvial soil indicated a strong decrease in growth due to declining tree condition from 2005 onwards, the same year that foliar Cd and Zn concentrations markedly decreased. Lower transpiration rates probably induced a lower uptake of dissolved trace metals. It is concluded that stand health and growth rate have a strong impact on the variation of foliar trace metal concentrations over time.

Arsenic mobility and speciation in a contaminated urban soil are affected by different methods of green waste compost application

Application of green waste compost (GWC) to brownfield land is now common practice in soil restoration. However, previous studies have demonstrated both beneficial and detrimental effects on arsenic and metal mobility. In this paper, trace element behaviour was investigated following GWC application, either as surface mulch to, or mixed into soil from a previously described brownfield site in the U.K. Significant differences in arsenic mobility were observed between treatments. Mulching caused most disturbance, significantly increasing soil pore water As, together with Fe, P, Cr, Ni and dissolved organic carbon, the latter was a critical factor enhancing As mobilization. Arsenate was the main inorganic As species in soil pore water, increasing in concentration over time. An initial flush of potentially more toxic arsenite decreased 4 weeks after compost application. Biological processes appeared to play an important role in influencing As mobility. The results point to the necessity for careful management of As-contaminated soils.

Remediation of petroleum contaminated soils by joint action of Pharbitis nil L. and its microbial community

The plot-culture experiments were conducted for examining the feasibility of Pharbitis nil L. and its microbial community to remedy petroleum contaminated soils. The petroleum contaminated soil, containing 10% (w/w) of the total petroleum hydrocarbons (TPHs), was collected from the Shengli Oil Field, Dongying City, Shandong Province, China. The collected soil was applied and diluted to a series of petroleum contaminated soils (0.5%, 1.0%, 2.0% and 4.0%). Root length, microbial populations and numbers in the rhizosphere were also measured in this work. The results showed that there was significantly (p<0.05) greater degradation rate of TPHs in vegetated treatments, up to 27.63-67.42%, compared with the unvegetated controls (only 10.20-35.61%), after a 127-day incubation. Although various fractions of TPHs had an insignificant concentration difference due to the presence of the remediation plants, there was a much higher removal of saturated hydrocarbon compared with other components. The biomass of P. nil L. did not decrease significantly when the concentration of petroleum hydrocarbons in soil was ≤2.0%. The trends of microbial populations and numbers in the rhizosphere were similar to the biomass changes, with the exception that fungi at 0.5% petroleum contaminated soil had the largest microbial populations and numbers.

Metal availability in a highly contaminated, dredged-sediment disposal site: field measurements and geochemical modeling

Two complementary approaches were used to characterize arsenic and metal mobilizations from a dredged-sediment disposal site: a detailed field study combined with hydrogeochemical modeling. Contaminants in sediments were found to be mainly present as sulfides subject to oxidation. Secondary phases (carbonates, sulfates, (hydr)oxides) were also observed. Oxidative processes occurred at different rates depending on physicochemical conditions and contaminant contents in the sediment. Two distinct areas were identified on the site, each corresponding to a specific contaminant mobility behavior. In a reducing area, Fe and As were highly soluble and illustrated anoxic behavior. In well-oxygenated material, groundwater was highly contaminated in Zn, Cd and Pb. A third zone in which sediments and groundwater were less contaminated was also characterized. This study enabled us to prioritize remediation work, which should aim to limit infiltration and long-term environmental impact.

Review: In situ and bioremediation of organic pollutants in aquatic sediments

Organic pollutants in sediments are a worldwide problem because sediments act as sinks for hydrophobic, recalcitrant and hazardous compounds. Depending on biogeochemical processes these hydrocarbons are involved in adsorption, desorption and transformation processes and can be made available to benthic organisms as well as organisms in the water column through the sediment-water interface. Most of these recalcitrant hydrocarbons are toxic and carcinogenic, they may enter the food-chain and accumulate in biological tissue. Several approaches are being investigated or have been already used to remove organic hydrocarbons from sediments. This paper provides a review on types and sources of organic pollutants as well as their behavior in sediments. It presents the advantages and disadvantages of traditional sediment remediation techniques in use, such as dredging, capping and monitored natural attenuation. Furthermore, it describes new approaches with emphasis on bioremediation, like biostimulation, bioaugmentation and phytoremediation applied to sediments. These new techniques promise to be of lower impact and more cost efficient than traditional management strategies.

Exposure of an anoxic and contaminated canal sediment: mobility of metal(loid)s

A derelict canal contains an estimated 9800 tonnes of anoxic sediment with highly elevated concentrations of trace elements. Lack of maintenance, reduced water levels and vegetation colonization threaten the stability of pollutants by removing existing waterlogged reduced conditions. A column leaching study of the sediment under increasingly oxidized conditions showed reductions in As mobility but increased heavy metal concentrations. In a reduced state, As mobility was higher (as a consequence of enhanced Fe and organic carbon solubility) whilst heavy metal concentrations in leachates were lower (due to markedly higher pH). Over 10 contiguous wetting and drying cycles, the consequences were profound; all trace elements were continuously leached with enhanced flushing of Fe, As, Zn and Cu. This raises concern over possible mobilization of pollutants to the wider environment, including groundwater. Options for management to stabilize contaminants are discussed that point to the importance of limiting water flow through the sediment.

Phytoremediation of contaminated soils and groundwater: lessons from the field

BACKGROUND, AIM, AND SCOPE

The use of plants and associated microorganisms to remove, contain, inactivate, or degrade harmful environmental contaminants (generally termed phytoremediation) and to revitalize contaminated sites is gaining more and more attention. In this review, prerequisites for a successful remediation will be discussed. The performance of phytoremediation as an environmental remediation technology indeed depends on several factors including the extent of soil contamination, the availability and accessibility of contaminants for rhizosphere microorganisms and uptake into roots (bioavailability), and the ability of the plant and its associated microorganisms to intercept, absorb, accumulate, and/or degrade the contaminants. The main aim is to provide an overview of existing field experience in Europe concerning the use of plants and their associated microorganisms whether or not combined with amendments for the revitalization or remediation of contaminated soils and undeep groundwater. Contaminations with trace elements (except radionuclides) and organics will be considered. Because remediation with transgenic organisms is largely untested in the field, this topic is not covered in this review. Brief attention will be paid to the economical aspects, use, and processing of the biomass.

CONCLUSIONS AND PERSPECTIVES

It is clear that in spite of a growing public and commercial interest and the success of several pilot studies and field scale applications more fundamental research still is needed to better exploit the metabolic diversity of the plants themselves, but also to better understand the complex interactions between contaminants, soil, plant roots, and microorganisms (bacteria and mycorrhiza) in the rhizosphere. Further, more data are still needed to quantify the underlying economics, as a support for public acceptance and last but not least to convince policy makers and stakeholders (who are not very familiar with such techniques).

Phytoremediation as a management option for contaminated sediments in tidal marshes, flood control areas and dredged sediment landfill sites

BACKGROUND, AIM AND SCOPE

Polluted sediments in rivers may be transported by the river to the sea, spread over river banks and tidal marshes or managed, i.e. actively dredged and disposed of on land. Once sedimented on tidal marshes, alluvial areas or control flood areas, the polluted sediments enter semi-terrestrial ecosystems or agro-ecosystems and may pose a risk. Disposal of polluted dredged sediments on land may also lead to certain risks. Up to a few years ago, contaminated dredged sediments were placed in confined disposal facilities. The European policy encourages sediment valorisation and this will be a technological challenge for the near future. Currently, contaminated dredged sediments are often not valorisable due to their high content of contaminants and their consequent hazardous properties. In addition, it is generally admitted that treatment and re-use of heavily contaminated dredged sediments is not a cost-effective alternative to confined disposal. For contaminated sediments and associated disposal facilities used in the past, a realistic, low cost, safe, ecologically sound and sustainable management option is required. In this context, phytoremediation is proposed in the literature as a management option. The aim of this paper is to review the current knowledge on management, (phyto)remediation and associated risks in the particular case of sediments contaminated with organic and inorganic pollutants.

MAIN FEATURES

This paper deals with the following features: (1) management and remediation of contaminated sediments and associated risk assessment; (2) management options for ecosystems on polluted sediments, based on phytoremediation of contaminated sediments with focus on phytoextraction, phytostabilisation and phytoremediation of organic pollutants and (3) microbial and mycorrhizal processes occurring in contaminated sediments during phytoremediation.

RESULTS

In this review, an overview is given of phytoremediation as a management option for semi-terrestrial and terrestrial ecosystems affected by polluted sediments, and the processes affecting pollutant bioavailability in the sediments. Studies that combine contaminated sediment and phytoremediation are relatively recent and are increasing in number since few years. Several papers suggest including phytoremediation in a management scheme for contaminated dredged sediments and state that phytoremediation can contribute to the revaluation of land-disposed contaminated sediments. The status of sediments, i.e. reduced or oxidised, highly influences contaminant mobility, its (eco)toxicity and the success of phytoremediation. Studies are performed either on near-fresh sediment or on sediment-derived soil. Field studies show temporal negative effects on plant growth due to oxidation and subsequent ageing of contaminated sediments disposed on land. The review shows that a large variety of plants and trees are able to colonise or develop on contaminated dredged sediment in particular conditions or events (e.g. high level of organic matter, clay and moisture content, flooding, seasonal hydrological variations). Depending on the studies, trees, high-biomass crop species and graminaceous species could be used to degrade organic pollutants, to extract or to stabilise inorganic pollutants. Water content of sediment is a limiting factor for mycorrhizal development. In sediment, specific bacteria may enhance the mobilisation of inorganic contaminants whereas others may participate in their immobilisation. Bacteria are also able to degrade organic pollutants. Their actions may be increased in the presence of plants.

DISCUSSION

Choice of plants is particularly crucial for phytoremediation success on contaminated sediments. Extremely few studies are long-term field-based studies. Short-term effects and resilience of ecosystems is observed in long-term studies, i.e. due to degradation and stabilisation of pollutants. Terrestrial ecosystems affected by polluted sediments range from riverine tidal marshes with several interacting processes and vegetation development mainly determined by hydrology, over alluvial soils affected by overbank sedimentation (including flood control areas), to dredged sediment disposal facilities where hydrology and vegetation might be affected or managed by human intervention. This gradient is also a gradient of systems with highly variable soil and hydrological conditions in a temporal scale (tidal marshes) versus systems with a distinct soil development over time (dredged sediment landfill sites).

CONCLUSIONS

In some circumstances (e.g. to avoid flooding or to ensure navigation) dredging operations are necessary. Management and remediation of contaminated sediments are necessary to reduce the ecological risks and risks associated with food chain contamination and leaching. Besides disposal, classical remediation technologies for contaminated sediment also extract or destroy contaminants. These techniques imply the sediment structure deterioration and prohibitive costs. On the contrary, phytoremediation could be a low-cost option, particularly suited to in situ remediation of large sites and environmentally friendly. However, phytoremediation is rarely included in the management scheme of contaminated sediment and accepted as a viable option.

PERSPECTIVES

Phytoremediation is still an emerging technology that has to prove its sustainability at field scale. Research needs to focus on optimisations to enhance applicability and to address the economic feasibility of phytoremediation.

Changes in the geochemistry and ecotoxicity of a Zn and Cd contaminated dredged sediment over time after land disposal

The management of dredged sediments is of environmental concern worldwide since they may be overloaded with myriads of pollutants. For inland waters' sediments, disposal on land is a common practice. For the long-term risks assessment of such a management, a better understanding of the fate of pollutants over time and an assessment of possible associated biological consequences are needed. Here, we studied the geochemical distribution of Fe, Mn, Zn and Cd in sediment dredged from the Scarpe canal (Nord-Pas-de-Calais Region, France). Analyses were carried out immediately after dredging and 12, 18 and 24 months after disposal in field conditions. In parallel, ecotoxicity of sediment leachates was assessed using standardized bioassays. The results reflected an initial oxidation of sulphides (first year) followed by changes explained by a reversible binding of metals to organic matter in winter and to Fe oxihydroxides in summer. The water-leachable fraction represented less than 2% of the total metal and its ecotoxicity was higher for deposited sediments than for the fresh one. After first year of disposal, sediment ecotoxicity remained stable. A long-term natural attenuation of metals within disposed sediment seemed unlikely since their speciation seemed to fluctuate seasonally without any time trend over years.

In situ removal of copper from sediments by a galvanic cell

This study dealt with in situ removal of copper from sediments through an electrokinetic (EK) process driven by a galvanic cell. Iron (Fe) and carbon (C) were placed separately and connected with a conductive wire. Polluted sediments were put between them and water was filled above the sediments. The galvanic cell was thus formed due to the different electrode potentials of Fe and C. The cell could remove the pollutants in the sediments by electromigration and/or electroosmosis. Results showed that a weak voltage less than 1V was formed by the galvanic cell. The voltage decreased with the increase of time. A slight increase of sediment pH from the anode (Fe) to the cathode (C) was observed. The presence of supernatant water inhibited the variation of sediment pH because H(+) and OH(-) could diffuse into the water. The removal of copper was affected by the sediment pH and the distribution of electrolyte in sediment and supernatant water. Lower pH led to higher removal efficiency. More electrolyte in the sediment and/or less electrolyte in the supernatant water favored the removal of copper. The major removal mechanism was proposed on the basis of the desorption of copper from sediment to pore solution and the subsequent electromigration of copper from the anode to the cathode. The diffusion of copper from sediment to supernatant water was negligible.

Assessing biological indicators for remediated anthropogenic urban soils

Anthropogenic urban soils, including brownfield soils, are currently characterised and evaluated using mainly physico-chemical properties. Our objective was to determine if biological indicators could provide a more comprehensive soil quality assessment relative to sustainability, identifying contamination issues, and effectiveness of remediation strategies. Plant, invertebrate and microbial assays and functional processes were evaluated at 10 brownfield/anthropogenic urban locations at different stages of remediation in northwest England. Extreme sites were discriminated on the basis of earthworm counts and a small number of indicators likely to be related to their activity. It was concluded that identifying a universally-applicable benchmark suite of biological indicators is very unlikely without considerable advancement of knowledge and technology.

Cycling and ecosystem impact of metals in contaminated calcareous dredged sediment-derived soils (Flanders, Belgium)

Significant areas in Flanders, Belgium exhibit moderate contamination with trace metals caused by disposal of contaminated dredged sediments. After disposal, the sediments develop into a soil-like material, on which vegetation is planted or develops spontaneously. Behaviour, cycling and ecosystem impacts of trace metals in calcareous dredged sediment disposal sites in Flanders is reviewed. Although soil physico-chemical properties favour a low metal bioavailability, pore water concentrations can be elevated compared to pore water in uncontaminated soils. While metal leaching is not considered to be of concern, several plants accumulate elevated levels of Cd and Zn in leaves. Also metal levels in soil dwelling organisms and small mammals, particularly Cd, are elevated compared to reference situations. This raises concern for an enhanced transfer of metals to the food chain. Future research should identify biological effects on organisms caused by the contamination. A comprehensive knowledge of metal behaviour in these sites is essential for developing appropriate management options for these sites.

Pedological characteristics of Mn mine tailings and metal accumulation by native plants

In southern China revegetation and ecological restoration of many abandoned Mn tailings has become a major concern. To determine the major constraints for plant establishment and evaluate the feasibility of remediation, a comparative study was conducted on Mn tailings and rhizosphere soils at the boundary of the tailings pond. Both tailings and rhizosphere soils had neutral to slightly alkaline pH and normal electrical conductivity. They were both enriched with organic matter (6.8-9.2%), total N (1.77-5.94 g kg(-1)), available P (41.78-73.83 mg kg(-1)) and K (146.7-906.9 mg kg(-1)), suggesting the tailings were a nutrient rich substrate for revegetation. Mn tailings were clay textured, while rhizosphere soils were silty loam or clay loam. The compaction and anoxic nature of Mn tailings were considered to be the major constraints for plant establishment. Total Mn (31903 mg kg(-1)), Cd (119 mg kg(-1)), Cu (126 mg kg(-1)) and Zn (2490 mg kg(-1)) in tailings were all at phytotoxic levels, but did not differ significantly from those in rhizosphere soils. In both tailings and rhizosphere soils, percentages of water- and DTPA-extractable metals were less than 1% and 2% of the total metal pools, respectively. Sequential extraction revealed that the majority of Mn, Cu and Zn were associated with the residual fraction, while the majority of Cd occurred as Fe-Mn oxides. The natural succession of plants around Mn tailings formed a distinctive metal-tolerant plant community, mainly comprising nine species such as Cynodon dactylon and Humulus scandens and so on. All species studied could be good candidates for revegetation of Mn tailings.

Cadmium and zinc in vegetation and litter of a voluntary woodland that has developed on contaminated sediment-derived soil

Vegetation that develops spontaneously on metal-contaminated soils presents an opportunity to evaluate both metal bioavailability and the risks posed to biota. The behavior of Cd and Zn in the species of a spontaneously developed woodland, colonizing a canal embankment, has been investigated. Nitric-acid-extractable metal concentrations in the sediment-derived substrate ranged between 5.0 to 376 mg kg(-1)dry wt. Cd and 83.0 to 784 mg kg(-1)dry wt. Zn. The woodland is dominated by Willow (Salix) species. Salix caprea selectively accumulated Cd in all stem tissues, in contrast to S. viminalis, which regulated tissue Cd content. Both species showed an effective regulation of tissue Zn. Cadmium uptake by S. caprea was correlated with differences in soil pH, while Zn uptake was not. There was no relationship between tissue metal concentrations and soil metal nitric acid-extractable concentrations. Other aspects of ecosystem function appeared unaffected by the elevated Cd flux in S. caprea; leaf litter organisms present represented all major groups and there was no accumulation of organic matter. The woodland represents a potentially sustainable option for remediating a low value site with difficult access that does not involve removal of the contaminated material to a landfill or making a permanent inert cover.