Leaching behaviour of Cd, Cu, Pb and Zn in surface soils derived from dredged sediments

Distribution and migration of heavy metals in the sediment-plant system: Case study of a large-scale constructed wetland for sewage treatment

Constructed wetlands are extensively applied in wastewater treatment and water ecosystem restoration. However, the characteristics of heavy metals accumulation and migration in a long-running large-scale constructed wetland for wastewater treatment remain unclear. In this study, the variation of heavy metals (Cu, Zn, Pb, Cd, Cr, and As) in the sediment-plant system of a wetland that has been operating for 14 years was quantified. Results show that the sediments of the constructed wetland were the sink for heavy metals. All heavy metals, except As, significantly increased (P < 0.05) in sediments within 0-40 cm depth, and Zn and Cr had leaked to 40-60 cm depth (P < 0.05). Along with the surface flow direction, heavy metal concentrations mostly showed a declining trend, and in comparison, Cu and Cr transported longer distances. Bioconcentration factors show that the two common wetland plants, Phragmites australis and Typha latifolia, exhibited obvious differences in enrichment performance of heavy metals, with the orders of Zn > Cr > Cd > Cu > Pb > As and Cd > Zn > Cr > Cu > As > Pb, respectively. The translocation factors of the two kinds of plants were less than 1 suggesting that they are suitable for phytostabilization. Redundancy analysis indicates that sediment organic matter was the primary environmental factor affecting the distribution and migration of heavy metals in the wetland system. The discrepancy in the migration characteristics of pollutants, especially heavy metals, should be seriously considered in the design and management of wetland systems, including highly-enrichment plants, appropriate hydraulic residence time, and effective surficial filling medium.

Interference between di(2-ethylhexyl) phthalate and heavy metals (Cd and Cu) in a Mollisol during aging and mobilization

Diffuse pollution of the soil by phthalates and heavy metals causes numerous concerns. Their respective fates when coexisting require further investigation. In this study, di(2-ethylhexyl) phthalate (DEHP) and Cd/Cu were used as subjects, focusing on their behavior in Mollisols under combined pollution based on their concentration, fractionation, and leaching. The results indicated that when the two pollutants coexist, the dissipation rate of DEHP in the soil decreased, and its half-life was extended from 30.81 to 40.53 (Cd) and 35.40 d (Cu). DEHP altered the distribution of Cd and Cu in the soil, and this effect persisted after most of the DEHP had degraded. Leaching tests showed that the interaction of DEHP with Cd and Cu hindered leaching during the first rainfall event, but as DEHP degraded and Cd/Cu stabilized, the trapped pollutants were gradually released in subsequent rainfall events. Additionally, to investigate the partitioning of pollutants between soil water and solid surfaces, a diffusion model of DEHP and metal ions on the surface of montmorillonite (high specific surface area adsorbents abundant in soils) was built using molecular dynamics simulations. Simulations revealed their density distribution on the clay surface increased synergistically, whereas their diffusion was antagonistic. This study provides basic data and theoretical support concerning the ecological risk assessment of combined phthalate and heavy metals pollution in soil.

Evaluation of Land Use Adaptation by Sequential Extraction of Soil Trace Elements at an Abandoned Gold and Copper Refinery Site in Northern Taiwan

This study site is located at an abandoned factory of mining, smelting, and refining of gold and copper in north Taiwan for more than one hundred years. The present study used soil background investigation out of the site and the sequential extraction procedures for arsenic and copper to assess the reutilization potential of brownfields at the site. The upper limit of background concentration out of the site was 300 mg/kg for arsenic and 700 mg/kg for copper. The soil arsenic within the site was mainly in the immobile fraction, such as forms fixed by layer silicates, that were very low risk for environmental releases. The soil copper in the abandoned sedimentation basin, gold refinery, and copper refinery was in the mobile fractions such as acid extractable, reducible, and oxidizable forms with higher release risk; therefore, except merely those three zones in the entire site with higher risk for environmental releases of copper, the release risks of trace elements are quite low in the rest of the areas, and land reuse without contact with soil or plant non-edible plants is possible. Therefore, in response to public demand for opening part of the site to promote local tourism development, appropriate control and isolation measures can be implemented to prevent the toxic elements from affecting human health through soil ingestion, skin contact, and other exposure pathways. In terms of pollution control, reducing dust inhalation is also an option to efficiently reduce health risks to an acceptable level and achieve the goal of sustainable land use at the contaminated site.

Effects of tubificid bioturbation on bioaccumulation of Cu and Zn released from sediment by aquatic organisms

To investigate the effects of bioturbation on absorption and enrichment of pollutants from sediments by aquatic organisms, microcosm systems similar to natural aquatic environment were established, and the release of Cu and Zn from the sediment, and their accumulation in some typical aquatic organisms, including submerged plants, floating plants and fish, with the presence of tubificids of different densities were measured. The results of this pilot study showed that the presence of tubificids promoted the migration of the trace metals from sediment to overlying water, especially when there were more worms and especially for Cu which is not easily released from the sediment. During the experiment, Cu in overlying water was mainly in particulate fraction. While for Zn, it was mainly in dissolved form in the early stage of the experiment, and then the dominant fraction gradually changed to particulate fraction. The bioturbation of tubificids also promoted the accumulation of both Cu and Zn in the aquatic organisms. In one system, different types of aquatic organisms showed different features for the accumulation of Cu and Zn. Meanwhile, with the presence of different intensity of bioturbation, the concentration of Cu or Zn in the same kind of organism was different. After a 30-day experiment, trace metal concentration in the aquatic organisms generally decreased in the order of floating plants (lesser duckweed) > submerged plants (Amazon sword) > small fish (zebrafish), and the concentration of Zn in the organisms was usually significantly higher than that of Cu in the same organism, especially in duckweed and zebrafish. However, the presence of tubificids and the density of them had more considerable effects on the uptake of Cu by the organisms, than on the uptake of Zn.

Leaching behaviors and speciation of cadmium from river sediment dewatered using contrasting conditioning

Chemical conditioning is an effective strategy for improved river sediment dewatering affecting both the dewatering efficiency and subsequent resource utilization of the dewatered cake. Two types of conditioning agents, polyaluminium chloride (PAC)/cationic polyacrylamide (PAM) (coagulation precipitation conditioning agent, referred to as P-P conditioning) and ferrous activated sodium persulfate (advanced oxidation conditioning agent, referred to as F-S conditioning) were examined. With increasing leach liquid to solid (L/S) ratio the concentration of Cd for the real time leachates from the dewatered cakes decreased, but the leaching ratio of Cd in both P-P and F-S dewatered cakes increased. With the same L/S, the leaching ratio was reduced for both types of conditioning, as compared to no conditioning, with the leaching ratio being least with F-S conditioning. The leaching ratio of Cd in the dewatered cake with L/S of 100 L kg^-1 was reduced from 21.3% of the total Cd present for the un-conditioned sediment to 12.5% upon P-P conditioning and 11.6% upon F-S conditioning. Furthermore, the different conditioning methods affected the Cd speciation in the dewatered cakes reducing the easy-to-leach speciation of exchangeable and carbonate-bound Cd species and increasing the potential-to-leach speciation of iron-manganese oxide and organically bound Cd species and also the difficult-to-leach species. Risk assessment indicates that the risk due to Cd leaching from the dewatered cakes at L/S of 100 L kg^-1 was reduced from high risk to medium risk after P-P and F-S conditioning with reduced bioavailability.

The vertical migration and speciation of the Pb in the paddy soil: A case study of the Yangtze River Delta, China

Migration of Pb in the soil can be enhanced by acidification and frequent change of environmental condition. The paddy soil, where the environmental condition such as redox fluctuates frequently due to soil submergence and drainage, may offer a favorable condition for Pb migrating to deeper soil and further contaminate groundwater by leaching or irrigation. To date, not much is known about how quickly Pb migrates in the soil and the relevant transformation of Pb. We use long-term soil profile monitoring, sequential extraction and isotopic measurement to examine the temporal change of concentrations and isotopic ratios of Pb associated with different soil components in the paddy soil profile in the Yangtze River Delta area during 2011-2017. We find that Pb vertical migration in the paddy soil is faster. Pb in the shallow soil may migrate downward up to 60 cm during six years. The migration of Pb is dominated by the carbonate, and secondarily influenced by Fe/Mn oxides. Our results also imply that the mechanism of Pb migration in soils is changing. The mechanism which is now characterized as the carbonate-dominating will be replaced as the Fe/Mn oxides-dominating in the near future as the carbonate in shallow soil is becoming depleted.

Prediction of long-term heavy metal leaching from dredged marine sediment applied inland as a construction material

Column leaching studies have been suggested as a reference for site-specific prediction of the long-term leaching characteristics of trace constituents in granular materials used as construction materials. In this study, the concept of the long-term leaching prediction using column studies is applied for dredged marine sediment impacted by heavy metals. The column studies show tailing of the liquid to solid ratio-dependent heavy metal leaching for sediment after heavy metal treatment by acid washing. A dual-mode first-order decay model, applied for the first time in this study for column leaching studies, is able to reproduce the leaching characteristics observed. A procedure for long-term leaching prediction using the dual-mode model is developed and applied to a virtual field scenario for which the sediment is beneficially used as a construction material. The prediction results show that by more accurately reproducing the column study results, the dual-mode model generally predicts greater long-term heavy metal loading to the underlying soil layer and longer duration of leaching than the single-mode model. The heavy metal leaching observed in the columns does not show any correlation with the sequential extraction procedure and toxicity characteristic leaching procedure (TCLP) results, suggesting that the column leaching test should be considered to be independent of such batch test procedures.

Leaching potential of geogenic nickel in serpentine soils from Taiwan and Austria

Serpentine soils may be natural sources of metal leaching and pollution. In this study, two contrasting serpentine soils from Taiwan and Austria were selected to evaluate the leaching potential of geogenic nickel (Ni). We applied selective sequential extractions and dissolution kinetics with three inorganic acids (HCl, HNO₃, and H₂SO₄) and three organic acids (citric, acetic, and oxalic acids) in concentrations ranging from 0.05 to 10 mM to determine the release rate of Ni in the soils with respect to pH and acid types. Chlorite and serpentine were the major Ni-bearing minerals in the studied soils. Ni was dominantly bound in unavailable forms within these silicate minerals, but smaller fractions of acid-soluble, Fe-Mn oxide-bound, and organically bound Ni represented more labile Ni sources in the soils. The release rate of Ni from the soils increased with decreasing pH in all acids. However, the organic acids caused stronger pH dependences than the inorganic acids, likely because of ligand-promoted dissolution. The maximum total rate of Ni dissolution occurred with citric acid in both soils. The dissolution of Ni strongly increased when the ionic strength in the background solutions increased. We observed marked differences in dissolution rates and ligand effects between the Austrian and Taiwanese soils, which reflect differences in labile Ni pools, especially in the organically bound fraction. Our results demonstrate that labile fractions control the leaching potential of Ni in serpentine soils and that Ni associated with soil organic matter may contribute to leaching at moderately acidic pH levels.

Understanding the remobilization of copper, zinc, cadmium and lead due to ageing through sequential extraction and isotopic exchangeability

Artificial infiltration facilities (AIFs) are useful to control urban runoff and regulate combined sewer overflows. Over the years, AIFs accumulate significant amounts of soakaway sediments and organic matter. The prolonged retention of soakaway sediments in AIFs is likely to cause metal remobilization due to ageing processes. The measurement of the individual consequence of ageing demands homogeneity in physical and chemical profiles of samples. This leads to assessment of metal remobilization in a single soil core through solid-phase extractions and isotopic exchangeability (E value). Depth-wise variation in the physicochemical properties and metal content of the underlying soil (below 1 m of AIFs) was created through 2 weeks of continuous leaching with artificial road runoff (ARR). Ten samples obtained from a 50-cm core by sectioning it at 5-cm intervals were subsequently incubated for 18 months. The results suggest that degradation of organic matter and changes in functional groups due to ageing govern metal remobilization. In general, the top segment showed significant alteration due to ageing. Post incubation, Zn increased dramatically in contrast to subdued Cu and Pb levels in exchangeable fractions with concomitant rise in organic-bound fractions. Isotopic exchangeability of Cd and Zn showed pronounced effect of ageing, although the effect of ageing was distinct in chemical partitioning and isotopic exchangeability of metals; a comparative study of short-term versus long-term incubation will benefit assessment of initial dynamics and final equilibrium. Consequently, the outcome from this work is a viable tool in risk prediction related to soakaway sediment accumulation in AIF.

Aging effect on the leaching behavior of heavy metals (Cu, Zn, and Cd) in red paddy soil

Aging effect can influence the fractions distribution and mobility of metals after they are added into soil. In this study, incubation and soil column experiments under simulated acid rain condition were conducted to evaluate aging effect on the leaching characteristic of Cu, Zn, and Cd in artificial polluted red paddy soil. Our results showed that aging effect reduced metal contents in exchangeable and HoAc soluble fractions. Power function was the most excellent to describe the variation of exchangeable fraction, while pseudo first- and second-order functions were more successful to describe the leaching characteristic of metals from soil columns. The leaching amount of the metals from the polluted soil only accounted for a small part of their total content in soil, and the leachability of Cu was the weakest. Both the exchangeable and HoAc soluble fraction were available as indicators to evaluate the leachability of metals in red paddy soil. The shorter time the soil was contaminated, the more amounts of metals released from the soil. The reduction of exchangeable fraction caused by aging effect was the main reason for the decrease of metal mobility in soil.

Evaluation of phytotoxicity of seaport sediments aged artificially by rotary leaching in the framework of a quarry deposit scenario

In the framework of an ecological risk assessment of seaport sediments for terrestrial ecosystems when deposited in quarries, we simulated the "ageing" of sediments exposed to rain. This experiment highlighted an inflection point at the solid/liquid ratio 1/25, after which the extraction of pollutants increases moderately. The raw sediments studied inhibited the germination of Lolium perenne and Armeria maritima (a halophytic species) seeds. Furthermore, they affected the early development of L.perenne. The same sediments, leached at a ratio of 1/25, presented a reduction of acute (germination) and chronic (growth) phytotoxicity. The bioconcentration factors of the metals studied decreased with the leached sediment, except for Cu which was still clearly identified in root parts. Thus rotary leaching tests and phytotoxicity bioassays can be used to provide an initial assessment of the ability of plants, particularly halophytes, to colonize deposits of dredged seaport sediments.

Assessment of the phytotoxicity of seaport sediments in the framework of a quarry-deposit scenario: germination tests of sediments aged artificially by column leaching

The aim of the Sustainable Management of Sediments Dredged in Seaports (SEDIGEST) project is to assess the risks of treated port sediments for terrestrial ecosystems when deposited in quarries. We simulated the "ageing" of these sediments up to the "moment" when plants can germinate. Sediments were leached by water percolating through a laboratory column. Sediments 1 and 2, taken from the port of Toulon (France), were dried and aired. Sediment 3, taken from the port of Guilvinec (France), was stabilised with lime. Phytotoxicity was evaluated on the three artificially aged sediments using germination and early development tests (48 h to 7 days) by Phytotoxkit F(TM) bioassays. The three dilutions tested were performed with the reference "ISO substrate" and with Lolium perenne sp. (rye grass), Sinapis alba (white mustard), and Lepidium sativum (watercress). The tests performed with sediments 1 and 2 showed (1) a decrease of their toxicity to the germination of the species selected following leaching and (2) that L. perenne was the most sensitive species. The tests performed with sediment 3 showed that it was improper for colonisation even after leaching simulating 16 months of ageing. These germination tests on aged sediments identified the effects of leaching and made it possible to appreciate the capacity of the sediments to allow colonisation by plants.

Potential mobility of heavy metals through coupled application of sequential extraction and isotopic exchange: comparison of leaching tests applied to soil and soakaway sediment

Artificial infiltration facilities (AIFs) that enhance groundwater recharge and regulate urban runoff are going to be an integral element of the urban infrastructure. However, AIFs provide a sink which trap pollutants that are likely to cause groundwater contamination. The current study aimed first to examine the mobility characteristics of Cu, Zn, and Pb through soil and soakaway sediment using an integrated analytical approach for column leaching with artificial road runoff (ARR) and then to differentiate the sorption patterns among different samples, (i.e., surface soil, underlying soil, and soakaway sediments) using mass balances. In addition, the study compares metal retention and release under continuous and intermittent flow conditions. Column leaching experiments were conducted using batches for 10 and 30 d under continuous flow condition and for 20 d of intermittent leaching. Heavy metal content and partitioning in soil and sediment used in columns were well characterized before and after leaching experiments. The results showed that a gradual increase in pH and decrease in dissolved organic carbon had pronounced effect on the mobilization of heavy metals. Pb showed the highest retention compared to Cu and Zn which implies that metal complexes play a pivotal role in metal transport. Labile fractions were found to be trapped by the solid materials for retention and their high concentration in ARR is a major concern from the pollution point of view through infiltration facilities. Results obtained in this study predict the risk associated with the release of retained heavy metal under changing environmental conditions in AIFs.

Quantification of uncertainty of experimental measurement in leaching test on cement-based materials

When mineral wastes are reused in construction materials, a current practice is to evaluate their environmental impact using standard leaching test. However, due to the uncertainty of the measurement, it is usually quite difficult to estimate the pollutant potential compared to other materials or threshold limits. The aim of this paper is to give a quantitative evaluation of the uncertainty of leachate concentrations of cement-based materials, as a function of the number of test performed. The relative standard deviations and relative confidence intervals are determined using experimental data in order to give a global evaluation of the uncertainty of leachate concentrations (determination of total relative standard deviation). Various combinations were realized in order to point out the origin of large dispersion of the results (determination of relative standard deviation linked to analytical measured and to leaching procedure), generalisation was suggested and the results were compared to literature. An actual example was given about the introduction of residue (meat and bone meal bottom ash--MBM-BA) in mortar, leaching tests were carried out on various samples with and without residue MBM-BA. In conclusion large dispersion were observed and mainly due to heterogeneity of materials. So heightened attention needed to analyse leaching result on cement-based materials and further more other tests (e.g. ecotoxicology) should be performed to evaluate the environmental effect of these materials.

Minerals as additives for decreasing the toxicity of Mediterranean contaminated dredged sediments

The management of dredged sediments is a priority issue in the Mediterranean sea where sediments are historically polluted. The aims of this study were to evaluate the toxicity of port sediment samples and the effect of three mineral additives (hematite, zerovalent iron (ZVI) and natural zeolite (NZ)) on sediment elutriate toxicity. Four sediments (A, B, C and D) were provided by port authorities after composting procedure; particle size, particulate organic carbon, metals and organic pollutants (TBT, PAHs, PCBs) were determined in whole sediments. Elutriates from these composted sediments were analyzed by determining toxicity level using oyster (Crassostrea gigas) larvae bioassay, metal and dissolved organic carbon concentrations. Toxicity, measured on undiluted elutriates (250 g/L), decreased as follows: A≥B>C∼D. The treatment of sediments with mineral additives (5%) revealed that hematite tends to decrease the elutriate toxicity in all samples, particularly in samples B and C. This effect may be related to metal concentration decrease in elutriates, in particular Cu and Zn, that have a significant toxic effect on oyster larvae. ZVI and NZ have a variable influence on elutriate toxicity. Results suggest that hematite may be a possible candidate for decreasing chemical concentration and improving the quality of elutriates. Hematite could be used for sediment stabilization prior to the deposit in a specific site or landfill.

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.

Colloid-facilitated transport of lead in natural discrete fractures

Colloid-facilitated transport of lead (Pb) was explored in a natural chalk fracture with an average equivalent hydraulic aperture of 139 microm. Tracer solution was prepared by adding montmorillonite (100 mg L(-1)) and/or humic acid (HA) (10 mg L(-1)) to modified artificial rainwater containing dissolved Pb (21.4 mg Pb L(-1)), naturally precipitated PbCO(3) particles (16.4 mg Pb L(-1)) and LiBr (39.0 mg L(-1)). We found that Pb is only mobile when associated with colloids. PbCO(3) particles were not mobile in the fracture. The addition of HA to the montmorillonite suspension increased the suspension's mobility and therefore promoted the colloid-facilitated transport of Pb. The increases in pH and sodium absorption ratio induced by the chalk-tracer solution interactions appeared to increase the dispersion and mobilization of colloids entering the fracture. The dominant colloid-facilitated transport of Pb reported in this study has significant implications for risk assessment of Pb mobility in fractured rocks.

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.

The leachability, bioaccessibility, and speciation of Cu in the sediment of channel catfish ponds

There have been growing concerns about the environmental impact of Cu applied in the catfish pond aquaculture. In this paper, sediments taken from three commercial catfish ponds were studied for content, leachability, bioaccessibility, and speciation of sediment-bound Cu. Results showed that the Cu was concentrated in the top 10 cm of the sediments and the peak Cu concentrations ranged from the background level to about 200 mg/kg. Toxicity characteristic leaching procedure showed only 1-8% of sediment Cu was leachable while bioaccessible Cu, evaluated by physiological based extraction test, accounted for up to 40-85% of total Cu. Due to the high redox potential in the surface sediments, acid-volatile sulfide was not a significant binding phase. The sequential extraction results showed that the residual phase (forms in lattices of primary and secondary minerals) was the major Cu fraction in the first two pond sediments but carbonate-bound, Fe/Mn oxide-bound and organically bound Cu, as well as the residual fraction, seemed equally important in the third pond.

Physicochemical and biological characterisation of different dredged sediment deposit sites in France

The aim of this work is to determine sediment properties, metal contents and transfers of Cd and Zn from dredged sediments to plants. To this end 10 deposit sites with different contexts were visited in France. The main agronomic characteristics and metal contents for surface soil layers were measured, the plant species present at the sites, such as Brassicaceae and Fabaceae, were listed, and the distribution of their root systems described. Soil characteristics such as available P (Olsen) varied between sites, with values ranging from 0.01 to 0.49 g kg(-1). Total contents and enrichment factors were studied, highlighting metal contamination in most of the sites. Despite carrying out principal component analyses, it was not possible to group deposits by age or geographical localisation. However, deposits could be distinguished as a function of proximity of industrial facilities, sediment grain size and carbonate content. Associations between metals were also highlighted: (1) Cd, Pb and Zn, and (2) Al, Cr, Cu and Fe. Consequently, we propose classifying them as technogenic anthrosols.

Evaluation of the phytotoxicity of contaminated sediments deposited "on soil": II. Impact of water draining from deposits on the development and physiological status of neighbouring plants at growth stage

As part of a study of the phytotoxic risk of spreading contaminated sediments "on soil", a laboratory experiment was carried out to assess the impact of water draining from sediments on peripheral vegetation. Drainage water was obtained in the laboratory by settling three sediments with different pollutants levels, and the supernatant solutions (respectively A1, B1, C1 drainage waters) were used as soaking water for maize (Zea maïs L.) and ryegrass (Lolium perenne L.). The physicochemical characteristics of the supernatant water, particularly metal contents, showed a pattern of contamination, with C1>A1>B1. The plants tested were grown on soil for 21 days, before being soaked for another 21-day period with drainage water (treatments) and distilled water (control). Biomass parameters (fresh weight, length, etc.), enzymatic activity [glutamine synthetase (GS), phosphoenolpyruvate carboxylase (PEPc)] and Zn, Cu, Cd and Cr contents were measured on both the shoots and roots of each plant. Biomass parameters were stimulated by C1, not affected by A1 and decreased with B1 for maize, whereas they increased for ryegrass in all the treatments. Compared to the control, GS activity was stimulated by C1 in the shoots of both plants and inhibited by treatments B1 and C1 in maize roots. PEPc activity in ryegrass was 1.5-5 times higher with contaminated water treatment, while contrasting effects were observed in maize plants. Both plants showed greater accumulation of chromium and zinc than cadmium and copper. Treatment A1 was found to be less active on plant growth and have a lower impact on the physiological status (enzymatic activities) of both plants. Treatment C1 stimulated the growth and physiological status of the plants, especially in shoots, with higher metal accumulation values in both plants. Treatment B1 was found to show more variable effects on growth indices, enzymatic activity and metal accumulation according to plant species.

Characterization of trace metal leachability from highway construction solid waste using the toxicity characteristic leaching procedure

The potential risk of soil, surface and ground water contamination by trace metals leached from highway construction solid wastes is a major environmental concern. The objectives of this study were to evaluate trace metal leachability, toxicity of the leachate to aquatic organism and environmental impacts of highway construction solid waste (HCSW). Potential leachability, defined as the maximum metal pool that may become available for leaching at a constant pH 4 decreased in the following order: Zn > Ni > As > Pb > Cu. Potential metal leachability was controlled mainly by solid phase distribution of metals in wastes. The kinetics of metal release were determined using the cascade leaching test (CLT) with water at pH 4. With the exception of zinc in the first fraction, metal concentrations in all consecutive leaching fractions remained below the maximum permissible level in water for human consumption. The result of toxicity test showed that the leachates were in concentrations that did not exceed toxic concentrations for aquatic toxicity tests. The results indicate low risk of surface and ground water contamination resulting from highway construction solid wastes.

Metal transfer to plants grown on a dredged sediment: use of radioactive isotope 203Hg and titanium

Improperly disposed of dredged sediments contaminated with metals may induce long-term leaching and an increase of metal concentrations in ground waters and vegetal cover plants. The objective of the study was to quantify the sediment-to-plant transfer of Cu, Pb, Hg and Zn with a particular focus on the pathway of Hg and to determine whether the establishment of vegetal cover modifies the metal availability. A pot experiment with rape (Brassica napus), cabbage (Brassica oleraccea) and red fescue (Festuca rubra) was set up using a sediment first spiked with the radioisotope 203Hg. Zinc concentrations (197-543 mg kg(-1) DM) in leaves were higher than Cu concentration (197-543 mg kg(-1) DM), Pb concentration (2.3-2.6 mg kg(-1) DM) and Hg concentration (0.9-1.7 mg kg(-1) DM). Leaves-to-sediment ratios decreased as follows: Zn > Cu > Hg > Pb. According to Ti measurements, metal contamination by dry deposition was less than 1%. Mercury concentration in plant leaves was higher than European and French thresholds. Foliar absorption of volatile Hg was a major pathway for Hg contamination with a root absorption of Hg higher in rape than in cabbage and red fescue. Growth of each species increased Cu solubility. Zinc solubility was increased only in the presence of rape. The highest increase of Cu solubility was observed for red fescue whereas this species largely decreased Zn solubility. Dissolved organic carbon (DOC) measurements suggested that Cu solubilisation could result from organic matter or release of natural plant exudates. Dissolved inorganic carbon (DIC) measures suggested that the high Zn solubility in the presence of rape could originate from a generation of acidity in rape rhizosphere and a subsequent dissolution of calcium carbonates. Consequently, emission of volatile Hg from contaminated dredged sediments and also the potential increase of metal solubility by a vegetal cover of grass when used in phytostabilisation must be taken into account by decision makers.

Assessment of acid neutralizing capacity and potential mobilisation of trace metals from land-disposed dredged sediments

In the past decades, large amounts of contaminated sediments have been dredged and spread out along rivers. The understanding of the fate and the behaviour of contaminants in dredged sediments is essential to deal with the management of contaminated sediments. Heavy metal leaching behaviour in dredged sediments of different ages since disposal, varying from 10 to 70 years, was assessed by pH(stat) leaching test and potentiometric titrations. Based on the pH(stat) titration results, two operationally defined pools of heavy metals, i.e., a labile and a slowly labile pool of heavy metals, were distinguished and quantified by mathematical description of leaching behaviour. The mathematical description of leaching behaviour during pH(stat) and potentiometric titrations allowed deduction of four groups of elements with a different type of leaching behaviour. Considering heavy metals, Zn, Cd and Ni displayed a leaching behaviour similar to acid neutralizing capacity (ANC), whereas the release of Cu, Pb and Cr was slower than ANC. However, no significant differences with regard to heavy metal leaching behaviour were found between sediments with different time since disposal on land. The elevated acid neutralizing capacity of the dredged sediments analysed in this study strongly decrease the risk of soil acidification with associated heavy metal leaching. The prediction of soil acidification over an extended period of time, together with the potential release of trace metals, was considered an important advantage of pH(stat) leaching tests compared to conventional leaching procedures.

Effects of bacterial activities on the release of heavy metals from contaminated dredged sediments

The potential impact of indigenous bacterial processes on the release of heavy metals from dredged sediment deposits was investigated. Batch re-suspension experiments were conducted in order to investigate the release of Zn, Cd, Cu and Pb from a polluted anoxic sediment submitted to oxidative perturbations. The concentrations of heavy metals, sulphate and dissolved organic carbon (DOC) were periodically recorded, and cell counts were performed to follow the evolution of several bacterial species. The specific effects of microbial processes were quantified by performing re-suspension assays on sterilised samples. Moreover, the effect of an initial acidification of the system was studied. The results showed that metal release was mainly due to oxidation of sulphide minerals contained in the sediment. Sulphur-oxidising bacteria such as Acidithiobacillus thiooxidans were identified to play a major role in the process, by enhancing the oxidation kinetic. However, the acid production resulting from these reactions was almost totally buffered by the dissolution of the calcite present in the sediment. Copper was released to a lesser extent, and a strong association with organic matter was observed. Lead was not observed in solution, because of its low solubility at neutral conditions and of its re-adsorption on the solid phase. The initial acidification of the system resulted in an faster growth of the acidophilic A. thiooxidans. A subsequent pH drop originating from microbial processes was then observed during the first stages of the experiment. As a consequence, drastic increases in metal (Zn, Cd) release were observed.

Phosphorus and copper leaching from dredged sediment applied on a sandy loam soil: column study

In this paper, downward movement of phosphorus and copper as dredged sediment applied on sandy loam soil was studied by column leaching experiments. Three sediment application rate, (i.e., 1, 2 and 5-cm depth of sediments) were applied to the top of the soil columns. Two and a half months leaching experiments were conducted, which include a 15-day un-watered period. Concentrations of phosphorus and copper in the leachate and the vertical distribution of Olsen-P and diethylenetriaminepentaacetic acid (DTPA) extractable Cu in the soil columns were determined. The results showed that, un-watered period could increase the downward movements of phosphorus and copper. Sediment application significantly increased Olsen-P concentration in the top 15 cm of the soil columns, but has not significantly affected that in the deeper soil layer. The 1-cm depth sediment treatment did not increase the DTPA extractable Cu concentration in the whole soil column. The 5-cm depth sediment treatment, however, significant increased the DTPA extractable Cu in the deeper soil layers. This study suggested that the application of dredged sediment laden with P and Cu on sandy loam soil might cause the significant downward movement of phosphorus and copper.

Evaluation of the phytotoxicity of contaminated sediments deposited "on soil". I. Impact of water draining from the deposit on the germination of neighbouring plants

As part of a study of the phytotoxic risk of spreading of contaminated sediments "on soil", we carried out a laboratory experiment assessing the impact of water draining from sediments in a deposit scenario on the peripheral vegetation. The plant tested were the Chinese cabbage (Brassica campestris L. var. chinensis), maize (Zea mays L.) and ryegrass (Lolium perenne L.). The draining water samples (A1, B1 and C1) were obtained after decantation in laboratory of the sediments. The classification of the water sampled in decreasing order of cumulative contamination was C1 > A1 > B1. The B1 and C1 water samples inhibited the germination of seeds tested to various extents. The percentage of seeds that did not germinate was 1.3-fold times higher for Chinese cabbage with B1 than for the control and 2.3-fold times higher for ryegrass with C1 than for the control. Seeds watered with B1 had lower moisture contents than the control: 10% lower for ryegrass and maize and 50% lower for Chinese cabbage. An increase (about 1.5 microg/ml) in total soluble protein (versus the control) was observed for all three plant species tested in the presence of C1. Glutamine synthetase activity was significantly (1.35 times) higher in ryegrass seeds in the presence of C1 than in the control. We also observed changes in the specific activity of phosphoenolpyruvate carboxylase, which increased for ryegrass and decreased for maize as the concentration of contaminants in the water increased. The results show (i) the necessity to use different plant species to evaluate the toxic effect of sediment deposited on soil on the vegetation, and (ii) that soon as on germination an evaluation of an impact is possible.

Temporal-spatial trends in heavy metal contents in sediment-derived soils along the Sea Scheldt river (Belgium)

The aim of this study was to survey the alluvial plains of the Sea Scheldt river in Belgium for the presence of old sediment-derived soils, and to appraise the heavy metal contamination at these sites. Historically, sediments of periodical dredging operations have been disposed in the alluvial plain without concern for the potential presence of contaminants. Up to 96% of the areas that were affected by sediment disposal (ca. 120 ha) was found to be polluted by at least one of the metals Cd, Cr, Zn or Pb. Concentrations of Cd, Cr and Zn were, in 10% of the cases, higher than 14, 1400 and 2200 mg/kg DM, respectively. Based on the Flemish decree on soil sanitation, Cu and Ni concentrations were of less environmental concern on any site. The pollution in the Sea Scheldt alluvial plain nevertheless is lower than for the Upper Scheldt alluvial plain. The sediment-derived soils in the most upstream part near Ghent were used for disposal of sediments from dredging operations elsewhere. Metal concentrations were explored and both spatial and temporal trend were analysed. The pollution levels encountered warrant caution as most of the soils affected by historical dredged sediment disposal are currently in use for pasture.

Mobility and toxicity of metals in sandy sediments deposited on land

A times series of laboratory experiments were conducted to investigate the effect of land deposition of contaminated sediments on the bioavailability and mobility of metals. Four sandy sediments were sampled at sites expected to have elevated levels of cadmium and zinc. The physical and chemical characteristics and ecotoxicity of sediments, pore waters, and leachates were evaluated after periods ranging from 1 to 45 days of land deposition. Cd and Zn retardation and leaching potential were calculated and this simulation gave good predictions of subsequently observed Cd and Zn mobility. The mobility and leaching of Cd and Zn in the sediments increased with decreasing pH and with decreasing content of organic matter. During the deposition an increase in sediment toxicity to plants and an increase in eluate toxicity to invertebrates were observed. A high rate of water flow through the sediment resulted in a lower toxicity enhancement of the sediments and a higher toxicity enhancement of the eluates. This result suggests that water flow through the sediment reduces the actual toxicity of the upper layer of deposited sediment but at the same time intensifies the risk of groundwater contamination.

Solubility of metals in an anoxic sediment during prolonged aeration

This work was conducted to study the evolution of the solubility of selected metals during the aeration of an anoxic sediment. Batch experiments were carried out for 76 days with a metal-polluted dredged sediment. The pH, Eh and concentration of Al, Cu, Fe, Hg, Pb and Zn were periodically recorded. Results showed that during the early stages of aeration, the solubility of metals increased rapidly but was then followed by fast re-adsorption. As a consequence, after 14 days most of the metals excepted Cu and Zn were present at low or undetectable concentrations in solution. Re-adsorption of Zn was observed to be much slower during the first two weeks, whereas solubilisation of Cu increased gradually during months after land disposal. According to speciation calculations, Cu solubilisation was in part due to complexation in solution by carbonates. In the case of Hg, although complexation by dissolved organic matter (DOM) could be expected, re-adsorption was the dominant process. However, more knowledge about the behaviour of the DOM present in anoxic sediments is needed in order to make more quantitative statements about the mobility of heavy metals contained in dredged material.

Cadmium and zinc uptake by volunteer willow species and elder rooting in polluted dredged sediment disposal sites

Salix species and Sambucus nigra L. (elder) naturally invade dredged sediment landfills and are commonly encountered on substrates contaminated with heavy metals. Foliar concentrations of Cd and Zn in four Salix species and elder were explored in the field. Metal contents in dredged sediment derived soils were elevated compared to baseline concentration levels reported for Flanders. To evaluate foliar concentrations, reference data were compiled from observations in nurseries, young plantations and unpolluted sites with volunteer willow vegetation. Willows grown on polluted dredged sediment landfills showed elevated foliar Cd and Zn concentrations (>6.6 mg Cd/kg DW and >700 mg Zn/kg DW). This was not the case for elder. For willow, a significant relation was found between soil total Zn or Cd and foliar Zn or Cd, regardless of age, species, or clone. Willows proved to be useful bioindicators. Results indicated a possible threat in long-term habitat development of willow brushwood from transfer of Cd and Zn to the food web.

Phytotoxicity of dredged sediment from urban canal as land application

Phytotoxicity of dredged sediment from Hangzhou section of the Grand Canal as land application was evaluated by pakchoi (Brassica chinensis L.) germination tests and pot experiments. Germination rates of pakchoi in the dredged sediment and in sediment-applied soils were both significantly higher than that in the soil controls, while the germination rate between the sediment-applied soils was no significant difference. In pot experiments, plant height and biomass were increased by the dredged sediment application rate in the rate of lower than 540 t ha(-1), but decreased when the application rate was over this rate. Concentrations of Zn and Cu in pakchoi were linearly increased with the increasing of the application rate of the dredged sediment. Both plant height and biomass of pakchoi in sediment-treated red soil were higher than that in sediment-treated paddy soil, regardless the application rate. The results suggest that plant biomass of pakchoi may be used as an indicator of the phytotoxicity of the dredged sediment. It also showed that red soil is more suitable to accept the dredged sediment than paddy soil, and 270 t ha(-1) is a safe application rate both in red soil and paddy soil.

Towards the characterisation of heavy metals in dredged canal sediments and an appreciation of 'availability': two examples from the UK

Canal sediments can act as sinks for a wide range of contaminants including heavy metals from various sources (e.g. industrial and waste water discharges). Dredging of canals is required to maintain navigational depth and prevent flooding. The sediments removed from canals are often disposed of to land, being deposited either straight on to the banks of the canal or, in recent years, in licensed disposal sites. The aim of this work was to investigate the nature of dredged sediment-derived soils and the heavy metals present in them. Two disposal sites in the United Kingdom (UK) were investigated and soil samples taken. A variety of analytical techniques were used, including Aqua regia digestion and sequential extraction, in order to assess the concentrations and associations of metals present. Diethylene triaminepenta-acetic acid extracts, performed to illustrate plant-available metal concentrations, reveal that up to 40% of the total extracted metals were in an 'available' form. Variations in metal concentrations with depth in the soil cores show a significant correlation with total organic carbon content.