Cd Species Research Articles

Synthesis and characterization of Mn2+ doped CdOZn3(PO4)2 nanocomposites

Undoped and Mn2+ doped CdOZn3(PO4)2 nanocomposites are fruitfully synthesized by chemical precipitation process at room temperature. The morphology, structure and spectroscopic properties of the prepared samples are characterized by X-ray diffraction (XRD), optical absorption, Scanning electron microscope (SEM) with EDS, Fourier transform infrared (FT-IR) spectroscopy, Photolumiscence (PL) and Electron Paramagnetic Resonance (EPR). XRD data confirms the cubic phase of CdO and monoclinic phase of Zn3(PO4)2. Strain and dislocation density are also calculated from XRD studies. Optical absorption spectrum of Mn2+ doped CdOZn3(PO4)2 nanocomposite shows different spin-forbidden DMSO-d6 bands which are the characteristics of octahedral site symmetry related to Mn2+. The crystal field parameter Dq and inter electronic repulsion parameters (B and C) are deliberate using optical absorption data. Surface morphology of sample is firm by Using Scanning electron microscopy (SEM) and the distribution of Zn, Cd, phosphate and oxygen species in the prepared sample is identified by EDS. PL studies recognize the white light emission. The ‘g’ value of the nanocomposite material synthesized is lesser than (negative shift) the free electron value (2.0023), which gives ionic nature to the bonding and confirms the presence of Mn2+ in distorted octahedral site symmetry.

Solid phase speciation of Zn and Cd in zinc smelter effluent-irrigated soils

Solubility of metal in contaminated soils is a key factor which controls the phytoavailability and toxic effects of metals on soil environment. The chemical equilibria of metal ions between soil solution and solid phases govern the solubility of metals in soil. Hence, an attempt was made to identify the probable solid phases (minerals), which govern the solubility of Zn2+ and Cd2+ in zinc smelter effluent-irrigated soils. Estimation of free ion activities of Zn2+ (pZn2+) and Cd2+ (pCd2+) by Baker soil test indicated that metal ion activities were higher in smelter effluent-irrigated soils as compared to that in tubewell water-irrigated soils. Identification of solid phases further reveals that free ion activity of Zn2+ and Cd2+ in soil highly contaminated with Zn and Cd due to long-term irrigation with zinc smelter effluent is limited by the solubility of willemite (Zn2SiO4) in equilibrium with quartz and octavite (CdCO3), respectively. However, in case of tubewell water-irrigated soil, franklinite (ZnFe2O4) in equilibrium with soil-Fe and exchangeable Cd are likely to govern the activity of Zn2+ and Cd2+ in soil solution, respectively. Formation of highly soluble minerals namely, willemite and octavite indicates the potential ecological risk of Zn and Cd, respectively in smelter effluent irrigated soil.

Synchrotron-based X-Ray Spectroscopy Studies for Redox-based Remediation of Lead, Zinc, and Cadmium in Mine Waste Materials.

Several studies have examined the effect of submergence on the mobility of metals present in mine waste materials. This study examines the effect of organic carbon (OC) and sulfur (S) additions and submergence time on redox-induced biogeochemical transformations of lead (Pb), zinc (Zn), and cadmium (Cd) present in mine waste materials collected from the Tri-State mining district located in southeastern Kansas, southwestern Missouri, and northeastern Oklahoma. A completely randomized design, with a two-way treatment structure, was used for conducting a series of column experiments. Two replicates were used for each treatment combination. Effluent samples were collected at several time points, and soil samples were collected at the end of each column experiment. Because these samples are highly heterogeneous, we used a variety of synchrotron-based techniques to identify Pb, Zn, and Cd speciation at both micro- and bulk-scale. Spectroscopic analysis results from the study revealed that the addition of OC, with and without S, promoted metal-sulfide formation, whereas metal carbonates dominated in the nonamended flooded materials and in mine waste materials only amended with S. Therefore, the synergistic effect of OC and S may be more promising for managing mine waste materials disposed of in flooded subsidence mine pits instead of individual S or OC treatments. The mechanistic understanding gained in this study is also relevant for remediation of waste materials using natural or constructed wetland systems.

Speciation and mobility of volatile heavy metals (Cd, Pb, and Tl) in fly ashes.

Speciation of volatile metals Cd, Pb, and Tl in fly ashes (FAs) produced from burning of hard coal in stocker-fired boilers (SFBs) was studied. Two grain fractions of fly ash collected in a multicyclone and battery cyclone of the systems of dust separation from three SFB units operating in various urban heating plants were analyzed. The characteristic feature of speciation of the three metals was a large share of labile fractions: Cd (av. 46.1%), Pb (av. 39.8%), and Tl (av. 21.6%). The fraction which most clearly reflected the different chemical properties of the investigated metals was the oxidizable fraction: F(4)-Cd-0%, F(4)-Pb-av. 10.0%, and F(4)-Tl-av. 30.2%. The importance of condensation of the volatile metal species on FA particles for shaping speciation of these metals was characterized using the normalized enrichment factor (NEF): Pb (2.3±0.8)>Tl (1.8±0.9)≈Cd (1.7±0.6). Speciation of heavy metals may also be important economically, because the level of mobility coefficients (K Cd=0.46, K Pb=0.40, and K Tl=0.22) in the case of fly ashes considerably enriched with toxic metals (Cd 4.8±3.4mg/kg, Pb 293±210mg/kg, and Tl 6.3±4.5mg/kg) may limit their utilization range.

Effects of biochars derived from chicken manure and rape straw on speciation and phytoavailability of Cd to maize in artificially contaminated loess soil

While biochar can reduce the bioavailability of heavy metals in acidic soils and reduce their risk of entering the food chain, conditions for alkaline soils such as loess soils with high pH values, high carbonate content and low organic matter content remain unclear. Pot experiments were conducted to assess the effects of four rates (1%, 5%, 10%, and 15% w/w) of biochars prepared at 600 °C from chicken manure and rape straw (CBC and RBC) on soil properties, Cd speciation and phytoavailability, and plant growth in Cd contaminated (20 mg kg−1) light sierozem using maize (Zea mays L.) as an indicator plant. Biochar additions significantly (P < 0.05) increased soil pH values, cation exchange capacity (CEC) and soil organic matter (OM). The results showed that Cd speciation turned somewhat into stable state as biochar application increased. When CBC and RBC was applied at the rate of 15%, the content of acid-extractable Cd decreased only by 16.3% and 11.64%, respectively. The uptake of Cd by maize shoots scarcely decreased with CBC and RBC amendment at the rate of 1% and 5%, respectively. Although it seemed that additions of more than 5% CBC or RBC significantly (P < 0.05) reduced Cd contents in maize shoots, maize growth was largely inhibited due to the high value of soil pH. These results could provide different implications for immobilization remediation of loess soils (e.g., light sierozem) contaminated with Cd.

Adsorption-solubility equilibria and speciation of Pb, Cd, and Zn in a savanna soil

Emission of contaminants into the environment is a common feature of industrialization. The danger of these substances has generated interest in their solubility and speciation. In this study, the nature of adsorption, solubility and speciation of Pb, Cd and Zn under high levels of accumulation were evaluated through a model sorption-solubility experiment. Equilibrium concentrations of metals and other soil characteristics were used as input parameters in a metal speciation model. The solubility controls of metal activities were evaluated through the use of activity/stability diagrams. The solubility of Pb, Cd and Zn was enhanced between pH 4.8 and 6.6. Increased solubility at this pH range indicates higher bioavailability and toxicity. Speciation results showed a predominance of free metal species in the soil solution. The activity diagrams used to test the stability of possible mineral bearing compounds in the soils did not show any particular solid phase as possible control of the activities of Pb, Cd and Zn in solution. This suggests that solubility controls might be due to the formation of metastable minerals which are not well crystallized.

Bioremediation of agricultural solid waste leachates with diverse species of Cu (II) and Cd (II) by periphyton

The aim of this work was to study the bioremediation of agricultural solid waste leachates with high-concentrations of Cu (II) and Cd (II) after washing the wastes with water and Na2EDTA solution (0.2M). Results indicate that Cu (II) and Cd (II) are mainly comprised of Cu2(OH)22+, Cu3(OH)42+, CuOH+, Cu(H2O)4(OH)2, Cd2+ and CdOH+ in the water-washed leachates and Cu(EDTA)2−, Cu(HEDTA)−, Cd(EDTA)2− and Cd(HEDTA)− in the Na2EDTA-washed leachates. Cu (II) removal efficiency by selected native periphyton from the water- and Na2EDTA-washed leachates were 80.5% and 68.4% respectively, and for Cd (II) it was 57.1% and 64.6%, because the periphyton was able to maintain a stable pH of the leachates and regulate its microbial composition and carbon metabolic capability to acclimate the chemical conditions of the leachates. This study provides a new biomeasure to treat leachates with high-concentration Cu2+ and Cd2+, and contribute valuable insights into the relationships between periphyton characteristics and heavy metals.

Leaching Behavior of Circulating Fluidised Bed MSWI Air Pollution Control Residue in Washing Process

In this study, air pollution control (APC) residue is conducted with water washing process to reduce its chloride content. A novel electrical conductivily (EC) measurement method is proposed to monitor the dynamic change of chloride concentrations in leachate as well as the chloride content of the residue. The method equally applies to various washing processes with different washing time, liquid/solid ratio and washing frequency. The results show that washing effectively extracts chloride salts from APC residues, including those from circulating fluidized bed (CFB) municipal solid waste incineration (MSWI). The most appropriate liquid/solid ratio and washing time in the first washing are found to be around 4 L water per kg of APC residue and 30 min, respectively, and washing twice is required to obtain maximum dissolution. The pH value is the major controlling factor of the heavy metals speciation in leachate, while chloride concentration also affects the speciation of Cd. Water washing causes no perceptible transfer of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from the APC residue to leachate. The chloride concentration is strongly related with electrical conductivity (EC), as well as with the concentrations of calcium, sodium and potassium of washing water. Their regression analyses specify that soluble chloride salts and EC could act as an indirect indicator to monitor the change of chloride concentration and remaining chloride content, thus, contributing to the selection of the optimal washing conditions.

In Situ Field-Scale Remediation of Low Cd-Contaminated Paddy Soil Using Soil Amendments

At present, the remediation of heavy-metal-polluted cropland soil is a considerable problem. In this study, in situ immobilization field experiment was conducted by planting rice (Oryza sativa L.) in low Cd-contaminated paddy soil to determine the optimal soil amendment that would reduce the accumulation of Cd in brown rice. GL (main component is alkaline residue), FG (main components are Si and Ca), and SH (main component is lime) were utilized as amendments. The remediation effects of the amendments on the soil and rice were investigated, and the potential mechanisms of reducing Cd availability to rice were analyzed. Amendment application significantly increased the soil pH value, reduced the DTPA-extractable Cd concentrations, and shifted Cd species from the exchangeable Cd fractions to the carbonate-bound, Fe-Mn oxides and residual fractions in paddy soil. For the plant, amendment application apparently increased the concentrations of Ca in rice plants, which could compete with Cd in root uptake. Besides, amendment application also effectively restricted the translocation of Cd from roots to shoots and consequently led to a notable decrease of Cd concentration in brown rice. These results demonstrated that the FG ameliorant could be effective in reducing Cd bioavailability and accumulation in rice grown on low Cd-contaminated paddy soils.

Arsenic, Lead, and Cadmium in U.S. Mushrooms and Substrate in Relation to Dietary Exposure.

Wild mushrooms can absorb high quantities of metal(loid)s, yet the concentration, speciation, and localization of As, Pb, and Cd in cultivated mushrooms, particularly in the United States, are unresolved. We collected 40 samples of 12 types of raw mushrooms from 2 geographic locations that produce the majority of marketable U.S. mushrooms and analyzed the total As, Pb, and Cd content, the speciation and localization of As in select samples, and assessed the metal sources and substrate-to-fruit transfer at one representative farm. Cremini mushrooms contained significantly higher total As concentrations than Shiitake and localized the As differently; while As in Cremini was distributed throughout the fruiting body, it was localized to the hymenophore region in Shiitake. Cd was significantly higher in Royal Trumpet than in White Button, Cremini, and Portobello, while no difference was observed in Pb levels among the mushrooms. Concentrations of As, Pb, and Cd were less than 1 μg g(-1) d.w. in all mushroom samples, and the overall risk of As, Cd, and Pb intake from mushroom consumption is low in the U.S. However, higher percentages of tolerable intake levels are observed when calculating risk based on single serving-sizes or when substrate contains elevated levels of metal(loid)s.

Determination of the Side-Reaction Coefficient of Desferrioxamine B in Trace-Metal-Free Seawater

Electrochemical techniques like adsorptive cathodic stripping voltammetry with competitive ligand equilibration (ACSV-CLE) can determine total concentrations of marine organic ligands and their conditional binding constants for specific metals, but cannot identify them. Individual organic ligands, isolated from microbial cultures or biosynthesized through genomics, can be structurally characterized via NMR and tandem MS analysis, but this is tedious and time-consuming. A complementary approach is to compare known properties of natural ligands, particularly their conditional binding constants, with those of model organic ligands, measured under suitable conditions. Such comparisons cannot be meaningfully interpreted unless the side-reaction coefficient (SRC) of the model ligand in seawater is thoroughly evaluated. We conducted series of potentiometric titrations, in non-coordinating medium at seawater ionic strength (0.7 M NaClO4) over a range of metal:ligand molar ratios, to study complexation of the siderophore desferrioxamine B (DFOB) with Mg and Ca, for which it has the highest affinity among the major seasalt cations. From similar titrations of acetohydroxamic acid in the absence and presence of methanesulfonate (mesylate), it was determined that Mg and Ca binding to this common DFOB counter-ion is not strong enough to interfere with the DFOB titrations. Stability constants were measured for all DFOB complexes with Mg and Ca including, for the first time, the bidentate complexes. No evidence was found for Mg and Ca coordination with the DFOB terminal amine. From the improved DFOB speciation, we calculated five SRCs for each of the five (de)protonated forms of DFOB in trace-metal-free seawater, yet we also present a more convenient definition of a single SRC that allows adjustment of all DFOB stability constants to seawater conditions, no matter which of these forms is selected as the 'component' (reference species). An example of Cd speciation in seawater containing DFOB illustrates the non-trivial use of different SRCs for polyprotic, polydentate organic ligands.

Effects of Amendments and Water Conditions on the Chemical Speciation of Cd and Pb in Contaminated Paddy Soil in a Mining Area

ABSTRACTLime, sepiolite and peat were used as amendments to study their effects on the chemical speciation of Cd and Pb in a polluted paddy soil under moist and water-logged conditions. The results showed that application of amendments reduced acid-extractable Cd (up to 12.56% and 5.07%) and Pb (up to 6.69% and 2.68%) in moist and water-logged soils, respectively. The reducible Cd and residual Cd was increased while the oxidizable Cd was decreased after adding amendments. However, the oxidizable Pb was increased while the residual Pb was decreased in amended soil, and the reducible Pb presented a different changing trend under moist and water-logged conditions. The soil pH was significantly correlated with different fractions of Cd and Pb in soil, which indicated a distinct influence of soil pH on the redistribution process of Cd and Pb fractions in amended soil.

Time‐Course Changes in Speciation and Solubility of Cadmium in Reduced and Oxidized Paddy Soils

Core Ideas The formation and dissolution of CdS were investigated in low‐ and high‐S soils. CdS reached 90% in the high‐S soil but did not exceed 35% in the low‐S soil. Cd dissolved rapidly at soil Eh &gt;435 mV (low‐S soil) and &gt;529 mV (high‐S soil). A prolonged flooding during rice (Oryza sativa L.) cultivation is recommended to immobilize soil Cd by the formation of CdS under anoxic conditions. The problem with this recommended practice is that it reduces rice yields compared with intermittent flooding, which is less effective at immobilizing CdS through reductive precipitation. This study investigated the solubility of Cd in relation to chemical speciation of Cd and S and soil redox potential through a time series of measurements during a 29‐d reduction period followed by a 20‐d oxidation period using X‐ray absorption fine structure spectroscopy and chemical extraction. Two Aquent soils with different S levels in water‐ and oxalate‐extractable fractions (hereafter low‐S and high‐S soils) were used to investigate the formation and dissolution of CdS. In the reducing period, the CdS proportion in the high‐S soil rapidly increased to 30% at Day 4 and reached 90% at Day 29, whereas CdS in the low‐S soil did not exceed 35%. In the following oxidizing period, CdS in the soils underwent oxidative dissolution but was not completely dissolved, remaining at &lt;20% of the total Cd. A two‐piece segmented linear relationship existed between HCl‐extractable Cd and soil Eh, and the regression slope was 28‐fold greater for the low‐S soil at Eh &gt;435 mV and 38‐fold greater for the high‐S soil at Eh &gt;529 mV. This result indicates that each soil has a threshold Eh value at which the solubility of Cd is drastically altered during the course of soil reduction and oxidation processes.

Alleviation of environmental risks associated with severely contaminated mine tailings using amendments: Modeling of trace element speciation, solubility, and plant accumulation.

Tailings are considered one of the most relevant sources of contamination associated with mining activities. Phytostabilization of mine spoils may need the application of the adequate combination of amendments to facilitate plant establishment and reduce their environmental impact. Two pot experiments were set up to assess the capability of 2 inorganic materials (calcium carbonate and a red mud derivate, ViroBindTM ), alone or in combination with organic amendments, for the stabilization of highly acidic trace element-contaminated mine tailings using Atriplex halimus. The effects of the treatments on tailings and porewater physico-chemical properties and trace-element accumulation by the plants, as well as the processes governing trace elements speciation and solubility in soil solution and their bioavailability were modeled. The application of the amendments increased tailings pH and decreased (>99%) trace elements solubility in porewater, but also changed the speciation of soluble Cd, Cu, and Pb. All the treatments made A. halimus growth in the tailings possible; organic amendments increased plant biomass and nutritional status, and reduced trace-element accumulation in the plants. Tailings amendments modified trace-element speciation in porewater (favoring the formation of chlorides and/or organo-metallic forms) and their solubility and plant uptake, which were found to be mainly governed by tailing/porewater pH, electrical conductivity, and organic carbon content, as well as soluble/available trace-element concentrations. Environ Toxicol Chem 2016;35:2874-2884. © 2016 SETAC.

A loss-of-function allele of OsHMA3 associated with high cadmium accumulation in shoots and grain of Japonica rice cultivars.

Excessive cadmium (Cd) accumulation in rice poses a risk to food safety. OsHMA3 plays an important role in restricting Cd translocation from roots to shoots. A non-functional allele of OsHMA3 has been reported in some Indica rice cultivars with high Cd accumulation, but it is not known if OsHMA3 allelic variation is associated with Cd accumulation in Japonica cultivars. In this study, we identified a Japonica cultivar with consistently high Cd accumulation in shoots and grain in both field and greenhouse experiments. The cultivar possesses an OsHMA3 allele with a predicted amino acid mutation at the 380(th) position from Ser to Arg. The haplotype had no Cd transport activity when the gene was expressed in yeast, and the allele did not complement a known nonfunctional allele of OsHMA3 in F1 test. The allele is present only in temperate Japonica cultivars among diversity panels of 1483 rice cultivars. Different cultivars possessing this allele showed greatly increased root-to-shoot Cd translocation and a shift in root Cd speciation from Cd-S to Cd-O bonding determined by synchrotron X-ray absorption spectroscopy. Our study has identified a new loss-of-function allele of OsHMA3 in Japonica rice cultivars leading to high Cd accumulation in shoots and grain.

Thermodynamic Equilibrium Calculations on Cd Transformation during Sewage Sludge Incineration.

Thermodynamic equilibrium calculations were performed to reveal the distribution of cadmium during the sewage sludge incineration process. During sludge incineration in the presence of major minerals, such as SiO2, Al2O3 and CaO, the strongest effect was exerted by SiO2 on the Cd transformation compared with the effect of others. The stable solid product of CdSiO3 was formed easily with the reaction between Cd and SiO2, which can restrain the emissions of gaseous Cd pollutants. CdCl2 was formed more easily in the presence of chloride during incineration, thus, the volatilization of Cd was advanced by increasing chlorine content. At low temperatures, the volatilization of Cd was restrained due to the formation of the refractory solid metal sulfate. At high temperatures, the speciation of Cd was not affected by the presence of sulfur, but sulfur could affect the formation temperature of gaseous metals.

Sorption, desorption, and speciation of Cd, Ni, and Fe by four calcareous soils as affected by pH.

The sorption, desorption, and speciation of cadmium (Cd), nickel (Ni), and iron (Fe) in four calcareous soils were investigated at the pH range of 2-9. The results indicated that sorption of Fe by four soils was higher than 80% at pH2, while in the case of Cd and Ni was less than 30%. The most common sequence of metal sorption at pH2-9 for four soils was in the order of Fe≫Ni>Cd. Cadmium and Ni sorption as a function of pH showed the predictable trend of increasing metal sorption with increase in equilibrium pH, while the Fe sorption trend was different and characterized by three phases. With regard to the order of Cd, Ni, and Fe sorption on soils, Cd and Ni showed high affinity for organic matter (OM), whereas Fe had high tendency for calcium carbonate (CaCO3). Results of metal desorption using 0.01M NaCl demonstrated that metal sorption on soils containing high amounts of CaCO3 was less reversible in comparison to soils containing high OM. In general, Cd and Ni desorption curves were characterized by three phases; (1) the greatest desorption at pH2, (2) the low desorption at pH3-7, and (3) the least desorption at pH>7. The MINTEQ speciation solubility program showed that the percentage of free metals declined markedly with increase of pH, while the percentage of carbonate and hydroxyl species increased. Furthermore, MINTEQ predicted that saturation index (SI) of metals increased with increasing pH.

Geogenic cadmium pollution and potential health risks, with emphasis on black shale

Cadmium (Cd) is a non-essential trace element that is toxic to humans. Previous studies of Cd in the environment have primarily focused on pollution resulted from anthropogenic sources, but little is known on naturally occurring sources of Cd. This paper aims to review the geochemical distribution of geogenic Cd and associated environmental risk. The source, accumulation, mobility, transportation, and health risk of Cd are discussed in a geo-environmental perspective, with an emphasis on black shale soils. Cadmium generally occurs in sulfides in black shale, and is easily released when exposed to oxygen and water. Leaching of these rocks tends to elevate Cd concentrations in aquatic systems, and may pose the potential to produce acid rock drainage (ARD) as well. Weathering of Cd-rich rocks also elevates soil Cd concentrations, and influence the geochemical species of Cd. Crops grown in these soils tend to accumulate higher Cd and threaten the food safety. Local inhabitant exposed to high geogenic Cd via food chains may experience Cd-related health risk. High Cd concentrations are observed in urine, and renal damage is also detected in Cd naturally enriched area based on low molecular weight proteins in urine. Overall, the findings in literature have provided with insights for potential health risk of Cd in areas with high Cd geochemical background levels, particular for the black shale exposed areas, more attentions should be paid on the geogenic Cd pollution, and suitable strategies of remediation and geo-environmental management for geogenic Cd pollution need further research.

Metal speciation and pollution assessment of Cd and Pb in intertidal sediments of Donghai Island, China

The speciation contents of Cd and Pb were investigated with the modified BCR method on the intertidal sediments of Donghai Island, China. For 24 surface samples of eight sections, the order was residual>exchangeable>oxidable>reducible for Cd and residual>reducible>oxidable>exchangeable for Pb; the four-fraction summation and single fraction concentration of Cd and Pb all decreased gradually from west to east because of different TOC; from high tidal zones to low tidal zones, the four-fraction summation, the residual fraction and its percentage all increased gradually due to the influence of the decreasing particle size of surface sediments. No accordant changing rule was found for the vertical distribution of Cd and Pb. About 32.0% of Cd and 36.9% of Pb for the nonresistant fraction presented the evidence of anthropogenic enrichment. The Risk Assessment Code suggested that a low risk for Pb and a medium risk for Cd to the intertidal environment.

Chemical speciation of Fe, Mn, Pb, Zn, Cd, Cu, Co, Ni and Cr in the suspended particulate matter off the Mejerda River Delta (Gulf of Tunis, Tunisia)

Fluxes of suspended particulate matter (SPM) and their associated metals were performed off the Mejerda River Delta during both the wet (March) and the dry (July) seasons in 2012, using sediment traps at study stations at depths of 10, 20 and 40 m. Fluxes nearest to the Mejerda outlet were more significant, especially during winter (36 g m−2 day−1), but dissipated further offshore, 24.5–6 g m−2 day−1 at the 20 m and 21.8–4.8 g m−2 day−1 at the 40 m stations. Many variations observed in seasonal and spatial metal fluxes are similar to those of SPM, in particular Pb and Zn, probably because they are associated with the mining activity characteristic of the Mejerda catchment. Chemical speciation reveals that most of the metals (20–100%) are bound to the residual fraction. The most toxic metals (Pb, Zn) are bound in part to the exchangeable fraction (20–50% for Pb and 5–15% for Zn) making them relatively bioavailable and therefore potentially toxic. While Cu and Cd fluxes are not always clearly established according to season, both metals are apparently sequestered deep in the sediment, bound especially to clays (40–80% for Cd and up to 100% for Cu).