Silicon-mediated enhancement of cadmium tolerance in maize (Zea mays L.) grown in cadmium contaminated soil

Pot experiments were performed to study the alleviative effects of exogenous silicon (Si) on cadmium (Cd) phytotoxicity in maize grown in an acid soil experimentally contaminated with Cd. Five treatments were investigated in the first trial consisting of a control (neither Cd nor Si added), Cd added at 20 or 40 mg kg(-1) Cd without or with Si added at 400 mg kg(-1) Si. A following-up trial was conducted with almost the same treatments as in the first trial except that Si was incorporated at 50 mg kg(-1) Si. The results showed that Cd treatment significantly decreased shoot and root dry weight, while addition of Si at both levels significantly enhanced biomass. Addition of Si at 400 mg kg(-1) Si significantly increased soil pH but decreased soil Cd availability, thus reducing Cd concentration in the shoots and roots and total Cd in the shoots. Moreover, more Cd was found to be in the form of specific adsorbed or Fe-Mn oxides-bound fraction in the Si-amended soil. In contrast, soil pH, available Cd and Cd forms were unaffected by addition of Si at 50 mg kg(-1) Si, but shoot Cd concentration in the Si-amended Cd treatments significantly decreased at both Cd levels used compared to the non-Si-amended Cd treatments. Total Cd in the shoots and roots was considerably and significantly higher in the Si-amended Cd treatments than in the non-Si-amended Cd treatments. The xylem sap significantly increased but Cd concentration in the xylem sap significantly decreased in the Si-amended Cd treatments compared with the non-Si-amended Cd treatments irrespective of Cd and Si levels used. The results suggest that Si-enhanced tolerance to Cd can be attributed not only to Cd immobilization caused by silicate-induced pH rise in the soils but also to Si-mediated detoxification of Cd in the plants.     

The landsnail Cepaea nemoralis regulates internal Cd levels when fed on Cd-enriched stinging nettle (Urtica dioica) leaves at low, field-relevant concentrations

We studied Cd accumulation in Cepaea nemoralis snails at low, but field-relevant Cd concentrations in the diet (Urtica dioica leaves). Six treatments of U. dioica plants were grown, resulting in leaf Cd concentrations between 0 and 2.6 microg g(-1) dw. Seven snails per treatment were fed for 38 days. Leaf Cd concentrations did not affect food consumption rates, and consequently Cd intake rates increased with increasing leaf concentrations. No differences were detected among treatments in the final soft tissue Cd concentrations and body burdens in the snails. Regression analyses showed no positive relationship between either snail Cd concentrations or body burdens and total Cd intake. This suggests a regulation of internal Cd concentrations at low food Cd concentrations. Our data suggest that Cd excretion via the mucus plays a substantial role in this regulation, in addition to Cd excretion via the faeces. Snail shells were no sinks for Cd.     

Potential contribution of arbuscular mycorrhiza to cadmium immobilisation in soil

The contribution of arbuscular mycorrhiza (AM) to immobilisation of Cd in substrate was studied in two experiments. In the first experiment, substrates prepared by cultivating tobacco, either non-mycorrhizal or inoculated with the AM fungus Glomus intraradices were enriched with a range of Cd concentrations, and Cd toxicity in the substrates was assessed using root growth tests with lettuce as a test plant. The tests revealed lower Cd toxicity in the mycorrhizal than in the non-mycorrhizal substrate, and the difference increased with increasing total Cd concentration in the substrates. In the second experiment, extraradical mycelium (ERM) of G. intraradices exposed in vivo to Cd was collected and analysed on Cd concentration. The ERM accumulated 10–20 times more Cd per unit of biomass than tobacco roots. While Cd concentrations were lower in the biomass of mycorrhizal plants than of non-mycorrhizal plants, Cd immobilisation by ERM did not affect the total Cd content in mycorrhizal tobacco.

It is concluded that mycorrhiza may decrease Cd toxicity to plants by enhancing Cd immobilisation in soil. The results therefore suggest a potential role of AM symbiosis in the phytostabilisation of contaminated soils, where high Cd availability inhibits plant growth.     

Metallothionein-like proteins in the freshwater oligochaete Limnodrilus udekemianus and their role as a homeostatic mechanism against cadmium toxicity

The purpose of this study was to determine if metallothioneins are present in the aquatic oligochaete Limnodrilus udekemianus and to determine the interplay between the presence of these proteins, cadmium (Cd) exposure, and Cd toxicity. The latter was geared specifically towards evaluating the role of metallothionein as a homeostatic mechanism against Cd toxicity. These issues are important for evaluating the usefulness of the quantification of metallothioneins as a biomonitoring tool. Worms in sediment were exposed to Cd under static conditions, with Cd initially added to the aqueous phase. Survival was monitored while respiration (as a measure of sublethal Cd effects) was determined immediately following exposure. Metallothioneins were separated from the cytosol by gel permeation high performance liquid chromatography (HPLC) while Cd levels were quantified in whole worms, cytosol and cytosolic fractions. Also, a Cd-saturation assay was used to determine the amounts of Cd bound to metallothionein and the total Cd-binding capacity of the metallothionein. Limnodrilus udekemianus has a metallothionein-like protein (an inducible cytosolic protein with an apparent molecular weight of approximately 15 kD that binds high levels of Cd and shows a red shift upon Cd binding). Sediment Cd levels above 60 microg/g were lethal to the worms (in 8-day exposures). Respiration rates at 13 and 41 microg/g Cd were not significantly different from controls, though cytosolic Cd levels were substantially increased in the 41 microg/g exposure. In this latter cytosol, Cd levels were significantly elevated in the low molecular weight pool (which includes metallothioneins) but not in the other pools, while the Cd-saturation assay also showed that worms in this group had significantly elevated levels of metallothionein-bound Cd. However, in all treatments the metallothionein was far from saturated by Cd. These observations indicate that no 'spill-over' of Cd was evident as lethal levels of Cd were approached. The overall cytosolic Cd distribution, and the degree of metallothionein saturation in Limnodrilus udekemianus thus do not appear to be good predictors of Cd toxicity in this species.     

Underlying mechanisms and effects of hydrated lime and selenium application on cadmium uptake by rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) seedlings

A pot experiment was conducted to investigate the effects of selenium (Se) and hydrated lime (Lime), applied alone or simultaneously (Se+Lime), on growth and cadmium (Cd) uptake and translocation in rice seedlings grown in an acid soil with three levels of Cd (slight, mild, and moderate contamination). In the soil with 0.41 mg kg^?1 Cd (slight Cd contamination), Se addition alone significantly decreased Cd accumulation in the root and shoot by 35.3 and 40.1%, respectively, but this tendency weakened when Cd level in the soil increased. However, Se+Lime treatment effectively reduced Cd accumulation in rice seedlings in the soil with higher Cd levels. The results also showed that Se application alone strongly increased Cd concentration in the iron plaque under slight Cd contamination, which was suggested as the main reason underlying the inhibition of Cd accumulation in rice seedlings. Se+Lime treatment also increased the ability of the iron plaques to restrict Cd uptake by rice seedlings across all Cd levels and dramatically decreased the available Cd concentration in the soil. These results suggest that Se application alone would be useful in the soil with low levels of Cd, and the effect would be enhanced when Se application is combined with hydrated lime at higher Cd levels.     

Alleviation of Cd toxicity by composted sewage sludge in Cd-treated Schmidt birch (Betula schmidtii) seedlings

We investigated alleviation of Cd toxicity and changes in the physiological characteristics of Betula schmidtii seedlings following application of composted sewage sludge to Cd-treated plants. Plants were grown under four test conditions: control, Cd treatment, sludge amendment, and Cd treatment with sludge amendment. B. schmidtii treated with Cd only accumulated the greatest amount of Cd in the leaves, but absorbed Cd was also highly concentrated in the roots. In contrast, Cd concentrations in the Cd and sludge amendment treated seedlings were the lowest in the roots. Since sludge amendment increased the growth of seedlings, it may have alleviated toxicity by dilution of Cd. Additionally, the absorbed Cd was more widely distributed since it was transported from the roots and accumulated in the stems and leaves of Cd and sludge treated plants. Cd treatment inhibited the growth and physiological functions of B. schmidtii seedlings, but sludge amendment compensated for these effects and improved growth and physiological functions in both Cd-treated and control plants. SOD activity in the leaves of seedlings was increased in the Cd-treated plants, but not in the Cd and sludge amendment treated seedlings. In conclusion, alleviation of Cd toxicity in response to sludge amendment may be related to a dilution effect, in which the Cd concentration in the tissues was effectively lowered by the improved growth performance of the seedlings.     

Differences in yield components and kernel Cd accumulation in response to Cd toxicity in four barley genotypes

A greenhouse hydroponics experiment was carried out to investigate genotypic difference in yield components in response to Cd toxicity, and kernel Cd concentrations and its relationship with Cd levels in roots and shoots during ontogenesis and as affected by shading and awn-removal. Root, shoot biomass and yield components of the four barley genotypes were impaired by increasing external Cd levels, with cv Wumaoliuling being most affected. Cadmium accumulation in roots and shoots increased with external Cd levels and differed significantly among genotypes. Meanwhile, 1 and 5 microM Cd treatments induced significant genotypic difference in kernel Cd concentrations, and Mimai 114, with the lowest Cd levels in roots and shoots, being the highest, whereas ZAU 3 being the lowest genotype. Shading had no significant effect on kernel Cd concentration, whereas awn-removal caused a significant decrease. Significantly negative correlation was discovered between Zn, Cu or Mn and Cd concentration in kernels, and there were positive relationships between Zn and Cu, Fe or Mn concentrations. Grain Cd concentrations were strongly correlated with both shoot and root levels. Regression equations between kernel- and shoot/root-Cd concentrations at different days of Cd exposure were established, allowing prediction of kernel Cd levels at harvest by measuring root- and shoot-Cd levels at early growth stage.     

Influence of cadmium on antioxidant capacity and four microelement concentrations in tomato seedlings (Lycopersicon esculentum)

Tomato (Lycopersicon esculentum) seedlings were grown in four cadmium (Cd) levels of 0-10 microM in a hydroponic system to analyze the antioxidative enzymes, Cd concentration in the plants, and the interaction between Cd and four microelements. The results showed that there was a significant increase in malondialdehyde (MDA) concentration, and superoxide dismutase (SOD) and peroxidase (POD) activities in the plants subjected to 1-10 microM Cd. This indicates that Cd stress induces an oxidative stress response in tomato plants, characterized by an accumulation of MDA and increase in activities of SOD and POD. Root, stem and leaf Cd concentrations increased with its exposure Cd level, and the highest Cd concentration occurred in roots, followed by leaves and stems. A concentration- and tissue-dependent response was found in the four microelement concentrations to Cd stress in the tomato leaves, stems and roots. Regression analysis showed that there was a significantly negative correlation between Cd and Mn, implying the antagonistic effect of Cd on Mn absorption and translocation. The correlation between Cd and Zn, Cu and Fe were inconsistent among leaves, stems and roots.     

Effect of soil characteristics on Cd uptake by the hyperaccumulator Thlaspi caerulescens

The influence of soil characteristics on the phytoremediation potential of Thlaspi caerulescens is not well understood. We investigated the effect of soil pH and Cd concentration on plant Cd uptake on one soil type, and the variation in Cd uptake using a range of field contaminated soils. On soils with total Cd concentrations of 0.6-3.7 mg kg(-1), T. caerulescens (the Ganges ecotype) produced greater biomass in the pH range 5.1-7.6 than at pH 4.4. The highest plant Cd concentration (236 mg kg(-1)) and Cd uptake (228 microg pot(-1)) were observed at pH 5.1. On soils with total Cd concentrations of 2.6-314.8 mg kg(-1), shoot Cd concentrations were 10.9-1,196 mg kg(-1). Multiple regression analysis indicated that higher Cd in soil, low pH (within the range of >5) and coarser texture were associated with higher Cd concentration and Cd uptake by T. caerulescens.     

Accumulation of Cd,Cu and Zn in shoots of maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) exposed to 0.8 or 20 nM Cd during vegetative growth and the relation with xylem sap composition

This work focuses on the exposure of maize plants to nanomolar concentrations of Cd, which is relevant for agricultural soils cropped with food and feed plants. Maize plants were cultivated in nutrient solution at 0.8 or 20 nM Cd during the vegetative growth stages. No significant hormesis or toxic effects of Cd were observed on maize growth, but a decrease in the allocation of Cd to shoots between the 0.8 and 20 nM Cd exposures revealed that the plants already responded to these low concentrations of Cd according to a shoot Cd excluder strategy. The Cd, Cu and Zn concentrations in shoots decreased with time as the result of an early decrease in the root/shoot ratio and of a decrease in the coefficient of allocation to aboveground for Zn and Cd at 20 nM. As a consequence, shoots of young plants were richer in micronutrients Cu and Zn but also in toxic Cd. The rate of delivery of Cd, Cu and Zn from xylem sap was successfully used to predict the time course of concentrations of Cd, Cu and Zn in the shoot. However, it overestimated the actual concentrations of Cd in the shoot, presumably because the reallocation of this trace element from shoots back to roots was not taken into account.     

The influence of metal speciation on the bioavailability and sub-cellular distribution of cadmium to the terrestrial isopod, Porcellio dilatatus

Cadmium is a non-essential toxic metal that is able to bioaccumulate in both flora fauna and has the potential to biomagnify in some food chains. However, the form in which cadmium is presented to consumers can alter the bioavailability and possibly the internal distribution of assimilated Cd. Previous studies in our laboratory highlighted differences in Cd assimilation among isopods when they were provided with a plant-based food with either Cd biologically incorporated into plant tissue or superficially amended with ionic Cd(2+). Cd is known for its high affinity for sulphur ligands in cysteine residues which form the basis for metal-binding proteins such as metallothionein. This study compares Cd assimilation efficiency (AE) in Porcellio dilatatus fed with food amended with either cadmium cysteinate or cadmium nitrate in an examination of the influence of Cd speciation on metal bioavailability followed by an examination of the sub-cellular distribution using a centrifugal fractionation protocol. As hypothesized the AE of Cd among isopods fed with Cd(NO(3))(2) (64%, SE=5%) was higher than AE for isopods fed with Cd(Cys)(2) (20%, SE=3%). The sub-cellular distribution also depended on the Cd species provided. Those isopods fed Cd(Cys)(2) allocated significantly more Cd to the cell debris and organelles fractions at the expense of allocation to metal-rich granules (MRG). The significance of the difference in sub-cellular distribution with regard to toxicity is discussed. This paper demonstrates that the assimilation and internal detoxification of Cd is dependent on the chemical form of Cd presented to the isopod.     

Symbiotic efficiency of autochthonous arbuscular mycorrhizal fungus (G. mosseae) and Brevibacillus sp. isolated from cadmium polluted soil under increasing cadmium levels

The effect of inoculation with indigenous naturally occurring microorganisms (an arbuscular mycorrhizal (AM) fungus and rhizosphere bacteria) isolated from a Cd polluted soil was assayed on Trifolium repens growing in soil contaminated with a range of Cd. One of the bacterial isolate showed a marked PGPR effect and was identified as a Brevibacillus sp. Mycorrhizal colonization also enhanced Trifolium growth and N, P, Zn and Ni content and the dually inoculated (AM fungus plus Brevibacillus sp.) plants achieved further growth and nutrition and less Cd concentration, particularly at the highest Cd level. Increasing Cd level in the soil decreased Zn and Pb shoot accumulation. Coinoculation of Brevibacillus sp. and AM fungus increased shoot biomass over single mycorrhizal plants by 18% (at 13.6 mg Cd kg(-1)), 26% (at 33.0 mg Cd kg(-1)) and 35% (at 85.1 mg Cd (kg(1)). In contrast, Cd transfer from soil to plants was substantially reduced and at the highest Cd level Brevibacillus sp. lowered this value by 37.5% in AM plants. Increasing Cd level highly reduced plant mycorrhization and nodulation. Strong positive effect of the bacterium on inocula, are important in plant Cd tolerance and development in Cd polluted soils.     

Characterization of cadmium uptake,translocation, and distribution in young seedlings of two hot pepper cultivars that differ in fruit cadmium concentration

The reasons why some cultivars of hot pepper (Capsicum annuum L.) accumulate low levels of Cd are poorly understood. We aimed to compare the characteristics of Cd uptake and translocation in low-Cd and high-Cd hot pepper cultivars by determining the subcellular locations and chemical forms of Cd, and its distribution among different plant organs. We conducted a hydroponic experiment to investigate the subcellular distribution and chemical forms of Cd in roots, stems, and leaves of a low-Cd (Yeshengchaotianjiao, YCT) and a high-Cd cultivar (Jinfuzaohuangjiao, JFZ). The results showed that the concentrations of Cd in almost all subcellular fractions of roots, and in all chemical forms in roots, were higher in YCT than in JFZ. Compared with YCT, JFZ had higher Cd concentrations in almost all subcellular fractions of stems and leaves, and higher Cd concentrations in almost all chemical forms in stems and leaves. Additionally, YCT had significantly higher total Cd accumulation but a lower Cd translocation rate compared with JFZ. In general, the results presented in this study revealed that root-to-shoot Cd translocation via the xylem is the key physiological processes determining the Cd accumulation level in stems and leaves of hot pepper plants. Immobilization of Cd by the cell walls of different organs is important in Cd detoxification and limiting the symplastic movement of Cd.     

Effect of liming on plant accumulation of cadmium under upland or flooded conditions

Liming a Cd-amended red earth (typical hapludalf) decreased plant shoot Cd content both in upland and in flooded conditions. The effect was due to the restriction of the total uptake of Cd from soil and the Cd transfer from roots to shoots following liming. Pot soil samples were sequentially extracted with 1 M ammonium acetate, 0.125 M Cu(II) acetate, and 1 m HNO3 to fractionate Cd into exchangeable, complexed, and acid-soluble forms. Results showed that soil flooding decreased exchangeable Cd and increased Cd in the complexed fraction. Liming also transferred Cd from exchangeable fraction to other lower available fractions depending on water regimes. With upland conditions, the Cd was transferred to the residual fraction, and to a lesser extent to the acid-soluble fraction, whereas in flooded soil the lost exchangeable Cd was recovered mainly in complexed fraction where the Cd was potentially more available for plants compared with that in residual and in acid-soluble fractions. Therefore, soil liming for controlling plant Cd uptake would be more favourable in upland rather than flooded conditions.     

Antioxidative response to Cd in a newly discovered cadmium hyperaccumulator, Arabis paniculata F

A hydroponic experiment was carried out to study the effect of cadmium (Cd) on growth, Cd accumulation, lipid peroxidation, reactive oxygen species (ROS) content and antioxidative enzymes in leaves and roots of Arabis paniculata F., a new Cd hyperaccumuator found in China. The results showed that 22-89 microM Cd in solution enhanced the growth of A. paniculata after three weeks, with 21-27% biomass increase compared to the control. Cd concentrations in shoots and roots increased with increasing Cd supply levels, and reached a maximum of 1662 and 8670 mg kg(-1) Cd dry weight at 178 microM Cd treatment, respectively. In roots, 22-89 microM Cd reduced the content of malondialdehyde (MDA), superoxide (O(2)(-1)) and H(2)O(2) as well as the activities of superoxide dismutase (SOD), guaiacol peroxidase (GPX), ascorbate peroxidase (APX) and glutathione reductase (GR). In leaves, the contents of MDA, O(2)(-1) and H(2)O(2) remained unaffected by 22-89 microM Cd, while 178 microM Cd treatment significantly increased the MDA content, 69.5% higher than that of the control; generally, the activities of SOD, catalase (CAT), GPX and APX showed an increasing pattern with increasing Cd supply levels. Our present work concluded that A. paniculata has a great capability of Cd tolerance and accumulation. Moderate Cd treatment (22-89 microM Cd) alleviated the oxidative stress in roots, while higher level of Cd addition (178 microM) could cause an increasing generation of ROS, which was effectively scavenged by the antioxidative system.     

Phosphorus and cadmium interactions in Kandelia obovata (S. L.) in relation to cadmium tolerance

This study focused on the cadmium (Cd) tolerance of mangroves with application of phosphate (P) in order to explore whether exogenous P can alleviate Cd stress on these intertidal species. Kandelia obovata (S. L.) seedlings were cultivated in rhizoboxes under different levels of Cd and P concentrations. The speciation distributions of Cd in the rhizosphere and non-rhizosphere sediments were examined by sequential extraction procedures; organic acid in plant tissues and soil solution was measured by high-performance liquid chromatography; Cd and P accumulation in the plants was also determined. Results showed that considerable differences existed in Cd speciation distributions between rhizosphere and non-rhizosphere sediments. Root activity influenced the dynamics of Cd, P application increased the organic acid content in root tissues, P also increased Cd accumulation in roots whilst lowering Cd translocation from root to the above-ground tissues, and a significant positive correlation was found between Cd and P in roots (r?=?0.905). It is postulated that Cd detoxification of K. obovata (S. L.) is associated with higher Cd immobilization in the presence of higher P and organic acid contents in root tissue.     

Phytoextraction with Brassica napus L.: a tool for sustainable management of heavy metal contaminated soils

Phytoextraction is a promising tool to extract metals from contaminated soils and Brassica napus L. seems to be a possible candidate species for this purpose. To select accessions with the ability to accumulate cadmium, hydroponically grown 21 day old seedlings of 77 B. napus L. accessions were exposed to 0.2 microM CdSO(4) for an additional 10 days. The effects of Cd on several parameters were quantified i.e.; shoot Cd concentration ([Cd](shoot)), total amount of Cd in shoots (Total Cd) and the shoot to root Cd concentration ratio (S/R ratio). Though generally natural variation was low for [Cd](shoot), Total Cd and S/R ratio, a number of accessions could be selected. Our results indicated that Total Cd and S/R ratio are independent parameters for Cd accumulation and translocation. The selected varieties were then tested in field experiments on two locations nearby metal smelters. The two locations differed in extractable soil Cd, Zn, Ca concentration and pH levels. On both locations B. napus L. accessions showed significant differences in [Cd](shoot) and Total Cd. Furthermore we found significant correlations between Cd and Zn accumulation in shoots. There were site-specific effects with respect to Cd accumulation in the B. napus L. accessions, however, two accessions seem to perform equally well on both sites. The results of the field experiment suggest that certain B. napus L. accessions are suitable for phytoextraction of moderately heavy metal contaminated soils.     

Soil Cd availability to Indian mustard and environmental risk following EDTA addition to Cd-contaminated soil

A pot experiment was conducted to investigate the influence of EDTA on the extractability of Cd in the soil and uptake of Cd by Indian mustard (Brassica juncea). Twenty levels of soil Cd concentration ranging from 10 to 200 mg kg(-1) were produced by spiking aliquots of a clay loam paddy soil with Cd(NO3)2. One week before the plants were harvested EDTA was applied to pots in which the soil had been spiked with 20, 40, 60...200 mg Cd kg(-1). The EDTA was added at the rate calculated to complex with all of the Cd added at the 200 mg kg(-1) level. Control pots spiked with 10, 30, 50... 190 mg Cd kg(-1) received no EDTA. The plants were harvested after 42 days' growth. Soil water- and NH4NO3-extractable Cd fractions increased rapidly following EDTA application. Root Cd concentrations decreased after EDTA application, but shoot concentrations increased when the soil Cd levels were >130 mg kg(-1) and Cd toxicity symptoms were observed. The increases in soil solution Cd induced by EDTA did not increase plant total Cd uptake but appeared to stimulate the translocation of the metal from roots to shoots when the plants appeared to be under Cd toxicity stress. The results are discussed in relation to the possible mechanisms by which EDTA may change the solubility and bioavailability of Cd in the soil and the potential for plant uptake and environmental risk due to leaching losses to groundwater.     

Chemical behavior of Cd in rice rhizosphere

Chemical behavior of Cd in rice rhizosphere as affected or not by Pb was investigated. The NH4OAc extractable Cd in the rhizosphere was distinctly lower than that in bulk soil. The depletion of Cd in the rhizosphere could not be simply attributed to Cd uptake by rice. The observed phenomena could be attributed to the decreasing pH in the rhizosphere and the complexing capabilities of soluble exudates for Cd. Extractable Cd increased in both the rhizosphere and bulk soil after the addition of Pb, which might be caused by the replacement of Pb for Cd. The extractable Cd in the non-rhizosphere varied with the distance from the root surface, especially within 0-1 mm, which was greatly affected by the combined effects of mass flow, activation and fixation, and had the lowest extractable Cd. Pb addition affected the distribution of extractable Cd in the non-rhizosphere, implying that the affinity of Pb for organic matter was greater than that of Cd. The difference of Cd species between rhizosphere and bulk soil demonstrated that the transformation of exchangeable Cd (EXC-Cd) to OM-Cd (bound to organic matter) and FMO-Cd (bound to iron and manganese oxide) occurred in the rice rhizosphere due to the exudations from the rice root, the activity of microorganisms on the root surface and the activation of Fe and Mn oxides. The interaction between Pb and Cd resulted in the content of EXC-Cd being higher in the presence of Pb, whereas the OM-Cd content was lower in the presence of Pb.     

Study on fractions of adsorbed Pb and Cd onto natural surface coatings

The speciation and extent of migration of adsorbed Pb and Cd in natural surface coatings (NSCs) were investigated using sequential extraction procedure to provide an understanding of distribution of the adsorbed Pb and Cd. Extractions were conducted on NSCs before and after Pb and Cd adsorption treatment under controlled laboratory conditions with initial Pb and Cd concentrations ranging from 0.2-2.5 mol/l. The Langmuir adsorption isotherms were applied to estimate equilibrium coefficients of Pb and Cd adsorption to NSCs components. The results showed that 58.50% of adsorbed Pb in average existed in tightly adsorbed form, and the remaining Pb was mostly present as solid oxides/hydroxides (34.00%) and exchangeable and soluble form (7.50%) in NSCs, respectively. Large amount of adsorbed Cd (70.51% in average) was present in exchangeable and soluble form, following a decreasing order in tightly adsorbed form (18.61%), solid oxides/hydroxides (9.87%), and easily oxidizable solids/compounds (1.01%), respectively. No Cd was found in strongly held oxides and precipitates. Compared to the distribution of adsorbed Pb in NSCs, Cd distribution showed that less migration of Cd from exchangeable and soluble form to solid oxides/hydroxides after adsorbed to NSCs, indicating fewer sites for Cd to adsorb to NSCs and less affinity of Cd to the NSCs. These percent distributions of metals provided an additional interpretation to that Pb adsorption to the NSCs greater than that of Cd, less retention of Cd than that of Pb and less roles attributed for Pb/Cd adsorption by organic materials in NSCs, which were observed based on the selective extraction techniques in the independent investigations.