Abstract
Lettuce, spinach, radish and carrot were grown on compost that had previously been contaminated at different concentrations of Cd, Cu, Mn, Pb and Zn. Control plants of each vegetable were also grown on unadulterated compost. The experiment was carried out under greenhouse conditions. Mature plants were harvested and their roots and leaves collected. Soil samples from each growing pot and plant materials were acid digested and analysed to determine total metal concentration. Flame-Atomic Absorption Spectroscopy (FAAS) was employed to determine metal concentrations in soil and plant samples (Mn and Zn), while Cd, Cu and Pb in plant materials were analysed by Differential Pulse Anodic Stripping Voltammetry (DP-ASV). Soil (BCR 146R and GBW 07310) and plant (tea leaves, INCT-TL-1) certified reference materials were used to assess accuracy and precision. The edible part of plants, i.e. the leaves of lettuce and spinach and the roots of radish and carrot, were also extracted using an in vitro gastrointestinal (GI) extraction to assess metal bioavailability. The results showed that the uptake of Cd, Cu, Mn and Zn by plants corresponded to the increasing level of soil contamination, while the uptake of Pb was low. Soil-to-plant transfer factor (TF) values decreased from Mn ≫ Zn > Cd > Cu > Pb. Moreover, it was observed from this investigation that individual plant types greatly differ in their metal uptake, e.g. spinach accumulated a high content of Mn and Zn, while relatively lower concentrations were found for Cu and Pb in their tissues. From the in vitro gastrointestinal (GI) study, results indicate that metal bioavailability varied widely from element to element and according to different plant types. The greatest extent of metal releasing was found in lettuce (Mn, 63.7%), radish (Cu, 62.5%), radish (Cd, 54.9%), radish (Mn, 45.8%) and in lettuce (Zn, 45.2%).
Introduction
Heavy metals in soils are derived from natural components or geological sources as well as from human activities or anthropogenic sources. The residence time of most heavy metals in soil is very long. There are many sources of heavy metals in soils including (Reichman Citation2002): Natural e.g. soil parent material, volcanic eruptions, marine aerosols, and forest fires; Agricultural e.g. fertilizers, sewage sludges, pesticides and irrigation water; Energy and fuel production e.g. emissions from power stations; Mining and smelting e.g. tailing, smelting, refining and transportation; Automobiles e.g. combustion of petroleum fuels; Urban/industrial complexes e.g. incineration of wastes and waste disposal; and, Recycling operations e.g. melting of scrap. Metal contamination issues are becoming increasingly common, the occurrence of heavy metals in soils, both natural and polluted, has been the subject of a number of studies (Muller and Anke Citation1994; Sanchez-Camazano et al. Citation1994; Dudka et al. Citation1996; Caussy et al. Citation2003; Cui et al. Citation2004).
Whereas metal contamination is widespread, the occurrence of heavy metals in agricultural soils is a major concern. Taken up by plants, heavy metals may enter the food chain in significant amounts. Hence, people could be at risk of adverse health effects from consuming vegetables grown in soils containing elevated metal concentrations. For instance, it is estimated that approximately half of human lead intake is through food, with around half originating from plants (Nasreddine and Parent-Massin Citation2002). Cadmium and lead are the elements of most concern because of their potential for toxicity or accumulation in plants and animals (Wolnik et al. Citation1983). According to the Environmental Protection Agency (EPA), lead is the most common heavy metal contaminant in the environment (Watanabe Citation1997) and may be toxic to organisms even when absorbed in small amounts.
Although metals such as zinc, copper and manganese are essential trace elements for plants and animals, they can also be dangerous at high exposure levels. For example, poisoning incidents with symptoms of gastrointestinal distress, nausea and diarrhoea have been reported after a single or short-term exposure to concentrations of zinc in water or beverages of 1000–2500 mg/l (WHO Citation2001). At high doses of certain metal compounds, of the order of several grams, chronic toxicity or carcinogenicity as well as fatality may occur.
Certain crops such as spinach, lettuce, carrot, radish, zucchini can accumulate heavy metals, e.g. Cd, Cu, Mn, Pb and Zn in their tissues (Sauerbeck Citation1991; Muller and Anke Citation1994; Hooda Citation1997; Bahemuka and Mubofu Citation1999; Cobb et al. Citation2000; Mattina et al. Citation2003; Hough et al. Citation2004; Zhou et al. Citation2005). Generally, uptake is increased in plants that are grown in areas with increased soil contamination. Among the metals, Cd and Zn are fairly mobile and readily absorbed by plants (Mench et al. Citation1994). In contrast, Cu and Pb are strongly adsorbed onto soil particles reducing their availability to plants (WHO Citation1998, Citation1989). In addition, they are bound to organic matter, as well as being adsorbed by carbonate minerals and hydrous iron and manganese oxides.
The fraction of heavy metals which can be readily mobilized in the soil environment and taken up by plant roots is considered the bioavailable fraction. The term “bioavailability” has been defined as the extent to which a chemical can be absorbed by a living organism and reach the systemic circulation (Kelley et al. Citation2002). Therefore, total metal concentrations in soil do not necessarily correspond with metal bioavailability. The bioavailability to plants of heavy metals depends on a number of physical and chemical factors in the soil. These include soil properties, e.g. pH, organic matter content, redox potential, cation exchange capacity (CEC), sulphate, carbonate, hydroxide, soil texture and clay content (Peijnenburg and Jager Citation2003). Apart from these factors, metal absorption by plants is influenced by the characteristics of the plants themselves (Hund-Rinke and Kordel Citation2003).
To assess the bioavailability of heavy metals from a contaminated soil to a crop plant, the selection of an extractant which simulates the plant-available fraction of the element is of importance (Helgesen and Larsen Citation1998). During recent decades, a large number of extractants have been applied for single extractions (McGrath Citation1996; Hooda Citation1997; Quevauviller Citation1998; Lu et al. Citation2003; Chojnacka et al. Citation2005) and sequential extractions (Tessier et al. Citation1979; McGrath Citation1996; Rauret et al. Citation1999; Mossop and Davidson Citation2003). Single extractions are mainly used to evaluate the exchangeable fraction of elements in soil. The method has been widely employed to predict toxicity deficiencies in crops and in animals eating such crops (Dean Citation2003). Sequential extractions are usually applied to assess metal association with the different solid-phase components in sediments (Sahuquillo et al. Citation2003).
The use of EDTA has proved to be a more reliable and consistent test in predicting the accumulation of metals in plants compared to DTPA, NH4NO3 and CaCl2 (Hooda Citation1997). In general, higher percentages are extracted with EDTA in comparison with DTPA. About 63% of the total Cd in soil was extracted by this medium (Mench et al. Citation1994). However, the ability of the extractants to assess the metal bioavailability to plants depends on the extractant used, the metal of interest, the plant species and soil-type variation used in the studies. Thus, not all methods are useful to study all heavy metals under different soil conditions.
Alternatively, sequential chemical extractions, which are more complicated, have been developed using a series of different reagents to extract metals from soil. The procedure under the designed SM&T program of the European Union consists of three main stages, plus a final (residual fraction) stage (Quevauviller Citation1998). Stage 1 involves the use of 0.11 mol/l acetic acid which can extract acid-soluble metals; stage 2, 0.1 mol/l hydroxyammoniumchloride, which extracts (reducible phase) metals bound to iron/manganese oxides; and stage 3, 8.8 mol/l hydrogen peroxide and 1 mol/l ammonium acetate (oxidizable phase), for organic matter bound to metals. The final stage (residual fraction), aqua regia extraction, has been employed to assess the residual fraction. Being operationally-defined procedures, sequential extraction inevitably gives results that are highly dependent on the given parameters of the procedure used, such as the extractant (pH, concentration, type), extraction time and temperature, methods of shaking and phase separation (Shiowatana et al. Citation2001).
There are a number of in vivo approaches available to evaluate the bioavailability of metals for humans, e.g. from soil, dust or food. However, each method has its limitations. Information obtained from in vivo studies can be difficult to interpret due to physiological discrepancies between humans and the experimental animals adopted. Such problems led to the development of in vitro systems, based on gastrointestinal (GI) extraction including the so-called physiologically-based extraction test (PBET). The technique measures the fraction of a metal which is solubilized from a sample under simulated gastrointestinal conditions and which therefore is available for absorption (Kelley et al. Citation2002). The simulated parameters representative of the human digestive tract include stomach and small intestinal pH and chemistry, soil-to-solution ratio, stomach mixing, and stomach emptying rates. Several in vitro methods have been developed (Miller et al. Citation1981; Crews et al. Citation1983; Ruby et al. Citation1993; Hack and Selenka Citation1996). All of the PBET models involve simulated gastric extraction with pepsin and with a mixture of pancreatin, amylase and bile salt in the intestinal stage. Researchers have shown that the in vitro study results can be correlated to bioavailability determined by in vivo studies (Ruby et al. Citation1996). The approaches can be simple, rapid and low in cost and may provide insights not achievable in whole animal studies (Miller et al. Citation1981).
This paper aims to (a) determine the mobility of metals in soil by assessing their uptake by a range of plants, and (b) assess an approach for assessing oral bioavailability of metals from plants.
Experimental
Apparatus and reagents
Soil (Cd, Cu, Mn, Pb and Zn) and plant (Mn and Zn) samples were analysed using an atomic absorption spectrometer (a Perkin-Elmer AAnalyst 100) with an air-acetylene flame.
A 757 VA Computrace (Metrohm, Herisau, Switzerland) in differential pulse anodic stripping voltammetry (DP-ASV) mode was used to determine Cd, Cu and Pb in plant samples. The ASV was equipped with a three-electrode system: a HMDE (Hanging mercury dropping electrode) as the working electrode, a Pt electrode as the auxiliary electrode, and a KCl saturated Ag/AgCl electrode as the reference electrode.
For the in vitro gastrointestinal extraction a shaking water bath (Grant Instruments Ltd., OLS 200, Cambridge, UK) was employed. Pepsin-A powder 1 Anson unit per g (lactose as diluent), amylase and pancreatin were provided by BDH Chemicals Ltd. (Poole, UK) and bile salts by Sigma-Aldrich Co. Ltd. (Dorset, UK).
All chemicals used were of analytical grade. Copper oxide, lead nitrate and zinc oxide were provided by BDH Chemicals Ltd. Manganese metal was provided by Merck and cadmium nitrate by Fisher Scientific UK Ltd. (Loughborough, Leicestershire). Concentrated hydrochloric acid and 30% hydrogen peroxide were provided by BDH Chemicals Ltd. and concentrated nitric acid by Fisher Scientific UK Ltd.
Certified reference materials (CRM) used in soil analysis were BCR 146R (No. 323) and GBW 07310 purchased from the Laboratory of the Government Chemist (LGC), Teddington, London; and, in plant analysis, tea leaves (INCT-TL-1) obtained from the Institute of Nuclear Chemistry and Technology, Warsaw, Poland.
Preparation of heavy metal contaminated soils
Contaminated soils spiked at two concentration levels (low and high) and control soils (unadulterated) were used for this study. A control soil is one to which no spiking of heavy metal has been made. The spiking was made for both individual metal and mixed metal experiments.
Before spiking, compost soil (Levington multipurpose compost) was passed through a 2 mm sieve and air dried for 48 h. For the low concentration level (10 times unadulterated concentration), a metal solution at the approximate levels (10, 150, 300, 10 and 100 µg/g for Cd, Cu, Mn, Pb and Zn, respectively) was prepared to spike into the soil. It is noted that Cd was spiked in the compost used for growing radish and carrot only. The soil was weighed, approximately 300 g for each mixing, and placed in a stainless steel tray. Then, the soil was thoroughly mixed with the metal solution to ensure homogeneity and air dried for a day to allow excessive water to evaporate. The high concentration level (50 times unadulterated concentration) was prepared in the same manner as described above.
Homogeneity test
Sub-samples of the contaminated soils (low and high concentration) were assessed for homogeneity prior to use for plant growth. Soil samples were acid digested (see below) and analysed by FAAS.
General procedure: Growing vegetable plants
Seeds of spinach, lettuce, radish and carrot (obtained directly from local markets) were germinated in plastic trays and the seedlings transplanted after two weeks, into individual plastic pots containing 100 g of metal contaminated soil. The plants were grown in soil contaminated at low and high concentrations for both individual and mixed metals with 5 pots per treatment. Three plants of spinach, lettuce, radish and carrot were also planted in unadulterated soil as control samples. The plants were frequently watered with distilled water; the unadulterated and low treatments were watered daily and the high treatments 2–3 times a week. The plants were grown under artificial light (Sodium lighting system, 150 Wm−2) with time intervals of 16 h daylight and 8 h dark. The temperature was within the range 12.1–25.3°C while the humidity varied between 38 and 89%. The mature plants (6–8 weeks growth) were harvested. Roots and leaves were separated and then thoroughly washed including a final rinse with distilled water and kept in a freezer at −18°C until analysis. Soil samples beneath each plant root were collected after harvesting the plant. The Cd, Cu, Mn, Pb and Zn determinations were carried out on plant (roots and leaves) and soil samples.
Soil digestion procedure
For determination of total heavy metal content of the soils, 1 g of each soil sample (oven dried at 70°C for 48 h) was accurately weighed into a digestion tube and 10 ml concentrated nitric acid: water, 1:1 v/v added. The sample was then heated at 95°C on a heating block (2006 Digestor, Foss Tecator) for 15 min without boiling. After cooling at room temperature for 5 min, 5 ml concentrated HNO3 was added and the sample were heated at 95°C for 30 min. Additional 5 ml aliquots of concentrated HNO3 was added until no brown fumes were given off. The solution was allowed to evaporate to <5 ml. After cooling, 2 ml of water and 3 ml of 30% H2O2 were added and heated (<b.p.) until effervescence subsided and the solution cooled. Additional H2O2 was added until effervescence ceased (but no more than 10 ml H2O2 was added). This stage was continued for 2 h at a temperature less than boiling point. Then, the solution was allowed to evaporate to <5 ml. After cooling, 10 ml concentrated HCl was added and the solution was heated at 95°C for 15 min. After cooling, the sample was filtered through Whatman No. 41 filter paper into a 100 ml volumetric flask, and then made up to the mark with distilled water.
Plant digestion procedure
Into a digestion tube, 1 g (accurately weighed) of plant sample (oven dried at 70°C for 48 h) was placed and 10 ml concentrated HNO3 added. The sample was then heated to 95°C on the heating block for approx. 1 h. After cooling, 5 ml of concentrated H2SO4 was added and the sample was heated to 140°C until charring first appears. After cooling, 5 ml of concentrated HNO3 was added and heated to 180°C. Further aliquots of HNO3 were added until the sample digest appeared clear or a pale straw colour. After cooling, 1 ml of 500 g l−1 H2O2 was added and heated to 200°C. This procedure was repeated until brown fumes cease to appear. After cooling, 10 ml of distilled water and 0.5 ml of concentrated HNO3 were added and heated to 200°C until white fumes were evolved. After cooling, 10 ml of distilled water and 1 ml of 500 g l−1 H2O2 were added and heated to 240°C until white fumes were evolved. Finally, the digest was cooled and filtered through a Whatman No. 41 filter paper into a 50 ml volumetric flask, then made up to the mark with distilled water.
In vitro gastrointestinal extraction of vegetable plants
This determination consists of two sequential processes, a gastric and an intestinal digestion, each one carried out employing simulated human conditions (enzymes, pH and temperature). In the first stage, approximately 1 g (accurately weighed) of plant samples (oven dried at 70°C for 48 h) was placed into a 50 ml Sarstedt tube and treated with 15 ml of pepsin (1% w/v) in saline (154 mmol L−1). The pH of the solution was adjusted to pH 1.8 with dilute HCl. The mixture was then shaken at 100 rpm in a thermostatic bath maintained at 37°C. After 4 h, the solution was centrifuged at 3000 rpm for 10 min and the supernatant was removed and its pH adjusted to 2.5 with 2 M HCl.
The second stage involved extraction with intestinal juices. To the gastric digest residue, 7.5 ml of a mixed solution of pancreatin (3% w/v) and amylase (1% w/v) in saline and 7.5 ml of bile salts (0.15% w/v) in saline were added. The sample pH was adjusted to 7.0 with saturated NaHCO3. The sample was then shaken at 100 rpm in a thermostatic bath maintained at 37°C. After 4 h, the solution was centrifuged at 3000 rpm for 10 min and the supernatant was removed. All extracts (gastric and intestinal) were analysed by either DP-ASV or FAAS. The resultant sample residue was further extracted by acid digestion employing the same procedures as for the vegetable extraction and analysed by DP-ASV for Cd, Cu and Pb and FAAS for Mn and Zn.
FAAS analysis
All soil and some plant extracts were analysed by FAAS. The measurements were made under operating conditions for the most sensitive absorption lines of individual elements, i.e. Cd, Cu, Mn, Pb and Zn at 228.8 nm, 324.8 nm, 279.5 nm, 217.0 nm and 213.9 nm, respectively with a hollow cathode lamp current at 7 mA and a slit width of 0.7 nm.
DP-ASV analysis
A 757 VA Computrace (Metrohm, Herisau, Switzerland) was used to determine the Cd, Cu and Pb concentration in plant samples. All electrodes and a measuring cell were thoroughly cleaned with distilled water. The measurement was done by pipetting a 1 ml portion of the plant extract into a measuring vessel containing 9 ml distilled water. Then, 1 ml of the electrolyte solution (NaOAc) was added. The electrode was immersed into the solution and then scanned from −1.0 V to 0.0 V. Nitrogen gas was used as a purging gas at 1 bar pressure and the measurements were carried out in Differential Pulse (DP) mode. The standard additions method was applied by analysing five calibration standard solutions of a 1 ppm stock solution of each metal. Quality control of the plant measurements was done by measuring CRMs (tea leaves and soils) every ten samples. A blank was analysed with each analytical batch.
Results and discussion
Analytical features of heavy metal determination
Separate standard calibration curves were established for FAAS with five standards, with concentrations ranging from 0.2–1.0 µg/ml and 2–10 µg/ml. It was found that the absorbance signal response of Cu, Mn and Pb followed a linear relationship. Hence, the regression used for the determination of these metals was y = mx + c. While for Cd and Zn a curved relationship was applied of the form y = ax2 + bx + c. All correlation coefficients were >0.994.
Detection limits, for FAAS, were calculated using the expression 3.Sblank / b, where Sblank is the standard deviation of at least seven replicate measurements of blanks and b is the slope of calibration curve. Typical detection limits were 0.010, 0.034, 0.039, 0.142 and 0.016 µg/ml for Cd, Cu, Mn, Pb and Zn, respectively.
The standard additions method was employed to simultaneously determine the Cd, Cu and Pb concentration of plant samples by DP-ASV. After analysis, 0.1 ml of standard solution was added directly to the solution in the cell and the analysis repeated. The potential was scanned from −1.0 V to 0.0 V. The peaks were measured at −0.560, −0.100 and −0.380 V for Cd, Cu and Pb, respectively. The formula for calculating the original solution concentration, which was done by the Metrohm 757 VA Computrace software, was 
Where i1 = sample peak height, i2 = sample + standard peak height, v = volume of standard addition (ml), V = original sample volume (ml), Cs = concentration of standard solution (mol/l), and C = concentration of the original sample (mol/l) (Shuman Citation1996).
Detection limits, for DP-ASV, were calculated using the expression 3.Sblank / b, where Sblank is the standard deviation of at least six replicate measurements of blanks and b is the slope of calibration curve. Typical detection limits were 0.0025, 0.0020, 0.0019, and 0.0016 µg/ml for Cd, Cu, Pb and Zn, respectively.
Total metal determination in soil
In order to assess the accuracy and precision of the method, soil CRMs were acid digested and analysed by FAAS. The values obtained for each CRM are presented in . The mean recoveries of the soil CRMs ranged from 117.6, 86.3–96.0, 87.7–102.7, 101.6–111.3, 80.4–99.2% for Cd, Cu, Mn, Pb and Zn, respectively.
Table I. Analysis of soil and plant certified reference materials by FAAS.
To undertake the homogeneity of the spiking process, prior to transplanting the seedlings, representative sub-samples of the spiked compost were digested and analysed by FAAS. The percent standard deviation (%RSD) of Cd, Cu, Mn, Pb and Zn of the spiked compost ranged from 4.9–8.6, 4.2–8.8, 1.5–5.9, 4.6–8.5 and 1.7–6.9, respectively (n = 8). This implied that the metals added in the compost were evenly distributed and suitable for plant growth.
To determine total metals in spiked soils, all soil samples were digested with concentrated HNO3 and 30% H2O2 and analysed for Cd, Cu, Mn, Pb and Zn by FAAS. shows the mean total metal concentrations (µg/g, dry weight) of compost used for growing lettuce, spinach, radish and carrot in different treatments. The total metal concentrations of the composts under the experiment varied from unadulterated-low-high level. Concentrations in the unadulterated treatment ranged from 0.0–0.4, 11.3–20.9, 45.3–65.0, 6.3–12.5 and 8.7–17.7 µg/g (dry weight) for Cd, Cu, Mn, Pb, and Zn, respectively; the low treatment ranged from 7.1–11.0, 170.7–336.3, 381.9–572.8, 22.6–33.2 and 93.5–181.7 µg/g (dry weight) for Cd, Cu, Mn, Pb and Zn, respectively; and the high treatment ranged from 39.6–70.6, 793.4–1626.1, 1829.3–1939.0, 58.0–92.4 and 490.3–581.8 µg/g (dry weight) for Cd, Cu, Mn, Pb and Zn, respectively.
Table II. Metal concentrations (μg/g) of compost for growing lettuce, spinach, radish and carrot in different treatments.
Total metal determination in vegetable plants
After plant materials had been digested, FAAS (Mn and Zn) and DP-ASV (Cd, Cu and Pb) were employed to determine total metal concentrations found in their tissues. shows total metal concentrations (µg/g, dry weight) found in lettuce, spinach, radish and carrot growing in different treatments of spiked composts. A quality control procedure for DP-ASV was included to assess the analytical stage.
Table III. Metal concentrations found in plants (μg/g, dry plant material).
This was done by analysing CRMs by DP-ASV analysis. The tea leaves CRM (INCT-TL-1) was included as representative of plant material. However the concentration of Cd and Pb within this CRM were close to the detection limit of DP-ASV, so it was necessary to use a soil CRM (BCR 146R) for these metals. The results are shown in . The mean recoveries were 99.5, 129.9 and 102.8% for Cd, Cu and Pb by DP-ASV, respectively. It is observed that the results (except for Cu) were in agreement with the certified values.
Table IV. Analysis of certified reference materials (Tea leaves, INCT-TL-1 and Soil BCR 146R) by DP-ASV: Quality control data
Uptake of heavy metal by vegetable plants
shows the mean concentrations of metals in roots and leaves of four vegetables grown on different treatments of contaminated soils. Lettuce and spinach represented the edible vegetable leaves (above ground) and radish and carrot represented edible vegetable roots (underground). Control plants were also produced by growing on uncontaminated soil. It was observed that each metal differed considerably in uptake from each other.
Cadmium
It should be noted that the Cd treatments were undertaken for only radish and carrot. From the results obtained, cadmium content in leaves of both radish and carrot were significantly higher than in their roots, except for the unadulterated treatment which was not significantly different. The highest Cd contents were found in Cd-H treatments for radish leaves, radish roots and carrot leaves with the concentrations of 184.4, 68.2 and 47.4 µg/g, respectively. The lowest Cd concentration was in carrot leaves (0.2 µg/g) in the unadulterated soil.
Copper
Copper uptake was significantly higher in roots than in leaves for lettuce and spinach from every treatment, i.e. unadulterated, Mix-L, Mix-H, Zn-L, Zn-H, Cu-L and Cu-H. In contrast, copper concentrations in radish and carrot leaves from every treatment (except the leaves of radish in Cd-H treatment) were higher than in roots. Copper concentrations were highest in lettuce roots (262.1 µg/g) in Cu-H treatment, followed by spinach roots in Cu-H treatment (150.1 µg/g) and Mix-H treatment (70.1 µg/g), and lettuce roots in Mix-L treatment (69.0 µg/g). The Cu content was lowest in radish roots (2.1 µg/g) in the Cd-L treatment.
Manganese
Manganese is the most available metal. The concentration of Mn in the plants ranged from 16.9–6631.6 µg/g. Plant leaves contain more manganese than their roots, except spinach leaves in the unadulterated and Cu-L treatment. Manganese concentrations were highest in spinach leaves (6631.6 µg/g) in Mix-H treatment, followed by radish leaves in Mix-H treatment (3821.8 µg/g), and spinach leaves in Mix-L treatment (1971.1 µg/g). The Mn content was lowest in radish roots (16.9 µg/g) in the Cd-H treatment.
Lead
Lead is the least available metal considered. Lead concentrations varied from the non-detectable level to 11.8 µg/g and significantly lower than other metals accumulated in the plants. This can be explained by the fact that lead binds to organic matter in soil, limiting uptake by the plants (Wang et al. Citation2004). Lead contents in every treatment, except in radish roots in Cd-H treatment (11.8 µg/g) and spinach roots in Mix-H treatment (11.1 µg/g), were lower than 10 µg/g.
Zinc
Zinc concentrations in leaves of radish, carrot and spinach were significantly higher than their roots for every treatment, except the spinach leaves in Zn-H and Cu-H treatments which were slightly lower. In contrast, zinc contents in lettuce leaves were lower than their roots for every treatment, except in the case of the Cu-H treatment which was not significantly different in concentrations between their leaves and roots. The highest zinc contents were found in Zn-H treatments for lettuce roots, lettuce leaves, spinach roots and spinach leaves with concentrations of 1440.6, 1171, 719 and 632 µg/g, respectively. This is in agreement with a previous study showing that lettuce and spinach accumulated zinc to a greater extent (Sauerbeck Citation1991). The Zn content was lowest in carrot roots (23.6 µg/g) from the Cd-L treatment.
Influence of plant species
indicates that individual plant types differ in their metal uptake. The Cu uptake was slightly low in lettuce and carrot, but relatively higher in spinach and radish. Manganese uptake was significantly higher in lettuce and spinach and slightly lower in radish and carrot. Zinc uptake was relatively high, whereas Pb tended to be taken up least by all the plants studies. Cadmium uptake was rather high in radish, but lower in carrot.
Transfer factors (TFs) of heavy metals from soil to plants
One approach to assess the mobility of metal by plants is to calculate the transfer factor (TF), as defined in the following equation (Sauerbeck Citation1991; Gray et al. Citation1999; Cui et al. Citation2004; Chojnacka et al. Citation2005): 
where C plant is the concentration of an element in the plant material (dry weight basis) and C total − soil is the total concentration of the same element in the soil (dry weight basis) where the plant was grown. The higher the value of the TF, the more mobile/available the metal is. The TF values of the metals for the plants studied are presented in .
Table V. Ratio of concentrations of metal in plants to metal in soil, Transfer Factor (TF) values.
The results indicate that the TF values for Cd, Cu, Mn, Pb and Zn for various vegetables varied greatly between plant types and soil treatments. The TF values for Cd were generally slightly higher than those for Cu, but lower than Zn. The TF values for Cu varied from 0.21 (lettuce leaves) to 1.22 (spinach roots) in the unadulterated soil and 0.01 (lettuce leaves, Cu-H treatment) to 2.17 (spinach roots, Zn-H treatment and radish roots, Cd-H treatment) in contaminated soil. The values for Pb were much lower than those for Cu, varying from 0.00 (carrot roots) to 0.35 (radish roots) in unadulterated soil and 0.00 (leaves and roots of lettuce, radish, carrot) to 0.94 (radish roots, Cd-H treatment). The TF values for Mn varied the most with values from 0.30 (radish roots) to 4.52 (spinach roots) in unadulterated soil and 0.16 (radish roots, Mix-L treatment) to 21.66 (lettuce leaves, Zn-H treatment). On the whole, the order of the transfer factor was Mn ≫ Zn > Cd > Cu > Pb. Typical metal transfer factors are 0.0–2.7, 0.01–2.17, 0.3–21.7, 0.0–0.9 and 0.48–5.07 for Cd, Cu, Mn, Pb and Zn, respectively.
Heavy metal bioavailability using in vitro gastrointestinal extraction
In order to assess oral bioavailability of metals in vegetables, the edible plant materials were extracted using an in vitro gastrointestinal extraction procedure. The experimental approach consists of two processes that simulate human digestion. The first stage is designed to mimic extraction in an acidic stomach using gastric fluid (pepsin), followed by stage two which involves extraction in the neutral small intestine using intestinal fluid (pancreation, amylase and bile salts). The sample residues left from this process were then acid digested (residual fraction). After the extractions were complete, all extracts (gastric, intestinal and residual) were analysed by either DP-ASV or FAAS. The CRM (tea leaves) was extracted and analysed in the same manner as the sample, in order to evaluate the approach. The in vitro results (gastric, intestinal and residual) were compared to the results obtained from acid digestion (HNO3 + H2O2) and are given in . It is noted that Pb was not investigated in this experiment due to its low accumulation in edible plant materials; hence it could be impossible to make meaningful measurements from the in vitro study.
Table VI. Total metal level (certified values and acid digestion) and concentrations obtained from PBET experiment for Tea CRM (INCT-TL-1).
Table VII. Total metal level (acid digestion) and concentrations obtained from PBET experiment for lettuce, spinach and radish.
Cadmium
Cadmium treatment for the in vitro gastrointestinal study was applied only to radish as no carrot samples were available. The cadmium solubility was highest (54.9%) in the gastric extraction. The cadmium concentrations were similar between the intestinal phase (24.8%) and the residual fraction (20.2%).
Copper
The solubility of copper greatly differs among different plants and extraction phases. The percent of soluble copper in lettuce was 12.6, 44.4 and 42.9 in gastric, intestinal and residual phase, respectively. For spinach, none of the copper was soluble in the neutral pH (intestinal phase), 24% found in acidic phase (gastric fluid) and most of the copper (76%) was left in the residual phase. About 3% of the copper in radish can be recovered during intestinal extraction, while 62.5 and 34.7% was found in the gastric and residual fractions, respectively.
Manganese
Most of the manganese in lettuce (63.7%) and radish (45.8%) was extracted in the acidic phase (gastric fluid), while it was significantly lower in spinach (16.5%). In contrast, manganese in the vegetables was extracted to the least extent in neutral pH (about 10–20%), i.e. in the intestinal fluid. The highest content of manganese in spinach (74.7%) was measured in the residual phase, and there was significantly lower concentrations in lettuce (25.9%) and radish (33.3%).
Zinc
The major content of zinc in lettuce (47.8%), spinach (87.7%) and radish (58.5%) remained in the residual fraction. Conversely, the least amount (less than 10%) were solubilized in the intestinal phase. In the gastric phase, only 7% of zinc in spinach was extracted, while 45.2% and 32.3% were recovered from lettuce and radish, respectively.
Conclusion
With the exception of lead, metal concentrations (Cd, Cu, Mn and Zn) in lettuce, spinach, radish and carrot depend on the concentrations of the (total) metal in the soils in which the plants were grown. Mn and Zn were easily mobilized from soils to plants; they tended to accumulate in the plants in high concentrations. Cadmium, Mn and Zn were more enriched in leaves, whereas Cu accumulated more in roots. Lead, although present in some treatments, had rather lower uptake by most vegetables. As expected the metal content in the unadulterated plants was significantly lower than those in the plants grown on contaminated soil treatments. The edible leaves of spinach contained more metals (Mn, Zn) than in their roots, while those of lettuce contained lower Zn than in their roots. The Cd, Mn and Zn contents were higher in the edible roots of radish, whereas they were relatively low in carrot roots.
Under the experimental conditions, the metal bioavailability from soil to the plant as indicated by the transfer factor values decreased in the order Mn ≫ Zn > Cd > Cu > Pb. This order of metal bioavailability was in accordance with previous findings (Sauerbeck Citation1991), except Mn which was not investigated.
An assessment of the metal bioavailability using in vitro gastrointestinal extraction was carried out. The results show that the bioavailability was independent of metal and plant type. However, it was generally observed that most of the metal was present in the insoluble residual phase which is not available for absorption. It was also found that minimal amount of metals were measured in the intestinal extraction, i.e. at neutral pH. For the gastric (acidic) extraction phase, metals in certain vegetables were solubilized indicating the potential for absorption, e.g. Mn in lettuce (63.7%), Cu in radish (62.5%), Cd in radish (54.9%), Mn in radish (45.8%) and Zn in lettuce (45.2%).
| Element | Certified values (mg/kg) | Acid digestion–FAAS (mg/kg) (n = 3) | % Recovery (n = 3) |
|---|---|---|---|
| Soil–BCR 146R (total)* | |||
| Cd | 18.8 ± 0.5 | 22.1 ± 0.6 | 117.6 ± 3.2 |
| Cu | 838 ± 16 | 776 ± 9 | 92.6 ± 1.1 |
| Mn | 324 ± 7 | 284 ± 4 | 87.7 ± 1.2 |
| Pb | 609 ± 14 | 619 ± 5 | 101.6 ± 0.8 |
| Zn | 3061 ± 59 | 2772 ± 26 | 90.6 ± 0.8 |
| Soil–BCR 146R (aqua regia soluble)** | |||
| Cd | 18.5 ± 0.4 | NA | NA |
| Cu | 831 ± 16 | 798 ± 8 | 96.0 ± 1.0 |
| Mn | 298 ± 9 | 306 ± 2 | 102.7 ± 0.7 |
| Pb | 583 ± 17 | 649 ± 12 | 111.3 ± 2.1 |
| Zn | 3043 ± 58 | 3019 ± 7 | 99.2 ± 0.2 |
| Soil–GBW 07310* | |||
| Cu | 22.6 ± 2 | 19.5 ± 0.1 | 86.3 ± 0.4 |
| Mn | 1010 ± 44 | 992 ± 16 | 98.2 ± 1.6 |
| Pb | 27 ± 3 | 30 ± 6.0 | 111.1 ± 22.2 |
| Zn | 46 ± 5 | 37 ± 2 | 80.4 ± 4.3 |
| * HNO3/H2O2 digestion; **Aqua regia digestion; NA, not applicable. | |||
| Mean concentrations (μg/g, dry weight) | |||||||
|---|---|---|---|---|---|---|---|
| Element | Control (n = 3) | Mix-L (n = 5) | Mix-H (n = 3) | Zn-L (n = 5) | Zn-H (n = 3) | Cu-L (n = 5) | Cu-H (n = 3) |
| (a) Soil batch #1, used for growing lettuce. | |||||||
| Cu | 11.3 ± 1.3 | 279.4 ± 4.4 | * | 13.4 ± 4.8 | 10.6 ± 1.4 | 207.9 ± 11.0 | 927.9 ± 24.9 |
| Mn | 45.3 ± 2.5 | 572.8 ± 23.2 | * | 48.4 ± 4.3 | 41.5 ± 7.5 | 51.6 ± 3.3 | 36.9 ± 5.5 |
| Pb | 6.6 ± 4.6 | 29.5 ± 2.2 | * | 8.8 ± 1.8 | 8.0 ± 8.0 | 8.0 ± 6.3 | ND |
| Zn | 17.7 ± 5.0 | 176.8 ± 4.0 | * | 117.0 ± 5.8 | 522.3 ± 10.4 | 15.7 ± 2.6 | 14.6 ± 0.4 |
| (b) Soil batch #2, used for growing spinach. | |||||||
| Cu | 17.4 ± 1.2 | 336.3 ± 7.2 | 1205.0 ± 57.8 | 16.1 ± 2.9 | 19.8 ± 0.0 | 239.1 ± 14.7 | 1626.1 ± 72.5 |
| Mn | 51.5 ± 2.1 | 508.3 ± 73.0 | 1939.4 ± 60.0 | 49.4 ± 2.6 | 56.1 ± 0.1 | 55.6 ± 4.6 | 44.9 ± 0.0 |
| Pb | 11.9 ± 4.0 | 33.2 ± 2.2 | 92.4 ± 9.0 | 14.2 ± 3.5 | 15.8 ± 7.9 | 12.6 ± 1.8 | 13.2 ± 4.6 |
| Zn | 13.8 ± 2.1 | 181.7 ± 4.7 | 581.8 ± 2.8 | 112.6 ± 6.0 | 531.9 ± 5.5 | 15.0 ± 1.6 | 13.4 ± 1.3 |
| Element | Control (n = 3) | Mix-L (n = 5) | Mix-H (n = 4) | Cd-L (n = 4) | Cd-H (n = 4) |
|---|---|---|---|---|---|
| (c) Soil batch #3, used for growing radish. | |||||
| Cd | 0.4 ± 0.3 | 10.4 ± 1.0 | 49.1 ± 1.0 | 11.0 ± 0.3 | 70.6 ± 3.2 |
| Cu | 16.7 ± 2.1 | 170.7 ± 3.3 | 793.4 ± 31.2 | 14.6 ± 0.0 | 16.7 ± 0.0 |
| Mn | 59.2 ± 0.1 | 381.9 ± 6.5 | 1829.3 ± 13.7 | 62.2 ± 4.2 | 60.0 ± 1.4 |
| Pb | 12.5 ± 0.0 | 22.6 ± 3.5 | 58.0 ± 3.1 | 14.1 ± 3.1 | 12.6 ± 5.1 |
| Zn | 8.7 ± 1.1 | 93.5 ± 1.8 | 490.3 ± 5.4 | 10.0 ± 1.0 | 13.1 ± 0.7 |
| Element | Control (n = 1) | Mix-L (n = 3) | Cd-L (n = 2) | Cd-H (n = 3) |
|---|---|---|---|---|
| (d) Soil batch #4, used for growing carrot. | ||||
| Cd | 0.0 | 10.8 ± 0.7 | 7.1 ± 1.7 | 39.6 ± 2.4 |
| Cu | 20.9 | 180.5 ± 4.3 | 18.8 ± 0.0 | 19.5 ± 1.2 |
| Mn | 65.0 | 388.3 ± 20.6 | 60.7 ± 2.1 | 68.1 ± 15.6 |
| Pb | 6.3 | 31.4 ± 6.3 | 25.1 ± 0.0 | 16.7 ± 3.6 |
| Zn | 13.3 | 111.8 ± 0.7 | 9.5 ± 1.6 | 10.8 ± 0.6 |
| *No plant survived and soils were not collected; ND, non-detectable. | ||||
| Control, no spiking; L, low treatment; H, high treatment; Mix-L, mixed metal at low level treatment; Mix-H, mixed metal at high level treatment. | ||||
| Mean concentration (μg/g) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Control | Mix-L | Mix-H | Zn-L | Zn-H | Cu-L | Cu-H | ||||||||
| Element | Leaves, n = 3 | Roots, n = 3 | Leaves, n = 4 | Roots, n = 4 | Leaves | Roots | Leaves, n = 4 | Roots, n = 4 | Leaves, n = 3 | Roots, n = 1 | Leaves, n = 4 | Roots, n = 4 | Leaves, n = 3 | Roots, n = 3 |
| (a) Lettuce grown on soil batch #1 | ||||||||||||||
| Cu | 2.4 ± 0.2 | 10.4 ± 7.7 | 13.5 ± 2.2 | No plants survived | 3.6 ± 1.5 | 25.4 ± 13.8 | 8.0 ± 0.7 | 21.1 | 7.1 ± 1.6 | 25.2 ± 4.2 | 12.8 ± 2.6 | 262.1 ± 36.9 | ||
| Mn | 114.3 ± 9.8 | 65.2 ± 12.3 | 1246.6 ± 161.9 | 148.9 ± 33.8 | 74.3 ± 9.3 | 900.1 ± 314.2 | 221.3 | 108.5 ± 24.7 | 88.9 ± 51.3 | 381.3 ± 71.4 | 331.3 ± 19.9 | |||
| Pb | 0.2 ± 0.2 | 1.0 ± 1.2 | ND | 0.3 ± 0.1 | 1.1 ± 0.4 | 1.7 ± 3.0 | ND | 2.1 ± 2.8 | 1.0 ± 0.7 | 4.3 ± 5.3 | 0.6 ± 1.0 | |||
| Zn | 31.3 ± 8.1 | 42.6 ± 17.1 | 154.3 ± 43.8 | 78.3 ± 22.9 | 97.5 ± 16.7 | 1171.0 ± 236.9 | 1440.6 | 30.2 ± 3.2 | 46.7 ± 15.5 | 54.7 ± 4.2 | 51.6 ± 5.3 | |||
| Control | Mix-L | Mix-H | Zn-L | Zn-H | Cu-L | Cu-H | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Element | Leaves, n = 3 | Roots, n = 3 | Leaves, n = 5 | Roots, n = 5 | Leaves, n = 1 | Roots, n = 1 | Leaves, n = 4 | Roots, n = 4 | Leaves, n = 3 | Roots, n = 3 | Leaves, n = 4 | Roots, n = 4 | Leaves, n = 3 | Roots, n = 3 |
| (b) Spinach grown on soil batch #2 | ||||||||||||||
| Cu | 4.6 ± 0.3 | 21.2 ± 5.1 | 20.5 ± 3.5 | 46.1 ± 5.4 | 29.6 | 70.1 | 6.0 ± 2.4 | 11.7 ± 4.7 | 7.5 ± 3.0 | 43.0 ± 18.9 | 17.9 ± 4.3 | 31.3 ± 15.0 | 32.3 ± 5.9 | 150.1 ± 50.3 |
| Mn | 180.7 ± 18.5 | 232.5 ± 44.9 | 1971.1 ± 324.4 | 882.7 ± 291.3 | 6631.6 | 901.5 | 246.6 ± 49.1 | 221.0 ± 43.4 | 233.3 ± 51.2 | 229.2 ± 71.4 | 206.0 ± 32.5 | 226.2 ± 64.1 | 468.4 ± 103.4 | 260.4 ± 72.7 |
| Pb | 0.2 ± 0.1 | 0.7 ± 0.4 | 1.7 ± 2.8 | 4.0 ± 3.8 | 5.2 | 11.1 | 1.9 ± 1.3 | 0.3 ± 0.5 | 0.3 ± 0.6 | 3.9 ± 4.3 | 3.6 ± 1.0 | 7.4 ± 6.8 | 1.0 ± 0.9 | 7.8 ± 0.5 |
| Zn | 70.2 ± 12.9 | 67.2 ± 5.8 | 233.5 ± 60.3 | 162.2 ± 36.5 | 638.5 | 275.9 | 225.9 ± 69.3 | 167.0 ± 32.0 | 632.0 ± 29.9 | 719.0 ± 175.6 | 45.6 ± 4.3 | 42.2 ± 4.6 | 54.9 ± 3.2 | 62.4 ± 9.8 |
| ND, non-detectable. | ||||||||||||||
| Mean concentration (μg/g) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Control | Mix-L | Mix-H | Cd-L | Cd-H | ||||||
| Element | Leaves, n = 3 | Roots, n = 3 | Leaves, n = 5 | Roots, n = 5 | Leaves, n = 1 | Roots, n = 1 | Leaves, n = 3 | Roots, n = 3 | Leaves, n = 4 | Roots, n = 4 |
| (c) Radish grown on soil batch #3 | ||||||||||
| Cd | 0.4 ± 0.4 | 0.9 ± 0.4 | 9.8 ± 1.8 | 5.7 ± 1.2 | 44.8 | 19.2 | 29.5 ± 12.8 | 8.8 ± 2.2 | 184.4 ± 6.4 | 68.2 ± 68.0 |
| Cu | 15.0 ± 7.1 | 10.7 ± 3.0 | 19.8 ± 4.3 | 7.2 ± 1.9 | 48.6 | 26.9 | 11.9 ± 9.1 | 2.1 ± 0.9 | 6.0 ± 3.6 | 36.3 ± 30.8 |
| Mn | 104.1 ± 37.0 | 17.6 ± 3.5 | 510.3 ± 114.8 | 60.5 ± 8.6 | 3821.8 | 758.9 | 139.4 ± 16.3 | 17.2 ± 6.7 | 134.7 ± 25.7 | 16.9 ± 19.8 |
| Pb | 3.4 ± 0.6 | 4.4 ± 4.2 | 0.8 ± 1.9 | ND | ND | ND | 2.7 ± 1.8 | ND | 0.9 ± 0.1 | 11.8 ± 13.1 |
| Zn | 32.2 ± 8.2 | 25.6 ± 5.9 | 121.7 ± 19.2 | 69.1 ± 7.1 | 599.3 | 500.3 | 26.9 ± 8.5 | 25.9 ± 6.7 | 62.6 ± 4.8 | 36.7 ± 37.1 |
| Control | Mix-L | Cd-L | Cd-H | |||||
|---|---|---|---|---|---|---|---|---|
| Element | Leaves, n = 1 | Roots, n = 1 | Leaves, n = 3 | Roots, n = 1 | Leaves, n = 2 | Roots, n = 2 | Leaves, n = 3 | Roots, n = 3 |
| (d) Carrot grown on soil batch #4 | ||||||||
| Cd | 0.2 | 1.0 | 8.3 ± 4.6 | 9.5 | 10.5 ± 3.2 | 8.0 ± 1.8 | 47.4 ± 6.8 | 27.4 ± 5.2 |
| Cu | 12.5 | 6.4 | 24.9 ± 12.7 | 15.5 | 10.1 ± 2.1 | 5.8 ± 0.9 | 16.3 ± 8.0 | 8.3 ± 7.7 |
| Mn | 198.0 | 34.4 | 1857.1 ± 716.6 | 271.0 | 165.5 ± 31.4 | 27.7 ± 3.0 | 342.6 ± 101.1 | 56.3 ± 19.7 |
| Pb | 0.7 | ND | 8.2 ± 11.3 | ND | ND | 0.5 ± 0.7 | 7.3 ± 8.8 | 1.2 ± 0.8 |
| Zn | 41.8 | 25.4 | 96.9 ± 10.8 | 52.6 | 40.0 ± 2.5 | 23.6 ± 4.5 | 43.2 ± 8.2 | 26.7 ± 5.0 |
| ND, non-detectable; Control, no spiking; L, low treatment; H, high treatment; Mix-L, mixed metal at low level treatment; Mix-H, mixed metal at high level treatment. | ||||||||
| Element | Certified reference material | Certified value μg/g | Measured μg/g | Mean recovery (%) |
|---|---|---|---|---|
| Cd | BCR 146R | 18.8 ± 0.5 | 18.7 ± 4.3+ | 99.5 ± 22.9+ |
| Cu | INCT-TL-1 | 20.4 ± 1.5 | 26.5 ± 2.7* | 129.9 ± 13.2* |
| Pb | BCR 146R | 609 ± 14 | 626 ± 71+ | 102.8 ± 11.7+ |
| * n = 17; + n = 12. | ||||
| TF values | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Control (n = 3) | Mix-L (n = 4) | Mix-H | Zn-L (n = 4) | Zn-H (n = 3) | Cu-L (n = 4) | Cu-H (n = 3) | ||||||||
| Element | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots |
| (a) Lettuce grown on soil batch #1 | ||||||||||||||
| Cu | 0.21 | 0.92 | 0.05 | No plants survived | 0.27 | 1.89 | 0.76 | 2.00 | 0.03 | 0.12 | 0.01 | 0.28 | ||
| Mn | 2.52 | 1.44 | 2.18 | 3.08 | 1.53 | 21.66 | 5.33 | 2.10 | 1.72 | 10.35 | 8.99 | |||
| Pb | 0.03 | 0.15 | 0.00 | 0.04 | 0.13 | 0.66 | 0.00 | 0.27 | 0.12 | 0.00 | 0.00 | |||
| Zn | 1.78 | 2.41 | 0.87 | 0.67 | 0.83 | 2.24 | 2.76 | 1.93 | 2.98 | 3.75 | 3.54 | |||
| Control (n = 3) | Mix-L (n = 5) | Mix-H (n = 1) | Zn-L (n = 4) | Zn-H (n = 3) | Cu-L (n = 4) | Cu-H (n = 3) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Element | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots |
| (b) Spinach grown on soil batch #2 | ||||||||||||||
| Cu | 0.26 | 1.22 | 0.06 | 0.14 | 0.02 | 0.06 | 0.37 | 0.72 | 0.38 | 2.17 | 0.07 | 0.13 | 0.02 | 0.09 |
| Mn | 3.51 | 4.52 | 3.88 | 1.74 | 3.42 | 0.46 | 4.99 | 4.47 | 4.16 | 4.09 | 3.71 | 4.07 | 10.44 | 5.80 |
| Pb | 0.02 | 0.06 | 0.05 | 0.12 | 0.06 | 0.12 | 0.13 | 0.02 | 0.02 | 0.25 | 0.28 | 0.59 | 0.08 | 0.60 |
| Zn | 5.07 | 4.86 | 1.29 | 0.89 | 1.10 | 0.47 | 2.01 | 1.48 | 1.19 | 1.35 | 3.03 | 2.81 | 4.10 | 4.66 |
| Control (n = 3) | Mix-L (n = 5) | Mix-H (n = 1) | Cd-L (n = 3) | Cd-H (n = 4) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Element | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots |
| (c) Radish grown on soil batch #3. | ||||||||||
| Cd | 0.91 | 2.25 | 0.95 | 0.55 | 0.91 | 0.39 | 2.68 | 0.80 | 2.61 | 0.97 |
| Cu | 0.90 | 0.64 | 0.12 | 0.04 | 0.06 | 0.03 | 0.81 | 0.15 | 0.36 | 2.17 |
| Mn | 1.76 | 0.30 | 1.34 | 0.16 | 2.09 | 0.41 | 2.24 | 0.28 | 2.25 | 0.28 |
| Pb | 0.27 | 0.35 | 0.19 | 0.00 | 0.00 | 0.00 | 0.19 | 0.01 | 0.07 | 0.94 |
| Zn | 3.69 | 2.93 | 1.30 | 0.74 | 1.22 | 1.02 | 2.68 | 2.58 | 4.78 | 2.81 |
| Control (n = 1) | Mix-L (n = 3) | Cd-L (n = 2) | Cd-H (n = 3) | |||||
|---|---|---|---|---|---|---|---|---|
| Element | Leaves | Roots | Leaves | Roots | Leaves | Roots | Leaves | Roots |
| (d) Carrot grown on soil batch #4 | ||||||||
| Cd | 0.00 | 0.00 | 0.77 | 0.88 | 1.48 | 1.14 | 1.20 | 0.69 |
| Cu | 0.60 | 0.31 | 0.14 | 0.09 | 0.54 | 0.31 | 0.83 | 0.43 |
| Mn | 3.04 | 0.53 | 4.78 | 0.70 | 2.73 | 0.46 | 5.03 | 0.83 |
| Pb | 0.11 | 0.00 | 0.26 | 0.00 | 0.00 | 0.04 | 0.44 | 0.07 |
| Zn | 3.13 | 1.90 | 0.87 | 0.47 | 4.23 | 2.49 | 4.01 | 2.48 |
| Control, no spiking; L, low treatment; H, high treatment; Mix-L, mixed metal at low level treatment; Mix-H, mixed metal at high level treatment. | ||||||||
| Physiologically-based extraction test (PBET), μg/g (dry weight) | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Phase I (Gastric) | Phase II (Intestinal) | Phase III (Residual) | |||||||
| Element | Certified values | Acid digestion, μg/g (dry wt) | Mean ± SD | % | Mean ± SD | % | Mean ± SD | % | Total Phase I + II + III |
| Tea leaves (n = 3) | |||||||||
| Cu | 20.4 ± 1.5 | 24.3 ± 0.1 | 1.3 ± 0.4 | 5.4 | ND | ND | 23.4 ± 8.2 | 94.6 | 24.8 ± 8.0 |
| Mn | 1570 ± 110 | 1541 ± 21 | 888 ± 56 | 52.1 | 80 ± 5 | 4.7 | 735 ± 80 | 43.2 | 1703 ± 146 |
| Mn was measured by FAAS and Cu was measured by DP-ASV; ND, non-detectable. | |||||||||
| Physiologically-based extraction test (PBET), μg/g (dry weight) | ||||||||
|---|---|---|---|---|---|---|---|---|
| Phase I (Gastric) | Phase II (Intestinal) | Phase III (Residual) | ||||||
| Element | Acid digestion, μg/g (dry wt) | Mean ± SD | % | Mean ± SD | % | Mean ± SD | % | Total Phase I + II + III |
| Lettuce leaves (n = 3) | ||||||||
| Mn | 1246.6 ± 161.9 | 768.4 ± 26.9 | 63.7 | 125.2 ± 7.7 | 10.4 | 312.2 ± 10.8 | 25.9 | 1205.8 ± 34.9 |
| Zn | 154.3 ± 43.8 | 80.3 ± 14.6 | 45.2 | 12.3 ± 5.8 | 6.9 | 84.9 ± 1.7 | 47.8 | 177.5 ± 20.1 |
| Spinach leaves (n = 2) | ||||||||
| Cu | 20.5 ± 3.5 | 4.0 ± 1.8 | 24.0 | ND | ND | 12.8 ± 3.1 | 76.0 | 16.9 ± 3.6 |
| Mn | 1971.1 ± 324.4 | 347.1 ± 98.7 | 16.5 | 182.6 ± 44.1 | 8.7 | 1567.7 ± 271.0 | 74.7 | 2097.5 ± 89.3 |
| Zn | 233.5 ± 60.3 | 16.7 ± 6.8 | 7.0 | 12.8 ± 15.1 | 5.3 | 211.2 ± 44.1 | 87.7 | 240.8 ± 67.5 |
| Radish roots (n = 3) | ||||||||
| Cd | 5.7 ± 1.2 | 3.0 ± 0.4 | 54.9 | 1.3 ± 0.5 | 24.8 | 1.1 ± 0.3 | 20.2 | 5.4 ± 1.1 |
| Mn | 60.5 ± 8.6 | 26.4 ± 1.5 | 45.8 | 11.9 ± 1.0 | 20.6 | 19.4 ± 3.0 | 33.7 | 57.7 ± 6.8 |
| Zn | 69.1 ± 7.1 | 21.2 ± 6.3 | 32.3 | 6.0 ± 2.4 | 9.2 | 38.5 ± 2.2 | 58.5 | 65.7 ± 10.4 |
| Mn and Zn were measured by FAAS, Cu and Cd were measured by DP-ASV; ND, non-detectable. | ||||||||
Related Research Data
Acknowledgements
The Royal Thai Government is acknowledged for financial support to one of us (MI). We also acknowledge the technical support of Gary Askwith, Jim Creighton and David Osborne (Northumbria University).
References
- Bahemuka, TE and Mubofu, EB. 1999. Heavy metals in edible green vegetables grown along the sites of the Sinza and Msimbazi rivers in Dar es Salaam, Tanzania. Food Chemistry, 66: 63–66. [Crossref], [Web of Science ®], [Google Scholar]
- Caussy, D, Gochfeld, M, Gurzau, E, Neagu, C and Ruedel, H. 2003. Lessons from case studies of metals: Investigating exposure, bioavailability, and risk. Ecotoxicology and Environmental Safety, 56: 45–51. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Chojnacka, K, Chojnacki, A, Gorecka, H and Gorecki, H. 2005. Bioavailability of heavy metals from polluted soils to plants. Science of the Total Environment, 337: 175–182. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Cobb, GP, Sands, K, Waters, M, Wixson, BG and Dorward-King, E. 2000. Accumulation of heavy metals by vegetables grown in mine wastes. Environmental Toxicology and Chemistry, 19: 600–607. [Crossref], [Web of Science ®], [Google Scholar]
- Crews, HM, Burrell, JA and McWeeny, DJ. 1983. Preliminary enzymolysis studies on trace element extractability from food. Journal of the Science of Food and Agriculture, 34: 997–1004. [Crossref], [Web of Science ®], [Google Scholar]
- Cui, Y-J, Zhu, Y-G, Zhai, R-H, Chen, D-Y, Huang, Y-Z, Qiu, Y and Liang, J-Z. 2004. Transfer of metals from soil to vegetables in an area near a smelter in Nanning, China. Environment International, 30: 785–791. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Dean, JR. 2003. Methods for environmental trace analysis, 76Chichester: John Wiley & Sons. [Crossref], [Google Scholar]
- Dudka, S, Piotrowska, M and Terelak, H. 1996. Transfer of cadmium, lead, and zinc from industrially contaminated soil to crop plants: A field study. Environmental Pollution, 94: 181–188. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Gray, CW, McLaren, RG, Roberts, AHC and Condron, LM. 1999. Cadmium phytoavailability in some New Zealand soils. Australian Journal of Soil Research, 37: 461–477. [Crossref], [Web of Science ®], [Google Scholar]
- Hack, A and Selenka, F. 1996. Mobilization of PAH and PCB from contaminated soil using a digestive tract model. Toxicology Letters, 88: 199–210. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Helgesen, H and Larsen, EH. 1998. Bioavailability and speciation of arsenic in carrots grown in contaminated soil. Analyst, 123: 791–796. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Hooda, PS. 1997. Plant availability of heavy metals in soils previously amended with heavy applications of sewage sludge. Journal of the Science of Food and Agriculture, 73: 446–454. [Crossref], [Web of Science ®], [Google Scholar]
- Hough, RL, Breward, N, Young, SD, Crout, NMJ, Tye, AM, Moir, AM and Thornton, I. 2004. Assessing potential risk of heavy metal exposure from consumption of home-produced vegetables by urban populations. Environmental Health Perspectives, 112(2): 215–221. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Hund-Rinke, K and Kordel, W. 2003. Underlying issues in bioaccessibility and bioavailability: Experimental methods. Ecotoxicology and Environmental Safety, 56: 52–62. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Kelley, ME, Brauning, SE, Schoof, RA and Ruby, MV. 2002. Assessing oral bioavailability of metals in soil, 2, 18Ohio: Battelle Press. [Google Scholar]
- Lu, A, Zhang, S, Shan, X-q, Wang, S and Wang, Z. 2003. Application of microwave extraction for the evalutation of bioavailability of rare earth elements in soils. Chemosphere, 53: 1067–1075. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Mattina, MI, Iannucci-Berger, W, Musante, C and White, JC. 2003. Concurrent plant uptake of heavy metals and persistent organic pollutants from soil. Environmental Pollution, 124: 375–378. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- McGrath, D. 1996. Application of single and sequential extraction procedures to polluted and unpolluted soils. Science of the Total Environment, 178: 37–44. [Crossref], [Web of Science ®], [Google Scholar]
- Mench, M, Vangronsveld, J, Didier, V and Clijsters, H. 1994. Evaluating of metal mobility, plant availability and immobilization by chemical agents in a limed-silty soil. Environmental Pollution, 86: 279–286. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Miller, DD, Schricker, BR, Rasmussen, RR and Campen, DV. 1981. An in vitro method for estimation of iron availability from meals. The American Journal of Clinical Nutrition, 34: 2248–2256. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Mossop, KF and Davidson, CM. 2003. Comparison of original and modified BCR sequential extraction procedures for the fractionation of copper, iron, lead, manganese and zinc in soils and sediments. Analytica Chimica Acta, 478: 111–118. [Crossref], [Web of Science ®], [Google Scholar]
- Muller, M and Anke, M. 1994. Distribution of cadmium in the food chain (soil-plant-human) of a cadmium exposed area and the health risks of the general population. Science of the Total Environment, 156: 151–158. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Nasreddine, L and Parent-Massin, D. 2002. Food contamination by metals and pesticides in the European Union. Should we worry?. Toxicology Letters, 127: 29–41. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Peijnenburg, WJGM and Jager, T. 2003. Monitoring approaches to assess bioaccessibility and bioavailability of metals: Matrix issues. Ecotoxicology and Environmental Safety, 56: 63–77. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Quevauviller, P. 1998. Operationally defined extraction procedures for soil and sediment analysis I. Standardization. Trends in Analytical Chemistry, 17: 289–298. [Crossref], [Web of Science ®], [Google Scholar]
- Rauret, G, Lopez-Sanchez, JF, Sahuquillo, A, Rubio, R, Davidson, C, Ure, A and Quevauvilleret, Ph. 1999. Improvement of the BCR three step sequential extraction procedure prior to the certification of new sediment and soil reference materials. Journal of Environmental Monitoring, 1: 57–61. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Reichman, SM. 2002. “The responses of plants to metal toxicity: A review focusing on copper, manganese and zinc”. Australian Minerals & Energy Environment Foundation. Available from: http://www.plantstress.com/Articles/toxicity_i/Metal_toxicity.pdf. Accessed 2004 October 1 [Google Scholar]
- Ruby, MV, Davis, A, Link, TE, Schoof, R, Chaney, RL, Freeman, GB and Bergstrom, P. 1993. Development of an in vitro screening test to evaluate the in vivo bioaccessibility of ingested mine-waste lead. Environmental Science and Technology, 27: 2870–2877. [Crossref], [Web of Science ®], [Google Scholar]
- Ruby, MV, Davis, A, Schoof, R, Eberle, S and Sellstone, CM. 1996. Estimation of lead and arsenic bioavailability using a physiologically based extraction test. Environmental Science and Technology, 30: 422–430. [Crossref], [Web of Science ®], [Google Scholar]
- Sahuquillo, A, Rigol, A and Rauret, G. 2003. Overview of the use of leaching/extraction tests for risk assessment of trace metals in contaminated soils and sediments. Trends in Analytical Chemistry, 22(3): 152–159. [Crossref], [Web of Science ®], [Google Scholar]
- Sanchez-Camazano, M, Sanchez-Martin, MJ and Lorenzo, LF. 1994. Lead and cadmium in soils and vegetables from urban gardens of Salamanca (Spain). Science of the Total Environment, 146/147: 163–168. [Crossref], [Google Scholar]
- Sauerbeck, DR. 1991. Plant, element and soil properties governing uptake and availability of heavy metals derived from sewage sludge. Water, Air and Soil Pollution, 57/58: 227–237. [Crossref], [Google Scholar]
- Shiowatana, J, Tantidanai, N, Nookabkaew, S and Nacapricha, D. 2001. A flow system for the determination of metal speciation in soil by sequential extraction. Environment International, 26: 381–387. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Shuman, LM. 1996. “Differential Pulse Voltammetry”. In Methods of Soil Analysis Part 3: Chemical Methods, Edited by: Sparks, DL, Page, AL, Helmke, PA, Loeppert, RH, Soltanpour, PN, Tabatabai, MA, Johnston, CT and Sumner, ME. 260Wisconsin: Soil Science Society of America, Inc.. editors [Crossref], [Google Scholar]
- Tessier, A, Campbell, PGC and Bisson, M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry, 51: 844–851. [Crossref], [Web of Science ®], [Google Scholar]
- Wang, X-P, Shan, X-Q, Zhang, S-Z and Wen, B. 2004. A model for evaluation of the phytoavailability of trace elements to vegetables under the field conditions. Chemosphere, 55: 811–822. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Watanabe, MA. 1997. Phytoremediation on the brink of commercialization. Environmental Science and Technology, 31: 182A–186A. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]
- Wolnik, KA, Fricke, FL, Capar, SG, Braude, GL, Meyer, MW, Satzger, RD and Bonnin, E. 1983. Elements in major raw agricultural crops in the United States. Cadmium and lead in lettuce, peanuts, potatoes, soybeans, sweet corn, and wheat. Journal of Agricultural and Food Chemistry, 31: 1240–1244. [Google Scholar]
- World Health Organization (WHO). 1989. Environmental Health Criteria for Lead, Available from: http://www.inchem.org/documents/ehc/ehc/ehc85.htm. Accessed 2004 September 13 [Google Scholar]
- World Health Organization (WHO). 1998. Environmental Health Criteria for Copper, Available from: http://www.inchem.org/documents/ehc/ehc/ehc200.htm. Accessed 2004 September 13 [Google Scholar]
- World Health Organization (WHO). 2001. Environmental Health Criteria for Zinc, Available from: http://www.inchem.org/documents/ehc/ehc/ehc221.htm. Accessed 2004 September 13 [Google Scholar]
- Zhou, DM, Hao, XZ, Wang, YJ, Dong, YH and Cang, L. 2005. Copper and zinc uptake by radish and pakchoi as affected by application of livestock and poultry manures. Chemosphere, 59: 167–175. [Crossref], [PubMed], [Web of Science ®], [Google Scholar]