The extraction by soil and absorption by plants of applied zinc and cadmium

In five consecutive years lettuce, spinach, spring wheat, endive and maize were grown in pots and the effects of native and soil-applied Zn and Cd on plant Zn and Cd concentrations were studied. The normal interactive pattern was antagonistic, Zn reducing plant Cd uptake, and conversely, but less so. Only in loam soil Zn and Cd were synergistic to some extent, plant Zn uptake increasing with applied Cd.When relating total soil Cd/Zn to plant Cd/Zn separate sets of data could be distinguished for loam and sandy soil, each fitting a straight line. The use of 0.1 M CaCl₂ instead of total extractable soil Cd/Zn makes the two sets of data to coalesce around a single straight line. All crops were found to show a positive linear relationship between 0.1 M CaCl₂-extractable soil Cd/Zn and plant Cd/Zn.     

The Effects of Drying Temperature on the Extractability of Metals from Dredged Sediments

Dredged sediments may contain considerable amounts of metallic and/or organic pollutants. The risk of metal transfer from sediment deposits to soils can be evaluated using chemical extraction procedures; however, the temperatures at which samples are dried before metal extraction vary widely from one study to another. This led us to investigate the impact of drying temperature on the extractability of metals from dredged sediments. First, water-, CaCl ₂ - and DTPA-extractions were performed on 12 dredged sediments collected in France, with analyses being carried out on both raw (i.e. not dried) samples and on samples dried at 105°C. Higher extractable Cd and Zn contents were recorded for the samples dried at 105°C than for the raw samples. In order to assess the effect of drying temperature on metal extractability, we selected one sediment and carried out CaCl ₂ -, HNO ₃ - and DTPA-extractions on the raw sample and on samples dried at three different temperatures. In general, increasing the drying temperature led to an increase in the extractability of Cr and Cu for all three extraction methods. The CaCl ₂ -extractability of Ni and Zn decreased as drying temperature increased, except for the sample dried at 105°C. The HNO ₃ - and DTPA-extractabilities of these two metals were highest when the sediments were air-dried and then decreased with increasing drying temperature. Similar behavior was observed for Cd, except in the case of HNO ₃ -extractability. As the aim of chemical analysis using selective extraction is to measure the bio-available fractions of metals in soils and sediments, we suggest that extraction should be carried out on raw samples in order to simulate plant growth conditions.     

Effects of biochar amendments on speciation and bioavailability of heavy metals in coal-mine-contaminated soil

An incubation experiment was executed on applying biochar as a soil remediation amendment to discuss an effect of the various addition rates on the speciation and bioavailability of heavy metals in mining-contaminated soil. The result showed that the content of Cd in soil was 9.51 times higher than the Huainan soil background values. The contents of Cu, Zn and As were 2.97, 1.60 and 1.42 times the background values, respectively, and the total contents of all heavy metals were higher than the standard values of soil environment quality GB15618-1995 set by the China Ministry of Environmental Protection. Speciation analysis indicated that Cu and Cd were mainly associated with the reducible fraction, while Zn and As were dominated by the residual fraction. After biochar was added to contaminated soil, the residual fractions of heavy metals increased, while the acid-soluble fractions reduced. According to the results of CaCl₂ extraction experiment, CaCl₂-extractable concentrations of Cu, Zn, As, and Cd were observed with a biochar dosage rate of 10%, which were 57.26%, 51.37%, 6.94% and 42.04% lower than those of control soil samples, respectively, but there were no obvious changes of CaCl₂-extractable As.     

Response of corn to micronutrients (Zn and Cu) on a saline soil. I. Growth and ionic relationships

Summary Response of corn to Zn and Cu on a salinized soil in pots has been studied. Zinc increased the dry weights of tops and roots at all levels of NaCl+CaCl₂. Increasing Zn level increased the weights considerably at 10 mM NaCl+CaCl₂. Copper increased the weight of tops at 10 mM NaCl+CaCl₂: it had little effects on plant weights at 50–125 mM NaCl+CaCl₂. The growth response of plant to low Cu was somewhat similar to that of higher Zinc.NaCl+CaCl₂ treatments, in general, increased Zn concentrations in tops as well as roots. At low Zn application, Zn concentrations in the tops were higher than those in the roots but at high Zn application, the concentrations of Zn in tops were similar to those in the roots.NaCl+CaCl₂ treatments increased Cu concentrations in the tops to a slight extent but had a depressive effect on those in the roots. Copper concentrations in the tops were, however, much below those in the roots. The greater retention of Cu in the roots remains to be explained.     

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

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

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

The influence of composting and maturation processes on the heavy-metal extractability from some organic wastes

Seven mixtures from four organic residues—aerobic sewage sludge, city refuse, peat residue and grape debris—were made up to study the influence of the composting and maturation processes on the extractability of Fe, Cu, Ni, Zn, Cd, Pb, Cr and Mn by a chelating agent (diethylenetriaminepentaacetic acid, DTPA) and a neutral salt solution (0·05m CaCl₂) from the residues.Composting and maturation increased the concentration of heavy metals in the materials due to the loss of weight of the materials during these processes. The metals also became more insoluble, and extractants employed removed larger amounts of metals from the raw composts than from the mature composts. In general, CaCl₂-extractable metals did not correlate significantly with the metal content of the composts. With DTPA-extractable metals, the only ones that showed significant correlations between the quantity of metal extracted by DTPA and the metal content of the samples in nearly all the composts studied were Pb and Zn.The load of ‘zinc equivalent’ increased with the composting and maturation processes, and the Cd/Zn ratio decreased with those processes.     

Phytoextraction of Cd and Zn from agricultural soils by Salix ssp. and intercropping of Salix caprea and Arabidopsis halleri

Contamination of agricultural topsoils with Cd above guideline values is of concern in many countries throughout the world. Extraction of metals from contaminated soils using high-biomass, metal-accumulating Salix sp. has been proposed as a low-cost, gentle remediation strategy, but reasonable phytoextraction rates remain to be demonstrated. In an outdoor pot experiment we assessed the phytoextraction potential for Cd and Zn of four willow species (Salix caprea, S. fragilis, S. × smithiana, S. × dasyclados) and intercropping of S. caprea with the hyperaccumulator Arabidopsis halleri on three moderately contaminated, agricultural soils. Large concentrations of Cd (250 mg kg⁻¹) and Zn (3,300 mg kg⁻¹) were determined in leaves of Salix × smithiana grown on a soil containing 13.4 mg kg⁻¹ Cd and 955 mg kg⁻¹ Zn, resulting in bioaccumulation factors of 27 (Cd) and 3 (Zn). Total removal of up to 20% Cd and 5% Zn after three vegetation periods were shown for Salix × smithiana closely followed by S. caprea, S. fragilis and S. × dasyclados. While total Cd concentrations in soils were reduced by up to 20%, 1 M NH₄NO₃-extractable metal concentrations did not significantly decrease within 3 years. Intercropping of S. caprea and A. halleri partly increased total removal of Zn, but did not enhance total Cd extraction compared to single plantings of S. caprea after two vegetation periods.     

Effects of liming on the extractability of Fe,Mn, Zn and Cu from a peat medium and the growth and micronutrient uptake of highbush blueberry plants

Summary Levels of extractable micronutrients in a peat and the growth and nutrient uptake of young highbush blueberry plants (Vaccinium corymbosum L cv. Blueray) were studied in a greenhouse experiment in response to liming and two rates of addition of Fe, Mn, Zn and Cu.Levels of extractable micronutrients showed different trends with liming depending upon the extractant used and the element being considered. Levels of 0.05M CaCl₂-extractable Fe, Mn and Zn decreased as the pH was raised whilst those of Cu first decreased and then increased again. There was a general decline in 0.1M HCl-extractable Fe, Mn and Cu with increasing pH but levels of Zn were not greatly affected. Levels of 0.005M DTPA extractable Fe, Mn Zn and Cu generally declined but those extractable with 0.04M EDTA were either unaffected or increased as the pH was raised. Levels of CaCl₂-extractable Mn and Zn were the same order of magnitude as those extractable with HCl, DTPA and EDTA. In contrast, the latter reagents extracted considerably more Fe and Cu than did CaCl₂.Dry matter yields of plants were increased as the pH was raised from 3.9 to 4.3 but then decreased markedly as the pH was raised further to 6.7. With increasing pH, concentrations of plant Fe generally increased those of Mn were decreased and those of Zn and Cu were not greatly affected except for a marked decline in plant Cu at pH 6.7.     

Evaluation of chemical methods for estimating available zinc and response of green gram (Phaseolus aureus roxb) to applied zinc in non-calcareous soils

Summary Studies were conducted in 22 non-calcareous soils (India) to evaluate various extractants,viz. (6N HCl, 0.1N HCl, EDTA (NH₄)₂CO₃, EDTA NH₄OAc, DTPA+CaCl₂ and 1M MgCl₂) to find critical levels of soil and plant Zn for green gram (Phaseolus aureus Roxb.). The order of extractability by the different extractants was 6N HCl>0.1N HCl>EDTA (NH₄)₂CO₃<EDTA NH₄OAc DTPA+CaCl₂>1M MgCl₂. Critical levels of 0.48 ppm DTPA × CaCl₂ extractable Zn, 0.80 ppm EDTA NH₄OAc extractable Zn, 0.70 ppm EDTA (NH₄)₂CO₃ extractable Zn, and 2.2 ppm 0.1N HCl extractable Zn were estimated for the soils tested. The critical Zn concentration in 6 weeks old plants was found to be 19 ppm. The 0.1N HCl method gave the best correlation (r=0.588^**) between extractable Zn and Bray's per cent yield, while with DTPA+CaCl₂, it was slightly low (r=0.542^**). The DTPA + CaCl₂ method gave significant (r=0.73^**) correlation with plant Zn concentration. The 0.1N HCl gave the higher correlation with Zn uptake (r=0.661^**) than DTPA (r=0.634^**) 6N HCl and 1M MgCl₂ method gave nonsignificant positive relationship with Bray's per cent yield. For noncalcareous soils apart from the common use of DTPA+CaCl₂, 0.1N HCl can also be used for predicting soil available Zn. The use of 0.1N HCl would be much cheaper than DTPA and other extractants used in the study.     

The Effect of Electrolyte Type and Concentration on the Release of Cd,Cu, Ni,and Zn in Some Contaminated Calcareous Soils

This paper reports the results of desorption experiments of cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn) from some contaminated calcareous soils under four electrolyte types (CaCl₂, MgCl₂, NaCl and Na₂SO₄) with different electrolyte concentrations (0.5, 4 and 10 mM). Among electrolytes, CaCl₂ significantly released more metals from soils. There was a negative relationship between total Cu and Zn content and percentage of Cu and Zn released (average of electrolyte concentrations) using CaCl₂ solution, indicating a higher Cu and Zn released when their total content was low. Generally, Cd, Cu, and Zn speciation was affected by both type of electrolytes and their concentrations, whereas Ni speciation stayed mostly stable and was almost unaffected by applied solutions. It can be suggested that beside competition with cations, chloro-complexation is important parameter in Cd release, while CuOH⁺, and to some extent ZnOH⁺ are important species affecting release of Cu and Zn. The distribution coefficient (K_d) values for each metals greatly varied with the types of electrolytes and electrolyte concentration. On the basis of average percentage of metal released under different electrolytes and concentrations the following sequences was found: Cd > Cu > Ni > Zn. The results are important in understanding the mobility of metals under different solutions and indicating that, Cd and Zn soils may pose a higher and lower mobility and ecological risk in contaminated calcareous soils, respectively.     

Long-Term Biomonitoring of Soil Contamination Using Poplar Trees: Accumulation of Trace Elements in Leaves and Fruits

Phytostabilization aims to immobilize soil contaminants using higher plants. The accumulation of trace elements in Populus alba leaves was monitored for 12 years after a mine spill. Concentrations of As and Pb significantly decreased, while concentrations of Cd and Zn did not significantly over time. Soil concentrations extracted by CaCl₂ were measured by ICP-OES and results of As and Pb were below the detection limit. Long-term biomonitoring of soil contamination using poplar leaves was proven to be better suited for the study of trace elements. Plants suitable for phytostabilization must also be able to survive and reproduce in contaminated soils. Concentrations of trace elements were also measured in P. alba fruiting catkins to determine the effect on its reproduction potential. Cadmium and Zn were found to accumulate in fruiting catkins, with the transfer coefficient for Cd significantly greater than Zn. It is possible for trace elements to translocate to seed, which presents a concern for seed germination, establishment and colonization. We conclude that white poplar is a suitable tree for long-term monitoring of soil contaminated with Cd and Zn, and for phytostabilization in riparian habitats, although some caution should be taken with the possible effects on the food web.

Supplemental materials are available for this article. Go to the publisher's online edition of International Journal of Phytoremediation to view the supplemental file.     

Effects of soil acidification on the chemical extractability of Fe,Mn, Zn and Cu and the growth and micronutrient uptake of highbush blueberry plants

Summary The effects of soil acidification and micronutrient addition on levels of extractable Fe, Mn, Zn and Cu in a soil, and on the growth and micronutrient uptake of young highbush blueberry plants (Vaccinium corymbosum L. cv. Blueray) was investigated in a greenhouse study.Levels of 0.05M CaCl₂-extractable Fe, Mn, Zn and Cu increased as the pH was lowered from 7.0 to 3.8. However, the solubility (CaCl₂-extractability) of Fe and Cu was considerably less pH-dependent than that of Mn and Zn. With the exception of HCl-and DTPA-extractable Mn, micronutrients extractable with 0.1M HCl, 0.005M DTPA and 0.04M EDTA were unaffected or raised only slightly as the pH was lowered from 6.0 to 3.8. Quantities of Mn and Zn extractable with CaCl₂ were similar in magnitude to those extractable with HCl, DTPA and EDTA whilst, in contrast, the latter reagents extracted considerably more Cu and Fe than did CaCl₂. A fractionation of soil Zn and Cu revealed that soil acidification resulted in an increase in the CaCl₂- and pyrophosphate-extractable fractions and a smaller decrease in the oxalate-extractable fraction.Plant dry matter production increased consistently when the soil pH was lowered from 7.0 to 4.6 but there was a slight decline in dry matter as the pH was lowered to 3.8. Micronutrient additions had no influence on plant biomass although plant uptake was increased. As the pH was lowered, concentrations of plant Fe first decreased and then increased whilst those of Mn, and to a lesser extent Zn and Cu, increased markedly.     

Effects of land irrigation with partially-treated wastewater on Frankia survival and infectivity

Phytoextraction of Cd by some populations of Thlaspi caerulescens which have the ability to co-hyperaccumulate Cd and Zn requires information about the distribution of both metals within the plant at the organ-level. This work was conducted to determine whether the distribution and solubility of Cd and Zn in Thlaspi caerulescens are affected by the age of plant and organ, and whether Cd and Zn have a common distribution in the plant in soils contaminated by both metals. A series of pot experiments were conducted where a Cd- and Zn-hyperaccumulating population was grown on soils contaminated by Cd and Zn. Temporal changes in metal concentration of roots and of shoots was recorded, along with the water and CaCl₂ solubility of metals in the plant organs. Also, leaves were grouped according to their age and their respective content of Cd and Zn was measured. Both metals were present at higher concentrations in leaves than in roots. The whole-plant content of Zn decreased with time while that of Cd increased or remained unchanged. At harvest, young leaves exhibited higher Cd concentration than older, but the reverse was true for Zn. Both metals were more soluble in dry leaves and senescent leaves than in fresh material, and Zn was more water-soluble than Cd. In conclusion, the distribution of Cd and Zn in the hyperaccumulator T. caerulescensvaried according to the organ and plant age, and Cd and Zn were shown to have a different distribution within the plant.     

Co-planting can phytoextract similar amounts of cadmium and zinc to mono-cropping from contaminated soils

Co-planting crops normally decreases the main crop yield due to the reduced soil surface area occupied by the main crop. However, in our previous experiments, co-planting Sedum alfredii, a shade-requiring, Cd and Zn-hyperaccumulating plant, with corn increased the biomass and metal phytoextraction of S. alfredii. This experiment was conducted to verify if co-planting another hyperaccumulator, Thlaspi caerulescens, with ryegrass (Lolium perenne) in a pot-trial could obtain a similar result. The soil was separated by two permeable nets with a 2 mm interface soil layer to obtain a shared rhizosphere zone. Soluble metal concentrations in the soil in different rooting zones were measured using 0.01 mol L^?1 CaCl₂ extraction. The results showed that the growth of T. caerulescens was significantly promoted by co-planting, with a growth increase of about 2-fold compared with monoculture growth. The total uptake of Cd and Zn by T. caerulescens was not decreased by co-planting, and resulted in similar phytoextraction rates for Cd (about 26.6% of the soil total Cd) and Zn (about 2.4% of the soil total Zn) when compared with monoculture, though the T. caerulescens population was decreased by 50% because of co-planting. Analysis of soil samples showed that T. caerulescens substantially reduced the concentrations of 0.01 mol L^?1 CaCl₂ extractable Cd and Zn throughout the soil, even in the interface area and the ryegrass rooting area. The ryegrass roots did not mobilize more metals for the co-planted T. caerulescens. Based on these results, existing grass on contaminated land could be partly left while planting metal hyperaccumulators for phytoremediation in order to reduce runoff from the contaminated soil. However a field scale trial would be required for these results to be verified.     

Soil washing of Pb, Zn and Cd using biodegradable chelator and permeable barriers and induced phytoextraction by Cannabis sativa

The feasibility of combined phytoextraction and in situ washing of soil contaminated with Pb (1750 mg kg^–1), Zn (1300 mg kg^–1), and Cd (7.2 mg kg^–1), induced by the addition of biodegradable chelator, [S,S] stereoisomere of ethylenediamine discuccinate ([S,S]-EDDS), was tested in soil columns with hemp (Cannabis sativa) as the phytoextracting plant. The addition of [S,S]-EDDS (10 mmol kg^–1 dry soil) yielded concentrations of 1026±442, 330.3±114.7 and 3.84±1.55 mg kg^–1 of Pb, Zn and Cd in the dry above-ground plant biomass, respectively. These concentrations were 1926, 7.5, and 11 times higher, respectively, compared to treatments with no chelator addition. Horizontal permeable barriers, composed of a 3 cm high layer of nutrient enriched sawdust and vermiculite mixture, and a 3 cm layer of soil, vermiculite and apatite mixture, were positioned 20, 30 and 40 cm deep in the soil. In chelator treatments, barriers placed 30 cm deep reduced leaching of Pb, Zn and Cd by 435, 4 and 53 times, respectively, compared to columns with no barrier, where 3.0, 4.3 and 3.3% of total initial Pb, Zn and Cd, respectively, was leached during 6-weeks water irrigation after chelator addition. Lower positioned barriers were almost equally effective in preventing leaching of Pb than barriers positioned closer to the soil surface, less effective for Cd, and did not prevent leaching of Zn. 2.53% of total initial Pb and 2.83% of Cd was washed from the contaminated soil and accumulated into the barrier. Combined method was less effective than simulated ex situ soil washing, where 14.2, 5.5 and 24.5% of Pb, Zn and Cd, respectively, were removed after 1-h extraction, but comparable effective to 48-h extraction. Abbreviations: BCF – bioconcentration factor; EDTA – ethylene diaminetetraacetate; HM – heavy metal; PP – phytoextraction potential; [S,S]-EDDS – [S,S]-ethylenediamine disuccinate.     

Copper uptake and phytotoxicity as assessed in situ for durum wheat (Triticum turgidum durum L.) cultivated in Cu-contaminated, former vineyard soils

This work assessed in situ, copper (Cu) uptake and phytotoxicity for durum wheat (Triticum turgidum durum L.) cropped in a range of Cu-contaminated, former vineyard soils (pH 4.2–7.8 and total Cu concentration 32–1,030 mg Cu kg⁻¹) and identified the underlying soil chemical properties and related root-induced chemical changes in the rhizosphere. Copper concentrations in plants were significantly and positively correlated to soil Cu concentration (total and EDTA). In addition, Cu concentration in roots which was positively correlated to soil pH tended to be larger in calcareous soils than in non-calcareous soils. Symptoms of Cu phytotoxicity (interveinal chlorosis) were observed in some calcareous soils. Iron (Fe)–Cu antagonism was found in calcareous soils. Rhizosphere alkalisation in the most acidic soils was related to decreased CaCl₂-extractable Cu. Conversely, water-extractable Cu increased in the rhizosphere of both non-calcareous and calcareous soils. This work suggests that plant Cu uptake and risks of Cu phytotoxicity in situ might be greater in calcareous soils due to interaction with Fe nutrition. Larger water extractability of Cu in the rhizosphere might relate to greater Cu uptake in plants exhibiting Cu phytotoxic symptoms.     

Variation in the uptake of Arsenic, Cadmium, Lead, and Zinc by different species of willows Salix spp. grown in contaminated soils

The experiment assessed the variability of in seven clones of willow plants of high biomass production (Salix smithiana S-218, Salix smithiana S-150, Salix viminalis S-519, Salix alba S-464, Salix ’Pyramidalis’ S-141, Salix dasyclados S-406, Salix rubens S-391). They were planted in a pots for three vegetation periods in three soils differing in the total content of risk elements. Comparing the calculated relative decrease of total metal contents in soils, the phytoextraction potential of willows was obtained for cadmium (Cd) and zinc (Zn), moderately contaminated Cambisol and uncontaminated Chernozem, where aboveground biomass removed about 30% Cd and 5% Zn of the total element content, respectively. The clones showed variability in removing Cd and Zn, depending on soil type and contamination level: S. smithiana (S-150) and S. rubens (S-391) demonstrated the highest phytoextraction effect for Cd and Zn. For lead (Pb) and arsenic (As), the ability to accumulate the aboveground biomass of willows was found to be negligible in both soils. The results confirmed that willow plants show promising results for several elements, mainly for mobile ones like cadmium and zinc in moderate levels of contamination. The differences in accumulation among the clones seemed to be affected more by the properties of clones, not by the soil element concentrations or soil properties. However, confirmation and verification of the results in field conditions as well as more detailed investigation of the mechanisms of cadmium uptake in rhizosphere of willow plants will be determined by further research.     

Physiologically based extraction of heavy metals in compost: Preliminary results

A physiologically based extraction test (PBET) was run for the extraction of six metals (Cu, Zn, Cd, Cr, Ni and Pb) in four composts containing high concentrations of heavy metals. An aqueous solution of pepsin plus citric, acetic, and malic acids buffered to pH 2 was used to simulate the gastric mixture, and an extraction of 1 h at 37 °C was run with a solid:liquid ratio of 1:100. The results were compared to those obtained using water and CaCl₂–DTPA solution. The PBET extracted far more metals than water, but less than CaCl₂–DTPA for Cu, Pb and Cr, while giving similar or slightly lower results for Cd, Zn, and Ni.     

Elevated Cd and Zn uptake by aspen limits the phytostabilization potential compared to five other tree species

Phytostabilization of metals using trees is often promoted, although the influence of different tree species on the mobilization of metals is not yet clear. This study examined effects of six tree species on the soil characteristics pH, organic carbon (OC) content and cation exchange capacity (CEC) and on the redistribution of cadmium (Cd) and zinc (Zn) on a polluted sandy soil. Soil and biomass were sampled in 10-year-old stands growing on former agricultural land. The tree species included were silver birch (Betula pendula), oak (Quercus robur and Quercus petraea), black locust (Robinia pseudoacacia), aspen (Populus tremula), Scots pine (Pinus sylvestris) and Douglas fir (Pseudotsuga menziesii). In the short period of 10 years, only aspen caused significant changes in the soil characteristics. Due to accumulation of Cd and Zn in its leaf litter, aspen increased the total as well as the NH₄OAc-EDTA-extractable Cd and Zn concentrations in the topsoil compared to deeper soil layers and to other tree species. Also, topsoil pH, OC content and CEC were significantly higher than under most of the other species. This caused rather low ‘bioavailable’ CaCl₂-extractable concentrations under aspen. Nevertheless, given the risks of aboveground metal dispersion and topsoil accumulation, it is recommended that aspen should be avoided when afforesting Cd and Zn contaminated lands.