Suitability of using diffusive gradients in thin films (DGT) to study metal bioavailability in mine tailings: possibilities and constraints

Background, aim, and scope: Diffusive gradients in thin films (DGT) have been recognized as a suitable tool to assess in situ metal bioavailability in soils. Mine tailings have some singular characteristics such as high heavy-metal concentrations, low pH, or absence of water retention capacity that may compromise the correct application of this technique whose applicability is known to be pH dependent. The goal of this study was to determine the response of DGT devices in heavy-metal-polluted mine tailings with different pH. In addition some experiments were performed in order to determine the effect of acidic pH and dissolved ions on the binding properties of the chelating resin. Materials and methods: We tested DGT devices on three different mine tailings: acid pH 3, acid tailing limed to pH 5.5, and neutral pH 7.2. The tailings showed high metal concentrations, e.g., 7,000mgkg−1 Pb, 9,000mgkg−1 Zn, and 380mgkg−1 Cu. Diffusive and Chelex resin gels were prepared according to previously published methods. Two chelating resins and diffusive gels thicknesses (0.4 and 0.7mm) were tested. Four DGT devices of each type were placed during 24h in pots (one device per pot) containing 1kg mine tailings in a climate chamber with humidity (50-90%) and controlled temperature conditions (night 16°C and day 23°C). Pots were irrigated with deionized water to field capacity, and then two different experiments were performed: (a) allowing free drainage and (b) maintaining the water saturation. In addition, we tested DGT devices in solutions at pH 3 with similar properties to the soil solution measured in the acid tailing. Eluted Zn, Cd, Pb, and Cu from the chelating resins were measured using inductively coupled plasma-optical emission spectrometer (ICP-OES; Vista-MPX Varian). Results and discussion: The metal concentrations taken up by the DGT devices were affected by the different pH values of the tailings. The highest metal concentrations measured with DGT (C DGT) were obtained in the pH 3 treatments (both saturated and free drainage). Significant differences for C DGT were observed between water-saturated and free drainage treatments in the acid pH 3 tailing. When limed pH 5.5 tailing and neutral pH 7 tailing were considered, these differences were lower and not significant. In pH 3 tailings low values for C DGT/C soil solution were obtained (<0.06), indicating that these soils have a low capacity to resupply depleted metals to the solution. The limed acid tailing and the neutral tailing showed values between 0.05 and 0.94 indicating a much more rapid resupply from the solid phase. Deployment under water-saturated conditions yielded much higher C DGT values than under free drainage, indicating the importance to adequately control the moisture content in these soils with poor water retention capacity. In solutions with pH 3 mimicking the soil solution composition of the tailings, a loss of the binding capacity of the resin of 50-60% and 60-80% for Zn in 0.7-mm DGT and 0.4-mm DGT devices, respectively, was observed. As a consequence, 0.7-mm DGT devices had better reliability to carry out in situ determinations in solutions with high metal concentrations and low pH. Conclusions: The use of DGT in mining soils can be a promising tool to study bioavailable metals concentrations in mine tailings but it has to be used carefully under acidic pH. Competition with other cations that are present at very high concentrations may hinder the accumulation of metals by the chelating resins, which should be tested under the conditions of the particular mine tailin

A Laboratory Study on Revegetation and Metal Uptake in Native Plant Species from Neutral Mine Tailings

Lygeum spartum, Zygophyllum fabago and Piptatherum miliaceum are typical plant species that grow in mine tailings in semiarid Mediterranean areas. The aim of this work was to investigate metal uptake of these species growing on neutral mine tailings under controlled conditions and their response to fertilizer additions. A neutral mine tailing (pH of soil solution of 7.1-7.2) with high total metal concentrations (9,100 and 5,200mg kg−1 Zn and Pb, respectively) from Southern Spain was used. Soluble Zn and Pb were low (0.5 and <0.1mg l−1, respectively) but the major cations and anions reached relatively high levels (e.g. 2,600 and 1,400mg l−1 Cl and Na). Fertilization caused a significant increase of the plant weight for the three species and decreased metal accumulation with the exception of Cd. Roots accumulated much higher metal concentrations for the three plants than shoots, except Cd in L. spartum. Shoot concentrations for the three plants were 3-14mg kg−1 Cd, 150-300mg kg−1 Zn, 4-11mg kg−1 Cu, and 1-10mg kg−1 As, and 6-110mg kg−1 Pb. The results indicate that neutral pH mine tailings present a suitable substrate for establishment of these native plants species and fertilizer favors this establishment. Metal accumulation in plants is relatively low despite high total soil concentration

Soil amendment with activated charcoal can reduce dieldrin uptake by cucumbers

Organochlorine pesticides (OCP) were once applied world wide but have been banned meanwhile in most countries because of their ecotoxicity, bioaccumulation and persistence. However, residues can still be present in soils even many years after applications have been stopped and taken up by crop plants. OCP accumulation from bound residues was found to be a particular problem in Cucurbitaceae plants. Two soil surveys performed in 2002 and 2005 in Switzerland revealed that OCP residues were taken up by cucumbers grown in soils that have been converted to organic production in the meantime. Even if legal tolerance values are not exceeded, this is a serious economic problem for the farmers affected by contaminated crops, because consumers of organically grown crops are only willing to pay the higher prices for these than for conventional products because they are particularly concerned about health and environmental quality and therefore expect pristine food. One approach to address the problem would be to increase the capacity of affected soils to bind OCP residues in order to prevent their uptake by the crops. In this study, we wanted to test the potential use of activated charcoal (AC) for this purpose. In addition, we wanted to assess the possibility of using OCP sorption in soil by Tenax® beads as a predictor for the phytoavailability of these compounds to cucumbers. We performed two pot experiments in which the cash crop cucumber (Cucumis sativus L.) was grown in soil with bound residues of dieldrin (70 µg/kg), pentachloroaniline (<0.01 µg/kg) and p,p-DDE. The soil was taken from a field under organic farming in which these residues were found in the 2005 survey. In the first experiment, cucumbers were grown for 12 to 13 weeks (until fruits were ripe) in soil into which AC had been mixed at concentrations of 200, 400, and 800 mg/kg and in untreated controls. In the second experiment, Tenax® beads were added to the soil and cucumbers, grown with and without AC amendment (800 mg/kg soil), were harvested after 4, 8, 10, 11, 12, and 13 weeks. Dieldrin was the only pesticide detected in the sampled cucumbers and extracted from soil by the Tenax beads. Dieldrin concentrations in the cucumbers were significantly reduced in the treatments with 400 and 800 mg/kg AC. Also significantly less dieldrin was sorbed by Tenax from the soil amended with 800 mg/kg AC than from the untreated control soil. More dieldrin was found to be sorbed by Tenax in the last 3-4 weeks of the experiment, particularly in the control soil, but this trend was not significant. The correlation between the amounts of Tenax-sorbed dieldrin and dieldrin accumulation in the cucumber fruits was significant in control soil and 800 mg/kg AC soil. Hence, Tenax appeared to be suited for the assessment of dieldrin solubility in soil and of phytoavailability to cucumbers

Antimony uptake by Zea mays (L.) and Helianthus annuus (L.) from nutrient solution

We investigated the extent of Sb uptake by maize (Zea mays) and sunflower (Helianthus annuus) from nutrient solutions containing concentrations from 3 to 24mg/L of potassium antimonate, with the aim of determining the potential of Sb to enter the food chain. The maximum shoot Sb concentrations in Z.mays and H.annuus were 41mg/kg and 77mg/kg dry weight, respectively. There was no significant difference in Sb uptake between species. The average bioaccumulation coefficients (the plant/solution concentration quotients) were 1.02 and 1.93 for Z.mays and H.annuus, respectively. Phosphate addition did not affect plant growth or Sb uptake. Antimony uptake by both Z.mays and H.annuus is unlikely to pose a health risk to animals and human

Response of Populus tremula to heterogeneous B distributions in soil

Background: Poplars accumulate inordinate amounts of B in their leaves and are candidate plants for the remediation of B contaminated soil. We aimed to determine the effect of heterogeneous B distribution in soil by comparing the growth and B accumulation of young Populus tremula trees growing in soil with heterogeneous and homogeneous B distributions. Methods: The first of two experiments focused on the tolerance and B accumulation of P. tremula under heterogeneous soil B distributions, while the second was designed to study fine root growth under such conditions in detail. Results: Growth and B accumulation of P. tremula were unaffected by the spatial distribution of B. Root and shoot growth were both reduced simultaneously when leaf B concentrations increased above 800mgkg−1. In the heterogeneous soil B treatments, root growth was more reduced in spiked soil portions with B concentrations >20mgkg−1. Fine root length growth was stronger inhibited by B stress than secondary growth. Conclusions: The root growth responses of P. tremula to B are primarily a systemic effect induced by shoot B toxicity and local toxicity effects on roots become dominant only at rather high soil B concentrations. Local heterogeneity in soil B should have little influence on the phytoremediation of contaminated site

Contribution of Ectomycorrhizal Fungi to Cadmium Uptake of Poplars and Willows from a Heavily Polluted Soil

Phytoextraction has been proposed in recent years as an environmentally and cost-efficient treatment technique for the remediation of heavy-metal contaminated sites. In particular, plants that are fast growing, metal accumulating, and economically interesting, such as sunflowers or trees, recently became more important in research on phytoextraction. Heavy metal uptake of trees can be strongly influenced by ectomycorrhizal fungi. We investigated the possibility of enhancing phytoextraction of Cd by willows (Salix viminalis) and poplars (Populus canadensis) in association with three well known ectomycorrhizal fungi (Hebeloma crustuliniforme, Paxillus involutus and Pisolithus tinctorius). A pot experiment was conducted using Cd polluted soil from a contaminated site. Four replicates of each combination of fungus and tree species, and controls without fungal inoculum, were set up. After a growth period of 11weeks, yields and Cd concentrations in roots, stems, and leaves were measured. In addition, the total Cd uptake, the transfer to roots, and the translocation to stems and leaves were calculated. The association of P. canadensis with P. involutus led to a highly significant increase of Cd concentrations, in particular in the leaves, which contained 2.74 ± 0.34mg Cd per kg dry matter. Compared to the control this is an enhancement of nearly 100%. The fungi also significantly enhanced the translocation from the roots to the leaves, leading to a concentration ratio (leaves/roots) of 0.32 ± 0.06 compared to 0.20 ± 0.02 of the control plants. Additionally, P. involutus significantly enhanced the total Cd extraction by P. canadensis. Similar effects were not observed by other fungi or in association with S. viminali

Modified micro suction cup/rhizobox approach for the in-situ detection of organic acids in rhizosphere soil solution

Root-soil interactions can strongly influence the soil solution chemistry in the rhizosphere. In the present study we propose a modification of the classical rhizobox/micro suction cup system to make it suitable for the collection and analysis of organic acids in the rhizosphere. In order to show the potential of the method, we tested the modified system with Lupinus albus L. as a model plant known to exude large amounts of citrate. The suction cups were installed through the transparent front plate of the rhizoboxes just after the emergence of cluster roots in order to allow optimal localized collection of soil solution. A small dead-volume allowed almost immediate stabilisation with formaldehyde of the sampled soil solutions in the collection container to prevent microbial degradation. The concentrations of organic acids were significantly larger in the rhizosphere soil solution of active cluster roots of Lupinus albus L. than in the bulk soil solution (about 400µM of citrate versus <0.05µM). We were able to follow the exudation process in-situ, which occurred during 2-3days. Also the concentrations of other organic acids and inorganic anions differed between the bulk soil and the rhizosphere of cluster roots, normal roots, and nodule

Uptake and allocation of plant nutrients and Cd in maize, sunflower and tobacco growing on contaminated soil and the effect of soil conditioners under field conditions

Contaminated land may in many cases still be used for agriculture, provided that crops are chosen appropriately, as the accumulation of contaminants varies greatly among cultivars and also plant parts. We aimed to determine whether maize (Zea mays), sunflower (Helianthus annuus) and tobacco (Nicotiana tabaccum) grown on a heavy-metal contaminated soil containing copper (540mg Cu kg−1), zinc (680mg Zn kg−1) and cadmium (1.4mg Cd kg−1) could be used to gradually remediate the soil, while producing valuable biomass. The soil was treated with either a normal fertiliser regime (control), elemental sulphur (S), or the biodegradable chelant NTA (nitrilotriacetic acid), to test how soil acidification or chelating organic compounds would affect the uptake and allocation of selected elements (Ca, Cd, Cu, Fe, K, Mg, Mn, P, S and Zn). The highest concentrations of Cd, Cu and Zn occurred in the leaves and/or roots, while seeds and grains contained much lower concentrations of these elements. All these concentrations, however, were still in the ranges considered normal for the respective plant parts grown on uncontaminated soil. While sunflower and maize could be safely used as food and feed, tobacco would better be used for bioenergy than for cigarette production because of its relatively high foliar Cd concentration. The two treatments (S and NTA) had only slight effects on the uptake and allocation of plant nutrients and Cd. Thus, there was little benefit of these treatments for phytoextraction purposes at this sit

Effects of heavy metal soil pollution and acid rain on growth and water use efficiency of a young model forest ecosystem

In a 4-year lysimeter experiment, we investigated the effects of topsoil heavy metal pollution (3,000mg kg−1 Zn, 640mg kg−1 Cu, 90mg kg−1 Pb and 10mg kg−1 Cd) and (synthetic) acid rain (pH3.5) on tree growth and water use efficiency of young forest ecosystems consisting of Norway spruce (Picea abies), willow (Salix viminalis), poplar (Populus tremula) and birch (Betula pendula) trees and a variety of understorey plants. The treatments were applied in a Latin square factorial design (contaminated vs uncontaminated topsoil, acidified rain vs ambient rain) to 16 open-top chambers, with 4 replicates each. Each open-top chamber contained two lysimeters, one with a calcareous, and the other with acidic subsoil. The four tree species responded quite differently to heavy metal pollution and type of subsoil. The fine root mass, which was only sampled at the end of the experiment in 2003, was significantly reduced by heavy metal pollution in P. abies, P. tremula and B. pendula, but not in S. viminalis. The metal treatment responses of above-ground biomass and leaf area varied between years. In 2002, the heavy metal treatment reduced above-ground biomass and leaf area in P. tremula, but not in the other species. In 2003, metals did not reduce above-ground growth in P. tremula, but did so in the other species. It appears that the responses in above-ground biomass and leaf area, which paralleled each other, were related to changes in the relative competitive strength of the various species in the two experimental years. S. viminalis gained relative to P. tremula in absence of metal stress, in particular on calcareous subsoil, while P. abies showed the largest increases in growth rates in all treatments. Above- and below-ground growth was strongly inhibited by acidic subsoil in S. viminalis and to a lesser degree also in P. abies. In P. abies, this subsoil effect was enhanced by metal stress. Acid rain was not found to have any substantial effect. Whole-system water use efficiency was reduced by metal stress and higher on calcareous than on acidic subsoi