Toxicity and metal bioaccumulation in Hordeum vulgare exposed to leached and nonleached copper amended soils.

Soil leaching has been proposed as a way to reduce soil-spiking artifacts (i.e., increased acidity, metal solubility) that occur in soils amended with trace metal salts. Leaching metal-spiked samples prior to ecotoxicity testing is therefore expected to reduce toxicity; however, leaching not only removes excess amounts of the trace metal being tested, but also reduces the concentrations of cations that could decrease the toxic effects of the metal of interest. To clarify these conflicting processes, the effects of leaching on toxicity and bioaccumulation of Cu, Ca, and Al were investigated using 14-d plant assays conducted on leached and nonleached, Cu-spiked soils. The median effective concentration (EC50) to root elongation ranged from 78 µg/g to 589 µg/g. Leaching was found to reduce toxicity by 1.2-fold to 2.1-fold. The Cu(2+) activity predicted toxicity better than root or shoot Cu concentrations, which were generally not affected by leaching. Plant uptake of Ca increased with increasing Cu dose in nonleached samples, which likely contributed to the hormesis-like response observed in these samples, whereas Ca uptake in the leached samples was more consistent with that of the control except at the largest Cu doses for which Ca uptake decreased. Surprisingly, Al uptake in the most acidic soil was greater in leached than nonleached samples, which may have contributed to the greater toxicity exhibited in this soil than was predicted by Cu(2+) activity. These findings have implications for predicting trace metal toxicity in nutrient-stressed, acidic soils. Environ Toxicol Chem 2013;32:1800-1809. © 2013 SETAC.

Comparing soil chemistries of leached and nonleached copper-amended soils.

Leaching metal-spiked samples has been proposed as a means to reduce the artifacts of the spiking procedure (e.g., salt effect, increased metal solubility) that can artificially increase metal bioaccessibility and toxicity in laboratory ecotoxicity tests. The effects on soil chemistry from leaching Cu-spiked samples were investigated by comparing chemistries of freshly spiked samples to samples that underwent the spike/leach procedure. Chemical parameters investigated included electrical conductivity (EC), pH, ethylenediaminetetraacetic acid- and CaCl(2) -extractable Cu, soil-solution Cu, Cu(2+) activity (estimated using Visual MINTEQ), and other solution parameters (dissolved organic carbon [DOC], Ca, Mg, Al). In leached samples, the electrical conductivity values of the spiked samples did not vary significantly from those of the control samples (p > 0.05), confirming that the leaching procedure had sufficiently minimized the salt effect. In the range of soil Cu concentrations where Cu ecotoxicity is expected, the pH in freshly spiked samples was as much as 0.52 units lower than the pH from leached samples at the same total-soil Cu concentration. The CaCl(2) -extractable fraction was up to 2.3-fold smaller in leached samples and inversely related to the pH of the spiked soil. Despite little to no difference in soil-solution Cu, up to 100-fold less Cu(2+) activity was observed in leached samples. Reduced Cu(2+) activity was related to less Al(3+) competition for DOC. Leaching resulted in solution chemistries that were more consistent with those of the control samples and reduced the artifacts of traditional soil-spiking procedures.

Optimizing the molarity of a EDTA washing solution for saturated-soil remediation of trace metal contaminated soils.

Three experiments were conducted to optimize the use of ethylenediaminetetraacetic acid (EDTA) for reclaiming urban soils contaminated with trace metals. As compared to Na(2)EDTA, (NH(4))(2)EDTA extracted 60% more Zn and equivalent amounts of Cd, Cu and Pb from a sandy loam. When successively saturating and draining loamy sand columns during a washing cycle, which submerged it once with a (NH(4))(2)EDTA wash and four times with deionised water, the post-wash rinses largely contributed to the total cumulative extraction of Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn. Both the washing solution and the deionised water rinses were added in a 2:5 liquid to soil (L:S) weight ratio. For equal amounts of EDTA, concentrating the washing solution and applying it and the ensuing rinses in a smaller 1:5 L:S weight ratio, instead of a 2:5 L:S weight ratio, increased the extraction of targeted Cr, Cu, Ni, Pb and Zn.

Modelling trace metal partitioning in forest floors of northern soils near metal smelters.

Trace metal (TM) mobility and toxicity varies with changing soil conditions. Geochemical models can account for the influence of soil characteristics on TM behaviour. We tested the effectiveness of the Stockholm humic model (SHM), and the NICA-Donnan model (NDM) to estimate partitioning coefficients (logKd) in 26 forest floor horizons of podzolic soils enriched in trace metals from deposition by metal smelters. We wanted to know if a consistent approach could be applied to model metal partitioning in forest floors without optimizing each individual soil. When optimized, the SHM reproduced the partitioning of Cd, Cu and Zn but not Pb. It was necessary to revise the affinity constants for the NDM to simultaneously simulate the partitioning of the four metals. Revised affinity constants for the NDM model based on a fixed definition of soil organic carbon, i.e., a fixed ratio of fulvic and humic acids per unit carbon, reproduced metal partitioning more effectively in an independent data set of 16 soils than the use of generic affinity constants available for these models. From the perspective of the applicability of these models to risk assessment, this result suggests geochemical models using affinity constants that have been verified and/or modified against multiple soils from a region can provide good estimates of metal partitioning on a regional scale.

Recycling EDTA solutions used to remediate metal-polluted soils.

The objective of this research was to investigate the recycling of ethylenediamine-tetraacetic acid (EDTA) used for the removal of trace metals from contaminated soils. We successfully used Na2S combined with Ca(OH)2 to precipitate the trace metals allowing us to recycle the EDTA. The results of batch and column leaching experiments show that both Ca-EDTA and Na-EDTA are powerful chelating agents with a similar soil remediation potential. The major advantage of Ca-EDTA is the preservation of soil organic matter. We found that Na2S was capable of separating the metals Cd, Cu and Pb from EDTA; however, the precipitation of Zn required the addition of Ca(OH)2. After reusing the reclaimed EDTA seven times, over a 14-day period, EDTA reagent losses ranged from 19.5% to 23.5%. Successive washing cycles enhanced the removal of trace metals from contaminated soils. The metal sulfide precipitates contain high concentrations of metals and could potentially be recycled.

Fluctuations of climatic conditions, elemental cycling and forest growth at the watershed scale.

The relationships between fluctuations in climatic conditions, forest productivity and elemental cycling were studied from 1994 to 1997 in a headwater catchment of the southern Laurentians dominated by sugar maple (Acer saccharum Marsh.) growing on podzolic soils. Annual budgets show that H+, K, and NO3 were retained in the watershed while Ca, Mg and Na were lost. The magnitude of the net annual budget for Ca, Mg and Na was correlated to annual variations in precipitation with the absolute budget value decreasing during dry years. Stemwood (r2 = 0.85) and total tree biomass production (r2 = 0.99) were correlated with mean annual temperature but fine roots and leaf litter were not. During the growing season, the pH of the organic horizons (FH) decreased as the volumetric water content of soil decreased. A positive association was also found between air temperature and H2O-soluble (r2 = 0.88) and PO4-extractable (r2 = 0.99) SO4 in the upper B horizon. On a multiyear scale, we suspect that the decrease in the storage of inorganic SO4 in the soil results from the cumulative effects of annual variations in climatic conditions superimposed on the long-term decrease in SO4 deposition from the atmosphere. These soil changes were reflected by a decline in SO4, Ca and Mg concentrations in the stream. The generalisation of the observed short-term patterns to longer time scales must be approached with caution. Yet, our results indicate that the associations between climatic variations and the biogeochemistry of the ecosystem occur at different spatial and temporal scales and integrate a broad range of chemical components and ecosystem compartments. This reflects the inherent complexity of natural systems and offers a vast palette of indicators of the response of terrestrial ecosystems to variations in the intensity of environmental factors such as climatic conditions.

Evaluating three trace metal contaminated sites: a field and laboratory investigation.

Selecting guidelines to evaluate elevated metals in urban brownfields is hindered by the lack of information for these sites on ecosystem structure and function. A study was performed to compare three trace metal-contaminated sites in the metropolitan Montreal area. The goal was to obtain an idea of the organisms that may be present on urban brownfields and to measure if elevated metals alter the presence and activity of the indigenous biota. Field and laboratory studies were conducted using simple methodologies to determine the extent to which microbial activity affected by trace metal content, to assess diversity of plant and soil invertebrate communities and to measure phytoaccumulation of trace metals. It was found that microbial activity, as measured by substrate-induced respiration (SIR) and nitrification, was not affected by the levels of soil Cd, Cu, Ni, Pb and Zn recorded on the sites. Seven of the 12 invertebrate groups collected were sampled on soils with similar Cd, Cu, Ni, Pb and Zn concentrations. Diversity of plant species increased as a function of the length of time the sites had been inactive. Levels of metals in plant tissue were influenced by soil characteristics and not by total soil Cd, Cu, Ni, Pb and Zn.

Trace metal speciation and bioavailability in urban soils.

Urban soils often contain concentrations of trace metals that exceed regulatory levels. However, the threat posed by trace metals to human health and the environment is thought to be dependent on their speciation in the soil solution rather than the total concentration. Three inactive railway yards in Montréal, Québec, were sampled to investigate the speciation and bioavailability of Cd, Cu, Ni, Pb and Zn. Soil solutions were obtained by centrifuging saturated soil pastes. In the soil solutions, up to 59% of the dissolved Cd was in its free ionic form. For Cu, Pb and Zn, organic complexes were the predominant species. Over 40% of Ni was present as inorganic complexes if the solution pH exceeded 8.1. Multiple regression analyses showed that pH and total metals in soil were significantly correlated with the activities of free metal ions, except for Cd(2+), which only had a weak correlation with soil pH. Free, dissolved and total soil metals were tested for their ability to predict metal uptake by plants in the field. However, none of these metal pools were satisfactory predictors. The results indicated that in these urban soils, trace metals were mainly in stable forms and bioavailability was extremely low.

Linking plant tissue concentrations and soil copper pools in urban contaminated soils.

Copper tissue concentrations of radish (Raphanus sativa cv. Cherry Belle), lettuce (Lactuca sativa cv. Buttercrunch) and ryegrass (Lolium perenne cv. Barmultra) grown in a greenhouse in urban contaminated soils are compared to total, soluble and free ion copper pools. The tissue concentrations of copper vary between 8.1 and 82.6 mg Cu kg(-1) dry tissue and the total soil copper content varies between 32 and 640 mg Cu kg(-1) dry soil. The linear regressions with cupric ion activity and total soil copper are both significant (p < 0.01), but cupric ion activity yields a higher level of statistical significance in every case. The results support the hypothesis that free metal in the soil solution is a better indicator of plant metal bioavailability than either total or soluble metal.