Laboratory Experiment pubs.acs.org/jchemeduc
Is the Total Concentration of a Heavy Metal in Soil a Suitable Tool for Assessing the Environmental Risk? Considering the Case of Copper David Fernández-Calviño, Paula Pérez-Rodríguez, Juan Carlos Nóvoa-Muñoz, and Manuel Arias Estévez* Department of Plant Biology and Soil Science, Section of Soil Science, University of Vigo, 32004 Ourense, Spain S Supporting Information *
ABSTRACT: National legislation concerning soil pollution by heavy metals in different countries is mostly based on the total heavy metal concentration levels allowed in different soils. As soil pollution is an issue of worldwide concern, here we develop a laboratory exercise for students in which they must check the suitability of a total metal concentration for soil pollution assessment. Undertaking two different Cu extractions promotes student discussion about soil pollution by heavy metals: Cu extracted with EDTA and Cu extracted with DTPA. These two extractions are commonly used by the soil science community to estimate the potentially available Cu for plants, which is compared with information about total Cu that is provided to the students. The results of this laboratory exercise demonstrate that the total concentration of Cu in the soil is not suitable for decision-making about whether a soil is polluted. However, establishing reference values from EDTA and DTPA extractions or EDTA/Total and DTPA/Total ratios may result in better indicators to establish the possible environmental risks from the presence of Cu in soils with which to make critical decisions about soil remediation strategies. KEYWORDS: Graduate Education/Research, Upper-Division Undergraduate, Environmental Chemistry, Testing/Assessment, Agricultural Chemistry, Laboratory Instruction
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INTRODUCTION Over the past few decades, many environmentalists have focused on the potential heavy metal pollution of soils, surface waters, and groundwater resources, which can generate environmental problems as well as food safety risks.1 These concerns have been discussed in many universities worldwide, where new knowledge about this problem along with possible solutions is generated and transmitted to students. From the viewpoint of teaching and learning chemistry, the problem of high levels of heavy metals has been addressed in this Journal and others. These reports included new instrumental techniques that allow the determination of levels of metals in soils and living organisms,2−4 remediation strategies5 that have been addressed using different complexing agents and surfactants,6 adsorption processes using bioadsorbents to immobilize metals in soil,7 phytoremediation,8 and electro-kinetic techniques and separation methods, including flotation and other methods.9,10 Copper is an essential nutrient for plants, and its concentration in natural soils is highly dependent on the parent material. The copper concentration is high in basic rocks (90−100 mg kg−1) and low in acidic rocks such as granite (10− 13 mg kg−1).11 Due to its necessity for plants and its industrial use, copper has been the subject of several experiments published in this Journal.12−14 Agricultural activities in the past © XXXX American Chemical Society and Division of Chemical Education, Inc.
few decades may have led to soil Cu increased to levels that could be considered phytotoxic. This Cu increase is a consequence of management practices such as the use of wastes and fertilizers or the application of composts and pesticides.15 In soils devoted to vineyards, the common application of Cu-based fungicides over decades has led to high Cu accumulations in the soil, reaching total values above 100 mg kg−1.16 Although Cu is not a very toxic heavy metal for human health, it is extremely toxic to other organisms such as annelids, algae, and soil microbes. As a general rule, its toxicity to these organisms exceeds the toxicity of Cd, Cr, Ni, Zn, Pb, Co, Mn, and Sn and is only exceeded by Hg toxicity.17 Toxicity limits for soils are usually established in terms of the total concentration of Cu. However, the establishment of such toxicity limits should depend on the availability and possible incorporation of the heavy metals into the food chain, which may cause serious problems from both environmental and public health perspectives. In fact, some of the possible remediation techniques mentioned above involve the immobilization of heavy metals in the soil (instead of their removal), as is the case when bioadsorbents are used. With that in mind, the Received: February 6, 2017 Revised: May 5, 2017
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DOI: 10.1021/acs.jchemed.7b00105 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Laboratory Experiment
Before commencing the laboratory experiment, the students should read the classical reference books,11,15 especially those chapters related to Cu, and an article that relates the presence of heavy metals in agricultural soils to food safety.1 Some questions related to these topics must also be answered by the students (see the Supporting Information).
aim of this laboratory exercise is to demonstrate to students how the total concentration of a heavy metal, in this case Cu, may not be a useful indicator for an environmental risk assessment. Regarding this, it is necessary to convey to the students a key thought: Cu behavior in the soil environment is more related to its fractionation than to its total concentration in the soil. This study was designed for graduate and postgraduate students in subjects related to soil science and the environment such as agricultural chemistry, chemistry of pesticides, or pollution of terrestrial ecosystems.
Lab Experiment
Students carry out extractions of the Cu potentially available for plants using EDTA20 (ethylenediamine tetraacetic acid) and DTPA21 (diethylenetriaminepentaacetic acid) as extraction solutions. The methods for the extraction of Cu from the soil with EDTA (10 g of soil mixed in a 50 mL centrifugation tube with 20 mL of a solution containing 0.02 M EDTA-Na2 and 0.5 M ammonium acetate) and DTPA (10 g of soil mixed in a 50 mL centrifugation tube with 20 mL of a solution containing 0.005 M diethylenetriaminepentaacetic acid and 0.1 M triethanolamine) were followed. The extractable Cu was measured by atomic absorption spectrophotometry. The two methods and timetable of laboratory activities are described in detail in the Supporting Information, which also reports results obtained by the students
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DESCRIPTION OF THE EXPERIMENT The laboratory exercise was carried out by 18 sophomore students divided into 9 groups of 2 students per group. The experiment was divided into three parts: prelab, lab, and postlab. Prelab Experiment
Four acidic soils (surface samples 0−20 cm) developed from two different parent materials were provided to the students: 2 soils developed from granite and devoted to vineyards, and 2 soils developed from amphibolite and devoted to maize (Table 1). The details of sampling and methods, such as pH
Postlab Experiment
Students must produce a final report that includes answers to the questions raised in the prelab experiments (see the Supporting Information), and answers to the same questions once the laboratory experiment has been completed, focusing on possible response changes. The lab report must also discuss the following topics to clarify and reinforce the objectives of this laboratory exercise: • Relationship between the total Cu concentration in the soil and its natural or anthropic origin • Comparison of the different methods for the estimation of potentially available Cu for plants • Possible changes in national legislation on heavy metal soil pollution to add extractions such as EDTA and DTPA, which are able to estimate the Cu potentially available by plants • Effects of Cu accumulation on agricultural sustainability and soil quality in crop production areas related to Cu origin and the most labile Cu fractions • The potential toxicity to soil microbes and the risk of Cu incorporation into the trophic chain are more related to the potentially bioavailable Cu for plants than the total Cu concentration
Table 1. Soil Characteristics Soil Sample
Parent Material
Use
pH
CuT, mg kg−1
1 2 3 4
Granite Granite Amphibolite Amphibolite
Vineyard Vineyard Maize Maize
4.3 5.6 4.8 4.6
55 112 110 56
determination and total Cu (CuT), are described in detail in the Supporting Information. The first activity proposed for the students is the analysis of the data shown in Table 1, where special attention must be focused on the total Cu concentration in soils. With these data, the students should decide which of these soils shows a higher potential for Cu phytotoxicity. The expected student answer would be to select soils 2 and 3, since both display a total Cu concentration greater than 100 mg kg−1 (Table 1). It would also be expected that students would have anticipated vineyard soils to be those with greater Cu concentrations, as they are more prone to anthropogenic sources compared to maize soils. However, how can we explain this if we know that natural soils developed from granite in northwest Spain usually show lower Cu concentrations (12−39 mg kg−1)18,19 than those found in soils devoted to vineyards? The explanation for the remarkable Cu enrichment observed in soils devoted to vineyards is that it is a direct consequence of the intensive application of Cu-based fungicides over decades.16 These Cu-based substances are frequently used by winegrowers to fight against vine diseases such as downy mildew (Plasmopara spp.), which can cause significant losses in grape quality and yield, especially in areas with high relative air humidity and mild temperatures. On the other hand, despite the high levels of Cu found in the soils developed from amphibolite, they are within the range of Cu concentrations found in natural soils developed from basic and ultrabasic parent materials (50−205 mg kg−1).18,19 Because national legislation about soil pollution only takes into account the total concentration of heavy metals (modifying the threshold values as a function of soil pH making them lesser in acidic soils), the first answer provided by the students seems to be adequate.
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HAZARDS EDTA causes serious eye irritation, is harmful if inhaled, and may cause damage to organs through prolonged or repeated exposure. DTPA causes skin irritation, causes serious eye irritation, may cause respiratory irritation. TEA is a flammable liquid and vapor that is harmful in contact with skin, causes severe skin burns and eye damage, may cause respiratory irritation, is toxic if inhaled, may damage fertility or the unborn child, and causes damage to organs through prolonged or repeated exposure. CaCl2 causes serious eye irritation. HCl may be corrosive to metals, causes skin irritation, and causes serious eye irritation. Glacial acetic acid is a flammable liquid and vapor that can cause severe skin burns and eye damage. Standard Cu solution may be corrosive to metals, causes skin irritation, causes serious eye irritation, and may cause respiratory irritation. B
DOI: 10.1021/acs.jchemed.7b00105 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
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RESULTS AND DISCUSSION An average of the results is presented in Table 2, where the relationships between the extractions with EDTA and DTPA
CONCLUSION The final conclusion is that, despite the similar total Cu concentrations found in the tested soils devoted to vineyards (soils 1 and 2) and maize (soils 3 and 4), soil 1 and especially soil 2 presented more risk than soils 3 and 4 for Cu incorporation into the trophic chain due to the higher bioavailability of the former soils, but this environmental risk is also extended to toxicity problems for soil microbes and plant behaviors. Therefore, national legislation concerning heavy metal soil pollution needs to incorporate labile Cu fractions rather than total Cu concentration, which would be a more realistic method for risk assessment. Setting more realistic thresholds for Cu-polluted soils may allow for better decisions in soil management and hence more effective actions for longterm soil sustainability.
Table 2. Concentration of Potentially Available Cu for Plants Determined via EDTA and DTPA Methods and EDTA/Total and DTPA/Total Ratios Soil Sample
EDTA, mg kg−1
DTPA, mg kg−1
EDTA/ Total
DTPA/ Total
1 2 3 4
12.0 46.0 2.6 3.7
9.0 31.0 1.4 1.7
0.22 0.41 0.02 0.07
0.16 0.28 0.01 0.03
Laboratory Experiment
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and the CuT are also presented. The results obtained by the students were satisfactory, and the standard deviations of the extractions were low (Tables S1 and S2, Supporting Information). Only 4 results were very different from those provided by the instructor. The first aspect to discuss about the bioavailable Cu extractions arises from the difference between the Cu extracted with DTPA and the Cu extracted with EDTA. Table 2 shows that the amount of bioavailable Cu extracted with EDTA was higher than that from DTPA. It is necessary to introduce to the students the concept that the discrepancies in Cu values may be attributed to the different pH values of the extraction solutions. Thus, as the diethylenetriaminepentaacetic acid (DPTA) solution pH is 7.0, whereas that of the ethylenediamine tetraacetic acid disodium salt (EDTA) is 4.65, the latter is probably more suitable for acidic soils.22 However, the contrasting Cu releases from soils upon EDTA and DTPA extractions could also be due to the effect of the different concentrations of both solutions, 0.005 M for DTPA and 0.02 M for EDTA, taking into account the fact that the metal removal efficiency of EDTA and DTPA increases with increasing concentration of the chelating agents in solution.23 Students’ data also show that the highest values of bioavailable Cu, after application of both the EDTA and DTPA methods, were found in soils 1 and 2 (Table 2), i.e., the soils devoted to vineyards (Table 1). It is also noticeable, and thus must be made clear to the students, that soil 1 shows a relatively high bioavailable Cu level despite having the lowest total Cu concentration of the assessed soils (Table 1). On the other hand, the low level of potentially available Cu found in the amphibolite-derived soils after extraction with EDTA and DTPA (Table 2) can be considered evidence supporting a natural origin of the Cu that occurs in those soils. These results can thus be considered a friendly exploratory tool for the students to discriminate the predominance of natural versus anthropogenic heavy metals in soils. With application of this single rule to the studied cases, the greater values of Cu extracted with EDTA or DTPA, up to 1 order of magnitude higher in soils 1 and 2, demonstrated that a significant portion of Cu in these soils arises from anthropic origins, whereas in soils 3 and 4, Cu arises largely from a natural origin (parent material). Similar conclusions may be reached from the EDTA/ Total and DTPA/Total ratios (Table 2). The soils devoted to vineyards (soils 1 and 2) showed ratios 1 order of magnitude higher than the soils devoted to maize (soils 3 and 4). Thus, it is important to clarify to the students that Cu origin (natural or anthropic) clearly governs Cu potential mobility and its possible incorporation into the trophic chain.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00105. Soil information, prelaboratory activities, laboratory activities, results for each student group, timetable of laboratory activities, details for carrying out a similar experiment at other universities, and postlaboratory activities (PDF, DOCX)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
David Fernández-Calviño: 0000-0003-0864-7039 Manuel Arias Estévez: 0000-0002-9162-1587 Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.jchemed.7b00105 J. Chem. Educ. XXXX, XXX, XXX−XXX
