Environ. Sci. Technol. 2008, 42, 2309–2315
Assessment of Heavy Metals Remobilization by Fractionation: Comparison of Leaching Tests Applied to Roadside Sediments GUSTAVO PÉREZ, MONTSERRAT LÓPEZ-MESAS, AND MANUEL VALIENTE* Centre Grup de Tècniques de Separació, Unitat de Química Analítica, Departament de Química, Universitat Autònoma de Barcelona, Facultat de Ciències, Edifici CN, 08193-Bellaterra, Barcelona, Spain
Received June 1, 2007. Revised manuscript received November 16, 2007. Accepted December 19, 2007.
The pollution emitted by traffic activities and road maintenance is an area of great interest as contaminants can be transported to roadside sediments and pose a risk to environmental and human health. In the present work, deposited pollution in roadside sediments has been assessed by sampling along a highly traveled highway in Barcelona and the surrounding area. The available amounts of the heavy metals was determined by applying different leaching tests and calculating the concentration enrichment ratio (CER) and the environmental concentration guideline values (ECG). To gain information on the heavy metals (HMs) fractionation, the sequential extraction scheme (SES), established by the Standard Measurement and Testing (SM&T), was implemented, and the results were compared with those obtained by single leaching tests. An anthropogenic enhancement of certain metals was observed after considering both the CER and ECG values. However, if only ECG values were considered, an overestimation of the anthropogenically enhanced pollutants was obtained due to disregarding geochemical and particle size variability. CER values provide a more realistic assessment by determining different levels of anthropogenic impact. Thus, CER values suggest a minimum anthropogenic apportion for metals such as Cd, Cr, and Ni, whereas different situations from significant to extreme anthropogenic contribution were observed for Zn, Pb, and Cu. These results have been complemented by other leaching tests that minimize the time-consuming environmental evaluation. In this study, HCl extraction produces suitable results for a quick screening since they correlate well with the corresponding SES: Cu(r2 ) 0.798), Pb(r2 ) 0.958) and Zn(r2 ) 0.901). Mild extractants have been observed to be limited to highly polluted samples due to their low leaching power. The information obtained following this procedure helps to identify hazardous areas that need a remedial strategy.
Introduction Among the environmental concerns of recent years (1), roadside sediments are of great interest due to the possible transmission of their pollutants to the environment which * Corresponding author phone: +34-935812903; fax: +34-935811985; e-mail:
[email protected]. 10.1021/es0712975 CCC: $40.75
Published on Web 02/28/2008
2008 American Chemical Society
may constitute a health hazard. High amounts of metals have been detected in these sediments, and their origin is linked to the combustion processes of vehicles, road surface degradation, application of road maintenance chemicals such as deicing salts, wearing of vehicles, and wearing of signposts and crash barriers (2). Pollution from these sources is emitted in a particulate form and, depending on climatic conditions, the coarsest particles may accumulate immediately at the asphalt border, mixing with natural components, and forming roadside deposited sediments (RDS). The metals pollution decreases in concentration with depth and with distance from the roadway (3, 4). From the environmental forensic point of view, the pollutant charge of roadside sediments can be easily characterized, related to nonpoint source pollution, and connected to vehicles emissions, thus acting as environmental registries of valuable information (5). Among the metals considered to pollute roadside sediments, we highlight Pb mostly produced by the combustion of former leaded fuels (6), Zn contained in ZnO used in the rubber compound for tires as a vulcanising agent and also in crash barriers (7), Cu as a component of brake linings and in many alloys present in cars, and Cd found in tires or in lubricating oils that can be spilled as a result of accidents (8). In order to assess the impact of trace elements in soils and sediments, the total metal content and its mobility and availability has to be considered due to the biogeochemical and ecotoxicological significance of a given contaminant. These contaminant characteristics are determined by its specific binding form and coupled reactivity rather than by its total concentration (9, 10). Consequently, there is a need for information about different issues such as the following: •The ease of remobilization in relation to the association strength to sediments. •How serious the hazard posed by these metals. •How nonpoint sources can be distinguished from point pollution sources. In this sense, the present work shows how leaching tests can be used to evaluate the mentioned available fractions (11). One of the most successful methods for studying labile fractions and pollutants partition among the different phase present in soils has been provided by sequential extraction schemes (SESs). This methodology is based on the process known as fractionation (12) where a sequential series of selective extractants with an increasing extractant power is employed to selectively dissolve or solubilize the different solid phase forms or mineralogical fractions (13–15). In this sense, SESs have become a common evaluative tool for the distribution or partitioning of heavy metals (HMs) in soils and sediments (16). SESs allow for a better insight into the mechanisms of HMs retention and release involved migration and decontamination, providing an evaluation of their availability, mobility, or persistence. Simultaneously, single nonselective extractions methods (leaching tests) that target groups of labile or mobile phases have also gained interest because this approach can provide a useful assessment for screening purposes to identify trace metal pollution (17). This minimizes the inconvenience of long SESs procedures. Furthermore, these leaching tests present some advantages against SESs, mainly related to their cost efficiency, ease of use, and a reduction of bias induced by sequential translation and accumulation of procedural errors. Single extractants differ by their dissolution power, including mild unbuffered extractants, that extract the fraction of easily exchangeable elements, acidic extractants that release the fraction remobilized by acidification proVOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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cesses, and finally the availability promoted by complexing reagents (18). Leaching tests are focused on mimicking soilto-plant transfer processes, reproducing those chemical reactions that can take place in soils and sediments on a particular environment (i.e, adsorption–desorption, dissolution-precipitation, reduction–oxidation, and complexationdecomplexation processes), and modifying the concentration of metals in soil solution (19). The aim of the present study is the characterization and assessment of the pollution degree of roadside sediments located along 20 km of a crowded Spanish highway. Such characterization includes the comparison of different assessment methods such as SES and single nonresidual extractions of the heavy metals Cd, Cr, Cu, Ni, Pb, and Zn, leading to evaluate the distribution of heavy metals between the different soil phases and to throw light on their mobility.
Experimental Section Study Site, Sample Collection, and Processing. This study looks at the initial 20 km of C-58 highway close to Barcelona city, Spain (Supporting Information, Figure SI-1). Within this transect, the traffic flow varies in the range of 10 000-20 000 vehicles/day (official data by Servei Català de Transit, Generalitat de Catalunya, Spain). A total of 13 samples (labeled M1-M13) representing road-deposited sediments alongside the C-58 motorway were collected after a long dry period. Additionally, three background samples were collected 500 m far away from possible contamination sources in order to collect soil background data. About 1 kg of each sample was collected by gently sweeping the road surface with a clean soft nylon broom, transferring the content of the plastic scoop to a coded polyethylene container, and transporting the samples to the laboratory. Samples were air-dried (30 °C) during 48 h and were passed through a 2 mm stainless steel sieve to remove large debris and homogenized by quartering with a riffler. The resulting subsamples were ground and sieved below 100 µm. Chemical Analyses. Soil edaphological parameters (pH, moisture content (%), organic matter (%O.M.), electrical conductivity (EC), and carbonate content (%CO3) were measured following the procedures described in the Official Methods of Soil Analysis envisaged by local governmental regulations (20). Major components were determined by X-ray fluorescence spectrometry using 56 geological international reference samples for calibration. Samples were diluted (1:40) with lithium tetraborate and melted in a radiofrequency inductive oven to obtain 30 mm diameter pearls. Range and average values of sample edaphological characteristics are detailed in Supporting Information Table SI-1. The different single leaching tests were based on the use of mild extractants (CaCl2, NaNO3), acid extractants (acetic acid and HCl) and complexing reagents (EDTA). Mild extractants aliquots were acidified to prevent growth of bacteria if not immediately analyzed. Sequential extraction was performed following the 3-step procedure from Standard Measurements and Testing (SM&T-SES), including the recommended modifications to improve the reproducibility of results (21). After each SES step, the suspension was centrifuged and the supernatant separated from the solid phase by filtering with a 0.22 µm filter (Millex-GS, Millipore, Ireland) to avoid the ICP-MS or ICP-OES nebulizer from fouling. The resulting extracts were stored in polypropylene bottles at 4 °C prior to analysis, except extracts from the second step, NaNO3 and CaCl2, which were analyzed immediately due to instability and degradation of the extracting reagent. Original sample treatment for total metal determination was the same as that used for residual fraction determination, that is, microwave digestion. All extractions and digestions were performed in triplicate including the 2310
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control of vessel, reagent, and procedural blanks. Three different aliquots of each sample were used to determine the moisture content to the proper correction to dry mass. Detailed information on the extractions and digestions experiments are summarized in Supporting Information Table SI-2. For QA/QC, two different CRMs were employed, including a sewage sludge amended soil (BCR CRM 483) (22) with both indicative values for CaCl2, NaNO3, and the SM&T-SES extractions and certified values for acetic acid and EDTA extractions. The second CRM employed was a lake sediment (BCR 701) (23) with certified values for the SM&T-SES. Prior to the application to sediment samples, single leaching tests and SM&T-SES procedures were validated by means of CRMs, obtaining a good traceability for corresponding HMs in each fraction/extraction (Supporting Information Table SI-3). No significant differences were observed (n ) 3) when applying F and t tests. Apparatus and Reagents. Major components were determined using a Philips PW2400 X-ray spectrophotometer equipped with Rh excitation tubes and a Philips radio frequency inductive oven (model PERL’X2, Holland). HMs were determined in each extract of the different leaching solutions by either a ThermoElemental inductively coupled plasma optical emission spectrometer (ICP-OES) (model Intrepid II XLS, Franklyn, MA) or a ThermoElemental inductively coupled plasma mass spectrometer (ICP-MS) (model PQExcell, Windsford, UK) for very low concentrations. Quantification of HMs was done using reagent-matched standard solutions, obtained by appropriate dilution of commercial stock solutions (Merck Darmstadt, Germany and J.T. Baker, North Kingstown, RI). Multielement standard solutions were used for ICP-OES and ICP-MS calibrations. Analytical grade reagents, supplied by Panreac, Barcelona, Spain, J.T. Baker, Phillipsburg, NJ, or Merck, Darmstadt Germany, were used throughout. All glassware and plastic containers were previously soaked overnight in 25% nitric acid and rinsed with double distilled water. Sample digestions for total determinations were performed in perfluoroalcoxy (PFA) vessels, with a CEM Corporation microwave laboratory unit (CEM Mars X, Mathews, NC). Conventional sequential extraction and single leaching tests were performed using a SBS end-overend mechanical shaker (model ABT-4, Barcelona, Spain). Extracts were separated from solid residues using a model C-5 centrifuge (Pacisa, Barcelona, Spain). Pollution Degree Assessment. Dutch target and intervention values, ECG, were taken as reference values to assess the contamination degree of roadside sediments (24). Three different environmental situations can be depicted from such reference: samples below target (there is a sustainable soil quality), above intervention (there is a severe case of soil contamination), and between both levels. However, these guidelines are based on total concentrations. Thus, by using different leaching extractants, more detailed information can be obtained in terms of mobile or available HMs pollution. Additionally, better insight of the overall pollution can be obtained by normalizing the different released amounts of HMs to the corresponding Dutch reference levels, thus allowing plotting the released HMs together using different leaching reagents. Furthermore, complementary reliable information can be obtained by considering the variation of metals concentration within the different geological substrates. In this sense, the proposed evaluation by CERs (25) is most appropriate to evaluate the anthropogenic contribution to the site contamination. CERs are calculated considering the concentration of a given element, Cn, in both target and background samples, normalized with respect to a lithogenic conservative element such as Al, which is accurately determined in each
TABLE 1. Statistical Summary of Total Content of Heavy Metals Including CER Values, Coded ECG Values, and Percentage of Samples per CER Class for Cd, Cr, Cu, Ni, Pb, and Zn in 13 Samples of Roadside Deposited Sediments from C-58 Highwaya Cd (mg · kg-1)
target intervention (mg · kg-1) mg · kg-1 (ECG) min CER mg · kg-1 (ECG) P25b CER mg · kg-1 (ECG) P50 CER mg · kg-1 (ECG) P75 CER mg · kg-1 (ECG) max CER CER < 2 2 < CER < 5 5 < CER < 20 % of samples per CER class 20 < CER < 40 40 < CER
0.8 12 3.42d 0.7 4.15d 1.35 4.46d 1.77 4.68d 2.34 9.3d 3.4 61.5 38.5 0 0 0
Cr
Cu
Ni
Pb
Zn
100 380 60c 0.37 71c 0.84 88c 1.15 117d 1.48 208d 2.24 92.3 7.7 0 0 0
36 190 83d 0.37 135d 0.96 216e 1.64 312e 2.25 5178e 42 53.8 30.8 7.7 0 7.7
35 210 21c 0.27 32c 0.87 36d 1.16 38d 1.25 1273e 35 84.6 0 7.7 7.7 0
85 350 179d 1.12 231d 3.08 283d 4.13 407e 6.62 1082e 9.06 15.4 46.1 38.5 0 0
140 720 218d 0.61 461d 2.01 542d 2.55 674d 2.89 966e 4.84 23.1 76.9 0 0 0
a Concentrations of Dutch target and intervention values refer to standard soil conditions (10% organic content and 25% fine fraction Zn2+ > Cu2+, has been found in previous SES studies on road and street sediments (37, 38). For the oxidizable fraction, the target limit for the metals considered is, again, surpassed. The known mobility of Cu, strongly associated with the organic matter, is less clear than that for Pb in the reducible fraction. However, the important amounts of Cu in this fraction are consistent with bacterial activity that increases mobilization when sediments receive water from runoff during rainfall events (37). Elements presenting a major residual distribution such as Cr, Ni, and Cd would only release substantial amounts under extreme conditions that may overcome target and intervention Dutch limits. These elements are bound to primary and secondary minerals, thus revealing the geological characteristics and becoming environmentally immobile. VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Correlation among Extractants. Unless particular assessment on contaminant phase association is needed, single extractions provide key information for mobility assessment. In this sense, the HCl extraction procedure reveals similar information to that obtained when using SES, providing an estimation of the total available amounts of HMs including acidic, reducible and oxidizable conditions (36) and improving the efficiency of SES by drastically reducing the timeconsuming experimental work. In this regard, considering the generally positive skewed data due to the nonnormal distribution of the anthropogenic enhanced HMs (Cu, Pb, and Zn), and after a log10 transformation, HCl released amounts were compared with the available amount released by SESs (ΣF1, F2, F3). The correlation of these two data quantities was statistically significant at p < 0,001 for Cu (r2 ) 0.798), Zn (r2 ) 0.901), and Pb (r2 ) 0.958) for the 13 roadside sediment samples. This fact indicates the utility of HCl leaching procedure as a fast screening test for HMs mobility. Distribution of Pollutants. A more correct assessment on the pollutants availability alongside the highway can be obtained from the leaching test data rather than using total content data (10). Mild extractants have not been considered here due to the low metal extraction. Plots in Figure 2 represent the ratio between the extracted amounts by acidic leachings for those HMs identified as anthropogenically enhanced, such as Cu, Pb, and Zn against the corresponding Dutch target value. An overestimation of HMs availability is observed when using total content data versus leaching test data. From these data, samples with the greatest ratio can be identified for the selected metals, that is, for Zn (M9, M3, M11, and M7), for Cu (M11 and M6), and for Pb (M1, M2, M3, and M11). This trend is also observed for Zn and Pb when dealing with the less acidic extractant (i.e., acetic acid 0.11 M) that produce a much lower metal release. This decreased release reveals only Zn to be the most mobile element in some of the samples (M3, M6, M7, and M9) since the rest of samples values for Cu, Pb, and Zn are below or close to the target value. However, for more acidic extractants, acetic acid 0.43 mol · L-1 and HCl, normalized values remain between target and intervention values, suggesting further investigation and possible restrictions in these areas. Moreover, some samples overcome the intervention value revealing the need for an urgent remedial strategy, i.e. samples M3, M11, M12, and M13 for Pb in HCl extraction. Figure 2 also (plots on the right) presents normalized data from SES and HCl test for Cu, Pb, and Zn. In this case, reducing environmental conditions represent the worst situation, especially for Pb, while the ratio for F1 and F3 contents are close to the target value. F2 contents indicate ratios above the target value, even above the intervention value for sample M3. Zn behaves in a similar way to Pb, but only samples M1 and M11, closest to the most polluted area, reflect a significant ratio between target and intervention limits. The case of Cu reflects the major availability of this element under oxidizing conditions, especially for those samples with higher pollution, and may be interpreted as the overall available pollution. A correlation among pollutants concentration and traffic density would be expected with an increasing content in the final sampling area, but this was not observed. In fact, samples M3 and M11 with a lower traffic density are more polluted. The continuous traffic retention accumulated during a whole year of road works, which involved stop and go traffic, road maintenance, and frequent accidents, would explain the contamination of these samples if compared with initial or final sampling points where traffic flow was steady despite its density. Furthermore, the relatively long dry period that occurred before the sampling could lead to a high ac2314
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cumulation of aging pollutants over the roadside sediments. Additionally, the abnormal enriched Zn sampling point at the initial part of the highway could be related to a highway exit with high truck traffic toward the closer industrial areas.
Acknowledgments Financial support for this study was provided by a research grant from the Spanish Ministry for Education and Science (CTM2006-13091-C02-02/TECNO). The Universitat Autònoma of Barcelona is acknowledged because of the scholarship provided to Gustavo Pérez to perform his PhD studies.
Supporting Information Available Figure of a map detailing the location of the highway and the sampling area. Tables containing the conditions and data results of extraction procedures, the results of samples edaphological characterization and QA/QC. This information is available free of charge via the Internet at http:// pubs.acs.org.
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