Heavy Metal and Trace Metal Analysis in Soil by Sequential Extraction: A Review of Procedures

Hindawi Publishing Corporation International Journal of Analytical Chemistry Volume 2010, Article ID 387803, 7 pages doi:10.1155/2010/387803 Review Article Heavy Metal and Trace Metal Analysis in Soil by Sequential Extraction: A Review of Procedures Amanda Jo Zimmerman1 and David C. Weindorf2 1 Louisiana State University Geology and Geophysics, Baton Rouge, LA 70803, USA 2 Louisiana State University AgCenter, Baton Rouge, LA 70803, USA Correspondence should be addressed to Amanda Jo Zimmerman, Received 2 October 2009; Revised 31 December 2009; Accepted 23 February 2010 Academic Editor: Alejandro Cifuentes Copyright © 2010 A. J. Zimmerman and D. C. Weindorf. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Quantification of heavy and trace metal contamination in soil can be arduous, requiring the use of lengthy and intricate extraction procedures which may or may not give reliable results. Of the many procedures in publication, some are designed to operate within specific parameters while others are designed for more broad application. Most procedures have been modified since their inception which creates ambiguity as to which procedure is most acceptable in a given situation. For this study, the Tessier, Community Bureau of Reference (BCR), Short, Galán, and Geological Society of Canada (GCS) procedures were examined to clarify benefits and limitations of each. Modifications of the Tessier, BCR, and GCS procedures were also examined. The efficacy of these procedures is addressed by looking at the soils used in each procedure, the limitations, applications, and future of sequential extraction. 1. Introduction Soils are the reservoir for many harmful constituents, elemental and biological, including heavy metals and trace metals, henceforth referred to as just metals [1]. Total metal content of soils is useful for many geochemical applications but often the speciation (bioavailability) of these metals is more of an interest agriculturally in terms of what is biologically extractable [2]. Speciation is defined by Tack and Verloo [3] as “the identification and quantification of the different, defined species, forms or phases in which an element occurs” and is essentially a function of the mineralogy and chemistry of the soil sample examined [4]. Quantification is typically done using chemical solutions of varying, but specific, strengths and reactivities to release metals from the different fractions of the examined soil [5]. In terms of bioavailability, various species of metals are more biologically available than others [6]. If bioavailability and the mobility of metals are related, then the higher the concentration of mobile toxic metals (Cu, Pb, Cd, and Al) in the soil column which increases the potential for plant uptake, and animal/human consumption [3, 7, 8]. Determination of metals in soil can be accomplished via single reagent leaching, ion exchange resins, and sequential extraction procedures (SEP), the latter under controversy. The number of available extraction techniques developed over the last three decades begs inquiry as to which technique is preferable over another. Moreover, the nonselectivity of the reagents used, handling of sediment prior to extraction, sediment-reagent ratio, and length of extraction all have an effect on data collected from SEP [3, 9] and can lead to inconsistent results even with the use of the same SEP. For true scientific collaboration to occur, a single SEP and set of standards would need to be adopted and applied across disciplines. The procedure adapted by Tessier et al. [4] is generally accepted as the most commonly used protocol followed closely by the BCR [5, 10, 11] but is still plagued by limitations discussed below. This paper examines five SEP techniques recently referenced in the literature by comparing fractions, reagents used, and length of extraction. Modifications to these procedures are also discussed as are the soils used in each case, limitations to, and applications of the SEP. 2 2. Sequential Extraction Procedures The theory behind SEP is that the most mobile metals are removed in the first fraction and continue in order of decreasing of mobility. All SEPs facilitate fractionation. Tessier et al. [4] named these fractions exchangeable, carbonate bound, Fe and Mn oxide bound, organic matter bound, and residual. These are also often referred to in the literature as exchangeable, weakly absorbed, hydrous-oxide bound, organic bound, and lattice material components, respectively [12]. Typically metals of anthropogenic inputs tend to reside in the first four fractions and metals found in the residual fraction are of natural occurrence in the parent rock [8]. The exchangeable fraction is removed by changing the ionic composition of water allowing metals sorbed to the exposed surfaces of sediment to be removed easily. A salt solution is commonly used to remove the exchangeable fraction. The carbonate-bound fraction is susceptible to changes in pH; an acid solution is used second. Metals bound to Fe and Mn oxides are particularly susceptible to anoxic (reducing) conditions so a solution capable of dissolving insoluble sulfide salts is used third. To remove metals bound in the organic phase, the organic material must be oxidized. The residual fraction consists of metals incorporated into the crystal structures of primary and secondary minerals. This fraction is the hardest to remove and requires the use of strong acids to break down silicate structures [4]. Most SEPs follow similar fractional degradation with little variation. Ure et al. [13] extracted the exchangeable and carbonate-bound fractions in a single step versus the two steps used in the Tessier procedure. The SEP used by the Geological Survey of Canada (GSC) divides the Fe and Mn oxide fractions into the amorphous oxyhydroxides and crystalline oxides, thereby increasing sequential fractionation from five to six steps [14]. Other SEPs with greater fractions include the procedure developed by Zeien and Brümmer [15] which included EDTA extractable, moderately reducible, and strongly reducible fractions for a total of seven; and that by Miller et al. [16] which consisted of nine fractions designed to test waste amended and agriculturally polluted sediments. The information needed from the SEP determines, to some extent, how the extraction is performed with respect to the final fraction, the residual. From a geochemical standpoint, total metal concentration is desired requiring the use of often dangerous reagents. From a biological or agricultural standpoint, less dangerous reagents may be utilized in lieu. The extraction conditions and reagents are listed in Table 1 for the five discussed SEPs. 2.1. Tessier Procedure. In the extraction procedure by Tessier et al. [4], 1 g of sample is placed in (...truncated)