Assessing soil quality in areas affected by sulphide mining. Application to soils in the Iberian pyrite belt (Sw Spain)

Minerals 2011, 1, 73-108; doi:10.3390/min1010073 minerals ISSN 2075-163X www.mdpi.com/journal/minerals/ Article Assessing Soil Quality in Areas Affected by Sulfide Mining. Application to Soils in the Iberian Pyrite Belt (SW Spain) Isabel González *, Emilio Galán and Antonio Romero Department of Crystallography, Mineralogy and Agricultural Chemistry, University of Seville, Apdo. 553, Seville 41071, Spain; E-Mails: [email protected] (E.G.); [email protected] (A.R.) * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +34-954-556-318; Fax: +34-954-557-140. Received: 1 September 2011; in revised form: 21 October 2011 / Accepted: 25 October 2011 / Published: 7 November 2011 Abstract: The characterization, evaluation and remediation of polluted soils is one of the present environmental challenges to be addressed in the coming years. The origin of trace elements in soils can be either geogenic or anthropogenic, but only the latter is interesting from a legal point of view. The hazard of the pollutants in the soils not only depends on their total concentration, but particularly on their availability. The mobility of the trace elements depends on their speciation, and it is also affected by several soil parameters. Mining activity is one of the most important anthropogenic causes of soil pollution. As a case study, this work is focused in the Riotinto mining area (Iberian Pyrite Belt, IPB, SW Spain). The IPB is one of the most important metallogenic provinces in the world and it has been exploited for thousands of years. The disposal of mining residues has produced important sources of contamination by trace elements and acidic waters affecting soils and rivers. In addition to these problems, the closure of mines in the Pyrite Belt at the end of the 20th Century has led to a great loss of employment, which has caused the development of an intensive agriculture of citrus fruits as a new source of income. The intensive growing of citrus fruits and the traditional subsistence agriculture have been developed surrounding the mining areas and on floodplains near to mining sites. The level of soil pollution has not been taken into account in these cases, nor has its impact on the health of the inhabitants of these areas. Therefore, it is of great interest to study the current state of the cultivated soils and the sources and types of contaminants derived from mining activity in order to program its decontamination, where appropriate, according to legislation. In order to know the present and future hazard posed by the soils chemical and mineralogical speciation has been carried out, given that the availability of a metal depends on the phase OPEN ACCESS Minerals 2011, 1 74 in which it is found. The results showed that mining activity has caused high levels of As, Cu, Pb and Zn in several cultivated soils. Moreover, Cu, Pb and Zn showed a high bioavailability. This suggests that at least other studies are necessary to preserve health in the inhabitants of this area. The methodology carried out in this work allowed to select potentially polluted areas where agricultural activities are not recommended. Keywords: soil contamination; trace elements; sulfide mining; Iberian Pyrite Belt 1. Introduction 1.1. Contamination and Contaminants The presence of harmful concentrations of some chemical elements and compounds in soil is a special type of degradation known as contamination. Contaminants are always found in anomalous (higher than usual) concentrations and generally have a harmful impact on some organisms. The origin of contaminants is either geogenic if they come from soil parent rocks, volcanic activity or mineral leaching, or anthropogenic if they come from hazardous industrial waste, agricultural or mining activities, or urban solid waste from households. However, anthropogenic contaminants are the only ones legally considered as true contaminants. The toxicity of a chemical element or compound is its ability to adversely affect biological functions. With the only exception of toxins (which are biogenic), toxic compounds and materials have a non-biological origin. Contaminants are mobilized by physical, chemical and biological vectors. They can leave soil by volatilization, dissolution, leaching or erosion and enter organisms when they are in a bioavailable (relatively soluble) form. Thus, the potential of an element––whether a contaminant or otherwise––passing from soil into its solution is known as “availability”. The more specific term bioavailability is the degree to which a contaminant in a potential source is free for uptake (movement into or onto an organism) [1]. Usually, only a small fraction of each potentially contaminating substance in a medium is bioavailable. Although contaminants are generally noxious, some may be harmless to specific organisms. Bioavailability is a function of (a) the specific physical and chemical form in which an element or substance occurs, and (b) the ability of organisms to absorb or ingest it. Bioavailable elements can be accumulated in an organism (i.e., bioaccumulation) at concentrations up to 3–5 orders of magnitude higher than those in their medium. 1.2. Heavy Metals and Trace Elements The periodic table includes about 70 metals, 59 of which are heavy metals (i.e., metals with an atomic weight exceeding the weight of iron, which is 55.85 g/mol). This definition excludes metals with a lower atomic weight, which however can also include contaminants. Such is the case for V, Mn and Cr, which have an atomic weight of 50.95, 54.44 and 52.01 g/mol, respectively. It also happens with other non-metal elements such as As, F and P. Therefore it is more accurate to refer to them as Minerals 2011, 1 75 trace elements, even though most inorganic contaminants are in fact heavy metals. Occasionally, soil is contaminated with high concentrations of major elements such as Na, Fe or Al. Trace elements are present at low concentrations (mg⋅kg−1) in the earth crust, soils and plants. Many of them (Table 1 [2]) are essential for plant, animal and human growth, but can be toxic above certain concentration levels. As a rule, trace elements are toxic if inhaled or ingested in large quantities and over a long period of time. For example, the deficiency and toxicity thresholds for Se, F and Mo differ from one another only by a few parts-per-million (ppm) [3]. Table 1. Micronutrients and macronutrients for optimum working of live organisms (collected from different sources by Siegel [2]). Trace elements that are essential micronutrients (a few mg or µg/day) As, Co, Cr, Cu, Fe, Mn, Mo, Se, V, Zn Other essential micronutrients F, I, Si Macronutrients (100 mg or more per day) Ca, Cl, Mg, P, K, Na, S Non-essential trace elements * Be, Cd, Hg, (Ni), Pb, Sb, (Sn), Ti * Metals in brackets can be essential. Most of trace elements in soil can be classified in five groups according to their chemical form in soil solution, namely: cations (Ag+, Cd2+, Co2+, Cr3+, Cu2+, Hg2+, Ni2+, Pb2+, Zn2+), native metals (Hg, V), oxianions (AsO43−, CrO42−, HSeO3−, SeO42−), halides (F−, Br−, I−) and organometals (Ag, As, Hg, Se, Te, Tl). These groups are not mutually exclusive and, in fact, some trace elements can fall into more than one. Typically, the concentrations of Cr, Ni, Pb and Zn in soil range from 1 to 1500 mg⋅kg−1; those of Co, Cu and As from 0.1 to 250 mg⋅kg−1; and those of Cd and Hg from 0.01 to 2 mg⋅kg−1 [4]. Table 2 lists other usual and anomalous geochemical concentrations of a variety of elements [5]. Table 2. Normal and anomalous geochemical concentrations of several trace elements in soils [5]. Element Normal range (mg/kg) Anomalous concentrations (mg/kg) As <5–40 Up to 2,500 Cd <1–2 Up to 30 Cu 2–60 Up to 2,000 Mo <1–5 10–100 Ni 2–100 Up to 8,000 Pb 10–150 10000 or more Se <1–2 Up to 500 Zn 25–200 10,000 or more From all of the trace elements found in soil, the following 17 are especially toxic and available at concentrations exceeding toxic levels: Ag, As, Bi, Cd, Co, Cu, Hg, Ni, Pb, Pd, Pt, Sb, Se, Sn, Te, Tl and Zn. Ten of them (Ag, As, Cd, Cu, Hg, Ni, Pb, Sb, Sn and Tl) are readily mobilized in concentrations exceeding considerably their geological levels due to human activities [6]. The latest US Environmental Protection Agency (EPA) list of priority pollutants includes the following 13 trace Minerals 2011, 1 76 elements: Sb, As, Cd, Cr, Cu, Hg, Ni, Ag, Pb, Se, Tl, Zn and Be, which was not included in previous lists. 1.3. Purifying Power of Soil Soil usually acts as a protective barrier for more sensitive media such as water and organisms by filtering, decomposing, neutralizing or storing contaminants. Hence it substantially reduces their bioavailability. The purifying ability of soil depends on its contents in organic matter, carbonates, and Fe and Mn oxyhydroxides, as well as on its clay content and mineralogical composition, cation exchange capacity, pH, Eh, texture, permeability and microbial activity. Therefore, the purifying power of soil has a limit for each substance. If this limit is exceeded, the soil becomes contaminated and is a source for contamination. The load capacity of soil shows its ability to control the adverse impacts of contaminants and render them inactive or harmless. Cheng et al. [7] proposed the Load Capacity of Agricultural Soils for Heavy Metals (LCASHM), which depends on soil properties, the type and contamination history, the presence of toxicity indicators and various environmental parameters. 1.4. The Origin of Trace Elements in Soil Trace elements present in soil can be geogenic or anthropogenic. The presence of the former is largely determined by their geoavailability, which is that portion of the total content of a chemical element or a compound in an earth material that can be liberated to the surficial or near-surface environment (or biosphere) through mechanical, chemical, or biological processes [8]. Geoavailable trace elements typically reach the soil when they are released from their parent rock because of weathering. Together with metals from volcanic emissions and mineral leachates, they constitute the so-called “geogenic metals”. The ease with which trace elements can be released from their parent rock depends on the structure and the thermodynamic stability of the rock forming minerals. Thus, highly polymerized structures are especially stable and resistant to destruction, and their stability decreases according to the following sequence of atomic bonds: Si–O > Al–O > X–O. The amount of geoavailable elements released from rocks into soil is negligible compared to those of anthropogenic origin. Weathering causes trace elements to concentrate in soil, but usually at levels below their toxic thresholds. Even if these levels are exceeded, there will be almost no risk of toxicity due to high stability and the low availability of these metals in soil. Ultrabasic rocks can contain large amounts of some trace elements such as Cr, Ni, Cu or Mn. On the other hand, acid igneous rocks and sedimentary rocks (sandstone and limestone) contain very small amounts of them. The highest levels of trace metals in soils––inherited from the parent rock––are typically those of Cr, Mn and Ni, followed by Co, Cu, Zn and Pb, and, far beyond As, Cd and Hg, which are usually present at very low concentrations. Anomalous high concentrations of trace elements in soil usually come from metal ores (sulfides, oxides). Metal mining and extraction produce tons of waste each year; most of it contains pyrite and other sulfides whose oxidation releases a large amount of trace elements into the environment, especially into soils. The upper layers of soil around mining and extraction areas can contain high concentrations of Cu, Ni, As, Se, Cd or Fe among other elements, depending on the particular ore. Minerals 2011, 1 77 Trace elements in soil can come from various other anthropogenic activities in addition to mining, namely: • Farming (irrigation, inorganic fertilizers, pesticides, manure, liming amendments and, especially, sewage sludge). • Electrical power production. Coal burning is one of the primary sources of metal deposition in soil. For example, thermal power plants typically release Pb, Ni and V contained in oil. • Industrial activities (particularly iron and steel making factories, which release metals contained in Fe and Ni ores). Heavy industrial areas are typical sources of As, Cd, Cr, Hg, Fe, Ni, Pb and Zn. • Household activities. Roughly 10% of all household waste consists of metals. Therefore, its burial can lead to the contamination of groundwater and its ashing to atmospheric contamination with volatile metals and their subsequent incorporation into soil. 2. Soil Contamination by Trace Elements 2.1. Physical State and Relative Availability of Soil Contaminants Contaminants in soil and sediments can be present in six different forms, namely [9]: particles (particulate contaminants), liquid films, adsorbed, absorbed, and solids or dissolved matter in pore water. The behavior of a contaminant in soil depends on its particular form. As a result, total concentration of trace elements in soil may represent their potential toxicity in relation to agreed thresholds, but not an actual risk. Therefore, the accurate assessment of their toxicity requires determining not only their concentrations, but also their physical and chemical forms, as well as their availability, which provides a direct measure of their actual hazardousness. Lastly toxicity of trace element concentration in the soil should also be tested against some specific organisms. Nevertheless the ease with which a potentially toxic substance can enter the food chain from soil depends on whether the metal is in free form in the interstitial solution or in solid form, and also on the way the metal is bound to soil particles and on its specific chemical form (i.e., its speciation). According to Kabata-Pendias [10], how strongly a metal is retained by soil conditions dictates its relative availability to plants and hence the risk of its reaching organisms (Table 3). Table 3. Relative mobility and phytoavailability of metals species in soils. Metal species and association Availability-mobility Simple or complex cations in solution phase Easy Exchangeable cations Medium Chelated cations Slight Metals adsorbed by soil particles Slight Metal compounds precipitated on soil particles Available after dissolution Metal bound or fixed inside organic substances Available after decomposition Metal bound or fixed inside mineral particles Available after weathering and/or decomposition Minerals 2011, 1 78 2.2. Contaminant Mobility in Relation to Soil Parameters The mobility of a metal depends not only on its chemical speciation, but also on various soil-related parameters such as pH, organic matter, carbonates and clay minerals. Thus, not all the exchange cations are equally available; rather, their availability depends on the particular minerals containing them in the exchange complex. For example, metals found in smectite or illite do not necessarily behave identically. Also, if the metal is found in its precipitated form, there is no reason why it has to behave identically, whether it is in a carbonate, sulfate or phosphate. Likewise, metals in sulfides—which are relatively easily oxidized and dissolved––will usually behave differently from metals in silicates, which are virtually stable in any medium. Usually trace elements show a very low mobility and accumulate within the top few centimeters of soil as a result. Therefore they are leached in very small amounts to the lower horizons. Hence an anthropogenic metal present at high concentrations in the upper soil horizon will usually be at much lower levels in deeper layers. However, this behavior is additionally dependent on the properties of the particular soil. The following soil variables are crucial with a view to assessing the sensitivity of soil to contaminant aggressions: • pH. Most metals tend to be more readily available in acid media, where they are less strongly adsorbed. As an exception, As, Mo, Se and Cr are more mobile in alkaline media. • Texture. Clayey soils are highly effective in retaining metals by adsorption (or in their exchange complex of clay minerals). On the other hand, sandy soils are unable to fix metals, which can thus easily reach the water table. • Clay minerals. Clay minerals differ in specific surface area and electrical charge. Thus, the higher the specific surface area of a silicate is, the higher its ability to adsorb metals [11]. However, absorption by clay minerals is negligible in soils containing large amounts of organic matter and/or iron oxyhydroxides which are more effective in this respect [12]. • Organic matter reacts with metals to form exchange complexes or chelates. Metals can be adsorbed to the extent of being stabilized as a result (Cu) or form highly stable chelates with organic matter (Pb, Zn). • Cation exchange capacity. The cation exchange capacity of soil depends on the clay minerals, organic matter, and valence and hydrated ionic radius of the metal. The greater the radius and lower the valence are, the less strongly the metal will be retained. • Redox potential. The redox potential dictates whether the metal is present in oxidized or reduced form. Eh–pH plots provide information about the stability of metal compounds and it is a convenient method for predicting the response of trace elements to a change in environmental conditions. • Carbonates. The presence of carbonates facilitates preservation of a highly alkaline medium and hence precipitation of trace elements. Also, Cd and other metals tend to be absorbed by carbonates. • Iron and manganese oxides and hydroxides. They are finely spread across soil and hence they are highly active. Their low crystallinity and small particle size make them highly sorptive for divalent metals especially Cu and Pb, and, to a lesser extent, Zn, Co, Cr, Mo, Ni and As. Minerals 2011, 1 79 • Salinity. An increased salinity can result in the mobilization and retention of trace elements via two different mechanisms. First, Na and K can replace metals at cation exchange sites, and then chloride and sulfate ions can form more stable compounds with previously released metals such as Pb, Zn, Cu and Hg. The salts usually make soil alkaline. 2.3. Assessing the Hazardousness of Metals in Soil The hazardousness (mobility and availability) of a contaminant can be assessed by using various chemical and mineralogical procedures. Whether a given element or compound is present in a water-soluble phase––and hence readily available to be transferred to a plant or living organism––can be checked by assessing its mobility via its water solubility. This process requires the extraction to happen with neutral or slightly acidic water. Another way of assessing the transferability of metals (particularly divalent cations) to plants is extracting soil with DTPA or 0.05 M EDTA at pH 7 [13]. There is a wide consensus about the fact that this process provides quite an accurate measure of the amount of metal that plants can absorb from soil under regular conditions [14]. These simple methods can only be used to discriminate between residual metal and metal extracted by water or a mild organic solvent. However there are more elaborated methods involving sequential chemical extractions to carry out speciation of elements in soil (i.e., operational speciation). Also chemical speciation is used to discriminate between oxidation states of an element, and functional speciation, which distinguishes between molecular forms. The plant available fraction of an element in soil depends on its chemical and functional speciation (i.e., of its distribution among its different species and chemical forms). Operational speciation, which is often referred to as simply “speciation”, is the most frequent choice for soils because it is the simplest analytically and can be virtually implemented in any laboratory with usual available means. However, proper speciation requires determining the total concentration of each metal and its different chemical species. The selectivity of each method used for this purpose depends largely on how fast the formation and dissociation reactions of the target species occur. Roughly, classical procedures used in this context that isolate and examine each constituent separately, are subject to two major shortcomings, namely: (a) complete isolation is very difficult or even impossible; and (b) the properties of a component in a complex mixture are not additive owing to the many interactions it can have with other components. Sequential extractions do not allow us to complete full speciation processes but they can be useful to characterize the behavior of a contaminating element as they allow its content in the fraction associated to each soil component related to its mobility and transferability to living organisms. In summary, sequential extraction procedures can provide rough estimates of the distribution of trace elements in different soil phases and their relative mobility. Sequential extraction procedures have been around for at least four decades. During that time they have been continuously modified and updated, or even replaced with others procedures providing us with improved results about specific elements and soil phases. Hall et al. [15] reviewed the most widely used speciation sequences for metals in soils and sediments. The procedure essentially involves obtaining various consecutive fractions by extracting procedures with different solvents to determine Minerals 2011, 1 80 the distribution of trace elements in a solid sample between its constituent phases. Such a distribution is usually called partitioning. López Julián and Mandado [16] critically reviewed available speciation methods of this kind and their operational protocols. One of the most popular protocols was initially proposed by Tessier et al. [17]. It has been the subject of a number of variations [18-21]. Briefly, the procedure is as follows: fractions 1 to 3 (F1–F3) are the results of extraction with ammonium acetate, a reductive solution made of hydroxylamine hydrochloride in acetic acid, and hydrogen peroxide in a nitric medium, respectively, whereas fraction 4 (F4) is the insoluble residue remaining after the three extractions (Table 4). Table 4. Sequential extraction scheme for assessment of partitioning of metals in sediments given by Tessier et al. [17] and modified by Galán et al. [21]. Fraction Procedure Acid (F1) NH4Ac 1 M (35 mL), pH 5, 1 h, 20 °C, continuous shaking (40 rpm) Reducing (F2) NH2OH.HCl 0.4 M in acetic acid 25% (20 mL), 6 h, 96 °C, manual shaking every 30 min. Oxidant (F3) HNO3 0.2 M (3 mL) + H2O2 30% (5 mL), pH 2 with HNO3, 2 h, 85 °C, manual shaking every 30 min.; add H2O2 30% (3mL), pH 2 with HNO3, 3 h, 85 °C, NH4OAc 3.2 M (5mL) v/v in HNO3 diluted in 20 ml with H2O2, 30 min., 20 °C, continuous shaking (40 rpm) Residue (F4) HF/HNO3/HCl 10/3/2.5 (20 mL), 2 h to dry F1 contains metals in the exchange complex and those forming or being absorbed in carbonates. The treatment used to obtain this fraction involves two steps: extraction with 1 M MgCl2 for 1 h to separate exchangeable cations (i.e., those which can be replaced with other more competitive cations in clay minerals, amorphous materials, etc.) and extraction with sodium acetate-acetic acid at pH 5 for 5 h to attack carbonates. F2 contains metals in reducible compounds (Mn oxides, and amorphous or low-crystalline Feand, occasionally, also Al-hydroxides). F3 is the oxidizable phase and contains organically bound (complexed, adsorbed and chelated) cations in addition to some sulfides. Finally, the residual fraction (F4) contains metals in the mineral structure of each extraction-resistant mineral. Overall, the performance of sequential extraction procedures is limited by factors such as (a) the selectivity of the reagents, (b) potential redistribution (or re-adsorption) of elements, (c) the experimental conditions used and (d) a scarcity of reference materials, which often prevents validation and precision assessment. A number of extractants are not fully selective for some metals present in specific geochemical phases. Such is the case of soil containing large amounts of iron hydrous oxides, which require repeating their extraction with hydroxylamine hydrochloride several times. Altering the pH to ensure that all metals associated to the target phases will be completely released [20,21] is also needed. In summary, sequential extraction protocols should be used with great caution, both when selecting the specific scheme to be applied and when interpreting the results. Some of the most important criteria for validating an analytical method (e.g., its theoretical background, cross-validation with other techniques and sample-independence of the results) cannot be applied to these procedures, which precludes validation of their operational results. However, they are bound to continue in use, constantly being improved and updated, until a universally accepted standard protocol is developed. Minerals 2011, 1 81 It will happen this way because the information they provide about distributions bears a direct relationship to the geochemical behavior and the bioavailability of chemical elements in nature [22]. Currently, analyses often include a mineralogical study of the fractions obtained from each extraction. This not only shows us the particular elements released, but also the specific minerals dissolved by each extractant, to be known, i.e., chemical-mineralogical speciation. Dold and Fontboté and Dold [23,24], developed a speciation scheme adapted to sulfide mining residues which involves six steps and mineralogical analysis of each extract in relation to the mineralogical composition of the sample. Indirect and semi-direct methods for characterizing solid phases by infrared, X-ray fluorescence spectrometry or energy dispersive spectrometry are being superseded by direct speciation methods based on more sophisticated techniques such as X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure spectroscopy (XANES), X-ray absorption near-edge structure spectroscopy (XAF) or X-ray absorption spectroscopy (XAS), all of which afford complete chemical speciation [25,26]. These techniques provide a more precise description of processes about solid surfaces, whether crystalline or not, and chemical species at atomic level. However, some analyses require using a synchrotron, which is usually inaccessible for many laboratories. Also, the routine work needed to process hundreds of samples makes these alternative procedures rather slow and expensive unless they are used to determine a single element. Another classical way of assessing the mobility of trace elements is by leaching, whether on site or at the laboratory. By using test columns or batch tests, one can run isotherms or adsorption-desorption curves and model the results as a function of pH or the amount of each contaminant and its nature with a view to predicting the behavior of a soil. This methodology can be used in combination with one of the previous alternatives. For example, adsorption-desorption isotherms can be used jointly with speciation results obtained from sequential extractions and metal solubility modeling conditions to predict the geochemical mechanisms governing metal leaching and mobility in soil. The software PHREEQC-2 [27] is among the most widely used for modeling in this context. Although all speciation methods (operational, functional, chemical, chemical-mineralogical, direct) and modeling of their results provide us with a broad picture of the potential “availability” of elements in soil and their ease of absorption by plants, each type of plant absorbs elements in a specific way. Thus, some elements are more readily absorbed, and some plants are more selective absorbers, than others. Chojnacka et al. [28] defined the so-called “transfer coefficient” for a metal as its concentration in plant tissue in relation to soil. This coefficient is maximal for Cd, Tl, Zn, Sn and Cu; intermediate for Ni; and minimal for Co, Cr, Pb, As and Sn. As a rule, the more persistent a given trace contaminant in soil is, the more stable it is and the lower its bioavailability and toxicity are. 3. Claim of Soils Contaminated by Trace Elements Whereas to claim a soil is contaminated is purely an administrative action, to actually demonstrate it is not an easy task. Within the context of this current work, contamination could be defined as the presence of an anomalous high level of a given element in the soil. However since no soil in the world is pristine and completely uncontaminated, it is quite difficult to determine if it has been contaminated or not. In fact, every soil has been under some kind of human pressure leading to a greater or lesser Minerals 2011, 1 88 Figure 2. Methodological approach followed in this study. Major elements were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) and minor elements by both ICP-OES and instrumental neutron activation analysis (INAA). Chemical speciation was done in 11 samples of agricultural soils, following the scheme originally proposed by Tessier et al. [17] as subsequently modified by Galán et al. [21]. The trace elements extracted by each solvent were determined spectrophotometrically. Mineralogical speciation was performed on 7 samples selected in terms of their trace metals content, iron oxides, and phyllosilicates. That was done using XRD to identify the mineral phases present in the solid residues left by the different extractants in Tessier’s protocol. For subsequent characterization, scanning electron microscopy (SEM-EDS) and microparticle induced X-ray emission (microPIXE) spectroscopy were also used. Soil sampling at random. Grinding, sieving, homogenizing Statistical treatment Discussion CONCLUSIONS pH Chemical analysis Major (ICP-OES) and trace (ICP-OES, INAA) Mineralogical analysis Global sample (DRX, Powder Method) <2µm fracción (AO, EG, 550 °C) Semiquantitative analysis Chemical Speciation (Tessier) Bioavailability. Acid water and EDTA extractions Characterization SEM-EDS and Próton Microbeam (microPIXE) XRD of solids residues alter each extraction Mineralogical Speciation <2 mm fraction Minerals 2011, 1 89 Bioavailability was assessed extracting the samples with a 0.05 M solution of EDTA in sodium form at pH 7 under continuous stirring for 1 h. Acid-extractable metals were obtained treating the samples with water acidified with HNO3 to pH 1 under continuous stirring for 1 h. Both extracts were spectrophotometrically analyzed. In order to confirm some relationships between elements with a similar geochemical behaviour in the soils, the results were subjected to correlation analysis and principal component analysis. Moreover, in order to facilitate spatial analysis of the data and mapping potentially dangerous zones for agricultural use, each sample was related to itself and to all others by using a Geographic Information System (GIS). Specifically, alphanumeric values were introduced as input into a database and georeferenced to obtain geographic location mapping outputs and develop interactive maps to display specific data for the sample concerned. That was done using the software ArcGIS v. 9.2. All information collected was recorded on a reference system used by the Spanish Geographic Institute (IGN) and Spain’s Army Mapping Service (SCE) for zone 30 in the 1950 European Datum (ED-50), which was made compulsory in our country by Decree 2303/1970. 5.3. Results and Discussion 5.3.1. Characterization of Agricultural Soils The soil samples studied were found to contain high proportions of gravel occasionally exceeding 50%. Therefore the soils were classified as sandy gravel. Because of their sandy nature, these soils have a low ability to absorb trace elements. In fact only those in Nerva zones, which were slightly more clayey, were able to retain greater amounts of the elements studied. Soil pH ranged from 4.6 to 7.6, the most acid samples being those collected very near the Peña del Hierro mine, Monte Sorromero and Tharsis (with pH < 5.5). These results are similar to those for contaminated soils in Mexico [58] and Mendips, England [86]. pH is a very important parameter since the concentration of hydrogen ions in soil influences the bioavailability of trace elements and nutrients. Also, it can be understood as a measure of soil deficiencies or alteration [87,88]. Most of the metals studied tend to be more readily available at an acid pH. As a rule, the contents in divalent metals such as Cu, Zn, Ni, Co, Cd and Mn decreased when increasing the pH [67,89]. The minerals found in the samples consisted mainly of quartz, phyllosilicates and feldspars. The latter are calcium plagioclases which explains the high Ca contents of some samples despite the absence of carbonates. There were also substantial amounts of hematite and goethite in some samples from Tharsis, Riotinto and Las Delgadas. The high content in iron oxides of some samples (5–13% Fe2O3)––particularly the ones form near the Tharsis and Sotiel mines, and from some sites in Riotinto ––where no hematite or goehtite was detected, suggest the presence of iron gels. This is an interesting finding since the high specific surface area and chemical reactivity of these materials play a major role in the sorption of trace elements (particularly metals such as Cu, Pb, and As) [90]. The <2 μm fraction consisted of illite, kaolinite and chlorite as major minerals, in addition to smectite in some samples from zones with altered volcanic rocks (Berrocal, Marigenta, Zalamea). Some leptosols (soils with shales as parent rocks) were found to contain chlorite-vermiculite mixed layers (Peña del Hierro, Riotinto) and illite-smectite mixed layers (Las Delgadas). Unlike other Minerals 2011, 1 90 contaminated soils [79], no jarosite was detected in any sample since their pH, above 3, precluded its formation. The soils were poor in alkaline and alkaline-earth metals. Their contents in CaO, K2O, MgO and Na2O never exceeding 5%. These results are consistent with those reported by Chopin et al. [76] for Tharsis soils, and those reported by Fernández Caliani et al. [77], Barba et al. [83] and López et al. [80] for other similarly contaminated soils in the Iberian Pyrite Belt. The spatial distribution of major elements in soils explains the existence of a zone with an increased abundance of iron and manganese associated to volcanic rocks (Figure 3), which is identified in the south of the study area [91]. Figure 3. Spatial distribution of Fe and Mn oxides of samples studied at Riotinto Mining District. The trace elements studied included As, Cd, Co, Ni, Cu, Pb and Zn, which are especially abundant in soils affected by sulphide mining [4]. All of them can be found in the US EPA priority pollutant list. The contents in As ranged from 19 to 994 ppm and peaked in the samples extracted from Tharsis (994 ppm) and Sotiel (882 ppm), which clearly exceeded the average value (227 ppm). This is logical if we consider these samples were under the direct influence of dumps and were thus mine soils. The contents in Pb ranged from 40 to 4,900 ppm. The highest values belong to the samples extracted from Tharsis (2,650 ppm) and Sotiel (4,890 ppm). Those of Zn ranged from 95 to 900 ppm and those of Cu from 27 to 1,160 ppm; the latter, however, were high in most samples and clearly exceeded the regional background level for this element [85]. The Co, Cr and Ni content, which were not too high, exceeded their respective regional background levels of Las Delgadas, Marigenta and Zalamea zones. The anomalous high levels they show were possibly geogenic and associated to the parent rock [84]. Minerals 2011, 1 91 Except for Cd, all the elements exceeded the levels for the 95 percentile in Andalusian soils and the 90 percentile for the geological domain of the Sub-Portuguese Zone [85]. In the agriculture soils, which were not directly under the influence of acid mining drainage, the trace elements contents exceeded the critical ecotoxic concentration established by Kabata-Pendias [10], except for Cd and Zn (Table 7). Also, the mean values for all elements were similar to those previously found in Tharsis (Iberian Pyrite Belt) by Chopin and Alloway [79]. Table 7. Minimum and maximum values (mean) for some soils close to mining exploitations and reference values. Data in mg/kg. Soils affected by mining exploitation As Cd Cu Pb Zn Cr South Korea [56] 23.3–75.9 (47.7) 2.6–6.1 (4.3) 40–99 (59) 289–948 (477) 275–568 (384) ----- Korea [47] 64.8–131 (99.6) ----- 49.6–50.3 (50) 2–11.3 (6.7) 153–322 (237) ----- China [48] ---- 4–24 (8) 11–100 (15) 11–380 (15) 33–1,600 (52) 48 Somerset, England [86] ----- 0.5–127 (2.1) 2.8–145 (12) 8–1,000 (52) 14–8,344 (132) Mexico [58] 7–1738 (569) ----- ----- 30–3,450 (312) ----- ----- “La Unión”, Spain [69] ----- 2–322 (41) 36–2,255 (332) 0–19,000 (8) 1,200–53,100 (12.5) “San Quintín”, Spain [94] ---- (3.76) (57.01) (393) (596) ---- “Tharsis” Spain [79] 2–1519 (94) ---- 20–3,090 (216) 18–6,350 (172) 45–870 (118) ------ “Riotinto” Spain [80] 12–204 (68) 0.3–3.2 (0.53) 47–586 (181) 34–605 (177) 72–795 (197) 48–776 (149) World normal values [4] 6 0.35 70 30 35 ------ Allowable values [10] 20 3 100 100 300 54 The factor analysis between trace elements suggests an association between Cu, Pb, Zn and As on the one hand, and Cr, Ni, Co and Fe on the other (Table 8). This is a logical result because of the presence of massive sulfides––and also, occasionally, veins of these elements––associated to the volcano sedimentary complex in the Sub-Portuguese Zone [91]. The principal component analysis confirmed these two associations, which were related to the potential origin of the elements. Thus, Cr, Ni and Co were essentially geogenic, whereas Pb, Zn, As and Cu were markedly anthropogenic. It should be noted that, unlike less contaminated soils [92-94], none of the trace elements were related to the phyllosilicate content found in the soil tested. Minerals 2011, 1 92 Table 8. Eigenvectors F1 and F2 after varimax rotation. Element Factor 1 Factor 2 Cu 0.840748 −0.09327 Cd 0.647392 −0.12501 Pb 0.916278 0.067878 Ni −0.16902 0.870814 Zn 0.882436 0.078646 As 0.910778 0.081364 Co 0.130555 0.892078 Cr −0.26457 0.883246 Fe 0.34058 0.843938 SiO2 −0.15414 −0.72585 Phillosilicates −0.35041 0.568918 Expl.Var 3.951898 3.938739 Prp.Totl 0.359263 0.358067 5.3.2. Chemical Speciation Overall, the sum of trace element contents in each sequential extraction was comparable to those in the total sample according to the INAA and the ICP-OES (Table 9). Although some samples exhibited very different values, they would be accepted if the combined metal contents of their different fractions were within a range of ± 10% in relation to those in the total sample [95,96]. Table 9. Chemical analyses of sequential extractions (mg/kg). Samples directly affected by waste rock piles or acid leachates are in bold. Fraction Location Sample As Fe Pb Zn Cd Cu Co Ni Total sample Peña Hierro RT-4-1 77 37,400 145 186 1 212 24 38 Peña Hierro RT-5-1 54 47,500 66 91 0 97 11 39 Peña Hierro RT-6-1 103 46,900 108 215 1 586 12 40 Las Delgadas RT-8-1 18 10,900 39 127 0 80 88 424 Las Delgadas RT-8-2 120 51,500 126 114 0 245 12 43 Nerva RT-20-1 77 51,800 598 795 3 495 15 47 Nerva RT-21-1 204 52,300 563 374 3 482 18 38 Nerva RT-21-2 197 48,400 603 388 1 526 16 41 Tharsis Rt-28 444 65,600 1,270 667 1 198 41 51 Tharsis Rt-29 994 88,100 2,650 897 1 443 70 54 Sotiel Rt-30 882 70,900 4,890 585 1 1160 19 48 Interchangeable Peña Hierro RT-4-1 2 0 2 109 0 6 0 1 Peña Hierro RT-5-1 1 1 5 9 0 4 0 0 Peña Hierro RT-6-1 1 0 1 0 0 20 0 0 Las Delgadas RT-8-1 1 0 1 0 0 2 0 5 Las Delgadas RT-8-2 1 0 14 20 0 9 0 0 Nerva RT-20-1 1 0 1 22 0 4 0 0 Nerva RT-21-1 1 0 1 14 0 8 0 0 Nerva RT-21-2 1 0 1 11 0 7 0 0 Tharsis Rt-28 5 0 35 211 1 0 N.D. 7 Tharsis Rt-29 8 0 114 337 1 10 N.D. 18 Sotiel Rt-30 20 0 83 15 1 33 N.D. 0 Minerals 2011, 1 93 Table 9. Cont. Fraction Location Sample As Fe Pb Zn Cd Cu Co Ni Labil/carbonates Peña Hierro RT-4-1 2 180 10 336 0 132 4 1 Peña Hierro RT-5-1 2 126 25 0 0 43 0 1 Peña Hierro RT-6-1 2 39 41 659 0 390 9 8 Las Delgadas RT-8-1 2 248 2 0 0 9 6 47 Las Delgadas RT-8-2 2 89 49 24 0 191 1 3 Nerva RT-20-1 2 28 94 1713 2 268 2 4 Nerva RT-21-1 2 27 48 428 0 96 2 1 Nerva RT-21-2 2 25 54 404 0 98 0 1 Tharsis Rt-28 20 180 162 26 1 17 N.D. 2 Tharsis Rt-29 26 2,747 457 202 1 92 N.D. 7 Sotiel Rt-30 42 1,371 1,574 342 1 455 N.D. 7 Fe-Mn Oxides Peña Hierro RT-4-1 1 15,174 22 24 2 72 5 2 Peña Hierro RT-5-1 1 20,484 11 8 3 36 1 2 Peña Hierro RT-6-1 1 24,779 19 32 4 276 2 3 Las Delgadas RT-8-1 1 30,348 9 9 5 26 17 39 Las Delgadas RT-8-2 1 20,861 18 7 3 68 1 2 Nerva RT-20-1 1 15,322 60 118 3 177 2 4 Nerva RT-21-1 1 14,106 70 58 2 174 3 2 Nerva RT-21-2 1 14,261 62 50 2 167 2 2 Tharsis Rt-28 194 47,984 1,259 3,414 0 214 N.D. 31 Tharsis Rt-29 513 70,834 2,781 5,713 1 390 N.D. 30 Sotiel Rt-30 246 42,384 3,877 2,740 1 1,380 N.D. 23 Organic matter Peña Hierro RT-4-1 1 1,789 1 13 0 3 1 2 Peña Hierro RT-5-1 1 1,774 1 7 1 1 7 3 Peña Hierro RT-6-1 1 1,384 1 6 0 5 0 2 Las Delgadas RT-8-1 1 2,803 1 5 1 1 3 16 Las Delgadas RT-8-2 1 1,137 1 4 0 2 0 2 Nerva RT-20-1 1 1,306 2 14 0 13 0 2 Nerva RT-21-1 1 1,730 3 9 0 6 0 2 Nerva RT-21-2 1 1,487 3 7 0 4 0 2 Tharsis Rt-28 15 2,892 99 79 0 57 N.D. 6 Tharsis Rt-29 39 2,947 158 66 0 126 N.D. 4 Sotiel Rt-30 24 2,861 292 180 0 17 N.D. 5 Residue Peña Hierro RT-4-1 71 20,257 110 0 0 0 14 32 Peña Hierro RT-5-1 50 25,114 25 67 0 12 2 33 Peña Hierro RT-6-1 99 20,698 47 0 0 0 1 27 Las Delgadas RT-8-1 13 75,601 26 113 0 43 62 317 Las Delgadas RT-8-2 116 29,413 44 59 0 0 9 36 Nerva RT-20-1 73 35,144 441 0 0 32 11 36 Nerva RT-21-1 200 36,437 442 0 0 198 12 33 Nerva RT-21-2 193 32,627 486 0 0 249 13 36 Tharsis Rt-28 98 28,157 1,210 861 0 24 N.D. 14 Tharsis Rt-29 366 14,135 644 961 0 61 N.D. 17 Sotiel Rt-30 245 17,395 1,291 112 0 110 N.D. 13 N.D.: not detected. Minerals 2011, 1 94 It should be noted that only Cd was present in high proportions in the exchangeable fraction. However its presence was not hazardous because of their low total content. All other trace elements were largely associated to iron oxyhydroxides and the residual fraction. These results confirm the presence of amorphous oxyhydroxides capable of incorporating large amounts of metal cations [24]. Figure 4. Sequential extraction of several trace elements in selected samples. Samples directly affected by mining wastes or leachates are highlighted. Arsenic and Ni were associated to the iron and manganese oxides fraction in the samples directly affected by mining wastes and to the residual fraction in all others. Hence it was scarcely present in the most mobile fractions (Figure 4). Copper was present in all fractions, but largely associated to labile fractions and iron and manganese oxides virtually in all samples. Lead and zinc were associated to iron and manganese oxides and in the insoluble residue, but also to labile fractions. The fact that Zn, Pb and Cu were partially associated to the exchangeable fraction that consisted of phyllosilicates and carbonates, suggests that their presence was a result of ion exchange in clay minerals or precipitation as oxides on their surface. As 0% 20% 40% 60% 80% 100% RT-4 RT-5 RT-6 RT-8 RT-20 RT-21 Rt-28 Rt-29 Rt-30 Samples Extracted % F1 F2 F3 F4 F5 Fe 0% 20% 40% 60% 80% 100% RT-4 RT-5 RT-6 RT-8 RT-20 RT-21 Rt-28 Rt-29 Rt-30 Samples Extracted % F1 F2 F3 F4 F5 Pb 0% 20% 40% 60% 80% 100% RT-4 RT-5 RT-6 RT-8 RT-20 RT-21 Rt-28 Rt-29 Rt-30 Samples Extracted % F1 F2 F3 F4 F5 Zn 0% 20% 40% 60% 80% 100% RT-4 RT-5 RT-6 RT-8 RT-20 RT-21 Rt-28 Rt-29 Rt-30 Samples Extracted % F1 F2 F3 F4 F5 Cu 0% 20% 40% 60% 80% 100% RT-4 RT-5 RT-6 RT-8 RT-20 RT-21 Rt-28 Rt-29 Rt-30 Samples Extracted % F1 F2 F3 F4 F5 Ni 0% 20% 40% 60% 80% 100% RT-4 RT-5 RT-6 RT-8 RT-20 RT-21 Rt-28 Rt-29 Rt-30 Samples Extracted % F1 F2 F3 F4 F5 Minerals 2011, 1 95 Similar results were previously reported for soils under the influence of mining waste in China and Korea [52,97], and the Pb-Zn mine in San Quintín (Spain) [94]. In conclusion, most of the elements studied were adsorbed in (or as structural component of) iron oxyhydroxides, or present in the residual fraction. Therefore, the present physical–chemical conditions of the soils facilitate the immobilization of the toxic elements studied, but their current levels pose a potential hazard that warrants assessment. 5.3.3. Availability and Bioavailability Tests The fraction extracted with acid water contained all elements studied, some at virtually negligible concentrations and others, including Fe, Mn, Zn, Cu and Pb, at very high levels (close to the 40% in some samples). As it can be noticed in Figure 5, As and Ni exhibited a high availability in all samples under the direct influence of a mine dump, maybe because both elements were associated to iron oxides. Figure 5. Acid water and EDTA extractions in selected samples without results for Fe, Mn, Pb, Zn and Cu. Samples directly affected by mining wastes or leachates are highlighted. The fraction extracted with EDTA also contained all elements. As Figure 5 shows, As was highly available, especially in soils near the mines in Tharsis (Rt28-29) and Sotiel (Rt-30), which contained EDTA 0 20 40 60 80 100 RT-4-1 RT-5-1 RT-6-1 RT-8-1 RT-8-2 RT-20-1 RT-21-1 RT-21-2 Rt-28 Rt-29 Rt-30 Samples Concentration (mg/kg) Ni Co Cd As Acid Water 0 20 40 60 80 100 RT-4-1 RT-5-1 RT-6-1 RT-8-1 RT-8-2 RT-20-1 RT-21-1 RT-21-2 Rt-28 Rt-29 Rt-30 Samples Concentration (mg/kg) Ni Co Cd As Minerals 2011, 1 96 high levels of trace elements. Similar results were previously obtained in agricultural soils and rice fields in Korea, where As, Cu and Zn were highly bioavailable and posed serious health hazards [47]. 5.3.4. Mineralogical Speciation The XRD results obtained from the different fractions did not allow us to assign the different trace elements to specific mineral phases because the fractions were very similar in mineralogical terms, and the detection of minor mineral phases was not possible with this method. The SEM study confirmed the presence of the major minerals detected by XRD (quartz, phyllosilicates, feldspars and iron oxides) in addition to accessory minerals such as pyrite, barite, zircon, rutile, monazite and ilmenite (Table 10). No association of any trace element to clay minerals was detected though. Micrographs revealed the presence of iron oxide aggregates containing substantial concentrations of Pb and smaller amounts of As and Sb (Figure 6). There were also isolated pyrite crystals transformed into iron oxides (hematite) associated to no trace element. Some samples additionally contained galena. Table 10. Mineralogical composition of total and <2 µm fraction by XRD and SEM-EDS. Samples directly affected by waste rock piles or acid leachates are in bold Location Sample Q Phy Fd Hem V K Ch I Others Peña Hierro RT-4–1 55 34 6 - Tr 15 - 83 Ilmenite, zircon, barite, rutile, monazite and cerianite Peña Hierro RT-5–1 45 44 6 - 5 35 - 60 Ilmenite, zircon, rutile and monazite Peña Hierro RT-6–1 57 38 Tr - 5 5 - 90 Rutile, monazite and ilmenite Delgadas RT-8–1 7 61 27 Tr Non quantifiable Pyrite Delgadas RT-8–2 52 43 Tr X Monazite, barite and rutile Nerva RT-20–1 55 40 Tr X 5 - 5 90 Barite, monazite, rutile, ilmenite Nerva RT-21–1 53 35 7 - 5 - 5 90 Zircon, barite, rutile, monazite Nerva RT-21–2 59 31 5 - 5 - 5 90 Pb antimonide, cassiterite, pyrite, Pb associated to sulphides. Tharsis Rt-28 56 34 5 Tr - 21 - 79 Fe oxy-hydroxides with Pb Tharsis Rt-29 76 24 5 - - 30 - 70 Fe oxy-hydroxides with Pb Sotiel Rt-30 80 16 4 - - 15 - 85 Fe oxy-hydroxides with Pb Q: quartz; Phy: Phyllosilicates; Fd: Feldspar; Hem: Hematite; V: Vermiculite: K: Kaolinite; Chl: Chlorite; I: Illita; Tr: <5%. Minerals 2011, 1 97 Figure 6. SEM image showing a polycrystalline aggregate composed of iron oxides, micas, quartz and apatite. The spectrum show Pb, Sb and As peaks. Figure 7. Compositional mapping of major and trace elements by means of microPIXE (sample 21). Selected samples were subjected to a microproton probe (PIXE) study in their <63 μm fraction. The results were used to produce distribution maps for major and trace elements. Some of these elements such as Al, were uniformly distributed across the study area (Figure 7), whereas others (Si, Mn, As, Cu, Pb and Zn) appeared accumulated in small granules. 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