Introduction

Mine tailings and soils are characterised by high levels of heavy metals, low pH, and low organic-carbon content. For these soils, and for any soils with the same characteristics, remediation is often difficult and the best obtainable result is merely reduction of risk by immobilisation of the metals. Source-separated municipal solid waste (SS-MSW) compost, mainly produced as “fertiliser” for agriculture, could be used to restore the organic fraction and to reconstitute the soil structure, but total concentration and fractionation of heavy metals in compost-amended soils should be carefully evaluated to predict element mobility in soil and availability to plants [1]. In recent years, treated red mud residues, by-products of alumina production, have been widely used for their metal-trapping property, both in composting processes [2] and in many environmental remediation activities [3, 4]. Added to contaminated soils they can neutralise low pH and reduce metal mobility, by different physicochemical mechanisms (including an increase of available adsorption sites).

A knowledge of trace element mobility and speciation in contaminated soils is an important aspect of environmental evaluation. Leachability of heavy metals (and other contaminants) can be evaluated by leaching tests that can be performed both in batch or column experiments enabling assessment of metal mobility in both “natural” and amended soils. Leaching properties are therefore an important criterion for management of contaminated soils and waste. Many leaching tests have been developed as standard methods for evaluating the potential impact of waste materials on the environment [5, 6], but there is still a considerable need for rapid screening tools for assessment of potential mobilities of contaminants.

Interesting studies show the possibility of using mathematical models, based on the linear adsorption isotherm, to determine the exchangeable metal pool of soils and sediments [7, 8]. Another important approach for evaluation of environmental risks related to contaminated soils is based on fractionation of metals from samples, step by step, by using different reagents or “extractants”. This approach, called “sequential extraction” should be more appropriately classified as “operationally defined fractionation” [9, 10], because the fraction of metal determined depends on the extractants and the operating conditions under which the extraction is carried out [11]. One of the first, and most applied, sequential extraction procedures is a five-step procedure published by Tessier et al. [12]. The metal fractions isolated by the five-steps are the exchangeable fraction, the carbonate fraction, the easily reducible fraction, the moderately reducible fraction, and the oxidisable fraction. Several other procedures followed the Tessier procedure, and a wide variety of sequential extraction procedures are now available in the literature, based on different sequences of extractants and/or different operating conditions. The wide variety of methods and the lack of reference materials have hindered the validation of the procedures and comparison of results obtained throughout the world. For this reason the Standards, Measurements and Testing Programme—SM&T (formerly BCR)—organised a series of intercomparisons on extractable trace metal determination and held a workshop on Sequential Extraction in Sediments and Soils (in Spain, in 1992) to discuss results from the intercomparisons and to establish a common procedural scheme. This led to optimization of a three-step procedure, continuously improved, and to the production of certified reference materials, certified for content of several metals in the three fractions resulting from applying the three steps procedure. The metal fractions isolated by use of the three-step procedure are the exchangeable/carbonate fraction, the easily reducible fraction, and the oxidisable fraction.

In the work discussed in this paper four different systems were studied: soil, soil and treated red mud, soil and compost, and soil and compost plus treated red mud. Two different leaching batch tests [5, 13] were applied on all four systems, using dilute sulfuric acid to “mimic” rain and EDTA solution (liquid/solid, L/S, ratio 10:1), according to conventional methods for measurement of “plant available” metals. In these tests we studied, first, the effect of adding treated red mud on metal leachability from a mine acidic soil, characterised by high heavy metal concentrations. Metal leachability was also evaluated after addition of both red mud and compost (the synergistic action of red mud and compost could be in fact exploited to achieve both metal trapping and an increase in organic carbon). The results from the leaching experiments were also used in a kinetic fractionation study based on the simplified assumption that the leaching reactions can be regarded as first-order reactions (explained in detail in the “Results and discussion” section). Finally, the data obtained in these kinetic studies were compared (in accordance with Ref. [5]) with results from the BCR three-step sequential extraction procedure [14].

Materials and methods

Reagents and materials

Sixty nine percent HNO3 Aristar (VWR, UK), 50% HF RPE (Carlo Erba, Italy), 70% HClO4 Analar (VWR), and ultrapure water (18.2 MΩ  cm at 25°C) obtained from a MilliQ Element system (Millipore, France), were used for preparation and dilution of samples and calibrating solutions. For leaching experiments a solution of 0.05 mol L−1 H2SO4 was diluted directly from 96% H2SO4 RPE (Carlo Erba) and 0.05 mol L−1 EDTA (ethylenediaminetetraacetic acid disodium salt) RPE (Carlo Erba) was prepared by dissolving an appropriate amount of the compound in water and adjusting the pH to 5.5 with NaOH 2 mol L−1. Acetic acid RPE (Carlo Erba), hydroxylammonium chloride RPE (Carlo Erba), 30% H2O2 Analar (VWR) and ammonium acetate Reagent Grade (Ashland, Italy) were prepared according to the BCR procedure [14]. Single-element ICP–MS standard solutions, 1000 mg L1 in nitric acid, Aristar (VWR), were used for preparation of calibrating solutions. Rhodium ICP–MS standard solution, 1000 mg L1, Aristar (VWR), was used as the internal standard to correct matrix interferences in ICP–MS analysis. The Nalgene (Labware, Italy) tubes used during sample handling were left in 1% HNO3 for 24 h and release of the studied elements was checked before their use. In addition, procedural blanks, inclusive of all potential contamination sources (impurities of reagents and contamination from materials used for sample handling like tubes and TFM (tetrafluormethaxil; Hoechst Chemical Company, Frankfurt, Germany) vessels), were always evaluated.

Instrumentation

A microwave system (Milestone 1200 Mega, Italy) was used for sample digestion. All measurements of trace element concentrations were made with a Perkin–Elmer Elan 6100 ICP–MS spectrometer (USA) equipped with a cross-flow nebulizer and a Perkin-Elmer Optima 2000 DV ICP–OES spectrometer (USA) equipped with a Scott-type spray chamber.

Samples

Soil samples were collected in the area of a mine dump approximately 70 km North-West of Rome (Italy) and were chosen for their high concentrations of heavy metals and relatively low reaction grade (pH 5.5). The red mud sample was kindly supplied by Virotec International (Australia). Virotec optimised the process of treatment of red mud with seawater, and patented this technology, and produce several products with the name Bauxsol [15–18]. The patented process is based on seawater treatment, which enables conversion of “soluble alkalinity” (above all from sodium hydroxide) into low-solubility minerals (essentially Ca and Mg hydroxides, carbonates and hydrocarbonates), thus reducing the pH of the mud (from pH>13 to pH<9). The material (kindly supplied by Professor McConchie as Bauxsol A3) used in the experiments described in this paper is not actually a commercial blend, but “ordinary” Bauxsol that has been washed with fresh water (after seawater or brine neutralisation: the Basecon process) to remove pore water salts and is used where it is important to keep the total dissolved salts content of treated water as low as possible.

Compost samples from source-separated municipal solid waste were supplied by CTI SCARL (Imola, Italy) and AMEK (Ferrara, Italy) and were produced by an innovative process (based on the use of vegetable active principles) for which a patent application has been made by CTI SCARL and AMEK at the composting Plant in Fossoli di Carpi (AIMAG, Modena, Italy). The raw material mainly comprises organic waste (mainly separately collected organic fractions from municipal solid waste and agro industrial wastes) and lignocellulosic matter (about 2:1 w/w). The composting process lasts 90 days. The final compost, after screening, underwent a curing phase for a further 8 months in a “static pile”.

Soil, compost, and red mud samples were air dried at 40°C and sieved at 2 mm before use in the experiments.

Determination of total content

For soil and treated red mud samples, approximately 0.5 g, accurately weighted, were digested with a mixture of 5 mL 69% HNO3, 2 mL 50% HF, and 1 mL 70% HClO4 in the TFM vessels with a microwave system. The working program used for microwave digestion was: 5 min at 250 W power, 10 min at 400 W, 10 min at 600 W, and 5 min at 250 W. Microwave digestion was followed by an open-vessel procedure. The samples were first evaporated smoothly nearly to dryness, in PFA vessels, and the residues were subsequently redissolved in 1 mL HClO4 and the solutions again evaporated nearly to dryness. The residues were then submitted twice to analogous treatment with 1 mL of HNO3. Finally, 2 mL HNO3 was added to each sample and the resulting solution was completely transferred to a 50-mL volumetric flask and diluted to volume with ultrapure water.

The same procedure was followed for the samples of compost except for the working program used for microwave digestion. A less drastic program was used, including steps of partial cooling of the solutions—2 min at 250 W power, 2 min at 0 W, 5 min at 250 W, 5 min at 0 W, 2 min at 400 W, 5 min at 0 W and 2 min at 500 W.

A procedure based on use of a mixture of HNO3, HF and HClO4 was used instead of mineralisation by use of aqua regia because it had already been found to give the best results (complete mineralisation) both for red mud and compost samples.

Sequential extraction

The BCR three-step sequential procedure (Table 1) was applied to the four different systems: (a) soil, (b) soil and treated red mud (8:2), (c) soil and compost (8:2), (d) soil and compost plus treated red mud (8:1:1).

Table 1 BCR three-step sequential extraction procedure
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The residues from step 3 were treated by the procedure used for determination of the total trace element content of the soil samples. The analytical performance of the laboratory in the sequential extraction procedure was evaluated by analysing four different aliquots of BCR 483 (sewage sludge amended soil from Great Billings sewage Farm, Northampton), to ensure adequate quality assurance. BCR 483 proposes certified values for Cd, Cr, Cu, Ni, Pb and Zn, extractable using 0.05 mol L1 EDTA and 0.43 mol L1 acetic acid, and indicative values for the optimised BCR sequential extraction procedure also [14]. The results obtained for BCR 483 are reported in Table 2 and, taking into consideration their relative standard deviation, almost always overlap the indicative values (the only exceptions are Cr in steps A and C, and Cd in step A). In particular, the results obtained were compared statistically with the indicative values for the reference material. Because the necessary information from the interlaboratory exercise (IE) was available [14], the statistical comparison was carried out by applying the F-test for the evaluation of the comparability of uncertainties, followed by the Student t-test for comparable uncertainties and the Cochran test for not-comparable uncertainties. All results (in a total of 22 sets of results) were statistically comparable with the indicative values. In particular, in 17 instances the results were statistically comparable at a level of α=0.05, in four instances at α=0.02 and in one instance at α=0.01.

Table 2 Results obtained from analysis of BCR 483 (mg kg−1)
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Because no indicative value was available for Mn for BCR 483 an indirect evaluation of laboratory performance was carried out by comparing the sum of extracted steps (plus residual) with the total concentration for the soil sample; the results reported in Table 3 can be regarded as satisfactory.

Table 3 Concentrations (total) of Ni, Mn, and Zn (expressed as mg kg−1, dry weight) in soil, compost, and red mud used in the experiments and Σ3 steps+residual for soil
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Leaching test

For the kinetic studies two different leaching tests were performed—2.5 g sample (soil) was used with dilute sulfuric acid (0.05 mol L1) or EDTA (0.05 mol L1), with an L/S ratio of 10. The tests were again performed on the soil sample and on the three soil/compost/red mud mixtures. Polyethylene tubes (previously washed in nitric acid) containing samples and extractant (dilute sulfuric acid or EDTA) were stirred using an end-over-end shaker at a speed of 30 rpm for a given time, different for each tube: 15, 30, 60, 100, 150, 240, 360, and 960 min. At the end of the chosen mixing time each tube was removed and the extract was separated from the solid residue by centrifugation at 5000g for 10 min and then filtered through filter membranes (0.2 μm Nalgene, Labware, Italy). For each sample 10 mL filtrate (after addition of 200 μL nitric acid) were kept at 4°C until analysis. Non-linear regression analysis of the leaching results was carried out using SigmaPlot 8.0, a software package produced by SPSS.

Results and discussion

Ni, Zn, and Mn, which were present at significantly high concentrations in the soil (approx. 300, 8000, and 12,000 mg kg−1, respectively; Table 3) were selected as the elements to be studied both in the leaching test and in sequential extraction experiments. Despite these high concentrations, less than 1% of all these elements (and also other analysed elements, for example Cd, Cu, Pb, Cr, etc.) was leached from the soil sample by sulfuric acid after 16 h. It was, consequently, impossible to study the possible effects of addition of compost and/or red mud to the soil. Also, for pH>4, H+ is practically unable to extract any metal [13] and the effect becomes evident even at pH<2. In the leaching test on the soil sample the pH changed from 3.2 (time 0) to 4.0 (after 16 h).

In contrast, interesting results were obtained with EDTA. Data for Mn, Ni and Zn are plotted as percentage leached versus time diagrams in Figs. 1a–c.

Fig. 1
figure 1

EDTA leaching of: 100% soil (filled circles) and (continuous line) non-linear regression study; 80% soil–20% red mud (filed diamonds); 80% soil–20% compost (filled triangles); 80% soil–10% red mud–10% compost (filled squares)

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EDTA is a strong chelating agent, capable of extracting metal cations from several soil compartments, but also of dissolving “fractions” with which trace elements are associated, e.g. carbonates, iron hydroxides and, partially, organic matter [19]. For this reason EDTA is not the best choice for identifying kinetic differences [5]. EDTA was chosen because it is generally used in methods for measurement of “plant-available” metals and because we needed for our experiments a strong chelating agent to evaluate “sequestrating” (refixation on a solid phase) capacity of red mud in the soil/red mud mixture. The concentration of EDTA chosen for use in the leaching tests was 0.05 mol L−1, in accordance with conclusions from studies by Fangueiro et al. [13] on the effect of several conditions and considering harmonisation proposals by the BCR. The EDTA can regarded as being in excess and the chelating ability in the leaching tests was ensured throughout the whole test period also considering that the pH varied in the range 4.8–6.0 for the 100% soil sample and in the range 5.9–7.1 for the soil–red mud mixture.

Addition of compost (20%) to the soil enhanced the organic carbon content from approximately 0.5 to 5%.

The “leachability” of these elements seems to be significantly affected (reduced) by compost amendment and, above all, by the addition of red mud. For the soil/red mud mixtures, after leaching for 16 h, the amount leached was reduced from about 100 to about 20% for Mn, from about 50 to about 30% for Ni, and from about 25 to about 10% for Zn.

For Ni and Zn the behaviour of the soil/red mud/compost mixture was similar to that of the soil/red mud mixture, implying that for these elements the effect of red mud and compost was quantitatively similar. For Mn it seems that red mud acts more powerfully in reducing the leachability of this element. This could be partially because under oxidising and neutral/slightly basic conditions (achieved by addition of red mud) manganese is readily immobilised as insoluble dioxide. So, on reducing from 20 to 10% the concentration of red mud in the mixture the prevalent effect is reduced and not compensated by 10% compost (as for Zn and Ni).

The effect of red mud has been already widely studied and reported in the literature [15–18, 20, 21] and the chemistry of seawater neutralization of Bauxite refinery residues (red mud) has been investigated in depth [16].

The use of relatively non-specific extractants (EDTA in this case) therefore suggests a kinetic approach because measurements of trace elements extracted at equilibrium cannot be related to their speciation. In accordance with the work of Bermond et al. [19], we studied the possibility of subdividing into a labile metal fraction (quickly extracted) and a non-labile metal fraction (less quickly extracted) the trace metals extracted by EDTA in the leaching test. This kind of approach is based on the simplified assumption that the leaching reactions can be regarded as first-order, and requires estimation by non-linear regression of the constants of an equation of the type [5, 19, 22]:

$$ {\text{y}} = {\text{a}}(1 - {\text{e}}^{ - {\text{k}}_1 {\text{t}}} ) + {\text{b}}(1 - {\text{e}}^{ - {\text{k}}_2 {\text{t}}} ) $$

where y represents the amount of metal extracted at time t; a and b represent the labile and non-labile amounts, respectively, and k1 and k2 are the kinetic constants associated with a and b, respectively, for a given metal.

The “kinetic” results were then compared with those obtained by use of the BCR sequential extraction procedure (Fig. 2) to evaluate if the kinetic approach, with a relatively rapid and simple procedure, could give adequate information about trace element mobility and speciation [5, 19, 22]. It must be stressed here that even if the effect of extractant volume/sample mass ratio (V/m) played a major role in the amounts of metal extracted [8] the results from the “kinetic” approach can be considered unaffected by V/m if the concentration of the extractant (in our case EDTA) is in excess [13].

Fig. 2
figure 2

Sequential extraction from 100% soil, 80% soil–20% compost, 80% soil–20% red mud and 80% soil–10% red mud–10% compost

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The sequential extraction results are expressed as a percentage of the amount of element removed from the soil in steps A, B, and C and the residues for Mn, Ni and Zn. There are no evident differences in the distribution of the studied elements in the different fractions obtained from the sequential extraction procedures.

The curves derived from the non-linear regression study for 100% soil sample (data expressed as a percentage of the element removed from the soil versus leaching time) are included in Fig. 1 and are in good agreement with the experimental data represented by the circle points. The kinetic data are reported in Table 4 and can be used for comparison with the sequential extraction results for the 100% soil sample.

Table 4 Synoptic table of kinetic data and fraction of metal extracted in the different steps of sequential extraction
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The comparison suggests the following conclusions, even if in this paper only one specific kind of soil was tested. For Zn and Ni it seems reasonable to assume that the kinetic constant a (labile metals) could be related to step A and part of step B; the kinetic constant b (scarcely labile/not-labile metals) could reasonably be related to the residual part of step B and to part of step C; finally the percentage difference (100−a−b) could be related to the “inert” (not extractable) fraction (part of step C and residue).

For Mn data from both sequential extraction and the kinetic approach give the strong indication that the “inert” fraction is practically negligible: the Mn in the residue, is in fact <2% in the sequential extraction procedure and the sum of the kinetic constants a+b is practically 100 (Table 4). It is even more evident for this element that the major part of the extractable fraction is in step B of the sequential extraction and this is logical because step B is related to Fe and Mn oxides. The kinetic approach gives the indication that approximately 55% Mn is in the “labile” fraction (constant a) and that approximately 45% is in the “scarcely labile/not-labile” fraction (constant b). This could be partially explained by considering that the sampled soil is significantly altered mineralogically by the weathering process, superimposition of tailings from mine activities and constant reoxidation phenomena leaving part of the element in a less labile form. Step B of the sequential extraction is however sufficient to extract both the labile and scarcely/not-labile forms of Mn in this sample.

It seems reasonable to conclude that it is difficult to define any exact borderline between “labile” and “scarcely labile/not-labile” fractions in the kinetic approach, because the extractants usually used are based on complexation mechanisms and/or modification of pH equilibria, whereas for step B of the sequential extraction the oxidoreductive mechanism is prevalent.

The same kinetic approach, used for the 100% soil sample, was inadequate for the complex mixture soil/compost/red mud (in different %). For such complex systems different mechanisms play an important role. At least two main mechanisms are important—extraction of metals from soil by EDTA and metal trapping by the red mud (sequestrating the metals from the liquid phase). The two mechanisms are regulated by different factors, pH conditioning both the EDTA extraction capacity and the coprecipitation/adsorption on red mud [21]. For the soil/red mud mixture it seems that complexation of metals by EDTA is kinetically favoured in the first 3–4 h, then coprecipitation/adsorption on red mud becomes prevalent.

Conclusions

The “kinetic fractionation method”, a relatively rapid and simple procedure, gives adequate information about trace element mobility, bioavailability and fractionation in soils. Comparison of this approach with data from sequential extraction of a sample of a contaminated soil from an abandoned mine gave interesting results. This kind of approach, based on the simplified assumption that the leaching reactions can be regarded as first-order reactions, was inadequate for the complex mixtures soil/compost/red mud. Substantial improvement of the theoretical basis of the approach seems necessary for such samples.

The synergistic action of red mud and compost, added to the contaminated soil to trap metals and increase organic carbon, was evident from results of the EDTA leaching tests. The leachability of Mn, Zn, and Ni, all present in large amounts in the soil studied, was significantly reduced by addition of red mud and compost. The following conclusions can reasonably be made. Addition of red mud indirectly reduces the mobility of metals in soil, sequestrating from the water/soil equilibria the leached metals and reducing soil acidity leading to neutrality. Addition of compost enhances the concentration of the organic matter in the soil, restoring the soil structure and consequently actively participating in reduction of metal mobility [23]. This evidence suggests interesting perspectives in soil-remediation activity [24].