Introduction

Sulphide minerals (like pyrite, pyrholite, marcasite, etc.), which can be found in old deposits after coal-, and gold minings, and in polymetallic ores, are transformed to the acid mine drainage (AMD). This is due to their modifications being liable to microbiological–chemical biodegradation by action of chemolithotrophic bacteria, namely genera Acidithiobacillus and Leptospirillum, under aerobic conditions (air and water) [1–4]. AMD is characterized by low pH value (pH ≤ 3), various dissolved metals (Fe, Al, Zn, Pb, etc.) and high concentration of SO 2−4 . The precipitation of ferric compounds from AMD is a process of formation of different solid phases at different pH values and sulphate concentrations [4–7].

Natural iron precipitates are formed from AMD from solid phases of ferric oxyhydroxides like ferrihydrite Fe5HO8·4H2O, schwertmannite Fe8O8(OH)6SO4, jarosite (NaKH3O)+Fe3(SO4)2(OH)6, goethite α-FeOOH or a mixture containing all three species [6–9]. They are frequently a by-product of metabolic processes in microorganisms containing iron (Fe2+) as an energy source [10]. The mixing and diluting of AMD with a fresh water stream, in drainage channels or in other places where mine water discharges, facilitate the formation of iron precipitates with various compositions [11]. Some of the resulting iron oxides are susceptible to external conditions (temperature, atmospheric effluence, etc.), and when these conditions are present, transformation of less stable oxides to the more stable forms takes place. Due to a different genesis, the iron precipitates vary in size and relatively high ratio of water molecules [12–14].

Knowledge of the thermal behaviour of iron precipitates has importance in final treatment of produced iron oxide materials and also in industrial application of products. Another possible application of iron precipitates is the detoxification of some pollutants based on the high surface area and the high reactivity of materials prepared by the homogeneous precipitation method of various metal salts with urea.

In this study, we used simple methods to treat natural iron precipitates (washing with de-ionized water, drying at room temperature) and to treat AMD using homogenous precipitation by urea to obtain synthetic iron precipitates [15]. By hydrolysis of AMD in the temperature range from 60 to 95 °C with urea, it is possible to prepare easily filterable relative pure ferric precipitates. Depending on the conditions of homogeneous precipitation, the resulting precipitates consist of various amorphous or crystalline phase (jarosite, schwertmannite, goethite, ferrihydrite, etc.). The physical properties of such treated natural iron precipitates and of prepared synthetic iron precipitates were investigated. High specific surface areas of natural and synthetic iron precipitates were examined. The iron precipitates can be used further for sorption and removal of phosphates, arsenic, chlorine hydrocarbons, and organic dyes from contaminated waste waters [16]. It was also examined that during heat treatment, the less stable forms of ferrihydrite, schwertmannite, and goethite transform to the more stable form—hematite.

Experimental

Preparation of samples

Natural iron precipitates were sampled from abandoned mine adits and settling pits from the mining area, Banská Štiavnica located near Hodruša, Slovakia. The natural iron precipitates were washed by de-ionized water and dried at room temperature. Synthetic iron precipitates were prepared by precipitation, using urea of AMD collected from the spoil dump of pyritized quartzite Šobov (Banská Štiavnica). The precipitation step was in the temperature range from 60 to 95 °C. The reaction was finished when molar ratio of urea to iron was 10:1, and the pH value was approx. 3.5 (more than 95 % of iron were selectively dropped out from AMD). The synthetic iron precipitate was then filtered and washed with de-ionized water. The last step of procedure consists of drying the precipitate at room temperature. Urea used for precipitation of AMD was of analytical-grade purity chemicals provided by Sigma-Aldrich, Germany.

Methods of samples characterization

Thermogravimetry (TG) and differential thermal analysis (DTA) and evolved gas analysis/mass spectrometry (EGA/MS) measurements were carried out by heating the iron precipitates in air (synthetic 30 mL min−1) by using SETARAM SETSYS Evolution-16-MS equipment. Gaseous products were analysed in the MID mode as Intensity (A) of individual selected fragments. The mass of each sample measured was 20 mg and the heating rate was 10 °C min−1 (from 20 to 1,100 °C).

Diffraction patterns were collected using a PANalytical X′Pert PRO diffractometer, equipped with a conventional X-ray tube (CoKα radiation, 40 kV, 30 mA, line focus), and a multichannel detector X’Celerator with an anti-scatter shield. We used conventional Bragg–Brentano geometry with 0.04 rad Soller slit, 0.5° divergence slit, 1° anti-scatter slit, and 15-mm mask in the incident beam, 5.5-mm anti-scatter slit, 0.04 rad Soller slit, and Fe beta-filter in the diffracted beam. In the case of collected or synthesized samples, we used a side-loaded sample holder, and the X-ray patterns were measured in the range from 10° to 95° 2Θ with steps of 0.0167° and 1,050 s counting per step. Samples after thermal analysis were spread on silicon zero-background sample holder, and X-ray patterns were measured in the range of 10° to 95° 2Θ with steps of 0.0167° and 250 s counting per step. XRD patterns were not pre-treated before interpretation as no background correction was needed. Qualitative analysis was performed using the HighScorePlus software package (PANalytical, The Netherlands, version 3.0e), Diffrac-Plus software package (Bruker AXS, Germany, version 8.0) and JCPDS PDF-2 database (release 2004).

To characterize the particle size and structural morphology of the collected and synthesized iron precipitates, a high-resolution transmission electron microscopy (HRTEM, Type JEM-3010, JEOL) at 300 kV accelerating voltage, equipped with EDX (Energy Dispersive X-ray) and the Philips XL 30 CP microscope equipped with EDX, Robinson, SE (Secondary Electron) and BSE (Back-Scattered Electron) detectors were used. For the observations by SEM in SE electrons, the samples were coated with thin conductive Au–10 % Pd alloy layer.

Specific surface area was determined by the B.E.T. (Brunauer–Emmett–Teller) method [17] from nitrogen adsorption–desorption isotherm acquired at liquid-nitrogen temperature using a NOVA 4200e instrument. A 12-h outgas at 80 °C was used before the measurement.

Results and discussion

Figures 1 and 2 depict the results of TG–DTA and the results of EGA/mass spectrometry (MS) detection of the gases evolved during air heating. Simultaneous measurements of thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried out at flow rates from 20 to 1,100 °C in 30 mL min−1 of air at the heating rate of 10 °C min−1. Gaseous products were analysed in the MID mode as intensity (A) of individual selected fragments.

Fig. 1
figure 1

EGA/MS, TG and DTA results of natural iron precipitate (sampled in-settling pit locality Banská Štiavnica, Slovakia) measured on heating in air

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Fig. 2
figure 2

EGA/MS, TG and DTA results of synthetic iron precipitate measured on heating in air

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Figure 1 depicts the results of thermal analysis of the natural iron precipitate compared with results of the synthetic iron precipitate (Fig. 2). From the TG–DTA curve (Fig. 1) up to 400 °C indicating two consecutive steps of the mass loss 25.6 %, which could be attributed to the release of physically adsorbed water, dehydroxylation reactions, and release of NO x and CO2, we can see these in the form of broad endothermic peak on DTA curve. The endothermic peak at 139 °C indicating the release of CO2 and the exothermic peak at 289 °C occur due to the burning of carbonaceous material resulting in the formation of CO2. Exothermic effect around 300 °C is often observed during the transformation of less stable ferrihydrite, goethite and schwertmannite to more thermally and chemically stable hematite [18, 19], which is confirmed by XRD measurements. The dehydroxylation reaction that occurs on heating goethite in air has usually been observed as an endothermic peak in the temperature range from 250 to 400 °C, depending on the particle size and crystallinity of the tested samples. Release of SO2 is observable from 811 °C to max. 925 °C with the mass loss of 1.8 %. An exotherm/endotherm pair in the range from 400 to 650 °C can be attributed to the formation of Fe2(SO4)3 followed by its decomposition to hematite and SO3, which can be compared with data observed by the Cornell et al. [18].

Figure 2 depicts the results of thermal analysis of the synthetic iron precipitate prepared by precipitation with urea. The TG–DTA curve up to 500 °C is characterized by two consecutive steps of the mass loss 22.6 %, which could be also attributed to the release of physically adsorbed water, dehydroxylation reactions and release of NOx and CO2. We can see endothermic double peaks at 138 and 223 °C on the DTA curve; this corresponds to the loss of adsorbed water and dehydroxylation reactions. Burning of carbonaceous material starts at approx. 200 °C with two maxima on MID_CO2 curve (264 and 318 °C) if compared with the natural iron precipitate (Fig. 1; 289 °C) connected with broad exothermic effect in the range from 250 to 500 °C. Release of SO2 that is observable from 550 °C to max. 800 °C indicates the mass loss of 9.5 %. This process is shifted to lower temperature (endothermic effect at 622 °C on DTA curve) in comparison with natural iron precipitate (Fig. 1; 925 °C). The transformation of goethite and schwertmannite to stable hematite is passing over 300 °C [20]. Goethite proceeds directly to hematite without any intermediate phase. The transformation temperature usually depends on the crystallinity. For example, as the crystallinity of goethite improved, the endothermic (DTA) peak temperature shifted from 260 to 320 °C. In addition, a double peak which is attributed to a two-phase transition of well-crystalline goethite to hematite developed. The double dehydroxylation peak has also been associated with high surface area samples, excess surface water and the water vapour pressure [18].

Figure 3 depicts the results of TG–DTA and the results of EGA/mass spectrometry (MS) detection of the gases evolved during air heating. The DTA curve of the another natural iron precipitate from location Hodruša (Slovakia) shows on heating air endothermic peaks at 127 and 268 °C due to the release of physically adsorbed water. The TG–DTA curve up to 450 °C indicating two consecutive steps of the mass loss 27.7 % could be attributed to the release of water, NO x and CO2. The exothermic peak at 316 °C occurs due to the burning of CO2. Continuing in heating from 567 °C to maximum 663 °C indicates the mass loss of 5.4 %, which is attributed to the release of SO2.

Fig. 3
figure 3

EGA/MS, TG and DTA results of natural iron precipitate sampled in old adit location, Hodruša, Slovakia measured on heating in air

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Powder X-ray diffraction (XRD) was used for determining the phase composition of the collected and synthesized samples. Weakly crystalline goethite was observed in natural sample (location Banská Štiavnica, Slovakia, Fig. 4) collected in mixture with ferrihydrite, goethite, and schwertmannite. Quartz was also identified as a minor phase, probably as a result of contamination during sampling of the natural environment. Quartz particles were partially covered with fine precipitate and could not be separated by a sieving procedure. The presence of quartz was also confirmed by SEM/EDS analysis.

Fig. 4
figure 4

Powder X-ray pattern of natural iron precipitate from settling pit locality, Banská Štiavnica, Slovakia

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In the case of synthetic iron precipitate (Fig. 5) obtained at laboratory recovery from typical AMD by homogenous precipitation using urea, the schwertmannite with goethite was observed, and the process of their formation is probably affected by the pH value (pH 2–3) [7].

Fig. 5
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Powder X-ray pattern of synthetic iron precipitate

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Transformation, after heating in air, to more stable hematite from less stable goethite, ferrihydrite, and schwertmannite in natural iron precipitate from the settling pit locality of Banská Štiavnica (Fig. 6) is confirmed by both the DTA and XRD measurements. Hematite is, in this case, accompanied by traces of quartz and cristobalite. Quartz has already been identified in the collected sample. The presence of cristobalite demonstrates that collected sample should contain some amorphous natural silica.

Fig. 6
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Powder X-ray pattern of natural iron precipitate after TA

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The same transformation takes place in the case of synthetic iron precipitate (Fig. 7). Less stable mixture of goethite and schwertmannite transforms to more stable hematite. Traces of cristobalite were identified also in this sample after TA. This is probably due to the use of natural water (taken from the pit) which could also be contaminated by amorphous silica.

Fig. 7
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Powder X-ray pattern of synthetic iron precipitate after TA

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Micrographs from scanning electron microscope and high-resolution transmission electron microscope of natural iron precipitate (Figs. 8, 10) and synthetic iron precipitate (Figs. 9, 11) confirm that the precipitates have agglomerated in few μm (ca. tenths of μm)-sized aggregates.

Fig. 8
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SEM micrograph of natural iron precipitate

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Fig. 9
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SEM micrograph of synthetic iron precipitate

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The specific surface area of natural iron precipitate (settling pit Banská Štiavnica) was 257.7 m2/g and that of synthetic iron precipitate was 75.7 m2/g, which is lower in comparison with that of the natural iron precipitate. It corresponds with micrographs from scanning electron microscope (Figs. 8, 9) and with micrographs from high-resolution transmission electron microscope (Figs. 10, 11). The samples have quite high specific surface area, and they could be inexpensive and efficient adsorbents for removal of select pollutants from water environments.

Fig. 10
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TEM micrograph of natural iron precipitate

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Fig. 11
figure 11

TEM micrograph of synthetic iron precipitate

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Conclusions

The TG, DTA and EGA/MS detection analyses were used for the characterization of thermal behaviour of iron precipitates. The release of carbonaceous material is shifted to higher temperature (318 °C) in the case of synthetic sample compared with the natural sample (289 °C). In the case of synthetic sample, the transformation runs in two steps: the first in the range from 250 to 350 °C, and the second in the range from 550 to 750 °C. A double peak which is attributed to a two-phase transition of well-crystalline goethite to hematite could be associated with high surface area samples, excess surface water and the water vapor pressure. Powder X-ray diffraction analysis confirmed that, in both cases, the less stable goethite, ferrihydrite and schwertmannite in the natural samples and goethite and schwertmannite in the synthetic samples were transformed after heating in air to more stable hematite. From electron microscopy, it is visible that both natural and synthetic samples have agglomerated into aggregates around tenths of μm. Potentially, such materials can be effectively used for both sorption and removal of phosphates, arsenic, chlorine hydrocarbons and organic dyes from contaminated waste waters or others like pigments.