Every year about 100 million tons of constructional aggregates are used in Austria [1
] and 3.2 billion tons in the EU [2
]. Besides natural rocks, industrial aggregates, e.g., steel slags, and recycled aggregates obtained from construction and demolition waste are used for construction purposes. In contrast to natural rocks, for industrial and recycled aggregates, not only geological, physical-technical, and chemical [3
] properties but also the environmental impact has to be assessed prior to application. According to the Recycling Construction Materials Ordinance (RCMO) the total contents of Cd, Cr, Mo, Tl, and W (industrial aggregates) and As, Pb, Cd, Co, Cr, Cu, Ni, Hg, and Zn (recycled aggregates) are regulated in Austria [4
] because these are considered as environmentally problematic.
For Austrian natural rocks, there is no regulation with regard to the environmental risks resulting from contaminants in the material. Data on the average total element contents of rocks in Austria are summarised in the Geochemical Atlas of the Republic of Austria [5
]. Data for fluorine are missing for Austria, but its mineralogy and geochemistry is generally known [6
] and related to the alkalinity of igneous rocks [7
]. These elements can either form distinct mineral phases or substitute for other major elements, which are summarized in Supplementary Table S1
In addition to the total concentrations of certain elements, leached amounts are also regulated for industrial and recycled aggregates. Elements whose leachable contents are regulated in Austria include Ba, Cd, Cr, Co, Cu, Ni, Mo, Tl, V, W, Cl, and F (Federal Ministry for Agriculture and Forestry, Environment and Waste Management) [4
]. As the environmental impact of chemical elements depends on their mineralogy and the hydrogeochemical conditions, all elements which are regulated in the RCMO, either with respect to total or with respect to leachable contents, are referred to as potentially environmentally problematic elements (PEPE) in this study. Leaching, i.e., the release of chemical elements from the solid into the aqueous phase, is experimentally determined by standardized leaching tests for which the samples are stirred (e.g., EN 1744-3 [8
]) or shaken (e.g., DIN 19529 [9
] or EN 12457-1 to -4 [10
]) with an aqueous solution or where the solution percolates through the material in a column test (e.g., EN 14405 [11
] or DIN 19528 [12
]). In most of these tests the pH, which has a high impact on the release of different elements, stabilizes due to the dissolution equilibrium of the material, whereas in pH dependence tests (e.g., EN 14429 [13
]) it is adjusted by acid and/or base addition. The effect of pH on the solubility of mineral phases in steel slags has also been investigated with respect to metal recovery by bioleaching [14
]. Besides the pH value, other factors, e.g., the particle size of the samples, the duration of the test, and the liquid to solid (L/S) ratio influence the leaching behaviour. Generally, it is assumed that the leaching of PEPE from natural rock is insignificant [15
]. Scientific data on the leachability of natural rocks is limited but they indicate that in general only small amounts of heavy metals and sulphur are released, the values normally being lower compared to secondary raw materials [16
]. However, for some elements (e.g., Zn, Ni and Co) higher leached concentrations were detected in natural rocks than in blast furnace slags [17
The mineralogy and mobility of chemical elements have long been studied with respect to the weathering of rocks and formation of supergene ore deposits, but not by standardized leaching tests used in waste management. More recent studies have dealt with Cr enrichment in a limonitic horizon of a soil formed by weathering of an ultramafic rock [18
]; release of Ni in ultramafic rocks from minerals like olivine ((Mg,Fe,Ni)2
) and secondary fixation in serpentine ((Mg,Fe,Ni)₆Si₄O₁₀(OH)₈) and smectite [19
]; and release of Mo from primary mineral phases and its adsorption onto iron hydroxides during granite weathering [20
]. However, the few studies which address the leachability of PEPE from natural rocks (e.g., [17
]) are still insufficiently linked to this fundamental mineralogical knowledge. Thus, the aim of this study is to investigate the leaching behaviour of PEPE like heavy metals from natural rocks and to link it to mineralogical data.
2. Materials and Methods
Crushed samples (0.15–1.00 kg) of four different rock materials used for road construction (A: quaternary gravel, B: diabase-greywacke mixture, C: serpentinite, and D: amphibolite breccia; NB: non-scientific trade names) were obtained in five different size fractions (<2 mm, 2–4 mm, 4–8 mm, 8–11 mm, and 11–16 mm) from commercial suppliers. Due to the non-disclosure agreement with the suppliers, the name of sample provider and sample provenance could not have been given. Sample designations refer to the particle size of the ’as received’ materials. For sample C, an additional new sample was obtained from the same supplier and location (Cn). Samples B and D were separated manually for mineralogical analysis because sample B turned out to be a mixture of diabase and greywacke (B1: diabase, B2: greywacke) and sample D consisted of red and green components (D1: amphibolite breccia red and D2: amphibolite breccia green). An additional sample (E: Mo-bearing gneiss) which is not used in road construction but interesting with respect to the relation between mineralogy and leachability of Mo, was taken during a field campaign in the Reichenspitze area, Hohe Tauern. The representativity of samples for any petrographic unit or production charge is not claimed, but also not relevant for the purpose of this study, which deals with the release mechanisms of PEPE.
One polished thin section of each sample (B1, B2, Cn
, D1, D2, and E) was prepared and characterized by optical microscopy in transmitted (Olympus BX40F4) and reflected light (Zeiss AXIO Scope A.1), the latter equipped with a digital camera (Zeiss AxioCam). The same samples were subsequently used for electron probe microanalyses (EPMA) using the JEOL JXA 8200 instrument installed at the Chair of Resource Mineralogy, Montanuniversität Leoben. The polished sections were carbon coated (EMITECH K950X), to minimize charging under the electron beam. Spot measurements and element maps of mineral phases were conducted using wavelength-dispersive spectrometers (WDS). The instrument was operated in high vacuum (<1.65 10−5
mbar), with 15 kV acceleration voltage, 10 nA beam current (on Faraday cup), and with beam diameter set to spot size (≈ 1 μm). For all quantitative analyses by EPMA, Kα lines were used (except for Mo, where Lα was used); the counting time was 20 s (40 s for Mo) for the peak and 10 s (20 s for Mo) for the background. For details and net lower limits of detection (LLD) see Supplementary Table S2
. The number of analyses performed for each mineral is given in Supplementary Tables S4 and S5
For chemical analyses of samples A, B, C, Cn
, D, and E, bulk sample material was comminuted in a jaw crusher to <0.5 mm, then split to subsample portions <50 g by coning and quartering. The subsamples were ground to <63 µm for analysis. The analyses were conducted at the Chair of Waste Processing Technology and Waste Management, Montanuniversität Leoben after total digestion according to EN 13656 [21
]. The chemical composition was determined according to EN ISO 17294-2 [22
] by inductively coupled plasma mass spectroscopy (ICP-MS, Agilent 7500ce). For samples Cn
and E additionally the sulphur content (inductively coupled plasma optical emission spectroscopy, ICP-OES, Varian Vista MPX CCD Simultan, based on EN ISO 11885 [23
]), the fluorine content after alkaline digestion (ion chromatography, IC, Dionex ICS 2000, Chromeleon software, EN ISO 10304-1 [24
]), and the Cr(VI) content (photometrically according to DIN 38405-24 [25
]) were determined. The modal mineral composition of the fractions <2 mm and 11–16 mm was investigated by X-ray powder diffraction (XRD) analysis. The powdered samples were analyzed using a PANalytical X’Pert Pro diffraction instrument (CoKα radiation (λ = 1.79Å), 40 mA, 45 kV) and the PANalytical HighScore Plus software package at the Institute of Applied Geosciences, Graz University of Technology. Diffractograms were automatically semi-quantified. The resulting net LLD are in the order of 2–5 wt %, meaning that minerals may be present in lower concentrations but are not detected by XRD.
Particle size specific leaching tests for samples A–D were conducted according to EN 12457-4 at the Chair of Waste Processing Technology and Waste Management, Montanuniversität Leoben, in order to assess the impact of the particle size on leaching. One hundred grams of non-comminuted sample material and 1000 g deionised water were shaken for 24 h (7 min−1
). Solids were removed by sedimentation and subsequent centrifugation. The pH and the conductivity of the leachate were determined and the anions were analyzed by ion chromatography (IC, Dionex ICS 2000) according to EN ISO 10304-1 [24
]. The total organic carbon (TOC) and the dissolved organic carbon (DOC) contents were determined according to EN 1484-3 [26
]. The leachate was analyzed for chemical composition according to ÖNORM EN ISO 17294-2 [22
] using ICP-MS (Agilent 7500ce).
and E were selected for pH-dependent leaching tests (ÖNORM EN 14429 [13
]). The samples were crushed to <1 mm and a liquid/solid (L/S) ratio of 10 was used in the test. The pH was adjusted to 8 final values between <2 and <12 by adding HNO3
and NaOH, respectively. The leaching tests were performed for 48 h and the electric conductivity and pH were monitored after 4 h, 44 h, and 48 h. The chemical composition of the leachates after centrifugation and filtration was analyzed by IC (Cl-
), ICP-MS, and photometry (Cr(VI)).
Additionally, percolation tests (according to EN 14405 [11
]) on samples Cn
and E were performed to evaluate the leaching behaviour as a function of the liquid/solid (L/S) ratio. The cumulative L/S ratio is the entire amount of water which has percolated the sample at a certain time, divided by the mass of the sample. The chemical evolution of the percolate reflects not only the behaviour in nature but also allows distinguishing between surface-wash off in the first fraction and continuous dissolution in the latter fractions. The samples (4 kg each) were crushed to <10 mm and the particle size distribution was determined by sieving analyses. Similar particle size distributions were achieved by re-mixing the size fractions in the same ratio. The samples were then densely packed into polyethylene (PE) columns (d = 11 cm). A dense packing was achieved by compressing the samples with a falling weight, yielding a sample height of 25 cm. The columns were closed on the top and at the bottom by caps with an outlet and with ceramic filters (pore size 10–16 µm). A crown-shaped polypropylene (PP) device was used to secure the caps from eventual expansion due to swelling processes. The columns were saturated for 3 d to achieve equilibration. During the tests, purified water was pumped in an upward flow through the columns with a flow rate of 0.99 mL min-1
. The electric conductivity, pH, and Eh of the leachate were analyzed every 5 min. A cumulative L/S ratio was calculated by dividing the cumulative mass of the percolate that has passed through the column at a certain time by the dry mass of the sample in the column (4 kg). Samples were taken after cumulative L/S ratios of 0.1, 0.2, 0.5, 1, 2, 5, and 10. The total experimental duration was 8 d, 58 min for sample Cn
(shortened duration due to technical problems) and 28 d, 4 h for sample E (scheduled duration).
The maximum leachable concentrations of the individual elements (derived from the pH dependence leaching tests) were considered as the available concentrations and used as the input data for hydrogeochemical modelling with LeachXSTM
(version 2.0.90) with the Orchestra geochemical modelling platform [27
]. The aim of the modelling was to identify the mechanisms which control the leaching in samples Cn
and E, i.e., dissolution/precipitation or desorption/adsorption equilibria. The thermodynamic database of Minteq, version 4.0 (minteq.v4.dat) [28
] was used. Additionally, minerals from the thermodynamic database from Lawrence Livermore National Laboratory [29
] and Thermoddem.dat [30
] as well as from additional literature were implemented. For sample C, the concentrations of 12 cations (Al, Ca, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, and Si) and 3 anions (Cl, CO3
(estimated), and SO4
) were implemented as primary entities. For sample E, the concentrations of 18 cations (Ag, Al, Ba, Ca, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Pb, Si, Sr, V, and Zn) and 5 anions (Cl, CO3
(estimated), F, NO3
, and SO4
) were used. For both samples, the adsorption onto hydrous ferric oxides (HFO, sample Cn
: 500 mg/kg, sample E: 300 mg/kg) was enabled as these phases usually form in natural systems under oxidizing conditions. The sum of the negative logarithm of electron concentration, pe, and pH was set to 12. This value was also chosen in another study for a steel slag [31
]. Those phases containing Cr, Cu, and Ni (for sample Cn) and Mo (for sample E) used for hydrogeochemical modelling are listed in Supplementary Table S3
Natural rocks compete with secondary raw materials, e.g., steel slags or construction and demolition (C and D) waste, for an application as an aggregate for concrete or asphalt or as a base layer in road construction. As this study was conducted within a research project focussing on EAF slag, the investigated rocks, i.e., potential natural aggregates, are compared with EAF slags but also other types of slags, i.e., industrial aggregates.
In comparison with EAF slags, all the observed rock samples are characterized by lower Cr and V contents [41
] and in the case of sample C also by significantly higher Ni contents [42
]. Spinels play an important role as host phases for Cr, both in the investigated rock sample C and in EAF slag [41
]. Olivine, which is a host phase for Ni in sample C, was also found in EAF slag as a host phase for V. Titanite, which was identified as the main V carrier in samples B and D, has also been found in basic oxygen furnace (BOF) slag [43
]. Melilite ((Ca,Na)2
]), whose hydration products were suggested to secondarily fix V in EAF slag (unpublished data), was not found in natural rock samples. Contrarily, molybdenite, which is the main Mo phase in sample E, has not been found in EAF slags, which are furthermore characterized by much lower Mo and S contents. Comparison of the leachable concentrations at natural pH of the investigated rock samples with that of EAF slags [44
] shows higher Ni leaching for sample C whereas leaching of V and Cr is in the same extremely low range for both types of materials. A comparison of the modelled leaching controlling mechanisms [45
] shows that the same processes, i.e., incorporation into spinels and adsorption onto iron hydroxides, control the leaching of chromium. In contrast, neither for the natural rock sample E nor for the EAF slag (unpublished data) was a well-fitting control mechanism for the leaching of Mo found. Further research is needed to better explain the leaching of Mo from both natural and industrial aggregates.
In summary, the leachability of PEPE in natural and industrial aggregates, is controlled by similar mineralogical mechanisms, e.g., the stable incorporation of Cr in spinels. No general statement for which material group (natural or industrial aggregate) releases less PEPE into the environment can be made, as specific materials and their mineralogy as well as the hydrogeochemical framework conditions, e.g., pH and Eh, have to be considered.