Enhancing the rate of ex situ mineral carbonation in dunites via ball milling

Advanced Powder Technology xxx (2016) xxx–xxx

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Original Research Paper

Enhancing the rate of ex situ mineral carbonation in dunites via ball milling Ioannis Rigopoulos a,b, Michalis A. Vasiliades c, Ioannis Ioannou b,d, Angelos M. Efstathiou c,⇑, Athanasios Godelitsas e, Theodora Kyratsi a,b,⇑ a

Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprus Nanotechnology Research Unit, University of Cyprus, 1678 Nicosia, Cyprus Department of Chemistry, Heterogeneous Catalysis Lab, University of Cyprus, 1678 Nicosia, Cyprus d Department of Civil and Environmental Engineering, University of Cyprus, 1678 Nicosia, Cyprus e Department of Geology and Geoenvironment, University of Athens, 15784 Zographou, Greece b c

a r t i c l e

i n f o

Article history: Received 11 September 2015 Received in revised form 31 December 2015 Accepted 11 January 2016 Available online xxxx Keywords: Ball milling CO2 sequestration Dunite Ex situ mineral carbonation CO2–DRIFTS

a b s t r a c t The investigation of potential options for CO2 sequestration is of vital importance for alleviating the ongoing climate problem. This paper presents an efficient method for enhancing the ex situ carbonation of dunites. The ball milling process was applied to a dunite from the Troodos ophiolite (Cyprus), in order to create a new type of material with enhanced CO2 uptake. Through CO2 chemisorption followed by temperature-programmed desorption (CO2-TPD) experiments, optimum ball milling conditions were found (12 h of wet ball milling with 50 wt% ethanol as process control agent), leading to an increase of CO2 uptake of dunite by a factor of 6.9. A further increase of CO2 uptake by 10% was accomplished after 4 h of additional ball milling with smaller balls. Additionally, CO2-TPD along with in situ DRIFTS studies indicated that the CO2 uptake of the dunitic materials can be substantially enhanced by the presence of H2O during CO2 chemisorption. The positive effect of H2O on CO2 chemisorption becomes much more evident after the ball milling process. Specifically, the CO2 uptake of the ball-milled sample (BM45) was enhanced by a factor of 5.8 (from 181.9 to 1047.5 lmol g
1), when CO2 chemisorption was performed in the presence of 20 vol% H2O. Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction The concentration of carbon dioxide (CO2) in the atmosphere has been increased by approximately 30% since the industrial revolution, primarily due to the widespread use of fossil fuels [1,2]. CO2 storage via mineral carbonation is an attractive method to mitigate the fast increase of CO2 in atmospheric levels, which is considered to be the main cause for the observed global warming [3–5]. Mineral carbonation is the chemical reaction of CO2 with lithotypes containing Mg and/or Ca-silicate minerals to form stable carbonates, such as magnesite (MgCO3), calcite (CaCO3) and dolomite (CaMg(CO3)2). These carbonate minerals are stable over geological timescales; hence this method provides a safe and

essentially permanent CO2 storage, and does not require longterm monitoring [6–8]. Ultramafic rocks are among the main sources of Mg and/or Ca-silicate minerals, which are abundant in ophiolite complexes worldwide. The main carbonation reactions of ultramafic lithologies are given below:

Mg2 SiO4 þ2CO2 ! 2MgCO3 þ SiO2 Forsterite

MgSiO3 þCO2 ! MgCO3 þ SiO2 Enstatite

Magnesite

ð2Þ

Silica

Mg3 Si2 O5 ðOHÞ4 þ3CO2 ! 3MgCO3 þ 2SiO2 þ2H2 O Serpentine

⇑ Corresponding authors at:. Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, Cyprus. Tel.: +357 22892267 (T. Kyratsi). Tel.: +357 22892776 (A.M. Efstathiou). E-mail addresses: [email protected] (I. Rigopoulos), mvasil04@ucy. ac.cy (M.A. Vasiliades), [email protected] (I. Ioannou), [email protected] (A.M. Efstathiou), [email protected] (A. Godelitsas), [email protected] (T. Kyratsi).

ð1Þ

Silica

Magnesite

Magnesite

Silica

ð3Þ

Mineral carbonation can be carried out either in situ, by injecting CO2 into specific geological formations [7,9,10], or ex situ in a chemical processing plant, after mining and pre-treating the rock material [4,11,12]. A few studies have tried to accelerate ex situ mineral carbonation by increasing the process temperature or

http://dx.doi.org/10.1016/j.apt.2016.01.007 0921-8831/Ó 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

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grinding the raw materials and dissolving them in acidic solutions [13,14]. The ball milling technique has also been recently used to enhance the CO2-storage capacity of rock materials [15–17]. The goal of this paper is to determine the effect of the ball milling process on the CO2 uptake of dunites. The latter are among the most important lithologies for the mineralization of CO2, taking into account that (i) they are rich in forsterite-rich olivine, which is the main source of Mg2+ cations, and (ii) they exhibit global abundance. It is worth noting that various works have investigated, so far, the CO2 sequestration of ultramafic rocks [18–23]. However, only a limited number of studies have focused on the influence of the ball milling process on the carbonation of olivine-rich materials [14,15], where none of them has investigated the surface chemistry of CO2 chemisorption (uptake) on these materials.

Polarized microscopy using a representative thin section was performed on the dunite under study, in order to determine its petrographic characteristics. Powdered samples were also prepared with a laboratory stainless steel pulverizer. The pulverized material was then sieved in order to acquire the 104–150 lm fraction, which was used as starting material for the ball milling experiments. This particular fraction was chosen to avoid internal mass transport resistances during CO2 chemisorption (uptake) and the subsequent CO2-TPD measurements (Section 2.6). For comparison purposes, a reference sample of forsterite (Alfa Aesar, Forsterite, naturally occurring mineral), of the same grain size fraction, was also experimentally analyzed in terms of CO2 adsorption/TPD.

2.2. Ball milling 2. Materials and methods 2.1. Sample selection, preparation and characterization A representative sample of dunite, collected from the western part of the Troodos mantle section (north of Mount Olympos; Fig. 1), was used for the preparation of ultramafic materials with enhanced CO2-storage capacity. The Troodos ophiolite (Fig. 1) is the most intact ophiolite worldwide. It was formed in a supra-subduction zone environment around 92–90 Ma ago (Cenomanian–Turonian), based on U–Pb isotopic dating of plagiogranites [25]. Its mantle section is divided into two units [26]. The eastern unit consists of spinel lherzolite with dunite bodies and zones of clinopyroxene-bearing harzburgite, while the western unit is principally composed of clinopyroxene-poor harzburgite and dunite. Above these mantle rocks, cumulate ultramafic and mafic lithotypes are found, which are cut by gabbroic intrusives; the upper massive gabbros locally include small plagiogranite bodies. Upwards, the sheeted dyke complex trends nearly N–S [27]. The overlying pillow lavas are traditionally divided into the ‘‘Lower” and ‘‘Upper” Pillow Lava units (LPL and UPL) [28].

The ball milling process was carried out in a Fritsch Pulverisette 6 planetary mono mill. The dunite was subjected to wet milling in an 80 mL tungsten carbide bowl, using deionized H2O or ethanol as process control agents (PCAs). According to Kleiv and Thornhill [29], dry ball milling leads to the agglomeration of olivine particles even after a few minutes of mechanical activation. Furthermore, Sandvik et al. [30] have shown that dry ball milling of olivine results in lower specific surface area compared to wet milling. Subsequently, all the ball milling experiments were conducted under wet conditions. The ball-to-powder mass ratio was 20:1 w/w, the PCA-to-powder mass ratio was 1:10 or 1:2 w/w (10 or 50 wt% PCA, respectively), and the rotation speed 300 rpm. The process was carried out using 30 tungsten carbide balls with a diameter of 10 mm. The material with the highest CO2 uptake was subjected to further wet ball milling in a 12 mL bowl using 50 balls with a diameter of 5 mm. In this case, the ball-to-powder mass ratio was also 20:1 w/w, while only ethanol was used as PCA (PCA-topowder mass ratio = 1:2 w/w). The goal was to investigate whether the size of the balls affects the CO2 uptake of the dunitic materials. The reason for using tungsten carbide bowl and balls was to avoid possible contamination of the material, taking into account the high hardness of dunitic rocks. The ball milling process was

Fig. 1. Simplified geological map of the Troodos ophiolite (modified after Pearce and Robinson [24]). Sample location is marked with the yellow square. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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I. Rigopoulos et al. / Advanced Powder Technology xxx (2016) xxx–xxx Table 1 Ball milling conditions of the studied dunite and textural properties of the starting material, the reference forsterite and the ball-milled samples. Sample code

SM15 (starting material) Reference forsterite BM30 BM31 BM41 BM42 BM26 BM27 BM34 BM35 BM44 BM36 BM38 BM39 BM45 BM40 BM46 BM58 BM59 *

Ball milling conditions

Textural properties

Milling time (h)

Type of milling

BET (m2 g
1)

Specific pore volume (cm3 g
1)

Avg pore diameter (nm)

– – 1 2 4 8 1 2 4 8 1 2 4 8 12 16 20 4* 8*

– – Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet Wet

6.8 1.0 24.5 36.9 45.9 52.2 29.5 38.0 47.7 32.5 25.2 30.3 35.7 41.5 51.9 49.9 64.6 48.9 47.3

0.011 0.003 0.048 0.055 0.059 0.052 0.097 0.104 0.109 0.089 0.081 0.104 0.121 0.170 0.173 0.157 0.179 0.141 0.162

5.4 9.0 6.5 4.9 4.2 3.9 11.2 9.3 7.9 9.0 11.4 12.2 11.8 13.7 11.4 10.9 9.3 9.8 12.1

(10 wt% (10 wt% (10 wt% (10 wt% (10 wt% (10 wt% (10 wt% (10 wt% (50 wt% (50 wt% (50 wt% (50 wt% (50 wt% (50 wt% (50 wt% (50 wt% (50 wt%

H2O) H2O) H2O) H2O) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol) Ethanol)

Additional hours of ball milling on sample BM45 using smaller vial and balls, see Section 2.2 for further details.

automatically interrupted every 5 min with 5 min stay in order to avoid heating of the test sample. This periodical interruption is significant in order to avoid possible phase transformations that could reduce the CO2-storage capacity of the rock material. Moreover, the interruption is essential in order to reduce the potential evaporation of PCA, especially during long hours of ball milling. The milled powders were left to dry out overnight. The ball milling conditions are summarized in Table 1.

2.3. Powder X-ray diffraction (PXRD) Powder X-ray diffraction analyses were also performed using a Bruker D8 Advance system in order to identify the mineral crystal phases of the studied ultramafic samples and investigate potential mineralogical transformations that might have occurred during the ball milling process. The analysis was carried out with a continual rotation of the sample and a step of 1°/min, within the angle range of 3–80° 2h. The ICDD PDF 2 database was used for the qualitative identification of the constituent mineral phases.

2.4. Surface texture The BET method (adsorption of N2 at 77 K) was carried out in a Micromeritics Gemini III Surface Area and Pore size Analyzer, in order to determine the specific surface area (m2 g
1), the specific pore volume (cm3 g
1) and the average pore diameter (nm) of the starting dunite, the ball-milled rock materials and the reference forsterite. Each measurement was taken after the sample was outgassed in dry nitrogen flow at 250 °C for 2 h.

2.6. Temperature-programmed desorption of carbon dioxide (CO2-TPD) Temperature-programmed desorption (TPD) of CO2 in He carrier gas was performed in a specially designed gas flow-system [31], in order to evaluate both the concentration (lmol g
1) of adsorbed CO2 on the basic sites of material (oxygen anionic sites), and the distribution of strength of CO2 adsorption (heterogeneity of the surface of adsorbent material). The latter is related to the temperature of appearance of the CO2 desorption rate maximum in the TPD trace [32]. The temperature of the samples under investigation (W = 0.7 g, starting material, ball-milled samples and reference forsterite) was initially increased to 500 °C under He gas flow and maintained at this temperature for approximately 1 h until the CO2 background signal (m/z = 44) in the mass spectrometer was reached. The feed was then switched to 5 vol% CO2/He gas mixture (50 N mL min
1) at 500 °C for 30 min, and the sample was then cooled for about 1 h to 50 °C under that mixture. It should be mentioned that the chemisorption temperature used was also varied (400 or 600 °C) in order to determine the temperature that favors the highest CO2 uptake (see Section 3.5). After the sample was cooled to 50 °C in the CO2/He flow, the feed gas was switched to He flow for approximately 15 min, until no signal of CO2 was detected in the mass spectrometer. The CO2-TPD experiment was then conducted by increasing the temperature of the powder sample from 50 to 950 °C (rate of temperature increase, b = 30 °C/min). Calibration of the CO2 signal (m/z = 44) of the mass spectrometer was carried out based on a certified calibration gas mixture of 985 ppm CO2 in He diluent gas. 2.7. In situ DRIFTS studies

2.5. Scanning electron microscopy The starting dunite and the materials produced via the ball milling process were characterized using a JEOL, JSM-6610 LV scanning electron microscope (SEM), equipped with a BRUKER type QUANTAX 200 energy dispersive spectrometer (EDS). The effect of the ball milling process on the studied rock type was determined using secondary electron images (SEI). EDS analysis was also performed in order to determine the chemical composition of the constituent minerals.

A Perkin–Elmer Spectrum GX II FTIR spectrometer, equipped with a high-temperature/high pressure controllable DRIFTS cell (Harrick, Praying Mantis) was used for performing in situ CO2– DRIFTS chemisorption studies. Approximately 100 mg of powder from each sample (starting material, or ball-milled sample with the highest CO2 uptake) were placed into the ceramic cup of the DRIFTS cell. Before any spectrum was recorded, the sample was pre-treated in situ under Ar at 700 °C for 1 h. Chemisorption of CO2 was performed at 500 °C using a 5 vol% CO2/He/Ar gas mixture

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(50 cc/min) for 30 min. Additionally, experiments were performed using a 5% CO2/10% H2O/He/Ar, or 5% CO2/20% H2O/He/Ar gas mixtures in order to investigate the influence of H2O on the CO2 uptake. Due to technical limitations in the formation of water vapor gas mixture above 20 vol% H2O/He, no experiments with such feed gas composition were conducted. Temperature-programmed desorption-DRIFT spectra were recorded after the adsorption of CO2 from the above mentioned mixtures, followed by a purge of the DRIFTS cell in Ar flow (15 min), and a temperature increase from 500 to 700 °C. A DRIFT spectrum (absorption mode) of the adsorbed phase was obtained after subtracting from the spectrum of the solid recorded under 5% CO2/Ar/He or 5% CO2/10% H2O/Ar/He or 5% CO2/20% H2O/Ar/He treatment that recorded under Ar/He or 10% H2O/Ar/He or 20% H2O/Ar/He treatment at the various temperatures of interest (500–700 °C). DRIFT spectra when necessary were smoothed to remove high frequency noise and further analyzed using the provided software (PerkinElmer Spectrum, v.10.03.02). 3. Results 3.1. Petrography The dunite under study is partially serpentinized, displaying porphyroclastic and cataclastic textures (Fig. 2). Its primary mineralogical composition includes olivine (forsterite) and disseminated Cr-spinel. The development of the secondary minerals serpentine, talc and chlorite imply that the dunite has been moderately affected by ocean-floor metamorphic processes. Intragranular and transgranular microcracks crosscut the rock, indicating that it has been mainly affected by brittle deformation.

more, a broadening of the peaks took place after ball milling. These modifications tend to become more obvious with increasing milling time, as demonstrated by the negative correlation between the duration of ball milling and the ratio of maximum intensity to mean background intensity (see inset of Fig. 3b). 3.3. Surface texture The specific surface area (BET, m2 g
1), the pore volume (cm3 g
1) and the average pore diameter (nm) of the starting rock material, the materials after ball milling, and the reference forsterite material are given in Table 1. The unmilled dunite (sample SM15) has higher specific surface area and pore volume compared to the reference forsterite. The variation of the aforementioned textural parameters with increasing ball milling time can be seen in Fig. 4. Taking into account that the ball milling process is severe with only 10 wt% PCA, we did not perform milling experiments longer than 8 h at this specific PCA concentration in order to avoid contamination of the material from the vial and balls. An increase of the content of ethanol from 10 to 50 wt% allowed to increase the duration of the ball milling process up to 20 h. 3.4. Scanning electron microscopy SEM images were acquired from the starting dunite and the new materials produced via the ball milling process. The unmilled dunite has large and angular particles (Fig. 5a), the size of which corresponds to the aperture size of the sieves (104–150 lm) that were used to acquire the fraction of the starting rock material. The particles of the ball-milled materials are significantly smaller, more rounded and uniform (Fig. 5b–f). Specifically, the particle size

3.2. Powder XRD (PXRD) Powder X-ray diffraction was used in order to determine the mineral phases that constitute the starting dunite (Fig. 3a). The detected minerals included forsterite, Cr-spinel, antigorite, lizardite and talc. The results are in agreement with the data acquired during the petrographic analysis. PXRD patterns were also obtained for the nanomaterials corresponding to the various ball milling conditions. Based on these diffraction patterns, it can be concluded that there is no mineralogical transformation due to the ball milling process. However, a substantial reduction in the intensity of all XRD peaks, which even led to the disappearance of some of these, was observed (compare Fig. 3a and b). Further-

Fig. 2. Photomicrograph (crossed polars) of the dunite under study, showing cataclastic texture (Ol: olivine, Serp: serpentine).

Fig. 3. Powder X-ray diffraction patterns of (a) the unmilled dunite, and (b) the rock material after 16 h of ball milling with 50 wt% ethanol. Inset shows the relationship between the ball milling time (h) and the ratio of maximum intensity (Imax) to mean background intensity (Io), for the materials produced with 50 wt% ethanol as PCA (the maximum intensity corresponds to the peak with the highest intensity in each XRD pattern, and the mean background intensity is the average of four background intensities).

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Fig. 4. (a) Specific surface area (m2 g
1), (b) specific pore volume (cm3 g
1), and (c) average pore diameter (nm) as a function of ball milling time (h) for samples with 10 wt% H2O, 10 wt% ethanol, and 50 wt% ethanol as PCA.

seems to decrease with increasing milling time (compare Fig. 5c– e). Additionally, the ball-milled samples usually contain agglomerated powder particles. The frequency of the latter significantly increases after many hours of ball milling (Fig. 5f).

3.5. CO2 chemisorption followed by TPD CO2 temperature-programmed desorption traces were acquired for the starting, ball-milled and reference materials. It should be pointed out that additional CO2-TPD experiments were performed in sample BM45 (Table 2), where the chemisorption temperature of CO2 was varied (400 or 600 °C instead of 500 °C). As can be seen in Fig. 6, the largest CO2 uptake occurred after CO2 chemisorption at 500 °C. Based on these results, the subsequent experiments over the other samples were performed using the adsorption temperature of 500 °C and following the procedure described in Section 2.6.

5

The quantity (lmol g
1 and mg g
1) of the basic sites which accommodate the adsorbed CO2 (see Table 2) was estimated after integrating the respective CO2 desorption trace (CO2 concentration vs time) and considering the appropriate material balance for a flow reactor. It should be noted that for this estimation, the time needed at 950 °C in He flow until no desorption of CO2 had been detected (not shown in Fig. 7) was considered. The temperatures at which maximum desorption rates (Tmax, °C) were observed, and the total CO2 uptake (or the equivalent desorbed amount) of all the samples under study are summarized in Table 2. In the unmilled dunite, the presence of different in strength surface basic sites is represented by four desorption peaks (peak maxima at 160, 654, 807 and 824 °C), with the largest one appearing at 654 °C as shown in Fig. 7a. Although the number of desorption peaks for the materials produced with the addition of 10 wt% H2O during the ball milling process has been increased compared to the unmilled dunite, the peak intensities do not differ significantly (Fig. 7b). For example, after 1 h of ball milling, five major peaks exist (Tmax = 135, 235, 646, 709 and 790 °C; Fig. 7b). Similarly, five main peaks are observed after 4 h of ball milling (Tmax = 135, 243, 556, 661, 756 °C; Fig. 7c). The peak intensities of the aforementioned milled samples are similar to those of the starting sample (compare Fig. 7a with Fig. 7b and c). The milled samples produced after ball milling with 10 wt% ethanol exhibit higher intensity desorption peaks. This increase is small after 1 h of ball milling (peak maxima at 129, 240, 342, 649, 705, 797, 872 °C; Fig. 7d). However, after 4 h of ball milling, the intensities of desorption peaks exhibit a noticeable increase; in this case, five main desorption peaks are observed (peak maxima at 207, 607, 732, 795 and 913 °C; Fig. 7e), with the highest one appearing at 732 °C. The materials produced with the addition of 50 wt% ethanol show substantially higher in intensity CO2 desorption peaks, compared to those produced with the addition of 10 wt% H2O (compare Fig. 7c and f). As can be seen in Fig. 7f, after 4 h of ball milling with 50 wt% ethanol, the presence of surface basic sites is principally represented by five desorption peaks (peak maxima at 202, 665, 703, 779 and 834 °C), with a small shoulder at the left part of the second peak (636 °C). It should be mentioned that the CO2-TPD traces after 4 h of ball milling with 10 or 50 wt% ethanol do not differ significantly (compare Fig. 7e with Fig. 7f). Specifically, the CO2 uptake of the materials produced with the addition of 10 and 50 wt% ethanol is 172.0 and 194.8 lmol g
1, respectively (see Table 2; samples BM34 and BM38). However, CO2 uptake values after 8 h of ball milling with 10 and 50 wt% ethanol are 171.7 and 260.7 lmol g
1, respectively (see Table 2; samples BM35 and BM39), implying that the addition of 50 wt% ethanol results in an increase of the concentration of basic sites with increasing milling time. On the other hand, the material produced after ball milling with 10 wt% ethanol does not show any further increase of CO2 uptake from 4 to 8 h of milling. The desorption peaks become significantly higher after 12 h of ball milling with 50 wt% ethanol (peak maxima at 134, 259, 664, 705, 772 and 831 °C; Fig. 7g); where maximum CO2 uptake has been obtained (see Table 2; sample BM45à,a). Regarding the reference forsterite sample (Fig. 7h), this exhibits five desorption peaks (peak maxima at 100, 193, 480, 620 and 770 °C), with the largest one appearing at 620 °C. However, the CO2 uptake estimated was the lowest one (0.26 mg CO2 g
1) among all samples investigated (Table 2). The correlations between the ball milling time and the estimated values of CO2 chemisorption from a 5% CO2/He gas mixture for the studied rock materials demonstrate that the ball milling technique substantially improves the ability of dunites to adsorb CO2 (Fig. 8). Emphasis should be placed on the fact that, during the ball milling process, the addition of 50 wt% ethanol results in higher CO2 uptake compared to the addition of 10 wt% ethanol or H2O. The use of 50 wt% ethanol leads to a fast increase of CO2

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Fig. 5. SEM images of the dunitic material under study: (a) SM15 (starting material), (b) BM30 (ball-milled for 1 h with 10 wt% H2O), (c) BM26 (ball-milled for 1 h with 10 wt % ethanol), (d) BM34 (ball-milled for 4 h with 10 wt% ethanol), (e) BM45 (ball-milled for 12 h with 50 wt% ethanol), and (f) BM46 (ball-milled for 20 h with 50 wt% ethanol). The magnification in (a) is significantly lower compared to (b–f).

uptake for the first 8 h of ball milling (Fig. 8). The addition of 10 wt % ethanol leads to a slightly slower increase of CO2 uptake, with the maximum value acquired after 4 h of ball milling (Fig. 8). A notably slower increase of CO2 uptake is observed with the addition of 10 wt% H2O as PCA (Fig. 8). As already mentioned (see Section 2.2), the material with the highest CO2 uptake (BM45) was subjected to additional wet ball milling in a 12 mL bowl using balls with a diameter of 5 mm. In fact, this sample was subjected to additional ball milling for 4 or 8 h. As can be seen in Fig. 9 and Table 2, the CO2 uptake of sample BM45 has been further enhanced after 4 h of additional ball milling (sample BM58); however, longer milling leads to a reduction of the CO2-storage capacity. 3.6. In situ DRIFTS The type of adsorbed carbonate species formed on the surface of the samples under investigation and their thermal stability were studied using in situ DRIFTS. It is worth mentioning that surface carbonation is the first important step prior to bulk carbonation (diffusion of surface carbonates into the subsurface of the solid material). Additionally, in situ DRIFTS were used in order to determine the influence of H2O on the CO2 uptake of the starting mate-

rial (SM15) and the ball-milled material with the highest CO2 uptake (BM45). Fig. 10a and b compares the DRIFT spectra (absorbance mode) of the unmilled and ball-milled dunite recorded in the 1800–1200 cm
1 range, after exposure of the samples to the 5% CO2/He/Ar gas mixture at 500 °C for 30 min, followed by an increase of the solid temperature to 600 and 700 °C under Ar flow (TPD run). The main IR bands of the unmilled dunite observed at 500 °C (Fig. 10a) correspond to unidentate (1590 cm
1) and carbonate ion (1445 cm
1) species, respectively (Table 3). The most intense IR bands of the ball-milled dunite observed at 500 °C (Fig. 10b) were detected at 1575 and 1438 cm
1, and also correspond to unidentate and carbonate ion species, respectively (Table 3). The main difference between the two samples is the relative population of these two types of carbonate species according to the relative intensities (or integral band areas). The spectrum of the unmilled sample recorded at 500 °C has an additional band at 1755 cm
1 (Fig. 10a), which corresponds to bridged bidentate carbonates [35]. The intensities of all infrared bands observed in both unmilled and milled dunite samples were decreased after the solid was heated in Ar flow to 600 and 700 °C. This implies that the adsorbed carbonate species tend to decompose with increasing temperature. More precisely, the band due to bridged bidentate carbonate was

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Table 2 Peak maximum desorption temperatures (Tmax) and amounts of adsorbed CO2 (lmol g
1 or mg g
1) (after chemisorption from 5 vol% CO2/He) estimated from CO2-TPD experiments for the starting dunitic material, the reference forsterite and the ball-milled samples.

y à

Sample code

Tmax1 (°C)

Tmax2 (°C)

Tmax3 (°C)

Tmax4 (°C)

Tmax5 (°C)

Tmax6 (°C)

Tmax7 (°C)

lmol CO2 g
1

mg CO2 g
1

SM15 (starting material) Reference forsterite BM30*,a BM31*,a BM41*,a BM42*,a BM26y,a BM27y,a BM34y,a BM35y,a BM44à,a BM36à,a BM38à,a BM39à,a BM45à,a BM45à,b BM45à,c BM40à,a BM46à,a BM58à,a BM59à,a

160 100 135 165 135 128 129 215 207 170 177 173 202 196 134 133 143 161 135 171 127

654 193 235 540 243 262 240 319 607 659 279 307 636 635 259 273 289 264 225 277 255

807 480 535 650 326 561 342 637 644 778 650 647 665 667 664 630 635 651 308 648 638

824 620 646 709 556 647 649 685 732 830 705 700 703 685 705 665 772 705 641 703 769

– 770 709 791 661 755 705 792 795 907 800 785 779 768 772 715 835 768 693 767 835

– – 790 – 694 – 797 854 913 – – – 834 826 831 774 – 830 769 819 –

– – – – 756 – 872 – – – – – – – – – – – – – –

40.1 5.8 61.5 70.6 86.3 129.6 82.6 125.3 172.0 171.7 113.4 129.4 194.8 260.7 278.1 256.0 213.4 266.7 275.6 305.3 284.6

1.76 0.26 2.71 3.11 3.80 5.70 3.63 5.51 7.57 7.55 4.99 5.69 8.57 11.47 12.24 11.26 9.39 11.73 12.13 13.43 12.52

* Nanomaterials produced with the addition of 10 wt% H2O. Nanomaterials produced with the addition of 10 wt% ethanol. Nanomaterials produced with the addition of 50 wt% ethanol. a The adsorption temperature was 500 °C during CO2 chemisorption. b The adsorption temperature was 400 °C during CO2 chemisorption. c The adsorption temperature was 600 °C during CO2 chemisorption.

Fig. 6. CO2 uptake (lmol g
1) versus the adsorption temperature (Tads, °C) used during CO2 chemisorption (see Section 3.5) for sample BM45.

significantly decreased when the unmilled dunite was heated to 600 °C and almost disappeared at 700 °C, a behavior also obtained for the other two types of carbonate species (Fig. 10a). On the other hand, this behavior was not observed in the ball-milled sample (Fig. 10b), where the thermal stability of all carbonate species was significantly larger compared to the unmilled sample. Fig. 11a and b presents DRIFT spectra (absorbance mode) recorded at 500 °C following a 30-min 5% CO2/He/Ar, 5% CO2/10% H2O/He/Ar or 5% CO2/20% H2O/He/Ar gas treatment at 500 °C and a subsequent 15-min Ar purge at the same temperature of the solid in the DRIFTS cell. It is clearly observed that the intensities of the IR bands recorded in the 1800–1200 cm
1 range substantially increase due to the presence of 10 vol% H2O in the adsorption gas mixture. The IR bands become notably larger due to a further increase from 10 to 20 vol% of the content of H2O in the adsorption gas mixture. Fig. 12 compares the DRIFT spectra of the starting material and the ball-milled material with the highest CO2 uptake recorded in the 1800–1200 cm
1 range (K–M units) after a 30 min exposure in the 5% CO2/20% H2O/He/Ar adsorption gas mixture at 500 °C.

The two major IR bands of the unmilled and milled samples correspond to unidentate and carbonate ion species (see Table 3). It is worth mentioning that the IR bands of the ball-milled sample are substantially larger compared to those of the starting dunite sample, implying that the former has a notably higher concentration of unidentate and carbonate ion adsorbed species. This result is in agreement with that of CO2-TPDs (see Fig. 7). However, as will be discussed next, the substantial increase of surface hydroxyl groups in the presence of gas-phase H2O can lead to the formation of another type of adsorbed carbonate species, that of hydrogen carbonate, CO2(OH)
, which gives rise to m(CO) vibrational mode frequencies very similar to those of the other two carbonate-type species previously mentioned [38]. The maximum increase in the CO2 uptake of sample BM45 due to the presence of H2O was investigated by performing additional CO2-TPDs. In particular, the chemisorption was performed from a 5% CO2/He or 5% CO2/20% H2O/He gas mixture at 500 °C for 30 min, followed by cooling of the sample to 200 °C under the same gas mixture followed by He purge (see Section 3.5). The CO2-TPD experiment was then conducted, and subsequently the quantity (lmol g
1 and mg g
1) of adsorbed CO2 was calculated (Table 4) according to the procedure described in Section 3.5. The difference in the quantity of CO2 uptake estimated for sample BM45 after chemisorption from the 5% CO2/20% H2O/He and 5% CO2/He gas mixtures was found to be 865.6 lmol g
1 (Table 4). 4. Discussion The method of ex situ mineral carbonation ensures the safe and permanent storage of CO2 [4,11,39]. However, the critical challenge is to accelerate the CO2 adsorption kinetics, in order to develop an economically viable commercial process [5]. This work has investigated a method to enhance the CO2-strorage capacity of dunites, which are among the most efficient sources of divalent cations that are essential for the carbonation process [20,40]. The studied dunite is a partially serpentinized lithology from the Troodos ophiolite complex.

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Fig. 7. CO2 temperature-programmed desorption (TPD) traces obtained under He flow for (a) the starting dunitic material (SM15), (b–g) representative ball-milled samples, and (h) the reference sample of forsterite. Adsorption conditions: 5 vol% CO2/He (50 N mL/min) at 500 °C for 30 min followed by cooling of the sample to 50 °C under the same mixture. Desorption conditions: QHe = 50 N mL/min, b = 30 °C/min.

The ball milling process was applied to the dunite in order to produce new materials with significantly higher CO2 uptake, compared to the unmilled dunite. The influence of the ball milling time, as well as of the type and content of PCA on the textural and morphological characteristics of the ball-milled materials were thoroughly examined. Initially, the specific surface area, pore volume and pore diameter increase due to the effect of the ball milling process (Fig. 4). However, these trends are different depending on the type and content of PCA. The increase of the content of ethanol PCA from 10 to 50 wt% leads to a smaller increase of specific surface

area with increasing ball milling duration (e.g. compare the values of specific surface area after 4 h of ball milling for 10 and 50 wt% ethanol; Fig. 4a). This is attributed to the fact that a higher content of PCA tends to reduce the intensity of the ball milling process, and consequently the reduction rate of particle size. On the other hand, the use of 10 wt% H2O resulted in the smallest increase of pore volume and pore diameter with increasing ball milling time (Fig. 4b and c), in accordance with Rigopoulos et al. [16]. This is also supported by SEM observations, which indicated that the use of ethanol during the ball milling process promotes the

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9

Fig. 7 (continued)

Fig. 8. CO2 uptake (lmol g
1) versus ball milling time (h) for the starting dunitic material and the dunitic ball-milled materials produced after the addition of 10 wt% H2O, 10 wt% ethanol, and 50 wt% ethanol as PCA.

Fig. 9. CO2 uptake (lmol g
1) versus ball milling time (h) for samples BM45, BM58 (additional ball milling of sample BM45 for 4 h) and BM59 (additional ball milling of sample BM45 for 8 h). The additional ball milling was performed using smaller vial and balls, compared to those used for the preparation of sample BM45 (see Section 2.2 for further details).

formation of smaller and more rounded particles compared to H2O (compare Fig. 5b and c). It should be mentioned that the initial large increase of specific surface area, pore volume and pore diameter becomes very small after only a few hours of ball milling, and these positive trends may in fact even become negative (e.g. see the correlations between pore volume and ball milling time for 10 wt% ethanol or H2O after more than 4 h of milling; Fig. 4b).

Based on SEM observations (Fig. 5), it is suggested that this behavior is due to the fact that agglomeration of rock secondary particles occurs after a few hours of ball milling [16,17]. The largest values of specific surface area and pore volume were obtained after 20 h of milling by using 50 wt% ethanol as PCA (see Fig. 4a and b). Regarding the maximum pore diameter value, it was obtained after 8 h of ball milling with the use of 50 wt% ethanol as PCA (see Fig. 4c). These observations are supported by the SEM images, which show that ball-milled samples contain significantly smaller, more rounded and uniform secondary particles, compared to the starting material (see Fig. 5). Through CO2-TPD experiments, it was proved that the ball milling process can substantially increase the CO2-storage capacity of dunitic lithologies. Specifically, the quantity of adsorbed CO2 was increased from 40.1 lmol g
1 in the starting material (SM15) to 278.1 lmol g
1 in the ball-milled material with the highest CO2 uptake (BM45) (see Table 2); hence an improvement by a factor of 6.9 was accomplished. This improvement was recorded after 12 h of ball milling with 50 wt% ethanol as PCA. However, longer ball milling does not lead to a further increase of CO2 uptake (see Fig. 8). The ball-milled material with the highest CO2 uptake was subjected to additional wet ball milling in a 12 mL bowl, using balls with a diameter of 5 mm. These experiments indicated that 4 h of additional ball milling with smaller balls (sample BM58) have led only to a small increase of CO2 uptake (by 10%) compared to sample BM45 (from 278.1 to 305.3 lmol CO2 g
1; see Table 2). On the other hand, additional ball milling for longer than 4 h resulted in a reduction of CO2 uptake (see Fig. 9). A comparison of the CO2 uptake of sample BM45 with that of ball-milled basaltic and pyroxenitic ophiolitic rocks measured after performing similar CO2-TPD experiments [16,17] implies that the ball-milled dunite exhibits the highest CO2 uptake. This result indicates that olivine-rich rocks are among the most promising materials for CO2 sequestration, and this is consistent with the literature [4,22,41]. However, it should be mentioned that the CO2 uptakes measured on these materials could be further increased, if aqueous carbonation is performed [42]. The number of surface basic sites increases with decreasing particle size of the material and the concomitant increase of specific surface area (Fig. 4a); thus the increase of CO2 uptake in the ballmilled dunitic samples is mainly attributed to the substantial increase of specific surface area caused by the ball milling process. On the other hand, special emphasis should be placed on the fact that many crystal defects tend to be created in the material due to the impact of ball milling. Therefore, during chemisorption of CO2, a number of these crystal defects comprise sites that promote

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Fig. 10. In situ DRIFTS spectra recorded in the 1800–1200 cm
1 range after exposing (a) the starting material (SM15), and (b) the ball-milled material with the highest CO2 uptake (BM45) in 5 vol% CO2/He/Ar gas mixture at 500 °C for 30 min and in Ar flow at 600 and 700 °C.

Table 3 Bands of carbonate species formed after exposing the starting material (SM15) and the ball-milled material with the highest CO2 uptake (BM45) in 5 vol% CO2/He/Ar, or 5 vol% CO2/20 vol% H2O/He/Ar gas mixture at 500 °C for 30 min. Sample code/chemisorption gas mixture and temperature

Band position (cm
1)/carbonate species

SM15/5%CO2/He/Ar – 500 °C

1755/bridged bidentate – – 1678/bidentate

BM45/5%CO2/He/Ar – 500 °C SM15/5%CO2/20%H2O/He/Ar – 500 °C BM45/5%CO2/20%H2O/He/Ar – 500 °C

References

1590/unidentate

1445/carbonate ion

–

1575/unidentate 1590/unidentate 1578/unidentate

1438/carbonate ion 1452/carbonate ion 1440/carbonate ion

– – 1304/ bidentate

Evans and Whateley [33]; Stark et al. [34]; León et al. [35] Evans and Whateley [33]; Prescott et al. [36] Evans and Whateley [33]; Stark et al. [34] Evans and Whateley [33]; Prescott et al. [36]; Kwon and Park [37]

Fig. 11. In situ DRIFTS spectra recorded in the 1800–1200 cm
1 range after exposing (a) the starting material (SM15), and (b) the ball-milled material with the highest CO2 uptake (BM45) in 5 vol% CO2/He/Ar, 5 vol% CO2/10 vol% H2O/He/Ar or 5 vol% CO2/20 vol% H2O/He/Ar gas mixtures at 500 °C for 30 min.

the formation of carbonate-type species (CO2 (g) + On
M CO3n
). Subsequently, these carbonates diffuse within the bulk of the particle resulting in the carbonation of the material. The aforementioned are in agreement with similar studies in other ophiolitic lithologies [16,17]. In addition, the rate of CO2 mass transport within the porous structure of the dunitic material has been probably enhanced due to the increase of pore volume after ball milling (see Fig. 4b); this is presumably an additional parameter that promotes the mineralization of CO2 [16,17,43]. It should also be mentioned that the starting dunitic material shows considerably higher CO2 uptake compared to the reference forsterite (see Table 2). However, the ratio of CO2 uptake (lmol g
1) to the BET specific surface area (m2 g
1) of these samples is similar (5.9 and 5.8 lmol m
2 for the starting dunitic material and the reference forsterite, respectively). Therefore, it is suggested that their notably different CO2 uptake is assigned to their different values of specific surface area. The desorption peaks of the ball-milled samples tend to shift to lower temperatures as the duration of ball milling increases. For

example, the highest desorption peak after 1 h of ball milling (sample BM26) is detected at 797 °C, while the same peak after 4 h of ball milling occurs at 732 °C (sample BM34) (see Fig. 7d, e and Table 2). This behavior is likely due to the fact that some of the crystal defects that are formed during the ball milling process comprise basic sites of lower strength. The modification of the minerals lattice during ball milling, as indicated by the XRD studies, might also be considered to influence the formation of carbonates which decompose at relatively lower temperatures, as a result of the modification of electron density in the oxygen anions of the respective oxygen sub-lattice. The aforementioned are in line with the results of similar CO2-TPD studies in basalts and pyroxenites [16,17]. In general, the relationships between the CO2 uptake and the duration of ball milling show a positive trend (see Fig. 8). The largest increase of CO2 uptake with increasing milling time is observed in the materials produced with the addition of 50 wt% ethanol, while the smallest increase occurs in the materials produced with the addition of 10 wt% H2O. Although the maximum

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Fig. 12. In situ DRIFTS spectra recorded in the 1800–1200 cm
1 range after exposing the starting material (SM15), and the ball-milled material with the highest CO2 uptake (BM45) in 5 vol% CO2/20 vol% H2O/He/Ar gas mixture at 500 °C for 30 min.

Table 4 Amounts (lmol g
1 or mg g
1) of adsorbed CO2 estimated after chemisorption from a 5 vol% CO2/He or 5 vol% CO2/20 vol% H2O/He gas mixture at 500 °C for 30 min followed by cooling to 200 °C under the same gas mixture estimated from CO2-TPD experiments for the ball-milled dunite with the highest CO2 uptake. Sample code

Gas mixture used during chemisorption

lmol

CO2 g
1

mg CO2 g
1

BM45 BM45

5 vol% CO2/He 5 vol% CO2/20 vol% H2O/He

181.9 1047.5

8.00 46.09

CO2 uptake was recorded in the sample produced with the addition of 50 wt% ethanol after 12 h of ball milling, the specific surface area shows a continual increase, even after 20 h of ball milling (Fig. 4a). These observations demonstrate that the specific surface area is not the only parameter that controls the CO2 uptake. This is in line with the XRD studies, where a substantial reduction of the peaks intensity occurs with increasing ball milling time (see inset of Fig. 3b). The aforementioned trend implies that the ball milling process modifies the crystal structure and morphology of the primary crystals present in the rock, and thus the characteristics (number density and strength) of the surface basic sites of the dunitic materials [15–17]. In situ CO2-DRIFTS studies indicated that unidentate and carbonate ion species are the major types of adsorbed CO2 formed on the surface of the dunitic materials after chemisorption from a 5% CO2/He/Ar gas mixture at 500 °C (see Fig. 10a, b and Table 3). The unmilled sample presented an additional infrared band assigned to bridged bidentate carbonates (see Fig. 10a and Table 3). All the aforementioned types of adsorbed carbonate species tend to decompose with increasing temperature. The fact that the surface coverage of carbonates did not reach the zero value, even when the temperature was increased to 700 °C (see Fig. 10a and b), revealed their high thermal stability. This result is in harmony with the CO2-TPD profiles (Fig. 7). Only in the unmilled sample, did the IR band assigned to carbonate ion species almost disappear at 700 °C (see Fig. 10a), indicating that this type of carbonate has become more thermally stable (enhanced binding strength) after the ball milling process. The intensities of the IR bands were substantially increased when the CO2 chemisorption was performed in the presence of 10 vol% H2O, while a further notable increase was observed when the concentration of H2O was increased to 20 vol% (see Fig. 11a and b). These results demonstrate that the concentration of carbonates increases with increasing concentration of H2O in

11

the CO2 adsorption gas mixture, indicating the important role of H2O in the mechanism of mineral carbonation. This result is in agreement with other published studies, which refer to the positive effect of H2O on the CO2 uptake of various materials [44–47]. The presence of H2O favors the adsorption capacity of dunites, likely due to the fact that the formation of hydroxyls on the surface promote the CO2 chemisorption via the formation of hydrogen carbonate species, CO2(OH)
[38,48]. The latter are expected to provide m(CO) vibrational frequencies very similar to the corresponding ones of the other types of carbonates previously mentioned. As mentioned earlier, the increase of the content of H2O in the gas mixture from 10 to 20 vol% contributes to a notable increase of CO2 uptake in the dunitic ball-milled material. However, it is placed emphasis on the fact that this improvement is much smaller in the unmilled dunite (compare Fig. 11a and b). Thus, it becomes evident that the positive role of H2O in the CO2 uptake of dunites can be considerably enhanced through the ball milling process. The CO2 uptake of the ball-milled dunite after using a 5% CO2/20% H2O/He gas mixture was found to be larger by 865.6 lmol g
1 compared to that estimated after using a 5% CO2/He gas mixture (see Table 4). Thus, it is evident that H2O plays a critical role for the remarkable enhancement of CO2-storage capacity of the dunitic ball-milled materials. The in situ DRIFTS experiments also showed that the quantity of both the unidentate and ionic carbonate species in the ball-milled dunite is significantly larger compared to the unmilled rock material (see Fig. 12), in agreement also with the CO2-TPD studies. Additionally, bidentate carbonate species are formed only in the ball-milled sample after CO2 chemisorption from the 5% CO2/20% H2O/He/Ar gas mixture at 500 °C (see Fig. 12; Table 3). Therefore, the ball milling process, in combination with the presence of H2O seem to favor the development of a small quantity of bidentate carbonate species.

5. Conclusions This study investigated the effect of the ball milling process on the carbonation of dunitic rocks. The experiments were carried out on a partially serpentinized dunite obtained from the Troodos ophiolite. The results show that 12 h of wet ball milling with 50 wt% ethanol as process control agent can increase the CO2 uptake of the starting rock material by a factor of 6.9 (from 40.1 to 278.1 lmol g
1). Although longer ball milling results in higher values of specific surface area, it does not lead to a further increase of CO2 uptake. This is assigned to the fact that prolonged milling tends to disturb the crystal structure of minerals contained in the dunite, and in turn the surface basic sites of the dunitic materials. A further but small increase of CO2 uptake by 10% (from 278.1 to 305.3 lmol g
1) was accomplished after 4 h of additional ball milling with smaller balls. CO2-TPD experiments, in combination with in situ DRIFTS studies, indicated that the CO2 uptake of the dunitic materials can be remarkably increased due to the presence of H2O during chemisorption of CO2 via the formation of hydrogen carbonate-like species. These experimental results also suggested that the positive impact of H2O on the CO2 uptake of dunites becomes more evident after the ball milling process. Specifically, the CO2 uptake of the ball-milled dunite (sample BM45) was remarkably enhanced by a factor 5.8 (from 181.9 to 1047.5 lmol g
1), when the CO2 chemisorption was performed in the presence of 20 vol% H2O. Hence, the results imply that the ball milling process, as well as the CO2 chemisorption in the presence of H2O could render the ex situ mineral carbonation of dunites a very promising methodology for the safe storage of CO2. The simplicity of the ball milling process, which only requires a mill, a vial and a

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I. Rigopoulos et al. / Advanced Powder Technology xxx (2016) xxx–xxx

number of balls, further implies that this could be a fundamental part of an efficient and technologically viable carbon sequestration strategy in the future. However, the high cost of ethanol should be taken into account for potential scale-up of this method; hence, further studies are required in order to find other efficient PCAs during the ball milling process. Additionally, future investigations will have to place emphasis on the degree of carbonation of ballmilled dunitic materials using the aqueous carbonation method. Acknowledgements This work was performed in the framework of the Project ‘‘Nanominerals”, funded by the University of Cyprus. We would like to thank the Cyprus Geological Survey Department (Dr. E. Georghiou Morisseau, Mr. Ch. Hadjigeorghiou and Mr. G. Hadjigeorghiou) for their help during the sampling procedure. We also thank Prof. K. Hatzipanagiotou from the Department of Geology of the University of Patras (Greece) for the preparation of the thin sections used for the petrographic analysis. References [1] U. Siegenthaler, H. Oeschger, Biospheric CO2 emissions during the past 200 years reconstructed by deconvolution of ice core data, Tellus 39B (1987) 140– 154. [2] C.D. Keeling, T.P. Whorf, M. Wahlen, J. van der Plicht, Interannual extremes in the rate of rise of atmospheric carbon dioxide since 1980, Nature 75 (1995) 666–670. [3] K.S. Lackner, C.H. Wendt, D.P. Butt, E.L. Joyce, D.H. Sharp, Carbon dioxide disposal in carbonate minerals, Energy 20 (1995) 1153–1170. [4] E.H. Oelkers, S.R. Gislason, J. Matter, Mineral carbonation of CO2, Elements 4 (2008) 333–337. [5] A.A. Olajire, A review of mineral carbonation technology in sequestration of CO2, J. Petrol. Sci. Eng. 109 (2013) 364–392. [6] W. Seifritz, CO2 disposal by means of silicates, Nature 345 (1990) 486–490. [7] J.M. Matter, P.B. Kelemen, Permanent storage of carbon dioxide in geological reservoirs by mineral carbonation, Nat. Geosci. 2 (2009) 837–841. [8] P.B. Kelemen, J. Matter, E.E. Streit, J.F. Rudge, W.B. Curry, J. Blusztajn, Rates and mechanisms of mineral carbonation in peridotite: natural processes and recipes for enhanced, in situ CO2 capture and storage, Annu. Rev. Earth Planet. Sci. 39 (2011) 545–576. [9] S.R. Gislason, D. Wolff-Boenisch, A. Stefansson, E.H. Oelkers, E. Gunnlaugsson, H. Sigurdardottir, B. Sigfusson, W.S. Broecker, J.M. Matter, M. Stute, G. Axelsson, Th. Fridriksson, Mineral sequestration of carbon dioxide in basalt: a pre-injection overview of the CarbFix project, Int. J. Greenh. Gas Con. 4 (2010) 537–545. [10] S.R. Gislason, E.H. Oelkers, Carbon storage in basalt, Science 344 (2014) 373. [11] S.J. Gerdemann, W.K. O’Connor, D.C. Dahlin, L.R. Panner, H. Rush, Ex situ aqueous mineral carbonation, Environ. Sci. Technol. 41 (2007) 2587–2593. [12] J. Pronost, G. Beaudoin, J. Tremblay, F. Larachi, J. Duchesne, R. Hébert, M. Constantin, Carbon sequestration kinetic and storage capacity of ultramafic mining waste, Environ. Sci. Technol. 45 (21) (2011) 9413–9420. [13] R.D. Schuiling, P. Krijgsman, Enhanced weathering: an effective and cheap tool to sequester CO2, Clim. Change 74 (2006) 349–354. [14] A.H. Haug, R.A. Kleiv, I.A. Munz, Investigating dissolution of mechanically activated olivine for carbonation purposes, Appl. Geochem. 25 (2010) 1547– 1563. [15] E. Turianicová, P. Balázˇ, L. Tucˇek, A. Zorkovská, V. Zelenˇák, Z. Németh, A. Šatka, J. Kovácˇ, A comparison of the reactivity of activated and non-activated olivine with CO2, Int. J. Miner. Process. 123 (2013) 73–77. [16] I. Rigopoulos, K.C. Petallidou, M.A. Vasiliades, A. Delimitis, I. Ioannou, A.M. Efstathiou, Th. Kyratsi, Carbon dioxide storage in olivine basalts: effect of ball milling process, Powder Technol. 273 (2015) 220–229. [17] I. Rigopoulos, M.A. Vasiliades, K.C. Petallidou, I. Ioannou, A.M. Efstathiou, Th. Kyratsi, A method to enhance the CO2 storage capacity of pyroxenitic rocks, Greenhounse Gas. Sci. Technol. 5 (2015) 1–14. [18] M. Andreani, L. Luquot, P. Gouze, M. Godard, E. Hoise, B. Gibert, Experimental study of carbon sequestration reactions controlled by the percolation of CO2rich brine through peridotites, Environ. Sci. Technol. 43 (2009) 1226–1231. [19] N. Koukouzas, V. Gemeni, H.J. Ziock, Sequestration of CO2 in magnesium silicates, in Western Macedonia, Greece, Int. J. Miner. Process. 93 (2009) 179– 186. [20] D. Wolff-Boenisch, S.R. Gislason, S. Wenau, E.H. Oelkers, Dissolution of basalts and peridotite in seawater, in the presence of ligands, and CO2: implications for mineral sequestration of carbon dioxide, Geochim. Cosmochim. Acta 75 (2011) 5510–5525.

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Please cite this article in press as: I. Rigopoulos et al., Enhancing the rate of ex situ mineral carbonation in dunites via ball milling, Advanced Powder Technology (2016), http://dx.doi.org/10.1016/j.apt.2016.01.007