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Ghassoul – Moroccan clay with excellent adsorption properties
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ScienceDirect Materials Today: Proceedings 5 (2018) S78–S87
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NanoOstrava_2017
Ghassoul – Moroccan clay with excellent adsorption properties Jonáš Tokarský* Nanotechnology centre, VŠB - Technical University of Ostrava, 17. listopadu 15, Ostrava - Poruba 708 33, Czech Republic
Abstract Ghassoul, Mg-rich clay consisting mainly of stevensite and containing also additional minerals (sepiolite, quartz, dolomite, gypsum, celestine) is mined in Morocco where the only known deposit in the world is located in Moulouya Valley in Fès-Meknès region. Ghassoul, formed by diagenetic transformation of dolomite in Tertiary fresh-water or brackish-water lacustrine environment, was used for centuries as a natural soap and shampoo. In 1884, the very first scientific report about this material was published by A. A. Damour who provided information about chemical composition of ghassoul. Over the next one hundred years, this material became a subject of geologists' interest until 1998 when the first study focused on practical applications of ghassoul was reported. Between 1998 and 2016, twenty-eight studies were published, twenty-four of which were focused on practical use. According to the studies, the ghassoul can be successfully used as an adsorbent of heavy metals and organic compounds, as a precursor of cordierite ceramics, and, recently, also as a component of functional composites for catalysis or photocatalysis. However, the greatest interest is still in adsorption. This review article is divided into three main sections. The first one provides information about chemical composition, crystallochemical formula, cation exchange capacity, specific surface area, and density of ghassoul. The second and the third section contain data obtained by various authors from experiments on adsorption of heavy metals (As, Cd, Cr, Cu, Hg, Mn, Pb, and Zn) or organic compounds (basic yellow 87, methyl violet, methylene blue, orange G, rhodamine B, metalaxyl, and tricyclazole), respectively. Dose of ghassoul, initial concentration of adsorbate, adsorption capacity, contact time, temperature, and pH are listed in two tables (the first one for heavy metals, the second one for organic compounds). Some of the articles contain tables comparing the adsorption capacity of ghassoul with adsorption capacities of other materials reported elsewhere. Comments on the comparisons are also provided in this article and it is shown that only few materials can compete with adorption capacity of ghassoul. © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of NanoOstrava2017. Keywords: Ghassoul; clay; stevensite; adsorption; dye; heavy metal.
* Corresponding author. Tel.: +420-597-321-519; fax: +420-597-321-546. E-mail address: [email protected] 2214-7853 © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of NanoOstrava2017.
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Nomenclature BY CTAB GHA MB MET MV OG RhB TRI
basic yellow 87 cetyltrimethylammonium bromide ghassoul methylene blue metalaxyl methyl violet orange G rhodamine B tricyclazole
1. Introduction Since ancient times, unusual clay with very good cleaning properties has been known in Morocco. The clay was used as a natural soap and shampoo not only by local inhabitants but also by other nations in the Mediterranean region, and nowadays is still being sold for this purpose. Its name “ghassoul” (or “rhassoul”) comes from the Arabic verb “rhassala” which means “to wash”. Ghassoul (GHA) is mined in the only known deposit in the world, Jbel Rhassoul (N 32°58'26.1"; W 4°19'57.6"), a mountain in Moulouya Valley located in Boulemane Province in FèsMeknès region, Morocco. The very first scientific report about the existence and chemical composition of GHA was written in 1843 by A. A. Damour [1] and ninety three years passed until the second report by Frey et al. appeared in 1936 [2]. Until the year 2016, several dozen articles on GHA were published [1-41] and their overview can be found in Fig. 1.
Fig. 1. Chronological overview of articles on GHA. Articles are divided into four groups according to the content. Corresponding reference to each article can be found at the top of the figure.
Two main periods (before and after the year 1998) are clearly recognizable. All articles starting with Damour’s report until the year 1997 [1-13] and four articles published later [15,23,24,37] were focused on explaining the origin of GHA and its phase composition. The initial opinion that the deposit was formed in evaporitic gypsum-rich environment was denied by later research revealing the origin of GHA in diagenetic transformation of dolomite in Tertiary fresh-water or brackish-water lacustrine environment. The deposit consists of two distinct stages, Formation Rouge (older stage) and Formation Intermédiaire (younger stage) [10-12,15] Although the main component of GHA, mineral ghassoulite, was relatively early recognized as stevensite [6], this determination was not generally adopted.
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Ghassoulite was identified as hectorite [7] or saponite [8] and despite very informative article published by Chahi et al. in 1997 [12] some uncertainity persisted until 2008 when thorough study performed by Rhouta et al. [23] proved that GHA really consists mainly of stevensite, a trioctahedral Mg-rich smectite. Short bristled fibres observed along the peripheries of stevensite particles were identified with high probability as sepiolite [23] which was in good agreement with previously described crystallization of this mineral on stevensite substrate [8,10,12]. In addition to stevensite and sepiolite, smaller or larger amount of quartz, dolomite, gypsum, and celestine can be also found in GHA [12]. Presence of celestine (SrSO4) is interesting and indicates a rock-forming action of bacteria in lacustrine environment in which the GHA was formed [42]. This composition relates to the younger stage of the deposit, the Formation Intermédiaire. Stevensite is only present in this stage, whereas older Formation Rouge contains illite, chlorite, and palygorskite [10,12]. Number of studies published in this "geological period" (1843 - 1997; see Fig. 1) is larger, some of them were omitted. The reader can find them in the literature quoted in presented articles [1-13]. The second period observable in Fig. 1 (1998 – 2016) is characterized by studies focused on practical applications of GHA. According to the area of utilization, three groups of articles can be identified: (1) GHA as adsorbent of heavy metals [14,18-21,30,31,40] and organic compounds [17,22,26,27,29,31,33,34,41], (2) GHA as precursor of cordierite ceramics [16,25,32,35,38], and (3) GHA as component of functional composites for catalysis [28] or photocatalysis [36,39]. This review aims to provide a summary of the structure and properties of GHA and of the current knowledge about GHA as adsorbent of heavy metals and organic compounds. 2. Structure and properties To provide a comprehensive picture of the structure and properties of GHA, five essential informations have been extracted from the literature: chemical composition, crystallochemical formula, cation exchange capacity, specific surface area, and density. The chemical composition of GHA is provided in Table 1. Table 1. Chemical composition of GHA as reported in literature (in wt.%). References to source articles are provided in the top line of the table. LOI – loss on ignition, NP – not provided, * - values reported in [40]
Al2O3
[1,7]
[17]
[18,19, 20]
[21]
[23]
[24,25, 30,35]
[26,29]
[29]
[34]
[40,41]
[43]
1.20
2.06
2.24
2.10
3.44
1.31
4.48
2.87
1.47
4.43*
2.21
CaO
1.01
1.84
1.46
1.88
12.73
0.14
1.88
12.13
0.34
5.38*
2.21
FeO
NP
NP
NP
NP
NP
NP
NP
NP
NP
NP
0.22
Fe2O3
1.40
0.84
1.35
0.86
1.64
0.82
1.92
1.44
0.43
1.87*
0.89
K2O
0.52
2.84
0.73
2.90
0.98
0.32
1.05
0.85
NP
1.15
0.67
Li2O
NP
0.28
NP
0.29
NP
NP
NP
NP
NP
NP
NP
MgO
28.00
23.63
25.03
25.14
23.85
24.73
27.44
24.64
32.46
20.08*
25.75
MnO
NP
0.41
NP
0.42
NP
NP
NP
NP
NP
0.02
<0.01
Na2O
NP
1.52
0.51
0.53
1.13
1.35
0.17
1.12
NP
0.48*
0.53
P2O5
NP
NP
NP
NP
NP
NP
NP
NP
NP
0.05
0.02
SiO2
55.00
64.57
57.49
65.89
53.64
56.53
58.16
53.31
57.94
48.97*
53.38
SO3
NP
1.91
NP
NP
NP
NP
NP
NP
NP
3.29
NP
SrO
NP
NP
NP
NP
NP
NP
NP
NP
NP
1.19
NP
TiO2
NP
NP
NP
NP
NP
NP
NP
NP
NP
0.26
0.17
LOI
NP
NP
8.31
0.00
NP
12.25
19.5
NP
7.36
7.00*
18.00
One can see distinc dominance of SiO2 and MgO, also Fe(III) is commonly present, as well as elements of I.A and II.A groups. Data in the last column were obtained from web site of the company Société Du Ghassoul Et De Ses Derivés Sefrioui Sarl, the exclusive exporter of GHA [43]. Double occurence of [29] is due to two chemical
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analyses in the article (raw GHA and fine fraction). Composition of GHA reported in [26] is similar to the composition of fine fraction reported in [29], however, the LOI is provided only in [26] (see seventh column in Table 1). Articles by Bentahar et al. [40] and Azarkan et al. [41] provide similar data, however, only in [41] the full list can be found (see tenth column). Values reported in [40] are marked with asterisk. Although the chemical composition is reported in eighteen sources (Table 1), only six articles provide crystallochemical formula of GHA [5,7,15,18,19,23], and both information can be found only in four of them [7,18,19,23]. In the following list, each crystallochemical formula (calculated per unit cell, i.e., per O20) has the same structure: (interlayer cations)(octahedral sheets)(tetrahedral sheets)O20(OH)4. (Ca0.14K0.10)(Mg5.72Fe0.14)(Si7.52Al0.18)O20(OH)4 [5], (Ca0.04Na0.26K0.08Mg0.24)(Mg5.14Al0.16Fe2+0.12Li0.20)(Si7.96Al0.04)O20(OH,F)4 [7], (Ca0.04Na0.04K0.06)(Mg5.48Al0.14Fe3+0.08Li0.18)(Si7.90Al0.10)O20(OH)4 [15], (Na0.16K0.16)(Mg5.84Fe0.18)(Si7.56Al0.44)O20(OH)4·8H2O [18,19], (Na0.25K0.20)(Mg5.04Al0.37Fe0.20•0.21)(Si7.76Al0.24)O20(OH)4 [23] (• is vacancy). Comparison with crystallochemical formula of stevensite from other deposit (Paterson, New Jersey, USA): (Mg5.76Mn0.04Fe3+0.04)(Si8.00)O20(OH)4 [44] shows difference in tetrahedral sheets. In the case of GHA, small Al tetrahedral substitutions are present in GHA. Despite the statement in Velde (1995) [45] that tetrahedral Al substitutions cannot occur in stevensite, Rhouta et al. [23] clearly proved (using 27Al and 29Si MAS NMR and FTIR) that the main constituent of GHA is the stevensite, however, with little or no short-range ordering of Si caused by the Al substitutions. Rhouta et al. [23] also drew attention to tetrahedral and octahedral coordination of Al reported by Chahi et al. [15] with the note that this crystallochemical formula containing Al0.14 in octahedra and Al0.10 in tetrahedra was proposed without precise knowledge of the Al coordination. The same problem definitely occured also in case of crystallochemical formula reported by Faust et al. [7] (claiming that the main component of GHA is hectorite). Following values of cation exchange capacity (CEC) of GHA (in meq/100g) can be found in the literature: 75.1 [7,24], 75.8 [30], 76.5 [18-20], 79.0 [29,33], 83.0 [40]. Taking into account CEC values of pure stevensite (36.0 [44], 41.0 [46]), the CEC of GHA is noteworthy. Only in [41], the comparable value 36.0 meq/100 g for pure GHA is reported. Generally, modification of GHA leads to decrease in CEC values. Some values are still higher than those of pure stevensite: 49.0 (homoionic Na+-GHA [17]), 60.0 (homoionic Na+-GHA calcined at 800 °C [23]), some are not: 39.9 (Al-pillared GHA [30]), 16.9 (GHA treated with CTAB [30]). However, the “protonated GHA”, i.e., GHA treated with H2O2 and HNO3, and further calcined at 600 °C, reported by El Mouzdahir et al. [21] exhibited the value 79.0 meq/100g similar to those reported in [29,33]. Specific surface area (SSA) values of GHA (in m2/g) were reported as follows: 104 [34], 115 [38], 119 [40,41], 133 [24,25,30,35], 134 [18-20], 137 [29,33]. Protonated GHA prepared by El Mouzdahir et al. [21] exhibited the SSA value 148 m2/g. For homoionic Na+-GHA, the SSA value 150 m2/g can be found [27]. Very exceptional value 414 m2/g for homoionic Na+-GHA precalcined at 250 °C was reported by Elmchaouri and Mahboub [17]. Modification of this Na+-GHA by Al pillaring led to SSA 571 m2/g, while improvement of the pillaring process by preadsorption of diethylamine even led to further increase in SSA and the value 624 m2/g was obtained [17]. The values 414 and 571 m2/g are reported also in article written by these authors five years later [28]. All the abovementioned SSA values were determined by nitrogen adsorption using BET mehod. Determination of SSA from titration curve using MB was performed by Bouna et al. [27] with the result 279 m2/g. This strong increase in comparison with 150 m2/g (measured using BET) was explained by the intercalation of MB molecules into the interlayer space [27]. Surprisingly, the density ρGHA is not mentioned very often and only two values can be found in the literature: 2.240 g/cm3 [18,19,25] and 2.320 g/cm3 [38]. 3. Adsorption of heavy metals Adsorption properties of GHA, the basis of its excellent cleaning ability, began to be systematically studied in 1998 when Ryachi and Bancheikh described the use of GHA for removal of Cd(II) a Cu(II) from wastewater [14]. In
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the following years, seven other articles on the use of GHA for adsorption of heavy metals have been published [1821,30,31,40]. Data from all these studies are summarized in Table 2. Primacy of Ryachi and Bencheikh [14] in the field of adsorption is a positive feature of their article which otherwise suffers from a considerable lack of information (dose of GHA, concentration of ions, contact time, temperature, see Table 2). Adsorption capacity of GHA for Cd(II) can be found both in text and in graph displaying adsorption curves for pH range 2.0 – 5.0 (the higher pH, the better adsorption). While data reported in [14] clearly shows that the adsorption capacity at pH 5.0 is 12.2 mg/g, value 1.22 mg/g is mentioned two times in the text. Author thinks that the latter value comes from the typing error, and, therefore, the value from the graph is provided in Table 2. Nine years later, two adsorption studies by Benhammou et al. were published [18,19]. The first one [18] was very extensive, adsorption of Cd(II), Cu(II), Mn(II), Pb(II), and Zn(II) on GHA was studied and pH (1.5 - 7.0) was found to be the dominant parameter. Amount of adsorbed metals increased with increasing pH. This observation was explained by strong protonation of aluminol and silanol groups at low pH restricting the number of possible binding sites on GHA [18]. While gradual increase in adsorption was observed for Cd(II), Mn(II), and Pb(II) throughout the whole studied pH range, rapid increase was detected in the case of Cu(II) and Pb(II) at pH > 5.0. Adsorption efficiency increased 12.4 times, 16.1 times, 3.5 times, 5.2 times, and 2.7 times for Cu(II), Pb(II), Zn(II), Cd(II), and Mn(II), respectively, at pH 7.0 in comparison with pH 1.5 [18]. Further experiments were performed at constant pH = 4 and the results are summarized in Table 2. Although the contact time was 120 min, the equilibrium was reached much earlier in all cases: ~ 10 min for Mn(II) and Zn(II), ~ 15 min for Pb(II), ~ 40 min for Cu(II), and ~ 60 min for Cd(II) [18]. Table 2. Data from studies on adsorption capacity of GHA for various heavy metals. mGHA - dose of GHA, cion - initial ion concentration, qGHA - adsorption capacity of GHA, t - contact time, T - temperature, NP - not provided, RT - room temperature. ion
mGHA (g)
cion (mg/l)
qGHA (mg/g)
t (min)
T (°C)
pH
Ref.
Cu(II)
NP
NP
7.1
NP
NP
5
[14]
Cu(II)
NP
NP
2.9
NP
NP
4
[14]
Cu(II)
0.02
9.5
8.4
120
25
4
[18]
Cd(II)
NP
NP
12.2
NP
NP
5
[14]
Cd(II)
NP
NP
8.8
NP
NP
4
[14]
Cd(II)
0.02
16.9
11.4
120
25
4
[18]
Pb(II)
0.02
31.1
26.4
120
25
4
[18]
Zn(II)
0.02
9.8
4.8
120
25
4
[18]
Mn(II)
0.02
8.2
5.0
120
25
4
[18]
Hg(II)
NP
100.3
15.3 (24.7)
NP
25
3.5 (4)
[19]
Hg(II)
NP
100.3
31.5
180
25
4
[19]
Cr(VI)
NP
104
(1.6)
NP
25
(2-5)
[19]
Cr(VI)
NP
104
0.5 (1.4)
960
25
3 (3)
[19]
Cr(VI)
0.25
520
0.5
240
RT
3
[20]
Cr(VI)
0.25
520
0.4
240
RT
4
[20]
Cr(VI)
0.25
520
0.7
120
RT
3
[20]
As(V)
0.5
1.0
0.020
1440
RT
4
[40]
As(V)
0.5
1.0
0.035
1440
RT
7
[40]
As(V)
0.5
1.0
0.075
1440
RT
11
[40]
As(V)
NP
50
1.076
1440
RT
7
[40]
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The second article published in the same year by Benhammou et al. [19] was focused on adsorption of Hg(II) and Cr(VI). In addition to pure GHA, Fe(II)-modified GHA was also used with aim to improve the adsorption of Cr via reduction of Cr(VI) to Cr(III). For description of the modification procedure the reader is referred to the article [19]. Values in round brackets in Table 2 ([19]) apply for Fe(II)-modified GHA. The first lines for Hg(II) a Cr(VI) in Table 2 contain data from test of effect of pH on adsorption capacity (range 1.5 - 7 was studied). Adsorption capacity increased with increasing pH up to 3.5 (4.0) and decreased at higher pH. The highest values reached are provided in Table 2. Data in the second lines were taken from the adsorption kinetics test. Here the pH values were fixed at 4 for Hg(II) and 3 for Cr(VI). GHA/solution ratio used for all adsorption experiments was 10.0 g/l and 1.0 g/l for Cr(VI) and Hg(II), respectively [19]. Since the volume of the solution is not provided in the article, the exact mass of GHA cannot be calculated. The ratios probably apply both for GHA and Fe-modified GHA, however, it is not explicitly stated in the article [19]. In 2007, Benhammou et al. reported study focused on adsorption of Cr(VI) on three various substrates: raw GHA, Al-pillared GHA, and organomodified GHA treated with CTA. For description of the pillaring and treating with CTA the reader is referred to the article [20]. Data from test of effect of pH (1.5 – 6) on adsorption capacity are summarized in the first two lines in Table 2. Adsorption capacity of pure GHA varied only slightly in the studied pH range (average value 0.5±0.1 mg/g was measured). On the other hand, Al-pillared GHA and CTA-GHA showed significantly higher adsorption capacities and different behavior in dependence on pH. Al-pillared GHA exhibited increase (0.5 mg/g at pH 1.5) up to pH 3.5 (maximum value 4.2 mg/g) followed by gradual decrease (3.5 mg/g at pH 6). Adsorption capacity of CTA-GHA was very high in comparison with pure GHA and Al-pillared GHA. The lowest value was measured at pH 1.5 (2.3 mg/g). Then a sharp increase was recorded (9.7 mg/g at pH 2) and, further, slight increase up to pH 6 (11.2 mg/g) was observed [20]. Data in the third line (Table 2, Ref. [20]) were obtained from adsorption isotherms. Similarly to the previous case, the adsorption capacity of pure GHA was significantly lower than for Al-pillared GHA and CTA-GHA for which 3.9 mg/g and 10.2 mg/g, respectively, was calculated using Dubinin–Kaganer–Radushkevich model. Equilibrium was reached within 30 min for all substrates. This article [20] contains a table with adsorption capacities of various materials reported elsewhere. Unfortunately, several errors can be found in the table, e.g., reported CTA-KLT with adsorption capacity 13 mmol/kg (Ref. 15 in [20]) is actually HDTMA-KLT with adsorption capacity 30 mmol/kg, CTA-modified montmorillonite, sepiolite, and palygorskite (Refs. 18 and 25 in [20]) are actually HDTMA-modified montmorillonite, sepiolite, and palygorskite, etc. Nonetheless, the table in [20] clearly shows that adsorption capacity of modified GHA is higher than seven of the eight listed materials. Only the HDTMA-modified montmorillonite (incorrectly denoted as CTAmontmorillonite) exhibits higher adsorption capacity (340 mg/g) [20]. El Mouzdahir et al. [21] studied the adsorption of Cd(II) and Pb(II) on two modified GHA samples. The first one, called “protonated GHA”, was prepared by treating the pure GHA with H2O2 and HNO3 followed by calcination at 600 °C. The second one, called “surface modified GHA”, was prepared by treating the protonated GHA with HNO3 (concentrations 0.5, 1.0, 1.5, 2.0, 3.0 mol/l). For description of the preparation processes the reader is referred to the article [21]. Since Table 2 contains only data for pure GHA, the adsorption capacities of these two modified GHAs are given here in the following text. Effect of pH (1.5 - 8) on the adsorption capacity was studied only for protonated GHA. Dose of adsorbent was 0.1 g, initial concentration of Cd(II) or Pb(II) was 0.5 mmol/l, temperature 20 °C, contact time 60 min. Adsorption capacity increased continually from pH 1.5 (adsorbed amounts of Cd(II) and Pb(II) were 7.5 mg/g and 38 mg/g, respectively) up to pH 7 (adsorbed amounts of Cd(II) and Pb(II) were 69.3 mg/g and 177.7 mg/g respectively). Adsorption capacity at pH 8 was similar to pH 7 [21]. Effect of contact time and initial concentration of ions was studied for both protonated GHA and surface modified GHA [21] under similar conditions. Only the contact time was longer (120 min) and pH was kept constant (6). For protonated GHA, adsorbed amounts of Cd(II) and Pb(II) were 45.6 mg/g and 114 mg/g, respectively. Graph provided in [21] showing curves for surface modified GHA also contains curves for protonated GHA because of comparison. Slightly lower values were obtained from these curves measured under the same conditions: 41 mg/g and 104 mg/g for Cd(II) and Pb(II), respectively [21]. However, the most important information is that for all HNO3 concentration used the surface modified GHA exhibited higher adsorption capacity in comparison with protonated GHA, and the higher HNO3 concentration, the higher adsorption capacity (although the difference between samples treated with 2.0 mol/l and 3.0 mol/l HNO3 was very small). Adsorption capacities for samples treated with 3.0 mol/l HNO3 were 107.2
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mg/g and 205.3 mg/g for Cd(II) and Pb(II), respectively [21]. El Mouzdahir et al. concluded that adsorption capacity of surface modified GHA is higher than of protonated GHA and that tha adsorption capacity of both samples was higher for Pb(II). In addition, this article [21] contains a table with adsorption capacities of various materials reported elsewhere. Only one material, prawn chitin, exhibits higher adsorption capacity (280 mg of Pb(II) per 1 g of the chitin) than both modified GHAs reported in this article [21]. Article published by Benhammou et al. in 2011 [30] contains similar information as their previous article published in 2007 [20]. The study on Cr(VI) adsorption on raw GHA, Al-pillared GHA, and organomodified GHA treated with CTA is repeated here (i.e., 0.7 mg/g, 3.9 mg/g, and 10.2 mg/g for pure GHA, Al-pillared GHA, and CTA-GHA, respectively). This article [30] was extended with preparation and characterization of modified GHA using various Al/GHA and CTA/GHA ratios, X-ray diffraction, infrared, and thermogravimetry analyses. Similarly to article [20], the table containing adsorption capacities of various materials reported elsewhere is included [30]. Unlike table in [20], correct surfactant HDTMA is reported for clays listed in this article [30], however, the table in [30] also contains several errors, e.g., Ref. 26 is actually Ref. 11, etc. Only two of eight listed materials can compete with CTA-GHA: HDTMA-modified montmorillonite (10.2 mg/g) and HDTMA-modified bentonite (18.2 mg/g) [30]. On the other hand, all eight materials exhibit higher adsorption capacity than pure GHA [30]. In 2012, El Fadeli et al. [31] reported interesting study on washing procedure applied to hair samples prior to analysis of trace elements. The analysis is used for evaluation of chronic intoxication of human body. However, it is necessary to remove the external contamination in order to assure that only elements incorporated in hair (and, therefore, reflecting the accumulation in organism) are recorded. Cu(II), Mn(II), Pb(II), and Zn(II) were monitored in this study [31]. Commonly used washing procedure (ultrasonication in 10% nitric acid) is very efficient but also very aggresive, affecting the integrity of hair. Fadeli et al. found that use of GHA in combination with this chemical procedure increases efficiency of the washing procedure and leads to less damage to surface of hair [31]. The latest article focused on adsorption of heavy metals on GHA was published by Bentahar et al. in 2016 [40]. Adsorption of As(V) was studied in pH range 2 - 12. Although the adsorbed amount of As(V) increased with increasing pH, it remained still very low, i.e., tenths of μg per 1 g of GHA (Table 2). However, the adsorption capacity calculated from the Langmuir isotherm reached 1.076 mg/g (Table 2). This article [40] also contains a table comparing the adsorption capacity of GHA with adsorption capacities of other materials reported elsewhere. Only pillared goethite and amorphous iron hydroxide, i.e., two of eleven listed materials, exhibits higher adsorption capacity for As(V) than GHA: 4 mg/g and 7 mg/g, respectively [40]. 4. Adsorption of organic compounds The article Effects of preadsorption of organic amine on Al-PILCs structures by Elmchaouri and Mahboub (2005) [17] is an exception from the studies summarized in Table 3. Its goal is not to study the adsorption as such but to use adsorption to improve the pillaring process in order to obtain more efficient GHA-based adsorbent with high SSA. Nevertheless, it is the very first report on some adsorption and GHA. According to authors, this improvement is highly valuable for selective adsorption and catalysis. The undisputed success of this work was discussed in section 2. Structure and properties. Since this article [17] does not fit elsewhere, it is included in this chapter, however, for the reasons mentioned above it is not included in Table 3. Briefly: homogeneity of Al-pillars distribution can be controlled by preadsorption (actually the intercalation) of diethylamine. Positive influence on the face-to-face stacking of the layers (resulting in more regularly ordered Al-pillared GHA) was observed. SSA 624 m2/g was obtained [17]. In 2007, the first study focused exclusively on adsorption of organic compound on GHA was published by El Mouzdahir et al. [22]. Authors studied the adsorption of methylene blue (MB) on GHA treated with HNO3, H2O2, and calcined at 600 °C (see also [21]). Such treated GHA was subsequently converted into homoionic Na-GHA [22]. For description of the modification procedures the reader is referred to the article [22].Since Table 3 contains only data for pure GHA, the adsorption capacity of the Na-GHA is given here. Adsorption experiment was carried out under following conditions: 0.1 g of the adsorbent in 50 ml of MB solution, contact time 120 min, temperature 27 °C, pH 7.2 [22]. For initial MB concentrations 10, 300, 500, 750, and 1000 mg/l the adsorption capacities 5.0, 109.6, 131.9, 133.9, and 134.5 mg/g, respectively, were measured. Kinetics study revealed that equilibrium is achieved within 5 min. This article [22] also contains a table comparing the adsorption capacity of GHA with four
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other materials reported elsewhere. Only activated carbon exhibits higher adsorption capacity (521 mg/g) than GHA [22]. Also the study published three years later by Elass et al. [26] was focused on MB adsorption on GHA. Effects of initial MB concentration, contact time, pH, and temperature on adsorption capacity of GHA were investigated. No significant influence of pH (3 - 11) and temperature (25 - 55 °C) on the adsorption was found [26]. Adsorption of MB on GHA (0.2 g in 100 ml of MB solution) was found to be dependent on the MB initial concentration (studied in the range 100 - 2000 mg/l). Strong increase in adsorption capacity was detected up to 450 mg/l for which the maximum adsorption 300 mg/g was reached. In the range 500 - 2000 mg/l, the value 300 mg/g remained nearly unchanged [26]. Adsorption in dependence on contact time was studied for MB initial concetrations 100 - 600 mg/l (Table 3). Equilibrium was reached within 30 min (initial concentrations 100 - 250 mg/l) or 120 min (initial concentrations 300 - 600 mg/l). Comparison of the adsorption capacity of GHA with other materials reported elsewhere is provided in this article and only one of ten listed materials (activated carbon from olive stones) exhibits slightly higher adsorption capacity (303 mg/g ) than GHA [26]. Table 3. Data from studies on adsorption capacity of GHA for various organic compounds. mGHA - dose of GHA, coc initial concentration of organic compound, qGHA - adsorption capacity of GHA, t - contact time, T - temperature, NP - not provided, RT - room temperature. organic compound
mGHA (g)
coc (mg/l)
qGHA (mg/g)
t (min)
T (°C)
pH
Ref.
MB MB MV MV MV MV MV RhB RhB RhB RhB BY BY BY BY BY MET MET TRI TRI
1 1 0.05 0.05 0.5 0.5 0.05 0.1 0.5 0.5 0.1 0.05, 0.1, 0.4 0.05 0.05 0.05 0.05 0.1 0.1 0.1 0.1
100, 300 400,500,600 500 500 500 200, 1000 1000 500 200, 400 600, 800 500 1000 1000 250 500 1000 0.5, 1, 2 3, 4, 5 0.5, 1, 2 3, 4, 5
100, 270 290-300 424, 451 456, 482 460, 494 199, 629 625 300, 363 186, 302 371, 391 332, 357, 374 535, 491, 125 380, 466, 517, 521 225, 245 399, 444 487 0.08, 0.17, 0.35 0.54, 0.73, 0.92 0.07, 0.13, 0.26 0.38, 0.50, 0.61
60-210 60-210 120 120 120 180 120 90 90 90 90 NP NP 10, 60 10, 60 10, 60 240 240 240 240
25 25 25 25 25, 55 25 25 25 25 25 25, 35, 45 NP NP NP NP NP 20 20 20 20
7 7 3, 5 7, 10 7.5 7.5 7.5 3, 10 7 7 7 NP 2, 4, 7, 11 NP NP NP NP NP NP NP
[26] [26] [29] [29] [29] [29] [29] [33] [33] [33] [33] [34] [34] [34] [34] [34] [41] [41] [41] [41]
Bouna et al. [27] studied adsorption of MB and orange G (OG) on homoionic Na-GHA. Since Table 3 contains only data for pure GHA, the adsorption capacity of the Na-GHA is given here. Adsorption test was performed at 25 °C, contact time is not provided [27]. In the case of cationic dye MB, the adsorption capacities 202 mg/g, 221 mg/g, 238 mg/g, and 240 mg/g were measured for pH 2, 9, 11, and 12, respectively. As would be expected, the adsorption of the anionic dye OG was negligible [27]. In 2011, Elass et al. [29] focused their attention to adsorption of methyl violet (MV). Similarly to their previous study on MB adsorption [26], effects of initial MV concentration, contact time, pH, and temperature on adsorption capacity of GHA were investigated [29]. Data are listed in Table 3. This article [29] also contains a table comparing the adsorption capacity of GHA with other materials reported elsewhere. None of the eight listed materials exhibits adsorption capacity comparable to GHA. The closest value (500 mg/g) is exhibited by activated carbon prepared from the common reed (Phragmites australis) [29]. The third study published by Elass et al. was focused on adsorption of rhodamine B (RhB) [33]. Similarly to their previous studies [26,29], effects of initial RhB concentration, contact time, pH, and temperature on adsorption capacity of GHA were investigated [33] and data are listed in Table 3. Maximum sorption capacity of GHA for RhB determined from Langmuir isotherm was 448 mg/g at 298 K [33]. No table comparing the adsorption capacity of
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GHA with other materials can be found in this article [33]. However, comparison provided in Khan et al. [47] shows than none of materials listed in [47] can compete with GHA. In 2013, Ajbary et al. [34] tested GHA as an adsorbent of another cationic dye, basic yellow 87 (BY). Effects of initial BY concentration, GHA dose, contact time, and pH on adsorption capacity of GHA were investigated [34] and data are listed in Table 3, however, without some important information omitted in the article (e.g., temperature, contact time for pH tests or pH in tests on influence of GHA dose). Decrease in adsorption capacity in dependence on increase in GHA dose can be explained by strong agglomeration of GHA particles at higher GHA concentration. Adsorption capacity of GHA for BY determined from Langmuir isotherm was 506 mg/g [34]. According to Ajbary et al., this value is significantly higher than adsorption capacities for BY onto other comparable materials, i.e., zeolites and clay minerals [34]. Azarkan et al. moved the area of interest from dyes to other type of organic compounds and the adsorption of two fungicides, metalaxyl (MET) and tricyclazole (TRI), on four different natural Northern Moroccan clays was investigated [41]. In addition to GHA, white bentonite from Zeghanghane in Nador Province in Oriental region, clay from the Dchiriyine in Tétouan Province in Tanger-Tétouan-Al Hoceima region, and clay from Targuist in Al Hoceima Province in Tanger-Tétouan-Al Hoceima region were tested. Data obtained from experiment focused on GHA are listed in Table 3. Significantly lower adsorption of these molecules compared to dyes (see Table 3) is due to their non-ionic character. Table 3 also shows that adsorption rate of MET is higher than adsorption rate of TRI. According to Azarkan et al. [41], this finding can be ascribed to polar carbonyl groups with strong acceptors of hydrogen bonds in the case of MET, while TRI has no polar functional groups. 5. Conclusions Important data on properties of GHA, a stevensite-rich clay from Morocco (i.e., chemical composition, crystallochemical formula, cation exchange capacity, specific surface area, and density), and data extracted from articles focused on study of adsorption of As(V), Cd(II), Cr(VI), Cu(II), Hg(II), Mn(II), Pb (II), Zn(II), basic yellow 87, methyl violet, methylene blue, orange G, rhodamine B, metalaxyl, and tricyclazole (i.e., dose of ghassoul, initial concentration of adsorbate, adsorption capacity of ghassoul, contact time, temperature, and pH) were listed in this article. Due to the negative charge of stevensite layers, GHA is suitable adsorbent for positively charged ions and molecules but it can be used also for adsorption of non-ionic organic compounds. Adsorption capacity of GHA is very high and – which is very important – tunable by various modifications (intercalation of Fe(II), CTA, or diethylamine, surface modification using CTA, H2O2, or HNO3, calcination at 600 °C, Al-pillaring). In the case of organic compounds, only activated carbon can compete with GHA. This is excellent result for natural clay. In the case of heavy metals, several other materials exhibited higher adsorption capacity (organomodified montmorillonite or bentonite, pillared goethite, amorphous iron hydroxide, and prawn chitin), but the adsorption capacity of GHA still exceeds most of the commonly used adsorbents. High specific surface area (104-137 m2/g for pure GHA, 148624 m2/g for modified GHA) and almost double the exchange capacity compared to pure stevensite from different deposits (75-83 meq per 100 g of pure GHA) are other benefits that predispose this material to extensive use in green applications, especially the wastewater treatment. Author hopes that this article is informative and useful to readers and that it will stimulate further interest in this unique material. Acknowledgements Author thanks Abdelhakim Safadi for his information on ghassoul without which this article would never be written. Author also thanks all scientists who have devoted themselves to research on ghassoul. Funding: This work was supported by the Ministry of Education, Youth and Sports of Czech Republic, grant numbers SP2016/63, SP2017/65 and National Programme of Sustainability (NPU II) project “IT4Innovations excellence in science – LQ1602” References [1] A.A. Damour, Annales Phys. Chim. 8(2) (1843) 316-321. (in French) [2] R. Frey B. Yovanovitch, J. Burghelle, Serv.Geol. Mem. 38 (1936) 195. (in French)
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[3] G. Millot, Geol. App. et prospection miniere, Nancy 11 (1949) 199-201. (in French) [4] A.-P. Jeannette, XIX Internat. Geol. Cong. Regional Mon. Ser. 3, Maroc No. 1 (1952) 371-383. (in French) [5] G. Millot, Compt. Rend. Hebd. Séances Acad. Sci. 238(2) (1954) 257-259. (in French) [6] M. Fleischer, Am. Miner. 40 (1955) 137-138. [7] G.T. Faust, J.C. Hathaway, G. Millot, Am. Miner. 44 (1959) 342-370. [8] N. Trauth, Sci. Geol. Mémoires, 49 (1977) 195. (in French) [9] A. Chahi, F. Risarcher, M. Ais, P. Duringer, in: Y.K. Kharaka, A.S. Maest (Eds.), Proceedings of 7th International Symposium on WaterRock Interaction (WRI-7), Park City UT, USA, 1992, pp. 627-629. [10] A. Chahi, J. Duplay, J. Lucas, Clays Clay Miner. 41(4) (1993) 401-411. [11] P. Duringer, M. Ais, A. Chahi, A., Bull. Soc. Geol. Fr. 166(2) (1995) 169-179. (in French) [12] A. Chahi, B. Fritz, J. Duplay, F. Weber, J. Lucas, Clays Clay Miner. 45(3) (1997) 378-389. [13] M. Benammi, Geobios 30(5) (1997) 713-721. [14] K. Ryachi, A. Bencheikh, Ann. Chim. Sci. Mat. 23(1-2) (1998) 393-396. [15] A. Chahi, P. Duringer, M. Ais, M. Bouabdelli, F. Gauthier-Lafaye, B. Fritz, J. Sediment. Res. 69(5) (1999) 1123-1135. [16] L. Nibou, A. Benhammou, A. Yaacoubi, J.P. Bonnet, B. Tanouti, Ann. Chim. Sci. Mat. 28(4) (2003) 83-90. (in French) [17] A. Elmchaouri, R. Mahboub, Colloids Surf. A 259(1-3) (2005) 135-141. [18] A. Benhammou, A. Yaacoubi, L. Nibou, B. Tanouti, J. Colloid Interface Sci. 282(2) (2005) 320-326. [19] A. Benhammou, A. Yaacoubi, L. Nibou, B. Tanouti, J. Hazard. Mater. 117(2-3) (2005) 243-249. [20] A. Benhammou, A. Yaacoubi, L. Nibou, B. Tanouti, J. Hazard. Mater. 140 (2007) 104-109. [21] Y. El Mouzdahir, A. Elmchaouri, R. Mahboub, A. ElAnssari, A. Gil, S.A. Korili, M.A. Vicente, Appl. Clay Sci. 35 (2007) 47-58. [22] Y. El Mouzdahir, A. Elmchaouri, R. Mahboub, A. Gil, S.A. Korili, J. Chem. Eng. Data 52 (2007) 1621-1625. [23] B. Rhouta, H. Kaddami, J. Elbarqy, M. Amjoud, L. Daoudi, F. Maury, F. Senocq, A. Maazouz, J.F. Gerard, Clay Miner. 43 (2008) 393-403. [24] A. Benhammou, B. Tanouti, L. Nibou, A. Yaacoubi, J.-P. Bonnet, Clays Clay Miner. 57(2) (2009) 264-270. [25] R. Bejjaoui, A. Benhammou, L. Nibou, B. Tanouti, J.-P. Bonnet, A. Yaacoubi, A. Ammar, Appl. Clay Sci. 49(3) (2010) 336-340. [26] K. Elass, A. Laachach, A. Alaoui, M. Azzi, Appl. Ecol. Environ. Res. 8(2) (2010) 153-163. [27] L. Bouna, B. Rhouta, M. Amjoud, A. Jada, F. Maury, L. Daoudi, F. Senocq, Appl. Clay Sci. 48 (2010) 527-530. [28] R. Azzallou, R. Mamouni, Y. Riadi, M. El Haddad, Y. El Mouzdahir, R. Mahboub, A. Elmchaouri, S. Lazar, G. Guillaumet, Rev. Chim. 61(12) (2010) 1155-1157. [29] K. Elass, A. Laachach, A. Alaoui, M. Azzi, Appl. Clay Sci. 54 (1) (2011) 90-96. [30] A. Benhammou, A. Yaacoubi, L. Nibou, J.-P. Bonnet, B. Tanouti, Environ. Technol. 32(4) (2011) 363-372. [31] S. El Fadeli, M. Chaik, A. Pineau, N. Lekouch, A. Sedki, Trace Elem. Electrol. 29(1) (2012) 22-27. [32] A. Benhammou, Y. El Hafiane, L. Nibou, A. Yaacoubi, J. Soro, A. Smith, J.-P. Bonnet, B. Tanouti, Ceram. Int. 39(1) (2013) 21-27. [33] K. Elass, A. Laachach, M. Azzi, Global Nest J. 15(4) (2013) 542-550. [34] M. Ajbary, A. Santos, V. Morales-Flórez, L. Esquivias, Appl. Clay Sci. 80-81 (2013) 46-51. [35] A. Benhammou, Y. ElHafiane, A. Abourriche, Y. Abouliatim, L. Nibou, A. Yaacoubi, N. Tessier-Doyen, A. Smith, B.Tanouti, Ceram. Int. 40 (2014) 8937-8944. [36] L. Bouna, B. Rhouta, F. Maury, A. Jada, F. Senocq, M.-C. Lafont, Clay Miner. 49 (2014) 417-428. [37] M. Thiry, A. Milnes, M. Ben Brahim, J. Geol. Soc. 172(1) (2015) 125-137. [38] G.L. Lecomte-Nana, Y. El Hafiane, A. Badaz, N. Tessier-Doyen, Y. Abouliatim, A. Smith, L. Nibou, B. Tanouti, J. Therm. Anal. Calorim. 122 (2015) 1245–1255. [39] J. Tokarský, K. Mamulová Kutláková, in: Proceedings of 7th International Conference on Nanomaterials - Research and Application (NANOCON 2015), Brno, Czech Republic, 2015, pp. 196-201. [40] Y. Bentahar, C. Hurel, K. Draoui, S. Khairoun, N. Marmier, Appl. Clay Sci. 119 (2016) 385-392. [41] S. Azarkan, A. Peñ, K. Draoui, C.I. Sainz-Díaz, Appl. Clay Sci. 123 (2016) 37-46. [42] D.M. Singer, E.M. Griffith, J.M. Senko, K. Fitzgibbon, I.H. Widanagamage, Chem. Geol. 440 (2016) 15-25. [43] Ghassoul: chemical analysis, www.ghassoul.org, 2004. (Online). Available: http://www.ghassoul.org/pages.php?lang=uk&ref=ghassoul_3_3. (Accessed: 12-Jun-2017). [44] G.T. Faust, K.J. Murata, Am. Mineral. 38 (1953) 973-987. [45] B. Velde, Origin and Mineralogy of Clays: Clays and the Environment, first. ed., Springer-Verlag, Berlin, Heideberg, New York, (1995). [46] N. Takahashi, M. Tanaka, T. Satoh, T. Endo, M. Shimada, Microp. Mater. 9 (1997) 35-42. [47] T.A. Khan, S. Dahiya, I. Ali, Appl. Clay Sci. 69 (2012) 58-66.
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NanoOstrava_2017
Ghassoul – Moroccan clay with excellent adsorption properties Jonáš Tokarský* Nanotechnology centre, VŠB - Technical University of Ostrava, 17. listopadu 15, Ostrava - Poruba 708 33, Czech Republic
Abstract Ghassoul, Mg-rich clay consisting mainly of stevensite and containing also additional minerals (sepiolite, quartz, dolomite, gypsum, celestine) is mined in Morocco where the only known deposit in the world is located in Moulouya Valley in Fès-Meknès region. Ghassoul, formed by diagenetic transformation of dolomite in Tertiary fresh-water or brackish-water lacustrine environment, was used for centuries as a natural soap and shampoo. In 1884, the very first scientific report about this material was published by A. A. Damour who provided information about chemical composition of ghassoul. Over the next one hundred years, this material became a subject of geologists' interest until 1998 when the first study focused on practical applications of ghassoul was reported. Between 1998 and 2016, twenty-eight studies were published, twenty-four of which were focused on practical use. According to the studies, the ghassoul can be successfully used as an adsorbent of heavy metals and organic compounds, as a precursor of cordierite ceramics, and, recently, also as a component of functional composites for catalysis or photocatalysis. However, the greatest interest is still in adsorption. This review article is divided into three main sections. The first one provides information about chemical composition, crystallochemical formula, cation exchange capacity, specific surface area, and density of ghassoul. The second and the third section contain data obtained by various authors from experiments on adsorption of heavy metals (As, Cd, Cr, Cu, Hg, Mn, Pb, and Zn) or organic compounds (basic yellow 87, methyl violet, methylene blue, orange G, rhodamine B, metalaxyl, and tricyclazole), respectively. Dose of ghassoul, initial concentration of adsorbate, adsorption capacity, contact time, temperature, and pH are listed in two tables (the first one for heavy metals, the second one for organic compounds). Some of the articles contain tables comparing the adsorption capacity of ghassoul with adsorption capacities of other materials reported elsewhere. Comments on the comparisons are also provided in this article and it is shown that only few materials can compete with adorption capacity of ghassoul. © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of NanoOstrava2017. Keywords: Ghassoul; clay; stevensite; adsorption; dye; heavy metal.
* Corresponding author. Tel.: +420-597-321-519; fax: +420-597-321-546. E-mail address: [email protected] 2214-7853 © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of NanoOstrava2017.
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Nomenclature BY CTAB GHA MB MET MV OG RhB TRI
basic yellow 87 cetyltrimethylammonium bromide ghassoul methylene blue metalaxyl methyl violet orange G rhodamine B tricyclazole
1. Introduction Since ancient times, unusual clay with very good cleaning properties has been known in Morocco. The clay was used as a natural soap and shampoo not only by local inhabitants but also by other nations in the Mediterranean region, and nowadays is still being sold for this purpose. Its name “ghassoul” (or “rhassoul”) comes from the Arabic verb “rhassala” which means “to wash”. Ghassoul (GHA) is mined in the only known deposit in the world, Jbel Rhassoul (N 32°58'26.1"; W 4°19'57.6"), a mountain in Moulouya Valley located in Boulemane Province in FèsMeknès region, Morocco. The very first scientific report about the existence and chemical composition of GHA was written in 1843 by A. A. Damour [1] and ninety three years passed until the second report by Frey et al. appeared in 1936 [2]. Until the year 2016, several dozen articles on GHA were published [1-41] and their overview can be found in Fig. 1.
Fig. 1. Chronological overview of articles on GHA. Articles are divided into four groups according to the content. Corresponding reference to each article can be found at the top of the figure.
Two main periods (before and after the year 1998) are clearly recognizable. All articles starting with Damour’s report until the year 1997 [1-13] and four articles published later [15,23,24,37] were focused on explaining the origin of GHA and its phase composition. The initial opinion that the deposit was formed in evaporitic gypsum-rich environment was denied by later research revealing the origin of GHA in diagenetic transformation of dolomite in Tertiary fresh-water or brackish-water lacustrine environment. The deposit consists of two distinct stages, Formation Rouge (older stage) and Formation Intermédiaire (younger stage) [10-12,15] Although the main component of GHA, mineral ghassoulite, was relatively early recognized as stevensite [6], this determination was not generally adopted.
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Ghassoulite was identified as hectorite [7] or saponite [8] and despite very informative article published by Chahi et al. in 1997 [12] some uncertainity persisted until 2008 when thorough study performed by Rhouta et al. [23] proved that GHA really consists mainly of stevensite, a trioctahedral Mg-rich smectite. Short bristled fibres observed along the peripheries of stevensite particles were identified with high probability as sepiolite [23] which was in good agreement with previously described crystallization of this mineral on stevensite substrate [8,10,12]. In addition to stevensite and sepiolite, smaller or larger amount of quartz, dolomite, gypsum, and celestine can be also found in GHA [12]. Presence of celestine (SrSO4) is interesting and indicates a rock-forming action of bacteria in lacustrine environment in which the GHA was formed [42]. This composition relates to the younger stage of the deposit, the Formation Intermédiaire. Stevensite is only present in this stage, whereas older Formation Rouge contains illite, chlorite, and palygorskite [10,12]. Number of studies published in this "geological period" (1843 - 1997; see Fig. 1) is larger, some of them were omitted. The reader can find them in the literature quoted in presented articles [1-13]. The second period observable in Fig. 1 (1998 – 2016) is characterized by studies focused on practical applications of GHA. According to the area of utilization, three groups of articles can be identified: (1) GHA as adsorbent of heavy metals [14,18-21,30,31,40] and organic compounds [17,22,26,27,29,31,33,34,41], (2) GHA as precursor of cordierite ceramics [16,25,32,35,38], and (3) GHA as component of functional composites for catalysis [28] or photocatalysis [36,39]. This review aims to provide a summary of the structure and properties of GHA and of the current knowledge about GHA as adsorbent of heavy metals and organic compounds. 2. Structure and properties To provide a comprehensive picture of the structure and properties of GHA, five essential informations have been extracted from the literature: chemical composition, crystallochemical formula, cation exchange capacity, specific surface area, and density. The chemical composition of GHA is provided in Table 1. Table 1. Chemical composition of GHA as reported in literature (in wt.%). References to source articles are provided in the top line of the table. LOI – loss on ignition, NP – not provided, * - values reported in [40]
Al2O3
[1,7]
[17]
[18,19, 20]
[21]
[23]
[24,25, 30,35]
[26,29]
[29]
[34]
[40,41]
[43]
1.20
2.06
2.24
2.10
3.44
1.31
4.48
2.87
1.47
4.43*
2.21
CaO
1.01
1.84
1.46
1.88
12.73
0.14
1.88
12.13
0.34
5.38*
2.21
FeO
NP
NP
NP
NP
NP
NP
NP
NP
NP
NP
0.22
Fe2O3
1.40
0.84
1.35
0.86
1.64
0.82
1.92
1.44
0.43
1.87*
0.89
K2O
0.52
2.84
0.73
2.90
0.98
0.32
1.05
0.85
NP
1.15
0.67
Li2O
NP
0.28
NP
0.29
NP
NP
NP
NP
NP
NP
NP
MgO
28.00
23.63
25.03
25.14
23.85
24.73
27.44
24.64
32.46
20.08*
25.75
MnO
NP
0.41
NP
0.42
NP
NP
NP
NP
NP
0.02
<0.01
Na2O
NP
1.52
0.51
0.53
1.13
1.35
0.17
1.12
NP
0.48*
0.53
P2O5
NP
NP
NP
NP
NP
NP
NP
NP
NP
0.05
0.02
SiO2
55.00
64.57
57.49
65.89
53.64
56.53
58.16
53.31
57.94
48.97*
53.38
SO3
NP
1.91
NP
NP
NP
NP
NP
NP
NP
3.29
NP
SrO
NP
NP
NP
NP
NP
NP
NP
NP
NP
1.19
NP
TiO2
NP
NP
NP
NP
NP
NP
NP
NP
NP
0.26
0.17
LOI
NP
NP
8.31
0.00
NP
12.25
19.5
NP
7.36
7.00*
18.00
One can see distinc dominance of SiO2 and MgO, also Fe(III) is commonly present, as well as elements of I.A and II.A groups. Data in the last column were obtained from web site of the company Société Du Ghassoul Et De Ses Derivés Sefrioui Sarl, the exclusive exporter of GHA [43]. Double occurence of [29] is due to two chemical
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analyses in the article (raw GHA and fine fraction). Composition of GHA reported in [26] is similar to the composition of fine fraction reported in [29], however, the LOI is provided only in [26] (see seventh column in Table 1). Articles by Bentahar et al. [40] and Azarkan et al. [41] provide similar data, however, only in [41] the full list can be found (see tenth column). Values reported in [40] are marked with asterisk. Although the chemical composition is reported in eighteen sources (Table 1), only six articles provide crystallochemical formula of GHA [5,7,15,18,19,23], and both information can be found only in four of them [7,18,19,23]. In the following list, each crystallochemical formula (calculated per unit cell, i.e., per O20) has the same structure: (interlayer cations)(octahedral sheets)(tetrahedral sheets)O20(OH)4. (Ca0.14K0.10)(Mg5.72Fe0.14)(Si7.52Al0.18)O20(OH)4 [5], (Ca0.04Na0.26K0.08Mg0.24)(Mg5.14Al0.16Fe2+0.12Li0.20)(Si7.96Al0.04)O20(OH,F)4 [7], (Ca0.04Na0.04K0.06)(Mg5.48Al0.14Fe3+0.08Li0.18)(Si7.90Al0.10)O20(OH)4 [15], (Na0.16K0.16)(Mg5.84Fe0.18)(Si7.56Al0.44)O20(OH)4·8H2O [18,19], (Na0.25K0.20)(Mg5.04Al0.37Fe0.20•0.21)(Si7.76Al0.24)O20(OH)4 [23] (• is vacancy). Comparison with crystallochemical formula of stevensite from other deposit (Paterson, New Jersey, USA): (Mg5.76Mn0.04Fe3+0.04)(Si8.00)O20(OH)4 [44] shows difference in tetrahedral sheets. In the case of GHA, small Al tetrahedral substitutions are present in GHA. Despite the statement in Velde (1995) [45] that tetrahedral Al substitutions cannot occur in stevensite, Rhouta et al. [23] clearly proved (using 27Al and 29Si MAS NMR and FTIR) that the main constituent of GHA is the stevensite, however, with little or no short-range ordering of Si caused by the Al substitutions. Rhouta et al. [23] also drew attention to tetrahedral and octahedral coordination of Al reported by Chahi et al. [15] with the note that this crystallochemical formula containing Al0.14 in octahedra and Al0.10 in tetrahedra was proposed without precise knowledge of the Al coordination. The same problem definitely occured also in case of crystallochemical formula reported by Faust et al. [7] (claiming that the main component of GHA is hectorite). Following values of cation exchange capacity (CEC) of GHA (in meq/100g) can be found in the literature: 75.1 [7,24], 75.8 [30], 76.5 [18-20], 79.0 [29,33], 83.0 [40]. Taking into account CEC values of pure stevensite (36.0 [44], 41.0 [46]), the CEC of GHA is noteworthy. Only in [41], the comparable value 36.0 meq/100 g for pure GHA is reported. Generally, modification of GHA leads to decrease in CEC values. Some values are still higher than those of pure stevensite: 49.0 (homoionic Na+-GHA [17]), 60.0 (homoionic Na+-GHA calcined at 800 °C [23]), some are not: 39.9 (Al-pillared GHA [30]), 16.9 (GHA treated with CTAB [30]). However, the “protonated GHA”, i.e., GHA treated with H2O2 and HNO3, and further calcined at 600 °C, reported by El Mouzdahir et al. [21] exhibited the value 79.0 meq/100g similar to those reported in [29,33]. Specific surface area (SSA) values of GHA (in m2/g) were reported as follows: 104 [34], 115 [38], 119 [40,41], 133 [24,25,30,35], 134 [18-20], 137 [29,33]. Protonated GHA prepared by El Mouzdahir et al. [21] exhibited the SSA value 148 m2/g. For homoionic Na+-GHA, the SSA value 150 m2/g can be found [27]. Very exceptional value 414 m2/g for homoionic Na+-GHA precalcined at 250 °C was reported by Elmchaouri and Mahboub [17]. Modification of this Na+-GHA by Al pillaring led to SSA 571 m2/g, while improvement of the pillaring process by preadsorption of diethylamine even led to further increase in SSA and the value 624 m2/g was obtained [17]. The values 414 and 571 m2/g are reported also in article written by these authors five years later [28]. All the abovementioned SSA values were determined by nitrogen adsorption using BET mehod. Determination of SSA from titration curve using MB was performed by Bouna et al. [27] with the result 279 m2/g. This strong increase in comparison with 150 m2/g (measured using BET) was explained by the intercalation of MB molecules into the interlayer space [27]. Surprisingly, the density ρGHA is not mentioned very often and only two values can be found in the literature: 2.240 g/cm3 [18,19,25] and 2.320 g/cm3 [38]. 3. Adsorption of heavy metals Adsorption properties of GHA, the basis of its excellent cleaning ability, began to be systematically studied in 1998 when Ryachi and Bancheikh described the use of GHA for removal of Cd(II) a Cu(II) from wastewater [14]. In
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the following years, seven other articles on the use of GHA for adsorption of heavy metals have been published [1821,30,31,40]. Data from all these studies are summarized in Table 2. Primacy of Ryachi and Bencheikh [14] in the field of adsorption is a positive feature of their article which otherwise suffers from a considerable lack of information (dose of GHA, concentration of ions, contact time, temperature, see Table 2). Adsorption capacity of GHA for Cd(II) can be found both in text and in graph displaying adsorption curves for pH range 2.0 – 5.0 (the higher pH, the better adsorption). While data reported in [14] clearly shows that the adsorption capacity at pH 5.0 is 12.2 mg/g, value 1.22 mg/g is mentioned two times in the text. Author thinks that the latter value comes from the typing error, and, therefore, the value from the graph is provided in Table 2. Nine years later, two adsorption studies by Benhammou et al. were published [18,19]. The first one [18] was very extensive, adsorption of Cd(II), Cu(II), Mn(II), Pb(II), and Zn(II) on GHA was studied and pH (1.5 - 7.0) was found to be the dominant parameter. Amount of adsorbed metals increased with increasing pH. This observation was explained by strong protonation of aluminol and silanol groups at low pH restricting the number of possible binding sites on GHA [18]. While gradual increase in adsorption was observed for Cd(II), Mn(II), and Pb(II) throughout the whole studied pH range, rapid increase was detected in the case of Cu(II) and Pb(II) at pH > 5.0. Adsorption efficiency increased 12.4 times, 16.1 times, 3.5 times, 5.2 times, and 2.7 times for Cu(II), Pb(II), Zn(II), Cd(II), and Mn(II), respectively, at pH 7.0 in comparison with pH 1.5 [18]. Further experiments were performed at constant pH = 4 and the results are summarized in Table 2. Although the contact time was 120 min, the equilibrium was reached much earlier in all cases: ~ 10 min for Mn(II) and Zn(II), ~ 15 min for Pb(II), ~ 40 min for Cu(II), and ~ 60 min for Cd(II) [18]. Table 2. Data from studies on adsorption capacity of GHA for various heavy metals. mGHA - dose of GHA, cion - initial ion concentration, qGHA - adsorption capacity of GHA, t - contact time, T - temperature, NP - not provided, RT - room temperature. ion
mGHA (g)
cion (mg/l)
qGHA (mg/g)
t (min)
T (°C)
pH
Ref.
Cu(II)
NP
NP
7.1
NP
NP
5
[14]
Cu(II)
NP
NP
2.9
NP
NP
4
[14]
Cu(II)
0.02
9.5
8.4
120
25
4
[18]
Cd(II)
NP
NP
12.2
NP
NP
5
[14]
Cd(II)
NP
NP
8.8
NP
NP
4
[14]
Cd(II)
0.02
16.9
11.4
120
25
4
[18]
Pb(II)
0.02
31.1
26.4
120
25
4
[18]
Zn(II)
0.02
9.8
4.8
120
25
4
[18]
Mn(II)
0.02
8.2
5.0
120
25
4
[18]
Hg(II)
NP
100.3
15.3 (24.7)
NP
25
3.5 (4)
[19]
Hg(II)
NP
100.3
31.5
180
25
4
[19]
Cr(VI)
NP
104
(1.6)
NP
25
(2-5)
[19]
Cr(VI)
NP
104
0.5 (1.4)
960
25
3 (3)
[19]
Cr(VI)
0.25
520
0.5
240
RT
3
[20]
Cr(VI)
0.25
520
0.4
240
RT
4
[20]
Cr(VI)
0.25
520
0.7
120
RT
3
[20]
As(V)
0.5
1.0
0.020
1440
RT
4
[40]
As(V)
0.5
1.0
0.035
1440
RT
7
[40]
As(V)
0.5
1.0
0.075
1440
RT
11
[40]
As(V)
NP
50
1.076
1440
RT
7
[40]
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The second article published in the same year by Benhammou et al. [19] was focused on adsorption of Hg(II) and Cr(VI). In addition to pure GHA, Fe(II)-modified GHA was also used with aim to improve the adsorption of Cr via reduction of Cr(VI) to Cr(III). For description of the modification procedure the reader is referred to the article [19]. Values in round brackets in Table 2 ([19]) apply for Fe(II)-modified GHA. The first lines for Hg(II) a Cr(VI) in Table 2 contain data from test of effect of pH on adsorption capacity (range 1.5 - 7 was studied). Adsorption capacity increased with increasing pH up to 3.5 (4.0) and decreased at higher pH. The highest values reached are provided in Table 2. Data in the second lines were taken from the adsorption kinetics test. Here the pH values were fixed at 4 for Hg(II) and 3 for Cr(VI). GHA/solution ratio used for all adsorption experiments was 10.0 g/l and 1.0 g/l for Cr(VI) and Hg(II), respectively [19]. Since the volume of the solution is not provided in the article, the exact mass of GHA cannot be calculated. The ratios probably apply both for GHA and Fe-modified GHA, however, it is not explicitly stated in the article [19]. In 2007, Benhammou et al. reported study focused on adsorption of Cr(VI) on three various substrates: raw GHA, Al-pillared GHA, and organomodified GHA treated with CTA. For description of the pillaring and treating with CTA the reader is referred to the article [20]. Data from test of effect of pH (1.5 – 6) on adsorption capacity are summarized in the first two lines in Table 2. Adsorption capacity of pure GHA varied only slightly in the studied pH range (average value 0.5±0.1 mg/g was measured). On the other hand, Al-pillared GHA and CTA-GHA showed significantly higher adsorption capacities and different behavior in dependence on pH. Al-pillared GHA exhibited increase (0.5 mg/g at pH 1.5) up to pH 3.5 (maximum value 4.2 mg/g) followed by gradual decrease (3.5 mg/g at pH 6). Adsorption capacity of CTA-GHA was very high in comparison with pure GHA and Al-pillared GHA. The lowest value was measured at pH 1.5 (2.3 mg/g). Then a sharp increase was recorded (9.7 mg/g at pH 2) and, further, slight increase up to pH 6 (11.2 mg/g) was observed [20]. Data in the third line (Table 2, Ref. [20]) were obtained from adsorption isotherms. Similarly to the previous case, the adsorption capacity of pure GHA was significantly lower than for Al-pillared GHA and CTA-GHA for which 3.9 mg/g and 10.2 mg/g, respectively, was calculated using Dubinin–Kaganer–Radushkevich model. Equilibrium was reached within 30 min for all substrates. This article [20] contains a table with adsorption capacities of various materials reported elsewhere. Unfortunately, several errors can be found in the table, e.g., reported CTA-KLT with adsorption capacity 13 mmol/kg (Ref. 15 in [20]) is actually HDTMA-KLT with adsorption capacity 30 mmol/kg, CTA-modified montmorillonite, sepiolite, and palygorskite (Refs. 18 and 25 in [20]) are actually HDTMA-modified montmorillonite, sepiolite, and palygorskite, etc. Nonetheless, the table in [20] clearly shows that adsorption capacity of modified GHA is higher than seven of the eight listed materials. Only the HDTMA-modified montmorillonite (incorrectly denoted as CTAmontmorillonite) exhibits higher adsorption capacity (340 mg/g) [20]. El Mouzdahir et al. [21] studied the adsorption of Cd(II) and Pb(II) on two modified GHA samples. The first one, called “protonated GHA”, was prepared by treating the pure GHA with H2O2 and HNO3 followed by calcination at 600 °C. The second one, called “surface modified GHA”, was prepared by treating the protonated GHA with HNO3 (concentrations 0.5, 1.0, 1.5, 2.0, 3.0 mol/l). For description of the preparation processes the reader is referred to the article [21]. Since Table 2 contains only data for pure GHA, the adsorption capacities of these two modified GHAs are given here in the following text. Effect of pH (1.5 - 8) on the adsorption capacity was studied only for protonated GHA. Dose of adsorbent was 0.1 g, initial concentration of Cd(II) or Pb(II) was 0.5 mmol/l, temperature 20 °C, contact time 60 min. Adsorption capacity increased continually from pH 1.5 (adsorbed amounts of Cd(II) and Pb(II) were 7.5 mg/g and 38 mg/g, respectively) up to pH 7 (adsorbed amounts of Cd(II) and Pb(II) were 69.3 mg/g and 177.7 mg/g respectively). Adsorption capacity at pH 8 was similar to pH 7 [21]. Effect of contact time and initial concentration of ions was studied for both protonated GHA and surface modified GHA [21] under similar conditions. Only the contact time was longer (120 min) and pH was kept constant (6). For protonated GHA, adsorbed amounts of Cd(II) and Pb(II) were 45.6 mg/g and 114 mg/g, respectively. Graph provided in [21] showing curves for surface modified GHA also contains curves for protonated GHA because of comparison. Slightly lower values were obtained from these curves measured under the same conditions: 41 mg/g and 104 mg/g for Cd(II) and Pb(II), respectively [21]. However, the most important information is that for all HNO3 concentration used the surface modified GHA exhibited higher adsorption capacity in comparison with protonated GHA, and the higher HNO3 concentration, the higher adsorption capacity (although the difference between samples treated with 2.0 mol/l and 3.0 mol/l HNO3 was very small). Adsorption capacities for samples treated with 3.0 mol/l HNO3 were 107.2
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mg/g and 205.3 mg/g for Cd(II) and Pb(II), respectively [21]. El Mouzdahir et al. concluded that adsorption capacity of surface modified GHA is higher than of protonated GHA and that tha adsorption capacity of both samples was higher for Pb(II). In addition, this article [21] contains a table with adsorption capacities of various materials reported elsewhere. Only one material, prawn chitin, exhibits higher adsorption capacity (280 mg of Pb(II) per 1 g of the chitin) than both modified GHAs reported in this article [21]. Article published by Benhammou et al. in 2011 [30] contains similar information as their previous article published in 2007 [20]. The study on Cr(VI) adsorption on raw GHA, Al-pillared GHA, and organomodified GHA treated with CTA is repeated here (i.e., 0.7 mg/g, 3.9 mg/g, and 10.2 mg/g for pure GHA, Al-pillared GHA, and CTA-GHA, respectively). This article [30] was extended with preparation and characterization of modified GHA using various Al/GHA and CTA/GHA ratios, X-ray diffraction, infrared, and thermogravimetry analyses. Similarly to article [20], the table containing adsorption capacities of various materials reported elsewhere is included [30]. Unlike table in [20], correct surfactant HDTMA is reported for clays listed in this article [30], however, the table in [30] also contains several errors, e.g., Ref. 26 is actually Ref. 11, etc. Only two of eight listed materials can compete with CTA-GHA: HDTMA-modified montmorillonite (10.2 mg/g) and HDTMA-modified bentonite (18.2 mg/g) [30]. On the other hand, all eight materials exhibit higher adsorption capacity than pure GHA [30]. In 2012, El Fadeli et al. [31] reported interesting study on washing procedure applied to hair samples prior to analysis of trace elements. The analysis is used for evaluation of chronic intoxication of human body. However, it is necessary to remove the external contamination in order to assure that only elements incorporated in hair (and, therefore, reflecting the accumulation in organism) are recorded. Cu(II), Mn(II), Pb(II), and Zn(II) were monitored in this study [31]. Commonly used washing procedure (ultrasonication in 10% nitric acid) is very efficient but also very aggresive, affecting the integrity of hair. Fadeli et al. found that use of GHA in combination with this chemical procedure increases efficiency of the washing procedure and leads to less damage to surface of hair [31]. The latest article focused on adsorption of heavy metals on GHA was published by Bentahar et al. in 2016 [40]. Adsorption of As(V) was studied in pH range 2 - 12. Although the adsorbed amount of As(V) increased with increasing pH, it remained still very low, i.e., tenths of μg per 1 g of GHA (Table 2). However, the adsorption capacity calculated from the Langmuir isotherm reached 1.076 mg/g (Table 2). This article [40] also contains a table comparing the adsorption capacity of GHA with adsorption capacities of other materials reported elsewhere. Only pillared goethite and amorphous iron hydroxide, i.e., two of eleven listed materials, exhibits higher adsorption capacity for As(V) than GHA: 4 mg/g and 7 mg/g, respectively [40]. 4. Adsorption of organic compounds The article Effects of preadsorption of organic amine on Al-PILCs structures by Elmchaouri and Mahboub (2005) [17] is an exception from the studies summarized in Table 3. Its goal is not to study the adsorption as such but to use adsorption to improve the pillaring process in order to obtain more efficient GHA-based adsorbent with high SSA. Nevertheless, it is the very first report on some adsorption and GHA. According to authors, this improvement is highly valuable for selective adsorption and catalysis. The undisputed success of this work was discussed in section 2. Structure and properties. Since this article [17] does not fit elsewhere, it is included in this chapter, however, for the reasons mentioned above it is not included in Table 3. Briefly: homogeneity of Al-pillars distribution can be controlled by preadsorption (actually the intercalation) of diethylamine. Positive influence on the face-to-face stacking of the layers (resulting in more regularly ordered Al-pillared GHA) was observed. SSA 624 m2/g was obtained [17]. In 2007, the first study focused exclusively on adsorption of organic compound on GHA was published by El Mouzdahir et al. [22]. Authors studied the adsorption of methylene blue (MB) on GHA treated with HNO3, H2O2, and calcined at 600 °C (see also [21]). Such treated GHA was subsequently converted into homoionic Na-GHA [22]. For description of the modification procedures the reader is referred to the article [22].Since Table 3 contains only data for pure GHA, the adsorption capacity of the Na-GHA is given here. Adsorption experiment was carried out under following conditions: 0.1 g of the adsorbent in 50 ml of MB solution, contact time 120 min, temperature 27 °C, pH 7.2 [22]. For initial MB concentrations 10, 300, 500, 750, and 1000 mg/l the adsorption capacities 5.0, 109.6, 131.9, 133.9, and 134.5 mg/g, respectively, were measured. Kinetics study revealed that equilibrium is achieved within 5 min. This article [22] also contains a table comparing the adsorption capacity of GHA with four
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other materials reported elsewhere. Only activated carbon exhibits higher adsorption capacity (521 mg/g) than GHA [22]. Also the study published three years later by Elass et al. [26] was focused on MB adsorption on GHA. Effects of initial MB concentration, contact time, pH, and temperature on adsorption capacity of GHA were investigated. No significant influence of pH (3 - 11) and temperature (25 - 55 °C) on the adsorption was found [26]. Adsorption of MB on GHA (0.2 g in 100 ml of MB solution) was found to be dependent on the MB initial concentration (studied in the range 100 - 2000 mg/l). Strong increase in adsorption capacity was detected up to 450 mg/l for which the maximum adsorption 300 mg/g was reached. In the range 500 - 2000 mg/l, the value 300 mg/g remained nearly unchanged [26]. Adsorption in dependence on contact time was studied for MB initial concetrations 100 - 600 mg/l (Table 3). Equilibrium was reached within 30 min (initial concentrations 100 - 250 mg/l) or 120 min (initial concentrations 300 - 600 mg/l). Comparison of the adsorption capacity of GHA with other materials reported elsewhere is provided in this article and only one of ten listed materials (activated carbon from olive stones) exhibits slightly higher adsorption capacity (303 mg/g ) than GHA [26]. Table 3. Data from studies on adsorption capacity of GHA for various organic compounds. mGHA - dose of GHA, coc initial concentration of organic compound, qGHA - adsorption capacity of GHA, t - contact time, T - temperature, NP - not provided, RT - room temperature. organic compound
mGHA (g)
coc (mg/l)
qGHA (mg/g)
t (min)
T (°C)
pH
Ref.
MB MB MV MV MV MV MV RhB RhB RhB RhB BY BY BY BY BY MET MET TRI TRI
1 1 0.05 0.05 0.5 0.5 0.05 0.1 0.5 0.5 0.1 0.05, 0.1, 0.4 0.05 0.05 0.05 0.05 0.1 0.1 0.1 0.1
100, 300 400,500,600 500 500 500 200, 1000 1000 500 200, 400 600, 800 500 1000 1000 250 500 1000 0.5, 1, 2 3, 4, 5 0.5, 1, 2 3, 4, 5
100, 270 290-300 424, 451 456, 482 460, 494 199, 629 625 300, 363 186, 302 371, 391 332, 357, 374 535, 491, 125 380, 466, 517, 521 225, 245 399, 444 487 0.08, 0.17, 0.35 0.54, 0.73, 0.92 0.07, 0.13, 0.26 0.38, 0.50, 0.61
60-210 60-210 120 120 120 180 120 90 90 90 90 NP NP 10, 60 10, 60 10, 60 240 240 240 240
25 25 25 25 25, 55 25 25 25 25 25 25, 35, 45 NP NP NP NP NP 20 20 20 20
7 7 3, 5 7, 10 7.5 7.5 7.5 3, 10 7 7 7 NP 2, 4, 7, 11 NP NP NP NP NP NP NP
[26] [26] [29] [29] [29] [29] [29] [33] [33] [33] [33] [34] [34] [34] [34] [34] [41] [41] [41] [41]
Bouna et al. [27] studied adsorption of MB and orange G (OG) on homoionic Na-GHA. Since Table 3 contains only data for pure GHA, the adsorption capacity of the Na-GHA is given here. Adsorption test was performed at 25 °C, contact time is not provided [27]. In the case of cationic dye MB, the adsorption capacities 202 mg/g, 221 mg/g, 238 mg/g, and 240 mg/g were measured for pH 2, 9, 11, and 12, respectively. As would be expected, the adsorption of the anionic dye OG was negligible [27]. In 2011, Elass et al. [29] focused their attention to adsorption of methyl violet (MV). Similarly to their previous study on MB adsorption [26], effects of initial MV concentration, contact time, pH, and temperature on adsorption capacity of GHA were investigated [29]. Data are listed in Table 3. This article [29] also contains a table comparing the adsorption capacity of GHA with other materials reported elsewhere. None of the eight listed materials exhibits adsorption capacity comparable to GHA. The closest value (500 mg/g) is exhibited by activated carbon prepared from the common reed (Phragmites australis) [29]. The third study published by Elass et al. was focused on adsorption of rhodamine B (RhB) [33]. Similarly to their previous studies [26,29], effects of initial RhB concentration, contact time, pH, and temperature on adsorption capacity of GHA were investigated [33] and data are listed in Table 3. Maximum sorption capacity of GHA for RhB determined from Langmuir isotherm was 448 mg/g at 298 K [33]. No table comparing the adsorption capacity of
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GHA with other materials can be found in this article [33]. However, comparison provided in Khan et al. [47] shows than none of materials listed in [47] can compete with GHA. In 2013, Ajbary et al. [34] tested GHA as an adsorbent of another cationic dye, basic yellow 87 (BY). Effects of initial BY concentration, GHA dose, contact time, and pH on adsorption capacity of GHA were investigated [34] and data are listed in Table 3, however, without some important information omitted in the article (e.g., temperature, contact time for pH tests or pH in tests on influence of GHA dose). Decrease in adsorption capacity in dependence on increase in GHA dose can be explained by strong agglomeration of GHA particles at higher GHA concentration. Adsorption capacity of GHA for BY determined from Langmuir isotherm was 506 mg/g [34]. According to Ajbary et al., this value is significantly higher than adsorption capacities for BY onto other comparable materials, i.e., zeolites and clay minerals [34]. Azarkan et al. moved the area of interest from dyes to other type of organic compounds and the adsorption of two fungicides, metalaxyl (MET) and tricyclazole (TRI), on four different natural Northern Moroccan clays was investigated [41]. In addition to GHA, white bentonite from Zeghanghane in Nador Province in Oriental region, clay from the Dchiriyine in Tétouan Province in Tanger-Tétouan-Al Hoceima region, and clay from Targuist in Al Hoceima Province in Tanger-Tétouan-Al Hoceima region were tested. Data obtained from experiment focused on GHA are listed in Table 3. Significantly lower adsorption of these molecules compared to dyes (see Table 3) is due to their non-ionic character. Table 3 also shows that adsorption rate of MET is higher than adsorption rate of TRI. According to Azarkan et al. [41], this finding can be ascribed to polar carbonyl groups with strong acceptors of hydrogen bonds in the case of MET, while TRI has no polar functional groups. 5. Conclusions Important data on properties of GHA, a stevensite-rich clay from Morocco (i.e., chemical composition, crystallochemical formula, cation exchange capacity, specific surface area, and density), and data extracted from articles focused on study of adsorption of As(V), Cd(II), Cr(VI), Cu(II), Hg(II), Mn(II), Pb (II), Zn(II), basic yellow 87, methyl violet, methylene blue, orange G, rhodamine B, metalaxyl, and tricyclazole (i.e., dose of ghassoul, initial concentration of adsorbate, adsorption capacity of ghassoul, contact time, temperature, and pH) were listed in this article. Due to the negative charge of stevensite layers, GHA is suitable adsorbent for positively charged ions and molecules but it can be used also for adsorption of non-ionic organic compounds. Adsorption capacity of GHA is very high and – which is very important – tunable by various modifications (intercalation of Fe(II), CTA, or diethylamine, surface modification using CTA, H2O2, or HNO3, calcination at 600 °C, Al-pillaring). In the case of organic compounds, only activated carbon can compete with GHA. This is excellent result for natural clay. In the case of heavy metals, several other materials exhibited higher adsorption capacity (organomodified montmorillonite or bentonite, pillared goethite, amorphous iron hydroxide, and prawn chitin), but the adsorption capacity of GHA still exceeds most of the commonly used adsorbents. High specific surface area (104-137 m2/g for pure GHA, 148624 m2/g for modified GHA) and almost double the exchange capacity compared to pure stevensite from different deposits (75-83 meq per 100 g of pure GHA) are other benefits that predispose this material to extensive use in green applications, especially the wastewater treatment. Author hopes that this article is informative and useful to readers and that it will stimulate further interest in this unique material. Acknowledgements Author thanks Abdelhakim Safadi for his information on ghassoul without which this article would never be written. Author also thanks all scientists who have devoted themselves to research on ghassoul. Funding: This work was supported by the Ministry of Education, Youth and Sports of Czech Republic, grant numbers SP2016/63, SP2017/65 and National Programme of Sustainability (NPU II) project “IT4Innovations excellence in science – LQ1602” References [1] A.A. Damour, Annales Phys. Chim. 8(2) (1843) 316-321. (in French) [2] R. Frey B. Yovanovitch, J. Burghelle, Serv.Geol. Mem. 38 (1936) 195. (in French)
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