materials Article
Porous Materials from Thermally Activated Kaolinite: Preparation, Characterization and Application Jun Luo, Tao Jiang, Guanghui Li *, Zhiwei Peng, Mingjun Rao and Yuanbo Zhang School of Minerals Processing & Bioengineering, Central South University, Changsha 410083, China;
[email protected] (J.L.);
[email protected] (T.J.);
[email protected] (Z.P.);
[email protected] (M.R.);
[email protected] (Y.Z.) * Correspondence:
[email protected]; Tel.: +86-731-888-305-42 Academic Editor: Thomas Fiedler Received: 11 May 2017; Accepted: 8 June 2017; Published: 12 June 2017
Abstract: In the present study, porous alumina/silica materials were prepared by selective leaching of silicon/aluminum constituents from thermal-activated kaolinite in inorganic acid or alkali liquor. The correlations between the characteristics of the prepared porous materials and the dissolution properties of activated kaolinite were also investigated. The results show that the specific surface area (SSA) of porous alumina/silica increases with silica/alumina dissolution, but without marked change of the BJH pore size. Furthermore, change in pore volume is more dependent on activation temperature. The porous alumina and silica obtained from alkali leaching of kaolinite activated at 1150 ◦ C for 15 min and acid leaching of kaolinite activated at 850 ◦ C for 15 min are mesoporous, with SSAs, BJH pore sizes and pore volumes of 55.8 m2 /g and 280.3 m2 /g, 6.06 nm and 3.06 nm, 0.1455 mL/g and 0.1945 mL/g, respectively. According to the adsorption tests, porous alumina has superior adsorption capacities for Cu2+ , Pb2+ and Cd2+ compared with porous silica and activated carbon. The maximum capacities of porous alumina for Cu2+ , Pb2+ and Cd2+ are 134 mg/g, 183 mg/g and 195 mg/g, respectively, at 30 ◦ C. Keywords: porous material; kaolinite; thermoactivation; adsorption; heavy metal ions
1. Introduction Due to their high toxicity and degradation resistance, accumulated heavy metal pollutants cause serious damage in aquatic and soil systems, and in the human body [1–3]. The pollution of aquatic environments, especially caused by heavy metals, is a problem of worldwide concern. The heavy metal ions in wastewater can be adsorbed by using porous materials with a fluffy structure and special functional groups [4,5]. The main adsorbents include activated carbon and porous mineral materials [6–8]. Activated carbon has the advantages of simple preparation, high efficiency and recyclability, despite its exorbitant price and short lifespan [4,5,7]. The layered aluminosilicates with high specific surface area and superior ion exchange ability can be satisfactorily used as mineral adsorption materials [9–11]. Currently, many researchers have been focusing on the exploitation of porous materials from natural-layered clays for adsorption of heavy metal ions, such as Cu(II), Pb(II), Cd(II) and Cr(III), Cr(VI) etc. [9,11,12]. The development of pillared interlayer clay (PILC) is based on exchangeable and expandable layered clays [13,14]. The PILC materials with high pore volume, tunable pore size and ordered structure can be prepared by inserting specific ions or ion-clusters in the interlayer of layered clays [14,15]. They are widely used in the fields of catalysis, selective adsorption and environmental protection [15,16]. To improve the adsorption capacities of layered clays, mechanical/thermal activation processes were performed to create the active sites and increase the surface area of clays [11,17]. Stefanova [17] has reported that a modified clay marl is obtained by thermal activation at 750 ◦ C, and this prepared Materials 2017, 10, 647; doi:10.3390/ma10060647
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product can be successfully used for removal of metal ions from water solutions in a wide range of concentrations. However, the adsorption capacities of modified clays are highly dependent on the activation temperature. If the temperature increases to some extent, the adsorption increases, while if the clay is activated at higher temperatures, the adsorption properties of modified clays may be decreased [11]. This is mainly caused by the crystal structure transformation during activation. Clay modified via inorganic acid treatment has also been exploited to remove heavy metal ions from aqueous solutions [18–20]. The surface area, porous structure and acidity of the clays can be improved after acid leaching [19]. Furthermore, acid treatments can replace exchangeable cations with H+ ions and Al3+ in tetrahedral and octahedral sites, resulting in the improvement of adsorption capacities [20]. However, the efficiency of treatments depends on the type of clays, exchangeable cations, and acid; as well as the treatment temperature and time [21]. It means that the restrictions on the scope of acid treatment become severe. For these reasons, mechanical/thermal activation followed by a selective leaching process has been developed further [11,22–29]. The clays first undergo mechanical or thermal treatment to activate silica or alumina before selective leaching [24–29]. This process enhances the dissolution of alumina/silica constituents, the pores are created on the surface of granules and between the layered microstructures after leaching, resulting in a superior surface area [24,26,28,29]. Okada et al. [24] reported that the porous silica material with specific surface area of 340 m2 /g can be obtained by acid leaching of the activated kaolinite (roasted at 600 ◦ C for 24 h) in 20% H2 SO4 liquor at 90 ◦ C for 0.5–5 h. Furthermore, the ions exchangeability of kaolinites modified via thermal roasting-acid leaching is superior to kaolinites modified only by thermal activation [22]. Obviously, mechanical/thermal activation followed by selective leaching is a more efficient process for the preparation of porous materials from natural layered clays than single acid modification or mechanical/thermal treatment. The adsorption properties of modified clays are closely correlated with dissolution of alumina/silica, which depends on their activation level. In this study, based on the premise of investigation of silica and alumina activation after roasting of natural kaolinite mineral, the porous alumina and silica materials were prepared separately from thermally activated kaolinite by selective alkali or acid leaching, and the correlations between their properties, including specific surface area, pore size and pore volume, and silica/alumina dissolution of kaolinite activated at various temperatures, were investigated. Lastly, the adsorption capacities of prepared porous materials for the heavy metal cations including Cu2+ , Cd2+ and Pb2+ were evaluated. 2. Materials and Methods 2.1. Materials Natural kaolinite raw ore with a purity of 91% was taken from the Guangdong province of China. From Table 1, the Al2 O3 and SiO2 contents were 38.8% and 42.2%, respectively. The XRD result in Figure 1 confirms that kaolinite is the predominant mineral, and the majority TiO2 -bearing impurity is rutile. Prior to the experiments, the kaolinite samples were ground to 70% of the particle size below 150 µm. Leaching reagents (HuiHong Reagent, Changsha, China), including sulfuric acid (H2 SO4 ) and sodium hydroxide (NaOH), used in this work were of analytical grade. Additionally, copper chloride (CuCl2 ), cadmium sulfate (CdSO4 ) and lead nitrate (Pb(NO3 )2 ) of analytic grade were used to prepare the aqueous solutions for adsorption tests. Table 1. Chemical composition of kaolinite raw ore/%. Al2 O3
SiO2
Fe2 O3
38.8
42.2
0.5
TiO2
CaO
MgO
K2 O
Na2 O
LOI 1
4.0
0.2
0.1
0.1
0.3
13.7
1
LOI: loss on ignition.
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Figure 1. XRD pattern of kaolinite raw ore.
Figure 1. XRD pattern of kaolinite raw ore.
2.2. Methods
2.2. Methods
2.2.1. Preparation of Porous Materials
2.2.1. Preparation of Porous Materials
Thermal activation followed by selective leaching was performed for the preparation of the
Thermal activation followed byThermal selectiveactivation leaching was was carried performed foran theelectric-heating preparation offurnace the porous porous alumina/silica materials. out in (U-Therm, Changsha, withactivation dimensionwas of 50carried cm × 20 cm 18an cm.electric-heating The pulverized kaolinite alumina/silica materials.China) Thermal out×in furnacesample (U-Therm, was loaded intowith the corundum cm×× 20 3 cm) heated isothermally at given temperatures Changsha, China) dimensionplates of 50(6cm cmand × 18 cm. The pulverized kaolinite sample was forinto 15 min. the activated products were cooled room temperature for the subsequent leaching for loaded theThen corundum plates (6 cm × 3 cm) andtoheated isothermally at given temperatures tests. At the beginning of the leaching trial, the aqueous leaching solution (sulfuric acid or caustic 15 min. Then the activated products were cooled to room temperature for the subsequent leaching soda liquor) and activated products were added to pots equipped with Teflon protective lining. The tests. At the beginning of the leaching trial, the aqueous leaching solution (sulfuric acid or caustic soda sealed pots were soaked in a glycerine bath and rotated at 30 rpm (for a schematic of the reactor liquor) and activated potswere equipped withatTeflon equipment refer toproducts [30]). Thewere acid added leachingtotests performed 120 °Cprotective for 90 min lining. in 20% The H2SOsealed 4 pots were soaked in a glycerine bath and rotated at 30 rpm (for a schematic of the reactor equipment liquor with liquid to solid ratio (L/S) of 10:1 mL/g, and the dissolution of silica was performed at 120 °C refer for to [30]). leaching tests were at Filtration 120 ◦ C for 90performed min in 20% H2 SO4 liquor 2 h inThe 120 acid g/L NaOH liquor with L/Sperformed of 10:1 mL/g. was immediately after with ◦ liquidleaching, to solid and ratiothe (L/S) of 10:1 mL/g, and the dissolution of silica was performed at 120 C for 2 h in residues, namely porous materials, were collected and dried. 120 g/L NaOH liquor with L/S of 10:1 mL/g. Filtration was performed immediately after leaching, 2.2.2. Adsorption Testsporous materials, were collected and dried. and the residues, namely In the adsorption tests, the various aqueous solutions were independently prepared by dissolving
2.2.2. the Adsorption Tests reagents including CuCl2, CdSO4 and Pb(NO3)2 in the distilled water. The initial concentrations
of Cu , Cd and Pb are 50 mg/L, 50 mg/L and 100 mg/L, respectively. The quantified adsorbent In the adsorption tests, the various aqueous solutions were independently prepared by dissolving (1.0 g/L of dose for Cu2+ adsorption and 0.4 g/L of dose for Cd2+ and Pb2+ adsorption) and aqueous the reagents including CuCl2 , CdSO4 and Pb(NO3 )2 in the distilled water. The initial concentrations of solution containing heavy metal ions were simultaneously added into the flask, which was oscillated 2+ and Cu2+ ,inCd Pb2+ are 50 mg/L, 50 mg/L and 100 mg/L, respectively. The quantified adsorbent an oven controlled crystal oscillator at a temperature of 30 °C for a time of 60 min. The agitation 2+ adsorption and 0.4 g/L of dose for Cd2+ and Pb2+ adsorption) and aqueous (1.0 g/L of dose for Cu speed was kept at 160 rpm. The solid-liquid separation was performed after adsorption via a solution containing heavy metal ions wereinsimultaneously added into the Based flask, on which was oscillated in centrifuge, and the heavy metal cations the supernatant were measured. the concentration ◦ C for a time of 60 min. The agitation speed an oven controlled at a temperature 30 supernatant, of heavy metalcrystal cationsoscillator in the simulated wastewaterofand the removal efficiency of the adsorbent heavy ions was separation calculated. was performed after adsorption via a centrifuge, and was kept at 160for rpm. Themetal solid-liquid the heavy metal cations in the supernatant were measured. Based on the concentration of heavy metal 2.2.3. Instrumental Techniques cations in the simulated wastewater and supernatant, the removal efficiency of the adsorbent for heavy X-ray diffraction patterns of samples were obtained by using an XRD (D/Max 2500; Rigaku, metal ionsThe was calculated. 2+
2+
2+
Tokyo, Japan) with a copper Kα X-ray source with the scanning angle ranging from 3° to 65° (2θ) in
2.2.3. 0.02° Instrumental (2θ) steps Techniques at scanning speed of 8°/min. Specific surface area (SSA) is an important index for assessing the adsorption capacity of porous
The X-ray diffraction patterns of samples were obtained by using an XRD (D/Max 2500; Rigaku, materials. The SSA of porous materials was determined by using the Brunauer-Emmett-Teller (BET) ◦ (2θ) in Tokyo, Japan)(QuadraSorb with a copper X-ray source with the scanning from 3◦ pore to 65size method SI, Kα Quantachrome Instruments, Boynton angle Beach,ranging FL, USA). The ◦ ◦ 0.02 distribution (2θ) steps at scanning speedwas of 8also /min. of porous materials determined based on the Barret-Joyner-Halenda (BJH) method. Specific surface area (SSA) is an important index for assessing the adsorption capacity of porous materials. The SSA of porous materials was determined by using the Brunauer-Emmett-Teller (BET) method (QuadraSorb SI, Quantachrome Instruments, Boynton Beach, FL, USA). The pore
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size distribution of porous materials was also determined based on the Barret-Joyner-Halenda (BJH) method. The microstructures of porous materials were characterized by a scanning electron microscope Materials 2017, 10, 647 4 of 12 (SEM; JSM-6360LV, JEOL, Tokyo, Japan). SEM images were recorded in the backscattered electron mode operating under a lowofvacuum conditionwere (0.5characterized Torr and 25 keV). The powder was coated with a The microstructures porous materials by a scanning electron microscope (SEM;of JSM-6360LV, Japan). SEM images were recorded in the backscattered electronJEOL, thin layer gold prior JEOL, to theTokyo, detection. A transmission electron microscope (TEM; JEM-3010, mode operating under a low condition (0.5 Torr and 25 keV).ofThe powder wasmaterials. coated with Tokyo, Japan) was also used forvacuum characterization of microstructures porous silica a thin layerspectrometry of gold prior to the detection. A XSP, transmission microscope (TEM; JEM-3010, JEOL, ICP-OES (IRIS Intrepid II Thermoelectron Elemental, Waltham, MA, USA) was used to Tokyo, Japan) was also used for characterization of microstructures of porous silica materials. measure the concentrations of heavy metal cations in the supernatant after adsorption. ICP-OES spectrometry (IRIS Intrepid II XSP, Thermo Elemental, Waltham, MA, USA) was used to measure the concentrations of heavy metal cations in the supernatant after adsorption. 3. Results and Discussion 3. Results and Discussion 3.1. Preparation of Porous Materials 3.1. Preparation of Porous Materials the phase formation and physicochemical properties of kaolinite During the thermal activation, are alteredDuring as thethe temperature increases As a result, dissolution of silica and thermal activation, the[31–33]. phase formation and the physicochemical properties ofalumina kaolinite from activated kaolinite is also different. It can be seen in Figure 2 that the alumina/silica dissolution are altered as the temperature increases [31–33]. As a result, the dissolution of silica and alumina from activated increases and then with the increase of activation temperature. from kaolinite activated kaolinite is initially also different. It candecreases be seen in Figure 2 that the alumina/silica dissolution from activated kaolinite from increases initially and then decreasesacid withliquor the increase activation For dissolution of alumina activated kaolinite in sulfuric (seen inofFigure 2a), only dissolution alumina fromwithout activated kaolinite in sulfuric liquor (seen in 18.7%temperature. alumina is For dissolved fromofraw kaolinite activation. This valueacid is markedly increased ◦ ◦ Figure 2a), only 18.7% alumina is dissolved from raw kaolinite without activation. This value is when the activation temperature is in the range from 650 C to 950 C. However, when the temperature markedly increased when the ◦activation temperature is in the range from 650 °C to 950 °C. However, ◦ increases from 850 C to 1000 C, the dissolution ratio of alumina decreases from 98.4% to 3.1%. when the temperature increases from 850 °C to 1000 °C, the dissolution ratio of alumina decreases For dissolution of silica from activated kaolinite in alkali liquor (in Figure 2b), the dissolution ratio from 98.4% to 3.1%. For dissolution of silica from activated kaolinite in alkali liquor (in Figure 2b), remains at a lower level of aboutat8%, when theoftemperature is below 900 ◦ C. When the temperature the dissolution ratio remains a lower level about 8%, when the temperature is below 900 °C. ◦ C, the dissolution of silica is improved and the value reaches 89.7% at 1150 ◦ C increases to 1000–1200 When the temperature increases to 1000–1200 °C, the dissolution of silica is improved and the value for 15reaches min. 89.7% at 1150 °C for 15 min.
Figure 2. Effects of activation temperature on dissolution of alumina (a) and silica (b).
Figure 2. Effects of activation temperature on dissolution of alumina (a) and silica (b).
From Figure 2, the appropriate activation temperatures for alumina and silica are different. The XRD activated atactivation 900 °C and 1150 °C for 15 min shown inand Figure 3. Compared Frompatterns Figureof2,kaolinite the appropriate temperatures forarealumina silica are different. ◦ ◦ with the raw kaolinite ore (Figure 1), the peaks of kaolinite disappear in the activated kaolinite, The XRD patterns of kaolinite activated at 900 C and 1150 C for 15 min are shown in Figure 3. meanwhile, wide and asymmetrical diffraction peakpeaks belonging to amorphous metakaolin the Compared withathe raw kaolinite ore (Figure 1), the of kaolinite disappear in theinactivated range of 15°–30° 2θ appears in kaolinite activated at 900 °C (Figure 3a) [32,33]. After acid leaching, kaolinite, meanwhile, a wide and asymmetrical diffraction peak belonging to amorphous metakaolin the main phases in acid leaching residue have no significant change (Figure 4a). However, the type in the range of 15◦ –30◦ 2θ appears in kaolinite activated at 900 ◦ C (Figure 3a) [32,33]. After acid of amorphous phase may be different due to leaching of the aluminum constituent. To verify this leaching, thethe main phases inresidue acid leaching residue have no g/L significant change (Figure change, acid-leached was further leached in 120 alkali solution at 120 °C for4a). 2 h However, with the type of 10:1 amorphous phase may be that different to silica leaching of the aluminum constituent. To verify L/S of mL/g. The results show 93.3%due of the is dissolved, while the value is only 8.0% ◦ C for 2 h this change, the acid-leached residue was further leached in 120 g/L alkali solution at 120 during alkali leaching of kaolinite activated at 900 °C (Figure 2b). Figure 4b also confirms that the with amorphous L/S of 10:1phase mL/g. The results show that 93.3% It ofmeans the silica dissolved, whileinthe value is only disappears after alkali leaching. thatisthe Al-O-Si bond metakaolin ◦ C (Figure by activation roasting of kaolinite is broken acid leaching, and 4b the also defected Si-O that 8.0% obtained during alkali leaching of kaolinite activated at 900during 2b). Figure confirms tetrahedron phase is in favor of silica dissolution. Asleaching. a consequence, the main of acid-leached the amorphous disappears after alkali It means thatphases the Al-O-Si bond in residue metakaolin are amorphous silica and quartz.
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obtained by activation roasting of kaolinite is broken during acid leaching, and the defected Si-O tetrahedron is in favor of silica dissolution. As a consequence, the main phases of acid-leached residue are amorphous silica and quartz. Materials 2017, 10, 647
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Figure 3. XRD patterns of kaolinite activated at 900 °C for 15 min (a) and at 1150 °C for 15 min (b).
Figure 3. XRD patterns of kaolinite activated at 900 ◦ C for 15 min (a) and at 1150 ◦ C for 15 min (b). Figure 3. XRD patterns of kaolinite activated at 900 °C for 15 min (a) and at 1150 °C for 15 min (b).
Figure 4. XRD patterns of leaching residues from activated kaolinite (a) residue from acid leaching of kaolinite activated at 900 °C for 15 min; (b) residue from alkali leaching of acid-leached residue; Figure 4. XRD patterns of leaching residues from activated kaolinite (a) residue from acid leaching of Figure 4. patterns of leaching leaching from activated (c) XRD residue from alkali of residues kaolinite activated at 1150 °Ckaolinite for 15 min.(a) residue from acid leaching kaolinite activated at 900 °C for 15 min; (b) residue from alkali leaching of acid-leached residue; ◦ C for 15 min; (b) residue from alkali leaching of acid-leached residue; of kaolinite activated at 900 (c) residue from alkali leaching of kaolinite activated at 1150 °C for 15 min.
Thefrom XRD alkali result leaching in Figure of 3b kaolinite also presents an amorphous (c) residue activated at 1150 ◦peak C forat1515°–30° min. 2θ appears in kaolinite
activated at 1150 °C in forFigure 15 min. means that an theamorphous primary silicon the kaolinite is converted into The XRD result 3bItalso presents peak in at 15°–30° 2θ appears in kaolinite amorphous silica after roasting at 1150 °C [32]. Due to the amorphous silica with superior alkali activated at 1150 °C for 15 min. It means that the primary silicon in the kaolinite is converted into The XRD result in Figure 3b also presents an amorphous peak at 15◦after –30◦ alkali 2θ appears inof kaolinite leaching behavior [32,34,35], satisfying silica be obtained leaching amorphous silica after roasting at 1150 °C [32].removal Due to can the amorphous silica with superior alkali ◦ activated at 1150 C for 15 min. It means that the primary silicon in the kaolinite is converted into activated kaolinite under suitable thermal treatment (Figure 2b). The XRD result in Figure 4c confirms leaching behavior [32,34,35], satisfying silica removal can be obtained after alkali leaching of ◦ that the main phases of alkali-leached residue are γ-Al 2 O 3 , mullite and quartz. The insoluble mullite amorphous silica after under roasting at 1150 Ctreatment [32]. Due to the with activated kaolinite suitable thermal (Figure 2b).amorphous The XRD resultsilica in Figure 4c superior confirms alkali quartz hinder theofdissolution of silica silica from activated and quartz. about 10% in kaolinite that the main phases alkali-leached residue are γ-Al 2O3be ,kaolinite mullite and The silica insoluble mullite leachingand behavior [32,34,35], satisfying removal can obtained after alkali leaching of activated activated at hinder 1150 °C fordissolution 15 min is not after alkali kaolinite leaching (Figure 2b).10% silica in kaolinite and quartz of dissolved silica from activated and about kaolinite under suitablethe thermal treatment (Figure 2b). The XRD result in Figure 4c confirms that Basedaton the°C above the generation ofalkali amorphous metakaolin and amorphous silica activated 1150 for 15analyses, min isresidue not dissolved after leaching (Figure 2b). the main phases of alkali-leached areisγ-Al and quartz. The insoluble 2 O3 , mullite under appropriate activation temperatures characterized by the superior acid dissolution of mullite Based on the above analyses, the generation of amorphous metakaolin and amorphous silica and quartz hinder the dissolution of silica from activated kaolinite and about 10% silica in kaolinite alumina and alkali dissolution of silica. It means that the porous alumina/silica materials can be under appropriate activation temperatures is characterized by the superior acid dissolution of ◦ prepared by C selective leaching of from activated of activated at 1150 for 15 min is not dissolved afterthat alkali leaching (FigureThe 2b).characteristics alumina and alkali dissolution of silica silica.orIt alumina means the porous kaolinite. alumina/silica materials can be prepared porous materials significantly impact on of the dissolution alumina/silica during leaching Based on the analyses, the generation amorphous metakaolin amorphous prepared by above selective leaching of silica or alumina from activatedofkaolinite. The and characteristics of silica process. prepared porous alumina and materials need firstly to be characterized. preparedHence, porousthe materials significantly impact onsilica the dissolution of alumina/silica during leaching
under appropriate activation temperatures is characterized by the superior acid dissolution of alumina process. Hence, the silica materials need firstly to be characterized. and alkali dissolution of prepared silica. It porous meansalumina that theand porous alumina/silica materials can be prepared by selective leaching of silica or alumina from activated kaolinite. The characteristics of prepared porous materials significantly impact on the dissolution of alumina/silica during leaching process. Hence, the prepared porous alumina and silica materials need firstly to be characterized.
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Characterization of Porous Materials 3.2.1. 3.2. Specific Surface Area 3.2.1.correlations Specific Surface Area the SSA of prepared porous alumina/silica materials and silica/alumina The between dissolution of activated kaolinite 5. Itporous can be alumina/silica seen in Figurematerials 5a that the The correlations betweenare theshown SSA in of Figure prepared andsilica dissolution ratio of raw kaolinite without thermal activation is only 5.5%. The corresponding silica/alumina dissolution of activated kaolinite are shown in Figure 5. It can be seen in Figure 5a that SSA 2 /g. There is no change, when compared with raw kaolinite’s SSA of of porous alumina is 11.0 mof the silica dissolution ratio raw kaolinite without thermal activation is only 5.5%. The corresponding 2 SSA of With porous is 11.0 m2/g. There is no change,the when compared with raw kaolinite’s SSA of 10.9 m /g. analumina increase in activation temperature, SSA of porous alumina initially increases 2/g. With an increase in activation temperature, the SSA of porous alumina ◦ 10.9 m initially increases with the silica dissolution. When the raw kaolinite is activated at 1150 C, the silica dissolution of with the silica dissolution. theand raw the kaolinite is activated at 1150 °C, the silica to dissolution of of activated kaolinite goes up toWhen 89.7%, SSA of porous alumina increases a maximum activated kaolinite goes up to 89.7%, and the SSA of porous alumina increases to a maximum of and 2 ◦ 55.8 m /g. However, at 1200 C, the silica dissolution of activated kaolinite decreases to 74.2%, 55.8 m2/g. However, 2at 1200 °C, the silica dissolution of activated kaolinite decreases to 74.2%, and the SSA drops to 11.4 m /g, accordingly. 2 the SSA drops to 11.4 m /g, accordingly.
Figure 5. Correlation between the SSA of porous materials and dissolution of activated kaolinite.
Figure 5. Correlation between the SSA of porous materials and dissolution of activated kaolinite. (a) Porous alumina; (b) porous silica. (a) Porous alumina; (b) porous silica.
As seen in Figure 5b, the alumina dissolution ratio of raw kaolinite without activation is 18.7%, with an SSA of 35.35b, m2the /g after acid leaching. With increasing activation without temperature, the SSAis of As seen in Figure alumina dissolution ratio of raw kaolinite activation 18.7%, 2 porous silica shares the same tendency of alumina dissolution. Its value attains a maximum of with an SSA of 35.3 m /g after acid leaching. With increasing activation temperature, the SSA of porous m2/g when thetendency alumina dissolution is 98.4% for kaolinite roasted at 850 °C for 15 min. This resultm2 /g silica280.3 shares the same of alumina dissolution. Its value attains a maximum of 280.3 shows that the SSA of porous silica is much higher than that of porous◦alumina. By further increasing when the alumina dissolution is 98.4% for kaolinite roasted at 850 C for 15 min. This result shows the activation temperature to 1000 °C, the alumina dissolution ratio decreases to 3.1%, and the SSA that the SSA of porous silica is much higher than that of porous alumina. By further increasing decreases to 3.0 m2/g.
the activation temperature to 1000 ◦ C, the alumina dissolution ratio decreases to 3.1%, and the SSA decreases 3.0Size m2 /g. 3.2.2. to Pore 3.2.2. PoreWith Sizethe dissolution of silicon or aluminum constituents, the pore size and their distribution in porous materials also changes. To determine the correlation between pore size of prepared porous
With theand dissolution of silicon aluminum constituents, the distributions pore size and distribution alumina silica dissolution of or activated kaolinite, the pore size of their porous alumina in obtained fromalso alkali leachingTo of determine kaolinite roasted at 900 °C, 1050 °C and 1150size °C of forprepared 15 min were porous materials changes. the correlation between pore porous considered, anddissolution the results are shown in Figure 6 andthe Table 2, size respectively. There of is only a single alumina and silica of activated kaolinite, pore distributions porous alumina peak from in each distribution for porous alumina Most◦ pores in the range ◦ C size obtained alkali leachingcurve of kaolinite roasted at materials. 900 ◦ C, 1050 C andare 1150 for 15 minofwere 5–10 nm, indicating the mesoporous structures of the prepared porous alumina (Figure 6). The BJH considered, and the results are shown in Figure 6 and Table 2, respectively. There is only a single pore sizes of various porous alumina are also similar. After alkali leaching of the kaolinite activated peak in each distribution curve for porous alumina materials. Most pores are in the size range of at 1150 °C for 15 min, the BJH pore size is 6.06 nm and the value is slightly lower than that of porous 5–10 nm, indicating the mesoporous of the prepared porous alumina (Figurekaolinite 6). The BJH alumina. The pore volume of porousstructures alumina decreases as the silica dissolution of activated pore sizes of various porous alumina are also similar. After alkali leaching of the kaolinite activated increases (in Table 2). The pore volume of porous alumina prepared from kaolinite activated at at ◦ −1 1150 1150 C for min, the BJH °C15 is 0.1455 mL·g . pore size is 6.06 nm and the value is slightly lower than that of porous alumina. The pore volume of porous alumina decreases as the silica dissolution of activated kaolinite increases (in Table 2). The pore volume of porous alumina prepared from kaolinite activated at 1150 ◦ C is 0.1455 mL·g−1 .
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Figure 6. BJH pore size distributions of porous alumina. (Porous alumina prepared via alkali leaching Figure 6. BJH pore size distributions of porous alumina. (Porous alumina prepared via alkali leaching of kaolinite activated at (a) 1150 °C; (b) 1050 °C and (c) 900 °C, for 15 min. of kaolinite activated at (a) 1150 ◦ C; (b) 1050 ◦ C and (c) 900 ◦ C, for 15 min. Figure 6. BJH pore size distributions of porous alumina. (Porous alumina prepared via alkali leaching of kaolinite activated atbetween (a) 1150 °C; 1050features °C and (c) °C, foralumina 15 min. and silica dissolution of Table 2. Correlation the(b) pore of 900 porous Table 2. Correlation between the pore features of porous alumina and silica dissolution of activated kaolinite. activated kaolinite. Table 2. Correlation between the pore features of porous alumina and silica dissolution of Activation Temperature/°C Silica Dissolution/% BJH Pore Size/nm Pore Volume/mL·g−1 activated kaolinite. Activation900 Temperature/◦ C
Silica Dissolution/% 8.9
BJH Pore Size/nm 6.54
Pore Volume/mL 0.2221·g−1
−1 Activation Temperature/°C Silica Dissolution/% BJH 6.54 Pore Volume/mL·g 1050 73.3 6.57Size/nm Pore 0.2221 0.2215 900 8.9 900 8.9 6.54 0.2221 1050 73.3 6.57 0.2215 1150 89.7 6.06 0.1455 1150 89.7 6.066.57 0.1455 1050 73.3 0.2215 89.7 0.1455 kaolinite The pore 1150 size distributions of porous silica prepared from6.06 acid leaching of activated areThe alsopore presented in Figure 7, which confirms thatprepared the pore size of various porouskaolinite silica size distributions of porous silica fromdistributions acid leaching of activated The pore size distributions of porous silica prepared from acid leaching of activated kaolinite materials are also similar, despite the size distributions of pores becoming smaller as the activation are also presented in Figure 7, which confirms that the pore size distributions of various porous silica are also are presented in Figure 7, the which confirms that the pore size distributions of various silica temperature increases, and pore being distributed more uniformly with decreasing materials also similar, despite the sizesize distributions of pores becoming smaller asporous the activation materials are also similar, despite the size distributions of pores becoming smaller as the activation temperature. As seenand in Figure 7, most poresdistributed are less thanmore 5 nmuniformly in size, which smaller than those temperature increases, the pore size being withare decreasing temperature. temperature increases, and the pore size beingand distributed more uniformly with decreasing of porous alumina. A large number of mesoporous micro-pores result in the great specific surface As seen in Figure 7, most pores are less than 5 nm in size, which are smaller than those of porous temperature. As seen Figure 7, size mostof pores are less than 5 nm inwhen size, which are smaller than those area (Figure 5b). TheinBJH porous silica increases thegreat activation temperature alumina. A large number ofpore mesoporous and micro-pores result in the specific surface area of porous alumina. large number of mesoporous andtrend micro-pores result in the the great specific surface increases, while theApore volume presents an opposite (in Table 3). When raw kaolinite ore (Figure 5b). The BJH pore size of porous silica increases when the activation temperature increases, area (Figureat5b). BJH pore the sizeprepared of porous silicasilica increases when activation is activated 850 The °C for 15 min, porous material hasthe a BJH pore sizetemperature of 3.05 nm while the pore volume presents an−1 opposite trend (in Table 3). When the raw kaolinite ore is activated increases, while the volume and pore volume of pore 0.1945 mL·g presents (Table 3).an opposite trend (in Table 3). When the raw kaolinite ore ◦ C for at 850 15 min,°Cthe porous silica material a BJHhas pore sizepore of 3.05 andnm pore is activated at 850 forprepared 15 min, the prepared porous silicahas material a BJH sizenm of 3.05 − 1 volume of 0.1945 mL (Table −1 (Table 3). and pore volume of·g0.1945 mL·g3).
Figure 7. BJH pore size distributions of porous silica (porous silica prepared via acid leaching of kaolinite activated at (a) 975 °C; (b) 850 °C and (c) 600 °C, for 15 min. Figure 7. BJH pore size distributions of porous silica (porous silica prepared via acid leaching of Figure 7. BJH pore size distributions of porous silica (porous silica prepared via acid leaching of kaolinite activated at (a) 975 °C; (b) 850 °C and (c) 600 °C, for 15 min. kaolinite activated at (a) 975 ◦ C; (b) 850 ◦ C and (c) 600 ◦ C, for 15 min.
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Table 3. Correlation between the pore features of porous silica and alumina dissolution of Table 3. kaolinite. Correlation between the pore features of porous silica and alumina dissolution of activated activated kaolinite. ◦
Activation Temperature/°C Temperature/ C Activation 600 600 850 850 975 975
Alumina Pore Size/nm ·g−1 −1 AluminaDissolution/% Dissolution/% BJH BJH Pore Size/nm Pore PoreVolume/mL Volume/mL·g 6767 1.53 0.1963 1.53 0.1963 98 3.05 0.1945 98 3.05 0.1945 79 3.06 0.1313
79
3.06
0.1313
3.2.3. Microstructure 3.2.3. Microstructure The The microstructure microstructure of of porous porous alumina alumina prepared prepared from from alkali alkali leaching leaching of of kaolinite kaolinite activated activated at at ◦ 1150 C for AsAs seen from Figure 8a the of porous alumina material via alkali 1150 °C for15 15min minwas wasobtained. obtained. seen from Figure 8a size the size of porous alumina material via leaching has anhas evident decrease compared with kaolinite raw ore. The alumina granules with awith size alkali leaching an evident decrease compared with kaolinite raw ore. The alumina granules of agglomerate and form spheres with diameters of 10–20 µm. These of a about size of5 µm about 5 μm agglomerate and form spheres with diameters of microstructures 10–20 μm. These porous alumina are consistent with conventional porous materials. Figure 8b,c shows that a large microstructures of porous alumina are consistent with conventional porous materials. Figure 8b,c number of pores with size of exist granules. granules present a shows that a large number of0.5–1 poresµm with sizebetween of 0.5–1the μmalumina exist between the These alumina granules. These multifaceted structure with an irregular spinel and there cracksand andthere interspaces on their granules present a multifaceted structure with surface, an irregular spinelare surface, are cracks and surfaces (Figure 8c,d). interspaces on their surfaces (Figure 8c,d). The Figure 9 that thethe profiles of The morphology morphology of of porous poroussilica silicawas wasalso alsoobserved. observed.ItItcan canbebeseen seeninin Figure 9 that profiles porous silica granules and porous alumina granules (Figure(Figure 8) are completely different.different. The particles of porous silica granules and porous alumina granules 8) are completely The of porous silica remain small lumps with sizes of 100–150 µm obtained by the superposition of platelets particles of porous silica remain small lumps with sizes of 100–150 μm obtained by the superposition with diameters 1–3 µm and thicknesses 10–20 nm.ofThe results thatindicate the sizethat of granules of platelets withofdiameters of 1–3 μm and of thicknesses 10–20 nm. indicate The results the size has no obvious change after leaching, while the silica platelets are injured. Figure 9b–d also shows of granules has no obvious change after leaching, while the silica platelets are injured. Figure 9b–d that large that number of pores with below 1 µm exist between thebetween silica platelets. For further also ashows a large number of sizes poresofwith sizes of below 1 μm exist the silica platelets. examination of the pore structure of silica platelets, HRTEMTEM images measured by using For further examination of the pore structure ofTEM silicaand platelets, andwere HRTEM images were ameasured transmission electron microscope. The results in Figure 10 confirm that the laminated structure by using a transmission electron microscope. The results in Figure 10 confirm that the of particlesstructure with a large number of shallow-hole micropores remains inmicropores the prepared porousin silica laminated of particles with a large number of shallow-hole remains the material, in the large specific surface area (Figure 5b).surface area (Figure 5b). preparedresulting porous silica material, resulting in the large specific
Figure 8. SEM images of porous alumina prepared via alkali leaching of kaolinite activated at 1150 °C Figure 8. SEM images of porous alumina prepared via alkali leaching of kaolinite activated at 1150 ◦ C for 15 min. (a) ×1000; (b) ×10,000; (a) ×30,000; (b) ×50,000. for 15 min. (a) ×1000; (b) ×10,000; (a) ×30,000; (b) ×50,000.
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Figure 9. SEM images of porous silica prepared via acid leaching of kaolinite activated at 850 °C for Figure 9. SEM images of porous silica prepared via acid leaching of kaolinite activated at 850 ◦ C for 15 min.9.(a)SEM ×100; (b) ×10,000; (c) ×30,000; (b) ×50,000. Figure images of porous silica prepared via acid leaching of kaolinite activated at 850 °C for 15 min. (a) ×100; (b) ×10,000; (c) ×30,000; (b) ×50,000. 15 min. (a) ×100; (b) ×10,000; (c) ×30,000; (b) ×50,000.
Figure 10. TEM (a,b) and HRTEM images (c,d) of porous silica prepared via acid leaching of kaolinite activated 850 (a,b) °C forand 15 HRTEM min. Figure 10.atTEM images (c,d) of porous silica prepared via acid leaching of kaolinite Figure 10. TEM (a,b) and HRTEM images (c,d) of porous silica prepared via acid leaching of kaolinite activated at 850 °C for 15 min. ◦ C for 15 min. at 850 2+, Cd2+ and Pb2+ in Aqueous Solution Using Porous Materials 3.3.activated Adsorption of Cu
3.3. Adsorption of Cu2+, Cd2+ and Pb2+ in Aqueous Solution Using Porous Materials The above results indicate that the porous alumina and silica materials with high activity and 3.3. Adsorption of Cu2+ , Cd2+ and Pb2+ in Aqueous Solution Using Porous Materials largeThe specific surface can bethat prepared fromalumina natural and kaolinite The adsorption of above resultsarea indicate the porous silicaore. materials with highproperties activity and 2+ andmaterials 2+, werewith Thespecific above results indicate that the porous alumina and high activity and prepared porous materials for heavy metal, including Cu2+ , Cdsilica PbThe investigated further, large surface area can be prepared from natural kaolinite ore. adsorption properties of 2+ large surface areawere can prepared from natural kaolinite ore. adsorption andspecific some porous other samples used formetal, comparison. TheCu comparative adsorbents are listedproperties in Table 4, of prepared materials forbe heavy including , Cd2+ and Pb2+The , were investigated further, 2+ , Cd2+ and Pb2+ , were investigated further, and some theporous adsorption results areheavy shown incomparison. Figure 11. The prepared for metal, including Cucomparative othermaterials samples were used for adsorbents are listed in Table 4, and the other adsorption results areused shown Figure 11. The comparative adsorbents are listed in Table 4, and some samples were for in comparison. Table 4. Various adsorbents used in11. the comparative adsorption experiments. and the adsorption results are shown in Figure Table 4. Various adsorbents used in the comparative adsorption experiments. Adsorbent Number Table Variousore adsorbents used in theAdsorbent comparative adsorption experiments. A Raw4.kaolinite Number B Activated kaolinite A Raw kaolinite ore at 850 °C for 15 min Number C Porous silica prepared via°C acid of activated kaolinite at 850 °C for 15 min B Activated kaolinite at 850 forleaching 15 min Adsorbent D Activated kaolinite °C for 15 minof activated kaolinite at 850 °C for 15 min Rawsilica kaolinite oreat 1150 CA Porous prepared via acid leaching ◦ for 15leaching Activated kaolinite 850 min EB Porous alumina prepared via of activated kaolinite at 1150 °C for 15 min D Activated kaolinite at at 1150 °CCalkali for 15 min C Porous silica prepared via acid leaching of activated kaolinite at 850at◦ C for °C 15 min F Activated carbon E Porous alumina prepared via alkali leaching of activated kaolinite 1150 for 15 min
D Activated kaolinite at 1150 ◦ C for 15 min Activated carbon E Porous alumina prepared via alkali leaching of activated kaolinite at 1150 ◦ C for 15 min It canFbe seen Activated in Figurecarbon 11 that the porous alumina material (E) possess satisfactory adsorption
F
2+, Cd2+ and Pb2+; while the other adsorbents, including raw kaolinite, activated capacity It canfor beCu seen in Figure 11 that the porous alumina material (E) possess satisfactory adsorption capacity for Cu2+, Cd2+ and Pb2+; while the other adsorbents, including raw kaolinite, activated
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It can be seen in Figure 11 that the porous alumina material (E) possess satisfactory adsorption Materials 2017, 10, 647 10 of 12 capacity for Cu2+ , Cd2+ and Pb2+ ; while the other adsorbents, including raw kaolinite, activated kaolinite kaolinite and and porous porous silica silica material, material, are are inferior inferior under under the the same same adsorption adsorption conditions. conditions. The The removal removal 2+2+, Cd2+ 2+ are 100%, 99.9% and 65.3%, respectively; while efficiencies of porous alumina for Cu and Pb 2+ 2+ efficiencies of porous alumina for Cu , Cd and Pb are 100%, 99.9% and 65.3%, respectively; while the of porous silica silica are only 14.5%,and respectively. For comparison, the removal removalefficiencies efficiencies of porous are8.66%, only 10.5% 8.66%,and 10.5% 14.5%, respectively. For 2+ 2+ the removal efficiencies of activated carbon for Cu and Cd are 53.3% and 27.1%, respectively. 2+ 2+ comparison, the removal efficiencies of activated carbon for Cu and Cd are 53.3% and 27.1%, 2+ and Cd2+ are The results show the show adsorption capacities of porous alumina for Cualumina higher 2+ are respectively. Thethat results that the adsorption capacities of porous for Cu2+ much and Cd 2 than of than activated with acarbon specificwith surface area of 450 marea /g. of The approximate adsorption muchthose higher thosecarbon of activated a specific surface 450 m2/g. The approximate 2+ is also displayed. capacity of porous alumina material and activated carbon for Pb The saturated 2+ adsorption capacity of porous alumina material and activated carbon for Pb is also displayed. The adsorption tests further indicate that the maximum capacities of prepared porous alumina material for saturated adsorption tests further indicate that the maximum capacities of prepared porous alumina 2+ 2+ 2+ ◦ Cu , Pb forand Cd 134 mg/g, 183 mg/g, mg/g 183 andmg/g 195 mg/g, respectively, at 30 C. at 30 °C. 2+ are 134 material Cu2+ , Pb2+are and Cd and 195 mg/g, respectively,
2+ and Pb2+ on various adsorbents. Figure11. 11.Adsorption Adsorptioncapabilities capabilitiesfor forCu Cu Cd2+ 2+2+,,Cd Figure and Pb2+ on various adsorbents.
Generally, the major factors affecting adsorption include cation exchange capacity, surface Generally, the major factors affecting adsorption include cation exchange capacity, surface charge, charge, specific surface area, and specific surface structure of adsorbents; as well as pH value of specific surface area, and specific surface structure of adsorbents; as well as pH value of aqueous aqueous solution, adsorption time and temperature, etc. [11,18,36]. Compared with the porous solution, adsorption time and temperature, etc. [11,18,36]. Compared2+ with2+the porous alumina material, alumina material, the adsorption capacities of porous silica for Cu , Cd and Pb2+ are unsatisfactory, the adsorption capacities of porous silica for Cu2+ , Cd2+ and Pb2+ are unsatisfactory, in spite of the in spite of the larger specific surface area. The spherical alumina granules with an irregular diamond larger specific surface area. The spherical alumina granules with an irregular diamond surface are surface are much more effective for adsorption of heavy metal ions than plate-like silica granules. much more effective for adsorption of heavy metal ions than plate-like silica granules. Besides, there Besides, there exist a large number of –Al–OH groups on the surface of alumina sheet due to exist a large number of –Al–OH groups on the surface of alumina sheet due to hydroxylation in hydroxylation in aqueous solution [22]. The heavy metal ions can be adsorbed or precipitated on the aqueous solution [22]. The heavy metal ions can be adsorbed or precipitated on the surface of porous surface of porous alumina granules by its reaction with hydroxyl groups [18]. Lastly, the dissolution alumina granules by its reaction with hydroxyl groups [18]. Lastly, the dissolution of silica may induce of silica may induce a charge imbalance that causes the introduction of cations to achieve the charge a charge imbalance that causes the introduction of cations to achieve the charge balance. The larger balance. The larger pore size and pore volume may also contribute to the adsorption of heavy metals. pore size and pore volume may also contribute to the adsorption of heavy metals. 4. Conclusions 4. Conclusions The preparation of porous alumina/silica materials via selective leaching of silica or alumina The preparation of porous alumina/silica materials via selective leaching of silica or alumina from activated kaolinite resulted from the generation of metakaolin or amorphous silica along with from activated kaolinite resulted from the generation of metakaolin or amorphous silica along with superior acid dissolution of alumina or alkali dissolution of silica, respectively, under the appropriate superior acid dissolution of alumina or alkali dissolution of silica, respectively, under the appropriate activation temperatures. activation temperatures. (1) The characterization of prepared porous materials indicates that their specific surface area (SSA) (1) increases The characterization of prepared porous of materials indicates that theirleaching specificsolution surface from area with the increasing dissolution alumina/silica in aqueous (SSA) increases with the dissolution of alumina/silica in aqueous leaching solution activated kaolinite. Theincreasing pore volume of porous alumina decreases with the increasing from activated kaolinite. The pore volume of porous alumina decreases with the increasing dissolution of silica, while there is no correlation between pore volume of porous silica and dissolution of silica,ofwhile therekaolinite. is no correlation between pore volume of porous silica and alumina dissolution activated The prepared porous alumina/silica materials belong to the mesoporous materials, despite the slight change of their pore size with the dissolution of activated kaolinite. (2) When the porous alumina is obtained via alkali leaching of kaolinite activated at 1150 °C for 15 min, the SSA, BJH pore size and pore volume are 55.8 m2/g, 6.06 nm and 0.1455 mL/g, respectively.
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alumina dissolution of activated kaolinite. The prepared porous alumina/silica materials belong to the mesoporous materials, despite the slight change of their pore size with the dissolution of activated kaolinite. When the porous alumina is obtained via alkali leaching of kaolinite activated at 1150 ◦ C for 15 min, the SSA, BJH pore size and pore volume are 55.8 m2 /g, 6.06 nm and 0.1455 mL/g, respectively. For the porous silica prepared via acid leaching of kaolinite activated at 850 ◦ C for 15 min, these values are 280.3 m2 /g, 3.06 nm and 0.1945 mL/g, respectively. The adsorption tests confirm that the prepared porous alumina has a superior adsorption of Cu2+ , Pb2+ and Cd2+ , with maximums of 134 mg/g, 183 mg/g and 195 mg/g, respectively. However, the porous silica has difficulty adsorbing the above-mentioned heavy metal ions.
Acknowledgments: The authors wish to express their thanks to the National Natural Science Foundation of China (Nos. 51234008 and 51174230) for financial support. This work was also financially supported by the Co-Innovation Center for Clean and Efficient Utilization of Strategic Metal Mineral Resources. Author Contributions: Guanghui Li and Tao Jiang conceived and designed the experiments; Guanghui Li, Jun Luo and Mingjun Rao performed the experiments, analyzed the data and wrote the paper; Zhiwei Peng and Yuanbo Zhang supervised experimental work and reviewed the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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