Effect of cations on thermal properties of montmorillonite

Transcription

1 Global Geology doi /j. issn Article ID Effect of cations on thermal properties of montmorillonite CHAI Mao 1 LI Chunxiao 2 JIANG Wei 3 and DU Fengzhi 4 1. College of Geo-Exploration Science and Technology Jilin University Changchun China 2. Hydrogeology Investigation Institute of Jilin Province Changchun China 3. The First Geological Survey of Jilin Province Changchun China 4. Regional Survey of Geology & Mineral Resources Jilin Province Changchun China Abstract Based on comparison and analysis on structural cations of montmorillonite in bentonite samples collected from several typical areas in Jilin Province relationships among type and quantity of interlayer /tetrahedral / octahedral cations and temperature and activation energy of removal of bound and hydroxyl water were investigated. The results show that the interlayer cations not only play decisive roles on removal temperature of bound water but also influence dehydroxylation temperature and activation energy of montmorillonite. Type of octahedral cations also has an effect on dehydroxylation process. Key words Montmorillonite cation interlayer water structural water 1 Introduction There is an important relationship between physical and chemical properties of bentonite and its fine structure. Physical and chemical properties of montmorillonite depend on its crystallographic properties. Main factors affecting mineral crystallographic chemistry of montmorillonite are composition of octahedra and the derivative layer charge density and structural characteristics as well as type and quantity of cations adsorbed between layers for charge compensation. Thermal properties of montmorillonite are also closely related to these fine structures. Most former studies on thermal properties of montmorillonite started from cation-exchange to achieve montmorillonite sample with specific cations. Subsequently thermal stability of interlayer cations and water and interlayer spacing was then investigated based on the specific montmorillonite sample He et al Li et al However few researchers took the relationship between stability of water and its fine structure into consideration. It is generally figured that replacement of octahedral cations leads to decrease of dehydroxylation temperature while the effects of interlayer cations on it are seldom took into account. In fact interlayer cations are adsorbed in the structure by negative charges arising from replacement of octahedral cations. Both octahedral and interlayer cations interact with oxygen atoms and balance the negative charges on oxygen. Therefore dehydroxylation temperature and activation energy are close related to both types of cations. This phenomenon has been demonstrated by literature He et al where there is obvious difference among the dehydroxylation temperature of the same raw soil sample after different cation exchange. The soil exchanged by Al 3 + Cr 3 + and Fe 3 + cations exhibited relatively low dehydroxylation temperature and end temperature was lower. While exchange of K + and Na + significantly increased dehydroxylation temperature and starting temperature was 20 Received 5 March 2013 accepted 2 April 2013

2 Effect of cations on thermal properties of montmorillonite higher and the end temperature increased to different degrees. The purpose of this paper is to investigate the effects of structural and interlayer cations on thermal properties of montmorillonite. 2 Experiment 1 Bentonite The samples are collected from different areas in Jilin Province. Bentonite used in the experiment is sodium bentonite from Liufangzi in Gongzhuling City sample 1 sodium bentonite sample 2 and calcium bentonite sample 3 from Jiutai and calcium bentonite from Panshi sample 4. Content of montmorillonite and information on associated minerals of these samples are shown in Table 1. Table 1 Characteristics of bentonite Sample Montmorillonite /% Associated minerals christobalite 2. 0% quartz 17. 0% christobalite 9. 1% quartz 7. 7% Feldspar 3. 0% christobalite 20. 6% quartz 23. 5% feldspar 3. 0% calcite 2. 0% quartz 2. 8% 2 Purification of montmorillonite Before measurement the soluble salts associated minerals and free SiO 2 were removed from raw soil samples respectively. Montmorillonite particles smaller than 2 μm were extracted so as to get samples with montmorillonite content higher than 97% which were used as samples. 3 Chemical analysis of the purified samples Chemical compositions of the purified samples were analyzed according to silicate analysis in reference Li Measurement of cation-exchange capacity and exchanged cations of the purified samples Cation-exchange capacity and exchanged cation of the purified samples were tested according to physical-chemical properties test method in reference Li Thermal analysis of purified samples Instrument types LCT -2B test conditions TG 10 mv DTA 100 mv heating rate 20 /min sample mass 10 mg. 3 Results and discussion 3. 1 Structure of montmorillonite Based on chemical analysis Table 2 and cation-exchange capacity Table 3 results structure formula and layer charge were calculated by structural formula calculation method Table 4 Lin Table 1 Chemistry analysis results of montmorillonite samples /% Sample Na 2 O K 2 O CaO MgO SiO 2 FeO Fe 2 O 3 MnO TiO 2 Al 2 O 3 LOS Total Table 2 Exchanged cation and cation-exchange capacity /mmol 100 g - 1 Sample K + Na + Ca 2 + Mg 2 + EC ECE

3 90 Chai M. Li C. X. Jiang W. et al. Table 4 Structure formula and layer charge of montmorillonite Sample Tetrahedron Si Al Octahedron Al Fe III Fe II Mg Layer charge It can be seen from Tables 2-4 that samples 1 and 2 are sodium bentonite while 3 4 are calcium bentonite. Compared to sample 1 the sample 2 exhibits higher cation-exchange capacity while the samples 3 and 4 are of similar capacity. According to Table 4 while the samples 1 and 4 belong to low layer charge montmorillonite sample 2 is attributed to high layer charge montmorillonite and the sample 3 is intermediate type montmorillonite Lin TG -DTA results Fig. 1 shows thermal analysis curves of four samples. DTA curve of montmorillonite can be divided into three regions. The first endothermic peak is caused by removal of interlayer water. The second exothermic / endothermic peak arises from removal of structural hydroxyl in montmorillonite. The exothermic peak at above 800 is related to collapse of montmorillonite structure and formation of new phases. In TG curves ratio of weight loss percentage at any temperature to that at 1000 is defined as weight loss ratio denoted as α. The curves of α versus temperature are shown in Fig. 2. Because of structural differences among montmorillonite samples whether for removal of bound water or removal of structural water relationships between α and t are slightly different. The α - t curve a within 200 corresponds to removal of bound water while α - t curve b at a- bove 600 is attributed to removal of hydroxyl water from montmorillonite Sarikaya et al According to Coat and Redfern 1964 reaction equation αa s = βb s + cc g Fig. 1 TG -DTA curves of 4 montmorillonite samples Fig. 2 Relationships between α and temperature Where C represents volatile components such as H 2 O and CO 2. Both types of dehydrations of montmorillonite can be described using this equation. The re-

4 Effect of cations on thermal properties of montmorillonite 91 action rate of A is expressed as dα dt = k 1 - α n 1 Where α is weight loss ratio shown in Fig. 2 and n represents reaction order k is rate constant at the same time k = Ae - E /RT 2 Where A is frequency factor and E is activation energy. Heating rate is denoted as β β = dt /dt 3 Because solid state adsorption-desorption process is approximately a zero order reaction therefore n = 0 the simultaneous equations of 1-3 follow ln [ ] ln 1 - α - 2 = - E T RT + ln AR βe Fig. 3 ( ) ( 1-2RT ) [ ] Calculated curves of samples E Plotting ln - ln 1 - α /T 2 versus 10 3 /T results in two lines a b where line a is calculated from interlayer water removal and line b from structural water removal as shown in Fig. 3. The slope of the lines is - E /R and the intercept is ln AR /βe 1-2RT /E. T represents Kelvin temperature β is heating rate R is gas constant E is activation energy A frequency factor. E is calculated from Fig. 3 listed in Table 5 Sarikaya et al The calculation results indicate that for removal of bound water activation energy of Na montmorillonite is obviously smaller than that of Ca montmorillonite while with respect to removal of hydroxyl water the activation energy of the former is higher than that of the latter. Table 6 shows hydration energies and ionic potentials of related cations. By comparison of hydration energies of interlayer cations it can be deduced that the smaller the hydration energy the lower the activation energy suggesting that it has to overcome hydration energy barrier to remove interlayer water. Therefore dehydration activation energy of montmorillonite corresponds to hydration energy of ions Relationship among E a dehydration temperature and interlayer cations exchanged and layer charge distribution Fig. 2 indicates that end temperature for interlayer water removal increases for samples 1-4 in turn and there is obvious difference among weight loss ratios. The interlayer cations of montmorillonite locate in complex triangle oxygen lattice and coordinate with lone pair electrons of H 2 O forming hydrated ions. For samples 1 2 interlayer cations are mainly sodium ions which coordination with water molecules is unidirectional. A layer of water is adsorbed in interlayer and weight loss ratio α is in the range of It can be seen from Table 6 that ions like K + and Na + are of small ionic potentials thus their hydration energies and binding force with water molecules are low. Consequently they display low dehydration end temperature. For samples 3 4 interlayer cations are predominated by Ca 2 + and Mg 2 + ions. Because of large ionic potential these cations separate from oxygen lattice at early hydration stage moving to layer containing bound water. They bidirectionally co-

5 92 Chai M. Li C. X. Jiang W. et al. ordinate with water molecules thus two layers of water are adsorbed in interlayer. α is in the range of Because of high hydration energy these ions strongly bind with water molecules. Therefore these samples exhibit high dehydration end temperature. As for sodium bentonite the dehydration temperature can be explained by number and location of interlayer charges. As listed in Table 4 sample 1 is of less layer charges all of which locates in octahedra. Negative charges caused by octahedra replacement distribute on ten basic oxygen atoms Coats et al thus strength of a single Na-O bond is low. Under isolation of bound water interlayer cations weakly bond to oxygen atoms. As a result binding forces between Na-H 2 O-O are weak. It is therefore difficult to remove bound water from sample 1 leading to low end temperature. While sample 2 is of higher layer charge part of which distribute in tetrahedra. Negative charges caused by tetrahedra replacement distribute on three basic oxygen atoms Coats et al A single Na-O bond is of high energy so the binding strengths of Na-H 2 O-O are higher than that of sample 1. At the same time based on Table 5 due to relatively high content of Ca in sample 2 the average hydration energy is high. Therefore it is difficult to remove bound water from sample 2 which thus exhibits high dehydration end temperature. Table 5 TG -DTA analysis results Sample First absorption of heat T / α Hydration energy of layer cation /kj mol - 1 E a /kj mol Sample Second absorption of heat T / α Potential energy of octahedral cation /kj mol - 1 ΔW /% E b /kj mol Table 6 Hydration energy and ionic potentials related cations after Coats et al Ion K + Na + Ca 2 + Mg 2 + Fe 2 + Fe 3 + Al 3 + Hydration energy /J mol Ionic potential Relationship among dehydroxylation temperature E b and interlayer /structrural cations and layer charge distribution Because of the negative charge originated from part replacement of octahedral Al 3 + by Mg 2 + or Fe 2 + cations are adsorbed in interlayer of montmorillonite and hydroxyl groups occur in the structure. Nonbridging oxygen interaction in the structure is mainly M-O-H M is structural cation. At the same time interlayer cation N interacts with oxygen through electrostatic force forming N-O-H N represents interlayer cation. Therefore the larger the ionic potential of M the bigger the interaction strength with O thus the higher the temperature and activation energy required to break bonds. Therefore with the increase amount of Al in octahedra substituted by Fe and Mg

6 Effect of cations on thermal properties of montmorillonite 93 the dehydroxylation temperature and activation energy decrease. The charge is balanced by the compensation effect of interlayer cations. Different distance and ionic potential lead to different interaction strength. The higher the ionic potential of N ion is the stronger the interaction for it with octahedral oxygen is. Therefore the binding strength between M and octahedral oxygen reduces and the energy required breaking the bond become smaller and the dehydroxylation temperature and activation energy is low. Generally speaking because the ionic potential of K + and Na + is small the binding strength which toward octahedral oxygen is weak while the interaction between M and octahedral oxygen is strong. Therefore it requires higher energy to destroy the structure consequently the the activation dehydroxylation is bigger and dehydroxylation temperature is higher. While Ca 2 + and Mg 2 + is on the contrary. Tables 3-5 show that there is higher Al content in samples 1 2. At the same time the interlayer cation is dominated by Na +. As a result the dehydroxylation temperature and E b is relatively high. Moreover the substitution amount and the activation energy of sample 1 are bigger than that of sample 2. While sample 3 and 4 exhibit higher replacement a- mount and the interlayer cation is dominated by Ca 2 + and Mg 2 + accordingly the dehydroxylation temperature and activation energy is low. 4 Conclusion temperature and activation energy of interlayer water removal. Type and amount of both octahedral and interlayer cations affect significantly the removal and activation energy of structural water in montmorillonite. References Coats A W Redfern J P Kinetic parameters from thermogravimetric data. Nature He H P Xie X D Guo J G et al Thermal stability of cation exchanged montmorillonite. Journal of Mineralogy and Petrology in Chinese with English abstract Li L Z Rock and mineral analysis 3 th Edition. Beijing Geological Publishing House in Chinese Li P Y Yuan W S Lin H F Discussion on factors influencing interlayer spacing variation of montmorillonite. Bull. Nanjing Inst. Geol. M. R. Chinese Acad. Geol. Sci in Chinese with English abstract Lin H F Calculation and significance of montmorillonite structure. Geological Research in Chinese Sarikaya Y Onal M Baran B et al The effection of thermal treatment on some of the physicochemical properties of a bentonite. Clays and Clay Minerals Wen Y K Shao J Ionic polarization introduction. Hefei Anhui Education Press in Chinese Wu X P Liao Z W Advance of clay minerals interlayer. Nature Magazine in Chinese with English abstract Type of interlayer cations plays a decisive role in