INDIAN INSTITUTE OF TECHNOLOGY

Transcription

1 INDIAN INSTITUTE OF TECHNOLOGY Madras, Chennai PROJECT REPORT [ TO ] ON METHODS FOR GEL SOLIDIFICATION TO FORM A BLOCK CARTRIDGE FOR WATER PURIFICATION GUIDED BY Prof. T. Pradeep DST-Unit on Nanoscience Department of Chemistry and Sophisticated Analytical Instrument Facility IIT-Madras, Chennai. SUBMITTED BY VED PRAKASH 3 RD YEAR, NISER Date of submission:

2 Declaration I hereby declare that this project entitled Methods for gel solidification to form a block cartridge for water purification is an authentic record of research work carried out by me under the supervision of Prof. T. Pradeep, DST-Unit on Nanoscience, at the Department of Chemistry and Sophisticated Analytical Instrument Facility, Indian Institute of Technology-Madras, Chennai, India. VED PRAKASH 3 RD YEAR, NISER 2

3 CERTIFICATE INDIAN INSTITUTE OF TECHNOLOGY Madras, Chennai DST-Unit on Nanoscience Department of Chemistry and Sophisticated Analytical Instrument Facility Certified that the project work described in this report entitled METHODS FOR GEL SOLIDIFICATION TO FORM A BLOCK CARTRIDGE FOR WATER PURIFICATION has been carried by under my supervision at the DST-Unit on Nanoscience, Department of Chemistry and Sophisticated Analytical Instrument Facility, Indian Institute of Technology, Madras, Chennai, India. Prof. T. Pradeep Date: , Place: Chennai. PROFESSOR DST-Unit on Nanoscience Department of Chemistry and SAIF IIT-Madras, Chennai. 3

4 ACKNOWLEDGEMENT I would like to thank Prof. T.Pradeep who has been a source of inspiration and constant guidance to me throughout my project his invaluable suggestions and ideas that was pivotal for my success. I would also like to thank Mr. Anshup, Mr. Antony & Mr. Shihab who have provided constant inputs leading to the success of my work. I would also like to thank Mr. Uday Shankar & Dr. Bhatt for technical the support. Without their helpful guidance the project wouldn t have been a success. I would also like to thank Mr. Sajanlal for helping me with SEM. I would also like to thank all my friends for supporting me throughout my project and making this internship a memorable experience. 4

5 ABSTRACT Clean potable water is a basic necessity of life. Water around us is generally unfit for direct consumption. From centuries, technologies have been developed all around the world for water purification. Fluoride ion is one of the main inorganic contaminant in water in many parts of the world. Excess consumption of fluoride causes fluorosis. Our aim is to develop a commercially viable method for efficient removal of F - from water. A very high performance material (gel) has already been prepared by the people working here. The challenge is to solidify the gel into a solid block for commercial use. 5

6 TABLE OF CONTENTS INTRODUCTION.7 EXPERIMENTAL SECTION.7 EXPERIMENT 1: Pressurization followed by freeze drying.7 EXPERIMENT 2: Effect of solvent.8 EXPERIMENT 3: Effect of water content..8 EXPERIMENT 4: Cement as a binder.10 EXPERIMENT 5: Silica gel as a binder 10 EXPERIMENT 6: CaO & sugar as a binder..12 EXPERIMENT 7: Chitosan & Montmorillonite as a binder. 12 EXPERIMENT 8: Chitosan as a binder 13 Future plans & prospects.15 References.15 6

7 INTRODUCTION The present work deals with solidifying a gel of AlOOH into solid water stable block which can be used as a cartridge in water purifiers for fluoride removal. The gel is composed of AlOOH nanoclusters networked together; with water trapped in the interstitial spaces. The gel is composed of around 90% water! The basic requirements of a good water filtration cartridge are: 1. It should have high performance. 2. It should be hydrophilic & non-toxic. 3. It should work in the neutral ph range 4. It should have high surface area. 5. It should be highly porous in order to allow water to pass through it. 6. It should be water stable so that it does not decay in water. 7. It should have good physical strength for its prolonged life. The first three goals had already been achieved by my senior colleagues working here. Fourth & fifth could possibly be achieved using freeze-drying technique. The sixth goal is achievable as most of the gels are irreversible, i.e., once they are dried, they do not re-dissolve. Further, the strength can be increased by adding some binder or cross-linking agent. Care has to be taken that the material does not lose its fluoride removal property & remains non-toxic. EXPERIMENTAL SECTION Experiment 1: Pressurization followed by freeze drying: A small sample was pressurized in a stainless steel container under N 2 atmosphere for 1 day at -10 o C at 50 atm. The sample was then freeze dried at a pressure of 60 Pa. This resulted in a fine powder. Another sample was mixed with excess of water & was allowed to settle down for an hour. The settled gel was filled into a PVC container & was sealed inside a hydrothermal bomb. The left-over space was filled with water. The whole setup was then placed in freezer at -10 o C for 4 days. The water content in the sample froze & during the process, it expanded. As the chamber was sealed, there was no space for expansion. This developed a very high mechanical pressure on the sample under frozen condition. Then, the sample was then freeze dried at 60 Pa. This process resulted in larger sized granules as compared to the earlier process. But still, every sample was a powder. The possible reasons for the disintegration of the sample into fine powder were: 1. There is not enough networking between the clusters. 2. Water content was very high. 7

8 The networking could possibly be enhanced by changing the solvent present in the gel. Water content can be reduced by evaporating the water in a hot air oven. EXPERIMENT 2 Effect of solvent: 500mg of sample was mixed with 5 ml of solvent & was then allowed to stand for air drying. S. No. Ethanol Water 0bservation % colloid % colloid % colloid % colloid No solubility Methanol % suspension % suspension % % No solubility acetone Homogeneous solid suspension % colloid % colloid % colloid % colloid From the above observations, we conclude that the gel interacts better with binary solvents rather than a pure solvent. This can be exploited in manufacture of finer grains for enhanced adsorption. EXPERIMENT 3 Effect of water content: Samples of gel were air dried in hot air oven at 60 o C until their water contents were reduced to required levels (90%, 80%, 70%, 60% & 50%). The samples were then frozen overnight at -10 o C & then freeze dried. It was observed that the average granule size increased as the water content was reduced. 8

9 Figure 1: SEM image of freeze dried gel. Micro-porous structures can be seen in the bulk. Figure 2: SEM image of freeze dried gel with reduced moisture (50%). Micro-porous structure content reduces as water content reduces. This indicates that it is better to freeze dry the gel with maximum quantity of water in it in order to impart maximum microporosity. 9

10 se Figure 3 All of the above observations showed that the material does not have enough cross links to form porous solid block. So, this indicates requirement of a suitable binder. EXPERIMENT 4 Cement as a binder: Cement was added to the freeze-dried powder (AlOOH) in different ratios to achieve the adequate strength. None of the ratios in acceptable range (up to 50 % cement by weight) provided the required strength. Possibly, AlOOH is not able to form networks with cement. EXPERIMENT 5 Silica gel as a binder: Silica is known to form strong networks. So possibly, it can be coupled with the powder to form the required structure. Different methods of silica gel synthesis were tried. The major concern in the synthesis was that the cost of starting material & synthesis should be low & processed material should be non-toxic. Synthesis using tartaric acid & sodium silicate: 50% sodium silicate solution was added drop-wise to 1N tartaric acid until a solid gel is formed. The gel was then aged in oven at 60 o C for 3 hrs. A milky white gel is obtained. The gel was then dried at 70 o C for 1 day. The gel contracted to form a hard solid mass with loose granules sticking to its outer surface. This was caused due to capillary stress. 10

11 Figure 4: silica gel synthesized using sodium silicate & tartaric acid. In order to reduce the capillary stress, the polarity of the liquid dispersed in the solid should be reduced. This can be done by solvent exchange. The solvent (presently water) was replaced by washing & soaking the gel in ethanol, then by chloroform & finally by DCM. The gel was then air dried at 40 o C to get dried & highly porous aerogel. Still, the aerogel didn t have enough strength to serve the purpose. *Silica gels of different ph were also synthesized by varying the strength of the acid taken. Synthesis using tetramethoxyortho silane (TMOS) [1] Another method of aerogel synthesis was tried using TMOS & NH 4 OH in methanol. This method resulted in much denser & stronger gel as compared to the earlier method but could not be converted into aerogel as the solvent could not be exchanged due to lower pore size. Figure 5: silica gel produced using TMOS Another method was tried in which the freeze dried powder was mixed with the starting materials for preparation of silica gel in ratios 90%, 80%, 70%, 60% & 50% by weight. The gel was allowed to form & age. Hot air oven drying & drying at reduced pressure were tried but none gave the adequate results. 11

12 EXPERIMENT 6 CaO & sugar as a binder: CaO & sugar powder were mixed together in different ratios (10%, 20%, 30%... 90%) with minimum quantity of water to form a thick paste. The paste was dried in oven at 50 o C. Some of the samples (1:1 sugar : CaO) became rock-solid on complete drying. They were water stable too. The only demerit of this material is that its ph is too high. EXPERIMENT 7 Chitosan & Montmorillonite as a binder: Chitosan, a cationic polymer combined by β-1-4 glucosidic linkage, is the main product obtained from the alkaline deacetylation of chitin. It is the major structural polysaccharide found in crustaceans, insects and lower plants. [2] Figure 7 Montmorillonite is clay. Various ratios of the two were tried for stability. A little stability was observed but not too strong. 12

13 Figure 4: Cross linking process of chitosan treated with glutaraldehyde. Glutaraldehyde (cross linking agent) was added to the samples in different quantities but no significant strength increment was observed. EXPERIMENT 8 Chitosan as a binder: 500 mg of 3% chitosan solution in 2N HCl was added to 600 mg of AlOOH powder to make a thick paste. The paste was cast into plastic mould & was then dried in oven at 50 o C. After complete drying, the sample became rock solid with a very high compressive strength. The material was then soaked in glutaraldehyde overnight in order to initiate cross linking in chitosan. The cross linked material was then oven dried at 50 o C for 10 hrs. It was then tested for water stability. Both cross linked as well as uncross linked, showed good water stability for 2 hrs. After that, the uncross linked material started decaying while the other one was still stable. The cross linked material started decaying after about 12 hrs of soaking. But still the mechanical strength was good. 13

14 Figure 14

15 The performances of the materials were tested & it was found that they retained the Fluoride removal capacity up to 75% (without optimization of the block synthesis process). Advantages of the technique: 1. Relatively very less amount of binder is required. (about 2.5% by weight) 2. Block preparation is being done at a low ph which is good for our material. 3. All the processing can be done at the room temperature. The vision forward: Future plans & prospects: 1. The acid used for dissolving chitosan was 2N HCl. This acid strength is too high to be neutralized or washed so that the ph is brought up to the neutral region. Experimentally it has been found that 0.15N HCl is good enough to make 3% chitosan solution. 2. Further easier methods of acid removal need to be developed. Presently, the method which is generally used is washing with distilled water until the ph is around 7. Acid neutralization using an alkali needs to be tested. This might impart further strength to the material as acid neutralization would result in precipitation of dissolved chitosan. 3. Use of better & non-toxic cross-linking agents need to be experimented. Presently the cross linking agent used is glutaraldehyde. It is known to be toxic. We cannot take the risk of using poisonous starting materials for synthesis of the block cartridge. Non-toxic cross linking agents like sodium citrate can be used for imparting the required strength to the block. 4. Water quantity & drying technique can be manipulated to impart greater strength as well as porosity to the block. Hot air drying introduces cracking in the block. Freeze drying or low pressure drying of the block with adequate water quantity may be of great use. 5. Evaporation of water takes a lot of time & generates a lot of capillary stress. Further water soluble binders have a possibility of re-dissolving in to water. Use of naturally occurring organic solvent soluble binders would be the possible solution. Care needs to be taken that once the block is dried, it should be sufficiently hydrophilic. 6. Chitosan is a costly polymer (about Rs.1000/kg). Different natural fibers such as hair, coconut shell fiber, silk, straw, banana tree fiber, palm leaf fiber etc. may be used as a substitute for chitosan. References: Effect of Crosslinking Agents on Chitosan Microspheres in Controlled Release of Diclofenac Sodium Polímeros: Ciência e Tecnologia, vol. 15, n 1, p. 6-12,