Luận văn thạc sĩ HUS evaluation of sterilization possibility in water environment of activated nano mno2 coated on calcined laterite

ĐẠI HỌC QUỐC GIA HÀ NỘI TRƯỜNG ĐẠI HỌC KHOA HỌC TỰ NHIÊN - Cao Việt EVALUATION OF STERILIZATION POSSIBILITY IN WATER ENVIRONMENT OF ACTIVATED NANO MnO2 COATED ON CALCINED LATERITE CHUYÊN NGÀNH: QUẢN LÝ CHẤT THẢI VÀ XỬ LÝ VÙNG Ơ NHIỄM (CHƯƠNG TRÌNH ĐÀO TẠO QUỐC TẾ) LUẬN VĂN THẠC SĨ KHOA HỌC GIÁO VIÊN HƯỚNG DẪN: PGS.TS TRẦN HỒNG CÔN Hà Nội - 2011 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Table of contents Abbreviation i List of Figures ii List of Tables iii Chapter INTRODUCTION 1.1 Water situation in general 1.2 Water sterilization 1.2.1 Boiling 1.2.2 Chlorine 1.2.3 Ozone 1.2.4 Ultraviolet light 1.2.5 Hydrogen peroxide 1.2.6 Solar disinfection 1.2.7 Photocatalysis on semiconductors 1.2.8 High speed water sterilization using one-dimensional nanostructures 1.3 Nanotechnology 1.4 Manganese dioxide 10 1.5 Laterite 11 Chapter 13 OBJECTIVES AND RESEARCH METHODS 13 2.1 Objectives 13 2.2 Materials and Research methods 13 2.2.1 Material and instruments 13 2.2.2 Research methods 14 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com 2.2.2.1 Synthesis of nano MnO2 adsorbents 14 2.2.2.3 Investigation of sterilizing capability of nano MnO2 adsorbents 15 Chapter 17 RESULTS AND DISCUSSION 17 3.1 Synthesis of nano MnO2 adsorbents 17 3.2 Investigation of sterilizing capability of nano manganese dioxide 23 3.2.1 Investigation in static condition 24 3.2.2 Investigation in dynamic condition 28 3.3 Mechanism of sterilization of MnO2 coated on calcined laterite in water 33 3.3.1 Investigation the influence of Mn2+ in sterilizing capability 33 3.3.2 Examine the mechanism of sterilization of MnO2 35 Chapter 38 CONCLUSION 38 REFERENCES 40 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Abbreviation MD Manganese Dioxide UV Ultraviolet DNA Deoxyribonucleic Acid SODIS Solar Disinfection CNT Carbon Nanotube AgNWs Silver Nanowires‟ TEM Transmission Electron Microscopy SEM Scanning Electron Microscope EPA Environmental Protection Agency E coli Escherichia coli BRM Bacteria removing material MPN Most probable number EBCT Empty Batch Contact Time i LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com List of Figures Figure 1: Nanoscale materials 10 Figure 3: Coating process 18 Figure 4: MnO2 nanoparticles with the magnification of 40000 times 19 Figure 5: MnO2 nanoparticles with the magnification of 60000 times 20 Figure 6: MnO2 nanoparticles with the magnification of 100000 times 21 Figure 7: Creation of adsorbent coating by nano MnO2 particles (100k) 22 Figure 8: Creation of adsorbent coating by nano MnO2 particles (200k) 22 Figure 9: Shaking equipment for static condition investigation 23 Figure 10: Column device for dynamic condition investigation 24 Figure 11: Samples in contact time‟s influence experiment 25 Figure 13: Samples in BRM/water ratio‟s influence experiment 27 Figure 14: Samples in BRM/water ratio‟s influence experiment 28 Figure 15: Model of column device 29 Figure 16: Samples in flow rate in BRM column‟s influence experiments 30 Figure 17: Influence of flow rate on bacteria sterilizing in BRM column 30 Figure 18: Samples in the experiments 32 Figure 19: Influence of column height on bacteria sterilizing in BRM column 32 Figure 21: Influence of Mn2+ in sterilizing capabilities 35 ii LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com List of Tables Table 1: Influence of contact time on bacteria sterilizing 24 Table 2: Influence of the ratio of BRM and water on bacteria sterilizing 27 Table 3: Influence of flow rate on bacteria sterilizing in BRM column 29 Table 4: Influence of column height on bacteria sterilizing in BRM column 31 Table 5: Influence of Mn2+ in sterilizing capabilities 34 iii LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 1: Introduction Chapter INTRODUCTION 1.1 Water situation in general Water is one of the world‟s most essential demands for human life, and the origin of all animal and plant life on the planet Civilization would be impossible without steady supply of fresh and pure water and it has been considered a plentiful natural resource because the sensitive hydrosphere covers about 75% of the Earth's surface Its total water content is distributed among the main components of the atmosphere, the biosphere, oceans and continents However, 97% of the Earth's water is salty ocean water, which is unusable for most human activities Much of the remaining 3% of the total global water resource, which is fresh-water, is locked away in glaciers and icebergs Approximately 20% of the freshwater resources are found as groundwater, and only 1% is thought to be easily accessible surface water located in biomass, rivers, lakes, soil moisture, and distributed in the atmosphere as water vapor [1] In the process of rapid development of science and technology, the demand for pure water is increasing to serve multifarious purposes in different types of industries Global water consumption raised six folds in the past century, double the rate of population growth In addition, the boom in world‟s population during recent decades, has contributed to the dramatically rising demand of pure water usage for both household and industrial purposes The high population density and industrialization speed have triggered the hydrosphere to be polluted with inorganic and organic matters at a considerable rate Moreover, to satisfy the food demand, a number of harmful chemicals such as pesticides and herbicides LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 1: Introduction are used in order to improve the productivity in agricultural production, which also causes the scarcity of clean resources [1] The contamination of ground water (mostly by toxic metal ions due to both natural and anthropogenic reasons) is also one of concerning issues on clean water It is necessary to assess the quality of water used in industry, household activities and drinking purpose Understanding of the importance of clean water in human life, many countries has gradually adjusted their environmental regulations more stringently to reserve clean water resources With the purpose of overcoming the water pollution problems, and to meet the stricter environmental regulations, scientists and researchers have focused on improving exist water purification processes and approaching to alternative water treatment technologies as well, so as to increase the efficiency of those decontamination methods It is surveyed that human awareness about the seriousness of water pollution has enhanced over the world People have also started realizing that water is not an unlimited resource, hence it needs to be protected and smartly used An ideal water treatment process should have the capability to mineralize completely all the toxic organic components without leaving behind any harmful by-products and to recover all toxic metals from wastewater In broader classification, biological, mechanical, thermal, chemical or physical treatments, or their combinations may be applied to purify contaminated water The choice of the proper water treatment processes depend on the nature of the pollutants presenting in water, and on the acceptable contamination level in treated water There are two main purposes of water treatment study – the reduction of contaminant level in the discharged stream to meet environmental standards, and LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 1: Introduction the purification of water to ultrapure water in order to be able to use in semiconductor, microelectronic and pharmaceutical industries Moreover, the cost or effectiveness of the water treatment processes also plays a significant role in choosing a particular one Biodegradation, adsorption in activated carbon, air stripping, incineration, ion-exchange, coagulation-precipitation, membrane separation, thermal and catalytic oxidation, oxidation by permanganate, chlorine, ozone and hydrogen peroxide are widely applied in conventional water treatment processes for organic and inorganic pollutant containing water Besides advantages, each process has their own shortcomings which are being improved gradually via new technologies [1, 2] 1.2 Water sterilization Water sterilization technology is useful in various ways for our daily life For example, it is used in water and sewerage systems treatment Methods commonly used for sterilization include chemicals, heat, ultraviolet (UV) radiation, and ozone Chemicals (chlorine, peroxide, etc.) are utilized extensively for sterilization because of their simplicity; however, they probably form unexpected effects, such as modifying the quality of the target In addition, sterilization by chlorine usually generates odorous substances and bio-hazardous materials [2] It is not totally accurate to assess whether water is of an appropriate quality only by visual examination Simple procedures such as boiling or the use of a household activated carbon filter are not sufficient for treating all the possible contaminants that maybe present in water from an unknown source Even natural spring water – considered safe for all practical purposes in the 1800s – must now be tested before determining what kind of treatment, if any, is needed Chemical LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 1: Introduction analysis, while expensive, is the only way to obtain the information necessary for deciding on the appropriate method of purification [3] Simple techniques for treating water at home, such as chlorination, filters, and solar disinfection, and storing it in safe containers could save a huge number of lives each year Sterilization is accomplished both by filtering out harmful microbes by and also adding disinfectant chemicals in the last step in purifying drinking water Water is disinfected to kill any pathogens which pass through the filters Possible pathogens include viruses, bacteria, including Escherichia coli, Campylobacter and Shigella, and protozoa, including Giardia lamblia and other cryptosporidia In most developed countries, public water supplies are required to maintain a residual disinfecting agent throughout the distribution system, in which water may remain for days before reaching the consumer Following the introduction of any chemical disinfecting agent, water is usually held in temporary storage often called a contact tank or clear well to allow the disinfecting action to complete [4] 1.2.1 Boiling Boiling is an easy, cheap and common way to eliminate contaminations and microorganisms in developing countries, but this method is only practical for small amounts When the water has boiled for – 10 all the pathogens have been killed and the water is safe to drink [2] The main disadvantage of this method is that it requires a continuous source of heat and appropriate equipment LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion Table 2: Influence of the ratio of BRM and water on bacteria sterilizing Sample BRM/polluted water (g/mL) 0.25/100 0.5/100 Bacteria colony (MPN/100ml) 154 21 Sample 1/100 Sample Sample 4 1.5/100 2/100 Sample Sample Figure 13: Samples in BRM/water ratio’s influence experiment 27 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion Figure 14: Samples in BRM/water ratio’s influence experiment In the first sample, the bacteria colony number is very high (154) at the ratio of 0.25/100 When the ratio increases to 0.5/100, the bacteria number decreases dramatically to 21 This happens as the amount of BRM in the second sample is higher than that of the first one, which leads to more chances for bacteria to contacting with BRM Figure 14 shows the optimal amount of BRM is 1.5g per 100ml water 3.2.2 Investigation in dynamic condition The parameters such as flow rate and the height of BRM column were tested to see their influence on the sterilizing capability 3.2.2.1 Influence of flow rate on bacteria sterilizing in BRM column The raw water was treated, diluted then transferred to a 2L tank The flow rate was controlled by input and output valves The flow rate of water column increased along the row of 1, 2.2, 2.8, 3, 4, ml/min (0.18, 0.39, 0.5, 0.53, 0.71, 0.88 mL/min.cm2) The diameter of column is 1.8cm; the height of material is 5cm 28 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion The results of this investigation are given in Table 3, Figure 16-17 Figure 15: Model of column device Table 3: Influence of flow rate on bacteria sterilizing in BRM column Sample Flow rate (ml/min.cm ) 0.18 0.39 0.5 Bacteria colony (MPN/100mL) 0 Sample Sample 0.53 0.71 0.88 12 56 140 Sample 29 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion Sample Sample Sample Figure 16: Samples in flow rate in BRM column’s influence experiments Figure 17: Influence of flow rate on bacteria sterilizing in BRM column The amount of total coliform in sample 4, and is 12, 56 and 140 MPN/100mL respectively Those results mean that the bacteria had not been killed effectively due to the lack of contact time between bacteria and MnO2 If the flow rate is 0.53mL/min.cm2, the analysis result shows that there is still 12MPN/100ml available Even the removal capacity increased remarkably, it 30 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion still did not meet the drinking water standard of Environmental Protection Agency [28] When the flow rate decreases to 0.5mL/min.cm2 or lower, the sterilizing capability is complete Figure 17 indicates that the slower the flow rate is, the better sterilizing is achieved For the optimal flow rate, 0.5 mL/min.cm2 will be chosen for the next experiments 3.2.2.2 Influence of column height on bacteria sterilizing in BRM column The raw water was treated, diluted then transferred to the 2L tank The flow rate was controlled by input and output valves The height of material column increased along the row of 1, 2, 3, 4, cm The diameter of column is 1.8cm; the flow rate is 0.5ml/min.cm2 The results are given in Table 4, Figure 18-19 Table 4: Influence of column height on bacteria sterilizing in BRM column Column height (cm) Bacteria colony (MPN/100mL) 250 Sample 100 Sample 10 Sample 31 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion Sample Sample Figure 18: Samples in the experiments Figure 19: Influence of column height on bacteria sterilizing in BRM column The amount of total coliform in the sample and were 250 and 100 MPN/100mL respectively The results mean that the bacteria had not been killed effectively because of the insufficiency of contact time between bacteria and MnO2 (the column height is not long enough) 32 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion If the height is 3cm, the analysis result shows that there is still 10MPN/100ml available Even the removal capacity increased remarkable but it still did not meet the EPA drinking water standard [28] When the height increases to 4cm or higher, the sterilizing capabilities is completely It is apparent that the height of BRM column and the flow rate of water strongly influence the sterilizing ability The sterilizing ability of column increases along with the increase of the BRM layer height In contrast, it decreases when the flow rate increases At current time, in many published reports, authors used parameter EBCT (Empty Batch Contact Time) for characterization of both of column filter parameters above EBCT = = From the results, V = πR2h = 3.14 x 0.92 x = 10.17 cm3 q = 2.8 mL/min So EBCT = = 3.63 mins In the case of the investigation, the minimum EBCT for safely bacterial sterilizing is 3.63 3.3 Mechanism of sterilization of MnO2 coated on calcined laterite in water 3.3.1 Investigation the influence of Mn2+ in sterilizing capability The experiment was performed to analysis the influence of Mn2+ on water sterilizing capability of MnO2 nanoparticles The four different concentrations which are ranging 0.1, and 10 ppm of Mn2+ were put into water samples with the available of MnO2 nanoparticles material 33 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion The processes were conducted in static condition (100mL waste water was treated by 0.5g BRM; contact time was 10 minutes; initial MPN in wastewater was 380) The results are shown in Table 5, Figure 20-21 Table 5: Influence of Mn2+ in sterilizing capabilities Sample 2+ Mn added (ppm) Bacteria (MPN/100ml) colony Sample 63 0.1 36 15 Sample Sample 10 Sample Sample Figure 20: Samples in the experiments 34 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion Figure 21: Influence of Mn2+ in sterilizing capabilities Figure 21 illustrates that the Mn2+ added in samples affects dramatically on the sterilizing capability, from 60 MPN/100ml of no Mn2+ to MPN/100ml of 10ppm Mn2+ Consequently, MnO2 added Mn2+ have better sterilization possibility than MnO2 itself In other words, ion manganese (II) has been shown to have effects on the sterilizing capability of BRM 3.3.2 Examine the mechanism of sterilization of MnO2 The mechanism of MnO2 for bacterial removal is still unclear Therefore, a mechanism of the process was proposed as follows: MnO2 attached (attacks) on bacteria cell as Figure 22: 35 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion Figure 1: Bacteria destruction - A healthy bacillus bacterial cell - Zooming in closer, MnO2 (light green) comes into contact with the cell wall The cell wall is vital to the bacteria because it ensures the organism can maintain its shape - As MnO2 molecules make contact with the cell wall, a reaction called an oxidative burst occurs which literally creates a tiny hole in the cell wall - A new hole created in the cell wall has injured the bacterium - The bacterium begins to lose its shape while MnO2 molecules continue creating holes in the cell wall - After thousands of MnO2 collisions over only a few seconds, the bacterial wall can no longer maintain its shape and the cell dies As can be seen from the previous results, ion manganese (II) has been shown to have effects on the sterilizing capability of BRM Therefore, the mechanism could be as follow: MnO2 + Mn2+ → [MnO2 Mn]2+ (1) [MnO2 Mn]2+ + nO2 → [MnO2.Mn].nO2 (2) [MnO2 Mn].nO2 → 2MnO2 + (n-1)O2 (3) 36 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 3: Results and Discussion Disintegration process of semi-product [MnO2.Mn].nO2 appeared trivalent or/and pentavalent Mn – high oxidation potential and very active species These species play as strong sterilization substances Therefore, in some circumstances, some raw water resources with Mn2+ pollution will have better performance in treating by nano MnO2 adsorbent 37 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 4: Conclusion Chapter CONCLUSION - The nano MnO2 solution was prepared as 2.2.2.1 The TEM images of solution clearly reveal the presence of a large quantity of MnO2 nanoparticles with lozenge shape and barked sphere with the diameters around 30nm - The BRM was prepared from calcined laterite and nano MnO2 solution The SEM images of material‟s surface shows the distribution of barked sphere shaped nano MnO2 all over laterite surface - Sterilizing possibilities is under the static condition was studied The detention time selected is 30 minute, and the preferable ratio of BRM/polluted water is 1.5g:100mL - Water can sterilize by means of use BRM as column filter All bacteria in water could safely exterminate when flowing through column filter with the minimum layer height of BRM 4cm and the maximum flow rate 0.5ml/min.cm2 or EBCT was guaranteed at least 3.6 minutes Both MnO2 nanoparticles solution and BRM were non toxic and economic, so they have a high potential to be applied for water sterilizing in water plants as well as at household scale - Ion manganese (II) affects the sterilizing capability of BRM It reacts with MnO2 to create semi-products [MnO2.Mn].nO2 which play as strong sterilization substances - The research obtains some positive results in creating a new material for wastewater sterilization, which may account for the global effort of saving clean water resources – which is currently one of the most concerning issues not only in Vietnam but also in other countries However, in order to apply those study 38 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com Chapter 4: Conclusion results in the real world, it is still required further investigations on the mechanism and the harms of manganese dioxide to water after treatment 39 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com REFERENCES 10 11 12 13 14 15 Van der Bruggen, B., C.E Isabel, and I.S Andrea, Chapter The Global Water Recycling Situation, in Sustainability Science and Engineering, Elsevier p 4162 Jan Davis, R.L., Engineering in emergencies – A practical guide for relief workers 2nd ed 2002 Water storage tips to assist in emergency preparedness 2010, Water Quality and Health Councils Conan, J., Water for life – community water security 2005 Keystone, J.S.R., S R., International travel health guide 2008 Block, S.S., Disinfection, sterilization, and preservation 2001: Lippincott Williams and Willkins Mark A Ratner, D.R., Nanotechnology: a gentle introduction to the next big idea 2003: Pearson Education Mishra, S.P., S.S Dubey, and D Tiwari, Inorganic particulates in removal of heavy metal toxic ions: IX Rapid and efficient removal of Hg(II) by hydrous manganese and tin oxides Journal of Colloid and Interface Science, 2004 279(1): p 61-67 Takamatsu, T., M Kawashima, and M Koyama, The role of Mn2+-rich hydrous manganese oxide in the accumulation of arsenic in lake sediments Water Research, 1985 19(8): p 1029-1032 Kawashima, M., et al., Phosphate adsorption onto hydrous manganese(IV) oxide in the presence of divalent cations Water Research, 1986 20(4): p 471475 Tripathy, S.S., J.-L Bersillon, and K Gopal, Adsorption of Cd2+ on hydrous manganese dioxide from aqueous solutions Desalination, 2006 194(1-3): p 1121 Kanungo, S.B., et al., Adsorption of Co2+, Ni2+, Cu2+, and Zn2+ onto amorphous hydrous manganese dioxide from simple (1-1) electrolyte solutions Journal of Colloid and Interface Science, 2004 269(1): p 11-21 Kanungo, S.B., S.S Tripathy, and Rajeev, Adsorption of Co, Ni, Cu, and Zn on hydrous manganese dioxide from complex electrolyte solutions resembling sea water in major ion content Journal of Colloid and Interface Science, 2004 269(1): p 1-10 Eliopoulos, D.G.a.M.E.-E., Geochemical and Mineralogical Characteristic of Fe-Ni and Bauxite Laterite Deposits of Greece Ore Geo Reviews, 2000 16: p 41-58 Bourman, R.P.a.C.D.O., A Critique of the Schellmann: Definition and Classification of Laterite Catena 2002 47: p 117-131 40 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com 16 17 18 19 20 21 22 23 24 Mamindy-Pajany, Arsenic adsorption onto hematite and goethite Comptes Rendus Chimie, 2009 12(8): p 876-881 S.V., D., Removal of arsenic from synthetic groundwater by adsorption using the combination of laterite and iron-modified activated carbon Journal of Water and Environment Technology 2008 6(1): p 43-54 Liu, K., Chen, Q., Hu, H., Yin, Z and B Wu, Pressure Acid Leaching of a Chinese Laterite Ore Containing Mainly Maghemite and Magnetite Hydrometallurgy In Press Mohapatra, M., Khatun, S and S Anand, Kinetics and Thermodynamics of Lead (II) Adsorption on Lateritic Nickel Ores of Indian Origin Chem Eng J., 2009 155: p 184-190 Yu, X., Zhu, L., Guo, B and S He, Adsorption of Mercury on Laterite from Guizhou Province, China J Environ Sci., 2008 20: p 1328-1334 Johnson, M.C., Wang, J., Li, Z., Lew, C M and Y Yan, Effect of Calcination and Polycrystallinity on Mechanical Properties of Nanoporous MFI Zeolites Mater Sci Eng., 2007 456: p 58-63 Sarikaya, Y., Sevinỗ, I and M Akinỗ, The Effect of Calcination Temperature on Some of the Adsorptive Properties of Fine Alumina Powders Obtained by Emulsion Evaporation Technique Powder Technol, 2001 116: p 109-114 EPA, Module 3: Characteristics of Particles - Particle Size Categories Cuong, L.M., Tổng hợp đánh giá khả hấp thụ asen, kim loại nặng hỗn hợp đồng kết tủa sắt hiđrôxit, mangan điơxit kích cỡ nanomet chất mang laterit cố định MnO2 có kích cỡ nanomet chất mang laterit 2010, HUS 25 Williams, D.B and C.B Carter, The Transmission Electron Microscope Transmission Electron Microscopy 2009, Springer US p 3-22 26 Goldstein, J., Scanning electron microscopy and x-ray microanalysis 2003: Kluwer Adacemic/Plenum Pulbishers 689 27 Thi, N.T., Nghiên cứu điều chế dung dịch Ag nano mật độ cao, ứng dụng chế tạo vật liệu tiệt trùng nước 2009 28 EPA, National Primary Drinking Water Regulations, in Total coliforms 41 LUAN VAN CHAT LUONG download : add luanvanchat@agmail.com ... Examine the mechanism of sterilization of MnO2 coated on calcined laterite in water There are two main purposes in this part: One is to examine whether the mechanism of sterilization of MnO2 coated. .. the investigation, the minimum EBCT for safely bacterial sterilizing is 3.63 3.3 Mechanism of sterilization of MnO2 coated on calcined laterite in water 3.3.1 Investigation the influence of Mn2+... sterilizing capability of nano manganese dioxide 23 3.2.1 Investigation in static condition 24 3.2.2 Investigation in dynamic condition 28 3.3 Mechanism of sterilization of MnO2 coated