wastewater treatment
Alfred Tolvtin
Sustainable Process Engineering, master's level
2019
Luleå University of Technology
I
A
cknowledgments
This thesis was carried out at Nanjing Tech University, China, and is the final task before receiving my master’s degree. When I moved to Luleå in the middle of August 2013, I never thought that five years could pass as quickly as it has. I thank my friends and classmates I’ve got to know these years for that. Thank you!
My thesis would not have been completed without the guidance from my supervisors. I firstly wish to thank Lixiong Zang at Nanjing Tech University for the opportunity to carry out my thesis in this area, and for all guidance I have gotten during the project. I also wish to thank my supervisor and examiner Liang Yu and Jonas Hedlund for all the help I received.Furthermore, I would like to thank Chongqing Wang for helping me with some characterization during my work.
Lastly, I want to thank the co-workers at Nanjing Tech University for the warm reception I got throughout my thesis work and SIDA-administrative authority for providing with a scholarship. Thank you!
II
A
bstract
Rhodamine B (RB) as a fluorescent dye has toxic effects in the environment, humanity and potentially harmful to ecological systems, therefore it needs to be removed. Adsorption is a simple and cost friendly approach, but the adsorption capacity of the reported adsorbents needs to improve.
Herein, the preparations of C@titania core-shell nanoparticles for the efficient elimination of RB from simulated textile wastewater was studied. The nanoparticles were prepared by first coating a layer of titania gel on ZIF-8 and HKUST-1 nanoparticles via hydrolysis of Titanium butoxide (TBOT) and followed by carbonization. XRD, FTIR, SEM, TEM, and N2 adsorption-desorption analysis was
used to explore the obtained products. The adsorption for RB in the simulated textile wastewater with different pH was investigated.
TEM pictures visibly illustrate the core-shell structures with a thickness of the titania layer of 14 to 25 nm. N2 adsorption-desorption results display the textural characteristics of the products with
mainly meso-pores and a relative high surface area of 107 and 43 m2 g-1 for C@titania core-shell
prepared from ZIF-8 and HKUST respectively. The equilibrium adsorption capacities have its climax at pH 7. The maximum RB adsorption capacities of the C@titania core-shell nanoparticles prepared from ZIF-8 and HKUST are 174.4 and 106.3 mg g-1, which is higher than the parental
carbons.
III
S
ammanfattning
Rhodamine B (RB) som fluorescerande färgämne har toxiska effekter i miljön, mänskligheten och potentiellt skadlig för ekologiska system, därför måste det tas bort. Adsorption är ett enkelt och kostnadseffektivt tillvägagångssätt, men adsorptionskapaciteten hos de rapporterade adsorbenterna behöver förbättras.
Här studerades beredningarna av C@titan kärnskal nanopartiklar för effektiv eliminering av RB från simulerad textilavlopp. Nanopartiklarna framställdes genom först belägger ett skikt titanoxidgel på ZIF-8 och HKUST-1 nanopartiklar via hydrolys av titanbutoxid (TBOT) och följt av karbonisering. XRD, FTIR, SEM, TEM och N2 adsorptions-desorptions analys användes för att utforska de erhållna produkterna. Adsorptionen för RB i den simulerade textilavloppet med olika pH undersöktes.
TEM-bilder illustrerar synligt kärnskalstrukturerna med en tjocklek av titanoxidskiktet 14 till 25. N2 adsorptions-desorptions resultatet visar de textuella egenskaperna hos produkterna med
huvudsakligen mesoporer och en relativt hög yt-area av 107 och 43 m2 g-1 för C@titan kärnskal
framställd från ZIF-8 respektive HKUST-1. Jämvikt adsorptionskapaciteten har sin klimax vid pH 7. Den maximala RB-adsorptionskapaciteten hos C@titan kärnskal nanopartiklar är 174,4 och 106,3 mg g-1, vilket är högre än föräldra-kolen.
IV
摘要
罗丹明 B（RB）作为一种荧光染料对环境，人类以及生态系统都具有极大的危害，因此，
罗丹明 B 的废水处理变得尤为重要。
吸附是一个简单并且经济适用的方法，并且以被广泛用于废水处理，但是已有报告中的
吸附剂都没有很好的吸附效果。
本课题致力于制备一种多孔 C@titania 核壳纳米材料用于有效的处理罗丹明 B 废水。并
对所制备的材料在不同条件下的吸附性能进行研究，例如废水 pH，RB 浓度等。此纳米材料
的制备主要是通过一步法直接将水解 TBOT 所得的二氧化钛凝胶涂覆在 ZIF-8和 HKUST-1纳
米颗粒表面，最后进行碳化，并利用 XRD，FTIR，TEM，N
2吸附-脱附等表征手段对所得材料
进行详细研究。
TEM 图中可以观察到明显的核壳结构，从 ZIF-8制得的 C@titania 核壳材料的 TEM 图可
以看出其二氧化钛涂层的厚度为14-25纳米。N
2吸附-脱附结果显示所得产品为明显的多孔
材料，从 ZIF-8和 HKUST-1制得的 C@titania 材料的比表面积分别为107和43 m
2g
-1。废水
pH 对所得材料的吸附性能具有较大影响，平衡吸附能力在 pH7时达到最高。从 ZIF-8和
HKUST-1制得的 C@titania 核壳纳米材料对 RB 的最大吸附能力分别是174.4和106.3 mg g
-1,
高于其对应的 MOF 直接碳化所制备的碳的吸附能力。
V
T
able of contents
Acknowledgments ... I Abstract ... II Sammanfattning ... III 摘要 ... IV Glossary ... VII Abbreviations ... VIII 1. Introduction ... 11.1 Background - pollution and environmental issues ... 1
1.1.1 An overview of the world’s ecological problems ... 2
1.1.2 General description of China ... 9
1.2 Wastewater treatments techniques ... 15
1.2.1 Adsorption ... 15
1.2.2 Flocculation technology ... 17
1.2.3 Advanced oxidation processes ... 19
1.3 Methodology in the present work ... 21
1.4 Objectives ... 22
1.5 Scope ... 22
2 Experimental methodology ... 23
2.1 Materials ... 23
2.2 Preparation of MOF nanocrystals ... 23
2.2.1 MOF ZIF-8 [162]... 23
2.2.2 MOF HKUST-1 [163] ... 23
2.3 Synthesis of C@titania core-shell nanoparticles ... 23
2.4 Characterization ... 24
2.5 Adsorption experiments ... 24
2.6 Adsorption models ... 25
3 Results and discussion ... 26
3.1 Preparation and characterization of MOF@titania core-shell nanoparticles ... 26
3.2 Preparation and characterization of C@titania core-shell nanoparticles ... 29
3.3 Adsorption of RB ... 33
VI
5 Future work ... 36
6 References ... 37
7 Appendix ... 48
VII
G
lossary
Adsorption – Adhesion of atoms or molecules to a solid surface.
Carbonization – Is a procedure where a material is heated without air to leave solid porous carbon. Catalysis – Is increasing the rate of a chemical reaction by adding a catalyst.
Catalyst - A material that is not used up by the chemical reaction, but lowers its activation energy. Diffuse sources – Widespread activities, with no discrete source that cause pollution.
Endocrine glands – Are of the endocrine system that stash their products, hormones, directly in the blood instead of through a duct.
Eutrophication – Overflow of nutrients in a lake or other body of water.
Hydrodynamic isolation – Can be used to isolate very small particles in an aqueous solution for an extensive period of time in microfluidics, to study their behaviour.
Membrane separation – A expertise which selectively separates substances by pores and/or minute gaps in the molecular construction of a continuous structure.
Nanomaterials – Describes substances of which a single unit is sized from 1 to 1000 nanometres. Oxidation – Any chemical reaction that includes the transfer of electrons.
Pathogenic microorganisms – Is an organism able to cause disease in its host
Phytoremediation – Approaches that use living plants to restore soil, air, and water which is contaminated with harmful pollutants.
VIII
A
bbreviations
1,3,5-H3BTC 1,3,5-benzenetricarboxylic acid
Å Ångström is a prefix which equal 10-10
AEOP Advanced electrochemical oxidation process
AOP Advanced oxidation process
As Arsenic
BET Brunauer-Emmett-Teller
BOD Biochemical oxygen demand
C2H2 Ethyne
CIP Ciprofloxacin
CO2 Carbon dioxide
COD Chemical oxygen demand
COF Covalent organic frameworks
Cu(NO3)2*3H2O Copper nitrate
CVD Cancer village density
CVN Cancer village number
CVN-r CVN distributed in areas with river water quality inferior to grade III
DDT Dichlorodiphenyltrichloroethane
EtOH Ethanol
F Fluorine
Fe-Mn Iron-manganese
FTIR Fourier-transform infrared spectroscopy
GAC Granular activated carbon
GC Gas chromatography
HACCP Hazard analysis of critical control points
HCl Hydrochloric acid
HKUST-1 Hong Kong University of Science and Technology-1
KBr Potassium bromide
M2M 2-Methylimidazole
MOF Metallic-organic frameworks
NaOH Sodium hydroxide
Zn(NO3)2 Zinc nitrate
Zn(NO3)2*6H2O Zinc nitrate hexahydrate
NH4+ Ammonium
NLDFT Non-local density functional theory
PAC Powdered activated carbon
PAHs Polycyclic aromatic hydrocarbons
PBDEs Polybrominated diphenyl ethers
PCBs Polychlorinated biphenyls
PCDDs Polychlorinated dibenzo-p-dioxines
IX
pH Measure of concentration of hydrogen ions
POPs Persistent organic pollutants
PVP Polyvinylpyrrolidone
RB Rhodamine B
Se Selenium
SEM Scanning electron microscopy
TBOT Titanium butoxide
TEM Transmission electron microscopy
TGA Thermogravimetric analysis
TiO2 Titanium dioxide
U Uranium
UV Ultraviolet light
UV–Vis Ultraviolet light-visible
VOCs Volatile organic compounds
WHO World health organization
XRD X-ray diffraction
1
1. I
ntroduction
B
ackground - pollution and environmental issues
Humanity are facing several major issues that are related to water quantity and/or water quality in the 21st century [1]. These difficulties are going to be even worse in the future as a result of climate
change, causing higher water temperatures, melting of glaciers and an intensification of the water cycle [2] with more possible droughts and inundations [3]. With regard to human health, the most direct and worst influence is the absence of improved sanitation and connected to it is the lack of safe drinking water that is presently affecting more than 33 % of the world’s population. Additional threats include for instance exposure to pathogens or to chemical toxicants via the food chain (for example the effect of watering plants with polluted water and of bioaccumulation of harmful chemicals by aquatic organisms, as well as seafood and fish) or during recreation (for instance, swimming in contaminated surface water) [4].
The report [4] reviewed the pollution of fresh-water resources, lakes, rivers and groundwater. Recently have several assessments reported that cover the various aspects of waterborne diseases in a comprehensive way, however chemical pollution was emphasised the most [5]. Over 33 % of the world’s available renewable freshwater is used up by domestic, agricultural and industrial purposes [6]. There is no wonder that chemical pollution of natural water has become a big public issue in most part of the world, because of these actions [7].
The chemical water pollutants can be separated into two types, the first are phosphorous species [8], the reasonably small amount of macro-pollutants, which normally take place at the milligram per litre level, which are also containing nutrients for instance nitrogen [9] and the second one is natural organic components [10]. The origins and influences of the common pollutants are fairly well understood, however scheming solid treatment technologies for the pollutants persists a scientific task [11]. An increased primary production of toxic algal blooms, oxygen reduction and biomass are result of excess of nutrients. Another issue are the increasing salt loads, which enter the surface water by road salt and too much irrigation [12, 13]. The direct use of drinking water and inhibit crop growth in agriculture are being prevented by high salt concentrations. The issue is most spoken of in coastal areas, for instance China and India, as a result of marine salt intrusion into groundwater in consequence of overexploitation of aquifers and sea level rise [14].
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is unaffected by severe groundwater pollution. The circumstances are also dire in regard to surface water quality, with a large proportion of observed river and lakes lately failed to meet the standards of water quality [18].
Qu Jiuhui & Fan Maohong published in 2010 their article “The Current State of Water Quality and Technology Development for Water Pollution Control in China” in the journal, Critical reviews in Environmental Science and Technology. In the article, they reported an overview of the present state of water pollution, in addition to the progress and potential future development of water pollution control technology in China. During China’s drastic economic development, water quality and water pollution have become substantial concerns with both surface and groundwater supplies which are suffering of damage. Contaminates in large amounts from both point and non-point sources which cannot be efficiently degraded by natural processes have been discharged into nation’s channels, the self-decontamination capacity is overpowering of bodies of water.
The current focus lies on the persistence in water supplies of poisonous chemicals and pathogenic microorganisms that affects the functioning of human endocrine glands. The common water pollution problem throughout the world has mostly been resolved. The present water supplies in China is going through a severe challenge, unwanted and poisonous substances are persistently reappearing as big quantities of the traditional pollutants are released into and accumulate in water bodies. Several surface and ground waters are threatening the safety of drinking water supplies by pollutants which are pesticides, arsenic, fluorine and random organic matter. As a result, pollution from heavy metals, nitrate, fluorine and random organic matter has become a most important water quality concern [19].
1.1.1 An overview of the world’s ecological problems
The water quality issue is a major problem, which people on our earth are facing in the 21st century.
Throughout the world, there are different kind of pollution and environmental problems, whereas the humanity’s health could be at risk depending on how serve the cause is. An overview of selected topics of chemical water pollution are shown in Table 1 [4].
Table 1. An overview of chemical water pollution [4]. Abbreviations: As, arsenic; F, fluorine; PCBs, polychlorinated biphenyls; PBDEs, polybrominated diphenyl ethers; DDT, dichlorodiphenyltrichloroethane; PAHs, polycyclic aromatic hydrocarbons; PCDDs, polychlorinated dibenzo-p-dioxines; PCDFs, polychlorinated dibenzofurans; Se, selenium; U,
uranium. Pollutant source Source type Pollutant types addressed Illustrative examples
Main water quality problems Major challenges Multiple (waste sites, spills, agriculture, combustion, and other) Globally distributed point and diffuse Persistent organic pollutants (POPs) PCBs, PBDEs, DDT, PAHs, PCDDs, PCDFs Bio-magnification in food chain, diverse health effects Phase out existing POPs, confine existing sources, prevent use of new POPs Agriculture Diffuse Pesticides Triazines,
chloraceanilides, DDT, lindane
3 Natural contaminants Geogenic contaminants Biogenic contaminants Diffuse Inorganic contaminants, cyano-toxins, taste and odour compounds
As, F, Se, U, micro-cystins, geosmin
Cancer, fluorosis, human health, aesthetics (taste and odour) Development of effective household treatment systems, control, eutrophication, consumer acceptance Mining Mostly point Acids, leaching agents, heavy metals
Sulphuric acid, cyanide, mercury, copper
Metal remobilization, acute toxicity, chronic neurotoxicity Acid neutralization, metal removal, introducing effective nontoxic reagents Hazardous waste
Point Diverse U, technetium, chromium, chlorinated solvents, nitro-aromatic explosives Long-term contamination of drinking water resources Containment of pollutants, monitoring of mitigation processes including natural attenuation Urban wastewater in industrialized countries Point Pharmaceuticals, hormones Diclophenac, 17 α-ethinylestradiol Eco-toxicological effects in rivers, feminization of fish Reduction of micropollutant loads from wastewater by polishing treatment Urban wastewater in developing and emerging countries
Point Microorganisms and viruses Cholera, typhoid fever, diarrhea, hepatitis A and B, schistosomiasis, dengue
Human health, child mortality, malnutrition Reduction of micropollutant loads from wastewater by polishing treatment The different areas report and exemplify several aspects of global water pollution, as well as important types of pollutant sources and pollutants in addition to different temporal and spatial scales of water pollution, ranging from long-term global POPs to long-term regional (e.g. geogenic pollutants, mining) to long-term local (e.g. unsafe waste sites) short-term regional (example agriculture) to short-term regional or even local (for instance wastewater) contaminants. The examples should also clarify that any mitigation and adaptation strategies to solve a given water pollution problem have their own technical, economic, political and societal boundary conditions [4].
1.1.1.1 POPs - global issue
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1. Persistent in the environment, which conclude that biological, photochemical and chemical transformation processes do not result in a substantial removal of the compound in any environmental section;
2. Inclined to long-range transport, therefore global distribution, even in isolated regions where the compound has not been used or disposed, due to its physical-chemical properties; 3. Bio-accumulative by food web; and
4. Poisonous to living organisms, counting human beings and animals [4].
Some outstanding POPs have been listed and divided into two international conventions with the aim to measure the POPs’ global presence and to decrease their emissions to the environment [20].
From a toxicological standpoint, POPs could threaten the health of both human beings and animals thanks to their harmful effects, as well as disruption of the endocrine, the reproductive and the immune system, in addition to their ability to create behavioural issues, cancer, diabetes and thyroid problems [4].
Speaking of global water pollution, POPs present a serious problem mainly due to their large bioaccumulation and bio-magnification potential in sea food webs [21, 22]. A bunch of observing surveys have reported critical concentrations of POPs in freshwater, sea fish, marine mammals and as a result in human milk and also human tissues of people who dependent of these food sources [23, 24].
Consequently, the impact of climate change on the distribution and the influence of POPs in the environment must to be addressed [25]. From an eco-friendly policy standpoint, the most critical actions of international community are to remove POPs that are currently in use, in order to increase source controls each and every time possible and also to ensure no new chemicals with POP features appear on the market [26].
1.1.1.2 Agriculture and quality of water
Currently there are three to seven million tons of pesticides annually produced. The use of the substance differs between 0.2 to 2 kg per hectare of arable land in developing against developed countries, correspondingly. The volume of chemicals necessary in order to control pests are dependent on the crop treated, what kind of pesticide that are being used, the used application method, in addition to geographic and climatic boundary conditions. Newly established agrochemicals do normally operate at lower doses in comparison to developed products, however toxic loads per dose of active ingredient differ extensively among different agrochemicals [4].
Annually there are numerous million tons of chemicals that are being used up for agricultural production to keep up and increase crop yields by controlling following insects; other pests; weeds and fungi [27]. The market has several thousands of diverse commercial products that contain roughly hundreds of different active chemicals components, which is available because of pesticides and related agrochemicals [28, 29].
5
Counting the portion of used pesticides that shows up at surface and groundwater [33] and creating effective mitigation measures [34, 35] beyond a case-by-case basis are ambitious as a consequence of the considerable spatial and temporal variability of pesticide losses [36].
Agricultural point sources consist of pesticide runoff from hard surfaces, regularly from storage facilities or farmyards during the usage of agrochemical products or unintentional leaks. Pesticides can either enter aquatic systems via sewage treatment plants or infiltrate into the nearby soil, which depends on connection to the sewer systems [4]. As an alternative, diffuse losses, consist of field runoff, drainage or leaching into the subsurface or spay drift are a worse issue and an extensive diversity of mitigation procedures have been estimated to reduce their impact on water resources [37].
Water pollution also take place in sewer and drainage systems from pesticide applications in non-agricultural or urban areas by increased runoff of pesticide-consisting rainwater over sealed surfaces, for instance roads and roofs [38]. From the standpoint of the overall environmental influences of extensive agriculture, a reduction of water and soil pollution by pesticides releases is considered a key factor within agriculture management follows to reduce ecological changes and to sustain biodiversity [39, 40]. Lastly, critical poisoning from direct pesticide exposure is a significant risk for workers within the field of agricultural. However, the impact of this exposure pathway is debated in Europe [41, 42] and North America, unintended exposure and deliberate ill use of agrochemicals seem more common in developing countries [43, 44, 45], which result in a projected poisoning of three million humans with as many as 20000 unintended deaths per year [46].
Besides distinct climatic or ecological conditions and grown corps, agricultural action in most developing countries is driven by the demand to accomplish or preserve food safety for increasing populations and the political or economic suggestions of this overarching objective [39]. Along with trends in the direction of urbanization and industrialization, these agricultural accomplishments are causing issues for the quality of water [47]. Pesticide use per hectare of cropland has increased over the modern years, although as documented for China, aids to crop yield were minimal [48].
In developing countries, capabilities and resources for observing pesticide concentration in water systems and evaluating the risk for human beings and the environment are usually restricted [49] and attitudes in the direction of implementation of regulations are scant [50]. The software used to observe pesticide occurrence and distribution demonstrates that the range of active components can still vary from those used in the developed countries [4].
1.1.1.3 Groundwater pollution by harmful waste locations and spills
6
year in water and on land by different kind of incidents including facility releases and transportation [4].
It is complex to evaluate the fluxes and number of poisonous chemicals from such polluted locations to the groundwater [54, 55]. In most situations of spills, abandoned facilities and waste disposal sites, their main pollutants are known [4]. Materials which are discarded are not enough mixed and characterized [55]. Most of the landfill materials cannot be predicted with accuracy, apart from some special predominant pollutant types [56]. For the reason that the hydrogeology of such locations is difficult, the dynamics of contaminant emission can only be measured dependably on case-by-case basis through combined non-stop on-site observation and arbitrary groundwater models [57].
Implementation of suitable cost-effective remediation plans are crucial, because valuation of human health risks to exposure of mixtures of chemicals is a result of the use of groundwater as a drinking water reserve and the persistence of pollutions [53, 58]. Common procedures for the active mitigation of groundwater pollutants from waste locations and spills are pump-and-treat approaches, phytoremediation, site excavation and permeable reactive barriers [59, 60]. The concepts for mitigation either aim at getting rid of the source of pollution or propose to catalyse reactions that lead to an immobilization or convert into unharmful and biodegradable products. The problem with remediation procedures is that they are either too expensive or ineffective, because they need treatment for a long time [59]. Up till now, approaches are focusing on abiotic degradation in natural attenuation or microbial are gradually being thought of as feasible long-term alternative treatment [61, 62]. The major scientific challenges and fundamentals for an in-depth evaluation of groundwater contamination by harmful waste locations and spills is therefore to measure the site-specific, related processes that regulate the transport and transformation behaviour of a given contaminant and its converted products [63].
1.1.1.4 Drinking water and medicines in wastewater
Public wastewater contributes drastically to the load of micro-contaminant into the marine surroundings [64]. The core issues are personal care items and pharmaceuticals compounds. Currently there are about 3000 pharmaceuticals are used in the United States and Europe, that consist of beta blockers, antibiotics, painkillers, contraceptives, antidepressants, lipid regulators and etc [65]. Approximately 30 new pharmaceuticals are started on the market every year within Germany and with 8 % of the global R&D expenses [66]. Averagely more than 300 new pharmaceutical compounds are started every year which can be projected, because of the global R&D expenses of approximately US$83 billion in 2007 [66]. Throughout the global the market of pharmaceuticals has 100000 tonnes per year [67] which equals to US$773 billion, where USA had the highest per capita US$676, followed by European countries that had a range between US$200-US$400 (which was UK and France) [66].
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microgram per litre are permitted by the analytical chemistry analysis, which is typical for aquatic systems and wastewater [64, 69]. The studied pharmaceuticals’ concentration of humans in raw sewerage, equal to more than a few micrograms per litre, which verifies that municipal wastewater is the core pathway for their release into the water bodies [70].
At present systems of wastewater, the pharmaceuticals are removed inadvertently through sorption to sludge and by biodegradation [71]. Pharmaceuticals’ degradation in those systems do not fully mineralise, but then they create metabolites by formation instead [4]. Consequently, speaking of the eco-toxicological impacts of the released wastewater, not only the parent composites, but also their wastewater-bone metabolites has to be taken into account. Fortuitously, if metabolites are more hydrophilic, they are projected to have a lesser eco-toxicological potential than their more hydrophobic parent composites, except another specific mode of action becomes important [72].
The core problems in wastewater sewer water related to pharmaceutical are linked to their eco-toxicological impacts, currently there is a developing concern for human health as a consequence of the presence of some of these composites in drinking water derived from direct or indirect drinkable reuse. Systems of indirect reuse is where waste-water derived pharmaceuticals and their metabolites can infiltrate to the aquifers via the riverbank. Fortunately, it seems that many of these composites are being blocked out by the riverbank [4]. However, reported evidence that a complete elimination of all potential pharmaceutical remains by riverbank filtration cannot be definite according to a recent report on residues of human pharmaceuticals in marine surroundings [73]. Additionally, synthetic organic chemicals are possibly the most thoroughly studied pharmaceuticals from a human toxicological standpoint. New pharmaceutical composite needs to go through thoroughly authorization, which is comprehensive information on pharmacokinetics, toxicology, pharmacology and clinical examinations [74]. Exposure to individual pharmaceuticals in drinking water seems rather low according to this report. Nevertheless, the long-term exposure to minor concentrations and mixtures of pharmaceuticals require more information [4].
8
Figure 1. Constants Kaw vs Kow are different organic water contaminations. The colour code
9 1.1.1.5 Water recycle and wastewater treatment
One of the main components for improving and maintaining public and ecosystem health, is that the mitigation of wastewater streams from household and industry. The removal processes of public wastewater are targeting nitrogen, phosphorous, pathogenic microbes and carbon. An elimination of nutrient will lead to a decrease of BOD of sewage water and hence a reduction in eutrophication of coastal areas and inland water bodies. Within the low-income countries there are more than 80 %, where the public wastewater is released without any treatment and contaminating lakes, coastal areas and rivers in comparison to industrialized countries, where the connectivity for public wastewater treatment plant is about 50 % to 95 % [1]. BOD is not only a foundation of the industrial wastewater there are also a point source of chemical contamination of organic composites and heavy metals. This issue has been reduced drastically in the industrialized countries by implementation of recycling of internal water, end-of-pipe treatment and recovery systems by advanced technologies, for instance advanced oxidation, membrane procedures or activated carbon. Industrial wastewater treatment efficiency of water is exceedingly flocculated, varying from about US$140 per m3 in Denmark to US$10 per m3 in the US [1] and much less in
low-income countries. The numbers are reliant on the kind of industrial activity. Currently, there are a significant potential for reuse of water, which would drastically decrease the removal of possibly contaminated water [4].
Ground and surface water bodies are common and well-known procedures for reuse of agriculture, drinking water and water recycling [77, 78]. These days, WHO is commending to guarantee water safety for agricultural reuse, by an agenda of risk management and integrating aspects of risk assessments. This include that multibarrier principle and HACCP is being relied on by the water safety plans [78]. Wastewater reuse for industrial and drinking water has becomes more known throughout the globe, with increasing water scarceness. For instance, using several advanced operations to recycle wastewater to acquire drinking water have been done in Namibia and Windhoek since 1973 [79]. Many other urban places who also have water issues are Australia, Singapore and California, industrial reuse or indirect portable or direct are being tested on large scales. Most of the operations depend on membrane technologies to deal with secondary wastewater sewage and to eliminate pathogens and micropollutants proficiently [79].
1.1.2 General description of China
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Table 2. The ten-point water pollution prevention and control action plan of the government of the people’s republic of china, which was released in the beginning of 2015 [81].
1 Take control of and reduce pollutant discharge.
2 Promote transformation of the economic structure (to lower pollution intensity). 3 Focus on protection of water resources through water saving.
4 Strengthen support for science and technology.
5 Allow market mechanisms to impact water and pollution levies. 6 Strict environmental law enforcement & supervision.
7 Strengthen the management of the overall hydrological cycle. 8 Ensure security of water for ecological and environmental purposes. 9 Confirm and implement the responsibilities for all parties involved. 10 Strengthen public participation and social supervision.
Whereas research and development are currently in progress in order to improve the present situation within China. It has also been improvements throughout the years, yet the situation still needs more improvements.
A general overview of China’s pollution and ecological problems can be shown in Figure 2 to Figure 3, however Hong Kong, Macao and Taiwan are not included in the following data [82].
Figure 2. Government statistics of China's water pollution - A) The six-class water quality classification system was used to rank the surface water. The six-class system for surface
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Figure 2 presents the quality of water from major river basins and near-shore coastal waters. Out of 208000 km of examined river reaches in China, quality of water in 31.4 % reaches falls into rank IV or poorer and are therefore not appropriate for potable use or human contact. The river reaches’ water quality is 14.9 %, which states that the reaches are inferior to class V, representing a total loss of potential for all potable uses or contact for humans [82].
Table 3. The standard six-class rating system for surface water quality [82].
Grade classification/applicable uses
I Pristine water sources (e.g. river headwaters and protected natural catchment areas). II Class A water source protection areas for centralized drinking supply.
III Class B water source protection areas for drinking supply and recreation. IV Industrial water supply and recreational water with no direct human contact. V Limited agricultural water supply.
VI Essentially useless.
Figure 3. Government statistics of China's water pollution - B) The five-class classification was used to rank the groundwater in six sub-areas of China, counting both shallow and deep
groundwater [83].
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Table 4. Distribution of “cancer village” at provincial level in China, together with groundwater and river water quality categorizations [83].
Province CVN CVD Groundwater River water classification (%) CVN-r IV-V class (%) I-III IV-V Interior to V Interior to III
HeiLongJiang 6 0.13 57.5 56.7 37.7 5.6 43.3 5 JiLin 1 0.05 27 67.5 24.4 8.1 32.5 0 LiaoNing 8 0.57 62 42.1 27.6 30.3 57.9 5 HeBei 40 2.21 63 48.6 21.4 30 51.4 34 BeiJing 1 0.63 43 52 7 41 48 1 TianJin 3 2.67 48.5 3.8 22 74.2 96.2 3 ShanDong 30 2.03 60 39.2 30.9 29.9 60.8 26 JiangSu 24 2.46 23 37.2 42.1 20.7 62.8 14 ShangHai 1 1.65 19 12.5 30.8 56.7 87.5 0 ZheJiang 20 2.01 30 66.2 18.9 14.9 33.8 2 FuJian 12 0.99 41 82.2 10.5 7.3 17.8 1 GuangDong 25 1.4 69 76 14 10 24 0 HaiNan 9 2.54 15 93.1 6.9 0 6.9 0 Inner Mongolia 9 0.08 55.4 40.4 40.5 19.1 59.6 1 ShanXi 15 0.99 51 48.5 24.2 27.3 51.5 13 HeNan 31 1.94 60 37.9 28 34.1 62.1 29 HuBei 17 0.94 46 77.4 11.1 11.5 22.6 1 HuNan 15 0.72 29 95.5 4.5 0 4.5 0 AnHui 19 1.4 90 70.5 17.8 11.7 29.5 16 JiangXi 18 1.09 66 91.2 4.2 4.6 8.8 0 GuangXi 2 0.08 35 92.5 7.3 0.2 7.5 0 ChongQing 11 1.37 69 74.7 25.3 0 25.3 1 Sichuan 6 0.13 57 83.2 12.7 4.1 16.8 2 ShaanXi 5 0.25 54 36.4 38.9 24.7 63.6 4 GuiZhou 3 0.17 48 74.4 3.9 21.7 25.6 1 YunNan 20 0.52 44 82.5 8.3 9.2 17.5 0 GanSu 0 0 50 63 12.3 24.7 37 0 NingXia 0 0 57 60 40 0 40 0 XinJiang 0 0 37 91.3 8.5 0.2 8.7 0 QingHai 0 0 - 100 0 0 0 0 Tibet 0 0 43 100 0 0 0 0
Groundwater’s percentage in grades IV and V is the ratio of wells with these rankings to the total observing wells of the province. The percentage of river water quality points to the ratio of control sections examined to the total control sections of the province in a particular water quality grade [82].
14
rapid, surficial water cycle, which result that it needs very long time for flushing [84]. An additional issue is that while groundwater pollution does influence certain regions, both shallow and deep groundwater is omnipresent nation-wide in China. The most vulnerable regions are the compactly settled North China Plain, where the drinkable groundwater (I-III) was found to take only 26.4 % of deep groundwater and 22.1 % of shallow groundwater [85], which is alike to the data from the [86].Furthermore, the less inhabited parts of northwest China could also have some influences from serious groundwater contamination in both deep and shallow aquifers Figure 3.
In order to finally assure the safety of drinking water the difficulty of accumulative effects of water pollution requires development and application of new technologies to control the pollution of surface and ground water. Approaches and various new technologies of development recently are being prioritized, which include ecological engineering technologies, biological technologies and the combination of the two for the progress and restoration of specific bodies of water and water quality in general. Redox techniques, absorption and flocculation technologies, electrochemical procedures and hydrodynamic isolation for ground water renewal, etc. have been included. Moreover, the strategic implementation of water pollution control and environmental improvements in China have been amplified by strict legislative and administrative regulation [19].
The current pollution in China over the past 30 years has the quality of surface water deteriorated considerably, especially in more developed regions in China. Pollution initiated by harmful substances in marine environments has made water quality issues even more difficult and eutrophication regularly take place in the bigger lakes and reservoirs. The loss of biodiversity along with a drop, in water function is the result of all the parameters. It has been detected repeatedly in big lakes of both harmful substances and traditional pollutants (e.g. COD), with more than 80 % of urban rivers showing substantial contamination levels, some appearing black materials and are unpleasant-smelling. On account of this, the quality of several drinking water origins has been considerably deteriorated, witheven more severe contamination obstructing scarcity by decreasing the accessibility of potable fresh water [19].
Previous investigation [80] states that pharmaceuticals have been found in water since the last 20 years. The health of human and fish could be endangered and also cause other environmental issues [87, 88, 89]. One of most common drugs identified in the water are CIP [90, 91]. The traditional treatment approaches are having a hard time to make the residues removed, due to CIP can inhibit bacteria [88, 92]. Therefore, a variety of physical methods have been developed such as, photocatalytic degradation [93, 94], oxidation [95, 96], chemical methods, adsorption [91, 97] and membrane separation [98]. Adsorption is the approach that has awakened the most research interest, thanks to its great productivity, easy to operate with and economic benefits [87, 88, 89, 90, 91, 92, 98, 97].
15
disadvantages of other technologies. Because of their large quantity, low cast and diverse properties, natural minerals have been widely examined and used in water treatment [19]. To enhance and adjust the elimination efficiency of the contaminants in difficult water circumstances, varied research on natural zeolites [107, 108] and their modified absorbents has been made in China. The outcomes indicate that these kinds of adsorbents are efficient for removing ammonia nitrogen [109], phosphate [110], heavy metals and organic pollutants from water. In order to increase their removal performance and adsorption ability for specific pollutants the modification of zeolites has lately drawn attention [19].
Different kind of material have been studied for CIP adsorption though these materials revealed low capacity, except the Fe-Mn binary oxide nanoparticles and nano-porous carbon [80]. The preparation by those materials are more complex in comparison to ZIF-8-derived carbon, whereas derived carbon material did also present great adsorption capacity [89]. By coating nanoparticles with a layer of silica with metals that could enhance the negative charge of the nanoparticles are worth investigating, due to it could make the performance of the adsorption to become greater [80].
Wastewater treatments techniques
1.2.1 Adsorption
Two vital and adequate phases in a catalytic reaction are adsorption of reactants and desorption of products. Whereas one vital purpose of the catalyst is to offer a surface with somewhat uncoordinated locations chemically fit for adsorbing reactants. Consequently, speaking in chemical terms, adsorption is the establishment of chemical bonds between the adsorbate and the adsorbent is being determined by tendency of an adsorbing surface atoms to increase their coordination numbers, which is to decrease their surface energy, which is presented in Figure 4.
Figure 4. schematic figure of adsorption on a surface [111].
16
attractive forces between adsorbent and adsorbate is Van der Waals, where the atomic distance is alike of a layer as Van der Waals. Chemisorption on the contrary is quite strong, selective adoption of chemically reactive gases on offered of locations of metal oxide surfaces or metal at rather higher temperatures. The interaction of adsorbent and adsorbate includes establishment of chemical bonds. Physisorption and chemisorption have some differences that are being presented in Table 5. Chemisorption by a reactive polyatomic or diatomic molecule usually happens by the physically adsorbed precursor state presented in Figure 4 subsequently dissociation to reactive atomic types Figure 4. One approving thing about dissociation is where the gain of energy owing to connections of atoms from the molecule with metals atoms surpasses the binding energy of the atoms in the molecule [111]. Various materials have been employed as absorbents, like porous carbon, MOF, zeolite and more, which are dependent on the mechanism of adsorption. Some adsorbents and their application in wastewater treatments are summarized below.
Table 5. summarized differences between physisorption and chemisorption [111].
So far, as described above, zeolite, porous carbon and porous composite compounds etc. adsorbents have been extensively used in wastewater treatment [112]. In addition, MOF (Metallic Organic Framework) as an emerging adsorbent has also been used for purification of water [113]. However, within most wastewater there are significant levels of large organic pollutants which are harmful or unwanted, since they create odour, bad flavour, foaming, etc. These materials are usually resilient to degradation by biological approaches and are not removed efficiently by adsorption using zeolites and MOFs due to their larger molecule size compared to the pore size of the adsorbents.
17 1.2.1.1 Sol-gel technique derived adsorbents
Within material chemistry there is a technique for creating small particles, which is sol-gel technique. This kind of procedure is mainly carried out for synthesis of metal oxides. The first thing of this procedure is to change the initial material or monomers to a sol, which is a colloidal solution, that is a precursor for the formation of a gel. The gel consists of polymers or isolated particles. Precursors that is mostly used are metal alkoxides or chlorides. The formation of colloids is created by hydrolysing the precursors and polycondensed. The main reason to use sol-gel approach is owing to its economic benefit and low-temperature procedure, that gives the possibility to be in charge of the composition of the created product. This kind of technique has a great variety of application in different fields, such as space, sensors, zeolite synthesis, energy, electronics, reactive material, separation methodology, medicine and optics [114].
Altogether, the sol-gel method consists of the transition of a solution system from a liquid (sol) to a solid (gel) phase, therefore, it has often been used for the preparation of adsorbent for wastewater treatment, for instance Qi et al [115] used the sol-gel technique to prepare magnetic expended graphite removal and recovery of Cr(VI) from wastewater. However, advantages of the sol-gel technique are enhanced bond between substrate and the top coat. The gel state provides the opportunity for material can be shaped into complex geometries. High-quality products are gained owning to the ceramic oxides precursors which are melted in suitable solvent for the sol-gel conversion. The process uses a low temperature of 200-600 °C. It is economical, easy and efficient approach to create good quality coating. On the other hand, the disadvantages of the method are owning to some restrictions, it cannot reach its full industrial potential. Some of them are having, low wear-tolerant, weak bonding, hard to attain control of porosity and high permeability. Thicker coating could result in a failure throughout the thermal process. Having a max coating thickness limit of 0.5 µm for a non-crack property. The technique has a con which is owning to the thermal mismatch and it is surface dependent [114].
In the present work, sol-gel technique will be used to prepare titania coating carbon materials for wastewater treatment.
1.2.2 Flocculation technology
Drinkable water in daily life is essential for humans and nature. In order to be able to meet the standard of drinkable, agricultural and industrial water the direct need to treat wastewater, especially slimes and sewage sludges from industrial and municipal wastes separately. This kind of waste are very dangerous and unwanted. Therefore, it is necessary to get rid of the pollutants before reusing the water. The treatment approach can be carried of either of coagulation or flocculation [116]. Owing to it is practical and have low cost. No matter the nature of the sample being treated and the overall applied treatment structure, coagulation-flocculation usually consists of a pre- or post-treatment step. The effectiveness of coagulation-flocculation strongly influences the overall performance, since the increase of the effectiveness of coagulation stage appears to be main factor for the improvement of the overall treatment effectiveness [117].
18
molecular weight polymeric materials is called flocculation. Meanwhile, for coagulation, if destabilisation is brought by charge neutralisation by adding of inorganic chemicals, it is a coagulation process. The agglomerates formed by coagulation are commonly loosely bound and compact, while the flocs are strongly bound, porous and larger size for the flocculation process. Also, not much of a change of surface charge achieved for the flocculation processes [116].
Flocculants are being used for purification of drinkable and industrial processed water. Their theoretical purpose is to get rid of suspended solids, which cause turbidity. Flocculants are as well used in gravity separation and flotation processes for industrial and municipal water purification. The case of wastewater, which containing primarily inorganic substances, anionic flocculants are chosen whereas for organic substances, cationic flocculants are most commonly used. Polymeric flocculants are used for wastewater treatment in the textile industries, petroleum refining, pulp and paper, chemicals, food processing and metals [116].
1.2.2.1 Factors affecting the flocculation process
The factors by polymer molecular weight of flocculation [120, 121, 122] is easiest described in teams through electrostatic patch and bridging mechanisms. In cases where bridging is the dominates irrespective of charge, there will be an increase in molecular weight which improves flocculation. However, at higher molecular weights, as the polymer gets adsorbed, it can further away from the particle surface and will slowly attain equilibrium. This will increase the collision number and particle radius and from now flocculation rate. Whereas anionic charge on polymer can hinder adsorption on a negative surface, its function as an extension of polymer chain by same charge repulsion, which helps its approachability. The flocculation efficiency decreases when molecular weight goes beyond an ideal weight, that makes the polymer molecules to create repulsion against themselves. Yet again, where molecular weight is less defined in cases where the electrostatic patch mechanism is rate controlling. Ideal flocculant concentration has been observed to be independent of molecular weight however, reliant on ionic strength [116].
1.2.2.2 Flocculation materials
The big diversity of materials which are being used in the flocculation process as agents can be divided roughly into two types polymeric and inorganic, whereas there are some inorganic substances that can exist in the polymeric form, for instances aluminium chloride, which is not take into account in this sorting. Materials which is synthetic can be anionic, cationic or nonionic. There has been a new development of a new class of polymeric flocculants, which is the graft copolymers that are being synthesized by synthetic and natural polymers [116].
Several of efficient flocculants such as nano-sized PACI polyaluminium chloride [123, 124], PFC polyferric chloride [125] and PASC polyaluminium silicate chloride [126] has been successfully created for supporting flocculation feasibility as adjusted for diverse grades of water. By use of an electrochemical approach [127, 128], created PACI with a large quantity of Al13, that is thought of
19
chroma and turbidity improved approximately 30 % when PACI with a large amount of Aluminium was used in the process of coagulation [116].
PFC was magnificently achieved by [130], which used the resources, small pieces of iron and ferrous sulphate and phosphorus as a stabilizer within a heating environment, whereas the product of the achieved liquid PFC has a 10 % Fe2O3 content. The product of PFC is able to be stored over
two years and yet to be stable. By using this kind of preparation approach, it is possible achieve a production of 3000 tons a year [131], whereas the product of PFC showed good quality and it were applied in several water supply plants, where it verified a high coagulation feasibility in algae-laden water with low turbidity and temperature. The stabilization method of activated silicate and polymerization approach of Al was used by [132] to prepare the PASC through a combination of polyaluminium with silicate. The product of PASC are also able to be stored over two years and yet be stable. By the preparation, the production of liquid PASC, for which the mole ratio of Al/Si and amount were able to yield 10 and 10 % individually, which are manufactured 3000 tons a year [133]. The product of PASC demonstrated very good coagulation feasibility with low temperature and low-turbidity water [116].
1.2.3 Advanced oxidation processes
Recent years a lot of work have been put into improving the purification feasibility of advanced oxidation processes (AOPs) for taking care of recalcitrant organic contamination within China. Mainly the goal of this reports has been to suggest innovative methods of the use by synergistic effects of these approaches in order to utilization and generation of *OH by a set of oxidation processes, that is Fenton oxidation, photocatalytic oxidation, electrochemical oxidation etc which can be shown in Figure 5 [19].
The advanced electrochemical oxidation processes (AEOPs) is a prevailing approach for water purification, which can generate powerful oxidizing agents, which is hydroxyl radicals of direct production or indirect generation through Fenton’s reagent. Those efficient and clean approaches for the mineralization of organic contaminations in wastewater and drinking water, have of late been established in China, where Table 6 and
Table 7 is a summary of the establishment [19].
Figure 5. The incorporation relation between the three usual advance oxidation methodologies
20
Table 6. Some cases of late development of AEOPs for water treatment within China [19].
Table 7. Some cases of late development of AEOPs for water treatment within China continued. Direct process, with some Ti-based lead dioxide electrodes with different conditions [19].
21 1.2.3.1 Advantages and limitations of AOP
Different AOP systems have pros and cons, which is UV/TiO2, Photo-Fenton, UV/H2O2 and
ozone-based systems reported by [134, 135, 136, 137, 138, 139, 140, 141, 142, 143] [144, 145]. Whereas [146, 147, 148, 149] stated this is the following advantages:
• Unlike predictable approaches that use strong oxidant species, under specific conditions AOPs can make fully mineralization available of contaminations;
• Used for the obliteration of rebellious substances resistant to other treatments, for instances biological systems;
• The conversion of recalcitrant composites and rebellious substances is given to biodegradation processes;
• This approach can be used along with other systems for post- or pre-treatment; • Has powerful oxidizing power with high rate of reaction;
• Supreme to reduce the concentration of composites created by other pre-treatment, for instance disinfection;
• The by-products formed can be minimized by adjusting the amount used of reactants; • In general, AOPs consume a lesser amount of energy in comparison with thermal
destruction systems of liquid wastewater and, • Makes in situ treatment available.
Nevertheless, AOPs cannot be indiscriminately used to any arbitrary residue system. AOPs limitations of integration by its conditions are being pointed out by [150, 151]:
• Most of systems are not readily industrial or scaled-up to commercial needs;
• The expenses could be high, generally by its consumption of electrical energy by radiating sources in the case of photo-oxidative degradation systems;
• Limitations during the conditions by high pollutant concentrations;
• Some cases require high monitoring or adjustment for pH and concentration of oxidant, which is for the procedure of disposal or post-processing of treated effluent (for instance photo-Fenton and Fenton);
• The system is limited due to pH changes, which can cause particle aggregation and modification of the surface characteristics of catalyst which is being used within heterogenous procedures, and iron compounds distribution in Fenton processes or even iron precipitation.
M
ethodology in the present work
In the present work, a novel core-shell porous C@TiO2 material will be developed for wastewater
treatment. The work includes for the preparation of several kinds of MOFs following the literature recipes. The following substances that were studied which could potentially enhance the photocatalytic activity of TiO2 coat are: Fe, Co, Ni, Cu, Zn, Ti5O9 (coupling as a semiconductor), Mn
with N, Co with N, Ni with N, Cu with N, Ag, Au and Pt. Whereas Fe, Cu, Ti5O9, Mn with N, Au and
Pt are reported to give the highest photocatalytic efficiency for TiO2 [152, 153, 154, 155, 156].
Therefore the existing MOFs which could be suitable for the potential enhancement of TiO2 are:
22
and Ni-Asp-bipy (Ni), because they contain the required substance for the enhancement activity of TiO2 [157, 158, 159, 160, 161]. This excludes the previously mentioned elements which are not
within the MOFs. Determine which MOF that should be used is dependent on the preparation procedure and the costs of the required materials. The plan is that Cu-based and Zn-based MOFs will be prepared in this project following the recipes in literature. In addition, the group has the experience on ZIF-8 (containing Zn) [80], therefore the feasibility is high.
Objectives
The objectives in this project are to:
• Preparation of several kinds of MOFs following the literature recipes.
• Coating TiO2 on MOFs by using the sol-gel technique and carbonization of the composites
to generate C@TiO2 nanomaterials.
• Characterization of the obtained nanomaterials by SEM, TEM, N2-Adsorption, FTIR, XRD
etc.
• Examining the adsorption and photocatalytic properties of the nanomaterial with, for instance, dye wastewater or common VOCs as models.
S
cope
The scope in this project was MOF-derived C@TiO2 core-shell nanomaterials prepared for textile
wastewater treatment. This kind of nanomaterials was expected to integrate adsorption property of porous carbons and photocatalytic property of TiO2. Furthermore, metal/metal oxides in MOFs may
promote the photocatalytic activity of TiO2. Therefore, combination of metal-organic
23
2 E
xperimental methodology
M
aterials
2-Methylimidazole (2-M2M) was bought from Shanghai Darui Fine Chemical Reagent Corporation. Zinc nitrate hexahydrate was attained by Xilong Chemical Industrial Corporation. NH3*H2O, NaOH,
Titanium butoxide (TBOT) was acquired from Shanghai Lingfeng Chemical Reagent Corporation with chemical purity. Ethanol (EtOH) was gotten from Wuxi Yasheng Chemical Reagent Corporation. Rhodamine B (RB) was bought from Macklin Biochemical Co., Ltd (Shanghai, China). Copper nitrate Cu(NO3)2*3H2O was acquired from Wako Pure Chemical Industries Ltd.
Polyvinylpyrrolidone (PVP, MW = 29000 g mol-1, Sigma-Aldrich) and 1,3,5-benzenetricarboxylic
acid (1,3,5-H3BTC) was obtained from Wako Pure Chemical Industries Ltd.
P
reparation of MOF nanocrystals
2.2.1 MOF ZIF-8 [162]
Zinc nitrate powder (2.34 g) was firstly dissolved in 16 mL of deionized water and additionally another 32.06 g of 2-M2M powder was dissolved in 112 mL of deionized water, which were mixed by vigorous stirring for three hours at ambient conditions. The products were attained by centrifugation, washing and freeze drying.
2.2.2 MOF HKUST-1 [163]
Copper nitrate powder (0.73 g) was firstly dissolved in 15 mL of distilled water and additionally another 1.5 g of PVP powder was dissolved in 15 mL of EtOH and 0.42 g of 1,3,5-H3BTC was
added slowly into the PVP-solution. After the solutions was completely dissolved, copper nitrate solution was added into the PVP and 1,3,5-H3BTC mixture followed by stirring vigorously for one
hour at ambient conditions. Furthermore, the solution was moved to a 50 mL teflon-lined stainless-steel autoclave then put into a 120 °C oven for 12 h. The products were attained by centrifugation, washing and freeze drying.
S
ynthesis of C@titania core-shell nanoparticles
A coating setup was used, that provided magnetic stirring and temperature controller. Firstly, 0.1 g of dried powder of MOF crystals (ZIF-8 or HKUST-1) was dispersed into 40 mL EtOH. A certain amount of TBOT was added dropwise into the solution by pump and mixed for one hour. Secondly, 70 mL of distilled water was added by pump at a flowrate of 1 mL min-1, and stirred for 24 hours.
24
C
haracterization
Scanning electron microscopy (SEM, Hitachi S-4800) was used to perceive the morphologies of the products before and after coating, likewise for carbonization. Transmission electron microscopy (TEM, JEOL JEM-2100) were used to inspect the morphologies and core-shell structures of the obtained nanoparticles. The Fourier transform infrared (FTIR) spectra were obtained by using Nexus 870 spectrometer. The samples were prepared to be pellets by mixing with KBr with a mass ratio of 1:100. FTIR spectra were collected in the wavenumber range of 4000–500 cm−1. X-ray
diffraction (XRD) patterns of the samples were recorded by using an X-ray diffractometer Bruker D8 Advance powder diffractometer with Cu Kα radiation target at 40 kV and 40 mA from 5° to 90°.
Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyser (Netzsch STA 409) in nitrogen with a ramp of 10 °C min−1. N2 adsorption-desorption isotherms were measured at
77 K using BELSORP MAX instrument. The sample was degassed at 300 °C for 4 h before the measurement. The total pore volume was estimated at a relative pressure of approximately 0.99 and the Brunauer-Emmett-Teller (BET) surface area was evaluated from the adsorption branches of the isotherms in the relative pressure range of 0.05–0.25. The pore size distribution was determined by a no linear density function theorem (NLDFT) equilibrium approach using the data from the desorption branch of isotherms.
A
dsorption experiments
All adsorption trials were performed at 30 °C in a sequence of 100 mL bottles containing 2.5 mg adsorbent and 50 mL different concentrations of RB solutions. To obtain adsorption isotherms, 2.5 mg carbon@titania nanoparticles were added to 50 mL of different concentrations of RB solutions starting from the concentration of 10 mg L-1 with an increase stepwise of 10 to 100 mg L-1 at pH
7.0. The solutions were stirred for 24 h for adsorption. The evaluation of the change of the RB concentration was carried out by taking about 3 mL solution from the mixture of adsorbent and RB solutions using a 5 mL syringe with filter (membrane with 0.2 μm pores) and then determined by UV–Vis spectrophotometer with a wavelength of 554 nm.
Investigation of the adsorption kinetics, 2.5 mg of carbon@titania nanoparticles were added to 50 mL of 10 mg L-1 RB solution at pH 7.0. At prearranged time intervals from 20 min to 24 h. About
3 mL solution was taken from the mixture using a 5 mL syringe and filter with 0.2 μm pore, and then the investigation of UV–Vis spectrophotometer was carried out.
The effect of pH on RB adsorption by the carbon@titania nanoparticles was investigated by regulating the pH of the mixtures to the prearranged values of pH using 0.1 M HCl or 0.1 M NaOH solutions. The concentration of RB was 10 mg L-1.
For comparison, the adsorption experiment was also carried out at the same adsorption conditions on ZIF-8 and HKUST-1 derived carbon without coating
The equilibrium adsorption quantity of RB (qe, mg g-1) was evaluated by use of equation (1).
𝑞𝑒=(𝐶0− 𝐶𝑒) × 𝑉 𝑚
(1)
where Co and Ce (mg L-1) are the starting and equilibrium RB concentrations, individually. V (L) is
25
A
dsorption models
Langmuir and Freundlich models were employed to clarify the adsorption activities of RB on the surface of the prepared materials, in that way assuming the adsorption mechanism.
Langmuir model describes that the monolayer adsorption system happens between the adsorbate and an ideal homogeneous surface, and not any interaction between adsorbed molecules [164, 165]. The linear equation is presented in equation (2):
𝐶𝑒 𝑞𝑒 = 1 𝐾𝐿× 𝑞𝑚+ 𝐶𝑒 𝑞𝑚 (2)
Where KL is Langmuir equilibrium constant (L mg-1) and qm (mg g-1) is the monolayer adsorption
capacity. RL is expressing separation factor, defined of Weber and Chakkravorti in equation (3):
𝑅𝐿= 1
1 + 𝐾𝐿× 𝐶0
(3)
Where C0 in this scenario is the highest initial solute, i.e. concentration of RB. The value of the
separation factor gives a suggestion for which kind of isotherm and nature of the adsorption system. According to the RL value, adsorption could be unfavourable (RL>1), linear (RL=1), favourable
(0<RL<1) or irreversible (RL=0) [165].
Freundlich model is an experimental equation founded on a heterogeneous surface of an adsorbent [166]. The famous logarithmic form of Freundlich is presented in equation (4):
𝑙𝑛𝑞𝑒 = 𝑙𝑛𝐾𝐹+𝑙𝑛𝐶𝑒 𝑛
(4)
Where KF and n are constants of Freundlich that are related to adsorption capacity and adsorption
26
3 R
esults and discussion
P
reparation and characterization of MOF@titania core-shell
nanoparticles
ZIF-8 and HKUST-1 nanoparticles were synthesised following literature [162, 163]. The obtained samples were characterised by XRD, SEM and FTIR. The XRD patterns in Figure 6 and Figure 8 show that the obtained MOF (ZIF-8 in Figure 6 and HKUST-1 in Figure 8) have high purity, without any impurity peak was detected. The SEM images of ZIF-8 (Figure 7) and HKUST-1 (Figure 9) show typical round- and octahedra -shaped morphologies of the corresponding crystals. The crystals of HKUST-1 and ZIF-8 have sizes of 2280 and 82 nm, respectively.
HKUST-1 and ZIF-8 ethanol suspension was mixed with a solution of deionized water and TBOT to coat a layer of titania gel on HKUST-1 and ZIF-8 individually. The XRD pattern and FITR spectra in Figure 6 and Figure 8 show that the peaks intensity of the HKUST-1 and ZIF-8 were weakened significantly after coating. The results indicated that the coating process will probably damage the structure of the MOF crystals. The absorption band attributed to the Ti-O-C vibrations of butoxy groups directly bonded titanium are not observed, owing to the overlapping of bands with the matrices of ZIF-8 and HKUST-1 [168, 169]. Moreover, the spectrum of FTIR demonstrates marked band at 1100 cm-1 for both HUKST-1 and ZIF-8 after coating. That can be recognised as Ti-O-C
group [170], which are proposing that titania gel are formed. In addition, the obtained ZIF-8@titania and HKUST-1@titania composites displays round- and octahedra- shaped morphologies as shown in Figure 12 and Figure 13, which is similar as the morphology of HKUST-1 and ZIF-8 before coating (see Figure 7 and Figure 9). The results indicated that the titania gel coating was probably only on surface of HKUST-1 and ZIF-8 crystals, respectively.
27
Figure 7. SEM pictures of ZIF-8 before (a) and after (b) carbonization. The carbonization condition was700 °C, 2 hours.
Figure 8. XRD and FTIR results of HKUST-1@titania before carbonization prepared with 1:2.5 mass ratio of HKUST-1 to TBOT at room temperature.
Figure 9. SEM pictures of HKUST-1 before (a) and after (b) carbonization. The carbonization condition was 600 °C, 2 hours.
a) b)
28
The curves of TGA for HKUST-1@titania and ZIF-8@ titania gel composites in Figure 10 and Figure 11 illustrates reduced amount of weight loss compared to HKUST-1 and ZIF-8 individually. The high amount of residues for the composites compared to the pure MOF samples are resulted from the titania. From the TGA curves, the content of titania in the prepare samples were 24 and 27 wt.% for ZIF-8 and HKUST-1, respectively. Therefore, the yields were estimated to be 3.2 and 3.7 wt.% based on the titania.
Figure 10. The curves of TGA for ZIF-8 and ZIF-8@ titania gel prepared with 1:1.2 mass ratio of ZIF-8 to TBOT at room temperature and TGA temperature was from 25 to 820 °C in N2.
Figure 11. The curves of TGA for HKUST-1 and HKUST-1@titania gel prepared with 1:2.5 mass ratio of HKUST-1 to TBOT at room temperature and TGA temperature from 25 to 820 °C
