INTRODUCTION
Research rationale
Pollution of air, water, and soil poses a significant global threat to both the environment and human health Water, which covers most of the Earth's surface and constitutes a large part of the human body, is vital for health and ecosystems Clean water is essential for human well-being, yet rapid industrial growth, population expansion, and urbanization have led to severe contamination of our natural resources The use of chemicals and fertilizers in agriculture and domestic settings contributes to life-threatening diseases, while the intense application of heavy metals in industries, such as dyeing and painting, exacerbates environmental issues Even low concentrations of heavy metals in water can be harmful, making their treatment crucial due to their persistence in the environment.
Various techniques have been developed to remove heavy metals, including traditional methods such as chemical precipitation, electrolysis, adsorption, and ion exchange Among these, adsorption stands out as an efficient technology for removing metal ions from aqueous solutions A diverse range of adsorbents, including activated carbon, water treatment sludge, zeolite, fly ash, and biomass, have been shown to effectively adsorb metal ions, demonstrating varying degrees of effectiveness in eliminating toxic pollutants from air, water, and soil.
More recently, one-dimensional (1-D) titanate nanotube (TNT) have been reported to be an attractive adsorbent to effectively adsorb a wide variety of metal ions including
Copper (Cu), lead (Pb), cadmium (Cd), and zinc (Zn) possess significant specific surface areas and layered structures, making them noteworthy in various applications Titanium nitride (TNT) is recognized as a modified structure in photocatalysis due to its unique electronic and mechanical properties, exceptional photocatalytic activity, and large specific surface area with high pore volume This makes TNT a promising material for the removal of metal ions from aqueous solutions.
In recent years, Graphene oxide (GO) has garnered significant attention globally due to its unique properties Graphene, a two-dimensional carbon nanomaterial composed of a single layer of sp² hybridized carbon atoms arranged in hexagonal patterns, exhibits remarkable mechanical, thermal, and electrical characteristics, with a theoretical specific surface area of 2630 m²/g GO, a functionalized form of graphene with various oxygen-containing groups, has been explored for diverse applications across physics, chemistry, biology, and material science Notably, graphene-based materials serve as effective adsorbents for pollutant removal, as GO's multiple functional groups and strong acidity enhance its adsorption capacity for basic compounds and cations Additionally, graphene's hydrophobic surface facilitates high chemical adsorption through robust π–π interactions.
Adsorption stands out as one of the simplest, fastest, and most effective methods for removing heavy metals among various treatment techniques In light of this, I focused on preparing a titanate nanotube/reduced graphene oxide composite to utilize as an efficient adsorbent.
Research's Objectives
The primary objective of this study is to develop and investigate a graphene based nano composite for the removal of toxic heavy metals from aqueous solution
Titanium nanotubes and GO were assembled in basic medium via microwave-assisted hydrothermal method
Estimation of the adsorption capacity of some heavy metal ions in aqueous solutions Characterization of material composites using techniques like XRD, TEM, SEM, FTIR, Raman Spectroscopy
Synthesizing the TNT and performing the adsorption experiments to evaluate its properties
The development of Titanate nanotubes combined with reduced graphene oxide is poised to greatly enhance wastewater treatment from mining activities in the country, improve domestic water efficiency, and support environmental protection efforts.
LITERATURE REVIEW
Overview of heavy metals
2.1.1 Definitions and sources of heavy metals
The origin of the term "heavy metal" is not clear An early use dates from 1817, when Gmelin divided the elements into nonmetals, light metals and heavy metals (Habashi F
2009) Light metals had densities of 0.860–5.0 gm/cm 3 ; heavy metals 5.308–22.000 (Gmelin L 1849) Heavy metals are divided into three types: toxic metals (Hg, Cr, Pb,
Metals such as zinc (Zn), copper (Cu), nickel (Ni), cadmium (Cd), arsenic (As), cobalt (Co), and tin (Sn), along with precious metals like palladium (Pd), platinum (Pt), gold (Au), silver (Ag), and ruthenium (Ru), as well as radioactive metals including uranium (U), thorium (Th), radium (Ra), and americium (Am), typically have a density greater than 5 g/cm³ (Bishop, 2002).
Heavy metals occur naturally in the earth but can become concentrated due to human activities Key sources of these contaminants include mining, industrial waste, vehicle emissions, lead-acid batteries, fertilizers, paints, and treated woods, with lead being the most common heavy metal contaminant.
Heavy metals are non-biodegradable and generally non-toxic in their elemental form, but they become hazardous to living organisms when present as cations, leading to accumulation over time (Tam & Wong, 1996) Approximately twelve heavy metals, including lead, mercury, aluminum, arsenic, cadmium, and nickel, are known to be toxic to humans While some heavy metals like iron, zinc, and copper are essential for health in trace amounts, excessive levels of these elements can be life-threatening (Foulkes, 2000) Other non-essential heavy metals, such as mercury and cadmium, can be highly toxic, particularly when they enter the food chain These toxic metals can enter the body through the respiratory system, gastrointestinal tract, and skin, and if their accumulation exceeds the body's ability to eliminate them, poisoning can occur (Foulkes, 2000) The toxicity of heavy metals manifests in various harmful effects on health.
(1) Some of heavy metal can be moved from low to higher toxicity in the form of some environmental conditions, such as mercury
(2) Accumulation and biological amplification of these metals through the food chain may damaging the normal physiological activity and ultimately endanger human health
(3) Toxicity of these elements may be at a very low concentration of about 0.1-10 mg.L -1 (Alkorta et al., 2004).
Heavy metal pollution in the world and Vietnam
2.2.1 In estuary, coastal and marine areas
Metal pollution in marine environments has surged in recent years, driven by global population growth and industrial expansion This heavy metal contamination poses a significant toxic threat to aquatic life in estuaries and coastal regions worldwide, while also presenting risks to human health.
Pb and Zn pollution poses significant risks to the Australian estuarine ecosystem, with contaminated sediments exhibiting alarming levels of 1000 µg/g Pb and 2000 µg/g Zn Research by Bryan et al (1985), as cited in Bryan & Langston (1992), revealed that inorganic lead concentrations in UK estuarine sediments varied from 25 µg/g in unpolluted areas to over 2700 µg/g in the Gannel estuary, heavily impacted by lead mining waste Additionally, lead compounds in these environments are likely linked to the historical use of leaded petrol.
Arsenic (As) concentrations have been detected in various estuaries and coastal regions globally, with levels in estuarine sediments ranging from 5 μg/g in the Axe estuary to over 1,000 μg/g in Restronguet Creek, Cornwall, where wastewater from mining activities significantly contributes to contamination.
Cadmium (Cd) concentrations in UK estuaries vary significantly, with levels as low as 0.2 μg/g in non-contaminated areas and reaching up to 10 μg/g in heavily polluted regions (Bryan & Langston, 1992) The Deule River in France exemplifies severe pollution due to waste from metallurgical plants, with sediment metal concentrations soaring to 480 mg/kg Additionally, heavy metal concentrations in sediments across global estuaries and coastal areas, particularly those with mangroves, range from low to high pollution levels, as noted by Tam & Wong (1995).
The concentration of lead (Pb) in the sediments of Sai Keng mangrove in Hong Kong was found to be 58.2 µg/g In a study by Zheng & Lin (1995), the Pb and cadmium (Cd) concentrations in the sediments of the Avicennia marina mangrove at Shenzhen Bay were recorded at 28.7 µg/g and 0.136 µg/g, respectively.
2.2.2 In acid sulfate soil areas
According to Astrom & Bjorklund (1995), acid sulphate soils are significant sources of heavy metal release, leading to aqueous solution pollution When these soils are exposed to oxygen, either through natural processes or artificial drainage, pyrite oxidation occurs, producing sulfuric acid and lowering the pH As the pH drops below 4, protons are released, which can attack clay minerals and dissolve metals, resulting in concentrations that may exceed those found in soils without acid sulphate.
Characteristics and hazards of some heavy metals
Arsenic distributed in many places of environment, they are ranked No.20 in the presence of many elements in the Earth’s crust, present less than Cu, Sn, but more than
Natural processes, particularly volcanic activity, are significant sources of arsenic emissions into the atmosphere, releasing approximately 17,150 tons of arsenic (Matschullat, 2000) This phenomenon is part of a broader context involving various elements such as Hg, Cd, Au, Ag, Sb, and Se (Bissen & Frimmel, 2003).
Sources of pollution due to human activity:
Mining ore (Cu, Ni, Pb, Zn), metallurgy releases into environmental a large amount of arsenic Approximately 62,000 tonnes of arsenic release into the environment every year from these activities
Burning of fossil fuels from the household, from the power plant
Use fungicides, herbicides, insecticides and industrial
Since when put to use DDT in 1947 and the organic pesticides containing organic arsenic compounds
The harmful effects of arsenic on human health:
The toxicity of arsenic varies significantly based on the compounds formed, particularly its valence, with trivalent arsenic being more toxic than pentavalent arsenic Inorganic arsenic, such as arsenic trioxide, has long been recognized for its harmful effects on humans, with lethal doses ranging from 50 to 300 mg, depending on individual susceptibility (Clark et al., 1997) Chronic arsenic poisoning manifests through symptoms like weakness, loss of reflexes, fatigue, gastritis, colitis, anorexia, weight loss, and hair loss Long-term exposure to arsenic through food or air can lead to serious health issues, including cardiovascular disease, nervous system disorders, brittle nails with horizontal white lines, liver dysfunction, and kidney damage (Bissen & Frimmel, 2003) Acute arsenic poisoning may result in nausea, dry mouth and throat, muscle cramps, stomach pain, and itchy skin.
Cadmium (Cd) is widely distributed in the Earth's crust, with an average concentration of about 0.1 mg/kg However, sedimentary rocks, particularly marine sedimentary rocks, can contain significantly higher levels, averaging around 15 mg/kg Each year, rivers contribute approximately 15,000 tons of Cd to the oceans In river and lake sediments, Cd concentrations can reach up to 5 mg/kg, while marine sediments typically show levels ranging from 0.03 to 1 mg/kg.
Cadmium (Cd) is extensively used in various industrial applications, including protective coatings for steel, stabilizers in PVC, and pigments in plastics and glass, as well as in numerous alloy components This widespread use contributes significantly to the release of cadmium into the environment.
The concentration of cadmium (Cd) in phosphate varies significantly based on the source of the phosphate rock For instance, phosphate fertilizer derived from North Carolina phosphate rock has a Cd concentration of 0.054 g/kg, while that from Sechura rock contains a lower concentration of 0.012 g/kg In contrast, phosphate fertilizer sourced from Gafsa phosphate rock has a Cd concentration of 0.07 g/kg.
The harmful effects of Cd on human health:
Cadmium is known to cause damage to the kidneys and bones at high doses Studied in
A study involving 1,021 men and women in Sweden found that cadmium (Cd) poisoning is linked to a higher risk of fractures in individuals over the age of 50 Itai-itai disease, a severe consequence of cadmium exposure, affects the kidneys and leads to symptoms such as bone pain, resulting in increased brittleness and a greater likelihood of fractures.
The average lead content in the lithosphere is estimated at 1.6 x 10^-3 weight percent, with land averaging around 10^-3 percent and normal fluctuations ranging from 0.2 x 10^-3 to 20 x 10^-3 percent Naturally, soil contains lead at an average concentration of 10 to 84 ppm.
Lead is used in batteries, the battery, in some instruments conductivity Some lead compounds are added in paint, glass, ceramics, such as colorants, stabilizers, binder
Improper recycling of lead waste can significantly elevate the levels of this toxic metal in the environment Additionally, the use of organic lead compounds like tetramethyl lead and tetraethyl lead in gasoline, particularly in developing countries, exacerbates the issue.
The harmful effects of lead on human health:
Lead (Pb) in the blood is linked to erythrocytes and accumulates in the bones, where it has a half-life of 20-30 years, compared to about a month in the blood The body’s ability to eliminate lead is slow, primarily excreting it through urine Organic lead compounds are particularly persistent and pose significant health risks, potentially leading to death.
Acute lead poisoning can manifest through symptoms like headaches, irritability, and nervous system disturbances Long-term exposure may result in memory loss, reduced cognitive abilities, and lower IQ, while also impairing hemoglobin synthesis, potentially leading to anemia Additionally, lead is recognized as a carcinogen linked to lung cancer, stomach cancer, and gliomas Furthermore, lead poisoning poses risks to reproductive health, including miscarriage.
Copper is naturally present in various minerals, including cuprite (Cu2O), malachite (Cu2CO3·Cu(OH)2), azurite (2CuCO3·Cu(OH)2), chalcopyrite (CuFeS2), chalcocite (Cu2S), and bornite (Cu5FeS4), as well as in numerous organic compounds.
(II) linked through oxygen to the inorganic agents such as H2O, OH-, CO3 2-
Copper in natural complexes primarily interacts with organic compounds such as phenolic and carboxylic agents The concentration of copper in lava rock ranges from 4 to 200 mg/kg, while in sedimentary rock, it varies from 2 to 90 mg/kg.
The harmful effects of Copper on human health:
Copper is essential for human development; however, excessive accumulation of this metal can be toxic According to Cumings (1948), as cited by WHO (1998), copper poses toxic risks for individuals with Wilson's disease, as their liver and brain exhibit significantly high levels of copper content.
Effects of heavy metal to environmental and human health
Environmental pollution from heavy metals has led to ecological imbalances and the decline of various organism populations worldwide The Severn Estuary, one of Britain's largest rivers, serves as a vital habitat for numerous fish species Over the decades, this river has been contaminated by heavy metals, including lead and cadmium, from multiple sources Such pollution has significantly contributed to the decline in fish populations However, recent reductions in water pollution levels have resulted in a resurgence of fish populations in the Severn Estuary.
Organisms in polluted environments exhibit a high capacity for accumulating pollutants, particularly metals, which poses health risks to consumers through the food chain A study by Ohi et al (1974), referenced by WHO (1985), measured lead levels in the blood, femur, and kidneys of pigeons from both rural and urban areas in Japan The findings revealed that pigeons from urban areas had significantly higher lead concentrations in their femurs, averaging between 16.5 and 31.6 mg/kg, compared to rural pigeons, which averaged only 2.0 to 3.2 mg/kg.
In the blood, lead levels also have similar trends from 0.15 to 0.33 mg.L -1 in urban areas, and from 0.054 to 0.029 mg.L -1
Recent studies have highlighted the severe health risks posed by arsenic exposure in countries like India, China, and Bangladesh, with millions of individuals at risk of arsenic poisoning In Vietnam, approximately 10 million residents in the Red River Delta are particularly affected by this issue.
In the Mekong Delta, between 500,000 and 1 million individuals are chronically poisoned by arsenic present in water from drilled wells Additionally, the accumulation of cadmium in the liver and kidneys of grazing animals in Australia and New Zealand poses significant risks, impacting both domestic meat consumption and international exports.
Some of treatment methods for the removal of heavy metals from aqueous
Various technological solutions exist for treating water pollution, but it is crucial to consider the effectiveness of these methods Key aspects to evaluate include the efficiency of the treatment process, the impact on water quality, and the sustainability of the solution implemented.
Ability to effectively apply in practice
Method of conducting is simple, easy to operate
Need of resources and energy to maintain the treatment process at a minimum
Sustainability is high; reduce long-term risk to aquatic environment
The processing time is fast
The ability to easily accept the method
Graphene, a carbon material known for its unique two-dimensional structure and excellent mechanical and thermal properties, has been utilized as a nanosorbent for heavy metal ion removal Zhao et al (2011) synthesized few-layered graphene oxide nanosheets using a modified Hummers method, demonstrating their effectiveness in adsorbing Cd²⁺ and Co²⁺ ions from aqueous solutions, with sorption efficiency influenced by pH and ionic strength due to the presence of oxygen-containing functional groups Additionally, Chandra et al (2010) reported that magnetite-graphene adsorbents with approximately 10 nm particle size exhibit high binding capacity for As³⁺ and As⁵⁺ ions, attributed to increased adsorption sites in the graphene composite Furthermore, Dana Fialova et al (2010) focused on isolating heavy metals using nanomaterials, achieving 99% cadmium adsorption on graphene surfaces after one hour of interaction, highlighting graphene's potential as an effective adsorbent for industrial applications, such as wastewater decontamination.
Graphene oxide (GO) is a chemically modified form of graphene, created through oxidation and exfoliation processes This monolayer material is characterized by its high oxygen content One of the primary applications of graphene oxide is in the development of thin membranes that facilitate the passage of water while effectively blocking harmful gases.
Graphene oxide (GO) demonstrates significant potential for metal ion complexation through electrostatic and coordinate interactions Research by Yang et al (2010) revealed that the interaction of Cu 2+ ions with GO in aqueous solutions leads to the folding of GO sheets and the formation of large aggregates The primary driving force behind this interaction is the coordination between Cu 2+ ions and oxygen atoms present on the GO surface Notably, GO exhibits a Cu 2+ adsorption capacity of 46.6 mg/g, which surpasses that of carbon nanotubes (28.5 mg/g) and activated carbon (4-5 mg/g).
The fabrication of reduced graphene oxide (rGO) and metal oxide composites has garnered significant attention in environmental remediation due to their ease of preparation, cost-effectiveness, and eco-friendliness Research by Chandra et al (2010) demonstrated the effectiveness of magnetite-graphene hybrids in removing arsenic species from drinking water in South Asia, achieving high binding capacities of 3.35-4.23 mg/g for As(V) and 6.21-7.81 mg/g for As(III) by reducing magnetite aggregation He et al (2010) enhanced the adsorption capacities for cationic dyes, methylene blue and neutral red, by covalently bonding surface-modified Fe3O4 nanoparticles to graphene oxide, reaching capacities of 190.14 mg/g and 140.79 mg/g, respectively Additionally, Zhang et al (2011) utilized these composites to effectively adsorb tetracycline, a prevalent antibiotic contaminant, with a maximum adsorption capacity of 95 mg/g on 6 nm Fe3O4-rGO.
In 2011, researchers developed rGO-MnO2 and rGO-Ag composites that effectively removed Hg (II) from wastewater, achieving a high distribution coefficient exceeding 10 l/g and demonstrating excellent performance in the uptake of heavy metal ions.
Phytoremediation is an eco-friendly approach that utilizes plants to address environmental issues by naturally mitigating contaminants in situ, eliminating the need for excavation and off-site disposal.
This method relies on the ability of various species, including aquatic plants, algae, fungi, and bacteria, to absorb and retain heavy metals from soil and water through a process known as biosorption.
The biological methods for handling heavy metals include:
Using anaerobic microorganisms and aerobic
Rahman et al (2011) explored the potential of common aquatic macrophytes for the phytoremediation of arsenic in contaminated water, highlighting its viability as a long-term solution Numerous aquatic plant species have been assessed for their ability to remove toxic elements from freshwater systems, with several, particularly macrophytes, demonstrating significant arsenic accumulation capabilities Notable candidates for phytoremediation include water hyacinth (E crassipes), duckweed (Lemna, Spirodela, and Wolffia), water fern (Azolla spp.), Hydrilla (H verticillata), and watercress (N officinale, N microphyllum), recognized for their hyperaccumulation of arsenic and favorable growth characteristics.
A number of studies revealed that phytoremediation of arsenic using aquatic macrophytes would be a good option to clean polluted water
Nanotechnology has advanced the use of nanomaterials as effective sorbents for wastewater treatment, particularly in removing heavy metal ions, thanks to their unique structural properties According to Lee et al (2012), for nanomaterials to be effective sorbents in this process, they must meet specific criteria.
The nanosorbents themselves should be nontoxic
The sorbents present relatively high sorption capacities and selectivity to the low concentration of pollutants
The adsorbed pollutants could be removed from the surface of the nano adsorbent easily
Sorbents have the potential for infinite recycling, and recent studies on various nanomaterials have demonstrated their effectiveness in removing heavy metal ions from aqueous solutions, showcasing a high adsorption capacity.
Titanium dioxide (TiO2) is a highly stable sorbent across a pH range of 2 to 14, exhibiting rapid adsorption and desorption processes Its nano-sized form offers distinct advantages, including a high surface area, increased surface atoms, enhanced reactivity, unique catalytic properties, and superior suspension stability compared to larger particles In a study by Doong et al (2012), titanate nanotubes (TNT) were successfully fabricated through an alkaline hydrothermal method and subsequently calcined at various temperatures, highlighting the versatility of TiO2 in advanced applications.
600 o C in air for 4h for removal of bisphenol A and Cu(II) ion The calcined TNT has good Cu (II) adsorption capacity
2.6 Overview of handling heavy metals in aqueous solution using Titanate nanotube / reduced graphene oxide composite
2.6.1 Scientific Basis of handling heavy metals in aqueous solution by rGO-TNT composite
The advancement of high technology has enhanced our understanding of Titanate nanotube/Graphene oxide (TNT/rGO) composites, enabling their application in addressing heavy metal pollution Researchers have extensively studied the role of TNT/rGO composites in mitigating water pollution, leading to a variety of solutions and results aimed at processing environmental contamination on Earth.
Heavy metals in drinking water pose serious health risks, including impaired growth, cancer, organ damage, nervous system issues, and even death Exposure to toxic metals like mercury and lead can trigger autoimmune disorders, leading to conditions such as rheumatoid arthritis and kidney diseases To combat heavy metal pollution in water, the treatment technology using TNT/rGO composites offers an effective solution.
2.6.2 Some research results of absorption of heavy metals in water by rGO-TNT composite
Extensive research has focused on graphene-based nanocomposites for the removal of toxic heavy metals from aqueous solutions Lee and Yang (2012) developed a flower-like TiO2 on graphene oxide (GO–TiO2) hybrid, achieving adsorption capacities of 88.9 mg/g for Zn2+, 72.8 mg/g for Cd2+, and 65.6 mg/g for Pb2+ at pH 5.6, surpassing the performance of either GO or TiO2 alone Similarly, Nguyen-Phan et al (2012) examined the adsorption capacity of reduced graphene oxide/titanate nanotube (rGO/TNT) hybrids in water purification, with a maximum dye uptake of 83.26 mg/g, significantly higher than the 75.36 mg/g observed with titanate nanotubes These results demonstrate that the hybrid materials exhibit nearly double the adsorption capacity compared to pure sheet and tubular titanates.
2.6.3 Prospects of technological rGO-TNT composite in removal of heavy metals in aqueous solution
METHODS
Materal
The chemicals used in this study were sourced without additional treatment ST01 Titanate was acquired from Ishihara Sangyo Ltd in Tokyo, Japan, while sodium hydroxide pellets and copper(II) nitrate pentahemihydrate were purchased from Riedel-de Haen in Seelze, Germany Hydrochloric acid (36.5-38.0%) was obtained from J.T Baker in Phillipsburg, NJ, and bisphenol A (99+% purity) was sourced from Aldrich Natural graphite powder was provided by Alfa Aesar Co Additionally, sulphuric acid, phosphoric acid, potassium permanganate, hydrogen peroxide, hydrochloric acid, and deionized water (with a resistivity of 18 MΩcm) were utilized for preparing all aqueous solutions and rinsing specimens.
X-ray powder diffraction (XRD) patterns were obtained using a Rigaku Ultima IV diffractometer (Cu Kα radiation), Raman spectra was collected using a JY Horiba HR-
The study utilized an 800 spectrophotometer, transmission electron microscope (TEM), microwave, rotary vacuum evaporator, centrifuge, stirrer, and atomic absorption spectrometer (spectra AA 100/200) from Varian, Australia, to assess the concentration of various metal ions in the solution.
Methods
Figure 3.1 Schematic of TNT synthesis
The 1-D nanostructured titanate nanotubes were synthesized by a hydrothermal method using ST01 TiO2 as the starting material In general, 300 mg of ST01 TiO2 powders were dispersed into 10 mL of 10M NaOH in each centrifuge tube (we prepared 6 tubes) After vigorous stirring by vortex machine, solutions were dispersed and washed by filtered water for 1hour They were synthesized into three tubes by scale The synthesized TNTs were then calcined at 150 o C The microwave is maintained at a power of 600w for 3 hours for the formation of titanate nanotubes Later, the products were added with 10ml 0.1M HCl and DI water (distilled water) and then cooled down to room temperature by shaking incubator at 25 o C for two nights Later the hydrothermal products were washed with 5ml 0.1M HCl and 20ml DI water
After thorough mixing with a Vortex machine to achieve a balanced solution, the mixture is centrifuged for 5 minutes to facilitate deposition The pH of the solution is monitored and should range between 6 and 8 The resulting white products are then filtered, washed with 20 ml of absolute ethanol, and dried in a drying oven at 60°C for 6 hours.
Figure 3.2 Schematic of GO synthesis
Graphene oxide (GO) is primarily synthesized through chemical oxidation and exfoliation of pristine graphite, utilizing methods such as Brodie, Staudenmaier, or Hummers, along with their variations This report details the preparation of GO from graphite powder using a modified version of the Hummer method, as established by Hummers and Offeman in 1958.
To synthesize the desired compound, combine 225 ml of H2SO4, 25 ml of H3PO4, and 2 g of natural graphite in a round-bottom flask, stirring the mixture using a magnetic stirrer Gradually add 5 g of KMnO4 while maintaining a temperature of 35°C for 10 hours Afterward, transfer the solution into 225 ml of deionized water in an ice bath to replace H2SO4 in the sheet, followed by the addition of 3 ml of H2O2 to reduce any remaining KMnO4 The solution will shift from deep brown to bright brown, indicating varying oxidation levels Finally, wash the mixture with 15 ml of 1M HCl and adjust the pH to the desired level for PO4^3-.
A sequential mixture of 5ml HPO4 2- buffer and 15ml DI water was performed, repeating the process two to three times until achieving a neutral pH The final solution was dried using a Rotary Vacuum Evaporator with a water bath set between 45 to 60°C, resulting in the production of graphene oxide sheets.
3.2.3 Synthesis of rGO-TNT Composite
TNT composites with graphene oxide (GO) were synthesized using an alkaline hydrothermal treatment method Initially, 50 mg of GO was dispersed in 10 mL of 1M NaOH overnight on a stirrer/hot plate to achieve a uniform dispersion Subsequently, 300 mg of ST-01 (TiO2) powder was gradually added to the GO dispersion while stirring for one hour to ensure thorough mixing The composite was then placed in a Teflon-lined container and heated in a microwave at 150°C and 800W under static conditions for three hours The resulting gray gel was washed with 0.1M HCl solution and stirred overnight in a shaking incubator at room temperature (25°C) Finally, the product was washed multiple times with deionized water, centrifuged until the pH reached approximately 7, and dried in an oven at 50°C for 40 hours to obtain the final rGO-TNT composite.
Adsorption of Cu (II) ion by TNT, rGO-TNT composites in aqueous solutions was investigated in centrifuge tubes
Figure 3 3 Adsorption experiment of Copper by TNT and rGO-TNT
To prepare a 1000 mL buffer solution that maintains stable pH levels upon the addition of acids or alkalis, 2.13g of ethane sulfonic acid (MES) was dissolved in 950 mL of deionized (DI) water, resulting in an initial pH of 4.02 Subsequently, 5 mL of sodium hydroxide (NaOH) and an additional 45 mL of DI water were added to adjust the pH to 5.0 ± 0.1.
In the TNT experiment, 10 mg of TNT composites were added to 20 mL solutions containing Cu (II) ions, with the pH maintained at 5.0 ± 0.1 using a buffer solution The mixture was stirred using a vortex machine and subsequently analyzed using atomic absorption spectroscopy (AAS) with a Perkin Elmer model 100.
In a study, 10 mg of TNT or GO-TiO2 hybrid structure powders were introduced into 20 mL of a heavy metal ion solution containing 20 mg/L of Cu²⁺, and the mixture was stirred thoroughly Following this, all samples underwent filtration using centrifugal filters and were subsequently analyzed using atomic absorption spectrometry (AAS, Perkin-Elmer model 100) Various removal experiments were conducted over different time intervals to assess the effectiveness of the hybrid structure powders in removing heavy metal ions, with the removal capacity (q, mg/g) calculated accordingly.
The adsorption of Cu(II) ions was evaluated using TNT and rGO-TNT as adsorbents The initial concentration (C0) and the filtered concentration (Cfiltered) of heavy metal ions were measured in mg/L using a centrifugal tube The sample volume (V) was expressed in liters, while the mass of the adsorbent (M) was recorded in grams.
Figure 3.4 Atomic adsorption spectroscopy (AAS)
It was similar experiments with others heavy metal
3.2.5 The method of determining the characteristics of the material
The characteristics of crystalline phase composites were analyzed using Raman spectroscopy and X-ray diffraction, while their functional structure was examined through Fourier transform infrared spectroscopy (FTIR) Morphological changes were studied using transmission electron microscopy (TEM) and scanning electron microscopy (SEM) Additionally, the absorption of heavy metals was assessed via atomic absorption spectroscopy (AAS).
Technique X-ray diffraction provides some essential information for research sample materials such as: The existence of the crystalline phase of qualitative, quantitative, lattice constants, lattice size, variable form, stretching the limits of the crystal lattice due to defects in the crystal lattice caused (Hanno zur Loye 2013)
The existence of phase qualitative, quantify are identified primarily based on location, intensity, area obtained from the diffraction signal collected
The crystal lattice constant can be calculated using the distance (d) between adjacent lattice surfaces obtained from X-ray diffraction diagrams, employing specific formulas for each crystal system For instance, in the cubic system, the equation takes a particular form to determine the lattice constant accurately.
The Miller indices h, k, l represent the orientation of the Bragg plane, while d hkl (Å) denotes the spacing between the atomic lattice planes, as observed in X-ray diffraction patterns To fully characterize the structure, it is essential to identify the parameters a, b, and c.
The size of particles of crystals obtained from XRD is calculated Scherrer formula:
The X-ray wavelength (λ in Å) is represented with K approximately equal to 0.9, while r denotes the mean size of the ordered crystalline domains measured in Å The line broadening (B size in radians) is determined at half the maximum intensity after accounting for instrumental line broadening Additionally, θB refers to the Bragg angle in this context.
X-ray diffraction was recorded on a Siemens D5000 at the Laboratory of Biomedical and Environmental Science, National Tsing Hua University
RESULTS
Characterization of GO and titanate nanotubes/rGO composite
The presence of rGO component in the rGO-TNT composite and the structural properties are confirmed by Raman spectroscopy and X-Ray diffraction technique
Figure 4.1 Raman spectra of GO, rGO-TNT materials
Figure 4.1 shows the Raman spectra of Graphene Oxide and rGO-TNT composite The Raman active 98 cm -1 , 138 cm -1 , 190cm -1 , 228 cm -1 , 276 cm-1, 396 cm -1 , 664 cm -1 and
The 865 cm⁻¹ modes in the composites correspond to the titanate structure, as noted by Habashi (2009) and Liu et al (2005) Notably, the composite peaks exhibit broader widths and a significant shift ranging from 98 cm⁻¹ to 228 cm⁻¹ The analysis focused on the most intense peak at 228 cm⁻¹, where the blue shift and broadening of the Raman spectra are influenced primarily by the size of the nanomaterial, along with defects and temperature variations.
Raman spectroscopy is essential for characterizing the electronic structure of carbon materials, as variations in Raman band intensity and blue shifts reveal insights into carbon-carbon bonds and defects The Raman spectra, illustrated in Figure 4.1, prominently display the characteristic D and G bands at 1346 cm⁻¹ and 1589 cm⁻¹, respectively.
The Raman spectra analysis reveals distinct characteristics of graphene oxide (GO) and reduced graphene oxide-titanium nitride (rGO-TNT) composites The D band, indicative of sp³ defects, and the G band, associated with sp² bonded carbons, are key features in this analysis The intensity ratio of the D band to the G band (I_D/I_G) in rGO-TNT samples is lower than that in GO, suggesting a reduced density of defects in the rGO Additionally, a blue shift of approximately 5 cm⁻¹ in the G band of rGO-TNT further confirms the presence of graphene in the composite These findings underscore the structural differences and enhanced properties of rGO-TNT compared to GO.
Figure 4.2 XRD patterns of GO
X-ray diffraction can also provide information on the crystal structure of the GO, TNTs and rGO-TNT Figure 4.2 shows a XRD pattern of GO with a sharp peak at about 2θ = 10.5 o corresponding to (002) reflection was observed, giving an interlayer spacing of 0.9 nm
Figure 4.3 XRD patterns of TNT and rGO-TNT composite
The crystal structures of the as-prepared TNT and RGO–TNT electrodes were analyzed using X-ray diffraction (XRD), revealing distinct peaks that indicate the formation of titanate nanostructures, specifically NaxH2-xTi3O7, under microwave-assisted hydrothermal conditions Notably, the rGO–TNT sample did not exhibit any visible peaks for graphene, a finding consistent with previous reports However, the presence of graphene in the rGO–TNT electrode was confirmed through Raman analysis, along with subsequent transmission electron microscopy (TEM) and scanning electron microscopy (SEM) imaging.
Morphology of TNT, GO and rGO-TNT composite
The morphological evolution of titanate in rGO-TNT composite was observed by TEM (transmission electron microscopy) images
Figure 4.5 TEM images of the synthesis TNT and rGO-TNT composite
The TEM images reveal that titanate nanotubes (TNT) exhibit tubular structures measuring approximately 200-250 nm in length before the attachment of reduced graphene oxide (rGO) Subsequent observations indicate numerous overlapping layers with high transparency, suggesting the formation of a very thin sheet-like structure Notably, no remnants of spherical titanate nanotube precursors were detected in the hybrid materials, confirming the effective interaction of titanium species within carbonaceous layers and their transformation into a sheet-like form Additionally, Raman spectroscopy analysis indicates a predominant presence of titanium and oxygen species, along with a minor amount of carbon, demonstrating the uniform distribution of these three components within the hybrid material.
Figure 4.5c Such characteristics demonstrate that the rGO sheets were assembled on the titanate nanotubes.
Application into removal of heavy metal ions
The absorption capacity of Copper ion by the TNT and rGO/TNT composite were examined
Table 4.1 The results of Cu (II) adsorption experiment by TNT, was observed by Atomic adsorption spectroscopy (AAS)
Figure 4.6 The adsorption of Cu(II) by TNT at pH=5 in aqueous solutionat room temperature
Figure 4.6 shows the absorbed amounts of Cu(II) ion by different concentration of TNT including 1ppm, 2ppm, 5ppm, 10 and 20ppm, and different absorption time (10,
The adsorption of copper ions by titanate nanotubes significantly increases with higher equilibrium Cu (II) concentrations, indicating a stronger adsorption capacity as the concentration rises.
The amount of heavy metal has reduced from 0.591 mg/L to 0.31 mg/L at the concentration is 20 ppm and mitigation under the next period
Table 4.2 The results of Cu (II) adsorption experiment by rGO-TNT, was observed by Atomic adsorption spectroscopy (AAS)
Figure 4.7 The adsorption of Cu(II) by rGO /TNT composite in aqueous solution at room temperature
The TNT/rGO composite demonstrates a significant capacity for Cu(II) ion absorption, as illustrated in Figure 4.7 The absorption rates were tested at various concentrations of Cu(II) ions (1 ppm, 2 ppm, 5 ppm, 10 ppm, and 20 ppm) over different time intervals (10, 30, and 180 minutes) Notably, the adsorption of copper ions by titanate nanotubes/reduced graphene oxide increased with higher equilibrium Cu(II) concentrations For instance, at 1 ppm, the absorption after 10 minutes was 0.015 mg/L, which slightly increased to 0.003 mg/L after 180 minutes In contrast, at a concentration of 20 ppm, the initial adsorption without the adsorbent was 0.591 mg/L, while the TNT/rGO composite achieved an adsorption capacity of 0.305 mg/L in 10 minutes and remained at 0.302 mg/L after 180 minutes Compared to TNT alone, the rGO/TNT composite exhibited superior adsorption capacities, highlighting the influence of pore structure, surface area, and surface functionality on adsorption efficiency.
DISCUSSION AND CONCLUSION
Discussion
This study presents a novel graphene-based nanocomposite designed for the efficient removal of toxic heavy metals from water The unique morphology, chemical structure, and electronic properties of graphene contribute to the advantages of this method, which include simplicity, cost-effectiveness, speed, and enhanced removal efficiency of TNT/rGO We synthesized titanate nanotubes using a hydrothermal method, produced graphene oxide via the Hummer method, and created a titanate nanotubes/reduced graphene oxide composite through hydrothermal synthesis The adsorption capacity of these materials was assessed, with XRD, TEM, and Raman spectroscopy confirming the structural morphology and the successful formation of titanate nanotubes measuring 200-250 nm Additionally, Raman spectroscopy and X-ray diffraction verified the presence of the rGO component in the rGO-TNT composite and its structural properties.
The experimental results indicate that the TNT/rGO composite is an effective adsorbent for heavy metals, reducing concentrations from 0.591 mg/L to 0.304 mg/L within 30 minutes at a 20 ppm concentration Comparisons between rGO/TNT and TNT revealed that rGO/TNT exhibited superior absorption capacity, attributed to its enhanced pore structure, surface area, and surface functionality Therefore, TNT/rGO emerges as a promising sorbent material for the removal of heavy metal ions from aqueous solutions, highlighting its significant potential in environmental pollution management due to its exceptional physicochemical properties.
Conclusion
In conclusion, the thesis demonstrates the morphology of TNT and rGO through SEM and TEM analysis, highlighting the effective adsorption of Cu ions by TNT/rGO composites These nanocomposites exhibit remarkable capacity for Cu (II) adsorption, achieving complete removal within just 30 minutes This significant finding suggests potential applications for the removal of other toxic chemicals from water and soil.
* There still lies a necessity in continuing the research on the adsorption of titanate nanotube/reduced graphene oxide on the other materials such as metals (Pb, As, Hg, )
* Researching the adsorption of heavy metal ions of TNT/rGO to large scale that may have high practical applicability
* Significant tests regarding the ability to remove other ions on TNT/rGO to be used for a variety of different contaminated water environment
Although every attempt has been made to make the review work presented in Section-
This report details the preparation of Graphene oxide (GO) from graphite powder using a modified Hummer method, acknowledging any unintentional omissions of important references The study suggests that future research could explore alternative methods to produce high-quality Graphene oxide Additionally, the current experiment focused on the adsorption of Cu(II) using TNT and rGO-TNT as adsorbents, but future studies may investigate the removal of various heavy metals.
Nanocomposite based other materials may be explored for its deployment in removal of toxic heavy metals from aqueous solution
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