The titanium in this region is mainly concentrated along the coast of Ninh Thuan. Of the more than 10 mines and titanium mineral deposits in this region, 3 have been explored and their reserves have been assessed; these are the Chum Gang, Bau Doi and Go Dinh mineral deposits. The reserves of the 3 explored mines are as follows: ilmenite is about 284.53 thousand tons. In the whole region, the forecast resource of ilmenite is over 4.3 million tons.
In general, nationwide, the total explored and assessed original ore reserves are 4,435 thousand tons of ilmenite and the forecast reserves are 19,600 thousand tons. The investigated and explored coastal placer ore reserves are 12,700 thousand tons of ilmenite and rutile, the forecast reserves are 15,400 thousand tons of ilmenite and rutile.
The results of the investigation and exploration over the past decades show that Vietnam's potential for titanium ore and associated minerals is among the largest in the world. The titanium ore mining industry in recent years has made significant contributions to the growth of the national economy, especially in poor provinces such as Ha Tinh and Binh Dinh.
Ilmenite is a raw material for the production of TiO 2 pigment , titanium metal and other titanium products, of which about 90% of titanium ore is used to produce TiO 2 pigment . However, in recent years, in Vietnam in particular and in the world in general, scientists have gradually turned their attention to TiO 2 produced from ilmenite ore as a photocatalyst.
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The research group of Associate Professor, Dr. Phan Dinh Tuan, Ho Chi Minh City University of Technology, has carried out the research topic: Research on production and refining technology of TiCl 4 from coastal sandstones of Vietnam and manufacturing TiO 2 powder . The project has built a technological process and equipment for chlorination of titanium slag and direct chlorination of rutile concentrate to produce TiCl 4 that meets standards, the manufactured TiO 2 has a purity of over 99%. The topic has been continued to be researched in the KC-02 program in the semi-industrial production phase.
The group of authors Nguyen Thi Dieu Cam prepared TiO 2 from Binh Dinh ilmenite ore by alkalization method with the ore decomposition agent being the solvent.
KOH solution. The concentration of KOH to decompose the ore was 80%, the concentration of H 2 SO 4 used to react with the solid obtained in the form of paste after alkalization was 4 M, the ratio between the mass of ilmenite ore and the volume of H 2 SO 4 acid was 1:9 and the temperature of calcination of TiO(OH) 2 precipitate was 600 o C. The obtained TiO 2 contained both anatase and rutile phases, in which the anatase phase was more dominant with a strong and sharp peak intensity, an average particle size of about 25 nm and a specific surface area of 47.8 m 2 /g. The results of the investigation of methylene blue decomposition on TiO 2 catalyst showed that the photocatalytic activity of TiO 2 prepared from Binh Dinh ilmenite ore with UV excitation light source was quite good [2] .
Author Nguyen Thi Lan, Quy Nhon University in her doctoral thesis prepared modified materials C, S, N-TiO 2 from Binh Dinh ilmenite ore for application in shrimp farming wastewater treatment. Ilmenite ore was dissolved with sulfuric acid, then Fe ions were removed from the solution using iron ingots combined with cooling and crystallization of FeSO 4 , then the TiOSO 4 solution was hydrothermally treated with thiourea and then heated at 500 o C for 5 hours to obtain modified materials C, S, N-TiO 2. The obtained material has an anatase phase structure, has the ability to strongly absorb visible light and gives higher photocatalytic efficiency than TiO 2 material due to limited recombination of photogenerated electron-hole pairs and narrow band gap energy. The results of the investigation of the decomposition of antibiotic TC on TiO 2 and 2TH-TiO 2 -500 catalysts showed that the efficiency of TC decomposition on 2TH-TiO 2 material reached 96% after 120 minutes of illumination [10].
Author Nguyen Tat Lam, University of Science/Vietnam National University, Hanoi, conducted a doctoral thesis on the preparation of TiO 2 and modified TiO 2 from ilmenite ore to apply as a catalyst to decompose some toxic organic compounds in the aquatic environment. He used HF acid as an agent to decompose ilmenite ore, combined with the use of NH 3 for hydrolysis and calcination at different temperatures to obtain TiO 2 and TiO 2 modified with S. The material has an anatase phase structure, has photocatalytic activity to decompose phenol in the aquatic environment[9].
The group of authors Le Thi Phuong Thao, Nguyen Thi Hoai Phuong, Tran Van Chinh studied the synthesis of TiO 2 from ilmenite ore by the ammonium sulfate method, as well as the study of the phase transformation and removal of S from the TiO 2 product . Ilmenite ore and (NH 4 ) 2 SO 4 were mixed together in a mass ratio of 1:7, then calcined at 700 o C for 2 hours. The solid after calcination was crushed, washed and dissolved with H 2 SO 4 solution . Finally, the solution after filtration and calcination was hydrolyzed to obtain TiO 2. However, the product contained S impurities (about 3.7% by mass). When calcined at 650 o C, the S impurities were removed and at 750 o C, the phase transformation from anatase to rutile began [11].
The group of authors Tran Van Chinh and Nguyen Thi Hoai Phuong studied the preparation of TiO 2 from Binh Dinh ilmenite ore by the hydrogen sulfate method, using KHSO 4 as an agent to decompose ilmenite ore. Specifically, ilmenite ore and KHSO 4 were mixed in a mass ratio of 1:7, calcined at 600 o C for 2 hours, the solid after calcination was crushed and washed, then dissolved with dilute H 2 SO 4 solution (concentration 5 - 10%). The TiOSO 4 filtrate solution after dissolution was hydrolyzed at 80 o C - 90 o C for 1 hour to form a white precipitate TiO(OH) 2 . Filtering, washing and calcining the precipitate at 600 o C obtained TiO 2 in the form of anatase [5].
TiO 2 is a traditional, popular, non-toxic semiconductor material and is widely used in environmental treatment fields. However, TiO 2 has a large band gap, so it is only active in the ultraviolet region. To improve the optical activity of TiO 2 nanomaterials in the visible light region, studies have shown a number of methods to achieve this goal.
- Firstly, modifying nano TiO 2 materials with elements that can narrow the band gap, thereby changing the optical properties of nano TiO 2 materials . Usually, transition metal elements such as Fe, Ag, Cu ... are used here.
- Second, TiO 2 is activated by inorganic or organic dyes (dye sensitizer), this method can also improve the optical properties of the composite material in the visible light region.
- Third, the electron pairs in the conduction band on the surface of metal nanoparticles oscillate in resonance with the electrons in the conduction band of nano TiO 2 as in metal-TiO 2 nanocomposite materials .
In addition, the surface modification of TiO 2 nanoparticles by other semiconductors can change the charge transfer ability of TiO 2 with the surrounding environment, thereby enhancing the application of these composite materials. Among these methods, the simplest method is to combine with transition metals to narrow the band gap, the most popular and widely used in practice is Fe
Ilmenite concentrate 52% is a by-product of the pure TiO 2 production industry , exported with low economic value (16.5 million/ton - announced price of Binh Dinh Minerals Company in the 4th quarter of 2020), so utilizing it will help increase the value of this by-product, contributing to reducing input costs, thereby creating a material with high economic efficiency.
On the other hand, ilmenite ore is also a natural source of TiO 2 and Fe 2 O 3 compounds . Combining these two oxides in composite materials can reduce the band gap energy, helping the material to show activity in the visible light region.
1.1.3.2. Graphene and TiO 2 composite materials
Many studies have used GO and RGO to modify TiO 2 for photocatalytic ability to treat the environment. Graphene has a large surface area, TiO 2 is known as a good photocatalytic substance in treating dyes and organic pollutants in the ultraviolet light region. In the publications, it has also been shown that graphene, GO or RGO can act as a coupling agent or
Stimulants for semiconductors such as TiO 2 , ZnO... when treating pollutants in the water environment [27].
Lui et al. [82] synthesized nano TiO 2 by hydrolysis method. The composite material consisted of graphene sheets surrounded by TiO 2 nanoparticles . The results showed that the photocatalytic efficiency of methyl blue (MB) treatment under ultraviolet light was enhanced in the presence of graphene. Graphene has good conductivity and charge transfer ability, which reduced the recombination ability of photogenerated electron-hole pairs of TiO 2 , thereby enhancing the photocatalytic efficiency. In addition, in the presence of graphene, the composite material also adsorbed MB more effectively and thereby increased the photocatalytic treatment efficiency.
As discussed in the previous section, GO and RGO have more defect sites than graphene. Zahra et al. [132] used graphene sheet for the synthesis of TiO 2 -graphene nanomaterials by sol-gel method. This composite material has outstanding photocatalytic activity, which is attributed to the fact that graphene is in the form of thin sheets, with a large surface area, increasing the adsorption capacity of the composite material. Thanks to its electrical conductivity, graphene sheet also acts as an electron transfer site. In short, the adsorption properties, electrical conductivity, as well as the large surface area of graphene are the decisive factors that increase the processing capacity of graphene-TiO 2 composite materials.
However, it can be seen that the studies mainly put TiO 2 nanoparticles on GO, RGO, graphene nanosheet substrates. There are very few studies published both domestically and internationally on putting TiO 2 nanoparticles on GNP substrates for environmental treatment applications, although GNP has outstanding advantages and potential for industrial scale applications. Most new studies focus on putting GNP into polymer materials to increase physical and mechanical properties and increase the electrical and thermal conductivity of composite materials [19].
The thesis chose to synthesize composite materials based on TiO 2 and Fe 2 O 3 on GNP directly from ilmenite and graphite. Because the precursors contain Ti and Fe in solution form, the thesis chose the hydrothermal method. The method has many advantages such as being favorable for the process of forming oxide crystals on GNP, increasing the ability to disperse, easy synthesis, and can be carried out on a large scale.
1.2. Heavy metal pollution in wastewater from explosives production.
1.2.1. Heavy metal pollution and Cr(VI) pollution
Heavy metals are metals with a density greater than 5 g/cm3 . They exist in the atmosphere (vapor form), hydrosphere (soluble salts), geosphere (insoluble solids, minerals, ores...) and biosphere (in the human body, animals and plants). Like many other elements, heavy metals can be essential or unnecessary for organisms, both plants and animals. Metals that are essential for organisms only mean "essential" at a certain concentration, if less or more, it will have the opposite effect. Unnecessary metals, when entering the body of organisms, even in trace form, can cause toxic effects. In the metabolism of organisms, those unnecessary heavy metals are often classified as toxic. For example, in the case of nickel, for plants, nickel is unnecessary and toxic, but for animals, nickel is very necessary at low concentrations.
Heavy metals in the environment are often not biodegradable but accumulate in organisms, participating in biological transformation to form toxic or less toxic compounds. They can also accumulate in abiotic systems (air, soil, water, sediments) and are transformed by changes in physical and chemical factors such as temperature, pressure, flow, oxygen, water, etc. Many anthropogenic activities also participate in the transformation of heavy metals and are the cause of affecting the geochemical and biological cycles of many animals and plants [3].
Nowadays, people are directly exposed to heavy metals in many ways and at different doses such as through the food chain or through long-term exposure to toxic metals in the environment. Poisoning is becoming more and more widespread, especially if waste disposal continues at the current level, it is difficult to hope that this growth will ever decrease. Some toxic heavy metals that people today are still regularly exposed to are mercury, chromium, cadmium, lead, etc.
In industry, chromium(VI) is used more than chromium(III), while chromium(VI) is more toxic.
+ Chromium (III) has little chance of being absorbed through the digestive tract, it combines with proteins of the skin surface layers to form stable complexes (Bidstrup PL, Wagg R.). That property may explain why Cr (III) does not cause dermatitis or chromium-induced ulcers. Chromium (VI) in its oxidized state is irritating and corrosive, it is easily absorbed through the digestive tract, through the skin and through the respiratory tract.
+ Exposure to Cr (VI) mainly affects the respiratory system and the skin. In addition to dermatitis and ulcers, there is also the possibility of lung cancer. Typical exposures are inhalation of dust during the processing of chromite ore, the manufacture of dichromates, lead, zinc chromates and inhalation of chromic acid mist (chromic anhydride) during chromium plating and surface treatment of finished metals (coloring, polishing, etc.).
In addition to blood samples of humans and animals, Cr is also found in teeth. Cr is also a metal involved in the metabolism of fatty acids. When animals are given Cr salt solutions, Cr is quickly and completely eliminated. However, when Cr salts are dissolved in water and administered subcutaneously, intravenously and endotracheally, Cr is retained in the lungs. In industry, exposure to Cr causes damage to the skin, nasal mucosa and lungs, can cause systemic poisoning and lung cancer [1].
The permissible concentration limit of Cr (VI) in industrial wastewater is 0.05 mg/L (column A), 0.1 mg/L (column B) according to QCVN 40:2011/BTNMT.
1.2.2. Heavy metal pollution in defense production wastewater
Hazardous waste generated from military activities (also known as special defense waste or special defense waste ...) is defined as a special type of waste, usually only generated from military activities, mainly from the activities of defense production, repair and technical support facilities of the army. The characteristic of this type of waste is that it often contains chemicals that are both dangerous, flammable and explosive, and toxic to the environment and human health [8].
Waste containing heavy metals generated from defense activities is classified into group 2 of special defense waste [8]. In the production of defense explosives, wastewater containing heavy metals is generated from three different sources:
The first source is from mechanical processing lines, surface treatment for shells and mechanical parts in defense production. This type of wastewater often contains additional highly toxic chemicals such as cyanide, nitrite, special grease and heavy metal ions such as Cr(VI), Pb(II), Cd(II), Ni(II)... [8].
The second source is from the production of primary explosives (primers or initiators) such as mercury fuminate (C 2 N 2 O 2 Hg), lead azotide (N 6 Pb), lead stypnate (C 6 HN 3 O 8 Pb). The wastewater from this activity often contains toxic organic substances with explosive properties such as stypnic acid (C 6 H 3 N 3 O 8 ), tetrazene, fuminate acid ..., organic solvents and heavy metal ions such as Pb, Hg ...
Table 1.3 lists some of the main components in the production of primary explosives and the waste generated from this activity. Through table 1.3, it can be seen that wastewater generated from primary explosives production technology, in addition to toxic organic substances that are difficult to decompose, also contains heavy metal ions such as Pb 2+ , Hg 2+ . These heavy metal ions have explosive components (Hg(ONC) 2 , Pb(N 3 ) 2 , lead stypnate), so it is required to separate heavy metals from wastewater in addition to the role