1 Introduction

TiO2 based materials have attracted significant interest in many fields of applications owing to their unique characteristic features. TiO2 is well known for its photocatalytic activity including water splitting and degradation of organic molecules in particular. H2 is considered to be a future clean fuel due to the depletion of fossil fuels. It is easily generated by water splitting catalyzed by TiO2 based catalysts [1,2,3]. Organic molecules such as textile dyes [4,5,6], pesticides [7, 8], pharmaceuticals [9,10,11] present as pollutants mainly in wastewater, are degraded by TiO2 based catalysts in the presence of UV and/or visible light. Further, such photocatalysis has been shown to be effective in antimicrobial activities [12,13,14]. Briefly, the photocatalytic activity of TiO2 could be explained as; upon exposure to UV light a photogenerated electron excites to the conduction band from the valence band leaving a hole in the valence band. The photocatalytic activity is caused by the photogenerated electrons and holes. TiO2 is promising in photovoltaic cells [15, 16] as electrons that are generated when the photosensitizer absorbs light are handed over to the conduction band of TiO2. These diffuse towards the counter electrode through the transparent conducting oxide electrode to complete the circuit [17]. Further, TiO2 is used as the anode material in Lithium ion batteries in electric vehicles, mobile electronics etc. due to its high working voltage that ensures a stable operation [18, 19]. TiO2 is used as a white pigment in paints, coatings, plastics and inks due to its unique light scattering ability especially of the Rutile phase which has the highest refractive index. Further, pigmentary TiO2 is inert, non-toxic, thermodynamically stable and inexpensive [20]. Moreover, TiO2 has been modified by do** metals [21], non-metals [22], and coupling with metal oxides [23] and non-metal oxides [34]. This would much broaden the scope and the applicability of using ilmenite as the raw material to prepare the titanium precursors. According to our knowledge, such synthesis methods have not been reported to compare their efficiency with the doped materials synthesized by existing chemical precursors. We believe that a simple co-precipitation method could be used to synthesize doped TiO2 from natural ilmenite. For example, a solution containing iron added to TiOCl2 can be easily precipitated by using ammonia. N doped TiO2 may be easily synthesized by using urea where it will act as the N source and the base to precipitate [104]. Moreover, do** can be done right after obtaining TiO2 as it has not been crystallized yet. In such instances, desired metal salts such as FeCl3, CuCl2, AgNO3 dissolved in deionized water could be added to TiO2 before heating and calcination. Overnight stirring followed by heating and calcination would result in doped TiO2. This method could be implemented in synthesizing non-metal doped TiO2 as well. For example, urea could be added at a proper concentration to just produced TiO2 to synthesize N doped TiO2. Such research projects have not been reported so far opening new avenues to research in this area. However, heterostructures have been prepared by using natural ilmenite as the raw material and the solution resulted after digestion as the precursor. Thambiliyagodage et al. report the synthesis of binary Fe2TiO5/TiO2 nanocomposites using only the precursor generated after the digestion of ilmenite [106] and ternary Fe2O3/Fe2TiO5/TiO2 composite using the precursor and externally added iron [107], and they have shown high photocatalytic activity on degradation of methylene blue under sunlight. The rate of photodegradation of methylene blue under sunlight (0.038 min−1) in the presence of 2 g/L, Fe2O3/Fe2TiO5/TiO2 is greater than the rate of photodegradation of methylene blue (0.028 min−1) under 300 W simulated sunlight in the presence of 1 g/L of the same ternary heterostructure synthesized by Iron nitrate, tetrabutyl titanate and hexamethylenetetramine [108]. The reason cannot be directly attributed to the difference in the dosage of the catalyst as Thambiliyagodage et al. have used normal scattered sunlight, while Bhoi et al. used simulated sunlight. Nanoflower like Fe2TiO5/TiO2 was obtained by treating ilmenite in NaOH hydrothermally by Fernando et al. and have been used for electrocatalytic H2 evolution [109]. Multi-shelled TiO2/Fe2TiO5 synthesized using TiCl4 and FeCl3 have shown to be effective in water oxidation under solar light [110]. Further, magnetic properties of Fe2TiO5 synthesized by natural ilmenite via co-precipitation method [111] and via oxidation [112] have been also studied. Activities of the composites synthesized from natural ilmenite and synthetic chemicals are not compared in this review as different conditions have been used by different researchers. Research projects should be elaborated in finding the potential new applications of products prepared by using ilmenite as the raw material. Leachates obtained from the dissolution of ilmenite should be chemically treated to produce more titanium derived products. Leachates could be treated in different methods including sol gel synthesis, spray pyrolysis, electrochemical deposition, hydrothermal synthesis etc. to produce nanomaterials with different morphologies that could have different properties which can be applied to various applications. Further, obtained leachate could be combined with other chemicals such as CuCl2, ZnCl2, FeCl3 to produce heterojunctions which could be used as visible active photocatalysts to degrade organic pollutants. However, given that ilmenite is a natural material available for low cost, converting it to a titanium precursor consumes a high amount of chemicals and energy. Chemicals like acids, bases and salts were supposed to use in very high concentrations. Further, as ilmenite is a macro material dissolution is less efficient. Therefore, techniques like milling have been used to reduce the particle size and further, to change the chemical composition to facilitate digestion. However, milling requires high temperatures and the instrument which add more depth to the cost of the process. Considering the above, it is evident that there is more space for innovative research in the field discussed in this review.

5 Conclusions

Ilmenite partially dissolves in strong acids such as hydrochloric acid, sulfuric acid and weak acids including oxalic acid, phosphoric acid, citric acid to yield titanium and iron in the leachate. Generally, with increasing acid concentration, leaching time and leaching temperatures dissolution of ilmenite has increased. Addition of a reducing agent such as Fe increases the dissolution of ilmenite, and pseudobrookite phase formed during the oxidation of ilmenite resulting in a higher titanium yield. Formation of the pesudobrookite phase is prevented when Na2S, ZnO and ZnS are added in the preoxidation increasing the yield of titanium and iron from acid leaching. Leaching of ilmenite in potassium hydroxide would produce potassium titanate which may need additional acid treatment step to enhance the further dissolution. Leachates obtained from digestion of ilmenite has been used as a precursor to synthesize TiO2, while the residue obtained after the treatment were also rich with TiO2. Therefore, natural ilmenite can be used to produce TiO2 which is an effective white colour pigment and TiO2 based nanomaterials that are efficient photocatalysts for generation of energy via water splitting and degradation of organic pollutants in wastewater.