llmenite and Biomass Composite Nanomaterials - A Review

Charitha Thambiliyagodage ^1,*, Madara Jayanetti ¹, Geethma Ekanayake ¹, Amavin Mendis ¹, Heshan Liyanaarachchi ², Supuni Wijayawardana ¹

1. Faculty of Humanities and Sciences, Sri Lanka Institute of Information Technology, Malabe, Sri Lanka

2. Department of Basic Sciences, Faculty of Health Sciences, Colombo Regional Center, The Open University of Sri Lanka, Nawala, Nugegoda, Sri Lanka

* Correspondence: Charitha Thambiliyagodage

Academic Editor: Samer H. Zyoud

Special Issue: Nanoparticles in the Catalysis

Received: April 11, 2024 | Accepted: May 30, 2024 | Published: June 13, 2024

Catalysis Research 2024, Volume 4, Issue 2, doi:10.21926/cr.2402006

Recommended citation: Thambiliyagodage C, Jayanetti M, Ekanayake G, Mendis A, Liyanaarachchi H, Wijayawardana S. llmenite and Biomass Composite Nanomaterials - A Review. Catalysis Research 2024; 4(2): 006; doi:10.21926/cr.2402006.

© 2024 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Water quality management research has advanced significantly in recent years due to a growing concern about increasing water contamination concerns [1,2,3,4,5,6,7]. Efforts to regulate and reduce exceedingly toxic and dangerous chemicals, such as dyes, are considered vital tasks by major environmental agencies and organizations, such as the United States Environmental Protection Agency (US EPA) [6,7]. Dyes are essential in several sectors of the dyeing and textile industries. More than 100,000 commercially available synthetic dyes are often utilized in numerous sectors. The dyes are typically obtained from coal tar and petroleum intermediates, with an annual output exceeding 7 × 10⁵ tons [8,9,10]. Dyes are also byproducts of other sectors, including food and beverage, printing, and leather industries [4]. The rapid development of this industry sector presents a significant environmental danger, with the World Bank estimating that 20% of industrial water pollution comes from this industry [5]. Out of 4000 dyes tested in an ETAD survey, 90% had LD₅₀ values above 2 × 10³ mg kg^-1. Diazo, direct, and basic dyes had the most significant levels of toxicity among all dyes studied [11]. The textile industry uses a considerable amount of potable water, approximately 200 liters per kilogram of textile produced. This leads to the discharge of untreated wastewater into water bodies and causing severe water pollution [12]. The aromatic structure of dyes enhances their stability against photodegradation, heat, biodegradation, and oxidizing agents [13]. Discharge of textile effluent is linked to a range of health ailments and environmental degradation [6,9,14,15]. Dye in water bodies is an aesthetic issue and hinders sunlight penetration, disrupting the environment [16]. Vibrant textile wastewater poses substantial health risks to people, animals, plants, and microorganisms and significantly impairs plant photosynthesis and aquatic life via eutrophication [6,17]. Various treatment options for textile wastewater include physical methods (such as adsorption, ion exchange, and membrane filtering), chemical methods (such as chemical precipitation, coagulation, flocculation, and oxidation), and biological processes (aerobic and anaerobic) [3,18].

Untreated or insufficiently treated wastewater effluents can initiate eutrophication and produce circumstances that promote the growth of water-borne pathogens or toxin-producing cyanobacteria [17,19]. Recreational water users who come into touch with the contaminated water are in danger. While certain microorganisms have helpful functions in wastewater systems, many are significant contributors to outbreaks of water-borne illnesses [19,20]. Waterborne diseases result from the spread of disease-causing microbes (such as bacteria, viruses, and protozoa) via contaminated water, directly or indirectly [21,22,23]. Water-borne pathogens are primarily transmitted through the fecal-oral route and reach wastewater treatment plants via excreted feces in raw sewage from infected individuals, contaminated drinking water, or food that has been in contact with polluted water. This can result in severe health issues [24,25,26].

Among the methods above used to treat wastewater, the biological process is regarded as rapid, simple, eco-friendly, and conducted only under moderate environments. Nevertheless, nanoparticles are considered the most efficient approach for removing dyes from wastewater, avoiding adverse effects and preserving water quality and environmental safety [27]. Nanotechnology has significant potential to enhance the efficiency of water purification and disinfection. Nanomaterials effectively eliminate organic and inorganic pollutants, heavy metals, and microbes from wastewater [28]. Various materials and nanocomposites have been identified for degrading and eliminating multiple contaminants [29]. Metal oxide nanoparticles are used for several applications, including the degradation of pigments, drugs, and wastewater treatment. Metal oxides are used to remove hazardous heavy metals from various samples and act as efficient adsorbents [6,27,30,31,32,33]. Moreover, nanotechnological advancements suggest that waterborne pathogens can be eliminated or deactivated using various nanotechnology products and processes such as nanobiosorbents, nanocatalysts, bioactive nanoparticles, nanostructured catalytic membranes, nanobioreactors, and nanoparticle-enhanced filtration [28,34,35,36,37,38]. Carbon-based materials such as activated carbon, graphene oxide, reduced graphene oxide, graphitic carbon nitride, and catalytically graphitized carbon are effective in many applications.

Ilmenite-biomass composite nanomaterials are an intriguing combination of materials science and renewable resources due to increased electrical conductivity, improved mechanical strength, biodegradability, and customized functionality through various chemical linkages. Ilmenite, a mineral composed of titanium and iron oxides, is renowned for its distinctive characteristics, such as its exceptional hardness, corrosion resistance, and remarkable conductivity. Biomass refers to a diverse array of organic resources obtained from plants and animals, which include qualities of environmental sustainability. When ilmenite and biomass are combined to form a composite nanomaterial, they exhibit synergistic features that provide significant advantages for various applications.

Several advantages of these composite nanomaterials include graphitic carbon networks and integrated materials used in energy storage, photocatalysis, wastewater treatment, and agriculture [39,40,41,42], graphene oxide and reduced graphene oxide based nanomaterials used in dye degradation and solar energy-based applications, graphitic carbon nitride-based composites, graphene oxide linked nanocomposites and sucrose-based graphitic carbon composites used in wastewater treatment and environmental remediation by removing methylene blue dye and photo-sterilizing microorganisms such as Escherichia coli [30,43], shrimp shell-based graphitic carbon-based composites used in ecological remediation systems [44] and activated carbon based nanocomposites with a high surface area and adsorption capacity used in photocatalyitic systems and environmental remediation (Figure 1) [45,46].

Figure 1 A diagram summarizing the applications of nanocomposites synthesized using biomass and ilmenite.

The current study outlines the usage, progress, and research applications of ilmenite and graphene oxide, activated carbon, graphitic carbon nitride, and sucrose-based nanomaterial.

2. Ilmenite and its Derivatives

Titania, the ninth abundant element on earth, naturally appears in the form of different minerals, including ilmenite, rutile, anatase, and leucoxene, of which ilmenite consists of both iron and titanium in the lattice abundantly [1,47,48,49,50]. Ilmenite is found in countries such as Australia, South Africa, America, India, Brazil, Sri Lanka, New Zealand, Vietnam etc. Though the main constituent of the mineral is FeTiO₃, other chemical compounds, including TiO₂, FeO, Na₂O, SiO₂, Al₂O₃, MnO, CaO, MgO, and V₂O₅ could also be present in minor quantities [1]. It is worth noting that value addition to ilmenite is economically favorable as it could be applied in many fields, including energy production, environmental remediation, and pigment production, which applies to many industries such as paint, cosmetics, food, pharmaceutical, textile, etc. The chemical formula of ilmenite is FeTiO₃ and appears in crystalline form in which a dense arrangement of Fe²⁺ and Ti⁴⁺ in octahedral coordination with oxygen results in hexagonal close packing. Fe²⁺ and Ti⁴⁺ account for a total of valency +6. Hence, only two out of three available oxygen octahedral positions are occupied, resulting in a neutral structure. However, due to the high stability of the lattice, the use of ilmenite for many applications is limited, and hence, breakdown of the lattice and conversion to more applicable compounds are needed. To be precise, breaking down the lattice was not reasonably feasible due to its high stability; hence, different treatment methods have been employed. Acid treatment is most prevalent, and ilmenite sand is treated mostly in strong acids such as hydrochloric acid and sulfuric acid [51,52,53,54]. Moreover, ilmenite was treated with alkaline solutions, mainly KOH [55], and with salts such as ammonium chloride [56], in which pretreatment with carbon was performed. The use of ilmenite alone and with carbonaceous compounds for different applications is discussed in this review in detail.

Hydrochloric acid leachate of ilmenite treated with ammonium hydroxide produced Fe₂TiO₅/TiO₂ [57] upon annealing at 800°C for 2 hours and the addition of FeCl₃ externally to the acid leachate and annealing at 600°C and 800°C led to the production of different binary and ternary metal oxide nanocomposites. Materials annealed at 600°C produced Fe₂O₃/Fe₂TiO₅ and Fe₂O₃/Fe₂TiO₅/TiO₂ composites while the materials annealed at 800°C produced Fe₂TiO₅/TiO₂ and Fe₂O₃/Fe₂TiO₅/TiO₂. All the composites were photocatalytically active to degrade methylene blue under sunlight in which Fe₂TiO₅/TiO₂ produced by annealing at 800°C degraded methylene blue more efficiently with a rate constant of 0.057 min^-1 compared to the others due to the favorable band alignment. More interestingly, nanocomposites obtained after annealing the materials at 800° are more effective than those produced at 600°C due to the high crystallinity of the photocatalysts [58]. Further, Usgodaarachchi et al. report the synthesis of TiO₂/Fe₃O₄ and TiO₂/Fe₂O₃ as well, in which the acid leachate obtained after the ilmenite digestion was bubbled with N₂ gas. Then FeSO₄ was added, which was afterward treated with NH₃ to obtain a precipitate. TiO₂/Fe₃O₄ and TiO₂/Fe₂O₃ were produced after annealing the obtained precipitate at 450°C and 800°C, respectively, and TiO₂/Fe₂O₃ resulted to produce the highest rate constant (0.044 min^-1) to degrade methylene blue under visible light. In addition to the generation of different metal oxide nanocomposites, the production of 99% pure TiO₂ spheres was also reported in the same study with a particle size of about 218 nm due to the Oswald ripening that occurred during the hydrothermal treatment [43]. Abdelgalil et al. reported the recovery of TiO₂ from ilmenite through roasting in ammonium sulfate and effects of roasting temperature, roasting time, ilmenite-to-ammonium sulfate mass ratio, ilmenite particle size, and second-stage roasting on iron removal and titanium loss leaching efficiency have also been a study where optimum roasting was obtained at roasting temperature of 500°C, time of 210 minutes, mass ratio of 1:7 with ilmenite particle size below 43 µm which produced 75.83% pure TiO₂ [59]. Namgyu et al. reported the oxygen transfer capacity of pseudobrookite derived from ilmenite in which they observed an increase in the oxygen transfer rate to 4.2% upon calcination of ilmenite due to the production of pseudobrookite phase in which the Fe²⁺ in FeTiO₃ was converted to Fe³⁺ in Fe₂TiO₅. Upon introduction of red mud, the oxygen transfer rate has increased to 4.9% due to the incorporation of more Fe, which facilitates the conversion of the rutile phase to pseudobrookite, creating a perfect pseudobrookite crystal. Meanwhile, they could increase the oxygen transfer rate to 6.0% through the incorporation of 15 wt% CuO, which was indicated by the cyclic voltammetry curves, which remained constant even after 200 cycles [60].

3. Graphitic Carbon Networks

Graphitic carbon networks are 3D networks of amorphous carbon containing crystalline graphitized zones within the matrix [61]. The crystalline regions within the matrix consist of sp² hybridized carbon atoms bonded to three other neighboring carbon atoms, and these regions are non-uniform and spread out in the matrix of amorphous carbon [62]. The amorphous carbon consists of a disordered distribution of carbon atoms of both sp² and sp³ hybridization with varying bond angles and lengths [63]. The properties of amorphous carbon vary depending on the spread of type hybridization. With the integration of other elements into the graphitic network, the properties of the graphitic network can be enhanced. A usual candidate for integration is nitrogen, to form graphitic carbon nitride which is derived from the pyrolysis of nitrogen-containing organic compounds [64]. Graphitic carbon nitride exhibits attractive electrochemical properties. The amount of graphitization varies from material to material, depending on the method of synthesis usually done through high-temperature catalytic carbonization in an inert atmosphere such as in nitrogen [65]. Hence, the production process minimizes environmental impact as no corrosive or toxic chemicals are utilized. A variety of naturally occurring organic precursors can be used in the synthesis of graphitic carbon. Carbohydrates such as sucrose and starch can be utilized in the creation of graphitic carbon [66,67]. Chitosan, a nitrogenous polysaccharide usually derived from crustacean sources such as shrimp, crab, and lobster shells, acts as an excellent precursor for the production of nitrogen-containing graphitic carbon networks [44,68]. Due to the inherent attractive properties of the material, graphitic carbon networks and integrated materials can be used in a wide variety of fields, such as energy storage, photocatalysis, wastewater treatment, and agriculture [39,40,41,42]. The wide availability, ease, and sustainability of production allow for effective, scalable commercialization.

4. Graphene Oxide-Based Composites

Ceylon graphite is a biomass that is very much known in the world as the purest graphite. Graphene, graphene oxides (GO), and reduced graphene oxides (rGO) are two-dimensional derivatives of graphite consisting of sp² carbon atoms organized into planer hexagonal lattice through σ bonds to 3 adjacent carbon atoms [69]. The bond lengths of the C-C bonds are 0.142 nm, while the sheet thickness is 0.34 nm [70]. The p electrons remain unbound in the lattice, allowing the formation of π bonds with surrounding atoms. The bonds formed are perpendicular to the frame of graphene [71]. Graphene exhibits immaculate physical and electronic properties such as charge-carrier mobility of 250 000 cm² V^-1 s^-1 at room temperature, thermal conductivity of 5000 Wm^-1 K^-1, and mechanical stiffness of 1 Tpa [72]. In graphene, the structure of carbon atoms remains aligned by offsetting the force by deformation and bending of the atomic surface, even when an external force is applied [73]. GO contains hydroxyl and epoxy functional groups, which randomly decorate the 2D carbon lattice. Carbonyl groups are present at the edge of the sheet but also as carbonyl defects within the lattice, most likely in the form of Carboxylic acids [74]. The functional groups present in GO provide excellent points for surface modifications to develop functionalized graphene oxide for various applications. The distribution of the unoxidized benzene ring region and an oxidized aliphatic six-membered ring region depends on the degree of oxidation and is randomly spread out in the GO sheet. While graphene is hydrophobic, GO is hydrophilic due to the oxygen-containing groups bound to the carbon lattice making it easily dispersible in water [43] GO contains both aliphatic (sp³) and aromatic (sp²) regions, which leads to a variety of interactions that can occur on the surface. There are 3 primary types of surface functionalization according to functional group bonding, covalent bonding, non-covalent bond interactions, and doping of elements [75]. Proper functionalization of graphene and graphene oxide prevents agglomeration during the reduction process. Both graphene and graphene oxide exhibit excellent mechanical, electrical, and thermal properties due to their unique structure and morphology. The chemical oxidation of graphene to produce graphene oxide is given in Figure 2.

Figure 2 Schematic representation of chemical oxidation of graphene to produce graphene oxide.

Over the years, many methods of producing graphene and graphene oxide are described. The preparation of graphene oxide usually follows two steps: oxidant intercalating oxidation followed by sheet peeling, which is utilized in multiple processes such as Brodie, Staudenmaier, and Hummer.

Brodie method involves dropping fuming nitric acid onto a physical mixture of graphite and NaClO₃ at which only small amounts of the oxidant are formed in situ. Delamination is only triggered by adding a base (NaOH) to deprotonate less acidic functional groups [76]. The Staudenmaier method is a modified Brodie method in which a mixture of HNO₃ and H₂SO₄ is used [77]. In contrast, Hummer method explains adding H₂SO₄ dropwise into a mixture of graphite and KMnO₄ at 65°C for 20 hours where 30% H₂O₂ was added subsequently to stop the reaction [78].

Through oxidation, the sp² hybridized structure breaks down, leading to defects in the lattice structure. The process expands the distance between sheets of graphene from 3.35 A° to about 6.8 A° in Graphite oxide, depending on the amount of water [79]. Exposing the oxidized product to low-power sonication in aqueous media results in the delamination of individual graphene oxide sheets. The sheets are stabilized in a slightly basic aqueous solution due to the surface's negatively charged, oxygen-containing groups [80]. At the same time, preparation of graphene relies on mechanical stripping, liquid phase stripping, chemical vapour deposition, epitaxial growth, and redox methods [81]. The different methods of graphene oxide preparation bring their advantages and disadvantages. The benefits of different methods include shorter reaction time, lower toxicity, reduction of pollution, yield improvements, and relative safety of the preparation process. The industrial production of graphene oxide is highly feasible, however, due to inefficiencies such as complicated purification steps and defects in the lattice, leading to an inferior product [82,83]. Both graphene and reduced graphene oxide act as a precursor for the production of reduced graphene oxide.

Reduced graphene oxide (rGO) production is initiated first by oxidizing graphite to graphite oxide, following the same processes as Brodie, Staudenmaier, and Hummer mentioned above. The ratio of C: O depends on the process utilized and the reaction time. Some oxygen-containing groups can be removed through reduction or thermal treatment to yield rGO. The oxidized product is exfoliated through sonication and reduced by a strong reducing agent such as hydrazine hydrate [84]. Due to the toxicity of hydrazine, alternatives such as NaBH₄, KI, and ascorbic acid are used [72] . Due to scalability, ascorbic acid is used in industrial production processes. The rGO can be used in multiple applications through further modifications downstream.

Usgodaarachchi et al. introduce a new photocatalyst fabricated from ilmenite and Ceylon graphite (r-GO/GO/α-Fe₂O₃/Fe₂TiO₅). Ilmenite's acid leachate was previously used to synthesize efficient photocatalyst nanocomposites. Fe₂TiO₅, with its narrow bandgap, is a crucial component due to its high photoexcitation efficiency under visible light. The incorporation of r-GO further enhances the photocatalyst's performance. The study employs the improved Hummers technique to produce graphene oxide (GO) and couples it with Fe₂TiO₅/Fe₂O₃ to create the composite. Finally, the photocatalytic activity of the synthesized nano photocatalyst is evaluated for the degradation of methylene blue (MB) under visible light, highlighting the need for materials that can harness a broader spectrum of solar energy for practical applications [43].

Ilmenite sand was subjected to leaching in concentrated HCl acid under refluxing conditions, yielding an acid leachate. This leachate was subjected to oxidation and subsequent treatment with FeSO₄.7H₂O and NH₄OH. Secondly, graphene oxide (GO) was prepared from natural graphite powder using the improved Hummer’s method, which included mixing with KMnO₄ and treatment with a combination of concentrated H₂SO₄ and H₃PO₄. The resulting solution was treated with 30% hydrogen peroxide and subsequent washing and drying to yield GO. Materials were treated with ascorbic acid to produce reduced graphene oxide. Finally, the photocatalysts were synthesized by dispersing GO powder in ultra-pure water and adding TiO₂-Fe₂O₃ (TF) in different weight ratios. After ultrasonication and hydrothermal treatment, the mixture was stirred with L-ascorbic acid solution to obtain materials designated as (0.1)r-GOTF, (0.2)r-GOTF, and (0.3)r-GOTF, depending on the percentage of GO relative to TF. The antibacterial activity of selected composite materials was tested against Escherichia coli. Band alignment of the metal oxide composites and elucidated visible-light-driven photodegradation mechanism of (0.3)r-GOTF are shown in Figure 3.

Figure 3 (a) Type II band alignment for Fe₂TiO₅ and α-Fe₂O₃ binary composite and (b) elucidated visible light-driven photodegradation mechanism of (0.3)r-GOTF. Figure Adapted from [43].

The composite incorporating 30% graphene oxide (GO) onto the Fe₂O₃/Fe₂TiO₅ photocatalyst, denoted as (0.3)r-GOTF, displayed enhanced efficiency in degrading methylene blue (MB) under visible light compared to the pure Fe₂O₃/Fe₂TiO₅ (TF). (0.3)r-GOTF exhibited a higher initial photodegradation rate of 0.033 min^-1 compared to 0.014 min^-1 for TF, indicating improved charge transport, separation, and adsorption and photodegradation capabilities. The composite's interaction between electrons and holes generated reactive oxygen species (O₂^•-, HO₂^•, and OH^•), facilitating MB degradation. Moreover, the (0.3)r-GOTF composite effectively inhibited Escherichia coli growth under visible light, although TF showed greater photo sterilization efficiency at 82.5% compared to (0.3)r-GOTF at 56.5%. The study indicates a cost-effective approach for creating GO-linked nanocomposites, emphasizing their potential in wastewater treatment and environmental remediation by removing MB dye and photo-sterilizing Escherichia coli [43].

5. Graphitic Carbon Nitride-Based Composites

Biomass can serve as a sustainable and renewable source for urea production, offering an environmentally friendly alternative to traditional fossil fuel-based feedstocks. A process reported by Zhang et al. involves biomass conversion into syngas using an entrained flow gasifier with pure oxygen from an air separation unit. The syngas is conditioned, and carbon dioxide is captured using a chemical absorption process for urea synthesis. A methanator has purified the syngas, removing CO and CO₂ traces. The resulting hydrogen and nitrogen mixture was compressed and used in loops for both ammonia and urea synthesis [85]. Another study was conducted in which urea was synthesized from biomass, where the procedure involved the removal of non-organic matter from the bio-mass feedstock stream, followed by cleaning, blending, and milling the cleaned bio-mass feedstock stream to produce a homogeneous blend with a consistent bulk density BTU content [86]. The availability of natural gas as feedstock for nitrogen fertilizer production continues to decline, while its price increases, encouraging the search for renewable feedstocks for the future green urea industry. Renewable feedstocks include waste biomass, hydrogen from solar PV-electrolysis, and the combination of these feedstocks with natural gas [87].

The pathway for green urea synthesis, aiming to minimize carbon footprint, requires a carbon-neutral source for CO₂. This ensures that the carbon balance remains intact throughout the urea production cycle. Instead of relying on fossil-fuel-derived CO₂, alternatives such as biomass combustion and gasification can be considered. By sourcing CO₂ via this method, the carbon utilized in urea synthesis originates from the natural carbon cycle, eliminating dependence on additional fossil fuels and ensuring sustainability. This approach maintains environmental integrity by minimizing emissions and aligning with green principles. Another work has reported that generated hydrogen from biomass gasification can directly be used in green ammonia production. At the same time, the CO₂ by-product would ideally be utilizable in a complementary urea production plant. This route marks the generation of H₂ from biomass as the most practical and environmentally --friendly option [88]. Urea, a precursor for graphitic carbon nitride, can be synthesized through environmentally friendly green synthesis, utilizing biomass as a sustainable source.

Graphitic carbon nitride is produced by precursors such as urea and melamine. Though they do not entirely originate from a biological material as such to be considered as biomass, graphitic carbon nitride produced from them combined with ilmenite has shown applications related to the environment. The organic n-type visible light active semiconductor photocatalyst [89] graphitic carbon nitride (g-C₃N₄) attracted a lot of interest in various applications due to its two-dimensional structure, sp² hybridization, composition devoid of metals, and excellent chemical and thermal stability. This material is interesting for several applications due to its unique properties, including its morphology, visible light absorption, tunable bandgap, surface chemical state, charge carrier separation efficiency, ease of synthesis process [90], and the main reason for the widespread applications as visible-light-driven photocatalyst.

Graphitic carbon nitride can be synthesized using various biomasses such as sucrose [91], cellulose [92], melamine [93], poly furfuryl alcohol [94], phenolic resins [95], and sawdust. Synthesis of g-C₃N₄ is very simple. The precursor, as mentioned above, is heated at tube furnace 550-700°C in an inert environment. g-C₃N₄ contains C and N conjugated layer structure which helps in electron flow through the material. It leads to the chemical and thermal stability of the composite. The band gap of g-C₃N₄ is about 2.7 eV which is active under visible and UV light. Nevertheless, the photoactivity performance of pure g-C₃N₄ is low due to its high charge carrier’s recombination, low absorption coefficient, and low specific surface area. This can be changed by doping g-C₃N₄ with metals or non-metals. Various types of metals such as K [96], Na [97], Rb [98], Au [99], Pd [100], Pt [101], Ag [102], Fe [103], Ti [104], Co [105], Cu [106], and Zn [107], and non-metals like B, O, P and S [108].

Natural ilmenite is composed of Ti and Fe oxides, and these metal ions act as catalysts for the graphitization process. Fe and Ti-doped g-C₃N₄ composites show enhanced electron flow and photocatalytic activity under energy sources like UV light [109], sunlight [110], and LED light. Therefore, Fe or Ti-doped g-C₃N₄ semiconductor materials are used in different industries such as wastewater treatment, water splitting, hydrogen production, removal of atmospheric CO₂, and conversion into an organic product. g-C₃N₄ composites show good adsorption capacity due to their high porosity. Hence, Fe-doped g-C₃N₄ composites are used for adsorption purposes. Ilmenite-derived metal oxide coupled g-C₃N₄ materials (g-C₃N₄/Fe₂TiO₅/Fe₂O₃) are used in photocatalysis and photosterilization under sunlight to remove methylene blue in wastewater. The study conducted by Thambiliyagodage et al. investigates a novel approach by integrating Fe₂TiO₅/Fe₂O₃ heterostructures with g-C₃N₄ to enhance visible range photocatalytic activity. The composite was synthesized by ilmenite sand was treated with concentrated HCl under refluxing conditions, followed by additional treatments with HCl and NH₃, which was then annealed at 800°C for 2 hours, resulting in the formation of Fe₂TiO₅/Fe₂O₃ (FF). Subsequently, g-C₃N₄ was synthesized by annealing urea at 550°C for 2 hours in an N₂ atmosphere. Ternary composites were created by altering the proportions of g-C₃N₄ relative to Fe₂TiO₅/Fe₂O₃, and vice versa, with the variable component set at 8%, 24%, and 40% concerning the fixed material. The composite containing 40% g-C₃N₄ with Fe₂TiO₅/Fe₂O₃, demonstrated the highest photocatalytic performance with a rate constant of 0.009 min^-1, more significant than Fe₂TiO₅/Fe₂O₃ nanoparticles. This higher photocatalytic efficacy across the fabricated composites resulted from increased visible light absorption, enhanced charge separation, and improved charge mobility. Furthermore, combining g-C₃N₄ with 40% Fe₂TiO₅/Fe₂O₃ displayed superior antibacterial effects (32.8%) against gram-negative Escherichia coli [30]. The z-scheme charge transfer that occurred in the composite synthesized is shown in Figure 4.

Figure 4 Possible band alignment of the synthesized nanocomposite indicating the z-scheme. Adapted from [30].

6. Sucrose-Based Graphitic Carbon Composites

Sucrose taken from sugar cane is biomass and converting sucrose to more ordered carbon through catalytic graphitization is known in scientific literature. Graphite can be acquired either through extraction from natural graphite or via catalytic graphitization of different carbon precursors, including biomass sources like sucrose, cellulose, and sawdust, and metals like Fe, Ni, Mn, and Ti are utilized as catalysts for the graphitization. Thambiliyagodage et al. focused on evaluating the feasibility of utilizing naturally occurring ilmenite to graphitize sucrose use as the carbon precursor. Fe₂TiO₅/TiO₂ is synthesized from ilmenite sand through digestion in HCl, precipitation with NH₃, drying, and annealing at 800°C. Such metal oxide composites mixed during the caramelization followed by annealing results in graphitic carbon composites with Fe, Fe₃C, TiO_2, and Fe₂O₃ nanoparticles embedded in the carbon framework. These composites are effective in adsorbing methylene blue with an adsorption capacity of 45.87 mg/g and degrading methylene blue under sunlight with a rate constant of 0.0048 min^-1. Synthesized nanocomposites could effectively degrade methylene blue as the conducting graphitic carbon network produced in the catalytic graphitization facilitates the electro-hole pair separation and migration from metal or metal oxide nanoparticles to the other charge-generating species and the pollutants. Further, the nanocomposites produced from ilmenite and sucrose effectively create inhibitory effects on Escherichia coli and Staphylococcus aureus as the model organisms [31].

7. Shrimp Shell-Based Graphitic Carbon-Based Composites

Shrimp shells are a naturally available biomass and a waste material abundant with N-based homopolysaccharide chitin. Mendis et al. reported the conversion of the inbuilt chitin in shrimp shells to chitosan, as shown in Figure 5. Ilmenite was subjected to acid digestion, followed by the addition of NH₃ to produce a mixture of metal oxide, which was then homogenized with the chitosan produced from shrimp shells in different proportions. Such obtained mixture was carbonized at 800°C in a N₂ atmosphere to produce N-rich graphitic carbon matrix in which TiO₂, Fe₂O_3, and Fe nanomaterials were immobilized. This composite is effective in degrading methylene blue under sunlight with a rate constant of 4.4 × 10^-3 min^-1 [44]. This further suggests that they are effective in environmental remediation once biomass is coupled with ilmenite-based metal oxides. A summary of the potential applications of composites derived from ilmenite coupled to different biomasses are tabulated in Table 1.

Figure 5 Production of chitosan from shrimp shells. Adapted from [44].

Table 1 A summary of the potential applications of composites derived from ilmenite coupled to different biomasses.

8. Activated Carbon

Activated carbon, or activated charcoal, is a biomass, processed to have a small, intricate network of low-volume pores of varying sizes, from obvious fractures and fissures to molecular dimensions that increase the surface area available for adsorption, and chemical reactions. It is a homogenous structure with a high degree of micro and mesoporosity and an extensive internal surface area [114]. The surface area varies greatly depending on the precursor and the condition of carbonization in synthesizing active carbon. An activation level sufficient for useful applications may be obtained solely from a high surface area, and the adsorption capacity depends on the porosity and surface chemistry. The unique structure of activated carbon gives it exceptional adsorption properties, enabling it to effectively remove a wide range of pollutants, contaminants, and impurities from gases, liquids, and solutions [115].

Depending on the source material, and the processing methods used to produce activated carbon, the physical and chemical properties of the end product differ significantly. This creates a matrix of possibilities for variation in commercially produced carbons, with hundreds of varieties available. Due to this, commercially produced activated carbons are highly specialized to achieve the best results for a given application. Despite such variation, there are three main types, powdered activated carbon (PAC), granular activated carbon (GAC), and extruded activated carbon (EAC) [116]. Particle size distribution of powdered activated carbons typically falls between 5 and 150 Å, with some commercially accessible sizes in between. Applications involving liquid-phase adsorption are the main usage for PACs. These have a versatile range of operations made from materials requiring less processing. Granular activated carbons have a particle size range of 0.2-5 mm and may be used in both liquid and gas phases. GACs are widely used because they are easy to handle and have a longer lifespan. They provide increased durability (hardness) and the potential for regeneration and reuse. Extruded activated carbon typically comes in cylindrical pellets with a diameter ranging from 1 to 5 mm. EACs are generally applied in heavy-duty activated carbon gas-phase processes through their extrusion method.

Besides these, bead-activated carbon (BAC), impregnated activated carbon, polymer-coated activated carbon, and woven activated carbon are also present [114]. Activated carbon can selectively adsorb and hold onto molecules according to their size, polarity, and chemical characteristics because of its adsorption processs based on Van der Waals forces, electrostatic interactions, and chemical bonding. Organic chemicals, volatile organic compounds (VOCs), heavy metals, chlorine, chloramines, pesticides, herbicides, and smells may all be effectively eliminated from air and water using activated carbon. Bituminous coal is one of the most common sources of activated carbon. It is widely used due to its availability and relatively low cost. Coconut shells, bamboo, hardwoods like oak and maple, peat, tea waste, paddy husk, coconut husk, sawdust, and palm kernel shells are starting materials to produce activated carbon [115].

Similar to graphite, activated carbon has a chemical structure mostly consisting of layers of sp²-hybridized carbon atoms. However, since it is porous, activated carbon, in contrast to graphite, it has an uneven and disorganized structure. High surface area and a complex network of pores, comprising macropores (greater than 50 nm), mesopores (between 2 and 50 nm), and micropores (less than 2 nm in diameter), are characteristics of the surface of activated carbon. These voids provide molecules with plenty of surface area to adhere to, enhancing activated carbon's high adsorption ability. Several variables, including the type of precursor material utilized, the activation process, the high temperatures used, and the particular treatment circumstances, can affect the chemical structure of activated carbon. Nevertheless, the significant component is still carbon, which has an uneven and porous structure that helps with its adsorption capabilities [114].

Activated carbon is synthesized through chemical or physical activation, which involves heating carbonaceous precursor materials at high temperatures in the presence of activating agents or gases. These processes create a network of pores within the carbon structure, providing a large surface area for molecules to adhere to. Physical activation involves the direct carbonization of the precursor material, followed by the activation of the resulting carbonized product through physical means. In this method, the precursor material is heated to high temperatures (700-1000°C) in the presence of steam or carbon dioxide gas. The gas reacts with carbonized material, etching away carbon atoms and creating pores in the structure. Physical activation typically yields activated carbon with a higher proportion of micropores and a larger surface area compared to chemical activation [45]. This method involves impregnating a carbonaceous precursor material with chemical activating agents such as zinc chloride, phosphoric acid, potassium carbonate, sodium hydroxide, potassium hydroxide, and other activating agents like Sulphuric acid, Phytic acid with the chemical formula C₆H₁₈O₂₄P₆, Di-ammonium hydrogen with a molecular formula of (NH₄)₂HPO₄ obtained from ammonium phosphate ((NH₄)₃PO₄) and ferric chloride. The impregnated precursor is then heated to high temperatures (400-900°C) in an inert atmosphere, causing carbonization and the development of a porous structure. The chemical activation process creates a network of pores within the carbon structure, resulting in activated carbon with a high surface area and adsorption capacity. Chemical treatment has been found to enhance the adsorption properties of activated carbon [46]. Microwave-assisted activation is a relatively newer method that utilizes microwave energy to heat and activate the carbonaceous precursor material. Microwave heating offers advantages such as rapid heating, uniform temperature distribution, and precise control over reaction conditions. This method can produce activated carbon with tailored properties by adjusting parameters such as microwave power, heating time, and precursor composition [117]. The chemical structure of activated carbon is shown in Figure 6.

Figure 6 Schematic representation of the structure of activated carbon with oxygen-containing functional groups located at the edges of broken graphite ring systems.

9. Potential Economic Applications

The remarkable characteristics and adaptability of ilmenite and biomass composite nanoparticles have multiple potential economic applications across several sectors.

9.1 Energy Storage Devices

Composite nanomaterials composed of ilmenite and biomass could be used to create sophisticated energy storage devices like supercapacitors and batteries [118]. Due to certain extraordinary qualities, including the large specific surface area, remarkable thermal conductivity, excellent mechanical strength, high electron mobility, electron conductivity, capacitance, energy density, charge-discharge, cyclic stability, and power conversion efficiency, graphene is an exceptional two-dimensional material with enormous potential for improving efficiency and energy-storage performance [119]. Nanocomposites incorporating graphene have higher mechanical robustness, thermal constancy, and microstructure properties [120]. Ilmenite nanoparticles may be useful in raising the energy density [121].

9.2 Water Purification

Water purification methods may benefit from the use of biomass and ilmenite nanoparticles in combination. Due to their superior adsorption capabilities, activated carbon and graphene oxide are frequently used to remove impurities and pollutants from water [122]. Additionally, ilmenite nanoparticles could help with catalytic reactions related to water treatment procedures. Adsorption is a reliable technique for cleaning water. Due to their superior physicochemical features, activated carbon, carbon nanotubes (CNTs), graphene oxide (GO), and reduced graphene oxide (rGO) are extensively investigated for the adsorptive removal of pollutants [123]. Increases in efficacy have been attained by structural alterations (vertically aligned CNTs) and by combining carbon nanomaterials with other nanoparticles (activated carbon-Fe₃O₄ nanoparticle composites). These innovative methods can potentially increase efficacy and efficiency [124]. The most common carbon material in its pure or mixed form is activated carbon. CNTs' hydrophilic surfaces make backwashing and rinsing possible. Utilizing carbon compounds in combination, such as GO-Ag, rGO-Ti, and MWCNT-ZrO₂, is a current attempt to increase the efficacy of water filtration. A further justification for the use of carbon nanoparticles as materials for water purification is their use as nanoelectrodes [125].

9.3 Catalysis

Ilmenite nanoparticles and nanomaterials generated from biomass have the potential to function as catalysts in a range of chemical processes. For instance, graphitic carbon nitride has demonstrated encouraging catalytic qualities. Mixing it with ilmenite nanoparticles might increase its catalytic activity for organic synthesis, hydrogen generation, and carbon dioxide conversion [126]. Utilizing natural ilmenite sand and waste product shrimp shells as raw materials, TiO₂-Fe₂O₃-Fe/nitrogen-doped graphitic carbon composites can be produced, and by adsorption, all of the composites successfully eliminated methylene blue according to [44]. It may be possible to assess how the manufactured photocatalysts affect the breakdown of various colors and organic contaminants like pesticides and medications. It is possible to create bimetallic nanocatalysts supported on graphitic carbon nitride. The ability to reduce the amount of noble metals used in these systems by changing the support materials or adding a different metal to the parent NPs is one of its benefits. Compared to currently available commercial catalysts, creating and improving bimetallic nanocatalysts may yield a new class of materials with improved, adjustable performance, thermal stability, and lower prices [127].

9.4 Environmental Remediation

The composite nanomaterials, such as soil and air, could be employed for environmental remediation purposes. Their adsorption capacity can support attempts to clean up the environment by assisting in removing poisons and pollutants from polluted areas [128]. Combining the current remediation techniques with the investigation of novel functional carbon nanomaterials (such as carbon nanotubes, graphene oxide, and graphene) from a variety of angles will provide a new space for understanding environmental phenomena and problems as well as methods to coexist with nature [127]. On the surface of both GO and CNTs, many adsorption processes are at work simultaneously. Depending on how surface functionalization and test circumstances alter, the mechanism is always intricate and occasionally unpredictable. Future research must focus on the catalysis and catalysis support roles of GO and CNTs. Thanks to the template effect of carbon nanotubes and graphene oxide nanosheets, efficient adsorption-catalysis integrative systems may be constructed [129].

9.5 Biomedical Applications

Carbon nanomaterials in biomedicine have garnered much interest for uses in tissue engineering, drug delivery, and biosensing [130]. Carbon nanomaterials have the intrinsic capacity to overcome the present limits in biomedical research, including poor biomolecule stability, limited sensitivity and selectivity of biosensors, and challenges with precise drug administration. These qualities include their distinctive structure and strong conductivity [131].

10. Environmental Assessment

Evaluating the environmental effect of ilmenite-biomass composite nanomaterials involves evaluating their lifespan, starting with extracting raw materials, and concluding with their disposal. Despite significant advancements in understanding the unique characteristics of testing and evaluating the fate and consequences of nanomaterials in the environment, there are still concerns about the accurate techniques for conducting dependable and resilient environmental assessments [132,133].

This assessment considers several elements, including resource depletion, energy consumption, emissions, human health and possible ecological concerns. In an Investigation of the environmental impacts of ilmenite and rutile processing routes in Australia, the most significant effect was climate change, with a value of 0.295 kg CO₂ eq. This impact is mostly attributed to the substantial electricity use during the ilmenite extraction process. Additionally, the significant presence of human toxicity-non cancer effects (4.42E-09 CTUh) and particulate matter (0.000155 kg PM_2.5 eq) is worth mentioning. The depletion of water resources is influenced by the chemical emissions from ilmenite ore, measured to be 0.00163 cubic meters of water equivalent [134].

10.1 Technical-Economic Evaluations

The economic feasibility of scaling up these technologies depends on factors such as raw material costs, energy consumption, labor expenses, yield and quality, scalability, market demand, regulatory compliance, and return on investment (ROI). Cost reductions, higher yields, and improved product quality would enhance the viability of large-scale production. Even though the method is used at the laboratory level, rigorous analysis and testing are needed to validate its economic feasibility in real-world scenarios.

10.2 Current Applications in the Industry

Although carbon-based nanocomposites show promising results in many fields such as environmental remediation, dye degradation, and filtration, they have not been adapted to industrial-scale applications. As the field advances with the rapid addition of new knowledge, biomass-derived carbonaceous metal oxide nanocomposites will be industrially used in the near future to combat pollution.

10.3 Scalability

In terms of scalability, several aspects must be taken into account. Scalability is contingent upon the productivity and scalability of the processes that produce biomass-derived nanomaterials and ilmenite nanoparticles. Creating scalable and reasonably priced synthesis pathways is essential for commercialization [131]. The availability of resources like biomass feedstock and ilmenite ore affects these technologies' capacity to scale. Large-scale production requires a reliable and long-lasting supply network [131]. Raising output requires adherence to legal requirements as well as environmental laws. Commercial profitability depends on maintaining environmental sustainability and safety throughout production [131]. Scalability also relies on the market demand for goods that use these technologies. Scalability efforts can be fueled by market research and the identification of promising applications with strong demand [131]. Although ilmenite and biomass composite nanomaterials have intriguing commercial uses, scaling will depend on efficiently overcoming technological, financial, and regulatory difficulties to fulfill market needs.

11. Opportunities

11.1 Enhanced Adsorption Properties

Due to its porous structure and wide surface area, ilmenite-based activated carbon has good adsorption capabilities. Its adsorption capability may be further increased by combining it with graphene oxide and other materials, making it appropriate for use in applications such as air and water filtration [135].

11.2 Improved Mechanical Strength

The structural integrity of the nanocomposite can be strengthened by graphene oxide, which is well-known for its remarkable mechanical qualities. This improvement can be beneficial for applications that need durable materials, including those in the building sector or the creation of lightweight and tough materials [136].

11.3 Tunable Electrical Conductivity

Modifying the composition and structure of graphene oxide may change its conductivity characteristics. Its incorporation into the nanocomposite may present chances for the creation of conductive materials with uses in energy storage, sensors, and electronics [137].

11.4 Photocatalytic Activity

Graphitic carbon nitride is a promising photocatalyst with prospective uses in solar energy conversion and environmental remediation. Its photocatalytic effectiveness may be increased by combining it with other materials like graphene oxide and activated carbon based on ilmenite. This will result in better performance in fields like wastewater treatment and solar energy harvesting [138].

12. Future Outlook

12.1 Activated Carbon - Ilmenite

Thus far, the synthesis of activated carbon-ilmenite composites remains unaccomplished, and the valuable properties of activated carbon have yet to be leveraged for the synthesis of composites with ilmenite, thereby hindering the exploration of their applications and potential uses.

12.2 Activated Carbon-graphitic Carbon Nitride (g-C₃N₄)

When compared to bulk g-C₃N₄, activated carbon/g-C₃N₄ composites demonstrate an extended visible light response and a greater separation rate of photogenerated electron-hole pairs. The morphological and opto-spectroscopic characterization of the synthesized composites, as well as photocatalysis experimental data, suggest that the significantly improved surface area and high photoinduced charge separation play a role in the high photocatalytic efficiency of activated carbon/g-C₃N₄ composites. These composites also demonstrate excellent recyclability and stability, making them potential candidates for use in environmental remediation [139].

According to a related study, composite photocatalysts of activated carbon and g-C₃N₄ demonstrate higher photocatalytic activity than g-C₃N₄. The incorporation of activated carbon broadened the absorption range of the catalyst towards visible light, improved the electron transfer efficiency, and suppressed the photogenerated electron-hole recombination. The composites continued to exhibit vigorous activity for carmine and rhodamine B degradation even after a few testing cycles [140].

12.3 Activated Carbon-Graphene Oxide

Research shows that combining activated carbon (AC) with graphene oxide (GO) yields an AC/GO composite that exhibits enhanced adsorption efficiency against pharmaceutical pollutants, including paracetamol and bisphenol A. The adsorption of both pollutants was a spontaneous, exothermic, and thermodynamically viable process by the thermodynamic studies for AC/GO. The combination of AC with GO moves towards obtaining new solid-phase materials with improved adsorbent abilities [122].

A second-generation method of desalinating water, known as capacitive deionization, uses porous electrodes made of activated carbon to temporarily store ions. Since porous carbon, which is used as electrodes in this technique, has intrinsic limitations like low capacitance and low electrical conductivity, optimizing electrode materials through careful engineering is essential to achieving hybrid electrodes, which would increase desalination performance. This study synthesized and evaluated several compositions of activated carbon and reduced graphene oxide (RGO) combination as brackish water desalination electrodes. This indicates that this simple approach may provide a viable means of creating the much-desired enhanced carbon network electrodes for capacitive deionization technology [141].

12.4 Graphene Oxide-Graphitic Carbon Nitride

Water treatment requires high selectivity and permeability nanofiltration membranes. Although the low permeability and poor selectivity limit its employment in nanofiltration, graphene oxide (GO) membrane has recently emerged as a potential nanofiltration material. To do this, scientists created laminated stack nanofiltration membranes and modified g-C₃N₄, which serves as a support material in GO membranes, using GO nanosheets with abundant functional groups. This study establishes a novel standard for applying stable and dependable nanofiltration membranes in aquatic settings by demonstrating the efficacy of GO-modified g-C₃N₄ as an additive for creating innovative GO membranes [142].

Given its potential use in organic degradation, piezocatalysis, which depends on the piezoelectric and piezopotential qualities of catalysts, is generating a lot of interest in water and wastewater treatment research. Reduced graphene oxide/graphitic carbon nitride-persulfate-ultrasound (rGO/CN-PDS-US), a new piezocatalytic oxidation system, was built to break down the antibiotics (sulfamethoxazole, SMX) in water. It was demonstrated that g-C₃N₄ had piezoelectric properties. This work produced an environmentally friendly and effective photocatalytic method for eliminating antibiotics from water, and the mechanism was anticipated to be advantageous for promoting electron transfer between g-C₃N₄, rGO, and PDS [143].

Despite coupling activated carbon, g-C₃N₄, and graphene oxide with each other, they have not yet integrated with ilmenite to form composites and test their suitability for practical applications. These unresolved research gaps still require attention.

13. Challenges

13.1 Synthesis Complexity

Considering the composition, morphology, and interfacial interactions between the components must be controlled, creating nanocomposites with many materials can be difficult. The synthesis parameters must be carefully optimized to achieve uniform dispersion and appropriate loading of each component [144].

13.2 Interfacial Compatibility

Optimizing the nanocomposite's synergistic effects and overall performance requires strong interactions and compatibility between its various components. It can be challenging to achieve enough interfacial bonding between materials with multiple characteristics and surface chemistries, and functionalizing or modifying the surface can be necessary [145].

13.3 Scale-Up Challenges

There are scalability, cost-effectiveness, and reproducibility issues when moving from laboratory-scale synthesis to industrial-scale manufacturing. Commercial feasibility depends on creating scalable synthesis techniques that minimize production costs while maintaining the required features of the nanocomposite [146].

13.4 Characterization and Quality Control

To correctly evaluate the nanocomposite's structural, morphological, and functional features, thorough characterization procedures are needed. Reliable and repeatable applications depend on the synthesized material's consistent quality and performance, which is ensured by strict quality control procedures [147].

14. Conclusions

TiO₂ isolated from natural ilmenite is a potential candidate that could be applied in many industries worldwide. Metal oxide nanocomposites, including Fe₂TiO₅/TiO₂, Fe₂O₃/Fe₂TiO₅/TiO₂, Fe₃O₄/TiO₂, Fe₂O₃/TiO₂ are evident to be effective in degrading methylene blue under sunlight and visible light. Ilmenite-derived metal oxides coupled to biomass-derived carbon-based materials produced nanocomposites such as r-GO/GO/α-Fe₂O₃/Fe₂TiO₅, g-C₃N₄/Fe₂TiO₅/Fe₂O₃, TiO₂/Fe₃C/Fe/Fe₃O₄-Graphitic Carbon, TiO₂/Fe₂O₃/Fe-N doped graphitic carbon have shown to be effective in photodegrading dyes in contaminated water. Also, they have been effective in inhibiting the growth of bacteria. However, ilmenite-based metal oxide coupled to activated carbon and two or three coupled carbon-based materials such as GO and g-C₃N₄, AC and g-C₃N_4, and AC and GO have not been investigated.

Author Contributions

Resources, C.T.; data curation, C.T.; M.J.; G.E.; A.M.; H.L.; S.W., writing—original draft preparation, C.T.; M.J.; G.E.; A.M.; H.L.; S.W., and writing—review and editing, C.T. and M.J.; supervision, C.T.; All authors have read and agreed to the published version of the manuscript.

Competing Interests

The authors have declared that no competing interests exist.