Facile synthesis of heterojunction of MXenes/TiO₂ nanoparticles towards enhanced hexavalent chromium removal

Abstract

Increasing the surface area and active sites of adsorbent for hexavalent chromium removal in water is considered an effective and efficient way to enhance reduction and adsorption performance. Here 2D/0D heterojunction of Ti₃C₂ MXenes/TiO₂ nanoparticles with different amounts of TiO₂ were synthesized using a one-step hydrothermal method and the reduction and adsorption of Cr (VI) at different pH values of composite material were analyzed. The results show that the Ti₃C₂/TiO₂ composite material allows significantly enhanced removal ability of toxic Cr (VI) in water, compared to pure Ti₃C₂ nanosheets. The effective removal of Cr (VI) in water was 99.35%. The Ti₃C₂/TiO₂ composite can not only rapidly and efficiently remove Cr (VI) from potassium dichromate solution by reducing Cr (VI) to Cr (III), but simultaneously adsorbs the reduced Cr (III). Therefore, the 2D/0D heterojunction of Ti₃C₂ MXenes/TiO₂ nanoparticles has potential application prospects in the reduction adsorption of Cr (VI).

Introduction

Chromium is a common heavy metal contaminant, and is usually present as trivalent chromium (Cr (III)), hexavalent chromium (Cr (VI)), and zero-valent chromium (Cr (0)) [1]. Hexavalent chromium ions are highly toxic [2]. Chromium compounds have potent oxidation potential and are harmful to the digestive tract, respiratory tract, skin, and mucous membranes of the human body. Chromium also has a carcinogenic effect, especially against the lung [3], [4]. Metallic chromium is invoked as Cr (0) for the manufacturing of steel. Cr (VI) and Cr (III) are commonly used in industrial processes, including metal processing, chrome plating, leather tanning, wood preservation and metal surface treatment [2]. Industries wastewater contains large amounts of residual Cr (VI), and discharged wastewater and waste gas are the main sources of environmental pollution. Leakage, improper storage or improper handling can release emormous amounts of chromium into the environment, where it can pollute the groundwater and cause great harm to animals and plants [5], [6]. Various materials and methods including physicochemical [7], electrocoagulation [8], permeable reactivity walls [9], [10], adsorption [11], [12], [13], [14], photocatalytic [15], [16], polymer [17], [18], [19], biosorption [20], bioreduction [21] have been developed to perform effective remediation of Cr (VI) and other heavy metal ions (Cu²⁺, Hg²⁺, Pb²⁺) in aquatic environment.

As Cr (III) is far less harmful than Cr (VI), reduction Cr (VI) into lower toxicity Cr (III) is the major approach for industrial emission control of Cr (VI) [22], [23], [24], [25]. The reduction precipitation method is widely used as an industrial treatment method for chromium-containing wastewater treatment, and the removal efficiency is over 90%. But this method also produces a large amount of secondary waste, and requires a large amount of acid and alkali to adjust the pH of the solution [26], [27], [28]. Since Cr (VI)/Cr (III) has a high standard reduction potential at low pH (e.g., E: Cr₂O₇²⁻/Cr³⁺ = 1.36 V) [29], Cr(VI) can be easily reduced by a reducing agent with a removal efficiency of over 90%. [30], [31]. Jun Zhang et al. [32] demonstrated the hydrothermal preparation of magnetic core-shell microspheres of Fe₃O₄@Zn_xCd_1−xS, and the efficiency of photochemical reduction of Cr(VI) under light irradiation was 90%. Although prepared material can rapidly separate the catalyst and the reaction solution, the required preparation method is complicated, and some synthetic materials are toxic, limiting use of this method for large-scale preparation. Additionally, the microspheres require activation by visible light, thus limiting the application efficiency. Dong-hyo Kim et al. [33] used a redox pair of Fe³⁺/Fe²⁺ to simultaneously and remove Cr (VI). In this system, Fe²⁺ fist converts Cr (VI) into Cr (III) and then forms Fe³⁺ with a removal efficiency of 78% to 100%. However, the system completely removed Cr (VI) only with phenolic substrate, which can cause secondary pollution.

With adjustable layer spacing and custom surface chemistry, superior structural stability, excellent electrical conductivity and environmental friendliness, a new two-dimensional material series called MXenes has drawn attention [34], [35], [36], [37]. MXenes were found in Hydrofluoric acid solution by etching the “A” layered component of the MAX matrix phase [37]. The MAX phase is a specific family of compounds, namely M_n+1AX_n (n = 1, 2, 3), where M represents an early transition metal, A is primarily a Group IIIA or IVA element, and X is C or N [36], [38]. After the removal of Al, the MXenes surface has a large number of surface-active groups (for example, F or OH). These groups not only provide direct ion exchange sites, but also can provide electrons as effective reducing agents to selectively remove toxic metal ions [39], [40], such as Mn (VII), Cr (VI), and Fe (III) [41]. Recently, Peng and coworkers found that titanium carbide with Ti surfaces covered with hydroxyl and intercalate titanium carbide have unique adsorption behavior for toxic Pb(II).[39] In our previous research, we reported the excellent performance of Ti₃C₂/TiO₂ nanoflowers for photocatalytic hydrogen production and oxygen production [42].

Recently, nanostructured materials have attracted widespread interest due to their high surface area and abundant surface-active sites [43]. Nano TiO₂ is low cost and toxicity have high thermal and chemical stability, has been widely utilized in photocatalysis, solar cells, lithium batteries, and for adsorption applications [44], [45], [46], [47], [48], [49], [50].One strategy to increase the ability to use this material is to create a 2D material, such as by forming a composite with MXenes [51], [52], [53], [54].The size of the particles affecting TiO₂ is a major factor affecting the material’s catalytic activity. However, most TiO₂ nanoparticles are relatively large (typically a few hundred nanometers). The larger size results in small specific surface area of the composite and relatively few surface active site, causing a weak adsorption capacity and a slow adsorption rate, thus greatly limiting application. Reducing the size of the TiO₂ nanoparticles not only effectively increases the BET surface area of TiO₂, but also increases the number of active sites in solution [55]. The MXenes phase has a larger specific surface area and an open layered structure, promoting adsorption in sewage treatment. Therefore, small sized Ti₃C₂/TiO₂ composites with distribution of TiO₂ nanoparticles on Ti₃C₂ nanosheets should have broad application prospects for adsorption. Therefore, the simple structure of growing of TiO₂ nanoparticles on Ti₃C₂ nanosheets by a simple hydrothermal synthesis method has broad adsorption prospects in the reduction adsorption of Cr (VI).

In this study, Ti₃C₂ was used as raw materials for metallic titanium (Ti) to prepare two-dimensional (2D) Ti₃C₂/TiO₂ composite material with TiO₂ nanoparticles (0D) using facile one-step hydrothermal oxidation method. These 2D Ti₃C₂ provided the reaction zone as a carrier, to significantly increase the specific surface area and active sites of TiO₂ heterostructures, allowing effective adsorption and reduction of Cr (VI) to Cr (III) under acidic conditions (with adsorption of Cr (III)). The adsorption and reduction of Cr (VI) on Ti₃C₂/TiO₂ composites under different pH conditions and the reaction mechanism of the system are examined.