Magnetic MXene-NH₂ decorated with persimmon tannin for highly efficient elimination of U(VI) and Cr(VI) from aquatic environment

Abstract

Herein, a magnetic MXenes based composite (Fe₃O₄@Ti₃C₂-NH₂-PT) was constructed by loading Fe₃O₄ nano-particles into the interlamellar spacing of persimmon tannin-functionalized Ti₃C₂-NH₂. The structure, morphology and physicochemical properties of the as-prepared adsorbents were probed by advanced spectroscopy techniques, while the impact of various experimental conditions like pH values, amount of adsorbent and contact time on the removal trend were examined by batch experiments. The elimination results revealed that Fe₃O₄@Ti₃C₂-NH₂-PT could be applied in a wide range of initial concentrations, and exhibited outstanding removal efficiency for U(VI) (104.9 mg/g, pH = 5.0) and Cr(VI) (83.8 mg/g, pH = 2.0). Meanwhile, the adsorption process was described well with the Langmuir isotherm and Pseudo-second-order kinetics models, which indicated that the monolayer chemical adsorption occurred during elimination of the two contaminants. The spectral analysis results manifested that elimination of U(VI) followed an inner-sphere configuration, whereas uptake of Cr(VI) was determined by electrostatic interaction and adsorption-reduction process. This work opened a new opportunity in designing MXenes based adsorbents in the application for environmental remediation.

Introduction

The rapid development of economy has created serious pollution of water body by various industrial pollutants, which brings a severe adverse impact on nature and living organisms [1]. Among these contaminants, uranium and chromium have attracted particular attentions. Uranium is an important raw material for nuclear power generation, which generally exists in contaminated water in the form of stable U(VI). Owing to its high stability, migration and radioactivity, hitherto the efficient treatment of uranium pollution in water still remains challenging [2]. On the other hand, Cr(VI) ions mainly exist as anions (HCrO₄⁻, CrO₄²⁻ and Cr₂O₇²⁻) in water environment, which are easily migrated, absorbed and accumulated in organism [3]. To safeguard water sources, it is urgent to develop highly efficient approach for elimination of uranium or chromium-contaminated from aquatic environment. Current remediation technologies for removal of these hazardous pollutants from wastewater include ultra-filtration [4], catalytic reduction [5], chemical precipitation [6] and adsorption [7]. Comparatively speaking, adsorption is considered as the most convenient and effective method in water pollution control due to its advantages in low energy consumption, ease of operation and cheap cost. So far, many kinds of different adsorbents, which including mesoporous carbons [8], hydrotalcite [9], SiO₂ [10], Fe₃S₄ [11], and so forth, have been studied to eliminate the two classic contaminants from water environments. However, the drawbacks of these traditional adsorbents can be obvious because of their high cost in regeneration, limited adsorption amounts and the hazard they cause to both the environment and practitioners. Hence, developing novel materials for water environmental purification is a task that must be pursued indefinitely.

MXenes are a family of two-dimensional (2D) transition-metal carbides and nitrides, its molecular structure is composed of transition metals (such as Ti, V, Nb, etc.) and carbon/nitrogen [12]. 2D MXenes have captured widespread attention in the environmental pollution remediation due to their high surface-volume ratio, unique inter-layer structure and abundant active functional groups. For instance, Kong et al. [13] developed an amino-functionalized MXenes for efficient removal of Cr(VI), and the experimental results found that amino groups and Ti₃C₂T_x nanosheets exhibited synergistic effects in the adsorption-reduction of Cr(VI). Zhang et al. [14] synthesized a carboxyl functionalized MXene nanosheets to elimination of uranyl ion from radioactive wastewater, and this adsorbent showed excellent removal ability for U(VI) with a high maximum adsorption amounts. Feng et al. [15] fabricated a MXene/PEI functionalized sodium alginate aerogel using a simple way, and this material exhibited an excellent ability to capture Cr(VI), and still maintained a high removal rate after five cycles. He et al. [16] investigated the adsorption efficiency of three-dimensional (3D) porous MXene gel towards uranium(VI), and this MXenes gel sorbent exhibited an excellent adsorption for uranium(VI) because of its 3D porous structure and abundant adsorption active sites. Nevertheless, while these MXenes based materials exhibited superior performance for targeted contaminants, they still existed some deficiencies during enrichment processes. Recent studies have pointed out that the narrow interfloor distance of MXenes is not beneficial to mass transfer and diffusion of target pollutants. Meanwhile, the insufficient of adsorption sites with special recognition function at the surface or interlayer of MXenes, which may result in poor selectivity to target pollutants. What's more, the separation and recycling of these MXenes materials from wastewater are still a challenging. Thus, it is imperative to exploit a fresh approach to construct a new type of MXenes materials for elimination of contaminants from water environment.

It is well known that natural organic matters are a primary electron source for metal ions adsorption-reduction in natural soils and waters [17]. Indeed, persimmon tannins are also an important category of natural organic matters, which is produced from the juice of young astringent persimmons after fermentation. More importantly, persimmon tannin is also a biological macromolecule, its molecular weight range is 7–20 kDa and the polymerization degree is 19–47 [18], [19], which have been applied as natural coagulants for adsorption and immobilization of inorganic or organic contaminants from wastewater. Meanwhile, the abundant adjacent hydroxyl groups with chelating ability, combined with an easy conversion into a solid materials make tannins good precursors for bio-adsorbents [20]. Moreover, magnetic nanoparticles have been intensively studied, not only for scientific interest, but also for technological applications. Particularly, Fe₃O₄ nano-particles have aroused considerable interest because they can help pollutant-loaded adsorbent to efficient separate from wastewater system by the extra magnet [21]. Based on the above considerations, it is of great significance to combine the advantages of MXenes, tannin and Fe₃O₄ to construct a new composite materials.

The aims of this investigation were as follows: (1) the influencing factors such as pH value, reaction time and concentration on removal efficiency were studied in detailed by batch experiments; (2) the classical kinetics, isotherms and thermodynamic formulas were taken into account to explore the interfacial reaction process between the adsorbates and adsorbents; (3) the relationship between the structural features and absorption properties of Fe₃O₄@Ti₃C₂-NH₂-PT was evaluated; (4) the reaction mechanism between the targeted pollutants and Fe₃O₄@Ti₃C₂-NH₂-PT was analyzed by the spectral analysis. Consequently, this magnetic persimmon tannin-functionalized MXenes was expected to be used in water purification and environmental remediation.