**1. Introduction**

Currently, with more rapid development of industrialization, more heavy metals in wastewater are derived from various industrial processes, such as mining, smelting, electroplating, tanning, and electrochemistry [1,2]. The heavy metals in wastewater would inevitably cause serious damage to the surface water, groundwater, and soil without proper treatment [2]. Among the reported heavy metals in wastewater, Cr(VI) is one of the most frequently detected in the natural environment, which was mainly derived from the chromium salt production and consumption process. The main existing oxidation states of Cr element in aqueous solution were Cr(III) and Cr(VI). The Cr(III) is a necessary trace element in our human body that is less harmful to the human body, compared with Cr(VI) [3,4]. Thus, more attention has been given to the treatment of Cr(VI)–polluted water/soil environment due to its potential threat to the ecological environment and human health [5]. Several methods have been proposed and investigated to remove Cr(VI) from

**Citation:** Wan, H.; Nan, L.; Geng, H.;Zhang, W.; Shi, H. Green Synthesis of A Novel MXene–CS Composite Applied in Treatment of Cr(VI) Contaminated Aqueous Solution. *Processes* **2021**, *9*, 524. https:// doi.org/10.3390/pr9030524

Academic Editor: Jose Enrique Torres Vaamonde

Received: 6 February 2021 Accepted: 6 March 2021 Published: 14 March 2021

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the water environment, including the adsorption method [6–9], reduction method [10], electrolysis method [11], ion exchange method [12], and membrane separation method [13]. Among the above–mentioned methods, the adsorption route has many inherent advantages for its lower energy consumption and relatively higher removal performance.

Chitosan (CS) is a kind of natural high molecular compound generating in the deacetylation of chitin. The CS is colorless and odorless, environmentally friendly with relatively higher adsorption properties [14] and stronger flocculation effect [15], that has already been widely applied in the chemical environmental protection [16,17], food hygiene [18,19], biomedicine [20], papermaking [21], and textile industries [22]. What is more, the hydroxyl group or amino group in the CS molecular would be beneficial to remove other pollutants for it can form into a hydrogen bond with a dye molecule (containing a -N=N- structure) or combine with the heavy metal ions under certain conditions [23]. However, the CS is not structurally stable under the acidic pH condition for it would degrade slowly in the acid solution, resulting in a decrease in the viscosity and molecular weight of the CS [24]. Therefore, the application of CS in the wastewater is obviously restricted for its unstable structure, while we should pay more attention to improve the stability of the CS and correspondingly increase its removal performance of pollutants from wastewater [25]. Several routes, including carboxymethylation modification [26], alkylation modification [27], and quaternization modification [27], etc., have been introduced to modify the structure of CS. A novel magnetic CS composite adsorbent was synthesized by the sol–gel method, proving its adsorption capacity of Cu2+ by the composite adsorbent could reach up to 216.6 mg/g [28]. The three–dimensional graphene oxide–CS composite was prepared and applied in adsorption of Uranium(VI) with an adsorption capacity of 384.6 mg/g at a pH of 8.3 [29].

The transition metal carbide or nitride (Mxene) is a kind of two–dimensional (2D) graphene–like nanomaterial newly developed in recent years [30]. Generally, the Mxene is mainly referred to as the transition metal carbides, nitrides, or borides, with the formula of *Mn+*1*XnTx*. In the formula, *M* is presenting the transition metal, *X* presents the non–metallic, including elements of carbon, nitrogen, and boron, *T* is presenting the functional groups (including -F, -O, and -OH) [31–33]. The novel Mxene phase is obtained by exfoliating the "A" component of the MAX phase in the HF solution, while the A in the MAX is presenting the elements for the Group IIIA or IVA, and X refers to the element of C or N [34]. The Mxene has been used as the adsorbent for the removal of heavy metal ions [35,36] or organic molecules [37] from the aqueous solution. The Mxene material is of relatively high structural stability, which should support and improve the structural stability of the CS [33]. It should be reasonable to expect the composite of Mxene and CS should be an ideal choice for the treatment of Cr(VI) contaminated water.

Herein, we synthesize and report a novel composite material of Mxene–CS. To the best of our knowledge, this is the first attempt to synthesize the Mxene–CS composite materials and used in the removal of Cr(VI) from the aqueous solution. We have studied the effects of pH, the Mxene–CS dosage, and the reaction time on the adsorption performance of Cr(VI) by synthesized Mxene–CS.

### **2. Experimental Section**
