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processes Article A Facile Synthesis of Hexagonal Spinel λ-MnO2 Ion-Sieves for Highly Selective Li+ Adsorption Fan Yang, Sichong Chen, Chentao Shi, Feng Xue * ID , Xiaoxian Zhang, Shengui Ju * and Weihong Xing College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China; yangfan930416@njtech.edu.cn (F.Y.); chensc@njtech.edu.cn (S.C.); shichentao@njtech.edu.cn (C.S.); 2637561934@njtech.edu.cn (X.Z.); xingwh@njtech.edu.cn (W.X.) * Correspondence:xuefeng@njtech.edu.cn(F.X.);jushengui@njtech.edu.cn(S.J.);Tel.:+86-25-8358-7182(F.X.) Received: 28 April 2018; Accepted: 14 May 2018; Published: 17 May 2018 Abstract: Ion-sieves are a class of green adsorbent for extraction Li+ from salt lakes. Here, we propose a facile synthesis of hexagonal spinel LiMn2O4 (LMO) precursor under mild condition which was first prepared via a modified one-pot reduction hydrothermal method using KMnO4 and ethanol. Subsequently, the stable spinel structured λ-MnO2 (HMO) were prepared by acidification of LMO. The as-prepared HMO shows a unique hexagonal shape and can be used for rapid adsorption-desorption process for Li+ adsorption. It was found that Li+ adsorption capacity of HMO was 24.7 mg·g−1 in Li+ solution and the HMO also has a stable structure with manganese dissolution loss ratio of 3.9% during desorption process. Moreover, the lithium selectivity (αLi ) Mg reaches to 1.35 × 103 in brine and the distribution coefficients (Kd) of Li+ is much greater than that of Mg2+. The results implied that HMO can be used in extract lithium from brine or seawater containing high ratio of magnesium and lithium. Keywords: LiMn2O4; λ-MnO2; ion-sieve; hydrothermal reaction; adsorption 1. Introduction Lithium and its compounds—known as “industrial monosodium glutamate” [1]—are widely used in significant fields such as batteries, ceramics, glass, alloy, lubricants, refrigerants and the nuclear industry [2,3]. The lithium reserves in China are the world’s second-largest, which are primarily distributed in the salt lakes of Qinghai and Tibet [4]. However, the ratio of magnesium to lithium in the salty brine is extremely high, making it difficult to extract and recover lithium using conventional separation technologies [5,6]. Compared with precipitation and solvent extraction methods, ion-sieve adsorption has many technical merits, such as excellent selectivity and relatively low cost [7,8], which is considered to be the most promising environmentally benign technology for extracting lithium from salt lakes [9,10]. Manganese series spinel ion-sieves are widely used in lithium ion adsorption, which primarily includes λ-MnO2, MnO2·0.3H2O and MnO2·0.5H2O, after removal of lithium by acidification from precursors LiMn2O4 [11], Li4Mn5O12 [12,13] and Li1.6Mn1.6O4 [14–16], respectively. LiMn2O4 (LMO) is commonly used adsorbent precursor8, which is fabricated through embedding the target Li+ in the Mn-O chemical skeleton to construct composite LixMnyOz. After extracting Li+ by acidification without damages in the structure, of λ-MnO2 (HMO) with regular vacancy [17]. The cubic spinel structures and adsorption-desorption relationship of HMO and LMO is shown in Figure 1. Oxygen atoms(O), Mn3+/Mn4+ and lithium atoms (Li) occupy 32 e, 16 d and 8 a of the Wyckoff site, respectively [18]. Then, lithium at the 8a position is extracted by hydrogen because of ion exchange process which can adsorption Li+ subsequently. Processes 2018, 6, 59; doi:10.3390/pr6050059 www.mdpi.com/journal/processesPDF Image | Sieves for Highly Selective Li Adsorption
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