The Mg-Al LDO adsorbent powders were produced by the calcination of the precursors in a muffle furnace with an accompanying heating rate of 10 ☌ min −1. The elemental ratios were confirmed by EDS analysis (as shown in Figure S3 and Table S1). White Mg-Al LDH precursors were created, and the precursor particles below 74 μm in size were selected by crushing and screening. The thick slurry was washed using deionized water until the filtrate was close to neutral, and then they were dried for 10 h in the oven at 70 ☌. The solution obtained was kept aging for 18 h at room temperature, and the supernatant was divided by filtering. The salt solution (A) and an alkaline solution (B) (NaOH) = 3 M (250 mL) were rapidly mixed, and a white slurry was formed synchronously. Based on the molar ratio, different masses of Mg (NO 3) 2♶H 2O and Al (NO 3) 3♹H 2O were mixed together to prepare 250 mL of a mixed metal nitrate solution (A). In addition, considering the standardization and rationality of the experimental design, the M 2+:M 3+ molar ratios of 2:1, 3:1, 4:1, 5:1, and 6:1 were chosen to fabricate the precursor. When the molar ratio of M 3+/ (M 2+ + M 3+) was between 0.2 and 0.4, a structured LDH could be acquired. LDO is becoming a popularly used adsorbent in wastewater treatment. The exchange between negative anions in the aqueous solution and the remaining anions in the interlayer can further improve the removal efficiency of the target ions. When LDHs are calcined, most of the interlayer anions are eliminated and then can be incorporated during rehydration to reconstruct the LDOs into their original layered structure, which is called “the structure memory effect.” It is possible to recover the layered structure of mixed oxides when they come in contact with water. In addition, LDHs that are calcined can undergo structural transformation and convert into metal oxides (LDOs) with thermal and chemical stability. Anions such as NO 3 − can be exchanged with CO 3 2−, SO 4 2−, and Cl −. Their laminate elements are characteristically available in variable types and various proportions changeable interlayer anions and controllable sizes also bring more possibility in terms of application of the materials. Their general formula can be given by (M 2+ 1−xM 3+ x(OH) 2) (A n−) x/n They are widely applied in adsorption, catalysis, energy storage, biology, medicine release, heavy metal ions, and functional materials. LDHs, also known as hydrotalcites, are a class of two-dimensional anionic intercalation materials with a bimetallic hydroxide laminate and exchangeable intercalation anions. Their theoretical specific surface areas are large, the hydrophilicity is strong, the interlayer charge density and reactivity is high, the economic consumption is low, and efficiency is satisfactory. Hydrotalcite-like compounds, inorganic functional materials with layered structures, are considered to be highly promising, highly efficient adsorbents due to several factors. By contrast, adsorption is gaining attention due to the fact that it does not harm the environment, has a straightforward process, is low in cost, and is highly efficient. However, it is worth noting that the methods described above are hard to practically apply to industrial wastewater due to prohibitive operating costs, inefficiency, limitations of treatment concentration, and a lack of suitable oxidants. Great effort has been made toward alleviating the dire situation of Cl − concentration in wastewater, including precipitation, evaporation and concentration, oxidation–reduction, and ion exchange. It was found that the electrostatic interaction between Cl − and the positively charged Mg-Al LDO laminate, coupled with the reconstruction of the layer structure, was what dominated the Cl − removal process. Moreover, the removal efficiency was greater than 90% even after 11 adsorption–desorption cycles. Under optimal conditions, more than 97% of the Cl − could be eliminated. The adsorption process was well matched to the pseudo-second-order kinetics model and the Freundlich model. The experimental results showed that a better porous structure endowed the Mg-Al LDO with outstanding adsorption properties for Cl −. The influence of calcination temperature, calcination time, adsorbent dosage, Cl − initial concentration, contact time, and adsorption temperature on Cl − elimination was investigated systematically. In this work, Mg-Al LDO adsorbents were produced by the calcination of the Mg-Al LDH precursor, which was constituted by improved coprecipitation. Sewage treatment with a double-layer hydroxide/oxide (LDH/LDO) is receiving increasing attention. The increasing threat of chloride ions (Cl −) has led researchers to explore efficient removal technologies.
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