The Performance and Mechanism of a Mg-Al Double-Layer Oxide in Chloride ion Removal from an Aqueous Solution

Abstract

The increasing threat of chloride ions (Cl−) has led researchers to explore efficient removal technologies. Sewage treatment with a double-layer hydroxide/oxide (LDH/LDO) is receiving increasing attention. In this work, Mg-Al LDO adsorbents were produced by the calcination of the Mg-Al LDH precursor, which was constituted by improved coprecipitation. The influence of calcination temperature, calcination time, adsorbent dosage, Cl− initial concentration, contact time, and adsorption temperature on Cl− elimination was investigated systematically. The experimental results showed that a better porous structure endowed the Mg-Al LDO with outstanding adsorption properties for Cl−. The adsorption process was well matched to the pseudo-second-order kinetics model and the Freundlich model. Under optimal conditions, more than 97% of the Cl− could be eliminated. Moreover, the removal efficiency was greater than 90% even after 11 adsorption–desorption cycles. 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.

Adsorption of nitrate from interflow by the Mg/Fe calcined layered double hydroxides

“…However, only 7.13% and 48.84% of removal efficiency and 6.58 mg/g and 8.65 mg/g of adsorption capacity were obtained with the Mg 2þ /Al 3þ ratios of 2:1 and 4:1 CLDHs. This difference can be attributed to the different interlayer spacing and electrical properties of CLDH caused by varying Mg 2þ /Al 3þ ratios (Xu et al 2022). At a low Mg 2þ /Al 3þ ratio, owing to the ionic radius difference between Mg 2þ (0.65 Å) and Al 3þ (0.50 Å), the excessive Al 3þ may lead to the disorder of CLDH layered structure, which will result in relatively poor adsorption performance (Jiang et al 2021).…”

“…For example, Jiang et al (2021) reported that with the rise in Mg 2þ /Al 3þ ratios from 2 to 5, the interlayer distance increased from 2.82 to 3.16 Å. However, with further increase of Mg 2þ /Al 3þ ratios, the unit charge density became too low, and the electrostatic attraction to anions will decrease consequently (Xu et al 2022). Ogata et al (2018) have reported that the 3:1 Mg/Al CLDH was more effective in removing nitrite and nitrate from water compared with 5:1 Mg/Al CLDH owing to the decrease in electric charge density with varying Mg 2þ /Al 3þ ratio from 3 to 5.…”

“…When adsorbent dosage increased from 1.5 g/L to 2.5 g/L, the removal efficiency of nitrate significantly increased from 38.59% to 62.08%. This can be attributed to the increase in the effective active sites available for adsorption with the increasing adsorbent dosage (Wang et al 2021;Xu et al 2022). However, an increase in dosage of the adsorbent from 2.5 g/L to 5.5 g/L only slightly improved the removal efficiency from 62.08% to 72.96%.…”

“…As the adsorbent dosage increased, the competition among nitrate molecules for empty adsorption sites enhanced (Mohammadi et al 2019). However, owing to the limited concentration of nitrate, some adsorption active sites cannot be entirely used, resulting in a relatively low unit adsorption capacity (Xu et al 2022). Taking into consideration the removal efficiency, adsorption capacity and cost-effectiveness, 2.5 g/L was considered as the optimal dosage for the following experiments.…”

“…The most commonly used CLDHs are Mg/Al CLDHs, which have been proven effective in the removal of various inorganic anions, including nitrate (Halajnia et al 2012), fluoride (Kang et al 2013), phosphate (Hong et al 2022), chloride (Xu et al 2022) and perchlorate (Yang et al 2012). For example, Halajnia et al (2012) synthesized Mg/Al CLDH and proved its excellent adsorption performance for nitrate with a rate constant of 0.623 g•mmol À1 •min À1 .…”