Adsorption mechanism of methylene blue by magnesium salt-modified lignite-based adsorbents

The rich pore structure and carbon structure of lignite make it a suitable adsorbent for effectively removing methylene blue (MB) from wastewater. This article reports the preparation of lignite-based adsorbents modified by magnesium salts, and the key factors and adsorption mechanism are analyzed to effectively improve the adsorption performance for MB. The results showed that the lignite was modified by magnesium salts, and the Mg²⁺ in the magnesium salts had a good binding effect on the oxygen-containing functional groups in the lignite. This improved the adsorption performance of the lignite-based adsorbents for MB. The Mg(NO₃)₂-modified lignite-based adsorbent showed the best adsorption performance and removal rate of MB (99.33%) when prepared with 8 wt % Mg(NO₃)₂. Characterization analysis showed that a "-COOMg" structure was formed between Mg²⁺ in the magnesium salts and the carboxylic acid functional group in the lignite, which was postulated to be the absorption site that promoted the adsorption performance for MB. It is speculated that the MB adsorption mechanism of this lignite-based adsorbent is ion exchange.