Recovering metal ions from oxalic acid leaching palygorskite-rich clay wastewater to fabricate layered mixed metal oxide/carbon composites for high-efficient removing Congo red.

This study developed a sustainable way to transform metallic residues in wastewater and spent adsorbents that adsorbed organic pollutants into novel high-efficiency adsorbents to treat water pollution again. The metal ions recovered from oxalic acid leaching palygorskite-rich clay wastewater was used to construct the hydrotalcite-like composites, after adsorbing organic pollutants, which was calcined and carbonized to convert into the mixed metal oxide/carbon composites (MMO/Cs). The fabricated MMO/Cs showed outstanding adsorption performance for the anionic azo dye Congo Red (CR). Especially, the MMO/C2 with the M/M molar ratio of 2, which adjusted by supplementing Mg, had ultra-high adsorption capacity and ultra-clean removal efficiency for CR. The adsorption capacity was as high as 3303 mg/g, and only 0.5 g/L MMO/C2 dosing treatment for 6 h could completely decolor and remove the 2000 mg/L CR aqueous solution. Moreover, MMO/Cs exhibited the ability to simultaneous remove CR and Methylene blue (MB) mixed dye contaminants, and demonstrated the excellent recyclability. This work provides a promising method for the high-value conversion of waste resources and the synthesis of high-efficiency adsorbents.