Circular conversion of waste rectorite@dye to efficient and pH-resistant heterogeneous silicate adsorbents for cyclic and complete dye removal

Abstract

A series of new heterogeneous silicate adsorbents were synthesized from waste dye-adsorbed rectorite (named as rectorite@dye) through conventional calcination, hydrothermal treatment and an upgraded oxidation-promoted conversion process. Compared with conventional calcination and direct hydrothermal treatment processes, the positive effect of oxidants such as hydrogen peroxide and calcium peroxide on improving the conversion efficiency of rectorite@dye waste and adsorption properties was studied. It was revealed that hydrogen peroxide promoted the conversion of waste rectorite@dye into a heterogeneous rectorite@carbon composite with a carbon content of 1.38% and nitrogen content of 0.44%. The optimal rectorite@carbon adsorbent can efficiently adsorb Methylene blue at 176.77 mg/g and Basic red 14 at 305.87 mg/g, which shows superior performance than rectorite@dye (44.26 mg/g for Methylene blue; and 25.26 mg/g for Basic red 14) and composite adsorbents prepared by conventional calcination (141.52 mg/g for Methylene blue; 213.03 mg/g for Basic red 14) or direct hydrothermal treatment (74.47 mg/g for Methylene blue and 88.34 mg/g for Basic red 14). The adsorption capacity of the adsorbent is pH-independent, and it can be used over a wide pH range. Moreover, the composite adsorbents can remove 99.99% of Methylene blue and 99.90% of Basic red 14 from an initial 100 mg/L dye solution, and thus clean water was obtained after treating dye-polluted water with the rectorite@carbon adsorbents. After each adsorption cycle, the waste adsorbent can be regenerated by a one-step calcination or hydrothermal process and reused, and the adsorption capacity of the regenerated adsorbent can still reach 198.43 mg/g (for calcination regeneration) and 268.35 mg/g (for hydrothermal regeneration) after 6 regenerations.