Ultrahigh-capacity adsorption of Rhodamine B by N, S co-doped carbon nanotube composites derived from coal

The release of organic dyes into the environment poses a significant threat to ecosystems and human health, making it necessary to develop efficient and economical dye removal technologies. Among the various treatment methods, adsorption is highly promising due to its simple operation and low operating cost. However, traditional adsorbents often face limitations such as low adsorption capacity and poor selectivity. In this study, N,S co-doped carbon nanotube composites (SCTNs/AC) were successfully synthesized using coal as the precursor, potassium carbonate as the activator, and thiourea as the nitrogen and sulfur source. The SCTNs/AC demonstrated outstanding adsorption performance for rhodamine B (Rh B). Experimental results revealed that SCTNs/AC removed 99.34 % of a 300 mg/L Rh B solution within 20 min at 323 K. After four consecutive adsorption–desorption cycles using ethanol as the desorption solvent, the removal efficiency remained as high as 98.89 %. The theoretical maximum adsorption capacity of SCTNs/AC was 3242.04 mg/g, significantly surpassing that of other adsorbents. The adsorption process adhered to the Langmuir isotherm and pseudo-second-order kinetic models, indicating that monolayer chemisorption was the predominant adsorption mechanism. Mechanistic analysis highlighted that the high adsorption efficiency of SCTNs/AC towards Rh B was attributed to the synergistic effects of pore diffusion, electrostatic interactions, π-π stacking, and hydrogen bonding. Specifically, micropores with diameters between 1 and 2 nm played a pivotal role in pore diffusion, while the incorporation of pyridine nitrogen substantially enhanced electrostatic interactions. This study not only offers an innovative approach for the high-value utilization of bituminous coal, but also provides both theoretical insights and technical support for the efficient removal of organic dyes from industrial wastewater, highlighting its environmental and economic significance.