Fast adsorption of low-concentration ammonia nitrogen by persulfate-modified carbon materials: structure influence, performance, and mechanism

Abstract

Carbon materials (CMs) have emerged as cost-effective adsorbents with significant potential for environmental remediation, yet their limited adsorption performance for ammonia nitrogen (NH₄⁺-N) restricts their application in water purification. Herein, persulfate (PS)-modified carbon nanotubes (MCNTs) were synthesized to enhance NH₄⁺-N adsorption from aqueous solutions. The MCNTs showed a well-developed mesoporous structure, higher graphitization degree, and more carbonyl (C=O) groups than pristine carbon nanotubes (CNTs). These structural modifications resulted in a remarkable adsorption performance, achieving both rapid adsorption equilibrium (≤3 min) and a 2- to 3-fold increase in adsorption capacity (compared to pristine CNTs). The adsorption rate constant of MCNTs was 2.58 mg/(g∗min) and the maximum adsorption capacity reached 15.49 mg/g. Mechanistic studies revealed that the enhanced mesoporous ratio of MCNTs significantly accelerated adsorption kinetics, whereas high graphitization degree and elevated C=O content contributed to their superior adsorption capacity and rate. Additionally, saturated MCNTs were effectively regenerated using a sodium hydroxide solution and maintained stable adsorption performance after 5-cycles. This modification strategy demonstrated excellent universal applicability across various CMs. To demonstrate practical applicability, the modified CMs (MCMs) were loaded on an ultrafiltration membrane and employed in a continuous flow adsorption system for treating aquaculture wastewater. The system reduced the NH₄⁺-N concentration in the effluent to below 0.3 mg/L within 80 min. This work not only provides a novel class of high-performance MCMs for NH₄⁺-N removal from wastewater but also elucidates the critical role of CMs' structure properties in governing adsorption performance.