Naphthenic acids removal using N-doped hemp fibers based mesoporous carbon from aqueous waste stream

There is a substantial need for removing the contaminants from aqueous waste stream using affordable, stable, and active adsorbents, such as heteroatom-doped carbonaceous materials. Heteroatom such as N-doped carbonaceous materials greatly improve the performance of carbon materials by enhancing their conductivity, basicity, oxidation stability, catalytic activity and adsorption capacity. In this study, hemp fibers (HFs) and N-aminoguanidine were utilized as carbon and nitrogen precursors to synthesize N-functionalized mesoporous carbon materials (N-HFCs) via simultaneous activation and carbonization with ZnCl₂. Higher BET surface area with a distinctive mesoporous structure and the covalent bond between N and C was developed in the prepared carbon, making N-HFCs suitable for adsorbing naphthenic acids (2-naphthoic acid and benzoic acid) from aqueous waste streams. Developed covalent bond helps to prevent the leaching of carbonaceous materials during adsorption study. The results showed that N-HFC-2 (ZnCl₂: N-HFs ratio = 2:1) exhibited a higher removal efficiency of naphthenic acids (2-naphthoic acid and benzoic acid) compared to nonfunctionalized porous carbon (HFC). Adsorption of 2-naphthoic acid and benzoic acid on the adsorbents followed the typical monolayer type of Langmuir adsorption model. The maximum adsorption capacity of HFC after 48 h was evaluated as 70 and 27 mg/g for 2-naphthoic acid and benzoic acid. Likewise, the maximum adsorption capacity of N-HFC-2 for 2-naphthoic acid and benzoic acid was found to be 71 and 33 mg/g. In the adsorption kinetic experiment, adsorption of 2-naphthoic acid and benzoic acid reached equilibrium within 1 h using both the adsorbents (N-HFC-2 and HFC). Adsorption kinetics were analyzed using pseudo-first-order and pseudo-second-order kinetic models and were found to follow a pseudo-second-order kinetic model. Although changing pH from 4.4 to 8.5 did not have any significant effect on the removal efficiency of 2-napthoic acid using HFC and N-HFC-2, the removal efficiency of benzoic acid was decreased from 94 to 60 % using HFC and increased from 98 to 100 % using N-HFC-2. Comparative evaluations demonstrated that the mesoporous carbonaceous materials derived from HFs are an attractive adsorbent for removal of such contaminants from contaminated aqueous streams.