Kinetics and mechanism studies of p-nitroaniline adsorption on activated carbon fibers prepared from cotton stalk by NH4H2PO4 activation and subsequent gasification with steam.

Activated carbon fibers (ACFs) were prepared for the removal of p-nitroaniline (PNA) from cotton stalk by chemical activation with NH(4)H(2)PO(4) and subsequent physical activation with steam. Surface properties of the prepared ACFs were performed using nitrogen adsorption, FTIR spectroscopy and SEM. The influence of contact time, solution temperature and surface property on PNA adsorption onto the prepared ACFs was investigated by conducting a series of batch adsorption experiments. The kinetic rates at different temperatures were modeled by using the Lagergren-first-order, pseudo-second-order, Morris's intraparticle diffusion and Boyd's film-diffusion models, respectively. It was found that the maximum adsorption of PNA on the ACFs was more than 510 mg/L, and over 60% adsorption occurred in first 25 min. The effect of temperature on the adsorption was related to the contacting time and the micropore structure of the adsorbents. And the increase of micropore surface area favored the adsorption process. Kinetic rates fitted the pseudo-second-order model very well. The pore diffusion played an important role in the entire adsorption period, and intraparticle diffusion was the rate-limiting step in the beginning 20 min. The Freundlich model provided a better data fitting as compared with the Langmuir model. The surface micrograph of the ACF after adsorption showed a distinct roughness with oval patterns. The results revealed that the adsorption was in part with multimolecular layers of coverage.