Adsorbents of 4-chlorophenol from long-flame coal activated by potassium hydroxide

Abstract

The purpose of the work was to evaluate the adsorption capacities of carbon materials (CMs) from long-flame coal with respect to 4-chlorophenol (CPh) and the influence of CMs preparation temperature under alkali activation at a low KOH/coal ratio (1 g/g). The CMs were obtained by heating coal with KOH to specific temperatures ranging from 350 to 8250C with a 1-hour holding time. Specific surfaces of all pores (SDFT), micropores (Smi), ultramicropores (Sumi), and supermicropores (Ssmi) were determined using nitrogen adsorption-desorption isotherms. The kinetics and isotherms of CPh adsorption as well as the maximum capacities (ACPh(m))were measured at 250C. Temperature was found to be a key factor in the formation of CMs nanoporosity and their ability to adsorb CPh. As the temperature increases, the SDFT, Smi, and Ssmi values increase monotonically, while Sumi exhibits an extreme change with a peak at 6000C. The adsorption kinetics followed the second-order model, reaching equilibrium within 2 hours. About half of CPh was absorbed in the first minute. The rate-determining step was identified as the interaction of CPh with the adsorption centers. The adsorption isotherms were best described by the Langmuir model. The ACPh(m) capacity was found to increase with temperature from 32.3 mg/g to 486 mg/g, a 15-fold increase, corresponding to an SDFT increase by 138 times. The ACPh(m) values exhibited an S-shaped dependence on SDFT, Smi, and Ssmi, indicating the non-uniform formation of adsorption centers with surface development. This was attributed to the molecular structure reorganization of CMs, which is also influenced by the activation temperature.