Alkali activated coals. Microporous structure and capability to adsorb phenol compounds

Abstract

The aim of the work is to compare the microporous structure characteristics of activated carbons (ACs) prepared from coals of different coals rank (CR) by alkaline activation (RKOH = 1 g/g, 800 °C) and to determine the ACs capability to adsorb phenol and 4-chlorophenol from aqueous solutions. Starting materials are coals with increasing carbon content (Cdaf = 80.0–95.6 %) selected as a CR criterion. ACs were obtained in argon in three stages: 1) thermoprogrammed heating (4 grad/min) to 800 °С; 2) isothermal exposure 1 h; 3) cooling, washing from alkali and drying. Based on low-temperature (77 K) nitrogen adsorption-desorption isotherms, integral and differential dependences of the specific surface area S (m2/g) and pore volume V (cm3/g) on the average pore diameter (D, nm) were calculated. They were used to define volumes of ultramicropores (Vumi), supermicropores (Vsmi) and micropores (Vmi). The total pore volume Vt was calculated from the nitrogen amount adsorbed at a relative pressure p/p0 ~ 1.0. The S values of ultramicropores (Sumi), supermicropores (Ssmi) and micropores (Smi) were similarly determined. The volumes and specific surfaces of different categories of pores were found to decrease with CR increase: volume Vt – from 0.59 to 0.23 cm3/g; Vmi – from 0.51 to 0.17 cm3/g; the ultramicropores volume – from 0.31 cm3/g to zero in anthracite AC. The supermicropores volume is almost independent on CR and varies in the wide range Vsmi = 0.15–0.22 cm3/h. The specific surface area is the maximum (S = 1547 m2/g) in AC from the coal of the lowest CR and decreases with coal metamorphism up to 322 m2/g. The micropores surfaces make dominant contributions to the S values: its portion is 94.7–99.4 %. For all ACs, the adsorption of phenol (Ph) and 4-chlorophenol (CPh) from aqueous solutions at 25 °C was studied. Adsorption kinetics and isotherms are best described by the pseudo-second order model and the Langmuir model (R2 ≥ 0.998). With increasing CR, the maximum adsorption capacities decrease from 3.113 to 1.498 mmol/g (Ph) and from 3.9 to 2.1 mmol/g (CPh), that is approximately ~2 times when the specific surface area decreases by ~5 times. The Ph and CPh specific capacities, characterizing the adsorption capacity of 1 m2 of surface, change little at Cdaf≤86 %, but markedly increase (2.3–2.5 times) for anthracite AСs. The Ph and CPh capacitances were determined to increase linearly (R2 ≥ 0.966) with increasing ACs specific surface area. Similar dependences were found on the Sumi and Smi parameters. The phenols were concluded to be equally adsorbed on the surface of pores of any size. A general trend was found for ACs from hard coals and anthracite: an increase in CR reduces the ACs microporosity and surface, decreases Ph and CPh capacities but increases specific capacities, i.e. concentrations of surface adsorption centers. The Ph and CPh adsorption was accepted to include the interaction of π-electrons of phenolic rings and π-electrons of graphene layers in ACs, the formation of complexes with surface groups and forming hydrogen bonds with OH-groups. Their contributions depend on adsorbate nature and change with the growth of fossil coals CR.