Direction of changes in porous structure and adsorption capacity during topochemical oxidation of coal activated by alkali

Abstract

The purpose of the work is to evaluate the influence of topochemical oxidation (H2O2, HNO3) of carbon prepared by alkali activation of coal on porosity and ability to adsorb 4-chlorophenol (CPh), Pb(II) cations and iodine. Carbons were oxidized at the reactant/carbon ratio of 1:1 (mol/mol, 250С, 24 h). Based on nitrogen adsorption-desorption isotherms, the volumes and specific surfaces of ultramicro- (Sumi), supermicro- (Ssmi) and other pores were evaluated. Kinetics and adsorption isotherms (250С) of CPh and Pb(II) were characterized; adsorption capacities of CPh, Pb(II) and I2 were determined. The H2O2-assisted modification was found to significantly increase Sumi (from 615 to 829 m2/g), but decrease Ssmi (from 515 to 494 m2/g). The HNO3-assisted modification slightly increases Sumi (from 615 to 651 m2/g), does not change Ssmi, but forms mesopores. The CPh adsorption is best approximated by the second-order kinetics, and isotherms are well fitted with the use of the Langmuir model. The H2O2 treatment increases the CPh capacity from 314 to 389 mg/g; and the НNO3 modification significantly decreases the CPh capacity (to 189 mg/g). Modifications reduce the iodine capacity by 1.11 times (H2O2) and 2.33 times (HNO3). The Pb(II) absorption was established to describe by the second-order kinetics equation; the adsorption isotherms obey Langmuir (R20.986) and Freundlich (R20.984) models. The Pb(II) capacity slightly increases after H2O2-assisted modification (from 87 to 95 mg/g), but increases sharply (from 87 to 298 mg/g) after HNO3-assisted treatment because of significant increasing OH-acidic groups concentration.