Congo red removal by HNO3-Modified resorcinol-formaldehyde carbon gels

Abstract

Resorcinol-formaldehyde (RF) carbon gel has gained much attention in various commercial applications including electro-catalysis, ion exchange resin and hydrogen and electrical energy storage owing to its highly sensitive nanostructure. In this work, the resorcinol-formaldehyde carbon gels were synthesised via a sol-gel method from polymerisation of resorcinol and formaldehyde with sodium carbonate (Na2CO3) as a catalyst. Different resorcinol to catalyst (R/C) ratios (mol/mol) of 1,000 (RC3) and 2,000 (RC4) were used, and the effect of nitric acid (HNO3) modification was investigated. The oxidised RC3 and RC4 were denoted as RC7 and RC8,. It was found that a further increase in R/C ratio above 1,000 caused a decline in surface area, pore volume and pore size. RC7 gave the highest surface area of 711.49 m2/g with pore size of 7.68 nm. The HNO3 modification step increased the surface area of RC7, but slightly decreased the surface area of RC8 probably due to crystallisation of the gel to graphite. Adsorptive study was carried out using congo red (CR) as an adsorbate. The carbon gels with higher surface area, pore volume and pore size exhibit a better CR adsorption performance. The HNO3-modified resorcinol-formaldehyde carbon gels gave a better performance for the removal of congo red. The kinetics data of congo red adsorption could be explained by the pseudosecond- order kinetic model. The maximum adsorption of congo red on resorcinol-formaldehyde carbon gel is comparable to other adsorbents and activated carbons.