Activation and adsorption performance of sewage sludge carbon for CO2: unusual enhancement effect of HF treatment

Abstract

To understand the dependence of activation processes of sludge carbons on CO₂ adsorption, five sludge carbons (SC₁–SC₅) were prepared by direct pyrolysis, pyrolysis–citric acid (CA) activation, pyrolysis–(CA + ZnCl₂) activation, pyrolysis–(CA + ZnCl₂ + KOH) activation and pyrolysis–(CA + ZnCl₂ + KOH)–HF activation, respectively. It was unexpectedly found that the features of SC₅ were very different from the others. Its specific surface area reached as high as 2654 mg g⁻¹, being 4.2, 12.0, 71.5 and 88.4 fold that of SC₄, SC₃, SC₂ and SC₁, correspondingly. Moreover, many ultramicropores and C–F groups were formed, in addition to the intended desilication. These microstructural changes lead to a great adsorption capacity of 215.2 mg g⁻¹ for CO₂, which was 2.5, 8.9, 26.9 and 40.6 fold that of SC₄, SC₃, SC₂ and SC₁, respectively. The significant increase of the surface area was attributed to the combined activation effect of HF chemical etching on SC and the expansion of d002 interlayers of SC due to the physical action of SiF₄ gas in HF activation. And, the enhancement of CO₂ adsorption was demonstrated to be due to the formation of extensive CO₂-philic expanded interlayer space-type micropores and C–F functional groups in SC₅ activated by HF.