Preparation of porosity-adjustable porous adsorbent materials derived from coal solid waste

Coal solid waste (CSW) was explored as precursor to prepare porous adsorption material (ACSW) by carbon dioxide activation. The effects of activation method (direct activation and pre-carbonization followed by activation), activation temperature, and activation time were systematically investigated. The burn-off characteristics of the CSW were measured, the ACSW were characterized by field emission scanning electron microscope, gas adsorption instrument and Raman spectrometer, and, in parallel, the adsorption ability was studied using methylene blue (MB) as a typical pollutant. The results showed that compared with direct activation, pre-carbonization reduced the carbon loss, increased the pore volume with pore size less than 1 nm, improved the carbon structural order and increased the MB adsorption performance. In addition, the evolution of the structure of the ACSW strongly depended on the activation temperature and time. With the increase of the activation time, the pore structure evolution underwent three stages: pore forming, expanding, and collapsing. Pore forming mainly involved clearing scattered graphite structures and amorphous carbon. Pore expanding mainly relies on consumption of regular graphite carbon crystal structure. At low temperature of 800 oC, micropores fully developed. As the activation temperature increased to 850-900 oC, pore expansion became apparent, mainly forming pores of 1-5 nm. Moreover, the MB adsorption capacity was positively correlated with the pore volume with pore size less than 5 nm. The burn-off can serve as an indicator for evaluating pore structure and the adsorption performance and when it was ~25.5%, the ACSW had the largest pore volume with pore size less than 5 nm and the highest MB adsorption capacity.