Preparation of carbon-based adsorbents from the pyrolysis of sewage sludge with CO2. Investigation of the acid washing procedure

Abstract

Carbon-based adsorbents were prepared through the physical activation of sewage sludge with CO2. The effect of the acid treatment was investigated, either before or after the physical activation. The post-washing increases the porosity due to the removal of the inorganic fraction and the improvement in the accessibility of the carbon fraction. The post-washing is essential for high activation temperatures since the high reactivity of CO2 leads to a high pore blockage. In fact, the material activated at 900°C has a very low surface area (43 m2/g), while after washing it reaches 654 m2/g. The effect of the acid treatment prior to the physical activation depends on temperature: at 600°C, it results in a huge pore development (mainly macropores), while at 900°C, the microporosity increases. The adsorption capacity is not directly related to the BET surface area. For both adsorbates, the uptake ability depends on the textural properties (it is favoured by mesoporosity for methylene blue (MB) and microporosity for phenol) and surface chemistry (favourable effect of oxygen- and nitrogen-containing functional groups for MB and unfavourable effect for phenol).