Upgrading of sewage sludge by demineralization and physical activation with CO2: Application for methylene blue and phenol removal

Abstract

Carbon-based adsorbents were prepared by the physical activation of sewage sludge with CO2 through an integrated method (without a precarbonization stage). The activation was conducted at different temperatures (600e1000 ºC), and the effect of the acid washing, performed either before or after the physical activation, was investigated. From the standpoint of textural properties, the optimum activation temperature is between 800 and 900 ºC; higher temperatures cause an important reduction in porosity. The acid post-washing significantly increases the porosity in the whole pore size range due to the removal of the inorganic fraction and pore unblocking. Both the activation temperature and the acid treatment strongly affect the surface chemistry. The uptake of methylene blue correlates significantly with textural properties (such as BET surface area), but also with surface chemistry (carbonyl groups). The combination of a moderate activation temperature (600 ºC) and acid post-washing results in a cost-effective material with a suitable methylene blue removal ability. On the contrary, the adsorption of phenol, favoured by materials with a high degree of microporosity, is maximized by acid pretreatment prior to activation at 900 ºC.