Structural and functional relationships of activated char briquettes from pyrolysis of sewage sludge for methylene blue removal

Activated char briquettes from pyrolysis residue of sewage sludge were prepared and characterized for methylene blue removal. The briquettes were prepared with different binder (phenolic resin) content (15–25 wt%), compression pressure (2.5–7.5 MPa) and thickness (3.5–14 mm), followed by CO₂ or H₂O activation under different temperatures. The effects of these parameters on the compositions, surface properties, apparent densities, mechanical properties, methylene blue adsorption kinetics and isotherms were systematically investigated. The results indicated that all of the prepared briquettes showed a characteristic of hierarchical porous structures, as well as high mechanical performance with impact resistant index much higher than 50. The increase of binder content from 15 to 25 wt% can result in marked increase of surface area (SBET), total pore volume (VT), apparent density and mechanical performance of the briquettes. According to the adsorption kinetics of methylene blue, the highest equilibrium adsorption capacity (qₑ,ₑₓₚ) for CO₂-activated and H₂O-activated briquettes were 194.6 and 203.6 mg/g. Both the increase of binder content and activation temperature markedly enhanced qₑ,ₑₓₚ, due to the increase of SBET and VT. qₑ,ₑₓₚ was much lowered by increasing the compression pressure or the thickness of briquettes, owing to the increased diffusion resistance and blockage of diffusion pathways inside briquettes. The adsorption kinetics data of the briquettes fit well into a pseudo-first order model system, suggesting physical adsorption of methylene blue onto briquettes surface. The adsorption isotherms of methylene blue indicated that both the CO₂-activated and the H₂O-activated briquettes followed a Langmuir isotherm model. The leaching tests of briquettes revealed that the leaching concentrations of Cd, Cr, Cu, Pb, Ni and Zn were much lower than the Chinese standard limits.