Preparation and sorption properties of materials from paper sludge

Three materials were prepared from paper sludge (PS) using different treatment processes and their sorption abilities for phosphate and methylene blue (MB) were determined. The samples were a powder sample prepared by heating PS in air (sample C), a pellet prepared by grinding, forming and heating PS in air (sample G) and a powder prepared by physical activation of PS in flowing wet nitrogen (sample A). The three samples were heated at 600-900 °C for 6 h. On heating at 700-800 °C, the organic fibers, limestone (CaCO₃), kaolinite (Al₂Si₂O₅(OH)₄) and talc (Mg₃Si₄O₁₀(OH)₂) in the original PS were converted to amorphous CaO-Al₂O₃-SiO₂ (CAS) and talc in sample C, while CAS was formed in sample G and activated carbon, CAS and talc was formed in sample A. On heating at 900 °C the CAS converted to gehlenite (Ca₂Al₂SiO₇) and anorthite (CaAl₂Si₂O₈). The specific surface areas (S_BET) of the three samples achieved maximum values of 23, 37 and 70 m²/g upon heating at 700, 600 and 600 °C, respectively. The S_BET value of the activated sample A was distinctly lower than usually reported for activated carbon. The samples C, G and A achieved maximum phosphate sorption capacities of 2.04, 1.38 and 1.70 mmol/g, calculated from the Langmuir model, upon heating at 700, 700 and 800 °C, respectively. The maximum sorption capacity for phosphate in sample C is attributed to the sorption by CAS, namely, adsorption on the alumina component and precipitation as Ca-phosphates. The MB multifunctional sorption capacity of sample A was 0.11 mmol/g. The phosphate and MB sorption rates show better correlation with a pseudo-second order model than with other models.