Pyrolysis of Paper Sludge and Utilization for Ionic Dye Adsorption

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Abstract
Paper sludge was pyrolyzed to synthesize an adsorbent for ionic dyes. Two reagents, Methylene Blue (MB) and Procion Red MX-5B (PR), were chosen as adsorbates. Pyrolysis of paper sludge was conducted at 600°C for 1 h under a nitrogen atmosphere, and the carbonized sample was washed with 1 M HCl for 30 min to remove inorganic salts (such as calcium) and to increase the carbon content. Scanning electron microscope (SEM) imaging showed that the particle size of the pyrolyzed paper sludge was obviously reduced when the acid-washing process was applied. Additionally, the specific surface area had increased from 13.25 to 193.86 m2/g. The isoelectric point was around pH 3, which meant the adsorbent revealed positive charge to adsorb anionic dye (PR) when pH was below 3. On the other hand, cationic dye (MB) was adsorbed well under high pH values because of the negative surface charge of the adsorbent. The Langmuir equation was adopted to determine maximum absorption capacity. For MB and PR, the maximum absorption capacities were 119.05 mg/g and 65.79 mg/g, respectively. The adsorption kinetic study revealed that MB and PR adsorptions fit the pseudo-second-order kinetic model well. The activation energies for MB and PR were 12.32 kJ/mol and 2.88 kJ/mol, respectively. The results presented indicate that paper sludge can be pyrolyzed to form a dye adsorbent for cationic and anionic dyes.

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