Effect of conditioners used for sludge dewatering on the adsorption performance of sludge-derived adsorbent

Converting waste activated sludge into adsorbent is a promising alternative for sludge valorization. While various conditioners are widely employed in sludge conditioning and dewatering processes, the influence of these conditioners on the properties of sludge derived adsorbents remains largely unexplored. Four conditioners, including FeCl₃, CaO, polyacrylamide, FeSO₄·7H₂O/Na₂S₂O₈, were used for sludge conditioning, and the sludge dewaterability and the adsorption performance of the resultant adsorbents were evaluated. The sludge dewaterability was markedly improved by the conditioners in a descending order as polyacrylamide, FeCl₃, FeSO₄·7H₂O/Na₂S₂O₈, and CaO. The type of conditioner highly influenced the adsorption ability of the sludge adsorbent: FeSO₄·7H₂O/Na₂S₂O₈ enhanced the adsorption ability, while others deteriorated it. Infrared spectra, pore size distribution, and micromorphology analysis of the adsorbents suggest that: FeCl₃ reduced mesopores through strong electrostatic interactions; CaO acted as a physical barrier for adsorption; polyacrylamide reduced the adsorbent surface area through hydrogen bond-driven bridging and potentially forming polymeric barrier; FeSO₄·7H₂O/Na₂S₂O₈ disrupted the sludge flocs and exposed abundant adsorption sites, promoting the adsorbent performance markedly. FeSO₄·7H₂O/Na₂S₂O₈ conditioning was thus beneficial to both sludge dewatering and adsorption. Adsorption of methylene blue by FeSO₄·7H₂O/Na₂S₂O₈-conditioned sludge adsorbent was best described by the pseudo-second-order model and Langmuir model, indicating the dominance of monolayer chemisorption. The adsorption mainly occurred in the mesopores of the adsorbent, with hydroxyl group and carbonyl in carboxyl group as the main binding sites. Both film and pore diffusion contributed to the adsorption rate, with pore diffusion serving as the main rate limiting step. Results highlight the beneficial effect of destructive conditioning method on sludge adsorbent performance.