Preparation of novel adsorbents based on combinations of polysiloxanes and sewage sludge to remove pharmaceuticals from aqueous solutions

Abstract

Novel adsorbents were developed by varying the ratios of polysiloxanes and sewage sludge. The adsorbents were synthesized by mixing various polysiloxanes with methyl (MK), methyl/phenyl (H44) or without functional groups with sewage sludge in a solvent and pyrolyzed under an inert atmosphere. The adsorbents were characterized using several analytical and functional techniques, and used for adsorption of diclofenac (DCF) and nimesulide (NM) from aqueous solutions. Nitrogen adsorption/desorption measurements showed type I isotherms, which are typically for microporous materials, and the specific surface areas (SBET) were found to be in the range of 48 m2 g−1 and 631 m2 g−1. The sludge content was the major determinant for a decrease in SBET and hydrophobicity, as compared with pure polysiloxane samples. Among the composite materials, H67S33-500 (sample with 67% of polysiloxane H44 and 33% of sludge pyrolyzed at 500◦C) had the highest SBET value of 487 m2 g−1 while M40T60-600 (sample with 40% of polysiloxane MK and 60% of TEOS and pyrolyzed at 600◦C) exhibited the highest SBET value of 631 m2 g−1 among the hybrid materials. Experimental variables such as initial pH of the adsorbate solutions was optimized for adsorptive characteristics of the novel adsorbents. The optimum pH for adsorption of DCF and NM onto the adsorbents were 7.0 and 9.0, respectively. The equilibrium of adsorption was investigated using Langmuir, Freundlich and Sips models. Sips isotherm model gave the best fit of the equilibrium data. DCF showed better affinity for adsorbents than NM—suggesting that hydrophobic sites on the surfaces played a key role in the adsorption process.