The role of sulfur-containing groups in ammonia retention on activated carbons

Two carbons with different contents of sulfur were prepared and oxidized either by heating in air or by chemical treatment. The materials were then tested as adsorbents of ammonia in dynamic conditions, at room temperature. Their chemical and structural features were analyzed by energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy, scanning electron microscopy, thermogravimetric analyses, potentiometric titration and sorption of nitrogen. It was found that not only oxygen-containing groups but also sulfur-containing groups enhance ammonia adsorption. In particular, sulfonic groups play a predominant role. In the presence of superoxide anions, they are converted into sulfates that react with ammonia to form ammonium sulfates. These salts are strongly retained in the micropores of the adsorbents.