Ammonia removal using activated carbons: effect of the surface chemistry in dry and moist conditions

The effect of surface chemistry (nature and amount of oxygen groups) in the removal of ammonia was studied using a modified resin-based activated carbon. NH3 breakthrough column experiments show that the modification of the original activated carbon with nitric acid, that is, the incorporation of oxygen surface groups, highly improves the adsorption behavior at room temperature. Apparently, there is a linear relationship between the total adsorption capacity and the amount of the more acidic and less stable oxygen surface groups. Similar experiments using moist air clearly show that the effect of humidity highly depends on the surface chemistry of the carbon used. Moisture highly improves the adsorption behavior for samples with a low concentration of oxygen functionalities, probably due to the preferential adsorption of ammonia via dissolution into water. On the contrary, moisture exhibits a small effect on samples with a rich surface chemistry due to the preferential adsorption pathway via Brønsted and Lewis acid centers from the carbon surface. FTIR analyses of the exhausted oxidized samples confirm both the formation of NH4+ species interacting with the Brønsted acid sites, together with the presence of NH3 species coordinated, through the lone pair electron, to Lewis acid sites on the graphene layers.

Ferredoxin-dependent glutamate synthase: involvement in ammonium assimilation in Haloferax mediterranei

Glutamate synthase (GOGAT) is one of the two important enzymes involved in the ammonium assimilation pathway glutamine synthetase (GS)/GOGAT, which enables Hfx. mediterranei to thrive in media with low ammonium concentration or containing just nitrate as single nitrogen source. The gene coding for this enzyme, gltS, has been sequenced, analysed and compared with other GOGATs from different organisms from the three domains of life. According to its amino acid sequence, Hfx. mediterranei GOGAT displays high homology with those from other archaeal halophilic organisms and with the bacterial alpha-like subunit. Hfx. mediterranei GOGAT and GS expression was induced under conditions of ammonium restriction. The GOGAT protein was found to be a monomer with a molecular mass of 163.78 kDa, which is consistent with that estimated by gel filtration, 198 ± 30 kDa. The enzyme is highly ferredoxin dependent: activity was only observed with one of the two different 2Fe–2S ferredoxins chromatographically isolated from Hfx. mediterranei. The enzyme also displayed typical halophilic behaviour, being fully stable, and producing maximal activity, at salt concentrations from 3 to 4 M NaCl, pH 7.5 and a temperature of 50 °C.