Silver nanoparticle-loaded graphitic carbon nitride/multiwall carbon nanotube composite with improved denitrification to nitrogen gas for the photocatalytic removal of aqueous ammonia nitrogen

Ammonia nitrogen (TAN) accumulated in the hydrosphere is an undesired nitrogen pollution and poses a great threat to both the ecological environment and human health. Its removal from water by the photocatalyst directly using solar energy is of strong interest in environmental remediation. In this study, graphitic carbon nitride (g- C 3 $C_3$ N 4 ) was combined with multiwall carbon nanotubes (CNTs) and loaded with silver nanoparticles (Ag (NPs) ${}_{\mathrm{(NPs)}}$ ) to hydrothermally synthesize a photocatalytic composite (Ag (NP) ${}_{\mathrm{(NP)}}$ -(g- C 3 $C_3$ N 4 )/CNT) with a high catalytic efficiency. Characterization analyses indicated even deposition of Ag (NPs) ${}_{\mathrm{(NPs)}}$ onto (g-C 3 N 4 )/CNT composite by photochemical reduction and the increased photoresponse property compared with raw g-C 3 N 4 and TiO 2 -based materials. The photocatalysis of the composite was evaluated in terms of different Ag (NP) ${}_{\mathrm{(NP)}}$ weight ratios, light intensities , photocatalyst dosages, solution pH values and light types. Results showed that a strong alkaline system induced a higher photocatalytic removal of TAN. The TAN removal ratio and rate constant at the optimal pH of 11 were 11.3 and 44.3 times those at a pH of 7. Compared with the process using g- C 3 $C_3$ N 4 alone (dosage = $=$ 0.5 g/L, initial TAN = $=$ 2.0 mg/L, pH = $=$ 11, Ag (NP) = 1 . 0 ${}_{\mathrm{(NP)}}=1.0$ wt%), Ag (NP) ${}_{\mathrm{(NP)}}$ -(g- C 3 $C_3$ N 4 )/CNT composite had effectively increased TAN removal and N 2 $N_2$ conversion ratios from 42.5% and 3.7% to 88.2% and 78.7 % upon 6-h Xe lamp irradiation (180 mW/cm 2 ). Ag (NP) ${}_{\mathrm{(NP)}}$ -(g-C 3 N 4 )/CNT composite also exhibited satisfactory stability in cyclical TAN removal. Moreover, the mechanisms of TAN removal were analyzed. This study highlights the application of the Ag (NP) ${}_{\mathrm{(NP)}}$ -(g-C 3 ${}_3$ N 4 )/CNT composite as a potential method for ammonia removal in environmental remediation.