Preparation of non-sintered fly ash filter (NSFF) for ammonia nitrogen adsorption

In accordance with China's goal of 'treating wastes with wastes, turning wastes into treasure', a non-sintered fly ash filter (NSFF) with sewage sludge as additive was prepared. It consists of 70.9% fly ash, 7% sewage sludge, 9% cement, 7.1% CaO, 1% NaHCO₃ and 5% sodium silicate solution. After mixing, 34 g/(100 g dry material) water was added, and then was granulated and steam cured under 80°C for 16 h. NSFF's main performance indexes include specific surface area (SSA) of 17.038 m² g^-1, filter media breaking rate (FMBR) of 2.2%, apparent density (AD) of 1140 kg m^-3, and porosity of 41.67%, meeting the Chinese Standard CJ/T 299-2008. This NSFF has a larger SSA and a lower AD comparing with the other similar non-sintered fly ash ceramsite products. Moreover, leaching toxicity of the NSFF has met the Chinese Standards for Hazardous Wastes (GB5085.3-2007). Therefore, the NSFF is effective and safe to use as a water treatment filter media. The NSFF's adsorption characteristics for ammonia nitrogen was investigated. Results showed that the optimized parameters for ammonia nitrogen adsorption are as follows, NSFF dosage at 5 g, initial ammonia nitrogen concentration of 225 mg L^-1, pH at 7, contact time of 12 h and temperature at 30°C. Under the optimum conditions, the adsorption capacity of NSFF for ammonia nitrogen was 4.25 mg g^-1. The adsorption process can be best described by Langmuir isotherm and pseudo-second-order kinetic model. The proposed adsorption mechanism include adsorption and cation exchange.