Effect of Fenton pre-oxidation on the physicochemical properties of sludge-based biochar and its adsorption mechanisms for ammonia nitrogen removal

Fenton pre-oxidation, as a promising technology for sludge deep dehydration, has been used intensively; however, the outputs (i.e., low-water-content sludge cakes) need to be further disposed for a full line sludge resource utilization. Carbonization presents a promising approach for disposing of the sludge cake within the process. Here, the physicochemical properties of sludge-based char obtained after Fenton pre-oxidation were studied. Both Fenton pre-oxidized sludge-based char (FSC) and raw sludge-based char (RSC) were produced within the temperature range of 400–600 ℃, and the adsorption performance of the prepared chars, exemplified by their ability to adsorb ammonia nitrogen (NH4+), was tested under various conditions. FSC exhibited superior char yield and adsorption performance compared to RSC, with chemical adsorption being the primary controlling factor. The optimum conditions for FSC were observed at 400 ℃, resulting in a char yield of 72.2 %, and an adsorption capacity of 221.7 mg/g. Notably, FSC demonstrated an increase in C-C, C-N, OC-O, C-O and CC functional groups in comparison to RSC, which suggests that pre-oxidation increases organic matter decomposition and chemical reactivity of the sludge. The introduction of Si-O-Si, Si-O-C and Fe-O-H functional groups in the FSC facilitated ion exchange with NH4+ in an aqueous solution. The key findings indicate that Fenton pre-oxidation not only enhances the adsorption performance of FSC but also reduces the required carbonization temperature for excellent-char preparation. These results demonstrated the potential to integrate sludge deep dewatering, including pre-oxidation, with char preparation.