Innovative chemical functionalisation of biosolids for removing heavy metals and enhancing ammonium recovery from wastewater

This study addresses ammonium removal from wastewater effluents. The utilisation of biosolids (stabilized sewage sludge) derived products as adsorbents was explored to contribute to sustainable sludge management. Biosolids were subjected to a mild acid (10% v/v HNO₃) and alkali (0.5 M NaOH) treatment at ambient conditions followed by pyrolysis at 450 °C. R-COOH functional group was introduced to the biochar surface by HNO₃ treatment followed by the ionic replacement of H⁺ with Na⁺ from NaOH treatment to form R-COO-Na group. The surface functional groups were studied via XPS, EDS, and FTIR analyses. The chemical treatment achieved on average about 75% removal of heavy metals from the product. The modified biochar showed a high ammonium adsorption capacity from both synthetic wastewater (17.03 mg g⁻¹) and real digester effluent (16.1 mg g⁻¹), higher than that of raw biosolids biochar (5.23 mg g⁻¹ in synthetic effluent). The N-laden biochar (composed of 1.6 wt% surface nitrogen) with reduced heavy metals content could be applied to land as a slow-release nitrogen fertiliser. Economic analysis reveals a net present value of $1 million and a payback period of 8 years. This approach potentially reduces environmental pollution and provides economic benefits via cost-effective waste treatment solutions.

Graphical abstract