Utilisation of industrial sludge-derived ferrous sulfate for hexavalent chromium mitigation in cement

Hexavalent chromium in Portland cement poses significant health risks and contributes to environmental pollution. Traditional reduction methods rely on costly, resource-intensive reducing agents derived from primary raw materials. This study presents a sustainable approach by synthesising ferrous sulfate from industrial iron-rich sludges and waste sulfuric acid to mitigate hexavalent chromium levels in Portland cement. Three types of iron-rich sludges, sourced from municipal wastewater treatment and steel manufacturing, were analysed using X-ray diffraction to identify their iron-bearing phases. Key parameters, including sludge grinding intensity, stirring rate, reaction time and temperature, were systematically optimised to enhance ferrous sulfate yields. Under optimal conditions, approximately 17 g of pure ferrous sulfate were produced from 10 g of sludge and 50 ml of 20% waste sulfuric acid, achieving a recycling efficiency of around 90%. Incorporating the synthesised ferrous sulfate into ordinary Portland cement reduced hexavalent chromium concentrations below 2 ppm, complying with Environmental Protection Agency, USA standards. This method demonstrates the feasibility of repurposing industrial waste to enhance cement’s environmental performance, offering a promising valorisation pathway for iron-rich sludges and waste acids. The approach is cost-effective and environmentally friendly, aligning with circular economic principles and promoting green innovations in industrial waste management.