Cobalt and nickel removal from urban and industrial wastewaters using sustainable synthetic carbonates

Urban and industrial wastewater often contains high concentrations of heavy metals, which are among the most harmful pollutants due to their toxicity, persistence, and negative effects on both biological systems and human health. This study investigates the efficiency of Co and Ni removal from aqueous solutions using synthetic Mg-carbonates produced through a CO₂-mineralization process, in which anthropogenic CO₂ reacts with MgCl₂ solutions. The open-high reactive structure of amorphous magnesium carbonates (AMCs) promotes rapid Co/Ni uptake through adsorption and ion-exchange mechanisms, offering a low-energy and sustainable remediation strategy for contaminated urban and industrial effluents. Batch experiments were conducted using Co and Ni solutions from 50 to 1000 mg/L, reacted with 0.1 g of AMCs for interaction times ranging from 20 minutes to 4 weeks at ambient pressure and temperature. The residual solutions were analyzed by ICP-AES to quantify removal efficiency and Mg release, whereas solid products were examined using SEM-EDS and XRPD to assess morphological and mineralogical transformations. The Co removal experiments provided a coherent dataset across analytical techniques and revealed a critical threshold of ~250 mg/L, marking the transition between two distinct removal regimes. Below the concentration 50–150 mg/L, morphological transformation is rapid and highly efficient, with AMC dissolution and reprecipitation as Co-carbonates occurring within a few hours and removal efficiencies reaching up to ~99% after four weeks. At concentrations ≥250 mg/L, removal remained significant (up to ~75%) but was characterised by slower kinetics and incomplete equilibrium within the experimental time. The correlation between the moles of Co removed and Mg released showed that at short times (0–3 h) the process was dominated by adsorption on AMC surfaces, whereas at longer times (24 h–4 w) ion exchange progressively prevailed, leading to nearly 1:1 Co–Mg stoichiometry. The transition between these mechanisms occurred earlier at higher Co concentrations. XRPD data confirmed structural reorganisation and the precipitation of new Co-bearing carbonate phases throughout the process. Preliminary Ni experiments indicate comparable trends, confirming that AMCs exhibit similar reactivity and removal mechanisms toward both metals. Overall, these results show that CO₂-derived synthetic Mg-carbonates offer an effective, low-energy and scalable solution for Co and Ni removal from contaminated waters, with clear relevance for urban and industrial water management. Their reactivity and sustainability, combined with the strategic value of Co and Ni as critical raw materials, highlight the potential of this approach for future resource-recovery applications.