Simultaneous Ca/Li separation and Ca-based layered double hydroxide synthesis from oilfield brine for heavy metal removal in electroplating wastewater

Oilfield brine, a by-product of oil and gas extraction, contains valuable resources such as Li + , K + and Ca 2+ , offering potential utilisation benefits but also presenting separation challenges. We developed a reaction-coupled separation strategy to separate Ca 2+ and Li + while simultaneously synthesising a calcium-based layered double hydroxide (Ca-based LDH) from the brine. This technology enables the recovery of over 90 % of Ca 2+ into the solid product, with 98.5 % of Li + retained in the liquid phase, achieving highly efficient separation of Ca 2+ and Li + . The resulting Ca-based LDH demonstrated excellent capacity for removing heavy metals, such as Cu 2+ , Ni 2+ and Zn 2+ , from electroplating wastewater. The removal capacities reached up to 384.2 mg/g for Cu 2+ , 319.3 mg/g for Ni 2+ and 367.9 mg/g for Zn 2+ . In both single-cation and mixed-cation wastewaters, metal concentrations were dramatically reduced to parts-per-billion (ppb) levels. Kinetic studies indicated rapid adsorption behaviour of Cu 2+ , Ni 2+ and Zn 2+ by the Ca-based LDH. Mechanistic analysis revealed that Ca 2+ ions in the LDH layers are isomorphously substituted by Cu 2+ , Ni 2+ and Zn 2+ from the solution, forming Cu-, Ni- and Zn-containing LDHs with enhanced structural stability. Additionally, the released Ca 2+ ions precipitate with CO₃ 2− , derived either from intercalated anions or carbonate in the water. Such conditions promote further substitution of Ca 2+ in the LDH layers by Cu 2+ , Ni 2+ and Zn 2+ , resulting in robust metal removal and ultralow residue concentrations in the treated wastewater. This work presents an efficient, economical and sustainable approach for the simultaneous separation and utilisation of oilfield brine resources and treatment of electroplating wastewater.