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

Abstract

Oilfield brine, a by-product of oil and gas extraction, contains valuable resources such as Li⁺, K⁺ and Ca²⁺, offering potential utilisation benefits but also presenting separation challenges. We developed a reaction-coupled separation strategy to separate Ca²⁺ 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 Ca²⁺ into the solid product, with 98.5 % of Li⁺ retained in the liquid phase, achieving highly efficient separation of Ca²⁺ and Li⁺. The resulting Ca-based LDH demonstrated excellent capacity for removing heavy metals, such as Cu²⁺, Ni²⁺ and Zn²⁺, from electroplating wastewater. The removal capacities reached up to 384.2 mg/g for Cu²⁺, 319.3 mg/g for Ni²⁺ and 367.9 mg/g for Zn²⁺. 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 Cu²⁺, Ni²⁺ and Zn²⁺ by the Ca-based LDH. Mechanistic analysis revealed that Ca²⁺ ions in the LDH layers are isomorphously substituted by Cu²⁺, Ni²⁺ and Zn²⁺ from the solution, forming Cu-, Ni- and Zn-containing LDHs with enhanced structural stability. Additionally, the released Ca²⁺ ions precipitate with CO₃²⁻, derived either from intercalated anions or carbonate in the water. Such conditions promote further substitution of Ca²⁺ in the LDH layers by Cu²⁺, Ni²⁺ and Zn²⁺, 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.