Recovery of high reactive alkaline-earth-oxide (CaO and MgO) from reverse osmosis reject desalination brine: Kinetics, equilibrium, cost-effectiveness and energy-consumption

Abstract

Reactive alkaline earth oxides (CaO and MgO) have recently received significant attention due to their low production costs, availability, and capacity for storing CO₂. This study examined the high content of bivalent ions (Ca²⁺ and Mg²⁺) in RO reject brine for producing reactive quicklime and magnesia via selective precipitation. Various precipitating agents were evaluated at various molar ratios to discuss the high purity and process efficiency. The oxalic acid dosage was evaluated at a molar ratio (C₂H₂O₄/Ca²⁺) of 2 which led to high recovery efficiency of calcium oxalate monohydrate (97.5 ± 0.5 %). The theoretical study using PhreeqCI3 via the Pitzer model was used to model the precipitation behavior at selected conditions. The alkaline caustic ash dosage was set at the molar ratio (NaOH/Mg²⁺) of 3, corresponding to the highest extent of precipitation of brucite (97 ± 0.5 %). Further, the calcination of the precipitated solids for 2 h at 900 °C and 500 °C produced the reactive quicklime and magnesia. The characterization of produced oxides at the optimized parameters was designated and discussed based on the composition, microstructure, and reactivity. The surface areas of the obtained oxides were 10.4 m²/g and 58.3 m²/g, respectively, according to the BET analysis. The energy consumption and production cost for generating high-purity oxides from desalination waste revealed that RO reject brine is an additional source for recovering alkaline earth oxides with high reactivities and allowed to minimize the environmental impacts resulting from desalination plants.