Selective precipitation of calcium ion from seawater desalination reverse osmosis brine

Abstract

The near zero liquid discharge (NZLD) approach, by recovering water and dissolved valuable salts, is the most attractive clean solution for the valorisation of brines from seawater desalination reverse osmosis (SWD-RO) plants. In this perspective, a key aspect is calcium removal/recovery, to avoid scaling problems in the successive advanced separation units for recovering other valuable salts. In this work sodium citrate (Na3C6H5O7), carbonate (Na2CO3) and hydrogencarbonate (NaHCO3) were tested as calcium precipitation reagents. Different pH, temperature, ionic strength and reagent molar ratio were tested to maximize the Ca2+ precipitation and minimize the Mg2+ loss. Aqueous solutions containing Ca and Mg ions with/without all major seawater electrolytes were used. The chemical basis of the precipitation processes were discussed based on the effective ion surface density (e.g. Slater rule), ion hydration and Eigen association process of the precipitate formation in the complex multicomponent brine. PhreeqC and Medusa equilibrium numerical codes were applied on some experimental data of the precipitation processes providing a good agreement between calculated and experimental values. Ca2+ removal efficiency higher than 90% coupled with an Mg2+ loss below 7% was obtained at 60 °C and controlled pH, by using NaHCO3. These results are very promising in view of designing a process for brines valorisation, thus mitigating the environmental problems related to SWD-RO brines disposal.