Mechanism studies of a CO2 participant softening pretreatment process for seawater desalination

• The mechanism of CO2 decalcification using NaOH as alkali source was studied.
• Mg(OH)2 and Ca(OH)2 firstly formed, and then dissolved but CaCO3 crystallized.
• The material conversion was analyzed with the ion concentration variation.
• The new enlightenment of magnesium hydrate as the new alkali source.

To explore the mechanism of the CO2 participant softening pretreatment process, a series of experiments and analyses have been conducted to study the process of decalcification with NaOH as alkali source and CO2 as precipitator, which includes the material conversion of precipitate, Ca2 +, Mg2 +, CO32 −, HCO3−, and OH− ion concentrations and pH in solutions. It is found that the decalcification pretreatment process is a complex reaction involving first formation of Mg(OH)2 and Ca(OH)2 before CO2 is injected, and then CaCO3 is nucleated and crystallized, following the dissolving of Mg(OH)2 and Ca(OH)2 with the continuous influx of CO2. MgCO3·3H2O is not formed until all the Ca(OH)2 gradually transformed to CaCO3, and it begins to dissolve as there is only CaCO3 left in the precipitates. Then the added amount of NaOH is changed to study the precipitate before CO2 is injected, and material transformation of deposit with bubbling CO2 is investigated. This further study provides a favorable evidence for a new enlightenment, that is, magnesium hydrate may work as the new alkali source for CO2 participant softening pretreatment technology, which has more economical value than sodium hydroxide.