Characteristics and mechanisms of dissolution (mixed Na2SO4-K2SO4, Na2SO4-Na/KCl) and deposition (Na2SO4, K2SO4, KCl, mixed Na2SO4-KCl) of salts in supercritical water

Supercritical water oxidation efficiently degrades organic waste but faces challenges from salt deposition due to the low solubilities of salts in supercritical water (SCW). This study investigated the characteristics and mechanisms of dissolution and deposition of mixed sulfate and chloride salts in SCW. At low water densities the common ion effect was observed in Na 2 SO 4 /K 2 SO 4 mixtures. Na + tended to occupy the junctions between SO 4 2- , while K + tended to occupy the cluster endings. Due to the weaker electrostatic interaction between K + (with larger ionic radius) and SO 4 2- , large and medium-sized clusters were more prone to decomposition in K + -rich solutions. K 2 SO 4 induced a “salting-in” effect on Na 2 SO 4 , whereas Na 2 SO 4 exerted a “salting-out” effect on K 2 SO 4 at high water densities. Both sulfates exhibited heterogeneous nucleation, depositing on heated sidewalls as nucleation sites, with Na 2 SO 4 proving more difficult to remove than K 2 SO 4 . In KCl/Na 2 SO 4 mixtures, homogeneously nucleated chloride salts created many preferential nucleation sites for Na 2 SO 4 deposition.