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 Na₂SO₄/K₂SO₄ mixtures. Na⁺ tended to occupy the junctions between SO₄^2-, while K⁺ tended to occupy the cluster endings. Due to the weaker electrostatic interaction between K⁺ (with larger ionic radius) and SO₄^2-, large and medium-sized clusters were more prone to decomposition in K⁺-rich solutions. K₂SO₄ induced a “salting-in” effect on Na₂SO₄, whereas Na₂SO₄ exerted a “salting-out” effect on K₂SO₄ at high water densities. Both sulfates exhibited heterogeneous nucleation, depositing on heated sidewalls as nucleation sites, with Na₂SO₄ proving more difficult to remove than K₂SO₄. In KCl/Na₂SO₄ mixtures, homogeneously nucleated chloride salts created many preferential nucleation sites for Na₂SO₄ deposition.