Coagulation of trace arsenic and cadmium from drinking water using titanium potassium oxalate

Coagulation is a simple and cost-effective water treatment method that does not work well in removing multiple cationic and anionic heavy metals simultaneously from drinking water. Titanium potassium oxalate (K₂TiO(C₂O₄)₂), a fur tanning reagent, was found to be able to efficiently remove arsenite (As(III)), arsenate (As(V)), and Cd simultaneously. A dose of 120 µmol/L K₂TiO(C₂O₄)₂ could remove more than 90% of As and Cd to meet the drinking water standards when their initial concentrations were 10 times their maximum concentration limits, whereas traditional coagulants, such as Fe₂(SO₄)₃ and Al₂(SO₄)₃, failed to meet the drinking water standards. Additionally, K₂TiO(C₂O₄)₂ coagulation consumes natural water hardness (Ca²⁺/Mg²⁺) to produce softer water and releases healthy K⁺ as a by-product. The mechanism study indicated that K₂TiO(C₂O₄)₂ reacted with natural calcium ions in drinking water to form calcium oxalate, while residual titanium was hydrolyzed with water to form hydrous titanium oxide. Arsenic was removed primarily via complexation with hydrous titanium oxide, while Cd was removed via the combined effect of adsorption by hydrous titanium oxide and mixed-crystal formation by calcium oxalate. This study provides an efficient coagulant for removing multiple heavy metals simultaneously, which can be applied in water treatment to provide safe and healthy drinking water.