“Switch to love, switch to kill” - The interaction of Co-doped La(OH)₃ and peroxymonosulfate achieved highly efficient As(III) removal in water

Materials combing catalytic and adsorption dual active sites are very important for arsenite (As(III)) due to its high mobility in water. Therefore, in this study, a bifunctional walnut-like Co-doped La(OH)₃ composites (Co-La) with independent catalytic sites and post-formed adsorption sites were synthesized via a simple hydrothermal method. Nearly 100% As(III) removal can be achieved in Co-La and peroxymonosulfate interaction systems (Co-La/PMS) within 10 min at the molar ratio of PMS to As(III) of 1.2. The results indicate that the combined Co-La adsorption system demonstrated superior performance compared to the Co-La adsorption alone. Precisely, the maximum removal capacity of the system was measured at 243.90 mg·g⁻¹, and the pseudo-first-order kinetic rate constant (k₁) was determined to be 1.0771 min⁻¹. These values were significantly greater than those observed for Co-La adsorption alone. Notably, the kinetic rate constant was 239 times faster, and the removal capacity was 4.6 times higher in the combined system. The fast kinetics of As(III) elimination can be elucidated by the production of reactive oxygen species (ROS) such as SO₄^·-, •OH, O₂^·- and¹O₂, which expedite the conversion of As(III) into arsenate (As(V) in this particular system. Meanwhile, the increase in adsorption capacity is ascribed to the fact that PMS can drive La(OH)₃ of Co-La to transform into transition states La₂SO₄(OH)₄, thus generating a new exchangeable adsorption site for As(V). A fixed membrane reactor assembled with Co-La was utilized to achieve a near-complete removal of As(III) within 3 min. The Co-La/PMS systems demonstrate exceptional effectiveness in eliminating arsenic from water across diverse real-water metrics, offering a unique perspective and technological progress in purifying water contaminated with arsenic.