Catalytic ozonation of humic substances with multi-layer graphene

Multi-layer graphene (MLG) was assessed as a catalyst in the ozonation process to remove humic substances (Hs) found in surface water. Ozonation experiments were carried out in jacketed reactor filled with 250 mL HS aqueous solution (concentration = 750 mg L− 1) and 0.1 g MLG (only for catalytic ozonation) fed with ozone (concentration of 22 mg L− 1, flow rate of 1 L min− 1). Two pre-treatments were applied to MLG (pre-ozonation and high-energy milling) and three pH values ​​at the beginning of the reaction (6; 9.5; and 12) were also assessed. The catalyst increased ozonation process efficiency in removing color (436 nm) and total organic carbon (TOC). The two pre-treatments enhanced MLG performance as catalyst at pH 9.5. Its performance was null, or negative, at the other pH values. The non-catalytic ozonation reaction at pH 9.5 allowed 90.9% color removal within 15 min. Removal efficiency increased to 95.2% (untreated MLG); 96.7% (ozonized MLG) and 97.3% (milled MLG) after MLG was added to the process. TOC removal rates within 60 min after ozone addition were 38.3% (in non-catalytic ozonation), 40.9% (untreated MLG), 66.0% (ozonized MLG) and 56.7% (milled MLG). The ozonation process based on using MLG as catalyst can remove HS and, consequently, increase water and wastewater treatment systems’ efficiency.