Advanced treatment of leachate secondary effluent by a combined process of MFPFS coagulation and sulfate radical oxidation

Leachate secondary effluent is usually unable to meet the national discharge permit in China and further treatment is a significant challenge because of the residual persistent pollutants. A novel magnetic composite coagulant of MFPFS was prepared from Fe3O4 nanoparticles and polymeric ferric sulfate (PFS). Landfill leachate secondary effluent was treated by MFPFS coagulation combined with sulfate radical oxidation due to Fe2+ activation. Higher COD and color removals were achieved using the MFPFS magnetic coagulant than by adding the coagulant PFS alone. Under the working conditions of 1:3 Fe3O4/PFS mass ratios, 1.2 g/L MFPFS dosage, 50 min sedimentation time, and without pH adjustment, the removals of COD and color reached up to 60% and 80%, respectively. In the oxidation process, Na2S2O8 was activated by Fe2+ to generate sulfate radicals to oxide the residual pollutants in the effluent from MFPFS coagulation. In general, higher temperature, higher dosage, and lower pH were in favor of the oxidation efficiencies. Under the working conditions of 2.4 g/L Na2S2O8 dosage, 0.3 Fe2+/S2O8 2- molar ratio, pH 3.0, and 60ºC temperature, the COD and color removals can reach up to 75% and 95%. After being treated by the combined process, no violation of Chinese effluent standards for landfill leachate was found in respect of each individual parameter, including COD, color, and other parameters like BOD, total nitrogen, and ammonia nitrogen. The combined process was a promising alternative treatment pathway for leachate secondary effluent.