Degradation Of Humic Acid From Landfill Leachate By Microwave Assisted Persulfate

There are lots of humic acid (HA) in landfill leachate, which is a high contribution to chemical oxygen demand (COD). HA is a topic recalcitrant pollutant and it can not only adversely affect appearance and taste, but can also be halogenated to form potentially carcinogenic chlorinated organic compounds. However, it is difficult to remove HA from landfill leachate by traditional biological processes.Advanced oxidation technologies, in which persulfate (PS) is used as oxidant, have come forth recently for the degradation of non-biodegradable contaminants. The conventional activation methods of PS consist of heat, UV and metal activation. Microwave (MW) heating presents several attractive advantages, such as shorter reaction times, higher yields, and cleaner reactions. A modified domestic MW furnace (800 W) is used to supply MW energy and activate PS to generate sulfate radical (SO₄^-). MW assisted persulfate (MW+PS) is investigated for the degradation of HA. Specific studies are as follows:(1) The effects of various operating conditions such as initial HA concentration, initial PS concentration, initial pH value, HCO₃^-/CO₃^2-and granular activated carbon (GAC) are examined. Degradation is evaluated by the decreases of absorbance at 254 nm (UV₂₅₄) and 400 nm (VIS₄₀₀)and total organic carbon (TOC). According to the results, nearly 100% of 100 mg/L HA is degraded by MW+PS process at 800 W in 30 min. Higher initial PS concentration could accelerate the HA degradation efficiency, while higher initial HA concentration could cause a lower degradation efficiency. The UV254 and VIS400 of HA kept high at pH 4 - 12.8. PS could not only oxidize functional groups of HA, but also mineralize it and the removal efficiency of TOC is about 98% under MW irradiation when PS dosage is 100 mmol/L. Methanol (MA) and tert-butyl alcohol (TBA) are applied as chemical probe compounds to detect the free radicals. The results show that both SO₄^- and·OH may exist in the MW+PS technology. Whether SO4?? or·OH playing a predominant role in HA degradation depends on the pH of the solution.(2) Humic substance is extracting from landfill leachate by resin. Experiments are carried out on the degradation of extracted HA by MW+PS technology. The results show that the removal efficiency of TOC and COD are 55% and 58.9% respectively. Furthermore, the degradation of UV254 and VIS400 are both more than 95% in 20 min when initial pH is 3, 7 and 13.(3) COD is one of the most important determination indicators of landfill leachate. However, the presence of high concentration chloride in leachate generates interference when detacting COD by national standard method. Compared potassium dichromate reflux method, rapid digestion-spectrophotometric method, basic potassium permanganate method with UV spectrophotometry method, it is found that UV spectrophotometry could not only shorten the measure time and improve the efficiency, but avoid the interference from chloride. UV spectrophotometry method is used to detect COD in landfill leachate.(4) The treatment of actual landfill leachate (cCOD = 3074.4 mg/L, cCl- = 5325 mg/L) by MW+PS is conducted compared with MW assisted hydrogen peroxide (H2O2) and peroxy-monosulfate (PMS). COD removal efficiencies are respectively 97%, 42% and 80% when the oxidants are 0.3 mol/L. The higher of the concentration is, the higher of the removal efficiency is. For H2O2 and PS, chloride ion in landfill leachate could improve COD removal slightly. While for PMS, chloride ion captures its radical and generates chlorine gas escaping out of the reactor, so that COD removal gets a bit decrease. COD removal efficiency of landfill leachate is all high in the pH value of 2, 7, 8.2, and 12. MW heating is more efficient than traditional water-bath heating when the temperature is above 85℃. GAC plays a synergism effect in the treatment of landfill leachate by MW+PS. When 0.1g/L GAC is added in MW+PS, COD removal is enhanced from 78% to 96% in 10 min compared with no GAC. PS dosage and GAC dosage are explored, and the results show that PS dosage plays the main role in MW+PS+GAC technology.