| Emerging contaminants in high strength wastewaters such as landfill leachate are causing environmental concerns due to their adverse impacts on the aquatic environment. Current treatment technologies are limited by high operating costs, low treatment efficiencies, or sensitive operating conditions. In this study, laboratory scale tests were conducted to evaluate the feasibility of iron-mediated oxidation processes---Fenton and hydrogen peroxide (H2O2) enhanced iron-mediated aeration (IMA)---to treat such wastewater for removal of organic content, in addition to reduction of ammonia nitrogen and electrical conductivity. Landfill leachate was selected as the model wastewater, due to recent disallowance of leachate for discharge to municipal sewer in Florida. First, the effects of initial pH, Fenton reagent doses, aeration, and number of dosing steps on chemical oxygen demand (COD) removal by the Fenton process were investigated. Also, a set of Taguchi orthogonal array experiments was conducted to investigate the effects of initial pH, iron grade, aeration rate, and H2O2 addition rate for COD removal on H2O2-enhanced IMA process. Second, preliminary comparison was made on these bases, and tests were conducted to study the interaction of coagulation and oxidation on overall processes. In Fenton treatment, oxidation played an important dual role, contributing to COD removal, while degrading organics to smaller, more soluble species less amenable to coagulation. In H2O2-enhanced IMA treatment of glyoxylic acid in simulated natural water, the known •OH radical scavenger, para-chlorobenzoic acid (pCBA), was found not to inhibit the decomposition rate of glyoxylic acid at pH 7.0-7.5, effectively ruling out hydroxyl radicals as the principal oxidant, suggesting that either H2O2 or ferryl species might play an important role. In addition, aeration was shown to slow the auto-decomposition of H2O2, enhance release of Fe 2+ from Fe0 corrosion, and slightly improve COD removal, suggesting that molecular oxygen took part in the oxidation of organics, and differentiating the H2O2-enhanced IMA process from the Futon reaction using Fe0. Tests showed that both processes could remove COD by >50%. However, H2O2-enhanced IMA process was shown, in addition, to simultaneously reduce ammonia nitrogen and electrical conductivity in the effluent, whereas the Fenton process removed little ammonia nitrogen and substantially increased electrical conductivity due to pH adjustment and FeSO4 input. Finally, kinetics tests of the H2O2-enhanced IMA process were conducted with respect to H2O2 decay, overall COD removal, and COD oxidation. Mathematical models utilizing measured reaction data were established and verified, which adequately predicted H2O2 concentration, overall COD removal rate, and COD oxidation removal rate under different conditions. |
