Investigation Of Removal Of Contaminants In Water By Fenton And Fenton-Like Oxidation

Fenton's reagent is largely researched (as a chemical for pre-oxidation, intermediate oxidation and post-oxidation in industrial and domestic wastewater treatment), which is a technology of higher-efficiency and covering a wide range of contaminant's removal. However, the optimum pH value range of Fenton oxidation is narrow (near pH 3) for high-efficiency oxidation by Fenton's reagent. Therefore, this study investigates the catalytic oxidation and the precipitation of Fenton's reagent near neutral pH condition as its potential application in water and wastewater treatment.Experiments are conducted at an initial pH of 7, in which malachite green (MG), humic acid (HA) and phosphate (PO₄^3-) are selected as target, respectively. This dissertation investigates the treatment efficiency, influencing factors and reaction mechanism of Fenton's oxidation (EDTA-Fe³⁺/H₂O₂) and Fenton's coagulation (Fe²⁺/H₂O₂) for removing organic and inorganic contaminants. Practical application of the Fenton reaction to phosphate removal is also investigated. The main research and results of this dissertation are shown below:Experiments about Fenton-like oxidation were investigated, which selected disodium ethylenediamine tetraacetate (EDTA) as promoter agent and MG as the target. The result indicated that the addition of EDTA significantly enhanced the MG decoloration in the process of Fe³⁺/H₂O₂. The MG decoloration is positively linear with respect to EDTA, Fe³⁺, H₂O₂ concentration, temperature and the initial pH value (pH≥5). The addition of promoter agent EDTA improved both the solubility of Fe³⁺ in solution and ferric catalytic decomposition of H₂O₂, which increased the formation of hydroxyl radical and intermediate iron species and increased the efficiency for the degradation of MG. The EDTA enhanced the decoloration of MG by Fe³⁺-H₂O₂ followed pseudo-first-order kinetic model, at 20℃and an initial pH of 7. Fenton-like reaction was accelerated in the presence of multiple constituents, which provided scientific basis for practical application.Based on the study of the MG decoloration efficiency in the process of EDTA-Fe³⁺/H₂O₂, the author investigated the mechanism about the MG decoloration by EDTA-Fe³⁺/H₂O₂. Some experiments are conducted at a temperature of 20℃and an initial pH of 7, such as the effect of the addition method of H₂O₂ on the oxidation, the effect of the presence of t-BuOH on the oxidation, the measurement of the electron paramagnetic resonance (EPR) spectra, the determination of H₂O₂ and ferrous concentration and the synergistic oxidation of K₂FeO₄/H₂O₂. Experimental results indicated that the multiple steps addition of H₂O₂ showed similar removal efficiency with those obtained by one step addition in the process of Fe³⁺-EDTA/H₂O₂; adding t-BuOH resulted in 50% decrease in the MG decoloration. These suggested that hydroxyl radical was not the only oxidant during the MG decoloration by Fe³⁺-EDTA/H₂O₂, and high-valent iron species was involved in the enhanced decoloration system. The data about ferrate simulation indicated that H₂O₂ (20mM) alone did not achieve MG decoloration, K₂FeO₄ (50μM) achieved 47.7% oxidation, and H₂O₂ (2.5mM)/K₂FeO₄ (50μM) achieved 95% oxidation. Obviously, synergistic oxidation of H₂O₂ and K₂FeO₄ occurred in the MG decoloration. EDTA could not enhance the MG decoloration by H₂O₂/K₂FeO₄, and the presence of t-BuOH could not inhibit the MG decoloration by H₂O₂/K₂FeO₄. These confirmed that high-valent iron species was the main oxidant and the inhibition of t-BuOH was minor in the decoloration of MG by H₂O₂/K₂FeO₄.Experiments on Fenton's coagulation was investigated at an initial pH of 7, in which both HA and phosphate were selected as target and the effect of Fe³⁺, H₂O₂ and target's concentration on the target's removal were studied. The results indicated an optimal molar ratio of [H₂O₂]0/[Fe²⁺]0 for HA or phosphate removal, and it was increasing with the increase in [HA]0 or [PO₄^3-]0. In this study, the ratio was 1:1³:1 and 0.5:1¹:1 for HA and phosphate, respectively. The optimal pH values for the removal of HA and P using Fe²⁺/H₂O₂ were 5⁷ and 4⁵, respectively. The residual soluble iron concentrations were also low at lower HA or phosphate concentrations after reaction. Chemical co-precipitation was considered as the dominant mechanism about HA and phosphate removal in the presence of Fe²⁺/H₂O₂.Practical application of the Fenton reaction to the phosphate removal is also investigated in secondary effluent. Experiments investigated the effect of water quality characteristic on the removal of phosphate. The EPR spectra of samples were measured, the efficiency of phosphate removal was compared, and economical aspect was also evaluated. The result indicated that the removal of phosphate with Fe²⁺/H₂O₂ was higher than that observed with ferric coagulation alone. The pH values after reaction using Fe²⁺/H₂O₂ were higher than using Fe³⁺ alone. However, slight difference of the pH values after reaction should not be the main reason for the difference in the phosphate removal of the two systems. This fact suggested that in situ formed Fe(III) having much affinity for phosphate. Chemical co-precipitation was considered as the dominant mechanism for phosphate removal in the presence of Fe²⁺/H₂O₂ in secondary effluent. And Fe²⁺/H₂O₂ had low cost advantage over Fe³⁺ alone. The EPR spectra tests in secondary effluents showed that Fe²⁺/H₂O₂ could produce an increasing hydroxyl radical concentration with a decrease in H₂O₂ dosage or phosphate concentration. This trend was similar to Fenton reaction at pH 3.This research provided data analysis of Fenton's reagent near neutral pH condition, and demonstrated its treatment efficiency and reaction mechanism. The results may be of significance in the potential application of Fenton's reagent in engineering.