Study On The Treatment Of Nitrobenzene Wastewater By Fe³⁺/MgO-Fenton Oxidation-Flocculation

Nitrobenzene is the high poisonous substance which is difficult to be biodegraded and can inhibit and poison biochemical reaction. Wastewaters contain Nitrobenzene come widely and can cause a large pollution, its popular treatment include biological, physical and chemical methods. These years, Advanced Oxidation Processes which can degrade organic pollutants effectively is becoming a hot field, creating hydroxyl radical which has a wonderful oxidant performance is the primary characteristic of AOP.Fenton Oxidation Methods is useful in the degradation of persistent pollutants, but there are also some shortcomings in the treatment by classical Fenton Oxidation Methods and its extend methods(Photo-Fenton, Electro-Fenton, Ultrasound-Fenton and so on), such as a low reaction rate, a rigorous reaction condition, a large energy consumption. Fe³⁺/MgO-Fenton Oxidation system which can obviously heighten the reaction rate and COD degradation rate in the treatment of Nitrobenzene wastewater on normal temperature is composed by classical Fenton reagent and Fe³⁺/MgO activator. As pH regulator and flocculation-Aid, Ca(OH)₂ is effective in the extended COD degradation. The significance of the factors(temperature, reaction time, H₂O₂ concentration, Fe²⁺ concentration, activator concentration) to the Fe³⁺/MgO-Fenton treatment is confirmed by orthogonal study; The best condition and temperature range of Fe³⁺/MgO-Fenton Oxidation—Flocculation process are confirmed by Single-factor Experiment. Repetitious use of activator is primarily studied.Compared with the homogeneous Fenton system, the reaction rate of Fe³⁺/MgO-Fenton system improves significantly. The time, 80min, within which the reaction basically achieve equilibrium has been shortened to 40min, this made the production efficiency increase 100%. With a good degradation rate of Nitrobenzene, Fe³⁺/MgO can also adsorb the smaller organic molecules created in the Nitrobenzene degradation and give a extended oxidation, this improve the degradation rate of COD markedly. Heightening temperature is useful to accelerating the reaction, but it can't thoroughly meliorate the reaction equilibrium of Fe³⁺/MgO-Fenton system, which means the improving of oxidation is unavailable. The degradation rate of Nitrobenzene and COD can meet 95% and 67% when the temperature change in the ideal range: 20～40℃. A period of 15 minutes is enough to achieve equilibrium when the temperature is 40℃.Compared with the homogeneous Fenton system, the anti-impacted capability of Fe³⁺/MgO-Fenton system improves significantly. With the increasing concentration ratio of H2O2 to Fe²⁺(the reaction condition become execrable), the difference between classical Fenton and Fe³⁺/MgO-Fenton become larger, the declining of the latter(degradation rate of Nitrobenzene and COD) is milder, which proves the validity of Fe³⁺/MgO to the anti-impacted capability of Fe³⁺/MgO-Fenton system. Fe³⁺/MgO can keep the strong oxidation ability when the concentration of Fe²⁺ is low and it also cause the analogous oxidation ability when concentration ratio of H2O2 to Fe²⁺ increases.With the improving of anti-impacted capability and oxidation, the use of Fe³⁺/MgO also lower the Fe²⁺ concentration. At the room temperature, 20℃, the suitable range of ratio of H₂O₂ to Fe²⁺ is 4 to 8, the best ratio is 6; this range is 4 to 12 in the Fe³⁺/MgO-Fenton system and the best ratio become 8. The range of ratio of H₂O₂ to Fe²⁺ become wider when Fe³⁺/MgO is used in Fenton system, at the same time, the rigorous reaction condition and the high Fe²⁺ concentration are all improved.In the Fe³⁺/MgO-Fenton system, the most important factor to the degradation rate of Nitrobenzene is the concentration of H₂O₂, while the use of Fe³⁺/MgO can only accelerate the reaction. According to the effect and cost, the best concentration ratio of H₂O₂ to COD is 1.25.At room temperature, 20℃, the best oxidation conditions of Fe³⁺/MgO-Fenton is: reaction time is 40 minutes, the concentration of H₂O₂ is 1000 mg/L(the concentration ratio of H₂O₂ to COD is 1.25), the concentration of Fe²⁺ is 130mg/L(the concentration ratio of H₂O₂ to Fe²⁺ is 8), the dosage of the catalyst is 2g/L. Dealing with the best conditions, the nitrobenzene and COD concentration in the production of nitrobenzene wastewater are 4.5mg/L and 254.8mg/L, at the same time, the degradation ratio of the nitrobenzene and COD meet 97.3% and 67.9%, this also meets the third grade of Wastewater Discharge Standard. While under the same conditions, the degradation rate of nitrobenzene and COD are only 77.0% and 39.5% when the wastewater is treated by classical Fenton.Obviously, Fe³⁺/MgO-Fenton system works under a adsorption oxidation mechanism, increasing the dosage of catalyst is useful to improve the oxidation when the dosage is not enough to cover the bottom of the container; After the critical dosage, the dosage increasing make a negative effect to the oxidation, the reason is: superfluous strong alkaline catalyst makes partial Fe²⁺ change into Fe(OH)3 and the deposition is adsorbed on the surface of catalyst, first ,this cause the reduction of Fe²⁺, on the other hand, some of the active points are covered. Study indicated, the dosage meet the best condition when the catalyst is just enough t cover the bottom of the container, in this experiment, the critical dosage is 2g/L.The re-use of the Fe³⁺/MgO catalyst is good. After repeated use, the trend of wastewater treatment effect is still gently downward. After 5 times reusing, the degradation rate of nitrobenzene and COD is still up to 96.2% and 66.4%.Flocculation after oxidation is not helpful for the degradation of nitrobenzene and the ideal pH range is 7 to 8. As a flocculation-Aid, Ca(OH)2 is helpful for the degradation of chroma and the accelerated sedimentation of floccule. The best flocculation pH is 8, under this condition, the floccule subside fast enough and the output is clear, also, the degradation rate of COD in this process can meet 68.4%.Dealing with the united technics, Fe³⁺/MgO-Fenton Oxidation—Flocculation, the total degradation rate of nitrobenzene and COD are 97.3% and 89.9%, the concentration of nitrobenzene and COD in the production are 4.5mg/L and 80.5mg/L.Treatment of organic wastewater contains high poisonous and bio-refractory substance will be a difficulty and a emphasis in environment fathering of our country for a long time. Compared with the Classical Fenton Oxidation, Fe³⁺/MgO-Fenton Oxidation has many advantages: a better effect, an accelerated reaction rate, a warmer condition need, a low energy consumption. Fe³⁺/MgO is an effective and environment-friendly catalyst because of its good reusing ability and recycling ability. There are also some demerits in the study. For example, the effect of initialized concentration of wastewater hasn't been studied, also, the composition in the oxidation production hasn't been analyzed, all of these need a extended study to perfect.