Construction Of Electro-iron-reduction Fenton Oxidation System And Degradation Of Dye Wastewater

Fenton oxidation is one of the advanced oxidation water treatment technologies.It has several advantages such as rapid reaction,easy operation and ideal organics degradation,but also has the disadvantages including the low utilization rate of H₂O₂,large amount of ferrous salt and large amount of iron sludge.This subject constructed an electro-iron-reduction Fenton oxidation system,which combined the electrochemical reduction with Fenton oxidation technology to regenerate Fe²⁺ by reducing Fe³⁺ at cathode.Thereby it can reduce the amount of iron salt,reduce the amount of sludge produced after the reaction and increases the utilization of H₂O₂.The outstanding feature of this method is that the electrochemical reduction and Fenton oxidation processes are carried out alternatively.The degradation targets in the study are 4-nitrophenol solution and dye wastewater.The former was used to explore the technical feasibility of the new process,while the latter was used to investigate the performance of the method on the treatment of actual industrial wastewater.Dye wastewater has the characteristics of complex composition,high salinity and high concentration of organic pollutants,which must be safely disposed to prevent environmental problems.The research content of this subject included two parts:（a）degradation of 4-nitrophenol by electro-iron-reduction Fenton oxidation method,focusing on whether the method has the ability of reducing the amount of ferrous salt and improving the ability of H₂O₂ utilization compared with the conventional Fenton oxidation method;（b）the treatment of dye wastewater by electro-iron-reduction oxidation method,investigating the effectiveness of this method in treating actual industrial wastewater compared with conventional Fenton oxidation method.The main findings of the study are as follows:（1）4-nitrophenol was degraded by electro-iron-reduction Fenton oxidation.The treatment was carried out by Fenton oxidation for 60 min,electro-reduction for 30 min（supply voltage of 1.2 V）and Fenton oxidation for 60 min sequentially.H₂O₂ was added by twice（50 mg/L for each time）.The best degradation was obtained with the molar ratio of H₂O₂/Fe²⁺ at 10:1 by the oxidation process.The degradation rate of 4-nitrophenol can reach 88.3% at 150 min.Compared with the ordinary Fenton oxidation method,when the molar ratio of H₂O₂ to Fe²⁺ is relatively high(low Fe²⁺ concentration),the electro-iron-reduction Fenton oxidation method can obtain a faster degradation rate of 4-nitrophenol;when the molar ratio of H₂O₂ to Fe²⁺ is relatively low(high Fe²⁺ concentration),the method can also obtain better degradation than the conventional Fenton oxidation method.（2）In the electro-iron-reduction Fenton oxidation method,the degradation of organic matter mainly depends on Fenton oxidation,and the introduction of electrochemical reduction of Fe³⁺ was to regenerate Fe²⁺.The graphite cathode modified by carbon nanotubes has a better property in reducing Fe³⁺.The electrochemical action was carried out at low voltage and anode potential.Therefore,the effect of anodic oxidation on the degradation of organic matter can be neglected.（3）The actual high-salt dye wastewater is used as the degradation target.When the amount of ferrous salt is 50% of the conventional Fenton oxidation method,the electro-iron-reduction Fenton oxidation presented the better degradation performance than the conventional Fenton oxidation method.The electro-iron-reduction oxidation method could effectively reduce the amount of ferrous salt by the regeneration of Fe²⁺,and the COD of the dye wastewater was significantly decreased,and the color becomes clear after degradation.The reaction process was carried out successively according to 45 min of Fenton oxidation,30 min of electrochemical reduction（the power supply voltage is 1.2 V）and 45 min of Fenton oxidation.H₂O₂ was added by twice（25000 mg/L for each time）.The results showed that the optimum ratio of H₂O₂ to Fe²⁺ is 12:1.Under this condition,the COD removal of 75.0% rate was achieved,showing the better degradation of organic matters.At the end of the treatment,the residual concentration of H₂O₂ is 1495.0 mg/L,indicating the Fenton reaction was carried out completely.While the optimum H₂O₂ and Fe²⁺ molar ratio for conventional Fenton oxidation is 6:1,presenting the COD removal rate of 68.0%,and the H₂O₂ residual concentration of 38.3 mg/L at the end of treatment.（4）Different molar ratios of H₂O₂ and Fe²⁺ have significant effect on Fenton oxidation and electro-iron-reduction Fenton oxidation.If the molar ratio is too high or too low,the Fenton reaction is unfavorable.When the molar ratio is too high,the reaction rate is slow due to the insufficient amount of Fe²⁺,and the degradation effect of organic matter is poor,while the low molar ratio is not beneficial to the effective utilization of H₂O₂.（5）More Fe²⁺ can be regenerated to participate in the Fenton reaction when extending the electroreduction time to 60 min.In this case,the molar ratio of H₂O₂/Fe²⁺ at 12:1 presented the COD removal rate of 78.4%.After the first-stage Fenton reaction,the Fe²⁺ decreased from the initial 6588.2 mg/L to 103.8 mg/L,and the Fe²⁺ concentration increased to 600.0 mg/L after 60 min electrochemical reduction;the H₂O₂ residual concentration at the end of the treatment was about 620.0 mg/L,and the Fenton reaction is more thorough.To sum up,compared with the conventional Fenton oxidation method,the electro-iron-reduction Fenton oxidation method can obtain the better organic matter degradation under the condition of lower Fe²⁺ dosage.This method can not only reduce the dosage of reagents,but also reduce the amount of iron sludge in the subsequent treatment process.The batch addition of H₂O₂ and avoiding the anodic oxidation of H₂O₂ are beneficial to improve its utilization efficiency.Therefore,the method has good application prospects in the field of liquid organic wastes treatment.