Research On Degradation Of Dyeing Wastewaters In Novel Fenton-Like Systems Based On Solid Oxidants

Printing and dyeing wastewater,due to their high toxicity and high COD content,causing serious damage to water and human body.Advanced oxidation technology is used in the oxidative degradation of organic wastewater because it produces strong oxidizing hydroxyl radicals（·OH）.As one of an advanced oxidation technology,Fenton technology is mainly composed of Fe²⁺ and H₂O₂.Its advantages are mild reaction conditions,simple equipment and simple operation,relatively low processing cost and wide range of application.The disadvantage is that the optimum pH range is narrow,only 2～4.In order to improve the applicability of Fenton reaction in a wide range of pH,there are currently studies on improving the catalyst in Fenton,using supported,iron type,modified,non-iron active site type catalysts instead of Fe²⁺.Although these catalysts can effectively broaden the pH of the Fenton reaction,most of the preparation methods are complicated.Therefore,in this paper,SPC and LDHs-H₂O₂ were prepared because their simple preparation methods.They act as oxidant to form a novel Fenton-like system for the oxidative degradation of methylene blue（MB）.The innovative achievements and conclusions obtained are as follows:（1）Sodium percarbonate was prepared by a modified low temperature crystallization method and used to construct Fe²⁺/SPC system.Using MB as the target pollutant,the effect of pH value,the dose of Sodium percarbonate and ferrous sulfate,reaction temperature,concentration of MB was investigated.The results showed that when methylene blue is degraded in SPC/Fe²⁺ system,the pH range can be effectively broadened to 2^-10 and the removal rate of MB can reach more than 97%.The best dosage of SPC and ferrous sulfate are 0.30 g·L^-1 and 0.75 g·L-t,respectively.The degradation and mineralization rate of MB,caculated as 99.6%and 64%,respectively.Fe²⁺/SPC system less affected by temperature.MB concentration can improve the removal rate.（2）The the main active species and their concentrations,the total iron ion and ferrous ion concentrations in the Fe²⁺/SPC system were determined,and the changes of pH in the system at different pH were observed.The results showed that the reaction mechanism is homogenous Fenton process by produce hydroxyl radical in the Fe²⁺/SPC system.The broadening mechanism of pH in Fe²⁺/SPC system was as follows:（1）The pH value of the solution is reduced by the consumption of OH-by Fe²⁺ in the solution and the acid intermediate produced during the reaction process.（2）The buffer effect of CO3/HCO3 can maintain a constant pH.Their synergy keeps pH at 2～5 to achieve the purpose of broadening the pH range.（3）The degradation rate of MB in Fe²⁺/SPC system accorded with the quasi-second-order reaction kinetics model by investigating the reaction rate constant and activation energy of MB degradation in Fe²⁺/SPC system.The reaction rate constant is 64.50×10-3（L·mol·s）^-1,the activation energy is 16.6 kJ·mol^-1.（4）LDHs-H₂O₂ was prepared by modified cryogenic crystallization method with LDHs as the carrier,and the Fe²⁺/LDHs-H₂O₂ system was constructed for the degradation of MB.The effects of pH value,dosage of LDHs-H₂O₂ and ferrous sulfate,temperature on the Fe²⁺/LDHs-H₂O₂ system were studied.The results showed that the system could broaden the initial pH to 2^-10 due to the precipitation of Fe²⁺.acidic intermediates and and the surface alkalinity of LDHs,MB could degrade efficiently in this system,after 10 min,the degradation efficiency was almost 100%.The optimal dosage of LDHs-H₂O₂ and ferrous sulfate were 0.50 g·L^-1 and 0.15 g·L^-1,the removal rate of MB can achieve 100%and the mineralization rate is 75.67%.The temperature has little effect on the system.（5）The reaction mechanism in Fe²⁺/LDHs-H₂O₂ system was clarified by determining the reaction composition in the system and the type and variation of active substances in the system,and determining the concentration of total iron and ferrous iron in the system.The results show that the reaction mechanism of the system is an heterogeneous Fenton reaction that produces hydroxyl radical.（6）The reaction rate constants of degradation of MB in Fe²⁺/LDHs-H₂O₂ system and activation energy have been investigated and found that the degradation of MB in Fe²⁺/LDHs-H₂O₂ system before 5min fits the pseudo-second-order reaction kinetics equation.Under the optimum reaction conditions.the reaction rate constant is 8.28×10-3 L-（mol·s）^-1.The calculated activation energy of the system is 87.40 kJ·mol^-1.（7）The LDHs filtered from the Fenton reaction and H₂O₂ were used to synthesize LDHs-H₂O₂,and used to build the new Fe²⁺/LDHs-H₂O₂ system to degrade MB.The results showed that the removal rate of MB could reach more than 75%,indicating that LDHs-H₂O₂ has a better stability.