Part Metal-catalyzed Double Oxidation System For Efficient Treatment Of Oilfield Contaminants

A variety of pollutants are present in oilfield production wastewater,such as polymers,organic compounds and heavy metals.Most of these pollutants are stable and complex polymers with benzene rings and are therefore difficult to treat.Conventional single oxidation systems mainly use electricity,light radiation and catalysts in one way to catalyse a single oxidant to produce highly active radicals to degrade pollutants.This advanced oxidation method degrades with limited oxidative degradation performance,narrow p H applicability range,easy loss of metal ions from the catalyst and poor stability.Therefore,in this study,an H₂O₂-Na₂S₂O₈double oxidation system based on a non-homogeneous zero-valent metal catalyst was constructed and applied to oilfield wastewater.The effects of oxidant dosage,p H and other factors on the degradation of oilfield pollutants were investigated,and the activation mechanism of active substances and the degradation mechanism of pollutants in this oxidation system were thoroughly explored.The main research findings are as follows:The Cu⁰catalyst was prepared by chemical reduction to activate the H₂O₂-Na₂S₂O₈system for the degradation of（hydroxypropyl guar gum）HPG.Under the optimal reaction conditions of45℃,p H 7,oxidant（H₂O₂:Na₂S₂O₈）ratio of 2:3,oxidant dosage of 10%and catalyst dosage of10%,the viscosity reduction rate of HPG was 81.00%and COD degradation rate was 61.36%after 25 min of reaction time,and the molecular weight of degraded HPG was reduced from the original 2.0×10⁶to 3.9×10³.These showed that Cu⁰-H₂O₂-Na₂S₂O₈had a good oxidative degradation effect.XRD,BET and SEM characterisation of the Cu⁰catalyst before and after the reaction were carried out.The results indicated that the Cu⁰during the reaction loses electrons to Cu⁺and Cu²⁺and the catalyst appears agglomerated,resulting in a decrease in pore volume and specific surface area after the reaction,thus causing a reduction in its catalytic performance.In the catalyst stability experiments it was found that the Cu⁰performance gradually decreased with increasing number of cycles,and when the catalyst was cycled five times,its viscosity reduction rate decreased to 74.24%.The kinetic study of the degradation of HPG showed that the Cu⁰-H₂O₂-Na₂S₂O₈-catalyzed degradation of HPG followed a quasi-level kinetic equation.In order to improve the stability of the above zero-valent catalysts,this thesis proposes the preparation of highly dispersed zero-valent metal catalysts using Mg-Al LDHs as a carrier to activate the H₂O₂-Na₂S₂O₈system for the degradation of HPG.Under the optimum reaction conditions,the viscosity reduction rate of HPG was 88.18%and the COD degradation rate was65.00%after 25 min of reaction.This shows that Cu⁰@LDHs-H₂O₂-Na₂S₂O₈has good oxidative degradation effect.The viscosity reduction rate was increased by 2.10%and the COD degradation rate was increased by 3.64%compared with Cu⁰-catalyzed degradation of HPG under the same conditions.The catalytic degradation of HPG by Cu⁰@LDHs catalyst was carried out for five cycles,and it was found that the degradation effect of Cu0 catalyst decreased by17.27%for 25 min of reaction.Cu⁰@LDHs The catalytic effect has decreased by 6.15%,the stability of Cu⁰@LDHs was improved substantially.In order to further expand the pore structure of the hydrotalcite-loaded zero-valent metal catalysts and promote the diffusion efficiency of pollutants in the catalyst hollow islands,a row of hydrotalcites was prepared in this thesis using micelles as a template,which was used as a carrier to reduce and prepare a micellar hydrotalcite-loaded zero-valent copper catalyst（Cu⁰@JLDHs）as a catalyst for the degradation of HPG in the H₂O₂-Na₂S₂O₈system.Under the optimal reaction conditions of 45℃,p H 7,oxidant（H₂O₂:Na₂S₂O₈）ratio of 1:4,oxidant dosage of 15%and catalyst dosage of 15%,the viscosity reduction rate of HPG was 89.31%,COD degradation rate was 72.10%and the molecular weight of degraded HPG was reduced from2.0×10⁶to 2.4×10³after the reaction time of 30 min.This indicates that Cu⁰@JLDHs-H₂O₂-Na₂S₂O₈It has good oxidative degradation effect.Under the same conditions Cu⁰@LDHs The catalytic degradation effect of HPG increased the viscosity reduction rate by4.39%and the COD degradation rate by 7.10%.The XRD,BET and SEM of Cu⁰@JLDHs before and after the reaction were characterized,and the results showed that the crystal structures of Cu₂O and Cu⁰did not change significantly during the reaction,and the pore structure of JLDHs had a good protection effect on the active sites,but there was still some agglomeration of the catalyst during the reaction,which reduced the reaction activity of the catalyst.By comparing Cu⁰@JLDHs The catalyst undergoes 5 cycles of catalytic degradation of HPG.The results showed that the reaction lasted for 25 minutes,Cu⁰@LDHs After five cycles of catalyst circulation,the degradation effect decreased by 6.15%,Cu⁰@JLDHs Decreased by 2.19%,Cu⁰@JLDHs Further improvement in stability.In this thesis,the reaction mechanism of the H₂O₂-Na₂S₂O₈double oxidation system was proposed by deter mining the catalyst valence change and crystal structure,the active oxidizing substance and combining the existing pollutant degradation pathways as well as the existing research results.The experimental results show that the effective active substance of the catalyst,elemental Cu,catalyzes the degradation of HPG in the H₂O₂-Na₂S₂O₈system in a cycle:Cu⁰-Cu⁺-Cu²⁺.The burst experiments showed that the activation process of H₂O₂in the study of Fenton system firstly activates to produce·OH,and then converts to·O₂^-,¹O₂through chain reaction.in the double oxidation system also SO₄^-·as the main radical does not directly participate in the oxidative degradation process in its entirety,but as the core radical converts H₂O₂,·OH,etc.to·O₂^-,and ¹O₂through self-dissociation or electron transfer.¹O₂to directly degrade pollutants.By attacking the sites of C-Y or C=Y（Y=O,N,S）groups,i.e.,by means of majorαcleavage causing HPG main branch chain breakage and hetero-ring opening for further mineralization to CO₂and H₂O.dual oxidation system degrades sulfonated lignite（SMC）mainly by oxidation,benzene ring,hetero-aromatic ring,olefins and carboxylic acid esters under the action of SET of SO₄^-·in the persulfate oxidation system,while It also destroys the sulfonated groups of SMC to form black precipitates,thus causing the two-phase separation solution to become clarified.The conversion of SO₄^-·attacking sulfonation groups to SO₄^2-was demonstrated by elemental analysis,UV spectral analysis,and ion analysis.The zero-valent metal-activated double oxidation system constructed in this study can rapidly degrade oilfield pollutants,which provides an idea to solve the reaction activity of catalysts and the mechanism of action of active substances and expands the scope of application of traditional advanced oxidation technology,and also proposes the mechanism of oxidative degradation of pollutants in the process of oilfield wastewater treatment,which is of great significance for the efficient treatment of highly stable and difficult to degrade oilfield wastewater.