Study On Adsorption-Photocatalytic Degradation Efficiency And Mechanisms Of Naphthol Blue Black By CeO₂@NiAl-LDHs

The large-scale production and use of dyes provide people with colors,followed by not only the huge economic incentive to the society but also the environmental problems caused by dyeing wastewater.Traditional wastewater treatment technologies for dyeing wastewater are difficult to achieve the desired effect.Adsorption and photocatalytic oxidation,which possess the advantages of high reaction rate,simple operation and green,have unique advantages in the field of dyeing wastewater treatment.Layered double hydroxide（LDHs）,as a novel remediation material,possesses strong adsorption capacity and can effectively remove organic pollutants.The synergistic effect of adsorbent-catalyst can be realized by combining the LDHs and photocatalytic semiconductor material.The semiconductor material CeO₂ was supported on the surface of NiAl-LDHs,which was synthesized by coprecipitation method,to synthesize CeO₂@NiAl-LDHs composite material with high adsorption capacity and photocatalytic activity.Meawhile,the physicochemical structure and UV absorption properties of the composite materials were examined and analyzed.Besides,Naphthol Blue Black（NBB）was selected as the target pollutant,the optimal reaction conditions and the adsorption-photocatalytic behavior of CeO₂@NiAl-LDHs for NBB removal were explored.Conclusively,the adsorption removal mechanism and photocatalytic oxidation path of CeO₂@NiAl-LDHs were investigated by combining characterization methods and chemical calculation.The main conclusions are as follows:（1）CeO₂@NiAl-LDHs with 10%CeO₂ had the optimal property and it showed a curd structure.The material characterization results indicated that the specific surface area of NiAl-LDHs and the utilization of UV light were improved by the blending of CeO₂.Besides,the surface of the materials was close to neutral in aqueous solution,which was beneficial to reduce the electrostatic repulsion between the material and NBB.（2）The saturation adsorption capacity of CeO₂@NiAl-LDHs reached 152.52mg/g when dosage was 0.20 g/L,initial concentration of NBB was 32 mg/L,adsorption time was 6 h and p H=7.It was found that the adsorption of CeO₂@NiAl-LDHs and NiAl-LDHs on NBB could be fitted by Langmuir model,indicating that the adsorption was controlled by the highly selective monolayer adsorption.The adsorption process of NBB by CeO₂@NiAl-LDHs and NiAl-LDHs was more in line with the pseudo-second-order kinetic model,and adsorption occured mainly through chemical forces.The adsorption was a spontaneous endothermic process,and the blending of CeO₂ was beneficial to enhance the spontaneity and force of NiAl-LDHs adsorption on NBB.（3）Under UV light,the reaction system had the best conditions when the catalyst dosage was 0.20 g/L,the initial concentration of NBB was 64 mg/L,the photocatalytic time was 6 h,and the p H=7,the photocatalytic degradation of NBB by CeO₂@NiAl-LDHs was 82.19 mg/g.The photocatalytic kinetics showed that the heterojunction formed between CeO₂ and NiAl-LDHs improved the efficiency of charge separation and transmission,the light absorption performance of NiAl-LDHs also was enhanced due to the incorporation of CeO₂.Therefore,the photocatalytic degradation rate of CeO₂@NiAl-LDHs was significantly higher than that of NiAl-LDHs.（4）Common anions and cations(such as Ca²⁺,Mg²⁺,Fe³⁺,NO₃^-)in water have a certain promoting effect on CeO₂@NiAl-LDHs adsorption-photocatalytic removal of NBB;CeO₂@NiAl-LDHs had good adsorption and photocatalytic activity as well as great stability,the adsorption-photocatalytic capacity of CeO₂@NiAl-LDHs to NBB was still more than 105 mg/g after five cycles.Compared with other high-efficiency composites,CeO₂@NiAl-LDHs not only had excellent efficiency,stability and reusability,but also was economical,thus it was a material with great potential in the dyeing wastewater treatment.（5）The adsorption forces of CeO₂@NiAl-LDHs to NBB mainly included electrostatic attraction,ligand exchange and ion exchange.By analyzing the band structure of CeO₂@NiAl-LDHs and the results of free radical trapping experiment,it was inferred that CeO₂@NiAl-LDHs is a direct Z-type heterojunction.Combined with quantum chemical calculations,it was inferred that the possible degradation pathways of NBB by CeO₂@NiAl-LDHs are as follows:ionization of NBB molecules in water—cleavage of C-S bond in NBB—cleavage of azo bond—cleavage of aromatic ring—oxidative decomposition of small molecule intermediates.