Removal Of Organic Pollution From Water By MgO And Its Composite Materials

With the development of industry,highly toxic organic wastewater is discharged into the environment.They cause serious harm to human and ecosystem,and an efficient solution is necessary.Among many methods,adsorption and photocatalysis are condised as potential applications due to high efficiency and feasibility.The wide application of the adsorbents is limited by high price of carbon-based adsorbents,the difficulty of recycling and non-selectivity.Also,the agglomeration and recycling and recycling of nano-powder photocatalyst limit the practical application of photocatalysis.Magnesium oxide（Mg O）has been used in the field of adsorption and photocatalysis owing to its abundant surface defects,high specific surface area,non-toxicity and low price.However,the photocatalytic ability of wide-gap Mg O（E_g=5.6 e V）subject to light absorption and excitation.To solve the above problems,the selective adsorption of reactive dyes was designed by using Mg O surface defects.Mg O was compounded with carbon material to improve its photocatalytic degradation performance for achieving efficient degradation of non-reactive dyes under visible light.（1）The Mg O nanosheet with abundant oxygen vacancies（F⁺-Mg O）was successfully prepared by high temperature treatment.The obtained F⁺-Mg O exhibits selective adsorption,separation,high cyclic stability,and recovery of reactive dyes from the dyes mixture.The X-ray photoelectron spectroscopy（XPS）,solid state electron paramagnetic resonance（EPR）,photoluminescence spectroscopy（PL）and temperature programmed reduction of hydrogen（H₂-TPR）testify the oxygen vacancies of the obtained F⁺-Mg O.Furthermore,the contrast experiments are used to verify the oxygen vacancy as the adsorption sites.A new closed-loop sustainable method for the separation and recovery of waste dyes based on selective adsorption was designed.（2）The high specific surface area of a photocatalyst is favorable for adsorption and catalysis,but is unfavourable for recycle.The wide band-gap of Mg O results in low photocatalytic performance.Here,the Mg O was grown on carbon fibers by hydrothermal combined with high temperature calcination to achieve simple recovery.The Mg O synthesized by high temperature calcination is with abundant oxygen vacancies in surface,and carbon fibers can effectively expand the absorption range and improve the utilization of light.At the same time,good conductorc of carbon fiber induce the electrons easily transfering to carbon fibers,effectively reducing the recombination rate of photogenerated electrons and holes.The influence of different magnesium salt dosages,hydrothermal temperature and calcination temperature on the synthesis of catalyst was studied.The optimum synthesis conditions were obtained as follows:magnesium salt dosage 7.5 mmol,hydrothermal temperature 180℃,calcination temperature 600℃.The degradation efficiency of 100 mg/L methylene blue solution within 60 min was 97.55%for Mg O/CFs synthesized under the optimal conditions.But the Mg O particles fall off easily from the carbon fibers.（3）In order to overcome the problem of Mg O shedding,Mg O/C nanofibers were prepared by electrostatic spinning.The optimum synthesis conditions were determined as follows:the mass ratio of magnesium citrate to magnesium acetate was 1:4 and the calcination temperature was 600℃.The Mg O prepared under the optimal conditions degraded 91.51%of 200 mg/L methylene blue solution within 50 min.The excellent photocatalytic performance is attributed to the presence of carbon in the material.After four cycles,the photocatalytic activity was still up to 82.33%,showing high cyclic stability.