Preparation Of Magnesium Oxide-Based Photocatalyst And Its Degradation Performance For Organic Dyes

Wastewater containing organic dyes poses a series of threats to public health and ecological systems because of their toxicity and biological accumulation.To date,various biological,physical and chemical protocols have been developed to handle dye-contaminated effluents.Among them,physical adsorption has demonstrated superior performance in removal of organic dyes from wastewater,but it generally requires an extra and tedious treatment after adsorption for regeneration of used adsorbents.Photocatalytic degradation is one of the most promising techniques for organic dyes and has received considerable attention in recent years due to its simplicity,cost-effectiveness,and environmental benign.Catalyst is the core of photocatalytic degradation,and its performance is closely related to the final degradation efficiency.Therefore,it is necessary to develop some approaches for preparation of efficient photocatalysts with simple procedures and low cost,and also the developed catalysts could be recycled for degrading organic dyes.In the current dissertation,a series of MgO-based photocatalysts with high efficiency for the degradation of organic dyes have been prepared via simple precipitation,and their physicochemical properties and morphology evolution have been systematically characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）,nitrogen adsorption and others.Much attentions have been focused on the influence of placing location in a muffle furnace during the calcination of MgO precursors at a high temperature on the adsorption performance of generated MgO.The catalytic performances of rose flower-like MgO and spherical-like Mg_0.8Zn_0.2O to degrade organic dyes under the irradiation of both UV and visible light have been studied in detail,and the related catalytic mechanisms have also been elucidated.The detailed contents are described as below.（1）Two types of MgO precursors were prepared by precipitation using Mg（NO₃）₂ and Na₂CO₃ as raw materials in the absence and presence of trace Na₂SiO₃ as inorganic additives,respectively.It was found that the placing location of MgO precursors in a muffle furnace during high temperature calcination had a pronounced effect on the final performance of generated MgO to the adsorption of Congo red in aqueous solution.For the MgO without Na₂SiO₃,its adsorption capacity demonstrated a gradual increasing trend with variation in the placing location from the most outer to the most inner position in used furnace.For the system with Na₂SiO₃,when MgO was located at the most outer and the most inner position in used furnace,both gave comparable efficiency to adsorb Congo red although the outer position illustrated a little bit superior effect to that from the inner one.As MgO was located in the middle of the furnace,its adsorption performance was the worst.After measuring the real temperature in used muffle furnace,the results illustrated that for MgO without Na₂SiO₃,its adsorption performance was closely related to the real temperature of used furnace.A higher temperature would result in a stronger adsorption capacity.For MgO with Na₂SiO₃,no close relationship was found between both.After charactering the physicochemical properties of generated MgO,it was found that for the system with Na₂SiO₃,the adsorption performance of MgO was closely related to its surface basicity.A stronger surface basicity would result in a stronger adsorption performance to Congo red.For the system without Na₂SiO₃,no correlation was observed between its surface basicity and adsorption performance.The results from FT-IR spectra indicated for the MgO particles without Na₂SiO₃,their adsorption performance was closely related to the adsorption bands of CO₃^2-stretching vibration with varying the placed location in calcination.The more intensive the bands occurred,the weaker the adsorption performance of generated MgO.On the contrary,for the MgO with Na₂SiO₃,the adsorption capacity of generated MgO from different locations was highly dependent on the intensity of the O–H stretching vibration in adsorbed H₂O and the stretching vibration of adsorbed or residual CO₃^2-over the MgO surface.The more abundant those bands,the stronger the adsorption performance of resulting MgO particles.（2）Rose flower-like MgO microparticles with excellent photocatalytic performance in degradation of various organic dyes were synthesized by a facile precipitation method via the reaction between Mg（NO₃）₂ and Na₂CO₃ at 70 ^oC.The morphology and component evolution of their precursors were studied in detail.It was found that agglomerates or rod-like particles with a formula of xMgCO₃·H₂O（x=0.75⁰.77 and y=1.87¹.96）were favorably formed after the initial mixture of the reactants.Owing to the chemical instability,they would turn into rose flower-like particles,which had a composition of xMgCO₃·Mg（OH）₂·zH₂O（x=0.84⁰.86,y=0.13⁰.23,and z=0.77¹.15）.After investigating the photocatalytic performance of obtained rose flower-like MgO,the results demonstrated that upon the UV light irradiation,various organic dyes with concentrations of 100 mg L^-1（e.g.,methylene blue,Congo red,thymol blue,bromothymol blue,eriochrome black T,and their mixture）could be thoroughly degraded in 90 min.Its performance was not only better than other recently developed photocatalysts（e.g.,N-doped TiO₂,P25 TiO₂,ZnO,WO₃,α-Fe₂O₃,BiVO₄ and g-C₃N₄）,but also superior to other morphologies of MgO particles（e.g.,nest-like,spherical,rod-like and trapezoidal products）.Various physicochemical characterization suggested that the excellent photocatalytic performance of rose flower-like MgO particles could be attributed to their stronger adsorption capacity to organic dyes in aqueous solution,which favored to the interactions between organic dyes and MgO and thus led to the degradation of studied dyes.Also,the developed MgO could be used for five cycles by maintaining its efficiency above 92.2%after the fifth cycle.（3）Spherical-like Mg_0.8Zn_0.2O microparticles composed of sheet-like structures were synthesized through the precipitation reaction among Mg（NO₃）₂,Zn（NO₃）₂ and Na₂CO₃ at 75 ^oC.The results demonstrated that as-prepared Mg_0.8Zn_0.2O could not only degrade various organic dyes with concentrations of300 mg L^-1（e.g.,methylene blue,Congo red,thymol blue and others）upon the UV light irradiation,but also degrade thoroughly 100 mg L^-1 of the above dye solutions in 90 min upon the visible light irradiation.The current work also illustrated the photocatalytic performance of Mg_0.8Zn_0.2O was better than the recently developed photocatalyts such as N-developed TiO₂,P25 TiO₂,ZnO and others in the degradation of different organic dyes upon both the UV light and the visible light irradiation,and its performance could be still maintained 98.3%after six recycles.We also found that the performance of developed Mg_0.8Zn_0.2O was much better when the solution of organic dyes possessed a lower（e.g.,pH=2）or a higher pH（e.g.,pH=12）value,whereas as the pH of studied solution was in the range of 6–8,the product demonstrated an inferior catalytic performance.For other catalysts such as MgO and ZnO,their catalytic performances demonstrated a gradual increasing trend with increase in the pH value,which suggested that the currently developed Mg_0.8Zn_0.2O was a superior catalyst for degradation of organic dyes in a strong acidic or alkaline solution.After studying the catalytic mechanism of Mg_0.8Zn_0.2O,it showed that upon the UV light irradiation,hydroxyl radicals（·OH）and superoxide radical anions（O₂·^-）played a leading role in degradation of organic dyes,whereas photogenerated holes（h⁺）had a little effect on the degradation.Upon the visible light irradiation,photogenerated holes（h⁺）and superoxide radical anions（O₂·^-）would prevent the catalytic degradation,and,on the contrary,hydroxyl radicals（·OH）promoted the thorough degradation of organic dyes in aqueous solution.