Synthesis Of TiO₂@Glucose Carbon Spheres?GCs? And Their Performance Evaluation For The Treatment Of Dye Effluents

Organic dyes are widely and frequently used in various industries as textile,printing and paper.During the manufacturing and dyeing process,a substantial amount of dyestuff is lost into water.The presence of these toxic and colorful organic compounds can reduce light penetration into water,affect photosynthesis of aquatic lives,create toxicity to fish and accumulate in the food chain,causing harm to the environment and human health.Thus,it is of great significance to develop cost effective and efficient technologies to remove dyes from aquatic environments.Adsorption has been proved to be an efficient wastewater treatment process because of its simplicity of operation,low costs,high flexibility,and insensitivity to toxic pollutants.But the adsorption process just accumulates the toxic organic contaminants from contaminated streams onto a solid material without causing damage to the molecules.Thus,secondary pollution may happen if the adsorbents are released into the environment.Moreover,the adsorbent can get saturated easily in the adsorption process,which requires extra complex regeneration or complete replacement.In recent years,heterogeneous photocatalysis has been an attractive research and development subject for water treatment owing to its high efficiency,proven capability to degrade recalcitrant organic contaminants and no secondary pollution.The irradiation of semiconductor with photos of energy equal to or higher than that if the band gap results in the excitation of electrons from the valence band?vb?to the conduction band?cb?,producing electrons in the conduction band and holes at the valence band edge.The electrons and holes can react with the adsorbed O2,H2 O,and surface hydroxide group to generate the highly reactive hydroxyl radicals??OH?and superoxide ions??O2-?,which can oxidize the organic compounds adsorbed onto or very close to the semiconductor,resulting in degradation of dye molecules.Hence,integrating photocatalysis and adsorption process can offer a good solution to overcome the shortcomings of a single adsorption method.On one hand,as the support material for the photocatalyst,adsorbent can increase the photodegradation rate by progressively allowing an increased concentration of dye molecules to come in contact with the semiconductor nanoparticles through means of adsorption.On the other hand,the photocatalyst can prevent the adsorbents from getting saturated by destroying dyes through photocatalytic oxidation,thus regenerating the adsorbent in situ.Therefore,in this paper,the author synthesized glucose carbon spheres?GCs?through a facile hydrothermal method using glucose as the carbon source.Then,GCs were used as the support material for TiO₂ to prepare regenerable TiO₂@GCs composite absorbent.By various of characterization,the crystalline morphology,crystalline structure,elemental composition,optical properties and the reusability of GCs and TiO₂@GCs were investigated.Meanwhile,their adsorption performance of four organic gyes?Malachite Green,Safranine T,Acid Fuchsine and Direct Deep Blue?were studied by the batch mode and fixed bed operations.The main conclusions are as follows:1.Hydrothermal synthesis of GCs and their adsorption performance for dyes?1?GCs were prepared through a facile one-step hydrothermal method using glucose as the carbon source.The morphology,surface functional groups and pore structure were characterized.The results indicate that GCs with regular spherical shapes and smooth outer surfaces were successfully formed with diameters between 0.5 and 1.0 ?m;Aromatization of glucose occurred during the hydrothermal process,and that –OH and C=O were the major oxygen-containing groups on the GCs surfaces;GCs are proved to be mesoporous material with the surface area and average pore size of 16.7 m2/g and 11 nm,respectively.Effects of initial dye concentration,GCs dosage,contact time and temperature on adsorption performance of GCs for Malachite Green?MG?were investigated.The results show that temperature caused little impact on adsorption,and the equilibrium adsorption quantities increased with increasing initial MG concentration,while increased first and then decreased with increasing GCs dosage.The kinetic data of MG adsorption fitted pseudo-second-order kinetic model well and the equilibrium data were best represented by Langmuir model.Thermodynamic analysis indicates that the MG adsorption onto GCs is spontaneous and endothermic.?2?The adsorption of Safranine T from aqueous solution onto GCs was investigated in the up-flow fixed-bed columns.The effects of bed depth,influent dye concentration and p H values were investigated.Breakthrough time and adsorption capacity of the GCs fixed-bed increased with increasing bed depth and solution p H values,whereas decreased with the increase in initial dye concentration and flow rate.The experimental data were in good agreement with BDST,Thomas and Yoon-Nelson models.The employed GCs bed saturated with Safranine T was regenerated through an acid cleaning process.As predicted,the adsorption–regeneration process can be conducted for at most five cycles until the GCs bed was exhausted.2.Controllable synthesis of regenerable TiO₂@glucose carbon microspheres?TiO₂@GCs?and adsorptive performance for dye removal?1?Hybrid TiO₂@GCs with controllable structures were prepared through a facile one step hydrothermal method using titanium tetrachloride?TiCl₄?as Ti source and GCs as support material.The morphology,element composition,crystalline structure,surface functional compositions,light response ability and pore structure were characterized,and a reasonable mechanism for the formation of TiO₂@GCs hybrid microspheres was also proposed.The results show that GCs can provide a good interface for guest nanoparticles,and the morphology of the as-synthesized TiO₂@GCs hybrid composites could be facilely controlled by varying the amount of TiCl₄.TiO₂@GCs exhibit an anatase phase of TiO₂,and GCs favor the formation of TiO₂ with smaller crystalline sizes and avoid TiO₂ nanoparticles from agglomeration.Compared with the pristine TiO₂ nanoparticles,TiO₂@GCs composites exhibit higher overall light response.The specific surface areas of TiO₂@GCs-1.0,TiO₂@GCs-2.5 and TiO₂@GCs-5.0 are 18.2,34.7 and 38.4 m2/g,and the average pore sizes are 11.9,6.3 and 6.5 nm respectively,indicating a mesoporous structure.Similar as GCs,–OH and C=O were the major oxygen-containing groups on the TiO₂@GCs surface.These functional groups provide ideal binding sites for the anchoring of the Ti?OH?x?4-x?+ species,which will form TiO₂ crystalline during the hydrothermal process.Adsorptive performance of TiO₂@GCs composites for acid fuchsin?AF?were evaluated,and the results show,after 2 h dark adsorption,the AF removal efficiency for the GCs,TiO₂@GCs-1.0,TiO₂@GCs-2.5 and TiO₂@GCs-5.0 composites was 30.7 %,40 %,55 % and 65 % respectively,while reached 45 %,85.2%,97.4% and 98.3% after in situ photocataytic regeneration,indicating UV illumination can effectively regenerate TiO₂@GCs composites.?2?TiO₂@GCs were prepared at different p H values via a facile one step hydrothermal method.The results indicate TiO₂@GCs synthesized at p H of 2.0 exhibit the best morphology,TiO₂ distribution and the largest specific surface area.Response surface methodology?RSM?was conducted to optimize the adsorptive removal of acid fuchsine.The results show initial AF concentration?C0?is the most important factor,followed by p H and TiO₂@GCs dosage?m?,and the optimal condition of adsorption for AF is p H=6.0,m= 0.23 g/L,C0=30.0 mg/L.Regeneration experiments show AF-loaded TiO₂@GCs can be efficiently regenerated by UV irradiation.?3?The adsorptive elimination of direct deep blue L-3RB?DDB?from aqueous solution through TiO₂@GCs was investigated in the up-flow fixed-bed columns.The effects of bed depth,influent dye concentration and p H values were investigated.Breakthrough time and adsorption capacity of the TiO₂@GCs fixed-bed increased with increasing bed depth,whereas decreased with the increase in initial dye concentration,p H and flow rate.Both Thomas and Yoon-Nelson models fitted the experimental data well.The DDB loaded TiO₂@GCs bed can be regenerated by simple UV irradiation,and ?OH and ?O2-are the main oxidative species dominate the degradation of DDB molecules.