Removal Of Cr(?) And Crystal Violet By RGO/TiO₂ Composites And Their Toxicity On Microbe

Dye molecules and heavy metal ions in wastewater often have strong biological toxicity,which may cause serious damage to ecological environment when they are discharged without treatment.Nanoscale TiO₂ can be used to remove these pollutions through photocatalytic reduction of heavy metal ions or photocatalytic oxidation decomposition of organic pollutions due to the strong reducibility/oxidation of photogenerated carriers.Thus,it can reduce the harm of these pollutants to environment.However,TiO₂ usually exhibits low utilization efficiency for light quantum because of the high recombination rate of photogenerated carriers and low light transmittance in high-chroma dye wastewater.In this paper,TiO₂ with special morphology and structure were synthesize and further coupled with graphene as to obtain composite with excellent photocatalytic property.Meanwhile,photobacterium phosphoreum,nitrifying bacteria,and activated sludge were used to evaluate the toxicity of crystal violet and Cr(?)and their photocatalytic products.The results indicate as follows:(1)Reduced graphene oxide modified TiO₂ nanoparticles(rGO/TNPs)and TiO₂nanosheets(rGO/TNSs)can be synthesized via hydrothermal and solvothermal route,respectively.The results of morphological and microstructural characterizations indicate that the crystal size of TNPs is only 5 nanometer and the thickness of TNSs is ca.1.1-1.2 nm.Besides,TiO₂ and rGO were combined through Ti-O-C bonds.In addition,it is found that the BET specific surface areas of rGO/TNPs and rGO/TNSs up to 141m²/g and 382 m²/g,respectively.(2)Photobacterium phosphoreum and nitrifying bacteria can effectively reflect the biotoxicity of Cr(?)and its reduced products.The results of photocatalytic experiments indicate that the photocatalytic activity of TNSs can be improved effectively by graphene,and the synthesized rGO/TNSs exhibit the best photocatalytic activity when the weight percentage of rGO is 7.5%.Besides,low p H value in solution is beneficial to the photocatalytic reduction of Cr(?)ions.After photocatalytic treatment of Cr(?)solution,the inhibition of both photobacterium phosphoreum and nitrifying bacteria in the produced solution was decreased weakened markedly,indicating the decreased of biotoxicity in the solution greatly.(3)Crystal violet even with very low concentration can strongly inhibit the bioluminescence of photobacterium phosphoreum.rGO/TNPs composites exhibit much higher photocatalytic activity than commercial P25 TiO₂ for the photocatalytic degradation of low concentration of crystal violet.Crystal violet molecules in high concentration solution can be removed by rGO/TNSs through adsorption-photocatalytic route due to its large adsorption capacity for crystal violet.In addition,it is found that the inhibition of photobacterium phosphoreum was markedly weakened in the solution after crystal violet was photocatalytic treated.(4)The solution containing both crystal violet and Cr(?)shows higher microbial toxicity than that of single pollutant.It is found that crystal violet molecules and Cr(?)ions in the mixed solution can be removed by rGO/TNSs in one step through photocatalytic degradation,and the product in resulting solution and rGO/TNSs have a little effect on bacteria and fungi in activated sludge.Namely,the photocatalytic products of the mixed solution have small impact on biological environment and rGO/TNSs have good biologically inert.