Preparation Of Ag₃PO₄ Based Catalysts And Their Applications In Pharmaceutical Wastewater Treatment

Due to the variety of the pharmaceutical products and the production scale,there are three kinds of pharmaceutical wastewater:（1）Biopharmaceutical wastewater,such as antibiotic wastewater:strong fluctuation in quantity,low C/N,high concentration of SS and sulfate,biological toxicity and high chroma;（2）Chemical pharmacy:lack of nutrition,hard to biodegrade,toxicity,high salt content;（3）Chinese patent medicine:containing sugar,glycosides,organic pigment,anthraquinone,tannins,Alkali content,cellulose,lignin and other organic matter.Pharmaceutical wastewater are discharged into water outside,they stay there for a long time and difficult to decompose.Most of pharmaceutical wastewater is highly toxic,could accumulate and enrich through the food chain,and finally harm to human health.There are three types of technologies for treating pharmaceutical wastewater,including physicochemical,chemical and biological process.Among them,the cost of adsorption are high,and likely to cause secondary pollution to the environment;Meanwhile,biological degradation take a long time,and is hard to bio-acclimate;Therefore,the photocatalytic oxidation can degrade wastewater quickly,which is reusable with no secondary pollution,non-poisonous,non-corrosiveness,in so many ways much better than another two methods.It has a widely applicable perspective in green-environmental protection.As a visible light driven semiconductor photocatalyst,Ag₃PO₄ has magnificent photooxidation capacity for water splitting and excellent performance for the decomposition of organic compounds.Using AgNO₃ as an electron capture agent,the quantum efficiency for water splitting under visible light can reach up to 90%,which is nowhere close in comparison by traditional photocatalysts such as TiO₂.In addition,the slight solubility and photocorrosion of Ag₃PO₄,leads to the unstable structure and decreased photocatalytic performance.Hence,this paper started with Ag₃PO₄ as the basic material,got three types of Ag₃PO₄-based photocatalysts.An enhanced photocatalytic performance and stability were obtained by Ag₃PO₄-based materials,which relieved the photocorrosion,solved the low-recycle-rate problem,and the possible routes and mechanics for degradation of tetracycline（TC）.Furthermore,we have investigated the selectivity of drugs and catalysts through treating the simulating pharmaceutical wastewater and pharmaceutical wastewater by three Ag₃PO₄-based photocatalysts.The results were recorded follows:（1）A nano-composite comprised of Ti³⁺-doped TiO₂ nanotubes（TNTs）and Ag₃PO₄quantum dots was synthesized by a two-step method.The effects of preparation and degradation conditions of catalyst on the photocatalytic degradation of TC were investigated.the active radical and degradation mechanism in the degradation process were explored.The results showed that:with NaBH₄ as a reducing agent,Ti³⁺was introduced into TNTs,this significantly enhanced the adsorption capability and the absorption of visible light of TNTs.Then,Ag₃PO₄ quantum dots depositing on TNTs were synthesized via a precipitation method with the average particle size of about 5 nm.Under visible light,about 90%of TC was removed in 8 min with the optimal conditions:the mass ratio of Ti³⁺-TNTs and Ag₃PO₄ was1:3,reduced temperature was 300 ^oC,C_CT=5.0 mg?L^-1,the dosage of catalysts were 0.50g?L^-1,pH=3.The stability of the nano-composite was examined by consecutively carrying out four round photocatalytic tests,with the removed of TC from 90（once used）to 80%（forth used）,which was more stable compared with pure Ag₃PO₄.In addition,under the existence of Ag⁰,highly efficient separation of electrons and holes resulted from the photocatalytic mechanism of‘‘Z”scheme have obtained,and which improve the photocatalytic performance of TC with a shorter time.（2）With the presence of AgCl could prevent Ag₃PO₄ from dissolution due to the smaller solubility of AgCl,and Fe（III）as a promoter,could promote the transfer and the separation of photoexcited electrons and holes.Herein,we have prepared a ternary composite of Ag₃PO₄/AgCl/Fe（III）through an in-situ anion-exchange and impregnation method by using ferric chloride as the sources of chlorine and iron,and investigated the effects of preparation conditions of catalyst on the photocatalytic degradation of TC,then,explored the active radical and degradation mechanism in the degradation process.The characterization results indicated that AgCl and Fe（III）were successfully loaded onto the surface of Ag₃PO₄ without any changes in its morphology and crystal structure.The removed of TC was 77%in 6 min with the optimum condition:C_TC=20.0 mg?L^-1,the dosage of Ag₃PO₄/AgCl/Fe（III）was 0.50 g?L^-1,pH=5.16,meanwhile,the degradation rate over Ag₃PO₄/AgCl/Fe（III）(0.4580 min^-1)was 2.3 times and 2.9 times that of Ag₃PO₄/AgCl and the pure Ag₃PO₄,respectively.With the removed of TC from 77（once used）to 70%（fifth used）,Ag₃PO₄/AgCl/Fe（III）was more stable compared with pure Ag₃PO₄.On the other hand,the presence of AgCl could prevent Ag₃PO₄ from dissolution due to the smaller solubility of AgCl,and the heterostructure and Fe（III）in Ag₃PO₄/AgCl/Fe（III）could promote the transfer and the separation of photoexcited electrons and holes,thus enhancing the photocatalytic activity.（3）It is inconvenient for subsequent recycling due to the recovery of Ag₃PO₄ is only 25%after photocatalytic degradation.In this paper,a new method was proposed to improve the photocatalytic performance and solved the problem of recycling simultaneously.SnFe₂O₄were prepared by hydrothermal（HT）and solvothermal methods（ST）,and adsorbed the positive Ag⁺due to its negative charge in the alkaline silver ammonia solution,and then,the SnFe₂O₄/Ag₃PO₄（Sn/Ag）composite photocatalyst was further prepared by precipitation method.The effect of different doping ratio on the degradation of TC and its photocatalytic mechanism were investigated.The degradation efficiency of TC was 80%in 8 min with the optimum condition:C_TC=10.0 mg?L^-1,the dosage of Sn/Ag₌1:5（ST）was 0.50 g?L^-1,pH=5.16,meanwhile,the degradation rate over Sn/Ag₌1:5（ST）(0.3088 min^-1)was 1.5 times and2 times that of Sn/Ag=1:5（HT）and the pure Ag₃PO₄,respectively.The degradation efficiency of TC by Sn/Ag=1:5（HT）from 80（once used）to 62%（fifth used）,which was more stable compared with pure Ag₃PO₄.After each experiment,the catalyst could be separated rapidly by an external magnetic field（95%）,with little loss of Ag₃PO₄.The heterojunction structure between SnFe₂O₄ and Ag₃PO₄ promotes the effective separation of photogenerated electrons and holes,and the high conduction band position of SnFe₂O₄opened the limitation of the formation of free radicals in the photocatalytic process of Ag₃PO₄.（4）Mentioned above,three different types of Ag₃PO₄-based photocatalysts were used to treat the simulated pharmaceutical wastewater and the actual pharmaceutical wastewater,then,compared of the photocatalytic performance and drug selectivity of three types of Ag₃PO₄-based catalysts in the treatment of pharmaceutical wastewater.The order for degradation of three drugs by three types Ag₃PO₄-based catalysts was:amoxicillin（AMO）>tetracycline（TC）>paracetamol（ACE）.Among them,the photocatalytic performance of simulated pharmaceutical wastewater by Ag₃PO₄/AgCl/Fe（III）was better than that of the other two catalysts,which due to the directly capture the photogenerated electrons by Fe（III）.According to the results of single and simulated pharmaceutical wastewater treatment,the degradation performance of drugs with different molecular structure was not alike,which may be related to the adsorption of pollutant and its own activity.In the decomposition process,each drug has a certain competitive effect in mixed simulated pharmaceutical wastewater.This paper presented three types of Ag₃PO₄ based catalysts,which alleviated the photocorrosion and recovery of Ag₃PO₄,and procides some new ideas for the pharmaceutical wastewater treatment by photocatalysis technology.