Study On Indium Leaching Mechanisms And Biorecovery Methods As Nanoparticles And Sustainable Process For Valuable Materials Recovery From Waste Liquid Crystal Displays

Indium is a precious metal resource that is mainly used for producing electrodes in LCDs.At present,the amounts of manufacturing and discarding LCDs are dramatically increasing.Herein,indium recovery from waste LCDs is a necessary tendency according to the demand for sustainable development.Compared with many advanced technologies for indium recovery from waste LCDs,using inorganic acids with high concentration for leaching indium cannot be replaced currently in industrial applications.Besides,purifying indium metal from recovery needs a high cost,but it could be solved using biosynthesis systems of nanoparticles that contain many necessary metal ions for culturing microorganisms.To solve these problems occurring in the recovery process of waste LCDs including the lack of commercial environmental technologies,and the application approaches of crude indium metal,this study explored the method and mechanism of bioleaching indium using Aspergillus niger,verified that oxide acid is the effective component in the fermentation broth,clarified the mechanism of leaching indium using oxide acid,simulated Cd S to build the biosynthesis method of In₂S₃ nanoparticles,furthermore investigated the biosynthesis method and application potential of In₂S₃ quantum dots,and finally formed a recovery process for the treatment and recovery of valuable materials from waste LCDs.The research was conducted as follows.?1?The bioleaching method of indium from waste LCDs was established using Aspergillus niger and its bioleaching mechanism was clarified.The order of indium bioleaching efficiency using Aspergillus niger in different methods was fermentation broth bioleaching>2 steps bioleaching>1 step bioleaching.The influences from decreasing initial p H,shaking speed,and sucrose addition on fermentation of Aspergillus niger were the decrease of p H value in the fermentation broth and the increase of carboxyl concentration from anaerobic respiration process and the oxidation degree of nutrients.Accordingly,when initial p H,shaking speed,and sucrose addition were optimized from 7.0,200 rpm,100 g/L to 4.0,and 125 rpm,50 g/L,respectively,the indium bioleaching efficiency improved from 12.3%to 100%using the fermentation broth from 15^th day under 70?within 90 min.Oxalic acid is the most effective component for the bioleaching of indium using Aspergillus niger comparing with gluconic acid,citric acid,and malic acid.?2?According to the experiment result from bioleaching using Aspergillus niger,the complex reaction mechanism was explained why oxalic acid showed better performance than inorganic acids in the indium leaching system from waste LCDs.The In³⁺leaching efficiency with the powder dosage of 50 g/L came up to 100%within 45 min,using 0.5 M oxalic acid and stewing at 70?.According to Arrhenius model,the apparent activation energy for indium leaching reaction was 43.622 k J/mol,indicating that the indium leaching rate was mainly affected by the ion diffusion process through the solid-liquid interface and the chemical reaction process.Ultrasonic cavitation can improve the powder dosage.In₂O₃ dissolution depended on the distributive H⁺concentration.Metal hydrolysis was a general negative phenomenon for ions loss in the acid leaching system.Oxalic acid can maintain an appropriate H⁺concentration to reduce the acid consumption from nontarget substances and to inhibit metal hydrolysis,and its C₂O₄^2-ions also could precipitate some metal impurities such as Ca²⁺and Sr²⁺in the leachate.?3?To synthesize a high-value product using crude indium metal,the biorecovery system of indium as In₂S₃ nanoparticles was established through precipitation method using Escherichia coli.The biosynthesis parameters of nanoparticles were based on the simulation of Cd S nanoparticles.Engineered Escherichia coli metabolized cysteine to produce S^2-and precipitate In³⁺that was adsorbed on the protein surface for stable distribution as In₂S₃nanoparticles.The average particle size of In₂S₃ nanoparticles was about 8 nm,its absorption peak of light was 303 nm,and its band gap width was 3.444 e V.The quantum restriction effect caused by its small size was obvious that can be used for catalytic degradation of pollutants.?4?Furthermore,the biorecovery system of indium as In₂S₃ quantum dots using Enterococcus faecalis was explored with its potential application.Based on the metabolic detoxification effect of bacteria on heavy metals in a culture medium,In₂?SO₄?was indium source,and S^2-was slowly produced by endogenous metabolism of Enterococcus faecalis,and formed quantum dots in situ precipitation with In³⁺that was absorbed for its distribution in the system.The abundant organic matter on the surface of quantum dots made it stable in the aqueous phase.The In₂S₃ quantum dots were a spherical polycrystalline structure of 3?5 nm.There were two fluorescence peaks in the samples,which are located at 393 nm and 460 nm respectively,and the fluorescence was obvious in bacteria.The appearance of samples after dialysis and purification was not significantly different from water,which lay a theoretical foundation for the application of biosynthetic In₂S₃ quantum dots for biomarker and fluorescence detection.?5?Combined with the above results,a low-cost,high-value,and green process was built for the recovery of valuable materials from waste LCDs.The valuable components in waste LCDs are polarizer and liquid crystal,indium,and glass substrate.The method of layer peeling of polarizer was established to shorten the soaking time of acetone to less than 30 min,and polarizer and liquid crystal were simultaneously recovered with reuse property.The limiting factors were investigated for the bioleaching of indium,and to develop a biological simulation technology using oxalic acid with a low concentration,response surface method for parameter optimization could significantly improve batch processing capacity and leaching efficiency.The glass from leaching residues of waste LCDs was analyzed for its reuse.Besides,the feasibility and applicability were also analyzed for the recovery of crude indium metal as nanoparticles.Finally,the research proposed a green process for the recovery of valuable materials.The achievements obtained from this present thesis research results lay a theoretical and technical foundation for the leaching and recycling of indium from waste LCDs and provide a guiding scheme for the recycling of valuable materials from waste LCDs.