Bioresources Of Indium From End-of-Life LCD Panels

With the continuously upgrading of the liquid crystal display panels(LCDs),the production and demand of LCDs have increased greatly.A large number of end-of-life LCD panels have become an inevitable electronic waste due to the development of scientific technology,and improper disposal of LCDs will cause serious pollution and damage to the environment,and the comprehensive resource treatment of end-of-life LCD panels is extremely important.The most valuable rare metal in the LCDs is indium(more than 0.03%).Compared with chemical leaching method,bioleaching has the advantages of simple operation,low energy consumption,less investment and environmental friendliness.However,it faces the problems of long processing period and low extraction rate.This work took the end-of-life LCD panels as the research object and explored a new process of bioleaching indium in LCDs,aiming to improve leaching efficiency of indium.In this study,the active aerobic sludge of the sewage treatment plant was used as the microbial source,and the microbial communities were domesticated in three pathways,which were sulfur-mediated pathway,iron-mediated pathway and mixed pathway respectively.The indium bioleaching efficiency from high to low were as follows:sulfur-mediated>mixed>iron-mediated.Under normal temperature and pressure conditions,only the sulfur-based specific microorganisms can act as a catalyst for the indium leaching process,which can penetrate the deep of the internal structure of the LCDs powder.The rare metal indium in the LCDs powder content of only 0.03%can be completely leached in six days.Sulfur-mediated pathway was a way to efficiently leaching indium.Microbial community analysis showed that the sulfur-based Acidithiobacillus was the dominant functional genus of indium leaching.Acidithiobacillus thiooxidans was equivalent to a catalyst for promoting indium leaching.From this study,we can speculate that the leaching mechanism of indium in sulfur-mediated pathways may involve the following:microorganism can secrete sulfoxidases,which can oxidize inert sulfur to form biosulfuric acid and then release H+,and In2O3 can be partly leached;microorganisms can secrete a large amount of extracellular polymeric substances(EPS)and form a biofilm leaching zone,which will facilitate H+ to penetrate into the interior of the LCD powder for corrosion;microorganisms may break some metal bonds by their enzymes.The multi-faceted leaching effects allowed indium to be fully leached in an efficient and environmentally friendly wayOptimize the conditions of the sulfur-mediated pathway by orthogonal experiments,such as the amount of LCDs powder,initial pH value,pre-culture time of microorganisms and amount of sulfur source addition.The optimized condition is were that 10 g/L,1.5,2 days,and 10 g/L respectively.The results also showed that biochar and yeast extracts had similar effects on the leaching process of indium,and the higher the content of biochar or yeast,the lower the indium leaching rate;the leaching of indium by L-cysteine at a content of 10-2 M promoted post-inhibition,while L-cysteine at a content of about 5×10-3 M completely inhibits the leaching of indium;the content was about 0.01 g/L humic acid can promote indium leaching;the tryptone content of about 1.2 g/L and below can promote indium leaching,and when it was less than 1.2 g/L,the less the amount of tryptone added,the less the promotion of indium leaching.In summary,humic acid and tryptone were the promoting factors in the process of bioleaching indium.The experiment proved that the suitable addition amount of humic acid was 0.05 g/L,and the suitable amount of tryptone was 2.0 g/LCombined parameters optimized by orthogonal experiment with the appropriate amount of organic matter,the indium leaching rate of humic acid and tryptone was studied.The results were as follows:microbial group with addition of tryptone(S+Tr+M)? microbial group with added humic acid(S+HA+M)>microbial group with added tryptone and humic acid(S+Tr+HA+M)>microbiome without added organic matter(S+M).S+Tr+M and S+HA+M can completely leaching the indium in end-of-life LCD panels in four days.The microbial community analysis found that the more the dominant functional genus Acidithiobacillus,the better the leaching effect of indium could not be inferred,and the leaching rate of indium can be improved by different promotion methods,such as Alicyclobacillus disulfidooxidans,although they did not have the function of directly leaching metal,they can promote the leaching of indium by assisting the leaching.The acidophilic fungus Acidomyces acidothermus also can secrete acidetolerant enzymes and extremozymes to further promote the leaching of indium.