Effects Of Aging On Environmental Behavior And Toxic Effects Of PbCrO₄ Pigments In Microplastics

As we all know,plastics widely exist in the natural environment.Due to various factors,plastics shatter into fragments and particles less than 5 mm in diameter,which are called microplastics.In the manufacturing process of plastic products,various chemical additives are often added to the base polymer to improve the polymer’s function or aging performance by providing heat resistance,oxidation resistance,and microbial degradation.These plastic additives such as pigments,plasticizers,stabilizers,flame retardants and antioxidants are released into the environment,causing great harm to the ecology,such as water bodies,plants,soil,microorganisms,etc.The main conclusions of this study are as follows:This article mainly takes low-density polyethylene microplastics containing lead chromate pigment as the research object.Mainly through a series of technical means such as micro-Raman,infrared spectroscopy,specific surface area（BET）,X-ray photoelectron spectroscopy（XPS）and scanning electron microscopy combined energy spectroscopy（SEM-EDS）,etc.PbCrO₄ microplastics with different aging cycles Perform characterization;study the leaching behavior of metal ionsCrand Pb in PbCrO₄ microplastics with different aging cycles;evaluate the toxic effect of microplastics leaching solution on Microcystis aeruginosa in freshwater algae;plant corn with soil containing PbCrO₄ microplastics;explore different Toxicity mechanism of microplastics in soil during aging cycle;analysis of the release rule of metal ions in soil and the degree of uptake by corn;the adsorption behavior of PbCrO₄ microplastics to Synechocystis extracellular polymer（EPS）as well as the fluorescence quenching titration of Ca²⁺to EPS,in the process of simulating Ca CO₃ crystal morphology,the concentration changes of metal ions Cr,Pb,and Ca²⁺are explored;during the mixed culture of microplastics and Synechocystis,the concentrations of the above ions and the Algae activity index parameters Algae cell growth density(OD₆₈₀),solution p H and PSII maximum quantum efficiency（F_v/F_m）,etc.Micro Plastics and clarify the mechanism of toxicity of Synechocystis additives.The main conclusions of this study are as follows:（1）From the elemental analysis and particle size distribution,it can be seen that the microplastics are fragmented during the aging process,and the oxidation reaction occurs on the surface of the polymer.As the aging time increases,the characteristic peak intensity of the Raman spectrum gradually increases.In infrared spectroscopy,the longer the aging time of the microplastics,the stronger the peak intensity of the carbonyl group,which indicates that the surface of the microplastics is oxidized to generate carbonyl functional groups.From the analysis of BET,SEM-EDS and XPS of PbCrO₄ microplastics,it can be seen that with the increase of aging time,the total color difference and specific surface area of microplastics show an increasing trend.Hexavalent chromium Cr（VI）is the main form of chromium in microplastics.The lead state in microplastics aged between 4 weeks is between Pb（II）and Pb（IV）,and the flat and smooth surface of microplastics after aging Gradually become rough and fragmented.（2）The aged PbCrO₄ microplastics release moreCrand Pb ions than unaged microplastics.Compared with neutral and alkaline environments,acidic conditions are conducive to the leaching ofCrand Pb.In addition,the use of high-concentration sodium chloride solution as the leaching solution also facilitates the leaching ofCrand Pb.Toxicity analysis results show that,due to the presence of PbCrO₄ in the leaching solution,only high concentration（>10μg g-1）of the leaching solution can significantly inhibit the photosynthesis of Microcystis aeruginosa cells.The leaching solution of aged microplastics has a strong inhibitory effect on the growth and photosynthesis of algal cells,and with the increase of the concentration of the leaching solution,the inhibitory effect is significantly enhanced.Prolonging the aging time will cause the microplastics to release more PbCrO₄ pigments,thereby causing higher toxicity to microalgae.（3）PbCrO₄ microplastics have certain adsorption capacity for Synechocystis EPS,and the aging microplastics have stronger adsorption capacity for EPS.Analysis of the Stern-Volmer fitting equation for the complexation of Ca²⁺with EPS,the modified Stern-Volmer fitting equation and the modified Hill fitting equation show that Ca²⁺has a strong complexation with peak A（humic acids）,but weaker with peak B（tryptophan）,moreover,Ca²⁺can be combined with multiple sites of tryptophan-containing substances,and only one type of humic substances participates in the complexation of Ca²⁺.The calcification of PbCrO₄ microplastics in Synechocystis biofilm EPS affects the leaching behavior ofCrand Pb,as follows:microplastics aged for 4 weeks promote the formation of Ca CO₃ precipitates in biofilm EPS,resulting in a lower Ca²⁺concentration in the algal fluid;Within 48 h,as the EPS concentration increased,theCrand Pb ion concentrations in the algal fluid gradually decreased;during the cultivation of Synechocystis,it can be seen from the changes in parameters such as OD₆₈₀ and F_v/F_m that high concentrations of Ca²⁺are not conducive to this growth of algae;during the cultivation process,the Ca²⁺concentration decreases with the reaction time,and when aging microplastics exist,the rate of decrease of Ca²⁺concentration is faster than that of unaged microplastics.（4）Three-dimensional fluorescence spectroscopy analysis showed that the fluorescence peaks of humic acid and fulvic acid in rhizosphere soil were stronger than those in non-rhizosphere soil.During corn growth,microplastics released moreCrin rhizosphere soil than in non-rhizosphere soil.PbCrO₄ also has a fluorescence quenching effect on soil DOM.The binding effect of DOM and PbCrO₄ in rhizosphere soil is higher than that in non-rhizosphere soil.As corn grows,PbCrO₄ microplastics accumulate in the corn.Through research,we found that the concentration ofCrand Pb in the underground part of corn is much higher than that in the above-ground part.The underground part of corn is also enriched in roots and stems.