Interaction And Degradation Mechanism Of Microplastics And Tetracycline In Microbial Electric Field

Plastic products are widely used in human daily life and industrial production because of their advantages of high strength,strong stability,light quality,strong corrosion resistance and low price.However,due to the low level of recycling and waste management,plastic waste remains in the environment in large quantities,resulting in a series of serious environmental problems.Bioelectric currents exist widely in various environmental media,especially in flooded paddy soils.The REDOX potential difference caused by such currents and the enrichment of electroactive microorganisms can affect the environmental behavior of microplastics(MPs)in soil.MPs usually have a large specific surface area and strong surface hydrophobicity,showing a high affinity for a variety of pollutants in the environment,especially for polar and highly hydrophobic tetracycline(TC).MPs in the environment can absorb organic pollutants in the environment through electrostatic,hydrophobic or hydrogen bonding,and even affect the chemical stability of organic pollutants.Therefore,this paper studied the effect of microbial electric field on degradation behavior of MPs in microbial electrochemical systems(MES),and further explored the interaction mechanism between MPs and TC,as well as the effect of MPs on degradation and transformation of environmental pollutants in MES.The main findings are as follows:(1)The MPs degradation experiments showed that mass loss of PLA in closed circuit(CC)group was higher than other types of MPs at 120 days(D),reaching 8.94%,which was 3.01 times and 3.54 times of that in open circuit(OC)group and non-electrode(NC)group,respectively,and showed strong correlation with soil total nitrogen(TN)and total organic carbon(TOC)content.The results of DFT calculation showed that the energy gap of the highest occupied molecular orbital(HOMO)and the lowest unoccupied molecular orbital(LUMO)(HOMO-LUMO gap)of the three polymers with periodic structure were 4.20,7.24 and 10.09 e V,respectively,and further decreased under the effect of electric field.This would increase the hydrolysis potential of PLA.Compared with OC group,the relative abundance of potential plastic-degrading bacteria on the surface of PLA and PVC in CC group increased by 1.92 times and 1.30 times,respectively,indicating that the microbial electric field promoted the microbial degradation of MPs in soil.The complexity of microbial co-occurrence network in the plasticsphere was lower than that in soil,and the determination process played a major role in the assembly of microorganisms in the plasticsphere.The plasticsphere community structure of CC group showed stronger xenobiotic biodegradation and metabolic capacity than that of soil,which further indicated that the electric field selectively enriched potential plastic-degrading microorganisms.(2)The MPs adsorption experiments showed that the adsorption kinetics of TC on the original and soil-exposed PLA,PVC and PE MPs conform to the pseudo-first order and pseudo-second order models.However,there is limited information on the adsorption behavior and antibacterial mechanisms of environmentally exposed MPs.In this experiment,the adsorption kinetics experiments of TC on the original and soil-exposed PLA,PVC and PE MPs showed that the adsorption capacity of MPs on TC increased after exposure to soil,and the adsorption capacity of PLA increased most obviously.Soil exposure increased the time for adsorption to reach equilibrium,and the adsorption rate was controlled by both intra-particle diffusion and membrane diffusion.The isothermal adsorption results of PE and PLA showed that the adsorption of TC on heterogeneous surface was affected by physicochemical adsorption.After exposure to soil,the adsorption capacity of MPs to TC increased by 88%(PLA),26%(PVC)and15%(PE),respectively.Soil soluble organic matter(DOM)promoted the desorption of TC from the surface of MPs,and TC morphology varied with p H value.Soil-exposed MPs showed potential to promote TC degradation without the addition of biological inhibitors.In addition,density functional theory calculation verified that PE and PVC adsorption TC mainly through physical effect,while PLA can form hydrogen bond with TC.(3)The MPs-TC composite degradation experiment showed that the addition of MPs promoted the degradation of TC in MES,and the TC degradation rate in Soil+TC group increased by 24%(PLA+TC)and 14%(PVC+TC),respectively.TN in CC group was consumed in large quantities along with TC degradation.TN content in soil may be an important factor limiting TC degradation in MES.The addition of MPs promoted the charge output of MES to 594 C(PLA+TC)and 502 C(PVC+TC),respectively,and these additional charge output increases did not come from the degradation of MPs.Through threedimension excitation emission matrix fluorescence spectroscopy analysis of DOM components,it was found that TC degradation accelerated the consumption of microbial available organic carbon in MES and the conversion of humus to low molecular weight organic matter in the system.Therefore,TC degradation may be inhibited in a carbon-poor environment,and the addition of MPs can increase the bioavailability of TC in MES.In addition to adsorption of TC in solution,MPs could also reduce the energy barrier for TC hydrolysis reaction,indicating that MPs promoted the transformation and degradation of TC in MES.The relative abundance of potential TC degrading bacteria such as Acinetobacter and Pseudomonas in the plasticsphere at 120 D was higher than that of the soil,indicating that MPs may be able to act as a continuous hotspot of TC degradation after the electric field disappears.The complexity of microbial co-occurrence network in soil and plasticsphere at 20 D was lower than that at 120 D.Although this situation might lead to unstable interspecific relationship of microbial community,it will improve the interspecific cooperation ability,which was also one of the reasons for the high voltage generation in the early stage of MES.The ability to degrade xenobiotic and metabolize carbohydrates and amino acids in the plasticsphere in the CC group was higher than that in the OC group,which further confirmed that the electric field promoted the improvement of the TC metabolic ability of the plasticsphere microbe in MES.Partial least squares path model fitting results showed that the promotion of MPs on TC degradation was mainly caused by environmental microorganisms.Secondly,MPs can affect the content of hydrolyzed products by reducing the hydrolysis reaction barrier,and further affect the degradation behavior of TC in MES.In soil,the electrochemical parameters of MES and microbial function exhibited a strong overall effect on TC degradation,and the microbial electric field mainly affected TC degradation through indirect effects.This study confirmed that microbial electric field can affect the degradation behavior of MPs.In the microbial electric field,the chemical stability of plastic substrate and the abundance of potential degrading bacteria on the surface played a major role in aging degradation of MPs.The interaction mechanism and influencing factors of environmental MPs and organic pollutants were revealed by soil exposure.It was found that soil exposure could promote the adsorption of PLA to TC by enhancing the hydrogen bond between PLA and TC.The introduction of MPs promoted the hydrolysis and transformation of TC in MES by reducing the reaction energy barrier,increase the bioavailability of TC,and generate higher cumulative charge output.This study focuses on the environmental behavior of MPs and organic pollutants under microbial electric fields,which is helpful to systematically assess their environmental risks.