Bioremediation Of PAHs-contaminated Soil Based On Plant-microbe Joint Mechanism And The Effect Of Microplastics

Polycyclic aromatic hydrocarbons(PAHs)are a typical class of persistent organic pollutants with carcinogenicity,teratogenicity and mutagenicity and accumulated in the soil.For PAHs-contaminated soil,the plant-microbe joint remediation combined the advantages of plant rhizosphere remediation and degrading bacteria bioaugmentation,which could improve the remediation effect.It is necessary to explore the role of plant root exudates in enhancing degrading efficiency of microorganisms.Microplastics(MPs),as a new pollutant,have been widely detected in various environmental media.It is necessary to clarify whether the accumulation of MPs in soil affects the natural attenuation and bioremediation of PAHs in soil,thereby affecting its ecological risk.In this study,soils J and Y with different physic-chemical properties were collected.Phenanthrene(PHE),pyrene(PYR)and benzo [a] pyrene(BAP)were used as representatives of PAHs to prepare PAHs-contaminated soils with different concentrations.The PAHs-degrading microbial consortium(composed of Brucella sp.,Ochrobactrum sp.,Sphingomonas sp.,Klebsiella sp.,and Acinetobacter sp.)in our laboratory was used to remediate the PAHs-contaminated soils with different properties and pollution levels to study the bioaugmentation effect.Ryegrass was used as a remediation plant to collect root exudates and the artificial root exudates as the reference group.The effects of different concentrations of root exudates on biostimulation and bioaugmentation of the in PAHs-contaminated soil were explored.One non-biodegradable low-density polyethylene(LDPE)and four biodegradable MPs,including polylactic acid(PLA),polybutylene succinate(PBS),poly-β-hydroxybutyric acid(PHB)and poly(butylene adipate-co-terephthalate)(PBAT)were selected as representative MPs to investigate MPs effect on the natural attenuation of PAHs in soil and the underlying mechanisms.The effects and mechanisms of two MPs(LDPE and PLA)on the bioaugmentation of PAHscontaminated soil were studied.The main conclusions are as follows:(1)The natural attenuation and bioaugmentation of PAHs in soils were affected the physic-chemical properties of the soils and the initial contamination levels.PAHs were more likely to dissipate in soil J than in soil Y.The residual contents of PAHs in soil J and soil Y were significantly reduced by bioaugmentation with exogenous PAHs-degrading consortium,indicating that exogenous bacteria promoted the degradation of PAHs in soil.The higher the initial contamination level of the soil,the higher the residual PAHs content after natural attenuation or bioaugmentation.(2)Biostimulation with different concentrations(40,80 and 120 mg C/kg)of ryegrass actual or artificial root exudates significantly improved the PAHs removal efficiency in PAHs-contaminated soil J.Bioaugmentation with PAHs-degrading consortium increased PAHs removal rate,which was higher than most biolstimulation efficiencies.The bioaugmentation effect was further enhanced by the addition of ryegrass actual or artificial root exudates and there was no significant difference between the two kinds of root exudates.All bioremediation measures promoted the growth of PAHs-degrading bacteria,affected the diversity,structure and function of soil bacterial communities to different extents and increased the absolute abundance of aromatic hydrocarbon degradation pathway genes,which might be the main reason for the promoting effect on PAHs degradation.(3)Different MPs showed different effects on PAHs natural attenuation and bacterial community in soil J.Non-biodegradable MP-LDPE significantly promoted the natural decay of PAHs,while biodegradable MPs(BPs)inhibited the decay process in the order of PLA < PHB < PBS < PBAT.Five MPs affected soil bacterial communities to different extents,selectively enriched some bacterial taxa and led to changes in Beta diversity and function.LDPE had a more significant effect on the soil bacterial community than the 4 BPs.The facilitating effect of LDPE on PAHs decay was attributed to the enhancements of PAHs-degrading genes and PAHs bioavailability,while the inhibitory effects of BPs were mainly due to the response of the soil bacterial community.(4)MPs affected the bioaugmentation of PAHs-contaminated soil J and the soil bacterial community therein.Non-biodegradable MP-LDPE significantly promoted bioaugmentation while biodegradable MP-PLA inhibited it.Two MPs increased the diversity of soil bacterial community,changed its structure and function to different extents,with the effect of LDPE more remarkable than PLA.Among bioaugmentation treatments,the addition of LDPE increased the abundance of PAHs-degrading genes,while PLA decreased them,which could explain their difference in PAHs removal efficiency.