Environmental Behavior Of Microplastic Particles In Soil/Solution-Plant Systems Under Combined Microplastic And Cd Conditions In Biogas Slurry

Biogas slurry is the product of full anaerobic fermentation of the biogas project,which can be used as an effective liquid fertilizer to provide moisture and nutrients to crops after harmless treatment.Long years of using biogas slurry containing contaminants may contaminate soil and harm crops.The compound contamination of microplastics(MPs)and heavy metals(HMs)that may be present in the biogas slurry is not clear,and the uptake of the compound contamination by crops in the biogas slurry-watered soil environment still needs to be further explored.The synergistic antagonism between the compound contaminants affects the toxicity of the contaminants,and the behavior of MPs determines the risk of MPs being taken up by plants when they are exposed to the co-contamination conditions.Therefore,in this study,we first investigated the contamination of MPs and HMs in biogas slurry from intensive pig farms in Yunnan;We then examined the impact of biogas slurry fertigation on growth physiology and possible uptake of MPs.by Salvia miltiorrhiza in a field;Finally,we used the model plant Arabidopsis thaliana to simulate the behavior of MPs particles in solution/soil under the possible co-contamination of MPs and Cd in biogas slurry under laboratory conditions,and the uptake of MPs by Arabidopsis thaliana under Cd stress was further clarified.The main study contents and results are as follows:(1)Biogas slurry from 11 intensive farms in Yunnan area was collected,and the contents of MPs and Cd,Cu,Zn in the biogas slurry samples were examined.The physicochemical properties(pH,electrical conductivity,total nitrogen,phosphorus and potassium,and organic matter)of the biogas were determined simultaneously.It was found that MPs contamination in biogas slurry was prevalent in biogas with high concentration and abundant types.The most common types of MPs were polyethylene(PE),polyvinyl chloride(PVC)and butadiene rubber(BR).There was no significant correlation between MPs contamination in biogas slurry and the physicochemical properties of biogas slurry.The concentration of Cd,Cu,Zn in biogas was low in Yunnan,and only the Zn content was correlated with the abundance of MPs,which may act as a carrier of Zn to promote its migration.(2)Biogas slurry was used as a fertilizer to irrigate Salvia miltiorrhiza.The growth(biomass,leaf area,yield)and physiological indicators(chlorophyll content,light rate system parameters,antioxidant enzymes and H2O2 content,malondialdehyde content,proline content)of Salvia miltiorrhiza,the accumulation of HMs in the plants,the soil properties and HMs content of the plots were investigated.The analysis found that the biogas slurry irrigation would promote the growth of Salvia at the early stage,there was no significant effect of biogas slurry on the growth of Salvia at the peak growth period and later stage,and the root yield of Salvia was increased by 22.53%by biogas slurry irrigation.The risk of Cd accumulation in Salvia plants and the excess of Cd elements in the soil due to slurry irrigation was small,even the Salvia plants could uptake Cu and Zn from the soil so that the Cu and Zn in the slurry was transferred to the plants through the slurry-soil-Salvia system and did not accumulate in the soil.Scanning electron microscopy of the root system of Salvia miltiorrhiza revealed particulate matter of unknown composition in the root system,but there is no direct evidence that Salvia miltiorrhiza can uptake MPs.(3)The possible co-contamination of Cd and MPs in biogas slurry irrigation plots was simulated in a laboratory setting,with the model plant Arabidopsis thaliana as the test plant.Growth(biomass)and physiological(root fatty acid content,root conductivity,malondialdehyde content,and proline content)indicators of Arabidopsis under a combined Cd and MPs condition were examined.The particles of MPs taken up by Arabidopsis were observed by scanning electron microscopy;Arabidopsis thaliana was found to uptake MPs through the middle column by Laser Confocal Microscopy.The analysis revealed that Cd stress disrupted the permeability of the Arabidopsis root membrane,allowing more and larger MPs to enter the plant root system and transfer to the shoot in the solution/soil-plant system.In this study,we investigated the non-negligible MPs contamination in biogas slurry,demonstrated that Cd stress drives the uptake of MPs by plants in a solution/soil-plant system in a laboratory environment.We explored the pathways of MPs uptake by plants under the coupling of MPs and Cd.This study provides data support for the environmentally sound resource utilization of methane,and in addition the possible existence of more biological risk of HMs and MPs combined pollution in the environment should receive more attention.