Effects Of Sludge Pretreatments On Microplastic Extraction And Their Physicochemical Characteristics

Microplastics(MPs)as new pollutants of environmental concern has gradually raised attention.In recent years,studies have shown that wastewater treatment plants are the main source of MPs.In this process,most of MPs in sewage are trapped in sludge,resulting in a high content of MPs in sewage sludge.At the same time,studies have shown that the surface characteristics of MPs after sludge treatment will change significantly,manifested as crushing,melting,particle size reduction and other phenomena.Compared with virgin MPs,highly fragmented and aging MPs in sludge may increase the enrichment effect of organic pollutants,heavy metals and pathogenic microorganisms,and thereby increasing the potential ecological risk of subsequent land use of sludge.At present,acid,alkali and other pretreatment methods are commonly used to improve the extraction and analysis of MPs.However,the effect of sludge pretreatment on extraction of MPs from sewage sludge and their physicochemical characteristics is not clearThis research firstly explored the effect of chemical pretreatment on sludge-based MPs,validated the MPs extraction in five solid matrices,sludge,cattle manure,soil,sediment and silica,and analyzed the reasons for limiting the MPs extraction in sludge At the same time,the effect of nine chemical pretreatments of acid,alkali and oxidation on the extraction of MPs from sludge was explored.Secondly,characterization methods such as FTIR,SEM,mass loss,and the adsorption capacity of MPs for Cd were used to evaluate the effects of chemical pretreatment on physicochemical characteristics of MPs,and PC A analysis and classification based on the response of six types of MPs to nine chemical pretreatments were made.On this basis,the transformation of MPs in sludge hydrothermal process was investigated.The effects of different hydrothermal treatment reaction conditions,temperature,time and reactant concentration on the physicochemical characteristics of MPs were discussed.The physicochemical characteristics of MPs in water and sludge were compared under the same hydrothermal conditions.Finally,the effects of different components of sludge,such as protein,polysaccharide and silica,on the hydrolysis of MPs were explored to reveal the mechanism of sludge promoting depolymerization of MPs.The main findings are as follows(1)Compared with the other solid matrices,cattle manure,soil,sediment and silica,sewage sludge showed the lowest MP extraction efficiency,especially for PET fibers The possible reason was that the EPS in the sludge floc produce a negative effect on the extraction of MPs by hindering their floating.Chemical pretreatments such as 30%H2O2 and Fenton can improve the MP extraction in sludge by destroying the flocculation structure of sludge so that organic matter can be released appropriately High concentration of acid and alkali pretreatments will lead to excessive dissolution of organic matter in sludge,thereby reducing the extraction efficiency of MPs.The relationship between the dissolved amount of organic matter in sludge and the MP extraction indicates that the degree of decomposition of sludge organic matter has an important influence on the improvement of MP extraction(2)Pretreatments affect the MP property such as size,surface morphology and adsorption potentials,especially for alkaline pretreatments.Compared with other pretreatments,30%H2O2 and Fenton pretreatments have little effect on the physicochemical characteristics of MPs.The pretreatments were classified into four main categories:Category P1(alkali pretreatment),Category P2(high concentration acid pretreatment),Category P3(low concentration acid pretreatment)and Category P4(oxidation pretreatment).The effect of the pretreatments on the MPs follows a decreasing sequence:Category P1>Category P2>Category P3>Category P4.The six MPs are divided into three main categories,Category M1(PET,PA and PMMA),Category M2(PS)and Category M3(PE and PP).The resistance of the MPs to the chemical pretreatments tends to increase as following:Category M1<Category M2<Category M3.Compared with the PE,PP and PS solely built of carbon atoms,the PET and PA with heteroatoms in their main chains are more susceptible to chemical pretreatments(3)Different hydrothermal reaction temperature,reaction time and reactant concentration have important influence on the physicochemical characteristics of MPs With the rise of hydrothermal temperature and the increase of reaction time,the degree of hydrolysis of MPs gradually intensified.Under the reaction conditions of 220? and 5h,the hydrothermal process was considered to be basically complete.Low MPs reactant concentration was beneficial to promote the hydrolysis of MPs.Under the same hydrothermal conditions,the degree of MPs decomposition is in the following decreasing order:PET>PS?PE.Compared with PE and PS whose main chain only contains carbon atoms,polyester polymer PET contains a large number of ester bonds which are easily broken by activation.Under the same hydrothermal conditions,sludge promoted the hydrolysis of MPs.Compared with the MPs in the pure water system,the size of the sludge-based MPs became smaller and the surfaces were more broken and rougher.At the same time,the content of organic matter released by the liquid phase products after hydrothermal treatment increased(4)In different hydrothermal media,the degree of hydrolysis of MPs is in the following decreasing order:sludge>protein>polysaccharide>silica? water.The results showed that the sludge system promoted the hydrolysis of MPs,in which the protein component in the sludge played a promoting role,while the polysaccharide component in the sludge played an inhibitory role.The possible reason is that the rate of protein hydrolysis is relatively fast,and the amino and carboxyl functional groups formed by the breakage of peptide bond are easily combined with the functional groups on the surface of MPs to promote decomposition.In the late stage of hydrolysis,polysaccharides are prone to group reorganization to form stable aromatized substances,which hinders the hydrolysis of MPs.The chemical properties of silica do not change after hydrothermal treatment;thus,it has little effect on the hydrolysis of MPsIn summary,acid and alkaline pretreatments will have a certain effect on the extraction and physicochemical characteristics of MPs.Oxidation pretreatment improves the MP extraction and has little effect on the physicochemical characteristics of MPs.Hydrothermal pretreatment affects the physicochemical characteristics of MP The presence of organic matter in sludge promotes the depolymerization of MPs Therefore,appropriate pretreatment methods should be used in the MPs analysis to avoid underestimation of MPs content or changes to the real physicochemical characteristics of MPs.The physicochemical characteristics of MPs have changed in the sludge treatment process,and the potential risks in the subsequent land use process need to be highly concerned.