Study On Preparation And Modification Of Eichhornia Crassipes Derived Biochar And Purification Mechanism Of Compound Pollutants In Water

With the reform and opening-up,the coastal economy of China has developed rapidly,and the problems of ecological destruction and environmental pollution in estuaries and coastal waters have become increasingly serious.The Pearl River Basin is the second largest river basin in China,and the Pearl River Estuary is the core area of the Guangdong-Hong Kong-Macao Greater Bay Area with dense population and developed industry.For example,the water environment pollution caused by heavy metals,persistent organic pollutants and microplastics and other major pollutants;due to the high content of nitrogen,phosphorus and other nutrients,Eichhornia crassipes wreak havoc,resulting in water ecological problems.These pollutants have a great impact on water quality degradation,and the current treatment technology of waterworks cannot effectively remove these pollutants,so there are great hidden dangers in regional water supply security.This study took the Pearl River Estuary as an example to study the treatment mechanism of ecological damage and compound pollution in the estuary and coastal waters.Firstly,the spatial distribution and biomass of Eichhornia crassipes in the Pearl River Estuary were investigated.Samples were collected on site for testing and analysis of pollutants such as microplastics,persistent organic pollutants and heavy metals.Combined with literature,the spatial distribution,migration and transformation rules of pollutants were analyzed,and their ecological hazards were assessed.Then,Eichhornia crassipes was prepared into Eichhornia crassipes biochar material and modified according to the characteristics of various pollutants.With modified Eichhornia crassipes biochar material as the core,the mechanism of pollutant removal was studied in the following three aspects:（1）the removal of Cr（VI）by nano-zero-valent iron modified biochar based material（C-nZVI-BC）with“sandwich”structure;（2）the application of“carbon coated cobalt tetroxide”modified biochar and advanced oxidation technology for the persistent organic pollutants degradation;（3）Eichhornia crassipes biochar membrane material is applied to the removal of microplastic combined pollutants.In order to provide theoretical support and reference for the treatment of complex estuarine water environment problems,this paper explored the removal technology and mechanism of new pollutants and compound pollutants by using modified biochar materials.In this context,the utilization of Eichhornia crassipes resources has certain practical significance,which provides technical support for the realization of the dual goals of pollution control and carbon fixation,and provides a guarantee for the safety of urban and rural water supply.The specific research content and the main conclusions are listed as follows:（1）The contents and spatial distribution of heavy metals,persistent organic pollutants（POPs）and microplastics（MPs）from Macao were sampled and analyzed.The levels of heavy metal pollution were Cd＜Cr＜Pb＜Cu.The content of Cd with an average value of 0.060μg/L.The Cr content with an average of 0.842μg/L.The contents of Pb with an average of1.024μg/L.The contents of Cu with an average of 3.138μg/L.Chemical oxygen demand（COD）and biochemical oxygen demand（BOD）are the integrative indexs for evaluating the degrees of organic pollution,COD_Cr and BOD₅ were 0.23～2.35 mg/L and 0.05～1.25 mg/L respectively.The abundance of MPs varied in range of 50～500 particles/kg d.w.The average abundance value of the MPs was 228.6 particles/kg d.w.in the reference area of Macao,five polymers,namely,PE,PP,PET,PA,and synthetic fibres,were identified and subsequently confirmed.On the whole,the spatial variations of contamination level in Macao’s coastal surface seawaters were mainly influenced by the water quality of the South China Sea during flood tides and by the upstream water from Humen and the Modaomen Estuary during ebb tides,the pollution is more serious in the eastern and southern regions of Macao,due to various factors,such as hydrodynamic and anthropogenic activities and the confluence of freshwater and saltwater.（2）The biochar（BC）exhibits a high surface area(278.56 m²·g^-1),honeycomb-like porous structure,a large number of oxygen-containing functional groups（OH,COOH,etc.）and heteroatoms（N,O,S）,and carbon mainly exists in the form of amorphous state.The BC anodes for lithium-ion batteries exhibited a higher initial reversible specific capacity of 697±4.3m Ah·g^-1 at the current density of 50 m A·g^-1 and rate capability of 229.7±0.9 m Ah·g^-1 at 3000m A·g^-1,as well as greater cyclic stability than the commercially used graphite.（3）We innovatively synthesized a“Sandwich”-like functionalized material（C-nZVI-BC）with biochar（BC）as carrier,nano zero valent iron（nZVI）as modifier,and chitosan（C）as stabilizer.The morphology and structure of C-nZVI-BC composites were characterized,and it was found that the addition of chitosan made nZVI uniformly distributed on the surface of BC,and effectively inhibited the oxidation and agglomeration of nZVI.The results showed that C-nZVI-BC showed higher Cr（VI）removal capacity（82.2 mg/g）and removal rate（97.34%）than nZVI-BC（64.51%）and BC（21.52%）.After 5 times of adsorption-desorption experiments,the adsorption capacity of Cr（VI）remained above 43.1 mg/g.The repair of Cr（VI）by C-nZVI-BC is a multiple synergistic mechanism,including complexation,electrostatic action,adsorption,reduction and precipitation.（4）A biochar heterogeneous catalytic material which can activate potassium persulfate（PMS）to degrade persistent organic pollutants（POPs）was synthesized by using biochar as a carrier.This material was prepared by in-situ loading combined with oxidizing calcination to obtain cobalt-trioxide modified biocarbon（Co₃O₄/C-BC）with carbon coated structure,which not only inhibited the leaching of Co ions,improved the stability of the composite material,but also provided more active sites for activating PMS and degrading POPs.Under the conditions of Co₃O₄/C-BC dosage of 0.3 g/L,PMS dosage of 1.0 mmol/L,reaction temperature of 30℃ and no change of pH value,POPs（bisphenol A,sulfanilamide,methyl orange and rhodamine B）wastewater of 20 mg/L could be completely degraded within 30 min.The degradation efficiency of Co₃O₄/C-BC/PMS remained above 92%of BPA after six cycles.The results show that SO₄^·-and HO^·exist in the Co₃O₄/C-BC/PMS system,and the contribution of SO₄^·-radical wasmore important than that of HO^·in BPA degradation.The possible degradation pathway of BPA in the Co₃O₄/C-BC/PMS system,including Heterolysis,hydroxylation,dihydroxylation,demethylation,carbonylation and ring opening,etc.The degradation of POPs by Co₃O₄/C-BC/PMS system is a synergistic process of adsorption and catalytic oxidation.（6）The system of“Eichhornia crassipes biochar membrane-biofilm”was constructed.Eichhornia crassipes biochar film has good hydrophilicity and stability.On the biofilm dominated by green algae,the total organic carbon（TOC）value was 0.622 mg C/L.The organic carbon is mainly non-polar carbon.Contains aromatic ring,carboxyl,methyl,methylene,carbonyl and other functional groups.Using the“Eichhornia crassipes biochar membrane-biofilm”system,the removal efficiency of microplastics-Cr（VI）or microplastics-tetracycline compound pollutants was realized for the first time,the removes most microplastics and Cr（VI）and tetracycline within 3 hours.