Preparation Of Nano-zero-valent Iron Modified Oyster Shell Filter Material And Its Adsorption And Removal Of LAS In Rural Gray Water

Water shortage is a serious global challenge.According to statistics,the proportion of rural grey water in the living area ranges from 50%to 90%.With the advancement of the timeliness of the concept of sustainable development of water resources,based on the concept of sewage collection,the treatment and reuse of gray water in rural areas has gradually become a new mode of rural water saving.Gray water is a high-quality reclaimed water,and the most typical pollutant anionic surfactant（LAS）in gray water poses an increasingly serious effect on the ecotope.Miscellaneous water standards and farmland irrigation standards have put forward strict standard limits for LAS.The research and development of methods to strengthen the removal of LAS in rural grey water is of good real meaning for utilizing the pattern of those resources and promoting sustainable environmental development.Based on the results of field surveys and reports refining on the gray water profile in typical water-deficient areas in Northwest China,adsorption technology has significant advantages in treating wastewater in rural areas.Therefore,in line with the original intention of recycling waste resources and preparing environmentally friendly materials,based on the green synthesis method,a modified oyster shell filter material（GT-nZVI/OS）loaded with nano zero valent iron（nZVI）was developed.At the same time,the preparation conditions of the composite material and the adsorption conditions of the target pollutants were optimized,the specific process of different preparation conditions affecting the synthesis efficiency of GT-nZVI/OS was explained,and the adsorption mechanism and degradation mechanism of LAS in a single gray water system were revealed.Finally,a good removal effect of the main pollutants in the simulated grey water was achieved,so that the effluent met the farmland irrigation water reuse standard.At the same time,the adsorbed GT-nZVI/OS material had no secondary pollution and had good regeneration performance.This thesis provided theoretical guidance for the subsequent research and development and application of LAS-containing wastewater treatment and reuse technology.The main research content and experimental results are listed as follows:（1）Through the experiment of the influence of key preparation factors on the physicochemical properties and adsorption efficiency of the modified filter material（GT-nZVI/OS）,it was found that the optimal conditions for the preparation of GT-nZVI/OS were:oyster shell particle size was 0.5 mm,the calcination temperature was 750℃,the ratio of iron source to oyster shell（m L/g）was 100:8,and the ratio of iron source to tea polyphenol extract was 1:2.The adsorption rate of modified GT-nZVI/OS to LAS was 91.86%,which was 56.13%higher than that of OS.The advantage of modification was obvious.It was worth noting that there was an optimal load between the carrier OS and nZVI.SEM images found that the surface of GT-nZVI/OS produced many circular mesoporous pores by calcination.The spherical nZVI was uniformly distributed on the surface and inside of the OS pores,with a particle size of about 20-30 nm.Fe content of the modified GT-nZVI/OS was increased from 0 to 25.4%.Although XRD and XPS results showed that the prepared GT-nZVI/OS was partially oxidized to Fe₂O₃ and Fe OOH,compared with pure nZVI,the characteristic peak vibration intensity of Fe⁰ was significantly weakened.The above-mentioned characterization results proved that the green synthesized nZVI with tea polyphenol extract（TPE）was loaded on the OS in situ,which was beneficial to improve the oxidation resistance of Fe⁰ and enhance the stability of the composite material.（2）The effect of different conditions on the adsorption of LAS in a single gray water system on the GT-nZVI/OS synthesized under the optimal preparation conditions was discussed.Those results showed that the point of zero charge(p H_PZC)of GT-nZVI/OS was 9.29.Under the optimal adsorption conditions（p H=6,LAS initial concentration=20 mg/L,dosage=0.08 g,T=308 K）,the removal rate of LAS by GT-nZVI/OS reached 93.80%,and the equalization adsorption ability was 12.91 mg/g.Specially,GT-nZVI/OS had a wider p H tolerance range than OS.The p H value was from 2 to 11.5,and the removal rate was only reduced by 6.99%.Dissolved organic matter will compete with the target pollutant for adsorption,and produce an inhibitory effect.However,anions had a slight effect on the removal of LAS by GT-nZVI/OS.In addition,cations had a better promoting effect.Therefore,GT-nZVI/OS could be effectively applied to the removal of anionic surfactants in common wastewater.（3）At different temperatures（288 K,298 K,308 K）,the pseudo second-order kinetic model and Langmuir isothermal adsorption model can be measured to represent those process of OS and GT-nZVI/OS to LAS,R²>0.995.Based on the Langmuir model,at 308 K,the maximum adsorption capacities of OS and GT-nZVI/OS were 14.35 mg/g and 36.34 mg/g,respectively.Those processes were endothermic spontaneously,and mainly dominated by the intra-particle proliferation and membrane spread principles.From the results of thermodynamic analysis,the mechanism of action of GT-nZVI/OS had changed from a single physical adsorption to a combined physical and chemical adsorption,with chemical adsorption being the dominant adsorption mechanism.（4）The surface and spectral characteristics of GT-nZVI/OS had changed significantly after adsorption.The conclusion showed that the change of the material’s micro morphology was due to the addition of target pollutants and the corrosion effect of nZVI.Analyzing the results of XPS,FTIR and XRD,it was found that·OH was produced during the adsorption process to promote the oxidative degradation of LAS.Furthermore,combined with mass spectrometry to identify the intermediate products and final products of LAS degradation,it was confirmed that physical adsorption and chemical adsorption occurred in GT-nZVI/OS in the process of removing LAS in a single system.In addition,due to the presence of Fe⁰ and Fe²⁺,a series of redox degradation had occurred.This may lay a theoretical foundation for the follow-up and deeper mechanism research.（5）Combining with the results of the investigation on the water quality of the rural gray water in the early stage,a comparative study was made on the removal effects of the two filter media OS and GT-nZVI/OS on the simulated rural gray water in the laboratory.It was found that the removal effect of GT-nZVI/OS on LAS,COD,TN,TP and NH₄⁺-N was significantly higher than that of OS.The average concentration of pollutants in simulated grey water after GT-nZVI/OS treatment for 120 min was:LAS1.37 mg/L,TN 8.83 mg/L,TP 0.007 mg/L,NH₄⁺-N 9.29 mg/L,COD 58 mg/L.The water quality was in line with the current"Farmland Irrigation Water Quality Standard"（GB 5084-2005）.