| The northeast black soil region has made great contributions to safeguarding China's food security,maintaining social stability,and ensuring economic development.With the continu ous increase in grain production and the continuous expansion o f production scale,the use of pesticides has increased year by year.Atrazine is the most widely used corn field herbicide in Northeast China,and its large application will cause the farmland environmental quality to decline,and at the same time cause harm to animal and plant health.Although previous studies have investigated and focused on the adsorption behavior of atrazine in various soil environments and the mechanism of microbial degradation,there are few reports on the adsorption characteristics and forms of atrazine on different particle size fractions of so il and whether there are differences in regularity and microbial degradation or not.In this paper,the adsorption characteristic s of atrazine on different particle size fractions of black soil were systematically investigated,and the differences in the forms and content of atrazine in different particle size fractions were determined.The conventional chemical analysis and 13C-NMR techniques were used to identify the differences.The content of organic matter in various particle size fractions of black so il and the key role of different types of carbon structures in the adsorption process.On this basis,the high-efficiency atrazine-degrading strain Arthrobacter sp.DNS10 obtained from the previous screening was used as the strain resource,and the survival of functional strains in different black soil particle size fractions and the removal of different forms of atrazine in the abo ve-mentioned fractions were investigated.The main results are:(1)Four different particle size fractions of black soil in northeast China were successfully extracted by continuous grading method,namely coarse sand(2000-200 ?m),fine sand(200-20 ?m),powder(20-2 ?m),clay(<2 ?m).The mass percentages of the above fractions in black soil were 44.04%,32.81%,14.79%,and 3.19%,respectively.The trend of organic carbon content in black soil particle size fractions mainly showed clay(28.8 g/kg)>coarse sand(27.3 g/kg)>fine sand(19.5 g/kg)>powder(16.5 g/kg).The percentages of carbon,hydrogen and nitrogen in each particle size fraction are 3.15%-4.09%,0.70%-1.38% and 0.60%-0.80%,respectively.The C/N value indicates that the tested black soil is less affected by human activities.The H/C value indicates that the organic carbon in the clay is the most aromatic.In addition,the organic c arbon of each particle size fraction is mainly O-alkyl carbon(59.13%-67.77%),followed by aryl carbon(17.81%-25.97%),and then alkyl carbon(7.73%-9.92%),the proportion of carboxyl carbon is relatively small(4.70%-6.86%).(2)The results of adsorption kinetics showed that atrazine(initial concentration of 20 mg/L)reached equilibrium at 4 h,4 h,8 h,and 2 h on coarse sand,fine sand,powder and clay.The maximum adsorption capacity of atrazine on each particle size fraction was clay(47.67 mg/kg)>coarse sand(44.08 mg/kg)>fine sand(42.67 mg/kg)>powder(37.12 mg/kg).The adsorption kinetics of each particle size fraction to atrazine was consistent with the quasi-secondary kinetic model,and the R2 values were all above 0.99.The results of isothermal adsorption test showed that both Langmuir model and Freundlich model can better fit the adsorption process of atrazine on different particle size fractions of black soil;the coefficient of KF related to adsorption capacity and adsorption strength is clay(1.83)>coarse sand(1.69)>fine sand(1.63)>powder(1.43),indicating that the adsorption and binding ability of the clay to atrazine is higher t han other fractions.In addition,by calculating the D-R model adsorption mean free energy(E),it is determined that the adsorption process of atrazine on the above-mentioned black soil particle size fractions is mainly physical adsorption.(3)This study found that the organic carbon content of each particle size fraction of black soil is positively correlated with the adsorption capacity and adsorption strength of each particle size fraction for atrazine.In addition,the correlation analysis between 13C-NMR spectral area integral and KOC further indicates that the surface adsorption of the atrazine on black soil fraction also has a coupling relationship with the structure of organic carbon,and atrazine is on the various particle size fractions of black soil.The adsorption capacity is positively correlated with the aromatic carbon content and negatively correlated with the alkoxy c arbon content.Besides,the correlation analysis between the effective polarity of each particle size fraction and KOC showed that the ability of each particle size fraction to adsorb atrazine was also negatively correlated with its effective polarity.In summary,combined with the mass percentage of the four particle size fractions,the organic carbon content,the structure and the polarity characteristics of each particle size fraction,and the adsorption capacity of each fraction for atrazine,etc.The c ontribution rate of the adsorption of atrazine to each particle size fraction of black soil was ranked as coarse sand(51.71%)>fine sand(30.78%)>powder(14.36%)>clay(3.14%).(4)The desorbing fraction,non-desorbing fraction and bound residue atrazine content of each particle size fraction were 5.89 mg/kg-11.95 mg/kg,2.63 mg/kg-4.85 mg/kg,1.78 mg/kg-2.19 mg/kg,respectively.The overall content trend is ranked as desorbing fraction>non-desorbing fraction> bound residue;the ratio of the desorbing fraction and t he non-desorbing fraction content of the particle size fractions to the total atrazine content is more than 80%,indicating that atrazine is mainly present by desorbing fraction and non-desorbing fraction.(5)The degradation test of atrazine on different particle size fractions of black soil by strain DNS10 showed that the content of atrazine in coarse sand,fine sand,silt and clay w ithout sterilization treatment decreased by 74.26%,50.43%,31.83% and 19.71% respectively.The content of atrazine decreased by 78.18%,57.49%,47.25% and 24.29% in the four particle size fractions after sterilization,respectively.The degree of reduction of atrazine in each particle size fraction after sterilization was significantly higher than that of each particle size fractio n that was not sterilized.It indicated that the sterilization of soil could promote the degradation of strain DNS10.The desorbing fraction,non-desorbing fraction and bound residue atrazine in each particle size component were 30.47%-91.95%,0.32%-64.71%,0.87%-31.85%,respectively,and the degradation rate decreased with particle size.However,the degree of degradation of each f orm of atrazine by strain DNS10 in the various particle size fractions of black soil is desorbing fraction>non-desorbing fraction>bound residue,and the smaller the particle size,the more difficult it is to degrade the adsorbed atrazine.(6)The results of Zeta potential measurement showed that the surface of different size fractions and the surface of strain DNS10 had net negative charge.The zeta potential of strain DNS10 was-5.17 m V,and the zeta potential of coarse sand,fine sand,powder and clay was-15.87 m V,-15.03 m V,-13.23 m V,-16.3 m V,indicating that there is electrostatic repulsion between particle size fractions and strain DNS10,and the affinity of strain DNS10 and each particle size fraction is ranked as powder>fine sand>coarse Sand>clay.The ATPase activity of each particle size fraction before and after atrazine degradation by strain DNS10 showed that the ATP content of each particle size fraction was 185.20 U/L,155.74 U/L,138.15 U/L and 114.96 U/L,respectively,after 10 days of degradation.The ATPase activity of each particle size fraction could be ran ked as coarse sand>fine sand>powder>clay,indicating that the ATPase metabolic activity of each particle size fraction decreased with the decrease of particle size.The results showed that after 10 days of degradation,the expression of trz N was 516.94 copies/g dry soil,449.07 copies/g dry soil,409.88 copies/g dry soil,48.47 copies/g dry soil and 538.44 copies/g dry soil,497.33 copies/g dry soil and 457.36 copies/g dry soil,73.21 copies/g dry soil,respectively,ranked as coarse sand>fine sand>powder>clay by size,indicating that the metabolic activity of strain DNS10 was ranked as coarse sand>fine sand>powder>clay in each particle size fraction,and the activity of strain DNS10 in each particle size fraction decreased with the decrease of particle size. |
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