Effects Of Natural Organic Matter On Adsorption Behavior Of Tetracycline And Quinolone Veterinary Antibiotics In Soils In Cold And Arid Regions

Natural Organic Matter?NOM?is a natural organic macromolecule substance formed by the decomposition of plant and animal residues.It is an important component of soil and can affect the migration and transformation of organic pollutants in soil.Antibiotics are widely used in daily life.At the same time,a large number of residues containing antibiotics enter the soil through some ways,which brings serious harm to human health and soil environment.It is important to study the effect of natural organic matter on the adsorption behavior of antibiotic organic pollutants in soil for the prevention and control of soil pollution.In this paper,Oxytetracycline?OTC?and Enrofloxacin?ENR?were selected as the target pollutants to study their adsorption kinetics,thermodynamics,influencing factors and the effects of natural organic matter on the adsorption behavior of the two pollutants in three typical soils?sierozem soil;loessial soil;dark loessial soil?in Northwest China by batch experiment.The influence factors such as initial concentration and ionic strength were discussed.The results are as follows:?1?Four kinds of organic matter HA?self-made?,HA?marketed?,FA?self-made?,FA?marketed?were analyzed by infrared spectroscopy,elemental analysis and ultraviolet spectroscopy.The results showed that four kinds of organic matter contained aromatic rings,carboxyl groups,hydroxyl groups,carbonyl groups,etc.The carbohydrate and carboxylic acid content in FA?marketed?was the highest and the fatty degree was also higher,while HA?marketed?had higher aromaticity.The polarity of FA?self-made?and FA?marketed?is stronger,HA?marketed?is next,HA?self-made?is weakest,and HA is stronger than FA in hydrophobicity.The physical and chemical properties of sierozem soil,loessial soil and dark loessial soil show that the pH is alkaline,the cation exchange capacity of loessial and dark loessial soil is higher than that of sierozem soil,the clay and silt content of loessial soil is higher than that of sierozem soil and dark loessial soil,and the organic matter content of dark loessial soil is the highest.?2?The adsorption experiments of OTC on sierozem soil,loessial soil and dark loessial soil show that the kinetic adsorption can be divided into two stages:the fast adsorption stage?0-2 h?and the slow adsorption stage?2 h later?on sierozem soil and loessial soil,the fast adsorption stage?0-1 h?and the slow adsorption stage?1 h later?on dark loessial soil.The adsorption equilibrium time of OTC on three soils was slightly affected by the addition of four kinds of organic matter HA?self-made?,HA?marketed?,FA?self-made?and FA?marketed?,but the adsorption capacity increased in varying degrees,with the growth rates of80.5%,50.0%,29.6%and 9.9%on sierozem soil,100.2%,82.6%,66.5%and 54.5%on loessial soil,respectively.56.8%,43.2%,38.5%and 15.5%on dark loessial siol,respectively.The partition coefficient K_d of OTC in solid phase increased significantly,indicating that organic matter contributed part of the adsorption capacity.Because HA has larger molecular weight,stronger hydrophobicity and more abundant groups than FA,HA has stronger adsorption capacity for OTC than FA.The adsorption kinetics of OTC conformed to the Pseudo-second-order kinetics,and the adsorption mechanism of OTC did not change after adding organic matter to three soils.Fitting the isothermal adsorption process of OTC on sierozem soil,loessial soil and dark loessial soil at different temperatures shows that the adsorption process of OTC on sierozem soil is higher fitted by Langmuir and Freundlich thermodynamic models.Fitting the adsorption process of OTC on loessial soil,dark loessial soil is higher than that fitted by Freundlich thermodynamic model,which indicates that the model is more suitable for explaining the adsorption process.Thermodynamic parameter analysis of OTC on sierozem soils and dark loessial soils showed that Entropy??H^??and Enthalpy??S^??were more than zero,while Gibbs free energy??G^??was less than zero,indicating that it is a spontaneous endothermic adsorption,which increases the degree of disorder during the process.The adsorption of OTC on loess soils showed that?H^???S^?and?G^?are less than 0,indicating that it is an exothermic reaction process with spontaneous decrease of disorder degree.With the increase of initial concentration,the adsorption capacity of OTC on sierozem soil,loessial soil and dark loessial soil increased,and the adsorption capacity of loessial soil to OTC was higher than that of sierozem soil and dark loessial soil.The adsorption of OTC by cations with different concentration and valence showed different effects.Except 0.01 mol/L Ca²⁺had a weak promoting effect on the adsorption of OTC by loessial soil and dark loessial soil,the other cations inhibited the adsorption of OTC to varying degrees.The effect of solution pH on the adsorption of OTC by sierozem soil and sierozem soil with four kinds of organic matter was not significant.When the solution is acidic,it is advantageous to the adsorption of OTC on loessial soil and dark loessial soil,but not advantageous to the adsorption reaction in alkaline.?3?The experimental results of ENR adsorption on sierozem soil,loessial soil and dark loessial soil show that the adsorption rate of ENR is faster in the process of kinetic adsorption,and the adsorption equilibrium can be reached within 15 minutes.The amount of ENR adsorbed by four kinds of organic matter HA?self-made?,HA?marketed?,FA?self-made?and FA?marketed?increased in varying degrees.The increase rates of ENR on sierozem soil soil were 346%,319%,283%,57.8%,on loessial soil were 7.4%,4.9%,1.8%,0.2%,and on dark loessial soil were 551%,506%,424%,108%,respectively.ENR increased greatly in the sorption of sierozem soil and dark loessial soil,and organic matter promoted the sorption significantly.Because of the large proportion of clay and silt,the influence of the particle size dominated adsorption of ENR onto loessial soil.The partition coefficient K_d of ENR in solid phase increases with the addition of organic matter.In the fitting of ENR adsorption kinetics process before and after adding organic matter in soil,the fitting parameter R² of Pseudo-second-order kinetic model is the largest,which indicates that the model can better explain the adsorption process of ENR on three kinds of soil,and the adsorption mechanism has not changed after adding organic matter.Fitting the isothermal adsorption process of ENR on sierozem soil,loessial soil and dark loessial soil at different temperatures showed that Freundlich thermodynamic model fitted R²higher,indicating that the thermodynamic adsorption process of ENR on three soils was multi-molecular layer adsorption;the adsorption thermodynamic parameters of ENR on sierozen soil,loessial soil and dark loessial soil indicated that?H^???S^?and?G^?are less than 0,showing that this adsorption process was a spontaneous exothermic reaction process with reduced chaotic degree.The effect of cations with different concentration and valence on ENR adsorption is similar to that of OTC adsorption.With the increase of pH,the adsorption capacity of ENR decreased.