The Adsorption And Degradation Of Benzalkonium Chloride In Soils And Their Microbial Response Characteristics

Quaternary ammonium compounds（QACs）,a category of cationic antibacterial agents,are composed of the positively charged central N⁺,four alkyl or aromatic groups,and negatively charged halogen ions.They were widely used in agriculture,industry,food,and health care industry due to their good bactericidal efficacy.And thus,QACs are widely detected in various environmental media due to their wide application and the limitated removal ratio of traditional wastewater treatment technology.Soil is one important sink of QACs,thus it is important to analyze the adsorption and degradation behavior of QACs in soil and its influence on soil microbial community structure and function for better understanding the environmental risk of QACs and maintaining soil health.In this study,benzalkonium chloride（BAC（C12））,a typical QACs,was selected as the target pollutant to explore its adsorption and degradation in two different topsoils（alkaline farmland soil HN and acidic forest soil XS）.And the effects of BAC（C12）on soil microbial community structure,nitrogen transformation,and resistance genes were analyzed,and those under the coexistence of Cd were further explored.The main results and conclusions are shown as follows.（1） The adsorption experiment of BAC（C12）showed that:The adsorption equilibrium time of BAC（C12）was 30-45 min in HN soil and 60-120 min in XS soil.With the increase of initial concentration,the equilibrium adsorption capacity（q_e）of BAC（C12）in the two soils increased,while the equilibrium rate constant（k）decreased.All the adsorption processes followed the pseudo second order kinetic equation,which proved that the adsorption mechanism was mainly chemical interaction.Under the same conditions,the q_e and K_oc of BAC（C12）in HN soil were higher than those in XS soil.The adsorption capacity of HN soil to BAC（C12）was significantly stronger than that of XS soil,which might be due to the higher p H value and CEC concentration,and the lower content of Fe and Al of HN soil.（2） The aerobic degradation of BAC（C12）showed that:BAC（C12）was hardly degraded in sterilized soil,and could be degraded in both HN and XS soils without sterilization,and its degradation followed the first-order kinetic,with half-life of 4.66 and17.33 days,respectively,which suggested that the biodegradation was the main dissipation way of BAC（C12）in soil.On day 203,the degradation ratios of BAC（C12）in HN and XS soil were 95.4%and 83.9%respectively.The high abundance of QACs-degrading microbes and low organic matter content led to the shorter half-life and higher degradation rate of BAC（C12）.Furthermore,the degradation kinetics of BAC（C12）in the presence of CdCl₂ showed that the half lives of BAC（C12）degradation in HN and XS soils were 4.63 days and 8.86 days,respectively,which indicated that the presence of Cd²⁺significantly promoted the degradation of BAC（C12）in acid XS soil,but had no significant effect on the degradation of BAC（C12）in alkaline HN soil.（3） The metagenome sequencing of BAC（C12）-loaded soils and the control groups on day 40 found that:The abundance of BAC（C12）degradation-related microorganisms（Pseudomonas,etc.）in HN control soil was more than twice that in XS soil,and that of these functional microorganisms increased significantly compared with the control.BAC（C12）caused significant changes in the abundance of Proteobacteria and Chloroflexi in both soils,and most of the bacteria with increased abundance were Gram-negative bacteria.Further analysis showed that the abundance of nitrogen fixation and ammonia oxidation-related microbes decreased,that of nitrification-related microbes slightly increased,and that of denitrification-related microbes significantly increased in HN soil.BAC（C12）may also induce multiple-resistance of soil microorganisms.Compared with the coontrol the abundance of antibiotic resistance genes（ARGs）in HN and XS loaded soils increased by 3.3%and 6.3%,respectively,in which sulfonamide ARGs increased by 36.26%and 6.12%,and tetracycline ARGs increased by 6.95%and 0.63%.（4） Further analysis was adopted to explore the effects of BAC（C12）and CdCl₂single and combined pollution on XS soil microbial community structure in 60 days,and the results showed that:At the initial stage,single and combined pollution of BAC（C12）and CdCl₂ reduced the diversity and abundance of soil microbial community（such as Crenarchaeota）.BAC（C12）single pollution and combined pollution significantly increased the abundance of BAC（C12）degrading-related microorganisms.The three treatments had different effects on the microbial flora related to nitrogen fixation,nitrification,denitrification and ammonia oxidation.The three treatments on nitrogen transformation ability of two soils were analyzed by soil urease activity measurement and quantification of nitrogen-cycling-related genes（nif H,AOA,AOB and nar G）.The results exhibited that:the three treatments had different effects on the microbial nitrogen cycling of the two soils.BAC（C12）single pollution promoted the urease activity of the two soils,enhanced the nitrogen fixation and ammonia oxidation of the two soils（the abundance of nif H,AOA and AOB increased）,and reduced the nitrate reduction of the two soils（the abundance of nar G decreased）.Generally,CdCl₂ alone enhanced the nitrogen cycling of XS soil（the abundance of four functional genes increased significantly）and the ammonia oxidation ability of HN soil,but inhibited the nitrogen fixation and nitrate reduction of HN soil.BAC（C12）and CdCl₂ had no significant effect on the urease activity of HN soil,but inhibited its soil nitrogen cycling.The abundance of four functional genes in XS soil increased significantly with time.（5） Quantitative analysis of QACs resistance gene（qac EΔ1）,two kinds of ARGs and one horizontal transfer element（int I）in soil demonstrated that:BAC（C12）and CdCl2single pollution and combined pollution had different effects on the resistance genes of the two kinds of soil microorganisms.In general,BAC（C12）single pollution led to a significant increase in the abundance of resistance genes in the two kinds of soil.The abundances of int I and qac EΔ1 in HN soil did not change significantly,while that of sul1increased and that of tet M decreased.Under the combined pollution of BAC（C12）and CdCl2,the abundances of int I,tet M and qac EΔ1 in XS soil decreased,while the that of sul1 in HN soil increased,which means that BAC（C12）and CdCl2 pollution could induce multiple resistance of soil microorganisms and increase the risk of resistance gene horizontal transmission.