| Nanoscale zero-valent iron(nZVI)particle is a kind of iron particle with diameter ranging between 1-100 nm.nZVI has small particle size,strong reduction capacity and large surface,which is widely used in the remediation of contaminated groundwater and soil.Bacteria,as an important part of water and soil ecosystems,will inevitably be exposed to nZVI.Different bacteria may have different responses to nZVI exposure due to their structural differences.However,there are few studies on the different effects of nZVI exposure on the activities of different bacteria and the related mechanisms.In this study,the toxicity of nZVI to Gram-negative representative Escherichia coli,and Gram-negative representative bacteria Staphylococcus aureus was detected by plate counting method.The adsorption behavior differences between nZVI and the two bacteria were revealed by ultradepth-of-field 3D video microscopy and Scanning electron microscopy.Zeta potential analyzer was used to test the surface potential of nZVI and two bacteria,and the electrostatic interaction between nZVI and two bacteria was analyzed.Fourier transform infrared spectroscopy and Excitation emission matrix spectroscopy were used to detect the spectral changes of cell membrane components.Besides,in order to elucidate the mechanism of inactivation of Escherichia coli and Staphylococcus aureus caused by nZVI adsorption,Live/dead fluorescent dye,DCFH-DA fluorescent dye and Catalase activity detection kit were used to detect membrane damage,reactive oxygen species and Catalase contents of the two bacteria.The research results are as follows:(1)Inactivation efficiency of Escherichia coli was 43.70%?73.70%?97.22%and 99.88%,while inactivation efficiency of Staphylococcus aureus was 32.75%,48.54%,54.5%and 81.82%after treatment with 50,100,200 and 500 mg/L nZVI for 60 min,respectively.The inactivation efficiency increased with the increase of nZVI dose.Inactivation efficiency of Escherichia coli was 24.29%,57.79%,75.54%and 85.93%,while inactivation efficiency of Staphylococcus aureus was 14.31%,23.56%,29.01%and 48.75%after treatment with 100 mg/L nZVI for 15,30,45 and 60 min,respectively.The inactivation efficiency increased with the prolongation of treatment time.Inactivation efficiency of Escherichia coli is higher than that of Staphylococcus aureus under the same nZVI treatment conditions.(2)Ultra-depth 3D video microscopy and Scanning electron microscopy test results show that Escherichia coli agglomerated rapidly and adsorbed a large amount of nZVI,the surface of the bacteria was significantly rough and severely damaged during nZVI treatment.While,Staphylococcus aureus clustered more slowly and adsorbed less nZVI around it,so its bacterial damage was less severe.The above results indicate that the adsorption capacity of nZVI to Escherichia coli is stronger than that of Staphylococcus aureus.(3)The surface charge of bacteria was measured by Zeta potential analyzer to reveal the effect of electrostatic action on the difference of adsorbability.Zeta potential analyzer test results found that the isoelectric point of nZVI is 5.6,the surface charge of Escherichia coli and Staphylococcus aureus is negative under weak acid and neutral conditions.When pH value is 3,5,7,9 and 11,the potential difference between nZVI and Escherichia coli surface was 23.17 mV,21.45 mV,14.69 mV,13.2 mV and 9.89 mV,while the surface potential differences between nZVI and Staphylococcus aureus were 41.73 mV,26.15 mV,18.76 mV,11.93 mV and 9.3 mV,respectively.With the decrease of pH value,the potential difference between nZVI and the two bacteria showed a trend of gradual increase.When pH value is 3,4 and 5,the inactivation efficiency of Escherichia coli is 99.56%,98.93%and 96.10%,while the inactivation efficiency of Staphylococcus aureus is 95.76%,62.40%and 55.06%.The inactivation efficiency of two bacteria at pH value 3,4 and 5 is higher than that at pH value 6.3,indicating that the electrostatic force affects the adsorption between nZVI and bacteria to a certain extent.On the other hand,at the same pH value,the surface potential difference between nZVI and Staphylococcus aureus was higher than that of Escherichia coli,but the adsorption between Staphylococcus aureus and nZVI was weaker than that of Escherichia coli,indicating that the electrostatic force was not the only factor leading to the adsorption difference of nZVI different bacteria.(4)In addition,the adsorption differences between the two bacteria and nZVI were revealed from the chemical interaction between the two bacteria and nZVI.Infrared spectrum analysis results show a significantly lower infrared absorption peak intensity on the protein,phospholipid and polysaccharide of Escherichia coli after nZVI treatment,infrared absorption peaks of protein and phospholipid moved to low wave number and polysaccharide absorption peak has not shift.These results indicated that nZVI chemisorbed absorbed on lipids and lipoproteins in Escherichia coli membrane,and the absorbtion was weak on polysaccharides.The infrared absorption peak intensity of protein,phospholipids and polysaccharides in Staphylococcus aureus system decreased slowly,and there was no low band shift phenomenon on characteristic absorption peak,which indicated that the surface components of nZVI and Staphylococcus aureus(mainly polysaccharides in the peptidoglycan layer)had a poor affinity and the adsorption between them was weak.Excitation-emission-matrix spectra analysis also confirmed the important component of the interaction between bacteria and nZVI was protein.(5)Escherichia coli membrane was damaged seriously,and intracellular ROS content increased significantly while the relative level of ROS/cell concentration increased significantiy during nZVI treatment and the relative level of Catalase/ROS decreased significantly.However,Staphylococcus aureus membrane was damaged weakly,and intracellular ROS increased weakly while the relative level of ROS/cell concentration increased significantly during nZVI treatment,and the relative level of Catalase/ROS decreased and then increased.These results indicate that nZVI adsorption can affect the bacterial inactivation process through cell membrane damage and intracellular oxidative stress response.In summary,nZVI has a stronger inactivation effect and stronger adsorption effect on Escherichia coli than Staphylococcus aureus.The difference of adsorption was mainly caused by the difference of bacterial surface components.Proteins and phospholipids on the outer layer of Escherichia coli adsorbed nZVI more easily than the peptidoglycan on the surface of Staphylococcus aureus.nZVI adsorption damaged bacterial membrane,which resulted in intracellular oxidative stress.This study clarified the mechanism of nZVI to different bacteria from the point of view of the interface interaction between nZVI and bacteria,and provided a theoretical basis for evaluating the biosafety of nZVI. |
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