Study On The Formation And Mechanism Of Siliver Nanoparticles Induced By Soil Humic Acid

There has been growing interest in the manufacture of silver nanoparticles（Ag NPs）due to their antibacterial and antiviral properties,accompanying by their numerous industrial applications including heterogeneous catalysis,cosmetics,microelectronics,and conductive inks and adhesives.Products containing Ag NPs will inevitably enter the environment phases during their production and application.In addition to engineered Ag NPs,researchers also discovered the natural generation of Ag NPs.Naturally occurring Ag nanoparticles have been widely observed in ore deposits,coal,soil,and environmental water.Determining the generation process of these natural Ag NPs helps to clarify the geochemical cycle of Ag⁺and Ag NPs.More studies have shown that organic matter plays an important role in the environmental chemical behavior of Ag NPs,and humic acids from different sources are very different in nature.However,the role of soil organic matter in the natural generation of Ag NPs is still unclear.Therefore,four humic acids from different sources were selected in this paper to focus on the effects of humic acid types and physicochemical properties on Ag⁺transformation and Ag NPs generation and the mechanism.Peat humic acid（PHA and PFA）and commercial humic acids（HA-1 and HA-2）were used as soil organic matter models,combining electron paramagnetic resonance spectroscopy（EPR）and attenuated total reflection-infrared（ATR-FTIR）,transmission electron microscopy（TEM）and X-ray photoelectron spectroscopy（XPS）and other spectroscopy methods to discuss the formation of Ag NPs by soil humic acid.On the basis of clarifying the difference and internal mechanism of humic acid to convert Ag⁺to Ag NPs,PHA was reduced by sodium borohydride（Na BH₄）to study the effect of soil humic acid on the formation of Ag NPs under different reducing conditions.It provides new ideas for understanding the formation of Ag NPs in soil.The main conclusions are provided following.（1）Soil humic acid can reduce Ag⁺to form Ag NPs under environment conditions,and the types and properties of soil humic acid determine the speed and size of Ag NPs generation.The reaction rates of the four types of humic acid follow the rules:PHA>PFA>HA-1>HA-2.TEM results show that the mean hydrodynamic diameter of Ag NPs associated with PHA and PFA were in range from 2.5 to 15 nm,which were smaller than that produced from HA-1 and HA-2 in the range from 20 to 120 nm.Different concentration of soil humic acid,Ag⁺and reaction temperature all have great influence on the formation of Ag NPs.Higher concentrations of humic acid(100 mg L^-1)and Ag⁺(1×10^-3M)can accelerate the formation rate,and higher concentrations of humic acid can stabilize Ag NPs with smaller particle sizes.A higher reaction temperature（90 ^oC）can accelerate the generation rate of Ag NPs,resulting in the formation of Ag NPs with smaller particles in a shorter time.（2）The rate of reduction of Ag⁺to Ag NPs by soil humic acid is different under different reduction conditions.Soil humic acid reduced by sodium borohydride can significantly accelerate the formation rate of Ag NPs.The higher the degree of humic acid reduction,the faster the rate of Ag NPs formation.This is mainly attributed to the reduction treatment of sodium borohydride resulting in changes in the content of phenolic functional groups and organic radicals in humic acid,which directly affects the formation of Ag NPs.（3）Soil humic acid reducing functional groups and organic radical content may play a role in the formation of Ag NPs.The functional groups and free radicals of four types humic acid were determined by ATR-FTIR and EPR,which proved that the content of phenolic hydroxyl and the difference of free radicals would affect the generation of Ag NPs.At the same time,the mechanism of Ag NPs autocatalysis has also been proposed.In the process of humic acid reduction of Ag⁺,Ag⁺and OH^-react to form colloidal Ag₂O,and Ag⁺may be deposited on the solid surface of Ag₂O.The adsorbed Ag⁺interacted with organic matter and then produced Ag⁰.XPS was used to further prove the occurrence of the autocatalytic process.Two different Ag states,i.e.,Ag₂O and Ag⁰ species.The obtained results indicate that Ag NPs mainly exist as Ag₂O,accompanied by minor amount of Ag⁰,which increased with the increasing reaction time.By studying the transformation of Ag⁺from soil humic acid of different sources,this paper makes clear that soil humic acid can promote the natural formation of Ag NPs,which lays a foundation for the exploration of the existence of Ag NPs in the environment and its geochemical process.