Stabilization Mechanism Of Persistent Free Radicals In Soil Organic Matter

Environmentally persistent free radicals（EPFRs）are known as an emerging contaminant with high environmental risk.They have attracted broad attention due to their wide distribution and high potential of environmental toxic effects in the environment.There are stable oxygen-centered radical species of EPFRs in soil organic matter.These species are either formed in the humification,increasing with the process,or formed in the interactions between humic substances and mineral particles.However,due to the complex chemical structure of humic substance,the mechanism of the formation and stabilization of EPFRs in humic substances are still unclear.Previous studies have demonstrated that the EPFRs in humic substance are stabilized by aromatic ring systems because the free electrons can be partially delocalized.However,in the complex soil environment,organic aromaticity can not clearly account for the distribution of EPFRs,and there may be other factors undiscovered..For example,the role of mineral particles in the formation and stabilization of EPFRs,whether organic matters with EPFRs can accumulate stably in soil particles with certain size,are still unknown.Therefore,this study investigated distribution and formation of EPFRs in the natural soil particles with different sizes and humic substances components to explore stability mechanism of EPFRs in HM-minerals based on soil particle size grading,separation of different humic substancesand demineralization of HM-minerals（the mixture ofhumin and inorganic minerals）methods..The experimental conclusions are as follows:（1）Two types of EPFRs are commonly found in the whole soil（natural soil）,namely non-organic EPFRs（g₁）and organic EPFRs（g₂）.In contrast to g₂,g₁has higher strength than that of homologous g₂,though it is barely noted.Experiments found that g₁ was non-organic EPFRs,which was probably derived from metal oxide-Si O₂combination.Compared with sand and clay,intensity of g₁in the silt is the strongest,which is independent of the sampling point and depth.In soil particles with different sizes,more quantities of g₂ were stabilized in clay,which was closely related to the large amount of organic carbon absorbed.The wrapping of organic matters and the formation of organic mineral bonds on clay can improve the anti-decomposition ability of organic matters and increase stability of EPFRs.（2）The total intensity of g₂in humic acid andfulvic acid（that is HA-FA）extracted from the whole soil was much higher than that in the whole soil,i.e.,5-56 times of that in the whole soil.This is most likely due to the shielding effect of iron in the whole soil,which is caused by the anti-ferromagnetic interaction between the d orbital electrons of iron and the EPFRs electrons.The iron content in the sample was significantly correlated with the g₂ intensity at the level of 0.01.The total intensity of g₂ in HM-minerals is 2-7 times of that in the whole soil,and the both present significant positive correlation,including the signal intensity and the carbon standardized intensity of g_2.This indicates that the g₂ signal in the whole soil is directly related to the component of HM-minerals,and the g₂ signal in HU-minerals is the dominant one.（3）g₂ loss ratio and carbon loss ratio in the demineralization of HM-minerals present significant positive correlation,which seems to indicate that g₂ loss is partly due to carbon loss.However,no EPFRs signal was found in the lost carbon.Previous studies indicated that the formation or disappearance of EPFRs were less likely to occur in the process of demineralization due to chemical reactions of organic matters.Therefore,the loss of g₂ in HM-mineral in the process of demineralization is likely due to the dissolution of active minerals,which caused the destruction of organic-inorganic complex structure and thereby partial loss of organic matter for the lack of binding with minerals.The quenching of EPFRs was resulted from the loss of mineral stabilization.（4）EPFRs retained after the demineralization of HM-minerals are not dependent on the structure of the organic and inorganic complex.They are probably derived from the HM chemical structure alone and stabilized by the aromatic ring system.This proportion of EPFRs varies significantly with the planted crops.They are relatively stable in the natural soil,but may have higher environmental risks.With the change of soil environment,their redox activity may be activated and cause harm.This experiment deepens the understanding of stability mechanism of EPFRs in soil organic matter.Not limited to the aromatics of organic matters,it starts from a new perspective,i.e.,the agglomeration of EPFRs in soil particles with different sizes,which breaks through the previous understanding of EPFRs in HM and unveils stabilizing effect of inorganic minerals on EPFRs in organic matters.