Study On The Hydroxyl Radicals Formed In Surface Soils And Sediments: Kinetics,influencing Factors And Mechanisms

Hydroxyl radical（HO^·）is a strong oxidative specie in natural environment（E=2.8V）.It can oxidize almost all biological macromolecules,organic and inorganic substances,and participate in a variety of environmental processes.HO^·is very likely to be involved in carbon cycle,oxygen cycle,and water cycle in soil and water.Surface soils and sediments such as farmlands and fishponds are sinks of organic pollutants,nutrients and organic carbon.Therefore,the occurrence of HO^·in soil could be importantly related to the transformation of organic matter,geochemical cycle of carbon,the degradation and detoxification of soil pollutants,as well as food safety.Hence,the apparent yield and formation mechanism of HO^·in surface soil and sediments were examined in this study.Firstly,the background levels and influencing factors for HO^·occurred in different types of surface soils and sediments were analyzed.Then,the effect of temperature on the formation kinetics of HO^·were explored,and the energy-demand for the occurrence of HO^·in dark were determined.Secondly,the effects of typical soil ameliorators and coordination remediation agents on the yields of HO^·were studied.Moreover,sulfamethoxazole was selected as a typical pollutant to study its degradation by soil/sedimental HO^·.Finally,the source of HO^·was confirmed,and quantum chemistry calculation was further performed to examine the relationship between the yields of HO^·and organic,mineral,as well as their complexes.The results of this study show that:（1）HO^·is widely exists in natural surface soils and sediments.The contents of HO^·in flooded soil like paddy fields and fish pond sediments were relatively high（3.96-5.08nmol/g）,which were still significantly lower than their contents found in underground.Yields of HO^·was positively correlated with the content of organic matter in samples with moisture less than 10%（R²=0.69）,suggested that the transition metal combined with organic matter may be the main source for the generation of HO^·;Yields of HO^·were significantly high in samples with moistures higher than 10%,implied the reductive condition was beneficial for the occurrence of HO^·.Yields of HO^·were positively correlated with temperature.The activation energy for HO^·generation was calculated as 28.63-55.59k J·mol^-1,indicated that the formation of HO^·in surface soils/sediments was a ubiquitous natural process.（2）Addition of Ca O in soils/sediments weakly promoted the formation of HO^·（<10times）.Addition of organic ligands such as EDTA-2Na could significantly promote the formation of HO^·（350 times）.The promotion effects were more significant in samples with higher moisture.Yields of HO^·stimulated by unstable complexes is much lower than stable complexes,suggested complexation is the basic mechanism in increasing the generation of HO^·in soils/sediments.The stimulated generation of HO^·would effectively degraded SMZ in the soil by 15%,implied the important roles of HO^·in the geochemical cycle of soil/sedimental pollutants.（3）Fe（II）and organic matter in the surface soil/sediment were the primary and important sources of HO^·.O₂^·-and H₂O₂ were the direct precursors of HO^·.Fe（II）and humus contained in the solid part of the soil would be released with the addition of ligand compounds.Coupled with the releasing processes,yields of HO^·achieved 4493.5n M in aqueous phases,while only 380.2 n M were formed by solid phase.Fe（II）complexes with lower VIP and higher VEA were more likely to induce HO^·formation,confirmed complexation played an important role in HO^·formation.Ligand compounds could promote HO^·generation by enhancing the electron transfer and Fenton/Fenton-like reactions.