Organic ligands activate the dark formation of hydroxyl radicals (HO) in surface soil/sediment: Yields, mechanisms, and applications.

Soil is an important sink for various pollutants. Recent findings suggest that soil and sediment would spontaneously form HO through Fenton or Fenton-like reactions under natural conditions. In this study, the effects and mechanisms of organic ligands (OLs) on the occurrence of HO in surface soil/sediment were experimentally and computationally examined. Results confirmed that HO generation was ND-12.92 nmol/g in surface soil/sediment, and the addition of EDTA-2Na would significantly enhance the yields of HO by 1.4-352 times. Moisture was the decisive factor of soil HO generation. The release of Fe(II) from solid into the aqueous phase was essential for the stimulation of HO in EDTA-2Na suspensions. Furthermore, complexation reactions between Fe(II) and OLs would enhance single electron transfer (SET) reactions and the formation of O. Interestingly, for specific OLs, their stimulations on SET and formation of O would depress HO generation. Provoking HO generation by OLs could be efficiently used to degrade sulfamethoxazole in rice field sediment. The study provided new knowledge on how commonly synthetic OLs affect the HO generation in surface soil/sediment, and it additionally shed light on the engineered stimulation of in-situ Fenton reactions in natural soil/sediment.