Amendment of organic acids significantly enhanced hydroxyl radical production during oxygenation of paddy soils

Abstract

Recently, hydroxyl radical (•OH) production during soil redox fluctuations has been increasingly reported, but the low efficiency of contaminant degradation is the barrier for engineering remediation. The widely distributed low-molecular-weight organic acids (LMWOAs) might greatly enhance •OH production due to their strong interactions with Fe(II) species, but it was less investigated. Herein, we found that LMWOAs amendment (i.e., oxalic acid (OA) and citric acid (CA)) significantly enhanced •OH production by 1.2 −19.5 times during oxygenation of anoxic paddy slurries. Compared with OA and acetic acid (AA) (78.4 −110.3 μM), 0.5 mM CA showed the highest •OH accumulation (140.2 μM) due to the elevated electron utilization efficiency derived from its strongest capacity for complexation. Besides, increasing CA concentrations (within 6.25 mM) dramatically enhanced the •OH production and imidacloprid (IMI) degradation (increased by 48.6%), and further decreased due to the extensive competition from excess CA. Compared to 0.5 mM CA, the synergistic effects of acidification and complexation induced by 6.25 mM CA rendered more formation of exchangeable Fe(II) that easily coordinated with CA, and thus significantly enhanced its oxygenation. This study proposed promising strategies for regulating natural attenuation of contaminants using LMWOAs in agricultural fields, especially soils with frequent occurrence of redox fluctuations.