Desorption of adsorbed lead(II) on hydrochar under conditions of soil pH and fluctuating redox potential induced by waterflooding

Hydrochar (HC) adsorbents for heavy metals (HMs) removal have received increasing attention in recent decades. However, existing studies demonstrate a paucity of data on the desorption performance of Pb(II) adsorbed on HC under the combined influence of soil pH and redox potential (Eh). In this study, a soil column experiment revealed that the soil Eh changed from an oxidative state (Eh 426-474mV)to a reductive state (approximately -200mV) in three soils with average pH values of 8.03, 6.53, and 5.53, respectively, after 70h of waterlogging. In the electrochemical experiment, X-ray photoelectron spectroscopy (XPS) revealed that the content of the three main oxygen-containing groups (OCGs), including C=O, C-O, and O-C=O on HC surfaces, decreased by 39.94%, 45.93%, and 74.11% at pH 4.0, 7.0, and 8.0, respectively, as Eh decreased from +200mV to -200mV. Under neutral and alkaline reducing conditions, the combined effects of hydrochar debris (HD) exfoliation from HC surfaces and OCGs reduction promoted the release of Pb(II) as HD-Pb(II) complexes; in contrast, under acidic conditions, Pb(II) existed predominantly as free Pb²⁺ ions. The soil column tests showed that Pb(II) release from HC-Pb(II) in acidic, neutral, and alkaline soils reached 1.04, 0.61, and 0.29mg L^-1, respectively, during waterlogging, with released concentration correlating with the pe+pH values in the soil environment. These results provide valuable insights into the fate of HC-adsorbed Pb(II) in soils under reducing conditions induced by waterlogging.