Visualizing Electron\nTransfer through Silver Nanoparticle\nFormation and Photothermal Imaging: A Case Study of Nanoscale Zerovalent\nIron

Electron release and transfer are pivotal to the efficiency\nof\nmultiple biogeochemical and pollutant processes. Despite substantial\nefforts to develop electron-transfer characterization techniques, <i>in situ</i> visualization of electron transfer remains challenging.\nThis study introduces an innovative strategy for mapping electron-transfer\ndistance using nanoscale zerovalent iron (nZVI) as a case study. In\nthis method, silver ions (Ag<sup>+</sup>) are employed as electron\ntraps, forming silver nanoparticles. These nanoparticles can then\nbe immobilized in the agarose-solidified gel used as a heterogeneous\nporous medium to simulate the underground aquifer and visualized through\nphotothermal imaging. <i>In situ</i> detection has unveiled\nremarkably extensive electron transfer, reaching distances of up to\ncentimeters from the nZVI source. Nationwide groundwater investigations\nhave further confirmed this long electron-transfer distance, ranging\nfrom 1.7 ± 0.4 to 9.6 ± 0.1 mm (agarose gel may not fully\nreplicate the complex and heterogeneous conditions of groundwater\nenvironments). Importantly, the efficiency of electron transfer from\nnZVI was primarily influenced by factors such as nZVI dosage and groundwater\nmatrix conditions, particularly the abundance of electron-shuttling\nmaterials like natural organic matter. Surprisingly, the inherent\nproperties of nZVI had a lesser impact on electron-transfer distance.\nOur work highlights the long-distance electron transfer from nZVI,\nrevealing a previously overlooked but ubiquitous nZVI remediation\nprocess.