Optimized synthesis of methanol-assisted nZVI for assessing reactivity by systematic chemical speciation approach at neutral and alkaline conditions

Nano zero valent iron (nZVI) of <20 nm gained recognition as one of the most promising nanomaterials for rapid environmental cleanup activities. This study demonstrates the formation mechanism, characterization and catalytic activity of <5% size distributed nZVI (12–20 nm) synthesized in aqueous-methanol solution. Dynamic light scattering proved that the solution containing 1:1.22 ratio of methanol and Deionized (DI) water provided the best control over production of smaller-sized homogenous particles. The peaks observed in UV–vis spectrometer, ATR-FTIR spectra and XRD verified the formation of pure Fe0 particles. The catalytic performance of MeOH45-nZVI was assessed using model pollutant Cr6+ at different aspects such as varying pH, initial concentration of MeOH45-nZVI and Cr6+. A detailed chemical speciation study was carried out to reveal the pH dependent multi-kinetic trend and Cr6+ reduction mechanism, especially at neutral and alkaline pH conditions. Regression equations were developed for Cr6+ reduction at every stage of reaction, considering significant Fe species; and the new model was assessed for quality of fit and adequacy. The passive layer on MeOH45-nZVI was evidenced with differential interference contrast (DIC) images and analyzed for chemical composition that further supported the proposed mechanisms. Also, chemical speciation study confirmed that acidic pH is more favorable not only in terms of accelerated kinetics, but also in lesser iron utilization/gm of Cr6+ removed. Furthermore, MeOH45-nZVI produced trace amount of Fe ions and sludge at the end of the reaction, favoring it as an ideal choice of packing material for point of use drinking water treatment and other environmental applications.