Synthesis of nanoscale zero-valent iron by one-pot route and study of its potential in passivating coexistent heavy metal anions and cations in soil

Nanoscale zero-valent iron (nZVI) was synthesized by a one-pot liquid-phase chemical method in the presence of FeSO₄ as the iron source and NaBH₄ as the reducing agent. The synthesized nZVI was characterized by scanning electron microscopy, X-ray diffraction, energy dispersive spectrometry, and Fourier transform infrared spectroscopy. Its ability to passivate Pb²⁺, Cd²⁺, and AsO₄ ^3- in soils was evaluated by inductively coupled plasma-atomic emission spectroscopy, and the passivation mechanism was explored based on adsorption thermodynamics and kinetics simulations. It was found that nZVI is spherical in shape with a diameter of 60-80 nm and exhibited a satisfactory magnetic response, favoring facile recycling under a magnetic field, which could be directly applied to passivate Pb²⁺, Cd²⁺, and AsO₄ ^3- in contaminated soils. The passivation ability for Pb²⁺, Cd²⁺, and AsO₄ ^3- depended on the drying conditions and the dosage of NaBH₄. Notably, nZVI prepared with 4 g of NaBH₄ under vacuum drying exhibited the strongest passivation ability. The adsorption of the tested heavy metals by nZVI conformed to the Langmuir isotherm model, and the correlation coefficients were 0.99 (Pb), 0.99 (Cd), and 0.93 (As), which indicated saturated monolayer adsorption. The corresponding maximum saturated adsorption amounts were 117.65 mg g^-1 (Pb), 45.45 mg g^-1 (Cd), and 6.82 mg g^-1 (As), respectively. Additionally, the adsorption by nZVI of the heavy metal ions under investigation followed the pseudo-second-order kinetic equation, referring to chemical adsorption, and the chemisorption percentages for Pb²⁺, Cd²⁺, and AsO₄ ^3- were 93.0%, 74.8%, and 32.9%, respectively. This could account for the difference in the adsorption capacity of nZVI for the tested heavy metal ions. Moreover, 19 consecutive days of desorption experiments demonstrated that nZVI/M (M represents Pb, Cd, and As; <i>i.e.</i>, Pb²⁺, Cd²⁺, and AsO₄ ^3-) possessed strong stability. Our data indicate that nZVI has the potential to be an excellent nano-adsorbent with good passivation performance for the rapid and efficient passivation of Pb²⁺, Cd²⁺, and AsO₄ ^3- in multi anion-cation co-contaminated soils.