Enhanced adsorption and reduction of Pb(II) from aqueous solution by sulfide-modified nanoscale zerovalent iron: characterization, kinetics and mechanisms

Nanoscale zerovalent iron (nZVI) was prone to aggregation and passivation, which limited its application. Here, sulfide-modified nZVI (S-nZVI) was synthesized to enhance Pb(II) removal, the transmission electron microscope and scanning electron microscope showed S-nZVI particles exhibited a core–shell structure, with Fe⁰ comprising the core and iron oxides in the shell, the generated iron sulfides (FeS_x) were distributed over the iron oxides shell. The water contact angle results suggested S-nZVI was more hydrophobic than nZVI, which could prevent the oxidation of the inner Fe⁰ core with aqueous solution, furthermore, the electrochemical experiments demonstrated the FeS_x improved the electron transfer efficiency from Fe⁰ core to contaminants, enhancing the reactivity of S-nZVI. Mechanism analysis confirmed that the electrostatic attraction, chemical precipitation, surface complexation and reduction occurred during the adsorption. S-nZVI with S/Fe molar ratio of 0.3 offered the best Pb(II) adsorption capacity, and higher pH value was favourable, the presence of co-existing cations (K⁺, Na⁺, Ca²⁺, Mg²⁺) all interfered the Pb(II) removal, followed as Ca²⁺ >Mg²⁺ > K⁺ > Na⁺. The pseudo-second-order kinetics and Langmuir model could well described the adsorption process, indicating that monolayer adsorption dominated the process. Thermodynamic results showed the adsorption was a spontaneous endothermic process, even after 4 cycles of recycling, the removal rate remained at 76.4 %, demonstrating S-nZVI had good reusability.