Flocculating microscale zero-valent iron (mZVI) improves its hydrodynamic properties for wastewater treatment

Abstract

Microscale zero-valent iron (mZVI) powder is dense due to the presence of metallic iron (Fe(0)). Herein, we show that the dense mZVI has several hydrodynamic problems in wastewater treatment unit operations (WTUO), and an mZVI modification method using inorganic floc is proposed and studied to solve the problems. Specifically, mZVI particles are flocculated using Fe(OH)2 or Al(OH)3 floc, forming a mZVI-floc combination ((mZVI)a(floc)b) in which mZVI is embedded in the floc. The (mZVI)a(floc)b is then characterized at hydrodynamic conditions simulating those of large-scale WTUO (e.g., mechanical mixing and gravitational settling). The study demonstrates that 1) the combination is better in mixedness during the mixing, in settleability during the settling and in fluidity in the concentrated form; 2) the mZVI remains embedded in or attached to the floc during the mixing. The study also provides physical and rheological data about the (mZVI)a(floc)b, such as size, bulk density and apparent viscosity. The improvement in hydrodynamic properties is attributed to the low particle density after the flocculation, which is less than one tenth to that of the bare mZVI. Comparative experiments show no reactivity inhibition of the floc on the embedded mZVI. The recommended floc/mZVI molar ratio for practical use is ≥0.5:1, with the influent pH adjusted to ∼6.2 (for Al(OH)3) or ∼8.9 (for Fe(OH)2); it is also suggested that these operating parameters can be determined on a case-by-case basis via bench- and/or pilot-scale experiments studying the treatment efficiencies and evaluating the overall treatment cost. The flocculation method makes mZVI more adaptable to the large-scale wastewater treatment applications (e.g., the treatment plant retrofitting). The work is also the 1st report about mZVI hydrodynamics for wastewater treatment and provides study methodology for this topic.