Enhanced separation of nanoscale zero-valent iron (nZVI) using polyacrylamide: Performance, characterization and implication

Applications of nanoscale zero-valent iron (nZVI) in wastewater treatment demand fast and complete separation of nZVI for its reuse and to ensure high-quality effluent. Gravity settling is a promising candidate for nZVI separation due to its low capital and operational cost. Herein, we report polyacrylamide-enhanced separation of nZVI under quiescent and dynamic conditions. A considerable fraction (20–60   mg/L) of bare nZVI remained in supernatant as stable colloids, even with extended settling time of 3   h and very low overflow rate (0.1   m 3 /m 2   h). The addition of low dose PAM (⩽10   mg/g-nZVI) increased the settling efficiency of nZVI by nearly 5 times and improved the effluent quality as well. The colloidal iron in the supernatant was reduced to less than 5   mg/L; the mean hydrodynamic size of nZVI increased by nearly 60 times, from 7.6   μm to 474   μm, as measured by optical microscope and particle size analyzer. Low dosage (⩽10   mg-PAM/g-nZVI) of PAM reduced the surface charge (zeta potential) of nZVI while overdosing PAM (⩾50   mg-PAM/g-nZVI) may recharge and re-stabilize the iron nanoparticles. No inhibition effect of PAM on nZVI reactivity was observed, likely due to its low surface coverage. The study demonstrates a reliable and cost-effective solution for nZVI separation and also proves that nZVI is fully compatible with conventional wastewater treatment processes.