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

• nZVI settling was studied under dynamic and quiescent conditions.
• Polyacrylamide (PAM) enhanced nZVI settling and improved effluent quality.
• PAM increased hydrodynamic size of nZVI by 60 times and reduced its surface charge.
• The inhibition effect of PAM on nZVI reactivity was negligible.
• PAM makes nZVI more applicable to the existing wastewater treatment facilities.

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 m3/m2 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.

Polyacrylamide increases the separation efficiency of iron nanoparticles and improves the effluent quality, providing a facile way to adapt iron nanoparticles in the existing wastewater treatment facilities.Figure optionsDownload as PowerPoint slide