Enhanced aggregation and sedimentation of nanoscale zero-valent iron (nZVI) with polyacrylamide modification

Nanoscale zero-valent iron (nZVI) settled slowly and incompletely in a nano-iron reactor (NIR) in wastewater treatment, and the effluent quality and processing capacity of nZVI were degenerated. Herein, three types of polyacrylamide (PAM), anionic-APAM (nZVI^APAM), cationic-CPAM (nZVI^CPAM), and nonionic-NPAM (nZVI^NPAM)) were applied to modify the nZVI (nZVI^PAM), which were proved to enhance aggregation and sedimentation in the gravity settling clarifier of NIR. PAM modification lead to aggregate by forming large agglomerates. The median sizes of aggregates were 32, 194, 168 and 133 μm respectively for nZVI, nZVI^CPAM, nZVI^NPAM, and nZVI^APAM. Under quiescent conditions, bare nZVI needed 5 min to reach sedimentation equilibrium, while nZVI^PAM just within 1 min nZVI^CPAM settled more quickly and completely than nZVI^NPAM and nZVI^APAM. The Fe concentration in the dynamic flow NIR effluent could keep a low level for 8 h for nZVI^PAM, while bare nZVI for 6 h. Iron concentration was 3.11, 0.037, 0.93, and 1.20 mg·L^-1 for nZVI, nZVI^CPAM, nZVI^NPAM, and nZVI^APAM after 8-h-reaction. Meanwhile, the reactivity of nZVI^PAM was kept much longer for lead removal in the NIR. Results demonstrated PAM modifications (especially CPAM) provided a reliable solution for nZVI aggregation and sedimentation in wastewater treatment.