Influence of humic acid on the colloidal stability of surface-modified nano zero-valent iron

To enhance colloidal stability of nano zero-valent iron (NZVI) used for groundwater remediation, the surfaces of such NZVI can be modified via coating with organic stabilizers. These surface stabilizers can electrostatically, sterically, or electrosterically stabilize NZVI suspensions in water, but their efficacy is affected by the presence of humic acid (HA) in groundwater. In this study, the effect of HA on the colloidal stability of NZVI coated with three types of stabilizers (i.e., polyacrylic acid (PAA), Tween-20 and starch) was evaluated. Differing stability behaviors were observed for different surface-modified NZVIs (SM-NZVI) in the presence of HA. Fluorescence spectroscopic analysis probed the possible interactions at the SM-NZVI-HA interface, providing a better understanding of the effect of HA on SM-NZVI stability. The adsorption of HA on the surface of PAA-modified NZVI via complexation with NZVI (rather than the PAA stabilizer) enhanced the electrosteric repulsion effect, increasing the stability of the particles. However, for NZVI modified with Tween-20 or starch, HA could interact with the surface stabilizer and apparently play a "bridge" role among the particles, which might induce aggregation of the particles. Therefore, the stability behavior of NZVI modified with Tween-20 or starch might have resulted from the combined effect of "bridging" and "electrosteric" exerted by HA. (C) 2012 Elsevier Ltd. All rights reserved.