Influence of calcium ions on the colloidal stability of surface-modified nano zero-valent iron in the absence or presence of humic acid

To decrease aggregation and enhance the mobility of nano zero-valent iron (NZVI) used for in-situ groundwater remediation, the surface of such NZVI must be modified using organic stabilizers, which can provide electrostatic repulsion, and steric or electrosteric stabilization. However, the stability of the nanoparticles can also be affected by groundwater components such as cations and humic acid (HA). In this study, the effect of Ca2+ on the colloidal stability of NZVI coated with three types of stabilizers (i.e., polyacrylic acid (PAA), Tween-20 and starch) was evaluated in the absence or presence of HA. Differing stability behavior was observed for different surface-modified NZVIs. The presence of Ca2+ exerted a slight influence on the settling of NZVI modified with PAA or Tween-20, in the absence or presence of HA. However, the presence of Ca2+ caused significant aggregation and sedimentation for starch-modified NZVI in the absence of HA, and induced an even higher degree of aggregation and sedimentation in the presence of HA. It is presumed that, in the absence of HA, starch-modified NZVI particles undergo attachment with each other via Ca2+ complexation with the coated starch molecules on the surface of the particles, thus enhancing the aggregation and the following sedimentation of starch-modified NZVI. However, in the presence of HA, spectroscopic analysis of the starch-modified NZVI aggregates indicated that the bridging interaction of HA with Ca2+ was the predominant mechanism for the enhanced aggregation.

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► Colloidal stability of three surface-modified NZVIs (SM-NZVI) was compared.
► The effects of Ca2+ on the stability of three SM-NZVIs were different.
► A synergistic effect of Ca2+ and HA on the settling of SM-NZVI was observed.
► The mechanisms for the particle stabilization or destabilization were probed.