Effect Of Copper Ions/Fulvic Acid On The Colloidal Stability, Transport And Reactivity Of Nanoscale Zero-valent Iron

Nanoscale zero-valent iron (NZVI) technology is quickly becoming the most widely used technology in environmental remediation. While the use and development of NZVI are understandably heralded as a promising environmental nanotechnology, fundamental questions remain on its long-term effectiveness, fate and health risks in the environment.This study investigated the interaction between Cu2+ and NZVI coated with three types of stabilizers (i.e., polyacrylic acid [PAA], Tween-20 and starch) by examining the Cu2+ uptake, colloidal stability and mobility of surface-modified NZVI (SM-NZVI) in the presence of Cu2+. The uptake of Cu2+by SM-NZVI and the colloidal stability of the Cu-bearing SM-NZVI were examined in batch tests. The results showed that NZVI coated with different modifiers exhibited different affinities for Cu2+, which resulted in varying colloidal stability of different SM-NZVI in the presence of Cu2+. The presence of Cu2+ exerted a slight influence on the aggregation and settling of NZVI modified with PAA or Tween-20. However, the presence of Cu2+ caused significant aggregation and sedimen-tation of starch-modified NZVI, which is due to Cu2+ complexation with the starch molecules coated on the surface of the particles. Column experiments were conducted to investigate the co-transport of Cu2+ in association with SM-NZVI in water-saturated quartz sand. It was presumed that a physical straining mechanism accounted for the retention of Cu-bearing SM-NZVI in the porous media. Moreover, the enhanced aggregation of SM-NZVI in the presence of Cu2+ may be contributing to this straining effect.This study also investigated the effect of fulvic acid (FA) on the colloidal stability and reactivity of NZVI at pH 5,7 and 9. The sedimentation behavior of NZVI differed at different pH. A biphasic model was used to describe the two time-dependent settling processes (i.e., a rapid settling followed by a slower settling) and the settling rates were calculated. Generally, the settling of NZVI was more significant at the point of zero charge (pHpzc), which could be varied in the presence of FA due to the adsorption of FA on the NZVI surface. More FA was adsorbed on the NZVI surface at pH 5-7 than pH 9, resulting in the varying sedimentation behavior of NZVI via influencing the electrostatic repulsion among particles. Moreover, it was found that there was a tradeoff between the stabilization and the reactivity of NZVIas affected by the presence of FA. When FA concentration was at a low level, the adsorption of FA on theNZVI surface could enhance the particle stabilization, and thus facilitating the Cr(?) reduction by providing more available surface sites. However, when the FA concentrations were too high to occupy the active surface sites of NZVI, the Cr(?) reduction could be decreased even though the FA enhanced the dispersion of NZVI particles. At pH 9, the FA improved the Cr(VI) reduction by NZVI. Given the adsorption of FA on the NZVI surface was insignificant and its effect on the settling behavior of NZVI particles was minimal, it was proposed that the FA formed soluble complexes with the produced Fe(?)/Cr(?) ions, and thus reducing the degree of passivation on the NZVI surface and facilitating the Cr(?) reduction.