The decreasing aggregation of nanoscale zero-valent iron induced by trivalent chromium

Abstract

Environmental contextNanoscale zero-valent iron is a promising material for environmental engineering and groundwater remediation. However, the environmental behaviour and fate of nanoscale iron that is essential for applications and risk assessment is still uncertain. We report a study on the aggregation behaviour and mobility of nanoscale iron in the aquatic environment using colloidal chemical methods. AbstractDespite high magnetisation, nanoscale zero-valent iron (nZVI) exhibits weak aggregation when treating hexavalent chromium (CrVI) (0.02mmol L–1) under anaerobic circumstances, which leads to the enhancement of its mobility in the aquatic environment. To elucidate such an unexpected phenomenon, the influences of different valences of chromium on the aggregation behaviour of nZVI were examined. Results indicate that trivalent chromium (CrIII) greatly decreases the aggregation of nZVI in acidic conditions (pH 5), while little influence is observed at a higher pH (pH 7). We suggest that such influences are mainly a result of precipitation on the surface of nZVI particles, which prevents the formation of chain-like aggregates. Accordingly, although the particles are highly magnetic (magnetite content >70%, saturation magnetisation=363 kA m–1), the magnetic attraction between aggregates and particles is not strong enough to promote further aggregation. Furthermore, the Cr(OH)3 shell blocks collisions between particles and greatly enhances their zeta-potential, which also assists in preventing aggregation. Our results suggest that heavy metals can significantly affect the environmental behaviours of nanoparticles.