The impact of natural organic matter on the surface reactivity of goethite nanoparticles: exploring mineralorganic contact time, pH, and ionic strength

Abstract

Hexavalent chromium, a toxic heavy metal generated by various industrial processes, can have dire consequences on groundwater resources. A potential low-cost and non-toxic remediation method is to use goethite nanoparticles, which sorb Cr(VI) onto its surface, thereby, sequestering the Cr(VI) in the subsurface. However, natural organics, which also sorb onto goethite, are present in all natural water sources, which consequently reduce the reactive surface area of goethite to sorb chromium. Previous investigations conducted by the Swindle lab have observed that natural organics sorb differently in a lab versus field setting. In the lab, organics formed aggregates on the surface of magnetite nanoparticles while in the field an even coating developed. In the previous work, the lab experiments had a mineral-organic time of only a few minutes, while in the field, the magnetite and organics were in contact for 30 days. There was also a considerable difference in ionic strength between the two previous studies. The aim of the current study was to determine if mineral-organic contact time and/or ionic strength can explain the differences observed in the previous studies. Goethite-chromate sorption experiments were conducted in the absence and presence of isolated organics, a natural water sample from the Cheney Reservoir, and a synthetic groundwater sample with mineral-organic contact times of 1 minute, 12 hours, and/or 24 hours. The isolated organic experiments resulted in decreasing amounts of chromium sorption as contact time increased. The natural water sample resulted in no chromium sorption and the synthetic water sample resulted in relatively small amounts of chromium sorption but only when a significant amount of time had passed. The results indicated that although mineral-organic contact time played a role in the isolated organics experiments, the pH in the natural and synthetic water experiments had such a large impact on chromium sorption that the roles of contact time and ionic strength were completely masked.