IMPACT OF NATURAL ORGANIC MATTER ON THE SURFACE REACTIVITY OF GOETHITE NANOPARTICLES: ROLE OF MINERAL-ORGANIC CONTACT TIME

Nanoscale minerals continue to be an object of scientific scrutiny due to their physical and chemical properties, their abundance and roles in natural systems, and the many potential applications for these materials. Research has demonstrated that the presence of NOM plays a key role in surface reactivity of nanoscale minerals in the environment. Several factors such as contact time, the nature of the NOM, and the aqueous chemistry of the system influence the interaction between mineral surfaces and organics.

A series of batch reactions were conducted using synthetic goethite nanoparticles to investigate the impact of contact time between the mineral surface and NOM on the ability of the iron oxide to sorb chromate from solution. The amount of time allowed for goethite-NOM interaction (contact time) was varied from several seconds, up to 24 hours prior to the addition of chromate. Previous goethite-NOM experiments were done to determine the time required for the NOM adsorption maxima to be reached.

Results from the NOM adsorption experiments revealed that maximum adsorption occurred after 30 minutes of mineral-NOM contact time. Results from the metal sorption experiments confirmed that chromate was readily sorbed onto the surface of goethite in the absence of NOM. Goethite nanoparticles exposed near simultaneously to chromate and NOM (short contact time) saw an ~85% reduction in the initial amount of chromate sorbed compared to the baseline results without NOM. The deficiency in chromate sorption decreased over time, but was still ~64% after 24 hours. When goethite-NOM contact time was increased to ~24 hours prior to the introduction of chromate (long contact time), the decrease in initial chromate sorption was more dramatic: however, after ~24 hours the total amount of chromate sorbed was nearly identical to the short contact time experiments.

The results of this investigation revealed that the presence of NOM in our experiments significantly reduced the surface reactivity of nanoscale goethite. However, while mineral-organic contact time played a large role in the intial amounts of chromate sorbed to the particle surfaces, it had little impact at later experimental times. Thereby indicating that factors other than mineral-organic contact time play a more signifcant role over longer time periods.