Effect of NOM on Sedimentation of nZVI

These videos show the effect on natural organic matter (NOM) on the settling of nano-sized zero-valent iron (nZVI) on aqueous carbonate buffer. 0 mg/L NOM is shown on the left, and 200 mg/L NOM is on the right. The NOM results in less aggregation of the primary particles and therefore slower settling. Some of the large aggregates in the 0 NOM video are immobile because they are stuck to the cuvette walls.

The experiments were performed to help with the interpretation of data that we published in Johnson et al. (2009) Natural organic matter enhanced mobility of nano zero-valent iron. Environ. Sci. Technol. 43(14): 5455-5460.

Early in that work, we collected conventional "sedimentation curves" (Fig. 4A), but we then found that calibration curves for these data (Fig. S13) were complex. To more fully characterize the sedimentation of nZVI, we obtained complementary data photographing cuvettes (Fig. 4B) serum bottles (Fig. S12), and glass slides (Fig. S14). The former were taken using a 10 mm by 2 mm thick cuvette, backlit using diffuse white light, and a Nikon D90 camera and 105mm macro lens. The full dimensions of the resulting images are approximately 7 x 4.6 mm.

Photographs of the cuvette were taken every 2 minutes, and a total of ~400 images were collected. To create the videos shown here, the images were combined and are displayed at a nominal rate of 0.25 seconds per image, which results in particle movement ~500 times faster than real time.

Both experiments were done with 1 g/L nZVI (Toda RNIP, dry powder) starting immediately after this material was first suspended in 100 mg/L sodium bicarbonate. At t = 0, the nominal size of the individual primary particles was ~40 nm.