DETERMINATION OF TRANSPORT BEHAVIOR OF CARBOXYMETHYL CELLULOSE STABILIZED NANO-SCALE ZERO VALENT IRON IN POROUS MEDIA THROUGH TWO-DIMENSIONAL EXPERIMENTS AND NUMERICAL MODELING

Nano-scale zero valent iron (nZVI) has shown potential for treating a wide variety of groundwater contaminants. However, due to agglomeration of the nZVI, delivery into subsurface contaminated zones is challenging. Stabilization of nZVI with polymer, such as carboxymethyl cellulose (CMC), can enhance the mobility of the iron particles in the subsurface. In this work, a set of laboratory-scale transport experiments and numerical simulations were performed to evaluate CMC stabilized nZVI transport in porous media. The transport experiments were conducted in a two-dimensional water-saturated glass-walled sandbox (55 cm x 45 cm x 1.3 cm in size) uniformly packed with silica sand. The experiments were designed to identify the effects of water specific discharge and CMC concentration on nZVI transport. Experiments were also performed using Lissamine Green B (LGB) dye as a non-reactive tracer to characterize the sand media. For the CMC stabilized nZVI transport experiments, nZVI was synthesized freshly before each experiment at a concentration of 1000 mg/L. The synthesized CMC-nZVI mixture was characterized using transmission electron microscopy, dynamic light scattering, and UV-visual spectrophotometry. The transport and movement of LGB, CMC, and CMC-nZVI was evaluated through analysis of the breakthrough curves at the outlet, retained nZVI in the sandbox, and the time-lapsed images captured using a light source and a dark box.

In all cases, the mass recovery of LGB was greater than 95 % indicating non-reactive transport in silica sand. The CMC mass recovery was also greater than 95 %; however, the mean residence time was significantly higher. The results also showed that increased concentration of CMC contributed to increased nZVI stability, but caused higher pressure drop in the sand box due to viscous effects, indicating that use of higher CMC concentration may limit the injection rates. The mass recovery of nZVI was lower (~ 30 %) due to attachment onto the sand surfaces. Transport of LGB, CMC, and nZVI was modeled using a multiphase flow and transport model considering LGB and CMC as solutes, and nZVI as a colloid. The simulation results matched the experimental observations and provided estimation for some transport parameters that can be used to predict CMC stabilized nZVI transport in similar porous media.