The ability of xanthan gum to act as a delivery vehicle for the transport in porous media of highly concentrated nano- and microscale zerovalent iron (NZVI and MZVI, respectively) slurries was investigated. Sand-packed column experiments were performed injecting iron suspensions at a concentration of 20 g/L, amended with xanthan gum (3 g/L), at different ionic strength values (6 x 10(-3) mM or 12.5 mM) in 0.46 m long columns. Breakthrough curves of iron, obtained by in-line continuous measurement of magnetic susceptibility, under each experimental condition showed that normalized elution concentration at the end of the injection (i.e., after 7 or 26 pore volumes) is higher for MZVI (>0.94) than for NZVI (>0.88). Additional susceptibility measurements along the column and pressure drop also confirmed that MZVI is more easily eluted than NZVI. Moreover, water flushing after the iron injection phase lead to recoveries of over 95% for MZVI, and over 92% for NZVI of the total injected iron mass. The tests proved that xanthan gum is an excellent stabilizing agent and delivery vehicle of ZVI particles and has a high potential for use in real scale remediation interventions.

A pilot injection test with guar gum stabilized microscale zerovalent iron (mZVI) particles was performed at test site V (Belgium) where different chlorinated aliphatic hydrocarbons (CAHs) were present as pollutants in the subsurface. One hundred kilograms of 56μm-diameter mZVI (~70gL(-1)) was suspended in 1.5m(3) of guar gum (~7gL(-1)) solution and injected into the test area. In order to deliver the guar gum stabilized mZVI slurry, one direct push bottom-up injection (Geoprobe) was performed with injections at 5 depths between 10.5 and 8.5m bgs. The direct push technique was preferred above others (e.g. injection at low flow rate via screened wells) because of the limited hydraulic conductivity of the aquifer, and to the large size of the mZVI particles. A final heterogeneous distribution of the mZVI in the porous medium was observed explicable by preferential flow paths created during the high pressure injection. The maximum observed delivery distance was 2.5m. A significant decrease in 1,1,1-TCA concentrations was observed in close vicinity of spots where the highest concentration of mZVI was observed. Carbon stable isotope analysis (CSIA) yielded information on the success of the abiotic degradation of 1,1,1-TCA and indicated a heterogeneous spatio-temporal pattern of degradation. Finally, the obtained results show that mZVI slurries stabilized by guar gum can be prepared at pilot scale and directly injected into low permeable aquifers, indicating a significant removal of 1,1,1-TCA.

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