Preparation, Characterization, and Applications of Polysaccharide-stabilized Metal Nanoparticles for Remediation of Chlorinated Solvents in Soils and Groundwater

Abstract

Removal of chlorinated solvents in groundwater and soils represents one of the most challenging environmental issues. Highly reactive palladized iron (Fe-Pd) nanoparticles offer the potential to migrate in the soil and rapidly degrade the contaminants in source zones and became an attractive alternative for chlorinated solvent remediation. However, due to their high surface energy, Fe-Pd nanoparticles prepared using current methods tend to agglomerate immediately to form large agglomerates, rendering them undeliverable to the targeted area. This study reports that select food-grade starch and carboxymethyl cellulose (CMC) can be used as green stabilizers to produce highly dispersible Fe-Pd nanoparticles, which are reactive and mobile in soil and suitable for in situ injection.

Transmission electron microscopy (TEM) showed that the average particle sizes of both starch- and CMC- stabilized iron nanoparticles were less than 20 nm. Fourier transform infrared (FTIR) spectroscopy results suggested that stabilizer molecules were adsorbed to iron nanoparticles resulting in a steric layer, and thereby, preventing the nanoparticles from agglomeration. The stabilized iron nanoparticles were mobile in the porous media. For example, the CMC-stabilized iron nanoparticles had a low sticking efficiency of 0.0025 in the sand. Meanwhile, the stabilized nanoparticles displayed remarkably greater reactivity than non-stabilized particles. Batch tests demonstrated that the CMC-stabilized nanoparticles degraded trichloroethene (TCE) 17 times faster than non-stabilized counterparts. Further studies showed that CMC may inhibit TCE degradation at a stabilizer-to-Fe molar ratio greater than 0.0124. Within the same homologous series, CMC of greater molecular weight resulted in more reactive nanoparticles. Through selecting the type of stabilizers and synthesizing conditions, the size of the stabilized ZVI nanoparticles were also controlled.

Two field tests carried out in California and Alabama confirmed the unprecedented soil mobility and dechlorination reactivity of the CMC-stabilized Fe-Pd nanoparticles. Groundwater samples from the Alabama site also showed significant promotion of enhanced biodegradation of chlorinated solvent contaminants up to 4 months after injection.

The feasibility of using CMC for synthesis of highly reactive Pd nanoparticles was also investigated in this study. The resultant CMC-Pd nanoparticles exhibited rather high catalytic activity for hydrodechlorinatino of TCE (kobs > 828 L*gPd-1*min-1) and hold the promise for future applications in chlorinated solvent remediation.