Guar gum enhances the stability and mobility of zerovalent iron nanoparticles in porous media

Injection of Nanoscale ZeroValent Iron (NZVI) is a promising technology for remediation of groundwater contamination and many successful reactivity studies have been reported using organic and inorganic pollutants. However, the efficiency of this process is significantly hindered by the rapid aggregation of the iron nanoparticles and their affinity with soil grains. The aim of this study is to enhance the colloidal stability of the nanoparticles by coating them with the “green” polymer guar gum, and assess the mobility of the stabilized suspensions in saturated porous media The study evaluated the properties of guar gum and its influence on the surface properties, particle size, aggregation, sedimentation and transport of the iron nanoparticles. RNIP (Reactive Nanoscale Iron Particles) from Toda Kogyo Corp. were dispersed at a concentration of 154 mg/L in 0.5 g/L guar gum solution. The aggregation behavior of the suspensions was compared to the ones of bare RNIP at different ionic strengths in the presence of NaCl and CaCl2. Column experiments were conducted to evaluate the mobility of the above suspensions in a representative sandy medium at various verisimilar aquifer conditions. High performance size exclusion chromatography (HPSEC), charge titration, and viscosity assessment showed that guar gum is a high molecular weight polymer which is nearly neutrally charged, rendering it suitable for steric stabilization of the iron nanoparticles. . Electrophoretic mobility measurements demonstrated the ability of guar gum to adsorb on the nanoparticles, forming a slightly negatively charged layer. Dynamic light scattering (DLS) experiments were conducted to estimate the particle sizes of the different suspensions and to determine the trend of aggregation at different ionic strengths. Guar gum showed the ability to reduce the hydrodynamic radius from 500 nm for bare particles to less than 200 nm and to prevent aggregation even at very high salt concentrations of 0.5 M NaCl and 3 mM CaCl2. Sedimentation of the different dispersions was evaluated by light absorbance measurements, confirming the improved stability of the dispersions in the presence of guar gum. Finally, column experiments proved the ability of guar gum to significantly increase particle elution. Values of C/C0 of up to 0.87 at the end of the column were observed, at relatively low fluid approach velocities.