Fe0 Nanoparticles Remain Mobile in Porous Media after Aging Due to Slow Desorption of Polymeric Surface Modifiers

Abstract

Nanosized zerovalent iron (nZVI) is used for in situ remediation of contaminated groundwater. Polyelectrolyte surface coatings are used to inhibit nZVI aggregation and enhance mobility in the subsurface for emplacement. The fate of nZVI is of interest given the uncertainties regarding the effects of nanomaterials on the environment, and depends in part on the stability of these surface coatings against desorption and biodegradation. This study measured the rate and extent of desorption of polyelectrolyte coatings used to stabilize nZVI, including polyaspartate (PAP MW = 2.5 kg/mol and 10 kg/mol), carboxymethyl cellulose (CMC MW= 90 kg/nol and 700 kg/ mol), and polystyrene sulfonate (PSS MW = 70 kg/mol and 1000 kg/mol). The initial adsorbed mass of polyelectrolyte ranged from 0.85 to 3.71 mg/m2 depending on the type and molecular weight (MW). Polyelectrolyte adsorption was confirmed by an increase in nZVI electrophoretic mobility. In all cases, desorption of polyelectrolyte was slow, with less than 30 wt% desorbed after 4 months. The higher MW polyelectrolyte had a greater adsorbed mass and a slower desorption rate for PAP and CMC. nZVI mobility in sand columns after 8 month of desorption was similar to freshly modified nZVI, and significantly greater than unmodified nZVI aged for the same time under identical conditions. Based on these results, polyelectrolyte modified nanoparticles will remain more mobile than their unmodified counterparts even after-aging. Other factors potentially affecting the fate of coated nZVI must be evaluated, especially the potential for biodegradation of coatings.