Transport, reactivity and fate of polyelectrolyte modified zero valent iron nanoparticles used for groundwater remediation in heterogeneous porous media

In situ ground water remediation using nano scale zero valent iron (nZVI) is proposed for treating chlorinated organic solvent (e.g. TCE, PCE) source zones. nZVI is surface modified with surfactant or anionic polyelectrolytes to achieve the mobility needed for emplacement in the subsurface. A fundamental understanding of how hydrogeochemical factors affect nZVI mobility and reactivity is needed to optimize treatment using nZVI, and to ensure that it is safely applied. This thesis determined the fate of the polyelectrolyte coatings used on nZVI, the effect of subsurface chemical and physical heterogeneity on nZVI transport, methods to enhance mobility for emplacement, and the effect of the nZVI mass loading and seepage velocity on the PCE reactivity with nZVI.;Groundwater pH over the environmental range of 6 to 8 affected the transport of anionic polyelectrolyte coated nZVI in the porous media by affecting both nZVI aggregation and deposition. There was more aggregation of both weak and strong polyelectrolyte modified nZVI at pH 6 than pH 8. The aggregate size of surface coated nZVI was >1.5 micron at pH 6 and affected transport. Transport of non-aggregating hematite particles was also lower at pH 6 than 8, indicating enhanced deposition at pH 6. This is attributed to pH dependent surface charge heterogeneity on the sand surface. Both aggregation and enhanced deposition onto clay particles reduced the mobility of surface modified nZVI at low pH. Enhanced deposition between surface modified nZVI and clay particles was confirmed with in a heteroaggregation study. Excess polymer (100 mg/L) mitigated particle aggregation and deposition onto the sand or clay surfaces at low pH and enhanced nZVI transport.;The PCE dechlorination rate constants determined from the batch studies with modified nZVI at field relevant application concentrations (1, 2, 5, 10, 15 and 20 g/L) ranged from 2 ∼ 4 x 10-4 Lh -1m-2. A nonlinear relationship between the pseudo first order reaction rate constant (kobs) and nZVI mass loading was observed below an nZVI concentration of 5 g/L. Increasing mass loading of nZVI increased the PCE dechlorination rate, however, the hydrogen evolution rate was higher at the lower nZVI dosage. Increasing the linear velocity from 7 cm/day to 113 cm/day increased reactivity nearly 3-fold which indicates mass transfer limitations may occur at low flow velocities. The nature of the limitation is not known.;The results of this thesis indicate that nZVI has the potential to be used as an alternative remediation technology for treating DNAPL source areas. Conclusions from these studies have several practical implications for the field application. First, adsorbed anionic polyelectrolyte surface coatings on nZVI are not readily desorbed therefore nZVI will remain potentially mobile in the aquifer after aging. Second, nZVI mobility can be strongly limited by pH and chemical heterogeneity of the aquifer material by promoting aggregation and deposition. Controlling surface charge heterogeneity on the surface of porous media by adding excess polymer in the injection solution is recommended to provide enhanced mobility under unfavorable transport conditions (e.g. low pH and high clay content). Finally, nZVI treatment is more effective at high mass loading (above 5 g/L) both for PCE dechlorination and for Fe 0 utilization efficiency.;Because the polyelectrolyte coatings dramatically affect transport, the loss of coating over time after emplacement will decrease nZVI mobility and the potential for unwanted migration from the source zone. The rate and extent of desorption of polyelectrolyte coatings used to stabilize nZVI was measured. The initial adsorbed mass of polyelectrolyte depended on the type and molecular weight (MW). Desorption of polyelectrolyte was slow and, in general the higher MW polyelectrolyte had a greater adsorbed mass and a slower desorption rate. Because the rate of desorption was so slow, nZVI remained potentially mobile even after 8 months.