Effect of particle age (Fe0 content) and solution pH on NZVI reactivity: H2 evolution and TCE dechlorination

Subsurface injection of nanoscale zerovalent iron (NZVI) has been used for the in situ remediation of chlorinated solvent plumes and DNAPL source zones. Due to the cost of materials and placement,the efficacy of this approach depends on the NZVI reactivity and longevity, selectivity for the target contaminant relative to nonspecific corrosion to yield H2, and access to the Fe0 in the particles. Both the reaction pH and the age of the particles (i.e., Fe0 content) could affect NZVI reactivity and longevity. Here, the rates of H2 evolution and trichloroethene (TCE) reduction are measured over the lifetime of the particles and at solution pH ranging from 6.5 to 8.9. Crystalline reactive nanoscale iron particles (RNIP) with different initial Fe0 weight percent (48%, 36%, 34%, 27%, and 9.6%) but similar specific surface area were studied. At the equilibrium pH for a Fe(OH)2/H2O system (pH = 8.9), RNIP exhibited first-order decay for Fe0 corrosion (H2 evolution) with respect to Fe0 content with a Fe0 half-life time of 90-180 days. A stable surface area-normalized TCE reduction rate constant 1.0 x 10(-3)L x hr(-1) x m(-2) was observed after 20 days and remained constant for 160 days, while the Fe0 content of the particles decreased by half, suggesting that TCE reduction is zero-order with respect to the Fe0 content of the particle. Solution pH affected H2 evolution and TCE reduction to a different extent. Decreasing pH from 8.9 to 6.5 increased the H2 evolution rate constant 27 fold from 0.008 to 0.22 day(-1), but the TCE dechlorination rate constant only doubled. The dissimilarities between the reaction orders of H2 evolution and TCE dechlorination with respect to both Fe0 content and H+ concentration suggest that different rate controlling steps are involved for the reduction reactions.