Pd-Fe bimetallic nanoparticles (BNPs) have substantially higher reactivity for reductive dehalogenation of chlorinated ethenes (e.g., trichloroethylene, TCE) compared to monometallic zero-valent iron nanoparticles, yet the rapid deactivation of BNPs in groundwater matrices limits their large-scale use in field applications. In spite of this shortcoming, the causes of BNP deactivation have not been clearly delineated. Stable carbon isotope fractionation measurements and product distribution analysis were used in this study to investigate the mechanisms of Pd-Fe deactivation in the presence of common groundwater solutes. Based on the apparent TCE degradation rates and at a constant solute concentration of 5 mM (except for humic acid, which was dosed at 20 mg/L), Pd-Fe BNPs exposed to SO42-, HPO42-, and humic acid solutions showed moderate declines in TCE dechlorination rates. Aging the bimetallic particles in Cl-, SO32-, HCO3-, and NO3- solutions, however, resulted in excessive or complete loss of TCE dechlorination reactivity. Analyses of the isotope fractionation associated with TCE hydrodechlorination (sTamc) as well as the yield of ethane over other dechlorination products suggest at least four distinctive causes of deactivation: (i) aging the BNPs in deionized water and humic acid induces surface passivation due to buildup of mineral or organic carbon deposits; (ii) SO32- and Cl- ions interact specifically with Pd sites and disable the catalyst functions; (iii) NO3- and HCO3- inhibits iron corrosion, thereby limiting the production of H-2 as the precursor of reactive hydrogen species, and (iv) selective deactivation Of surface sites involved in ethene hydrogenation was observed for BNPs aged in SO42- and HPO42- solutions. The findings suggest the Pd-on-Fe configuration of the bimetallic particles is susceptible to deactivation in a broader range of groundwater chemistry than previously expected. (C) 2016 Elsevier B.V. All rights reserved.