Transformation of nitrate and humic acid by nano-scale zero valent iron particles

Abstract

Chemical reduction of nitrate, which is a major water contaminant regulated in most countries, by nano-scale zero valent iron (NZVI) particles has attracted great attention recently due to its high potential for nitrate control. However, a proper kinetic model for nitrate reduction by NZVI has not been proposed especially in iron-limiting condition and furthermore, the practical applicability for water treatment, e.g., the effect of other components in natural water bodies has hardly been concerned. In this study, kinetic models for nitrate removal and product generation were proposed and evaluated for different NZVI dose, pH and ionic strength. And the effects of humic acid (HA) on nitrate reduction by NZVI were investigated. HA is adapted as the representative inhibitor and/or promoter because it is the most widely distributed natural organic matter which significantly affects the performance of water treatment processes. Property of HA was also investigated for better understanding of the effects on the performance of NZVI. NZVI for this study was prepared via chemical reduction by borohydride and it showed average particle size of 16.7 nm and BET surface area of $17.623 \plusmm 0.042 $m^2$/g$. Nitrate removal was described successfully by adsorption kinetic equations and unreacted nitrate at NZVI surface was partially recovered in this study. Parameters of adsorption kinetics showed good correlation with reaction conditions. Pseudo first order reaction kinetic equation, which has been commonly adopted, provided poor fit but the rate constant showed good correlation with reaction conditions. Adsorption models considering catalyst deactivation provided better prediction of nitrate profiles, while global deactivation model predicted equilibrium and concentration dependent deactivation model predicted residual activity. Ammonia generation was described separately because the retardation of ammonia generation has been reported and also observed in this study. ...