Kinetics of Reductive Immobilization of Rhenium in Soil and Groundwater Using Zero Valent Iron Nanoparticles

Abstract

Abstract

This study reports experimental results and modeling analysis of reductive removal of perrhenate (a surrogate for radioactive pertechnetate) from water using a class of starch-stabilized zero valent iron (ZVI) nanoparticles. Thermodynamic analysis of the reduction pathway indicated that reduction of Re(VII), and possibly Tc(VII), is highly favorable with a ΔG of=−316.66 kJ/mol for Re(VII) and −448.66 kJ/mol for Tc(VII). The final reduced product was ReO₂, which is much less mobile and less bioavailable than ReO₄⁻. Starch-stabilized ZVI nanoparticles were able to rapidly and completely reduce and remove perrhenate at ambient temperatures. The reaction rate constant increased from 0.351 h⁻¹ at 15°C to 0.428 h⁻¹ at 45°C and a fairly low activation energy level of 5.13 kJ/mol was determined. A surface blocking effect on the ZVI nanoparticles was evident due to surface precipitation of the resulting ReO₂. As a result, the pseudo first order rate constant decreased from 0.356 to 0.230 h⁻¹ when the initial Re(VII) concentration was increased from 5 to 20 mg/L; complete Re(VII) reduction and precipitation were achievable in all cases. Accordingly, a more general and sounder pseudo nth order model was proposed to interpret reaction kinetics. Starch-stabilized ZVI nanoparticles were nearly four times more effective in reducing Re(VII) than commercial iron powder. Results revealed great potential of starch-stabilized ZVI nanoparticles for possible reductive immobilization of pertechnetate in soil and groundwater.