Development of a sulfidized zerovalent iron-geopolymer composite for the reductive immobilization of ReO₄

A geopolymer waste form has become a suitable approach for the immobilization of the volatile technetium (Tc) due to the low curing temperature (<60 °C). However, the low retention and the high mobility of the anionic technetium (TcO₄^-) remain challenging due to the charge repulsion stemming from the negative charges of the geopolymer surface and the anionic TcO₄^-. Herein, a geopolymer composite containing sulfidized nanoscale zerovalent iron (S-nZVI) was developed to reductively immobilize ReO₄^- (used as a non-radioactive surrogate for TcO₄^-). With low S-nZVI addition (≤4.0 wt%), S-nZVI particles are homogeneously dispersed in the geopolymer paste, resulting in stable mechanical strength of the S-nZVI/GP composite (>33.02 MPa). Moreover, the S-nZVI/GP composite showed a lower leaching rate of Re than the geopolymer without adding S-nZVI under the same leaching conditions. Apparently, the presence of S-nZVI enhanced the reductive condition of geopolymer slurry, converting ReO₄^- to a lower oxidation state species including ReS₂ during geopolymerization, and causing the decrease of Re leachability from 1.04 × 10^-7 to 3.42 × 10^-11 cm² s^-1. This new type of S-nZVI/GP composite is expected to expand the application of a geopolymer binder in the radioactive waste management field of the nuclear industry.