Sulfide minerals are important in immobilizing toxic contaminants in reducing environments. Iron sulfide (FeS) is ubiquitous in anoxic conditions and is a good scavenger of various organic and redox sensitive contaminants and heavy metals. Despite its contaminant removal capabilities, FeS has not been used as a practical adsorbent in contaminant removal due to its rapid oxidation under atmospheric conditions. To increase its applicability, we developed a method of modifying FeS by the addition of NaBH4 to form the less oxygen-sensitive NaBH4-FeS. We conducted oxidation tests using laboratory batch testing and real-time synchrotron X-ray absorption spectroscopy (XAS). The Fe K-edge XAS results showed that the oxidation rate of NaBH4-FeS was eight times slower than that of unmodified FeS while maintaining comparable contaminant removal capacities for Cr(VI) and As (III). The results of mechanistic density functional theory (DFT) calculations demonstrated that the oxidation of FeS occurred through electron transfer from sulfur of FeS to an oxidizing agent of oxygen, and that the hydride ion provided by NaBH4 retarded electron transfer from the FeS surface.