Role of reactor type on Cr(VI) removal by zero-valent iron in the presence of pyrite: Batch versus sequential batch reactors.

This study was conducted to understand the role of application sequence of pyrite and zero-valent iron (Fe) (simultaneous vs. sequential) on chromium (VI) removal by Fe. In batch experiments, pyrite and Fe were homogeneously mixed in batch reactors maintained at a constant total solids loading of 2 g L. In sequential batch experiments, however, the first reactor containing variable doses of pyrite was operated for 20 min, and the liquid fraction from the first reactor was then subsequently loaded into the second reactor containing a fixed Fe dose of 1.2 g L. The batch reactors achieved much higher Cr(VI) removal efficiency than sequential batch reactors under similar operating conditions due to discrepancies in Fe redox cycling activities between these two systems. In batch reactors, the Fe particles deposited on pyrite surface due to electrostatic attraction between negatively charged pyrite and positively charged Fe, thus, rendering the overall solids surface charge neutral at optimum pyrite and Fe doses. As a result, the whole system behaved like a composite material, with pyrite functioning as a support material for Fe. This stimulated Fe redox cycling more effectively to generate new Fe(II) sites on Fe for enhanced Cr(VI) removal relative to Fe only system. In sequential batch reactors, however, the Fe redox cycling activity was limited, but significantly increased with increasing pyrite dose in the first reactor. Overall, our results indicate that the stimulatory effect of pyrite on Cr(VI) removal by Fe may be much higher if the reactors are operated in batch mode.