Metalloporphyrin-catalyzed reduction reactions of hexavalent chromium.

摘要

Previous studies have demonstrated that the reduction of oxidized organic and inorganic contaminants could be catalyzed by electron shuttle systems, which generally were biological organic macrocycle complexes with transition metals. Metalloporphyrins (MPs) and their derivatives are well known electron shuttles for many biogeochemical systems. The objective of this research was to study the catalytic capabilities of selected MPs for the reduction of hexavalent chromium (Cr(VI)) in the presence of reducing agents. Zero valent iron (ZVI) was chosen as the primary electron donor in the experimental systems. Protoporphyrin IX (Proto) and Uroporphyrin I (Uro) are naturally occurring porphyrins produced during heme biosynthesis. MPs were prepared by inserting Co(II) or Fe(II) to the dissolved porphyrins. These four synthesized MPs and Vitamin B12 (VB12) were applied to Cr(VI) reductions by micro-sized ZVI (m-ZVI), nano-sized ZVI (n-ZVI), and n-ZVI immobilized in Ca-alginate gel beads. The kinetic data were analyzed using pseudo-first order models, and the catalytic capability was evaluated by the comparison of these reactions to those without a catalyst.Different concentrations of MPs or VB12 were added to catalyze Cr(VI) reduction by 1.7 g/L m-ZVI or 0.1 g/L bare/immobilized n-ZVI at pH 7. No significant catalytic effects were found for Cr(VI) reduction by m-ZVI in the presence of 20 muM Proto-Co or Proto-Fe. At the same concentration, Uro-Co and Uro-Fe slightly accelerated the reaction by approximately 7% and 4%, respectively. VB12 (20 muM) dramatically increased Cr(VI) reduction by m-ZVI, approximately 20% in 200 min. For Cr(VI) reduction by a more reactive form of m-ZVI, VB12 catalysis was not obvious in the first and second runs of a reuse test using it. However, the VB12 significantly catalyzed Cr(VI) reduction in the third reuse cycle. This result indicates that the catalyst (VB12) may be more important for long-term remediation when using reactive reductants.Small amounts of VB12 (0.1 muM) made Cr(VI) reduction by n-ZVI reach completion approximately three times faster than when only using the n-ZVI alone. Encapsulation of n-ZVI in Ca-alginate gel beads hindered the Cr(VI) reduction rate by a factor of 8 at pH 6 and a factor of 3 at pH 7. Upon adding 5 muM VB12 to the reaction at pH 7, the reaction rate was significantly enhanced. Cr(VI) (100 muM) was totally reduced in 20 min, which was faster than without VB12 (150 min), as well as when using bare n-ZVI (50 min). Interestingly, n-ZVI gel beads became more reactive after being kept in the anaerobic chamber three months, which may be due to the enlargement of pore sizes, crack on the beads surface, or F e2+ produced by Fe0 hydrolysis. Furthermore, the n-ZVI gel beads were reused multiple times at pH 6. After four reuse cycles, the beads were nearly completely broken, but they collected a lot of precipitated products. Therefore, using this kind of material for in-situ remediation may be beneficial from an aesthetic standpoint.VB12 immobilized in sol-gel matrices provided a more moderate catalysis than the free VB12, but the reductions by m-ZVI or n-ZVI were greatly accelerated compared to those not adding VB12. The collective results of this research indicate that metalloporphyrins and related compounds can facilitated electron transfer and enhanced Cr(VI) reductions by ZVI under certain conditions. VB12 is the most promising catalysts, followed by Uro-Co and Uro-Fe. The VB12 immobilized sol-gel only slightly inhibited the overall VB12 catalytic capability and still significantly catalyzed Cr(VI) reduction.