| Chromium (Cr) can exist in the environment in many valence states ranging from -2 to +6. Hexavalent chromium (Cr(VI) and trivalent chromium (Cr(III) are the most stable forms of Cr and therefore are the most common in the environment. Cr(VI) is very toxic and has been considered a hazardous material and danger to society for many years. One of the main locations of Cr(VI) contamination in the environment is in groundwater aquifers. A simulation of this system coupled with a reduction-precipitation remediation method was utilized. In this research, n-ZVI was used in a supported silica sol-gel matrix by itself and with additions of the reaction catalysts: cobalt-protoporphyrin complexes (Co-PPIX) and cobalt-uroporphyrin complexes (Co-Uro).;Experimental results demonstrated that Co-Uro and Co-PPIX additions to the supported n-ZVI system each increased the rate constants compared to the supported n-ZVI alone. The concentration data for the reaction kinetics was fitted as two pseudo-first order reactions, a kfast for the initial decline and a kslow once the initial steep decline was complete. The Co-PPIX increased the slow reaction rate constant by a factor of 2.4 from 0.0080±0.0013 min-1 to 0.0192±0.0027 min -1 when added to the supported n-ZVI at an n-ZVI:Co-PPIX ratio of 100:1. The addition of Co-Uro also increased the slow reaction rate constant, although not as much as the Co-PPIX addition at the 100:1 addition rate, as it was increased by a factor of 2.0 to 0.0161±0.0015 min-1. The Co-Uro and Co-PPIX additions were studied across different addition rates in the supported n-ZVI system. Addition ratios were studied as molar ratios of 1500:1, 750:1, 500:1, and 100:1 for n-ZVI:Catalyst concentrations inside each sol-gel. The addition rates of 1500:1 and 750:1 had negligible effects on the reaction rate constants of the reduction system, but the addition ratios of 400:1 and 100:1 had an increased effect. The Co-Uro additions in the 1500:1, 750:1, and 400:1 addition ratios produced increases in the ks rate constants by factors of 1.1, 1.3, and 1.5 to 0.0088±0.0008 min -1, 0.0103±0.0029 min-1, and 0.0123±0.0030 min-1. The effects for the addition rates of 1500:1 and 750:1 in the systems with Co-PPIX additions were negligible as the rate constants increased slightly or none at all to 0.0080±0.0015 min-1 and 0.0079±0.0009 min-1. The addition rate of 400:1 for the system with Co-PPIX additions did have a measurable effect as the rate constant increased by a factor of 1.4 to 0.0110±0.0027 min -1. Ultimately, through the course of this research, it was determined that the catalyst effect for Co-Uro is greater than that of Co-PPIX at smaller ratios such as 1500:1 and 750:1, but at higher ratios such as 100:1, the Co-PPIX acts as a stronger catalyst. |
