ENHANCEMENT EFFECT OF REACTIVE MINERALS IN CEMENT/FE(II) SYSTEM FOR DEGRADATION OF TRICHLOROETHYLENE

Abstract

Ferrous iron (Fe(II)) in combination with Portland cement is effective in dechlorinating trichloroethylene (TCE). However there is no clear evidence about the component in cement solely responsible for TCE dechlorination. The experimental studies were conducted to enhance the formation of responsible reactive mineral in cement/Fe(II) system for dechlorination of TCE. The study was conducted in four different stages. First different cement hydration minerals suspected for dechlorination of TCE, such as ettringite (AFt) and monosulfate (AFm) were synthesized separately in laboratory. This study developed the synthesis methods for four different minerals namely Al-ettringite, Al-monosulfate, Fe-ettringite and Fe-monosulfate. The synthesized minerals were characterized using X-ray diffraction (XRD) and the peak intensity of minerals was compared with standard reference. The surface morphologies of AFt and AFm were analyzed using scanning electron microscopy with energy-dispersive spectrometry (SEM-EDS). The TCE dechlorination ability of these minerals in combination with Fe(II) was investigated. It was found that these minerals in pure form in presence of Fe(II) do not have TCE dechlorination capacity. In second part of research the reactivity of different alpha-hematite (α-Fe2O3) systems for dechlorination of trichloroethylene (TCE) in presence of Fe(II) and CaO was investigated. Initially different experiments were conducted to investigate the reactivity of pure and doped α-Fe2O3. It was found that presence of elements like Si, Cu, and Mn in α-Fe2O3 had a significant effect on TCE reduction rate of α-Fe2O3 associated with Fe(II); however, the reduction kinetics was less than that of α-Fe2O3/Fe(II) (Bayferrox®-110M, used in previous study). Further studies were carried out and α-Fe2O3 was synthesized in manner similar to that of Bayferrox®-110M. This synthetic α-Fe2O3 showed improved reactivity and was found to follow pseudo-first-order kinetics when used in TCE reduction experiments. The preliminary end products analysis showed that TCE degradation was probably via β-elimination pathway. Detailed investigations of α-Fe2O3 systems were carried out using XRD, XRF (X-ray fluorescence) and SEM-EDS. The results demonstrated that the TCE reduction rate of α-Fe2O3/Fe(II) was strongly dependent on the other elements present in iron powder used to synthesize α-Fe2O3. It was suspected that these multi-elements in α-Fe2O3 helped to improve its conduction property. Current findings suggest that the α-Fe2O3 not in pure but combined with other elements could be thought as reactive system for TCE reduction. Further the trichloroethylene (TCE) reduction rate of four different types of cements, ordinary Portland cement (OPC), two types of calcium aluminate cements (CAC) with higher Al2O3 (UAC-50), and higher Fe2O3 (ISTRA-40), and calcium sulfoaluminate cement (CSA), were evaluated in presence of Fe(II). The OPC and CSA cements individually showed dechlorination capacity for TCE, following pseudo–first–order kinetics, but both types of CAC cements did not show reduction capacity. Further combined systems of CSA and OPC were found to enhance the rate of dechlorination compared to OPC or CSA used alone. The cement mixtures were optimized to get fastest reduction kinetics. The iron oxide and sulfate originally present in cement during manufacturing were found to play a significant role together. The end products of the dechlorination reaction were acetylene, ethylene and ethane suggesting β–elimination as the major pathway. The X-ray diffraction (XRD), scanning electron microscope with energy dispersive spectroscopy (SEM–EDS), analyses were used to identify the solids that were produced during TCE reduction experiments. Ettringite crystals were abundant in all the reactive cement samples. However the formation of ettringite was not seen in nonreactive cement samples. It is speculated that Fe-ettringite associated with Fe(II) could be one of the reactive species in TCE reduction. Finally the research was carried out to improve the TCE reduction efficiency of ordinary Portland cement. Knowing the fact that ettringite could be one of the reactive minerals for TCE dechlorination the experiments were designed to improve the formation of ettringite in OPC. It was suspected that the dissolution rate of calcium sulfate plays significant role in formation of ettringite crystals. Experiments were conducted in which three different types of calcium sulfate were mixed with clinker powder. Thus obtained three cements namely cement with calcium sulfate dihydrate (CaSO4.2H2O), calcium sulfate hemihydrate (CaSO4.0.5H2O) and calcium sulfate anhydrate (CaSO4.0.001H2O) were checked for their TCE reduction rate in presence of Fe(II). From the TCE experiments it was found that cement with anhydrate form of gypsum had highest TCE degradation capacity followed by cement containing hemihydrate and dihydrate form of gypsum. Formation of mineral ettringite was highest in cement with anhydrate form of gypsum and least in cement with dihydrate form.