Identification and characterization of agent for reductive dechlorination in mixtures of ferrous iron and Portland cement

Abstract

Degradative Solidification/Stabilization (DS/S) is a modification of conventional S/S in which organic pollutants are destroyed while inorganic pollutants are immobilized. Fe(II) is used in Fe(II)-based DS/S as a reductant to degrade organic pollutants, especially chlorinated aliphatic hydrocarbons. Experimental studies were designed to identify and characterized an active agent in Fe(II)-based DS/S. PCE was chosen as a model chlorinated aliphatic hydrocarbon. First, XRD and elemental analyses were conducted to identify a potential active agent in a mixture of Fe(II) and Portland cement extract (PCX). Results of XRD analysis indicated that a potential active agent is likely to be green rust chloride, which is a layered Fe(II)/Fe(III) hydroxide with anionic interlayer. This XRD analysis result strongly supports the hypothesis that the active agent in Fe(II)/PCX was also formed in Fe(II)/Cement slurry system. The elemental analysis showed that the solids produced by Fe(II)/PCX consisted of 44% of calcium, 31% of chloride, and 10% of iron on a molar basis. Second, a potential active agent was characterized in terms of its surface area, its ability to degrade PCE, and the ageing effect on its ability to degrade PCE. The degradation kinetics followed a pseudo-first-order rate law. First-order rate constants for solids produced with FeCl₂/PCX were 0.013 (mM Fe(II) · day)⁻¹ and 0.0061 L · (day· g of solid) ⁻¹. First-order rate constants for solids produced with FeSO₄/PCX were 0.014 (mM Fe(II) · day)⁻¹and 0.0060 L · (day· g of solid) ⁻¹. Fe, Al, and SO₄²⁻ had strong relationships with activity. The activity was somewhat related with Cl⁻ and weakly related with Ca. When solids formed from Fe(II)/PCX were aged, their activity for PCE degradation was reduced. However, ageing of cement extract prior to forming solids did not have any effect on the activity of the solids. The solids produced by Fe(II)/PCX had large specific surface area (400 m²/g) as measured by the EGME procedure. The k[] value of solids produced by FeCl₂/PCX was 1.50 x 10⁻⁵ (L/(d·m²)) and the k[] value for solids produced with FeSO₄/PCX was 1.38 x 10⁻⁵ (L/(d·m²)).