Synergistic effect of nano-sized mackinawite with cyano-cobalamin in cement slurries for reductive dechlorination of tetrachloroethylene

https://doi.org/10.1016/j.jhazmat.2016.02.074Get rights and content

Highlights

  • Complete degradation of PCE was observed in nFeS-Cbl(III)-cement at pH 12.

  • PCE was completely degraded to non-chlorinated organic compounds by this system.

  • Co redox couple and Ca species in cement played a pivotal role for PCE reduction.

  • Increases in Cbl(III) concentration, cement ratio, and pH enhanced PCE degradation.

  • Efficiency of the system for PCE reduction was good even at high concentration of PCE.

Abstract

Experiments were conducted to investigate the reductive dechlorination of tetrachloroethylene (PCE) by nano-Mackinawite (nFeS) with cyano-cobalamin (Cbl(III)) in cement slurries. Almost complete degradation of PCE by nFeS-Cbl(III) was observed in cement slurries in 5 h and its degradation kinetics (kobs-PCE = 0.57 h−1) was 6-times faster than that of nFeS-Cbl(III) without the cement slurries. PCE was finally transformed to non-chlorinated organic compounds such as ethylene, acetylene, and C3-C4 hydrocarbons by nFeS-Cbl(III) in cement slurries. X-ray photoelectron spectroscopy and PCE degradation by cement components (SiO2, Al2O3, and CaO) revealed that both the reduced Co species in Cbl(III) and the presence of Ca in cement played an important role for the enhanced reductive dechlorination of PCE. The increase in the concentration of Cbl(III) (0.005–0.1 mM), cement ratio (0.05–0.2), and suspension pH (11.5–13.5) accelerated the PCE degradation kinetics by providing more favorable environments for the production of reactive Ca species and reduction of Co species. We also observed that the degradation efficiency of PCE by nFeS-Cbl(III)-cement lasted even at high concentration of PCE. The experimental results obtained from this study could provide fundamental knowledge of redox interactions among nFeS, Cbl(III), and cement, which could significantly enhance reductive dechlorination of chlorinated organics in contaminated natural and engineered environments.

Introduction

Tetrachloroethylene (PCE) is a prevalent organic contaminant found in soil and groundwater environments due to its extensive usage in industrial processes [1]. Because PCE is toxic, carcinogenic, and mutagenic to human beings and animals, many efforts have been made to effectively remove PCE from contaminated soil and groundwater. Zero-valent iron (ZVI) and iron-bearing soil minerals (e.g., green rust, magnetite, and pyrite) have been widely studied as reactive reductants to degrade PCE effectively [2], [3], [4], [5]. In particular, Mackinawite (FeS), one of the most abundant iron-bearing soil minerals in subsurface environments [6], [7], has shown a remarkable reductive degradation of PCE by reactive triple bondFeOH and triple bondFeSH surfaces under anaerobic conditions [8], [9], [10]. However, both the relatively slow reaction (>1 month) for PCE reduction and the production of toxic by-products such as cis-dichloroethene (DCE) and 1,1-DCE, remain issues to be resolved during the reductive degradation of PCE by FeS [11].

Recent studies have indicated that addition of cobalt (Co) to a FeS suspension could enhance the dechlorination kinetics of PCE. Jeong et al. showed 10-times faster dechlorination kinetics of PCE by FeS with Co(II), than that by FeS alone, due to the formation of reactive sulfide species (CoS) on the FeS surface [12]. Our previous study further enhanced the dechlorination kinetics of PCE by addition of a transition-metal coenzyme i.e., cyano-cobalamin (Cbl(III)), to a nano-Mackinawite (nFeS) suspension [13]. It is well known that Cbl(III) is an efficient electron transfer mediator (ETM) that contains a trivalent cobalt (Co(III)) in the center of a tetrapyrrole ring, which can act as a redox-active metal [13], [14], [15]. The combination of nFeS and Cbl(III) showed a remarkable reactivity toward PCE due to partial reduction of Co(III) to Co(II), and Co(I) by Fe2+, S2− and Sn2− on the nFeS surface. This led to enhanced electron transfer from the nFeS surface to PCE by the Co redox couple (Co(III)/Co(II)) [1], [13].

Interestingly, the dechlorination kinetics of PCE by nFeS with Cbl(III), was dramatically improved at pH > 12 (i.e., 35 μM of PCE was fully degraded by nFeS-Cbl(III) within one day). This indicates that the nFeS-Cbl(III) system could be most productively applied to alkaline environments contaminated by PCE. The solidification/stabilization (S/S) technology causes high alkalinity (>pH 12) due to the hydration processes of ordinary Portland cement, and could be a good remediation technology for application of the nFeS-Cbl(III) system. Indeed, the S/S technology has been successfully applied to around 24% of the Superfund sites in the United States for in situ remediation of organic and inorganic contaminants over the last few decades. It has been reported as the ‘best demonstrated available technology’ (BDAT) for 57 regulated hazardous wastes [16], [17]. Furthermore, a modified S/S technology, degradative solidification/stabilization (DS/S), was developed by adding Fe(II) to cement slurries to promote reductive degradation of chlorinated organics [18], [19], [20], [25]. Therefore, the high reactivity of nFeS-Cbl(III) at strongly alkaline pH may enhance successful implementation of DS/S for PCE remediation. However, there is currently only limited knowledge about DS/S of PCE using nFeS-Cbl(III).

In this study, the dechlorination kinetics of PCE by nFeS-Cbl(III) in cement slurries was examined using batch kinetic experiments. Transformation products for the reductive dechlorination of PCE by nFeS-Cbl(III) in cement slurries were monitored to understand the degradation pathways. X-ray photoelectron spectroscopy (XPS) analysis was conducted to investigate the oxidation states of active redox elements (Fe, S, Co, etc.) on nFeS surfaces with Cbl(III) in cement slurries. Furthermore, the effect of the main components of cement (silicon oxide: SiO2, aluminum oxide: Al2O3, and calcium oxide: CaO) on the PCE dechlorination kinetics, was investigated to determine the key factors for enhancing PCE degradation. Finally, the effects of environmental factors such as Cbl(III) concentration, cement ratio, suspension pH, and PCE concentration on the PCE dechlorination kinetics were examined.

Section snippets

Chemical reagents

The chemical reagents used in this study were cement (Portland cement); silicon oxide (SiO2, 99.5%, Sigma); aluminum oxide (Al2O3, 98%, Sigma); calcium oxide (CaO, 99%, Sigma); cyano-cobalamin (Cbl(III), vitamin B12, 99%, Sigma); PCE (99 +%, Sigma); trichloroethylene (TCE, 99.5 +%, Sigma); cis-dichloroethene (cis-DCE, 99 +%, TCI); trans-dichloroethene (trans-DCE, 98 +%, TCI); 1,1-dichloroethene (1,1-DCE, 99 +%, Sigma); and vinyl chloride (VC, 99 +%, Supelco). A mixture of gases methane (1%), ethane

Reductive dechlorination of PCE by nFeS-Cbl(III) in cement slurries

Fig. 1 shows the dechlorination kinetics of PCE by nFeS-Cbl(III) in cement slurries at pH 12. The control (DDW) showed >95% of PCE recovery during 5 h reaction, indicating that no significant loss of PCE occurred by sorption to the vial wall or by volatilization from the reactor. Early termination of PCE removal (15% in 5 h) occurred for samples containing nFeS with and without cement slurries. In contrast, 40% of PCE was removed by a nFeS-Cbl(III) suspension in 6 h, and the rate constant (kPCE)

Conclusions

The results from this study demonstrated that nFeS-Cbl(III) in cement slurries can significantly improve the degradation kinetics of PCE, with production of non-chlorinated by-products. PCE was initially transformed to TCE via hydrogenolysis and later transformed to non-chlorinated organic compounds (ethylene, acetylene, and C3-C4 hydrocarbons) via reductive β-elimination, alkyl-radical reaction, and isomerization. The results from XPS analysis and addition of cement oxides, revealed that both

Acknowledgments

This research was supported by the Korean Ministry of Environment, GAIA project (173-111-036), other GAIA projects (RE201402059), and the National Research Foundation of Korea (NRF) funded by the ministry of Education (2012-C1AAA001-M1A2A2026588).

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