Elsevier

Chemical Engineering Journal

Volume 256, 15 November 2014, Pages 205-211
Chemical Engineering Journal

A novel and efficient method for dechlorination of hexachlorobenzene using a sodium carbonate/glycerol system

https://doi.org/10.1016/j.cej.2014.06.092Get rights and content

Highlights

  • We develop a novel and efficient BCD system for organochlorine compounds destroy.

  • We firstly report that weak base can effectively catalyzed HCB dechlorination.

  • We extend usage of glycerol to chlorinated POPs treatment.

  • We reveal the kinetic and propose dechlorination pathway of HCB in the system.

Abstract

In this report, we developed a novel base-catalyzed dechlorination (BCD) system using sodium carbonate/glycerol (Na2CO3/glycerol). Hexachlorobenzene (HCB) was used as a standard organochlorine compound to investigate the dechlorination pathways and detoxification mechanism of the system. Our results indicated that the system was efficient for the dechlorination of HCB, as well as organochlorine compounds such as Endosulfan and DechloranePlus. Approximately 99.12% of HCB was destroyed within 3 h at 250 °C with a dechlorination efficiency (DE) of 95.96%. The base-catalyzed dechlorination of HCB occurred in a stepwise process. About 2.63% of chlorobenzenes were volatilized from the reactor and distributed in the condensate water and exhaust gas, which should be treated prior to emission. Additional reactions occurred that competitively consumed Na2CO3 with BCD process, and may reduce the dechlorination capacity of the Na2CO3/glycerol system. We also proposed a HCB dechlorination mechanism and possible additional reactions that occur in this system. The developed BCD system is an efficient and promising system for the detoxification of organochlorine compounds. To the best of our knowledge, this is the first report to show that a weak base can also effectively catalyze HCB dechlorination, and thus increases our understanding of BCD technology.

Introduction

Base-catalyzed dechlorination (BCD) processes are promising for the disposal of organochlorine pesticides [1] and have been commercially applied in some developed countries [2], [3]. A typical BCD process involves treatment of liquid and solid wastes in the presence of a reagent mixture consisting of a high boiling point hydrocarbon (as a hydrogen donor), such as paraffin oil, sodium hydroxide, and a proper catalyst. The mixture is heated to 200–400 °C for ∼0.5 h or longer until the organochlorine wastes are effectively destroyed [4], [5]. Several factors, including catalysts, reaction temperature, and, especially, base and hydrogen donors significantly influence waste destruction and dechlorination efficiencies [4], [6], [7], [8].

Some mature BCD systems, such as NaOH/paraffin oil and KOH/polyethylene glycol (PEG), have been developed and are widely used [4], [9], [10], [11]. When a NaOH/paraffin oil system is used in a BCD process, the temperature required is usually higher than 320 °C, and the process requires several hours [4], [6]. However, the KOH/PEG system can be used at much lower temperatures (70–200 °C) to achieve similar destruction and dechlorination efficiencies [12], [13]. Both of the H-donors (paraffin oil or PEG) of the two systems were expensive industrial products and not readily available. Besides, both of the systems used strong bases (NaOH or KOH) as base materials during the BCD process, which may erode facilities and harm operators. Thus, new BCD system with less expensive and safe reagents is required to meet industry demands for organochlorine compound treatment.

Glycerol is the main byproduct of biodiesel synthesis; one volume unit of glycerol is generated for every 10 volume units of biodiesel produced [14], [15], [16]. As the production of biodiesel, large amounts of raw glycerol has become a significant environmental and economic concern [15], [17], [18], [19]. New uses for glycerol are required to avoid accumulation of this material. Because of the large amount of impurities, crude glycerol cannot be directly used in food or pharmaceutical products, and its purification is expensive [20]. Raw glycerol has been used as an inexpensive substrate for production of various valuable products, including 1,3-propanediol, ethanol, animal feed, and hydrogen [17], [19], [21]. In this study we evaluated the use of glycerol in a new field.

Specifically, we developed a novel and efficient BCD system based on glycerol and a weak base Na2CO3. Additionally we investigated the influence factors, the removal and dechlorination kinetics, and proposed the dechlorination pathways and mechanism of HCB in the novel system. This study increased our understanding of BCD technology and provided a promising BCD system for organochlorine compound treatment.

Section snippets

Chemicals and materials

HCB (purity 99%) was purchased from Beijing HengYe ZhongYuan Chemical Co., Ltd. Glycerol and Na2CO3 were obtained from Tianjin GuangFu Chemical Co., Ltd. Hexane (pesticide residue grade) was obtained from J.T. Baker Inc., USA. High-purity, zero-valent iron, nickel, graphite powder (all were 74 μm in diameter), calcium hydroxide, potassium chromate, and silver nitrate were purchased from Beijing Chemical Works. Chlorobenzene standard (12-mix) was obtained from AccuStandard, Inc., USA.

BCD reaction process

All

Primary characterization of the reaction system

Bases were important for the BCD process, and different bases may lead to significant differences in the removal and dechlorination efficiencies [4]. In our primary experiments, calcium hydroxide, sodium carbonate, sodium hydroxide and potassium hydroxide were tested for the dechlorination of HCB with glycerol as an H-donor. The results indicated that the Na2CO3/glycerol system showed higher DE than the Ca(OH)2/glycerol system and NaOH or KOH/glycerol system in our experimental condition (Fig. 1

Conclusion

We developed a novel and efficient method for HCB dechlorination based on a Na2CO3/glycerol system. The system destroyed 99.12% of HCB in 3 h, with a dechlorination efficiency of 95.96%. The base-catalyzed dechlorination of HCB occurred in a stepwise process. The novel system is promising for organochlorine waste treatment, particularly because it makes use of abundantly available and less expensive glycerol as the H-donor and Na2CO3 as the base, and thus reduced the operational cost of BCD

Acknowledgements

The authors are grateful to the China Postdoctoral Science Foundation (Project No. 2013M530050) for financial support.

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