Recovery of Pentachlorophenol from Aqueous Solution via Silicone Rubber Membrane

Jun Sawai, Ko-Ichi Sahara, Tamotsu Minami, Mikio Kikuchi

Faculty of Applied Bioscience, Kanagawa Institute of Technology, Atsugi, Japan.

DOI: 10.4236/aces.2012.23044                

Abstract

Although pentachlorophenol (PCP) has been widely employed as a biocide for over 60 years, its production and use are currently severely curtailed in many countries due to its extreme toxicity. In recent years, the contamination of both soil and surface waters by PCP has become a concern. In this study the permeation characteristics of PCP penetrating silicone rubber membranes (SRM) were studied, in order to determine the feasibility of separation of PCP from water via the permeation and chemical desorption (PCD) method. It was found that efficient separation and recovery of PCP could be obtained using an acidic feed solution and an alkaline recovery solution. The permeation rate of PCP into the SRM was optimized when the feed solution was maintained at a pH of 4 or lower. The SRM thickness did not significantly affect the permeation rate, indicating that the rate determining step for the process is the initial movement of the PCP into the SRM. The activation energy for the penetration process was determined to be quite high, and thus thermal controls will play an important role in the recovery of PCP by this method. The membrane distribution coefficient (m_c) for PCP moving into SRM was large and showed a strong correlation to permeation rates reported previously, confirming that PCD is a suitable technique for the separation and recovery of PCP from aqueous solution.

Keywords

Persistent Organic Pollutants (POPs); Soil Pollution; Membrane Separation; Polymer Membrane; Partition Coefficient

1. Introduction

Pentachlorophenol (PCP), or its sodium salt, has been extensively used since the 1930’s as a herbicide, algaecide, germicide, fungicide, molluscicide, defoliant and wood preservative, due to its action as a potent biocide [1-3]. In addition to its innate toxicity, technical or commercial grade PCP also contains approximately ten percent impurities, consisting of several potentially hazardous chlorinated aromatic compounds, primarily the more highly chlorinated dibenzo-p-dioxin and dibenzofuran congeners [4]. In the 1970s, the toxicity of PCP towards the liver and kidney was confirmed and its reproductive and developmental toxicities were also reported [5]. Following this, between 1978 and 1984, many countries either restricted or banned the production and use of PCP, due to its potential adverse effects on human health [6]. In the 1990s, the endocrine disrupting effects of PCP were also recognized [7,8]. Although it is now largely banned, PCP is still commonly found as a contaminant in air, water and soil worldwide, due to its widespread use in the past [6,9]. Remediation of contaminated sites is complicated by the fact that chlorinated phenols, such as PCP, are chemically stable. Although there have been attempts to remove PCP from soil by bioremediation, such treatment requires a very long duration and typically does not produce acceptably clean sites [10-12]. Thus the development of improved treatment technologies for the remediation of PCP-contaminated soil and water is of interest.

We have previously investigated the permeation and chemical desorption (PCD) methodology for the separation and recovery of pollutants, using nonporous materials such as silicone rubber membranes (SRM) [13,14]. In the PCD method, two solutions with different chemical properties are separated by a nonporous membrane. The compound to be recovered (in the so-called feed side solution) has significant affinity for the membrane material and penetrates through the membrane. Upon exiting to the recovery side solution, this same compound is chemically modified such that it no longer has an affinity for the membrane and is thus trapped. To date, this technique has been demonstrated to be effective in the recovery of various contaminants including iodine, phenols and anilines [13-18]. Both 4-substituted phenols and anilines have been recovered from aqueous solutions using either NaOH or HCl, respectively, for neutralization [15]. A comparison of the relative efficiencies of the PCD and pervaporation (PV) methods has been reported, using a tube-type apparatus. The removal rates of phenols by the PCD method were much greater than those by the PV method, demonstrating the efficient separation and recovery of compounds with low-volatility via PCD [15]. The rate at which phenols and anilines permeate into the SRM, the most important step in the PCD method, has been found to be well correlated to their concentration in the membrane [16]. Livingston et al. have also described a membrane aromatic recovery system (MARS) for recovering anilines and phenols using an SRM [17,18]. The MARS process operates on a very similar principle to that of the PCD method. Recently, the successful scale-up and operation of the MARS process following pilot-plant trials have been reported [19].

In this study the permeation characteristics of PCP through SRMs using the PCD method were investigated, as a first step in developing the technology to separate and remove PCP contamination in water and soil.

2. Materials and Methods

2.1. PCD Method

Figure 1 illustrates the basic principles of the PCD method. A solution of PCP dissolved in an acidic solvent is placed in the feed cell, while the recovery cell is filled with an alkaline solution. Dissolved PCP molecules in the feed solution, being protonated at the hydroxyl group and thus uncharged, will tend to penetrate into the hydrophobic SRM. Once these PCP molecules permeate the membrane and emerge in the alkaline recovery solution, the hydroxyl group of the molecule is deprotonated to produce the charged phenolate anion (ROH → RO⁻). This charged phenolate species is poorly adsorbed by the SRM, thus does not tend to migrate back to the feed cell. As a consequence, PCP dissolved in an acidic solution in the feed cell, with an alkaline solution in the recovery cell, is eventually concentrated to the recovery cell.

Conflicts of Interest

The authors declare no conflicts of interest.

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