Feasibility and energy consumption analysis of phenol removal from salty wastewater by electro-electrodialysis

https://doi.org/10.1016/j.seppur.2019.01.001Get rights and content

Highlights

  • Phenol was removed from model solution using an electromembrane process.

  • Energy consumption for phenol removal was affected by the competing anions.

  • Increased phenol concentration in feed water led to lower energy consumption.

Abstract

The discharge of phenol-containing wastewater has become a crucial and health threatening environmental issue. In this paper, an electro-electrodialysis (EED) process was investigated to remove phenol molecules in the form of phenoxide ions, and simultaneously desalt the laboratory-prepared salty wastewater. The influence of applied voltage, sodium sulfate and phenol concentrations on the separation performance and energy consumption of the EED process was studied. More than 90% of the phenol could be removed, illustrating the feasibility of the EED process. The specific energy consumption for phenol removal was primarily influenced by the concentration and characteristics of the competitive anions, such as sulfate and hydroxide ions in the feed solution. An increase of phenol concentration reduced the specific energy consumption, while a higher salt concentration resulted in higher energy consumption. The EED process is more energy-efficient for treating wastewater with low salt concentrations and high phenol concentrations. These results can significantly enhance the application potential of this emerging separation process.

Introduction

Phenol is considered as one of the most common and hazardous organic pollutants even at very low concentrations [1], [2]. It is toxic and carcinogenic to human and aquatic lives, and is potentially accumulated in the environment [3], [4]. Wastewater containing phenol could be released from a broad range of industries, such as coal gasification, tanning, oil refineries, and paint manufacturing. In particular, wastewater discharged from coal-gasification and petroleum processing contains a high concentration of phenol [2], [5]. In such a toxic condition, microorganisms hardly survive and hence are inactivated during biological treatment [6], [7]. Beside, inorganic salts such as sodium sulfate are inevitably present in water together with organic pollutants, making chemical oxidation treatment difficult [8], [9], [10]. Therefore, pretreatment processes are required to remove most of the phenol, prior to employing any conventional methods (e.g., biological treatment).

To date, separation of phenol from wastewater can be implemented through several techniques, including solvent extraction [11], [12], adsorption [13], [14], [15], steam distillation [16], Fenton oxidation [17], [18], and membrane technology [3], [19], [20]. Among these, membrane technology has been widely utilized for wastewater treatment, demonstrating the advantages of high-efficiency, smaller footprint, continuous operation, lower capital and operating costs, and no addition of chemicals. Except the normally used activated carbons, some new adsorbents are explored for phenol removal from aqueous solution [13], [14], [15]. The related regeneration should be considered and the structure of the adsorbent may be destroyed during this procedure [15]. For phenol removal from wastewater by extraction, organic solvents such as hydrocarbons and oxygenated compounds, and ionic liquid were also used as solvent [11], [12]. However, the stability, volatility and flammability of the organic solvents have restricted their widely industrial applications. The liquid membrane systems, which could be deemed a membrane based extraction, exhibit significantly high mass transfer efficiency and has been intensively studied membrane processes for phenol-containing wastewater treatment [3], [19], [20]. The separation principle of these liquid membrane processes is mainly based on the solubility of phenol in different solution phases and the steric hindrance from the porous membranes.

Electromembrane processes such as Electrodialysis (ED) have been widely used for the removal or concentration of ions or charged molecules in wastewater. These processes rely on the migration of charged species through ion exchange membranes under an applied electrical potential, without the introduction of additional chemicals. They are considered to be more environmentally friendly compared to the aforementioned processes [21], [22]. However, phenol is uncharged in acidic and neutral conditions, restricting its ability to transfer across ion exchange membranes under an electric potential difference. It was reported that phenol transfer through ion exchange membranes is only due to diffusion and convection during the ED process [10]. As a weak acid, phenol can dissociate into phenoxide ions and protons if the solution is adjusted to alkaline conditions. The percentage dissociation (α) in this system can be determined from the Henderson–Hasselbalch relationship [23]:α=Ph-Ph-+Ph×100%=1-11+10pH-pKawhere the dissociation constant (pKa) of phenol is 10 [24], Ph- and Ph are the molar concentrations of phenoxide ions and phenol molecules in the solution, respectively. It can be estimated that the theoretical dissociation percentage (α) is higher than 90% when pH is greater than 11, meaning that most of the phenol molecules in the system are in ionic state and can potentially be removed by an electrodialysis process.

The adjustment of pH is typically achieved by adding sodium hydroxide directly into the system, which would result in an increased ion concentration in the system. A typical electromembrane process that has the ability to regulate pH is bipolar membrane electrodialysis (BPED), where water splitting occurs on the bipolar membranes [25], [26]. It has the advantages of relatively high current efficiency and low energy consumption. However, the high costs and unstable performance of bipolar membranes have limited the development of BPED processes [26], [27]. To overcome these disadvantages, an extended electrodialysis process called electro-electrodialysis (EED) is employed in this work for phenol removal from model wastewater containing sodium salts.

An EED stack is comprised of one or two ionexchange membranes, assembling into a two-camber or three-chamber module. Hence, only conventional ion exchange membranes are used in this process, potentially leading to stale and reliable operations. Solutions containing phenol are introduced into to the cathode chamber (i.e., the chamber adjacent to cathode). The hydroxide ions generated by water splitting from the cathode reaction (Eq. (2)) are effectively utilized to raise the pH of the solution to achieve phenol dissociation (Eq. (3)), allowing the dissociated phenoxide ions to migrate through the anion exchange membrane under an applied electrical potential.2H2O + 2e → H2↑ + 2OHC6H5OH ⇄ C6H5O + H+

Therefore, no chemical addition is required in this process, making EED an environmentally friendly process.

EED has been investigated for the recovery and concentration of organic acids, such as hydriodic acid, lactic acid, and citric acid [28], [29], [30], [31], [32], as well as for the generation of sulfuric acid and sodium hydroxide from sodium sulphate salt [33]. However, to the best of our knowledge, its application in organic acid separation from salty solutions has not been performed systematically in the literature. In this work, the feasibility to remove phenol from salty wastewater using an EED process is studied. The influence of the applied voltage, as well as the initial phenol and salt concentrations on phenol removal is investigated. As salt ions also migrate through the ion exchange membranes during EED, the effect of salt and phenol concentrations on energy consumption for phenol removal is studied in this work.

Section snippets

Materials

Phenol (AR, analytical reagent grade) and sodium sulfate (anhydrous, analytical reagent grade) were purchased from Tianjin Kermel Chemical Co Ltd. The anionexchange membranes (AEM) were obtained from Shandong Tianwei Membrane Technology Co Ltd. The main properties of the AEMs are listed in Table 1.

Experimental setup

The electro-electrodialysis system used in this study is shown in Fig. 1. A two-chamber cell was used with an AEM to separate the anode and cathode chambers. The effective membrane area was 78.2 cm2.

Influence of voltage

The variation of percentage of phenol removal with operation time under different voltages is presented in Fig. 2. The results were obtained with a feed solution containing 100 mg/L phenol and 1000 mg/L sodium sulfate. The rate of phenol removal increased with the applied voltage, with more than 90% of the phenol removed after 120 min for voltages higher than 9 V. This could be attributed to the higher voltages applied, leading to a faster rate of ion migration. It can be also seen that the

Conclusion

The EED process was investigated to remove phenol from salty wastewater by converting phenol into phenoxide ions, through pH adjustment achieved by the hydroxide ions generated by the cathode reaction. This process could successfully remove phenol in an ionic state (i.e., as phenoxide ions) and transform phenoxide ions into uncharged phenol molecules in the anode chamber. Meanwhile, the salts could be transformed into acid and base in the anode and cathode chamber, respectively; and could

Acknowledgement

We are grateful for financial support from the Special Science and Technology International Cooperation Projects of Key Research and Development Plan of Hebei Province (17393601D), and Tianjin Key Research Program of Application Foundation and Advanced Technology (14JCZDJC38900).

References (37)

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