Elsevier

Journal of Membrane Science

Volume 566, 15 November 2018, Pages 190-196
Journal of Membrane Science

Cr(VI) recovery from chromite ore processing residual using an enhanced electrokinetic process by bipolar membranes

https://doi.org/10.1016/j.memsci.2018.07.079Get rights and content

Highlights

  • An improved electrokinetic system was used to recycle Cr(VI) from the COPR.

  • The current density had some effects on the cell voltage and COPR pH.

  • The recovery efficiency of Cr(VI) in different system was higher than 82.0%.

  • The CE increased and the SEC decreased in more COPRCs equipped system.

Abstract

An electrokinetic (EK) system improved by a bipolar membrane (BPM) (BPM-EK) was used to recycle hexavalent chromium (Cr(VI)) from the chromite ore processing residual (COPR) in the form of H2CrO4. The results showed that electrolyte concentrations in anode chamber (AC) and cathode chamber (CC) influenced the cell voltage and that 0.6 mol/L HNO3 in the AC and 1.0 mol/L NaNO3 in the CC were the optimal electrolyte concentrations. The current density also affected cell voltage, pH value of the COPR chamber (COPRC), Cr(VI) recovery efficiency, current efficiency (CE), and specific energy consumption (SEC). The optimal current density was 3.0 mA/cm2. Compared to traditional EK processes, a higher CE and a lower SEC were obtained when two and three COPRC were equipped in the BPM-EK system and the Cr(VI) recovery efficiencies of all COPRCs were higher than 82%. With the increase in the number of equipped COPRC from one to two and three, CE increased from 0.60% to 1.67% and 2.30% and SEC decreased from 0.906 to 0.433 and 0.395 kW h/g, respectively. Experimental results showed that the BPM-EK process is an effective method for the recovery of Cr(VI) from COPR.

Introduction

Chromium salts are important chemical feedstock used in various industrial processes such as the production of pigments, catalysts, and tanning agents [1]. The chromite ore processing residual (COPR) is a byproduct of chromium salt production. China is one of the major chromium salt-producing countries. More than 6,000,000 t of untreated COPR was discharged in the past 30 years [2] and 200,000 to 300,000 t is discharged annually in China [3]. Untreated COPR is a significant threat to the environment and human health because of its high toxicity, mutagenicity, and carcinogenicity [2]. Till 2009, more than 100 pollution accidents caused by the deposition of COPR occurred in China [4]. On 13 August 2011, a serious COPR contamination accident occurred in Qujing in Yunnan Province and discharged 5000 t of untreated COPR, produced by Lvliang Chemical Industry Co., Ltd, into the mountains and the Nanpan River [5]. COPR contamination is also a concern in other countries including the United States, Pakistan, India, Scotland, Japan, and England [6], [7], [8]. As one form of chromium, hexavalent chromium (Cr(VI)) is the main component in COPR and 500–1000 times more toxic than trivalent chromium (Cr(III)) [9]. If ingested, Cr(VI) can be absorbed by human tissue 53 times than Cr(III) [10]. Therefore, recently, more attention has been paid to remove Cr(VI) from COPR before it is discharged into the environment.

Several methods, such as pyro-based reduction and hydro-based reduction [11], [12], microbial detoxification [2], [13], and chemical leaching [1], [14] have been used to remove Cr(VI) from COPR. However, chemical reduction methods are often costly and require large quantities of chemical reagents. Although Cr(VI) is reduced to Cr(III), it is not completely removed from COPR. The efficiency of microbial methods is often affected by environmental conditions. During the process of chemical leaching, inorganic acids are exhausted and temperatures higher than indoor temperature are required. Thus, it is essential to explore a more effective method for removing Cr(VI) from COPR.

Electrokinetic (EK) techniques have been used to remove heavy metals from soil and sludge since the 1980s due to their high removal efficiency and environmental friendliness [15], [16]. The main mechanisms of EK systems include electromigration, electroosmosis, and electrophoresis [17]. By applying a voltage between the anode and the cathode, H+ and OH- can be generated on the surface of two electrodes through the electrolysis of water. When large amounts of H+ pass through the soil chamber, heavy metals desorb from the soil via ion-exchange reactions with H+ and migrate to the oppositely charged electrode chamber under the electric field force to complete heavy metal removal. Conversely, when OH- reaches the soil chamber, precipitation of heavy metals increases in the soil, which is not favourable to remove heavy metals [18], [19]. However, Cr(VI) mainly exists as CrO42- in alkaline conditions with high mobility [20]. This means that, in theory, when OH- exists in COPR, the ion-exchange reactions between OH- and Cr(VI) can occur, indicating that the EK system can be used to remove Cr(VI) from COPR under alkaline conditions.

However, only one soil/sludge chamber can be handled in a traditional EK system, which results in a low current efficiency (CE) and high specific energy consumption (SEC), which limit the practical engineering applications of EK systems. In previous studies, the chelators [21], citric acid, and polyaspartic acid [17] were added to the soil to increase the solubility of metals, increase CE, and decrease SEC. However, these processes need large amounts of chemical reagents, which potentially threaten the environment and increase the cost. In our previous study [22], an EK process combined with a bipolar membrane (BPM) (BPM-EK) was used to remove copper and nickel from sludge. Compared with the traditional EK process, a higher CE and lower SEC were obtained because the sludge chamber could be equipped in the system and the position of the dissociated water shifted from the electrode (water dissociation voltage 2.057 V) to the BPM (water dissociation voltage 0.828 V). Since BPM has the ability to dissociate water into H+ and OH-, if the BPM-EK process can be used to treat COPR, Cr(VI) can not only be removed from COPR but also be recycled as chromic acid (H2CrO4). To our knowledge, there have been no studies that used the BPM-EK systems to recover Cr(VI) from COPR.

The main objective of this study is to evaluate the ability of the BPM-EK process to recover Cr(VI) from COPR. The specific objectives are (1) to investigate the effects of electrolyte concentrations on the voltage because voltage has an important effect on energy consumption (EC); (2) to evaluate the effect of current density on Cr(VI) recovery efficiency, pH value of COPR chamber (COPRC), and cell voltage for obtaining an optimal current; (3) to evaluate the Cr(VI) recovery efficiency, CE, and SEC under two- and three-COPRC equipped BPM-EK systems; (4) to analyse the SEC, CE, and recovery mechanisms.

Section snippets

Materials

All the chemical reagents used in the study were analytical grade and purchased from the Sinopharm Chemical Reagent Co. (Shanghai, China). The anion-exchange membrane (IONSEP EDI) was purchased from Hangzhou Iontech Environmental Technology Co. Ltd. (Hangzhou, China). A DuPont Nafion® cation-exchange membrane (NRE212) was supplied by Shanghai Hesen Electric Co. (Shanghai, China). The bipolar membrane (BP-1E) was supplied by Astom Co. (Tokyo, Japan). Distilled water was used throughout the study.

Effects of electrolyte concentrations in AC and CC on cell voltage

The electrolyte concentrations in AC, CC, and CAC have a significant effect on cell voltage and thus on energy consumption. Thus, the effects of electrolyte concentrations on cell voltage were investigated in this section. The conditions of 0.8 mol/L HNO3 in AC, current 0.01 A, NaNO3 concentrations of 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, and 2.0 mol/L in CC and CAC were chosen to investigate the effect of NaNO3 concentrations on cell voltage. The results are shown in Fig. 2. Cell voltage gradually

Consumption analysis

To investigate the CE and SEC of the improved EK process by BPM, a traditional EK system similar to the system in Fig. 1a was used to recycle Cr(VI). The main difference between the traditional EK process and the system in Fig. 1a was that BPM was replaced by an AEM to guarantee the generation of OH- by water dissociation on the surface of the cathode that can reach the COPRC. Both systems had nearly the same CE (0.59% and 0.60%, respectively) and SEC (0.901 and 0.906 kW h/g). In other words,

Conclusion

In this study, the recovery of Cr(VI) from COPR was investigated using the BPM-EK process. The current density had an effect on the Cr(VI) recovery efficiency, CE, and SEC. The optimal current density was 3.0 mA/cm2. Compared to the one-COPRC equipped system, the two- and three-COPR systems had higher CE and lower SEC values. The CE increased from 0.60% to 1.67% and 2.30% and SEC decreased from 0.906 to 0.433 and 0.395 kW h/g, respectively. The recovery efficiencies of Cr(VI) from all COPRCs

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 21507069), the Natural Science Funds (Grant No. 2017J01573) of Fujian Province, China, a Leading Foundation grant (Grant No. 2017Y0026) from the Fujian Province Department of Science and Technology, China, and a Major Projects grant (Grant No. 2015YZ0001-1) from the Fujian Province Department of Science and Technology, China.

References (27)

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