Removal of chromium(III) from aqueous waste solution by liquid–liquid extraction in a circular microchannel

A new circular microchannel device has been proposed for the removal of chromium(III) from aqueous waste solution by using kerosene as a diluent and (2-ethylhexyl) 2-ethylhexyl phosphonate as an extractant. The proposed device has several advantages such as a flexible and easily adaptable design, easy maintenance, and cheap setup without the requirement of microfabrication. To study the extraction efficiency and advantages of the circular microchannel device in the removal of chromium(III), the effects of various operating conditions such as the inner diameter of the channel, the total flow velocity, the phase ratio, the initial pH of aqueous waste solution, the reaction temperature and the initial concentration of extractant on the extraction efficiency are investigated and the optimal process conditions are obtained. The results show that chromium(III) in aqueous waste solution can be effectively removed with (2-ethylhexyl) 2-ethylhexyl phosphonate in the circular microchannel. Under optimized conditions, an extraction efficiency of chromium(III) of more than 99% can be attained and the aqueous waste solution can be discharged directly, which can meet the Chinese national emission standards. doi: 10.2166/wst.2017.371 s://iwaponline.com/wst/article-pdf/76/9/2313/208837/wst076092313.pdf Jian Hong Luo Jun Li (corresponding author) Lei Guo Xin Hua Zhu Shuang Dai Xing Li Department of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P.R. China E-mail: lijun@scu.edu.cn


INTRODUCTION
Chromium is a relatively common element and occupies the 21st position on the index of elements occurring most commonly in the earth crust (Massumi et al. ).It enters the environment as a result of effluent discharge from steel works, electroplating, tanning industry, oxidative dyeing, chemical industries and cooling water towers ( Jadwiga et al. ).Cr(III) is considered as indispensable for the metabolism of glucose, lipids and proteins in living organisms and is also mutagenic, carcinogenic and teratogenic (Jadwiga et al. ).According to World Health Organization guidelines for drinking water, the permissible level of chromium is 0.05 mg/L (Narin et al. ).Water treatment processes have been widely investigated from the standpoint of environmental conservation.The conventional process used to remove hexavalent chromium is either its reduction and subsequent precipitation as chromium hydroxide (Argo et al. ) or ion exchange (Costa et al. ).These processes are not completely satisfactory and have several disadvantages such as reduction of hexavalent chromium, neutralization of acidic solution and precipitation, and instability of the ion exchange membrane due to powerful oxidation of hexavalent chromium.Thus, there is a need for the development of low cost, easily available materials that can remove and recover hexavalent chromium economically.
Liquid-liquid extraction technique has been widely used in the fields of chemical engineering, analytical chemistry, environmental sciences, and biology (Hamber et al. ).Disadvantages of conventional contactors, including the large space requirements and high energy input, motivates the development of an extraction process which effectively uses raw materials and requires low energy input.These objectives can be met by carrying out extraction in microfluidic devices.Compared to the conventional liquid-liquid extractor, the microfluidics technique with a high interfacial area, short transport path, easy to enlarge for industrialization and the higher production strength has been evaluated as an attractive alternative due to the microscale effect (Singh et al. ).Furthermore, the mass transfer performance of different types of microstructured reactors was investigated and the mass transfer rate was reported to be about two or three orders of magnitude higher than those of conventional extraction equipment (Tang et  There are more and more examples where microreactors are integrated into pilot plants or even into large-scale production lines (Schwalbe et al. ).In this paper, we will focus on the various benefits this technology can offer for advanced solvent extraction.

Materials and equipment
The diluent used in this work was kerosene produced by Luo yang Zhongda Chemical Company (China).The (2ethylhexyl) 2-ethylhexyl phosphonate is employed as an extractant is produced by Luo yang Zhongda Chemical Company (China) (AR grade).Chromium(III) nitrate nonahydrate and sodium hydroxide were purchased from Ke Long Chemical Company (China) (AR grade).Deionized water was produced by an Aquapro making-water machine (ABZ1-1001-P) in our laboratory.The solution's pH was measured with a pH meter from Shanghai Precision & Scientific Instrument Co., Ltd.

Parameters that could affect the extraction process
The experiments were carried out in home-built microchannels with a circular section of inner diameter 1 mm, which were home-built.The channels had a T-junction at the inlet.The experiments were carried out in a T-junction straight microchannel of length 6 m.A schematic diagram of the experimental setup used for the microchannel study is shown in Figures 1 and 2. The two fluids were pumped into the microchannel using peristaltic pumps (Lead Fluid, BT100S).After extraction, the two phases were collected in a reservoir.
The temperature was measured by the main channel located in the water bath.All experiments were carried out at near atmospheric pressure.
In order to study Cr(III) removal efficiency and advantages of the extraction process, the parameters studied to optimize extraction and separation conditions are: inner diameter of the channel from 1 to 2 mm, total flow velocity from 0.275 to 1.348 m/s, extractant volume fraction from 30 to 100%, phase ratios organic/aqueous (O/A) from 1:1 to 5:1, reaction temperature from 303.15 to 343.15 K, pH value of waste water from 1.86 to 3.5.
Extraction efficiency is defined as follows: × 100% (1) E: efficiency of extraction process; Distribution coefficient (D) is calculated as: where R is the phase ratio between the organic and aqueous phases.

Analysis
Samples at the outlet were collected and analyzed for Cr(III) concentration.
The percentage of Cr(III) was determined by colorimetric spectrophotometry using a UV-visible spectrophotometer (type UV-1100, Mapada, China).

Effect of the inner diameter of the channel
The effect of inner diameter of the main channel on the extraction efficiency is shown in Figure 3 for a channel with L ¼ 6 m.By increasing the inner diameter of the main channel, the extraction efficiency is observed to decrease.The smaller channel size induces the stronger internal circulation and, therefore, enhances the convective mass transfer.The stronger internal circulation inside the droplets renews the interface, which augments the concentration gradient of (2-ethylhexyl) 2-ethylhexyl phosphonate between two phases, thus intensifying the diffusive penetration through the interface.As a consequence, higher effective interfacial area and extraction efficiency are obtained in the microchannel with smaller inner diameter.Tang et al. () reported the same trend.

Effect of total flow velocity
In this work, we investigated the extraction efficiency under different experimental conditions.Firstly, we investigated the influence of total flow velocity on the extraction efficiency.Figure 4 shows the effect of total flow velocity on the extraction efficiency at two fixed volume flow rate ratios.The total flow velocity does not affect the extraction efficiency of Cr(III) obviously.However, as the total flow velocity value reaches 1.08 m/s, the extraction of Cr(III) decreases with the increasing total flow velocity.
Reynolds number increases by reducing the microchannel diameter at the same contact time, and volumetric mass transfer coefficient should improve in theory (Tang et al. ).However, when the contact time is decreased in the microchannels, the velocities of the continuous and dispersed phase increase.This amounts to less energy available for generating dispersion, leading to a coarser dispersion and a

Effect of phase ratio
As can be observed from the Figure 5, the extraction efficiency is increased with the phase ratio increasing from 1:1 to 5:1.This is as expected because increasing the phase ratio will enhance the amount of the extractant, which leads to increase in extraction efficiency.

Effect of initial pH of aqueous waste solution
As the cationic extractant (3) Therefore, the extraction efficiency of chromium(III) increases rapidly as the initial pH of aqueous waste solution rises in the extraction system, as shown in Figure 6.However, considering the economic cost factor, the pH value of 4.5 for extraction is appropriate.

Effect of reaction temperature
Figure 7 shows the effect of reaction temperature on extraction distribution ratio (logD).The distribution ratio (D) increases as the temperature rises.It can be seen from Figure 7 that a linear relationship between logD and T À1 10 À1 is obtained in this experiment.From the famous van't Hoff equation (Out et al. ), the standard enthalpy change (ΔH) value 8.318 × 10 À3 (J/mol) can be calculated, which shows that the extraction of chromium(III) with (2-ethylhexyl) 2-ethylhexyl phosphonate is endothermic.

CONCLUSIONS
Based on the results of removing chromium(III) from aqueous waste solution by solvent extraction of (2-ethylhexyl) 2-ethylhexyl phosphonate in the circular microchannel, the following specific conclusions can be drawn.Solvent extraction in the circular microchannel is an effective method to remove chromium(III) from aqueous waste solution with (2-ethylhexyl) 2-ethylhexyl phosphonate as extractant.
The optimal process conditions are as follows.The inner diameter of the channel is 1 mm, the total flow velocity is 1.081 m/s, the phase volume ratio is 3:1, the initial pH of aqueous waste solution is 4.5, the reaction temperature is 30 W C, the (2-ethylhexyl) 2-ethylhexyl phosphonate volume fraction is 50%.
The thermodynamic ΔH value of the extraction reaction is 8.318 × 10 À3 (J/mol) at the temperature 303 K.

Figure 2 |
Figure 2 | Photograph of the microextraction experiment process.

Figure 8
Figure 8 shows the effect of the (2-ethylhexyl) 2-ethylhexyl phosphonate volume fraction on the extraction efficiency.It can be seen that the amount of extractant increases as (2-ethylhexyl) 2-ethylhexyl phosphonate volume concentration in the solvent phase increases.Therefore, the number of free extractant molecules taking part in the extraction reaction will also increase.However, when the (2-ethylhexyl) 2-ethylhexyl phosphonate concentration increases to a certain value, as the extraction reaction reaches equilibrium, the extraction efficiency will remain almost unchanged.

Figure 5 |Figure 6 |
Figure 5 | The extraction efficiency (E) versus the phase ratio.Condition: initial pH of waste water ¼ 3.37, volume concentrations of the extractant ¼ 30%, inner diameter of the channel ¼ 1 mm, total flow velocity ¼ 0.8122 m/s, reaction temperature ¼ 60 W C.
be replaced when (2-ethylhexyl) 2-ethylhexyl phosphonate reacts with chromium(III).The reaction mechanisms proceed in accordance with the cation exchange (Wang et al. ; Van et al. ) and chelation, and the extraction reaction can be described as follows (Luo et al. ), where HA corresponds to (2-ethylhexyl) 2-ethylhexyl phosphonate: