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Separation and recovery of phenols from an aqueous solution by a green membrane system
Journal of Cleaner Production 251 (2020) 119675
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Separation and recovery of phenols from an aqueous solution by a green membrane system Xiang Mei a, *, Junhui Li a, Chenchen Jing a, Chenhong Fang a, Yang Liu b, Yong Wang c, Juan Liu a, Shuqi Bi a, Ying Chen a, Yanyan Xiao d, Xu Yang d, Yifan Xiao a, Shuai Wu a, Yang Ding a a
College of Biology and the Environment, Nanjing Forestry University, Nanjing, 210037, China Division of Water and Wastewater Engineering, Nanjing Jinling Petrochemical Engineering Co., Ltd., Nanjing, 210042, China State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing, 210023, China d Nanjing Haiyi Environmental Protection Engineering Co., Ltd., Nanjing, 211200, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2018 Received in revised form 19 November 2019 Accepted 10 December 2019 Available online xxx
Oil-refinery alkali residue wastewater (ORARW) contains large amounts of phenol and its derivatives, which are highly toxic and valuable but difficult to biodegrade. In this study, using vegetable oil as the liquid membrane phase, a green polypropylene-hollow-fiber supported liquid membrane (SLM) system was constructed to simultaneously separate and recover phenol, m-cresol and o-cresol from aqueous solutions. After screening several types of vegetable oil, linseed oil was selected as the liquid membrane phase. Based on a single SLM module and using a NaOH solution as the stripping reagent with feed phase and stripping phase hydraulic retention times (HRTs) of 7.8 and 6.9 min, respectively, the phenol separation and recovery rates were 92.9% and 88.8%, respectively. The separation rates for m-cresol and ocresol were both greater than 96%, their recovery rates were approximately 92%, and the membrane separation flux of every phenolic compound exceeded 0.40 g/m2$h. When two SLM modules were operated in series at room temperature for 3 h, the separation efficiency for phenols increased by approximately 9% compared with that obtained with a single SLM module. After applying two SLM modules in series to treat ORARW with an initial concentration of phenols of 4800e5200 mg/L, the simultaneous separation and recovery rates of phenols were near 95% and 92%, respectively. This work concluded that vegetable oil can be used as a nontoxic and renewable green liquid membrane phase and that two SLM modules in series efficiently separated and recovered phenols from aqueous solutions. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: M.T. Moreira Keywords: Phenols Vegetable oil Supported liquid membrane Separation and recovery Oil-refinery alkali residue wastewater
1. Introduction Phenols and their derivatives are some of the most common organic pollutants in industrial wastewater (Cui et al., 2017). Among the many phenolic compounds, phenol is an important raw material in chemical industries, and cresol is the simplest derivative of phenol. Both phenol and cresol are highly toxic and difficult to biodegrade. These phenolic substances are mainly produced by oil refining, coking, and processes to create synthetic resins, medicine, and preservatives. The concentration of phenols in wastewater depends on the type of technology and production process, e.g., 6e500 mg/L in refinery wastewater, 28e3900 mg/L in coking
* Corresponding author. E-mail address: [email protected] (X. Mei). https://doi.org/10.1016/j.jclepro.2019.119675 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
plant wastewater, 9e6800 mg/L in coal chemical plant wastewater and 2.8e1220 mg/L in petrochemical industry wastewater (Busca et al., 2008; Serrano et al., 2017). For the treatment of these highconcentration phenolic wastewaters, physical and chemical treatment methods such as waste residue adsorption (Yang et al., 2019) and solvent separation (Gai et al., 2019) have the advantages of simple operation and high efficiency, but the adsorbent and the solvent are high-cost, difficult to reuse and tend to generate secondary pollution. Some researchers adopted catalytic methods to degrade phenols in petroleum refinery wastewater (Ani et al., 2018), but the catalysts are usually unstable and the problems of catalyst recycling and degradation have not been solved (He et al., 2019). The multi-stage treatment of industrial phenolic wastewater has been carried out by biological methods in combination with other technologies (Bahri et al., 2018; Guo et al., 2019), because
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microbial activity is easily limited by the influent concentration if only biological treatment is used (Werkneh et al., 2018). From the perspective of resource recovery and reducing the burden for the subsequent treatment, it is important that the phenols in industrial wastewater be separated and recovered through a novel approach before the wastewater is discharged into the environment. The supported liquid membrane (SLM) process is a new extraction technology for the treatment of wastewater containing organic compounds (Mahdavi et al., 2016) and metals (Bhatluri et al., 2015). The process mainly consists of a liquid membrane phase, feed phase, stripping phase and support material. Compared with solvent extraction, the SLM process has the advantages of a large mass-transfer impetus, high speed, high selectivity and minimal extractant loss. Additionally, the extraction and stripping processes are realized in the same equipment, so the capital investment and operating cost are low. The separation of different components is more efficient, and solutes can be transported against the concentration gradient. However, the disadvantage of the SLM process is that the liquid membrane is unstable, which is related to the carrier concentration (Nakano et al., 2006), the type and solubility of the solvents and the molecular structure of the carrier (Deblay et al., 1991; Neplenbroek et al., 1992). Regardless of the mechanism, the main factors that cause the loss of the liquid membrane phase are multifaceted, and thus the choice of the support material and extractant is critical. The SLM extraction technique has been used to treat single phenolic compounds in wastewater, and the liquid membrane phases used have mostly been carrier-solvent systems, such as the trioctylphosphine oxide (TOPO)-kerosene system (Zidi et al., 2011), bis(2-ethylhexyl) sulfoxide (BESO)-kerosene system (Yang et al., 2015), tributyl phosphate (TBP)-kerosene system (Shen et al., 2009) and Cyanex 923 (Reis et al., 2007). In these systems, the solvents, such as chloroform, carbon tetrachloride, dichloromethane, benzene, toluene and pyridine, are considered hazardous chemicals. Ionic liquid membranes and hydrocarbon compounds have also been used for the separation of bisphenol A (BPA) (Gupta et al., 2014; Panigrahi et al., 2013). The experimental results showed that the addition of TBP to kerosene improved the extraction rate of phenol by SLM in a short time, and the stability of the flat polyvinylidene-fluoride membrane was enhanced (Zidi et al., 2010). The SLM with TOPO as the extractant was confirmed to be better than that with TBP, and the effective contact area of a hollow-fiber SLM is larger than that of a flat membrane (Zidi et al., 2011). Most organic solvents are toxic, easily lost, unstable and cause secondary pollution in SLM processes. Vegetable oil is a nonhazardous and inexpensive chemical that could be used as the liquid membrane phase in a SLM process. Vegetable oils are naturally produced, readily available, nontoxic, low-cost and renewable resources. Venkateswaran et al. (2006) used natural vegetable oil in the treatment of phenol in an aqueous solution. Kazemi et al. (2014) constructed an SLM with a TBP-sesame oil system and obtained a higher mass transfer rate and good stability with suitable concentrations of stripping reagent and carrier. Peydayesh et al. (2014) investigated the effects of various factors on the separation rate and membrane stability of phenolic compounds. An edible oil was used as the liquid membrane phase, and good results were achieved. This study presents experiments performed using vegetable oil as the liquid membrane phase in an SLM system constructed to separate and recover phenols from an aqueous solution. A single SLM module and two SLM modules in series were investigated to simultaneously separate and recover phenol, m-cresol and o-cresol from oil-refinery alkali residue wastewater (ORARW). The purpose of this study was to establish a novel, green SLM system for the efficient separation and recovery of phenols from phenolic wastewater.
2. Materials and methods 2.1. Experimental materials The vegetable oils used in this study are commercially available from East Sea Grain and Oil Industry (Zhangjiagang) Co., Ltd., China and included sunflower oil, rapeseed oil and linseed oil. The simulated phenolic wastewater was prepared using distilled water. The ORARW was produced after catalytic wet air oxidation (CWAO) at an oil refinery located in Nanjing, China. The wastewater had a poor water quality with a brown color and high concentration of free alkali. The chemical oxygen demand (COD) and concentrations of volatile phenols, total dissolved solids (TDS) and mineral oil were ca. 50,500e57,500 mg/L, 4800e5200 mg/L, 48,900 mg/L and 31 mg/L, respectively, and the pH was 13.4e13.5. The main phenolic compounds were the toxic phenol and cresol, and the concentrations of phenol, m-cresol and o-cresol were about 1700 mg/L, 1600 mg/L and 1600 mg/L, respectively. The hydrophobic polypropylene (PP) hollow-fiber microporous membrane was obtained from Hangzhou Haotian Membrane Technology Co., Ltd., China. The other chemicals used in this study, including phenol (AR), m-cresol (AR), o-cresol (AR), NaOH (AR) and acetic acid (GR), were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Methanol and acetonitrile, which were of chromatographic purity, were obtained from TEDIA Company, Inc., USA. 2.2. Experimental system The parameters of the hollow-fiber membrane modules are shown in Table 1. The membrane module was completely impregnated with vegetable oil for 12 h. After impregnation, the excess oil was wiped off the module, and then the membrane was sandwiched between the feeding and stripping cells, as illustrated in Fig. 1. In the experiment with a single SLM module, 500 mL of a phenols solution was used as the feed phase and run through the tubes, and 250 mL of a NaOH solution was used as the stripping phase and run through the shells. The effect of the vegetable oils, NaOH concentration, hydraulic retention time (HRT) of the feed and stripping phases, and temperature on the separation and recovery of phenols was investigated (Fig. 1(a)). An attempt was also made to separate and recover phenols from ORARW using SLM modules in series (Fig. 1(b)). The wastewater was continuously circulated in both SLM modules with separate stripping solutions in each SLM module to allow the transport of the phenols from the feed phase to the stripping phase. Samples were taken from the feed and stripping solutions at regular time intervals and analyzed. 2.3. Experimental design Three types of vegetable oils, sunflower oil, rapeseed oil and
Table 1 Parameters of the hollow-fiber membrane modules. Parameter
Value
Empty bed volume (mL) Number of membrane fibers Effective length of fiber (mm) Membrane surface area (m2) Internal diameter of fiber (mm) Outer diameter of fiber (mm) Wall thickness of fiber (mm) Porosity (%) Pore size (bubble point method) (mm) Gas flux (m3/(m2$h))
186 800 265 0.275 320e350 400e450 80e100 64.8 0.38 3
X. Mei et al. / Journal of Cleaner Production 251 (2020) 119675
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Fig. 1. Schematic diagram of the supported liquid membrane (SLM) systems. (a) Single SLM module; (b) Two SLM modules in series.
Table 2 Operation conditions of the SLM system. Condition Number of membrane Caq Cstr (mol/L) modules (mg/L)
HRTaq (min)
HRTstr (min)
Operation time (h)
Temperature pH ( C)
Mineral oil concentration (mg/L)
I
1
7.8
6.9
4
25
5.9
e
II
1
25
5.9
e
1
0.10
5.6, 6.9, 8.1 6.9
4
III
5.2, 7.8, 10.3 7.8
4
15, 25, 30
6.0
e
IV
1
0.10
7.8
6.9
4
25
5.9e6.0
e
V
2
0.10
7.8
6.9
3
25
6.0
e
0.10
7.8
6.9
3
25
0.10
7.8
6.9
3
25
2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 32 10.0, 11.0 7.0e8.0 31, 391, 702, 977
a
VI
2
VII
2
a
2930~ 2990 2970~ 3050 2800~ 3010 2840~ 3350 2890~ 3010 4970~ 5200 5090~ 5200
0.01, 0.03, 0.05, 0.10, 0.20 0.10
ORARW was diluted to some extent when used.
linseed oil, were screened. Three 50-mL beakers were washed and dried, and 25 mL of a 2800 mg/L simulated phenols solution and 10 mL of vegetable oil were added to each beaker. Then, the beakers were placed in an incubator at 25 C. The speed of the magnetic stirrer was 350 rpm. After 3 h of extraction and static stratification, a water sample was taken to determine the remaining phenols content in the feed solution and calculate the partition coefficient. The experiment was repeated three times. After the screening of the vegetable oils, the extraction performance of the SLM system and the treatment of the ORARW by two SLM modules in series were studied. The operation conditions are shown in Table 2. Under conditions I ~ V, the SLM extraction experiments were carried out with a feed-phase volume (Vaq) of 500 mL, stripping-phase volume (Vstr) of 250 mL, feed-phase concentration (Caq) of ca. 3000 mg/L and unchanged mass ratio of phenol:m-cresol:o-cresol ¼ 1:1:1. Under conditions VI and VII, ORARW as the feed phase was treated by two SLM modules in series.
510 nm, the mineral oil concentration was determined by ultraviolet spectrophotometry, and the COD was measured by a standard method (APHA, 1998). The density and viscosity of the vegetable oils were determined, respectively, by a petroleum densitometer (measuring range 0.90e0.95 g/cm3) from Hejian Zhenyan Instrument Co., Ltd., China and a Pinkevitch viscometer (capillary inner diameter 1.2 mm, constant 0.1490 mm2/s2) from Shanghai Shenyi Glass Products Co., Ltd., China. The TDS was determined using a TDS measuring instrument (HI 983,301) from HANNA Co., Ltd., Singapore. An Agilent high-performance liquid chromatography (HPLC) system was also used to determine the concentrations of phenol, mcresol and o-cresol. An SB-C18 column (4.6 mm 150 mm 5 mm) was selected. The column temperature was 30 C, the UV detection wavelength was 278 nm, the mobile phase was acetonitrile (1% acetic acid)-water (1% acetic acid) (30:70), the flow rate was 0.7 mL/ min and the injection volume was 40 mL.
2.4. Analytical methods
2.5. Evaluation index
The concentration of volatile phenols in the wastewater was analyzed by the 4-aminoantipyrine spectrophotometric method at
2.5.1. Separation rate (S) In the liquid membrane separation process, the separation rate
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(S) reflects the separation effect of the SLM on the phenolic compounds and is expressed by Eq. (1).
C0 Ct 100% C0
S¼
3.1. Screening of vegetable oils
(1)
where C0 is the initial concentration of phenolic compounds in the feed phase (mg/L) and Ct is the concentration of phenolic compounds in the feed phase at time t (mg/L). 2.5.2. Recovery rate (R) In the stripping process, the NaOH solution reacts with phenolic compounds to form sodium phenates. The concentrations of sodium phenates were determined by HPLC after the addition of hydrochloric acid. The recovery effect can be expressed by the recovery rate (R) (Eq. (2)).
R¼
Cstr Vstr 100% Caq Vaq S
(2)
where Cstr is the concentration of phenolic compounds in the NaOH solution (mg/L), Vstr is the volume of NaOH solution (mL), Caq is the initial concentration of phenolic compounds in the feed phase (mg/ L), and Vaq is the volume of the feed solution (mL). 2.5.3. Membrane separation flux (J) As an important evaluation index of the SLM extraction performance, the membrane separation flux (J) of a phenolic compound was calculated by Eq. (3).
J¼
M AT
3.2. Effect of stripping solution concentration
2.5.4. Partition coefficient (P) The partition coefficient (P) (Eq. (4)) can be used to evaluate the extraction performance of extractants and is an important indicator in the vegetable oil screening.
Corg Caq
(4)
where Corg is the concentration of phenolic compounds in the organic phase after reaching the extraction equilibrium (mg/L) and Caq is the concentration of the remaining phenolic compounds in the feed phase after reaching the extraction equilibrium (mg/L). 2.5.5. Hydraulic retention time (HRT) The hydraulic retention time of the feed phase (HRTaq) and the hydraulic retention time of the stripping phase (HRTstr) were calculated by Eqs. (5) and (6), respectively.
HRTaq ¼
VM Qaq
In this study, the viscosities and densities of commercially available vegetable oils, including sunflower oil, rapeseed oil and linseed oil, were measured. Based on the main physical and chemical properties (Chang et al., 2014; Hu et al., 2008), liquidliquid extraction experiments were carried out. After allowing static stratification, the concentration of the remaining phenols was determined by taking samples from the feed solution, and the partition coefficient was calculated. The experimental results are shown in Table 3. The melting points of the three types of vegetable oil are lower than that of water, so the vegetable oils exist in a state of liquid aggregation, which is a prerequisite for use in an SLM process. The three vegetable oils are nonpolar substances with dielectric constants less than 15, and they are suitable as extractants of the weakly polar phenol, m-cresol and o-cresol according to the theory of ‘similarity and intermiscibility’. Additionally, they are safe and stable with high flash points. Based on the partition coefficient results, linseed oil has a stronger affinity for phenolic compounds than sunflower oil or rapeseed oil, which might be a result of the viscosity differences among the vegetable oils. The viscosity order of the three vegetable oils is linseed oil < sunflower oil < rapeseed oil. The viscosity is usually the main factor affecting the flow resistance and thickness of a liquid, and the higher the viscosity is, the larger the resistance is. A higher viscosity is not beneficial for the internal transport of phenolic compounds in vegetable oil. Therefore, linseed oil was found in this study to be more suitable as an extractant.
(3)
where M is the mass of a phenolic compound extracted by the supported liquid membrane (g), A is the membrane area (m2), and T is the operation time (h).
P¼
3. Results and discussion
(5)
Different concentrations of the NaOH solution, which was used as the stripping phase, were tested. The NaOH solution is essential to the separation and recovery of the phenolic compounds and the SLM stability. The stripping efficiency was studied in the NaOH solution concentration range of 0.01 moL/L~0.20 moL/L. Fig. 2 shows that the separation rates and membrane separation fluxes of the phenolic compounds increased as the NaOH concentration increased from 0.01 moL/L to 0.10 moL/L and decreased for NaOH concentrations above 0.10 moL/L. When the effective contact area of the feed solution and SLM and the liquid membrane phase content were constant, a low NaOH concentration led to a failure to react with the phenolic compounds in a timely and effective manner. In contrast, an excessively high NaOH concentration caused excessive alkalinity and increased the surface tension between the vegetable oil and membrane filaments, which resulted in the emulsification of the vegetable oil, the loss of the liquid membrane phase and a reduction in the separation rate. Meng et al. (2015) observed similar results for phenol transport through a polymer-inclusion membrane with N,N-di (1-methylheptyl) acetamide as the carrier from an aqueous solution. Thus, the concentration of NaOH could not be too high or too low, and a 0.10 moL/L NaOH solution was used as the stripping phase to separate the phenolic compounds in this experiment. 3.3. Effect of HRT
V HRTstr ¼ M Qstr
(6)
where VM is the empty bed volume of the membrane module (mL), Qaq is the flow rate of the feed solution (mL/min), and Qstr is the flow rate of the stripping solution (mL/min).
In the experiment, a hydrophobic hollow-fiber membrane was used as the support material. When the feed solution and stripping solution flow through the different sides of the membrane, a boundary layer forms. The thickness of the boundary layer varies with the flow rate and affects the mass transfer (Lin et al., 2016). To
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Table 3 Comparison of properties of vegetable oils. Property
Linseed oil
Rapeseed oil
Sunflower seed oil
Water
Melting point ( C, 1 atm) a Dielectric constant (25 C) a Density (g/cm3, 25 C) b Viscosity (mPa$s, 25 C) b Flash point ( C, 1 atm) a Partition coefficient b
19 z3 0.927 48.90 222 12.94 ± 0.68
9 z3 0.916 79.97 283 9.82 ± 1.10
15 z3 0.921 62.44 121 11.07 ± 0.57
0 78 0.997 0.89
a b
References: (Chang et al., 2014; Hu et al., 2008). Data from actual measurements.
Fig. 2. Effect of the stripping solution concentration on the separation of phenolic compounds under condition I (Table 2).
achieve a better mass-transfer effect, the feed-phase HRTaq was set at 5.2 min, 7.8 min and 10.3 min, and the stripping-phase HRTstr was set at 5.6 min, 6.9 min and 8.1 min. The experimental results are shown in Fig. 3. Fig. 3(a) shows that the separation rates of phenol, m-cresol and o-cresol increased when HRTaq increased from 5.2 min to 7.8 min and decreased as HRTaq increased from 7.8 min to 10.3 min. Since the inner diameter of the hollow-fiber membrane is on the micron scale, when an increase in the feed phase flow rate and a reduction in HRTaq cause decreases in the boundary layer thickness and masstransfer resistance, the phenolic compounds enter the membrane phase more quickly than before, and the separation rate is improved. However, an excessively large flow rate increases the feed solution pressure on the membrane and the shear force between them, resulting in instability and the loss of the liquid membrane phase. Fig. 3(c) shows that the variation in HRTstr had a slight influence on the separation rates of m-cresol and o-cresol and almost no influence on the separation rate of phenol because the boundary layer resistance of the feed phase was much greater than that of the stripping phase in the total mass-transfer resistance (Peretti et al., 2002). Therefore, the appropriate HRTaq and HRTstr were 7.8 min and 6.9 min, respectively. Under these two HRTs, the membrane separation fluxes of m-cresol and o-cresol were also the highest (Fig. 3(b) and (d)).
3.4. Effect of temperature The temperature is an important influencing factor in the separation and recovery of phenolic compounds by the SLM system.
The maximum tolerable temperature of the polypropylene hollowfiber membrane used in this experiment was 45 C. In consideration of the membrane life and power consumption, the operation temperature was set at 15 C, 25 C and 30 C. The experimental results are shown in Fig. 4. Fig. 4(a) clearly shows that the SLM separation rates of phenol, m-cresol and o-cresol improved with the increasing temperature. At 30 C, the separation rate of phenol was 92.9%, and those of mcresol and o-cresol were both greater than 96%. The membrane separation fluxes of phenol, m-cresol and o-cresol were 0.437 g/ m2$h, 0.499 g/m2$h and 0.428 g/m2$h, respectively (Fig. 4(b)). The increase in the temperature enhanced the molecular thermodynamic movement and reduced the viscosity of the vegetable oil, both of which are conducive to reducing the mass-transfer resistance. Additionally, phenolic compounds are weak acids, and their reactions with NaOH release a small amount of heat. The two roles above could not offset each other, but the elevated temperature could increase the chance of intermolecular association, thus improving the separation rates of phenolic compounds. Based on the comprehensive analyses of the effects of the stripping solution concentration, HRT and temperature, an experiment was carried out under the optimized conditions, and the results are shown in Table 4. Under the conditions of Vaq ¼ 500 mL, Caq ¼ 2800e3000 mg/L, HRTaq ¼ 7.8 min; Vstr ¼ 250 mL, Cstr ¼ 0.10 moL/L, HRTstr ¼ 6.9 min, and T ¼ 30 C, the separation and recovery rates of phenol were 92.9% and 88.8%, respectively. The separation rates for m-cresol and o-cresol were both greater than 96%, and their recovery rates were approximately 92%. The membrane separation fluxes of the phenolic compounds all exceeded 0.40 g/m2$h.
3.5. The stability of the SLM system When the feed solution and the stripping solution flowed on the two sides of the membrane, the membrane pore blockage and the pressure difference probably resulted in the loss of the liquid membrane phase of the supported liquid membrane. To investigate the stability of the SLM system in the separation and recovery of phenolic compounds, the experiment was performed 5 times consecutively without refilling with linseed oil, and the SLM system operated for 4 h in each batch of experiments. The separation and recovery results of the phenols are shown in Fig. 5. It can be seen from Fig. 5(a) that the separation rates of phenols by the SLM system are all over 90% and the recovery rates were stable at about 88% in the consecutive experiments. There was little difference among the repeated experimental results, indicating that the loss of the liquid membrane phase was very slight. For all repeated batches of experiments, the separation fluxes of phenols were all above 1.140 g/m2$h (Fig. 5(b)), which shows that the SLM system can operate steadily without refilling with linseed oil, and an increase in the number of operation cycles had little effect on the stability of the system. The slightly higher recovery effects in the
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Fig. 3. Effect of the HRT on the separation of phenolic compounds under condition II (Table 2).
Fig. 4. Separation of phenolic compounds at different temperatures under condition III (Table 2).
Table 4 Separation and recovery of phenolic compoundsa. Parameter
phenol
m-cresol
o-cresol
Initial quantity (mg) Separation rate (%) Recovery rate (%) Separation flux (g/m2$h)
497 92.9 88.8 0.420
553 96.7 92.1 0.486
466 96.4 91.8 0.408
a Linseed oil as the liquid membrane phase, phenols solution as the feed phase, and NaOH solution as the stripping phase.
first batch of experiments were probably because there was a little residual linseed oil in the shell or the tube of the membrane
Fig. 5. Stability of the SLM system under condition IV (Table 2).
module. In addition, when a single membrane module was in operation, the difference between the recovery rate and the separation rate of the phenols was less than 5%, and when the separation rate was increased to about 98% by using two membrane modules in series, the difference between the recovery rate and the separation rate was reduced to 3%. The results show that the SLM system has the advantages of high stability, a high recovery rate and low consumption. 3.6. Two SLM modules in series In this experiment, two SLM modules were connected in series
X. Mei et al. / Journal of Cleaner Production 251 (2020) 119675
to increase the contact time with the wastewater, because the contact time affects the mass-transfer efficiency. Fig. 6(a) shows that two SLM modules in series have a higher efficiency than a single SLM module for the separation of phenolic compounds under the same operating conditions. After 3 h, the separation rates of phenol, m-cresol and o-cresol were approximately 90.0%, 97.1% and 98.1%, respectively. As seen in Fig. 6(b), the separation rates of the phenolic compounds with the SLM modules in series increased by approximately 9% compared with that obtained with a single SLM module. The separation rates of m-cresol and o-cresol were similar, and those of phenol were slightly lower. As shown in Fig. 6(c), the membrane separation fluxes of the phenolic compounds gradually decreased with the extension of the operation time, and those obtained with the SLM modules in series decreased by approximately 0.20e0.40 g/m2$h compared with those obtained with a single SLM module (Fig. 6(d)). The order of the octanol-water partition coefficients is mcresol > o-cresol > phenol. Huddleston et al. (1998) confirmed that the partition coefficient of materials in a vegetable oil-water system is similar to that in an octanol-water system. Phenol is a hydrophilic substance, and its partition coefficient is relatively small, resulting in minimal solubility in a liquid membrane phase and a low separation rate. However, a solute partition coefficient in a solvent that is too high can easily led to solute retention in the organic phase, and then the solute is not easily extracted by the stripping phase. Therefore, a moderate partition coefficient is more conducive for improving the separation rate. The test results showed that two SLM modules in series are more conducive to phenolic compound separation, and these SLM modules were applied to real industrial wastewater.
7
3.7. Application to real industrial wastewater 3.7.1. Effect of pH Phenolic compounds are weak electrolytes that dissociate in water, and they exist in different forms in aqueous solutions with different pH values. That is, the pH affects the presence of phenolic compounds in aqueous solutions. According to the principle of hollow-fiber SLM extraction, a solute in the molecular state can be easily extracted by the extractants in the membrane phase, while the solubility of a solute in the ionic state in the feed solution is lower. The alkaline scrubbing of the oil products leads to the strong alkalinity of ORARW. Therefore, most of the phenolic compounds were in the ionic state, which is not conducive for their separation and recovery. To ensure the separation of the phenolic compounds by the SLM system, the ORARW pH was adjusted by hydrochloric acid from the actual value (13.4e13.5) to 2.0e11.0 to determine the optimum pH for ORARW treatment by the SLM system. Fig. 7 shows that the separation rates and fluxes of the phenols were constant at approximately 95% and 0.8 g/m2$h, respectively, when the ORARW pH was less than 8.0. When the pH was 8.0e9.0, the separation rate began to decrease, and it continued to decrease for pH values above 9.0. Similarly, Yang et al. (2015) reported that phenol (PhOH) was transported from an aqueous solution through an SLM impregnated with bis(2-ethylhexyl) sulfoxide (BESO) in kerosene. The partition coefficient was constant when the phenolcontaining wastewater pH was 0.7e8.5 and began to decrease for pH values above 8.5. Finally, the partition coefficient was near zero for a pH close to 13.8. Therefore, to ensure the separation of the phenolic compounds in ORARW and other biochemical treatments, the ORARW pH was adjusted to neutral.
Fig. 6. Separation of phenolic compounds by a single SLM module and two SLM modules in series under condition V (Table 2).
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separation rates of the phenolic compounds tended to decrease with an increase in the mineral oil concentration. When the mineral oil concentration was near 400 mg/L, the inlet pressure increased to 0.02e0.04 MPa, and when the mineral oil concentration was approximately 700 mg/L, the inlet pressure was more than 0.08 MPa, which increased the necessary membrane cleaning frequency. The SLM modules were easily damaged when the mineral oil concentration was near 1000 mg/L. Therefore, the pretreatment of the ORARW is necessary to reduce the mineral oil content.
Fig. 7. Effect of the pH on the separation of phenols with two SLM modules in series in the treatment of ORARW under condition VI (Table 2).
3.7.2. Effect of mineral oil content Mineral oil is present in ORARW and enters the pipe along with the feed solution; thus, membrane blocking easily occurred because of the large molecules and high viscosity of the mineral oil. To understand the effect of the mineral oil concentration on the SLM service life, experiments with SLM modules in series were carried out. The mineral oil concentration in the ORARW increased to 977 mg/L, 702 mg/L, 391 mg/L and 31 mg/L upon adding oil extracted from the ORARW. The slight deviations in the initial concentrations of phenol, m-cresol and o-cresol made the separation rates and fluxes of the phenolic compounds somewhat different, but their variation trends were the same. Fig. 8 shows that small amounts of mineral oil had little effect on the separation of phenolic compounds from the ORARW, which indicates that the antipollution ability of the SLM was stronger. However, the
3.7.3. Techno-economic analysis As a cleaner production technology, the SLM system has the technical advantages of recycling resources, reducing pollution and improving the recovery efficiency. The conventional treatment of phenolic wastewater not only wastes resources and has inferior wastewater treatment effects but also costs more. The application of SLM technology in the ORARW treatment process (Fig. 9) shows that SLM system can efficiently recover phenols before the biotreatment, and the economic benefits of the SLM technology also cannot be ignored. Thus, a preliminary economic evaluation can be performed for the SLM technology. This research was performed only on the laboratory scale, and thus, it is difficult to calculate the costs completely and accurately when scaling the process up to the industrial scale, especially the costs of the equipment investment and its maintenance, energy, and labor. Therefore, in this preliminary economic evaluation of the SLM technology, only the costs of chemicals and power consumption in the ORARW treatment were estimated. An oil refinery (Nanjing, China) produces 100 tons of ORARW every day and its density is approximately 1 kg/L, so the volume of wastewater is 1.0 105 L. If the volume ratio of wastewater, the liquid membrane phase and the stripping phase was 250:1:10 in actual production, the volumes of the liquid membrane phase and stripping phase were 400 L and 4.0 103 L, respectively. In addition, the vegetable oil used as an extractant can be recycled, so the daily consumption of vegetable oil was 0.4 L (0.37 kg) based on a
Fig. 8. Effect of the mineral oil concentration on the separation of phenolic compounds in the treatment of ORARW under condition VII (Table 2).
X. Mei et al. / Journal of Cleaner Production 251 (2020) 119675
9
Fig. 9. Schematic diagram of ORARW treatment process containing a green SLM system. Inside the dotted box are the existing treatment units of the refinery wastewater; the blue part is the SLM system for the separation and recovery of phenols.
Table 5 Daily costs and profits of the SLM system. Items
a
Phenols Vegetable oil Sodium hydroxide Hydrochloric acid Phosphoric acid Power (Pump & Mixer & Drying) Gross profit ($/d) a b c d
Price ($/kg)
Output (kg)
Consumption (kg)
Profit ($/d)
Cost ($/d)
1.138 b 11.376 c 0.256 b 0.025 b 0.882 b 0.078 ($/kWh) e
500.5 e e e e e e
e 0.37 194.6 1000.0 280.4 531.6 (kWh) e
569 e e e e e 202
e 4 50 25 247 41 e
d
Major daily cost and benefit items, where $ is US dollars. Chemical prices refer to international trade market prices. The price of linseed oil refers to www.cngrain.com. The electricity price is the unit peak price in Jiangsu province, China.
loss rate of 0.1%. The density of vegetable oil (as linseed oil) was 0.927 kg/L. In order to keep the stripping phase concentration of 0.10 moL/L as determined by the experiment, NaOH should be added according to the amount of phenolic compounds recovered every day. When the concentration of phenols in the ORARW was 5.5 g/L, the recoverable phenols from the wastewater were up to 550.0 kg every day. According to the experimental results, assuming that the recovery rate of phenols was 91%, the recovered amount of phenols was 500.5 kg every day. In the pretreatment stage, the pH was adjusted from about 13 to 7 by hydrochloric acid (containing 36.5% HCl). In the final recovery stage, the amount of added phosphoric acid (containing 85% H3PO4) was dependent on the amount of produced sodium phenolate (ArONa). The power consumption was mainly for pumping and stirring the fluids of the SLM system as well as drying the produced phenols. Four centrifugal pumps with a rated flow rate of 6.3 m3/h and a rated power of 0.55 kW per motor consumed 52.8 kWh every day, and three electric mixers with a rated power of 0.40 kW per motor consumed 28.8 kWh every day. Assuming that the recovered wet phenols have a moisture content of 50%, that is to say, there is 500.5 kg of water that needs to be evaporated every day, the daily power consumption for product drying is estimated to be 450 kWh. A list of the operation costs and profits of the SLM system for one day is shown in Table 5. Consequently, the profits of the phenols recovered by the SLM system are higher than the costs. In this oil refinery, the present flow rate of the refinery comprehensive wastewater is 12,000 m3/d. In the absence of the SLM system, the refinery comprehensive wastewater treatment system needs to treat an additional 500.5 kg of phenols every day. Only taking power consumption as an example, according to the statistical analysis of the operation energy consumption of wastewater treatment plants in China (Chu et al., 2018), approximately 1.3 kWh of electricity is required for the aerobic removal of 1 kg of COD. Thus 1609 kWh of electricity, namely a $126 electricity fee, is
required for the aerobic degradation of 500.5 kg of phenols every day. That is to say, when ORARW after recovery of phenols by the SLM system enters the refinery comprehensive wastewater treatment system, the electricity fee of $126 spent in the treatment of this part of phenolic wastewater will be saved every day. Based on the green SLM system, the benefit from the recovery of phenols in ORARW is $202 every day (Table 5). Thus, the two items add up to a total benefit of $328 every day. Therefore, the SLM technology is not only environmentally sustainable but also technically-economically feasible. 4. Conclusions The use of vegetable oil, especially linseed oil, as a liquid membrane phase to separate and recover phenols from an aqueous solution is green and environmentally friendly. In the single SLM module process, when a 0.10 moL/L NaOH solution was used as the stripping phase, the separation and recovery rates of phenol were 92.9% and 88.8%, respectively, with an HRTstr of 6.9 min, HRTaq of 7.8 min, pH of 2.0e8.0 and initial concentration of phenols 2800 mg/L~3100 mg/L. The separation rates of m-cresol and ocresol were both greater than 96%, and their recovery rates were approximately 92% under the same conditions. Two SLM modules in series were used to treat ORARW with an initial concentration of phenols of 4800 mg/L~5200 mg/L, and satisfactory results were obtained. Therefore, the SLM can be effectively used for the separation and recovery of phenols from wastewater to realize the resource reuse of pollutants and conform to cleaner production requirements. Credit author statement Xiang Mei: Conceptualization, Methodology, Project administration, Formal analysis, Writing - Original Draft, Writing - Review
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& Editing; Junhui Li: Investigation, Data Curation, Formal analysis, Writing - Original Draft; Chenchen Jing: Investigation, Validation, Data Curation; Chenhong Fang: Investigation, Validation; Yang Liu: Resources, Investigation; Yong Wang: Resources, Formal analysis; Juan Liu: Investigation, Formal analysis, Writing - Original Draft; Shuqi Bi: Investigation, Validation; Ying Chen: Investigation, Validation; Yanyan Xiao: Resources, Investigation; Xu Yang: Investigation, Writing - Review & Editing; Yifan Xiao: Investigation; Shuai Wu: Investigation; Yang Ding: Writing - Review & Editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Graduate Research & Practice Innovation Program of Jiangsu Province of China (SJCX17_0251), the College Student Innovative Training Project of Nanjing Forestry University of China (CSITP), and the Priority Academic Program Development of Jiangsu Higher Education Institutions of China (PAPD). References American Public Health Association, 1998. Standard Methods for the Examination of Water and Wastewater, twentieth ed. (Washington, DC, USA). Ani, I.J., Akpan, U.G., Olutoye, M.A., Hameed, B.H., 2018. Photocatalytic degradation of pollutants in petroleum refinery wastewater by TiO2- and ZnO-based photocatalysts: recent development. J. Clean. Prod. 205, 930e954. Bahri, M., Mahdavi, A., Mirzaei, A., Mansouri, A., Haghighat, F., 2018. Integrated oxidation process and biological treatment for highly concentrated petrochemical effluents: a review. Chem. Eng. Process. 125, 183e196. Bhatluri, K.K., Manna, M.S., Ghoshal, A.K., Saha, P., 2015. Supported liquid membrane based removal of lead(II) and cadmium(II) from mixed feed: conversion to solid waste by precipitation. J. Hazard. Mater. 299, 504e512. Busca, G., Berardinelli, S., Resini, C., Arrighi, L., 2008. Technologies for the removal of phenol from fluid streams: a short review of recent developments. J. Hazard. Mater. 160, 265e288. Chang, S.H., 2014. Vegetable oil as organic solvent for wastewater treatment in liquid membrane processes. Desalin. Water Treat. 52, 88e101. Chu, X.-X., Luo, L., Wang, X.-C., Zhang, W.-S., 2018. Analysis on current energy consumption of wastewater treatment plants in China (in Chinese). China Water & Wastewater 34 (7), 70e74. Cui, P., Mai, Z., Yang, S., Qian, Y., 2017. Integrated treatment processes for coalgasification wastewater with high concentration of phenol and ammonia. J. Clean. Prod. 142, 2218e2226. Deblay, P., Delepine, S., Minier, M., Renon, H., 1991. Selection of organic phases for optimal stability and efficiency of flat-sheet supported liquid membranes. Separ. Sci. Technol. 26, 97e116. Gai, H., Qiao, L., Zhong, C., Zhang, X., Xiao, M., Song, H., 2019. A solvent based separation method for phenolic compounds from low-temperature coal tar. J. Clean. Prod. 223, 1e11. Guo, C., Cao, Q., Chen, B., Yang, S., Qian, Y., 2019. Development of synergistic extraction process for highly efficient removal of phenols from coal gasification wastewater. J. Clean. Prod. 211, 380e386. Gupta, S., Chakraborty, M., Murthy, Z.V.P., 2014. Performance study of hollow fiber supported liquid membrane system for the separation of bisphenol A from aqueous solutions. J. Ind. Eng. Chem. 20, 2138e2145. He, L., Niu, Z., Miao, R., Chen, Q., Guan, Q., Ning, P., 2019. Selective hydrogenation of phenol by the porous Carbon/ZrO2 supported NieCo nanoparticles in subcritical water medium. J. Clean. Prod. 215, 375e381. Hu, L., Toyoda, K., Ihara, I., 2008. Dielectric properties of edible oils and fatty acids as
a function of frequency, temperature, moisture and composition. J. Food Eng. 88, 151e158. Huddleston, J.G., Rogers, R.D., 1998. Room temperature ionic liquids as novel media for ‘clean’ liquideliquid extraction. Chem. Commun. 16, 1765e1766. Kazemi, P., Peydayesh, M., Bandegi, A., Mohammadi, T., Bakhtiari, O., 2014. Stability and extraction study of phenolic wastewater treatment by supported liquid membrane using tributyl phosphate and sesame oil as liquid membrane. Ind. Eng. Chem. Res. 92, 375e383. Lin, Z., Zhang, Z., Li, Y., Deng, Y., 2016. Recyclable magnetic-pickering emulsion liquid membrane for extracting phenol compounds from wastewater. J. Mater. Sci. 51, 6370e6378. Mahdavi, H.R., Arzani, M., Peydayesh, M., Mohammadi, T., 2016. Pertraction of Llysine by supported liquid membrane using D2EHPA/M2EHPA. Chem. Eng. Process. 106, 50e58. Meng, X., Gao, C., Wang, L., Wang, X., Tang, W., Chen, H., 2015. Transport of phenol through polymer inclusion membrane with N,N-di(1-methylheptyl) acetamide as carriers from aqueous solution. J. Membr. Sci. 493, 615e621. Nakano, M., Takahashi, K., Takeuchi, H., 2006. A method for continuous operation of supported liquid membranes. J. Chem. Eng. Jpn. 20, 326e328. Neplenbroek, A.M., Bargeman, D., Smolders, C.A., 1992. Supported liquid membranes: instability effects. J. Membr. Sci. 67, 121e132. Panigrahi, A., Pilli, S.R., Mohanty, K., 2013. Selective separation of Bisphenol A from aqueous solution using supported ionic liquid membrane. Separ. Purif. Technol. 107, 70e78. Peretti, S.W., Tompkins, C.J., Goodall, J.L., Michaels, A.S., 2002. Extraction of 4nitrophenol from 1-octanol into aqueous solution in a hollow fiber liquid contactor. J. Membr. Sci. 195, 193e202. Peydayesh, M., Kazemi, P., Nabavi, M.S., Mohammadi, T., 2014. Phenolic wastewater treatment by supported liquid membrane using different cooking oils as liquid membrane. Chem. Eng. Commun. 201, 1593e1605. Reis, M.T.A., Freitas, O.M.F.D., Ismael, M.R.C., Carvalho, J.M.R., 2007. Recovery of phenol from aqueous solutions using liquid membranes with Cyanex 923. J. Membr. Sci. 305, 313e324. ~ as, B., Rodríguez-Gutierrez, G., FernandezSerrano, A., Fermoso, F.G., Alonso-Farin Bolanos, J., Borja, R., 2017. Olive mill solid waste biorefinery: high-temperature thermal pre-treatment for phenol recovery and biomethanization. J. Clean. Prod. 148, 314e323. Shen, S.F., Smith, K.H., Cook, S., Kentish, S.E., Perera, J.M., Bowser, T., Stevens, G.W., 2009. Phenol recovery with tributyl phosphate in a hollow fiber membrane contactor: experimental and model analysis. Separ. Purif. Technol. 69, 48e56. Venkateswaran, P., Palanivelu, K., 2006. Recovery of phenol from aqueous solution by supported liquid membrane using vegetable oils as liquid membrane. J. Hazard. Mater. 131, 146e152. Werkneh, A.A., Rene, E.R., Lens, P.N.L., 2018. Simultaneous removal of selenite and phenol from wastewater in an upflow fungal pellet bioreactor. J. Chem. Technol. Biotechnol. 93, 1003e1011. Yang, X., Duan, H., Shi, D., Yang, R., Wang, S., Guo, H., 2015. Facilitated transport of phenol through supported liquid membrane containing bis(2-ethylhexyl) sulfoxide (BESO) as the carrier. Chem. Eng. Process. 93, 79e86. Yang, X., Tang, W., Liu, X., Du, H., Wu, Y., Zhang, J., 2019. Synthesis of mesoporous silica from coal slag and CO2 for phenol removal. J. Clean. Prod. 208, 1255e1264. Zidi, C., Tayeb, R., Ali, M.B.S., Dhahbi, M., 2010. Liquideliquid extraction and transport across supported liquid membrane of phenol using tributyl phosphate. J. Membr. Sci. 360, 334e340. Zidi, C., Tayeb, R., Dhahbi, M., 2011. Extraction of phenol from aqueous solutions by means of supported liquid membrane (MLS) containing tri-n-octyl phosphine oxide (TOPO). J. Hazard. Mater. 194, 62e68.
Glossary ORARW: oil-refinery alkali residue wastewater SLM: supported liquid membrane HRT: hydraulic retention time (min) HRTaq: feed phase HRT (min) HRTstr: stripping phase HRT (min) Vaq: feed phase volume (mL) Vstr: stripping phase volume (mL) Vorg: organic phase volume (mL) VM: empty bed volume of membrane module (mL) Caq: feed phase concentration (mg/L) Cstr: stripping phase concentration (mg/L) COD: chemical oxygen demand (mg/L) TDS: total dissolved solids (mg/L)
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Separation and recovery of phenols from an aqueous solution by a green membrane system Xiang Mei a, *, Junhui Li a, Chenchen Jing a, Chenhong Fang a, Yang Liu b, Yong Wang c, Juan Liu a, Shuqi Bi a, Ying Chen a, Yanyan Xiao d, Xu Yang d, Yifan Xiao a, Shuai Wu a, Yang Ding a a
College of Biology and the Environment, Nanjing Forestry University, Nanjing, 210037, China Division of Water and Wastewater Engineering, Nanjing Jinling Petrochemical Engineering Co., Ltd., Nanjing, 210042, China State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing, 210023, China d Nanjing Haiyi Environmental Protection Engineering Co., Ltd., Nanjing, 211200, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2018 Received in revised form 19 November 2019 Accepted 10 December 2019 Available online xxx
Oil-refinery alkali residue wastewater (ORARW) contains large amounts of phenol and its derivatives, which are highly toxic and valuable but difficult to biodegrade. In this study, using vegetable oil as the liquid membrane phase, a green polypropylene-hollow-fiber supported liquid membrane (SLM) system was constructed to simultaneously separate and recover phenol, m-cresol and o-cresol from aqueous solutions. After screening several types of vegetable oil, linseed oil was selected as the liquid membrane phase. Based on a single SLM module and using a NaOH solution as the stripping reagent with feed phase and stripping phase hydraulic retention times (HRTs) of 7.8 and 6.9 min, respectively, the phenol separation and recovery rates were 92.9% and 88.8%, respectively. The separation rates for m-cresol and ocresol were both greater than 96%, their recovery rates were approximately 92%, and the membrane separation flux of every phenolic compound exceeded 0.40 g/m2$h. When two SLM modules were operated in series at room temperature for 3 h, the separation efficiency for phenols increased by approximately 9% compared with that obtained with a single SLM module. After applying two SLM modules in series to treat ORARW with an initial concentration of phenols of 4800e5200 mg/L, the simultaneous separation and recovery rates of phenols were near 95% and 92%, respectively. This work concluded that vegetable oil can be used as a nontoxic and renewable green liquid membrane phase and that two SLM modules in series efficiently separated and recovered phenols from aqueous solutions. © 2019 Elsevier Ltd. All rights reserved.
Handling editor: M.T. Moreira Keywords: Phenols Vegetable oil Supported liquid membrane Separation and recovery Oil-refinery alkali residue wastewater
1. Introduction Phenols and their derivatives are some of the most common organic pollutants in industrial wastewater (Cui et al., 2017). Among the many phenolic compounds, phenol is an important raw material in chemical industries, and cresol is the simplest derivative of phenol. Both phenol and cresol are highly toxic and difficult to biodegrade. These phenolic substances are mainly produced by oil refining, coking, and processes to create synthetic resins, medicine, and preservatives. The concentration of phenols in wastewater depends on the type of technology and production process, e.g., 6e500 mg/L in refinery wastewater, 28e3900 mg/L in coking
* Corresponding author. E-mail address: [email protected] (X. Mei). https://doi.org/10.1016/j.jclepro.2019.119675 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
plant wastewater, 9e6800 mg/L in coal chemical plant wastewater and 2.8e1220 mg/L in petrochemical industry wastewater (Busca et al., 2008; Serrano et al., 2017). For the treatment of these highconcentration phenolic wastewaters, physical and chemical treatment methods such as waste residue adsorption (Yang et al., 2019) and solvent separation (Gai et al., 2019) have the advantages of simple operation and high efficiency, but the adsorbent and the solvent are high-cost, difficult to reuse and tend to generate secondary pollution. Some researchers adopted catalytic methods to degrade phenols in petroleum refinery wastewater (Ani et al., 2018), but the catalysts are usually unstable and the problems of catalyst recycling and degradation have not been solved (He et al., 2019). The multi-stage treatment of industrial phenolic wastewater has been carried out by biological methods in combination with other technologies (Bahri et al., 2018; Guo et al., 2019), because
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microbial activity is easily limited by the influent concentration if only biological treatment is used (Werkneh et al., 2018). From the perspective of resource recovery and reducing the burden for the subsequent treatment, it is important that the phenols in industrial wastewater be separated and recovered through a novel approach before the wastewater is discharged into the environment. The supported liquid membrane (SLM) process is a new extraction technology for the treatment of wastewater containing organic compounds (Mahdavi et al., 2016) and metals (Bhatluri et al., 2015). The process mainly consists of a liquid membrane phase, feed phase, stripping phase and support material. Compared with solvent extraction, the SLM process has the advantages of a large mass-transfer impetus, high speed, high selectivity and minimal extractant loss. Additionally, the extraction and stripping processes are realized in the same equipment, so the capital investment and operating cost are low. The separation of different components is more efficient, and solutes can be transported against the concentration gradient. However, the disadvantage of the SLM process is that the liquid membrane is unstable, which is related to the carrier concentration (Nakano et al., 2006), the type and solubility of the solvents and the molecular structure of the carrier (Deblay et al., 1991; Neplenbroek et al., 1992). Regardless of the mechanism, the main factors that cause the loss of the liquid membrane phase are multifaceted, and thus the choice of the support material and extractant is critical. The SLM extraction technique has been used to treat single phenolic compounds in wastewater, and the liquid membrane phases used have mostly been carrier-solvent systems, such as the trioctylphosphine oxide (TOPO)-kerosene system (Zidi et al., 2011), bis(2-ethylhexyl) sulfoxide (BESO)-kerosene system (Yang et al., 2015), tributyl phosphate (TBP)-kerosene system (Shen et al., 2009) and Cyanex 923 (Reis et al., 2007). In these systems, the solvents, such as chloroform, carbon tetrachloride, dichloromethane, benzene, toluene and pyridine, are considered hazardous chemicals. Ionic liquid membranes and hydrocarbon compounds have also been used for the separation of bisphenol A (BPA) (Gupta et al., 2014; Panigrahi et al., 2013). The experimental results showed that the addition of TBP to kerosene improved the extraction rate of phenol by SLM in a short time, and the stability of the flat polyvinylidene-fluoride membrane was enhanced (Zidi et al., 2010). The SLM with TOPO as the extractant was confirmed to be better than that with TBP, and the effective contact area of a hollow-fiber SLM is larger than that of a flat membrane (Zidi et al., 2011). Most organic solvents are toxic, easily lost, unstable and cause secondary pollution in SLM processes. Vegetable oil is a nonhazardous and inexpensive chemical that could be used as the liquid membrane phase in a SLM process. Vegetable oils are naturally produced, readily available, nontoxic, low-cost and renewable resources. Venkateswaran et al. (2006) used natural vegetable oil in the treatment of phenol in an aqueous solution. Kazemi et al. (2014) constructed an SLM with a TBP-sesame oil system and obtained a higher mass transfer rate and good stability with suitable concentrations of stripping reagent and carrier. Peydayesh et al. (2014) investigated the effects of various factors on the separation rate and membrane stability of phenolic compounds. An edible oil was used as the liquid membrane phase, and good results were achieved. This study presents experiments performed using vegetable oil as the liquid membrane phase in an SLM system constructed to separate and recover phenols from an aqueous solution. A single SLM module and two SLM modules in series were investigated to simultaneously separate and recover phenol, m-cresol and o-cresol from oil-refinery alkali residue wastewater (ORARW). The purpose of this study was to establish a novel, green SLM system for the efficient separation and recovery of phenols from phenolic wastewater.
2. Materials and methods 2.1. Experimental materials The vegetable oils used in this study are commercially available from East Sea Grain and Oil Industry (Zhangjiagang) Co., Ltd., China and included sunflower oil, rapeseed oil and linseed oil. The simulated phenolic wastewater was prepared using distilled water. The ORARW was produced after catalytic wet air oxidation (CWAO) at an oil refinery located in Nanjing, China. The wastewater had a poor water quality with a brown color and high concentration of free alkali. The chemical oxygen demand (COD) and concentrations of volatile phenols, total dissolved solids (TDS) and mineral oil were ca. 50,500e57,500 mg/L, 4800e5200 mg/L, 48,900 mg/L and 31 mg/L, respectively, and the pH was 13.4e13.5. The main phenolic compounds were the toxic phenol and cresol, and the concentrations of phenol, m-cresol and o-cresol were about 1700 mg/L, 1600 mg/L and 1600 mg/L, respectively. The hydrophobic polypropylene (PP) hollow-fiber microporous membrane was obtained from Hangzhou Haotian Membrane Technology Co., Ltd., China. The other chemicals used in this study, including phenol (AR), m-cresol (AR), o-cresol (AR), NaOH (AR) and acetic acid (GR), were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Methanol and acetonitrile, which were of chromatographic purity, were obtained from TEDIA Company, Inc., USA. 2.2. Experimental system The parameters of the hollow-fiber membrane modules are shown in Table 1. The membrane module was completely impregnated with vegetable oil for 12 h. After impregnation, the excess oil was wiped off the module, and then the membrane was sandwiched between the feeding and stripping cells, as illustrated in Fig. 1. In the experiment with a single SLM module, 500 mL of a phenols solution was used as the feed phase and run through the tubes, and 250 mL of a NaOH solution was used as the stripping phase and run through the shells. The effect of the vegetable oils, NaOH concentration, hydraulic retention time (HRT) of the feed and stripping phases, and temperature on the separation and recovery of phenols was investigated (Fig. 1(a)). An attempt was also made to separate and recover phenols from ORARW using SLM modules in series (Fig. 1(b)). The wastewater was continuously circulated in both SLM modules with separate stripping solutions in each SLM module to allow the transport of the phenols from the feed phase to the stripping phase. Samples were taken from the feed and stripping solutions at regular time intervals and analyzed. 2.3. Experimental design Three types of vegetable oils, sunflower oil, rapeseed oil and
Table 1 Parameters of the hollow-fiber membrane modules. Parameter
Value
Empty bed volume (mL) Number of membrane fibers Effective length of fiber (mm) Membrane surface area (m2) Internal diameter of fiber (mm) Outer diameter of fiber (mm) Wall thickness of fiber (mm) Porosity (%) Pore size (bubble point method) (mm) Gas flux (m3/(m2$h))
186 800 265 0.275 320e350 400e450 80e100 64.8 0.38 3
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3
Fig. 1. Schematic diagram of the supported liquid membrane (SLM) systems. (a) Single SLM module; (b) Two SLM modules in series.
Table 2 Operation conditions of the SLM system. Condition Number of membrane Caq Cstr (mol/L) modules (mg/L)
HRTaq (min)
HRTstr (min)
Operation time (h)
Temperature pH ( C)
Mineral oil concentration (mg/L)
I
1
7.8
6.9
4
25
5.9
e
II
1
25
5.9
e
1
0.10
5.6, 6.9, 8.1 6.9
4
III
5.2, 7.8, 10.3 7.8
4
15, 25, 30
6.0
e
IV
1
0.10
7.8
6.9
4
25
5.9e6.0
e
V
2
0.10
7.8
6.9
3
25
6.0
e
0.10
7.8
6.9
3
25
0.10
7.8
6.9
3
25
2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 32 10.0, 11.0 7.0e8.0 31, 391, 702, 977
a
VI
2
VII
2
a
2930~ 2990 2970~ 3050 2800~ 3010 2840~ 3350 2890~ 3010 4970~ 5200 5090~ 5200
0.01, 0.03, 0.05, 0.10, 0.20 0.10
ORARW was diluted to some extent when used.
linseed oil, were screened. Three 50-mL beakers were washed and dried, and 25 mL of a 2800 mg/L simulated phenols solution and 10 mL of vegetable oil were added to each beaker. Then, the beakers were placed in an incubator at 25 C. The speed of the magnetic stirrer was 350 rpm. After 3 h of extraction and static stratification, a water sample was taken to determine the remaining phenols content in the feed solution and calculate the partition coefficient. The experiment was repeated three times. After the screening of the vegetable oils, the extraction performance of the SLM system and the treatment of the ORARW by two SLM modules in series were studied. The operation conditions are shown in Table 2. Under conditions I ~ V, the SLM extraction experiments were carried out with a feed-phase volume (Vaq) of 500 mL, stripping-phase volume (Vstr) of 250 mL, feed-phase concentration (Caq) of ca. 3000 mg/L and unchanged mass ratio of phenol:m-cresol:o-cresol ¼ 1:1:1. Under conditions VI and VII, ORARW as the feed phase was treated by two SLM modules in series.
510 nm, the mineral oil concentration was determined by ultraviolet spectrophotometry, and the COD was measured by a standard method (APHA, 1998). The density and viscosity of the vegetable oils were determined, respectively, by a petroleum densitometer (measuring range 0.90e0.95 g/cm3) from Hejian Zhenyan Instrument Co., Ltd., China and a Pinkevitch viscometer (capillary inner diameter 1.2 mm, constant 0.1490 mm2/s2) from Shanghai Shenyi Glass Products Co., Ltd., China. The TDS was determined using a TDS measuring instrument (HI 983,301) from HANNA Co., Ltd., Singapore. An Agilent high-performance liquid chromatography (HPLC) system was also used to determine the concentrations of phenol, mcresol and o-cresol. An SB-C18 column (4.6 mm 150 mm 5 mm) was selected. The column temperature was 30 C, the UV detection wavelength was 278 nm, the mobile phase was acetonitrile (1% acetic acid)-water (1% acetic acid) (30:70), the flow rate was 0.7 mL/ min and the injection volume was 40 mL.
2.4. Analytical methods
2.5. Evaluation index
The concentration of volatile phenols in the wastewater was analyzed by the 4-aminoantipyrine spectrophotometric method at
2.5.1. Separation rate (S) In the liquid membrane separation process, the separation rate
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(S) reflects the separation effect of the SLM on the phenolic compounds and is expressed by Eq. (1).
C0 Ct 100% C0
S¼
3.1. Screening of vegetable oils
(1)
where C0 is the initial concentration of phenolic compounds in the feed phase (mg/L) and Ct is the concentration of phenolic compounds in the feed phase at time t (mg/L). 2.5.2. Recovery rate (R) In the stripping process, the NaOH solution reacts with phenolic compounds to form sodium phenates. The concentrations of sodium phenates were determined by HPLC after the addition of hydrochloric acid. The recovery effect can be expressed by the recovery rate (R) (Eq. (2)).
R¼
Cstr Vstr 100% Caq Vaq S
(2)
where Cstr is the concentration of phenolic compounds in the NaOH solution (mg/L), Vstr is the volume of NaOH solution (mL), Caq is the initial concentration of phenolic compounds in the feed phase (mg/ L), and Vaq is the volume of the feed solution (mL). 2.5.3. Membrane separation flux (J) As an important evaluation index of the SLM extraction performance, the membrane separation flux (J) of a phenolic compound was calculated by Eq. (3).
J¼
M AT
3.2. Effect of stripping solution concentration
2.5.4. Partition coefficient (P) The partition coefficient (P) (Eq. (4)) can be used to evaluate the extraction performance of extractants and is an important indicator in the vegetable oil screening.
Corg Caq
(4)
where Corg is the concentration of phenolic compounds in the organic phase after reaching the extraction equilibrium (mg/L) and Caq is the concentration of the remaining phenolic compounds in the feed phase after reaching the extraction equilibrium (mg/L). 2.5.5. Hydraulic retention time (HRT) The hydraulic retention time of the feed phase (HRTaq) and the hydraulic retention time of the stripping phase (HRTstr) were calculated by Eqs. (5) and (6), respectively.
HRTaq ¼
VM Qaq
In this study, the viscosities and densities of commercially available vegetable oils, including sunflower oil, rapeseed oil and linseed oil, were measured. Based on the main physical and chemical properties (Chang et al., 2014; Hu et al., 2008), liquidliquid extraction experiments were carried out. After allowing static stratification, the concentration of the remaining phenols was determined by taking samples from the feed solution, and the partition coefficient was calculated. The experimental results are shown in Table 3. The melting points of the three types of vegetable oil are lower than that of water, so the vegetable oils exist in a state of liquid aggregation, which is a prerequisite for use in an SLM process. The three vegetable oils are nonpolar substances with dielectric constants less than 15, and they are suitable as extractants of the weakly polar phenol, m-cresol and o-cresol according to the theory of ‘similarity and intermiscibility’. Additionally, they are safe and stable with high flash points. Based on the partition coefficient results, linseed oil has a stronger affinity for phenolic compounds than sunflower oil or rapeseed oil, which might be a result of the viscosity differences among the vegetable oils. The viscosity order of the three vegetable oils is linseed oil < sunflower oil < rapeseed oil. The viscosity is usually the main factor affecting the flow resistance and thickness of a liquid, and the higher the viscosity is, the larger the resistance is. A higher viscosity is not beneficial for the internal transport of phenolic compounds in vegetable oil. Therefore, linseed oil was found in this study to be more suitable as an extractant.
(3)
where M is the mass of a phenolic compound extracted by the supported liquid membrane (g), A is the membrane area (m2), and T is the operation time (h).
P¼
3. Results and discussion
(5)
Different concentrations of the NaOH solution, which was used as the stripping phase, were tested. The NaOH solution is essential to the separation and recovery of the phenolic compounds and the SLM stability. The stripping efficiency was studied in the NaOH solution concentration range of 0.01 moL/L~0.20 moL/L. Fig. 2 shows that the separation rates and membrane separation fluxes of the phenolic compounds increased as the NaOH concentration increased from 0.01 moL/L to 0.10 moL/L and decreased for NaOH concentrations above 0.10 moL/L. When the effective contact area of the feed solution and SLM and the liquid membrane phase content were constant, a low NaOH concentration led to a failure to react with the phenolic compounds in a timely and effective manner. In contrast, an excessively high NaOH concentration caused excessive alkalinity and increased the surface tension between the vegetable oil and membrane filaments, which resulted in the emulsification of the vegetable oil, the loss of the liquid membrane phase and a reduction in the separation rate. Meng et al. (2015) observed similar results for phenol transport through a polymer-inclusion membrane with N,N-di (1-methylheptyl) acetamide as the carrier from an aqueous solution. Thus, the concentration of NaOH could not be too high or too low, and a 0.10 moL/L NaOH solution was used as the stripping phase to separate the phenolic compounds in this experiment. 3.3. Effect of HRT
V HRTstr ¼ M Qstr
(6)
where VM is the empty bed volume of the membrane module (mL), Qaq is the flow rate of the feed solution (mL/min), and Qstr is the flow rate of the stripping solution (mL/min).
In the experiment, a hydrophobic hollow-fiber membrane was used as the support material. When the feed solution and stripping solution flow through the different sides of the membrane, a boundary layer forms. The thickness of the boundary layer varies with the flow rate and affects the mass transfer (Lin et al., 2016). To
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Table 3 Comparison of properties of vegetable oils. Property
Linseed oil
Rapeseed oil
Sunflower seed oil
Water
Melting point ( C, 1 atm) a Dielectric constant (25 C) a Density (g/cm3, 25 C) b Viscosity (mPa$s, 25 C) b Flash point ( C, 1 atm) a Partition coefficient b
19 z3 0.927 48.90 222 12.94 ± 0.68
9 z3 0.916 79.97 283 9.82 ± 1.10
15 z3 0.921 62.44 121 11.07 ± 0.57
0 78 0.997 0.89
a b
References: (Chang et al., 2014; Hu et al., 2008). Data from actual measurements.
Fig. 2. Effect of the stripping solution concentration on the separation of phenolic compounds under condition I (Table 2).
achieve a better mass-transfer effect, the feed-phase HRTaq was set at 5.2 min, 7.8 min and 10.3 min, and the stripping-phase HRTstr was set at 5.6 min, 6.9 min and 8.1 min. The experimental results are shown in Fig. 3. Fig. 3(a) shows that the separation rates of phenol, m-cresol and o-cresol increased when HRTaq increased from 5.2 min to 7.8 min and decreased as HRTaq increased from 7.8 min to 10.3 min. Since the inner diameter of the hollow-fiber membrane is on the micron scale, when an increase in the feed phase flow rate and a reduction in HRTaq cause decreases in the boundary layer thickness and masstransfer resistance, the phenolic compounds enter the membrane phase more quickly than before, and the separation rate is improved. However, an excessively large flow rate increases the feed solution pressure on the membrane and the shear force between them, resulting in instability and the loss of the liquid membrane phase. Fig. 3(c) shows that the variation in HRTstr had a slight influence on the separation rates of m-cresol and o-cresol and almost no influence on the separation rate of phenol because the boundary layer resistance of the feed phase was much greater than that of the stripping phase in the total mass-transfer resistance (Peretti et al., 2002). Therefore, the appropriate HRTaq and HRTstr were 7.8 min and 6.9 min, respectively. Under these two HRTs, the membrane separation fluxes of m-cresol and o-cresol were also the highest (Fig. 3(b) and (d)).
3.4. Effect of temperature The temperature is an important influencing factor in the separation and recovery of phenolic compounds by the SLM system.
The maximum tolerable temperature of the polypropylene hollowfiber membrane used in this experiment was 45 C. In consideration of the membrane life and power consumption, the operation temperature was set at 15 C, 25 C and 30 C. The experimental results are shown in Fig. 4. Fig. 4(a) clearly shows that the SLM separation rates of phenol, m-cresol and o-cresol improved with the increasing temperature. At 30 C, the separation rate of phenol was 92.9%, and those of mcresol and o-cresol were both greater than 96%. The membrane separation fluxes of phenol, m-cresol and o-cresol were 0.437 g/ m2$h, 0.499 g/m2$h and 0.428 g/m2$h, respectively (Fig. 4(b)). The increase in the temperature enhanced the molecular thermodynamic movement and reduced the viscosity of the vegetable oil, both of which are conducive to reducing the mass-transfer resistance. Additionally, phenolic compounds are weak acids, and their reactions with NaOH release a small amount of heat. The two roles above could not offset each other, but the elevated temperature could increase the chance of intermolecular association, thus improving the separation rates of phenolic compounds. Based on the comprehensive analyses of the effects of the stripping solution concentration, HRT and temperature, an experiment was carried out under the optimized conditions, and the results are shown in Table 4. Under the conditions of Vaq ¼ 500 mL, Caq ¼ 2800e3000 mg/L, HRTaq ¼ 7.8 min; Vstr ¼ 250 mL, Cstr ¼ 0.10 moL/L, HRTstr ¼ 6.9 min, and T ¼ 30 C, the separation and recovery rates of phenol were 92.9% and 88.8%, respectively. The separation rates for m-cresol and o-cresol were both greater than 96%, and their recovery rates were approximately 92%. The membrane separation fluxes of the phenolic compounds all exceeded 0.40 g/m2$h.
3.5. The stability of the SLM system When the feed solution and the stripping solution flowed on the two sides of the membrane, the membrane pore blockage and the pressure difference probably resulted in the loss of the liquid membrane phase of the supported liquid membrane. To investigate the stability of the SLM system in the separation and recovery of phenolic compounds, the experiment was performed 5 times consecutively without refilling with linseed oil, and the SLM system operated for 4 h in each batch of experiments. The separation and recovery results of the phenols are shown in Fig. 5. It can be seen from Fig. 5(a) that the separation rates of phenols by the SLM system are all over 90% and the recovery rates were stable at about 88% in the consecutive experiments. There was little difference among the repeated experimental results, indicating that the loss of the liquid membrane phase was very slight. For all repeated batches of experiments, the separation fluxes of phenols were all above 1.140 g/m2$h (Fig. 5(b)), which shows that the SLM system can operate steadily without refilling with linseed oil, and an increase in the number of operation cycles had little effect on the stability of the system. The slightly higher recovery effects in the
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Fig. 3. Effect of the HRT on the separation of phenolic compounds under condition II (Table 2).
Fig. 4. Separation of phenolic compounds at different temperatures under condition III (Table 2).
Table 4 Separation and recovery of phenolic compoundsa. Parameter
phenol
m-cresol
o-cresol
Initial quantity (mg) Separation rate (%) Recovery rate (%) Separation flux (g/m2$h)
497 92.9 88.8 0.420
553 96.7 92.1 0.486
466 96.4 91.8 0.408
a Linseed oil as the liquid membrane phase, phenols solution as the feed phase, and NaOH solution as the stripping phase.
first batch of experiments were probably because there was a little residual linseed oil in the shell or the tube of the membrane
Fig. 5. Stability of the SLM system under condition IV (Table 2).
module. In addition, when a single membrane module was in operation, the difference between the recovery rate and the separation rate of the phenols was less than 5%, and when the separation rate was increased to about 98% by using two membrane modules in series, the difference between the recovery rate and the separation rate was reduced to 3%. The results show that the SLM system has the advantages of high stability, a high recovery rate and low consumption. 3.6. Two SLM modules in series In this experiment, two SLM modules were connected in series
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to increase the contact time with the wastewater, because the contact time affects the mass-transfer efficiency. Fig. 6(a) shows that two SLM modules in series have a higher efficiency than a single SLM module for the separation of phenolic compounds under the same operating conditions. After 3 h, the separation rates of phenol, m-cresol and o-cresol were approximately 90.0%, 97.1% and 98.1%, respectively. As seen in Fig. 6(b), the separation rates of the phenolic compounds with the SLM modules in series increased by approximately 9% compared with that obtained with a single SLM module. The separation rates of m-cresol and o-cresol were similar, and those of phenol were slightly lower. As shown in Fig. 6(c), the membrane separation fluxes of the phenolic compounds gradually decreased with the extension of the operation time, and those obtained with the SLM modules in series decreased by approximately 0.20e0.40 g/m2$h compared with those obtained with a single SLM module (Fig. 6(d)). The order of the octanol-water partition coefficients is mcresol > o-cresol > phenol. Huddleston et al. (1998) confirmed that the partition coefficient of materials in a vegetable oil-water system is similar to that in an octanol-water system. Phenol is a hydrophilic substance, and its partition coefficient is relatively small, resulting in minimal solubility in a liquid membrane phase and a low separation rate. However, a solute partition coefficient in a solvent that is too high can easily led to solute retention in the organic phase, and then the solute is not easily extracted by the stripping phase. Therefore, a moderate partition coefficient is more conducive for improving the separation rate. The test results showed that two SLM modules in series are more conducive to phenolic compound separation, and these SLM modules were applied to real industrial wastewater.
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3.7. Application to real industrial wastewater 3.7.1. Effect of pH Phenolic compounds are weak electrolytes that dissociate in water, and they exist in different forms in aqueous solutions with different pH values. That is, the pH affects the presence of phenolic compounds in aqueous solutions. According to the principle of hollow-fiber SLM extraction, a solute in the molecular state can be easily extracted by the extractants in the membrane phase, while the solubility of a solute in the ionic state in the feed solution is lower. The alkaline scrubbing of the oil products leads to the strong alkalinity of ORARW. Therefore, most of the phenolic compounds were in the ionic state, which is not conducive for their separation and recovery. To ensure the separation of the phenolic compounds by the SLM system, the ORARW pH was adjusted by hydrochloric acid from the actual value (13.4e13.5) to 2.0e11.0 to determine the optimum pH for ORARW treatment by the SLM system. Fig. 7 shows that the separation rates and fluxes of the phenols were constant at approximately 95% and 0.8 g/m2$h, respectively, when the ORARW pH was less than 8.0. When the pH was 8.0e9.0, the separation rate began to decrease, and it continued to decrease for pH values above 9.0. Similarly, Yang et al. (2015) reported that phenol (PhOH) was transported from an aqueous solution through an SLM impregnated with bis(2-ethylhexyl) sulfoxide (BESO) in kerosene. The partition coefficient was constant when the phenolcontaining wastewater pH was 0.7e8.5 and began to decrease for pH values above 8.5. Finally, the partition coefficient was near zero for a pH close to 13.8. Therefore, to ensure the separation of the phenolic compounds in ORARW and other biochemical treatments, the ORARW pH was adjusted to neutral.
Fig. 6. Separation of phenolic compounds by a single SLM module and two SLM modules in series under condition V (Table 2).
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separation rates of the phenolic compounds tended to decrease with an increase in the mineral oil concentration. When the mineral oil concentration was near 400 mg/L, the inlet pressure increased to 0.02e0.04 MPa, and when the mineral oil concentration was approximately 700 mg/L, the inlet pressure was more than 0.08 MPa, which increased the necessary membrane cleaning frequency. The SLM modules were easily damaged when the mineral oil concentration was near 1000 mg/L. Therefore, the pretreatment of the ORARW is necessary to reduce the mineral oil content.
Fig. 7. Effect of the pH on the separation of phenols with two SLM modules in series in the treatment of ORARW under condition VI (Table 2).
3.7.2. Effect of mineral oil content Mineral oil is present in ORARW and enters the pipe along with the feed solution; thus, membrane blocking easily occurred because of the large molecules and high viscosity of the mineral oil. To understand the effect of the mineral oil concentration on the SLM service life, experiments with SLM modules in series were carried out. The mineral oil concentration in the ORARW increased to 977 mg/L, 702 mg/L, 391 mg/L and 31 mg/L upon adding oil extracted from the ORARW. The slight deviations in the initial concentrations of phenol, m-cresol and o-cresol made the separation rates and fluxes of the phenolic compounds somewhat different, but their variation trends were the same. Fig. 8 shows that small amounts of mineral oil had little effect on the separation of phenolic compounds from the ORARW, which indicates that the antipollution ability of the SLM was stronger. However, the
3.7.3. Techno-economic analysis As a cleaner production technology, the SLM system has the technical advantages of recycling resources, reducing pollution and improving the recovery efficiency. The conventional treatment of phenolic wastewater not only wastes resources and has inferior wastewater treatment effects but also costs more. The application of SLM technology in the ORARW treatment process (Fig. 9) shows that SLM system can efficiently recover phenols before the biotreatment, and the economic benefits of the SLM technology also cannot be ignored. Thus, a preliminary economic evaluation can be performed for the SLM technology. This research was performed only on the laboratory scale, and thus, it is difficult to calculate the costs completely and accurately when scaling the process up to the industrial scale, especially the costs of the equipment investment and its maintenance, energy, and labor. Therefore, in this preliminary economic evaluation of the SLM technology, only the costs of chemicals and power consumption in the ORARW treatment were estimated. An oil refinery (Nanjing, China) produces 100 tons of ORARW every day and its density is approximately 1 kg/L, so the volume of wastewater is 1.0 105 L. If the volume ratio of wastewater, the liquid membrane phase and the stripping phase was 250:1:10 in actual production, the volumes of the liquid membrane phase and stripping phase were 400 L and 4.0 103 L, respectively. In addition, the vegetable oil used as an extractant can be recycled, so the daily consumption of vegetable oil was 0.4 L (0.37 kg) based on a
Fig. 8. Effect of the mineral oil concentration on the separation of phenolic compounds in the treatment of ORARW under condition VII (Table 2).
X. Mei et al. / Journal of Cleaner Production 251 (2020) 119675
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Fig. 9. Schematic diagram of ORARW treatment process containing a green SLM system. Inside the dotted box are the existing treatment units of the refinery wastewater; the blue part is the SLM system for the separation and recovery of phenols.
Table 5 Daily costs and profits of the SLM system. Items
a
Phenols Vegetable oil Sodium hydroxide Hydrochloric acid Phosphoric acid Power (Pump & Mixer & Drying) Gross profit ($/d) a b c d
Price ($/kg)
Output (kg)
Consumption (kg)
Profit ($/d)
Cost ($/d)
1.138 b 11.376 c 0.256 b 0.025 b 0.882 b 0.078 ($/kWh) e
500.5 e e e e e e
e 0.37 194.6 1000.0 280.4 531.6 (kWh) e
569 e e e e e 202
e 4 50 25 247 41 e
d
Major daily cost and benefit items, where $ is US dollars. Chemical prices refer to international trade market prices. The price of linseed oil refers to www.cngrain.com. The electricity price is the unit peak price in Jiangsu province, China.
loss rate of 0.1%. The density of vegetable oil (as linseed oil) was 0.927 kg/L. In order to keep the stripping phase concentration of 0.10 moL/L as determined by the experiment, NaOH should be added according to the amount of phenolic compounds recovered every day. When the concentration of phenols in the ORARW was 5.5 g/L, the recoverable phenols from the wastewater were up to 550.0 kg every day. According to the experimental results, assuming that the recovery rate of phenols was 91%, the recovered amount of phenols was 500.5 kg every day. In the pretreatment stage, the pH was adjusted from about 13 to 7 by hydrochloric acid (containing 36.5% HCl). In the final recovery stage, the amount of added phosphoric acid (containing 85% H3PO4) was dependent on the amount of produced sodium phenolate (ArONa). The power consumption was mainly for pumping and stirring the fluids of the SLM system as well as drying the produced phenols. Four centrifugal pumps with a rated flow rate of 6.3 m3/h and a rated power of 0.55 kW per motor consumed 52.8 kWh every day, and three electric mixers with a rated power of 0.40 kW per motor consumed 28.8 kWh every day. Assuming that the recovered wet phenols have a moisture content of 50%, that is to say, there is 500.5 kg of water that needs to be evaporated every day, the daily power consumption for product drying is estimated to be 450 kWh. A list of the operation costs and profits of the SLM system for one day is shown in Table 5. Consequently, the profits of the phenols recovered by the SLM system are higher than the costs. In this oil refinery, the present flow rate of the refinery comprehensive wastewater is 12,000 m3/d. In the absence of the SLM system, the refinery comprehensive wastewater treatment system needs to treat an additional 500.5 kg of phenols every day. Only taking power consumption as an example, according to the statistical analysis of the operation energy consumption of wastewater treatment plants in China (Chu et al., 2018), approximately 1.3 kWh of electricity is required for the aerobic removal of 1 kg of COD. Thus 1609 kWh of electricity, namely a $126 electricity fee, is
required for the aerobic degradation of 500.5 kg of phenols every day. That is to say, when ORARW after recovery of phenols by the SLM system enters the refinery comprehensive wastewater treatment system, the electricity fee of $126 spent in the treatment of this part of phenolic wastewater will be saved every day. Based on the green SLM system, the benefit from the recovery of phenols in ORARW is $202 every day (Table 5). Thus, the two items add up to a total benefit of $328 every day. Therefore, the SLM technology is not only environmentally sustainable but also technically-economically feasible. 4. Conclusions The use of vegetable oil, especially linseed oil, as a liquid membrane phase to separate and recover phenols from an aqueous solution is green and environmentally friendly. In the single SLM module process, when a 0.10 moL/L NaOH solution was used as the stripping phase, the separation and recovery rates of phenol were 92.9% and 88.8%, respectively, with an HRTstr of 6.9 min, HRTaq of 7.8 min, pH of 2.0e8.0 and initial concentration of phenols 2800 mg/L~3100 mg/L. The separation rates of m-cresol and ocresol were both greater than 96%, and their recovery rates were approximately 92% under the same conditions. Two SLM modules in series were used to treat ORARW with an initial concentration of phenols of 4800 mg/L~5200 mg/L, and satisfactory results were obtained. Therefore, the SLM can be effectively used for the separation and recovery of phenols from wastewater to realize the resource reuse of pollutants and conform to cleaner production requirements. Credit author statement Xiang Mei: Conceptualization, Methodology, Project administration, Formal analysis, Writing - Original Draft, Writing - Review
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& Editing; Junhui Li: Investigation, Data Curation, Formal analysis, Writing - Original Draft; Chenchen Jing: Investigation, Validation, Data Curation; Chenhong Fang: Investigation, Validation; Yang Liu: Resources, Investigation; Yong Wang: Resources, Formal analysis; Juan Liu: Investigation, Formal analysis, Writing - Original Draft; Shuqi Bi: Investigation, Validation; Ying Chen: Investigation, Validation; Yanyan Xiao: Resources, Investigation; Xu Yang: Investigation, Writing - Review & Editing; Yifan Xiao: Investigation; Shuai Wu: Investigation; Yang Ding: Writing - Review & Editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Graduate Research & Practice Innovation Program of Jiangsu Province of China (SJCX17_0251), the College Student Innovative Training Project of Nanjing Forestry University of China (CSITP), and the Priority Academic Program Development of Jiangsu Higher Education Institutions of China (PAPD). References American Public Health Association, 1998. Standard Methods for the Examination of Water and Wastewater, twentieth ed. (Washington, DC, USA). Ani, I.J., Akpan, U.G., Olutoye, M.A., Hameed, B.H., 2018. Photocatalytic degradation of pollutants in petroleum refinery wastewater by TiO2- and ZnO-based photocatalysts: recent development. J. Clean. Prod. 205, 930e954. Bahri, M., Mahdavi, A., Mirzaei, A., Mansouri, A., Haghighat, F., 2018. Integrated oxidation process and biological treatment for highly concentrated petrochemical effluents: a review. Chem. Eng. Process. 125, 183e196. Bhatluri, K.K., Manna, M.S., Ghoshal, A.K., Saha, P., 2015. Supported liquid membrane based removal of lead(II) and cadmium(II) from mixed feed: conversion to solid waste by precipitation. J. Hazard. Mater. 299, 504e512. Busca, G., Berardinelli, S., Resini, C., Arrighi, L., 2008. Technologies for the removal of phenol from fluid streams: a short review of recent developments. J. Hazard. Mater. 160, 265e288. Chang, S.H., 2014. Vegetable oil as organic solvent for wastewater treatment in liquid membrane processes. Desalin. Water Treat. 52, 88e101. Chu, X.-X., Luo, L., Wang, X.-C., Zhang, W.-S., 2018. Analysis on current energy consumption of wastewater treatment plants in China (in Chinese). China Water & Wastewater 34 (7), 70e74. Cui, P., Mai, Z., Yang, S., Qian, Y., 2017. Integrated treatment processes for coalgasification wastewater with high concentration of phenol and ammonia. J. Clean. Prod. 142, 2218e2226. Deblay, P., Delepine, S., Minier, M., Renon, H., 1991. Selection of organic phases for optimal stability and efficiency of flat-sheet supported liquid membranes. Separ. Sci. Technol. 26, 97e116. Gai, H., Qiao, L., Zhong, C., Zhang, X., Xiao, M., Song, H., 2019. A solvent based separation method for phenolic compounds from low-temperature coal tar. J. Clean. Prod. 223, 1e11. Guo, C., Cao, Q., Chen, B., Yang, S., Qian, Y., 2019. Development of synergistic extraction process for highly efficient removal of phenols from coal gasification wastewater. J. Clean. Prod. 211, 380e386. Gupta, S., Chakraborty, M., Murthy, Z.V.P., 2014. Performance study of hollow fiber supported liquid membrane system for the separation of bisphenol A from aqueous solutions. J. Ind. Eng. Chem. 20, 2138e2145. He, L., Niu, Z., Miao, R., Chen, Q., Guan, Q., Ning, P., 2019. Selective hydrogenation of phenol by the porous Carbon/ZrO2 supported NieCo nanoparticles in subcritical water medium. J. Clean. Prod. 215, 375e381. Hu, L., Toyoda, K., Ihara, I., 2008. Dielectric properties of edible oils and fatty acids as
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Glossary ORARW: oil-refinery alkali residue wastewater SLM: supported liquid membrane HRT: hydraulic retention time (min) HRTaq: feed phase HRT (min) HRTstr: stripping phase HRT (min) Vaq: feed phase volume (mL) Vstr: stripping phase volume (mL) Vorg: organic phase volume (mL) VM: empty bed volume of membrane module (mL) Caq: feed phase concentration (mg/L) Cstr: stripping phase concentration (mg/L) COD: chemical oxygen demand (mg/L) TDS: total dissolved solids (mg/L)