Phenol contaminated petroleum refinery wastewater presents a great threat on water resources safety. This study investigates the effect of microwave irradiation on removal of different concentrations of phenol in an attempt for petroleum refinery wastewater treatment. The obtained results show that the MW output power and irradiation time have a significant positive effect on the removal efficiency of phenol. The kinetic reaction is significantly affected by initial MW output power and initial phenol concentrations. Response surface methodology (RSM) was employed to optimize and study the interaction effects of process parameters: MW output power, irradiation time, salinity, pH, and H2O2 concentration using central composite design (CCD). From the CCD design matrix, a quadratic model was considered as an ultimate model (
The main recalcitrant organic material found in petroleum refinery wastewater (PRWW) effluent is phenol due to its high water solubility behavior (86 g/L) and resistance to conventional physicochemical treatment methods, for example, oil separation, coagulation, and flocculation [
Over the past few decades, advanced oxidation processes (AOP) have received increasing attention for the destruction of phenolic pollutants commonly found in wastewaters [
MW energy belongs to nonclassical source of energy, with separate bands of electromagnetic radiation and frequencies ranging from 300 MHz to 300 GHz [
The aim of this work is to use MW irradiation in batch mode as AOP to study the kinetics of phenol removal from PRWW sample to optimize the operating conditions. Due to the reasonable cost and high oxidizing power of hydrogen peroxide as a homogeneous oxidizing agent, response surface methodology (RSM) was applied to optimize and enhance the process of phenol removal throughout a combination system of MW/H2O2 to enhance the removal efficiency. The experimental design matrix was developed through a central composite design (CCD) using the studied variables: MW output power, irradiation time, salinity, pH, and initial hydrogen peroxide concentrations.
Analytical grade phenol (>98% purity) and hydrogen peroxide (30%, w/v) were purchased from Sigma Aldrich, USA, while all other chemical reagents employed in this study were of analytical grade. A stock solution containing 1000 mg/L phenol was prepared and then diluted to the required concentration using the authentic wastewater solution, according to the experimental conditions. The authentic wastewater (synthetic water) solution used in this work was prepared according to the physicochemical characteristics of petroleum refinery effluents collected from wastewater treatment plant at Cairo Oil Refining Company (CORC) in Egypt at different dates and time intervals (data not shown) and was composed of NaCl 0.48 g, KCl 0.019 g, MgSO4 0.074 g, Na2SO4 0.009 g, CaCl2 0.12 g, MgCl2 0.04 g, NaHCO3 0.18 g, and CaSO4 0.03 g, dissolved in 1000 mL deionized water (18.2 M Ω cm).
A modified domestic MW oven (Electrolux, Model EMM2005) with frequency 2450 MHz and maximum output power of 800 Watt was used to supply MW irradiation as shown in Figure
Schematic diagram of the MW reactor for PRWW treatment.
HPLC instrument model Agilent 1200 series equipped with autosampler and photodiode array detector (set at full scan range 190–400 nm) was used to analyze the phenol concentrations under the following conditions: C8 reversed phase (
All the experiments were conducted using MW/H2O2 system in a 250 mL capacity quartz flask reactor with a working volume of 25 mL. The phenol solutions with different concentrations (10, 25, and 50 mg/L) were irradiated at different MW power levels, 100, 150, 300, 450, and 600 Watt, and at different irradiation times intervals ranging from 1 to 40 min.
The initial salinity of the solution range (200–1000 mg/L) was adjusted by adding NaCl; initial pH range (pH 5–9) was adjusted by 0.1 M NaOH or HCl and the initial H2O2 concentrations range (1 to 10% v/v) was employed to decontaminate authentic PRWW containing different concentrations of phenol. After the MW removal process, the phenol concentration in the samples was determined using HPLC. The listed removal efficiency by MW was the arithmetic average of the results derived from duplicate experiments.
The kinetic study was carried out at different MW powers (100–600 W) at a fixed initial phenol concentration of 10 mg/L and at different concentrations of phenol (10, 25, and 50 mg/L) at a fixed MW output power of 450 W. Three different kinetic models were tested for the obtained data to elucidate the removal processes ( zero-order kinetic model [ first-order kinetic model [ second-order kinetic model [
The validity of each model was determined by the following statistical error functions. The sum of the squares of errors (ERRSQ) [ The hybrid fractional error (HYBRID) [
The design of experiments was intended to reduce the number of experiments with a wide range of combinations of independent variables. In the present study, CCD with five independent variables, each with three levels (coded as −1, 0, and +1 for low, medium, and high levels, resp.), was used for the experimental design model (Table
The considered levels of independent variables for the phenol removal by CCD.
Independent variables | Symbol | Variable levels | ||
---|---|---|---|---|
−1 | 0 | +1 | ||
( |
|
300 | 450 | 600 |
( |
|
10 | 20 | 30 |
( |
|
200 | 600 | 1000 |
( |
|
5 | 7 | 9 |
( |
|
1 | 5.5 | 10 |
Design matrix for the phenol decomposition process by CCD.
Run number |
|
|
|
|
|
Phenol decomposition % | |
---|---|---|---|---|---|---|---|
MW power (Watt) | Time (min) | Salinity (mg/L) | pH | H2O2 (%) | Predicted | Experimental | |
1 | 600 | 10 | 1000 | 5 | 1 | 44.39 | 47.05 |
2 | 600 | 30 | 1000 | 9 | 1 | 36.25 | 38.90 |
3 | 600 | 10 | 1000 | 9 | 1 | 27.09 | 17.60 |
4 | 600 | 30 | 200 | 9 | 10 | 48.23 | 51.45 |
5 | 300 | 10 | 200 | 9 | 10 | 47.36 | 45.47 |
6 | 450 | 20 | 1000 | 7 | 5.5 | 54.69 | 68.49 |
7 | 600 | 10 | 200 | 5 | 10 | 50.17 | 55.13 |
8 | 300 | 10 | 1000 | 5 | 10 | 55.51 | 50.74 |
9 | 450 | 20 | 600 | 5 | 5.5 | 61.19 | 69.79 |
10 | 300 | 30 | 1000 | 5 | 1 | 49.65 | 59.44 |
11 | 600 | 10 | 1000 | 5 | 10 | 56.60 | 51.24 |
12 | 300 | 30 | 200 | 9 | 1 | 42.37 | 46.79 |
13 | 600 | 30 | 1000 | 5 | 1 | 57.86 | 56.43 |
14 | 300 | 10 | 200 | 9 | 1 | 31.50 | 42.70 |
15 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 52.65 |
16 | 600 | 30 | 1000 | 9 | 10 | 55.32 | 59.92 |
17 | 300 | 10 | 1000 | 9 | 1 | 24.02 | 27.21 |
18 | 600 | 10 | 200 | 5 | 1 | 50.47 | 57.31 |
19 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 57.76 |
20 | 300 | 30 | 200 | 5 | 10 | 65.84 | 66.90 |
21 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 51.08 |
22 | 600 | 30 | 200 | 5 | 10 | 63.17 | 65.84 |
23 | 450 | 20 | 600 | 7 | 10 | 68.50 | 67.44 |
24 | 300 | 30 | 200 | 5 | 1 | 59.87 | 59.12 |
25 | 600 | 20 | 600 | 7 | 5.5 | 50.04 | 55.97 |
26 | 450 | 20 | 600 | 7 | 1 | 55.22 | 63.61 |
27 | 300 | 10 | 200 | 5 | 1 | 44.68 | 30.39 |
28 | 600 | 30 | 1000 | 5 | 10 | 68.57 | 59.21 |
29 | 300 | 10 | 200 | 5 | 10 | 52.17 | 58.85 |
30 | 600 | 30 | 200 | 9 | 1 | 41.67 | 44.81 |
31 | 600 | 30 | 200 | 5 | 1 | 64.98 | 58.57 |
32 | 450 | 20 | 600 | 9 | 5.5 | 47.97 | 46.70 |
33 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 55.90 |
34 | 300 | 10 | 1000 | 5 | 1 | 35.51 | 32.43 |
35 | 300 | 20 | 600 | 7 | 5.5 | 49.84 | 51.24 |
36 | 300 | 10 | 1000 | 9 | 10 | 52.39 | 52.58 |
37 | 600 | 10 | 1000 | 9 | 10 | 47.68 | 55.38 |
38 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 53.91 |
39 | 600 | 10 | 200 | 9 | 10 | 39.55 | 30.50 |
40 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 48.94 |
41 | 450 | 10 | 600 | 7 | 5.5 | 49.44 | 57.21 |
42 | 300 | 30 | 1000 | 5 | 10 | 68.14 | 70.31 |
43 | 300 | 30 | 1000 | 9 | 1 | 33.85 | 20.30 |
44 | 450 | 30 | 600 | 7 | 5.5 | 60.85 | 60.42 |
45 | 300 | 30 | 1000 | 9 | 10 | 60.72 | 61.00 |
46 | 300 | 30 | 200 | 9 | 10 | 56.72 | 54.63 |
47 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 54.68 |
48 | 450 | 20 | 600 | 7 | 5.5 | 56.97 | 51.46 |
49 | 600 | 10 | 200 | 9 | 1 | 31.48 | 28.20 |
50 | 450 | 20 | 200 | 7 | 5.5 | 55.73 | 49.27 |
The CCD containing a total of 50 experiments with eight replicates at the central points, to estimate the experimental error, was employed to investigate the selected variables effect: initial MW power output, irradiation time (min), salinity, initial pH, and initial hydrogen peroxide concentration (
The trends in the removal of phenol under different MW irradiation power and time intervals experiments are shown in Figure
The removal efficiency for phenol pollutant gradually increased with the increase of the studied MW output power and irradiation time within the range 100–600 W and 1–40 min, respectively. This indicates that MW output power and irradiation time exert a positive effect on phenol removal, that is, enhancing phenol removal efficiency. Similar observation was reported by Papadaki et al. [
Also, the higher removal efficiency at MW output power (≥300 W) and irradiation time (≥10 min) (Figure
Removal of 10 mg/L phenol under different MW irradiation powers (100–600 Watt) and time intervals (1–40 min).
In this study, with the increase of MW irradiation power and time up to 300 Watt and 10 min, respectively, the temperature of authentic wastewater increases to 90°C (as recorded by temperature controller) and consequently water transparency leading to deeper penetration of MW radiation to phenol molecules and results increment of MW electromagnetic field absorption; the dipoles within the phenol pollutant attempt to realign themselves according to the applied MW field. This generates internal friction, resulting in high energy absorption by phenol that induced polarization of phenol and thermal removal by pyrolysis to their elemental constituents. This means that phenol removal by MW alone is highly significant. Chien [
Furthermore, it can be seen that the removal rate of phenol increases until attaining approximately highest removal values at MW output power of 450 Watt for all studied range of MW power while with higher MW irradiation power (600 W), more heat could be generated; but no significant removal values were attained. Therefore, in view of cost effectiveness and MW power saving, the fixed output MW power of 450 W was chosen for further experiments as an optimum MW power to study the effect of different initial phenol concentrations (10–50 mg/L) on their removal by MW treatment system.
The profiles of phenol illustrating the removal efficiency with time at different concentrations (10–50 mg/L) at a fixed MW output power 450 W are depicted in Figure
Effect of initial phenol concentration on removal efficiency by MW at 450 W.
The kinetic study of phenol removal was analyzed using the experimental data obtained with various initial MW power (100–600 W) and different initial phenol concentrations (10–50 mg/L). The correlation coefficient
(a) Kinetic model parameters for decomposition of 10 mg/L phenol at different initial MW output powers (Watt). (b) Reaction rate constants (
Initial MW output power | 100 | 150 | 300 | 450 | 600 |
---|---|---|---|---|---|
Zero-order kinetics | |||||
|
0.97 | 0.97 | 0.87 | 0.94 | 0.75 |
|
0.065 | 0.065 | 0.099 | 0.119 | 0.114 |
|
76.57 | 76.57 | 50.35 | 42.19 | 44.01 |
ERRSQ | 12.12 | 0.52 | 4.94 | 13.05 | 38.74 |
HYBRID | 33.76 | 2.22 | 23.979 | 19.86 | 215.54 |
First-order kinetics | |||||
|
0.96 | 0.97 | 0.98 | 0.98 | 0.97 |
|
0.008 | 0.008 | 0.013 | 0.017 | 0.017 |
|
88.87 | 87.74 | 51.73 | 41.01 | 41.51 |
ERRSQ | 19.74 | 2.54 | 4.33 | 10.52 | 3.67 |
HYBRID | 52.173 | 8.21 | 17.45 | 14.67 | 23.252 |
Second-order kinetics | |||||
|
0.97 | 0.98 | 0.99 | 0.99 | 0.97 |
|
0.001 | 0.001 | 0.002 | 0.003 | 0.003 |
|
111.11 | 100.00 | 52.63 | 40.00 | 40.00 |
ERRSQ | 28.40 | 8.29 | 16.759 | 11.58 | 19.68 |
HYBRID | 71.852 | 23.52 | 61.377 | 16.74 | 97.469 |
Half-life for zero-order:
Initial phenol concentrations | 10 mg/L | 25 mg/L | 50 mg/L |
---|---|---|---|
Zero-order kinetics | |||
|
0.94 | 0.97 | 0.97 |
|
0.119 | 0.314 | 0.633 |
|
42.19 | 39.83 | 39.48 |
ERRSQ | 13.05 | 66.98 | 27.25 |
HYBRID | 19.86 | 33.93 | 63.65 |
First-order kinetics | |||
|
0.98 | 0.99 | 0.99 |
|
0.017 | 0.018 | 0.018 |
|
41.01 | 38.51 | 38.94 |
ERRSQ | 10.52 | 47.91 | 26.97 |
HYBRID | 14.67 | 25.55 | 56.98 |
Second-order kinetics | |||
R2 | 0.98 | 0.99 | 0.99 |
|
0.003 | 0.001 | 0.0005 |
|
40.00 | 36.36 | 40.00 |
ERRSQ | 11.58 | 57.47 | 23.94 |
HYBRID | 16.74 | 29.15 | 33.85 |
Half-life for zero-order:
Comparison between experimental and theoretical removal efficiency on 10 mg/L phenol at different MW irradiation power (100–600 W) (symbols, experimental results; line, theoretical results).
100–150 Watt
300–600 Watt
Comparison between experimental and theoretical removal efficiency of phenol at different initial phenol concentrations (10–50 mg/L) (symbols, experimental results; line, theoretical results).
Obviously, from the results listed in Tables
Zhao et al. [
RSM was successfully used in optimizing the studied parameters (i.e., output MW power, irradiation time, salinity, pH, and initial hydrogen peroxide concentration) for removal of phenol at 10 mg/L concentration by MW/H2O2 operation. The experimental design matrix derived from CCD and the experimental and predicted percentage removals of phenol (response) are shown in Table
The quality of the models fitted was judged from coefficients of correlation
ANOVA test for the phenol removal efficiency.
Source | Sum of squares | df | Mean square |
|
|
---|---|---|---|---|---|
Model | 5564.95 | 20 | 278.25 | 4.23 | 0.0002 |
|
0.34 | 1 | 0.34 |
|
0.9430 |
|
1107.51 | 1 | 1107.51 | 16.84 | 0.0003 |
|
9.21 | 1 | 9.21 | 0.14 | 0.7109 |
|
1483.81 | 1 | 1483.81 | 22.56 | <0.0001 |
|
1498.65 | 1 | 1498.65 | 22.79 | <0.0001 |
|
122.19 | 1 | 122.19 | 1.86 | 0.1833 |
|
8.18 | 1 | 8.18 | 0.12 | 0.7268 |
|
7.61 | 1 | 7.61 | 0.12 | 0.7362 |
|
14.11 | 1 | 14.11 | 0.21 | 0.6466 |
|
59.17 | 1 | 59.17 | 0.90 | 0.3507 |
|
0.91 | 1 | 0.91 | 0.014 | 0.9071 |
|
19.16 | 1 | 19.16 | 0.29 | 0.5935 |
|
67.63 | 1 | 67.63 | 1.03 | 0.3189 |
|
121.29 | 1 | 121.29 | 1.84 | 0.1849 |
|
2.14 | 1 | 2.14 | 0.033 | 0.8580 |
|
37.24 | 1 | 37.24 | 0.57 | 0.4578 |
|
4.58 | 1 | 4.58 | 0.070 | 0.7938 |
|
5.78 | 1 | 5.78 | 0.088 | 0.7690 |
|
313.38 | 1 | 313.38 | 4.77 | 0.0373 |
|
140.03 | 1 | 140.03 | 2.13 | 0.1552 |
Residual | 1907.00 | 29 | 65.76 | ||
Lack of fit | 1850.33 | 22 | 84.11 | 10.39 | 0.0020 |
Pure error | 56.67 | 7 | 8.10 | ||
Cor total | 7471.95 | 49 |
The value of the determination coefficient
ANOVA analysis (Table
The empirical predicted quadratic model for response (phenol removal %) in terms of process variables is plotted in three-dimensional (3D) diagrams (Figure
3D surface plot with counterdiagram presenting the effect of output MW power, irradiation time, salinity, pH, and initial H2O2 dosage (v/v) on phenol decomposition efficiency.
Figure
Figure
Figure
Figure
Figure
These observed positive interactions were further confirmed, substantiated by analyzing the
The increase in phenol removal efficiency with increase of hydrogen peroxide concentration may be due to the increase of
The effect of each factor was further assessed by the use of perturbation plots (Figure
Perturbation plots showing the optimum values of the tested variables.
Furthermore, the optimization process was carried out to determine the optimum value of phenol removal efficiency, using the Design Expert 8.0.1.7 software. Accordingly, the optimum working conditions for maximum phenol removal percentage under MW/H2O2 treatment process were presented in Table
Optimum values of the process parameter for maximum phenol decomposition efficiency under MW process.
Treatment Process | Input power (Watt) | Time (min) | Salinity (mg/L) | pH | H2O2 (%) | Phenol decomposition % | |
---|---|---|---|---|---|---|---|
Experimental | Predicted | ||||||
MW Process | 439 | 24.22 | 574.10 | 5.10 | 10.00 | 75.70 | 72.90 |
The removal efficiency of phenol under MW treatment processes under different operating conditions increases with increasing MW output power and irradiation times. Also, the phenol removal (%) at different initial phenol concentrations (10–50 mg/L) was almost unchanged under studied MW treatment process (ranged from
A kinetic study confirmed that the overall phenol removal rate follows zero-order kinetic model at low MW power (100 and 150 W) and first-order kinetics at higher MW power (300–600 W) for initial phenol concentrations (10–25 mg/L). However, kinetic reaction shifted to second-order kinetic model for initial phenol concentration of 50 mg/L at 450 W MW power.
A combined process of H2O2 with MW improves the removal efficiency of phenol as proved by RSM. Where, MW/H2O2 system improves the generation of hydroxyl radicals from H2O2 due to the excitation of molecules to higher vibrational and rotational levels. Optimization of the process variables for phenol removal using RSM by employing CCD design matrix of experiments was as follows: MW power output of 439 Watt, irradiation time of 24.22 min, salinity of 574.10 mg/L, pH 5.10, and initial H2O2 concentration of 10.00% (v/v) with predicted and experimental percentage removal of phenol of 72.90% and 75.70%, respectively.
Finally, it can be proposed that MW radiation with H2O2 is an effective treatment method for the removal of phenol from petroleum refinery wastewater in a batch system. Future work is recommendable in an attempt to apply this treatment system in a continuous reactor.
Advanced oxidation process
Microwave
Response surface methodology
The central composite design
Watt
Initial phenol concentration (mg/L)
Phenol concentration at any time (mg/L)
Correlation coefficient
Zero-order rate constant (mg/min)
First-order rate constant (
Second-order rate constant (
The sum of the squares of errors
The hybrid fractional error function
Independent variables
Predicted response.
Variables for the axial points
Interaction coefficient
The constant model coefficient.
The authors declare that there is no conflict of interests regarding the publication of this paper.