Effective degradation of oil pollutants in water by hydrodynamic cavitation combined with electrocatalytic membrane Open Access

Oil is a very important energy in modern society, but its large utilization in recent years has inevitably caused oil spill and waste. Oil pollutes seas, rivers and terrestrial environments, which will finally harm human health.^1,2 Oily sewage contains a variety of oxygen-consuming pollutants which consume and decrease dissolved oxygen in water. This pollution leads to deterioration of water quality and impacts the survival of aquatic organisms. The issue of oil pollution is becoming increasingly serious and has become an active area of research in recent years.^3,4 Due to the characteristics of oily sewage, it is necessary to conduct effective treatment to solve the oil pollution as soon as possible.⁵ At present, the conventional methods to treat oil wastewater include physical methods,⁶ chemical methods,⁷ biological methods,⁸ advanced oxidation processes (AOPs)⁹ and other combined methods.¹⁰ Although the traditional methods are widely used to degrade organic pollutants in water, these methods suffer from low efficiency and high cost.¹¹ 

As a new water treatment technology, hydrodynamic cavitation (HC) involves the formation of bubbles within a liquid under the critical conditions of high pressure.¹² These bubbles collapse to produce energy and ·OH,^13,14 and oil pollutants are degraded into CO₂ and H₂O.¹⁵ Simultaneously, the solubility of oil in water is increased by the dissolution effect of HC,¹⁶ making it more convenient to be degraded. Pandit et al.¹⁷ found that HC could degrade fatty oils and had the advantage of requiring only simple equipment and processing capabilities well suited in terms of throughput for application in industry. Sunita et al.¹⁸ used HC and its combination with process-intensifying additives (such as H₂O₂ and ozone) to treat effluent from the pesticide industry. The results showed that the combined process was effective and the HC+H₂O₂ technique was cost-effective and reduced power consumption. Considering HC requires simple experimental devices and has a large sewage-treatment capacity, but the organics degradation rate of HC is relatively low. To overcome this shortcoming, HC combined with electrocatalytic membrane (ECM) takes advantage of ECM-induced degradation for the highly efficient treatment of oil wastewater. ECM employs the method of membrane separation and electrocatalytic oxidation. Electrocatalytic oxidation is a novel AOP technique that uses a catalyst to promote electrochemical reaction. Under this condition, water and O₂ can be converted into intermediate products, including ·OH, O₂^- and H₂O₂, without additional chemicals,¹⁹ and the oil pollutants are gradually degraded into H₂O and CO₂.²⁰ Guan et al.²¹ made use of ECM reactor to treat phenolic wastewater. Under the optimized conditions, the chemical oxygen demand (COD) removal rate reached 82.31%, and the energy consumption was only 0.32 kWh/kg when the COD was 10 mM. Thus, ECM reactor has been demonstrated to be a low-energy-consumption device. Liu et al.²² found that nano-TiO₂ modified ECM degraded aqueous tetracycline at a removal rate approaching 100%. Thus, ECM technique is a highly efficient method to treat organics wastewater. Because ECM technique has the advantages in their low energy consumption, low cost and environmental friendliness. In addition, they are simple to operate and require no further processing.

This work studies the degradation performance of oil wastewater using a combined method of HC and ECM. The HC plays a role of dissolution and preliminary degradation of oil pollutants. Then the ECM is initiated to degrade the dissolved oil completely. The operating parameters, including the electrode spacing, current density, residence time, pH and solution temperature, were controlled to optimize the experimental process, and the degradation products of oil wastewater were analyzed.

The electrocatalytic membrane was provided by the Dalian University of Technology, which was fabricated by high temperature pyrolysis and carbonization of carbonaceous material. The detailed specifications of the electrocatalytic membrane were as follows: average pore size 0.45 μm, porosity 46.7%, thickness 5 mm. The oil sample SF 15W-40 was purchased from the Great Wall Sinopec Lubricating Oil Company. The chemical reagents including Na₂SO₄, NaOH, HCl and CCl₄ were purchased from Kermel Chemical Reagent Company. All the chemical reagents were of analytically pure grade (AR) and used without further purification. Gas chromatography mass spectrometry (GC-MS) was performed using an instrument model 4000MS from Varian Inc., USA. An infrared spectral colorimetry spectrometer (model No. JDS-105U) from Jilin Jiguang Technology Company was also used.

The schematic of the experimental device combining HC and ECM is shown in Fig. 1. The oil was used to prepare 30 L of oil wastewater with a concentration of 90 mg/L, which was initially poured into the water tank. HC is a closed system, and the treated oil wastewater could return to the water tank after cavitation via the cavitation reactor. The cavitation reactor is composed of a plate with 49 holes and each hole diameter of 1 mm. After a certain time, oil sewage enters through control valve V3 into the middle tank, and the ECM begins to execute. In this process the oil wastewater in the middle tank is pumped into the permeate tank through the ECM module by a peristaltic pump.

The HC was conducted at various inlet pressures and temperatures to disperse and initially degrade oil wastewater. Then after the HC, the ECM began to degrade oil wastewater under certain conditions of electrode spacing, current density, residence time, pH and temperature. In order to increase the conductivity of the solution, a certain amount of Na₂SO₄ was added. The oil concentration in the wastewater was measured by infrared spectral colorimetry spectrometry, and the oil removal rate was calculated through the oil concentration before and after treatment.

The analysis of oil composition and content in water was performed using GC-MS. The cleaning process of ECM employed a peristaltic pump with positive pressure to pump ethanol to rinse the ECM for 0.5 h.

Using an original oil concentration of 90 mg/L and temperature of 35°C, the effect of inlet pressure on the removal performance of oil wastewater by HC was determined. The results are shown in Fig. 2. In the initial stage of HC, the oil concentration at different inlet pressures increases with the increasing cavitation time. When the time reaches 2 h, the oil concentration in water reaches a maximum. Under the inlet pressure of 0.25 MPa, 0.30 MPa and 0.35 Mpa, the maximum oil concentrations were 214 mg/L, 215 mg/L and 223 mg/L, respectively. After 2 h, the oil concentration in water gradually decreases with the increasing cavitation time. When the cavitation time was 5 h, the oil concentrations at inlet pressures of 0.25 MPa, 0.30 MPa and 0.35 MPa were 148 mg/L, 136 mg/L and 167 mg/L, respectively. These results indicate that under the same conditions, the oil concentration in water is increased with greater inlet pressure at the initial stage. When the oil concentration reaches the maximum value, it does not decrease more quickly at higher inlet pressure. When the pressure is 0.30 MPa, the decrease of oil concentration is more obvious, indicating the removal rate of oil pollutants is greater. There are two main reasons for this. Firstly, the oil in the water mainly contains suspended oil and dissolved oil, and at the initial stage the mechanical effect of cavitation is larger than the cavitation degradation effect. With greater pressure, there is a stronger mechanical effect, so the solubility of suspended oil increases. Thus, when the initial inlet pressure increases, the oil concentration in water is higher. While in the second stage when the oil concentration reaches the maximum value, the degradation effect of ·OH free radical predominates over the mechanical effect of HC, and the oil concentration decreases.

However, this relationship does not imply that with higher inlet pressure, there is better degradation effect by HC. The cavitation number is often used to describe the cavitation effect, and cavitation number C_V is defined as $Cv=(P0−Pr)/0.5ρV02$⁠, where P₀ is recovery pressure of downstream fluid, P_r is vapour tension of fluid under experimental temperature, ρ is density of fluid and V₀ is average velocity of fluid. Thus, observing the formula of cavitation number, the cavitation number has no direct connection with inlet pressure. The change of inlet pressure will influence cavitation number through recovery pressure of downstream fluid (P₀) and average velocity of fluid (V₀). So the relationship between inlet pressure and cavitation number is not simple linearity. When the cavitation number is smaller, the cavitation effect is stronger and more conductive to degrade organic matters. As the inlet pressure increases, the flow rate of orifice plate increases correspondingly, and the cavitation number decreases, so the cavitation effect is enhanced, which enhances the cavitation reaction. However, with an increase in inlet pressure and flow rate, the speed of cavitation bubbles also increases via the cavitation zone. The growth time of cavitation bubbles is shortened, so there is not enough time to develop into large bubbles and the bubbles fastly collapse at external high pressure, resulting in weaken of the cavitation effect.

The solution temperature greatly influences the cavitation effect, so the effect of solution temperature on the removal of oil pollutants in water by HC was studied under an initial oil concentration of 90 mg/L and inlet pressure of 0.30 Mpa. The results are shown in Fig. 3. The temperature has a large influence on the oil removal efficiency at 35°C, and the oil concentration reached a maximum value after 2 h. In contrast, at 30°C the required time was 3.5 h, and at 25°C the required time was 4 h. Thus at higher temperature, the dissolved oil concentration reaches a maximum value with shorter cavitation time, and the removal effect of oil is greater. At 25°C, 30°C and 35°C, the maximum oil concentrations were 209 mg/L, 214 mg/L and 215 mg/L, respectively, and fell to 179 mg/L, 167 mg/L and 139 mg/L after 5 h. Known from the definition of cavitation number, the temperature directly affects the vapour tension of fluid (P_r) and further influences cavitation number. When the solution temperature is high, the saturated vapor pressure is correspondingly high, making the cavitation number small and the cavitation effect strong. So with the increase of saturated vapor pressure, the cavitation is easier to produce. In addition the diffusion speed of molecular also accelerates in high temperature, so it is more conducive to the chemical reaction. However, the content of dissolved gas is reduced in solution with increased temperature, which is not beneficial to the cavitation. Thus, high temperature is not necessarily optimal.

The above studies show the amount of dissolved oil reaches the maximum at a concentration of 215 mg/L at the inlet pressure 0.30 MPa, solution temperature 35°C, cavitation time 2 h. And then, with the increase of cavitation time, the oil concentration decreases slightly, but the removal rate of oil pollutants is still low. Inspired by this finding, after the treatment of HC, the ECM technique was conducted to further degrade oil pollutants in water, which maybe an effective method to treat oil wastewater.

The electrode spacing greatly influences the degradation of oil pollutants, so the effect of electrode spacing on the oil removal rate of the ECM was studied. The specific experimental conditions were as follows: solution temperature 30°C, pH 7, residence time 3.8 min, current density 2.0 mA/cm². The results are shown in Fig. 4. With increased electrode spacing, the oil removal rate and COD removal rate both increase and then decrease. When the electrode spacing was 40 mm, the oil removal rate was 98.67% and COD removal rate was 91.57%. When the electrode spacing was small, electrons accumulated on the surface of the electrolytic electrode, and the flotation phenomenon of microbubbles could be observed. The microbubbles generated by the electrode affect this system through two ways. Firstly, the microbubbles will influence convection and diffusion rate of electrons, resulting in reduction of current efficiency. Secondly, the microbubbles cover the electrode surface and influence the contact of oil pollutants and electrode, resulting in reduction of degradation efficiency. These microbubbles are highly dispersed in water, which involve water-gas-particle phase hybrid system. The flocs are formed by fine particles adhered to microbubbles, and the apparent density of flocs is less than that of water. Since the electrode spacing is small, the flocs could not easily float and lead to blocking of membrane pores, resulting in decline of degradation reaction. On the other hand, when the electrode spacing is excessively large, the convection and diffusion rate of electrons decrease, and the degradation reaction rate decreases.

The effect of current density on the degradation of oil pollutants was investigated and the results are shown in Fig. 5. The specific experimental conditions were as follows: solution temperature 30°C, pH 7, residence time 3.8 min, electrode spacing 40 mm. With increasing current density, the oil removal rate and COD removal rate first increase and then decrease. The lowest oil concentration in water occurred at a current density of 2 mA/cm², for which the oil removal rate and COD removal rate were the largest, reaching 98.67% and 91.57%. This is because current density represents the electrocatalytic reaction rate. When the current density increases, the velocity of electrons accelerates, increasing oxidative groups concentration such as ·OH and reaction rates. Thus, this mechanism is conducive to oil removal and increases the removal rate. While the current density is too large, the electrocatalytic reaction is too intense. Thus a large number of bubbles are generated on the electrode surface, the oil pollutants cannot reach the electrode surface, and the oil removal rate is inevitably decreased.

Fig. 6 shows the effect of residence time on the removal performance when the experimental conditions were as follows: solution temperature 30°C, pH 7, current density 2.0 mA/cm², electrode spacing 40 mm. As shown in Fig. 6, when the residence time is less than 3.8 min, the oil removal rate and COD removal rate increase quickly with increasing residence time. This is because at longer residence time, the ·OH on the surface of the electrode persists longer. Thus the catalytic effect is more prominent as more oil is degraded, improving the oil removal rate. When the residence time is greater than 3.8 min, the oil removal rate increases slowly. Since the residence time is too long, the majority of oil molecules closed to the ECM are removed. At low oil concentrations, polarization phenomena are substantial, resulting in reduction of oil molecules diffused to membrane surface, which leads to oil removal rate increases slowly.

The pH is an important factor that affects the electrocatalytic reaction. The effect of pH on the oil removal rate was studied at 30°C, residence time 3.8 min, current density 2.0 mA/cm² and electrode spacing 40 mm. The results are shown in Fig. 7. The oil removal rate and COD removal rate first increase and then decrease with increasing pH, and the optimal oil removal rate is 98.81% at pH 6. Formulas (1) and (2) show that for each unit increase in pH value, the potential E(·OH/OH^-) is reduced by 0.05915 V. With increasing pH, the cavity oxidizes OH^- into ·OH, increasing the oxidation capacity and oil removal rate. When the pH is greater than 6, the production of ·OH radical increases rapidly, leading to self-quenching. The oxidation rate slows the reaction speed and decreases the oil removal rate. As the pH continues to increase, ·OH radical is quenched more prominently. Large amounts of OH^- and SO₄^2- adsorb onto the membrane surface and affect the adsorption of oil on the membrane, resulting in a decline in the oil removal rate.

The effect of solution temperature on oil degradation was investigated at current density 2 mA/cm², pH 6, residence time 3.8 min and electrode spacing 40 mm. The results are shown in Fig. 8. As the temperature rises, the amount of removed oil pollutants first increases and then decreases. At 30°C, the maximum oil removal rate of 98.81% and COD removal rate of 94.77% are reached. As the temperature increases, the molecular movement in the solution accelerates, and the conductivity of the electrolyte Na₂SO₄ increases. Thus the electrocatalytic reaction is accelerated, and the removal rate increases correspondingly. When the temperature exceeds a certain value, the dissolved oxygen decreases, leading to decrease ·OH radicals, which slows down oxidation rate and decreases removal rate. Furthermore, due to increasing temperature, the hydrogen evolution reaction and oxygen evolution reaction accelerate, which produce a lot of bubbles. In the presence of bubbles, organic pollutants are inhibited close to the ECM, so the removal rate is reduced.

The effect of membrane cleaning on the oil removal rate and flux was investigated at current density 2 mA/cm², pH 6, residence time 3.8 min, electrode spacing 40 mm and temperature 30°C. The results are shown in Fig. 9 and Fig. 10. Before the membrane cleaning, the initial flux was 75.0 L/(m²·h), and with time the flux decreased. At 8.3 h the flux fell to 31.3 L/(m²·h), and the oil removal rate decreased from 99.02% to 93.74%. As the processing time extends, the ECM may suffer membrane fouling which need to be cleaned. After cleaning the membrane, the flux recovered from 31.3 L/(m²·h) to 72.8 L/(m²·h), which was essentially restored to the initial flux. In the meantime, the oil removal rate could still reach very high percentages about 98.72% after cleaning, indicating that the ECM can sufficiently recover membrane performance with cleaning for reuse.

With the inlet pressure 0.30 MPa, temperature 35°C, oil concentration 90 mg/L and cavitation time 5 h, the oil wastewater was treated by HC and the water samples were subjected to GC-MS analysis. The results are shown in Fig. 11. There are many chromatographic peaks in original oil wastewater, and the maximum peak position is approximately 26 min, mainly because the oil components are complex. With the extension of cavitation time, the intensity and amount of total peaks are reduced. There are 65 peaks in the original oil wastewater, including the largest number of alkanes occupied 42.37% of the total organic matters and 49.36% of the total peaks area. After 5 h HC treatment, the number of chromatographic peaks are 45, which means the peaks are reduced by 20 relative to the original water sample. Although the number of alkanes decrease to a certain extent, the degradation effect is not obvious. The reduced alkanes only contribute 28.45% of total organic matters and 35.49% of the total peak area, which indicates HC has certain effect on the removal of oil pollutants in water.

With the current density 2 mA/cm², pH 6, residence time 3.8 min, electrode spacing 40 mm and solution temperature 30°C, the oil wastewater was then treated by ECM and the water samples were subjected to GC-MS analysis. The results are shown in Fig. 12. The ECM significantly reduces the peak intensity and peak number, and the peak number has fallen to 10. By analysis the degradation products are mainly alkanes. The ECM can effectively reduce the concentration of oil pollutants, and the large amount of organic matters is decomposed into H₂O and CO₂. The results prove that the ECM promotes the removal of oil wastewater through electrocatalytic effect.

In summary, HC offers favorable dispersion and dissolution performance of oil pollutants in water, and the oil concentration in water increases from 90 mg/L to 215 mg/L at temperature 35°C, inlet pressure 0.30 MPa and cavitation time 2 h. At this point, the degradation effect of HC begins to exert. With cavitation time of 5 h, the oil concentration decreases from the highest value of 215 mg/L to 139 mg/L, indicating that HC has a certain effect on the degradation of oil wastewater.

After the HC process, the ECM is continued to treat oil wastewater. Under the conditions of current density 2 mA/cm², pH 6, residence time 3.8 min, electrode spacing 40 mm and temperature 30°C, the oil removal rate reaches 98.81%. By GC-MS analysis, the kinds and content of organic matters decrease, and the degradation products are mainly alkanes. After membrane cleaning, the used ECM could recover the flux and removal performance.

The combined technology of HC and ECM could improve the degradation efficiency of oil pollutants and is an effective method for removing oil pollutants in water. This technique combines the individual advantage of HC and ECM, which has considerable potential for the removal of organic pollutants from water in practical applications.

This work was supported by the National Natural Science Foundation of China (No. 51478461).