Filtration of oil from oily wastewater via hydrophobic modi ﬁ ed quartz sand ﬁ lter medium

To improve the hydrophobicity and lipophilicity of a quartz sand ﬁ lter medium, two coupling agents, DN101 and KH570, were employed. The ﬁ lter medium surface wettability and oil removal ef ﬁ ciency before and after modi ﬁ cation were investigated, and the characteristics are summarized. The test results show that, after modi ﬁ cation by the grafting of an organic long-chain coupling agent to the ﬁ lter medium surface, the lipophilic to hydrophilic ratio increased from 1.31 (UQS) to 12.09 (MQD-Ti) and 5.11 (MQD-Si), and the oil removal ef ﬁ ciencies of MQD-Ti and MQD-Si improved by 21.7% and 6.9%, respectively. The stronger hydrophobicity resulted in higher quality factor values of 0.668 m (cid:1) 1 and 0.548 m (cid:1) 1 for MQD-Ti and MQD-Si, respectively, compared to 0.533 m (cid:1) 1 for UQS. This means that improving the ﬁ lter medium surface hydrophobicity and oil removal ef ﬁ ciency via ﬁ lter medium surface modi ﬁ cation is effective.


INTRODUCTION
Numerous industries, such as the food, metal, transport, textile and petrochemical industries, produce large amounts of oily wastewater ( Jamaly et al. ), which has a complex composition and a wide range of concentrations and is hazardous to the environment in various ways (Mueller et al. ). Current common oily wastewater disposal methods include gravity separation, dissolved air flotation, chemical treatment, coalescence, membrane treatment and biological treatment (Kota et al. ; Li et al. ; Mazumdar et al. ; Ngang et al. ). However, there are numerous issues: the processing efficiencies of these methods are low; the processing facilities require a large area, are expensive, or require complex operations; there are strict requirements on the concentration of the oily wastewater inflow; and the equipment is susceptible to clogging. Filtration is also commonly applied in secondary and tertiary sewage treatment (Hamoda et al. ; Kratochvil et al. ). After this treatment, the oily wastewater meets relevant standards and can be discharged or recycled.
A deep bed filter may remove particle sizes smaller than the filter pore size (Takahashi et al. ; Bai & Tien ) due to adsorption to the surface of the filter medium. Therefore, the physical chemical interaction between the pollutant and filter medium is the major factor influencing pollutant removal (Bai & Tien ). In general, if the granular filter medium surface has strong hydrophobicity, it also has strong lipophilicity and a superior capability to remove hydrophobic pollutants such as oil. On the other hand, if the filter surface has weak hydrophobicity, it has weak lipophilicity and an inferior oil removal capability (Shin & Chase ). Therefore, to improve oil-water separation efficiency from oily wastewater by the filtration process, the filter medium surface can be modified to improve its hydrophobicity. water and anhydrous ethanol as a solvent to hydrolyze the silane coupling agent KH550, and a quartz sand filter was soaked in the hydrolysate for modification. This method reduced the water wetting weight in the packed bed from 1.5589 g to 0.0607 g and effectively improved the oil removal efficiency. However, this method has disadvantages such as the large agent dosage requirement, environmental unfriendliness, and a low primary modification rate, which make it unsuitable for industrial mass production.
In this work, two different coupling agents, isopropyl dioleic (dioctylphosphate) titanate (DN101) and 3-methacryloxypropyl trimethoxy silane (KH570), were selected, and an improved modification method was employed to modify the quartz sand filter medium, where the coupling agent dosage was less than 10% of that used in the method proposed by Wei et al. (). This has advantages such as a low agent dosage requirement and suitability for mass production. In the following sections, the unmodified quartz filter medium and the filter media with the DN101 modification and KH570 modification are denoted as UQS, MQD-Ti and MQD-Si, respectively. In this paper, the wettability and oil removal performance of the three filter media were investigated. Here, the oil removal performance includes the oil removal efficiency and water head loss across the filter bed (Shin ). The oil removal efficiency (E) is calculated as where C in and C out denote the inlet and outlet oil concentrations of the wastewater, mg/L.
To conduct comprehensive research on the filter bed oil removal performance, the quality factor (QF) defined by Brown () and Kulkarni et al. () was used: where H is the water head loss, m.

Materials and reagents
The quartz sand filter medium is from Henan Songxing Filter A certain amount of the preprocessed quartz sand filter medium was placed in the mixer, mixed at 300 rev/ min and heated at 60 C for 5 min. Next, 1.2% of the filter medium weight of the DN101 alcoholysis solution (for MQD-Si, 1.5% KH570 hydrolysis solution) was sprayed slowly from the inlet onto the filter surface. The filter was kept in the dry state, mixed for another 70 min (50 min for MQD-Si), then cooled and soaked in water for 24 h, washed three times and dried at 100 C to a constant weight to prepare the MQD-Ti (or MQD-Si) filter medium.

Modification mechanism
Both the alcoholyzed DN101 and hydrolyzed KH570 contain hydroxyl groups. The condensation reaction between these hydroxyl groups and the hydroxyl groups on the quartz sand filter surface grafts the organic long-chain functional group of the coupling agent to the filter surface. The reaction equation is as follows (Zhang et al. ): In Equation (4), RY represents .

Wettability test
In a porous packing bed with a solid grain filling, when the capillary rise reaches the equilibrium state, the relation between the liquid weight in the capillary bed and the wet- where θ is the static contact angle of the wetting liquid on the filter surface, ; ɡ is the gravitational acceleration, m/s 2 ; r s is the effective radius of the porous packing bed, m; R is the inner diameter of the packing bed, m; ε is the packing bed porosity, %; w e is the liquid mass that enters the capillary when the capillary rise reaches the equilibrium state, g; and γ GL is the surface tension of the wetting liquid, mN/m.
As it is difficult to measure r s , the contact angle cannot be calculated directly via Equation (5). Yang & Chang () proposed a concept of the lipophilic to hydrophilic ratio (LHR) as follows: where θ O and θ W are the wetting contact angles of oil and water, respectively, for a single sample.
Substituting Equation (5) into Equation (6): where w eO and w eW are the liquid mass that enters the capillary when the capillary ascent reaches the equilibrium state, g.
In this paper, cyclohexane represents the oil phase, and

Filtration experimental setup
The filtration apparatus is shown in Figure 1 Figure 3 shows filtration efficiency for the three filter media.

Filtration performance
In the first 9 h, the removal efficiency essentially stabilizes.  improvements, respectively, over that of UQS. After 9 h, the oil removal efficiency starts to decline. This is because after oil is adsorbed on the filter surface, the channels between the filter media are blocked. When the face velocity is fixed, the water head loss in the filter bed increases (as shown in Figure 4), and the media velocity increases. Therefore, the water flow shear force on the oil adhering to the filter surface increases, and the service cycle is then complete. Chu et al. ). Therefore, the water channels in the filtering bed are reduced, which leads to an increase in the water head loss. In general, MQD-Ti has the highest filtering bed water head loss, while UQS has the lowest. It can be seen from Figures 3 and 4 that the stronger the hydrophobicity of the filter medium, the more entrapped oil accumulates in the filter bed, the proportion of voids in the filter bed then becomes smaller and the water head loss increase rate becomes faster. Therefore, the MQD-Ti filtering bed water head loss increase rate is the highest, while UQS has the slowest increasing rate.
A combined performance measure of the oil removal efficiency and water head loss is given by the quality factor in Equation (1). Figure 5 shows the quality factor values of the three filter media. Before 6 h, the quality factor values of the three filter media were stable. The average value for MQD-Ti was 0.668 m À1 , far exceeding those of the other two. The quality factor value of MQD-Si was slightly higher than that of UQS, 0.575 m À1 versus 0.564 m À1 . This means that as the LHR increases, the quality factor increases faster. After 6 h, the quality factor values of the three filters started to decline, with MQD-Ti and MQD-Si exhibiting faster declining rates than UQS. This is because when more oil is entrapped in the MQD-Ti and MQD-Si filter beds, the filter channel blockage is more

XPS analysis
The XPS spectra of the three filters are shown in Figure 8.

CONCLUSION
As the coupling agent contains a long-chain organic functional group and a short chain alkoxy capable of hydrolysis or alcoholysis, a condensation reaction occurs between the hydrolysis-or alcoholysis-generated hydroxyl groups and the hydroxyl groups on the quartz sand filter medium surface. The long-chain organic functional group is grafted to the filter medium surface, and thereby, the hydrophobicity and lipophilicity of the filter medium are increased. FTIR and XPS analyses show that after modification, organic functional groups unique to the coupling agent appear on the filter medium surface. This indicates that the modification is successful. After the surface modification, the oil removal efficiency and quality factor are effectively improved; the oil removal efficiencies of MQD-Ti and MQD-Si improve by 21.1% and 7.5% over that of UQS. To summarize, the hydrophobicity modification effectively improves the oil removal efficiency.