# Some Theoretical Aspects of Tertiary Treatment of Water/Oil Emulsions by Adsorption and Coalescence Mechanisms: A Review

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## Abstract

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## 1. Introduction

## 2. Adsorption Isotherms

#### 2.1. Langmuir Isotherm (LI)

_{e}(mg/L) is the equilibrium concentration, q

_{e}(mg/g) is the equilibrium adsorption capacity of the adsorbent, q

_{m}(mg/g) is the maximum adsorption capacity, and K

_{L}= k

_{a}/k

_{d}is the Langmuir adsorption (equilibrium) constant.

_{m}, K

_{L}) using linear regression. The boundaries of linearity are also the boundaries of validity of the LI.

_{L}(Equation (3)).

_{L}corresponds to the adsorption processes according to the following criteria [23]:

_{L}> 1: The adsorption is unfavorable (an increase in Gibbs free energy of adsorption).

_{L}> 0: The adsorption is favorable (a decrease in Gibbs free energy).

_{L}= 1: It characterizes a linear adsorption (unoccupied sites at the adsorbent are randomly occupied by adsorbate proportionally to their concentration, and only one reaction site is occupied by one species).

_{L}= 0: The desorption process is irreversible.

#### 2.2. Brunauer–Emmet–Teller (BET) Isotherm

_{e}(mg/L) is an equilibrium concentration, q

_{e}(mg/g) is the equilibrium adsorption capacity of the adsorbent, q

_{m}(mg/g) is the maximum adsorption capacity, c

_{s}is the saturation concentration of the solute in water, and C is a constant defined as:

_{m}

^{S}(J/mol) is the molar enthalpy of adsorption related to the first layer, ΔH

_{m}

^{L}is the molar enthalpy of condensation, and g

_{0}is the entropic factor. The BET isotherm consists of two adjustable parameters (q

_{m}, C). Because Equation (4) did not give good fit of the experimental data, some authors proposed keeping c

_{s}as an adjustable parameter [27,28]. In this case, there are three adjustable parameters in the BET isotherm, and the equation gives a better fit of the experimental data; however, large discrepancies between the calculated c

_{S}and the experimental values were found. This resulted in attempts to redefine the meaning of c

_{s}in Equation (4). For instance, Miller et al. [29] defined c

_{s}as the concentration at which the adsorbent is saturated by adsorbate.

_{s}, they directly modified the BET isotherm derivation to the form expressed by Equation (6).

_{S}is the equilibrium adsorption constant for the first (adjacent) layer, and K

_{L}is the equilibrium adsorption constant for the upper layers. These two constants define the constant C; C = K

_{s}/K

_{L}. This model was successfully applied for various adsorption processes, including oil adsorption [31,32,33].

#### 2.3. Freundlich Isotherm (FI)

_{F}(L/mg) is the Freundlich adsorption constant.

#### 2.4. Dubinin–Radushkevich (D-R) Isotherm

^{2}kJ

^{−2}) is a constant related to the adsorption energy, and ε (kJ mol

^{−1}) is the adsorption potential (also called the Polanyi potential). The Polanyi potential for gas adsorption corresponds to the change in the Gibbs free energy of an adsorbent after adsorption of 1 mol of gas [38].

_{s}is the saturation concentration of the solute in water, similar to the BET isotherm.

#### 2.5. Error Analysis

## 3. Thermodynamics of Adsorption

^{o}

_{ad}), the isosteric enthalpy of adsorption (ΔH

^{o}

_{ad}), and the isosteric entropy of adsorption (ΔS

^{o}

_{ad}), are calculated from adsorption (equilibrium) constants, particularly from the Langmuir adsorption constant, in order to characterize the thermodynamics of adsorption at various temperatures [17]. However, the use of those constants should be made with caution. The first problem is that the equilibrium constant in the original Langmuir isotherm is not dimensionless, whereas the equilibrium constant K

^{0}in Equation (13) has no dimension [17].

^{0}(T) on 1/T. The thermodynamic parameters are expressed in J/mol. It is evident that both terms on the right side of the Equation (15) are dimensionless, and thus, K

^{0}must be dimensionless as well. This means that if K

_{L}values obtained from the LI and Equation (15) are used, then K

_{L}must first be transformed into a dimensionless parameter. The values of K

_{L}published in the literature have various units (L/g, L/mg, L/mol, L/mmol) [49,50,51], and thus, some consensus is needed on how to transform the dimensional K

_{L}into the non-dimensional K

_{L}. Recently, some approaches addressing this issue were proposed. The first approach is based on classical thermodynamics. The K

_{L}value involved in the LI for gases is also not dimensionless (the unit is Pa

^{−1}or its equivalent); however, if it is used as the equilibrium constant K

^{0}in Equation (13), K

_{L}is multiplied by the standard reference pressure, which is commonly 1 bar. This does not change the value of K

_{L}but makes it dimensionless. A similar principle was applied for the K

_{L}constant in the LI for the adsorption of liquids. Let us consider solute A in liquid phase (in solution) A (liquid) and adsorbed at sorbent A (solid). In the equilibrium A(liquid)↔A(solid), the equilibrium constant can be expressed as K = a

_{solid}/a

_{liquid}, where a

_{solid}and a

_{liquid}are activities in the related phase at equilibrium. The equilibrium constant of the A(liquid)↔A(solid) reaction is given by Equation (16) [52,53]:

_{solid}and c

_{liquid}are the concentrations in two phases, c

^{0}

_{solid}and c

^{0}

_{liquid}are the same concentrations in the selected standard states, and γ

_{solid}and γ

_{liquid}are activity coefficients. For simplicity, in dilute solutions of nonelectrolytes, γ

_{solid}, γ

_{liquid}≈ 1. If the influence of the activity coefficients is neglected and the validity of Equation (16) is respected, the last remaining problem is the determination of values related to the standard concentrations. Unfortunately, there is no generally accepted consensus in the scientific community in the case of adsorption from liquid solutions. The simplest choice is to set standard concentrations of 1 mol/L or 1 mol/kg for both components, depending on how a concentration in equilibrium is expressed. More practically, the concentration can also be 1 mmol/L or 1 mmol/kg. In this case, c

_{e}must also be expressed in the same units. This approach can be applicable for true solutions; however, it is not physically correct to define a standard state for emulsions in this way. Other methods that consider different standard states or just nullified the dimension have been published [54,55,56,57,58] and critically discussed [59].

_{ML}= k

_{a}/k

_{d}(Equation (18)) as the dimensionless parameter, unlike the common LI, where K

_{L}depends on the units used (it is reciprocal to the unit in which a concentration of solute in equilibrium is expressed). Equation (17) also predicts that q

_{m}is reached when c

_{e}= c

_{s}instead c

_{e}→∞ as a result of the original Langmuir isotherm (Equation (2)).

_{s}in general into the models (including BET and D-R) is questionable. It is evident that parameter c

_{s}, which may be significantly higher than c

_{e}, strongly influences the values obtained from the fitting of experimental data by the modified LI. Second, this model is not applicable for substances that do not form true solutions, such as emulsions, for which the saturation state cannot be unambiguously defined. This means that the dimensionless K

_{ML}would also be calculated from Equation (18) for the adsorption of emulsions because no standard state is required; however, experimental data q

_{e}= f(c

_{e}) cannot be fitted by Equation (17) because the c

_{S}value is undefinable for emulsions. It may be kept as an adjustable parameter, however, without any physical meaning. The modified BET isotherm (Equation (6)), as proposed by Ebadi et al. [30], looks to be a good approach for evaluating the adsorption of liquids and emulsions because (i) it is a physically plausible model that lacks the involvement of any speculative parameters, (ii) it is applicable for multilayer adsorption, (iii) it does not require the use of the constant c

_{s}, which is an undefinable parameter for emulsions, and (iv) it enables calculations of equilibrium constants and adequate thermodynamic parameters.

## 4. Kinetics of Adsorption

#### 4.1. Pseudo-First-Order Kinetic Model (PFOM)

_{1}(min

^{−1}) is the pseudo-first-order rate constant, q

_{e}is the amount of adsorbed species per mass of adsorbent in equilibrium (mg/g), and t is time (min). An integrated form of Equation (1) gives Equation (20):

_{e}and k

_{1}from nonlinear fitting. On the other hand, the linear form given by Equation (21) is mostly applied and cited in the literature.

_{0}(mg/L) is the initial concentration, c

_{e}(mg/L) is the concentration in equilibrium, V (L) is the volume of the investigated liquid and m (g) is the mass of the sorbent. The units mentioned here are the most common units referred to in the literature, but different units can be used as well.

#### 4.2. Pseudo-Second-Order Kinetic Model (PSOM)

_{2}is the pseudo-second-order rate constant. Unlike k

_{1}, which has always the dimension reciprocal to time, the constant k

_{2}may have various dimensions (mg/g.min, g/g.min, mmol/g.min, etc.), depending on the definition of q [20,63].

_{e}and k

_{2}.

_{e}from changes in the bulk concentration (c

_{e}), particularly in batch systems, where the initial concentration changes significantly over the sorption experiment.

_{0}is the initial molar concentration of the solute, q

_{m}is the maximum sorption capacity of the adsorbent, M

_{w}(g/mol) is the molar weight of the solute, V (L) is the volume of the solution, c

_{e}is the equilibrium molar concentration of the solute and θ

_{e}is the equilibrium coverage fraction. From the general model (Equation (26)), Azizian derived the PFOM (Equation (20)) and PSOM (Equation (25)) as special cases of the general model (Equation (26)).

^{2}values strongly depends on the selection of data. If the data set does not include enough values related to the short duration of the experiment but consists of many more data for long periods, the long-term data have a higher weight on the statistical parameters. Canzano [71] showed that the PSO model better fits data including measurements from systems that approach an equilibrium. Another problem where mathematical procedures can overlap the physical reality is associated with a selection of the equation used from a fitting. As mentioned above, an integrated form of Equation (5) can be expressed by four different equations. It was shown that application of those equations to the same set of data leads to significantly different R

^{2}values in the range from 0.862 to 1. On the basis of this finding, the authors proposed, first, to minimize the number of data points nearest to equilibrium and, second, to use nonlinear fitting by Equation (24) [71].

#### 4.3. Diffusional Models

#### 4.3.1. Douven’s Model

_{e}is the total number of adsorbed entities at equilibrium, D

_{eff}is the effective diffusion coefficient, and R

_{p}is the radius of the adsorbed entity.

_{1/2}) needed for an adsorption of half of the full adsorbent capacity and showed that the PSO model is equivalent to Equation (28):

#### 4.3.2. Weber–Morris Intra-Particle Diffusion (IPD) Model

_{ipd}(mg/g.min) is the rate constant for intra-particle diffusion, and C (mg/g) is a constant related to the boundary layer thickness. This model does not distinguish among various processes that can simultaneously occur during solute sorption and considers just a single rate-limiting process of diffusion.

#### 4.4. Adsorption Kinetics in Flow-through Systems (Fixed Bed Adsorption)

- A mass transfer from the bulk of the fluid to the surface of the adsorbent through the boundary layer around the particle.
- An internal diffusion through the pores of the adsorbent.
- An adsorption onto the surface of the adsorbent.

_{b0}(mg/L) is the initial concentration of oil in the bulk solution, ξ

_{m}is the batch capacity factor, τ (s) is the time constant characterizing diffusion through a boundary layer, V (L) is the volume of the solution (a volume of a reactor), ε

_{b}is the bulk porosity, K

_{f}(cm/s) is the mass transfer coefficient of the boundary layer, and a

_{f}(cm

^{2}/cm

^{3}) is the external surface area of sorbent per volume of sorbent given by the relation a

_{f}= 3/r

_{p}, where r

_{p}(cm) is the particle (sorbent) radius. The adjustable parameters τ

_{f}and K

_{f}are obtained from a fit of experimental data by Equation (32).

#### 4.5. Bed Depth Service Time (BDST) Model

_{b}(min)) at the breakthrough point in the depth (X, (cm)) of the packed column. In this model, intra-particle diffusion is neglected, and direct adsorption of solutes on the adsorbent surface is assumed. The original BDST model was proposed by Bohart et al. [80] in the following form (Equation (33)):

_{0}(mg/L) is the dynamic bed capacity, v (cm/h) is the linear flow rate, defined as the ratio of the volumetric flow rate Q

_{vol}(cm

^{3}/h) to the cross-sectional area of the bed, S

_{c}(cm

^{2}); c

_{0}and c

_{b}(mg/L) are the initial and breakthrough concentrations of the solute, respectively, and k

_{ads}(L/mg.h) is the adsorption rate constant.

^{2}/s), breakthrough curve, solute distribution parameter (Dg), Biot number (Bi), and Stanton number (St).

## 5. Interfacial interactions and Wettability of Surfaces

^{2})) or the surface tension (γ (N/m)). Except for intramolecular (covalent, ionic) bonds, intermolecular interactions, generally called Van der Waals interactions, contribute to both internal and interfacial energy. The energy of those interactions strongly decreases with the distance between interacting bodies (~1/r

^{6}) and with temperature (1/T) [17]. The range of the forces is approximately from 0.15 to 1 nm, and the energy depends on the type of interactions: (i) dipole–dipole interactions ~5 to 25 kJ/mol, hydrogen bonds ~10 to 40 kJ/mol, (ii) ion–dipole interactions ~40 to 600 kJ/mol, (iii) ion-induced dipole interactions ~2 to 10 kJ/mol, and (iv) London (dispersion) interactions ~0.5 to 40 kJ/mol. These interactions are time-independent and act instantaneously; therefore, the time dependence of an adsorption process is governed by additional, mostly diffusional phenomena [96].

#### 5.1. A Droplet Placed at a Smooth and Rough Surface in Air

_{SV}is the interfacial tension at the solid–vapor interface, γ

_{SL}is the interfacial tension at the solid–liquid interface, and γ

_{LV}is the surface tension of the liquid, which is in equilibrium with a saturated vapor phase.

_{adv}and Θ

_{rec}are advancing and receding contact angles, respectively.

#### 5.2. A Droplet Placed at a Smooth and Rough Surface in Water

_{OA}, γ

_{WA}, and γ

_{OW}are the oil/air, water/air, and oil/water interface tensions, respectively, and Θ

_{O}, Θ

_{W}, and Θ

_{OW}are the contact angles of oil in air, water in air, and oil in water, respectively. Equations (38)–(40) indicate that a hydrophilic surface in air is also oleophilic in air because γ

_{OA}<< γ

_{WA}, and hydrophilic surfaces in air behave as oleophobic in water, as is evident from Equation (40) [107].

_{OW*}and Θ

_{OW}are the contact angles on the oil droplet on the rough surface and the smooth surface in the water surroundings, respectively. The Wenzel state corresponds to the situation where an oil droplet is attached to a rough, superhydrophobic surface, which has pores filled with air, and capillary forces are strong enough to suck adjacent oil into the pores [106]. In this case, the surface is fully wetted by oil. The Cassie state corresponds to the situation where valleys on the surface are filled with water, which suppresses the penetration of oil into those cavities, and oil droplets can only be in contact with the “pin” objects. This situation leads to an enhanced oleophobicity of such surfaces [107]. The capability or incapability of water to fill cavities at a given surface plays an important role in the final wettability of the surface by oil. This capability is given by the surface free energy and surface topology. Superhydrophobic surfaces are characterized by a static contact angle of water of over 150°, and the rolling angle is below 6–10°. Superhydrophobic or superoleophilic behavior is a consequence of a combination of the chemical structure of the material and controlled roughness [108,109,110,111].

## 6. Oil/Water Separation Techniques according to the Wettability of Solid/Oil/Water Interfaces

#### 6.1. Free (Stratified) Oil Separation from Water (Oil Spills): Superhydrophobic and Superoleophilic Surfaces

#### 6.2. Oil/Water Emulsions—Separation of Oil Droplets by Membrane Filtration. Superoleophobic and Superhydrophilic Surfaces

#### 6.3. Oil/Water Emulsions—Separation of Oil Droplets by Sorption (Preferable in Batch Systems). Superhydrophobic and Superoleophilic Surfaces

#### 6.4. Oil/Water Emulsions—Separation of Oil Droplets by Demulsification-Coalescence (Deep-Bed Filtration). Balanced Hydrophobic and Oleophilic Surfaces

_{ad}) between different phases can be calculated by the Young–Dupre equation (Equation (43)) [119]:

_{Ly}is the interfacial tension between the dispersed liquid (y-oil) and the surroundings (water, or y-air). The dependence of the ratio of W

_{ad}(water/iso-octane) on the surface energy has a sigmoidal character, and the curve characterizes two distinguished regions: the region with low surface energy in which the adhesion of iso-octane droplets is stronger than the adhesion of water (below ~25–27 mN/m) [119] and the region with high surface free energy (over 30 mN/m) where the adhesion of water is stronger than the adhesion of oil. Materials with high-energy surfaces are not suitable for oil adsorption because they cannot replace adsorbed water. For this reason, the surfaces of coalescing media should be hydrophobic and oleophilic, which enable good wetting and sufficient contact time, thereby promoting the coalescence of droplets [119].

^{−6}M, at which the sand surface has an opposite charge to that of the emulsion droplets. Below this concentration, the deposition efficiency is reduced due to the electrostatic repulsion between the negatively charged droplets and the sand. Above this concentration, the deposition is again reduced due to the surfaces of sand and droplets being positively charged.

## 7. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**The most important factors affecting the adsorption process (adapted from: Muftah H. El-Naas [16]).

**Figure 2.**Schematic representation of the different types of pores. (adapted from: Muftah H. El-Naas [16]).

**Figure 3.**SEM micrograph of a common as-produced LDPE pellet (

**A**), SEM micrograph (

**B**) and profilometry image (

**C**) of LDPE powder prepared by grinding (unpublished results).

**Figure 4.**Steps involved in the overall adsorption process (adapted from: Muftah H. El-Naas [16]).

**Figure 5.**A schematic representation of the movement of the adsorption zone and the related breakthrough curve (adapted from: Muftah H. El-Naas [16]).

**Figure 6.**Schematic diagrams of a liquid droplet on various surfaces. (

**a**) Flat surface, (

**b**) Wenzel state, (

**c**) Cassie–Baxter state.

**Figure 8.**Influence of the solid surface energy on the work of adhesion and wetting of iso-octane and water.

**Figure 9.**Influence of substrate surface energy on oil-in-water emulsion separation performance for (

**a**) pore size < dispersed droplet size, pressure drop 2.2 bar and (

**b**) pore size ≥ dispersed droplet size, pressure drop 1.1 bar.

The sum of the squares of the errors (SSE) | SSE = ${\sum}_{\mathrm{i}}^{\mathrm{N}}{\left({\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{cal}}-{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}\right)}^{2}$ |

The sum of the absolute errors (SAE) | SAE = ${\sum}_{\mathrm{i}}^{\mathrm{N}}\left({\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{cal}}-{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}\right)$ |

The average relative error (ARE) | ARE $=\frac{100}{\mathrm{N}}(\frac{{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{cal}}-{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}}{{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}}$) |

The hybrid fractional error function (HYBRID) | HYBRID = $\frac{100}{\mathrm{N}-\mathrm{p}}$ ${\sum}_{\mathrm{i}}^{\mathrm{N}}\left(\frac{{\left({\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{cal}}-{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}\right)}^{2}}{{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}}\right)$ |

Marquardt’s percent standard deviation (MPSD) | MPSD = $100\times \sqrt{\frac{1}{\mathrm{N}-\mathrm{p}}{\sum}_{\mathrm{i}}^{\mathrm{N}}{\left(\frac{{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}-{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{cal}}}{{\mathrm{Q}}_{\mathrm{a}\mathrm{i}\mathrm{meas}}}\right)}^{2}}$ |

Sorbent | Emulsion | Outputs | Models | Ref. |
---|---|---|---|---|

Optipore L493, Lewatit AF5, Amberlite IRA958 (synthetic resins), batch system | Synthetic PW emulsion, Octane 95, nonionic surfactant, 25–50 ppm | Below 2 ppm | Langmuir, Freundlich, Flor–Huggins isotherms, Toth, Dubinin–Radushkevich isotherms, PFO, PSO models, intra-particle diffusion model | [19] |

Calcium alginate hydrogel modified by maleic anhydride, batch system | Crude oil, sonication, no surfactant, 1 M NaCl, 100–500 ppm | 80% removal efficiency | Freundlich, BET isotherms, PFO and PSO models, intra-particle, diffusion model | [33] |

Fe_{3}O_{4} magnetite nanoparticles grafted in silica (SiO_{2}), batch system | Gasoline oil, 500–4000 ppm | >90% removal efficiency | Langmuir, Freundlich isotherms | [82] |

Iron Oxide/ Bentonite Nano Adsorbents, batch system | Diesel oil, 66 to 170 ppm, non-ionic surfactant | 67% removal efficiency | Langmuir, Freundlich, Toth | [83] |

Thermally reduced graphene and graphene nanoplatelets, batch system | 200 ppm and adjusted salinity | TRG: 1550 mg oil/g GNP: 805 mg oil/g | Langmuir, Freundlich, Dubinin–Radushkevich, Tempkin isotherms | [84] |

Zeolitic imidazolate, batch system | Soybean oil, 5 wt% | 6633 mg/g | Langmuir, Freundlich isotherms, PFO and PSO | [85] |

Hydrophobic silica aerogels, batch system | Vegetable oil, motor oil 10W30, light crude oil, Tween 80 | Freundlich isotherm | [86] | |

https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/lanthanum (accessed on 26 February 2021), zirconium and cerium embedded chitosan/gelatin, batch system | 4 wt% cutting oil | Langmuir, Freundlich, Dubinin–Radushkevich, Tempkin isotherms | [87] | |

Activated carbon, bentonite, deposited carbon, batch system | Produced wastewater from Gamasa Petroleum Company, 600–1012 ppm | Up to 98% removal efficiency | Langmuir, Freundlich isotherms | [88] |

Polyether polysiloxane, batch system | Oil-flooding-produced water from the Daqing oil field, 400 ppm | 90% removal efficiency | Langmuir, Freundlich isotherms | [89] |

Regranulated cork, flow system | Sunflower oil and saponified matter, 200 ppm | <15 ppm | Freundlich isotherms and linear partitioning models | [79] |

Magnetic nanosorbent polydimethylsiloxane, zinc oxide, batch system | 2 wt% diesel, span 80 | 96% removal efficiency | PFO and PSO, intra-particle diffusion models | [90] |

Hydrophilic hierarchical carbon with TiO_{2} nanofiber membrane, batch system | Engine oil, cooking oil, hexane, toluene, 1000 ppm, Surfactant SDS 200 ppm | 95.4% removal efficiency | BET isotherm | [91] |

Sunflower pith, flow system | Artificial reservoir brine, 0.1, 2.0, and 20.0 g/L of oil | Over 99% removal efficiency | PFO, PSO, Modified logistic model | [92] |

Magnetic ZnFe2O4–Hydroxyapatite Core–Shell Nanocomposite, flow system | Produced water containing oil from 100 to 10,000 ppm | 98% removal efficiency | Thomas–BDST model, Yoon–Nelson model | [93] |

Oleophilic natural organic-silver nanocomposite, batch system | Motor oil, 200–1000 ppm | NA | Langmuir, Freundlich, Temkin isotherms, PFO and PSO | [94] |

Chitosan/Mg-Al hydroxide composite, batch system | 4 wt.% cutting oil | 78% removal efficiency | Langmuir, Freundlich, Dubinin–Radushkevich, Tempkin isotherms | [95] |

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Sobolčiak, P.; Popelka, A.; Tanvir, A.; Al-Maadeed, M.A.; Adham, S.; Krupa, I.
Some Theoretical Aspects of Tertiary Treatment of Water/Oil Emulsions by Adsorption and Coalescence Mechanisms: A Review. *Water* **2021**, *13*, 652.
https://doi.org/10.3390/w13050652

**AMA Style**

Sobolčiak P, Popelka A, Tanvir A, Al-Maadeed MA, Adham S, Krupa I.
Some Theoretical Aspects of Tertiary Treatment of Water/Oil Emulsions by Adsorption and Coalescence Mechanisms: A Review. *Water*. 2021; 13(5):652.
https://doi.org/10.3390/w13050652

**Chicago/Turabian Style**

Sobolčiak, Patrik, Anton Popelka, Aisha Tanvir, Mariam A. Al-Maadeed, Samer Adham, and Igor Krupa.
2021. "Some Theoretical Aspects of Tertiary Treatment of Water/Oil Emulsions by Adsorption and Coalescence Mechanisms: A Review" *Water* 13, no. 5: 652.
https://doi.org/10.3390/w13050652