The process of surface adsorption as an effective, efficient and economic method has been widely studied for water purification and has been used in various fields for a long time. Also, in recent years, a large number of new absorption processes were developed to increase the efficiency of removing organic and inorganic pollutants from water. Here, advanced surface absorption methods including surface absorption/catalytic oxidation processes, surface absorption/catalyst reduction, and surface absorption processes with new types of adsorbents such as biomimetic adsorbents, which are specially designed for water purification in terms of composition, structure and properties, have been reviewed. Is.

Surface adsorption process

Before explaining the process of surface absorption and its importance in the field of water and wastewater treatment, it is necessary to present some information about organic pollutants. The term organic pollution or organic pollution is used when we are faced with a large volume of organic compounds. This type of pollution can be found in domestic sewage, municipal sewage, industrial effluents and agricultural sewage.

Industrial wastewater is also referred to as industrial wastewater, such as food processing, paper and pulp production, agriculture and aquaculture. During the process of decomposition of organic pollutants, the rate of consumption of oxygen dissolved in water may be higher than the amount replaced for it, which causes oxygen deficiency and causes severe consequences for aquatic organisms.

Wastewater containing organic pollutants can contain large amounts of suspended solids, which reduces the available light for photosynthetic organisms and changes the conditions of the river bed, making the habitat of many vertebrates and aquatic organisms unsuitable. Organic pollutants include poisons and pesticides, chemical fertilizers, hydrocarbons, phenols, plastics, biphenyls, detergents, oils, greases, pharmaceuticals, proteins and carbohydrates.

Organic and toxic pollutants cause many environmental problems for our surroundings. The most common organic pollutants are called persistent organic pollutants (POPs). They have received much attention due to their toxicity, durability, ability to be transported in the long term, and also due to their ability to accumulate biologically in the body of animals, they can travel long distances and remain in the body of living organisms. POPs are carbon-based chemical compounds and mixtures that contain industrial chemicals such as polychlorinated biphenyls (PCBs), dibenzo-p-dioxins and chlorinated dibenzofurans (PCDD/Fs), and some organochlorine toxins (OCPs) such as hexa Chlorobenzene (HCB) or dichloro-diphenyl-trichloroethane (DDT), dibenzo-p-dioxins and dibenzo-p-furans.

PCDD/Fs are released into the environment as byproducts of various processes such as waste incineration or metal production. Many of these compounds are used in large quantities and due to their stability, they can accumulate and multiply in the body of living organisms.

So far, many effective techniques for removing organic and highly toxic compounds from water have received much attention from researchers. A number of these methods, such as coagulation, filtration with coagulation, sedimentation, weighting, surface adsorption, ion exchange, reverse osmosis and advanced oxidation processes, have been used to remove organic pollutants from polluted water and wastewater. Researchers have realized that these methods have limitations.

Because in most cases, they require a lot of initial capital and operating costs. On the other hand, ion exchange and reverse osmosis methods are more attractive processes because some of these pollutants can be recovered by removing them from the produced wastewater. However, reverse osmosis, ion exchange and advanced oxidation processes do not seem to be economically viable due to high initial capital and operating costs.Among the possible methods for water purification, the process of surface absorption using solid adsorbents has shown that it can be one of the most effective methods for purifying and removing organic pollutants in wastewater treatment. Compared to other methods, surface absorption has advantages such as the simplicity of the design and the need for a small initial capital regarding the initial costs and the land required for the construction of the design. Surface absorption process has been widely used for industrial wastewater treatment to remove organic and inorganic pollutants and has attracted the attention of many researchers. In recent years, the research on cheap adsorbents that have good capacities in binding to pollutants has intensified.

Materials that are available at the place of the process, such as natural materials, agricultural and industrial waste, can be used as cheap absorbents. The activated carbon produced from these materials can be used as an absorbent for water and wastewater treatment.Surface adsorption is the process of adsorption of atoms or molecules in a liquid or gas in contact with a solid surface. This absorption occurs by adhesion forces.

Surface adsorption is divided into two types, chemical and physical, physical surface adsorption begins with van der Waals forces and ends with strong ionic and metallic forces, and covalent force is chemical surface adsorption that is created by performing a reaction.In physical absorption, the greater the temperature difference between the solid surface and the absorbent material, the sooner the absorption occurs because the thermal energy of the material is the driving force for absorption on the surface.

Surface adsorption isotherms

Measurement of surface absorption is done at constant temperature. The standard for measuring the amount of adsorbed on the adsorbent is in terms of pressure for gas adsorbed and in terms of concentration for liquid adsorbed. Adsorption isotherms are mathematical relations that show the amount of substance absorbed on the surface. The four known adsorption isotherms are:

Langmuir adsorption isotherm
BET adsorption isotherm
Temkin adsorption isotherm
Freundlich adsorption isotherm

Adsorption is the process of atoms, ions, biomolecules, or gas, liquid, or dissolved solid molecules sticking to the surface. In this process, a thin layer of adsorbed material is created on the surface of the adsorbent. This process is present in many physical, biological and chemical systems. Due to the fact that nano materials have a high surface-to-volume ratio, in recent years, with the expansion of studies in the nano field, the phenomenon of absorption has been investigated and welcomed more than before.

Physical surface absorption

In physical adsorption, due to van der Waals forces, there is no type of electron transfer. The absorption power depends on the visible physical properties of the adsorptive, so the physical properties of the adsorbent have no effect on the absorption process. In physical surface adsorption, molecules are stuck to the surface of the adsorbent material by van der Waals forces. The amount of adsorption energy is usually less than 50 kJ/mol. This type of absorption happens quickly.Because the activation energy is close to zero and the resulting heat is very low, this process is mainly visible at low temperatures. By increasing the pressure, absorption can be increased to the extent that absorption takes place in several layers. In physical absorption, all substances can act as absorbers or absorbers. As a result of all these features, physical absorption is widely used in industry.

Chemical surface adsorption

In chemical adsorption, the absorbed molecules are kept on the surface of the material with chemical bonds. In the process of forming a bond with the adsorbent, the molecules that are chemically absorbed undergo a change in the internal electron configuration. The bonds inside some molecules are stretched, weak, and even the bonds of some of them are broken. So it can be said that in this absorption electron transfer is done between the particle and the surface.

In this absorption, the activation energy is high, the absorption speed is low, and the heat of absorption is high due to the formation of chemical bonds. Also, the nature of absorption depends on the properties of the absorbent and absorbent. Increasing the pressure also slightly changes the amount of absorption.

The difference between physical and chemical absorption

As mentioned, the types of surface absorption can be divided into two categories: physical and chemical. There are several differences between physical and chemical adsorption, which include:

In physical adsorption, weak van der Waals forces cause adsorption, but in chemical mode, chemical bonds cause adsorption.

Enthalpy in the physical state is lower than the enthalpy of the chemical process.

Physical absorption occurs at a temperature lower than the boiling point of the absorbent, but in the chemical state, absorption can also occur at higher temperatures.

In physical absorption, the amount of absorption per unit area increases by increasing the pressure on the absorbent body. But in the chemical state, with increasing pressure on the absorbing body, the amount of absorption per unit area decreases.

In the physical state, the amount of absorption depends on the properties of the absorbent material. In case of chemical absorption, the amount of absorption depends on the properties of both bodies (absorbent and absorbed).

The activation energy is not so much involved in the physical state; While it is involved in the chemical absorption of activation energy.

Physical absorption takes place in several layers. But in the chemical state, it leads to a maximum of one layer.

Description of surface adsorption

Among the distinguishing points of surface studies from volume, it can be said that the surface is very quickly covered with absorbent material. We justify this sentence with the following relation:

where P is the pressure, T is the temperature, M is the mass and N is the number of particles. Also, k is Bollensmann’s constant and Z is the number of collisions per unit area per unit time.

Factors affecting absorption power

The absorption power of a substance depends on the following factors
Type of absorbed and absorbent material
The state of absorbed and absorbent matter
contact surface

Calculation of surface properties of non-ideal two-component liquid mixtures


After studying and predicting the surface tension of two-component mixtures, in this chapter we have calculated the surface properties of some mixtures. These properties provide useful information about the thickness of the surface layer, the structure and behavior of the surface phase of solutions. Also, in this section, some of these surface properties (relative surface Gibbs adsorption, mole fraction of components on the surface and thickness of the surface layer (as a new parameter)) and surface thermodynamic parameters (enthalpy and entropy of the surface) in non-ideal two-component mixtures are introduced in this section. we pay

The composition of the interface of a two-component mixture is usually different from its composition in the bulk phase, and the absorption of a chemical species can lead to a substantial reduction in the surface tension of the system [154-155]. Surface absorption is one of the important surface parameters that provides us with valuable information about the surface behavior of liquid mixtures. The surface adsorption of a compound at the boundary between two phases leads to a change in the composition and concentration of the interface layer and the bulk phase adjacent to it. In fact, predicting the amount of substance that is absorbed on the surface is one of the main goals in surface chemistry. The Gibbs adsorption isotherm equation is one of the most important equations in surface chemistry that was used in this research [35].

The concentration of the components in the bulk and surface phases of the solution is different in non-ideal mixtures. In order to measure the concentration of components in the surface phase, quantitative theories have been presented. The best model presented in this field (calculation of mole fraction of components on the surface), developed Langmuir modelIt was presented in 2001 by Amigo and his colleagues. This model is used to analyze the surface tension of some two-component liquid mixtures and calculates the surface concentration by relating the surface tension of binary mixtures to the mass composition [36-37].

Another surface property that we will describe and determine in this chapter is the calculation of the thickness of the surface layer in the studied two-component mixtures. So far, several methods (taking into account the limitations) have been presented to determine this surface property. In this project, using the method presented by Dr. Bagheri and his colleagues in 2011, the thickness of the surface layer of two-component mixtures was determined [38-39].

Surface enthalpy and entropy and standard surface Gibbs free energy changes are among the surface thermodynamic properties that are calculated and analyzed for the systems studied in this chapter by using surface tension values ​​at different temperatures.

First, we will give a full description of how to determine these surface properties for the studied two-component mixtures.

2-4- Calculation of the mole fraction of components on the surface using the UNIFAC model and the developed Langmuir model

With the development of Spro-Perrosnitz model and UNIFAC model, it has been possible to calculate the concentration of components on the surface in addition to predicting the surface tension of mixtures.

In addition to this method, it is possible to calculate the mole fraction of components on the surface by using another new model called the extended Langmuir model, which we will describe first.

4-2-1- Extended Langmuir (EL) model

In 2001, Amigo and his colleagues[36] presented an equation for the relationship between the surface tension of the mixture and the intermolecular interaction in the mass by correcting the Langmuir isotherm. The reason for using the Langmuir equation and correcting it to investigate the surface tension behavior of two-component mixtures was that the Langmuir isotherm is limited in the investigation of the surface tension of two-component mixtures in high mole fractions. The developed Langmuir model has been used to study the surface tension of many two-component mixtures and good results have been obtained.

The details of how to obtain the relationships of this model are given in different sources; Therefore, in order to avoid repeating those relationships, we will express the final equations used in the project [156, 154, 36]:

In the above relationship, the surface pressure is [2] which is the amount of reduction in the surface tension of the solvent (component with higher surface tension).

) when component B (as solvent) is added, it is defined as follows:



The surface tension difference of pure components is calculated through the following equation:

α is the adjustability parameter that is influenced by the interaction between species A and B and is the value in the case that this interaction causes very small (negligible) changes in the structure and adhesion force during mixing. β is another adjustable parameter that indicates the relationship between the surface composition and the mass of the mixture and expresses the amount of solvent reluctance [3] to migrate to the surface.

In relation (1-4) and the volume fraction are types A and B, which are expressed as follows:

which are the molar volumes of species A and B in this regard.

After obtaining the values ​​of α and β from fitting equation (1-4), the value can be calculated from the following equation which is one of the results of the EL model:

We also have:

In this model, according to the results obtained from equation (1-4) and calculating β and putting it in equation (4-5), the value is calculated and according to the above relationship and considering that, the values ​​can be deducted Get the moles of components on the surface.

In addition to calculating the mole fraction of components on the surface using the results of the EL model, different molecular interpretations can be presented with α and β values ​​[157,36]:

a) Whenever and, from the point of view of the EL model, the target system is ideal and the partial volume of each component is the same in both phases (bulk and surface) and intermolecular interaction has no effect on the surface tension value.

b) If it is and, the concentration of component B in the surface phase is higher, and in fact, the solvent resistance has increased.

c) If it is and, in this case, the surface of the mixture is almost similar to the mass in terms of components.

d) In the case where and, in this case, there is a special interaction between molecules A and B, the stronger the interaction, the larger the value of α (this interaction can be due to the size of the molecule, symmetry, or due to steric hindrance) arise).

EL model has been successful in many systems and has provided good results. But despite the advantages such as relating the surface tension of the mixture to the properties of the solution mass, there are criticisms such as extracting parameters through correlating experimental data and not presenting results with a specific trend. In addition, it may not be correct to separate the complexity of structural effects and intermolecular interactions in the solution by two separate factors of intermolecular forces (α) and solvent repulsion factor (β). Another drawback of the EL model is its inability to justify the behavior of systems that exhibit aneotropic behavior

4] (Systems whose surface tension difference is less than one unit have an aneotropic behavior) [36].

4-3- Investigating the Gibbs relative adsorption process

In the case where we have a pure liquid, the surface free energy is reduced as much as possible. This process occurs by reducing the surface area through the application of internal attraction forces on the surface molecules. But in the situation where there are two or more components in the solution, the process of minimizing the surface is different and more complicated than its pure state, because there are different molecules in the solution that have different gravity, shape and molecular volume. In this case, the molecules that have a stronger force field tend to the solution mass, and those that have a weaker force field tend to the surface, and this causes us to have a different concentration of each component (solvent) on the surface and mass.

This different concentration of a dissolved component on the surface or, in a more precise sense, the amount of molecules (moles or number of molecules, weight, volume) that accumulate in a unit surface (1m2) is called absorption [1].

4-3-1-Gibbs adsorption isotherm and relative adsorption

Predicting the amount of substance adsorbed on the surface is one of the main goals in surface chemistry. Adsorption and surface free energy are related through Gibbs adsorption law. If it is equal to the number of absorbed moles (additional moles) per surface unit:

Using the definition of internal surface energy from a thermodynamic point of view, the Gibbs-second theorem and the definition of absorption under constant temperature conditions, we arrive at a relation for the Gibbs adsorption isotherm [159,158,1]:

Equation (4-8) in a two-component system is expressed as follows:

Subordinate 1 is related to solvent and subordinate 2 is related to solute. According to Gibbs phase law, the chemical potential of each component on the surface and mass are equal, that is, in equation (4-9) we have two unknowns and which makes it almost impossible to solve. In order to reduce the number of unknown parameters, we place the Gibbs separator plate in a place where it is, and in fact, in this case, the additional concentration of the solvent at the level becomes zero, therefore:

In this case it can be said that is called relative absorption of Gibbs[5]. The chemical potential of the soluble species is determined as follows:

By inserting equation (4-13) into equation (4-11), we will get an expression that enables calculation by having surface tension and activity data (in fact, activity coefficient):

Equation (14-4) called Gibbs adsorption isotherm is one of the most important equations in surface chemistry. According to this equation, the numerical value in different systems can be positive, negative or zero according to the surface tension changes with the activity of the components. Positive values ​​indicate the absorption of the solute on the surface (such as surfactants in water), negative values ​​indicate the removal of the solute from the surface (salt solution) and the zero value of the same distribution of the solute on the surface and the mass of the solution (solvents with near-ideal behavior) ) is showing. In general, the absorption process becomes meaningful when we have a system with more than one component; In this case, the solute is absorbed on the surface with a lower surface tension, this process continues until the surface layer is saturated with solute molecules [159].

4-4- Calculating the thickness of the surface layer in two-component mixtures

In this section, it has been tried to present a new equation for measuring the thickness of the surface layer [6] of mixed solvents. According to the Gibbs theory, the volume of the surface layer is assumed to be zero, the first problem that comes to mind in obtaining this equation is to challenge this assumption, because really the part that is considered as the surface layer is the arrangement, configuration[7] and The density of its molecules is different and it does not have a volume equal to zero and has a surface area and thickness (Figure 1-4).

In the following, before presenting the method of obtaining the equation for calculating the thickness of the surface layer, we will review the previous works, which are also very limited [1, 160]. In 1996, Yang and Lee [8] [165] presented a method to calculate the interfacial thickness in two completely immiscible liquids. Their method is a combination of surface tension dependence with pressure and Kahn-Hilliard theory [9]. For the first time, they expressed the volume of the interfacial layer in terms of the interface area and the thickness of the interfacial layer (l):

In 2003, Panayato [10]]161[, presented a relationship based on the relative Gibbs equation and the Flory-Huggins theory [11] to calculate the surface thickness of polymer mixtures. In solving this equation, the Flory-Huggins interaction parameter must be calculated.

In 2009, Doillard [12]]162[, with the help of ellipsometry and surface tension data, was able to obtain a method to calculate the surface thickness of pure species. Doillard’s results show that the thickness of the surface layer is between 1.1 and 2.4 times the size of a molecule.

In 2010, Dr. Uzirian and his colleagues [163], from Bu Ali Sina University, presented an equation for measuring the surface thickness of two-component systems in the dilute region (x<0.1). brought.

In 2011, Dr. Bagheri and his colleagues [38] presented a method to provide an equation for determining the thickness of the surface layer, which is similar to Panayato’s (2003) method, with the difference that instead of the Flory-Hagenis theory [13], they used the UNIFAC model to calculate the activity coefficient. The method of obtaining related relations is described below.

In this model, in addition to equation (4-14), another expression of relative Gibbs absorption is provided:

In the previous section we had that. Also, for component i, the following relationship exists between the number of total moles, the number of moles in the mass and the number of moles of species on the surface:

According to the definition of concentration, for the number of moles of component i on the surface we have:Here, V is the total volume of the solution. By multiplying both sides of equation (17-4), we have:

If we subtract equation (4-19) from equation (4-18), the following result is obtained:

In the next step, we divide the sides of the equation (4-20) by the area of ​​the surface, and according to the definition of the incremental function of the surface, we will have:

The resulting new term is another expression of Gibbs relative absorption, , . According to this matter, the equation (20-4) can be rearranged as follows [1, 161].

From where it is, we will have:

If we consider and as the volume component corresponding to component 2 in the surface phase and bulk phase, respectively, we will have:

In the above equations, the number of moles of component i and and is the molar volume of the components.

For a homogeneous surface layer, if t is the thickness of the surface layer, the area of ​​the surface layer and the volume of the surface layer, we will have the relationship (Figure 1-4).

Assuming that the molar volume in the mass and the surface is considered the same, the number of moles of component 2 on the surface is determined from the following equation [161]:

or (4-26)

And similarly we have:

By integrating equations (4-14), (4-23), (4-26) and (4-27) and rearranging, we will have:

Or in other words we will have:

Equation (29-4) is a valuable relationship that relates the thickness of the surface layer to the properties of pure components (molar volume), concentration of components (on the surface and mass) and surface tension. It is possible to calculate these parameters using Sprue-Prosnitz or EL and UNIFAC model. On the other hand, if the thickness of the surface layer can be determined through an independent laboratory method (such as ellipsometry [14]), the relative absorption of Gibbs or the concentration of components on the surface can be experimentally calculated using equation (29-4). 162-161 [.

5-4- Surface thermodynamic functions of two-component mixtures

The effect of temperature on the behavior of the surface and volume properties of aqueous solutions is often used to determine information about the structural effects of the solute on the solvent structure [46]. In this project, the following surface thermodynamic functions were determined for the studied systems:

A) Surface formation entropies

The entropy of surface formation per surface unit ( ) is calculated from the thermodynamic equations of Clapeyron[15], which have been changed in the direction of liquid surfaces [164,46]:

In this regard, it shows the entropy changes per unit area due to the formation of the interface, and according to equation (4-30), the entropy of the surface is equal to the negative of the temperature coefficient of surface tension.

b) surface enthalpy

Since the surface enthalpy ( ) is equal to the sum of the surface free energy required to increase the surface (surface tension (s)) and the latent heat (q) to maintain isothermal conditions: the surface enthalpy per unit surface is calculated through the following equation] 165-164 [:

c) Standard Gibbs free energy changes

Changes in the standard Gibbs adsorption energy, which represents the energy required for the movement of a soluble molecule from the mass to the surface, have been determined using the results of the EL model and the following equation for some studied systems [37]:

In this equation, α and β are the adjustable parameters of the EL model, v1 and v2 are the molar volumes of species 1 and 2.

Gibbs free energy can be used to quickly evaluate an adsorption process. Negative values ​​indicate that absorption occurs spontaneously and is thermodynamically stable, and cases with positive values ​​indicate that this process is an unfavorable and spontaneous transition [166].

6-4- Calculation of surface properties of two-component water/alcohol mixtures (high polarity system)

In this section, we have calculated the surface properties of two-component mixtures of aliphatic alcohols (methanol, ethanol, 1-propanol and 2-propanol) with water in the temperature range (293.15-323.15) K with a temperature step of 5 K [96]. Experimental surface tension data of two-component mixtures of water/alcohols have been collected from the article of Alvarez and his colleagues [67].

The values ​​of species activity coefficients in the liquid mass phase were determined using the UNIFAC model. The values ​​of relative Gibbs absorption, mole fraction of particles on the surface and thickness of the surface layer were calculated for each of the water/alcohol systems at all temperatures. The calculated values ​​of surface properties are presented by using the models introduced in tables 1-4-1 to 4-4-7 respectively for water/methanol, ethanol, 1-propanol and 2-propanol systems at different temperatures.

The largest changes in relative Gibbs absorption occur at low concentrations of solutes, and in this concentration range, there are very few surface tension data for liquid mixtures at different temperatures, and as a result, we need computational methods to estimate these surface tension data. In order to solve this problem, by using the adjustable coefficients of the appropriate FLW equation (Equation (1-56)), which is obtained from fitting the experimental data for each two-component system, it is possible to determine the surface tension values ​​for each mole fraction of the case. Calculate the requirement in low concentrations. For this purpose, the surface tension values ​​of the studied two-component mixtures at low concentrations (x2<0.5) have been calculated (tables 1-1-4 to 4-4-7). The results for the surface properties of the desired two-component systems can be seen in graphs 1-4 to 4-7.

1Extended Langmuir model

[2] Surface pressure

[3] lyophobicity

[4] Aneotropic

[5] Relative Gibbs Adsorption

[6] Thickness of surface layer

[7] Configuration

[8] Yang & Li

[9] Cahn-Hilliard

[10] Panayitou

[11] Flory–Huggins

[12] Douillard

[13] Flory-Huggins

[14] ellipsometry

[15] Clapeyron