ADSORPTION OF CERTAIN ANIONS AND CATIONS IN WATER RESOURCES USING A MODIFIED LOCAL KAOLIN SORBENT

ABSTRACT

This research paper investigated the colloid-chemical properties of adsorption of some anions (chlorine, sulfur) in water resources of sorbent materials obtained by modifying AKS-30 and AKS-70 brands of kaolin, which are considered local raw materials of our republic. The colloidal-chemical properties of these sorbents were studied in terms of their adsorption of certain heavy metal ions (Si, So, Sr, Fe, Zn, Ni) from water resources. Static conditions for the separation of metal ions from water resources using modified kaolin adsorbents were determined and recommendations for their use in production were given. Also, the surface area of the adsorbents (Θ), energy, solution concentration, chemical potential of substances, surface tension, and Gibbs energy (∆G) were calculated based on the Dubinin-Radushkevich equations.

It was shown that the adsorption isotherms of chlorine and sulfur ions of composite sorbents obtained by modifying Angren kaolin are linearly related based on the BET equation, and the process of separation of various ions from solutions occurs in a uniform manner according to the Dubinin-Radushkevich and Langmuir equations.

АННОТАЦИЯ

В данной научной работе исследованы коллоидно-химические свойства сорбционных свойств некоторых анионов (хлора, серы) водных ресурсов сорбционными материалами, полученными путем модификации каолина марок АКС-30 и АКС-70, являющегося местным сырьем нашей республики. Изучены коллоидно-химические свойства полученных сорбентов по сорбционным свойствам некоторых ионов тяжелых металлов (Si, So, Sr, Fe, Zn, Ni) водных ресурсов. Определены статические условия выделения ионов металлов из водных ресурсов с использованием модифицированных каолиновых адсорбентов и даны рекомендации по их использованию в производстве. Также на основе уравнений Дубинина-Радушкевича рассчитывалась степень заполнения площади поверхности адсорбентов (Θ), концентрация растворов, химический потенциал веществ, поверхностное натяжение, энергия Гиббса (∆G).

Показано, что изотермы адсорбции ионов хлора и серы композиционными сорбентами, полученными путем модификации ангренского каолина, линейно связаны на основе уравнения БЭТ, а процесс разделения различных ионов из растворов происходит единообразно согласно уравнениям Дубинина-Радушкевича и Ленгмюра.

Keywords: Local raw materials, kaolin, modification, sorbent, sorption, anions, heavy metal ions, colloid-chemical properties.

Ключевые слова? Местное сырье, каолин, модификация, сорбент, сорбция, анионы, ионы тяжелых металлов, коллоидно-химические свойства.

Introduction. In recent years, attention to surface phenomena related to colloidal chemistry has rapidly developed, but insufficient discussion and analysis of the obtained results have been reflected. The adsorption process has proven to be an effective and efficient method in oil and gas processing, organic synthesis, animal and vegetable oil purification, medical production, water resource purification, and other industrial sectors. At the same time, the adsorption method is widely used in chemical analysis and scientific research [1-4]. Unlike physical and physicochemical methods (distillation, crystallization, dialysis, etc.), the process of adsorption is distinguished by the dependence of the chemical nature of the substances that make up the system's phases. In this case, the value of the adsorption potential (ε) plays an important role in adsorbing disperse systems [5-7]. The adsorption potential has a maximum value at the boundary between the adsorbent and the adsorption volume and is zero at the boundary between the adsorption volume and the gas phase [8-12]. Based on this, new composite adsorbents with specific compositions are developed using local raw materials and industrial waste. Their morphology, molecular structure, physic chemical properties, and sorption mechanisms are studied using modern analytical methods.

Methods and materials. Angren kaolin and its modified sample with 0.1 N sulfuric acid were used in the research. To study the surface energy and porosity of the adsorbents, their surface was cleaned under high vacuum and in an inert gas flow.

Expression of adsorption isotherms was performed by the dependence of vapor pressures on temperatures. Determination of adsorption isotherms of deposits on modified Angren kaolin adsorbents was carried out in the quartz spring device of Mak-Ben-Bakra. The adsorbents were vacuumed by heating under vacuum conditions at a temperature of 150-160^oC for 8-10 hours. In the Mak-Ben-Bakra device, up to 1.33∙10-3 mm.cm. column was created using the device's for-vacuum and pumps.

The temperature in the adsorption column was controlled with an accuracy of 0.2-0.3°C by heaters enclosed in the device body. The equilibrium of the adsorption process was determined by the end of the elongation of the quartz spirals.

Results and discussion. Nowadays, adsorbents are widely used globally for removing organic substances from wastewater. Therefore, studying the physicochemical and sorption properties of sorbents with magnetic properties is of great importance. The process of adsorbing organic substances from aqueous solutions depends on the nature of the surface and the porosity of the adsorbent. Based on this, new composite adsorbents with specific compositions are developed using local raw materials and industrial waste. Their morphology, molecular structure, physicochemical properties, and sorption mechanisms are studied using modern analytical methods.

The composite adsorbents created based on the modified kaolin grades AKS-30 and AKS-70 were tested for purifying wastewater containing heavy metal ions. The obtained results are presented in Table-1. According to the table, the sorbents demonstrated the ability to effectively remove heavy metal ions from wastewater within the range of 93-98.0%, meeting the permissible limits set by state standards.

Table 1.

The efficiency of cleaning some heavy metal ions from aqueous solutions (E, %)

The value of the adsorption potential decreases as the substance absorbed by the adsorbent surface moves away (ε₁>ε₂>ε₃). The maximum adsorption potential is observed near the surface of the adsorbent and is determined by the formula:

ε=

ε=

Scientific literature demonstrates changes in adsorption potential in relation to the amount of adsorbed substance. Therefore, the change in adsorption potential of the adsorbent was investigated corresponding to the amount of chloride ion adsorption in modified Angren kaolin (Fig.1). According to the change in the adsorption potential of chloride ions on the modified Angren kaolin, the adsorption potential at the initial part of adsorption at V_ads=0.0078 cm³/g is equal to=7856.8 J/mol, and with the saturation of the adsorbent surface, at V_ads=0.0298 cm³/g, the adsorption potential sharply decreases to=865.9 J/mol. To determine the surface area of a solid (adsorbents and catalysis), the selected adsorbate must be chemically inert. The saturated vapor pressure of the adsorbate should be sufficiently high at the experimental adsorption temperature, meaning that the relative pressure range (P/Ps=0.01–0.6) should allow for the measurement of adsorption with the required accuracy. Typically, the amount of adsorption is measured starting from a temperature of 293 K at the boiling point of substances in the liquid phase. Figure-2 below shows the correspondence of the adsorption isotherms of chlorine and sulfur ions with composite sorbent materials obtained by modifying Angren kaolin to the equation of the BET theory.

Figure 1. Characteristic diagram of the adsorption potential based on the amount of chloride ion adsorption in modified Angren kaolin

As can be seen from the data, the application of the BET theory equations in the adsorption of chlorine and sulfur ions on modified kaolin adsorbents across all relative pressure ranges is observed in a straight line. The adsorption of chloride ions on adsorbents consists of two straight-line substrates. In one of them, this was observed at a relative pressure in the range of 0.10–0.25. This diversity can be due to several reasons. The area of filling the monolayer capacity of the adsorbent is determined by the constant value C. In such cases, a straight line is observed in the pressure range of 0.05<P/Ps < 0.20-0.35 in the range of 60-150 points. If the constant C has a smaller value, then the capacitance of the monolayer occurs in the lower relative pressure range. The second reason may be the presence of micro pores in the adsorbent, where adsorption is high at low relative pressures, which makes a significant contribution to the amount of adsorption. It should also be noted that calculations using BET theory allow for reliable results if the adsorption isotherm belongs to type II or IV.

Sorption isotherms were obtained for the adsorption process of chlorine and sulfur ions by new sorbent materials created based on modified Angren kaolin according to Langmuir's equation. The adsorption isotherms according to the Langmuir equation   are shown in Figure 3 (a, b). It has been established that the adsorption process of Cl and C ions modified kolin adsorbents occurs according to the Langmuir equation according to experiments.

Figure 2. The expression of the adsorption isotherms of chlorine and sulfur ions on modified Angren kaolin in the coordinates of the linear form of the BET equation

Figure 3. Isotherms of adsorption of modified Angren kaolin sorbents:

 1-700^oC; 2-500^oC; 3-300^oC; 4-200^oC

The sorption isoterms for chlorine and sulfur ions of new sorbent materials based on modified AKS-30 and AKS-70 kaolins were obtained using the Dubinin-Radush-kevich equation (Fig.4.)

Figure 4. Isotherms of adsorption of chlorine and sulfur ions in sorbents based on modified Angren kaolin

The adsorption isotherms obtained from the experiments showed that the amount of adsorbed substance during the separation of various ions from solutions using porous sorbents occurs with the same value found based on the Dubinin-Radushkevich and Langmuir equations.

The interaction of heavy metal ions in wastewater with modified kaolin-based adsorbents was studied using a static method. In this process, the solution-to-sorbent mass ratio was 1:100, 1:150, and 1:200; the particle size was 0.4 mm; the process temperature was 293 K; and the stirring speed of the reaction mixture was 400 rpm. Under practical application conditions, the porosity of adsorbents plays a significant role. Adsorbents generally exhibit four types of porosity: non-porous, uniformly large porous, uniformly small porous, and non-uniform porous adsorbents. The degree of surface filling (Θ) of the adsorbents is expressed using the following equation:

Θ = exp[-(F/E) ⁿ]

Here, F is the decrease in free energy, expressed as F = RTln*P_s/P ga, E - is the adsorption energy; Θ - is the degree of filling of the adsorption phase, defined as Θ=; n- is an integer (1, 2, 3,); P- is the equilibrium pressure; P_s- is the saturated vapor pressure; a- is the specific adsorption. It is recommended to determine the adsorption energy using a graph constructed using the equation lga = lg- 0.434 (F/E)ⁿ The above-mentioned Dubinin-Radushkevich equation for the adsorption process involving solutions can be expressed as follows:

 , or

where a is the specific adsorption at a specific temperature (T) and concentration (C, mol/g); C_s - concentration of saturated solution, mol/m³; V* - molar volume of the adsorbent, m³/mol; V_a - relative adsorption volume, m³/g; E - is the adsorption energy; R - is the universal gas constant.

Depending on the selectivity of the adsorbent, the composition of the solution, the chemical potential of the substances, the surface tension, and the surface area of the interphase separation change during the sorption process. The chemical potential (Mi) of a solution containing component i is given by the following equation:

μi = μ_i° + RT ln a_i^*

Here, a_i is the activity of the solution, and it is equal to the value of C_i divided by the coefficient of activity f. This equation can also be expressed with respect to the adsorption volume, V_a, then,

μ_iads = μi°_iads + RT lna_iads^*

When interphase equilibrium is established during adsorption, the value of their chemical potential is also equal, i.e., Mi = Mi,ads.

Then, compared to the above equations

μ_i + RT ln Ci fi = μ°_i,ads + RT ln C_i,ads f_iads
-(μ°_i,ads - μ_i) = RT ln (C_i,ads f_i,ads )/ (C_i f_i) = -ΔG_i = const

Therefore, in the process of adsorption from solutions, the decrease in the amount of standard free energy is always constant, and the value of dG does not depend on the concentration of the component molecules in the phases and the interaction of the component molecules in the solution. Therefore, dF can be expressed as follows.

−ΔGi = μ°_i,ads − μ_i° = RT ln (C_i,ads f_i,ads )/ (C_i f_i)  = RT ln K_i

from which,

(C_i,ads f_i,ads )/ (C_i f_i) = K_i

In practice, this equation is an isotherm equation for the adsorption of component i from aqueous solutions. The adsorption isotherm of heavy metal ions (Cu²⁺, Zn²⁺, Cr⁶⁺, Ni²⁺) in aqueous solutions by modified kaolin sorbent materials is shown in Figure 5.

The equilibrium state of the metal ion adsorption process in aqueous solutions is proportional to the surface area of the liquid phase adsorbents and is expressed by the following equation:

da / (dθ * dτ) = β_ads(a_muv − a_τ)    or    dC / (dθ * dτ) = β_c (C_τ− C_muv)

The driving force of the process is a certain unit of time t and the difference of concentrations (C_τ − C_muv) when equilibrium is established. β_ads and β_c in the equations are called mass transfer velocities. Based on the inverse value of β, i.e., mass transfer 1/β_ads or 1/β_c = R_c, the above equations can be written as follows:

or     

Figure 5. The adsorption isotherms of heavy metal ions (Cu²⁺, Zn²⁺, Cr⁶⁺, Ni²⁺) by  modified kaolin adsorbents (1 - Cu²⁺; 2 - Zn²⁺; 3 - Cr⁶⁺; 4 - Ni²⁺)

The value of β directly depends on the interaction of the liquid with the adsorbent during the adsorption process, as well as on the size of the adsorbent particles and its porosity structure.

Conclusion. The experiments conducted on the adsorption isotherms of chlorine and sulfur ions with new sorbent materials based on modified Angren kaolin belong to type 1. The adsorbent has a micro porous structure and accounts for 60% of the total porosity of the adsorbent. Thus, based on the research results, it was found that adsorption of chlorine and sulfur ions in water resources using modified adsorbents yielded positive results, and due to the presence of sorption and ion exchange properties, it is possible to widely use them in wastewater treatment.

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