THE USE OF NITRIC ACID AS A PERLITE ACTIVATOR IN ADSORBING METHYLENE BLUE

ABSTRACT


Introduction
Industrial activities in modern times that are growing rapidly can cause environmental problems. This is due to the increasing amount of waste generated due to increased production. Liquid waste discharged without treatment becomes a serious problem in life because it can reduce water quality and become toxic to water. Synthetic dyes contain compounds with complex aromatic molecular structures, making it difficult to decompose naturally when discharged into the environment [1] . Methylene blue (C16H18N3SCl) is one of the synthetic dyes often used in the textile, paper, rubber, pesticide, varnish, leather, plastic, and paint industries [2] . Untreated dye waste can inhibit sunlight penetration, photosynthesis activity, the growth of aquatic biota also mutations, allergies, poisoning, and cause cancer [3] . Indo. J. Pure App. Chem. 6 (1), pp. 09- 16,2023 One method commonly used in dye effluent treatment is adsorption. This is because it is practical, easy to do, and the processing costs are low, so this method is right for removing dye contaminants [4] . Adsorption is a process in which molecules from the gas or liquid phase are bound to a solid or liquid surface. The substance that binds molecules is called an adsorbent, while the molecules bound to the adsorbent are called adsorbates [5] . Adsorption is divided into two types, namely physical and chemical adsorption. The main difference between the two types of adsorption lies in the reaction rate process that occurs on the surface of the adsorbent and the activation energy. Physical adsorption (physisorption) involves weak Van Der Waals forces between the adsorbent and the adsorbate. Chemical adsorption (chemisorption) involves the exchange or co-use of electrons between adsorbate molecules and the adsorbent surface [6,7,8] . Factors that affect the adsorption process include adsorbate concentration, pH, contact time, and temperature [9] , Perlite is an amorphous volcanic rock obtained at 900 -1000°C. Perlite is a material with a high percentage of silica, porous, durable, cheap, and easy to obtain. This makes perlite as one of the materials that can be used as an adsorbent. It is because the silanol group which is formed on the surface of perlite by silicon atoms. The silicon atoms at the surface tend to maintain their tetrahedral coordination with oxygen. They complete the coordination at room temperature by attachment to monovalent hydroxyl groups, forming silanol groups. The surface of perlite can become negatively or positively charged according to the pH of the solution medium. This can affect the adsorption capacity [10] . Nitric acid in this research use to activate the functional group in perlite, to increase the perlite performance. Many studies have discussed the ability of perlite as an adsorbent, such as for victoria blue removal [11] and methyl violet removal [12] . For this reason, research was conducted on the adsorption ability of perlite on methylene blue, as an alternative to addressing the problem of water pollution and its impact on the environment.

Perlite Preparation
A total of 25 g of perlite was crushed until smooth and filtered with a 120 mesh sieve. After that, perlite was soaked in 100 mL of 5% HNO3 solution for 1 hour, then dried in an oven at 80℃ temperature for 1 hour. Furthermore, perlite was calcined in a furnace for 2 hours at 500℃ temperature, then cooled at room temperature.

Maximum Wavelength of Methylene blue
The 2 ppm methylene blue from 10 ppm standard solution was measured for absorbance using a UV-Vis spectrophotometer in the wavelength range of 400 -800 nm. The maximum wavelength of methylene blue is obtained from the highest absorbance result.

Calibration Curves of Methylene Blue
Calibration curves were made by varying methylene blue standard solutions of 0; 0.5; 1; 1.5; 2; and 2.5 ppm from a 10 ppm standard solution. Each solution was measured for absorbance at the maximum wavelength. The measurement results were used to calculate the straight-line equation and methylene blue concentration.

Effect of Methylene blue Adsorption on Optimum pH
Adsorption measurements with pH variations were carried out using a 10 ppm solution of methylene blue as much as 250 mL. Then the solution was set at pH 3, 5, 7, 9, and 11. Then each solution was added with 0.25 g perlite. The solution was then stirred using a magnetic stirrer for 3 hours. The solution was then centrifuged at 1500 rpm for 10 minutes. The solution was then measured for absorbance at the maximum wavelength.

Effect of Methylene Blue Adsorption on Optimum Contact Time
Adsorption measurements with variations in contact time were carried out using a 10 ppm solution of methylene blue as much as 250 mL. The methylene blue solution used was set at the optimum pH. Furthermore, 0.25 g of perlite was added to the solution and stirred using a magnetic stirrer. Methylene blue solution was measured at contact times of 10, 30, 60, 90, 120, 180, and 240 minutes at the maximum wavelength. Previously, the solution was centrifuged at 1500 rpm for 10 minutes.

Effect of Methylene blue Adsorption on Initial Ion Concentration
Adsorption measurements with concentration variations of 10, 20, 30, 40, 50, 60, 70, and 80 ppm methylene blue as much as 250 mL were carried out by adjusting the solution with the optimum pH, then adding 0.25 g perlite into each solution and stirring using a magnetic stirrer until the optimum contact time. The solution was then centrifuged at 1500 rpm for 10 minutes. The solution was measured for absorbance at the maximum wavelength.

Activated Perlite
Perlite is an amorphous volcanic rock that can be found in the Mediterranean Sea region. Perlite has a high SiO2 content of 74.33% and Al2O3 as much as 12.75% [13] . The activated perlite was then analyzed for its functional groups. Analysis of functional groups of perlites is performed with Fourier Transform Infrared Spectrometer (FTIR) instrument to show the functional groups contained in pearlite material. FTIR spectra of perlite are shown in Figure 2. The peak observed at wave number 1010.7 cm -1 is due to asymmetric stretching vibrations on the Si-O-M group (M = Al or Si), while at wave number 786.96 cm -1 is the result of stretching vibrations of the silanol group (Si-OH) [14] . The wave number 424.34 cm -1 shows the bending vibrations of siloxane groups (Si-O-Si) [15] .

Effect of Optimum pH
The relationship between adsorption capacity (qt) with pH variation can be seen in Figure 3. The highest adsorption capacity (qt) of methylene blue is at pH 9 with an adsorption capacity of 2.594 mg/g. Methylene blue dye in solution will dissociate into cations. At low pH, the perlite surface is positively charged due to excess H+ ions [16] which creates a repulsion force from the perlite thus limiting the cations from methylene blue to fill the perlite surface. The increase in pH makes perlite negatively charged so that methylene blue cations are more easily adsorbed to the perlite surface [14] . This occurs because of the electrostatic interaction of methylene blue cations with the negatively charged perlite surface. Electrostatic attraction force can increase along with the increase of negative charge on the adsorbent surface [15] . The adsorption ability of perlite is supported by its high silica content. Silica atoms on the perlite surface form silanol groups that can adsorb methylene blue [10] . Oxygen atoms (O -) which are quite reactive from silanol groups and perlite siloxanes are able to bind N + atoms on methylene blue, resulting in an adsorption reaction. The following is a simple reaction equation when adsorption occurs.
-MO -+ dye + M-O-dye [10] Figure 4. shows that the longer the contact time, the more methylene blue molecules are adsorbed. The number of methylene blue molecules adsorbed from the 10 th minute increases until the 120 th minute, this is because there are many active sites on the surface of the adsorbent that have not been filled, allowing methylene blue to interact with perlite. As a result, the adsorption reaction takes place quickly and continuously, until the 180 th minute tends to stabilize due to decreased adsorption sites [17] . Figure 4. shows that at a contact time of 180 minutes, the adsorption capacity has been optimum at 2.6387 mg/g. Thus 180 minutes is the ideal adsorption time perlite.

Adsorption Kinetics of Methylene Blue
Adsorption kinetics expresses the process of adsorbate absorption by adsorbent as a function of time as seen from the adsorption rate. The adsorption rate can be known from the adsorption rate constant (k) and the reaction order resulting from an adsorption kinetics equation. Determination of adsorption kinetics is done through the most appropriate adsorption reaction kinetics equation approach by comparing the coefficient of determination (R 2 ) or linearity of the reaction kinetics used [18] . There are two types of adsorption kinetics equations used, namely pseudo first-order according to the following equation.
= + [19] The curve for determining the kinetics model of methylene blue adsorption with perlite is presented in Figure 5. Based on the curve above, it can be concluded that the adsorption kinetics follow pseudo second-order due to the resulting R 2 value close to 1. This indicates that the speed of perlite absorption of methylene blue per unit time (dq/dt) is directly proportional to the square of the adsorbent capacity that is still empty (qe-qt), so that in the initial process of adsorption has a profile of reducing the concentration of the solution quite a lot, then the adsorption speed continues to decrease until an equilibrium condition is reached. The kinetic model obtained is used to determine the value of the adsorption rate constant (k) and the maximum adsorption capacity (q). Calculation of methylene blue adsorption kinetics as show on Table 1. Table 1. Calculation of methylene blue adsorption kinetics Pseudo first-order Pseudo second-order k (g/mg.min) q (mg/g) R 2 k (g/mg.min) q (mg/g) R 2 0,0023 1,1768 0,1146 0,0055 3,3102 0,992

Adsorption Isoterms of Methylene Blue
Adsorption equilibrium states an isotherm equation that explains how the relationship between the amount of substance adsorbed by the adsorbent with pressure or concentration at equilibrium with a fixed temperature [19] . The isotherm model used in this study is the Langmuir isotherm that according to the following equation.
= + . [20,21,22]  The Langmuir adsorption isotherm is defined as the maximum adsorption capacity that occurs due to a monolayer onto a surface with a finite number of identical. The second isotherm model is the Freundlich model which assumes that there is more than one surface layer (multilayer) and is heterogeneous. This isotherm model also explains that adsorption occurs physically, where more absorption occurs on the adsorbent surface. Freundlich isotherm equation follows. log = log ! + " log & [23] The relationship between methylene blue concentration at equilibrium time (Ce) and adsorption capacity at equilibrium time (Qe) is shown in Figure 6. The curve for determining the adsorption isotherm model is presented in Figure 7. Based on the curve above, it can be concluded that the adsorption isotherm follows the Langmuir isotherm model due to the resulting R 2 value is higher than the Freundlich isotherm model. This indicates that the adsorption process of methylene blue is chemical, and the distribution of active sites from the perlite surface is homogeneous [24] . This adsorption has active sites that can only accommodate one adsorbed ion on the adsorbent surface. If all active sites have bound the adsorbate, the adsorption process will stop and has experienced equilibrium because the interaction of methylene blue molecules with adsorbents is only in a single layer or monolayer [25,26] . The statement that methylene blue adsorption is chemical is supported by the adsorption energy (E) value obtained. The calculation of adsorption energy is based on the calculation of Gibbs free energy, namely: where R is the general gas constant (8.314 J/mol.K), T states the temperature (Kelvin), and K is the Langmuir and Freundlich constants (L/mg). The energy generated in the Langmuir isotherm is 1349.2707 kJ/mol. If the adsorption energy is >20.9 kJ/mol, the adsorption includes chemical adsorption [27] . The results of the adsorption isotherm can be seen in Table 2 and Table 3. From the calculation of Langmuir isotherm, it was seen that the maximum adsorption capacity (qmax) is 3,783 mg/g, it is the optimum result of methylene blue adsorption in this research.

Conclusion
Based on the research that has been done, the conclusions are as follows: 1. The best perlite performance in adsorbing methylene blue at pH 9 and contact time 180 minutes, with a maximum adsorption capacity of 3.873 mg/g. 2. The adsorption process of methylene blue follows the second-order pseudo kinetics model and Langmuir isotherm.