Adsorption isotherm and kinetic study of Bengkulu Kaolinite in removing ferrous cation by continuous operation

Iron is a chemical element that is found in groundwater that can be suspended in the form of ferrous cations. Iron presence that exceeds the threshold can cause detrimental effects such as piping corrosion, bad smells, and health problems. According to observations, groundwater in Sukarame, Bandar Lampung, has smells and colors that indicate iron metal contamination. Hence, it is necessary to research reducing the iron content. The method used in this research is the adsorption method using kaolin adsorbent that has been activated by physical activation at 700°C. The adsorption was conducted continuously with various reactor heights of 20 cm, 15 cm, and 10 cm. The results showed that the most significant reduction in efficiency was found in the adsorption using reactors with a height of 20 cm by 92%. The adsorption phenomenon using a reactor height of 20 cm followed the Langmuir isotherm model (R2 = 0.9255) with an adsorption capacity of 17.03 mg/g. The kinetic data correlated well with the pseudo-second-order equation (R2 = 0.9313). It can be concluded that kaolin adsorbent can reduce iron levels, in which the adsorption using reactors with a height of 20 cm is the most effective.


Introduction
Groundwater is one of the sources for fulfilling the essential consumption of the Indonesian people.Based on Badan Pusat Statistik data, in 2019, around 42% of the water sources used by the community came from groundwater.Iron is a chemical element that is found on earth, including in groundwater.Iron metal can be suspended in water in the form of ferrous cations.Iron presence that exceeds the threshold can cause detrimental effects such as piping corrosion, brown color turning of the water, bad smells, and health problems [1].According to observations, groundwater in Sukarame, Bandar Lampung, has smells and colors that indicate the water is contaminated with iron metal.For this reason, it is necessary to research reducing the iron contained in groundwater.
There are many ways to reduce the amount of iron in a substance, including oxidation, ion exchange, lime softening, adsorption, and filtration.According to earlier studies, the adsorption process 1311 (2024) 012020 IOP Publishing doi:10.1088/1755-1315/1311/1/012020 2 may be a good option for treating wastewater because it is simple, easy to carry out, and relatively inexpensive.The adsorption method efficiently removes heavy metals from groundwater since the adsorbent may be recycled [2], [3].Adsorption is carried out by using an adsorbent that engages in ion exchange, physical interaction, and chemical interaction with the heavy metal.Suitable and recommended adsorbents are widely accessible, simple to regenerate, and low-cost [4].
Kaolin is one of the minerals that can be used to make adsorbents Al2O3.2SiO2.2H2O is the chemical formula for the mineral kaolinite, a kaolin clay component.With one sheet of octahedral alumina on one side and one sheet of octahedral silica on the other, it is physically classified as a 1:1 phyllosilicate.An eight-hybrid bond stack is created when hydrogen bonds between silica-oxygen and alumina-oxygen bind the two layers together.Depending on the pH of the solution, the charge on the alumina's surface and edges can trigger protonation and deprotonation of the hydroxyl groups [5].Kaolin is employed as an alternative absorbent due to its abundance, high porosity, large surface area, and reduced cost compared to other adsorbents.
A batch system is typically used in laboratory research and is restricted to processing small amounts of wastewater in studies of metal ions in groundwater.Most treatment approaches require more information than can be obtained under batch conditions.The contact times in the batch system are too brief to bring the column system to equilibrium, which is its main drawback.In contrast, the continuous system can adjust to the ongoing process and has low operating costs [6].This study focused on the efficiency of an adsorption column employing kaolinite as an adsorbent for ferrous cation removal by a continuous operation based on previous research to examine adsorbent performance [7], [8].

Materials
This study considered a groundwater sample from Bandar Lampung City's Sukarame District and the raw material kaolin from Bengkulu City.All experiments were prepared with distilled water.

Equipment and Instrumentation
This research was conducted at the Environmental Engineering Laboratory, Institut Teknologi Sumatera.The equipment used in this research was a continuous reactor with a column diameter of 4.5 cm and column heights of 10 cm, 15 cm, and 20 cm, vial bottle, 10 mesh sieve, furnace reactor, agate mortar and oven (Fishcer Scientific model 655F) as well as glassware laboratory equipment.The instruments used in this study were a UV-Vis spectrophotometer (Genesis 20), an infra-red spectrophotometer (FTIR, Shimadzu Prestige-21), and a Scanning Electron Microscope (SEM, Jeol JED-2200).

Method 2.3.1 Groundwater sampling.
The groundwater used in the research was taken from Sukarame Bandar Lampung.Soil water is taken using bottles that have been previously cleaned and washed, after which water is put in a bottle of 1 liter.The groundwater that has been captured is then taken to the Environmental Engineering Laboratory and tested for the concentration of iron in the groundwater using a UV-Vis spectrophotometer.

Kaolinite Preparation and Activation.
Kaolin obtained from Bengkulu City was washed and rinsed with distilled water to remove sand and other impurities.The kaolin was then put in an oven at 110℃ for 24 hours.The dried kaolin was ground using an agate mortar and then sieved using a 10-mesh sieve.The kaolin was then activated using physical activation at a temperature of 700℃ for 30 minutes, resulting in activated kaolin (AK).The obtained activated kaolin was characterized using an FTIR spectrophotometer between 4000 and 400 cm -1 and Scanning Electron Microscopy (SEM).

Adsorption of Fe Metals in Groundwater.
The adsorption experiment was carried out by flowing a sample of artificial groundwater solution into the reactor tube that has been filled with kaolin adsorbent with a height variation of 20 cm, 15 cm, and The adsorption capacity (qe) and removal percentage (R %) were calculated as Eq. ( 1) and Eq. ( 2), respectively.
Here,  0 and   are the initial and equilibrium concentrations of Cr(VI) in mg/L, respectively, and m is the mass of adsorbent in g/L.

Kaolinite Preparation and Activation
Natural kaolin contains many impurities, such as small rocks and soil.Thus, preparation was done by washing it using distilled water, then drying and sieving to obtain kaolin adsorbent.The activation of kaolin aims to increase the surface area by physical activation at a temperature of 700 ℃.The activated kaolin adsorbent is shown in Figure 2.

Characterization of Kaolin Adsorbent 3.2.1 FTIR Spectroscopy
Its surface chemistry is the primary factor in influencing an adsorbent's adsorption capability.Figure 3 shows the results of characterizing functional groups on the AK adsorbent using a Fourier-transform infrared spectroscopy (FTIR) device.

Figure 3. IR Spectra of AK Adsorbent
As can be observed in Figure 3, there is a high absorption with a wave number of 1028 cm-1, which is indicative of the strain vibration of Si-O, a typical uptake of kaolinite materials.This condition relates to an examination of the mineral combination kaolinite, which revealed that SiO2 makes up the majority, up to 46,66% [9].The absorption with a wave number of 790,2 is attributed to the vibration of C-H, and absorption at a wave number of 2132 indicates the presence of O-H vibration from the hydroxyl compound.The OH ions can be found since the silica tetrahedral and aluminum octahedral sheets are not symmetrical on either side of the kaolin mineral's unit cell.As a result, the basal plane of OH ions in the following layer confronts the basic plane of oxygen atoms in the crystal unit [9].This analysis provides the surface functional groups that will participate in the adsorption activities on the kaolin's surface.

Morphology Structure
Scanning Electron Microscopy (SEM) was used to characterize the minerals' morphology, geometry, and distribution patterns in some of the selected samples [10].SEM analysis was performed in this study utilizing a JEOL model JEDD-2200.SEM is an important test since it can reveal the adsorbent's microstructure.The SEM images acquired are shown in Figure 4.It can be seen from Fig. 4 that heterogeneous pseudohexagonal layered sheets of varying sizes make up the majority of typical kaolin shapes.A transparent surface is likewise depicted in Fig. 4 due to the physical activation's decrease of hydrocarbon molecules.

Figure 4. SEM microphotograph of AK adsorbent
Kaolin particles are clear without impurities such as organic carbon and matter, probably due to physical treatment.The most stable kaolinite phase was created during this phase.As a result, the kaolinite lattice develops a more porous structure [11].The surface takes on a more porous structure so that negative charges on the structure's base surface will attract cations.The discovered particles had lustrous, dustfree surfaces and were spherical in shape.This condition made the adsorption better than the raw kaolin, which contains many impurities.Activation at this temperature has achieved the dehydroxylation of kaolinite [12].The particles were millimeter-sized or smaller, containing pores of various diameters.Following the isotherm results (Table 2), it is stated that a chemical reaction is likely to occur in the pores of this natural kaolin.

Adsorption of Fe Metal in Groundwater 3.3.1 Fe removal efficiency.
The concentration of Fe metal in groundwater taken from the source is as much as 1.5 mg/l.This amount is higher than the standard of clean water by the Ministry of Health, which is 1 mg/L.A high iron level in the water can cause a bad smell, taste, yellowish color, deposition on the pipe wall, high turbidity, and danger to human health [13].It means that treatment should be performed to lower the groundwater's iron level and be safe to use as clean water.The treatment chosen is adsorption using kaolinite.Kaolinite was used as it is found abundantly in Indonesia, especially on Sumatra Island, including Bengkulu.Kaolinite also has the advantages of being a relatively lower price than other adsorbents, safe to be used, and easy to find [14].This study used a continuous system of adsorption through columns.The system differs from a batch system that directly contacts the fixed amount of solution with adsorbent.In the continue system, the solution contacts the adsorbent by flowing through the solution to the adsorbent bed.Column system has the advantage of having higher capacity than batch system to be more applicable for mass scale [15].The Fe metal removal efficiency is indicated by the ratio of influent to effluent concentration on the Fe metal removal efficiency curve.The results showed that the greatest reduction in efficiency was found in the adsorption using reactors with a height of 20 cm by 92%, followed by 90% for reactors with a height of 15 cm and 78% for reactors with a height of 15 cm as seen in Figure 5.By 78 to 92% removal efficiency, the final Fe concentration in groundwater reached 0.12 -0.33 mg/L.It can be concluded that adsorption can be an alternative treatment process for the groundwater before safe use.

Figure 5. Iron removal efficiency
The first 150 minutes of contact time with the AK adsorbent saw the greatest removal of Fe from groundwater.This condition may be because the Fe ions were successfully bonded to the adsorbent surface.After that, efficiency fell as Fe ions began filling the adsorbent surface's pores.The performance of 20 cm reactors is better than that of 10 and 15 cm reactors.The 20 cm reactor has a higher kaolinite column as a result.A broader specific surface area can be bonded to metal ions when there is a higher adsorbent concentration.Similar findings showed that greater iron ions were adsorbed in a higher adsorbent bed of a continuous reactor [16], [17].

Modelling of adsorption kinetic data.
Adsorption kinetics estimates the rate at which the adsorbent gets coated with the adsorbed solution.The rate at which a chemical reaction occurs is the order of the rate of a chemical reaction or chemical process.An adsorption pattern emerges from the interaction of the ferrous cation with the activated kaolin.The kinetics of Fe(III) ion adsorption by activated kaolin adsorbent was investigated using the data from the adsorption pattern.Testing the applicability of kinetics to experimental data can be done using kinetic models.According to equations ( 3) and ( 4), the kinetic experimental data were fitted using the Ho and McKay pseudo-second-order kinetic model and the Lagergren pseudo-first-order equations, respectively.Lagergren pseudo-first-order model equation, Ho and McKay's pseudo-second-order model equation, By fitting ln (qe-qt) versus t and (t/qt) versus t, respectively, linear equations can be used to assess the applicability of those two models.The relationship between contact time and iron metal uptake by kaolin in reactors with varying heights of 20, 15, and 10 cm and an initial Fe (II) concentration of 1.5 mg/L is used to characterize the adsorption kinetics.To better understand the adsorption kinetic process and look for the model that fits the experimental data the best, the results and kinetic parameters are reported in Table 1.To assess the best-fit model, correlation coefficients (R 2 ) were obtained.Both models demonstrated a considerable amount of linearity in the data fitting.However, the pseudo-first-order model on running kaolin with heights of 20 cm, 15 cm, and 10 cm had a low-value correlation coefficient (R 2 ).Compared to a pseudo-second-order model, it can be shown why this model is inapplicable to the kinetics of ferrous cation adsorption onto kaolinite.Compared to the pseudo-firstorder model, the pseudo-second-order model more closely matches the experimental results [18].Running kaolin with a reactor height of 20 cm produced the highest correlation coefficient (R 2 ) value of a pseudo-second-order model (R 2 = 0.9313).This condition demonstrates unequivocally that the data fits the pseudo-second-order model, with the amount of adsorbate at equilibrium (qe) measured at 22.85 x 10-3 (mg/g) and the adsorption rate constant (k2) measured at 1.1841 mg/g.min.With chemisorptions involving valence forces through electron sharing or exchange between kaolinite adsorbent and ferrous cation, the better match with this model suggests that the adsorption process is regulated by interaction [19].

Adsorption Isotherms.
An adsorption isotherm is crucial for figuring out the adsorption potential.The variation utilized in this isotherm is a reactor with a height of 20 cm since the adsorption kinetics model used is second-order pseudo at that height.The amount of iron adsorbed by kaolin and the process by which it occurs is determined by the determination of this adsorption isotherm.The experimental data for iron metal adsorption from aqueous solution onto kaolinite was connected with the Freundlich and Langmuir isotherms at starting Fe concentrations of 1.5 to 4.5 mg/L.The values of the adsorption isotherm parameters and the correlation coefficients are displayed in Table 2.The linear coefficient (R 2 ) for the Freundlich isotherm is 0.8514, whereas the linear coefficient (R 2 ) for the Langmuir isotherm is 0.9255.According to the findings, the Langmuir isotherm model matches the experimental data more closely than the Freundlich isotherm.The greatest quantity of iron that kaolin can absorb is 17.03 mg/g, which is the Qm value.This study demonstrates that chemical reactions are involved in the interaction between iron and kaolin [19].

Conclusions
The greatest reduction in efficiency was found in the adsorption using reactors with a height of 20 cm by 92%, followed by 90% for reactors with a height of 15 cm and 78% for reactors with a height of 15 cm.The adsorption phenomenon using a reactor height of 20 cm followed the Langmuir isotherm model (R 2 = 0.9255) with an adsorption capacity of 17.03 mg/g.The kinetic data correlated well with the pseudo-second-order equation (R 2 = 0.9313).It can be concluded that kaolin adsorbent can reduce iron levels, in which the adsorption using reactors with a height of 20 cm is the most effective.