Effectiveness of cellulose acetate composite membrane with the addition of zeolite additives to reduce TDS levels in peat water using a cross-flow reactor

The need for clean water continues to increase along with population and industrial growth, which causes a water crisis. Quantitatively, peat water has the potential to be managed as a source of clean water using membrane filtration. This study aims to compare the composition of the cellulose acetate-zeolite composite membrane in reducing TDS levels in peat water using a cross-flow reactor. Cellulose acetate-zeolite composite membranes were prepared by phase inversion technique using 15 wt.% cellulose acetate (CA), with variations in zeolite concentration (5%; 10%; 15%; 20% and 25%), contact time (20, 40 and 60 minutes), the reactor pressure was 1.5 bar, and the TDS concentration in the feed water was 1,201 mg/L. Membrane characterization was done through morphological analysis using a Scanning Electron Microscope (SEM) and tensile test analysis. Membrane performance is determined through permselectivity and permeability testing. The results showed that the highest flux value was obtained on a 5% cellulose acetate-zeolite membrane at the 20th minute, namely 78.8 L/m2.hour. The best composition was obtained on a 25% cellulose acetate-zeolite membrane with a tensile strength value of 54.089 MPa and a rejection value of 92%. Morphological characterization showed that the pore size of the cellulose acetate-zeolite composite membrane decreased after use because TDS particles covered it.


Introduction
Water is an important substance to support human life.The need for clean water continues to increase along with population and industry growth which can cause a water crisis to occur [1].Quantitatively, peat water can be managed as a source of clean water.Peat water is the surface water of peat soil characterized by a brownish-red color, containing organic matter and relatively high iron content, pH 3-5, and low hardness [2].Peat water can be hazardous to health if consumed.The high acidity of the water can damage teeth and cause stomach aches.Meanwhile, the high organic content in peat water can cause an odor [3].Therefore, it is necessary to treat peat water so that it meets quality standards and can be used for daily needs.
Peat water treatment methods that have been carried out are conventional (coagulation and flocculation), adsorption and filtration using membranes.Conventional peat water treatment has been carried out, and the results obtained are that this method can remove color, organic matter content and has succeeded in converting peat water into water that meets drinking water requirements.However, some of the equipment is difficult to obtain economically and technologically, and the people living in the peatland area must be better off [4][5][6][7].Peat water treatment with adsorption techniques has also been carried out using a combination of cationic surfactant-modified zeolite, granular activated carbon, and IOP Publishing doi:10.1088/1755-1315/1352/1/012044 2 limestone as absorbents.The results obtained can remove color and organic matter in peat water but require a long processing time [8].
Peat water treatment can also be done using a membrane filtration method.Membrane technology has advantages such as being environmentally friendly, economical, and easy to manufacture [9].For example, a cellulose acetate membrane made using the phase inversion technique can reject organic substances in peat water up to 90% [10].The materials used in the manufacture of membranes include organic and inorganic polymers.The organic polymer materials often used are cellulose acetate, polysulfone, polyethersulfone and polyamide.Using cellulose acetate-based membranes in water treatment is known to reject color, TSS, TDS and organic matter so that water quality meets permissible quality standards.However, the cellulose acetate membrane still has a weakness; namely, the membrane's mechanical properties are less strong [11][12][13].
Using inorganic materials such as zeolite as a mixture in organic membranes has been carried out.The results showed that the membrane was able to reduce turbidity in peat water and improve the membrane's mechanical properties.Thus, it is necessary to modify membranes made from organic polymers by adding inorganic materials [14,15].Modifying membranes based on organic polymers (cellulose acetate) with the addition of inorganic materials (zeolite) has been carried out and applied in dead-end flow.The results showed that the resulting membrane could reject TDS, TSS, turbidity, pH, Fe metal and benzophenone-3.In addition, the membrane performance increases with the addition of zeolite [16][17][18].On the other hand, a performance study of Portland zeolite-clay-cement composite membranes has been carried out in peat water treatment with cross-flow.The study results show that the composite membrane can reject organic compounds in peat water.Furthermore, applying membranes in cross-flow can minimize the buildup or fouling of the membrane [19].
In cross-flow, the permeate produced is less than in dead-end flow.However, using cross-flow can help remove cake that accumulates on the membrane during the filtration process so that the lifetime of the membrane can be extended.In addition, cross-flow can be used continuously and operates continuously at relatively high solids loads by minimizing the risk of clogging.Cross-flow is used to reduce pollutant buildup on the membrane by sweeping material off the surface of the membrane so that premature fouling can be prevented [20].
This study examines the effect of zeolite concentration as an additive in cellulose acetate membranes in reducing Total Dissolved Solid (TDS) levels in peat water using a cross-flow reactor.The resulting cellulose acetate-zeolite composite membrane was analyzed for its mechanical properties in the form of tensile strength of the membrane.The performance of composite membranes in reducing TDS water in peat water was studied by analyzing the permeability and permselectivity of the membrane.Furthermore, the best composite membranes were characterized by morphology using a Scanning Electron Microscope (SEM) before and after use.

Materials
The materials used in this study included cellulose acetate polymer (acetyl 39.7 wt% average Mn ~50,000 by GPC) obtained from Sigma-Aldrich; Zeolite; 5N HCl solution; Acetone (99% analytical purity); and peat water obtained from Sukarame Baru Village, Kualuh Hulu District, North Labuhan Batu Regency, Indonesia.All reagents and solvents used are of analytical grade, purchased from commercial sources, and used without further purification.

Zeolite Activation.
Zeolite activation was carried out using a 5N HCl solution.10 g of zeolite was added to 100 mL of 5 N HCL solution and stirred with a magnetic stirrer for 1 hour.The mixture is then allowed to stand for 24 hours and filtered.The filtered zeolite was washed with hot distilled water and dried in an oven at 105°C for 3-4 hours to obtain activated zeolite.The activated zeolite was then crushed using a mortar and sieved using a 200-mesh sieve.

Preparation of 15% Cellulose Acetate Solution.
The 15% cellulose acetate solution was prepared by referring to Manik [21].A mass of cellulose acetate powder is put into a beaker glass and added to 250 mL of acetone.The solution formed was stirred for 24 hours using a magnetic stirrer at room temperature.This cellulose acetate solution is then used to manufacture cellulose acetate composite membranes.

Preparation of Cellulose Acetate-Zeolite Composite Membrane.
The manufacture of cellulose acetate-zeolite composite membranes was carried out by referring to Manik [21].The previously prepared 15% cellulose acetate solution was added with activated zeolite according to the predetermined variations (5%; 10%; 15: 20%; and 25%).The mixture was then stirred using a magnetic stirrer for 24 hours until it became homogeneous, then allowed to stand for ± 24 hours to remove air bubbles.After being allowed to stand, the mixture was poured into a petri dish and allowed to stand for 7 minutes before being immersed in distilled water.The formed membrane was removed from the petri dish and dried at room temperature.The composition of the cellulose acetate-zeolite composite membrane can be seen in Table 1.

Membrane Application in
The Cross-flow Reactor.The application of a cellulose acetate-zeolite composite membrane in a cross-flow reactor was carried out based on Rambe [22] with a few modifications.This study conducted sampling every 20 minutes for one hour to calculate the resulting flux and rejection values.The cellulose acetate-zeolite composite membrane was placed in a water nut (permeate exit hole) in the order of rubber, coarse gauze, fine gauze, membrane, fine gauze, coarse gauze, and cover.Installation of the membrane is done tightly to prevent leakage.

Analysis of Tensile Strength of the Membrane
Analysis of the tensile strength of the composite membrane is carried out by involving the deformation of the membrane under a certain pressure.Tensile strength analysis was performed using an autograph.
In the tensile strength analysis, stress and strain values will be obtained.Stress expresses the magnitude of the force (F) exerted on the material being tested against the area of the material (A).The strain is the ratio of the change in the length of the material to its initial length due to the application of pressure, initially due to a force in the direction parallel to the change in length.Young's Modulus of the composite membrane can be calculated based on the stress and strain data.Determination of the values of stress, strain, and Young's Modulus is carried out using Equations ( 1) to (3). (3)

Performance Analysis of Cellulose Acetate-Zeolite Composite Membranes
Membrane performance is assessed based on the permeability and permselectivity of the membrane.Membrane permeability is assessed based on membrane flux, the permeate volume obtained during membrane operation per unit time, and membrane surface area unit.The permselectivity of the membrane is measured based on the percentage of rejection (R), namely the concentration of solutes that can pass through the membrane.The flux and rejection percentage on the cellulose acetate-zeolite composite membrane can be calculated using Equations ( 4) and (5).
Where: Jv = volume flux (L/m 2 .h)V = permeate volume (L) A = surface area (m 2 ) t = time (hours) Cp = concentration of solute in the permeate Cf = concentration of solutes in the feed water

Analysis of Total Dissolve Solid (TDS) Levels
Analysis of TDS levels was carried out using the gravimetric method [23].The peat water sample is put into a filter equipped with a suction pump and filter paper.After filtering the sample, the filter paper was rinsed using 10 mL of distilled water three times.The rinsed filter paper is transferred to a cup of known mass.The filter results were evaporated to dryness in a water bath.Then, the cup containing dry filter paper was put into the oven at 18 o C for 1 hour.The cup is then put into the desiccator, and its mass is weighed with an analytical balance.

Morphological Analysis Using a Scanning Electron Microscope (SEM)
Morphological analysis of the cellulose acetate-zeolite composite membrane was carried out on the composite membrane, which had the best TDS rejection percentage value.Morphological analysis of composite membranes before and after use was carried out using a Scanning Electron Microscope (JEOL JSM-6510LA) with a magnification of 10,000 times.

Tensile Strength Analysis Results
Tensile strength analysis was conducted on a cellulose acetate-zeolite composite membrane with 20 mm x 16 mm x 0.2 mm dimensions.The value of the tensile strength of a material is seen based on the load value, namely stress, and strain, when the material is broken.Based on these data, the value of Young's modulus is the stiffness of a material, where the material can return to its original shape when given a load.The results of the tensile strength analysis of the cellulose acetate-zeolite composite membrane in this study can be seen in Table 2. Table 2 shows that Young's modulus value of the composite membrane increases as the concentration of the zeolite used increases.Composite membranes with the highest mechanical strength were obtained using 25% zeolite, with a maximum load value of 173.09N and Young's modulus of 551.9 N/mm 2 .Increasing the concentration of zeolite in the composite membrane will cause the structure of the composite membrane to become denser.The denser the composite membrane structure indicates that the distance between the molecules in the composite membrane will be denser, resulting in a stronger tensile strength.The addition of zeolite will provide more resistance so that the resulting composite membrane has a stronger tension [24].In this study, the five types of composite membranes produced followed Hooke's law theory, where the amount of strain is inversely proportional to Young's modulus.In contrast, Young's modulus is directly proportional to the stress applied to the composite membrane.
Figure 1 shows the relationship of stress to strain on a cellulose acetate-zeolite composite membrane.In the analysis of the tensile strength of the composite membrane with the addition of 5% zeolite (Figure 1a), the composite membrane broke when given a load of 17.275 MPa.The composite membrane experienced a stretch of 0.236 mm/mm, elongating 4.725 mm.The value of the maximum load the composite membrane can accept increases when the zeolite concentration increases.When using 25% zeolite (Figure 1b), the composite membrane broke when given a load of 54.089 MPa.When given a maximum load, the composite membrane stretches by 0.098 mm/mm, elongating 1.973 mm.

Performance Analysis Results of Cellulose Acetate-Zeolite Composite Membrane
The peat water used in this study has a TDS level of 1,201 mg/L.This value has exceeded the threshold set in Government Regulation no 82 of 2001, namely a maximum of 1,000 mg/L [25].In this study, the performance of the cellulose acetate-zeolite composite membrane in reducing TDS levels of peat water was carried out by analyzing the permeability and permselectivity of the composite membrane.Permeability is the speed of a particle penetrating the membrane.Permeability is expressed by flux, where the flux can be obtained by calculating the volume of permeate that has passed through a unit area of the membrane in a certain time due to pressure-pushing force [26].The permeability or flux of the membrane is affected by the number and size of the membrane pores, the interaction between the membrane and the feed solution, and the operating pressure used.The smaller the pore size, the smaller the resulting membrane flux [14].In addition, the membrane flux is also influenced by the composition of the membrane material, including the type of polymer chosen, the concentration of the polymer solution used, and the addition of additives used to improve the membrane's mechanical properties [27].
In this study, the effect of adding zeolite to the cellulose acetate composite membrane on membrane flux is shown in Figure 2. Based on Figure 2, it can be seen that the membrane flux decreased as the concentration of zeolite used increased.In addition, the longer operating time of the cross-flow reactor also causes a decrease in the membrane flux value.Longer operating time will reduce membrane flux due to fouling or pollutant buildup [10].The highest membrane flux was obtained on a 5% cellulose acetate-zeolite composite membrane at the 20th minute of 78.8 L/m 2 .hour,while a 25% cellulose acetate-zeolite membrane obtained the lowest membrane flux at the 60th minute of 12 L/m 2 .hour.Besides permeability, membrane performance can also be measured through membrane permselectivity.Membrane permselectivity is the ability of a membrane to separate or pass certain species from feed water.Membrane permselectivity is measured by looking at a membrane's rejection percentage (R).Figure 3 shows the percentage of TDS rejection on composite membranes with various zeolite concentrations.Based on Figure 3, it is known that the percentage of TDS rejection increases with increasing operating time of the crossflow reactor.In addition, the higher concentration of zeolite in the cellulose acetate-zeolite composite membrane also increased the percentage of TDS rejection of peat water [28].The best TDS rejection percentage was obtained on a 25% cellulose acetate-zeolite composite membrane in the 60th minute, namely 92%, with the resulting permeate concentration of 102 mg/L.

Morphology of the Cellulose Acetate-Zeolite Composite Membrane
Morphological characterization of cellulose acetate-zeolite composite membranes was carried out on the best composite membranes before and after use, namely composite membranes with 25% zeolite.The morphology of the cellulose acetate-zeolite composite membrane before and after use is shown in Figure 4.In Figure 4a, the composite membrane has a pore size ranging from 0.1 to 0.2 µm, so that it can be classified as an ultrafiltration membrane [29].After use, the pores on the membrane surface appear to be reduced (Figure 4b).This is due to fouling or accumulation of pollutant material in the membrane's pores after the membrane is used.These pollutant materials are pollutants retained in the membrane's pores, causing compaction of the membrane [30].The occurrence of fouling on the composite membrane that has been used will reduce the value of the membrane flux.This is by the membrane flux analysis shown in Figure 2, where the lowest flux was obtained on a 25% cellulose acetate-zeolite composite membrane after 60 minutes of use.

Conclusion
The best cellulose-acetate composite membrane was obtained using 25% zeolite, with a tensile strength value of 58.089 MPa and a TDS rejection percentage of 92%.The concentration of TDS in the resulting permeate is 102 mg/L.The resulting composite membrane is classified as an ultrafiltration membrane with some visible pores on the surface of the membrane before use.These pores are then closed after the membrane is used, which indicates the accumulation of TDS particles on the surface of the membrane.After processing using a 25% cellulose acetate-zeolite composite membrane, the TDS parameter of peat water complies with Government Regulation 82 of 2001, namely a maximum of 1,000 mg/L.

Figure 1 .5th
Figure 1.Relationship of strain to stress on a cellulose acetate-zeolite composite membrane; (a) Addition of 5% zeolite; (b) Addition of 25% zeolite.

Figure 2 .
Figure 2.Comparison of the flux values of each cellulose acetate-zeolite composite membrane.

Figure 3 .
Figure 3. Percentage of TDS rejection on each cellulose acetate-zeolite composite membrane.

Table 1 .
Composition for the manufacture of cellulose acetate-zeolite composite membranes.

Table 2 .
Tensile Test Results of Cellulose Acetate-Zeolite Membrane.