Synthesis of Sodium Lauryl Sulfate ( SLS ) and Hexadecyltrimethylammonium Bromide ( HDTMA-Br ) Surfactant-Modified Activated Carbon as Adsorbent for Pb 2 + and N 03 "

Received: 27th May 2020 Revised:1st November 2020 Accepted: 12th November 2020 Online: 30th November 2020 The adsorption efficiency and selectivity of activated carbon as an adsorbent for ions can be improved. One way is to convert activated carbon into surfactant modified activated carbon (SMAC). The surfactants used in this study were the anionic surfactant Sodium Lauryl Sulfate (SLS) and the cationic surfactant hexadecyltrimethylammonium bromide (HDTMA-Br). This research aims to synthesize SMAC to obtain a material with a surface charge and absorb ions better than activated carbon. This research consisted of four stages. The first step was the carbonization of rice husks using a pyrolysis reactor at 400°C for 1 hour. The second stage was carbon activation using 30% ZnCl2 and microwave radiation for 5 minutes and 400 W. The third stage was the modification of activated carbon and characterization by FTIR, SEM, SAA. The fourth stage was the adsorption of Pb cations and nitrate anions by carbon, activated carbon, and SMAC. Several variables were applied, such as the type of surfactant, time, and method of modification. There are three ways of modification: (1) method A, in which activated carbon is brought into contact with SLS then HDTMA-Br. (2) Method B in which activated carbon was contacted with HDTMA-Br then SLS. (3) Method C in which activated carbon was brought into contact with SLS together with HDTMA-Br. All variables were investigated. The results showed that the optimum time for making SMAC for both surfactants was 4 hours, the optimum concentrations of SLS and HDTMA-Br were 60 and 300 ppm, respectively. SMAC made by the C method was the most effective at adsorbing Pb2+ and N03 with adsorption capacities of 1.376 and 0.896 mg/g, respectively. The success of SMAC synthesis was evidenced by the S=0 and (CH3)3N groups in the FTIR spectra. The SMAC surface area is smaller than activated carbon, 14.472 m2/g, but the surface morphology is smoother and more homogeneous.


Introduction
Activated carbon derived from coconut shells is more profitable than other agricultural wastes because it has a high density and purity, is harder, and is almost ash-free [1]. Coconut shell contributes to the world ' s pollution problem, reaching 3.18 million tons per year and representing more than 60% of the national waste volume [2]. Therefore, the use of coconut shells to become activated carbon is a means to reduce coconut shell waste. In general, activated carbon is made in 2 Previous research [ 5 ] reported that at a concentration ratio of ZnCl2: biomass of 1: 5 , most pores of activated carbon were classified as micropore structures ( < 2 nm), while at a ratio of 2: 5 and 3 : 5 , the structure became mesoporous (2-50 nm). Meanwhile, the production of activated carbon from pumpkin seed shells with ZnCl2 activator resulted in a surface area of 1, 564 m 2 / g at an optimum temperature of 500°C [6].
Many studies have currently used the microwave heating method because it has many advantages compared to furnace heating. Heating with microwaves requires a relatively short time, directly imparting energy to the carbon via molecular interactions, making it more energy-efficient [ 7 ]. Activated carbon from spruce seeds was prepared using microwave radiation in as little as 5 minutes [8].
Research on modification of activated carbon with surfactants has been carried out. Agustinus et al. [ 9 ] reported that activated carbon ' s adsorption power prepared from coal to Cr(VI) metal ions increased twofold due to modification with cationic surfactant ethylenediamine from 52.09 % to 100%. Lee et al. [10] also proved that modified activated carbon using anionic surfactants, which are sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (SDBS), and sodium octanoate (SO), increased the efficiency and absorption capacity of the ammonium ion cation compared to carbon.

Procedure
Carbon was obtained from coconut shell carbonization through the pyrolysis process at 400°C for 1 hour. The carbon obtained was put into 100 mL of ZnCl2 30 % and irradiated with microwaves for 5 minutes with a power of 400 W. The activated carbon was then washed with distilled water until a constant pH was obtained. Then, the activated carbon was dried at 105°C for 1 hour. The activated carbon was then contacted with SLS surfactant at concentrations of 40 , 50 , 60, 70 , and 80 ppm with a contact time of 2-10 hours, with HDTMA-Br at concentrations of 200, 250 , 300 , 350 , and 400 ppm with a contact time of 3 -7 hours, to obtain the optimum surfactant concentration and time. Synthesis of SMAC was carried out by three methods. Method A was that activated carbon was contacted with SLS at the optimum concentration and time and then contacted with HDTMA-Br. Method B was that activated carbon was contacted with HDTMA-Br at the optimum concentration and time then contacted with SLS. Whereas method C was carried out in which the two surfactants were mixed then contacted with activated carbon. The amount of SLS absorbed by activated carbon was determined by the MBAS method, while the amount of HDTMA-Br absorbed was determined by the bromine phenol blue method. The formed SMAC was characterized using FTIR, SEM, SAA and kinetic data to determine the rate constants of the reaction, then SMAC was applied to the adsorbent Pb 2+ and N 03 \

. Coconut Shell Carbonization
Coconut shells were carbonized at 400°C for 1 hour in a pyrolysis reactor [11]. Carbonization is carried out to produce carbon, where in this process, volatile compounds are removed so that the initial pore structure is formed. The resulting heat energy breaks down the coconut shell ' s carbon complex molecules into carbon, gas, and tar. The results of this study obtained carbon of 25.11 %. The reactions that occur during carbonization are presented in Figure 1.
The synthesis of activated carbon from coconut shells was carried out using ZnCl2 activator and microwave radiation. Then, activated carbon was modified with anionic surfactant Sodium Lauryl Sulfate (SLS) hexadecyltrimethylammonium bromide (HDTMA-Br) to obtain a better SMAC than activated carbon. The synthesized SMAC was used to adsorb Pb 2+ and N 03~.
FTIR characterized materials to determine functional groups, SEM to determine surface morphology, and Gas Sorption Analysis (GSA) to determine specific surface area, total pore volume, and pore diameter. The concentration of Pb 2+ before and after being adsorbed by SMAC was determined by atomic absorption spectrophotometer (AAS), while the nitrate groups were analyzed using UV-Vis. The purpose of this study was to determine the optimum contact time and concentration in the adsorption process of SLS anionic surfactant and HDTMA-Br cationic surfactants, to determine the most effective method of modification with SLS and HDTMA-Br to absorb Pb 2+ and N 03 " .

Materials and Equipment
The materials used were coconut shell, ZnCl2,  Figure 1. Carbonization reaction [12]

. Carbon activation
ZnCl2 activator is Lewis acid, which acts to bind impurities, hydrocarbons and open the pore structure formed from the carbonization. Activation can increase the carbon structure ' s lattice structure so that the more activated carbon structure is formed, the pore volume increases [ 13 ]. ZnCl2 reagent is widely used as an active reagent because it produces a high surface area and a high yield of activated carbon [ 14 ]. The mechanism of activation with microwaves can be explained by dipolar polarization and ionic conduction. Some solvents have permanent dipoles. Dipoles rotate when exposed to an electric field, which causes friction and heat in the material. Ion-containing solutions can also generate heat by ionic conduction [ 15 ]. As a result, heating using microwaves, the time required is shorter. The characteristics of the activated carbon produced are also different from conventional heating. Acquisition of activated carbon in this study was 89.8 % of carbon.  Figure 3 . The relationship between contact time and the adsorption efficiency of SLS and HDTMA-Br Activated carbon is hydrophobic, so surfactants can easily interact on its surface through hydrophobic attraction [16]. Surfactants change the surface characteristics of activated carbon from slightly polar to polar. The hydrophobic alkyl chains of the anionic surfactant (SLS) and long HDTMA-Br are oriented to the activated carbon ' s surface. Meanwhile, the SLS hydrophilic groups, which are negatively charged, and the positively charged hydrophilic groups of HDTMA-Br are oriented to the aqueous solution. As a result, the SMAC-SLS surface is negatively charged, while the SMAC-HDTMA-Br surface is positively charged. Some important control parameters in the adsorption of SLS and HDTMA-Br on activated carbon are the concentration of SLS and HDTMA-Br and contact time. This study determines the optimum conditions for SLS and HDTMA-Br adsorption on activated carbon to form the optimum SMAC. The surfactant adsorption mechanism on the activated carbon surface is presented  the surface of the activated carbon is weak. After all, it is only physical adsorption, so that the adsorbate is easily released. It is also thought that the pore corresponding to the molecular size of the SLS has been covered, resulting in decreased adsorption capacity.

. Synthesis of SLS and HDTMA-
Modification with HDTMA-Br showed that during the contact time between 3 to 4 hours, there was an increase in the number of HDTMA-Br molecules adsorbed by activated carbon. However, after 4 hours to 7 hours of contact time, the adsorption efficiency decreased, which means that the optimum contact time for HDTMA-Br adsorption on activated carbon was 4 hours. At 4 hours of contact time, activated carbon can adsorb HDTMA-Br with an adsorption efficiency of 89.04 %. However, this result is smaller than previous studies, with different bases and activators, which showed up to 99 -7 % adsorption [20]. It can be explained that at the contact time of 4 hours, the pores of activated carbon corresponding to the size of HDTMA-Br were covered by HDTMA-Br surfactants. At the contact time after 4 hours, the efficiency of activated carbon in adsorbing HDTMA-Br molecules decreased by about 1.6%. This is because the functional group bonds formed between the activated carbon and the HDTMA-Br molecule are weak (physical adsorption), so they are easily released.

. Determination of the optimum contact time for surfactant adsorption
The relationship between contact time and adsorption efficiency is presented in Figure 3 .

. Determination of the optimum surfactant concentration
The The surface morphology of carbon activated carbon and surfactant modified activated carbon were obtained from Scanning Electron Microscopy (SEM). Sample micrographs with magnifications of 2500 x and 5000 x, respectively, are shown in Figure 6. Comparison of carbon and activated carbon micrograph shows that the cavities and pores of activated carbon are more homogeneous and well developed on the external surface. The pores are seen to be uniformly developed in a mixture of oval and circular patterns. The pores formed have a large enough diameter that ranges from 2.19 -6.39 pm. This indicates that the carbon activation was successful because the inactivated carbon did not show any pores. This indicates that the activation process, which involves ZnCl2 as an activating agent, helps create more pores in the activated carbon, increasing surface area, and pore volume [   surfactants ' adsorption tends to increase in the concentration range of 40 to 60 ppm, with the adsorption efficiency also increasing. The curve tends to flatten out after 60 ppm concentration. This shows that the activated carbon surface is starting to saturate because the pores on the surface of the activated carbon, which correspond to the surfactant molecules, have been covered by the surfactant molecules. The curve in Figure  4 (b) shows that the amount of HDTMA-Br adsorbed on activated carbon increases in the 200 to 300 ppm area. At concentrations after 300 ppm, the adsorption tends to flatten out, which indicates that HDTMA-Br molecules have covered the carbon surface. When the curve starts to flatten out, which is called the optimum concentration of HDTMA-Br adsorption, the efficiency is 90.69 %. This efficiency is lower than the previous study, which used different basic materials and activators, which reached 99.7 % [20].   are visible. This indicates that the surfactant has been adsorbed by activated carbon. SEM images also show that SMAC ' s surface morphology is more homogeneous and smoother than that of carbon and activated carbon. Theoretically, the presence of surfactant interactions with the activated carbon surface provides more sites for adsorbing heavy metals or organic compounds [21].

. Gas Sorption Analysis (GSA)
The pore and surface characters obtained from the Gas Sorption Analyzer (GSA) are presented in Table 1.  (SMAC) adsorption as proposed by [ 23 ]. Both surfactants occupy the pores of the activated carbon so that the surfactants are spread on the surface of the activated carbon in the form of their monomers.  Table 1 shows that activated carbon activated by ZnCl2 produces a larger surface area, volume, and pore radius than carbon. This shows that the preparation of activated carbon using ZnCl2 activator using microwaves for 5 minutes with 400 -Watt power is a fast and effective activation method. Surfactant-modified activated carbon shows a reduction in the BET-specific surface area or pore volume. In line with SEM analysis, Figure 7 shows that activated carbon pores are lower because it contains SLS and HDTMA-Br surfactants. According to the definition by IUPAC, the adsorbent pores are classified into three groups: micropores ( <2 nm diameter), mesoporous (2-50 nm), and macropores ( > 50 nm). Figure 7 shows that the pore size distribution of carbon, activated carbon, and SMAC is mostly at diameters of more than 20 A or mesopore.

. Adsorption Mechanism of Cationic and Anionic
Surfactants by Activated Carbon Figure 8 shows that the activated carbon modified by methods A and B occurs electrostatic interactions between each surfactant [22]. This makes SMAC less effective at adsorbing ions. In contrast to methods A and B, there was a competition between SLS and HDTMA-Br surfactants in method C, or there was competitive

. SMAC application for Pb 2+ and N 03~a dsorption
This adsorption compares the specific absorption of Pb 2+ and N 03~a nions on activated carbon modified by anionic-cationic surfactant and activated carbon before being activated. Pb 2+ and N 03 _ cations were adsorbed by SMAC with initial concentrations of 60 ppm and 36 ppm, respectively, with a contact time of 60 minutes. The absorption comparison test results between activated carbon, the results of the modification of activated carbon to SMAC with methods A, B, and C against Pb 2+ and N 03 ' are presented in Figure 9 .  Modification of activated carbon with HDTMA-Br and SLS surfactants causes activated carbon to have positive and negative charges on the activated carbon ' s surface. It can then be used as an adsorbent for Pb 2+ and N 03 " ions through ion exchange processes and electrostatic interactions [18]. Figure 9 shows that for all adsorbentsactivated carbon, SMAC-A, SMAC-B, and SMAC-Cthe adsorption capacity for the Pb 2+ cation is always higher than the adsorption of N 03 anions. This is because the initial concentration of Pb 2+ is higher than the N 03 " ion, where the higher the initial concentration of the adsorbate, the adsorption capacity also increases [ 24 ]. Meanwhile, the adsorption capacity of nitrate ions in the four samples did not differ significantly. This is because the nitrate concentration used is too low and because the radius of N 03~h ydrated ( 5.1 A) is bigger than the Pb 2+ cation (2.61 A). This results in lower nitrate anions being adsorbed by activated carbon. Figure 9 also shows that the adsorption capacities of Pb 2+ and N 03 cations are higher at SMAC (C). The increase in the adsorption capacity of Pb 2+ cations and N 03 " anions occurs due to modification of the surfactant molecules on the activated carbon surface, which can multiply the active sites to bind or absorb ions. This is evidenced by a decrease in activated carbon ' s specific surface area after surfactant modification, from 47.512 m 2 / g to 14.472 m 2 / g [ 25 ]. The ion adsorption process does not only occur in pores but also occurs on charged surfaces. At SMAC-C, the adsorption capacity for N 03ã nions was 1.376 mg / g and Pb 2+ cations 0.896 mg / g. Adsorption on SMAC-C is competitive adsorption between SLS and HDTMA-Br so that both adsorbates occupy the entire surface of the activated carbon.

. Conclusions
The optimal contact time for both SLS and HDTMA-Br adsorption on activated carbon was 4 hours with the optimum concentration of 60 ppm of SLS surfactant and 300 ppm of HDTMA-Br. The most effective method of modifying activated carbon with SLS and HDTMA-Br to absorb Pb 2+ cations and N 03~a nions is method C, with adsorption capacities of 1.376 mg / g, 0.896 mg / g, respectively. Surfactant modified activated carbon (method C) showed the presence of S = 0 and (CH 3 ) 3 N + functional groups in the FTIR spectra with wavenumbers of 1097 cnr 1 and 1460 cm 1 , respectively. SMAC ' s surface