Title Science and Technology Indonesia e-ISSN:2580-4391 p-ISSN:2580-4405 Vol. 6, No. 4, October 2021 Research Paper ZnAl-Humic Acid Composite as Adsorbent of Cadmium(II) From Aqueous Solution Jeri Rahmadan1, Veronika Parhusip2, Neza Rahayu Palapa2,4, Tarmizi Taher3, Risfidian Mohadi2, Aldes Lesbani2,4* 1Magister Programme, Faculty of Mathematics and Natural Sciences, Sriwijaya University, Palembang, 30139, Indonesia2Graduate School, Faculty of Mathematics and Natural Sciences, Sriwijaya University, Palembang, 30662, Indonesia3Department of Environmental Engineering, Faculty of Mathematics and Natural Sciences, Institut Teknologi Sumatera, Lampung, 35365, Indonesia4Research Center of Inorganic Materials and Complexes, Faculty of Mathematics and Natural Sciences, Universitas Sriwijaya, Palembang, 30139, Indonesia *Corresponding author: aldeslesbani@pps.unsri.ac.id AbstractThe modified ZnAl composites with humic acid (HA/ZnAl composite) were a coprecipitation method used as an adsorbent to removecadmium(II) in an aqueous solution. The synthesized material was characterized using SEM-EDX, FTIR, and XRD. From the analysisusing XRD, there is a widening peak at an angle of 2\ (24°), FTIR analysis there is a new peak at 1000 cm−1 and 1220 cm−1 and SEManalysis show that there is a change in surface morphology due to more aggregated particles. The experimental results on HA/ZnAlcomposite followed the PSO model, where the cadmium(II) removal process was chemical adsorption. The adsorption isothermfollows the Langmuir model where adsorption occurs in a monolayer and maximum adsorption capacity was obtained 38.76 mg/ghigher than pristine. Thermodynamics of the adsorption process of cadmium(II) in an aqueous solution occurred spontaneouslywhere G < 0 at all temperatures and endothermic properties were tested. The performance of the HA/ZnAl composite showed thatstrong potential as an adsorbent of low cost, high efficiency, easy operation, and good reusability. The cadmium(II) was absorbed onthe surface of HA/ZnAl composite by surface complexation and chelating interactions, and surface complexation was the main routeof cadmium(II) removal. In addition, HA/ZnAl composite has excellent treatment efficiency in actual aqueous solution. KeywordsLDH, Humic Acid, Composite, Cadmium(II), Adsorption Performance, Regeneration Received: 24 May 2021, Accepted: 18 August 2021 https://doi.org/10.26554/sti.2021.6.4.247-255 1. INTRODUCTION Heavy metal accumulation is a threatening contaminant in wa- terpollutionbecauseof itshigh toxicity. Oneofpollution isone of the most severe water pollution issues worldwide because of the high toxicity and easy accumulation of heavy metals. Cadmium(II) (Cd(II)) is one of the heavy metals that has re- ceived widespread attention today because of its high activity and is widely used in typical heavy metal elements (Li et al., 2019). There are many ways to remove Cd(II) from aqueous solutions, including adsorption, ion exchange, chemical precip- itation, membrane ltration, electrochemical treatment, plant and microbial methods (Zhu et al., 2019). However, those methods have limitations. For example, chemical precipitation, such as chemical precipitation, is generally expensive and en- vironmentally unfriendly because of the amounts of chemical reagents for regulating the pH (Su et al., 2019). Adsorption is popular because it has the advantages of easy operation, high eciency, and reusability (Siregar et al., 2021). In addition, the adsorbent has a high potential for water treat- ment. Activated carbon, zeolite, and silica gel are the adsor- bents widely used in removing Cd through the adsorption process. However, these materials have several disadvantages, are challenging to modify, and their structure tends to be rigid (Gu et al., 2019). On the other hand, synthetic materials are easier to modify and exible. This uniqueness makes them interesting to study (Yao et al., 2019). LDH is a material that can be easily modied (Tichit et al., 2019). LDH is a synthetic anionic clay mineral having the general formula [M2+(1−x)M3+x(OH)2](An−)x/n•mH2O where M2+ and M3+ represent divalent metal cations and trivalent metal cations, respectively (Bukhtiyarova, 2019). LDH is a stack of positive layers separated by interlamellar spaces con- sisting of anions and water molecules (Mishra et al., 2018). Whereas space within the anions has the role of counterbal- ancing and exchanging species symbolized by A−n, including nitrate, carbonate, sulfate, organic acid anions, and inorganic anions (Sun et al., 2019). Furthermore, LDH has a large sur- face area, good ion exchange capacity, layered structure, easy modication, high exibility, and acid-base properties (Mostafa and Bakr, 2019). However, LDH has several disadvantages, https://crossmark.crossref.org/dialog/?doi=10.26554/sti.2021.6.4.247-255&domain=pdf https://doi.org/10.26554/sti.2021.6.4.247-255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 including peeling o during application and not reusing (Ruan et al., 2016). So it is necessary to improve the structure of LDH in adsorbent applications by using supporting materials that can increase the layer integrity of LDH (Qu et al., 2019). Many researchers have modied LDHs as heavy metal adsor- bents in aqueous environments and exhibited great adsorption performance. Adsorbent Fe3O4/MgAl-LDH showed a large adsorption capacity as adsorbent of Pb and Cd metals in an aqueous solution (Sun et al., 2019). Lyu et al. (2019) reported that Pb2+ and Cd2+ adsorption using chitosan/MgAl compos- ite produced a maximum adsorption capacity of 33.3 mg/g and 140.8 mg/g in an aqueous solution. Dye adsorption using CuAl/biochar composite, with a maximum capacity of 470.96 mg/g (Palapa et al., 2020). Humic acid (HA) is a natural and multifunctional macro- molecular organic compound consisting of a carbon skeleton with a high aromatic character and having a functional group containing mostly oxygen atoms. The humic acid functional groups have dierent abilities as ligands in the formation of complexes with metal cations. Extensive sources and low price of HA consider and has been considered as an adsorbent for heavy metals. Recently, HA was included as a support material to increase the surface area of the material (Shi et al., 2020). Moreover, it has a large adsorption capacity and surface area (Yang et al., 2019). Advanced utilization of humic acid (HA) in a structural improvement of materials was reported in LDH composites. This material is used in removing water contami- nants through the adsorption method. The materials that can be used for LDH modication are carbon-based, metal oxides, and synthetic polymer materials that can increase the integrity of the adsorbent to a certain extent and limit applications in wastewater treatment (Zhanget al., 2020; Normah et al., 2021). While other materials such as natural polymer materials have various sources, wide varieties, and low cost. Therefore, the modied LDH material as a metal ion adsorbent has a much greater potential than other supporting materials. As promis- ing adsorbent (Basu et al., 2019). Shi et al. (2020) wrote that humic acid/Mg-Al LDH composites as Cd2+ metal adsorbent resulted in an adsorption capacity of 155.28 mg/g. ZnAl-LDH was used as a raw material in this paper due to the ability of anionic species to intercalate and there is a large interlayer space (high porosity), the ability of anions to be exchanged between the positively charged layers, and the struc- tural water resistance. however, ZnAl-LDH has a weakness so that it needs to be modied to increase its integrity as an adsorbent (Palapa et al., 2021). This paper will modify LDH by being composited with a carbon-based material, namely humic acid. This paper, synthesize HA/ZnAl composite, was proposed and researched. Considering the wide range of raw material sources and the sample preparation method, it may be a promising adsorbent with broad application prospects for Cd(II) adsorption. This research aimed to synthesize HA/ZnAl composite ex- plore its adsorption performance. First, I observed the surface properties of HA/ZnAl composite with a series of characteri- zation methods. Then, the adsorption capacity of HA/ZnAl composite was studied by batch Cd(II) adsorption experiments. In this paper, to see the performance of HA/ZnAl composite adsorbent in absorbing metal Cd(II) in aqueous solution, it will be discussed and investigated in batch adsorption equip- ment, by covering various conditions such as the eect of pH, time, concentration, adsorption temperature, regeneration and eectiveness of the adsorbent so that it can be reused. 2. EXPERIMENTAL SECTION 2.1 Material and Instrumentation The synthesis of materials in this study used HCl (by Merck), Al(NO3)3•9H2O (by Merck) humic acid (by Merck), NaOH (by Sigma Aldrich), Zn(NO3)2•6H2O (by Merck), CdCl2 (by Merck), 1,10-phenanthroline monohydrate (by Merck), ac- etate buer (by Merck), ethanol (by Merck), and acetone (by Merck). In this paper, characterization material using XRD the Rigaku Miniex-6000 diractometer, the FTIR analysis using Shimadzu FTIR Prestige-21, and the SEM analysis us- ing SEM Quanta-650 Oxford instrument. Spectrophotometer UV-Visible Biobase BK-UV 1800PC was used to measure the concentration of the solution. 2.2 Preparation of ZnAl-LDH and HA/ZnAl Composite The preparation HA/ZnAl composite was made by developing previous research using the coprecipitation method (Shi et al., 2020). The preparation of ZnAl-LDH was carried out by Karami et al. (2019). The materials used in this synthesis included 10 mL of 0.75 M Zn (NO3)2.6H2O and 10 mL of 0.25 M Al (NO3)3•9H2O (3:1) mixed with vigorous stirring for one hour until homogeneous. Then the resulting mixture was added with 1 g of humic acid and then stirred continuously until the solution was homogeneous. Then the mixture was adjusted to pH 10 using a 2 M NaOH solution slowly until a precipitate was formed in the solution. Then the mixture was stored at 65°C for three days. Finally, the composite was washed and dried at 40°C. 2.3 Batch adsorption experiments and Regeneration Study In this paper, to determine the adsorption performance of ZnAl-LDH and HA/ZnAl composites various experiments need to be carried out. These included making a solution of 1.634 g CdCl2 with 1000 mL of water to obtain 1000 mg/L as a Cd(II) stock solution. Standard solutions are prepared by diluting the Cd(II) stock solution. Measurement of the stan- dard curve was carried out by measuring each solution that had been complexed with a 1,10-phenanthroline solution and then measured using a UV-Vis spectrophotometer at the max- imum wavelength obtained. A batch system carried out the Cd(II) adsorption in aqueous solutions. The Cd(II) adsorp- tion process was carried out by varying the pH of the initial solution from 2 to 9, contact time from 10 to 200 minutes, initial Cd(II) concentrations of 5 mg/L and 10 mg/L. The thermodynamic parameters are determined by concentration variation of (9, 12, 15, and 20) mg/L, 0.02 g of adsorbent, 20 © 2021 The Authors. Page 248 of 255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 mL of Cd(II), and temperature variations at 30-60◦C. After that, the mixture was centrifuged at a constant stirring speed of 400 rpm then the mixture is ltered. The resulting super- natant was then measured to determine Cd(II) concentration using a UV-Visible spectrophotometer after complexing with 1,10-phenanthroline. The process of regenerating uses the adsorption-desorption process repeatedly in 5 cycles. First, as much as 0.02 g of adsorbent was added 20 mL of Cd(II) with a concentration of 10 mg/L, stirred for 2 hours. The adsorbate residue was measured using a UV-Vis spectrophotometer, and the adsorbent was dried for desorption using water, sodium hy- droxide, hydrochloric acid, acetone, and ethanol. Finally, a des- orption process was carried out with several solvents to obtain a suitable solvent. In this paper, to determine the adsorption performance of ZnAl-LDH and HA/ZnAl composites various experiments need to be carried out. These included making a solution of 1.634 g CdCl2 with 1000 mL of water to obtain 1000 mg/L as a Cd(II) stock solution. Standard solutions are prepared by diluting the Cd(II) stock solution. Measurement of the standard curve was carried out by measuring each so- lution that had been complexed with a 1,10-phenanthroline solution and then measured using a UV-Vis spectrophotometer at the maximum wavelength obtained. A batch system carried out the Cd(II) adsorption in aqueous solutions. The Cd(II) adsorption process was carried out by varying the pH of the ini- tial solution from 2 to 9, contact time from 10 to 200 minutes, initial Cd(II) concentrations of 5 mg/L and 10 mg/L. The thermodynamic parameters are determined by concentration variation of (9, 12, 15, and 20) mg/L, 0.02 g of adsorbent, 20 mL of Cd(II), and temperature variations at 30-60◦C. After that, the mixture was centrifuged at a constant stirring speed of 400 rpm then the mixture is ltered. The resulting super- natant was then measured to determine Cd(II) concentration using a UV-Visible spectrophotometer after complexing with 1,10-phenanthroline. The process of regenerating uses the adsorption-desorption process repeatedly in 5 cycles. First, as much as 0.02 g of adsorbent was added 20 mL of Cd(II) with a concentration of 10 mg/L, stirred for 2 hours. The adsorbate residue was measured using a UV-Vis spectrophotometer, and the adsorbent was dried for desorption using water, sodium hydroxide, hydrochloric acid, acetone, and ethanol. Finally, a desorption process was carried out with several solvents to obtain a suitable solvent. 3. RESULTS AND DISCUSSION The structural morphologies of ZnAl-LDH and HA/ZnAl composite were investigated using SEM with a particle size of about 10.00 mm, which had been successfully prepared (Figure 1a and 1b) HA/ZnAl composite Figure had a rougher surface than ZnAl-LDH, Overall, and the composite presented a stacked layered structure with a large amount of debris, and this is due to the presence of humic acid. EDX image (Figure 1c and 1d) of HA/ZnAl composite and the elemental map- ping presented shows that the HA/ZnAl composite contains zinc, aluminum, oxygen, carbon, and nitrogen of 13.3%, 6.6%, Figure 1. Morphologies and Compositions of ZnAl-LDH (a,c) and HA/ZnAl Composite (b,d) 46.9%, 24.6%, and 5.8% for ZnAl-LDH pristine the content of zinc, aluminum, oxygen, and carbon were 48.3%, 38.2%, 4.9%, and 4.2% respectively. The mapping images show that oxygen, zing, and aluminum have an excellent correlation due to using the same raw material. The carbon distribution is heteroge- neous, with more in the upper left and right sides and less in the middle. According to Koesnarpadi et al. (2015) the eect of foreign carbon may cause this. Nitrogen has a homogeneous distribution and good correlation with other elements, but the overall content was lower. The rough surface and debris may both create new adsorption sites, which increase the adsorption of Cd(II). Figure 2. XRD Patterns and FTIR Spectra of ZnAl-LDH (a) Humic Acid (b) and HA/ZnAl Composite (c) The XRD and FTIR characterization of ZnAl-LDH, hu- mic acid, and HA/ZnAl composite was shown in Figure 2. Figure 2a ZnAl-LDH pristine has diraction peaks at 10.29°, 20.07°, 29.59°, 32.12°, 34.02°, 48.06° and 60.16° that can be indexed to the (003), (006), (101), (012), (015), (107) and © 2021 The Authors. Page 249 of 255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 (110) based on the diraction peaks the formation of the ZnAl material structure is following JCPDS 48-1023 le. Figure 2b shows that the diraction peaks at 12.36° (001), 20.01° (110), 24.88° (002), and 26.64° (111) are typical peaks of humic acids. According to Abate and Masini (2005) the diraction peak at 20-26° indicates that the humic acid contains a lot of carbon and oxygen. Figure 2c for HA/ZnAl composite all the diraction peaks matched well with those of ZnAl-LDH pristine, in Figure 2c the diraction peak around 24° is the characteristic peak of the HA/ZnAl composite material which has an amorphous diraction pattern (Shi et al., 2020). This may be related to the addition of modied humic acid. The results of material analysis using FT-IR spectrum are shown in Figure 2. The characteristic peaks of humic acid in the com- posite were detected around the peaks at 1000 cm−1 and 1220 cm−1 indicating C-O and -COOH groups. According to Shi et al. (2019) the characteristics of humic acids are character- ized by the presence of a carboxyl group (-COOH). The band at 3448.72 cm−1 indicated the O-H region extends from the material layer and water molecules between layers. The peak at 1703 cm−1 indicates a COO-carbonyl group, the peak at 1000 cm−1 indicated the C-C group. The characteristic band of the -OH group in the layer and peak of the H2O bending vibration is 1637.7 cm−1 (Deng et al., 2015). The peak at 1500-1800 cm−1 indicates the COO- group. The peak at 2088.8 cm−1 is the C-O vibration (Sun et al., 2019). The peak of 1250 cm−1 indicated the C-OH group. The peak at 1117.8 cm−1 corresponds to the –COO– vibration and the C–O strain vi- bration, respectively (Shi et al., 2019). In addition, the bands at 671.3 cm−1 and 558.8 cm−1, located in the 450 cm−1 –750 cm−1 region, all represent Al-O and Zn-O vibrations. Overall this conrms that humic acid has been successfully loaded into LDH. 3.1 Eect of Solution pH and Eect of Initial Concentration The adsorption process in this paper controls the pH of the so- lution as the main parameter. Because it can aect the surface charge of the adsorbent and the ionization behavior of the so- lution. The Cd(II) solution with a concentration of 5 mg/L for ZnAl-LDH and 10 mg/L for HA/ZnAl composite where the pH was adjusted with HCl (0.1N) and NaOH (0.1N). In Figure 3a Cd(II) removal increased signicantly with increasing pH of the initial solution, Highest adsorption capacity at pH 6.5 for LDH pristine and pH 6 for composites the removal eciencies can reach up to 68.01% and 90.77% respectively. This may be due to the deprotonation of functional groups in the HA/ZnAl composite through the formation of hydrogen bonds or water bridges, electrostatic interactions or ion exchange, coordination bonds, and chelate ring structures. At low pH, the adsorbed Cd(II) is very small, and a Cd(II)-HA/ZnAl composite com- plex is formed. This is due to a large number of Cd(II) H+ ions in the system causing the composite to tend to be protonated, which results in strong hydrogen bonds between functional groups in protonated composites, both intermolecular (Terdki- atburana et al., 2008). Prevent the adsorption of metal ions on Figure 3. pH Amount of ZnAl and HA/ZnAl Composite (a) Initial Cd(II) Concentration (b) the composite surface due to electrostatic repulsion between equal charges. In this paper, the initial concentration of metal ions plays an important role in adsorbent absorption. The eect of initial concentrations of Cd(II) (9, 12, 15, and 20 mg/L) was tested using ZnAl-LDH and HA/ZnAl composite at pH 6.5 and pH 6. Figure 3b The resulting adsorption capacity increased at low concentrations. While the adsorption capacity gradually decreased with increasing Cd(II) concentration. This process indicates that thevacantactivesitesonbothadsorbentsarelled at critical concentrations. However, the equilibrium adsorp- tion capacity continued to increase as the Cd(II) concentration increased. the maximum absorption of Cd(II) using ZnAl pristine is 19.92 mg/g while HA/ZnAl composite is 21.882 mg/g. 3.2 Adsorption Kinetics and Isotherms The eect of adsorption time on Cd(II) adsorption in aque- ous solution using ZnAl-LDH and HA/ZnAl composite was through variations in the contact time of the adsorbate with the adsorbent. A total of 0.02 g of the sample was added to an erlenmeyercontaining20 mLof Cd(II) with aconcentration of 5 mg/L for ZnAl-LDH and 10 mg/L for HA/ZnAl composite whose pH value had been adjusted according to the provisions, with time variation 10-200 minutes. The adsorption ability of all concentrations increased with increasing time from 0 to 120 minutes for the HA/ZnAl composite occurred rapidly in the rst stage. while the second stage is slower and tends to equilibrium, this tendency in the early stages, This process occurs because the higher accessibility of the carboxyl group results in many vacant sites for adsorption on the surface of the adsorbent. The optimal equilibrium time was 120 minutes for ZnAl-LDH and 90 minutes for HA/ZnAl composite. Adsorption kinetics of Cd(II) onto ZnAl-LDH and HA/ ZnAl composite was investigated using pseudo-rst-order and pseudo-second-order, shown in equations (1) and (2) log (Qe-Qt) = log Qe– k1 2.303t (1) t Qt = ( 1 k2Qe2 ) + 1 Qe t (2) © 2021 The Authors. Page 250 of 255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 Table 1. Parameters Kinetic for Cd(II) Adsorption onto ZnAl-LDH and HA/ZnAl Composite Kinetics Models Parameters Adsorbent ZnAl-LDH HA/ZnAl Composite PFO QeExp (mg/g) 4.148 8.61 QeCalc (mg/g) 4.797 7.509 K1 (min−1) 0.021 0.02 R2 0.928 0.979 PSO QeExp (mg/g) 4.148 8.61 QeCalc (mg/g) 5.359 10.267 K2 (min−1) 0.003 0.002 R2 0.99 0.994 Figure 4. Time Adsorption of Cd(II) by ZnAl-LDH and HA/ZnAl Composite where Qe (mg/g) and Qt (mg/g) are the adsorption ca- pacity of ZnAl-LDH and HA/ZnAl composite for Cd(II) at equilibrium and time t (min), respectively; k1(1/min) and k2 (g/mg.min) are the rate constants of the pseudo-rst-order and pseudo-second-order models, respectively. The results of the two-parameter kinetic model are shown in Table 1. both adsorbents provide a larger correlation coef- cient (R2) in the pseudo-second-order model compared to the pseudo-rst-order model. From the results obtained, the pseudo second orderkinetics model is more suitable to describe the adsorption of Cd(II) on ZnAl-LDH and HA/ZnAl-LDH composite. According to Niu et al. (2011) this proves that the rate-limiting step for ZnAl-LDH and HA/ZnAl composite adsorption of Cd(II) is most likely chemical adsorption. In addition, the resulting Qe value for the LDH pristine adsorbent is smaller than the Qe value for the composite adsorbent, the adsorption process can involve a series of functional groups on the surface of the composite, including hydroxyl groups, carboxyl groups, etc. These results indicate that composite material exhibits a large adsorbent potential compared to LDH pristine. The isotherm data analysis has shown that the isotherm pa- rameters were calculated using the Langmuir isotherm model, formulated in Equations (3). The hypothesis of the Langmuir Figure 5. Adsorption Isotherm of Cd(II) by using ZnAl-LDH (a) and HA/ZnAl Composite (b) adsorption model, the adsorbent has several properties, includ- ing uniform adsorption sites, monolayeradsorbent surface, and no lateral interactions between the adsorbed molecules. Ce Qe = Ce Qm + 1 QmKL (3) The equilibrium concentration of Cd(II) and equilibrium adsorption capacity of ZnAl-LDH and HA/ZnAl composite materials are respectively Ce (mg/L) and Qe (mg/g). The max- imum adsorption of the Langmuir model is Qm (mg/g). Lang- muir’s constant is KL (L/mg). The isotherm ttings parameters are shown in Figure 5 and Table 2. The maximum adsorption capacity of the HA/ZnAl composite increased signicantly up to 38.76 mg/g at 60°C. The maximum adsorption capacity of HA/ZnAl composite is higher than LDH pristine. Thus, the HA/ZnAl composite showed high eectiveness in adsorption. This nding conrms that the composite material improves the absorption performance of heavy metals related to the syn- ergistic eect of humic acid which contains many functional groups such as carboxyl and ZnAl-LDH in the composite be- cause of the surface area of the HA/ZnAl composite is higher than LDH pristine. © 2021 The Authors. Page 251 of 255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 Table 2. Parameters and Coecient of Langmuir Isotherms Models of Cd(II) Adsorption onto ZnAl-LDH and HA/ZnAl Composite Adsorbent Adsorption Isotherm T (◦K) Adsorption Constant Qmax KL R2(mg/g) (L/mg) ZnAl-LDH Langmuir 303 19.920 0.259 0.989 313 19.305 0.344 0.985 323 19.802 0.353 0.977 333 22.727 0.349 0.997 303 21.882 0.609 0.998 HA/ZnAl 313 20.325 0.842 0.969 Composite 323 21.834 0.929 0.964 333 38.760 0.408 1.000 Table 3. Thermodynamic Parameters for the Adsorption of Cd(II) onto ZnAl-LDH and HA/ZnAl Composite T (◦K) Concentration ZnAl-LDH HA/ZnAl Composite (mg/L) ΔH(J/mol) ΔS (J/mol.K) ΔG (kJ/mol) ΔH(J/mol) ΔS (J/mol.K) ΔG (kJ/mol) 303 20 11.724 0.043 -1.377 29.329 0.105 -2.568 313 -1.81 -3.621 323 -2.242 -4.674 333 -2.675 -5.726 3.3 Thermodynamic Parameter Adsorption Calculation of thermodynamic parameters using the following Van’t Ho equation (4) and (5). The value of enthalpy (ΔH) and entropy (ΔS) can be calculated as the slope and intercept value of 1/T concerning ln Qe/Ce as shown in equation (4). ln Qe Ce = ΔS R − ΔH RT (4) Figure 6. Van’t Ho Linearity Equation for the Adsorption of Cd(II) onto ZnAl-LDH (a) and HA/ZnAl Composite (b) The value of the Gibbs free energy change (ΔG) is cal- culated from the enthalpy value (ΔH) and entropy (ΔS) is presented in equation (5). ΔG = ΔH −TΔS (5) Table 3 describes the adsorption process that occurs en- dothermically, where the adsorption process occurs followed by the capture of heat from the environment to the system, this is indicated by the enthalpy value (ΔH) which is positive. According to Rakić et al. (2015) endothermic process, the ad- sorbate species displaces more than one water molecule to be adsorbed. The value of ΔH also describes the type of adsorp- tion. The low enthalpy (ΔH) shows that Cd(II) is adsorbed through a physical interaction process (Juleanti et al., 2021). ΔS positive value indicates an increase in randomness on the surface of the adsorbent with the solution during the adsorp- tion process, this is because of the physical bond between the Cd(II) molecule and the active site of the ZnAl-LDH and HA/ZnAl composite decreases with increasing temperature. ΔG negative value indicates that the adsorption process occurs spontaneously by the ZnAl-LDH and HA/ZnAl composite. Overall it can be concluded that the adsorption of Cd(II) is aected by changes in the temperature of the solution. 3.4 Adsorbents Performance The most important indicator to evaluating the performance of adsorbent is by carrying out the desorption and reusability process. The adsorbent desorption process was carried out us- ing several eluents for Cd(II) desorption from the ZnAl-LDH and HA/ZnAl composite as shown in Figure 7a. The results obtained that the percentage of desorption in acid solution was as high as possible according to the LDH charge to be positive. However, the electrostatic attraction of the Cd(II)-adsorbent molecules and H-bonds weakened. Based on Pearson’s clas- © 2021 The Authors. Page 252 of 255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 Table 4. Comparison of Cd(II) Absorption using Several Types of Adsorbents Adsorbent Experimental t/min Adsorption Capacity RefCondition (mg/g) HA/MgAl-LDH pH = 5.5 80 155.28 (Shi et al., 2020) Fe3O4/Mg–Al–CO pH = 9.0 300 45.6–54.7 (Shan et al., 2015) Mg/AlCO3–LDH pH = 4.0 60 61.4–70.2 (Shan et al., 2015) CQDs/ZnAl-LDH pH = 6.0 20 12.6 (Rahman et al., 2018) FeMnMg-LDH pH = 6.5 250 59.99 (Zhou et al., 2018) MGO/MgAl-LDH pH = 6.5 240 45.05 (Huang et al., 2018) Kiwi Biochar/MgFe-LDH pH = 6.5 120 25.8 (Tan et al., 2019) Humic Acid (HA) pH = 5.0 60 8,8 (Abate and Masini, 2005) Biochar W/HA pH = 7.5 30 167.3 (Park et al., 2017) MRSA/ZnFe-LDH pH = 8.5 240 70.99 (Moaty et al., 2017) Chitosan/TiFe-LDH pH = 8.5 60 98 (Mahmoud et al., 2017) Co-Fe LDH Nanoparticle pH = 8.0 360 65-94 (Moaty et al., 2017) HA/Fe-Mn-OLB pH = 6.0 720 67.11 (Guo et al., 2019) ZnAl-LDH pH = 7.0 120 22.72 This Study HA/ZnA Composite pH = 6.0 90 38.76 This Study sication of hard-soft electrophile, Cd(II) is categorized as a soft electrophile. Therefore, increasing the number of depro- tonated carboxylic groups with increasing pH, which is a weak nucleophile, will lower adsorption power (Liu et al., 2008). In the base condition, the desorption process on the material is also higher due to hydrophobic interactions, and the OH− ions formed so that they have a higher anity for anion exchange to occur. Figure 7. Desorption of Adsorbent (a) and Regeneration of Adsorbent (b) The results of the regeneration of each adsorbent on the Cd(II) can be seen in Figure 7b. As a similar result of increas- ing surface area properties after the formation of HA/ZnAl composite, thus adsorption of Cd(II) was higher than starting materials. The eect of the number of additives in this ex- periment follows the general trend, the greater the number of additives, the higher the adsorption eciency. Nevertheless, humic acid has a good adsorption eect. This is because humic acid contains many hydrophilic functional groups, such as hy- droxyl and carboxylic groups. The presence of this functional group can increase the solubility of humic acid, which increases the ability of humic acid to adsorb. Therefore, HA/ZnAl com- posite has good adsorption eciency due to its large specic surface area. This may be due to the addition of humic acid to create new adsorption sites. According to Rosset et al. (2020) the decrease in adsorption capacity in the regeneration process after repeated cycles, is due to the progressive loss of solid crys- tallinity during the reconstruction process in layered materials and is caused by the incorporation of residual organic species. Basedonseveral typesofadsorbents thathaverecentlybeen found to remove Cd(II) in aqueous solutions in recent years and will compare the adsorption capacity with the adsorbents produced in this paper. In Table 4 the adsorption capacity of most of the adsorbents for Cd(II) is between 0 and 150 mg/g. The maximum adsorption capacity of pure ZnAl-LDH was 22.72 mg/g, while the maximum adsorption capacity of HA/ZnAl composite was 38.76 mg/g. Overall, HA/ZnAl composite materials are very promising to be used as heavy metal removal adsorbents in aqueous solutions. 4. CONCLUSIONS ModicationofZnAl-LDHusinghumicacidasasupportmate- rial to improve structural stability and adsorption performance has been successfully carried out using a simple coprecipita- tion method to remove Cd(II) in an aqueous solution. The characterization results suggest that HA/ZnAl composite by SEM-EDX, XRD, and FTIR has a stacked layered structure, a large specic surface area, and many carbon-containing func- tional groups and the morphology of HA/ZnAl composite showed heterogeneity with some aggregates of LDH. The sur- face area characterization of HA/ZnAl composite increased the surface area increase greater than LDH pristine. From the results obtained, highest adsorption capacity at pH 6.5 for LDH pristine and pH 6 for composite the removal eciencies © 2021 The Authors. Page 253 of 255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 can reach up to 68.01% and 90.77% respectively. The kinetic parameters corresponding to PSO dynamics proved that the adsorption process of Cd(II) on HA/ZnAl composite was most likely chemical adsorption. The equilibrium of Cd(II) adsorp- tion using ZnAl pristine was reached at 120 minutes and the equilibrium of the HA/ZnAl composite was reached at 90 min- utes. The adsorption isotherm follows the Langmuir model where adsorption occurs in a monolayer and maximum adsorp- tion capacity was obtained 38.76 mg/g higher than pristine. Thermodynamic analysis showed that the adsorption of Cd(II) on ZnAl-LDH and HA/ZnAl composite was a spontaneous and endothermic process. Additionally, Cd(II) was absorbed on the surface of HA/ZnAl composite by surface complexa- tion and chelating interactions, and surface complexation was the main route for Cd(II) removal. The HA/ZnAl composite was high structural stability on the Cd(II) readsorption process until ve cycles process. All results illustrated that the prepared HA/ZnAl composite has an excellent adsorption performance and is promising for a potential application. 5. ACKNOWLEDGEMENT The research/publication of this article was funded by DIPA of Public Service Agency of Universitas Sriwijaya 2021. SP DIPA-023.17.2.677515 /2021, On November 23, 2020. In accordance with the Rector’s Decree Number: 0014/ UN9/ SK.LP2M.PT/2021, On Mei 25, 2021. REFERENCES Abate, G. and J. C. Masini (2005). Inuence of pH, ionic strength and humic acid on adsorption of Cd (II) and Pb (II) onto vermiculite. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 262(3); 33–39 Basu, H., S. Saha, I. A. Mahadevan, M. V. Pimple, and R. K. Singhal (2019). Humic acid coated cellulose derived from rice husk: A novel biosorbent for the removal of Ni and Cr. Journal of Water Process Engineering, 32; 100892 Bukhtiyarova, M. (2019). Areview on eect of synthesis condi- tions on the formation of layered double hydroxides. Journal of Solid State Chemistry, 269; 494–506 Deng, L., Z. Shi, and X. Peng (2015). Adsorption of Cr (VI) onto a magnetic CoFe2O4/MgAl-LDH composite and mechanism study. Rsc Advances, 5(61); 49791–49801 Gu, S., X. Kang, L. Wang, E. Lichtfouse, and C. Wang (2019). Clay mineral adsorbents for heavy metal removal from wastewater: a review. Environmental Chemistry Letters, 17(2); 629–654 Guo, J., C. Yan, Z. Luo, H. Fang, S. Hu, and Y. Cao (2019). Synthesis of a novel ternary HA/Fe-Mn oxides-loaded biochar composite and its application in cadmium (II) and arsenic (V) adsorption. Journal of Environmental Sciences, 85; 168–176 Huang, Q., Y. Chen, H. Yu, L. Yan, J. Zhang, B. Wang, B. Du, andL.Xing(2018). Magneticgrapheneoxide/MgAl-layered double hydroxide nanocomposite: one-pot solvothermal synthesis, adsorptionperformanceandmechanismsforPb2+, Cd2+, and Cu2+. Chemical Engineering Journal, 341; 1–9 Juleanti, N., N. R. Palapa, T. Taher, N. Hidayati, B. I. Putri, and A. Lesbani (2021). The Capability of Biochar-Based CaAl and MgAl Composite Materials as Adsorbent for Re- moval Cr (VI) in Aqueous Solution. Science and Technology Indonesia, 6(3); 196–203 Karami, Z., M. Jouyandeh, J. A. Ali, M. R. Ganjali, M. Aghaz- adeh, S. M. R. Paran, G. Naderi, D. Puglia, and M. R. Saeb (2019). Epoxy/layered double hydroxide (LDH) nanocom- posites: Synthesis, characterization, and Excellent cure fea- ture of nitrate anion intercalated Zn-Al LDH. Progress in Organic Coatings, 136; 105218 Koesnarpadi, S., S. J. Santosa, D. Siswanta, and B. Rusdi- arso (2015). Synthesis and characterizatation of magnetite nanoparticle coated humic acid (Fe3O4/HA). Procedia Envi- ronmental Sciences, 30; 103–108 Li, M., S. A. Messele, Y. Boluk, and M. G. El-Din (2019). Isolated cellulose nanobers for Cu (II) and Zn (II) removal: performance and mechanisms. Carbohydrate Polymers, 221; 231–241 Liu, J.-F., Z.-S. Zhao, and G.-B. Jiang (2008). Coating Fe3O4 magnetic nanoparticles with humic acid for high ecient removal of heavy metals in water. Environmental Science & Technology, 42(18); 6949–6954 Lyu, F., H. Yu, T. Hou, L. Yan, X. Zhang, and B. Du (2019). Ecient and fast removal of Pb2+ and Cd2+ from an aqueous solution using a chitosan/Mg-Al-layered double hydroxide nanocomposite. Journal of Colloid and Interface Science, 539; 184–193 Mahmoud, R., S. A. Moaty, F. Mohamed, and A. Farghali (2017). Comparative study of single and multiple pollutants system using Ti–Fe chitosan LDH adsorbent with high per- formance in wastewater treatment. Journal of Chemical & Engineering Data, 62(11); 3703–3722 Mishra, G., B. Dash, and S. Pandey (2018). Layered double hydroxides: A brief review from fundamentals to application as evolving biomaterials. Applied Clay Science, 153; 172–186 Moaty, S. A., A. Farghali, M. Moussa, and R. Khaled (2017). Remediation of waste water by Co–Fe layered double hy- droxide and its catalytic activity. Journalof the TaiwanInstitute of Chemical Engineers, 71; 441–453 Mostafa, M. S. and A.-S. A. Bakr (2019). Adsorptive re- moval of Cd (II) from contaminated water via hexavalent molybdenum-containing layered double hydroxide: Ni/Mo- LDH. Energy Sources, Part A: Recovery, Utilization, and Envi- ronmental Eects, 41(18); 2257–2265 Niu, H., D. Zhang, S. Zhang, X. Zhang, Z. Meng, and Y. Cai (2011). Humic acid coated Fe3O4 magnetic nanoparticles as highly ecient Fenton-like catalyst for complete mineraliza- tion of sulfathiazole. Journal of Hazardous Materials, 190(3); 559–565 Normah, N., N. R. Palapa, T. Taher, R. Mohadi, H. P. Utami, and A. Lesbani (2021). The Ability of Composite Ni/Al- carbon based Material Toward Readsorption of Iron (II) in © 2021 The Authors. Page 254 of 255 Rahmadan et. al. Science and Technology Indonesia, 6 (2021) 247-255 Aqueous Solution. Science and Technology Indonesia, 6(3); 156–165 Palapa, N. R., T. Taher, B. R. Rahayu, R. Mohadi, A. Rachmat, and A. Lesbani (2020). CuAl LDH/Rice husk biochar com- posite for enhanced adsorptive removal of cationic dye from aqueous solution. Bulletin of Chemical Reaction Engineering & Catalysis, 15(2); 525–537 Palapa, N. R., T. Taher, A. Wijaya, and A. Lesbani (2021). Modication of Cu/Cr Layered Double Hydroxide by Keg- gin Type Polyoxometalate as Adsorbent of Malachite Green from Aqueous Solution. Science and Technology Indonesia, 6(3); 209–217 Park, C. M., J. Han, K. H. Chu, Y. A. Al-Hamadani, N. Her, J. Heo, and Y. Yoon (2017). Inuence of solution pH, ionic strength, and humic acid on cadmium adsorption onto acti- vated biochar: experiment and modeling. Journalof Industrial and Engineering Chemistry, 48; 186–193 Qu, J., L. Sha, C. Wu, and Q. Zhang (2019). Applications of mechanochemically prepared layered double hydroxides as adsorbents and catalysts: A mini-review. Nanomaterials, 9(1); 80 Rahman, M. T., T. Kameda, S. Kumagai, and T. Yoshioka (2018). A novel method to delaminate nitrate-intercalated MgAl layered double hydroxides in water and application in heavy metals removal from waste water. Chemosphere, 203; 281–290 Rakić, V., V. Rac, M. Krmar, O. Otman, and A. Auroux (2015). The adsorption of pharmaceutically active compounds from aqueous solutions onto activated carbons. Journal of Haz- ardous Materials, 282; 141–149 Rosset, M., L. W. Sfreddo, O. W. Perez-Lopez, and L. A. Feris (2020). Eect of concentration in the equilibrium and kinetics of adsorption of acetylsalicylic acid on ZnAl layered double hydroxide. Journal of Environmental Chemical Engineering, 8(4); 103991 Ruan, X., Y. Chen, H. Chen, G. Qian, and R. L. Frost (2016). Sorption behavior of methyl orange from aqueous solution onorganicmatterandreducedgrapheneoxidesmodiedNi– Cr layered double hydroxides. Chemical Engineering Journal, 297; 295–303 Shan, R.-r., L.-g. Yan, K. Yang, Y.-f. Hao, and B. Du (2015). Adsorption of Cd (II) by Mg–Al–CO3-and mag- netic Fe3O4/Mg–Al–CO3-layered double hydroxides: ki- netic, isothermal, thermodynamic and mechanistic studies. Journal of Hazardous Materials, 299; 42–49 Shi, M., Z. Zhao, Y. Song, M. Xu, J. Li, and L. Yao (2020). A novel heat-treated humic acid/MgAl-layered double hydrox- ide composite for ecient removal of cadmium: Fabrication, performance and mechanisms. Applied Clay Science, 187; 105482 Shi, Z., Z. Tang, and C. Wang (2019). Eect of phenanthrene on the physicochemical properties of earthworm casts in soil. Ecotoxicology and Environmental Safety, 168; 348–355 Siregar, P. M. S. B. N., N. R. Palapa, A. Wijaya, E. S. Fitri, and A. Lesbani (2021). Structural stability of Ni/Al layered double hydroxide supported on graphite and biochar toward adsorption of congo red. Science and Technology Indonesia, 6(2); 85–95 Su, J., C. Gao, T. Huang, Y. Gao, X. Bai, and L. He (2019). Characterization and mechanism of the Cd (II) removal by anaerobic denitrication bacterium Pseudomonas sp. H117. Chemosphere, 222; 970–979 Sun, X., J. Dong, Z. Li, H. Liu, X. Jing, Y. Chi, and C. Hu (2019). Mono-transition-metal-substituted polyoxometa- late intercalated layered double hydroxides for the catalytic decontamination of sulfur mustard simulant. Dalton Trans- actions, 48(16); 5285–5291 Tan, Y., X. Yin, C. Wang, H. Sun, A. Ma, G. Zhang, and N. Wang (2019). Sorption of cadmium onto Mg-Fe Layered Double Hydroxide (LDH)-Kiwi branch biochar. Environ- mental Pollutants and Bioavailability, 31(1); 189–197 Terdkiatburana, T., S. Wang, and M. Tadé (2008). Com- petition and complexation of heavy metal ions and humic acid on zeolitic MCM-22 and activated carbon. Chemical Engineering Journal, 139(3); 437–444 Tichit, D., G. Layrac, and C. Gerardin (2019). Synthesis of lay- ered double hydroxides through continuous ow processes: a review. Chemical Engineering Journal, 369; 302–332 Yang, Y.-J., B. Wang, X.-J. Guo, C.-W. Zou, and X.-D. Tan (2019). Investigating adsorption performance of heavy met- als onto humic acid from sludge using Fourier-transform infrared combined with two-dimensional correlation spec- troscopy. EnvironmentalScienceandPollutionResearch, 26(10); 9842–9850 Yao, L., Z. Zhao, L. Zhao, Z. Zhang, M. Shi, and M. M. Xu (2019). Evaluation of Adsorption of Cadmium onto Ferrihydrite-Humic Acid Coprecipitation. Science of Ad- vanced Materials, 11(9); 1232–1240 Zhang, W., J. Luo, Y. Huang, C. Zhang, L. Du, J. Guo, J. Wu, X. Zhang, J. Zhu, and G. Zhang (2020). Synthesis of a novel dispersant with topological structure by using humic acid as raw material and its application in coal water slurry preparation. Fuel, 262; 116576 Zhou, H., Z. Jiang, S. Wei, and J. Liang (2018). Adsorption of Cd (II) from aqueous solutions by a novel layered dou- ble hydroxide FeMnMg-LDH. Water, Air, & Soil Pollution, 229(3); 1–16 Zhu, Y., W. Fan, T. Zhou, and X. Li (2019). Removal of chelated heavy metals from aqueous solution: A review of current methods and mechanisms. Science of the Total Envi- ronment, 678; 253–266 © 2021 The Authors. Page 255 of 255 INTRODUCTION EXPERIMENTAL SECTION Material and Instrumentation Preparation of ZnAl-LDH and HA/ZnAl Composite Batch adsorption experiments and Regeneration Study RESULTS AND DISCUSSION Effect of Solution pH and Effect of Initial Concentration Adsorption Kinetics and Isotherms Thermodynamic Parameter Adsorption Adsorbents Performance CONCLUSIONS ACKNOWLEDGEMENT