Palm oil mill effluent (POME) precipitation using ammonium-intercalated clay coagulant

Clay intercalation has been completed to improve coagulation ability using ammonium ions intercalant via multi-step intercalation. The intercalated clay was confirmed by Scanning Electron Microscope-Energy Dispersive Spectroscopy analysis of expanded lamellar and reduction impurities. Fourier Transform Infra-Red analysis confirmed the sharp and strong peak adsorption at 1448 cm -1 as ammonium (NH 4+ ) bending vibration, and X-Ray Diffraction analysis confirmed the peak shifting to smaller 2θ at 10.08° as increasing basal spacing because of ammonium ion intercalated. The Palm Oil Mill Effluent (POME) coagulation was carried out using contact time and coagulant dose variations to determine the optimum conditions, reaching 45 minutes of coagulation and 0.4 g coagulant was used. Furthermore, the turbidity, free fatty acid, and total suspended solids were measured to reach the reduction values of 93%, 49.7%, and 73.7%, respectively. The reusable study of ammonium-intercalated clay confirmed the stability of the three cycles of coagulation used.


Introduction
Oil extracted from the palm (Elaeis Guineensis Jacq.) is included as a vital commodity due to its users around the world. Indonesia is one of the largest countries producing palm oil and exporter of raw materials. The data of the Ministry of Agriculture of the Republic of Indonesia revealed that in 2015 there were 11.3 million hectares of palm plantations and it could produce up to 37.5 million tons of palm oil [1]. The palm oil industry has become an over-productive sector that impacts the country's financial balance. Secondary products also produce the massive palm oil production; one of the largest is palm oil mill effluent (POME), annually disposed of up to 50 million m 3 in Indonesia and Malaysia [2].
Visually POME is the brown viscous liquid categorized as non-toxic waste but generating the disturbing odors and containing various soluble materials that can impact environmental quality [2]. As reported in literature, POME acutely pollutes the water body for containing 25 mg/L of Biological Oxygen Demand (BOD), 50 mg/L of Chemical Oxygen Demand (COD), 8 mg/L of fat and oil, 20 mg/L of suspended solid, and 40 mg/L of total solid [3]. Therefore, it is not highly suggested to dispose the POME directly into the ecosystem as it can affect the soil and water bodies system, especially vegetation [4].
Several methods have recently been improved for oil-based waste treatment, such as Fenton oxidation, membrane separation, adsorption, flocculation, and coagulation [5,6].
Considering its effectiveness and general use, the coagulation has been selected to improve for being easily separated from the final product. Natural materials have become interesting to improve as the coagulant candidate, considering their non-toxic character and abundance in nature [7]. Clay is the best candidate for natural coagulant improvement due to the high potency of modification [8][9][10]. Recent work has observed the coagulation capacity of clay as a coagulant aid of Poly-Aluminum Chloride (PAC) to car wash waste with 90 % removal capacity [11]. The modified organo-clays as coagulant aid of alum removed the pesticide waste up to 82% [12]. However, there is a need for the updated work in using the claybased material as an individual coagulant.
Clay and clays mineral have been popular for a long time in view of their wide range of applications. All clay-based materials based are abundant as their formation during the tecto-volcanic process spreads on Earth [13]. There are two general groups of clay minerals with specific features that are appropriate for contamination problems, those are swelling clay generally formed in the continental area, and non-swelling clay is generally formed in the oceanic area [14,15]. Based on the swelling character, clay is formed by the composition of 2:1 or the sandwich layers of octahedral alumina layer and covered by tetrahedral silica layers. This then can make it to have a high ability to absorb water as it contains varying exchangeable actions; thus, it causes the clay to have coagulant potency [8,16,17].
This study focused on the modification of West Java clay started with the first step of multi-step intercalation of sodium and then was exchanged by the ammonium intercalant. Ammonium is suitable for clay adaptation in organic pollutant treatment by hydrophobic interaction and effectiveness in increasing the clay interlayer [18,19]. This application has brought innovation on the clay-modified application in specific palm oil waste management. The clay-intercalated was tested in the POME coagulation according to the turbidity, free fatty acid (FFA), and total suspended solid (TSS) reduction parameters. Furthermore, the coagulant was used in three coagulation cycles to determine the POME coagulation's stability structure and capacity.

Instrumentation and chemicals
The pure clay was imported from the West Java region, and the precursor chemicals with pure grade quality including sodium chloride (NaCl), ammonium chloride (NH4Cl), sodium hydroxide (NaOH), hydrochloride acid (HCl), and ethanol 95% were purchased from Sigma-Aldrich. Meanwhile, phenolphthalein was purchased from Merck and the distilled water was purchased from Bratachem. The palm oil mill effluent (POME) was imported from the palm oil production outlet in Palembang, South Sumatera.
The instrumentation for supporting data was conducted to X-Ray Diffraction (XRD) Rigaku Mini-flex600 in the condition of scanning sample at a speed of 1 deg/min in the 2θ scan range of 10-90°, Fourier Transform Infra-Red (FTIR) Perkin-Elmer UATR Spectrum 2 with a scan range of 400 -4000 cm -1 , Scanning Electron Microscope-Energy Dispersive Spectroscopy (SEM-EDS) JEOL JSM 6510-LA using scanning energy 20 kV and 20,000 magnifications, and Eutech TN-100 Turbidimeter.

Clay intercalation
Clay intercalation was conducted in the literature [20,21] with modification. The pure clay was intercalated by the multistep incorporation of sodium followed by ammonium through the following steps: mixing a saturated NaCl solution (333 mL) with the clay (100 g) for 2 hours, and then diluting it with distilled water for 10 minutes ratio of 1:2 of clay mixture to distilled water. The clay was then filtrated and re-dissolved in the saturated NaCl solution (333 mL) for 2 hours. The clay residue was separated and washed three times with boiled distilled water. The precipitated clay was put into a furnace of 200°C for 12 hours and denoted as C-Na.
Ammonium intercalation was executed by mixing the C-Na (50 g) with a saturated NH4Cl solution (165 mL) for 2 hours. The mixture was diluted using distilled water by a ratio of 1:2 of clay mixture to distilled water. The clay precipitate was filtrated and re-dissolved with saturated NH4Cl solution (165 mL) for 120 minutes. The precipitate was filtrated and washed with boiled distilled water, before being put into a furnace at 200 ºC for 12 hours. This material was denoted as C-NH.

Determination of optimum condition
The optimum condition was configured by measuring the initial pH and turbidity of the pure POME. The optimum time was determined by mixing 0.1 g of clay-based coagulant with 10 mL of POME, and then was processed at the rapid stirring of 100 rpm in variation of times for 5, 10, 20, 30, 45, and 60 minutes. Moreover, the determination of coagulant doses was completed by varying the doses in a variation of 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 g of clay-based coagulant by an identic process of the previous step. The solution was aged for 5 minutes to precipitate the flocs. The final turbidity and pH were then measured.

Free fatty acids (FFA) analysis
The FFA analysis was measured using the National Standard of Indonesia number 7709-2012 (based on the titrimetric method). The process was initiated by preparing 5 g of POME before and after coagulation by 0.1, 0.2, 0.3, 0.4, and 0.5 g claybased coagulant. Each sample was added with boiled ethanol 95% (50 mL) and phenolphthalein indicator (three drops). The mixture was titrated with the standardized NaOH solution. The NaOH solution used was measured as the FFA level calculation. The FFA reduction level was measured by the following Equation 1 in the percentage of [22]: V NaOH is denoted as the total NaOH solution used at titration (mL), Mrpalmic acid is denoted as a mass relative to Palmitic acid chemical structure (g/mol), and mPOME is denoted as the total mass of POME solution used (g).

Total suspended solid (TSS) analysis
The TSS analysis was measured using the National Standard of Indonesia number 6989.3-2019 based on the gravimetry method. The solution of POME before and after coagulation with an optimum condition was filtrated in the sterile filter paper, followed by drying in the oven for 1 hour. The total weight of the sample filtrated was weighed. The TSS reduction was measured based on the following Equation 2: (2) W 1 and W 0 are denoted as total mass after and before coagulation, respectively (g), and Vsample is denoted as the volume of sample used (mL).

Reusable study
0.4 g clay-based coagulant was added to 10 mL of POME and rapidly stirred at 100 rpm for 45 minutes. After the precipitate separated, the turbidity was measured. The residue subsequently was separated, cleaned using distilled water and dried to be used at the next coagulation. The whole mechanism was conducted for three cycles to analyze the stability of the coagulant agent.

Results and Discussion
The characterization of morphology and particle composition was confirmed by SEM-EDS (see Figure 1). Pure clay was observed as a compact material and the intercalation process expanded the lamellar structure of clay. The intercalation process triggered the tactoids disaggregation and reduce the impurities of Ti + particles as proven in the SEM analysis in Figure 1(b) [23,24]. Table 1 describes the claybased coagulant's detailed compositional analysis. According to the literature, the high-temperature treatment and the excess amount of intercalant in the solution have affected the structural transformation of the broadening layer and cation formation on the surface material [20]. Furthermore, the transformation of functional group and bonding interaction was detected by FTIR analysis (Figure 2). The shifting of the transmittance band from 3626 to 3624 cm -1 and 3396 cm -1 indicated the slightly increasing vibration of the O-H group interlayer. The ammonium intercalant was detected by a higher intensity band at 1448 cm -1 as NH4 + bending vibration and at 3253 cm -1 as NH4 + -Si-OH stretching (this value was superimposed on the O-H stretching vibration) [25,26]. The high and sharp peak at 1005 cm -1 indicated the characteristic of silica tetrahedra vibration. Moreover, the triple difference of the band shown at 874 cm -1 , 796 cm -1 , and 518 cm -1 confirmed the alumina-silica octahedral.
The XRD analysis determines the crystallinity transformation (see Figure 3(a)). The XRD orderliness of PC and C-NH showed some changes indicating the structural transformation. Based on the analysis, montmorillonite existing was indicated by the sharp and highest peak of 2θ at 20.08° and declared the dominant mineral in this West Java Clay structure. Kaolinite and quartz were noticed at 11° and 27.5°, respectively [27]. The kaolinite mineral disappeared after intercalation due to the disintegration between the detached platelet [28]. Moreover, the slight increase in quartz peak was affected by the re-structure of the intercalation process after calcination. Those were caused by the calcination process using 200°C of temperature [27,[29][30][31]. According to Figure 3(b), the detailed diffractogram area between 10° and 11° strength proves interlayer lifting due to ammonium ion (NH4 + ) intercalation [32]. The ammonium ion had a bigger ionic radius (1.43Å) than the sodium ion (1.02Å); thus, affecting the expansion of layer distance [33]. An optimum condition observation was reached with varying coagulation times and coagulant doses. The optimum coagulation time was analyzed by determining the duration resulting from the highest turbidity reduction and the pH changes to understand the impact of clay-based coagulant (see Figure 4). As shown in Figure 4, the highest turbidity reduction reached pH 7.8 for 45 minutes of coagulation. The literature confirmed that clay-based coagulant is effective in the neutral pH range [34]. Furthermore, the optimum dose was reached with a ratio of 0.4 g/10 mL POME (see Figure 5). An anomaly on turbidity removal increased after the optimum dose due to the saturated capacity on neutralization and bridging of floc formation. Thus, the excess clay caused turbidity in the coagulation system [35].
Since determining the optimum condition parameter, the C-NH coagulant was used as the main coagulant in the next step of POME coagulation. The reduction of FFA and TSS was analyzed and shown in Figures 6 and 7, respectively. FFA reduction was included as an urgent parameter to avoid generating undesirable odors, hydrolysis, and triggering the oxidation process of the organic content of POME in the water bodies [36,37]. These processes became the reason eutrophication occurred. The decrease in FFA level meant the level reduction/precipitation of POME via neutralization by the structure charges of clay coagulant. A negative charge from the surface layer of clay and a positive charge from the ammonium intercalant can overcome the neutralization processes.
As shown in Figure 6, the optimum reduction of FFA levels after POME coagulation using C-NH reached 0.4 g doses. The increasing doses had no significant effect on FFA reduction with an assumption that the ammonium group can reach the saturated capacity in neutralization to form floc. The free fatty acid (palmitic acid) coagulation occurred when it interacted with the positive interlayer charges of NH4 + intercalant at a neutral pH range (at 7.8) hydrophobically and finely spread over the swelling interlayer clay materials [38]. The expected mechanism was figured out in Figure 9. Data shown in Figure 7 compares the TSS level contained in the POME solution before and after coagulation (using 0. Standards, the final TSS levels of POME treated by clay-based coagulant fall under the standard quality. Furthermore, the regeneration trial has concluded that clay-based coagulant was effective up to three cycles application by the evidence of 83% turbidity reduction of the third cycle reusable as shown in Figure 8. Table 2 describes the comparison of several coagulant abilities for POME removal. This work informed that the ammonium-intercalated clay performed appropriately in POME removal by coagulation. The proposed mechanism for this coagulation process was completed by neutralizing the POME charge by NH4+ groups, followed by bridging to flocculate due to the outstanding characteristic of clay as a precipitate agent [39,40].

Conclusion
The pure clay of the West Java area was intercalated using ammonium ions and confirmed by several analyses, including the SEM-EDS for the composition and swelling property changes, the FTIR for the functional group transformation, and the XRD for crystallinity changes. Moreover, this modified clay was found effective in flocculating the POME under the value regulated in Wastewater Quality Standards (Regulation of the State Minister of Environment Number 5/2014) by reduction of turbidity up to 93%, then FFA and TSS levels up to 23.1 mg/L (49.7%) and 62.5 mg/L (73.7%) 238 mg/L, respectively. The reusable process has proven that clayintercalated has a stable structure to apply three cycles.