Studies on the intercalation of calcium – aluminium layered double hydroxide - MCPA and its controlled release mechanism as a potential green herbicide

: The intercalation of 2 - methyl - 4 - chlorophenox - yacetic acid ( MCPA ) herbicide into the interlayer matrix of calcium – aluminium layered double hydroxide ( CaAl LDH ) host has been successfully done via the co - precipitation method to form CaAl - MCPA nanocomposite, proposing an eco - friendly alternative with an adjusted delivery system for herbicide application. The intercalation process is supported by powder X - ray di ﬀ raction analysis with an expanded interlayer spacing from 8.6 to 19.6 Å for nano - composite pH 13, which is due to the inclusion of larger size anion in the interlayer. Next, the absence of a nitrate peak at 1,326 cm − 1 and the presence of a newly formed peak at 1,416 cm − 1 in the Fourier transformed infrared spectroscopy analysis also con ﬁ rmed the process of the intercalation. The signi ﬁ cant decrease in nitrogen content to 0.50% indicates the intercalation of MCPA using the carbon, hydrogen, nitrogen, sulphur analyser. The release rate of the MCPA anion in the aqueous solutions is initially rapid, followed by the slow release in the order of phos - phate > carbonate > chloride and followed the pseudo - second - order kinetic model. Hence, the conducted studies exhibit the successful intercalation of the MCPA herbicide anion and its controlled release mechanism as a potential hybrid green herbicide.


Introduction
Many herbicides have been applied extensively to impede the growth of unwanted plants in the agriculture sector, leading to a high concentration of agrochemicals affecting the quality of soil and water.As a result, the soil pH tends to be slightly increased and enhances the process of herbicide sorption to remain on the soil surface [1].For instance, 2-methyl-4-chlorophenoxyacetic acid, known as the MCPA, refers to a growth inhibitor belonging to the phenoxy acid group, as shown in Figure 1 [2].It is a selective agent to control dicotyledonous weeds in diverse agricultural applications [3], and the herbicide is commercially available due to its cost-effectiveness.Once the MCPA is applied, the chemicals are released directly into the environment.The chemicals released are not considered a threat as it has short residual life, approximately 1-4 months in the soil [1].However, MCPA has high water solubility, which is responsible for groundwater contamination and endangers aquatic species [1,4].Therefore, a proper application of herbicide needs to be addressed and developed using innovative techniques to minimize environmental pollution.
Recently, flexible inorganic material called the layered double hydroxide (LDH) was examined as a possible host and controlled release mechanism in agriculture.LDH is a two-dimensional anionic clay, also known as hydrocalumite is made up of positively charged layers, compensated by negatively-charged anions that accommodate the interlayer space together with water molecules [5].It is also known as brucite-like sheets formed by OH groups stacked to each other [6].The LDH has a general formula of where M 2+ is a divalent cation such as Ni 2+ , Ca 2+ , Zn 2+ and M 3+ is a trivalent cation like Al 3+ , Mn 3+ , Cr 3+ , respectively, and A n− an interlayer anion such as NO 3

−
, Cl − , and CO 3 2− [7].This material has high anion exchange capacity, less toxicity, flexible interlayer space, and a suitable matrix for controlled release behaviour [6,[8][9][10].This research aims to explore the potentiality of calcium-aluminium (CaAl) LDH as a carrier host for the intercalation of MCPA anion via a co-precipitation method and act as a slow release of agrochemicals to the environment.The characterization of CaAl LDH and its nanocomposite, CaAl-MCPA, was carried out using various instrumentations.This includes Fourier transformed infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD), carbon, hydrogen, nitrogen, sulphur (CHNS) analyser, and field emission scanning electron microscopy (FESEM) techniques, followed by the controlled release of the MCPA anion run by UV-Vis instrument.Yet again, LDH is interesting because it serves different functional components to improve its functionality and performances.Different anion loading in the layered structure has different purposes.Therefore, LDH has widespread applications in catalysts [11], corrosion inhibitors [12], fire retardants [13], drug delivery [14], and agriculture [15].To date, there are only few studies of hybrid LDH modified with organic anions especially MCPA.In 2017, a study related to reduce the negative impacts on the environment through the intercalation of MCPA into zinc-LDH achieved a percentage loading of 50.2% (w/w) [16].Recently, the intercalation of MCPA into the Mg-Al LDH showed effective weed control effect and slow release of MCPA in different types of soil [17].In contrary to the usage of different cations to synthesize the LDH host, this article introduces Ca-Al cations instead to accommodate MCPA anion inside the layered material to find out more about the interaction between the inorganic host and the herbicide.

Materials and methods
The chemicals employed for the inorganic host comprise of aluminium nitrate nonahydrate (Al(NO 3 ) 3 •9H 2 O) and calcium nitrate tetrahydrate (Ca(NO 3 ) 2 •6H 2 O) obtained from R & M Chemical.Meanwhile, the guest anion used includes 2-methyl-4-chlorophenoxyacetic acid (MCPA) from Sigma Aldrich and absolute ethanol (CH 3 CH 2 OH) from HmbG.The other reagent used was 2 M of sodium hydroxide (NaOH) from QreC, and all the reagents employed were prepared using deionized water (DI) without further purification.

Preparation of CaAl-MCPA by coprecipitation method
First, the synthesis of the CaAl LDH host was carried out via a direct co-precipitation method from the mixed aqueous solution of 0.025 M Al(NO 3 ) 3 •9H 2 O as well as 0.10 M of Ca(NO 3 ) 2 •6H 2 O with respect to nitrogen gas' constant purge.MCPA was dissolved in 30 mL of 90% absolute ethanol before being added into the cation mixture [16].Subsequently, 0.025 M of the dissolved MCPA anion was added into the intense stirred mixture with dropwise addition of 2 M NaOH at a reaction pH of 11-13, followed by an aging process performed in an oil bath shaker for 18 h at 65°C.Later, the precipitate was centrifuged for 25 min and thoroughly rinsed with DI water before being dried for 72 h at 80°C.Finally, the generated nanohybrid was ground into a fine powder and stored in a vial for sample analysis.

Sample characterization
A Bruker D8 Advance XRD diffractometer was used to record a PXRD pattern in the 2θ range between 5°and 90°for the characterization approach, using CuKα radiation at 40 mA and 40 kV (λ = 1.54059Å).Within the wavenumber range of 4,000-650 cm −1 , FTIR spectra were used on a Perkin-Elmer 100 infrared spectrophotometer in ATR mode.The surface morphology of the materials was observed by FESEM using Supra 55VP.The MCPA herbicide-controlled release from the interlayer host was conducted in various salt solutions of NaCl, Na 2 CO 3 , and Na 3 PO 4 at a concentration of 0.01 M each by utilizing a 0.3 mg sample.The collected amount of MCPA released into the aqueous solutions was assessed by Perkin Elmer UV-Vis Spectrophotometer Lambda 35 at a predetermined time and fitted to several kinetic model equations of zeroth order, first order and pseudo-second order.
3 Results and discussion LDH with the basal spacing of 8.6 Å, which corresponds to the nitrate peak as the counter anion [18,19].However, the resulting nanocomposites at pH 11 and 12 have only expanded from 8.6 to 8.7 Å.Therefore, it may be suggested that MCPA anion did not intercalate during the reaction and nitrate ions remain in the interlayer region.However, the diffraction peaks for both nanocomposites are sharp and slightly shifted to a lower 2θ angle = 10.15°,indicating that the gallery height has slightly increased.It is possible that the insignificant expansion of the gallery height is due to the orientation of the nitrate anion, which is slightly tilted to the surface layers and held by electrostatic forces [10].When the electrostatic interaction increases between the counter anion and the surface layer, the value of basal spacing will decrease due to stronger electrostatic attraction [20].Moreover, a small fraction of carbonate contamination can be observed around 2θ angle = 30-38°for all nanocomposites and the LDH host, which is caused by the interference of atmospheric carbon dioxide during the purging process of nitrogen gas [21].

The PXRD analysis
Further, as the pH increased to 13, the diffraction peaks of CaAl-MCPA nanocomposite portrayed broad reflection and were deviated to a lower 2θ angle = 4.5°, causing the basal spacing to increase its height to 19.6 Å.The deviation of diffraction peaks also can be attributed to the polymorphic transformation as the nitrate ion leaves the interlayer region [22].The major increase in basal spacing signifies the successful intercalation of larger MCPA anions as they accommodated the interlayer of CaAl LDH through the intercalation process.However, there is a probability that the broad peaks produced may be due to the presence of carbonate contamination that likely intercalated together with MCPA anion in the interlayer.Thus, it is notable that the MCPA anion can be intercalated in the interspacing at higher pH of 13 compared to the nitrate ion.
The three-dimensional molecular size of the MCPA anion and its theoretical spatial orientation arrangement were made using Chemdraw 3D Pro software, as illustrated in Figure 3.According to the PXRD analysis in Figure 2, the value of basal spacing for CaAl-MCPA synthesized at pH 13 is 19.6 Å.Since the thickness of CaAl LDH is 4.81 Å [23], the interlayer region of CaAl-MCPA can be calculated by subtracting the total basal spacing of the nanocomposite, giving its value of 14.79 Å.As a result, the value 14.79 Å is the area allocated for the spatial orientation of MCPA anion as they displaced into the intergalleries of CaAl LDH.Theoretically, the orientation of MCPA anions was likely to orientate in a vertical manner adjacent to the positively charged brucite layer of CaAl LDH.Hence, the value obtained for the interlayer CaAl LDH is consistent to the enlargement of the CaAl-MCPA after the MCPA anion has intercalated into the layered structure.

FTIR analysis
The FTIR spectra of MCPA herbicide, CaAl LDH and its nanocomposites, CaAl-MCPA, synthesized at various pH are displayed in Figure 4. Based on the broad absorption peak in the CaAl LDH host at 3,460 cm −1 , the peak is denoted by the stretching vibrations of the O-H functional group caused by the adsorbed interlayer water of the hydroxyl of the layered structure [24].Other than that, the small peak at 1,632 cm −1 belongs to the deformation of water molecules, followed by the strong peak of nitrate, which occupies the counter anion at 1,326 cm −1 [25].
The nanocomposite CaAl-MCPA at reaction pH 11 and 12 exhibits broad absorption around 3,452-3,454 cm −1 dominated by the O-H stretching vibration of the adsorbed interlayer water.The weak bands at 1,643 and 1,645 cm −1 can be assigned to the bending of water in the interlayer.It should be highlighted that both of the nanocomposites possessed high intensity of nitrate peak at 1,353 and 1,354 cm −1 [26], indicating that the interlayer region is accommodated by nitrate ion as the guest.Thus, it is possible that intercalation of MCPA anion did not occur, which is parallel to the PXRD analysis in Figure 2.
At a higher pH reaction of 13, the broadband centred at 3,524 cm −1 is corresponded to the O-H stretching vibration of the water molecules in the interlamellar region as well as the bending of water molecules at 1,645 cm −1 .Further, the significant peak of the nanocomposite is the newly formed peak at 1,416 cm −1 after the absorption band has slightly shifted from the original position due to the interaction process of the MCPA anion and the positively charged surface layers.According to the spectra of pure MCPA anion, the carboxylic acid bands at 1,743 and 1,247 cm −1 have disappeared for the nanocomposite CaAl-MCPA at pH 13.This is due to the ionization of carboxylate anion to prove that the intercalation of MCPA is in the anionic form [27].The antisymmetric peak around 1,366 cm −1 is probably due to the presence of carbonate in the interlayer.Therefore, the FTIR analysis for the nanocomposites is consistent with the PXRD pattern,  which supported the intercalation process of MCPA into the interlayer region of CaAl LDH.

Elemental analysis
The elemental analysis of the CaAl LDH and CaAl-MCPA nanocomposite (pH 13) was obtained by the CHNS analyser, shown in Table 1.Based on the results, the carbon content for CaAl-MCPA has increased from 0.16 to 16.02% after the MCPA intercalated into the interlamellar gallery.Additionally, the content of nitrogen in the CaAl-MCPA after the intercalation process has significantly reduced to 0.50%, implying that the nitrate ions have been exchanged with acetate ions of MCPA.The low content of nitrate is in good agreement with the absence of nitrate peak in the diffractogram of FTIR based on Figure 2. The loading percentage of MCPA intercalated in the composite is estimated to be 19.63%.In spite the lower MCPA loading into the CaAl LDH, the intercalation process of MCPA is not neglected.Thus, the elemental analysis has proven the successful intercalation of MCPA into the interlayer space.

Surface morphology and surface area
The surface morphology of the CaAl LDH and CaAl-MCPA nanocomposite (pH 13) was observed under 20,000× magnification as shown in Figure 5.For the LDH host, the image formed shows the typical morphology of the layered material, which displays aggregated hexagonal plate-like with non-uniform particles of different sizes and shapes [10].After the MCPA anion had intercalated into the interlayer region of CaAl LDH, the structure changed into a smaller size of aggregated plate-like porous materials with higher surface area.This is consistent with the enlargement of basal spacing in the PXRD analysis in Figure 2 to prove the intercalation of MCPA in the layered material.CaAl LDH compared to carbonate and chloride [28].

The release profile of CaAl-MCPA into salt solutions
Hence, phosphate ion is intercalated into the layered material while releasing acetate ion into the salt solution.
The saturated release of the MCPA anion from the nanocomposite is arranged in the order of PO   2. Based on the R 2 values, the intercalated CaAl-MCPA curve closely follows the pseudo-second-order kinetic model, as evidenced by the R 2 value approaching 0.9999.This high R 2 value suggests that the experimental data fits the theoretical model very well, and supports the conclusion that the reaction follows pseudo-second-order kinetics.The adsorption of CaAl-MCPA suggested that it undergoes chemisorption between the adsorbate and adsorbent, which in this case are MCPA and interlayer of CaAl LDH.Moreover, the obtained value could imply that the release of MCPA anion from CaAl-MCPA was caused by the dissolution of inorganic LDH, followed by ion exchange between the MCPA anion and the anions in the release medium [24].
Based on Table 2, the half-time for the release of MCPA anion in chloride, carbonate, and phosphate aqueous solutions was calculated to be 35.21,12.34, and 0.40 min, respectively.The lower affinity of chloride anion for the CaAl LDH interlayer is responsible for the longest half-time.In contrast, the release of MCPA in phosphate  solution has the shortest half-time recorded due to higher exchange capability for MCPA and tends to bind in the interlayer of the CaAl host, releasing MCPA anion easily into the aqueous solution.Hence, the kinetic release of the CaAl-MCPA shows good correspondence to the half-time observed and the MCPA release in different media.

Conclusion
The potential green herbicide, CaAl-MCPA, was successfully synthesized via the simplest co-precipitation method to intercalate MCPA herbicide into the interlayer space of CaAl LDH at pH 13.The comparison of CaAl-MCPA with lower pH shows that the reaction at pH 13 is proved by the expansion of basal spacing from 8.6 to 19.6 Å using PXRD.The presence of a new peak at 1,416 cm −1 including the disappearance of nitrate peak around 1,326 cm −1 in the layered structure of CaAl LDH, also supported the inclusion of MCPA herbicide in the interlamellar.Besides that, the nitrogen content of the nanocomposite has decreased significantly from 3.05 to 0.50%, with an increase in carbon content to 16.02% after MCPA was intercalated into the structure.The release of MCPA herbicide from the interlayer region of CaAl LDH is in the order of PO 4 3− > CO 3 2− > Cl − and obeyed the pseudo-second-order kinetic model.Hence, such a material can be used as a potential carrier host to lessen the chemical concentration of herbicide used for the environment.

Figure 2
Figure 2 displays the PXRD patterns for CaAl LDH host, MCPA anion and the intercalated nanocomposites at different pH of 11-13.The diffractogram for CaAl LDH shows that the host is in a well-ordered layered structure and possesses high crystallinity at 2θ = 10.25°and21°.The obtained CaAl LDH is similar to the typical features of

Figure 2 :
Figure 2: PXRD patterns of CaAl LDH host and its nanocomposites at various pH.

Figure 4 :
Figure 4: ATR-FTIR spectra of CaAl LDH host and its nanocomposites at various pH.

Figure 6
Figure 6 displays the release profiles of MCPA anion from the CaAl-MCPA nanocomposite into various salt solutions of NaCl, Na 2 CO 3 , and Na 3 PO 4 that were carried out at a concentration of 0.01 M in 300 min timeframe.pH values do not perfectly mimic real soils and can be considered in a future set of measurements in buffer solutions.Both Naphosphate and carbonate buffers offer a wide pH-range to work with.As shown in Figure 6, a rapid release was observed from the initial time for carbonate (CO 3 2− ) and phosphate (PO 4 3− ) solutions and a slow and stagnant release for chloride (Cl − ) solution.The release of MCPA from the interlayer LDH is faster in PO 4 3− solution due to the higher density of phosphate that caused an increase in electrostatic attraction during the ion exchange process between phosphate anion with the surface layer of

4 3 −
> CO 3 2− > Cl −depending on the anions available in the solutions.It is notable that the charge density of the anion is important in determining the release rate of the anion[29].

3. 6 Figure 7
Figure7displays the release profiles of MCPA fitted into several kinetic models consisting of zeroth order, first order and pseudo-second order.The values of linear correlation coefficient, R 2 obtained from the fitting, the rate constant, k, and the half-time, t recorded for the MCPA released are summarized in Table2.Based on the R 2 values, the intercalated CaAl-MCPA curve closely follows the pseudo-second-order kinetic model, as evidenced by the R 2 value approaching 0.9999.This high R 2 value suggests that the experimental data fits the theoretical model very well, and supports the conclusion that the reaction follows pseudo-second-order kinetics.The adsorption of CaAl-MCPA suggested that it undergoes chemisorption between the adsorbate and adsorbent, which in this case are MCPA and interlayer of CaAl LDH.Moreover, the obtained value could imply that the release of MCPA anion from CaAl-MCPA was caused by the dissolution of inorganic LDH, followed by ion exchange between the MCPA anion and the anions in the release medium[24].Based on Table2, the half-time for the release of MCPA anion in chloride, carbonate, and phosphate aqueous solutions was calculated to be 35.21,12.34, and 0.40 min, respectively.The lower affinity of chloride anion for the CaAl LDH interlayer is responsible for the longest half-time.In contrast, the release of MCPA in phosphate

Figure 6 :
Figure 6: Release profile of MCPA from CaAl LDH in various aqueous media.

Figure 7 :
Figure 7: Fitting release data from CaAl-MCPA to zeroth-order, first-order and pseudo-second-order kinetic model.

Table 1 :
Chemical composition of CaAl LDH and its nanocomposite,

Table 2 :
Linear correlation coefficient, rate constant, and half-time for pseudo-second order of CaAl-MCPA Linear correlation coefficient (R 2 ) Pseudo-second order