3.1 Development and characterisation of composite coagulant
This section discusses the results regarding the development of finding the optimum composite coagulant of PACTPPg. The first stage of PACTPPg development is the combination of both coagulants of PAC and TPP as described in the methodology according to different weight ratios (Table 2). The best weight ratio of TPP/Al was determined based on the coagulation performance of the coagulant in treating leachate sample. This section also presents the structure, morphological, and physicochemical characteristics of PAC and TPP to further understand the properties of PACTPPg. Some analyses of PAC and TPP were characterised using its original powder form, i.e., Fourier-transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). Meanwhile, scanning electron microscopy/energy-dispersive X-ray (SEM-EDX), pH, ζ potential, isoelectric point, molecular weight, particle size, and conductivity tests were performed using its liquid form after dissolving the coagulants at the respective concentrations. Similar characterisation tests were also performed on the optimum PACTPPg. Before further discussions regarding the composite coagulant, the individual coagulants of PAC and TPP were analysed first to determine their best properties in the synthesis of PACTPPg.
3.1.1 Structure and morphological characteristics of PAC and TPP
The characteristics of PAC and TPP coagulants are discussed in this study in terms of physical-chemical properties, structure, and morphology. For the structure and morphological characterisation, 10% concentration of PAC was prepared and analysed quickly using the wet method of the SEM-EDX instrument. The analysis needs to be done immediately because PAC tends to dilute quickly in distilled water, which would likely result in an idle image. Figure 2 shows the appearance of 10% PAC at 300× magnification. Based on the observation, it shows that PAC has a long-connected structure like polydimethyldiallylammonium chloride (PDM) in (Appendix), which is likely to portray its polymeric chain properties with a mean diameter of 41.3 μm. It also has heterogeneous shapes deposited on the surface, which might be the reason for the uneven appearance. The state of PAC here also represents a non-compact texture by showing such void spaces surround the polymer (Figure 2). By using EDX, chlorine (Cl), sodium (Na), nitrogen (N), and aluminium (Al) were detected on the surface particles of PAC with respective weight and atomic percentages (Appendix). Based on the observation of the EDX spectrum, there are two noticeable elements found in PAC, i.e., Cl and Na.
Next, the SEM-EDX analysis was done on the TPP coagulant. It could be observed that the image of TPP had distinct differences from the PAC coagulant. The morphologies of 1% TPP solution and raw TPP are depicted in Figures 3 and 4, respectively. Figure 3 illustrates the liquid form with 150× magnification. It was observed that the TPP solution has fibrous-like characteristics, much likely to resemble its sticky and adhesive properties with a mean strand size of 241.5 μm. The morphology of tapioca peel in the study is virtually similar to the tapioca starch solution as studied by Azizan [50] (Appendix). However, a lower contribution of fibrous components and strands could be observed. The less fibrous component is due to the less starch content in the peels compared to the starch from the root tuber itself. Small particles were also deposited on the strands (yellow circles) that depicted the undissolved starch of TPP, which resulted in fewer strands. Figure 4 shows the image of raw TPP coagulant with 3,000× magnification. Based on the observation of the image, TPP exhibits smooth globular particles and polygonal-shaped starch granules in the form of agglomerated particles with a mean diameter of 9.0 μm. This occurrence is in good agreement with the previous findings by Versino et al. [37] and Shaylinda [25], who studied the morphological observation on the raw tapioca starch (Appendix). This result verifies that the morphological structure of the starch extracted from the agro-waste of tapioca peel is almost similar to the starch from the tapioca tuber.
The surface morphologies of starch are usually in granule shape, solid surface, and non-pores. However, they are distinguishable from the perspective of particle size distribution according to a comparison study done by Choy et al. [19] on the starch of rice, wheat, corn, and potato. The images obtained for TPP in the current work are also found to be in close agreement with the characterisation of cassava peel reported by Asharuddin et al. [44]. Consequently, several elements could be found on the surface of TPP particles through EDX (Appendix). Several elements were noticeable, including carbon (C), iron (Fe), calcium (Ca), and Al. Silica (Si) and potassium (K) were also discovered, but in the slight amount (Appendix). The properties of Si could promote the coagulating process by accelerating the precipitation formation [17, 44]. Besides, the content of Si particles would trigger the coagulant to perform effectively, which is achieved by inducing the ionic exchange and heteroaggregation of oppositely charged colloids in wastewater [54].
Further analysis was done to evaluate the structural bonding formation of both PAC and TPP coagulants. In this study, FTIR spectroscopy was used to investigate the bonding characteristics by examining the present spectral bands. According to Kakoi et al. [55], pollutant removal could depend on the ionisation degree of the functional groups of the polymer structure in the added coagulants. The main identified spectral bands of both PAC and TPP are shown in Tables 3 and 4, respectively, whereas the extensive analysis is available in the Appendix. According to Versino et al. [37], FTIR spectroscopy could reflect the changes of spectral band characteristics, which resemble the physical and chemical interactions when two or more substances are synergised together. Therefore, the potential interactions between PAC and TPP are expected in this study. The spectral bands of raw PAC and TPP were observed individually for comparison purpose. Meanwhile, further discussions of FTIR results for the composite coagulant PACTPP are presented in section 3.2.1. Figure 5 shows the FTIR spectra of raw PAC and TPP coagulants.
Based on the infrared spectral database (Appendix), the IR spectra of PAC show that strong absorption bands could be observed in several regions, which indicated particular functional groups. A very broad intensity was detected at 2500-3100 cm-1 (i.e., specifically at 3013.07 cm-1), which denoted the existence of carboxylic acid compounds with O-H hydrogen-bonded stretching. Furthermore, alkenes and aromatic ring compounds with C=C-H asymmetric stretching could also be identified (Table 3). The characteristics of C≡C stretching vibration could be observed as well at the wavenumber of 2163.44 cm-1 in the 2260–2100 cm-1 absorption range. Moreover, the peak wavenumber at 1635.14 cm-1 might belong to several compounds, such as alkenes, aldehydes, amines-primary, and amides with respective stretching vibrations and bends (Table 3). The strong intensity of C-O stretching was also detected at 1050–1150 cm-1 absorption range with 1097.96 cm-1 as the peak wavenumber that resembled the alcohol compound. Other than that, a strong and broad band with 1097.96 cm-1 peak was observed at the range of 1110–1000 cm-1, which could be related to the silicate compound and the asymmetric stretching vibration of Al-OH-Al [11]. According to Yang et al. [11], the peaks of 557 cm-1 and 975 cm-1 are also related to Al-O vibration and Al-OH-Al stretching, respectively, in the spectra of PAC.
Table 3 Main functional groups of PAC from FTIR spectrum analysis
Functional Group
|
Absorption Range (cm-1)
|
Vibration Type
|
Carboxylic Acids
|
3100–2500
|
O-H stretching
|
Alkenes
|
3100–3000
|
C=C-H asymmetric stretching
|
1675–1600
|
C-C=C symmetric stretching
|
1680–1620
|
C=C non-conjugated
|
Aromatic Rings
|
3100–3000
|
C=C-H asymmetric stretching
|
Alkynes
|
2260–2100
|
C-C≡C-C
|
Aldehydes
|
1750–1625
|
C=O stretching
|
1750–1590
|
C=O conjugated
|
Amines-Primary
|
1640–1560
|
N-H bending
|
Amides
|
1670–1600
|
C=O stretching
|
1640–1550
|
N-H bending
|
Alcohols
|
1150-1050
|
C-O stretching
|
Silicates
|
1110–1000
|
Si-OR stretching,
asymmetric stretching vibration of Al-OH-Al
|
More chemical groups and bonds could be observed following respective wavenumbers (cm-1) in TPP compared to PAC (Table 4 and Appendix). The broad band of 3500–3200 cm-1 with the peak at 3275.75 cm-1 might be allocated to the O-H group in polymeric compounds, e.g., carboxylic acids, phenols, and alcohols, as well as the O-H group of free hydroxyl groups existed in the peel [44]. The peaks at both wavelengths between 3200–3300 cm-1 and 2100–2260 cm-1 with 3275.75 cm-1 and 2168.01 cm-1, respectively, indicated a similar group of alkyne compounds with different types of vibration. The strong intensity was identified in ≡C-H stretching in the CH stretching, whereas the medium intensity was observed in the -C≡C- of C≡ stretching vibration. The H-C-H asymmetric and symmetric stretching might also be found in the 2850–2960 cm-1 range that belonged to the alkanes of alkyl groups with the peak wavenumber of 2931.24 cm-1. Another main group existed that built up the structure of TPP was identified in alkene compounds with non-conjugated C=C bending, which was reflected in the wavelength of 1620–1680 cm-1 (i.e., specifically, the peak at 1640.37 cm-1). The peak at the wavenumber of 1640.37 cm-1 might also belong to the amines-primary group with the N-H bend stretching found in PAC as well.
Table 4 Main functional groups of TPP from FTIR spectrum analysis
Functional Group
|
Absorption Range (cm-1)
|
Vibration Type
|
Alkynes
|
3300–3200
|
≡C-H stretching,
CH stretching vibrations
|
2260–2100
|
C≡ stretching,
-C≡C-
|
Alkanes,
Alkyl Groups
|
2960–2850
|
C-H,
H-C-H asymmetric and
symmetric stretching
|
Carboxyl
|
1750–1680
|
C=O stretching
|
Alkenes
|
1680–1620
|
C=C
non-conjugated
|
Amines-Primary
|
1640–1560
|
N-H bend stretching
|
Alcohols
|
1300–1000
|
C-O stretching,
–COOH
|
1150–1050
|
C-O stretching
|
Silicates
|
1110–1000
|
Si-OR stretch,
asymmetric stretching
|
Secondary Cyclic Alcohols
|
990–1060
|
C-OH stretching
|
Alkyl Halides
|
600–800
|
C-Cl stretching
|
The alcohol structures found in PAC were also identified in TPP, with both peak wavenumbers at 1077.04 cm-1 and 1103.78 cm-1, respectively. Besides the alcohol compound, the secondary cyclic alcohol compound was also observed in the absorption range of 990–1060 cm-1, with 998.73 cm-1 peak wavenumber. The bonds related to silicate compounds were also observed at the peaks of 1077.04 cm-1 and 1103.78 cm-1 in the range of 1110–1000 cm-1 [11]. Meanwhile, the deep peaks observed between 600 and 800 cm-1 indicated the C-Cl stretching of alkyl halide compounds through two identified peak wavenumbers at 763.29 cm-1 and 706.86 cm-1, respectively, with strong intensity. According to Chen et al. [15], the discovered peaks in the range of 1077.04–1103.78 cm-1 could be attributed to the asymmetric stretching vibration of Fe-OH-Fe or Al-OH-Al. Meanwhile, the bending vibration of Si-O-Al or Si-O-Fe bonds might exist due to the peaks that appeared around 998.73 cm-1 [15]. The results and trends obtained for the wavenumbers of TPP are also in agreement with the characterisation studies of tapioca peel using FTIR analysis by Asharuddin et al. [44]. The structural properties of TPP are also almost similar to other natural peels, such as citrus and mango peels [44].
Hydroxyl (O-H), hydrogen bonding, carboxyl (C=O), amino, or amide (-NH2) groups are also the preferred groups for the coagulation-flocculation process. At the wavelength range of 3500–3200 cm-1, the bonded O-H in polymeric compounds, e.g., alcohol, carboxylic acids, phenols, and O-H groups of free hydroxyl groups might present to induce the flocculating activity [44]. The identified groups are in agreement with the FTIR analysis done on watermelon seeds [57], banana pith [55], and Cecropia obtusifolia seeds [58] for the coagulating features, as shown in Table 5. Misau & Yusuf [58] studied the coagulant properties of crushed watermelon seeds and concluded that the presence of proteins observed through the spectra of 3447, 1845, 1740, 1647, 1559, 1541, and 1419 cm-1 helped in the water treatment (Table 5). The existence of protein properties in a coagulant is beneficial in order for the coagulant to work efficiently as the proteins behave like silica elements that help in the destabilisation mechanism, as reported by Fatombi et al. [54] who developed a coagulant from the extract of Cocos nucifera for water purification. According to Shak & Wu [58], broader and fewer absorption bands would be found in inorganic coagulants compared to narrower and intense bands in organic compounds, which resulted in more absorption bands. This outcome supports that the bigger range of wavelength numbers would enhance the potential of natural materials to facilitate more contaminant removal in water treatment, as claimed by the study of Kakoi et al. [55] who used banana pith powder.
Banana pith had proven the potential to treat turbid water by removing 98.5%, 54.3%, 96.03%, 98.9%, 88.7%, 100%, 100%, 92%, 81%, 100%, and 60% of turbidity, COD, suspended solids, sulphates, nitrates, Cu, Cr, Fe, Zn, Pb, and Mn, respectively [55]. Another study that used banana peel indicated that the functional groups, i.e., the carboxylic acid (C=O), hydroxyl (-OH), and aliphatic amines (N–H), showed both positively and negatively charged species [56]. These groups might promote the charge neutralisation mechanism by neutralising the negative impurity charge in the leachate sample. On the other hand, in a study done by Shak & Wu [58], the functional groups of O-H stretching, CH2 groups in fatty acids (i.e., symmetrical and asymmetrical stretching), and C=O stretching in C. obtusifolia seed were identified as the coagulating agents to help in the removal of 87% suspended solids and 55% COD. In conclusion, from the structure and morphological characterisation studies of PAC and TPP, it is proven that both materials have coagulating properties, especially for TPP as a newly-discovered natural coagulant. Table 5 shows the FTIR analysis of natural materials in water treatment and its detection in TPP.
Table 5 Functional groups identified in several natural materials
Natural Materials
|
Absorption Range/ Wavelength Number (cm-1)
|
Identified Functional
Groups
|
Detected/Not Detected (N.D.) in TPP
|
Watermelon Seeds
[57]
|
3770–3650
|
AlO-H stretching vibration
|
N.D.
|
|
3447.68
|
N-H group stretching
|
N.D.
|
2930–2820
(2925.49 & 2854.13)
|
CH2 asymmetric stretching
CH2 symmetric stretching
|
Detected
|
2360.54
|
Si-H (silane)
|
Detected
|
1845.28
|
C=O (5-membered β-lactones)
|
N.D.
|
1740.89
|
C=O (Carboxyl)
|
Detected
|
1654.27
|
–C=C– symmetric stretching of alkenes
|
Detected
|
1647.87
|
N-H bending (1° amines)
|
Detected
|
Banana Pith
[55]
|
3369.4
|
O-H group vibration
|
Detected
|
|
2925.8
|
C-H stretching
|
Detected
|
1645
|
Asymmetric stretching,
carboxylic COO-double bond, deprotonated carboxylate
|
Detected
|
1384.8
|
Symmetrical or asymmetrical stretching of ionic carboxylic groups (COOH), pectin
|
Detected
|
1247
|
C-O stretching of ketones, aldehydes, and lactones or carboxyl groups
|
Detected
|
1029.9
|
-C-O-C and -OH of polysaccharides
|
Detected
|
848.6
|
Amine groups
|
Detected
|
C. obtusifolia Seeds
[58]
|
3281
|
O-H stretching
|
Detected
|
|
2923 & 2853
|
CH2 groups in fatty acids (symmetrical and asymmetrical stretching of C-H)
|
Detected
|
1800 & 1600
|
C=O bond stretching
|
Detected
|
3.1.2 Physicochemical characteristics of PAC and TPP
For the physical and chemical characteristics of respective coagulants, 10% PAC and 1% TPP were analysed in their liquid form. These characteristics are essential to analyse as they have major control over the particles’ surface charges and polyelectrolyte charge density. Table 6 shows the results of physicochemical characteristics in terms of pH, ζ potential, isoelectric point, molecular weight, particle size, and conductivity parameters. Based on the observation, these findings verified that PAC has the acidic properties with pH 3.36 and high ζ potential value at +20.5 mV. The determination of ζ potential is essential to confirm the role of the charge neutralisation mechanism [59]. A higher ζ potential would be beneficial for better coagulation performance due to the superior charge neutralisation process taking place [60, 61].
The disseminated PAC coagulant during the treatment process would neutralise the stabilised negative particles (-18.73 mV) of raw leachate effectively, as discussed earlier in section 4.2. On the contrary, the TPP solution practically has a neutral feature with pH 6.33 and negative ζ potential value at -0.68 mV. This result shows an agreement with other starch-based coagulants that have approximate pH and ζ potential values in previous studies [62]. Based on the findings of Azizan [50], the ζ potential of tapioca starch was -0.559 mV with pH 6.8, whereas Ong [63] discovered the ζ potential values of -3.12 mV for tapioca flour and -4.37 mV for sago starch. Similarly, the use of psyllium husk and tobacco leaf as the primary coagulant and coagulant aid showed high negative charges of -1.92 and -3.57 mV as identified by Al-Hamadani et al. [61] and Rusdizal et al. [62], respectively.
Table 6 Physicochemical characteristics of PAC and TPP
Coagulant
|
pH
|
ζ Potential (mV)
|
Isoelectric Point (pH)
|
Conductivity
(mS/cm)
|
Molecular Weight (g/mol)
|
Particle Size (d.nm)
|
PAC (10%)
|
pH 3.36
|
+20.5
|
pH 8.90
|
71.5
|
8.55 × 104
|
6.152 × 102
|
TPP
(1%)
|
pH 6.33
|
-0.68
|
pH 7.25
|
0.786
|
5.67 × 106
|
4.079 × 104
|
The low ζ potential also indicates that tapioca is polymerised-anionic carbohydrates that have carboxylic substitutions [64]. The low ζ potential of TPP would cause low capability in neutralising the colloids of the same-charged leachate particles. In this case, it also would cause the charge reduction in PACTPP as PAC would combine with TPP that has a low negative charge. However, the addition of TPP polymer with higher molecular weight (5.67 × 106 g/mol) and bigger particle size (4.079 × 104 d.nm) would help to cater the weakness of PAC to become more dense and compact. TPP as a starch-based coagulant can also perform coagulation by charge neutralisation to remove any positively charged pollutants and accomplish sweep flocculation through the adsorption-bridging mechanism during treatment [65]. Subsequently, the isoelectric point is interrelated with the parameters as mentioned earlier. The ζ potentials of 0 mV in PAC and TPP coagulants were determined at pH 8.90 and pH 7.25, respectively.
The isoelectric point of TPP also shows the agreement with previous research of natural coagulant extracted from C. Nucifera at pH 7.5, as studied by Fatombi et al. [54]. The identification of ζ potential and isoelectric point provides some ideas on the conditions of coagulants to work effectively during the treatment process.
Meanwhile, on the conductivity parameter, PAC is characterised with high value that portrays its capability of flowing electric current compared to TPP, which is due to the concentrations of cations and anions, as well as dissolved inorganic constituents [66]. Despite the superiority of PAC, TPP is dominant in terms of molecular weight and particle size of 5.67 × 106 g/mol and 4.079 × 104 d.nm, respectively. These properties show that the presence of TPP could complement the disadvantages of PAC to become an effective coagulant [67, 68]. Oladoja [59] also stated that the operation of natural polymeric coagulants could predominantly occur via the adsorption-bridging mechanism, where its molecular weight needs to be distinctively high to perform efficiently. Next, the physical and chemical characterisation was continued with the analysis of XRD for the determination of crystalline phases in the solid coagulants.
In PAC, the tridecamer of Al13 or AlO4Al12(OH)247+ is widely reported to be the most efficient and predominant species due to its higher positive charge among others: the monomers Al3+, Al(OH)2+, Al(OH)2+, Al(OH)3(am), and Al(OH)4-, a dimmer (Al2(OH)24+), and a trimer (Al3(OH)45+) [69, 70]. Nevertheless, the peak of Al13 species in PAC products could not be proven without any modification; therefore, it needs respective reagents to separate the Al13 precipitate [50]. High-purity Al13 products could be obtained by using sulphate precipitation and consequent barium nitrate metathesis operations or the SO42-/Ba2+ deposition-replacement method from the medium-high concentration of PAC [69, 71]. The result for XRD analysis is presented through the peaks that occurred from the analysis spectra. Figure 6 displays the spectra of raw PAC coagulant in the range of 2-theta (θ) scale.
Based on the observation in Figure 6, strong crystalline peaks are detected at the angle of 15°, 17°, 18°, 23°, 24°, 25°, 26°, 27°, 28°, 30°, 31°, 32°, 33°, 35°, 39°, 42°, 44°, 47°, 50°, and 52°, which possibly present the bonding of aluminium chloride hexahydrate (AlCl36H2O) [72]. The pattern of raw PAC here could be noted as semi-porous and irregular, which resembled its unwell-formed crystalline structure, unlike the Al13-(SO4)n precipitate that primarily consists of well-formed crystalline solid when analysed by 27Al-NMR spectroscopy [71]. The characteristic of well-formed crystalline solid is due to the separation and purification processes that cause the precipitate to contain more Al13 than PAC itself. Despite the difficulty in detecting this main species, some studies stated that a strong Al13 signal could appear in the range of 2θ from 5° to 25° of low angles in the XRD spectrum of the original PAC [69]. Thus, through the characterisation by using XRD, it could be described that PAC has the semi-formed crystalline phase. Nonetheless, PAC contains highly charged positive atoms, which made PAC as the preferred inorganic polymer coagulant. Next, the raw TPP was also characterised to determine its physicochemical properties by using XRD. The spectral pattern of TPP is presented in Figure 7. Based on the observations, the XRD of the starch shows non-sharp peaks at low angles of 2θ, at 15°, 17°, 18°, 19°, and 23°, respectively.
Based on the obtained curve fitting routine and finding areas under the curve for each group, the components such as potassium iron silicate (K2Fe2Si0.29O4.58), aluminium potassium silicon (K8Al8.02Si37.98), and silicon oxide (SiO2) were distinctly detected in the analysis. The distinct peaks could be observed from starch-based materials if the element is crystalline, which would reveal a certain extent of particle uniformity as a biopolymer. In general, all starch could be classified as low crystalline polymers with sharp crystalline peaks that are visible across the same region of 10°–30° [73]. The spectra in Figure 7 also consists of broad patterns (25°–60°) without any distinguished sharp peaks that indicate its amorphous properties in nature [74]. These findings are in line with the study done by Oladayo et al. [74] that investigated the properties of biofilms prepared using cassava starch and starch-keratin blend. Comparisons were also made with the diffraction peaks obtained by the starch of potato, corn, wheat, and rice [73], and tapioca starch [50], which showed similar patterns of the graph (Appendix). By referring to the spectra obtained, it could be noted that tuber-based starch has lower particle uniformities according to the lower percentage of crystallinity compared to cereal-based starch. In this study, the tapioca peel was obtained from the tapioca tuber itself; therefore, it could be concluded that its level of crystallinity is also minimal.
The low crystallinity level might also be attributable to the high polymerisation of amylose molecules and a small presence of proteins and lipids in the composition of TPP [73]. The results from this characterisation are also in agreement with the study done by Zayadi et al. [47], in which the pattern of diffraction peaks in natural materials appeared within the same curve area of low angles. In the study of Zayadi et al. [47], the potential of agro-wastes, i.e., cassava peel, banana peel, and coconut coir as the medium for metal and nutrient removals were investigated. Cassava peel was chosen as the promising natural material as most of the significant compounds were discovered at the foremost diffraction peaks (Table 7). However, the cassava peel here was prepared without discarding the outer brownish layer, which might attribute to the dissimilar compound obtained. Table 7 shows the significant compounds found in the cassava peel in comparison with TPP according to its peaks. Overall, it could be concluded that TPP has major amorphous properties, in which the crystallinity of the starch might not give much impact on the treatment performance if used as a primary coagulant [73].
Table 7 Comparison of compounds based on XRD peaks on cassava peel and TPP in this study
Agro-waste
|
Peak (°)
|
Compound
|
Agro-waste
|
Peak (°)
|
Compound
|
Cassava Peel
[47]
|
15°
|
Iron chloride hydrate
|
Tapioca peel powder (TPP)
|
15°
|
Potassium iron Silicate (K2Fe2SiO.29O4.58)
|
17°
|
Diosgenin
|
17°
|
Siderite (Fe(CO3))
|
L-lactide-poly (ethylene glycol)
|
18°
|
Calcium aluminium silicide (CaAlSi)
|
4-chlorophenyl sulfone
|
19°/23°
|
Aluminium potassium silicon (K8Al8.O2Si37.98)
|
17°/23°
|
Starch (maize)
|
26°
|
Carbon (C)
|
26°
|
Carbon
|
27°
|
Silicon oxide (SiO2)
|
27°
|
Diclone
|
|
|
Silicon oxide
|
3.2 Characterisation of composite coagulant (PACTPPg)
The conventional coagulation-flocculation treatment of landfill leachate had been done earlier using all the prepared different weight ratios of composite coagulants. Based on the observation, PACTPPg has been designated as the best optimum weight ratio according to the obtained removal percentages. In this section, the study of PACTPPg is emphasised by examining its structure, morphology, physical, and chemical characteristics using SEM-EDX, FTIR, XRD, pH, ζ potential, isoelectric point, molecular weight, particle size, and conductivity analyses.
3.2.1 Structure and morphological characteristics of PACTPPg
Morphological property is an in-depth key parameter for the visual analysis of a composite material [17]. In complementary to the morphological characteristics of individual PAC and TPP, PACTPPg is considered as the combination of both coagulants, as could be seen in Figure 8. In brief, these characteristics are evidence of a favourable composition process to develop a new coagulant. Figure 8 exhibits the SEM image of the PACTPPg solution of TPP/Al = 3.71 with 200× magnification. It is observed that the coagulant has heterogeneous and uneven shapes with a bigger mean size of 142.3 μm, which could increase the surface area to initiate more spots for the adsorption-bridging between particles to take place [17]. The increment of size is about 70.1% compared to the PAC solution with 300× magnification in the earlier analysis (section 3.1.1). PACTPPg also shows a further compact and occupied state of coagulant, which occurs due to the addition of TPP that has altered PAC in the new present form. The longer-connected and multi-branched structure describing the fibrous properties originated from TPP has also been discovered in PACTPPg. The multi-branched structure formed does not only contribute to larger fractal dimension and size of the polymer but would also improve the bridging effects and the aggregating abilities [17, 58, 75].
Next, the EDX analysis was done to identify the elemental compositions of PACTPPg. The EDX recorded the elements, i.e., carbon (C), aluminium (Al), silica (Si), chlorine (Cl), potassium (K), sodium (Na), calcium (Ca), and iron (Fe) on the surface of PACTPPg’s particles. Based on the observation of the EDX spectrum, C has the notable increment compared to the C element found in PAC, which resulted from the addition of organic substance throughout the process. These results also conclude that the addition of TPP increased the weight of some elements in the new PACTPPg. Furthermore, Niu et al. [76] and Zhu et al. [77] stated that the increase of polymerisation degree to form polymer chains or three-dimensional networks would improve the stability of the composite coagulant. The formation of macromolecular compounds, such as Al-O-Si or Al-Si-Fe complexes, might be responsible for the better performance of PACTPP [6, 76]
Previously, the FTIR analysis was done on the sole coagulants of PAC and TPP. Based on the results, it is noted that the existed dominant functional groups would help in the coagulation-flocculation process. TPP also showed such exceptional characteristics that proved the capability of the natural coagulant developed from agro-waste. FTIR analysis was also done on PACTPPg to examine the information of conceivable inter- or intramolecular interaction in the compound through the molecular geometry and functional groups [37, 78]. According to the wavenumber analysis in PACTPPg, various chemical groups and bonds could be detected, including the new groups that did not appear in the individual PAC and TPP coagulants. The main identified spectral bands are shown in Figure 9 and Table 8, where the extensive analysis could be viewed in Appendix.
Based on the observation in Figure 9, it is clear that the spectrum of PACTPPg is slightly similar to TPP but not the complete combination of PAC and TPP spectra, which has broader and denser intense bands, especially in the range of 3500–2500 cm-1. The broader and denser intense band is due to the intermolecular stretching vibration of dissociative –OH [9, 13, 75], since PACTPPg was prepared through the dilution of PAC and TPP in distilled water, and then dried for the adsorption spectrum analysis. The functional group consists of –OH is one of the important factors to indicate the efficiency of coagulants [13]. Next, it is observed that the peak of 3231.46 cm-1 belongs to the wide-ranging bands of 3600–3100 cm-1 and 3500–3100 cm-1 that respectively assigned to alcohols, phenols, and amines-secondary functional groups [25, 44], which appear with strong O-H stretching and medium N-H bend intensities, respectively. Compared to PAC and TPP, some adsorption bands, i.e., the chemical bonding of carboxylic acids (3400–2400 cm-1) with peaks at 3231.46 cm-1, 2518.62 cm-1, and 2511.76 cm-1, the bonding of alkenes (1675–1600 cm-1), and amines-primary (1640–1560 cm-1) with a peak at 1629.79 cm-1 also existed [11]. Table 8 shows the main functional groups of PACTPPg from FTIR spectra analysis.
Table 8 Main functional groups of PACTPPg from FTIR spectra analysis
Functional Group
|
Absorption Range (cm-1)
|
Vibration Type
|
Alcohols, Phenols
|
3600–3100
|
Hydrogen-bonded O-H stretching
Strong and broad intensity
|
Amines-secondary
|
3500–3100
|
N-H bending
Medium intensity
|
Carboxylic acids
|
3400–2400
|
Hydrogen-bonded O-H stretching
|
|
3300–2500
|
O-H stretching
Medium intensity
|
Ketones
|
1750–1625
|
C=O
Newly discovered group
|
Aldehydes
|
1750–1625
|
C=O
Newly discovered group
|
Alkenes
|
1675–1600
|
C-C=C symmetric stretching
Medium intensity
|
Amines-primary
|
1640–1560
|
N-H bending
Medium intensity
|
Alkanes
|
1370–1350
|
C-H rocking mode
Newly discovered group
Medium intensity
|
Phosphates
|
1200–1100
|
P=O stretching
Strong intensity
|
Silicates
|
1110–1000
|
Si-OR stretching
Strong and broad intensity
|
The carboxylic acid group has the same stretching vibration as alcohols-phenols, which is hydrogen –OH. Despite the bending of water adsorbed, it could be considered that PACTPPg contained more absorbed, crystallised, and polymerised water than others by having stronger intensity peaks. It is also an essential factor for coagulants to work effectively [8, 15]. The intensity peaks of PACTPPg are also stronger than PAC and TPP coagulants, which indicate that the coagulant consists more free radical form of dissociative –OH [11, 15]. Other than that, the existence of bonding groups of alcohol-phenols (3600–3100 cm-1), amines-secondary (3500–3100 cm-1), 1°-2° amines amides (3400–3250 cm-1), aromatics (1500–1400 cm-1), sulphates (1450–1350 cm-1), nitro groups (1400–1300 cm-1), alkyl halides (1300–1150 cm-1), and phosphates (1200–1100 cm-1), as presented in Table 8 and Appendix were observed due to the absorption bands of the groups that appeared only in TPP. Meanwhile, only two functional groups observed due to the source of PAC, i.e., amines-primary (1640–1560 cm-1) and amides (1640–1550 cm-1), with the same peak at 1629.79 cm-1. The amines-primary and amides groups respectively indicate the similar type of vibration of N-H bending with medium intensity.
Next, near the 1400 cm-1 wavelength, the absorption peaks appeared on PACTPPg and TPP, but it did not appear in PAC. According to Chen et al. [13], who developed a new composite coagulant of polyferric-aluminium-silicate-sulphate, it could be deduced that the peaks were assigned to Al-O-Al bond stretching and bending vibration. Simultaneously, the peaks between 1359.32 and 1151.24 cm-1 (Appendix) could indicate the sign of Si-O-Si bonds [9]. Silicate stretching (Si-OR) with a strong and broad intensity was also determined in the ranging band of 1110–1000 cm-1, with the discovered peaks at both 1078.08 cm-1 and 1018.07 cm-1. Phosphate stretching (P=O) was also discovered at the peak of 1151.24 cm-1. Meanwhile, in the region of 1125–1000 cm-1, the spectra of PACTPPg were almost similar to the TPP spectra. Besides, it could be noted that the addition of heated TPP did not diminish the existence of the main bonds in PAC, e.g., carboxyl, amine, and amide that helped in the coagulating process [25]. Other additional bonds of C-O bonds probably existed as well when the compositing process had started. According to Shaylinda [25], the C-O bonds were highly detected in the spectra of tapioca starch, which could stimulate the new bonds in PACTPPg.
The remarkable findings in the analysis of PACTPPg could happen due to the peaks that appeared around 1750–1625 cm-1 and 1370–1350 cm-1, which are unobservable in TPP and PAC spectra. According to these experimental findings, it could be suggested that some new chemical compounds, probably from the functional groups of ketones (C=O), aldehydes (C=O), and alkanes (C-H rock) formed when heated TPP was introduced into PAC, as shown in Figure 9 and the highlighted row of Table 8. The new chemical compounds indirectly indicate that PACTPPg did not just involve a simple physical mixing, but also the chemical synthesis with some reactions occurred between these two coagulants [10, 14].
3.2.2 Physicochemical characteristics of PACTPPg
Regardless of PACTPPg: TPP/Al = 3.71 becomes the optimum composite coagulant, physical and chemical characterisation tests were carried on all ten ratios of PACTPP to observe their properties in becoming the ideal coagulant for the treatment of raw stabilised leachate. Table 9 shows the properties of different weight ratios of PACTPP (a–j) in terms of pH, ζ potential, particle size, and conductivity parameters. It could be seen that the addition of TPP into PAC during the composite process affected the outcome solution. It was noted that TPP/Al = 64.94 had the lowest values of ζ potential and conductivity at +1.81 mV and 9.13 mS/cm, respectively, and the biggest particle size of 6.019 × 104 d.nm. These properties are certainly contributed by the utmost amount of TPP in PACTPPa that has the highest molecular weight. Combining polymerised coagulants with the higher molecular weight of anionic polyelectrolytes can enhance the aggregating ability of impurities during the treatment process [79]. The ζ potentials of PACTPP (a–j) also remained in the positively charged strength even though the particular amount of TPP had their anionic charges. This condition is favourable since the anionic polymer should have a weak anionic charge to hinder the weakening of the high-charge strength of the composite coagulant [25].
The decreasing ratio of TPP/Al from 64.94 to 0.65 increased the values of ζ potentials, decreased the size of particles, and improved the ability to pass an electric current through the measurement of conductivity in all final products of PACTPP. However, this is in contrast to the readings of the pH values in this characterisation study. Based on the results, all the developed composite coagulants are in the acidic condition, but not in the ascending nor descending orders until the end.
Table 9 Properties of different weight ratios of PACTPP (a–j)
PACTPP
(a–j)
TPP/Al
|
pH
|
ζ Potential
(mV)
|
Particle Size
(d.nm)
|
Conductivity (mS/cm)
|
a: 64.94
|
3.62
|
+1.81
|
6.019 × 104
|
9.13
|
b: 29.22
|
3.58
|
+2.24
|
5.909 × 104
|
12.15
|
c: 17.32
|
3.40
|
+2.67
|
5.801 × 104
|
15.17
|
d: 11.36
|
3.38
|
+3.10
|
5.693 × 104
|
18.20
|
e: 7.79
|
3.35
|
+3.22
|
5.304 × 104
|
22.30
|
f: 5.41
|
3.41
|
+3.33
|
4.916 × 104
|
26.40
|
g: 3.71
|
3.45
|
+3.45
|
4.528 × 104
|
30.50
|
h: 2.44
|
3.53
|
+3.52
|
4.029 × 104
|
32.57
|
i: 1.44
|
3.55
|
+3.59
|
3.532 × 104
|
34.64
|
j: 0.65
|
3.56
|
+3.66
|
3.035 × 104
|
36.70
|
Based on Table 9, it is observed that the pH values of PACTPP decreased from TPP/Al = 64.94 until TPP/Al = 7.79, and increased again at TPP/Al = 5.41 with pH 3.41 until TPP/Al = 0.65. The shift in pH certainly happened due to the changing point phase in weight ratio, where PAC started to have a higher proportion compared to TPP in PACTPPf. Next, regarding the satisfactory performance of PACTPPg, the molecular weight and isoelectric point tests were further done to emphasise its superiority. Table 10 displays the physical and chemical characteristics of PACTPPg in terms of pH, ζ potential, isoelectric point, molecular weight, particle size, and conductivity parameters, and also its comparison with PAC and TPP coagulants.
Table 10 Physicochemical characteristics of PACTPPg
Coagulant
|
pH
|
ζ Potential (mV)
|
Isoelectric Point (pH)
|
Conductivity
(mS/cm)
|
Molecular weight (g/mol)
|
Particle Size (d.nm)
|
PACTPPg
TPP/Al = 3.71
|
pH 3.45
|
+3.45
|
pH
9.81
|
31.5
|
1.59 × 107
|
4.528 × 104
|
PAC
(10%)
|
pH 3.36
|
+20.5
|
pH 8.90
|
71.5
|
8.55 × 104
|
6.152 × 102
|
TPP
(1%)
|
pH 6.33
|
-0.68
|
pH 7.25
|
0.786
|
5.67 × 106
|
4.079 × 104
|
Based on the findings in Table 10, PACTPPg was characterised by the acidic properties at pH 3.45, with a slight pH increase from the individual 10% PAC solution at pH 3.36. Meanwhile, a significant drop of ζ potential value was noticed in the PACTPPg compared to PAC by having a difference of 17.05 mV. Zeta potential studies are not just useful for the characterisation of particle surfaces, but also in the applications of determining the colloidal stability and in the ion adsorption studies [64]. Similarly, the addition of TPP into PAC also reduced the conductivity value of the composite coagulant. This brief conclusion is also in agreement as to the study by Shaylinda [25], who indicated that the low conductivity of heated tapioca flour altered the high conductivity of prehydrolysed iron coagulant during the composite process. Similarly, Moussas & Zouboulis [8] also recorded a massive reduction of conductivity value throughout the combination of polyacrylamide as a non-ionic polymer with polyferricsulphate. Therefore, it could be stated that the decline of conductivity of the final product is a typical outcome.
The isoelectric point test was also conducted on PACTPPg using the titration method. Based on the observation, the point where PACTPPg achieved 0 mV was at pH 9.81, PAC at pH 8.90, and TPP at pH 7.25. The relevance of result is supported by the previous findings in the study by Azizan [50] that also reported the increment of isoelectric point in its composite coagulant. As discussed earlier, the parameters of ζ potential and isoelectric point give the fundamental properties of coagulants’ surface chemistry, which would further react during the adherence and contact with their surroundings [67]. These parameters affect the coagulation by counteracting the charges in raw leachate samples and give the expected optimum pH range to perform effectively [50]. However, based on the observation in the optimum pH (Appendix), the optimum pH range is between pH 4–7, which is much lower than the isoelectric point of PACTPPg at pH 9.81. The difference between the optimum pH and the isoelectric point happened due to the miscellaneous strength of positive charges of the composite coagulant (PACTPP (a–j)) compared to the raw leachate (-18.73 mV). For instance, the ζ potential of PACTPPa at +1.81 mV resulted in the optimum leachate pH for treatment at pH 4; meanwhile, PACTPPj was characterised with +3.66 mV and achieved the optimum leachate pH at pH 6.
Another equally important discovery is the increment of molecular weight and particle size of PACTPPg with 1.59 × 107 g/mol and 4.528 × 104 d.nm, respectively. The higher molecular weight of PACTPPg compared to PAC and TPP is the result of the convergence of both materials into a single product. This characteristic would enhance the aggregation capacity besides reducing the residual of aluminium concentration in the generated sludge [13]. Furthermore, the contact of the high molecular weight of coagulant and the linear structure of cationic organic matter with wastewater contaminants would result in the improved adsorption-bridging mechanism during coagulation-flocculation practice [14]. Teh et al. [67] also stated that the high molecular weight polymer could improve the aggregating ability via adsorption and bridging mechanism [67, 79].
The next characterisation test was the XRD analysis. The XRD spectra of PACTPPg as the composite coagulant is shown in Figure 10. There is a slight difference in the spectra between PACTPPg and PAC (Figure 6) and a distinct diffraction shift angle with TPP (Figure 7). As discussed previously, the spectra of PAC present the combination of both signs of crystallinity and irregular phase. Meanwhile, TPP shows no distinguished sharp peaks that indicate its amorphous properties in nature. Noticeably, the interpenetration of TPP has slightly altered the spectra pattern of PAC in generating PACTPPg. The newly generated composite coagulant was detected with several sharp and clear traces of crystal peaks around the angle of 15°–52°, as well as the broad and low intense peaks at 55°–90° of the diffraction angles. The distinct crystalline peaks were detected at 15°, 17°, 23°, 24°, 26°, 28°, 30°, 33°, 35°, 40°, 41°, 44°, 47°, 50°, and 52° that practically pose as the bonding of aluminium chloride hexahydrate (AlCl36H2O) [72]. Besides, based on the attained curve fitting routine of XRD spectra, the components of aluminium chloride hydroxide (AlCl3.6H2O), potassium aluminium chloride oxide (KAlCl2O), chlormayenite (Ca12Al12O32C12), wadalite (Ca12(Al10.6Si3.4O32)Cl5.4), and potassium aluminium chloride silicate hydrate (K12Al10Si10O40Cl2.8H2O) were distinctly detected in the analysis (Appendix).
These polymeric complexes imply that to some extent, the introduction of TPP into PAC resulted in the formation of new compounds, which play an essential role in the stability and coagulation function [11, 13]. These compounds are shifted compared to the standard crystals, which is an indicator of lattice contraction or expansion according to the study of surface chemistry. The shifted compounds also could be the reason for the increment of the molecular weight of the product as discussed previously with 1.59 × 107 g/mol [11]. The abundance of characteristic peaks in the spectra data suggested that certain adsorbing chemical compounds existed [14]. Meanwhile, some of the detected compounds did not exist in the particular PAC and TPP, which demonstrated that PACTPPg is a novel polymer [14].
Furthermore, based on the spectral data, it is suggested that the existing polymeric hydroxide hydrate complexes that contain typical functional groups such as –OH, Al-OH-Al, Al-O-Al, Si-O, and Fe-OH-Fe play a significant role in the coagulation performance compared to the standard crystals [11, 13]. The broad and unrecognisable peaks with weak intensities signify that PACTPPg has little evidence of having an unstructured phase. The peak that resembles TPP could not be recognised probably due to the high concentration of PAC that obscures it in the new reagent.
Similar shifts in the diffraction angles were reported by Azizan [50] when preparing the new composite coagulant of PACTSb made from polyaluminium chloride and commercial tapioca starch (TS) with the optimum concentrations of 90 and 20 g/L for PAC and TS, respectively. According to Azizan [50], the low crystalline properties of TS caused the well-recognised peaks in PACTSb to diffract clearly, similar to the characteristics of PAC. The unstructured phase also occurred due to the relatively weaker peaks of TPP in the deterioration of the ordered structure, which had been remarkably demolished by the graft chain [80]. Meanwhile, a study done by Yang et al. [11] prepared polymeric ferric aluminium sulphate chloride (PFASC) for papermaking wastewater treatment also validated the presence of Al or Fe hydroxide hydrate compounds based on the XRD analysis. Therefore, the XRD results showed that PACTPPg encompassed possible complexion compounds of Al, Fe, Si, Pb, Cl, and other ions in the inorganic-organic interpenetration networks, rather than a simple mixture of raw materials that are reliable to the analyses of FTIR spectroscopy [11, 13, 14].
This finding is also in agreement with the study conducted by Li et al. [14] that analysed the XRD spectra of PFM-PDMDAAC, a novel inorganic-organic composite coagulant made from poly-ferric-magnesium (PFM), polydimethyldiallylammonium chloride (PDMDAAC), FeSO4, and MgSO4 as the raw materials. The formation of a new substance with better coagulation properties occurred when the reactions took place between all the respective reagents [14]. In general, the characterisation of PACTPPg through every analysis mentioned above, has answered the early curiosity behind its effectiveness. The intersection point of all respective characteristics of PACTPPg becomes the reason for the composite coagulant to be determined as the optimum weight ratio for the wastewater treatment of the leachate sample.