4.1 Nitrate Biosorption
4.1.1 Preliminary Screening of Biosorbent
4.1.1.2 XRD Analysis of Passion Fruit Peel Based Biosorbents
Figure 4.5: X-ray Diffraction Patterns of Pre-sorption and Post-sorption PFP Samples: (a) Raw PFP; (b) Heat-treated PFP, (c) Acid-treated PFP; (d) Heat- and Acid-treated PFP; (e) Heat- and Acid-treated PFP (After Nitrate Sorption)
As a result of heat treatment, the peaks had decreased sharpness and shifted slightly towards higher 2θ, as it tends to alter the cellulose structures of biomass (Wu et al., 2017). In biochar samples, the broad peaks between 15 and 35° of 2θ corresponded to turbostratic graphite plane (002), which is an amorphous carbon structure (Ahmad et al., 2016). Between 40 and 45°, the characteristic peaks representing graphite-turbostratic plane (100) also disclosed the presence of a trace amount of graphite-like microcrystalline components in the ash (Ahmad et al., 2016).
These results comply with the findings by Zhao et al. (2018). The amorphous phase that emerges naturally in all biosorbents or as a result of heat, acid or both treatments is represented by the peak broadening and the lack of distinctively sharp diffraction peaks overall. This also indicates the contents of hemicellulose and lignin (Islam et al., 2018). The decrease in crystallinity denotes that the biosorption was favoured with the protrusion of functional group into aqueous medium.
The thermal stability of cellulosic components of the PFP exhibited via the FTIR and XRD analyses agrees with the inferences made by Corradini et al. (2009) as regards their study on Curauá fiber. At a pyrolysis temperature as high as 500 °C, degradation of hemicellulose (240 – 310 °C), cellulose (310 – 360 °C) and lignin (200 – 550 °C) was presumed with varying degrees to each other of the biosorbent candidates, as the complex and specific reactions might have overlapped between 220 and 360 °C. Kim et al. (2015) explored cellulosic decomposition by mixing lignocellulosic materials with polymers. Glycosidic bonds between Carbons 1 and 4 of the adjacent glucose monomers are broken when the temperature is below 350
°C, whereas the C–O bonds of cellulose and volatile components such as CO and
CH4 are broken at temperatures above 400 °C. Besides, higher relative thermal stability of lignin can be associated to its high extent of condensation and may precipitate residual phenolic compounds. The composition of ash can be also indicated by the peaks corresponding to several other crystal structures in the biosorbent and biochars, such as quartz and sylvite around 26°. Besides, it is believed there was trace amounts of miscellaneous inorganic components which were adversely affected with liquid phase adsorption (Azargohar and Dalai, 2005).
Acidic treatment eliminated the amorphous regions in the XRD spectra, whose disorder and voids are deficit in the more ordered regions, and targeted specifically by the acids (Bensah and Mensah, 2013; Huntley et al., 2015). The primary cell walls were disintegrated and the cellulose, hemicellulose, and lignin enclosed within the plant cells were liberated (Huntley et al., 2015). The increased sharpness of all peaks at the 2θ values mentioned in the raw samples (around 15°, 17°, 23°, 35° and 65°) corresponds to increased crystallinity. With suitable temperature parameters, diluted acids can effectively remove hemicellulose and enhance the yield of cellulose (Knappert et al., 1981; Huntley et al., 2015). Sahoo et al. (2018) demonstrated the effect of dilute acid treatment on sisal fiber and wild rice grass, where the amorphous portions of the hemicellulose and lignocelluose are more affected than the crystalline fraction. As an evidence, the crystallinity of PFP increased from 10.1118% to 12.1997% after acid treatment, as shown in Table 4.4.
The molecules coming out of the crystallized areas that are typically unable to pack adequately well due to sterical problems such as entanglement and ends, could reorganize sufficiently to crystallize. In contrast, in PFPBC sample, acid treatment
decreased crystallinity index as the fraction of molecules initially compacted in the crystalline zones was dissolved.
Table 4.4: Crystallinity Index (CrI) and Crystallinity Percentage (%) of All PFP Samples
Pre-sorption Nitrate-sorption
PFP Sample CrI Crystallinity
Percentage (%) CrI Crystallinity Percentage (%)
Raw PFP 0.8296 10.1118 0.8063 9.6303
Acid-treated PFP 0.7879 12.1997 0.9115 8.8223
Raw PFPBC 0.7976 17.8604 0.7988 16.8740
Acid-treated PFPBC 0.4857 10.8450 0.5952 11.6892
The XRD spectra of pre-sorption and nitrate-sorption acid-treated PFPBC samples in Figures 4.6 revealed their similar crystal structures with the mixture of amorphous and crystalline natures, where the amorphousness was predominant.
The carbon crystallinity was converted into amorphous nature to varying extents throughout biosorption. This indicated that the adsorption regime of nitrate ions into macropores and micropores of the biochar was majorly via chemisorption by altering the structure of the carbon in the biosorption reaction (Ahmad et al., 2016).
Meanwhile, the crystallinity indices of all samples are shown in Table 4.4 and found to be lower than other typical cellulose materials, at 70 – 80 % (Reddy and Yang, 2009). This is due to the coincidentally greater amorphous portion and cellulose crystallinity, resulting in greater permeability to water and other chemicals (Zain et al., 2014). According to Zhao et al. (2018), the sharp peaks that show the crystalline phases of cellulose is ascribed to the increase in intermolecular hydrogen bonds and Van der Waals forces, as more free NH2 groups within the molecular structure involved in the biosorption reaction. Then, when the peak intensity declines,
crystalline chains were formed, where it involved broad bonds of weakly crystallized biosorbent or biochar particles. The XRD patterns of acid-treated PFPBC show a little change in peak intensities at 2θ near to 40°, 34° and 30°.
From the results in Figure 4.5, it can be deduced that nitrate adsorption uptake depends on the type of biosorbent and biochar. In addition, the nitrate adsorption performance of biosorbent is largely affected by the morphological changes of the biosorbent surface such as the specific surface area increment and the average pore size decrement after heat treatment. Apart from that, the chemical properties of the biosorbent such as net surface charge, acidic and protonated surface functional groups change. As a result, acid-treated PFPBC was the most eligible biosorbent candidate to proceed to the interaction effect study with adsorption uptake of 5.179 mg NO3/g acid-treated PFPBC.
4.1.2 Interaction Effect of Operating Conditions on Nitrate Biosorption