4.2 Phosphate Biosorption
4.2.1 Preliminary Screening of Biosorbent
4.2.1.2 XRD Analysis of Longan Seed Based Biosorbents
Once bound to the carboxylate groups on the biosorbent and biochar surface, the adjacent primary and secondary amine groups can also adsorb phosphate via hydrogen bonding and electrostatic attraction (Aneesh and Jishna, 2017). From Figure 4.11, the vibration bands that specific for O–P–O bonds of adsorbed and/or intact phosphates appeared in acid-treated LS and other samples can be identified.
These include 1,165 cm-1 (HPO42- group, P–O–H in-plane and out-of-plane deformation modes), 1,028 and 1,100 cm-1 (v3, P–O asymmetric stretching vibrations), 960 cm-1 (v1 sym, P– O stretch), 603 cm-1 (v4, P–O stretch), as well as 560 cm-1 (v4, P–O stretch and P– O bending) (Arshadi et al., 2015). Given the varying natural composition of phosphorus and phosphates in the biomass, the change in the intensity of stretching peaks of –OH between 2,350 and 2,370 cm-1 after biosorption establishes the fact about the role and effect of phosphate group in the biomass in phosphate biosorption. The leaching of phosphate from biomass can be reasoned by the abundance of polyphosphate granules adjoining or confined in the vacuoles (Yang et al., 2017).
2016). The peaks with 2θ around 15° and 17° tend to overlap and form a single broad peak owing to a high full-width at half-maximum (Ornaghi Júnior et al., 2014; Liu et al., 2018). A mixture of polymorphs was formed with the aforementioned cellulose Iβ structures and plane (002) for cellulose II as regards the peaks detected at 2θ nearly 22° (Oliveira et al., 2017). Similar diffraction peaks were seen in pomelo (Citrus grandis) albedo peel at 2θ of 18° and 22° (Zain et al., 2014) and mandarin (Citrus unshiu) peel at 2θ of 16° and 24° (Hiasa et al., 2014).
All in all, the primary constituents are cellulose, which is randomly distributed, lignin and hemicellulose, which are amorphous macromolecules.
Figure 4.12: X-ray Diffraction Patterns of Pre-sorption and Post-sorption LS Samples: (a) Raw LS; (b) Heat-treated LS, (c) Acid-treated LS; (d) Heat- and Acid-treated LS; (e) Heat- and Acid-treated LS (After Phosphate 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 38 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.
Acid 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). As shown in Figure 4.11, the increased sharpness of all peaks at the 2θ values mentioned in the raw samples (around 15°, 17° and 23°) 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 presented in Table 4.9, the crystallinity of LS decreased from 9.9716% to 9.4704% after acid treatment because the fraction of molecules initially compacted in the crystalline zones was dissolved. In contrast, heat treatment increased the crystallinity of LS from 9.9716% to 11.9449%. 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.
Table 4.9: Crystallinity Index (CrI) and Crystallinity Percentage (%) of All LS Samples
Pre-sorption Phosphate-sorption
Sample CrI Crystallinity
Percentage (%) CrI Crystallinity Percentage (%)
Raw LS 0.9649 9.9716 0.6810 8.7926
Acid-treated LS 0.7943 9.4704 0.6076 10.6867
Raw LSBC 0.6418 11.9449 0.3636 12.4342
Acid-treated LSBC 0.7315 9.2783 0.3649 11.5845
The XRD spectra of pre-sorption and phosphate-sorption acid-treated LS samples in Figures 4.12 reveal 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 phosphate 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).
The crystallinity indices of all samples are as shown in Table 4.9 and it is 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 LS show a little change in peak intensities at 2θ near to 26°.
From the results in Figure 4.10, it can be deduced that phosphate adsorption uptake depends on the type of biosorbent and biochar. In addition, the phosphate 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 surface such as net surface charge, acidic and protonated surface functional groups also changes. Based on the experimental results, acid- treated LS was the most eligible biosorbent candidate to proceed to the interaction effect study with adsorption uptake of 3.393 mg PO4/g acid-treated LS.
4.2.2 Interaction Effect of Operating Conditions on Phosphate Biosorption