fine granules and are further modified for efficient biosorption (Pehlivan et al., 2013). This is mainly to change the surface groups by clearing away, concealing or exposing more binding sites (Rao and Bhargavi, 2013). The longer the pretreatment period, the further the biosorption uptake can be improved.
Table 2.8 summarizes the acid and alkali treatments of biomasses for heavy metal, nitrate and phosphate ions removal with high adsorption rates. Functional groups such as amino, carboxyl, phosphate, hydroxyl and carbonyl groups are very common for surface adsorption. Depending on the polarity of target contaminants, as well as the solution pH, oppositely charged surface groups or ligands are usually favoured due to the electrostatic attraction (Khowala, 2012; Wang et al., 2015;
Dongre, 2018).
Table 2.8: Application of Chemically-treated Biosorbents for Biosorption
Biosorbent
Type Type of
Treatment Biosorbate Removal
Efficiency Mechanism Reference Termitomyces
Clypeatus Acid and alkali Cr 100% Physical adsorption, ion exchange, complexation, electrostatic attraction
Khowala, 2012
Aspergillus
Niger 0.5 N NaOH Pb (II),
Ni (II) 80%, 60% Not mentioned Rao and
Bhargavi, 2013 Chitosan-
Graphite Composite
20% Graphite- doping
NO3 - 42.9% Intra-particle diffusion Dongre, 2018
Pine needle Lanthanum nitrate hexahydrate (La(NO3)3∙ 6H2O)
PO4 3- 85% Intra-particle diffusion Wang et al., 2015
2.7.2 Effect of Initial Concentration of Nitrate and Phosphate
The heterogenous mass transfer of contaminant is highly dependent on the initial concentration of nitrate and phosphate ions (Ogata et al., 2014; Battas et al., 2019).
The biosorption uptake of the biosorbent increases with the concentration of contaminant ion up to the saturation threshold. Nonetheless, in the same abovementioned occurrence, the biosorption efficiency of the biosorbent lowers because of the complete ionic interaction with all available binding sites (Ogata et al., 2014). Within the increment range of initial concentration between 10 and 90 mg/L, 6.73 and 9.05 times of increment in adsorption uptake of nitrate and phosphate were observed respectively. The nitrate removal rate of the modified corn stalk declined from 95.10% to 71.20%, whereas the phosphate removal rate still maintained at the plateau of over 90%, at 90 mg/L initial concentration (Wang et al., 2018).
2.7.3 Effect of Particle Size of Biosorbent
The contact between biosorbent surface and the liquid phase is crucial in nitrate and phosphate biosorption. Generally, the rate of anion biosorption is relatively high at low biosorbent particle sizes. This is because of the high number of active sites that are available on high exposed area of biosorbent surface. Larger contaminant molecules may be difficult to enter small pores, which reduce adsorption independently of other causes. When the particle size of biosorbent is increased, a longer interaction time is required to obtain similar results, as diffusion must occur through the aggregates.
In a study by Battas et al. (2019) using local clay from Morocco, highest nitrate adsorption capacity was observed with smaller clay particle size. Due to more specific sites of adsorption onto the surface of clay particles, maximum adsorption capacity was recorded at 5.1 mg NO3/g for a granulometry of the biosorbent of 110 μm mesh size. It then decreased to 4.24 mg nitrate/g from 200 to 400 μm of particles, and then finally to 3.96 mg nitrate/g when the mesh size exceeded 400 μm (Battas et al., 2019). The particle size of biosorbent also had a profound effect on the adsorption of phosphate, where higher efficiency was seen in smaller grains than larger ones (53 ± 6% for 200 – 500 µm; 29 ± 1% for > 1,000 μm grains) (Vieira et al., 2019). Given the same mass, larger specific surface areas were provided by smaller particles than larger ones for the contact with the phosphate ions. The results agreed with the findings of Palágyi et al. (2013), where with smaller grain size, the higher adsorption capacity was observed with smaller grain size owing to higher specific surface area. Moreover, Zou and Rezaee (2016) demonstrated that, specific surface area and pore volume increased with decreasing particle size. The pore size distribution of small pores (10 nm) were reduced, and new connection of closed pores was introduced to the surface of particles.
Nevertheless, Mitra et al. (2018) postulated that, removal efficiency was not improved by smaller particle sizes due to the aspect ratio, which had to be below a critical threshold to achieve highest adsorption capacity. The outcome from simulation analysis also revealed the optimum heat exchanging geometry at a specific particle size.
A vast majority of the previous studies pertaining to nitrate and phosphate biosorption concluded that the biosorption uptake and removal efficiency had reached the maximum or remained at a plateau between 100 and 150 μm (Battas et al., 2019; Jendia et al., 2020).
2.7.4 Effect of pH
The pH of the solution affects the solubility of contaminants and the surface charge of the biosorbents (Alagha et al., 2020). At low pH, the hydrogen and hydronium ions are strongly correlated to the active ligands of the biosorbents. Thus, anion biosorption could be enhanced with bindings between oppositely charged anions and protonated biosorbent surface (Alagha et al., 2020). With lower number of hydrogen and hydronium ions at higher pH, more exposed and free active sites of the functional groups gives an overall negative charge which leads to decreased anion biosorption (Dongre, 2018). In Dongre’s study (2018), the effect of pH value on the effectiveness of nitrate and phosphate sorption onto chitosan adsorbents was found to increase along with a decreasing initial pH and highest at an optimal pH 4. The nitrate removal efficiency was recorded as high as 95%, and the phosphate removal efficiency was recorded as high as 48%.
2.7.5 Effect of Dosage of Biosorbent
The biosorption process of nitrate and phosphate strongly depends on the dosage (Soumya et al., 2015; Alagha et al., 2020). At a given initial concentration of biosorbates, when the biosorbent dose increases, the biosorption rate increases.
This is due to the increased surface area that increases the number of available binding sites (Soumya et al., 2015; Alagha et al., 2020). Nevertheless, the adsorption uptake per unit mass of biosorbent was found to decrease at high biosorbent dosage. This was well observed in the study of Soumya et al. (2015) and Alagha et al. (2020) on the biosorption of nitrate and phosphate. The lower the biosorbent concentrations, the higher the uptake of nitrate and phosphate ions per unit mass of the biosorbent. In contrast, under high biosorbent concentrations, the biosorption uptake of contaminants per unit mass is relatively low. This was mainly due to the inadequate solute for complete distribution onto the available binding sites, as well as possible interaction between binding sites, the ratio of adsorbate to binding site becomes lower (Soumya et al., 2015; Alagha et al., 2020). In addition, it is also observed that an excessively high dosage of biosorbent will not improve the process. Instead, it stabilized at higher dosages of biomass due to the formation of aggregates that lowers the effective surface area for biosorption (Soumya et al., 2015: Alagha et al., 2020).