Impact of Initial Concentration, Adsorbent Dose, and Ionic Strength on Batch Adsorption of Metals and Anions and Elucidation of the Mechanism
Deepak Cusain
Durban University of Technology
Shikha Dubey and Yogesh Chandra Sharma
IIT(BHU), Varanasi
Faizal Bux
Durban University of Technology
CONTENTS
- 7.1 Effect of Initial Concentration on Adsorption of Metals and Anions..........240
- 7.2 Effect of Adsorbent Dose on the Adsorption of Metals and Anions............242
- 7.3 Effect of Ionic Strength on the Adsorption of Metals and Anions...............245
This chapter includes the effects of parameters such as ionic strength and adsorbent dose on the batch adsorption process. These factors usually affect in the same way for most metal anions, e.g. increase in percentage removal with increase of adsorbent dose and decline in initial concentration of the metals and anions. The ionic strength effect depends on the mechanism of adsorption, i.e. whether the adsorption process undergoes inner sphere complex formation or external sphere complex formation.
Effect of Initial Concentration on Adsorption of Metals and Anions
The percentage removal declines with increase of initial concentration, e.g. in removal of vanadium (Anirudhan et al. 2009), chromium (Maleki et al. 2015), iron (Al-Anber 2010), copper (Liu. Zhu. et al. 2013), zinc (Zhang, Li, et al. 2010), gallium (Chan 1993), cadmium (Pal and Pal 2017), cesium (Dwivedi et al. 2013; Zong et al. 2017), lead (Venkateswarlu and Yoon 2015a; Moradi et al. 2017; Huang et al. 2011), uranium (Yang, Liu, et al. 2017), fluoride (Zhu et al. 2016; Dhillon et al. 2015; Jin et al. 2016), and nitrate (Zhao and Feng 2016; Banu and Meenakshi 2017a, b; Srivastav et al. 2014; Ganesan, Kamaraj, and Vasudevan 2013; Hu et al. 2015). The decline in percentage removal is attributed to the limited number of active sites, which become saturated after a certain concentration (Huang et al. 2013) or after changing the ratio of surface active sites to total metal ions (Chou et al. 2010).
However, adsorption capacity improved with increase of initial concentration of vanadium with chitosan Zr(IV) composite (Zhang et al. 2014), amine-modified poly(-glycidyl methacrylate)-grafted cellulose (Anirudhan et al. 2009), Ti-doped chitosan bead (Liu and Zhang 2015), manganese on multiwalled carbon nanotubes (Ganesan, Kamaraj, Sozhan, et al. 2013), cobalt (Anirudhan et al. 2016), nickel (Mohammadi et al. 2014; Saleh. Ibrahim, et al. 2017), copper (Liu, Zhu, et al. 2013), zinc (Zhang, Li, et al. 2010), strontium (Chen and Wang 2012; Zhang, Liu, Jiang, et al. 2015), cesium with magnetic Prussian blue (Jang and Lee 2016), copper with a hexacyanoferrate polymer composite (Dwivedi et al. 2013) or Fe,O4 @WO, (Mu et al. 2017), lead (Mahmoud, Abdou, and Ahmed 2016; Pourbeyram 2016), uranium (Yang, Liu, et al. 2017), fluoride (Zhu et al. 2016; Dhillon et al. 2015; Jin et al. 2016). nitrate (Zhao and Feng 2016; Banu and Meenakshi 2017a, b; Srivastav et al. 2014; Ganesan, Kamaraj, and Vasudevan 2013; Hu et al. 2015), and perchlorate (Yang, Gao, Chu, etal. 2012).
The escalation of adsorption capacity with rise in initial concentration is attributed to increased collision frequency (Verma and Dutta 2015), increased utilization of active sites (Huang et al. 2013), and increased concentration gradient between liquid and solid phase (Anirudhan et al. 2016), which declined the mass transfer resistance (Dwivedi et al. 2013; Zong et al. 2017) and led to enhanced mass transfer (Hamdaoui 2017; Kannamba et al. 2010). However, increase in adsorption capacity is not linear, and the adsorption capacity increased up to a point; afterwards it declined (Zha et al. 2014) or equilibrated (Guo. Jiao, et al. 2017; Zhang, Xia. et al. 2015; Dwivedi et al. 2013; Mu et al. 2017; Dolatyari et al. 2016; Srivastav et al. 2014) (Figure 7.1) or declined insignificantly (Zhou et al. 2013; Yang, Gao, Chu, et al. 2012). The limited number of adsorption sites was held responsible for saturation or declination of adsorption capacity (Zha et al. 2014).
The concentration of gallium or selenium can be increased up to a point, and after that, precipitation or aggregation was observed. Gallium concentration of more than 1 mM causes it to form aggregates in addition to adsorption with poly-y-glutamate (Hakumai et al. 2016). Similarly, the removal of selenite with increase in initial concentration from 30 to 100-1000 mg/1 causes precipitation in addition to the adsorption phenomenon (Zhang, Fu, et al. 2017).
FIGURE 7.1 General effect of initial concentration on the percentage removal and adsorption capacity of inorganic contaminants (solid line curve represents adsorption capacity, and dashed line curve represents percentage removal).
The change in concentration causes a change in the mechanism of adsorption of uranyl ions on carbon nanofibers (Sun et al. 2016). The increase in concentration at pH 4.5 during adsorption of U(VI) on carbon nanofibers led to the transformation of the inner sphere complex to an outer sphere complex. This is evidenced by EXAFS spectra and its corresponding Fourier transform analysis (Sun et al. 2016). This was due to the change in speciation with concentration (Huynh et al. 2017).
The change in concentration of uranium led to a change in the structure of Mg/ Al LDH adsorbent after modification (Ma, Huang, et al. 2015). The concentration of less than 50 ppm of uranyl causes no change in basal spacing (dbasal) of the adsorbent. However, on changing the concentration from 50 to 120 ppm, a new peak emerged at 0.89 (dbasal). On further raising the concentration, the peak becomes more dominant than the peak at 0.82 nm (dbasal). However, the peak corresponding to the structure of polysulfide Mg/Al LDH did not change on increasing the concentration. The unchanged peaks in the FT-IR spectrum at 668/669cm'1 (v (M-O)) and 447 cm’1 (6 (M-O-M) indicate the stability of the Mg/Al LDH as an adsorbent after adsorption.
The study of the effect of concentration is also responsible to give better isotherm results; the maximum adsorption capacity was difficult to achieve without saturation and plateau (karmakar et al. 2016). The isotherm study at low concentration gave erroneous results; hence, an initial fluoride concentration of 1400 mg/1 was used in the isotherm study for the adsorption of fluoride with aluminum fumarate. The change in the concentration affects the thermodynamic parameters in adsorption of cobalt with multiwalled carbon nanotube/iron oxide composites (Wang et al. 2011). The standard change in entropy and enthalpy declined with the increase in initial concentration of cobalt on multiwalled carbon nanotube/iron oxide composites.
The modification of adsorbent from calcium silicate hydrate to magnetic calcium silicate hydrate affected the variation of percentage removal with uranyl ion concentration (Zhang, Liu, Wang, et al. 2015). The adsorption efficiency of calcium silicate hydrate and magnetic calcium silicate hydrate is the same in the range of concentration of 200-2000 mg/1. The decrease in adsorption capacity with increasing concentration (3000-5000 mg/1) is higher in the case of magnetic calcium silicate hydrate as compared to pristine calcium silicate hydrate.
The percentage removal of selenite in the range of 1-10 mg/1 did not change with magnetic graphene oxide (Fu et al. 2014), but in the case of selenate with magnetic graphene oxide, it decreased with an increase in the initial concentration from 1 to 10 mg/1. However, graphene oxide without magnetism depicted different results, and the percentage removal of selenate with graphene oxide remained nearly constant (c.a. 30%) with an increase in the initial concentration from 1 to 10 mg/1.
The increase of adsorption capacity (12.32-47.48 mg/g) of cadmium on the TMU-16-NH, metal organic framework occurred with a slight decline of percentage removal (98.6%-95.6%) on increasing the initial concentration (50-200 mg/1) (Roushani et al. 2017). In spite of the increase in adsorption capacity, the optimum concentration was chosen to be 50 mg/1 due to higher remainder cadmium concentration at higher initial concentration.
The increase in initial concentration of ferric ions on adsorption with eggshell and hexavalent chromium on aluminum magnesium mixed hydroxide led to a decline in the adsorption rate constant (Yeddou and Bensmaili 2007; Li et al. 2009).
Effect of Adsorbent Dose on the Adsorption of Metals and Anions
The percentage removal increased with the increase in adsorbent dose for removal of vanadium with amine-modified poly(glycidyl methacrylate)-grafted cellulose) (Anirudhan et al. 2009), zinc chloride-activated carbon (Namasivayam and Sangeetha 2006), chromium with a metal-organic framework (Maleki et al. 2015), titanium cross-linked chitosan composite (Zhang, Xia, et al. 2015), manganese with sodium dodecyl sulfate-modified alumina (Khobragade and Pal 2016), manganese oxide-coated zeolite (Taffarel and Rubio 2010), cobalt with polyaniline/ poly pyrrole copolymer nanofibers (Javadian 2014), ferrous ions (Shokry and Hamad 2016), ferric ions (Al-Anber 2010; Bhattacharyya and Gupta 2009), nickel with charcoal ash (Katal, Hasani, et al. 2012), zinc (Zhang, Li, et al. 2010), arsenic (Alijani and Shariatinia 2017; Dehghani et al. 2016; Nashine and Tembhurkar 2016; Mandal et al. 2013; Roy et al. 2014; A. Saleh et al. 2016), selenium (Katneda et al. 2014; Adio et al. 2017), strontium (Wen et al. 2014; Zhao et al. 2014; Yu, Mei, et al. 2015; Zhang, Liu, Jiang, et al. 2015), cadmium (Venkateswarlu and Yoon 2015b; Yan, Zhao, et al. 2015; Roushani et al. 2017; Chen, Shah, et al. 2017; Beyki et al. 2017; Yakout et al. 2016), uranium (Dolatyari et al. 2016; Tan, Wang, Liu, Sun, et al. 2015), fluoride (Zhang, Li, et al. 2012), nitrate (Zhao and Feng 2016; Srivastav et al. 2014; Suriyaraj et al. 2015; Banu and Meenakshi 2017b; Ebrahimi-Gatkash et al. 2017), and perchlorate (Yang, Gao, Chu, et al. 2012). The high percentage removal with the increase in adsorbent dose is attributed to the high rate of superficial adsorption (Hamdaoui 2017) and increased availability of unsaturated sites (SenthilKumar et al. 2011).
However, the increase in adsorbent dose does not lead to a linear increase of the percentage removal, and the percentage removal equilibrated (Figure 7.2.) or slowed
FIGURE 7.2 General effect of adsorbent dose on the percentage removal and adsorption capacity of inorganic contaminants (solid line curve represents adsorption capacity, and dashed line curve represents percentage removal).
down after a particular dose, e.g. in the case of removal of nickel (Fouladgar et al.
2015) . fluoride (Zhang. Li. et al. 2012), and nitrate (Zhao and Feng 2016; Suriyaraj et al. 2015; Bagherifam et al. 2014). The percentage removal of nickel with y -A12O3 (Fouladgar et al. 2015) first increased with the increase in adsorbent dose. However, after a particular adsorbent dose, the percentage removal decreases with increasing adsorbent dose. The decrease in removal is attributed to the screening of active sites by particle interaction or aggregation. The aggregation happened due to hydrogen bonds of alumina with water (Fouladgar et al. 2015). However, the adsorption of nickel with charcoal ash after optimal dose increased slowly; the reason is attributed to the near saturation of the adsorbent (Katal, Hasani, et al. 2012).
Similarly, the percentage removal of cadmium increased on raising the adsorbent dose up to a point, and afterward there was no increase in percentage removal on increasing the adsorbent dose (Venkateswarlu and Yoon 2015b; Yan, Zhao, et al. 2015; Roushani et al. 2017; Chen, Shah, et al. 2017; Beyki et al. 2017; Yakout et al.
2016) . The increase in percentage removal is attributed to increase in the number of available sites. The equilibrated state of percentage removal is attributed to adsorbent particle aggregation, which leads to a decrease in surface area and increase in diffusion length (Beyki et al. 2017). The decline in the percentage removal of As(III and V) with the increase in adsorbent dose after optimal dose is attributed to the concentration gradient of arsenic, in addition to aggregation and clumping of the adsorbent. The aggregation and clumping of the adsorbent lead to reduced surface area, decrease in adsorption sites (Yazdani et al. 2016), and increased diffusion path length (A. Saleh et al. 2016).
The percentage removal of fluoride increased with increase in adsorbent dose, and reached equilibrium up to a certain adsorbent dose, and afterward additional fluoride adsorption did not occur (Zhang, Li, et al. 2012) or substantial removal of adsorption did not happen (Mohan et al. 2012; Liu, Cui, et al. 2016). The increase in the removal efficiency is attributed to the increase in the surface area and the high number of unsaturated active sites.
The increase in adsorbent dose led to a decline in the adsorption capacity, e.g. chromium (Maleki et al. 2015; Zhang, Xia, et al. 2015), ferric ions (Bhattacharyya and Gupta 2009), copper (Hamdaoui 2017), zinc (Zhang, Li, et al. 2010), strontium (Wen et al. 2014; Zhao et al. 2014; Yu, Mei, et al. 2015; Zhang, Liu. Jiang, et al. 2015), cesium (Zong et al. 2017; Liu, Xie. et al. 2017; Al Abdullah et al. 2016; Mu et al. 2017), uranium (Dolatyari et al. 2016; Tan, Wang, Liu, Sun, et al. 2015), fluoride (Chen, Shu, et al. 2017; Dhillon et al. 2015), nitrate (Zhao and Feng 2016; Srivastav et al. 2014; Suriyaraj et al. 2015; Banu and Meenakshi 2017b; Ebrahimi-Gatkash et al. 2017), and perchlorate (Yang, Gao, Chu, et al. 2012). The decline in adsorption capacity of chromium (Maleki et al. 2015), copper (Ngah and Fatinathan 2010) and cesium (Zong et al. 2017; Liu, Xie, et al. 2017; Al Abdullah et al. 2016; Mu et al.
2017) with the increase in the adsorbent dose was due to the presence of the unsaturated adsorption sites and aggregation.
The decline in the adsorption capacity of copper on increase in the dose of the adsorbent (Hamdaoui 2017) is also attributed to the decrease in the concentration gradient between the solute concentration in the solution and adsorbed on the surface of the adsorbent. The particle concentration effect is also supposed to be responsible for the decline in adsorption capacity after optimum removal. The higher solid content in the adsorption system blocks the adsorption sites to the adsorbates. The blocking of adsorption sites occurs by blocking or by electrostatic interferences (Sen and Gomez 2011). The increase in adsorbent dose (bentonite) caused increase in pH of the solution. This is attributed to the negative charge on the surface of the adsorbent, which results in the adsorption of hydronium ions on the surface of the adsorbent and leads to increase in pH of the solution.
The decline in adsorption capacity was not linear and not universal with increase in adsorbent dose. In the case of removal of zinc and cadmium with magnetic hydroxyapatite, a maximum adsorbed amount is reached, and after that, the adsorbed amount decreased (Feng et al. 2010). This was due to the increase of vacant sites on the adsorbent. The increase in percentage removal is attributed to the increase in adsorption sites. The adsorbent employed after maximum percentage removal did not further increase the removal. The adsorbent employed after equilibrium is attained between the adsorbent and zinc and remained unused. So, the adsorption capacity declined after optimum capacity since the mass of the adsorbent was not considered in the calculation of the removal capacity (Zhang, Li, et al. 2010).
Similarly, the adsorption capacity of uranyl ions with Fe3O4@C@layered double hydroxide composite increased with increase in dose from 0.005 g to 0.01 g/50 ml and then declined afterward (Zhang, Wang, et al. 2013). The increase is attributed to the presence of more binding sites (Tan, Wang, Liu, Wang, et al. 2015; Verma and Dutta 2015).The decline is attributed to agglomeration, which led to decrease in effective surface area (Zhang, Wang, et al. 2013). However, the saturation of percentage removal is achieved at 0.1 g/50 ml at which c.a. 90% adsorption is achieved.
The percentage removal of manganese with iron-impregnated pumice also increased with increase in the adsorbent dose up to 15 g/1, and afterward it saturated (Cifgi and Meric 2017). However, the adsorption capacity declined, but in the case of pumice (nonimpregnated), the adsorption capacity increased on increase in the adsorbent dose from 5 to 10 g/1 and nearly saturated after that.
The nonsignificant increase in adsorption capacity after a particular dose in removal of selenite with nano zero-valent iron is attributed to agglomeration of the adsorbent (Xia et al. 2017). The percentage removal of strontium increased, and adsorption capacity declined with increase in adsorbent dose (Wen et al. 2014; Zhao et al. 2014; Yu, Mei, et al. 2015; Zhang, Liu, Jiang, et al. 2015), attributed to the fact that the surface active sites were exposed fully at low adsorbent dose, and the increase of the adsorbent dose led to particle aggregation and caused the decrease in total surface area and reduction of diffusion path length (Wen et al. 2014).
The adsorbent dose needs to be increased to maintain the same percentage removal with rise in initial concentration, e.g. to maintain the 100% removal efficiency at initial concentrations of 10,40 and 100 mg/1, the doses of sodium dodecyl sulfate-modified chitosan were 0.45, 0.9, and 1.35 g/1 (Pal and Pal 2017). The optimum dose for the composite varied from its individual units. The adsorbent dose required for arsenic removal by hematite was much higher (4 g/1) than that of its composite with multiwalled carbon nanotubes (0.2 g/1) (Alijani and Shariatinia 2017). The percentage removal obtained with hematite was even much lower, i.e. 16%-23%, as compared to the iron and multiwalled carbon nanotube composite, i.e. >80%.
The adsorbent dose of Na-montmorillonite at less than 1 g/1 for strontium removal suggested that the percentage increase was high with amplification of adsorbent dose, but after 1 g/1, the ascent was not steep and near to equilibration (Yu, Mei, et al. 2015). The reason for the large increase was attributed to the increase in functional groups. The decline in amplification of removal was attributed to particle aggregation.
Effect of Ionic Strength on the Adsorption of Metals and Anions
Adsorption declined (Wang et al. 2011; Manohar et al. 2005; Gu et al. 2016), increased (Marco-Lozar et al. 2007), and remained unaffected (Dolatyari et al. 2016; Pan et al. 2017) with increase in ionic strength and varied from case to case (Figure 7.3). Perchlorate (Svecova et al. 2011; Dolatyari et al. 2016; Pan et al. 2017), sodium nitrate (Zhong et al. 2016), potassium nitrate (Guo, Jiao, et al. 2017), sodium chloride, calcium nitrate (Zeng et al. 2015), and calcium chloride (Huang, Yang, et al. 2015) salts have been used to vary the ionic strength of the system. The decrease of percentage removal with ionic strength was taken as macroscopic
IONIC STRENGTH -------■
FIGURE 7.3 General effect of ionic strength on percentage removal and adsorption capacity.
evidence for outer sphere complexation (Li, Li, et al. 2012; Min et al. 2015; Guo, Jiao, et al. 2017; Zhong et al. 2016) or the electrostatic nature of adsorption (Zeng et al. 2015), and a process independent of ionic strength was suggested to form the inner sphere complex (Huang, Wu, et al. 2015).
The increase in the ionic strength primarily increases the thickness of the electrical double layer around the adsorbent (Manohar et al. 2005; Wang et al. 2011; Chen and Wang 2006), which leads to a decline in the adsorption capacity. In addition, it also affects the activity coefficient of the metal ions (Wang et al. 2011; Guo, Jiao, et al. 2017; Chen and Wang 2006; Kara et al. 2017), electrostatic interaction (Zhao et al. 2014), accumulation of surface charge (Guo, Jiao, et al. 2017) and competitive sorption (Svecova et al. 2011; Dolatyari et al. 2016; Pan et al. 2017), which, in turn, affect the transfer of metal ions from the solution to the surface of the adsorbent.
The ionic strength influences the thickness and interfacial potential of the double layer, which, in turn, affects the adsorption of the adsorbate. However, the adsorption of copper on multiwalled carbon nanotubes was not affected by ionic strength, as the adsorption is governed by the inner sphere complex formation (Sheng et al. 2010). It is also postulated that the background electrolyte concentration, in turn, affects the ionic strength applied to predict the adsorption. On the basis of the triple-layer model (Hayes and Leckie 1987), P-plane adsorption occurred when ionic strength easily affects the adsorption process; otherwise it follows the o-adsorption process (Sheng et al. 2010). Hence, the adsorption of copper on multiwalled carbon nanotubes participates in o-plane complex reaction. Increase in ionic strength as a result of increasing base in the solution reduced the removal of copper with y-Al2O, (Fouladgar et al. 2015). The decrease in Gibbs free energy of the hydrated ion solution is postulated to be the reason for this. It decreases the interaction between the cation in the solution and the adsorbent sites having negative charges, in addition to the promotion of formation M+-0H~ ion pairs (Fouladgar et al. 2015). The reduction in removal is also attributed to the competition of positive ions with the adsorbate, screening of electrostatic interaction, and reduction of activity coefficient of copper ions (Hamdaoui 2017).
The effect of ionic strength varied with the species of selenium on titanate nanotubes (Sheng, Linghu, et al. 2016). The adsorption of selenite on titanate nanotubes is independent of the ionic strength and occurred with decrease in pHzpc; both these phenomena led to the formation of the inner sphere complex, whereas the adsorption of selenate increases with decrease in ionic strength, and the pHzpc of the adsorbent remains unaffected and suggested to form the outer sphere complex.
Similarly, adsorption of As(V) on hydrous cerium oxide increased with increase in ionic strength, but adsorption of As(III) remained unaffected. The increase of ionic strength led to the increase in zeta potential for hydrous cerium oxide, suggesting a decline in negative charge on increase in ionic strength (Li, Li, et al. 2012). The dominant As(III) species was neutral As(III) species, i.e. H,AsO„ and the dominant As(V) species was charged As(V) species, i.e. HAsO42" at pH 7. The increase or unchanged behavior of arsenic species As(III)) is attributed to inner sphere complex formation. However, this is not the case always; the adsorption of As(III and V) on magnetite remains unaffected by the increase in ionic strength (Liu, Chuang, et al. 2015).
The ionic strength effect on adsorption also depends on pH. The adsorption of cesium with graphene oxide declined the adsorption capacity at pH lower than 6, but its effect was not significant at pH higher than 6 (Tan et al. 2016). This is attributed to the follow-up of the adsorption process by outer sphere complexation at pH lower than 6 and inner sphere complexation at pH higher than 6. Similarly, the adsorption of strontium with sodium rectorite tremendously decreased, was not significantly affected, and was unaffected at pH below 9.5, after 9.5, and after 10.5, respectively, with increase in ionic strength (Zhao et al. 2014). The reduced electrostatic interaction with increasing ionic strength led to the small number of binding sites and, hence, decline in adsorption capacity.
The germanium catechol complex (Ge(cat)3)2- adsorption on activated carbon increased with increase in ionic strength at pH 10. The increase in adsorption is attributed to the reduction of repulsion between the adsorbate and the adsorbent. The effect is more effective in another adsorbent with high acidic character. However, the adsorption capacity of both the adsorbents reached the same value (Marco-Lozar et al. 2007).
The surface modification of 0-zeolite with ethylene diamine changes the mechanism of adsorption by complex formation (Liu, Yuan, et al. 2017). The adsorption of nickel on p-zeolite decreased on increase in ionic strength. However, when the surface of adsorbent is modified with ethylene diamine, then the adsorption process is not affected by the change in ionic strength. The reason stated for this behavior is that the adsorption of nickel on P-zeolite is governed by the outer sphere complex, whereas in the latter case, it is governed by the inner sphere complex.
The adsorption of sulfate on ferrihydrite decreased with increase in ionic strength (Gu et al. 2016). The decline of the outer sphere complex proportion occurred with increasing ionic strength. The decline of the outer sphere complex formation is attributed to the competition from background electrolyte and electrical double layer contraction. The higher ionic strength causes a more pronounced electrical double layer contraction, and this leads to a decline in electric potential (positive) at the adsorbent plane.
In addition, the proportion of outer sphere complex did not remain linear and varied with ionic strength (Gu et al. 2016). The proportion of sulfate as the outer sphere complex via adsorption on ferrihydrite decreases with increase in pH at ionic strength 0.5, but at an ionic strength of 0.02 and 0.1 M, the outer sphere complex formation reached a maximum at pH 5, and afterward it declined. The rise in pH led to two antagonistic conditions promoting and impeding the formation of the outer sphere complex. The rise in pH led to the decline of inner sphere complexation, which causes generation of new sites for outer sphere complexation. In addition, the surface becomes less negatively charged and hence declines the prospects of outer sphere complex formation. At lower ionic strength (0.02 and 0.1 M), both factors are comparable and results in the maximum proportion of outer sphere complexation along with the rise of pH. The high ionic strength (0.5 M) depresses the outer sphere complex formation, and hence, the maximum for outer sphere complexation was absent at high ionic strength. In addition to this, the maximum was not also present at high initial concentration, as most of the active sites are engaged, and the number of active sites that vacated on the decline of inner sphere complexation is inadequate to show any significant increase in the active sites in comparison to the adsorbate. Hence, no significant change in outer sphere complexation is observed.
The decline in adsorption of strontium on modified sawdust on raising the sodium nitrate concentration is attributed to the decrease in strontium species with increase in sodium nitrate concentration estimated by speciation software (Cheng et al. 2012). The decrease in adsorption capacity of strontium with SBA-15 is attributed to the formation of an ion pair between strontium and nitrate (Zhang, Liu, Jiang, et al. 2015). Uranium adsorption with attapulgite depends on ionic strength, but after surface modification with chitosan, it becomes independent of ionic strength (Pan et al. 2017).
