Strontium

Strontium is found in nature in the +2 oxidation state only (Watts and Howe 2010). Strontium can find its way into environment by natural sources such as weathering of rocks. Anthropogenic sources are milling, use of phosphate fertilizers, and pyrotechnic devices. In addition, strontium-90 is an abundant radionuclide in nuclear fission and can be accidentally released into the environment (RSC 2020i). The high ingestion of the stable strontium led to a decline in the serum levels of calcitriol, which led to an adverse effect on calcium absorption (Armbrecht et al. 1998). An overview of the experimental parameters and optimized conditions from batch adsorption experiments for strontium is presented in Table 3.6.

Effect of pH

The adsorption of strontium increases on increasing the pH from acidic toward basic region (Zhao et al. 2014; Chen and Wang 2012; Zhang, Liu, Jiang, et al. 2015). The removal of strontium with graphene oxide-hydroxyapatite composite weakly depends on the pH (Wen et al. 2014). However, percentage removal increases with a rise in pH from 2 to 11. The protonation at a low pH was held responsible for low percentage removal.

In some cases, the increase in percentage removal was not uniform (Chen and Wang 2012; Zhao et al. 2014). The percentage removal in the case of Na-montmorillonite increased with a rise in pH (Zhang, Liu, Jiang, et al. 2015). Further increasing the pH of strontium solution after the attainment of maximum removal did not lead to any significant change in adsorption capacity (Yu, Mei, et al. 2015). Similarly, the adsorption increased very fast from pH 1 to 6 on SBA-15, and afterward, the rate of increase was not significant till pH 10 (Zhang, Liu, Jiang, et al. 2015). The protonation of the adsorbent at lower pH and deprotonation at higher pH caused the change in electrostatic force of attraction, leading to a change in the adsorption behavior with pH.

The pH dependency of strontium adsorption on Na-rectorite and Na-montmori lion ite was attributed to three factors, i.e., speciation of Sr(II), surface property of the adsorbent, and functional group dissociation (Zhao et al. 2014; Yu, Mei, et al. 2015). The strontium hydrolysis constants were log pl =-13.29 and log p2 = -28.51. Strontium was present as Sr2+ between 3 and 12. In addition, Sr(OH)~ was present in negligible amount at lower pH. The increase in pH led to a decrease in =SOH2+ and an increase in =SO~ sites. This led to an increase in adsorption due to the deprotonation.

The percentage removal also declined in some cases on increasing the pH after maximum removal at a certain pH (Liu, Meng, Luo, et al. 2015). The adsorption

TABLE 3.6

Summary of Parameters and Optimized Conditions for Batch Adsorption of Strontium

Adsorbent

Adsorbate

Surface Area

(m2/g), Pore Volume (cmVg),

Pore Size (nm) pHzpc

Experimental Conditions

Adsorption

Capacity (mg/g)

Thermodynamic Parameters

Kinetic

Model and

Curve Fitting

Isotherm

Model and

Curve

Fitting

Maximum

Adsorption

Conditions

References

Graphene oxidehydroxyapatite nanocomposite

Sr

Surface

area=91.85

pH =2-11

Dose = 0.5 g/1

Concentration = 20.60. and 100mg/l (kinetic study)

Contact time = up to 48 h Temperature = 25 °С

702.18

Pseudo

second-order

model.

linear, high r2 and qe close

No

comparison with first Older

Langmuir

Maximum at pH 11, but isotherm at pH 7

Contact time = 2h

Wen et al.

(2014)

Hydrophilic ion-imprinted polymer

Sr

pH =2-8

Dose = 0.4 g/1

Concentration = 3-8 mg/1

(kinetic study)

Contact time = up to 240 min

Temperature = 25 °C-45°C

135.28

Pseudo

second-order

model

Langmuir

pH = 6

Contact

time = 60 min

Temperat ure = 45 °C

Liu. Meng.

Luo. et al.

(2015)

Niobium-doped

tungsten oxide

Sr

Surface area = 153

pH =0-7

Dose = 2 g/1

Concent ration = 60-180 mg/1 (isotherm study) and 90 mg/1 (kinetic study)

Contact time =up to 180 min

Temperature = 298 K

Pseudo

second-order

model

Freundlich

pH = 4

Contact

time = 60 min

Liu. Mu. et al.

  • (2015)
  • (continued)

Batch Adsorption Process of Metals and Anions

TABLE 3.6 (Continued)

Summary of Parameters and Optimized Conditions for Batch Adsorption of Strontium

Adsorbent

Adsorbate

Surface Area

(m2/g), Pore

Volume (cm3/g), Pore Size (nm) pHzpc

Experimental Conditions

Adsorption Capacity (mg/g)

Thermodynamic Parameters

Kinetic

Model and

Curve Fitting

Isotherm

Model and

Curve

Fitting

Maximum

Adsorption

Conditions

References

Na-rectorite

Sr

Surface

area = 11.9

pH =2-12

Dose =0.1-1 g/1

Concentration = 10 mg/l

(most studies)

Contact time = up to 24 h

Temperature = 293-3 33 K

10.78-14.28

AH= 1.31-132 kJ/mol

AS =90.30

AG = negative

Pscu do-second-order model, linear

Langmuir model, linear

pH =10

Dose = 0.6 g/1 (not maximum)

Equilibrium achieved in 5 h. but using the 24-h contact period

Zhao ct a).

(2014)

Temperature = 333

K

Remediation of Major Toxic Elements

Na- Sr

pH =2-12 10.93

AH= 15.08-15.25

Pseudo-

Freundlich,

pH>9

Yu, Mei, ct al.

montmorillonite

Dose = 0.1-1.2 g/1

Concentration = 10 mg/l (most studies)

Contact lime = up to 12 h

Temperature = 293-3 33 K

kJ/mol

AS = 119.33

AG = negative

second-order model, linear (no comparison)

linear

Dose = 1.2 g/1, but 0.5 g/1 used in most studies

Contact time= 12h, but equilibrium achieved in 6h

(2015)

Temperature = 333

K

Dolomite

Sr

pll=2.5-8.5

AH = -16.68

Pseudo-

Langmuir

pH = 5.5

Ghaemi et al.

Dose= 10 g/1

kJ/mol

second-order

model.

Contact

(2011)

Agitation speed = 200rpm Concentration = 10-50 mg/l (isotherm study)

Contact lime = up to 360 min

Temperature = 293-333 K

AS = 18.60

AG = negative

kinetic

model, linear

non-linear

time= 120 min

Temperature = 293

K

oo

(continued)

оо

4-

TABLE 3.6 (Continued)

Summary of Parameters and Optimized Conditions for Batch Adsorption of Strontium

Surface Area

Isotherm

(m2/g), Pore

Adsorption

Kinetic

Model and

Maximum

Volume (cm3/g),

Capacity

Thermodynamic

Model and

Curve

Adsorption

Adsorbent

Adsorbate

Pore Size (nm) pHzpc

Experimental Conditions (mg/g)

Parameters

Curve Fitting

Fitting

Conditions

References

FciO4 particle-

Sr

pH =4-9

12.59

Langmuir

pH=9

Cheng et al.

modified .sawdust

Dose = 0.4-5 g/1

model

Dose = 0.25 and 0.4

(2012)

Concentration = 5-50 mg/1

g/1 in terms of

(isotherm study)

adsorption

Contact time = up to

capacity

120 min

Contact

Temperature = 20 °C

time = 30 min

Magnetic chitosan

Sr

pH =3.3-8.2

2.28

Langmuir

pH = 8.2

Chen and

bead

Dose =0.67-10 g/1

model.

Contact lime = 6h

Wang

Agitation speed = 150rpm

linear

(2012)

Concentration = 5-300

mg/1

Contact time=up to lOh

Temperature = 30°C

Activated carbon

Sr

Surface area =135

pH =4-8

12.2

AH = 44.70 kJ/mol

pH = 6

Ka^an and

from textile

Dose =0.33-13.66 g/1

AS= 195.84 J/mol

Dose= 13.33 g/1

Kiitahyah

sewage sludge

Concentration = 10-90

К

Initial

(2012)

mg/1

AG = negative

concentration = 10

Contact time =up to

mg/1

360 min

Contact

Temperature = 20°C-60°C

lime = 5 min

Batch Adsorption Process of Metals and Anions

TABLE 3.6 (Continued)

Summary of Parameters and Optimized Conditions for Batch Adsorption of Strontium

Adsorbent

Adsorbate

Surface Area

(m2/g), Pore Volume (em’/g),

Pore Size (nm) pHzpc

Experimental Conditions

Adsorption Capacity (mg/g)

Thermodynamic Parameters

Kinetic

Model and Curve Fitting

Isotherm

Model and

Curve

Fitting

Maximum

Adsorption

Conditions

References

Sodium trititanate

Sr

Surface

area =29.37

Pore

volume = 0.0012

Pore size = 9.89

pH = 2-6

Dose = 0.1-2 g/1 (isotherm and pH studies)

Concentrations 10-500 mg/1 (isotherm study) and 10-30 mg/1 (kinetic study)

Contact timc = up to 15 h Temperature = 298-318 K

79.37-108.7

AH =14.23 kJ/mol

AS = 30.79 J/molK

AG = positive (may be due to value of KI taken as qc/ce)

Pseudo-second-order model, linear

Langmuir model, linear

pH =6 Temperature = 318

K

Guan et al.

(2011)

Potassium tetratitanate

SBA-15 (Santa

Barbara

Amorphous-15)

Sr

Sr

Surface

area=38.35

Pore

volume = 0.044

Pore size = 4.59

do

pH=l-10

Concentration=5-80 mg/1 Contact time = 5-260min Temperature =ambient temperature

  • 88.50-111.1
  • 17.67

AH= 11.97 kJ/mol AS = 24.57 J/molK AG = positive (may be due to value of KI taken as qe/ce)

Pseudo-second-order model, linear

Pseudo-second-order model, linear

Langmuir model, linear

Langmuir, linear

pH =6

Temperature = 318

K

pH =6-10

Contact

time= 100 min

Guan et al.

(2011)

Zhang. Liu. Jiang, et al. (2015)

H2()2-modified attapulgite

Sr

Surface

area = 174.4

Pore

volume = 0.2556

pH = up to pH 11

Dose = 60 g/1 (isotherm study)

Agitation speeds 145 rpm

86.8 pg/g

Pseudo-sccond-ordcr model, linear

Langmuir, linear

Contact time = 8h Temperature = 40°C

Liu and Zheng (2017)

Concentration = IO"5 to

Remediation of Major Toxic Elements

8 x10 s mg/1

Contact time=up to 24h

Temperature =25°C-5O°C

ooof hydrophilic ion-imprinted polymer based on graphene oxide, i.e., RAFT-IIP, increased slightly from pH 2 to 3, and afterward, it increased up to pH 6. The adsorption capacity decreased after pH 6. The low percentage removal at low pH was attributed to proton competition.

Zeta Potential

The change of percentage removal with pH is akin to change in the zeta potential (Liu, Mu, et al. 2015). The decrease in zeta potential occurred with the simultaneous increase in adsorption. In addition, the stagnant behavior of zeta potential led to no further increase in percentage removal after pH 4. The simultaneous variation in adsorption with respect to the change in zeta potential suggests that the adsorption mechanism was electrostatic in nature.

The zeta potential varies from case to case. The zeta potential of graphene oxidehydroxyapatite composite was negative and became more negative with an increase in pH (Wen et al. 2014). In the case of niobium-doped tungsten oxide, the zeta potential became more negative with a rise in pH up to 4, and afterward, it became stagnant (Liu, Mu, et al. 2015).

Effect of Coexisting Ions

Cadmium and lead slightly influence the removal of strontium, whereas magnesium, aluminum, and sodium have no effect on strontium adsorption (Wen et al. 2014). The reason for this phenomenon is attributed to the difference in radii by less than 15% for lead and cadmium.

Humic acid increased the adsorption of strontium at pH lower than ca. 8.3, but at higher pH, it caused a decline in percentage removal (Zhao et al. 2014). Similarly, the adsorption of strontium with Na-montmorillonite increased in the presence of humic acid below pH 7, and an antagonistic effect was noticed at pH higher than 7 (Yu, Mei, et al. 2015).

Effect of Adsorbent's Modification

The treatment of adsorbent increased the removal efficiency (Wen et al. 2014; Liu and Zheng 2017). The strontium adsorption on graphene oxide by Wen et al. (2014) was more than that previously reported on graphene oxide. This is attributed to the pre-oxidization process and the residing functional groups of graphene oxide. The modification of attapulgite with H2O2 also increased its adsorption capacity (Liu and Zheng 2017). The H2O2 modification after treatment led to a decrease in agglomeration and an increase in surface area (Liu and Zheng 2017). The FT-IR spectra also showed a blueshift after H2O2 treatment. This was taken as the evidence of formation of smaller particles (Liu and Zheng 2017).

In addition, composite formation and doping led to an increase in percentage removal. The composite of graphene oxide and hydroxyapatite had adsorption capacity two times that of hydroxyapatite and nine times that of graphene oxide (Wen et al.

2014). The doping of niobium in tungsten oxide increased the adsorption capacity of strontium (Liu, Mu, et al. 2015). This was attributed to the degradation of crystallinity of tungsten oxide on addition of niobium.

Mechanism

The ion exchange mechanism of strontium adsorption on graphene oxide-hydroxy-apatite nanocomposite was estimated by XPS analysis (Wen et al. 2014). The Ca 2p peak’s intensity (347.4 and 350.9eV) declined after strontium adsorption on graphene oxide-hydroxyapatite nanocomposite. This was attributed to the replacement of calcium ion with cadmium by the ion exchange phenomenon. The ionic radius of strontium (0.125 nm) is comparable to that of calcium (0.103 nm). Firstly, H+ ion liberated from the solid surface into the solution; afterward, exchange of ions occurred; and finally, the complex was formed.

The ion exchange mechanism for the adsorption of Sr2+ on Na-rectorite was estimated by the determination of exchanged Na+ ion in the solution (Zhao et al. 2014). The amount of Na+ ion present in the solution was 1.25 times that of Sr2+ adsorbed. However, at pH 10.5 and above, Na+ had no change with the amount of Sr2+ adsorbed. This was attributed to the formation of outer-sphere complex at low pH and inner-sphere complex at higher pH. The same case was reported in strontium adsorption with Na-montmorillonite (Yu, Mei, et al. 2015). The theoretical diffuse-layer model (DLM) model via Visual Minteq 3.0 was also used to explain the experimental data (Yu, Mei, et al. 2015). The FT-IR analysis was also used to ascertain the functional groups involved in the adsorption process (Chen and Wang 2012).

Desorption

The strontium regeneration was achieved by means of lowering the pH, e.g., the use of hydrochloric acid (Liu, Meng, Luo, et al. 2015; Zuo et al. 2019). The adsorbent, i.e., RAFT-ПР (hydrophilic ion-imprinted polymer based on graphene oxide), underwent five adsorption-desorption cycles without any significant loss of capacity (Liu, Meng, Luo, et al. 2015).

 
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