Chromium

The maximum permissible level of chromium in drinking water recommended by the World Health Organization and Bureau of Indian Standards is 0.05 mg/1, whereas the USEPA has suggested the maximum level of chromium in drinking water at 0.015 mg/1 (Kumari et al. 2015).

Chromium(VI) stays in the solution in the form of dichromate (Cr2O72-), hydrogen chromate (HCrO4‘), or chromate (CrO42-) ions (Li et al. 2009). They vary in solution with a change in pH and concentration of the solution (Li et al. 2009). The equations for their interconversion are as follows:

H2CrO4 <=> H+ + HCrO4 (3.1)

HCrO4 <=> H+ + СгОГ

  • (3.2)
  • 2HCrO) <=> H2O + Cr2O7
  • (3.3)

HCrO4~ is the most dominant species at pH lower than 6.8 between 0.05 and 300 mg/1, and above 6.8, CrO42- is the only dominant species. However, above 300 mg/1, Cr2O72- is the dominant species between pH ca. 1 and 6.8 (Li et al. 2009). An overview of the experimental parameters and optimized conditions from batch adsorption experiments for chromium is presented in Table 3.2.

Effect of pH

Metal’s adsorption on pH is largely dependent on three factors, i.e. the type of the functional group, ionic state of the functional group, and metal chemistry in the solution (Matheickal et al. 1999; Adeli et al. 2017).

The percentage removal of Cr(VI) declined with an increase in pH from acidic to alkaline region (Qiu et al. 2014; Sun et al. 2014; Sakulthaew et al. 2017). However, the adsorption of Cr(III) increased with an increase in pH of the solution with aminated iron oxide/mesoporous silica nanocomposite, with maximum adsorption at pH 5.4; on further enhancing the pH, Cr(III) precipitated as insoluble Cr(OH), (Egodawatte et al. 2015).

The large amount of adsorption of Cr(VI) at low or acidic pH can be elucidated by the species of chromium and the nature of adsorbent’s surface (Mor et al. 2007). The high protonation of the amine groups in polyaniline-coated ethyl cellulose aids in the high adsorption of Cr(VI), and at higher pH values, the hydroxide ions on the surface of adsorbent lead to repulsion between the adsorbate and adsorbent (Qiu et al. 2014). The decline in percentage removal in the case of hexavalent chromium with protonated titanate nanotubes at pH larger than 5 was attributed to the change in the species of chromium. The changed species (CrO42-) carried divalent charge as compared to monovalent charge on species (HCrO4‘) at lower than pH 5 (Wang, Liu, et al. 2013). The chromium species having divalent charge consumes two amino groups, which leads to a decline in the adsorption capacity of chromium.

The protonation helped in the adsorption of chromium(VI) at lower pH in the case of poly(glycidyl methacrylate)-grafted copolymer (Duranoglu et al. 2012) and amino-functionalized magnetic cellulose nanocomposite (Sun et al. 2014). However, at a very low pH (lower than 2 or 2.79), the decline in percentage removal is attributed to the formation of the neutral species H2CrO4, which does not offer any electrostatic attraction (Wang, Liu, et al. 2013; Sun et al. 2014). However, with carbon nano-onions the decline in percentage removal from pH 3.4 to 2 was attributed to blockage of the active sites of adsorption (Sakulthaew et al. 2017).

The point of zero charge also played a major role in the effect of pH on adsorption of chromium (Duranoglu et al. 2012). There is very little difference in the percentage removal of Cr(VI) on decreasing the pH from 7 to 2 using mesoporous magnetite (Kumari et al. 2015). The reason for this is the low pHzpc of the adsorbate (2.11). At pH higher than 2.11, the adsorbate surface is negative; thus, attraction is less between the adsorbate and adsorbent. Hence, electrostatic force of attraction for adsorption is inhibited (Kumari et al. 2015). So, the role of electrostatic interaction in the adsorption of chromium can be ascertained by adsorption capacity below and above the adsorbent’s point of zero charge.

Effect of Coexisting Ions

The adsorption capacity of Cr(VI) decreased in the presence of coexisting ions (Wang, Liu, et al. 2013; Huang et al. 2013). The adsorption of chromium on amino-functionalized titanate nanotubes and protonated titanate nanotubes declined after the addition of chloride, nitrate, and phosphate (Wang, Liu, et al. 2013). The declination effect of anions was more for sulfate as compared to chloride, nitrate, and phosphate. The higher surface charge on sulfate was attributed to this. Phosphate exists as monohydrogen phosphate under experimental conditions, i.e., at pH 5.4. Hence, sulfate has a higher negative effect. The cations (Fe3+, Cu2+, Zn2+, K+, Na+, Ca2+, Mg2+) did not show any significant effect on the adsorption of chromium(VI) with titanium-cross-linked carbon nanocomposites (Zhang, Xia, et al. 2015), and among anions, sulfate had a more pronounced effect than monovalent ions.

The presence of Cr(VI) in the solution had a promotional effect on the adsorption of fluoride with mesoporous alumina (Li, Xie, et al. 2016). The Cr(VI) adsorbed on the surface of mesoporous alumina led to the formation of a new surface hydroxyl group (=CrOH or =CrOH2+), which acted as a new adsorption site for fluoride (F~). However, the addition of fluoride ions in the solution decreased the adsorption of

TABLE 3.2

Summary of Parameters and Optimized Conditions for Batch Adsorption of Chromium

Surface Area Ki netic

Adsorbent

Adsorbate

(m2/g), Pore Volume (cm’/g), Pore Size (nm)

pHzpc

Experimental Conditions

Adsorption Capacity (mg/g)

Thermodynamic Parameters

Model and

Curve

Fitting

Isotherm

Model and

Curve Fitting

Maximum

Adsorption

Conditions

References

MWCNT/iron

Сг(ПІ)

92

4

pH = 3-7

15.24

pH =6

Gupta et al.

oxide

Dose = 0.1-2 g/1

Dose = 0.4 g/1

(2011)

composite

Agitation

Agitation

(batch and

speed=0-150 rpm

speed = 150 rpm

column

Concentration = 20 mg/1

Contact

study)

Contact time= 10-60 min

time = 60 min

Temperature =25°C

Treated and

Cr(VI)

pH =1-7

Second-

High R2 for

pH =2

Gupta et al.

activated

Dose = 0.5-5 Ji g/1

order

both

Dose = 4 g/1

(2011)

carbon slurry

Concentration = 10-100

model.

Langmuir

Concentration =

waste (batch

mg/1 (isotherm study)

linear

and

100 mg/1

and column

Contact time= 10-120 min

Freundlich

Contact

study)

Temperature = 303-313 K

models.

time=70min

linear

Temperature =

303 K

Lignin

Сг(ПІ)

Surface

2.3

pH =1.5-6

17.97

Pseudo

Langmuir

pH = 5

Wu et al.

area=21.7

Dose = 1-7 g/1

seco nd-

two-surface

Dose = 5 g/1

(2008)

Pore sizc = 14.7

Concentration = 0.1 -2.5

order

model

mmol/1 (isotherm study)

model.

Contact time = 24h

linear

Temperature = 20°C

Aluminum

Cr(VI)

Zeta

pH= 1-10

105.3-112.0

AH = 17.74 kJ/mol

Pseudo-

Langmuir

pH = 5

Li et al.

magnesium

potential

Dose = 2 g/1

AS = 85.41 J/molK

second-

model, linear Contact

(2009)

mixed

(+48 mV)

Agitation speed = 200 rpm

AG = negative

order

time= 150 min

hydroxide

Concentration = 20-200

model.

Tern pe rature = 40 °С

mg/1

linear

Contact time = 5-180min

Temperature =20°C-40°C

Batch Adsorption Process of Metals and Anions

TABLE 3.2 (Continued)

Summary of Parameters and Optimized Conditions for Batch Adsorption of Chromium

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

Graphene

Cr(VI)

pH= 1-12

21.57

AH =-11.737

Pseudo-

Langmuir.

pH = 2

Wu et al.

modified with

Dose = 2-20 g/1

kJ/mol

second-

linear

Dose = 8 g/1

(2013)

СТАВ

Agitation speed = 150 rpm

AS= 59.92-

order

Time = 1 h

Concentration = 20-100

60.33 J/K mol

model.

Temperature =

mg/1 (isotherm study)

AG = negative

linear

293 K

Contact time = 2-120min

Temperature = 293-313 K

Fe,O4

Cr(VI)

Surface area = 11

2.11

pH= 2-7

6.64-8.90

Endothermic

Pseudo-

Redlich-

pH =4

Ku mari et al.

nanos pho res

Pore size = 24.1

Dose= 1-3 g/1

second-

Peterson

Dose = 2 g/1

(2015)

Concentration = 10-100

order

model.

Contact time = 48 h

mg/1 (isotherm study)

model.

non-linear

Temperature = 45°C

Contact time = up to 72 h

non-linear

Temperature =25°C-45°C

Dolomite

Cr(VI)

Surface

pH = 2-12

10.01

AH =-13.21

Pseudo-

Freundlich,

pH =2

Albadarin

area = 4.63

Agitation

kJ/mol

first-order

non-linear

Agitation

ct al. (2012)

Pore

speed = 100-200 rpm

AS =-22.47 J/mol

model.

speed = 200 rpm

volume = 0.0064

Concentration = 5-50 mg/1

К

non-linear

Contact time = 70 h

Pore size

(isotherm study)

AG = negative

Temperature = 20°C

= 15.97A

Contact time = up to 90h

Temperature =20°C-60°C

Amino

Cr(VI)

Surface

pH = 2

86.4 and 63.3

Fellenzet al.

functionalized

area=774 and

Dose= 1 g/1

(2017)

MCM-41

517

Concentrations 130 mg/1

Pore volume = 0.4

Contact time = 24h

and 0.3

Temperature =25°C

Pore size = 2.5

and 2.4

Remediation of Major Toxic Elements

TABLE 3.2 (Continued)

Summary of Parameters and Optimized Conditions for Batch Adsorption of Chromium *■

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

Mesoporous carbon mesospheres

Surface

area= 1121

Pore volume = 2.7

Pore size = 95

pH = 1-9

Dose = 0.2 g/1

Agitation speed = 100 rpm

Concentration = 5-100

mg/1

Contact time = 0-24h

Temperature = 15°C-45°C

156.3

AH =42.5 kJ/mol

AS= 206.3 J/mol К AG = negative

Pseudo-second-order model.

linear

Langmuir.

linear

pH = 3

Contact time =24 h

Temperature = 45°C

Zhou et al.

(2016)

Activated carbon

Cr(VI)

Surface

area= 1126

pH = 2-8

Dose = 25-250 mg Agitation

speed=50-250 rpm Concentration = 1 mg/1 Contact time = 0-24 h Temperature = ambient temperature

1.8

Pseudo-second-order model.

linear

Langmuir.

linear

pH = 3

Dose = 75 mg

Agitation

speed = 200 rpm

Contact time = 4h

Ihsan ullah. Abu-Sharkh. et al. (2016)

Acid-mod ¡tied activated carbon

Cr(VI)

Surface

arca= 1420

do

2.02

Pscudo-sccond-order model, linear

Langmuir, in spite of low

R2

do

Ihsanullah.

Abu-Sharkh. et al. (2016)

CNT

Cr(VI)

Surface

area= 156

do

1.02

Pseudoseco nd-order model, linear

Langmuir model, linear

pH = 3

Dose = 200 mg

Agitation

speed = 150 rpm

Contact timc = 4h

Ihsanullah, Abu-Sharkh. et al. (2016)

Acid-modified

CNT

Cr(VI)

Surface

area= 170

do

0.96

Pseudo

second-

Langmuir model, linear

do

Ihsanullah.

Abu-Sharkh.

order et al. (2016)

model, linear

Batch Adsorption Process of Metals and Anions

Summary of Parameters and Optimized Conditions for Batch Adsorption of Chromium

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

Mesoporous

Cr(VI)

Surface

pH =6

9.19

AH = -32.96

Pscudo-

Freundlich

Contact time = 12h

Li, Xie. et al.

alumina

area = 211.4

Dose = 1 g/1

kJ/moI

second-

Temperature = 25°C

(2016)

(simultaneous

Pore

Agitation speed = 120rpm

AS = -115J/moIK

order

adsorption

volume = 0.54

Concentration = 100 mg/1

AG = positive

with fluoride)

Pore size = 8.4

Contact time = up to 24 h Temperature = 25 °C-40°C

Chitosan

CrtVI)

Surface

pH =2-6

60.24

Pseudo-

Langmuir,

pH = 4.5

Li. Li. et al.

powder

area = 6.69

Dose = 1 g/1

second-

linear

Contact

(2015)

Agitation speed = 150rpm

order

lime = 20 min

Concentration = 100 mg/1

model.

Contact time = up to 24 h Temperature = 25 °C

linear

Chitosan fibers

Cr(VI)

Surface

do

131.58

Pseudo-

Freundlich.

pH = 4.5

Li. Li. et al.

area = 0.489

second-order model,

linear

linear

Contact time = 8 h

(2015)

Polypyrrole/

Cr(VI)

pH =2-6

Column

Bhaumik

Fe,O4

Do sc = 2-6 g

Yoon-Nelson and

et al. (2013)

composite

Concentration = 50-150

Thomas models

(column

mg/l

study)

Bed depth = 30 cm

Bed diameter =2 cm

Flow rate = 3 ml/min

Temperature = ambient

temperature

Breakthrough capacity = l 12-125 mg/g

Remediation of Major Toxic Elementschromium on the adsorbent (mesoporous alumina). The reason for the decline was attributed to the competition for active sites and the competitive effect of chromium with fluoride’s internal diffusion.

Effect of Surface Modification

The amino-functionalization increased the adsorption capacity of iron oxide/ mesoporous composite (Egodawatte et al. 2015). The increase in the shell thickness of poly(m-phenylenediamine) around Fe3O4 led to an increase in the amino groups, which was attributed to the increased adsorption capacity (Wang, Zhang, et al. 2015).

The increase in polyaniline coating on ethyl cellulose increased the removal capacity and the rate of adsorption (Qiu et al. 2014). The increase in polyaniline coating increased the amount of protonated amine groups and led to increased Cr(VI) adsorption via electrostatic attraction. In addition, the increase in the polyaniline loading enhanced the hydrophilic property of polyaniline/ethyl cellulose composite; hence, a shorter time period was enough as compared with ethyl cellulose for chromium(VI) removal by polyaniline-coated ethyl cellulose.

Effect of Material

The adsorption capacity of chitosan nanofibers was more than that of chitosan powders, in spite of the reduction in the number of functional groups in chitosan nanofibers. The reason for this is the instability of the virgin fibers (powder) in the aqueous system (Li, Li. et al. 2015).

The adsorption of Cr(VI) varied with the Mg/Al ratio used for the synthesis of aluminum-magnesium mixed hydroxide (Li et al. 2009). The adsorption of Cr(VI) with the aluminum-magnesium mixed hydroxide increased with an increase in the Mg/Al ratio up to 3. On increasing the ratio from 1 to 2, the percentage removal increased from 90.2 to 97.0. However, there was only a slight increase in the percentage removal from 97.0 to 98.4 (molar ratio 3); on further increasing the ratio to 4, there was a diminutive decrease in percentage removal to 98.2. The particle size decreased and zeta potential increased with an increase in the Mg/Al ratio up to 3. The high zeta potential and lower size were attributed to the high percentage removal. The lower size was expected to have larger surface area, and the high zeta potential was estimated to enhance the interaction between the adsorbate and adsorbent. However, the surface area was not reported in the study.

Mechanism

The van der Waals force was postulated to be the chief reason for the adsorption of Cr(VI) on activated carbon. The small pore size and low pore volume led to slow mass transport rate (Zhou et al. 2016). However, the high adsorption capacity of mesoporous carbon was attributed to considerable surface area, high pore volume, and pore diameter. The mesopores offer an unobstructed path for Cr(VI), due to which Cr(VI) diffuse into the activated sites without any hindrance, which leads to high adsorption capacity.

The confirmation of adsorption along with the uniform distribution of chromium(VI) on adsorbent can be ascertained by energy-dispersive X-ray spectroscopy (EDS) spectrum analysis (Cao, Qu, Yan, et al. 2012). The FT-IR analysis was used to depict the mechanism of adsorption by emergence or disappearance and change in the intensity of the peak. The FT-IR analysis of poly(m-phenylenediamine)-coated iron oxide after chromium adsorption suggested oxidation of benzenoid amine units to quinoid imine units (Wang, Zhang, et al. 2015), which can be predicted by the change in the intensity of peak corresponding to quinoid imine (increase) and benzenoid amine (decrease). The results of XPS also supported the FT-IR results. In addition to oxidation or conversion of the group, FT-IR was also used to predict the involvement of functional group participating in the adsorption process. The FT-IR analysis of polyaniline-coated ethyl cellulose after adsorption suggests the participation of -OH groups via declination in the intensity at 1053 cm-1 along with the oxidation of half-oxidized emeraldine base to fully oxidized pernigraniline base form (Qiu et al. 2014). The declination in the intensity along with the oxidation process suggests the participation of both polyaniline and ethyl cellulose in the Cr(VI) reduction.

The XPS and XANES analyses were also used to predict the mechanism of adsorption. The ion exchange behavior on adsorption of chromium on hematite can be ascertained by XPS and XANES spectrum analyses (Cao, Qu, Yan, et al. 2012). XPS indicates the reduction in the peak of the O-H than that of hematite, which suggests the hydroxyl group exchange with chromate. The O K-edge XANES spectrum analysis supports this observation by depicting the declination in the t2g-to-eg ratio. The eg are sensitive to the ligands linked to O atom, and their intensity (eg) increases when hydroxide ions are replaced with chromate ions.

The XPS analysis also helped in the determination of the oxidation state of the chromium adsorbed on the surface of the adsorbent (Egodawatte et al. 2015). The hexavalent chromium adsorbed may undergo reduction to trivalent chromium (Liu et al. 2012) or partial reduction (Fan et al. 2012; Zhang, Xia, et al. 2015). The XPS spectra of the sample after the adsorption of hexavalent chromium on iron oxide hollow microspheres depict XPS peaks corresponding to trivalent chromium. This indicates that after the adsorption of Cr(VI), it has been reduced to trivalent chromium (Liu et al. 2012). The increase in pH after the adsorption of Cr(VI) with polyaniline-coated ethyl cellulose was attributed to the consumption of protons (Qiu et al. 2014) and was suggested as supporting evidence for the reduction of chromium. The adsorption on the basis of XPS, FT-IR, and change in pH was suggested to follow protonation of amino groups, followed by complexation, then reduction of hexavalent chromium along with consumption of protons, and finally adsorption of reduced form via electrostatic interaction.

Similarly, the chromium(VI) adsorption on magnetite nanospheres was postulated to follow multiple steps, i.e. complexation and hydrolysis followed by adsorption. The magnetite surface in water has = Fe-OH surface sites. The pH of the solution was postulated to alter the charge on the = Fe-OH group by protonation and deprotonation. At pH lower than pHzpc, protonation occurs, which leads to positive charge on the group. However, when the pH is more than pHzpc, deprotonation occurs, leading to negative charge on the surface. Upon deprotonation, the Fe-O~ site acts as a Lewis base and can coordinate with metal ion (Kumari et al. 2015).

Along with the change in the oxidation state in chromium during adsorption, the change in complexation is also postulated during the adsorption of hexavalent chromium with mesoporous alumina (Li, Xie, et al. 2016). The adsorption of Cr(VI) is postulated to be started with the formation of outer-sphere complex, w'hich upon dehydration leads to the formation of more stable inner-sphere complex.

RAMAN spectra can also be used as supporting evidence with XPS to ascertain the mechanism of hexavalent chromium (Fellenz et al. 2017). The surface of the amino-functionalized MCM-41 after Cr(VI) adsorption contains corresponding peaks of -NH2 (400 eV) and -NH3+ (402 eV) in the sample. The quantity of -NH3+ is reported to be more than that of the -NH, group, and the electrostatic attraction between -NH3+ and negatively charged species of chromium at pH 2 leads to generation of -NH, group. The Cr(VI) reduction into Cr(III) is reported with transfer of proton from -NH3+ on the surface of adsorbent to the aqueous solution. So, surface functional groups on adsorbent’s surface play a role in the reduction of Cr(VI) to Cr(III). The Raman spectra also confirmed the presence of both Cr(VI) (860cm_|) and Cr(III) species (500cm’1) on the adsorbent after adsorption.

The physical mixture of iron oxide and mesoporous silica nanocomposite was also tested to nullify the hypothesis that the adsorption of trivalent chromium on aminated iron oxide/mesoporous silica nanocomposite was not due to the physical mixture of iron oxide and mesoporous silica. The physical mixture exhibited a lower adsorption capacity (0.37 mmol/g) as compared to the composite (2.08 mmol/g) (Egodawatte et al. 2015). To ascertain that both silanol and amino groups aid in the adsorption, the experiment was conducted with adsorption of copper, chromate, and arsenate. The adsorption of copper was much more as compared to chromate and arsenate, in spite of chromate and arsenate ions having electrostatic advantage over copper ions with protonated amine surface (Egodawatte et al. 2015). This suggests that both silanol and amine functional groups contribute to the adsorption.

Desorption

Desorption of the chromium species was conducted with sodium hydroxide (Wang, Zhang, et al. 2015; Sun et al. 2014; Mohamed et al. 2017; Huang et al. 2013; Zhang, Xia, et al. 2015; Fan et al. 2012; Daneshvar et al. 2019). The adsorption efficiency declined after desorption (Huang et al. 2013; Zhang, Xia, et al. 2015). However, during desorption of chromium from amino-functionalized magnetic cellulose nanocomposite, there was not any significant decline in the adsorption capacity of the adsorbent after successive desorption (Sun et al. 2014).

 
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