Removal of Heavy Metals by using Carbon Nanotube

By

Ashish Gadhave

M.Tech

• Introduction» Heavy Metals in

Wastewater» Other Adsorbents

» Carbon Nanotubes

Heavy Metals in Wastewater • Extensive industrialization and improper disposal are

prime factors responsible for release of heavy metals into environment.

• In India, only 60% of wastewater is being treated (CPCB, 2011)

• Almost all heavy metals are toxic to living beings

e.g. Cd- nausea, cancer

Pb- gastrointestinal disorder, abdominal pain

Ni- cancer of lungs, bones, weakness, headache

Other Adsorbents• Many adsorbents have been studied for removal of heavy

metals Activated CarbonHuskOlive Stone wasteMordeniteCrab shell

• BUT low adsorption capacity• Researchers are putting efforts to investigate new

adsorbent

Carbon Nanotubes

• With emergence of nanotechnology, research has been initiated to exploit the unusual and unique properties of carbon nanotubes (CNTs).

• It is first invented by Dr. Ijima in 1991.• Made by rolling up of graffin sheet to form

CNT.• Two types

A. SWCNTs

B. MWCNTs

Adsorption Properties of CNTs

• Highly porous and hollow

• Large specific surface area, light mass density and strong interaction between CNTs and pollutant

• Adsorption properties mainly depend on adsorption sites

Adsorption sites

1)Internal Sites

2)Interstitial Channels

3)Grooves

4)External Surface

Continues…• Adsorption reaches equilibrium much faster on

external sites than on internal sites under same conditions of temperature and pressure.

• Fraction of opened and unblocked nanotubes can considerably influence the overall adsorption capacity.

• The opened CNTs provide more adsorption sites than closed ones

Functionalization• Functionalization plays very important role in

adsorption properties of CNTs

• Functionalization adds –OH, -C=O, -COOH groups.

• Functionalization aims for easy processing.

SEM image.A. Non functionalized B. Functionalized

CNTs Characterization• There is no direct correlation between metal ion

adsorption capacity of CNTs and BET surface area, pore volume

• Surface total acidity influences the adsorption capacity of CNTs [Report table no 1].

• Adsorption of heavy metals onto the CNTs are mainly controlled by the strong interactions between the metal ions and hydrophilic surface functional groups

Metal Ion

Adsorbent SA PV MPD STA STB qmax

Pb (II)

CNT/HNO3/ Xylene Fe

47 0.18 3.4 1.63 14.8

CNT/HNO3/ Benzene Fe

62 0.26 3.2 1.65 11.2

Ni (II)SWCNTs 577 1.15 7.98 0.54 0.23 9.22

SWCNT/NaOCl 397 0.46 4.62 4.42 0.35 47.85

Cd (II)CNT/HNO3 154 0.58 3.6 4.04 5.1

Ag-MWCNT 101 0.27 10.98 4.69 0.1 16.95

Zn (II)SWCNTs 590 1.12 7.6 11.23

SWCNT/NaOCl 423 0.43 4.12 43.66

SA = BET surface area (m2/g), PV = pore volume (cm3/g), MPD= mean pore diameter (nm), STA= surface total acidity (mmol/g), STB = surface total basicity (mmol/g), qmax = maximum adsorption capacity (mg/g).

Adsorption PerformanceAdsorption Isotherm

Adsorption MechanismEffect of pH

Adsorption Isotherm• The metal ion adsorption equilibrium are

commonly correlated with the Langmuir or the Freundlich equations.

• Several researchers depicted that metal ion adsorption on CNTs can be well fitted in Langmuir equation.

• whereas some researchers reported that sorption of heavy metals on CNTs can be correlated with both Langmuir and Freundlich equations

Metal Ion

Adsorbent parameters Initial conc. of metal ion

Qmax (mg/g)

Pb (II) CNTs/MnO2 pH= 7, t= 2hr 30 ppm 78.74

CNTs/ HNO3 pH= 5, T= 298K 80 ppm 35.6

Cu (II) Dispersed MWCNT pH= 5.6 10 ppm 67.8

Undispersed MWCNTs pH= 5.6 10 ppm 51.3

Cd (II) Amino modified MWCNTs

pH= 6, T= 318K 5 ppm 31.45

Activated alumina-CNT

pH= 7.5 250 ppm 229.9

Ni (II) SWCNT/NaClO T= 298K 60 ppm 47.86

Qmax= adsorption capacity, t= contact time, T= Temperature

Adsorption Mechanism

Possible Adsorption Reactions• Step I Protonation and deprotonation of CNTs:

CNT-OH + H+ ↔ CNT-OH2+

CNT-OH ↔ CNT-O- + H+

• Step II Adsorption of divalent metal ions on CNTs

CNT-OH2+ + M2+ ↔ [CNT-OHM2+]2+ + H+

CNT-O- + M(OH)n2-n ↔ [CNT-O-M(OH)n

2-n]1-n

Effect of pH• pH plays very important role in adsorption of metal ions.• When the solution pH is higher than pHPZC (a pH value,

called ‘point of zero charge’, at which the net surface charge is zero), the negative surface charge provides electrostatic interactions that are favourable for adsorbing metal ions.

• The decrease of pH leads to neutralization of surface charge, thus, the adsorption of metal ions should decrease.

• pH also affects metal ion species and competing complexation reactions, and influences adsorption capacity

Future Work• Much progress has been made over the last few years in

adsorption applications of CNTs.

• In spite of high costs, using CNTs as adsorbents maybe advantageous in future because the high adsorption capacities of CNTs compared to other media may offset their high cost.

• There are still a lot of works to do to enhance CNT adsorption properties in future.

• The surface modification to enhance the dispersion property of CNTs in solution can greatly increase the interaction of CNTs with metal ions.

• The practical use of CNTs as sorbents in water and wastewater treatment depend upon the continuation of research into the development of a cost-effective way of CNT production and the toxicity of CNTs and CNT related materials

Conclusion• CNTs can be used as effective adsorbent for removal of

heavy metal ions.• The adsorption capacities of metal ions to different CNTs

follow roughly the order: Pb2+ >Ni2+ >Cu2+ >Cd2+

• The adsorption mechanism appears mainly attributable to chemical interaction between the metal ions and the surface functional groups.

• Process parameters such as surface total acidity, pH and temperature play a key role in determining sorption rate of metal ion onto CNTs.

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