Carbon nanotubes: a review on green synthesis, growth mechanism and application as a membrane filter for fluoride remediation

Figures & data

Figure 1. Health effect of fluoride ion on people in the Dugda Bora district, East Shoa Ethiopia. Photos taken from ‘Aanaa Dugdaa Booraa; shawaa Bahaa; Oromiyaa’.

Figure 2. Fluoride distribution (12).

Table 1. Fluoride remediation experience in Ethiopia (12).

Method	Advantage	Disadvantage	Remark
Al2(SO4)3 with CaO	High availabilityEasily obtainedEasily synthesizedEasily appliedEasy to operateEfficientLess costly	Mud productionNon-recyclableHigh initial costLess feasible for F^- > 10 mg/l and F^- < 3 mg/lLess applicable in rural area	Almost accepted by all
Bon Char	Readily availableCheapRemove F^- easily	Bad color, odor, tasteUnpleasantCultural conflictScarcity of raw bones	Almost rejected by all
Natural Clay	Readily availableCheap	InefficientEasily operatedLess manpower	Less preferred
Activated Alumina	Effective	ExpensiveDifficult to operate	Practiced

Figure 3. Setup for arc discharge technique.

Figure 4. Laser ablation scheme.

Figure 5. Experimental CVD equipment setup.

Figure 6. The FESEM images of CNTs obtained by green method using walnut extract (28).

Figure 7. Diffusion ways of CNTs growth with metal catalyst: (a) VLS and (b) VSS mechanism.

Figure 8. Interaction strength between catalyst particles and substrate CNTs growth: (a) tip-growth (b) base-growth model.

Table 2. The objectives of few characterization techniques of CNTs.

Techniques	Objectives	Ref.
SEM	Diameter, length defects and impurity analysis	(45), (46)
TEM	Analysis of number of layers and distance between them	(45)
EDS	Analysis of elemental composition and functionalization	(47)
Raman spectroscopy	Quality and purity analysis	(48)
FTIR	Functional group identification	(49)
UV-VIS spectroscopy	Analysis of dispersion state	(49)
XPS	Elemental composition, purity	(50)
TG	Purity and functionalization	(50)
Boehm techniques	Oxygen containing functional group	(51)

Figure 9. Flow chart for size boundary, pressure range and types of material separated in filtration technology (59).

Table 3. Driving forces and their related membrane separation processes.

Cause	Processes
Pressure gradient	MF, UF, NF and RO
Chemical potential gradient	PV, dialysis, gas separation, vapor permeation, liquid membrane
Electrochemical potential gradient	ED, membrane electrophoresis, membrane electrolysis

Figure 10. Principle of reverse osmosis.

Figure 11. Charge balance mechanism by Donnan effect.

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