English abstract
The rising burden on the issue of water scarcity necessitates a cost-effective, efficient
water treatment technology in order to provide high-quality clean water and meet
global water demand. Membrane distillation (MD) is a relatively new alternative
separation method that may be utilized for desalination. This technology is promising
to replace currently dominating pressure driven membranes-based desalination
methods. However, most of current studies aim at improving the MD membrane in
terms of cost, durability and salt rejection. This work attempts to address some of the
bottlenecks concerning the commercialization of these membranes in the desalination
industry by focusing on the utilization of cheap raw materials for the synthesis of
nanoparticles and the membrane. Functionalized Carbon Nanotubes (f-CNTs) were
derived from local waste biomass and utilized as an additive to improve the efficacy
of the membrane used in an MD unit configuration.
Local waste date palm stems (Phoenix dactylifera) were used to prepare activated
carbon (AC) using KOH and two metal chloride salts (NaCl and ZnCl2) as activation
agents. The carbonization was conducted at 600 oC for 2hr under nitrogen flow,
followed by activation at 750 oC for 2 hr under carbon dioxide gas flow. AC was
characterized using a scanning electron microscope (SEM), Attenuated total
reflectance Fourier transform infrared (ATR FT-IR), Thermogravimetric analysis
(TGA). In addition, surface morphology was characterized by Iodine adsorption and
Brunauer-Emmett-Teller (BET), at different carbon-to-activation-agent-ratios. FTIR
spectra results showed a reduction in AC-NaCl bands compared to other AC, which
indicates less functional surface groups. At 750 oC, the TGA analysis showed that
carbon yield as AC-ZnCl2 > AC-NaCl > AC-KOH. However, among all samples, AC NaCl at 1:2 ratio was the best in terms of iodine removal. This treated AC sample
exhibited about 18.3 % maximum iodine removal, which indicates the high surface
area and porosity with 550.44 m2
/g, 348.74 m2
/g, and 201.69 m2
/g BET, micropores
and mesopores surface areas, respectively. In conclusion, the local Omani date palm
waste stems can be used for AC production with a well porous structure using cheap
and environmentally friendly salts as activation agents. Furthermore, the produced AC
attained better adsorption characteristics among other alternatives.
The current work addresses the utilization of AC as a supporting substrate combined
with NaCl as a novel green catalyst for the synthesis of carbon nanotubes (CNTs) via
a catalyzed chemical vapor deposition (cCVD) method. The effect of different AC NaCl ratios on CNT growth was investigated. The nano-particle yield was estimated
and samples were characterized by BET, Electron microscopy (SEM and TEM), XRD,
FTIR, and TGA analyses. The asymmetrical porous structure and high surface area of
the AC clearly offer excellent uniform NaCl dispersion properties on the surface,
resulting in a high catalyst-transition metal free-yield of CNTs forest growth. The
results showed higher mass yield in the order AC-NaCl 1:2 > AC-NaCl 1:1 > AC NaCl 1:3 ratios. AC and NaCl are excellent choices as substrate and catalyst
combinations for the synthesis of metal-free MWCNTs as they are cost-effective and
environmentally friendly.
Acid functionalization was conducted using an acid mixture of H2SO4:HNO3 at
different volume ratios to improve the CNTs dispersity and to eliminate contaminants
from their surfaces. Functionalized carbon nanotubes (f-CNTs) were then employed
to prepare composite membranes for saline water desalination via direct contact
membrane distillation (DCMD). Different f-CNT loadings were tested. All produced
membranes were subjected to various characterization tests including porosity, SEM,
ATR-FTIR, TGA, Contact Angle (CA) and Tensile strength tests.
The results showed that the inclusion of f-CNT structure could improve
hydrophilicity, porosity, thermal stability, and mechanical strength of the composite
membranes. The DCMD process was also improved using f-CNTs compared to other
membranes as the f-CNTs offered additional pathways for water vapor transport. In
the DCMD experiments, the highest permeate flux reached was 19.6 kg/m2
.hr and >
99% salt rejection at 80 oC feed and 20 oC permeate temperature. This was achieved
with 78.8o
contact angle, 56% porosity and thermal stability higher by more that to 11
°C compared to pristine PVDF. Overall, the results show that PVDF-f-CNTs
membranes offer a potential and encouraging alternative to existing membranes.
Central composite design (CCD) was applied to analyze and optimize the
performance of PVDF-f-CNT 0.03% membrane. CCD is the most common design
used in response surface method (RSM). The MD performance was modeled using
the quadratic regression model as a function of the operating conditions (feed
temperature, flow rate and concentration) was studied. Feed temperature had higher
x
impact on vapor flux for DCMD compared with the other variables as it was the most
significant factor (p-value < 0.05) followed by feed flow rate (p-value =0.0830). On
the other hand, feed concentration had the least effect on the flux (p-value =0.1683).
Increasing feed temperature improves the evaporation rate in the feed side which
consequently creates higher vapor pressure difference, therefore, improving MD flux.
Optimization of operating conditions was conducted to maximize permeate flux
within the range. A desirability of 0.776 was achieved at a feed temperature of 80 oC,
feed flow rate of 75mL/min and 0.4 M feed concentration in which a flux of 19.472
kg/m2
.hr was obtained. Another optimization study was conducted at wider ranges and
maximum flux of 31.078 kg/m2
.hr could be found at 90 oC feed temperature,
80mL/min flow and 0.4 M NaCl feed concentration. A permeate flux of 29.7 kg/m2
.hr
was achieved by the validation experiments, which indicates the reasonable accuracy
of the predicting model.
The inclusion of waste-biomass-based carboxylated CNTs increased the MD process
performance by enhancing the interactions between water and the membrane and
boosting vapor permeability while preventing liquid from penetrating the membrane
pores. This work contributes in paving the way for enhancing the MD process
performance and promoting its commercial application potential.
