robust water desalination membranes against degradation using high loads of carbon nanotubes

by:Ultimate     2020-11-30
Anti-chlorine reverse osmosis (RO)
Film is used more
Carbon nanotubes-polyamide (MWCNT-PA)nanocomposite.
Separation performance of these membranes after chlorine exposure (4800 ppm·h)
Remain unchanged (99. 9%)
But in the absence of MWCNT, this ratio dropped sharply to 82%.
It was observed that the surface roughness of the film changed significantly by adding MWCNT.
In addition, films containing MWCNT grades greater than 12. 5 wt.
% Significantly improved the degradation ability of chlorine-resistant exposure, and the water flux increased while maintaining the salt inhibition performance.
In order to understand the high chemical stability of MWCNT, molecular dynamics and quantum chemical calculations were carried out.
PA nano-composite film and revealed the high activation energy required for PA chloride.
The results of this paper confirm the unique potential of carbon nanoparticles embedded in polymer complexes in high-efficiency reverse osmosis seawater desalination technology.
Big-current technology
Scale water desalination of seawater or slightly salty water is mainly based on reverse osmosis (RO)
Membrane, has been improving since 1960.
The construction of the RO film mainly depends on the film composite material (TFC)
System, where the active layer is a polymer film deposited on the top of a high-permeability porous substrate.
The active layer of the TFC membrane is responsible for the rejection of solvent ions, and now they are usually composed of aromatic polyamide (PA).
Given the importance of RO-
Based on the purification system, considerable efforts have been made to reduce the operating costs and energy consumption of desalination plants.
Therefore, it is very important to develop a new type of RO film, which has improved water permeability and salt retention within the active layer, and is robust to chemical and biological degradation processes.
In this case, a feasible alternative is to add carbon nanotubes (CNT)
To form a polymer nano-composite film into a polymer matrix, even using carbon nanotubes-Based on membrane.
At present, chlorine degradation is one of the main disadvantages of reducing the operating life of PA-operation
Based on RO membrane, because in order to prevent biological contamination in the process of seawater desalination, or for disinfection in the food separation system, chlorine compounds are usually added to the water supply.
PA deteriorated due to chlorine, resulting in the passage of salt and water, thus reducing the performance life of the membrane;
For many years, the potential mechanism of chlorine-induced degradation has been studied.
Most of the proposed pathways involve several chemical steps and structural reorganization, including reversible and irreversible chloride, and few factors are found to be critical to the degradation of PA membranes, such as free chlorine concentration, pH and exposure time.
The latest research shows that the degradation process is mainly due to the break of the amide bond produced after the amine chain chloride nitrogen.
In fact, the whole process is divided into two stages: the absorption of chlorine, strongly dependent on the concentration of free chlorine, and the subsequent break of the amide bond promoted by the presence of oh ions (OH).
A small amount of carbon nanoparticles, such as CNT or graphene, have been proposed to be incorporated into composite films as an alternative to improving their chlorine-resistant degradation capabilities.
However, the mechanism of PA degradation reduction still needs to be understood in detail in the presence of different quantities of carbon nano-material loads.
We recently reported a tfc ro membrane of an active layer made of MWCNT
PA nano composite film.
The activity layer that is included. 15 wt.
The nano tube with a diameter of % between 12 and 15 nm, we found that compared with other composite ro films, MWCNT not only improves the membrane in terms of water flow, salt rejection and anti-biocontamination, but also increases its tolerance to chlorine exposure.
We propose that the PA matrix adopts a unique structure in which the first two layers of aromatic structure are preferentially oriented around the nanotubes.
This polymer nano-structure is combined with rigid CNT to produce a stronger membrane compared to ordinary PA.
After hydration, the PA matrix of these nano-composite films is mechanically constrained, thus forming a large gap in a small part that can accommodate water bags.
Thus, the CNT enhanced PA membrane showed lower chlorine absorption compared to the normal PA, as shown in the XPS analysis.
In this paper, we study MWCNT-
Pa ro composite film, by first evaluating the water flux and salt retention properties before and after exposure to water hypochlorite salt (NaClO)
As a solution for MWCNT load function (
From normal PA to 20 wt. % MWCNT load)
Use harsh operating conditions comparable to industrial processes (i. e.
High salinity water separation test under high pressure CrossFlow conditions).
In addition, computer simulation of MWCNT-
PA is used to clarify the resistance mechanism of chlorine in these complex membranes.
The results obtained show that due to the presence of an increasing number of MWCNT in PA substrates, this is a feasible protection mechanism, as well as the stabilization of large diameter MWCNT that hinders the chloride of PA structures. Thus, MWCNT-
PA composite film provides an alternative way to improve the performance of current RO technology, especially in applications where the membrane is critical to the robustness of chlorine degradation.
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