The use of cellulose in the form of filter paper is known for long. Still, we see limited use in water purification since most of the impurities flow through a standard filter paper. What happens if we use cellulose micro- or nanofibers instead to make paper or flat sheets? How do these sustainable materials perform in water purification applications? Our guest writer, Assoc. Prof. Aji Mathew, shares her thoughts.
MAKING NANO-SIZED EXCLUSION MEMBRANES
Cellulose nanofibers based paper, membranes or flat sheets take biobased water cleaning technology to a new level regarding functionality, versatility, and performance. Microfibrillated cellulose has a high aspect ratio and forms robust and stable networks once dried from water. The network strength comes from the physical entanglements as well as H-bonding.
Unfortunately, this network has low permeability even towards water molecules, which limits the use in water purification. However, the pore structure and permeability in these networks can be controlled by changing the diameter of nanofibers, solvent exchange to low polar solvents or use of polymer phase as binder, to name a few. This approach usually leads to size exclusion membranes just as in the case of filter paper but with pore sizes in the nanometer range. (React Funct Polym, 2015, 86, 209-214 (DOI:10.1016/j.reactfunctpolym.2014.09.014)
Purifying water by ion exchange
Recent research has confirmed the excellent potential of microfibrillated and nanofibrillated cellulose in water purification. Not only by acting simply as a filter material but also as an ion exchanger. Cellulose nanofibers once isolated have large surface area and a high amount of OH that can convert to anionic or cationic anchoring sites for water contaminants by suitable modifications (see the recent review by Carpenter Environmental Science and Technology 49 (2015) 5277-5287). The functional sites on the cellulose backbone anchor contaminants, such as metal ions, dyes, pesticides, and nitrates, via ion exchange or surface adsorption.
The graph below compares the contaminant capture performance of regular cellulose with microfibrillated cellulose and its modified versions. Unmodified MFC improves the copper adsorption significantly, and the efficiency is even better when the fibers are modified. These cellulose micro- and nano-fibers once made into membranes or porous filters opens up the possibility to capture small-sized contaminants at low concentrations with low pressure and low energy.
The mechanism of purification in microfibrillated or nanofibrillated cellulose based membranes is attributable to the capability of the microfibrillated or nanofibrillated cellulose and/or nanochitin (with or without functionalization) to selectively adsorb contaminants from water while it permeates through the porous medium. The inherent hydrophilicity of microfibrillated or nanofibrillated cellulose also provides additional benefits regarding antifouling and high water flux through the ion exchange membranes.
Figure: Adsorption of the different types of cellulose and MFC
The next generation of biobased ion exchange membranes for water cleaning from microfibrillated cellulose can be a reality in industrial and domestic applications. Especially where long-term stability and membrane reuse are not the main priority.