Mechanism of sodium-hypochlorite-induced degradation of cellulose acetate and the enhancement of its degradation resistance by chemical modification

Abstract

Cellulose acetate, when used in the form of hollow fibers in the ultrafiltration stage of water treatment, is occasionally treated with sodium hypochlorite to remove organic particles such as humic acids. However, prolonged use of sodium hypochlorite reduces the strength of the membrane and facilitates its breakage. The present study was designed to reveal the degradation mechanism of cellulose acetate caused by aqueous sodium hypochlorite and to improve its resistance to this chemical. Filaments of cellulose acetate, prepared using a nonsolvent-induced phase separation method, were exposed to 2000 ppm aqueous sodium hypochlorite at 25°C for 13 days to allow for evaluation of the changes in their tensile strength, elongation at break, molecular weight, degree of substitution, and chemical structure. The tensile strength, elongation at break, and molecular weight decreased as the duration of exposure to sodium hypochlorite increased. No significant changes in the degree of substitution were observed by one-dimensional hydrogen 1 nuclear magnetic resonance, and cleavages of both glycoside bonds and carbon–carbon bonds were detected by two-dimensional nuclear magnetic resonance, revealing that base-catalyzed hydrolysis of ester groups did not play an important role in degradation. The chemical modifications of the cellulose esters, such as the introduction of pentanoyl, stearoyl, and benzoyl groups, were studied in accordance with the degradation mechanism, and cellulose esters with bulky substituents such as benzoyl groups were found to exhibit improved chlorine resistance. Perbenzoylated cellulose, which exhibited high resistance to sodium hypochlorite, is considered to be a potential membrane material for the filtration of foulant-rich raw water.

Exploration of advanced cellulosic material for membrane filtration with outstanding antifouling property

“…59 However, the CA membrane deteriorates and its strength decreases after washing with sodium hypochlorite solution, which is used to efficiently remove foulants. 41 Furthermore, introducing bulky substituents such as benzoyl or stearoyl groups can prevent the deterioration of CA by washing it with sodium hypochlorite solution. 41 Conversely, these bulky substituents have a large number of carbon atoms, which makes the polymer hydrophobic.…”

“…Benzoyl groups can be used to improve chlorine resistance. 41 However, introducing benzoyl groups makes cellulose derivatives hard but brittle. 61 If the polymer used for the membrane is brittle, it can break during membrane formation.…”

“…The chlorine resistance test was performed according to Hashizume et al 's method. 41 The test piece used for the tensile test was formed by casting 0.1 g mL −1 concentration of a cellulose derivative in dichloromethane solution on a glass plate, air-drying and peeling it off from the glass. The resulting film was processed and prepared in the same manner as in the tensile test to obtain the test pieces.…”

“…However, when CA membranes are exposed to sodium hypochlorite for a long time, and the oxidation reaction by hypochlorite causes CA main-chain scission, membrane embrittlement, and easy destruction of the membrane. 41…”

“…However, when CA membranes are exposed to sodium hypochlorite for a long time, and the oxidation reaction by hypochlorite causes CA main-chain scission, membrane embrittlement, and easy destruction of the membrane. 41 Additionally, sodium hypochlorite treatment of polymer membranes for desalination of seawater lowers sodium chloride removal performance. 39,42,43 To improve the fouling problem, much research has been conducted on the types of foulants that reduce the permeation rate of membranes.…”