DOI: 10.14457/tu.the.2020.73
title: Chemical modification of chitosan and its applications as antiscalants and antimicrobial agents for water cooling system

description: Quaternized carboxymethyl chitosan (QCMC), a polyampholyte with the presence of both negative charge and positive charge on two sides of chitosan backbone, has attached vast attentions for its high potential in applications, such as water purification, drug delivery, cancer treatment, antioxidant, moisture-absorption and retention agents in cosmetic applications. However, highly toxic monochloroacetic agents, as well as hard conditions are required to produce QCMC. A promising “green” alternative method to introduce negative charges to chitosan is to directly oxidize chitosan by (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl (TEMPO) in a presence of co-oxidants, such as NaOCl/NaBr, NaOCl2, or O2/laccase. Although TEMPO is well-known as effective catalyst for “green” oxidation of primary alcohols, its co-oxidants impose concerns for their impacts to the environment as well as their cost. Hence, this study aims to develop a “green”, halide-free, and cost-affordable process to prepare charged chitosan derivatives, such as quaternized chitosan (QCs), oxidized chitosan (OCs), and chitosan polyampholyte (CPAs). In addition, insights into the oxidation reaction mechanisms are investigated. Chitosan was initially grafted by glycidyl trimethyl ammonium chloride (GTMAC) in aqueous solutions at 60 °C for 24 h to obtain QCs. The quaternization degree (QD) and solubility of QCs was optimized by varying molar ratios of GTMAC and NH2 groups on chitosan. Oxoammonium cation (TEMPO+), a main oxidant formed in situ in the oxidation reaction of TEMPO and co-oxidants, was synthesized by ozone oxidation of TEMPO at 4-5 °C in 0.05M H2SO4 solution. The conversion mechanism of TEMPO to TEMPO+ was investigated by UV-Vis spectroscopy and 2D UV-Vis spectroscopy correlation analysis. The optimized conditions of TEMPO+ formation, the stoichiometry molar ratio of TEMPO+ and the consumed TEMPO under different supplied ozone contents and reaction times were determined by UV-Vis spectroscopy and iodometric titration. The quaternized chitosan with high water solubility was then oxidized by freshly prepared TEMPO+ solution at 4-5 °C, under nitrogen atmosphere. The oxidation degree (ODs) was optimized by varying TEMPO+ contents, reaction times, initial QC concentrations, and pH of the solutions. The proposed two-step TEMPO+ and one-step TEMPO/ozone oxidation were conducted in parallel with ozone/H2O2 oxidation and NaIO4 oxidation for soluble QC to compare the oxidation efficiency and the extents of side reactions. Native chitosan and ethylene glycol (EG) microwave pretreated chitosan (MW chitosan) were also oxidized by these oxidation procedures with a purpose to collect soluble fraction of the oxidized products for further applications. The charged chitosan products, with a focus on soluble fraction which is more important for the intended application in this study, were characterized by ATR-FTIR, synchrotron-based transmission FTIR, 2D FTIR correlation analysis, 1H NMR, 2D-NMR, synchrotron-based XPS, GPC, TEM, Zeta sizer, and conductivity titration. The percentage of different fractions of carboxylic acid (intra, inter-molecular hydrogen bonding, and free COOH) and carboxylate, interacted with different counter ions, were determined by quantitative FTIR spectroscopy. Quaternized chitosan is successfully prepared with different QD values of 11, 34, 92, and 153 %, depending on the molar ratio of GTMAC/NH2, in which the latter two show high solubility in water in a large range of pH. The conversion percentage of TEMPO to TEMPO+ is in a direct proportion with supplied ozone contents. The optimized conditions to synthesize TEMPO+ (70 % of conversion) from ozone oxidation of TEMPO are 1 :1 molar ratio of TEMPO/ozone, in 45 min of stirring, 0.05 M H2SO4, and at 4-5 °C. The stoichiometric molar TEMPO+/consumed TEMPO is determined as 1:1 was reached in the first two min of reaction, and remained for over four h of reaction, indicating an effectiveness of ozone as an oxidizing agent for TEMPO. CPAs structure are found in the form of polymeric particles due to the self-folding and self-assembly depending on the oxidation degree. The optimized conditions for oxidation reaction are observed at 1.25 mmol/g of TEMPO+ content, 60 min of reaction time, 15 mg/mL of initial QC, and pH 3-4 or pH=6-7, at 4-5 °C, under nitrogen atmosphere, which give up to 100 % of oxidation degree determined by conductivity titration. In the oxidation of NaIO4 and ozone/H2O2, a higher depolymerization and formation of hemiacetals are found in parallel with higher free carboxylate contents, implying lower contents of self-folding structures of CPAs. Different charged-induced chitosan (QC and CPAs) were then applied as scale inhibitors for CaCO3 and silica, and as antibacterial agents for Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 6538. The results show that the performances of CPAs are higher than the current commercial ones, in terms of weight inhibition at the same dosage of 20 ppm. The scale crystallites form in a presence of CPAs tends to be adherent together due to the predominant mechanism of charge neutralization, leading to flocculation of the scale crystals. Similar to blank sample, calcite and aragonite are two morphology types found in the samples with CPAs, but with more distortion of crystals compared to blank sample and the extent of distortion enhanced by increasing CPAs concentration. CPAs at a 40 ppm dosage show higher performance in maintaining highly soluble silica in solution after 48 h, which is dropped from 500 ppm initial concentration to 300 ppm after 24 h and likely unchanged up to 48 h. The performance is comparable to dendrimer polyaminoamines generation 1 and 2, common commercial anti-colloidal silica agents. Additionally, CPAs are found as effective antibacterial agents for both E. coli and S. aureus in which the second one is more sensitive with CPAs. The performance of minimum concentrations of CPAs for effects on E. coli and S. aureus are 7.81 and 15.63 ppm, respectively, which is comparable with 1.95 ppm of commercial biocide isothiazolone.