Investigation of chitosans for water treatment: Effects of feedstock and synthesis on contaminant removal

Chitosan, a biopolymer derived from chitin, has attracted significant attention for use in water treatment applications because of its abundant surface functional groups and affinity for a wide range of contaminants. However, contaminant removal performance can vary with the chitin source and synthesis method, as documented in the literature. This study reviews synthesis methods and key physicochemical characteristics of chitosan relevant to contaminant removal and experimentally evaluates the removal of phosphate and nitrate by four commercial chitosans derived from crustacean shells and mushrooms. The chitosans were characterized using scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and zeta potential analysis. Batch adsorption experiments were conducted under varying pH conditions, and the results were analyzed using Freundlich, Langmuir, and Temkin isotherm models to assess adsorption behavior and infer removal mechanisms. The Langmuir model provided the best fit for phosphate adsorption for most chitosans and pH conditions, whereas the Freundlich model more accurately described nitrate adsorption in most cases; however, model performance varied across chitosans and pH conditions. Langmuir-derived phosphate adsorption capacities at unadjusted pH conditions ranged from 17.6 to 41.6 mg/kg, while nitrate adsorption capacities ranged from 59.8 to 129.8 mg/kg. Mushroom-derived chitosan consistently exhibited lower phosphate and nitrate removal than the three crustacean-shell-derived chitosans, and flake-form chitosan showed reduced phosphate removal. Langmuir adsorption capacities decreased as initial solution pH increased, with the lowest values observed at pH 11. Among the four chitosans evaluated, no consistent relationship was found between contaminant removal and molecular weight, degree of deacetylation, or zeta potential. These findings demonstrate that source and physical characteristics contributed to differences in contaminant removal performance among the chitosans tested, and should be considered when selecting a commercial chitosan product for water treatment applications.