Adsorption properties of cross-linked cellulose-epichlorohydrin polymers in aqueous solution

•Cellulose was cross-linked with epichlorohydrin at variable composition.•Tunable physical properties are evidenced by materials characterization methods.•Cross-linked cellulose has greater uptake than cellulose with carboxylate anions.•Cross-linked polymers show molecular selective uptake in solution at equilibrium.•Micropore domains of cross-linked polymers are active adsorption sites. Cellulose was cross-linked with epichlorohydrin (EP) at variable levels (CLE-0.5, CLE-2 and CLE-4), where CLE-i denotes the cellulose to EP mole ratios. The cross-linked products were characterized by TGA and FT-IR spectroscopy, pH at the point of zero charge (pHpzc), water swelling, and dye-adsorption methods employing two types of dyes [phenolphthalein (phth) and p-nitrophenol (PNP)]. The characterization methods provide evidence of cross-linking of cellulose in accordance with variations in surface area, PZC, available surface hydroxyl groups, and thermal stability when compared against pristine cellulose. The pHpzc of the sorbent materials was ∼6.5 indicating a negative surface charge occurs above pHpzc. The cross-linked polymers possess greater swelling properties relative to pristine cellulose. Detailed adsorption studies were carried out at pH 9 for cellulose and CLE-i with five types single component carboxylate anions [2-hexyldecanoic acid (S1), trans-4-pentylcyclohexanecarboxylic acid (S2), 2-dicyclohexylacetic acid (S3), adamantane carboxylic acid (S4), and cyclohexane carboxylic acid (S5)] at 295K. The uptake properties of PNP with cellulose and CLE-i were also compared at pH 5 and 9, respectively. CLE-2 had the highest uptake of PNP (Qm=1.22×10−1mmol/g, pH 9) and S1 (Qm=4.27mg/g) while cellulose and CLE-4 had the strongest binding affinity (1.43L/mmol and 5.90×10−2L/mg), respectively. Uptake of PNP by CLE-0.5 at pH 5 (Qm=5.30×10−2mmol/g) was higher than uptake at pH 9 (Qm=3.11×10−2mmol/g). Sorption of CLE-4 with S1, S2 and S3 showed that relative uptake of the surrogates had the following order: S3>S2>S1, where S2 had the strongest binding affinity to CLE-i. CLE-2 had the highest sorption capacity towards Si in an equimolar mixture with evidence of molecular selective uptake. At pH 9, low uptake was mainly related to electrostatic repulsion between the negatively charged sorbent surface and the carboxylate head groups of Si.