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2. Experimental procedure
2.1. Materials
The cellulose raw material (cotton linter pulps) with the weight-average molecular weight (Mw) of about 10.0 × 104 g/mol was supplied by Hubei Chemical Fiber Co. Ltd. (Xiangfan, China) [29]. The Ca(NO3)2·4H2O and (NH4)2HPO4 (Sinopharm Chemical Reagent Co., Ltd), NaOH, HCl, NH3·H2O and urea (Shanghai Chemical Reagent Co. Ltd., China). CuCl2, SrCl2 and CdCl2 were obtained from Aladdin Reagent Company. All the reagents were of analytical grade and used without further purification.

2.2. Preparation of cellulose/HAP nanocomposite hydrogels
Ca(NO3)2·4H2O and (NH4)2HPO4 were used as raw materials to prepare HAP. Briefly, a Ca(NO3)2 aqueous solution was added dropwise into a (NH4)2HPO4 aqueous solution with vigorously stirring to keep the molar ratio of Ca/P at 1.67, and NH3·H2O solution was used to regulate the solution pH to about 10.0. The stirring process was remained for 2 h after Ca(NO3)2 solution was added completely, and the aging process of 12 h was conducted at 25 °C without stirring. Then, the resultant sample was filtered and rinsed with absolute ethanol, and the obtained dried product was ground by a pestle before calcinations at 600 °C to get HAP nanoparticles. Scheme 1 shows the fabrication process of cellulose/HAP nanocomposite hydrogels. Briefly, required amounts of HAP nanoparticles were added into 7% NaOH and 12% urea aqueous solutions, which were stirred vigorously for 6 h with the following ultrasonic treatment a KQ5200 ultrasonic instrument (200 W, Kunshan Ultrasonic Instrument Co., Ltd., Kunshan, China) for 20 min at room temperature. The blend solvents were pre-cooled to −12.5 °C. Then, the desired amounts of shredded cotton linter pulp (cellulose source) were added in the above NaOH/urea/HAP solvents immediately at room temperature. Cellulose was completely dissolved in 5 min with the stirring speed at about 1500 rpm to obtain the cellulose/HAP blend solutions, which were centrifuged in a high-speed refrigerated centrifuge (H1850R, Cence Company, China) to degas at 5 °C and 2000 rpm for 10 min, and then sucked in an injector with the following crosslinking at 60 °C. The obtained hydrogels were washed with running water and then exhaustively with deionized water to remove the urea. The nanocomposite hydrogels were denoted as CHG20-1, CHG10-1 and CHG5-1 according to the weight ratios of cellulose to HAP of 20:1, 10:1 and 5:1. Pure cellulose hydrogel was prepared similarly without the addition of HAP, which was coded as CG (Scheme 2).

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