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Water contaminations by toxic heavy metals have emerged as a focus of global concern due to the limitations of the worldwide fresh water, which could threat the environment and human beings severely as a result of their non-biodegradability and potential persistent nature effects [1], [2]. Especially, the removal of toxic heavy metals such as Cu2+, Cd2+ and Pb2+ has become very important. Numerous methods have been utilized for the removal of heavy metals ions such as chemical precipitation, membrane filtration and electrocoagulation [3]. However, the above methods have multiple disadvantages such as relative high cost, ineffective removal of trace-level heavy metals, and the problem of different secondary pollutants [4]. Compared with the above methods, adsorption has been regarded as one of the most promising pathway due to high efficiency, facile operation, wide range of usage, low cost and abundant resources from mineral or polymer materials [5]. Among different adsorption materials types, hydrogels have attracted high attention because of their extraordinary advantages such as facile synthesis, high hydrophilicity and swelling ratio, imbibing capability, wide application and the presence of functional groups such as -COOH and –OH in their 3D network structure, which could interact with heavy metal ions effectively in the solutions [6], [7]. Moreover, the exploit of effective, renewable and low-cost absorbents has become a hot research spot in recent years [8], [9].

Natural polymeric adsorption materials have become one of the important alternatives as a result of their excellent properties including complexation, flocculation, separation and adsorption of a wide range of pollutants such as heavy metal ions and organic dyes [10], [11], [12], [13], [14]. As the most abundant natural polymer, cellulose is a low cost, renewable and biodegradable natural resource, which could bind heavy metal ions in the solutions through abundant -OH groups from its molecular chain for waste water treatment [15], [16]. However, the adsorbability of pure cellulose-based materials such as hydrogels is usually not enough, so the improvement of their adsorbability has become more and more important and urgent [17], [18], [19], [20], [21], [22]. The combination of natural polymers with inorganic materials has been proposed for to develop composites with improved properties.

Hydroxyapatite (Ca10(PO4)6(OH)2, HAP) nanoparticles, as another nature occurred inorganic material, have gained widespread attention in the water treatment field because of their advantages including low water solubility, low cost and high adsorption affinity to many kinds of heavy metals ions in aqueous solutions [23], [24], [25]. HAP nanoparticles could be used to absorb Cu2+, Cd2+, Zn2+ and Pb2+ effectively through surface absorption and bivalent ion exchange [1], [26], [27], [28]. However, HAP nanoparticles could form aggregates easily due to their high surface reactivity in aqueous solutions, which could reduce the surface reactivity towards target heavy metal ions. The utilization of supporting materials such as biochar could restrict the self-agglomeration of HAP nanoparticles effectively to improve the adsorbability to heavy metals ions from wastewater [1], [26]. Thus, the major goal of this work was to construct nanocomposite hydrogels to combine the advantages of HAP nanoparticles and cellulose for the restriction of the aggregation of HAP nanoparticles and the improvement of cellulose properties to broaden their applications. To the best of our knowledge, no previous studies have focused on the preparation of cellulose/HAP nanocomposite hydrogels for the removal of heavy metal ions (Cd2+, Sr2+ and Cu2+) from a novel NaOH/urea aqueous solvent. In this work, HAP nanoparticles were synthesized through a facile chemical precipitation method with the following calcination at 600 °C. HAP nanoparticles were first dispersed in the NaOH/urea aqueous solvent with a following ultrasonic processing to avoid their aggregation. Moreover, the viscous cellulose solution could make it easier for HAP nanoparticles dispersion through the hydrogen bonding interaction between HAP and the dissolved cellulose chains. In our findings, the HAP nanoparticles could improve the properties of cellulose hydrogels such as mechanical property and adsorbability towards heavy metal ions (Cd2+, Cu2+ and Sr2+), which could broaden the applications of cellulose materials from the NaOH/urea aqueous system.

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