Superadsorbent with three-dimensional networks: From bulk hydrogel to granular hydrogel

•Four polymerization strategies for preparation of granular hydrogel are reviewed.•Potential of hydrogel as the adsorbent for pollutant removal is discussed.•Extensive literature information using this novel adsorbent is summarized.•Future perspectives in this direction are proposed.

Hydrogels are 3D networks of polymer chains that are crosslinked via either physical or chemical bonds. A strong repulsive force is generated among those abundant functional groups with the same charge, which enables the hydrogels to be initially applied as the superabsorbent materials. Recent studies have found that the hydrogel adsorbents show superhigh adsorption capacities, fast adsorption rate, wide pH-independence, easy regeneration and reusable ability, by which the hydrogels are given the appellation “superadsorbent”. The limitations of traditional bulk hydrogel make granular hydrogel have received much attention in recent years. Therefore, the purpose of this review is to organize the scattered available information on the focus of polymerization strategies for the formation of granular hydrogels, with the scope limited to the crosslinked hydrogels via polymerization using one or more vinyl monomers. Furthermore, some of the important applications of hydrogel as the superadsorbents for removing various pollutants since 2000 are summarized and discussed, regardless of its form in either bulk or granular. An extensive list of hydrogel adsorbents has been compiled and selected studies with adsorption capacities for heavy metals including radioactive and rare earth metals, dyes and eutrophic substances as available in the literature are presented, with the key advancements of this type of novel adsorbents are briefly addressed. The facile synthesis and energy savings offer them a lot of promising benefits for commercial purposes in the future.

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