Evaluation of anion adsorption properties of nanocomposite polymer hydrogels containing layered double hydroxides

• Agarose-LDH nanocomposite hydrogel, a 3D composite, shows anion adsorption ability.
• LDH nanoplates in the hybrid can have comparable adsorption ability to powder LDH.
• Decarbonation treatment effectively restored the adsorption ability of the hybrids.

The feasibility of nanocomposite polymer hydrogels as anion adsorbents was explored by using agarose-layered double hydroxide (LDH) hybrids. Also, to obtain fundamental data for possible applications such as slow drug release of more complex hybrid hydrogels such as ionic polymer–LDHs, the absorption ability of LDH highly dispersed in an uncharged polymer, agarose, was evaluated. An aqueous colloidal dispersion of LDH containing lactate was mixed with agarose and water. Then, the mixture was boiled to dissolve the agarose and cooled to form a gel. It was confirmed by SEM that LDH nanoplates were uniformly dispersed in the resulting nanocomposite hydrogels. LDH restacked from the colloidal dispersion by evaporation of water was used for comparison. Samples were treated with an acetate-buffer/NaCl solution to replace lactate anions in LDHs with Cl− anions and to exclude contaminating carbonate anions from LDHs. Adsorption ability for SO42−  was evaluated as the amount of adsorbed anion per unit quantity of LDH by normalizing the amounts of adsorbed anion by the Al content (mol/mol). The adsorption ability of the LDH nanoplates in the hybrid gels was lower than that of the restacked LDH when the LDH contents were low. When LDH/agarose ratio was lower, agarose polymer probably covered the LDH nanoplates more, which may have hindered anion access to the LDH nanoplates. When the LDH content was increased so that the ratio of LDH to agarose approached 1:1 by weight, the adsorption ability of the LDH nanoplates was about the same as or slightly greater than that of the restacked LDH. In addition, when LDH:agarose = 1:1, anion selectivity among SO42− , I− and HPO42−  was about the same between the hybrid gel and the restacked LDH.

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