Application of a hybrid material formed by layered zinc hydroxide chloride modified with spiropyran in the adsorption of Ca2+ from water

The excess of metal ions in water is associated with drawbacks in industrial processes due to the precipitation of insoluble compounds in pipes and cooling systems. Therefore, investigating alternatives for the removal of ions is especially important and can be achieved with the use of adsorbing materials. In this paper, a novel hybrid material formed by zinc hydroxy chloride (ZHC) functionalized with molecules of the spiropyran class is presented as an alternative for the removal of calcium ions from water. Characterization techniques, including XRD, FTIR, and EDS allowed to identify the spiropyran molecules covering the surface of ZHC in the merocyanine form, forming active sites for calcium chemisorption. The material, described as ZHC/MC24, showed good adsorption capacities that reached 19 mg g−1 at pH 8.0. ZHC/MC24 presented a negatively charged surface, according to zeta potential analyses. Shifting the pH to alkaline values increased the negative potential, which reflected in greater calcium adsorptions. The isotherm investigation showed that the adsorption occurs in a monolayer process, and a first-order adsorption model was found to describe the process. Control tests performed with the ZHC without spiropyran showed low adsorption capacities, inferior to 2 mg g−1, evidencing the importance of the functionalization with spiropyran to the proposed application. [Display omitted] •Zinc hydroxide chloride was modified with spiropyran molecules.•Spiropyran molecules adsorbed to the surface of the ZHC in the merocyanine form.•The hybrid compound adsorbed 10 times more Ca2+ than the unmodified material.•The Ca2+ adsorption kinetics followed a first-order profile.•The Ca2+ adsorption isotherm followed a type-I behaviour.