In-situ growth of 2D magnesium hydroxide on zirconium-based metal organic frameworks for phosphate removal: An experimental and theoretical exploration of adsorption behavior

Two serious challenges of water eutrophication and phosphorus resources exhaustion make the P recovery imperative. In this work, a novel phosphate adsorbent UiO-66-NH2@Mg(OH)2 (UNM) has been synthesized by the in-situ growth of magnesium hydroxide nanosheets around UiO-66-NH2. The batch adsorption experiments were carried out by controlling the single variate (such as pH, initial concentration, contact time, temperature and interfering anions) to explore the adsorption behaviors. And the adsorption dynamics and isotherms fit well with the pseudo-second-order (PSO) and Langmuir models, combined with the thermodynamics results, indicating a spontaneous and endothermic monolayer chemisorption process. Besides, the maximum phosphate adsorption capacity of UNM was calculated as 408.07 mg/g (≈130.39 mg P/g) at pH = 6.0, T = 298 K. A series of characterizations (XRD, FT-IR, SEM, EDS, BET, TGA, XPS, ICP-MS et. al) and quantum chemical analysis methods (ESP, ALIE and Hirshfeld surface) was applied to predict the reactive sites. The possible mechanism can be deduced as two continuous steps: the physical attraction by weak interactions and the chemical adsorption with an ion-exchange mechanism. The UNM also presented high removal efficiency in the environmental water samples and showed satisfactory reusability and excellent chemical stability in the recycling process. From the desorption effluents, we successfully recycled the struvite with 14.31% P content. Therefore, the UNM will be regarded as a promising phosphate adsorbent in practice and this work will provide inspiration and guidance for the design and preparation of high-efficiency P-removal adsorbents.