Efficient phosphate adsorption by a composite composed of Mg6Al2(CO3)(OH)16·4H2O LDH and Chitosan: kinetic, thermodynamic, desorption, and characterization studies

Abstract

A composite composed of Mg6Al2(CO3)(OH)16·4H2O Layered Double Hydroxide (Mg-Al-CO3 LDH) and chitosan was synthesized and used as an adsorbent for phosphate removal from aqueous solutions. The composite adsorption capacity (106.3 mg(PO4)/g) was about twice higher than its constituents (chitosan 45.2 mg(PO4)/g and Mg-Al-CO3 LDH 54.9 mg(PO4)/g). Kinetic, equilibrium, thermodynamic, and desorption studies were performed, and the adsorbent was characterized before and after adsorption experiments by scanning electron microscopy and energy dispersive spectrometry, X-ray diffraction, Fourier transform infrared, and Brunauer-Emmett-Teller analyses. The results showed that the composite has a similar structure to Mg-Al-CO3 LDH, with an initial surface area of 28.47 m2/g, and satisfactory phosphate adsorption. The phosphate removal by the composite was favored at slightly acid conditions (pHZCP = 6.5 ± 0.3) at room temperature and was better fitted by the pseudo-second-order kinetic model (R2 = 0.98) and Freundlich isotherm model (R2 = 0.98). This indicates a favorable process occurring with a multilayer covering a heterogeneous surface and an adsorption rate limited by forces acting by exchanging or sharing electrons. In addition, thermodynamic data showed a spontaneous and exothermic (ΔH = -5.77 kJ/mol) process, with positive entropy (ΔS = 12.77 kJ/mol·K). After the adsorption experiments, the surface area of the adsorbent was 81.70 m2/g, which together with other results indicates that the high phosphate adsorption capacity presented by the composite may have been influenced by the partial dissolution of the adsorbent followed by its reprecipitation. The loaded composite was feasible (65.9 %) to be desorbed by a sodium hydroxide solution (NaOH, pH 12 ± 0.2) after 30 min of the experiment. The results emphasized that the Mg-Al-CO3 LDH/Chitosan composite was efficient in phosphate removal and could be successfully used for this purpose.