Magnetic Hydrogel Beads as a Reusable Adsorbent for Highly Efficient and Rapid Removal of Aluminum: Characterization, Response Surface Methodology Optimization, and Evaluation of Isotherms, Kinetics, and Thermodynamic Studies

Biopolymers such as alginate and gelatin have attracted much attention because of their exceptional adsorption properties and biocompatibility. The magnetic hydrogel beads produced and used in this study had a core structure composed of magnetite nanoparticles and gelatin and a shell structure composed of alginate. The combination of the metal-ion binding ability of alginate and the mechanical strength of gelatin in magnetic hydrogel beads presents a new approach for the removal of metal from water sources. The beads were designed for aluminum removal and fully characterized using various methods, including Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy–energy-dispersive X-ray spectroscopy, vibrating sample magnetometry, microcomputed tomography, and dynamic mechanical analysis. Statistical experimental designs were employed to optimize the parameters of the adsorption and recovery processes. Plackett–Burman Design, Box–Behnken Design, and Central Composite Design were used for identifying the significant factors and optimizing the parameters of the adsorption and recovery processes, respectively. The optimum parameters determined for adsorption are as follows: pH: 4, contact time: 30 min, adsorbent amount: 600 mg; recovery time: reagent 1 M HNO3; and contact time: 40 min. The adsorption process was described by using the Langmuir isotherm model. It reveals a homogeneous bead surface and monolayer adsorption with an adsorption capacity of 5.25 mg g–1. Limit of detection and limit of quantification values were calculated as 4.3 and 14 μg L–1, respectively. The adsorption process was described by a pseudo-second-order kinetic model, which assumes that chemisorption is the rate-controlling mechanism. Thermodynamic studies indicate that adsorption is spontaneous and endothermic. The adsorbent was reusable for 10 successive adsorption–desorption cycles with a quantitative adsorption of 98.2% ± 0.3% and a recovery of 99.4% ± 2.6%. The minimum adsorbent dose was determined as 30 g L–1 to achieve quantitative adsorption of aluminum. The effects of the inorganic ions were also investigated. The proposed method was applied to tap water and carboy water samples, and the results indicate that magnetic hydrogel beads can be an effective and reusable bioadsorbent for the detection and removal of aluminum in water samples. The recovery values obtained by using the developed method were quantitative and consistent with the results obtained from the inductively coupled plasma optical emission spectrometer.

The equations of the pseudo-first order, pseudo-second order and intra-particle diffusion models can be shown as follows, respectively: (Equation S1) log (q e -q t ) = logq e -k 1 2.303 t (Equation S2) t q e C (Equation S3) where q e is the amount of the adsorbed aluminum at equilibrium (mg g -1 ), q t is the amount of aluminum adsorbed at time t (mg g -1 ), k 1 is the pseudo first order rate constant (min -1 ), k 2 is the pseudo second order rate constant (g mg -1 min -1 ) and k id is the intraparticle diffusion rate constant (mg g -1 min -1/2 ), t is the time (min) and C (mg g -1 ) represents the boundary layer thickness.
The equations of Freundlich, Langmuir and Dubinin-Radushkevich (D-R) isotherm models can be shown as follows, respectively: S5) In Freundlich isotherm model, q e is the amount of aluminum adsorbed by the adsorbent (mg g - 1 ), C e is the equilibrium concentration of aluminum (mg L -1 ), K F (mg g -1 ) and n (dimensionless) are Freundlich constants related to the adsorption capacity and intensity of adsorption, respectively.Freundlich isotherm was obtained by plotting ln C e versus ln q e .
In Langmuir isotherm model, C e is the equilibrium concentration of aluminum (mg L -1 ), q e is the adsorption capacity at equilibrium (mg g -1 ), Q m is maximum adsorption capacity (mg g -1 ) and K L is the Langmuir adsorption constant (L mg -1 ).Langmuir isotherm was obtained by plotting Ce/qe versus Ce.
For D-R isotherm model, q e is the amount of aluminum adsorbed by the adsorbent (mol g −1 ), q m is the maximum sorption capacity (mol g -1 ), k is the activity coefficient related to sorption energy (mol 2 kJ -2 ), R is the gas constant (J mol -1 K -1 ), T is the temperature (K), ε is the Polanyi potential (J mol -1 ), C e is the equilibrium concentration of aluminum (mol L -1 ) and E is the sorption energy represents the energy required for moving one mole of the solute from infinity to the surface of the adsorbent (kJ mol -1 ).D-R isotherm was obtained by plotting ln q e versus ε 2 .Parameters of the equations were calculated using the slope and intercept of the plots.
Gibbs free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) were calculated from the following equations: , (Equation S7) where K c is the equilibrium constant, C s is the amount of aluminum adsorbed by adsorbent (mg g -1 ), C e is the equilibrium concentration of aluminum (mg L -1 ), R is the gas constant (8.314J mol -1 K -1 ) and T is the temperature (K).
Limit of detection (LOD) and limit of quantification (LOQ) values were calculated using the equations; where σ is the standard deviation of the responses of blank solution and S is the slope of the calibration curve.Table S1.Effect of type and concentration of reagent on recovery of aluminum (concentration of reagent: 1 mol L -1 , volume: 5 mL, recovery time: 60 min, agitation speed: 150 rpm).