Nitrate removal from aqueous solutions by magnetic cationic hydrogel: Effect of electrostatic adsorption and mechanism

Excessive nitrate (NO₃^-) is among the most problematic surface water and groundwater pollutants. In this study, a type of magnetic cationic hydrogel (MCH) is employed for NO₃^- adsorption and well characterized herein. Its adsorption capacity is considerably pH-dependent and achieves the optimal adsorption (maximum NO₃^--adsorption capacity is 95.88 ± 1.24 mg/g) when the pH level is 5.2-8.8. The fitting result using the homogeneous surface diffusion model indicates that the surface/film diffusion controls the adsorption rate, and NO₃^- approaches the center of MCH particles within 30 min. The diffusion coefficient (D_s) and external mass transfer coefficient (k_F) in the liquid phase are 1.15 × 10^-6 cm²/min and 4.5 × 10^-6 cm/min, respectively. The MCH is employed to treat surface water that contains 10 mg/L of NO₃^-, and it is found that the optimal magnetic separation time is 1.6 min. The high-efficiency mass transfer and magnetic separation of MCH during the adsorption-regeneration process favors its application in surface water treatment. Furthermore, the study of the mechanism involved reveals that both -N⁺(CH₃)₃ groups and NO₃^- are convoluted in adsorption via electrostatic interactions. It is further found that ion exchange between NO₃^- and chlorine occurs.

La3+/La(OH)3 loaded magnetic cationic hydrogel composites for phosphate removal: Effect of lanthanum species and mechanistic study

In this study, La³⁺(ion)/La(OH)₃-W/La(OH)₃-EW-loaded magnetic cationic hydrogel (MCH) composites were fabricated in situ and characterized to investigate the effects of lanthanum species on phosphate adsorption. The corresponding maximum P adsorption capacities of MCH-loaded La³⁺(ion) (MCH-La³⁺(ion)), La(OH)₃-W (MCH-La(OH)₃-W), and La(OH)₃-EW (MCH-La(OH)₃-EW) were 70.5 ± 2.67, 69.2 ± 3.5, and 90.2 ± 2.9 mg P/g, respectively. Furthermore, for MCH-La(OH)₃-EW, the P adsorption capacity was maintained relatively stable and high at pH 4.5-11 because of the ligand exchange, electrostatic interactions, and Lewis acid-base interactions. The enhanced adsorption of P was achieved over a wide pH range, as well as in the presence of competing anions (including Cl^-, NO₃^-, SO₄^2-, HCO₃^- and SiO₄^4-). Moreover, the exhausted MCH-La(OH)₃-EW could be easily regenerated by a NaOH-NaCl desorption agent with above 72% adsorption capacity remained during five recycles. The column adsorption capacity of MCH-La(OH)₃-EW reached ∼3500 bed volumes (BV) (∼67.7 mg P/g) as the concentration of P decreased from 5 mg/L to 0.1 mg/L. The ATR-IR, Raman, and XPS deconvolution results revealed that both MCH and lanthanum compounds, including La³⁺(ion), La(OH)₃-W and La(OH)₃-EW, contributed to the phosphate adsorption because of the electrostatic interactions between -N⁺(CH₃)₃ and phosphate, as well as the formation of LaPO₄·xH₂O.

Characterization and adsorption properties of a lanthanum-loaded magnetic cationic hydrogel composite for fluoride removal

In this study, a novel lanthanum-loaded magnetic cationic hydrogel (MCH-La) was synthesized for fluoride adsorption from drinking water. The adsorption kinetics, isotherms, and effects of pH and co-existing anions on fluoride uptake by MCH-La were evaluated. FTIR, Raman and XPS were used to analyze the fluoride adsorption mechanism of MCH-La. Results showed that MCH-La had positive zeta potential values of 23.6-8.0 mV at pH 3.0-11.0, with the magnitude of saturation magnetization up to 10.3 emu/g. The fluoride adsorption kinetics by MCH-La fitted well with the fractal-like-pseudo-second-order model, and the adsorption capacity reached 93% of the ultimate adsorption capacity within the first 10 min. The maximum fluoride adsorption capacity for MCH-La was 136.78 mg F(-)/g at an equilibrium fluoride concentration of 29.3 mg/L and pH 7.0. Equilibrium adsorption data showed that the Sips model was more suitable than the Langmuir and Freundlich models. MCH-La still had more than 100 mg of F(-)/g adsorption capacity at a strongly alkaline solution (pH > 10). The adsorption process was highly pH-dependent, and the optimal adsorption was attained at pH 2.8-4.0, corresponding to ligand exchange, electrostatic interactions, and Lewis acid-base interactions. With the exception of both anions of HCO3(-) and SiO4(4-), Cl(-), NO3(-), and SO4(2-) did not evidently prevent fluoride removal by MCH-La at their real concentrations in natural groundwater. The fluoride adsorption capacity of the regenerated MCH-La approached 70% of the fresh MCH-La from the second to fifth recycles. FTIR and Raman spectra revealed that C-O and CO functional groups on MCH contributed to the fluoride adsorption, this finding was also confirmed by the XPS F 1s spectra. Deconvolution of C 1s spectra before and after fluoride adsorption indicated that the carboxyl, anhydride, and phenol groups of MCH were involved in the fluoride removal.