Highly Efficient Magnetic Carbon Microspheres for Reusable Sulfonamide Removal from Water

Abstract

We report a facile hydrothermal synthesis of magnetic carbon microspheres (MCMs) using glucose and Fe₃O₄ nanoparticles. Calcination at 600 °C for 1 h yields a material with an exceptional BET surface area of 1228 m²/g and a pore volume of 0.448 m³/g. The MCMs adsorb sulfonamide with a maximum capacity of 24.6 mg g⁻¹ at pH 4.0, decreasing to 19.2 mg g⁻¹ at pH 10.0. Adsorption follows a Langmuir isotherm and pseudo‑second‑order kinetics, and the adsorbent retains 18.31 mg g⁻¹ after four regeneration cycles, demonstrating robust reusability.

Background

Antibiotic contamination, especially sulfonamides, poses a significant threat to aquatic ecosystems and human health. Traditional carbon adsorbents are difficult to recover from water, limiting their practical use. Incorporating magnetic nanoparticles into carbon matrices creates magnetic carbon composites that can be effortlessly retrieved with an external magnet. While such materials have been explored for dyes and phenols, their application to sulfonamide removal remains underdeveloped.

Methods

Chemicals and Materials

All reagents were analytical grade (Sinopharm, Nanjing Chemical Reagent Co., Ltd., Xilong Chemical Co., Ltd.). Distilled water was used throughout.

Fe₃O₄ Nanoparticle Synthesis

FeCl₃·6H₂O (1.35 g) and NaAc·3H₂O (3.60 g) were dissolved in 40 mL ethylene glycol, transferred to a 100 mL Teflon‑lined autoclave, and heated at 200 °C for 8 h. The product was washed with water and ethanol.

Preparation of MCMs

0.1 g Fe₃O₄ nanoparticles were dispersed in 60 mL water with glucose, stirred, and hydrothermally treated at 200 °C for 11 h. The resultant microspheres were washed, impregnated with 40 % ZnCl₂, dried, and calcined under N₂ at 600 °C for 1 h.

Characterization

MCMs were examined by TEM, FE‑SEM, VSM, BET analysis, and zeta potential measurement. Equations for adsorption capacity and kinetic models were applied as described in the original study.

Adsorption Experiments

Sulfonamide solutions (50 mL) were mixed with MCMs (1 g L⁻¹) and shaken at 300 K and 120 rpm. After equilibrium, the adsorbent was magnetically separated and the sulfonamide concentration was measured at 258 nm. Regeneration involved NaOH (0.1 M) desorption, ultrasonic agitation, and rinsing to pH 7.

Results and Discussion

Microstructure

TEM images (Fig. 1) confirm ~200 nm Fe₃O₄ cores uniformly coated with a carbon shell, forming spherical MCMs.

Spectroscopic Analysis

FT‑IR shows characteristic Fe–O stretching (~574 cm⁻¹) and –OH broad bands (~3462 cm⁻¹). XRD confirms the cubic magnetite phase.

Surface Area and Porosity

BET analysis (Fig. 3) reveals a surface area of 1228 m²/g and a pore volume of 0.448 m³/g for the 600 °C/1 h calcined sample (MCM‑d). ZnCl₂ impregnation and higher calcination temperatures are critical for achieving this high porosity.

Magnetic Properties

VSM data (Fig. 4) show a saturation magnetization of 42.3 emu g⁻¹, sufficient for efficient magnetic separation while maintaining a superparamagnetic response.

Adsorption Isotherm

Langmuir fitting yields a maximum capacity Q_m = 27.86 mg g⁻¹, indicating monolayer adsorption. Freundlich parameters (K_F = 3.06 L g⁻¹, 1/n = 0.476) confirm favorable adsorption. The Langmuir model provides a higher R², suggesting a homogeneous surface.

Adsorption Kinetics

Pseudo‑second‑order kinetics (R² = 0.999) outperform pseudo‑first‑order, implying chemisorption controls the process.

Effect of pH

Adsorption decreases from 24.22 to 12.48 mg g⁻¹ as pH rises from 4 to 12, due to protonation state changes of sulfonamide and surface charge inversion of the MCMs.

Effect of Temperature and Ionic Strength

Higher temperatures and added KCl reduce adsorption capacity, likely due to competition and thermal desorption.

Reusability

After four regeneration cycles, MCMs retain 73.23 % removal efficiency (18.31 mg g⁻¹) and exhibit no morphological change (Fig. 7b). The adsorbent can be reused at least five times with acceptable capacity loss.

Conclusions

We have developed a magnetic carbon microsphere with an unprecedented surface area (1228 m²/g) and pore volume (0.448 m³/g) that effectively removes sulfonamide from water. The material follows Langmuir adsorption, pseudo‑second‑order kinetics, and can be regenerated with NaOH, retaining high performance over multiple cycles. These findings provide a practical pathway for designing recyclable adsorbents for pharmaceutical contaminants.