Advances in Functional Magnetic Nanomaterials for Water Pollution Control

The application of magnetism in water treatment processes has enhanced efficiency across various stages, including coagulation, flocculation, sedimentation, and filtration, representing a field with significant potential. This Special Issue of Magnetochemistry presents a collection of cutting-edge research, comprising eight original articles and two comprehensive reviews, focused on the application of functional magnetic nanomaterials in water pollution control. The contributions are broadly categorized into three thematic areas, namely the adsorption of dyes, heavy metals, and other aquatic pollutants, with several studies addressing multiple topics synergistically.

The article “Synthesis of a Novel Magnetic Biochar from Lemon Peels via Impregnation-Pyrolysis for the Removal of Methyl Orange from Wastewater” by Samah Daffalla from King Faisal University (Saudi Arabia) and the University of Khartoum (Sudan), Enshirah Da’na from Al-Balqa Applied University (Jordan), Amel Taha from King Faisal University (Saudi Arabia) and Al-Neelain University (Sudan), and Mohamed R. El-Aassar from Jouf University (Saudi Arabia) reported a novel magnetic biochar (MLPB) synthesized via the impregnation–pyrolysis of agricultural waste (lemon peels) integrated with iron oxides (Fe₃O₄/α-Fe₂O₃) [1]. Based on the natural porous structure of lemon peels, MLPB achieved a specific surface area of 111.45 m³ g⁻¹ and a mesoporous structure with an average pore diameter of 4.65 nm, providing abundant adsorption sites for methyl orange (MO). Thus, at pH 4, MLPB achieved 90.87% MO removal with a maximum capacity of 17.21 mg g⁻¹. The uniform dispersion of magnetic iron oxide nanoparticles enabled facile magnetic separation, overcoming the recovery limitation of conventional biochar. It retained over 88% adsorption capacity after four regeneration cycles, with adsorption kinetics dominated by chemisorption (pseudo-second-order model).

In a paper entitled “Macroscopic and Microscopic Levels of Methylene Blue Adsorption on a Magnetic Bio-Based Adsorbent: In-Depth Study Using Experiments, Advanced Modeling, and Statistical Thermodynamic Analysis” by Mohamed A. Ali, Aliaa M. Badawy, Ali Q. Seliem, Hazem I. Bendary, Eder C. Lima, M. Al-Dossari, N. S. Abd EL-Gawaad, Glaydson S. dos Reis, Mohamed Mobarak, Ali M. Hassan, and Moaaz K. Seliem from Badr University in Cairo (Egypt), Beni-Suef University (Egypt), El-Shorouk Academy (Egypt), the Federal University of Rio Grande do Sul (Brazil), King Khalid University (Saudi Arabia), Åbo Akademi University (Finland), and the Swedish University of Agricultural Sciences (Sweden), a magnetic bio-based adsorbent (ANZ/TC/MNPs) was developed by combining H₂O₂-activated zeolite, turmeric carbohydrate polymer, and magnetite nanoparticles (MNPs) [2]. This “trinity” design yielded exceptional methylene blue (MB) adsorption capacity (307.73 mg g⁻¹ at pH 8.0, 25–55 °C), surpassing most conventional adsorbents. Leveraging its high adsorption capacity and easy recovery via magnetic separability, the material maintained 74% removal efficiency after five cycles.

The paper “A Novel Magnetic Nano-Adsorbent Functionalized with Green Tea Extract and Magnesium Oxide to Remove Methylene Blue from Aqueous Solutions: Synthesis, Characterization, and Adsorption Behavior” by Wenchao Lin, Yaoyao Huang, Shuang Liu, Wei Ding, Hong Li, and Huaili Zheng from Chongqing University (China) and Chongqing Technology and Business University (China) employed a hydrothermal method to synthesize GTE-MgO-Fe₃O₄ NPs, synergistically loading green tea extract (GTE) and magnesium oxide (MgO) onto Fe₃O₄ nanoparticles, which endowed the adsorbent with abundant functional groups and increased surface area [3]. The adsorbent of high stability exhibited outstanding MB removal (>99.8%) across a wide pH range (≥4) and ionic strength, achieving a maximum capacity of 174.93 mg g⁻¹. The adsorption process followed the Freundlich isotherm and pseudo-second-order kinetics, indicating multilayer interactions and electron transfer. The superparamagnetic property enabled rapid solid–liquid separation (<30 s), with the material retaining >76% of its initial oxidative capacity through six consecutive reaction cycles without intermediate regeneration treatments.

The study “Magnetic CuFe₂O₄ Nanoparticles Immobilized on Modified Rice Husk-Derived Zeolite for Chlorogenic Acid Adsorption” utilized agricultural waste (rice husk) to synthesize a hydrothermal zeolite precursor (Z18), which was then modified with 3-aminopropyltriethoxysilane (APTES) and trimethylchlorosilane (TMCS) and immobilized with magnetic copper ferrite (Z18:CuFe₂O₄) [4]. This significantly enhanced affinity for chlorogenic acid (CGA), achieving 89.35% adsorption within 1 h and 95.65% equilibrium removal at 36 h, which doubled the capacity of unmodified material (51.42%). The material maintained >85% mass recovery via magnetic separation due to its integrated ferrite phase, demonstrating exceptional reusability without centrifugation. The study was developed by Tainara Ramos Neves, Letícia Ferreira Lacerda Schildt, Maria Luiza Lopes Sierra e Silva, and Vannyla Viktória Viana Vasconcelos from the Federal University of São Carlos and Embrapa Instrumentação (Brazil), Corrado Di Conzo from Polytechnic of Turin (Italy), Francesco Mura and Marco Rossi from Sapienza University of Rome (Italy), Gaspare Varvaro and Maryam Abdolrahimi from Università degli Studi di Genova (Italy), Simone Quaranta from the Italian National Research Council (Italy), Sandra Aparecida Duarte Ferreira from the Federal University of Espírito Santo (Brazil), and Elaine Cristina Paris from Embrapa Instrumentação (Brazil).

In the article entitled “reparation of the New Magnetic Nanoadsorbent Fe₃O₄@SiO₂-yl-VP and Study on the Adsorption Properties of Hg(II) and Pb(II) in Water” by Dun Chen from Xinjiang Education Institute (Urumqi, China), Jianxin Chen from Xinjiang Hongyuan Construction Group Co., Ltd. (Urumqi, China), and Wanyong Zhou and Amatijan Sawut from Xinjiang University (Urumqi, China), a core–shell superparamagnetic adsorbent, Fe₃O₄@SiO₂-yl-VP, was prepared by grafting 4-vinylpyridine (VP) onto allyl-functionalized silica-coated Fe₃O₄ nanoparticles [5]. The silica interlayer prevented core oxidation, while the pyridine groups provided abundant coordination sites for heavy metal adsorption, achieving maximum Langmuir capacities of 85.06 mg g⁻¹ for Hg(II) at pH 5 and 73.78 mg g⁻¹ for Pb(II) at pH 7. Adsorption equilibrium was reached within 30 min, conforming to both pseudo-first-order (initial diffusion-controlled phase) and pseudo-second-order (coordination-driven chemisorption) kinetic models. The material retained >90% removal efficiency through eleven adsorption–desorption cycles, demonstrating exceptional stability and magnetic recoverability.

Another paper, authored by Sangeetha Jayakumar, Barid Baran Lahiri, and Arup Dasgupta from the Indira Gandhi Centre for Atomic Research (Kalpakkam, India) and the Homi Bhabha National Institute (Mumbai, India), is titled “Preparation, Characterization, and Application of Citrate-Functionalized Cobalt-Doped Iron Oxide Nanoparticles for Rhodamine Dye and Lead Ion Sequestration” [6]. The citrate coating conferred a negative zeta potential while preserving superparamagnetic properties, enabling rapid magnetic separation. Deprotonated carboxylate groups (-COO⁻) facilitated the electrostatic adsorption of cationic pollutants, achieving 93.7% rhodamine B removal and 90% Pb(II) uptake.

The paper “Sustainable Phosphate Remediation via Hierarchical Mg-Fe Layered Double Hydroxides on Magnetic Biochar from Agricultural Waste” by Xiuling Li, Lei Xin, Yuhan Peng, Shihao Zhang, Delong Guan, and Jing Song from Hechi University (China) reported a hierarchical adsorbent, MBC@LDH, engineered by hydrothermally anchoring Mg-Fe layered double hydroxides (LDHs) onto magnetic biochar (MBC) derived from mulberry branches [7]. The composite leveraged synergistic mechanisms, including hydrogen bonding, inner-sphere complexation, and interlayer anion exchange, to achieve 99.22% phosphate removal with a Langmuir monolayer capacity of 7.72 mg g⁻¹ at pH 6 and 25 °C. Its integrated design enabled rapid magnetic separation and sustained 87.83% adsorption efficiency after three regeneration cycles, demonstrating robust reusability for phosphate recovery applications.

In the paper “The Efficient Degradation of Oxytetracycline in Wastewater Using Fe/Mn-Modified Magnetic Oak Biochar: Pathways and Mechanistic Investigation” by Yujie Zhou, Yuzhe Fu, Xiaoxue Niu, Bohan Wu, Xinghan Liu, Fu Hao, and Yuheng Liu from Hebei Medical University (China), Zichuan Ma from Hebei Normal University (China), and Hao Cai from Hebei GEO University (China), an Fe/Mn-modified magnetic oak biochar (FMBC) was prepared via one-pot hydrothermal co-precipitation, synergizing biochar’s porous electron-transfer capability with catalytic Fe₃O₄/MnFe₂O₄ mixed crystals [8]. This design with H₂O₂ achieved 98.3% oxytetracycline (OTC) degradation at pH 4.0, exhibiting a pseudo-first-order rate constant (4.88 min⁻¹), significantly exceeding single-component counterparts. Crucially, built-in phenol–quinone moieties in biochar acted as electron mediators, accelerating Fe³⁺/Fe²⁺ and Mn⁴⁺/Mn³⁺/Mn²⁺ redox cycles to overcome Fe²⁺ regeneration bottlenecks in classical Fenton systems. The composite enabled >98% magnetic recovery within a short time and retained >96% degradation efficiency after five cycles, resolving nanoparticle aggregation and secondary pollution challenges while sustaining catalytic activity.

A review in this Special Issue, entitled “Exploring the Utilization of Magnetic Composite Materials for High-Risk Contaminant Removal from Wastewater by Adsorption and Catalytic Processes” by Oana-Georgiana Dragos-Pinzaru, Nicoleta Lupu, Horia Chiriac, and Gabriela Buema from the National Institute of Research and Development for Technical Physics (Romania), comprehensively evaluated magnetic layered double hydroxide (LDH) materials as adsorbents for wastewater treatment, emphasizing their efficacy in removing heavy metals and dyes through adsorptive processes [9]. These materials demonstrated substantial adsorption capacities for metallic and organic pollutants while also exhibiting significant removal potential for pharmaceuticals, phenolic compounds, phytohormones, and fungicides. The review subsequently explored catalytic water purification methods, examining both monometallic and composite Fe₃O₄-based materials for degrading organic contaminants including dyes, phenols, and pharmaceuticals. Finally, the study assessed multifunctional materials capable of simultaneously removing diverse pollutant types in wastewater systems.

The final contribution to this Special Issue was a review entitled “Research Progress of Magnetic Flocculation in Water Treatment”, which contextualized magnetic flocculation technology as an efficient and environmentally sustainable solution addressing the limitations of conventional flocculants, including high dosage requirements, excessive sludge production, and limited recyclability, in wastewater treatment applications [10]. This technology leveraged magnetic seeds to enhance particle agglomeration and sedimentation, offering distinct advantages over traditional methods through shorter processing times, reduced energy consumption, and lower operational costs. The review systematically examined critical operational parameters governing magnetic flocculation efficiency, specifically forces acting on magnetic seeds, seed particle size distribution, and magnetic seed material selection. It further classified three predominant categories of magnetic flocculants and documented their respective implementation cases across diverse wastewater streams. Notably, the review highlighted the synergistic integration of magnetic flocculation with complementary technologies, including sulfate radical-based advanced oxidation processes, response surface methodology (RSM), and artificial neural network (ANN) optimization, to enhance contaminant removal efficacy in targeted application scenarios. This review was developed by Zhihao Hu, Kun Wu, Zihan Wang, Kinjal J. Shah, and Yongjun Sun from Nanjing Tech University (China).

This Special Issue clearly demonstrates the transformative potential and versatility of functional magnetic nanomaterials in addressing diverse water pollution challenges, providing critical insights into material mechanisms while showcasing significant application efficiencies that establish a robust foundation for future advancements. We extend our sincere gratitude to all contributing authors for their dedicated efforts and outstanding submissions, which offer valuable perspectives on the rapidly evolving landscape of magnetic materials research. We invite scholars across environmental science, materials science, chemical engineering, and related disciplines to engage with these findings published in Magnetochemistry. We would also like to express our profound appreciation to the editorial team for their unwavering support in realizing this collection. Building upon these innovations, continued exploration is encouraged to further expand the impactful application of these nanomaterials in safeguarding global water resources.