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This review covers the preparation, characterization, and application of magnetic adsorbents obtained from carbon-based sources and their application in the adsorption of both inorganic and organic pollutants from water. Different preparation routes to obtain magnetic adsorbents from activated carbon, biochar, hydrochar, graphene, carbon dots, carbon nanotubes, and carbon nanocages, including the magnetic phase incorporated on the solid surface, are described and discussed. The performance of these adsorbents is analyzed for the removal of fluoride, arsenic, heavy metals, dyes, pesticides, pharmaceuticals, and other emerging and relevant water pollutants. Properties of these adsorbents and the corresponding adsorption mechanisms have been included in this review. Overall, this type of magnetic adsorbents offers an alternative for facing the operational problems associated to adsorption process in water treatment. However, some gaps have been identified in the proper physicochemical characterization of these adsorbents, the development of green and low-cost preparation methods for their industrial production and commercialization, the regeneration and final disposal of spent adsorbents, and their application in the multicomponent adsorption of water pollutants.

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High concentrations of fluoride (F−) in drinking water represent a public health threat, and consequently, effective and sustainable methods are required to improve the water quality, mainly in developing and low-income countries. This study focused on the thermodynamics of fluoride adsorption on bone char regenerated with NaOH for water defluoridation. A detailed analysis of the number of fluoride adsorption/desorption cycles, their impact on the performance and surface chemistry of bone char using different NaOH concentrations, and modeling of the adsorption mechanism using statistical physics theory was carried out. The results showed that 0.075 mol/L NaOH was effective in recuperating the defluoridation properties of bone char with a regeneration efficiency higher than 90% during five adsorption/desorption cycles. Bone char regeneration efficiency decreased up to 64% after ten adsorption/desorption cycles with a maximum fluoride adsorption capacity of 0.18 mmol/g. NaOH restored the bone char surface properties for ligand exchange of the fluoride anions via the hydroxyapatite functionalities contained in this adsorbent. It was calculated that around 0.25–0.46 mmol/g hydroxyapatite ligand exchange sites of regenerated bone char samples could be involved in the fluoride adsorption, which was also expected to be a mono-ligand mechanism. The reduction in defluoridation properties of bone char during the regeneration cycles was attributed to the decrease in the ligand exchange capacity as well as the deactivation and blocking of some functional groups of hydroxyapatite, which limited their participation in consecutive adsorption processes. This study contributes to the optimization of the recycling and reuse of bone char for fluoride removal from water to reduce the operating defluoridation costs, thus enhancing the application of this technology in low-income areas where fluorinated water represents a threat to public health.

A rich variety of ferrihydrite/hematite nanocomposites (NCs) with specific size, composition and properties were obtained in transformation reactions of 2-line ferrihydrite. Transmission electron microscopy observations showed that the NCs consist of clusters of strongly aggregated nanoparticles (NPs) similarly to a "plum pudding", where hematite NPs "raisins" are surrounded by ferrihydrite "pudding". The arsenic adsorption performance of the NCs can be modified for As(III) and As(V) removal, which depends on the NCs composition and the size of hematite NPs. It was found that adequate ferrihydrite/hematite NCs adsorb As(III) faster than ferrihydrite. Otherwise, for As(V), the adsorption exhibited by NCs is slower that for pure ferrihydrite. The data of adsorption isotherms fitted better to the Freundlich model for both As(III) and As(V) adsorption. For all the samples, the model revealed that chemical adsorption is the favorable adsorption process. Additionally, for the ferrihydrite/hematite NCs, the capacity of adsorption for As(III) is higher than for As(V). The high affinity of the NCs for the adsorption of As(III) makes them a good alternative for arsenic removal.