Differentiating nanomaghemite and nanomagnetite and discussing their importance in arsenic and lead removal from contaminated effluents: A critical review

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Benites, D.
Chelle-Michou, C.
Fontboté, L.
Kouzmanov, K.
Laurent, O.
Torró, L.
Vallance, J.
Valverde, P.E.
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Arsenic and lead heavy metals are polluting agents still present in water bodies, including surface (lake, river) and underground waters; consequently, the development of new adsorbents is necessary to uptake these metals with high efficiency, quick and clean removal procedures. Magnetic nanoparticles, prepared with iron-oxides, are excellent candidates to achieve this goal due to their ecofriendly features, high catalytic response, specific surface area, and pulling magnetic response that favors an easy removal. In particular, nanomagnetite and maghemite are often found as the core and primary materials regarding magnetic nanoadsorbents. However, these phases show interesting distinct physical properties (especially in their surface magnetic properties) but are not often studied regarding correlations between the surface properties and adsorption applications, for instance. Thus, in this review, we summarize the main characteristics of the co-precipitation and thermal decomposition methods used to prepare the nano-iron-oxides, being the co-precipitation method most promising for scaling up processes. We specifically highlight the main differences between both nano-oxide species based on conventional techniques, such as X-ray diffraction, zero and in-field Mössbauer spectroscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism, the latter two techniques performed with synchrotron light. Therefore, we classify the most recent magnetic nanoadsorbents found in the literature for arsenic and lead removal, discussing in detail their advantages and limitations based on various physicochemical parameters, such as temperature, competitive and coexisting ion effects, i.e., considering the simultaneous adsorption removal (heavy metal–heavy metal competition and heavy metal–organic removal), initial concentration, magnetic adsorbent dose, adsorption mechanism based on pH and zeta potential, and real water adsorption experiments. We also discuss the regeneration/recycling properties, after-adsorption physicochemical properties, and the cost evaluation of these magnetic nanoadsorbents, which are important issues, but less discussed in the literature. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
The authors thank the Fondo Nacional de Desarrollo Cient?fico, Tecnol?gico y de Inno-vaci?n Tecnol?gica (PROCIENCIA-CONCYTEC), project number: 177-2020-FONDECYT (PROCIEN-CIA), project CLEAN NANOMAGNETIC. The APC was funded by PROCIENCIA. Acknowledgments: Edson C. Passamani is also thankful to FAPES and CNPq for their financial support in the infrastructure of Ufes?s laboratory under his supervision. We finally thank Jean-Marc Greneche for supporting us with the in-field M?ssbauer measurement of NPEDTA samples.
Palabras clave
Water purification, Arsenic, Contaminated effluents, Lead, Nano-Fe3O4, nano-?-Fe2O3