Abstract
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.
Highlights
Water remediation for cleaning heavy metal ions (e.g., As, Pb, Cd, etc.) by employing magnetic nanohybrids is a hot and emerging topic that still requires more investigation as suggested in this review that compiled some of the most relevant papers reported in the literature
NPs should be well understood to facilitate the entire removal procedure, i.e., magnetic structures of nanohybrids need to be comprehended because these nanoadsorbents should be magnetically manipulated using an applied field either in bare or with functionalized surface adsorption properties
Two of the most common Fe-oxide NPs suggested to be used in the magnetic remediation process are nanomagnetite and nanomaghemite, but they have different magnetic and catalytic properties and, still need further investigation
Summary
Licensee MDPI, Basel, Switzerland.Attribution (CC BY) license (https://creativecommons.org/licenses/by/ 4.0/).Bulk magnetite (Fe3 O4 ) and maghemite (γ-Fe2 O3 ) are common iron-oxides (Fe-oxides) found in nature as minerals and in the corrosion industry [1,2]. However, with the appearance of nanotechnology and advances in new routes of synthesis (including physical, chemical, and biosynthesis), many researchers have tried to prepare either pure ‘nanomagnetite’ (nano-Fe3 O4 ) or nanomaghemite (nano-γ-Fe2 O3 ) or a mixture of these phases forming the core–shell-like nanosystems [3,4,5,6,7,8,9,10]. Moreover, it should be emphasized that these materials can exist in a ferrofluid form, magnetic nanopowders, solid magnetic matrices, core–shell-like structures, and magnetic colloid solutions. However, in a nanoscale
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