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Liosis, C. Heavy Metal Adsorption Using Magnetic Nanoparticles. Encyclopedia. Available online: https://encyclopedia.pub/entry/17354 (accessed on 27 July 2024).
Liosis C. Heavy Metal Adsorption Using Magnetic Nanoparticles. Encyclopedia. Available at: https://encyclopedia.pub/entry/17354. Accessed July 27, 2024.
Liosis, Christos. "Heavy Metal Adsorption Using Magnetic Nanoparticles" Encyclopedia, https://encyclopedia.pub/entry/17354 (accessed July 27, 2024).
Liosis, C. (2021, December 20). Heavy Metal Adsorption Using Magnetic Nanoparticles. In Encyclopedia. https://encyclopedia.pub/entry/17354
Liosis, Christos. "Heavy Metal Adsorption Using Magnetic Nanoparticles." Encyclopedia. Web. 20 December, 2021.
Heavy Metal Adsorption Using Magnetic Nanoparticles
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This entry is adapted from the peer-reviewed paper 10.3390/ma14247500

Research on contamination of groundwater and drinking water is of major importance. Due to the rapid and significant progress in the last decade in nanotechnology and its potential applications to water purification, such as adsorption of heavy metal ion from contaminated water, a wide number of articles have been published. An evaluating frame of the main findings of recent research on heavy metal removal using magnetic nanoparticles, with emphasis on water quality and method applicability, is presented. A large number of articles have been studied with a focus on the synthesis and characterization procedures for bare and modified magnetic nanoparticles as well as on their adsorption capacity and the corresponding desorption process of the methods are presented.

adsorption contamination magnetic nanoparticles heavy metals

1. Introduction

Nowadays, water issues, such as exhaustion of resources and quality of drinking water, have attracted the interest not only of researchers but also of national and international organizations and governments [1]. Terms such as water stress and water scarcity are now subjects of research on a daily basis. The first term refers to situations where the quantity of available water is not sufficient for agricultural, industrial, or domestic uses. It takes into account several physical aspects related to water resources, such as water quality, environmental flows, and water accessibility [2][3][4]. On the other hand, the second term refers to the volumetric abundance of water supply [5][6][7], which is the ratio of human water consumption to available water supply for a specific area [8][9]. The main factors that cause water crisis in the long run are population growth, expansion of industrial activities, urbanization, climate change, depletion of aquifers, and water contamination [10][11][12]. It is obvious that the coverage of global demands for safe drinking water in the near future is utopian, if we consider that water quantity is nearly constant due to the hydrologic cycle in various forms, such as seawater, groundwater, surface water, and rainwater [13][14]. However, climate change will affect the quality and quantity of potentially available drinking water due to increased flooding, more severe droughts, and enhanced toxicity of chemical contaminants in the environment [15][16]. Of more immediate concern is the efficiency of existing water treatment methods due to increasing pollution resulting from the anthropogenic activities [17]. Thus, purification of water from polluted sources is essential to enable the utilization of sustainable global water [18][19].
Water pollutants are categorized into point source, where the pollution originates from a single and identifiable source, and nonpoint source, where pollutants originate from a variety of sources [20]. Based on their main physicochemical characteristics, they can also be classified into radioactive, thermal, microorganism, nutrient, suspended solid and sediment, and organic and inorganic pollutants [21]. Inorganic pollutants consist of heavy metals, fertilizers, sulphides, ammonia, oxides of nitrogen, acids, and bases [22][23][24]. Since water quality is improving with increasing advances in technologies [17] and water purification is highly required to prevent toxic effects and disruption of ecological balance, this particular research review focuses on water contamination by heavy metals and ways of heavy metal removal.
Pollutants can contaminate water through both natural processes and anthropogenic activities [25][26]. Their concentrations in water depend on the local geological, hydrogeological, and geochemical characteristics of the aquifer. A significant burden on water quality and, consequently, public health is the increasing concentrations of heavy metals due to their toxicity, persistence, and bioaccumulative nature [25][27]. Unlike organic contaminants, heavy metals, which are toxic and mostly carcinogenetic, are not biodegradable and tend to accumulate in organisms [28]. Hence, exposure to toxic heavy metal through drinking water has long been a critical public health concern across the world [29]. Elements whose density exceeds 5 g/cm3 fall into the category of heavy metals [30] and are listed in Table 1 based on their toxicity, which is related to the maximum contaminant level (MCL) [31]. Specific differences in metal ion toxicities arise from differences in solubility, absorbability, transport, chemical reactivity, and the complexes that are formed within the body [32].
Table 1. Metal toxicity [31][33].
Heavy Metals MCL (mg/L) × 102
Zinc (Zn) 80
Nickel (Ni) 20
Copper (Cu) 25
Chromium (Cr) 5
Arsenic (As) 5
Cadmium (Cd) 1
Lead (Pb) 0.06
Mercury (Hg) 0.003
Heavy metals are classified into essential (Zn, Cu, Fe, and Co) and nonessential (Cd, Hg, As, and Cr) based on their toxicity. At low concentrations, essential heavy metals are harmless, unlike nonessential metals, which are highly toxic [34][35]. It is worth noting that for nonessential heavy metals, toxicity is very high, even at low concentrations [36][37]. Water contamination by heavy metals occurs in anthropogenic activities or in natural processes. Sources of contamination include metal corrosion, atmospheric deposition, soil erosion of metal ions, leaching of heavy metals, sediment resuspension, and metal evaporation from water resources to soil and groundwater. Phenomena such as weathering and volcanic eruptions fall into the category natural contamination [38]. In contrast, the major sources resulting from anthropogenic activities are mining wastes, landfill leaches, municipal wastewater, urban runoff, industrial wastewater, electroplating, and electronic and metal finishing industries [39][40]. Moreover, due to an increasing production of metals from technological activities, the problem of waste disposal has become of paramount importance; hence many aquatic environments face metal concentrations that do not meet water quality criteria designed to protect the environment, animals, and humans [41]. Metals such as Cu, Cd, Cr, Hg, and Ni have the ability to produce reactive radicals, resulting in DNA damage, lipid peroxidation, depletion of protein sulfhydryls, and other effects [42]. In this context, the availability and release of pollutants from anthropogenic sources needs to be considered [43]. The source of pollution and the effect of each heavy metal on human health are presented in Table 2.
Table 2. Heavy metal characteristics [16][33][44][45][46].
Heavy Metal Human Health Impacts Common Sources
Arsenic Skin damage, circulatory system issues, protein coagulation, nerve inflammation, muscle weakness, carcinogenicity Naturally occurring, electronic production, agricultural applications, nonferrous smelters, metallurgy, coal-fired and geothermal electrical generation, tanning, pigments, antifouling paints, light filters, fireworks, veterinary medicine
Cadmium Kidney damage, carcinogenicity, DNA damage, gastrointestinal irritation,
hyperactivity, renal failure		 Naturally occurring, various chemical industries, agricultural applications (phosphatic fertilizers), pigments, anticorrosive metal coatings, plastic stabilizers, alloys, coal combustion
Chromium Allergic dermatitis, diarrhoea, nausea, vomiting, headache, neurotoxicity, kidney and liver damage Naturally occurring, steel manufacturing metallurgy, refractory, chemical industries, plating, pigments, textile and leather tanning, passivation of corrosion of cooling circuits, wood treatment
Copper Gastrointestinal issues, liver and kidney damage, anorexia, Wilson’s disease Household plumbing systems, naturally occurring, chemical and pharmaceutical equipment, pigments, alloys
Lead Kidney damage, reduced neural
development, carcinogenicity,
high blood pressure		 Lead-based products (batteries), household plumbing systems, antiknock agents, pigments, glassware, ceramics, plastic, alloys, sheets, cable sheathings, solder
Mercury Kidney damage, nervous system damage, carcinogenicity, gingivitis, stomatitis, gastrointestinal issues, abortions Fossil fuel combustion, electronic industries, fluorescent light bulbs, electrical and measuring apparatus, catalysts, pharmaceuticals, dental fillings, scientific instruments, rectifiers, oscillators, solders
Nickel Allergic dermatitis, nausea, chronic asthma, coughing, carcinogenicity, hair loss Paper products, fertilizer plating, electroplating, batteries, arc welding, rods, pigments for paints and ceramics, surgical and dental prostheses, moulds for ceramic and glass containers, computer components, catalysts
Zinc Depression, lethargy, neurological signs, increased thirst, hyperactivity, physical dysfunction Mining, coal, waste combustion, steel processing, agricultural applications (phosphatic fertilizers), anticorrosion coating, batteries, cans, PVC stabilizers, medicines and chemicals, rubber industry, paints, soldering and welding fluxes
However, the removal of heavy metal ions from water has been a vital and challenging issue for several decades [47][48], without reaching the heart of the problem. On the other hand, in recent years nanotechnology has been integrated with several novel techniques for the removal of heavy metals from water systems to improve removal efficiency [49][50][51].
Several water purification methods for heavy metals exist, such as chemical precipitation and coagulation, flocculation, electrochemical methods, photocatalytic degradation, membrane filtration, ion exchange, bioremediation, and adsorption, just to mention few [16][52][53]. The present work focuses on the adsorption methods developed during the last decade, where nanotechnology contributed quite a lot to great progress. In this context, adsorption capacity and reuse efficiency of magnetic nanoparticles for capturing heavy metals in water environments are investigated. In addition, the findings are analysed based on certain characteristics, such as viability method, time of purification, water quality after purification (adsorption capacity), and nanoparticle reusability.
The main interest in the adsorption method is the interaction between nanoparticles and adsorbents, which depends on their physicochemical properties [54]. The term physicochemical covers the particle size, surface area, surface charge, agglomeration, morphology, surface coating, and so forth. Particles with sizes below 100 nm are defined as nanoparticles [55], and their applications vary according to their size [54][56][57]. Magnetic, mechanical, optical, and electrical properties affect the formation and aggregation of nanoparticles [44]. Aggregation of nanoparticles is affected by surface charge, particle size, and composition. Nanoparticles without magnetic properties have limited applicability in water purification due to the difficulty of separation from the aqueous solution [58]. Magnetic nanoparticles present advantages due to their large surface area, size, and shape-dependent catalytic properties and can be separated from the aqueous solution with the use of a magnet field [48][59]; thus many methods have been investigated for their potential application in both environmental and biological fields [60][61][62].
Our focus was on studies that have been published during the last decade. The selection of relevant research articles was performed in several stages. In the first stage, the search was based on article title, abstract, and keywords. The terms for the search were (a) water purification, (b) magnetic nanoparticles, (c) heavy metals, and (d) reusability. In the second stage, we selected articles according to purification method (adsorption process). The final stage included the expansion of the bibliography using the remaining articles’ reference lists.

2. Main Findings during the Last Decade

Restrictions on the use of bare hematite, magnetite, and maghemite nanoparticles force the majority of researchers to synthesize modified nanoparticles from iron oxide. A large number of parameters apart from those of the magnetic nanoparticle synthesis, which have been analysed above, affect the adsorption efficiency of heavy metals, among them being pH, contact time, temperature, adsorbent dose, and initial ion concentration [58][63]. pH is a factor that is involved not only at this stage of the method but also during the synthesis of nanoparticles; almost in all cases, the pH values are different between these two stages. pH is directly related to the competition ability of hydrogen and metal ions to the adsorbent surface active sites, for by increasing the pH value due to the formation of soluble hydroxylated metal complexes, the metal uptake capacity decreases [64], the maximum adsorption capacity that is observed varies from pH 2 to 9, and most researchers achieve optimum adsorption capacity at pH 5–7. The adsorbent dosage is directly related to the adsorbent capacity since it determines the contact areas between the adsorbent and the adsorbate [65]. Moreover, when the adsorbent dose is increasing, the number of available binding sites is also increasing; but as the equilibrium point of adsorption is reached, the efficiency does not reflect the available sites and remains steady [64]. A critical point also exists for the contact time parameter, since initially, removal rates increase rapidly with time; then they gradually decrease due to the availability of the binding sites until the equilibrium is reached [64]. Experiments show that even for the same adsorbent but for different pollutants, the ideal contact time may vary significantly, as in some cases it is between 30 min to 12 h [66]. The factor that has been investigated less is the effect of coexisting cations/anions, as in previous parameters, the initial ion concentration does not differ; thus removal rates increase with increasing initial concentrations until a point where the rates remain unaffected [64]. Research shows that these cations/anions that are contained in water have no significant influence on adsorption capacity [67][68], since the adsorbent surfaces suggest multisurface adsorption active sites. Moreover, differences in the radius of heavy metal ions have significant influence on adsorption efficiency [69], ions with a smaller radius have higher mobility in aqueous solutions, and therefore, they have a lower tendency to adsorb on magnetic nanoparticles. Additionally, the temperature that is used during the adsorption process varies between 15 and 45 °C, although in some rare occasions, it reaches 70 °C.
Every research method must be evaluated according to the criteria that have been set from the beginning. At this phase of the literature review, the results of heavy metal adsorption in aquatic solutions using bare or modified magnetic iron oxide nanoparticles are summarized. In order to be able to evaluate the methods through the experimental results since 2010, many factors must be taken into consideration. Initially, many researchers used different nanoparticle compounds for various pollutants, but also observed a large selectivity of the initial parameters (i.e., adsorbent dosage, contact time, pH, temperature), which creates difficulties in the categorization of the experimental results. Despite that, the main purpose of every work is the removal of heavy metals; thus adsorption capacity is the main factor that has been focused on in each research. Hence, Table 3 provides useful information, such as the time of equilibrium/contact time, pH, adsorption capacity or removal efficiency, and temperature during the adsorption process, which could lead to safe extractions for the applicability of the method, which is based on the findings during the last decade. Additionally, the findings are listed in chronological order. Due to several nanoparticle compounds, the equilibrium time is not constant but has large dispersion.
Table 3. Adsorption capacity, removal efficiency, and system conditions.
Year of Publish Magnetic
Nanoparticle		 Heavy
Metal Ion		 Adsorption
Capacity (mg/g) 1		 Removal Efficiency (%) Time (min) pH Tempe-rature (°C) Number of
Cycles		 Ref.
2010 Amino-functionalized Fe3O4 Cu(II) 25.77   5 6 25 15 [70]
Fe3O4 As(V) 44.1     3 25   [68]
Fe3O4 As(III) 49.8     7 25   [68]
MnFe2O4 As(V) 90.4     3 25   [68]
MnFe2O4 As(III) 93.8     7 25   [68]
CoFe2O4 As(V) 73.8     3 25   [68]
CoFe2O4 As(III) 100.3     7 25   [68]
Amino-functionalized Fe3O4@SiO2 Cu(II) 0.69 mmol/g       45 4 [71]
Amino-functionalized Fe3O4@SiO2 Cu(II) 0.60 mmol/g       35 4 [71]
Amino-functionalized Fe3O4@SiO2 Cu(II) 0.47 mmol/g       25 4 [71]
Amino-functionalized Fe3O4@SiO2 Pb(II) 0.54 mmol/g       45 4 [71]
Amino-functionalized Fe3O4@SiO2 Pb(II) 0.45 mmol/g       35 4 [71]
2010 Amino-functionalized Fe3O4@SiO2 Pb(II) 0.37 mmol/g       25 4 [71]
Amino-functionalized Fe3O4@SiO2 Cd(II) 0.33 mmol/g       45 4 [71]
Amino-functionalized Fe3O4@SiO2 Cd(II) 0.27 mmol/g       35 4 [71]
Amino-functionalized Fe3O4@SiO2 Cd(II) 0.20 mmol/g       25 4 [71]
GA–APTES-NPs Cu(II) 61.07 75.3 15 4–5.3 20 ± 0.1 3 [72]
Fe3O4 Pb(II) 36   30 5.5   5 [67]
EDA-MPs-10 Cr(VI) 61.35   60 2.5 35   [73]
EDA-MPs-8 Cr(VI) 60.98   60 2.5 35   [73]
EDA-MPs-6 Cr(VI) 49.5   60 2.5 35   [73]
EDA-MPs-4 Cr(VI) 36.63   60 2.5 35   [73]
EDA-MPs-2 Cr(VI) 32.15   60 2.5 35   [73]
Fe3O4-γ-Fe2O3 As(III) 3.69 96   2     [74]
Fe3O4-γ-Fe2O3 As(V) 3.71 96   2     [74]
Fe3O4-γ-Fe2O3 Cr(VI) 2.4 99   2     [74]
Fe3O4 Pb(II) 63.33     6     [75]
Fe3O4 Ni(II) 52.55     6     [75]
Fe3O4@SiO2 Pb(II)   97.34   6   5 [76]
Nanoiron Ni(II) 11.53     5 25   [77]
EDA-NMPs Cr(VI) 136.98   30 2.5 35   [78]
DEDA-NMPs Cr(VI) 149.25   30 2.5 35   [78]
TETA-NMPs Cr(VI) 204.08   30 2.5 35   [78]
TEPA-NMPs Cr(VI) 370.37   30 2 35   [78]
Magnetite NPs Cr(VI)   82 20 2     [79]
Fe3O4-γFe2O3 As(ΙΙΙ) 4.75 91 180 6.5     [80]
Fe3O4-γFe2O3 As(V) 4.85 92 180 6.5     [80]
2011 γ-Fe2O3 Hg(ΙΙ) 140     8     [66]
Iron oxide-coated perlite (IOCP) As(V) 0.39   5 6.5–7     [81]
γ-Fe2O3 onto ball-milled expanded perlite carrier As(V) 4.64     7     [82]
NiFe2O4 Cu(II) 55.83           [83]
NiFe2O4 Cr(VI) 36.95           [83]
NiFe2O4 Ni(II) 37.02           [83]
EDTAD-treated Fe3O4 Pb(II) 99.26     5.5 30   [84]
EDTAD-treated Fe3O4 Cd(II) 48.70     6 30   [84]
Fe3O4@SiO2-MIIP Cu(II) 24.2         5 [85]
Fe3O4@SiO2-NIP Cu(II) 5.2           [85]
Fe3O4–TW Ni(II) 38.3         5 [86]
γ-Fe2O3@Fe3O4 Cr(VI) 74.07   30   35   [87]
γ-Fe2O3@Fe3O4 Cr(VI) 78.13   30   25   [87]
γ-Fe2O3@Fe3O4 Cr(VI) 83.33   30   15   [87]
Polyrhodanine-coated
γ-Fe2O3		 Hg(II) 179         5 [66]
Polypyrrole/F3O4 Cr(VI) 169.49   30–180 2 25 3 [88]
Polypyrrole/F3O4 Cr(VI) 204.08   30–180 2 35   [88]
2011 Polypyrrole/F3O4 Cr(VI) 238.09   30–180 2 45   [88]
Iron oxide-modified sewage sludge Pb(II) 42.4     6 25 ± 0.1   [89]
Iron oxide-modified sewage sludge Cu(II) 17.3     6 25 ± 0.1   [89]
Iron oxide-modified sewage sludge Cd(II) 14.7     7 25 ± 0.1   [89]
Iron oxide-modified sewage sludge Ni(II) 7.8     7 25 ± 0.1   [89]
SH-mSi@Fe3O4 Hg(II) 260     6.5 25 6 [90]
SH-mSi@Fe3O4 Pb(II) 91.5     6.5 25 6 [90]
Fe3O4@SiO2 Hg(II)   98         [91]
MWCNT/IO/CD Cu(II)   59   5.5 25.15   [92]
CS-co-MMB-co-PAA
hydrogel		 Pb(II) 163.90     5.5 25   [93]
CS-co-MMB-co-PAA
hydrogel		 Cd(II) 135.51     5.5 25   [93]
CS-co-MMB-co-PAA
hydrogel		 Cu(II) 152.42     5.5 25   [93]
MWCNT/nano-iron oxide Cr(III)   82   5     [94]
MWCNT/nano-iron oxide Cr(III)   88   6     [94]
Nano-Fe3O4 Cu(II) 8.90     5 25   [95]
PEI-grafted magnetic porous Cu(II) 157.8   10 6–7.5   4 [96]
PEI-grafted magnetic porous Zn(II) 138.8   10 6–7.5   4 [96]
PEI-grafted magnetic porous Cd(II) 105.2   10 6–7.5   4 [96]
Fe3O4-coated boron nitride nanotubes As(V) 0.96   720 9 25 4 [97]
2012 Fe3O4-C Pb(II) 126     6     [98]
Pectin-coated iron
oxide		 Cu(II) 48.9           [99]
Fe3O4@ZrO2 Cr(III) 24.5     8–9     [100]
MNPs–Ca-alginate
immobilized
P. chrysosporium		 Pb(II) 176.33       35 5 [101]
AF-Fe3O4 Cu(II) 523.6   120 7     [102]
AF-Fe3O4 Cd(II) 446.4   120 7     [102]
AF-Fe3O4 Pb(II) 369.0   120 7     [102]
Fe3O4@APS@AA-co-CA Cd(II) 29.6   45 5.5 25 4 [103]
Fe3O4@APS@AA-co-CA Zn(II) 43.4   45 5.5 25 4 [103]
Fe3O4@APS@AA-co-CA Pb(II) 166.1   45 5.5 25 4 [103]
Fe3O4@APS@AA-co-CA Cu(II) 126.9   45 5.5 25 4 [103]
Fe3O4–SiO2-poly As(III) 84±5   120 6 30   [104]
Fe3O4–SiO2-poly Cu(II) 65±3   120 6 30   [104]
Fe3O4–SiO2-poly Cr(III) 77±3   120 5.3 30   [104]
Acid-coated Fe3O4 As(V) 16.56           [105]
Acid-coated Fe3O4 As(III) 46.06           [105]
γ-Fe2O3 functionalized with citrate ions Ni(II) 0.57 mmol/g   15 6 ± 0.5     [106]
Fe3O4@CTAB As(V) 23.07   2 6   5 [107]
Fe3O4-γ-Fe2O3 Cr(VI) 6     4 10   [107]
2012 Fe3O4-γ-Fe2O3 Cr(VI) 6.9     4 22   [107]
Fe3O4-γ-Fe2O3 Cr(VI) 7     4 50   [107]
Fe3O4–PEI800–MMT Cr(VI) 8.77           [108]
Fe3O4–PEI25000–MMT Cr(VI) 7.69           [108]
MPTS-CNTs/Fe3O4 Hg(II) 65.52     6.5 25 ± 0.2   [109]
MPTS-CNTs/Fe3O4 Pb(II) 65.40     6.5 25 ± 0.2   [109]
rGO–Fe(0)–Fe3O4 As(III) 44   60 7 25   [110]
rGO–Fe3O4 As(III) 21   60 7 25   [110]
Nanomagnetite (NMT) Cu(II) 14.3   70   45   [111]
Nanomagnetite (NMT) As(V) 6.5   120   45   [111]
Water-soluble Fe3O4
nanoparticles		 Pb(II) 96.8   60 7 18   [112]
Water-soluble Fe3O4
nanoparticles		 Cr(VI) 41.5   90 7 18   [112]
TF-SCMNPs Hg(II) 207 93.76 15 6 22.5   [113]
M-MIONPs Hg(II)   98.6 4 9 25   [114]
Fe3O4-RGO–MnO2 As(III) 14.04     7 25   [115]
Fe3O4-RGO–MnO2 As(V) 12.22     7 25   [115]
MWCNTs/Fe3O4 Pb(II) 41.77     5.3   5 [116]
MWCNTs/Fe3O4-NH2 Pb(II) 75.02     5.3   5 [116]
γ-Fe2O3 Cu(II) 71.42     6 ± 0.1 25 ± 1   [117]
γ-Fe2O3 Zn(II) 111.11     6 ± 0.1 25 ± 1   [117]
γ-Fe2O3 Pb(II) 84.95     6 ± 0.1 25 ± 1   [117]
Fe3O4/CS/PAA Cu(II) 193           [118]
2013 Fe3O4/SiO2 Pb(II) 14.65     4 27   [119]
Fe3O4/SiO2 Pb(II) 16.83     4 50   [119]
Fe3O4/SiO2 Pb(II) 17.65     4 70   [119]
Fe2O3–Al2O3 Cu(II) 4.98   60 6   4 [120]
Fe2O3–Al2O3 Pb(II) 23.75   60 6   4 [120]
Fe2O3–Al2O3 Ni(II) 32.36   60 6   4 [120]
Fe2O3–Al2O3 Hg(II) 63.69   60 6   4 [120]
Mixed magnetite–
hematite		 Pb(II) 617.3   60 7 25   [121]
Mixed magnetite–
hematite		 Cr(III) 277.0   120 7 25   [121]
Mixed magnetite–
hematite		 Cd(II) 223.7   1440 7 25   [121]
Chitosan-coated MnFe2O4 (CCMNPs) Cu(II) 22.6     6   5 [122]
Chitosan-coated MnFe2O4 (CCMNPs) Cr(VI) 15.4     6   5 [122]
γ-PGA/Fe3O4 MNPs Cr(III) 162.6 99.66 120 6 30   [123]
CDpoly-MNPs Pb(II) 64.5   45 5.5–6 25 4 [124]
CDpoly-MNPs Cd(II) 27.7   45 5.5–6 25   [124]
CDpoly-MNPs Ni(II) 13.2   45 5.5–6 25   [124]
3D flowerlike a-Fe2O3 As(V) 41.46   120       [125]
3D flowerlike a-Fe2O3 Cr(VI) 33.82   120       [125]
Magnetite nanorods Pb(II) 112.86   60 5–6 25   [126]
Magnetite nanorods Zn(II) 107.27   60 5–6 25   [126]
2013 Magnetite nanorods Ni(II) 95.42   60 5–6 25   [126]
Magnetite nanorods Cd(II) 88.39   60 5–6 25   [126]
Magnetite nanorods Cu(II) 76.10   60 5–6 25   [126]
Maghemite nanotubes Pb(II) 71.42           [126]
Maghemite nanotubes Zn(II) 86.95           [126]
Maghemite nanotubes Cu(II) 111.11           [126]
EDTA-modified chitosan/SiO2/Fe3O4 Cu(II) 0.495 mmol/g   720 5 25 12 [127]
EDTA-modified chitosan/SiO2/Fe3O4 Pb(II) 0.045 mmol/g   720 5 25 12 [127]
EDTA-modified chitosan/SiO2/Fe3O4 Cd(II) 0.040 mmol/g   720 5 25 12 [127]
Fe3O4@mesoporous SiO2 core-shell Pb(II) 128.21         6 [128]
Fe3O4@mesoporous SiO2 core-shell Cu(II) 51.81         6 [128]
Fe3O4/GO Cr(VI) 32.33     4.5 20   [129]
Hollow nestlike
α-Fe2O3 spheres		 As(V) 75.3 88 120       [130]
Hollow nestlike
α-Fe2O3 spheres		 Cr(VI) 58.6 67 120       [130]
α-Fe2O3 nanofibers Cr(VI) 16.17       25 4 [131]
Fe3O4@SiO2–NH2 Pb(II) 243.9   180 5.2 25 5 [132]
Cyanex-301-coated SPION Cr(VI) 30.8     2 23   [133]
2014 S-doped Fe3O4@C Cu(II) 54.7     5 35 4 [134]
Fe3O4@SiO2-QTPA Cd(II)   95.29 720   25   [135]
Fe3O4@SiO2-QTPA Zn(II)   92.37 720   25   [135]
Fe3O4@SiO2-QTPA Cu(II)   91.06 720   25   [135]
Glycine-functionalized
maghemite nanoparticles		 Cu(II) 625     6.5 25   [136]
Maghemite (γ-Fe2O3) Pb(II) 10.55     7.5 45   [137]
Maghemite (γ-Fe2O3) Zn(II) 4.79     7.5 45   [137]
Maghemite (γ-Fe2O3) Cd(II) 1.75     7.5 45   [137]
α-Fe2O3 Cr(VI) 17   300       [138]
3-MPA-coated SPION Cr(VI) 45     1 25   [139]
EDA-Fe3O4 NPs Cr(VI)   98 120 2   6 [140]
M-FeHT As(V) 1.2813   15 9 25   [141]
M-FeHT As(III) 0.1213   15 9 25   [141]
Fe3O4-GS Cr(VI) 17.29   240 1–3.5   5 [142]
Fe3O4-GS Pb(II) 27.95   120 6–7   5 [142]
Fe3O4-GS Hg(II) 23.03   120 6–7     [142]
Fe3O4-GS Cd(II) 27.83   120 6–7     [142]
Fe3O4-GS Ni(II) 22.07   120 6–7     [142]
Fe3O4@CPS Cu(II) 53.6     5 25 3 [143]
Fe3O4@CPS Cd(II) 87.1     6 25 3 [143]
Fe3O4@CPS Pb(II) 25.2     6 25 3 [143]
MWCNT
CoFe2O4–NH2		 Pb(II) 140.1     6   5 [144]
Fe3O4/talc Cu(II)   72.15         [145]
Fe3O4/talc Ni(II)   50.23         [145]
2014 Fe3O4/talc Pb(II)   91.35         [145]
Magnetite nanoparticles Cr(VI) 121.9   60 5.5     [146]
CoFe2O4-rGO Pb(II) 299.4   80 5.3 25   [147]
CoFe2O4-rGO Pb(II) 274.7       35   [147]
CoFe2O4-rGO Pb(II) 253.2       45   [147]
CoFe2O4-rGO Hg(II) 157.9   60 4.6 25   [147]
CoFe2O4-rGO Hg(II) 105.1       35   [147]
CoFe2O4-rGO Hg(II) 90.49       45   [147]
Graphene Oxide−MnFe2O4 Pb(II) 673     5     [148]
MnFe2O4 Pb(II) 488     5 60   [148]
Graphene Oxide−MnFe2O4 As(III) 146     6.5 60   [148]
MnFe2O4 As(III) 97     6.5 60   [148]
Graphene Oxide−MnFe2O4 As(V) 207     4 60   [148]
MnFe2O4 As(V) 136     4 60   [148]
Water-soluble Fe3O4 Hg(II)   >99 10 7 25 3 [149]
Fe3O4@C Pb(II) 90.7 96.3 10       [150]
Fe3O4@C Hg(II) 83.1 98.1 10       [150]
Fe3O4@C Cd(II) 39.7 93.8 10       [150]
Fe3O4@silica xanthan gum Pb(II) 21.32     6 20 22 [151]
2015 Fe3O4-NTA Cu(II) 40.24   35 5     [152]
Magnetic chitosan/cellulose hybrid microspheres by
embedding γ-Fe2O3		 Cu(II) 88.21       30   [153]
Magnetic chitosan/cellulose hybrid microspheres by
embedding γ-Fe2O3		 Cd(II) 61.1       30   [153]
Magnetic chitosan/cellulose hybrid microspheres by
embedding γ-Fe2O3		 Pb(II) 45.86       30   [153]
Mesoporous CoFe2O4 Pb(II) 32.11   480 5     [154]
MAMNPs Cd(II) 91.5   100 6   3 [155]
MAMNPs Hg(II) 237.6   100 6   3 [155]
MAMNPs Pb(II) 118.5   100 6   3 [155]
Fe3O4/MMT NC Pb(II) 263.15   2       [69]
Fe3O4/MMT NC Cu(II) 70.92   2       [69]
Fe3O4/MMT NC Ni(II) 65.78   2       [69]
Fe3O4@SiO2 core-shell
nanoparticles		 Cd(II) 179           [156]
Fe3O4@SiO2 core-shell
nanoparticles		 Pb(II) 156           [156]
Fe3O4 Ni(II) 362.318     8   5 [157]
Magnetic PAAAM/ PAMPS Zn(II) 289.12       25   [158]
Magnetic PAAAM/ PAMPS Cd(II) 385.2       25   [158]
Hollow magnetite
nanospheres		 Cr(VI) 6.64     4 25   [159]
Hollow magnetite
nanospheres		 Cr(VI) 7.31     4 35   [159]
Hollow magnetite
nanospheres		 Cr(VI) 8.90     4 45   [159]
Hollow magnetite
nanospheres		 Pb(II) 13.40     5 25   [159]
2015 Hollow magnetite
nanospheres		 Pb(II) 14.11     5 35   [159]
Hollow magnetite
nanospheres		 Pb(II) 18.47     5 45   [159]
SPION As(V) 0.91 mmol/g   60 3.8     [160]
SPION surface-coated with 3-mercaptopropionic acid (3-MPA) As(V) 1.92 mmol/g   60 3.6     [160]
Fe3O4@SiO2 Cd(II) 24.8   180 7 25 5 [161]
PPY/γ-Fe2O3 Cr(VI) 209   15 2   4 [162]
PANI/γ-Fe2O3 Cr(VI) 196   35 2   4 [162]
PPY/γ-Fe2O3 Cu(II) 171   15 5.5   4 [162]
PANI/γ-Fe2O3 Cu(II) 107   35 5.5   4 [162]
EDTA-Fe3O4 Pb(II) 508.4   40 4.2 45   [163]
EDTA-Fe3O4 Hg(II) 268.4   50 4.1 45   [163]
EDTA-Fe3O4 Cu(II) 301.2   90 5.1 45   [163]
EDTA-Fe3O4 Pb(II) 548.1       35   [163]
EDTA-Fe3O4 Hg(II) 242.2       35   [163]
EDTA-Fe3O4 Cu(II) 289.4       35   [163]
EDTA-Fe3O4 Pb(II) 481.2       25 5 [163]
EDTA-Fe3O4 Hg(II) 203.1       25 5 [163]
EDTA-Fe3O4 Cu(II) 246.1       25 5 [163]
Fe3O4@SiO2/Schiff Cu(II) 97.2   60 5     [164]
Fe3O4@SiO2/Schiff Zn(II) 81.6   60 5     [164]
PMMA-gft-Alg/Fe3O4 Pb(II) 62.5     5 50   [165]
PMMA-gft-Alg/Fe3O4 Cu(II) 35.71     5 50   [165]
SH-HMSMCS Hg(II)   95   6.2   5 [166]
Ppy–Fe3O4/rGO Cr(VI) 293.3     3 45   [167]
Ppy–Fe3O4/rGO Cr(VI) 226.8     3 30   [167]
Ppy–Fe3O4/rGO Cr(VI) 180.8     3 20   [167]
Co0.6Fe2.4O4 Pb(II) 80.32   30   45.18   [168]
Co0.6Fe2.4O4 Pb(II) 70.22       35.15   [168]
Co0.6Fe2.4O4 Pb(II) 44.58       25.15   [168]
Fe3O4/Mg–Al–CO3 Cd(II) 54.7       50   [169]
Fe3O4/Mg–Al–CO3 Cd(II) 50.5       40   [169]
Fe3O4/Mg–Al–CO3 Cd(II) 45.6       30   [169]
2016 NMag–CS Cu(II) 123.4   120 5.5     [170]
NMag–CS Pb(II) 114.9   120 5.5     [170]
NMag–CS Cr(VI) 116.2   120 5.5     [170]
NMag–CS Cd(II) 112.3   120 5.5     [170]
NMag–CS Ni(II) 109.8   120 5.5     [170]
Amino-functionalized Fe3O4@SiO2 Zn(II) 169.5   120 5 ± 0.1 25   [171]
Fe3O4 Cu(II) 37.04     4–6     [172]
Fe3O4 Pb(II) 166.67     4–6     [172]
Fe2O3 Cu(II) 19.61     5–6   4 [172]
Fe2O3 Pb(II) 47.62     3–4   4 [172]
Fe2O3@SiO2 As(III) 77.7   180 7 30   [173]
2016 Fe2O3@TiO2 As(V) 99.5   180 6 30   [173]
Fe3O4@SiO2-SH Hg(II) 132     6 25 5 [174]
Fe3O4-Aspa Ni(II) 87.183   70 6 40   [175]
Fe3O4-Aspa Ni(II) 58.582   70 6 35   [175]
Fe3O4-Aspa Ni(II) 34.602   70 6 30 6 [175]
Henna-Fe3O4 Cu(II) 28.90   20 5.2     [176]
Fe3O4/Bent-2.0 Pb(II) 81.5   30       [177]
Fe3O4/Bent-2.0 Cd(II) 21.7   30       [177]
Fe3O4/Bent-2.0 Cu(II) 19.6   30       [177]
Fe3O4 Pb(II)   100 30 5 25   [178]
Fe3O4 Cr(VI) 34.9     2 45   [179]
Fe3O4 Pb(II) 53.1     5 45   [179]
Fe3O4 Cr(VI) 26.8     2 35   [179]
Fe3O4 Pb(II) 52.8     5 35   [179]
Fe3O4 Cr(VI) 20.2     2 25 2 [179]
Fe3O4 Pb(II) 52.9     5 25 2 [179]
a-Fe2O3 Cd(II) 127.23     6 20   [180]
a-Fe2O3 Cd(II) 146.41     6 30   [180]
a-Fe2O3 Cd(II) 158.48     6 40   [180]
GO/Fe3O4 Pb(II) 65.96   180 3 20   [181]
GO/Fe3O4/LA Pb(II) 53.06   180 3 20   [181]
GO/Fe3O4/LA/EDTA Pb(II) 161.80   180 3 20   [181]
Amine-functionalized Fe3O4 Cu(II)   85 30 7     [182]
Fe3O4@SiO2@TiO2 Cu(II) 125   30       [183]
Fe3O4@SiO2@TiO2 Zn(II) 137   30       [183]
Fe3O4@SiO2@TiO2 Cd(II) 148   30       [183]
Fe3O4@SiO2@TiO2 Pb(II) 160   30       [183]
Rice straw/Fe3O4 NCs Pb(II)   91.18         [184]
Rice straw/Fe3O4 NCs Cu(II)   75.54         [184]
Cys-Fe3O4 Pb(II) 183.5   120 6   5 [185]
Cys-Fe3O4 Cd(II) 64.35   120 6   5 [185]
Nano-Fe3O4@Nano-SiO2 Pb(II) 1100 μmol/g           [186]
Nano-Fe3O4@Nano-SiO2 Cu(II) 300 μmol/g           [186]
Nano-Fe3O4@Nano-SiO2 Cd(II) 150 μmol/g           [186]
Nano-Fe3O4@Nano-SiO2 Hg(II) 100 μmol/g           [186]
a-Fe2O3 As(V) 38.48       20   [187]
Fe3O4-MnO2 Pb(II) 208.17       25   [188]
Fe3O4-MnO2 Cu(II) 111.90       25   [188]
Fe3O4-MnO2 Cd(II) 169.90       25   [188]
Fe3O4-MnO2 Zn(II) 100.24       25   [188]
Fe3O4-MnO2 Ni(II) 55.63       25   [188]
Fe3O4@SiO2-EDTA Cu(II) 37.59   10 5.3 30   [189]
Fe3O4@SiO2-EDTA Pb(II) 114.94   10 5.3 30   [189]
Fe3O4@SiO2-EDTA Ni(II) 32.15   10 5.3 30   [189]
Fe3O4@SiO2-EDTA Cd(II) 50.25   10 5.3 30   [189]
Fe3O4–FeB Cr(VI) 38.9     6.3     [190]
Fe3O4–TiO2 As(V) 11.434 μg/g   40 6.3 25   [191]
Fe3O4–TiO2 As(III)   93   11 25   [191]
Chitosan MWCNT/Fe3O4 Cr(VI) 360.1   30 2 45   [192]
2016 Chitosan MWCNT/Fe3O4 Cr(VI) 348.2       35   [192]
Chitosan MWCNT/Fe3O4 Cr(VI) 335.6       25   [192]
AMGO Cr(VI) 123.4     2   5 [193]
2017 Fe3O4@MnO2 Pb(II) 666.67   120   25   [194]
Fe3O4-SO3H Pb(II) 108.93     7 25   [195]
Fe3O4-SO3H Cd(II) 80.9     7 25   [195]
PGMA-MAn
Copolymer@Fe3O4		 Pb(II) 53.33   20 5 25   [196]
PGMA-MAn
Copolymer@Fe3O4		 Cu(II) 48.53     5     [196]
Fe3O4/CTAB Cr(VI)   95.77 720 4 25 ± 0.1   [197]
Fe3O4@Cu3(btc)2 Pb(II) 215.05         4 [198]
Fe3O4@Cu3(btc)2 Hg(II) 348.43         4 [198]
Fe3O4-CS-L Zn(II) 256.41   45 6 25 5 [199]
Fe3O4-CS-L Cd(II) 156.99   45 6 25 5 [199]
Fe3O4-CS-L Pb(II) 128.63   45 6 25 5 [199]
Fe3O4-mSiO2 Cu(II) 84.4           [200]
Fe3O4-mSiO2 Cd(II) 80.5           [200]
Fe3O4-mSiO2 Zn (II) 72.6           [200]
γ-Fe2O3@CS Cd(II) 15.2   60 5 20 ± 0.1 5 [201]
L-Cyst-Fe3O4 Pb(II) 18.8     2 45 5 [202]
L-Cyst-Fe3O4 Cr(VI) 34.5     6 45 5 [202]
MDA-Fe3O4 Pb(II) 333.3     5 30   [203]
ZrO2-Fe3O4 As(III) 113.48     7   5 [204]
Fe3O4@SiO2@CS-TETA-GO Cu(II) 324.7   16 6 30 6 [205]
CoFe2O4@SiO2–SH Hg(II) 641.0     8 25 5 [206]
CoFe2O4@SiO2–SH Hg(II) 628.9     8 35   [206]
CoFe2O4@SiO2–SH Hg(II) 591.7     8 45   [206]
EDTA-functionalized CoFe2O4 (EDTA-MNP) Cu(II) 73.26     6 50   [207]
IMSA Pb(II) 133.73           [208]
IMSA As(V) 21.61           [208]
CoFe2O4@SiO2 Cd(II) 199.9     7   5 [209]
CoFe2O4@SiO2 Cu(II) 177.8     7   5 [209]
CoFe2O4@SiO2 Pb(II) 181.6   30 7   5 [209]
TEPA chitosan/CoFe2O4 Cu(II) 168.067   50 5 30   [210]
TEPA chitosan/CoFe2O4 Pb(II) 228.311   50 5 30   [210]
2018 Lantana camara capped iron
nanoparticles		 Ni(II) 227.2     6 60   [211]
NA-FeOx Cd(II) 11.3   60       [212]
NA-FeOx Cu(II) 12.3   60       [212]
Graphene oxide-Fe3O4 Pb(II) 373.14   10 6     [213]
PAE-AAm-g-MNPs Cu(II) 30.34     6   7 [214]
Fe3O4/LDH-AM Cu(II) 64.66   240     5 [215]
Fe3O4/LDH-AM Cd(II) 74.06   240     5 [215]
Fe3O4/LDH-AM Pb(II) 266.6   180     5 [215]
TEPA-GO/MnFe2O4 Pb(II) 263.2     5.5 30 4 [216]
GO/MnFe2O4 Pb(II) 133.3           [216]
MnFe2O4–BC Cd(II) 181.49     7 25 5 [217]
2018 CoFe2O4@SiO2 Hg(II) 149.3     7 25 5 [218]
CoFe2O4@SiO2 Hg(II) 144.9     7 35   [218]
CoFe2O4@SiO2 Hg(II) 131.6     7 45   [218]
p-BNMR@Fe3O4 Pb(II) 249.5     5.5 25   [219]
Aminated-Fe3O4 Cr(VI) 19.5     3.5   3 [220]
Aminated-Fe3O4 Pb(II) 21.2     3.5   3 [220]
Fe3O4/poly(C3N3S3) Pb(II) 232.6     6 25 7 [221]
Fe3O4/poly(C3N3S3) Hg(II) 344.8     6 25 7 [221]
rGO-PDTC/Fe3O4 Cu(II) 113.64     5 25 5 [222]
rGO-PDTC/Fe3O4 Cd(II) 116.28     6 25 5 [222]
rGO-PDTC/Fe3O4 Pb(II) 147.06     6 25 5 [222]
rGO-PDTC/Fe3O4 Hg(II) 181.82     6 25 5 [222]
d-MoS2/Fe3O4 Hg(II) 425.5   20     5 [223]
CHT/ALG/Fe3O4@SiO2
(8 beads)		 Pb(II) 243.77     4.2 20   [224]
CHT/ALG/Fe3O4@SiO2
(1 bead)		 Pb(II) 228.73     4.2 20   [224]
2019 Fe3O4/BC/AC Pb(II) 169.78     5 25 5 [225]
Fe3O4@FePO4 Cd(II) 13.51   15 7     [226]
Fe3O4@SiO2-NH-MFL Pb(II) 150.33   0.5 5     [227]
Fe3O4@SiO2-NH-MFL Cu(II) 70.7   0.5 5     [227]
DTT-Fe3O4@Au As(III)   68.8   5     [228]
rGO-poly(C3N3S3)/Fe3O4 Pb(II) 270.3   60 6 25 15 [229]
rGO-poly(C3N3S3)/Fe3O4 Hg(II) 400   60 6 25 15 [229]
APTES-Fe3O4 (3 wt%) As(V) 14.6   210 2 25   [230]
Fe2O3-SiO2-PAN Cr(III) 4.36           [231]
Fe2O3-SiO2-PAN Cu(II) 7.20           [231]
Fe2O3-SiO2-PAN Zn(II) 5.06           [231]
Fe2O3-SiO2-PAN Ni(II) 2.60           [231]
biochar-MnFe2O4 Pb(II) 154.94     5 25   [232]
biochar-MnFe2O4 Cd(II) 127.83     5 25   [232]
CoFe2O4@SiO2-EDTA Hg(II) 103.3   360 7 25 5 [233]
Fe3O4-CS@BT Cr(VI) 62.1     2 25 5 [234]
Fe3O4-CS@BT Cr(VI) 48.3     4 25   [234]
Fe3O4-CS@BT Cr(VI) 36.4     6 25   [234]
CMC/SA/graphene
oxide@Fe3O4		 Cu(II) 55.96     5 30   [235]
CMC/SA/graphene
oxide@Fe3O4		 Cd(II) 86.28     6 30   [235]
CMC/SA/graphene
oxide@Fe3O4		 Pb(II) 189.04     5 30   [235]
Fe3O4/NaP/NH2 Pb(II) 181.81   480 5–6 60 10 [236]
Fe3O4/NaP/NH2 Cd(II) 50.25   240 5–6 70 10 [236]
Fe3O4 ECSBNC Cu(II) 90.90           [237]
Fe3O4 ECSBNC Cr(VI) 83.33           [237]
MNPs-COOH Pb(II) 0.855 mmol/g     6 25   [238]
MNPs-COOH Cu(II) 0.660 mmol/g     6 25   [238]
2019 MNPs-COOH Cd(II) 0.518 mmol/g     6 25   [238]
MNPs-COOH Ni(II) 0.441 mmol/g     6 25   [238]
CMC-Fe3O4 Pb(II) 152.0         6 [239]
Fe3O4-loaded CS NPs Cd(II) 97.76     5   5 [240]
CuFe2O4 Cd(II) 157.7     2     [241]
CoFe2O4 Pb(II) 63.1     12     [241]
HP-β-CD-GO/Fe3O4 Cu(II) 17.91     6   5 [242]
HP-β-CD-GO/Fe3O4 Pb(II) 50.39     5   5 [242]
Bentonite/CoFe2O4@MnO2-NH2 Cd(II) 115.79 98.88         [243]
2020 Fe3O4@SiO2@GLYMO(S) Cd(II) 80.64   55 7   5 [244]
Fe3O4@SiO2@GLYMO(S) Pb(II) 93.5   55 7   5 [244]
MGO Pb(II) 200   30 5     [65]
MGO Cr(III) 24.33   30 6     [65]
MGO Cu(II) 62.89   30 6     [65]
MGO Zn(II) 63.69   30 7     [65]
MGO Ni(II) 51.02   30 8     [65]
Proanthocyanidin-
functionalized Fe3O4		 Cu(II) 18.8   30 8   5 [245]
Proanthocyanidin-
functionalized Fe3O4		 Cd(II) 20.9   30 8     [245]
Proanthocyanidin-functionalized Fe3O4 Pb(II) 21.5   30 8     [245]
Ggh-g-PAcM/Fe3O4 Cu(II) 224.8       30   [246]
Ggh-g-PAcM/Fe3O4 Hg(II) 213.8       30   [246]
Fe3O4-GO hybrid (9:1) Pb(II) 107.56     6     [247]
Fe3O4-GO hybrid (5:1) Pb(II) 151.22     6     [247]
M-45 OA Pb(II) 42.553     6     [247]
M-45 OA Zn(II) 42.919     6     [248]
M-45 OA Cd(II) 42.373     7     [248]
M-55 OA Pb(II) 41.841     6     [248]
M-55 OA Zn(II) 42.735     6     [248]
M-55 OA Cd(II) 42.017     7     [248]
M-55+ Pb(II) 40.816     6     [248]
M-55+ Zn(II) 40.816     6     [248]
M-55+ Cd(II) 39.216     7     [248]
BNNF@Fe3O4 Pb(II) 203.75   50   25   [249]
Fe3O4@SiO2-NH2 Cd(II)   93       5 [250]
RH + iron oxide NPs As(V) 82   60     5 [251]
MnFe2O4 Zn(II) 454.4   120 6 25 3 [252]
CoFe2O4 Zn(II) 384.6   120 6 25 3 [252]
SiO2/CuFe2O4/PANI Cu(II) 285.71   300 5.3 30 4 [253]
MWCNT/γ-Fe2O3 Cr(VI) 208.1   150 4     [254]
MWCNT-PEI/γ-Fe2O3 Cr(VI) 352.3   150 4     [254]
Fe3O4 Cr(VI) 201.55   50     6 [255]
Fe3O4@Z-NCNT/PC Pb(II) 789.87   20 5.5   10 [256]
FO-BC-450 Cd(II) 151.3     2 25   [257]
2020 FO-BC-450 Cu(II) 219.8     2 25   [257]
FO-BC-450 Pb(II) 271.9     2 25   [257]
Fe2O3@SiO2@SH Hg(II)   98         [258]
γ-Fe2O3 coated Bacillus subtilis Cd(II) 32.6     4 30   [259]
Fe3O4-HBPA-ASA Cu(II) 136.66     8 25 5 [260]
Fe3O4-HBPA-ASA Cd(II) 88.36     8 25 5 [260]
Fe3O4-HBPA-ASA Pb(II) 165.46     8 25 5 [260]
Crystalline iron oxide nanoparticles (IO-NPs) Pb(II) 9.206     6 40   [261]
Crystalline iron oxide nanoparticles (IO-NPs) Ni(II) 9.666     6 40   [261]
Crystalline iron oxide nanoparticles (IO-NPs) Cu(II) 8.355     6 40   [261]
Crystalline iron oxide nanoparticles (IO-NPs) Zn(II) 9.106     6 40   [261]
Pec-g-PHEAA/Fe3O4 Cu(II) 248.6           [262]
Pec-g-PHEAA/Fe3O4 Hg(II) 240.2           [262]
Fe3O4 fibre Pb(II) 16.78         6 [263]
Fe3O4 powder Pb(II) 15.80         6 [263]
Iron oxide (MNPs) grafted (HPG) Cu(II) 0.700 mg/mg   120 9 20   [264]
Iron oxide (MNPs) grafted (HPG) Ni(II) 0.451 mg/mg   120 9 20   [264]
Nano-CI Pb(II) 1900 μmol/g   30 6     [265]
Nano-CI Cu(II) 2250 μmol/g   30 7     [265]
Nano-CI Cd(II) 850 μmol/g   30 7     [265]
Nano-CIC Pb(II) 2700 μmol/g   30 4     [265]
Nano-CIC Cu(II) 4250 μmol/g   30 6     [265]
Nano-CIC Cd(II) 1800 μmol/g   30 6     [265]
Nano-CIS Pb(II) 2600 μmol/g   10 4     [265]
Nano-CIS Cu(II) 4700 μmol/g   10 6     [265]
Nano-CIS Cd(II) 1900 μmol/g   10 6     [265]
1 Cases with unit of adsorption capacity that is different from mg/g are listed next to the respective value.
A crucial stage in the evaluation of the applicability of the experimental methods is the possibility of reducing the production cost and time. This reduction could be achieved by reusing the existing nanoparticles; thus the stages of synthesis and characterization could be avoided. Desorption processes may occur either by thermal treatment or through suitable desorbing agents and are necessary for recycling [70], so the nanoparticles could be used again. Among the selected articles, all have employed desorbing agents during the desorption process. The performance of the desorption process for magnetic nanoparticles is directly related to the size, coating, magnetic behaviour [266] and pH of the solution [267], while other critical factors are the kind of regenerative solutions (i.e., NaOH, HCl) but also their concentration. For example, 2 M of HNO3 has a desorption efficiency of Cr (VI) equal to 73%, but the efficiency drops to 20% when 0.1 M of HNO3 is employed [136]. Additional benefits of the desorption process are the limited cost of desorbing agents and the time of the process, which could be achieved in less than 1 h. The usage of the desorption process is enhanced by the ease of collection, which comes from the selectivity of the paramagnetic nanoparticles assisting the technique, because they could be readily separated from the solution when a magnetic field is applied; thus iron oxide nanoparticles are more preferable to other nanoparticles with no magnetic cores. Additionally, computational fluid dynamics could be employed at this stage, reducing the cost of the materials. The major advantage of computational water treatment methods compared with an experimental method is that the steps of synthesis and characterization of magnetic nanoparticles are not time-consuming since they do not exist. The aims of microfluidic mixing and driving simulations for water purification from heavy metal ions are to achieve rapid mixing and desired guidance of nanoparticles [48][51][59][268].
Nonetheless, the effectiveness of the adsorption–desorption process is evaluated by the efficiency of heavy metal adsorption after each recycling. An ideal adsorbent is considered to be one that simultaneously possesses high adsorption capability and high desorption efficiency [269]. Of critical importance is the effectiveness of the process in several studies, in which up to five cycles of adsorption without a significant decrease in efficiency have been achieved [270]. In addition, high adsorption and desorption efficiencies equal to 98.4% after seven cycles [214] and 98% (constant) for over 15 cycles have been reported [70]. The deterioration of active binding sites on the surface absorbent during recycling results in a decrease in efficiency. It should be noted, however, that in some cases after the desorption process, the adsorption efficiency did not decrease, but instead, it increased in the next cycle [271]; this phenomenon is based on the increase in the positively charged surface of nanoparticles, which leads to increased electrostatic attraction forces between the iron oxide nanoparticles and the pollutant. Recycling of the adsorbent is important to obtain the process that enhances the viability of the adsorption method.
The recycling efficiency and adsorption capacity for each cycle are presented in Table 4 and Table 5, respectively. We must mention here that there are very few articles that have investigated the adsorption capacity. This fact has a negative impact on the applicability of the method. During the literature review were recorded cases where the recycling effectiveness was measured with adsorption capacity instead of efficiency. Additionally, both adsorption and desorption efficiency decreased through the regeneration cycles and due to the difficulty in reversing adsorption [272].
Table 4. Recycling efficiency (%).
Pollutant 1st Cycle 2nd Cycle 3rd Cycle 4th Cycle 5th Cycle Ref.
Pb(II) 90.12 88.05 85.65 81.35   [270]
Cu(II) 93.70       58.66 [99]
Pb(II)         90 [101]
Cr(VI) 55 88   90   [271]
Pb(II) 97.34       90 [76]
Hg(II)         ≥96 [66]
Pb(II)         90 [106]
Pb(II) 93.5       89.3 [244]
Cu(II) 80.64       73.3 [244]
As(V) 95       56 [251]
Cd(II)       76.4   [257]
Cu(II)       80.4   [257]
Pb(II)       70.2   [257]
Hg(II) >90       ~75.5 [233]
Cd(II) 98.8 95.1 91.7 84.6 78.3 [201]
Cd(II) 99.96     97.25   [209]
Cu(II) 88.05     84.15   [209]
Pb(II) 90.79     87.12   [209]
Pb(II) 96.2       86.4 [163]
Hg(II) 95.1       85.9 [163]
Cu(II) 96.5       87.6 [163]
Table 5. Recycling adsorption capacity (mg/g).
Pollutant 1st Cycle 2nd Cycle 3rd Cycle 4th Cycle 5th Cycle 6th Cycle Ref.
Cr(VI) 132.56 130.62 127.52 125.97 124.42 121.71 [255]
Cu(II) 197.5 196.5 195 194.4     [253]

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