Modeling the sorption of metal ions from aqueous solution by iron-based adsorbents

Recovery of Ag(I) from Wastewater by Adsorption: Status and Challenges

Untreated or inadequately treated silver-containing wastewater may pose adverse effects on hu-man health and the ecological environment. Currently, significant progress has been made in the treatment of Ag(I) in wastewater using adsorption methods, with adsorbents playing a pivotal role in this process. This paper provides a systematic review of various adsorbents for the recovery and treatment of Ag(I) in wastewater, including MOFs, COFs, transition metal sulfides, metal oxides, biomass materials, and other polymeric materials. The adsorption mechanisms of these materials for Ag(I) are elaborated upon, along with the challenges currently faced. Furthermore, insights into optimizing adsorbents and developing novel adsorbents are proposed in this study.

Harnessing synergy: Integrating agricultural waste and nanomaterials for enhanced sustainability

This paper aims to explore the cooperative use of agricultural waste and nanomaterials to improve environmental sustainability. The introduction highlights global environmental challenges and the objectives of integrating the two are highlighted. Valorization of agricultural waste is considered to reduce waste generation, while nanomaterials act as conversion catalysts that help to increase the efficiency of waste conversion and environmental remediation. In addition, synergistic approaches are discussed, including the combination of agricultural waste and nanomaterials, as well as the concept of enhanced resource management. The paper also presents case studies that demonstrate the success of such synergistic applications in pollution control and environmental remediation. Despite the challenges and risks, this approach can provide new ways to create more sustainable and resilient environments through the integration of resources, interdisciplinary cooperation and policy support.

Effects of Fe(III) (hydr)oxide mineralogy on the development of microbial communities originating from soil, surface water, groundwater, and aerosols

Microbial Fe(III) reduction is a key component of the iron cycle in natural environments. However, the susceptibility of Fe(III) (hydr)oxides to microbial reduction varies depending on the mineral's crystallinity, and the type of Fe(III) (hydr)oxide in turn will affect the composition of the microbial community. We created microcosm reactors with microbial communities from four different sources (soil, surface water, groundwater, and aerosols), three Fe(III) (hydr)oxides (lepidocrocite, goethite, and hematite) as electron acceptors, and acetate as an electron donor to investigate the shaping effect of Fe(III) mineral type on the development of microbial communities. During a 10-month incubation, changes in microbial community composition, Fe(III) reduction, and acetate utilization were monitored. Overall, there was greater reduction of lepidocrocite than of goethite and hematite, and the development of microbial communities originating from the same source diverged when supplied with different Fe(III) (hydr)oxides. Furthermore, each Fe(III) mineral was associated with unique taxa that emerged from different sources. This study illustrates the taxonomic diversity of Fe(III)-reducing microbes from a broad range of natural environments.

Enhanced removal of aqueous Cr(VI) by the in situ iron loaded activated carbon through a facile impregnation with Fe(II) and Fe(VI) two step method: Mechanism study

In this study, a novel in situ iron-loaded activated carbon (AFPAC) was prepared by a FeSO₄/K₂FeO₄ impregnation and oxidation combination two-step supported on activated carbon for enhanced removal of Cr(VI) from aqueous solutions. Cr(VI) removal efficiency greatly increased by AFPAC more than 70% than that of fresh activated carbon (AC), which is due to rich iron oxides formed in situ and the synergistic effect between iron oxides and activated carbon. Cr(VI) adsorption behaviors on AFPAC under different water quality parameters were investigated. The maximum monolayer adsorption capacities for Cr(VI) by AFPAC are as high as 26.24 mg/g, 28.65 mg/g, and 32.05 mg/g at 25 °C, 35 °C and 45 °C at pH 4, respectively. Density functional theory (DFT) results showed that the adsorption energy of K₂Cr₂O₇ on the surface of FeOOH was - 2.52 eV, which was greater than that on the surface of bare AC, and more charge transfer occurred during the adsorption of K₂Cr₂O₇ on the surface of FeOOH, greatly promoting the formation of Cr = O-Fe. Cr(VI) removal by AFPAC included electrostatic attraction, redox reaction, coordinate complexation, and co-precipitation. Cr(VI) adsorption process on AFPAC consisted of the three reaction steps: (1) AFPAC was fast protonation and Cr₂O₇^2- would electrostatically attract to the positively charged AFPAC surface. (2) Cr₂O₇^2- was reduced into Cr₂O₃ by the carbons bond to the oxygen functionalities on activated carbon and the redox reaction process of FeSO₄ and K₂FeO₄. (3) The inner-sphere complexes were formed, and adsorbed on AFPAC by iron oxides and then co-precipitation.

A facile modification of cation exchange resin by nano-sized goethite for enhanced Cr(VI) removal from water

A novel macroporous strong acidic cation exchange resin (D001) modified by nano-sized goethite (nFeOOH@D001) was fabricated by using a facile ethanol dispersion and impregnation method, and its efficiency for Cr(VI) removal was tested thereafter. Due to the dispersing effect of ethanol, FeOOH particles of 20-150 nm were coated on the D001 surfaces. The nFeOOH@D001 obtained a Cr(VI) removal efficiency and capacity of 80.2% and 7.4 mg/g respectively, 5 times and 8 times higher than that of the pristine D001. The Cr(VI) removal by nFeOOH@D001 followed the pseudo second-order kinetics and the Langmuir adsorption model. Column experiments also demonstrated that the nFeOOH@D001 exhibited a much better ability to remove Cr(VI) as compared to the D001. Additionally, the nFeOOH@D001 showed a potential for reusability and renewability. The adsorbed nFeOOH@D001 could be easily desorbed by 0.1 M acetic acid and a reuse efficiency of 92.7% could be maintained after 4 desorption-adsorption cycles. The used nFeOOH@D001 could be eluted by 0.1 M HCl to remove nFeOOH, and the renewed D001 could be recoated by nFeOOH and achieved a regeneration rate of 97.8% for Cr(VI) removal. The above results indicated that nano-sized goethite modification is a promising method to endow D001 with the ability to remove Cr(VI) from water.

Sustainable nanotechnology based wastewater treatment strategies: achievements, challenges and future perspectives

Nanotechnology is being an emerging science for wastewater treatment requires more research emphasis and depth knowledge. For wastewater treatment, different forms of nanomaterials are used based on the type of contaminants and treatment efficiency desired. With the development in the field of nanomaterials, novel and emerging nanomaterials are coming into existence. The nanomaterials used for wastewater treatment can be carbon, single-walled carbon nanotubes, multiple walled carbon nanotubes, covalent organic frameworks, metal and metal oxide- based nanoparticles. Graphene based nanoparticles, their oxides (GO) and reduced graphene oxide (rGO) find tremendous applicability to be used in wastewater treatment purposes. Due to the introduction of graphene oxide nanoparticles in the adsorbent materials, their adsorption capacities have get enhanced and such materials have also improved the mechanical stability of the adsorbent. Ferric oxide shows greater adsorption capacities for organic pollutants. Furthermore, magnetic nano-powder confers a low adsorption capacity for phenols. Pyrrolidone reduced graphene oxide (PVP-RGO) nanoparticles have been used as adsorbents for the elimination of inorganic target contaminant copper, with great adsorption (1698 mg/g). The present study comprehensively reviews nanotechnology as a wastewater treatment strategy besides enlightening its safety issues and efficiency. The novelty of this article is that it highlights the overview of recent applications of various types of nanomaterials and research works releated to it. Such an approach will be helpful to get insights into technological advances, applications and future challenges of nanotechnology implementation for wastewater treatment.

Adsorption behaviors of the pristine and aged thermoplastic polyurethane microplastics in Cu(II)-OTC coexisting system

In this work, the hypothesis that thermoplastic polyurethane (TPU) microplastics (MPs) could form complex toxic pollution by absorbing both antibiotics and heavy metals simultaneously was proposed. The unique features of the adsorption of Cu(II) and oxytetracycline (OTC) on the pristine TPU and photo-aged (aged) TPU MPs in single and coexisting system were investigated, which included the kinetics, isothermal equilibrium and thermodynamics. The possibly synergistic or competitive effects between Cu(II) and OTC were also evaluated. The results showed that the adsorption process of Cu(II) and OTC could be described well by pseudo-second-order kinetic equation. The entire process could be divided into two stages: internal diffusion and external diffusion. The Sips model could give good fitting for the isothermal adsorption equilibrium. The thermodynamic parameters depicted the endothermic nature of adsorptions and the process was spontaneous. In the coexisting system, synergistic or competitive effects depended critically on the ratio of concentrations (Cu(II) vs OTC). When the ratio was 1:1, Cu(II) significantly enhanced the adsorption of OTC, while OTC showed a weak effect on Cu(II) adsorption. The synergies could be attributed to the formation of Cu(II)-OTC complex and the bridging effect of Cu(II). Overall, the adsorption capacity of aged TPU was higher than that of pristine TPU, which was due to the differences in morphological characteristics and functional groups. FTIR studies revealed that ester carbonyl and acylamino groups in the TPU may be involved in the adsorption of Cu(II) and OTC.

Predicting breakthrough of vanadium in fixed-bed absorbent columns with complex groundwater chemistries: A multi-component granular ferric hydroxide-vanadate-arsenate-phosphate-silicic acid system

Granular ferric hydroxide (GFH) is often used for fixed bed adsorbent (FBA) columns in groundwater purification units around the world to remove arsenate contaminations. Groundwater can contain also other toxic (e.g., antimonite and vanadate) and non-toxic oxo-anions (phosphate and silicic acid) that are known to affect FBA lifetimes. Therefore, understanding the breakthrough of toxic compounds intended for removal by FBA is essential to their design, and is important to predict accurately breakthrough curves (BTCs) for FBAs in waterworks to plan future operating costs. Rapid small-scale column tests (RSCCT) and pilot-scale FBA were used to simulate vanadate BTCs for complex groundwater chemistries. The BTCs were simulated successfully using a homogeneous surface diffusion model (HSDM) combining equilibrium chemical adsorption and kinetic mass transfer. Adsorption parameters for various groundwater compositions were predicted using the CD-MUSIC surface complexation model, which was set up for the first time for akaganéite-based granular ferric hydroxide with a competitive multi-solute system. The results indicated that V(V) is less prone to competitive adsorption effects, and use of the homogeneous surface diffusion model to predict the BTCs requires then the kinetic mass transfer Biot number to be used as the only fitting parameter. On the other hand, a concentration overshoot could be observed for the two weaker absorbed oxo-anions arsenate and phosphate because of displacement by the vanadate. Results of pilot scale test column BTCs of vanadate for three waterworks with different groundwater compositions could be favorably extrapolated with a unique Freundlich constant k_F of 3.2 derived on basis of the multi-solute CD-MUSIC model, and a unique Biot number of 37 fixed for all three different test sites.

Adsorption of Cu (II) and Cd (II) from Wastewater by Sodium Alginate Modified Materials

Natural macromolecule adsorbing materials are alternatives for remediation of heavy metals’ polluted waters. In this study, sodium alginate composite gel (SACL) was synthesized from sodium alginate (SA), polyethylene glycol oxide (PEO), and nanomaterials to remove wastewater Cu (II) and Cd (II). The adsorption efficiency of SACL was analysed in relation to the contact time, initial concentrations of metal ions, temperature, adsorbent dosage, and solution pH. Three models, i.e., kinetic model, isothermal adsorption model, and thermodynamic model, were used to fit the experimental data. Our results showed that the highest removal rates of Cu (II) and Cd (II) from wastewater were 96.8% and 78%, respectively, under the condition of the best liquid-solid ratio of 12.5 ml·g−1, and the contact time of 180 min (25°C). Overall, the SACL adsorption of Cu (II) and Cd (II) was spontaneous. The adsorption kinetics and the isothermal adsorption were fitted well with the pseudo-second-order kinetic equation and Langmuir equation, respectively. Combined with SEM-EDS and FTIR analysis, results suggested that SACL adsorbs wastewater Cu (II) and Cd (II) mainly through chemical reaction on its surface area. Altogether, this work concludes on SACL as an efficient and ecofriendly adsorbent for wastewater Cu (II) and Cd (II).

Preparation and characterization of poly aniline modified chitosan embedded with ZnO-Fe3O4 for Cu(II) removal from aqueous solution

Poly aniline modified chitosan embedded with ZnO/Fe₃O₄ nanocomposites were synthesized using a precipitation method and applied to the removal of Cu(II) from aqueous solution. The synthesized nanocomposite was characterized by FT-IR, XRD, FESEM, TEM, EDS, TGA, BET and zeta-potential analyses. The adsorption batch experiments were conducted as a function of five effective parameters including pH, contact time, initial concentration of copper, temperature, and adsorbent dosage using a central composite design (CCD) in response surface methodology (RSM). Contour and surface plots were used to determine the interaction effects of main factors and optimum conditions of process. The regression equation coefficients were calculated and the data confirmed the validity of second-order polynomial equation for the removal of Cu(II) with novel absorbent. Analysis of variance (ANOVA) showed a high coefficient of determination value (R²) for copper removal being 0.99. The optimum level of the pH, temperature, initial concentration of copper, adsorbent dosage and contact time for maximum Cu(II) removal (94.51%) were found to be 6.5, 31 °C, 82 mg L^-1, 0.81 g L^-1, and 51 min, respectively. It was confirmed from XPS and EDS analyses that heavy metal ions were present on the surface of nanocomposite after adsorption. The adsorption equilibrium data fitted well with the Langmuir isotherm model and the adsorption process followed the pseudo-second-order and intra-particle diffusion kinetic model. The saturated adsorption capacity is found to be 328.4 mg/g. Thermodynamics analysis suggests that the adsorption process is endothermic, with increasing entropy and spontaneous in nature. Further recycling experiments show that nanocomposite still retains 95% of the original adsorption following the 5th adsorption-desorption cycle. The effects of coexist cation ions on the adsorption of Cu(II) was also investigated under optimal condition. All the results demonstrate that nanocomposite is a potential recyclable adsorbent for hazardous metal ions in wastewater.

Lead Remediation Using Smart Materials. A Review

The nanoparticles have been prepared and employed as excellent adsorbents for the sequestration of heavy metal ions and hazardous impurities from the aqueous media. The surface morphological, textural and structural properties of nanoparticles have been modified, which are capable and potentially useful for the remediation of metal ions. Several metals (oxides, doped, nanocomposites of Fe, Ti, Zn, SiO2, SiC, Mo, Co, Ni, Zr, Mn, Si, S, Al, Cu, Ce, graphene, CNTs) were reported an efficient adsorbents for the removal of lead (Pb) ions from aqueous media and polluted water. The present review focuses on different kinds of nanoparticles such as metal oxides, carbon based and host supported employed for removal of Pb ions under varying experimental conditions such as pH, temperature, contact time and concentrations. The preparation strategies, physicochemical properties and adsorption are also discussed. Based on studies, it was found that the smart materials are affective adsorbents for the purification of wastewater containing Pb ions and could possibly extended for the remediation of other heavy metal ions.

Influences of As(v), Sb(iii), and Hg(ii) ions on the nucleation and growth of akaganeite

The physico-chemical properties of akaganeite are known to be modified when formed in the presence of ions, but there are no reports on these effects for arsenic, antimony and mercury.

Flotation in Water and Wastewater Treatment

Flotation constitutes a separation process that originated from mineral processing. Nowadays, wider applications have been found and compared to flotation for water and wastewater treatment. Stress in the present review paper was mainly applied to heavy metal ions recovery by flotation and the respective mechanism followed, being either ion, precipitate, or sorptive flotation. In the latter case, the use of adsorbents is included (such as powdered activated carbon, zeolites, and goethite), as well as various biosorbents. The flotation of the following metals was reviewed: copper, zinc, nickel, lead, iron, chromium, arsenic, gold, and others. The bubble generation method could be applied for typical dispersed-air flotation column, electroflotation, or dissolved-air flotation; the latter being the most appropriate established technique in water treatment. The role of particle size (for example, studying flotation of salt-type mineral fines) was also examined.

Magnetic iron oxide nanoparticles for the collection and direct measurement of adsorbed alpha-emitting radionuclides from environmental waters by liquid scintillation analysis

Radioactive contamination, be it from accidental or intentional release, can create an urgent need to assess water and food supplies and the environment, and monitor human health. In the event of such an emergency, rapid and efficient methods may be needed to assess contamination levels in scores of samples within a short time frame. Internalized exposure to radionuclides that decay by alpha (α) emission can be especially hazardous, given the strongly ionizing nature of the α particle. Unfortunately, the determination of α-emitting radionuclides using traditional radioanalytical methods is typically labor and resource intensive and time consuming. In an effort to devise methods that are fast, require little labor and laboratory expendables, and minimize the use of toxic or corrosive reagents, researchers at PNNL have evaluated superparamagnetic nanoparticles as extracting agents for α-emitting radionuclides from chemically unmodified and acidified (pH 2) aqueous systems. It is demonstrated that bare magnetite nanoparticles exhibit strong affinity for two representative α-emitting radionuclides (241Am and 210Po) from two representative aqueous matrices (river and ground water). Furthermore, use of the superparamagnetic properties of these nanomaterials to concentrate the analyte-bearing solids from the bulk aqueous solution has been demonstrated. The nanoparticle concentrate can be either directly dispensed into a scintillation cocktail, or first dissolved and then added to a scintillation cocktail as a solution for an α-emission assay by liquid scintillation analysis. Despite the severe quenching caused by the metal oxide suspensions in the cocktail, the authors have demonstrated that modern liquid scintillation analyzers can report accurate α activity count rates; the upper limits of nanoparticle suspension concentrations in a cocktail are reported for cases wherein normal instrument count mode and a quench correction protocol are used. Discussions are provided on the presented sample processing and analysis method, the improvement (lowering) of minimum detectable activity concentrations using the nanoparticle-based assay method, and the quenching effects of nanoparticle suspensions in a scintillation cocktail.

In situ synthesis of hierarchical structured cotton fibers/MnO2 composites: a versatile and recyclable device for wastewater treatment

Hierarchical structured cotton fiber–MnO₂ composites were prepared by a new two-step strategy “ion exchange–redox reaction”.

Evaluating the long-term performance of low-cost adsorbents using small-scale adsorption column experiments

This study investigated a novel method of predicting the long-term phosphorus removal performance of large-scale adsorption filters, using data derived from short-term, small-scale column experiments. The filter media investigated were low-cost adsorbents such as aluminum sulfate drinking water treatment residual, ferric sulfate drinking water treatment residual, and fine and coarse crushed concretes. Small-bore adsorption columns were loaded with synthetic wastewater, and treated column effluent volume was plotted against the mass of phosphorus adsorbed per unit mass of filter media. It was observed that the curve described by the data strongly resembled that of a standard adsorption isotherm created from batch adsorption data. Consequently, it was hypothesized that an equation following the form of the Freundlich isotherm would describe the relationship between filter loading and media saturation. Moreover, the relationship between filter loading and effluent concentration could also be derived from this equation. The proposed model was demonstrated to accurately predict the performance of large-scale adsorption filters over a period of up to three months with a very high degree of accuracy. Furthermore, the coefficients necessary to produce said model could be determined from just 24 h of small-scale experimental data.

New trends in removing heavy metals from wastewater

With the development of researches, the treatments of wastewater have reached a certain level. Whereas, heavy metals in wastewater cause special concern in recent times due to their recalcitrance and persistence in the environment. Therefore, it is important to get rid of the heavy metals in wastewater. The previous studies have provided many alternative processes in removing heavy metals from wastewater. This paper reviews the recent developments and various methods for the removal of heavy metals from wastewater. It also evaluates the advantages and limitations in application of these techniques. A particular focus is given to innovative removal processes including adsorption on abiological adsorbents, biosorption, and photocatalysis. Because these processes have leaded the new trends and attracted more and more researches in removing heavy metals from wastewater due to their high efficency, pluripotency and availability in a copious amount. In general, the applicability, characteristic of wastewater, cost-effectiveness, and plant simplicity are the key factors in selecting the most suitable method for the contaminated wastewater.

Adsorption behavior and mechanism of perfluorooctane sulfonate on nanosized inorganic oxides

Adsorption of perfluorooctane sulfonate (PFOS) on manufactured nanoparticles (NPs) is critical for understanding their transport and fate in aquatic environments. In this study, the adsorption behavior of PFOS on nanosized Al2O3, Fe2O3, SiO2 and TiO2 was examined in terms of adsorption isotherms and influences of pH, ionic strength and heavy metallic cations. The nano-oxides had much higher adsorption capacities than bulk particles due to higher surface hydroxyl density. PFOS adsorption showed strong pH dependence due to different species of surface hydroxyl groups on nano-oxides. Besides electrostatic interaction, sulfonic group of PFOS possibly formed hydrogen bonds on the surface of nano-oxides. Because of the bridging effect in the co-adsorption process, the coexisting PFOS and heavy metallic cations greatly enhanced their adsorption onto the nano-oxides. Comparative adsorption of different perfluorinated sulfonates indicated the possible formation of bilayer PFOS adsorption on the nano-oxides, leading to the enhanced Cu(II) adsorption on the sulfonic groups of PFOS on the surfaces through electrostatic interaction.

Sorption of Cr(III) and Cr(VI) to High and Low Pressure Synthetic Nano-Magnetite (Fe3O4)Particles

The binding of Cr(III) and Cr(VI) to synthetic nano-magnetie particles synthesized under open vessel conditions and a microwave assisted hydrothermal synthesis techniques was investigated. Batch studies showed that the binding of both the Cr(III) and Cr(VI) bound to the nano-materials in a pH dependent manner. The Cr(III) maximized at binding at pH 4 and 100% binding. Similarly, the Cr(VI) ions showed a maximum binding of 100% at pH 4. The data from the time dependency studies showed for the most part the majority of the binding occurred within the first 5 minutes of contact with the nanomaterial and remained constant thereafter. In addition, the effects of the possible interferences were investigated which showed some effects on the binding of both Cr(III) and Cr(VI). However, the interferences never completely eliminated the chromium binding. Isotherm studies conducted at room temperature showed the microwave synthesized nanomaterials had a binding capacity of 1208 ± 43.9 mg/g and 555 ± 10.5 mg/g for Cr(VI) and Cr(III), respectively. However, the microwave assisted synthesized nanomaterials had capacities of 1705 ± 14.5 and 555± 10.5 mg/g for Cr(VI) and Cr(III), respectively. XANES studies showed the Cr(VI) was reduced to Cr(III), and the Cr(III) remained as Cr(III). In addition, the XANES studies indicated that the chromium remained coordinated in an octahedral arrangement of oxygen atoms.

A novel adsorbent of Na(2)Ta(2)O(6) porous microspheres with F(-) gradient concentration distribution: high cationic selectivity and well-regulated recycling

Pyrochlore Na2Ta2O6 porous microspheres with F(-) gradient concentration distribution were first prepared, which showed an excellent selectivity toward cationic dyes as an adsorbent. These dyes were regenerated rapidly by adding to NaAc solution. After then, the adsorbent still showed a high adsorption capacity. Optionally, the effective recycling of the adsorbents was achieved by UV light illumination, free of secondary environmental contamination. The rate of adsorption reaction followed the pseudo second-order kinetics, and the sorption isotherm well fitted to the Freundlich isotherm model. Eventually, the adsorption reaction for the absorbents was found to be a spontaneous and endothermic process.

Nanocrystalline Akaganeite as Adsorbent for Surfactant Removal from Aqueous Solutions

The present study presents the effective use of nanocrystalline akaganeite for the adsorption of an anionic (SDS), a cationic (CTAB), and a nonionic (tween80) surfactant from wastewater. Equilibrium experiments, as well as thermodynamic analysis, were performed. The maximum SDS adsorption occurs at the lowest pH value (5), the opposite is observed for CTAB (pH = 11), while for tween80, the change of pH value did not affect the adsorption. The equilibrium data could be described by Freundlich and Langmuir isotherms. The maximum adsorption capacity at 25 °C (pH = 8) was 823.96 mg/g for SDS, 1007.93 mg/g for CTAB, and 699.03 mg/g for tween80. The thermodynamic parameters revealed the exothermic and spontaneity nature of the process. Also, FTIR measurements established that surfactants are adsorbed on the surface of akaganeite, replacing adsorbed water.

Heavy metal removal from water/wastewater by nanosized metal oxides: a review

Nanosized metal oxides (NMOs), including nanosized ferric oxides, manganese oxides, aluminum oxides, titanium oxides, magnesium oxides and cerium oxides, provide high surface area and specific affinity for heavy metal adsorption from aqueous systems. To date, it has become a hot topic to develop new technologies to synthesize NMOs, to evaluate their removal of heavy metals under varying experimental conditions, to reveal the underlying mechanism responsible for metal removal based on modern analytical techniques (XAS, ATR-FT-IR, NMR, etc.) or mathematical models, and to develop metal oxide-based materials of better applicability for practical use (such as granular oxides or composite materials). The present review mainly focuses on NMOs' preparation, their physicochemical properties, adsorption characteristics and mechanism, as well as their application in heavy metal removal. In addition, porous host supported NMOs are particularly concerned because of their great advantages for practical application as compared to the original NMOs. Also, some magnetic NMOs were included due to their unique separation performance.