Simultaneous aggregation and oxidation of nZVI in Rushton equipped agitated vessel: Experimental and modelling

Nanoscale zero-valent iron immobilized by ZIF-8 metal-organic frameworks for enhanced removal of hexavalent chromium

nZVI is considered to be a promising material for environmental remediation. However, the drawbacks of easy agglomeration and low activity severely limit its application. In this work, nZVI/ZIF-8 was obtained by in-situ reduction of nZVI in the presence of performed ZIF-8. The reactivity of the as-obtained nZVI/ZIF-8 nanocomposites was investigated by removing hexavalent chromium (Cr(VI)) from wastewater. The as-obtained nZVI/ZIF-8 nanocomposites showed a superior activity for Cr(VI) removal, with an optimum activity (91.27%) achieved over 0.25 nZVI/ZIF-8 (i e., the mass ratio of ZIF-8 to nZVI was 0.25), higher than that of nZVI (64.55%), and this could be owned to the excellent dispersion of nZVI in nZVI/ZIF-8 and the high specific surface area as compared with the bare nZVI. The results of XPS characterization, quenching experiment analysis and kinetics fitting indicated that the Cr(VI) elimination was a surface-dominated chemical reduction process. Besides, more than 99.00% Cd(II), Cu(II), Cr(VI) and Pb(II) was removed from wastewater over nZVI/ZIF-8 nanocomposites, and negligible zinc ion was detected in the aqueous solutions. The results of our finding demonstrate that the introduction of MOFs is an effective strategy in developing a highly efficient nZVI-based nanocomposites system, and also highlight the promising role of using nZVI/MOFs in heavy metal treatment for practical wastewater.

Metallic iron (Fe0)-based materials for aqueous phosphate removal: A critical review

The discharge of excessive phosphate from wastewater sources into the aquatic environment has been identified as a major environmental threat responsible for eutrophication. It has become essential to develop efficient but affordable techniques to remove excess phosphate from wastewater before discharging into freshwater bodies. The use of metallic iron (Fe⁰) as a reactive agent for aqueous phosphate removal has received a wide attention. Fe⁰ in-situ generates positively charged iron corrosion products (FeCPs) at pH > 4.5, with high binding affinity for anionic phosphate. This study critically reviews the literature that focuses on the utilization of Fe⁰-based materials for aqueous phosphate removal. The fundamental science of aqueous iron corrosion and historical background of the application of Fe⁰ for phosphate removal are elucidated. The main mechanisms for phosphate removal are identified and extensively discussed based on the chemistry of the Fe⁰/H₂O system. This critical evaluation confirms that the removal process is highly influenced by several operational factors including contact time, Fe⁰ type, influent geochemistry, initial phosphate concentration, mixing conditions, and pH value. The difficulty in comparing independent results owing to diverse experimental conditions is highlighted. Moreover, contemporary research in progress including Fe⁰/oxidant systems, nano-Fe⁰ application, Fe⁰ material selection, desorption studies, and proper design of Fe⁰-based systems for improved phosphate removal have been discussed. Finally, potential strategies to close the loop in Fe⁰-based phosphate remediation systems are discussed. This review presents a science-based guide to optimize the efficient design of Fe⁰-based systems for phosphate removal.

Two-step ball milling-assisted synthesis of N-doped biochar loaded with ferrous sulfide for enhanced adsorptive removal of Cr(Ⅵ) and tetracycline from water

Nitrogen-doped biochar loaded with FeS (FeS@NBC_BM) was synthesized by two-step ball milling processes. Characterization results revealed that N-doping process successfully introduced pyridinic, pyrrolic, and graphitic N structures, and FeS was subsequently embedded in N-doped biochar (NBC_BM). The resultant FeS@NBC_BM presented predominant adsorption capacity for Cr(VI) (194.69 mg/g) and tetracycline (TC, 371.29 mg/g) compared with BC (27.28 and 37.89 mg/g) and NBC_BM (71.26 and 81.26 mg/g). In addition, the Cr(VI)/TC elimination process by FeS@NBC_BM was basically stable with multiple co-existing ions with slight decrease on adsorption performance after three desorption-regeneration cycles. Most importantly, FeS@NBC_BM was found to achieve Cr(VI) elimination not only by electrostatic attraction, ion exchange and complexation, but also by electrons-triggered reduction provided by different species of N, Fe²⁺ as well as S(Ⅱ). Meantime, pore filling, hydrogen bonding, and π-π stacking interactions were demonstrated to contribute to TC adsorption. These results suggested the co-modification of N-doping and FeS loading by ball milling as an innovative decorating method for biochar to adsorptive purification of Cr(VI) and TC-contaminated water.

Interaction of norfloxacin and hexavalent chromium with ferrihydrite nanoparticles: Synergistic adsorption and antagonistic aggregation behavior

The co-existence of hexavalent chromium (Cr(VI)) and norfloxacin (NOR) can be detected in natural environments. However, the interaction of the co-existing Cr(VI), NOR and ferrihydrite nanoparticles (FNPs, a ubiquitous natural iron oxide nanoparticle) is lacking investigation. Figuring out this interaction could help us better predict the transport and fate of the relevant contaminants. Here, the adsorption and aggregation of FNPs in the presence of Cr(VI) and NOR were investigated. Comparing to FNPs interaction with Cr(VI) or NOR alone, the co-existence of Cr(VI) and NOR could lead to a synergistic effect to increase their adsorption onto FNPs. This observation can be attributed to the complexation between Cr(VI) and carboxyl or amino groups from NOR. Furthermore, the aggregation of FNPs could be accelerated by Cr(VI) through charge neutralization since the adsorption of Cr(VI) could decrease the surface potential of FNPs (positive charge). However, the presence of NOR will increase the surface charge, and thus stabilize FNPs. In general, the aggregation state of FNPs in the presence of co-existing Cr(VI) and NOR depends on their ratio. Overall, these understandings help us predict the transport and fate of FNPs and the associated contaminants in natural environments.

Enhancing nitrate removal efficiency of micro-sized zero-valent iron by chitosan gel balls encapsulating

The activity of micro-sized zero-valent iron (MZVI) material for nitrate removal in neutral pH and low C/N ratios water needs to be improved. In this study, micro-sized zero-valent iron@chitosan (MZVI@CS) material was synthesized through embedding MZVI particles into chitosan (CS) gel by sol-gel method, and was used for deep removal of NO₃^--N in the absence of organic carbon sources and neutral pH. The NO₃^--N removal rate of MZVI@CS was 0.37 mg-N·L^-1·d^-1 (dosage of 1%, initial pH = 7, 25 °C, initial nitrate concentration = 15 mg-N·L^-1), which was 11.33 times higher than that of MZVI. The apparent activation energy (E_a) of MZVI@CS with nitrate was 38.23 kJ·mol^-1. MZVI@CS can remove nitrate effectively at a low concentration (15 mg-N·L^-1). A stable denitration rate (0.37-2.28 mg-N·L^-1·d^-1) could be maintained under weak acidic, neutral and alkaline conditions (pH = 5-9). More than 80% of reduced nitrate was converted to N₂, and only a small amount was converted to NH₄⁺ or NO₂^-. The gel structure of MZVI@CS eliminated the agglomeration between MZVI particles while the forming of Fe-CS chelates reduced the formation of iron oxide and solved the problems of passivation, hence successfully strengthened the NO₃^--N removal efficiency of MZVI. Therefore MZVI@CS has great application potential in NO₃^--N deep removal of water bodies with neutral pH and low C/N ratios.

Influence of aggregation and sedimentation behavior of bare and modified zero-valent-iron nanoparticles on the Cr(VI) removal under various groundwater chemistry conditions

Aggregation behaviors of bare, and sodium polyacrylate (PAA) and starch modified zero-valent-iron nanoparticles (nZVI), as well as their effects on the Cr (VI) removal were investigated by simulating the groundwater. Results showed that increased concentration of PAA (1-6 wt%) and starch (0.1-0.6 wt%) alleviated the aggregation of modified nZVI (abbreviated as P-nZVI and S-nZVI), while there was an optimum dosage of 4 wt% PAA and 0.3 wt% starch for the Cr (VI) removal, respectively. Moreover, as one of the fundamental water chemistry parameters, Ca²⁺ (0, 5, and 10 mg L^-1) greatly promoted the aggregation of modified nZVI, and decreased the Cr (VI) removal efficiency by them via forming bidentate bridging structure (between Ca²⁺ and PAA) or complexes (between Ca²⁺ and starch). Additionally, fulvic acid (FA) (0, 2, 5, and 10 mg L^-1) decreased the Cr (VI) removal by P-nZVI because of the significantly improved electronic repulsion. However, FA enhanced the aggregation of S-nZVI, but diminished its performance on Cr (VI) removal due to the bridging effect between FA and starch. The present study was of great importance in predicting the migration of nZVI and contaminants removal under complex geological conditions in groundwater.

Mechanistic insight into the adsorption of mercury (II) on the surface of red mud supported nanoscale zero-valent iron composite

Recently, nanoscale zero-valent iron (nZVI) particles have been efficiently used in the remediation of many heavy metals, yet potential agglomeration and loss of nZVI remain a critical area of research. In this study, we used red mud as a stable supporting medium to develop red mud modified nZVI to form (RM-nZVI) composite. We assessed its sorptive/reductive removal of mercury (Hg²⁺) from aqueous solutions. The RM-nZVI was synthesized through the reduction of ferric iron by sodium borohydride (NaBH₄) in the presence of red mud. Morphological characterization of RM-nZVI confirmed its diffusion state with lesser aggregation. The RM-nZVI has the BET surface area, pore diameter, and pore volume as 111.59 m²g^-1, 3.82 nm, and 0.49 cm³g^-1, respectively. Adsorption of mercury (Hg²⁺) by RM-nZVI exhibits pH-dependent behavior with increased removal of Hg²⁺ with the increase in pH up to 5, and the removal rate decreased gradually as the pH increased from 5 to 10. Extensive characterization of RM-nZVI corroborated the evidence that the removal of Hg²⁺ was initially by rapid physical adsorption, followed by a reduction of Hg²⁺ to Hg⁰. The adsorption data were best fitted with Langmuir isotherm with R² (correlation coefficient) > 0.99 with high uptake capacity of 94.58 (mg g^-1). The novel RM-nZVI composite with enhanced sorptive and reductive capacity is an ideal alternative for removing Hg²⁺ from contaminated water.

Antifungal Potential of Green Synthesized Magnetite Nanoparticles Black Coffee-Magnetite Nanoparticles Against Wilt Infection by Ameliorating Enzymatic Activity and Gene Expression in Solanum lycopersicum L

Tomato plants are prone to various biotic and abiotic stresses. Fusarium wilt is one of the most devasting diseases of tomatoes caused by Fusarium oxysporum f. sp. lycopersici, causing high yield and economic losses annually. Magnetite nanoparticles (Fe3O4 NPs) are one of the potent candidates to inhibit fungal infection by improving plant growth parameters. Spinach has been used as a starting material to synthesize green-synthesized iron oxide nanoparticles (IONPs). Various extracts, i.e., pomegranate juice, white vinegar, pomegranate peel, black coffee (BC), aloe vera peel, and aspirin, had been used as reducing/stabilizing agents to tune the properties of the Fe3O4 NPs. After utilizing spinach as a precursor and BC as a reducing agent, the X-ray diffraction (XRD) pattern showed cubic magnetite (Fe3O4) phase. Spherical-shaped nanoparticles (∼20 nm) with superparamagnetic nature indicated by scanning electron microscopy (SEM) monographs, whereas energy-dispersive X-ray gives good elemental composition in Fe3O4 NPs. A characteristic band of Fe-O at ∼ 561 cm-1 was exhibited by the Fourier transform infrared (FTIR) spectrum. X-ray photoelectron spectroscopy (XPS) results confirmed the binding energies of Fe 2p3/2 (∼710.9 eV) and Fe 2p1/2 (∼724.5 eV) while, Raman bands at ∼310 cm-1 (T2 g ), ∼550 cm-1 (T2 g ), and 670 cm-1 (A1 g ) indicated the formation of Fe3O4 NPs synthesized using BC extract. The in vitro activity of BC-Fe3O4 NPs significantly inhibited the mycelial growth of F. oxysporum both at the third and seventh day after incubation, in a dose-dependent manner. In vivo studies also exhibited a substantial reduction in disease severity and incidence by improving plant growth parameters after treatment with different concentrations of BC-Fe3O4 NPs. The increasing tendency in enzymatic activities had been measured after treatment with different concentrations of NPs both in roots and shoot of tomato plants as compared to the control. Correspondingly, the upregulation of PR-proteins and defense genes are in line with the results of the enzymatic activities. The outcome of the present findings suggests that Fe3O4 NPs has the potential to control wilt infection by enhancing plant growth. Hence, Fe3O4 NPs, being non-phytotoxic, have impending scope in the agriculture sector to attain higher yield by managing plant diseases.

Synergistic removal of Cr(VI) by S-nZVI and organic acids: The enhanced electron selectivity and pH-dependent promotion mechanisms

The effects of organic acids on hexavalent chromium (Cr(VI)) removal by reduced iron-based materials have been extensively studied. Nevertheless, the promotion mechanism from the perspective of the electron transfer process is still unclear. Herein, sulfidated nanoscale zero-valent iron (S-nZVI) and the selected organic acids, citric acid (containing both -OH and -COOH groups) and oxalic acid (containing only -COOH groups), showed significant synergistic promotion effects in Cr(VI) removal. The FeS and FeS₂ on S-nZVI surface could enhance the Cr(VI) reduction as the reductive entity and electron conductor. Furthermore, even though the reactivity of FeS with Cr(VI) is higher than that with FeS₂, the Cr(VI) removal efficiency by FeS₂ was much higher than that by FeS with organic acids. Under neutral and alkaline conditions (pH 6.0-8.0), organic acids promoted the diffusion, adsorption and complexation of Cr(VI) on S-nZVI surface, thus enhancing the electron selectivity towards Cr(VI). However, when the solution pH changed to acidic conditions (pH 4.0), organic acids facilitated the dissolution of Fe(II) ions from S-nZVI and enhanced the electron utilization towards Cr(VI) via the fast Fe(III) reduction process. This study provided a new insight into the Cr(VI) removal, which was beneficial to understand the application boundaries of S-nZVI for Cr(VI) remediation.

Highly efficient removal of Cr(VI) from aqueous solution by pinecone biochar supported nanoscale zero-valent iron coupling with Shewanella oneidensis MR-1

Nanoscale zero-valent iron (nZVI) has been extensively used to remove various pollutants. However, the rapid deactivation due to aggregation and surface passivation severely limits its practical application. In this study, a novel composite with nZVI supported by pinecone biochar (nZVI-PBC) was successfully synthesized and used for the removal of high concentration Cr(VI) from aqueous solution in the presence of Shewanella oneidensis MR-1 (MR-1). The results showed that the nZVI-PBC coupling with MR-1 (nZVI-PBC/MR-1) exhibited an excellent removal performance for high concentration Cr(VI) compared to the nZVI-PBC alone. Under optimal conditions, 100 mg/L Cr(VI) could be removed completely by nZVI-PBC/MR-1 within 48 h, while only 39.50% of Cr(VI) was removed by nZVI-PBC alone. The improvement of Cr(VI) removal is due to the dissolution of the surface passivation layer of nZVI-PBC, formation of sorbed Fe(II) in the presence of MR-1, and an important role of extracellular polymeric substance (EPS) derived from MR-1. X-ray photoelectron spectroscopy (XPS) and Cr K-edge X-ray absorption near-edge structure spectra (XANES) confirmed that most Cr(VI) was reduced to insoluble Cr(III) and formed Cr₂O₃, Cr_xFe_1-x(OH)₃ and FeCr₂O₄ precipitates, and a small amount of unreduced Cr(VI) was immobilized through adsorption and complexation. The results suggest that nZVI-PBC/MR-1 can effectively overcome the limitations of nZVI and achieve highly efficient removal of high concentration Cr(VI).

An efficient, economical, and easy mass production biochar supported zero-valent iron composite derived from direct-reduction natural goethite for Cu(II) and Cr(VI) remove

In this study, a novel biochar-supported zero-valent iron (ZVI) composite was synthesised by a one-pot co-pyrolysis reduction method, and was used to remove Cu(II) and Cr(VI). The raw materials for the composite were derived from natural bagasse/straw and goethite. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, Fourier-transform infrared (FTIR) spectroscopy, thermogravimetry (TG), and Brunauer-Emmett-Teller (BET) analysis were used to characterise the biochar and biochar-supported ZVI composites. Batch removal experiments on the effects of the initial pH and citric acid concentrations were performed as well as kinetic studies and isotherm experiments. The composite materials showed better Cu(II) and Cr(VI) removal performance than single biochar and mineral. The removal of Cu(II) and Cr(VI) is pH-dependent, and proceeds via heterogeneous multilayer chemisorption. Electrochemical analysis revealed that straw biochar-supported ZVI composite exhibited greater electrical conductivity and electron transfer rate than pure biochar and ZVI. FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) elucidated the uptake mechanism, showing that Cu(II) and Cr(VI) were easily adsorbed onto the biochar surface and were then reduced by ZVI. These results indicate that biochar-supported ZVI composite is effective for heavy metal remediation, which is economical, environment-friendly, and suitable for mass production.

A comparative study on high-efficient reduction of bromate in neutral solution using zero-valent Al treated by different procedures

Bromate, a toxic by-product of bromide-containing drinking water after disinfecting with ozone, has attracted much attention in the past two decades. Traditional methods to activate zero-valent metals for reducing bromate are to eliminate their surface oxide layer by acid washing. In this work, for the first time, zero-valent Al (ZVAl) was surface treated by the following procedures including soaking, soaking and freeze-drying, soaking and heat-treating, and γ-Al₂O₃ covering Al particle surfaces (GCAP). It was found that all of above surface treated ZVAls have an obvious high efficiency for bromate reduction relative to pristine ZVAl. The bromate reduction rate is GCAP > soaking Al > freeze-drying Al > soaking and heat-treating Al > pristine Al, and using GCAP just 30 min is taken to completely reduce bromate to bromide in neutral solution. Mechanism analyses revealed that Al surface treating or covered by fine γ-Al₂O₃ phase can promote the hydration and breakage of Al surface passive oxide layer, resulting in a fast contact of inner Al with outside ions, leading to a high reduction rate of bromate in neutral solution. XPS analyses indicated that there are no bromate or bromide ions adsorbed on Al particle surfaces, implying that there is a high direct donating efficiency of electrons from inner Al to bromate ions in solution. Furthermore, GCAP has a good reusability and >90% bromate can be reduced even it was reused up to 4 cycles.

Simply closing the reactor improves the electron efficiency of zerovalent iron toward various metal(loid)s removal

The controlled corrosion of zerovalent iron (ZVI) is crucial for the favorable performance of ZVI toward metal(loid)s removal, and dissolved oxygen (DO) plays an important role in the process of ZVI corrosion. However, few efforts have been made to control the concentration of DO in real practice. In this study, we found that the electron efficiency and the specific removal capacity of ZVI toward the removal of four metal(loid)s were increased by 1.2-9.1 times and 1.2-3.6 times, respectively, by simply closing the reactor, while the removal kinetics of metal(loid)s was slightly influenced. The rate constants obtained under open condition were always greater than those obtained under closed condition, and the removal amounts of metal(loid)s by ZVI at the reaction equilibrium under closed condition were nearly equivalent to those under open condition. Compared with the case under open condition, the consumption-redissolution process of DO was decelerated under closed condition, and the rapid corrosion of ZVI was alleviated subsequently. Although closing the reactor is simple, it does contribute much to the favorable electron efficiency of ZVI toward metal(loid)s sequestration and can be easily adopted in real practice. © 2021 Water Environment Federation PRACTITIONER POINTS: Closing the reactor promoted the selectivity of ZVI towards four metal(loid)s removal. The consumption-redissolution process of DO and corrosion of ZVI were decelerated by closing the reactor. Metal(loid)s were reduced to lower valence by ZVI under closed condition. Effect of DO was different when ZVI was applied to remove different metal(loid)s.

A novel preparation of S-nZVI and its high efficient removal of Cr(VI) in aqueous solution

The chitosan-stabilized biochar supported S-nZVI (CS@BC/S-nZVI) composite with low aggregation and superior antioxidation were successfully synthesized by liquid-phase reduction method for the outstanding removal of Cr(VI) from wastewater and characterized by SEM, BET, FTIR, XRD, and XPS. The optimized synthesis parameters of CS@BC/S-nZVI were determined as a 0.14 molar ratio of S/Fe and a 0.25 mass ratio of BC/Fe. The CS@BC/S-nZVI possessed a specific surface area of 199.246 m²/g and an average pore size and pore volume of 1.186 nm and 0.272 cc/g. The CS@BC/S-nZVI could remain reductive activity after Cr(VI) removal and present a remarkable tolerance to the coexisting ions during Cr(VI) removal. The adsorption data were fitted well by the pseudo-second order model and the Langmuir model. The removal of Cr(VI) by CS@BC/S-nZVI was an exothermic process with prominent Cr(VI) removal capacities of 244.07 mg/g at 120 min and 221.84 mg/g at 15 min at 25 ℃. Further mechanism analysis proved that the binding of Cr(VI) to CS@BC/S-nZVI was mainly a synergistic effect of reduction and electrostatic attraction. Overall, these findings shed new light on the research of a novel S-nZVI compound and revealed the potential practical application of CS@BC/S-nZVI in the future heavy metal removal from wastewater.

Modified nanoscale zero-valent iron in persulfate activation for organic pollution remediation: a review

Under the action of different activators, persulfate can produce sulfate radicals (SO₄·^-) with strong oxidizing ability, which can destruct many organic compounds. Meanwhile, persulfate is widely used in groundwater and soil remediation because of its fast reaction and wide application. With the high specific surface area and reactivity of nanoscale zero-valent iron (nZVI), it can enhance the degradation efficiency of the persulfate system on organic pollutants in soil and water as a persulfate activator. However, nZVI is easy to get oxidized and has a tendency to aggregation. To solve these problems, a variety of nZVI modification methods have been put forward and put into to applications in the activation of persulfate. This article will give a systematic introduction of the background and problems of nZVI-activated persulfate in the remediation of organic pollution. In addition, the modification methods and mechanisms of nZVI are summarized, and the applications and progress of modified nZVI-activated persulfate are reviewed. The factors that affect the removal of organic compounds by the activation system are discussed as well. Worldwide, the field studies and full-scale remediation using modified nZVI in persulfate activation are yet limited. However, the already known cases reveal the good prospect of applying modified nZVI in persulfate activation to organic pollution remediation.

Vitamin C mediates the activation of green tea extract to modify nanozero-valent iron composites: Enhanced transport in heterogeneous porous media and the removal of hexavalent chromium

Both green tea (GT) extract and vitamin C (VC) were used for the reduction of Fe³⁺ to Fe⁰ using a green synthesis method. Modified nanozero-valent iron (GT-nZVI@VC nanocomposites) was successfully obtained and characterized as α-Fe⁰-iron oxide/VC by multiple analytical methods. The GT-nZVI@VC nanocomposites showed better transportability than nZVI, in that transport behavior was slightly dependent on various ratios of sand/soil in water-saturated heterogeneous porous media. Breakthrough curves of GT-nZVI@VC nanocomposites in paddy soil exhibited "blocking effects" and were well described using a first-order straining coefficient (k₂) on site 2 obtained from a two-site kinetic attachment model. In particular, GT-nZVI@VC (VC/Fe = 0.6) showed higher Cr(VI) removal (especially reducibility) in both paddy soil and water compared to that of nZVI and VC. It is likely that the synergistic effects of VC (ascorbic acid) and tea polyphenols can increase the released free electrons into solution, favoring the high reduction of Cr(VI) into Cr(III) (i.e., FeOCr₂O₃, Cr(OH)₃ and Cr₂O₃), where Cr(III) is prone to be immobilized by the nanocomposites in soil. This research highlights that VC can mediate the activation of GT extract to successfully modify nZVI, which could be beneficial for efficient transport in subsurface and remediation of Cr(VI)-contaminated soil and underground water.

Facile green synthesis of zero-valent iron nanoparticles using barberry leaf extract (GnZVI@BLE) for photocatalytic reduction of hexavalent chromium

In this study, zero-valent iron (GnZVI) was synthesized using barberry leaf extract (GnZVI@BLE). The physicochemical properties of the final products were characterized by FT-IR, SEM, TEM, and EDS techniques. The results of TEM analysis showed that the obtained iron zero-valent nanoparticles with a diameter between 20 and 40 nm and shell-core structures were successfully synthesized. The results of FT-IR confirmed the presence of various functional groups. The photocatalytic activity of synthesized nanoparticles was investigated by reduction of hexavalent chromium. Laboratory data showed that the presence of GnZVI@BLE as a nanocatalyst in the photocatalytic process could be reduction the hexavalent chromium (Cr (VI)). Photocatalytic data revealed that, when the dosage of nanoparticles was 0.675 g/L, the reduction efficiency of hexavalent chromium was 100%. The kinetics of the reaction follows a pseudo-second-order equation. The constant of reaction rate was 0.4 at pH 2 and 0.5 g/L concentration of GnZVI@BLE.

Nanoparticles-EPS corona increases the accumulation of heavy metals and biotoxicity of nanoparticles

Microbial extracellular polymeric substances (EPS) coating nanoparticles (NPs) surface can form NPs-EPS corona, which significantly affect the adsorption of NPs to toxic substances and alter the ecotoxicological effect of NPs. In this work, the EPS coronas on TiO₂ NPs (TNPs) and CeO₂ NPs (CNPs) were characterized and the adsorption characteristics of NPs with and without EPS corona to five heavy metals were investigated in single-metal and multiple-metal systems. The results of spectral analysis showed that NPs-EPS corona exhibited new crystalline phases and abundant functional groups. Moreover, 42 and 13 proteins were identified in the TNPs-EPS and CNPs-EPS coronas, respectively. The rates of Cd²⁺, Pb²⁺, Cu²⁺, Ni²⁺ and Ag⁺ adsorption by NPs-EPS corona increased to values that were 6.7-7.6, 4.4-5.1, 4.2-5.5, 3.9-4.9 and 8.5-8.8 times those of NPs without EPS corona, respectively, in single-metal system. NPs-EPS coronas are effective in absorbing Ag⁺, Pb²⁺ and Cu²⁺ compared with Cd²⁺and Ni²⁺ in multiple metal adsorption. These results indicated that NPs-EPS corona effectively adsorb and remove heavy metals by forming NPs-EPS-metal complexes and inducing precipitation. However, NPs-EPS corona can enhance the toxicity of NPs by accumulating highly-toxic heavy metals in aquatic environments.

Computational study and optimization experiment of nZVI modified by anionic and cationic polymer for Cr(VI) stabilization in soil: Kinetics and response surface methodology (RSM)

Nanoscale zero-valent iron (nZVI) modified by cationic polyquaternium-7 (M550-nZVI) or anionic carboxymethyl cellulose (CMC-nZVI) were freshly synthesized, and followed by the successful applicability for the stabilization of Cr(VI) in soil. Scanning electron microscope (SEM) showed that the sizes of M550-nZVI and CMC-nZVI were 42-170 nm and 66-200 nm, respectively. X-ray diffraction (XRD) confirmed the presence of Fe⁰ and Fe₃C in the as-synthesized composites. The kinetics were well fitted with pseudo-second order model (R² > 0.99), indicating that the process was principally chemical reduction. Additionally, we observed that M550-nZVI had better resistance to oxidation than that of CMC-nZVI. Besides, RSM experiments showed that acetate ion (AA) could promote the Cr(VI) removal but humic acid ion (HA) and carbonate ion (CA) resulted in negative effects. Moreover, the modeling predication revealed that the optimum Cr(VI) removal of 92.44% by CMC-nZVI was available, being 22.52% higher than that of M550-nZVI. In conclusion, this work demonstrated that the inoxidizability of M550-nZVI had a dominant advantage, while CMC-nZVI had the more excellent reactivity than M550-nZVI. We believe that our conducted research work will open the new avenues for effective removal of heavy metals from the soil.

Green synthesis of iron-based nanoparticles using Chlorophytum comosum leaf extract: methyl orange dye degradation and antimicrobial properties

Nowadays, green synthesis methods have gained growing attention in nanotechnology owning to their versatile features including high efficiency, cost-effectiveness, and eco-friendliness. Here, the aqueous extract of Chlorophytum comosum leaf was applied for the preparation of iron nanoparticles (INPs) to obtain spherical and amorphous INPs with a particle size below 100 nm as confirmed by TEM. The synthesized INPs managed to eliminate methyl orange (MO) from the aqueous solution. The concentration of MO can be easily checked via ultraviolet-visible (UV-Vis) spectroscopy throughout the usage of INPs at the presence of H2O2. The synthesized INPs exhibited MO degradation efficiency of 77% after 6 h. Furthermore, the synthesized INPs exhibited antibacterial activity against both Gram-negative and Gram-positive bacteria. The prepared INPs have an impressive effect on Staphylococcus aureus at concentrations below 6 μg/ml. Overall, the synthesized INPs could considerably contribute to our combat against organic dyes and bacteria.

Ultrasound-assisted catalytic reduction of Cr(VI) by an acid mine drainage based nZVI coupling with FeS2 system from aqueous solutions: Performance and mechanism

Nowadays, nanoscale zero valent iron (nZVI) has been extensively applied for the decontamination of various pollutants, but passivation of nZVI severely affects its reactivity in use. In this study, ultrasound (US)-assisted catalytic reduction of Cr(VI) by an acid mine drainage based nZVI (AMD-nZVI) coupling with FeS₂ system was systematically examined. Results show that the presence of FeS₂ and US induced a synergistic enhancement of Cr(VI) removal by AMD-nZVI. Nearly 98% of Cr(VI) removal was achieved by AMD-nZVI/FeS₂/US process within 60 min under optimal reaction conditions. Several coexisting substances with lower concentration including Pb(II), Ni(II), bisphenol A (BPA) and 2,4-diclorophenol (2,4-DCP) could be effectively removed in simultaneous manner with Cr(VI) removal. The inhibitory order of water matrix species on Cr(VI) removal was NO₃^- > PO₄^3- > HCO₃^- > Ca²⁺ > Mg²⁺ > Cl^-, and a serious suppression effect was induced by humic acid (HA). Addition of ethylene diamine tetra-acetic acid (EDTA) and citric acid (CA) could enhance Cr(VI) removal rate. An enhanced reaction mechanism was proposed, which involved the regeneration of more Fe²⁺ and H⁺ by AMD-nZVI/FeS₂/US process, leading to the reduction of Cr(VI) by AMD-nZVI and FeS₂ into Cr(III) species inculding Cr₂O₃ and Cr(OH)₃. This study well demonstrates that AMD-nZVI/FeS₂/US process is considered as a potential candidate for the remediation of Cr(VI) in real wasterwater.

Degradation of azo dye Acid Red 14 (AR14) from aqueous solution using H2 O2 /nZVI and S2 O8 2- /nZVI processes in the presence of UV irradiation

Azo dyes are mostly toxic and carcinogenic and cause harm to humans and the environment. This study was conducted to investigate the degradation of azo dye acid red 14 (AR14) from aqueous solution using hydrogen peroxide (H₂ O₂₎ /nano zerovalent iron (nZVI) and persulfate (S₂ O₈ ^2- )/nZVI processes in the presence of ultraviolet (UV) irradiation. This experimental study was carried out in a laboratory-scale batch photoreactor with a useful volume of 1 L. The nZVI was synthesized by the sodium borohydride (NaBH₄ ) reduction method. In these processes, the effects of parameters including initial pH, H₂ O₂ concentration, S₂ O₈ ^2- concentration, nZVI dose, concentration of AR14 dye, and reaction time were studied. The results showed that decolorization increased by increasing the nZVI dosage, H₂ O₂ and S₂ O₄ ^2- concentrations, and reaction time, or decreasing dye concentration and pH. However, a too high oxidant concentration (H₂ O₂ and S₂ O₄ ^2- ) could inhibit the degradation. The experimental conditions for degradation of AR14 by UV/S₂ O₈ ^2- /nZVI and UV/H₂ O₂ /nZVI processes were as follows: [H₂ O₂ ] = 10 mM, [S₂ O₈ ^2- ] = 8 mM, AB14 dye = 100 mg/L, pH = 3, and nZVI dose = 0.05 g. Under these conditions, the highest removal efficiencies of AR14, chemical oxygen demand (COD), and total organic carbon (TOC) for the UV/S₂ O₈ ^2- /nZVI process were 93.94%, 86.5%, and 81.6%, respectively, while these values were 89.3%, 79.57%, and 72.9% for the UV/H₂ O₂ /nZVI, respectively. Also, the average oxidation state (AOS) was decreased from 2.93 to 2.14 in the effluent of the UV/S₂ O₈ ^2- /nZVI process and from 2.93 to 2.2 for the UV/H₂ O₂ /nZVI process. The results showed that the ratio of biochemical oxygen demand (BOD₅ ) to COD in the effluents of the UV/S₂ O₈ ² /nZVI and UV/H₂ O₂ /nZVI processes after 90 min was 0.63 and 0.74, respectively. These findings suggest biodegradability improvement. PRACTITIONER POINTS: Photocatalytic degradation of azo dye Acid Red 14 (AR14) was achieved using H₂ O₂ /nZVI and S₂ O₈ ^2- /nZVI processes in the presence of UV irradiation. Effects of operating parameters on photocatalytic degradation AR14 dye were evaluated in the UV/H₂ O₂ /nZVI and UV/S₂ O₈ ^2- /nZVI processes. Biodegradability and mineralization studies of AR14 dye photocatalytic degradation were performed for the UV/H₂ O₂ /nZVI and UV/S₂ O₈ ^2- /nZVI processes.

ZnO Nano-Particles Production Intensification by Means of a Spinning Disk Reactor

Zinc Oxide is widely used in many industrial sectors, ranging from photocatalysis, rubber, ceramic, medicine, and pigment, to food and cream additive. The global market is estimated to be USD 3600M yearly, with a global production of 10 Mt. In novel applications, size and shape may sensibly increase the efficiency and a new nano-ZnO market is taking the lead (USD 2000M yearly with a capacity of 1 Mt and an expected Compound Annual Growth Rate of 20%/year). The aim of this work was to investigate the possibility of producing zinc oxide nanoparticles by means of a spinning disk reactor (SDR). A lab-scale spinning disk reactor, previously used to produce other nanomaterials such as hydroxyapatite or titania, has been investigated with the aim of producing needle-shaped zinc oxide nanoparticles. At nanoscale and with this shape, the zinc oxide particles exhibit their greatest photoactivity and active area, both increasing the efficiency of photocatalysis and ultraviolet (UV) absorbance. Working at different operating conditions, such as at different disk rotational velocity, inlet distance from the disk center, initial concentration of Zn precursor and base solution, and inlet reagent solution flowrate, in certain conditions, a unimodal size distribution and an average dimension of approximately 56 nm was obtained. The spinning disk reactor permits a continuous production of nanoparticles with a capacity of 57 kg/d, adopting an initial Zn-precursor concentration of 0.5 M and a total inlet flowrate of 1 L/min. Product size appears to be controllable, and a lower average dimension (47 nm), adopting an initial Zn-precursor concentration of 0.02 M and a total inlet flow-rate of 0.1 L/min, can be obtained, scarifying productivity (0.23 kg/d). Ultimately, the spinning disk reactor qualifies as a process-intensified equipment for targeted zinc oxide nanoparticle production in shape in size.

Preparation of Activated Carbon Supported Bead String Structure Nano Zero Valent Iron in a Polyethylene Glycol-Aqueous Solution and Its Efficient Treatment of Cr(VI) Wastewater

Nanometer zero-valent iron (nZVI) has been widely used in the treatment of heavy metals such as hexavalent chromium (Cr(VI)). A novel composite of bead string-structured nZVI on modified activated carbon (nZVI–MAC) is prepared here, using polyethylene glycol as the stable dispersant rather than traditional ethanol during the loading process. The microstructure characterization shows that nZVI particles are loaded on MAC with a bead string structure in large quantity and stably due to the addition of hydroxyl functional groups on the surface by polyethylene glycol. Experiments on the treatment of Cr(VI) in wastewater show that the reaction process requires only 20 min to achieve equilibrium. The removal rate of Cr(VI) with a low concentration (80–100 mg/L) is over 99% and the maximum saturation removal capacity is up to 66 mg/g. The system converts Cr(VI) to trivalent chromium (Cr(III)) through an oxidation-reduction effect and forms an insoluble material with iron ions by coprecipitation, which is then adsorbed on the surface of the nZVI–MAC. The process conforms to the quasi-second order adsorption kinetics equation (mainly chemical adsorption process).