Mathematical modelling of simultaneous nitrate and dissolved oxygen reduction by Cu-nZVI using a bi-component shrinking core model

Superior reduction and immobilization of Cr(VI) in soil utilizing sulfide nanoscale zero-valent iron supported by phosphoric acid-modified biochar: Efficiency and mechanism investigation

A novel strategy was proposed to remediate Cr(VI)-contaminated soil via phosphoric acid-modified biochar supported sulfide nanoscale zero-valent iron (SnZVI@PBC). Results of characterizations revealed that FeSX shell existed in outer layer of nZVI to prevent its oxidation after sulfidation modification, and SnZVI was effectively dispersed owing to the support of PBC, accelerating the electron transport for Cr(VI) reduction. The SnZVI@PBC presented pH-dependence and fast capture for Cr(VI) with outstanding binding amount of 335.55 mg/g. More importantly, the Cr(VI) content declined from 1300.75 to 223.30 mg/kg with conversion into stable Cr(III) in soil after 42 d of remediation with 2.0 % SnZVI@PBC under 60 % moisture content. Furthermore, leaching experiments showed that SnZVI@PBC could effectively immobilize Cr(VI), decreasing its migration and harmful risks to plants and human. Particularly, the fractions of exchangeable and carbonate-bound Cr decreased by 96.77 % and 83.60 %, which transformed to relatively stable fractions. Interestingly, the presence of humic acid, and the freezing-thawing/wetting-drying process promoted the immobilization performance of SnZVI@PBC for Cr(VI). SnZVI@PBC could alleviate the migration and poisonousness of Cr(VI) in soil primarily via reduction, co-precipitation, pore filling, and electrostatic attraction. Overall, SnZVI@PBC could be considered as a feasible amendment with superior reducing capacity and immobilization performance for Cr(VI)-contaminated soil.

Accelerated Cr (VI) removal by a three-dimensional electro-Fenton system using green iron nanoparticles

Green-synthesized iron nanoparticles (GAP-FeNP) were used as particle electrodes in a three-dimensional electro-Fenton (3DEF) process to accelerate the removal of hexavalent chromium [Cr (VI)]. Removal was evaluated by varying the pH (3.0, 6.0, and 9.0) and initial Cr (VI) concentrations (10, 30, and 50 mg/L) at 5 and 25 min. These results demonstrated that GAP-FeNP/3DEF treatment achieved more than 94% Cr (VI) removal under all tested conditions. Furthermore, it was observed that Cr (VI) removal exceeded 98% under pH 9.0 in all experimental parameters tested. The results of the response surface methodology (RSM) determined two optimal conditions: the first, characterized by a pH of 3.0, Cr (VI) concentration at 50 mg/L, and 25 min, yielded a Cr (VI) removal of 99.7%. The second optimal condition emerged at pH 9.0, with Cr (VI) concentrations of 10 mg/L and 5 min, achieving a Cr (VI) removal of 99.5%. This study highlights the potential of the GAP-FeNP to synergistically accelerate Cr (VI) removal by the 3DEF process, allowing faster elimination and expansion of the alkaline (pH 9.0) applicability. PRACTITIONER POINTS: The required time for >99% of Cr (VI) removal by the GAP-FeNP/3DEF process was shortened from 25 to 5 min. EF process with GAP-FeNP reduces the time necessary for Cr (VI) removal, which is 67% faster than conventional methods. EF process using GAP-FeNP removed >94% of Cr (VI) after 25 min for all initial Cr (VI) concentrations and pH treatments. Cr (VI) removal by the GAP-FeNP/3DEF process was >98% at a pH of 9.0, widening the solution pH applicability.

Mechanistic insights into sulfate and phosphate-mediated hexavalent chromium removal by tea polyphenols wrapped nano-zero-valent iron

Nano zero-valent iron via green synthesis (g-nZVI) has great potential in removing toxic hexavalent Cr(VI) from industrial wastewater. Sulfate and phosphate in wastewater can influence Cr(VI) removal by g-nZVI. In this study, the Cr(VI) removal kinetics by different g-nZVI materials were investigated with the existence of sulfate and/or phosphate, and the corresponding mechanisms were first revealed using multiple characterizations, including X-ray absorption near-edge spectra (XANES) and X-ray photoelectron spectroscopy (XPS). The results showed that Cr(OH)₃ was the dominant species initially formed on the surface of g-nZVI particles before transforming to Cr₂O₃ during the reaction of g-nZVI with Cr(VI). Sulfate in wastewater can promote the reduction from Cr(VI) to Cr(OH)₃ by g-nZVI, because sulfate triggers the release of Fe(II) and tea polyphenols (from tea extracts) from the g-nZVI surface due to the corrosion of Fe⁰ core, which is in line with an obvious increase in pseudo-second-order rate constant (k₂) and subtle change in Cr(VI) removal capacity (q_e). However, phosphate impedes the g-nZVI corrosion and inhibits q_e because of the inner-sphere complexation of phosphate onto g-nZVI decreasing the released Fe(II) for Cr₂O₃ production. When sulfate and phosphate coexisted in contaminated water, the inhibition effect of phosphate in Cr(VI) removal by g-nZVI was stronger than the promotion of sulfate. Accordingly, q_e value of g-nZVI declined from 93.4 mg g^-1 to 77.5 mg g^-1, while k₂ remained constant as the molar ratio of phosphate/sulfate increased from 0.1 to 10 in water. This study provides new insights into applying g-nZVI in efficient Cr(VI) removal from contaminated water with enrichment of sulphates and phosphates.

Enhanced selective nitrate-to-nitrogen reduction by aerosol-assisted iron-carbon composites: Insights into the key factors

In this study, an aerosol-assisted Fe⁰/C (carbon supported zero-valent iron) composite was prepared and evaluated, which could effectively remove nitrate and exhibit high nitrogen selectivity. The results show that the selectivity of nitrogen for freshly prepared Fe⁰/C composites could reach 52.2% when pH at 7, compared to that of 7.7% for traditional nZVI. Meanwhile, the removal efficiency of nitrate was slightly increased from 63.5% to 69.9%. Furthermore, a variety of methods such as SEM, TEM, XRD, XPS, BET, FTIR and TGA were used to characterize the Fe⁰/C composites before and after reaction. Hence, the following key factors were determined for the effective conversion from nitrate to nitrogen: the surface of zero valent iron particle should be protected from oxidation and its genuine characteristics are well retained; the reaction should be controlled under an anaerobic condition; and the carbon as the carrier to support iron particles is very important; lower initial pH favors nitrogen generation. Various materials including aged Fe⁰/C composites, Fe⁰/SiO₂ (SiO₂ supported zero-valent iron) composites and nZVI particles in the deoxygenated and oxygenated systems were assessed for comparison.

Electrochemical water softening as pretreatment for nitrate electro bioremediation

Electro bioremediation is gaining interest as a sustainable treatment for contaminated groundwater. Nevertheless, the investigation is still at the laboratory level, and before their implementation is necessary to overcome important drawbacks. A prevalent issue is the high groundwater hardness that generates scale deposition on electrodes that irreversibly affects the treatment effectiveness and their lifetime. For this reason, the present study evaluated a novel and sustainable approach combining electrochemical water softening as a preliminary step for electro bioremediation of nitrate-contaminated groundwater. Batch mode tests were performed at mL-scale to determine the optimum reactor configuration (single- or two-chambers) and the suitable applied cathode potential for electrochemical softening. A single-chamber reactor working at a cathode potential of -1.2 V vs. Ag/AgCl was chosen. Continuous groundwater softening under this configuration achieved a hardness removal efficiency of 64 ± 4% at a rate of 305 ± 17 mg CaCO₃ m^-2_cathode h^-1. The saturation index at the effluent of the main minerals susceptible to precipitate (aragonite, calcite, and brucite) was reduced up to 90%. Softening activity plummeted after 13 days of operation due to precipitate deposition (mostly calcite) on the cathode surface. Polarity reversal periods were considered to detach the precipitated throughout the continuous operation. Their implementation every 3-4 days increased the softening lifetime by 48%, keeping a stable hardness removal efficiency. The nitrate content of softened groundwater was removed in an electro bioremediation system at a rate of 1269 ± 30 g NO₃^- m^-3_NCC d^-1 (97% nitrate removal efficiency). The energy consumption of the integrated system (1.4 kWh m^-3_treated) confirmed the competitiveness of the combined treatment and paves the ground for scaling up the process.

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.

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.

Metal Nanoparticles Formation from Nickel Hydroxide

In this study, the mechanism of nickel nanoparticle formation from its hydroxide was analyzed. Metallic nickel nanoparticles were obtained through the hydroxide’s reduction under hydrogen. Nickel hydroxides were produced from nickel (II) nitrate hexahydrate and NaOH by deposition under various initial conditions. The influence of washing treatment on the dispersion of obtained nickel powders was studied. The washing procedure of precipitates was carried out by centrifugation, ultrasonic treatment, and decantation. X-ray diffractometry, transmission electron microscopy, low-temperature nitrogen adsorption, infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy methods were used for nanoparticle characterization. Based on the resulting data, a model of the Ni(OH)₂ aggregate structure after deposition was proposed. The number of nickel hydroxide particles required to form one nickel nanoparticle was estimated, and a model of its formation was proposed.

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.

Modification of Lignocellulosic Materials with a Mixture of m-DMDHEU/Choline Chloride to Remove CrO4 2-, NO3 -, and H2AsO4 - in Aqueous Solution

A new denaturation agent is the mixture of 4,5-dihydroxy-1,3-bis(methoxymethyl)imidazolidin-2-one (m-DMDHEU)/choline chloride (CC) introduced to modify three kinds of lignocellulosic materials containing different lignin contents in the following order: cotton used in medicine < sawdust from acacia auriculiformis wood < powder from the coconut shell. The modification process is carried out through two main steps: 0.2 N NaOH solution with 70% v/v ethanol and 30% v/v water was applied to remove lignin and activate the initial raw materials, and then delignified materials were modified with m-DMDHEU/CC by using a parched heat supply method after chemical impregnation. Structural characterictics and physicochemical properties of modified materials were tested and dissected by scanning electron microscopy, Fourier transform infrared spectroscopy, solid-state ¹³C nuclear magnetic resonance spectroscopy (solid-state ¹³C CP-MAS NMR), specific surface area, and pH at the point of zero charge (pH_PZC). The ability to adsorb and exchange anions of modified materials was referred and examined by using aqueous solutions containing CrO₄ ^2-, NO₃ ^-, and H₂AsO₄ ^- anions in different conditions. The results revealed that anionite lignocellulosic materials could separate these anions with very good efficiency and better than strong anion exchange resin (GA-13) in the same conditions; outlet water could meet the permissible drinking and living water standards; and the m-DMDHEU cross-link bridge also was a good bridge to connect CC to cellulose chain beside other common urea cross-link bridges.

The mechanism of nitrate-Cr(VI) reduction mediated by microbial under different initial pHs

To date, comparatively little research is known about the role of pH conditions in bioremediation of Cr(VI) contaminated aquifers. This study explored microbial Cr(VI) reduction and denitrification under different initial pHs. The underlying mechanism was also investigated. When testing 50 mg/L-N nitrate and 10 mg/L Cr(VI), complete contaminants removal was observed at initial pH 10.0 and 11.0, and only 10 %-30 % of removal achieved under other conditions, which might be ascribe to the significant up-regulation of functional genes narG (8.31 and 10.46 folds) and azoR (24.90 and 15.96 folds) at initial pH 10.0 and 11.0. Metagenomic sequencing showed that alkali tolerant bacteria played major roles in the NO₃^--Cr(VI) reduction (i.e. Pannonibacter increased by 13.08 % and 25.24 % at initial pH 10.0 and 11.0), and metabolic pathways of Degradation and Energy were found of increased abundant. Furthermore, a significative study suggested that potential interspecies cooperation existed at initial pH 11.0 to facilitating the simultaneous removal of contaminants, and Pannonibacter indicus might be an important participant in the degradation of contaminants. The results of this study will fully understand the metabolic patterns of bacteria under alkaline conditions, expand the range of available functional bacteria, and enhance the practical aspects of co-contaminants remediation.

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).

On the Micromixing Behavior of a Spinning Disk Reactor for Metallic Cu Nanoparticles Production

The interest in producing copper nanoparticles is high, mainly due to their enhanced properties, a wide range of possible and different applications, and the possibility for their use in the framework of catalysis. The purpose of this study is to synthesize copper nanoparticles by chemical reduction of copper sulfate with L-ascorbic acid and sodium borohydride capable to eliminate free radicals providing an antioxidant environment promoting pure copper nuclei formation. The micromixing conditions necessary for the nanoparticles production are provided by a spinning disk reactor (SDR). Relevant operational parameters, such as the disk speed velocity and the position of the reactant injectors, will lead to different product outcomes. The latter was checked by means of a dynamic light scattering instrument (DLS). At the end, depending on the adopted operating conditions, the SDR was able to produce particles between 16 nm and 39 nm, with a particle size distribution (PSD) characterized by a narrow, monomodal plot. In comparison to the smallest particles obtained in a stirred reactor tank, that is, 132 nm, the obtained results appear to be very promising.

A physical-based interpretation of mechanism and kinetics of Cr(VI) reduction in aqueous solution by zero-valent iron nanoparticles

The aim of this paper is to show the results obtained by investigating the reduction of hexavalent Chromium [Cr(VI)] by iron nano-particles in aqueous solution, interpreted in light of the particle-grain model. The diffusional and geometric parameters that govern and describe the reacting system were estimated from the evidences deriving from the characterization and the experiments conducted, allowing assumptions based on physical principles. Such procedure rendered the particle-grain model a valid choice for the interpretation of the results obtained. The model, used in its dimensionless form, was tested according to a preliminary procedure aimed at analyzing the sensitivity of the system, by varying within wide ranges the ratio between the reaction rate, the diffusive mass transfer rate, and the particle-grain radius, to show how reliable its potential application may be. Subsequently, a non-linear regression procedure was used to estimate the two main parameters of the model that affect the reduction process: (i) the diffusion coefficient within the solid layer produced along with the reaction, D^p_c (6.02 E-13 m² s^-1), and (ii) the kinetic constant of the surface reaction, k_c (0.21 m s^-1). The values found for the parameters were perfectly in line with theoretical considerations and experimental evidences.