Graphene oxide-supported nanoscale zero-valent iron composites for the removal of atrazine from aqueous solution

Interactions of graphene oxide with the microbial community of biologically active filters from a water treatment plant

With widespread occurrence and increasing concern of emerging contaminants (CECs) in source water, biologically active filters (BAF) have been gaining acceptance in water treatment. Both BAFs and graphene oxide (GO) have been shown to be effective in treating CECs. However, studies to date have not addressed interactions between GO and microbial communities in water treatment processes such as BAFs. Therefore, in the present study, we investigated the effect of GO on the properties and microbial growth rate in a BAF system. Synthesized GO was characterized with a number of tools, including scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectrometry. GO exhibited the characteristic surface functional groups (i.e., C-OH, C=O, C-O-C, and COOH), crystalline structure, and sheet-like morphology. To address the potential toxicity of GO on the microbial community, reactive oxygen species (ROS) generation was measured using nitro blue tetrazolium (NBT) assay. Results revealed that during the exponential growth phase, ROS generation was not observed in the presence of GO compared to the control batch. In fact, the adenosine triphosphate (ATP) concentrations increased in the presence of GO (25 μg/L - 1000 μg/L) compared to the control without GO. The growth rate in systems with GO exceeded the control by 20 % to 46 %. SEM images showed that GO sheets can form an effective scaffold to promote bacterial adhesion, proliferation, and biofilm formation, demonstrating its biocompatibility. Next-generation sequencing (Illumina MiSeq) was used to characterize the BAF microbial community, and high-throughput sequencing analysis confirmed the greater richness and more diverse microbial communities compared to systems without GO. This study is the first to report the effect of GO on the microbial community of BAF from a water treatment plant, which provides new insights into the potential of utilizing a bio-optimized BAF for advanced and sustainable water treatment or reuse strategies.

Changes of atrazine dissipation and microbial community under coexistence of graphene oxide in river water

The coexistence of herbicide atrazine (ATZ) and the nanomaterial graphene oxide (GO) in natural water bodies will be an inevitable scenario due to their widespread application and consequent release into aquatic ecosystems. But the dissipation of ATZ with GO and the response of the microbial community to their combination are still not clear. Here, we investigated the dissipation dynamics and transformation of ATZ with and without GO in river water after 21-d incubation. In the presence of GO, ATZ residue significantly decreased by 11%-43%; the transformation of ATZ markedly increased by 11%-17% when ATZ concentrations were not above 1.0 mg∙L-1. The direct adsorption of ATZ on GO, mainly via π-π interactions, proton transfer and hydrogen bonding, contributed 54%-68% of the total increased ATZ dissipation by GO. ATZ and ATZ+GO exerted effects of similar magnitude on microbial OTU numbers with an increase of bacterial diversity. The coexisting GO increased the relative abundance of ATZ-degradation bacteria and Chitinophagales, thus improving ATZ transformation. This work indicated that the coexistence of GO at environmentally relevant concentrations can effectively reduce ATZ residues and promote the transformation of ATZ to degradation products in river water; nevertheless, the potential risk of GO acting as an ATZ carrier should be given more prominence.

GO accelerate iron oxides formation and tetrabromobisphenol A removal enhancement in the GO loaded NZVI system

Tetrabromobisphenol A (TBBPA) is an emerging persistent organic pollutant, which is very difficult to remove by common methods. In this study, the GO-load nanoscale zero-valent iron (NZVI/GO) was fabricated and optimized to improve the reaction rate and removal efficiency for TBBPA reliably and efficiently. The results showed that GO-load significantly reduced the self-aggregation of NZVI and the aggregate size decreased by 50.00% (1400-700 nm). Meanwhile, GO significantly improved the reaction rate k_obs (1.11 ± 0.11 h^-1) of TBBPA in the NZVI/GO system compared to the NZVI (0.40 ± 0.08 h^-1) system, and this increment was more pronounced (177.5%) when the mass ratio of NZVI-to-GO reached 1.0 than other mass ratios. Furthermore, X-Ray Diffraction and X-ray photoelectron spectroscopy analysis suggested that the Fe²⁺ transformation was changed and enriched by the GO. Only magnetite (Fe₃O₄) was detected on the surface of NZVI, whereas the maghemite (γ-Fe₂O₃), hematite (α-Fe₂O₃), and Fe₃O₄ were detected on the interface of NZVI/GO, which further performed the complexation adsorption through the -OH of TBBPA. This specific complexation adsorption is another potential accelerated removal mechanism for TBBPA and intermediates within the NZVI/GO system. This research has put forward a new perspective for widening the application of TBBPA removal using the synergistic effect between GO and NZVI.

Graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) for antimony removal: The role of active oxygen species and reaction mechanism

Sulfidated nano zero-valent iron (S-nZVI) was used to remove various pollutants from wastewater. However, the instability, poor dispersibility, and low electron transfer efficiency of S-nZVI limit its application. Herein, graphene oxide supported sulfidated nano zero-valent iron (S-nZVI@GO) was successfully synthesized using graphene oxide (GO) as a carrier. The properties of S-nZVI@GO were characterized by scanning electron microscopy coupled to X-ray photoelectron spectroscopy (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) concerning the surface morphology, crystalline structure, and elemental components. S-nZVI@GO displayed an excellent capacity for antimony (Sb) removal under aerobic conditions (96.7%), with a high adsorption capacity (Q_max = 311.75 mg/g). It maintained a high removal rate (over 90%) during a wide pH range (3-9). More importantly, S-nZVI@GO activated the molecular oxygen in water via a single-electron pathway to produce •O₂－ and H₂O₂, and then oxidized trivalent antimony (Sb(III)) to pentavalent antimony (Sb(V)) and further separated it by synergistic adsorption and co-precipitation. Therefore, S-nZVI@GO shows excellent potential for Sb contamination remediation.

Insights into the mechanism underlying remediation of Cr(VI) contaminated aquifer using nanoscale zero-valent iron@reduced graphene oxide

Currently, there is an urgent need to develop functional nanomaterials for highly effective environmental remediation. However, the long-term effect of remedial materials upon their injection into contaminated aquifer has frequently been overlooked. Here, the remediation of Cr(VI) contaminated aquifer by reduced graphene oxide (rGO) supported nanoscale zero-valent iron (nZVI@rGO) was investigated from a long-term perspective. The performances of nZVI@rGO samples with different rGO loadings in the removal of aqueous Cr(VI) were evaluated in batch experiments. The electron transfer properties different nZVI@rGO samples were investigated by measuring their corrosive potentials using the steady-state Tafel polarization curves. The results show that the electron transfer efficiency between Cr(VI) and nZVI@rGO is enhanced owing to the large reactive conjugated structure of rGO. Besides, the surface passivation of nZVI is effectively retarded due to the uniform accommodation of Cr(III) precipitates on rGO. The structure and composition of nZVI@rGO before and after Cr(VI) removal were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The characterization results revealed that most Cr(VI) ions (∼90%) will be reduced to Cr(III) precipitates on nZVI@rGO as the passivation product. Accordingly, Cr(VI) ions tend to react more readily at less blocked regions on the surface of rGO, and a layer-by-layer passivation model on nZVI@rGO surface is proposed. Our results provide new insights into the mechanism underlying the long-term remediation of Cr(VI) contaminated aquifer using nZVI@rGO, which helps design new materials and approaches for practical in-situ remediation engineering.

A novel lignin hydrogel supported nZVI for efficient removal of Cr(VI)

A novel hydrogel-supported nanoscale zero-valent iron (nZVI) composite (nZVI@LH) was synthesized by ion exchange and in-situ reduction. The removal efficiency was tested, and the mechanism was also explored. The nZVI@LH at the precursor Fe(II) ion concentration of 0.1 mol/L presented an enhanced Cr(VI) removal capacity of 310.86 mg/g Fe⁰ at pH 5.3, which was 11.6 times more than that of the pure nZVI. The removal efficiency of the composite at pH 2.1 was more than double compared with alkaline or neutral conditions. Scanning electron microscopy (SEM) suggested that the nZVI particles were uniformly immobilized in the lignin hydrogel. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) provided evidence supporting the removal mechanism. According to the XPS results, the high removal capacity of the composite was attributed to chemical reduction/precipitation (69.7%), surface sorption (19.7%), and swelling uptake (10.6%). The pseudo-first-order reduction kinetics and pseudo-second-order kinetic model were employed to simulate the kinetic data, which supported the mechanism that chemical reduction and surface sorption could simultaneously remove Cr(VI). The electron acceptor and electron donor affected the reaction rate, and the presence of humic acid significantly inhibited the reaction. The present study demonstrated that lignin hydrogel acted as a carrier to prevent aggregation of nZVI particles. nZVI particles loaded on lignin hydrogel showed high reactivity and high degree of utilization compared with bare-nZVI. These results exhibited the great potential of nZVI@LH in practical water treatment due to its high activity.

Nitrate Removal by Zero-Valent Metals: A Comprehensive Review

Nitrate is a widespread water contaminant that can pose environmental and health risks. Various conventional techniques can be applied for the removal of nitrate from water and wastewater, such as biological denitrification, ion exchange, nanofiltration, and reverse osmosis. Compared to traditional methods, the chemical denitrification through zero-valent metals offers various advantages, such as lower costs, simplicity of management, and high efficiencies. The most utilized material for chemical denitrification is zero-valent iron (ZVI). Aluminium (ZVA), magnesium (ZVM), copper (ZVC), and zinc (ZVZ) are alternative zero-valent metals that are studied for the removal of nitrate from water as well as from aqueous solutions. To the best of our knowledge, a comprehensive work on the use of the various zero-valent materials that are employed for the removal of nitrate is still missing. Therefore, in the present review, the most recent papers concerning the use of zero-valent materials for chemical denitrification were analysed. The studies that dealt with zero-valent iron were discussed by considering microscopic (mZVI) and nanoscopic (nZVI) forms. For each Fe0 form, the effects of the initial pH, the presence or absence of dissolved oxygen, the initial nitrate concentration, the temperature, and the dissolved ions on the nitrate removal process were separately evaluated. Finally, the different materials that were employed as support for the nanoparticles were examined. For the other zero-valent metals tested, a detailed description of the works present in the literature was carried out. A comparison of the various features that are related to each considered material was also made.

Enhanced Slow Sand Filtration for the Removal of Micropollutants from Groundwater

Remote areas, where centralized water supply cannot reach, rely heavily on decentralized supply systems such as slow sand filters (SSFs). Groundwater used to be a reliable water source; yet, the advent of micropollutants (MPs) has raised concerns over its quality. In this study, an enhanced slow sand filtration utilizing graphene oxide (GO)-coated sand prepared via a simple thermal method was employed to remove two representative MPs, atrazine (ATZ) and atenolol (ATL), from real groundwater for drinking water treatment. The removal of ATZ and ATL was studied in a bench-scale enhanced SSF using GO-coated sand in comparison with the conventional plain sand. The results showed that the GO-coated sand performed better in the removal of ATZ, ATL, and total organic carbon (TOC), as well as turbidity reduction. Moreover, in order to study the role of the schmutzdecke in MPs' removal small lab-scale columns with and without schmutzdecke growth were set up. The results indicated the enhanced removal capacity of the coated sand toward ATZ, ATL, and TOC could mainly be attributed to the GO coating layer, not the schmutzdecke. Hence, if the coated sand is to be used in field SSFs for the removal of organic contaminants, the schmutzdecke growing phase might not be needed. A preliminary techno-economic analysis was performed to evaluate the practicability of enhanced SSF and GO was found to dominate the overall cost. For a community-level or a household-level SSF, the extra cost using GO-coated sand may be $0.34 and $3.25 per m³ of water if the GO price is $10 and $100 per kg, respectively.

Application of porous styrene resin loaded carboxymethyl cellulose-stabilized nano-zero-valent iron for highly efficient hexavalent chromium removal

The results of this study provide a new idea for the design of efficient Cr(vi) removal materials based on nZVI.

The efficient removal of bisphenol A from aqueous solution using an assembled nanocomposite of zero-valent iron nanoparticles/graphene oxide/copper: Adsorption isotherms, kinetic, and thermodynamic studies

In this study, nanoparticles of zero-valent iron (nZVI) along with graphene oxide (GO) and copper (Cu) was synthesized to apply as a promising adsorbent for the rapid removal of bisphenol A (BPA) from aqueous solution. The characteristics of nZVI-GO-Cu were analyzed by field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), X-Ray Diffraction (XRD), and vibrating sample magnetometer (VSM). The average particle size of nZVI-GO-Cu was found to be 20.89 nm. The effective experimental variables such as pH, adsorbent dosage, contact time, initial BPA concentration, and temperature were surveyed to assess optimum conditions. Results revealed that the maximum removal percentage was obtained at pH of 7, adsorbent dosage of 0.2 g, contact time of 10 min, the BPA concentration of 10 mg/L, and a temperature of 35 °C as optimum conditions. Experimental data were fitted to the Langmuir and pseudo second-order models with a coefficient of determination (R²) equal to 1 and 0.995, respectively. The obtained maximum adsorption capacity (q_max) of the Langmuir isotherm was 21.59 mg g^-1. Thermodynamic parameters under the various temperatures confirmed that the adsorption process was endothermic (ΔH = 17,459.4 J/mol and ΔS = 61.23 J/mol/K) and spontaneous (ΔG < 0). As a conclusion, nZVI-GO-Cu can be selected as an efficient adsorbent for the treatment of aqueous media from BPA and the other pollutants, due to its low-cost, high removal efficiency (97%), and rapid adsorption with the minimum time of 10 min compared with the other adsorbents.

Efficient Removal of 2,4-DCP by Nano Zero-Valent Iron-Reduced Graphene Oxide: Statistical Modeling and Process Optimization Using RSM-BBD Approach

In this study, nano zero-valent iron-reduced graphene oxide (NZVI-rGO) composites were synthesized to remove 2,4-dichlorophenol (2,4-DCP) as an efficient adsorbent. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicated that NZVI particles were successfully loaded and dispersed uniformly on rGO nanosheets. Fourier transform infrared spectroscopy (FTIR) analysis showed that the interaction between NZVI-rGO and 2,4-DCP promoted the adsorption process. A three-level, four-factor Box-Behnken design (BBD) of the response surface methodology (RSM) was used to optimize the influencing factors including NZVI-rGO dosage, 2,4-DCP initial concentration, reaction time and initial pH. A statistically significant, well-fitting quadratic regression model was successfully constructed to predict 2,4-DCP removal rate. The high $F$ value (15.95), very low $P$ value (<0.0001), nonsignificant lack of fit, and appropriate coefficient of determination ( $R^2=0.941$ ) demonstrate a good correlation between the experimental and predicted values of the proposed model. The analyses of variance reveal that NZVI-rGO dosage and reaction time have a positive effect on 2,4-DCP removal, whereas the increase of contaminant concentration and initial pH inhibit the removal, whereas the effect of contaminant concentration and initial pH is in reverse, where the change of NZVI-rGO dosage has the greatest effect. The optimum condition is1.215 g/L of NZVI-rGO dosage, 20.856 mg/L of 2,4-DCP concentration, 4.115 of pH, and 8.157 min of reaction time. It is verified by parallel experiments under the optimum condition, achieving the removal efficiency of100%.

Citrate iron complex induced dramatically enhanced oxidation of atrazine with bimetallic Bi/Fe0: Reactivity, oxidation and mechanism

The oxidative degradation of atrazine (ATR) using bimetallic Bi/Fe⁰ nanoparticles cooperated with citric acid (CA) and sodium citrate (NaCA) without extra addition of H₂O₂ or another oxidant was conducted. Almost 73% of ATR was removed in Bi/Fe⁰+NaCA + CA buffer system in 3 h, and the bimetallic Bi/Fe⁰ performs high stability and long service life in the buffer system according to the results of cyclic degradation experiments. The citrate iron complex of Fe(II)[Cit]^- played the key role for the degradation process since it could quickly react with the generated H₂O₂ to produce free radicals in the Bi/Fe⁰+NaCA + CA system, which broadened the applicable pH range of the traditional Fenton reaction and promoted the oxidative degradation process of ATR. The possible degradation pathways of ATR were also investigated. In the Bi/Fe⁰+NaCA + CA buffer system, twelve kinds of ATR intermediate products were detected, of which the main products were dechlorination products and alkyl oxidative products. Due to the pH controllable of the Bi/Fe⁰+NaCA + CA system, it could reduce the acidity impact on the environment and makes the additional impact on the environment lower. Therefore, this work provides a new strategy for the degradation of ATR.

Nano zero valent iron encapsulated in graphene oxide for reducing uranium

Nano zero-valent iron (Fe⁰) has been widely used to remove Uranium (U(VI)). In order to enhance the performance of Fe⁰ toward U(VI) removal, the Fe⁰ was assembled into graphene oxide (GO) sheets via 3-aminopropyl triethoxysilane (APTES) as Fe⁰/APTES-GO composites. The Fe⁰/APTES-GO composites were triumphantly prepared, characterized and analyzed by means of Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) together with Energy Dispersive Spectrometer (EDS), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). SEM and TEM-EDS results manifested that Fe⁰ particles were encapsulated into rolled-up GO, which greatly improved the stability of Fe⁰. Batch experiment showed that only a small amount of Fe²⁺ was leached in the first two leaching cycles of Fe⁰/APTES-GO composites. The removal capacity of Fe⁰/APTES-GO composites was up to 1357.99 mg/g at pH = 4.1 and T = 50 °C, which was mainly attributed to the reducing activity of Fe⁰ and an abundance of functional groups (i.e., -COOH, C-OH and -OH) on the Fe⁰/APTES-GO composites. The electrostatic potential (ESP) from the calculation also supported that U(VI) tended to be reduced at the back side of the GO-Fe⁰ cluster.

Removal of atrazine from aqueous solutions onto a magnetite/chitosan/activated carbon composite in a fixed-bed column system: optimization using response surface methodology

In this study, a magnetite/chitosan/activated carbon (MCHAC) composite is proposed as an efficient adsorbent for the removal of atrazine from aqueous solutions. The prepared composite was characterized using Fourier-transform infrared (FTIR) and X-ray diffraction (XRD) methods. Response surface methodology (RSM) coupled with composite central design (CCD) were used to optimize the effects of the four independent variables, pH, initial concentration of atrazine (C 0), bed depth (H), and flow rate (Q), which influence the adsorption process. The experimental results modeled using response surface methodology (RSM) coupled with central composite design (CCD) (RSM-CCD) indicated a quadratic relationship with p < 0.0001 for adsorption capacity at saturation (q s) and fraction of bed utilization (FBU). The results of the experiments performed under the optimized conditions, pH = 5.07, C 0 = 137.86 mg L-1, H = 2.99 cm and Q = 1.038 mL min-1, showed a q s value of 62.32 mg g-1 and FBU of 72.26%, with a deviation value of less than 0.05 from the predicted q s and FBU values. The obtained breakthrough curves were fitted with four mathematical models, Thomas, Bohart-Adams, Yan and Yoon-Nelson, in order to determine the limiting step of the mass transfer of the atrazine adsorption onto the composite. A desorption study of the composite revealed the high reuse potential for MCHAC, thus, the prepared material could be used as a low-cost and efficient adsorbent for the decontamination of polluted wastewater.