Iron Sludge Research Articles

Recent advances in application of heterogeneous electro-Fenton catalysts for degrading organic contaminants in water.

Over the last decades, advanced oxidation processes (AOPs) have been widely used in surface and ground water pollution control. The heterogeneous electro-Fenton (EF) process has gained much attention due to its properties of high catalytic performance, no generation of iron sludge, and good recyclability of catalyst. As of October 2022, the cited papers and publications of EF are around 1.3 × 10-5 and 3.4 × 10-3 in web of science. Among the AOP techniques, the contaminant removal efficiencies by EF process are above 90% in most studies. Current reviews mainly focused on the mechanism of EF and few reviews comprehensively summarized heterogeneous catalysts and their applications in wastewater treatment. Thus, this review focuses on the current studies covering the period 2012-2022, and applications of heterogeneous catalysts in EF process. Two kinds of typical heterogeneous EF systems (the addition of solid catalysts and the functionalized cathode catalysts) and their applications for organic contaminants degradation in water are reviewed. In detail, solid catalysts, including iron minerals, iron oxide-based composites, and iron-free catalysts, are systematically described. Different functionalized cathode materials, containing Fe-based cathodes, carbonaceous-based cathodes, and heteroatom-doped cathodes, are also reviewed. Finally, emphasis and outlook are made on the future prospects and challenges of heterogeneous EF catalyst for wastewater treatments.

Effect and mechanism of norfloxacin removal by guava leaf extract in the ZVI/H2O2 system

To overcome the bottlenecks of the conventional zero-valent iron Fenton-like (ZVI/H2O2) process, such as low reagent utilization, low applicable pH, and iron sludge contamination, guava leaf extract (GLE) was used as a green promoter to enhance ZVI/H2O2 process in this study. Compared with the ZVI/H2O2 system, the removal rate and kobs of norfloxacin by the ZVI/H2O2/GLE system were increased by 33.76% and 2.19 times, respectively. The experimental investigation of the mechanism showed that the attack of reactive oxygen species was the main pathway for the removal of pollutants, and three types of reactive oxygen species (1O2, O2−,·OH) generations in the ZVI/H2O2/GLE system were effectively promoted by the introduction of GLE. The reactivity improvement was mainly due to the decrease of pH. At the same time, the chelation of iron ions by GLE promoted the Fe(III)/Fe(II) cycle on the catalyst surface was also a minor mechanism to improve the reactivity. This study provides a crucial reference for the practical application of guava leaf to promote the ZVI/H2O2 process in environmental pollution control.

Switching the primary mechanism from a radical to a nonradical pathway in electrocatalytic ozonation by onsite alternating anode and cathode

Concurrently elevating the degradation efficiency of pollutants and realizing the reduction of iron sludge in Fe-based catalytic ozonation is important but still challenging. Herein, we developed an electrocatalytic ozonation (ECO) system with iron plate cathode and graphite felt anode (ECO-Fe-cathode), which was free from added chemical reagents. Unlike the iron plate as a sacrificial anode in the ECO (ECO-Fe-anode) system, this delicately designed system shows a much higher degradation rate of ibuprofen (kobs = 1.490 min−1) than that of the ECO-Fe-anode system (kobs = 0.345 min−1). Simultaneously, the effluent was totally limpid without the corrosion of iron plates and the formation of iron sludge in the ECO-Fe-cathode system. Unexpectedly, the generation of singlet oxygen (1O2) which is indirectly generated by the single-electron transfer derived from superoxide ion (O2•-) is the primary reactive oxygen species (ROS) in the ECO-Fe-cathode system, which is different from the ECO-Fe-anode system with hydroxyl radicals (•OH). Moreover, linear sweep voltammetry (LSV) was applied to reveal the oxygen evolution reaction (OER) performance of the iron plate and graphite felt, and the results showed that graphite felt as anode has better electrocatalytic performance. The electrochemical analysis and density functional theory (DFT) calculation revealed that ozone adsorbed on the iron plate surface is more conducive to facilitating and triggering subsequent reactions. Finally, the different degradation pathways of ibuprofen in both systems were proposed. This work represents a fundamental breakthrough toward the design of an efficient and harmless ECO system for wastewater treatment.

Sustainable and Reagentless Fenton Treatment of Complex Wastewater.

Conventional Fenton treatment is fundamentally impractical for large-scale applications, as the consumption of Fe(II), H2O2, and pH regulators and the accumulation of iron hydroxide sludge are very costly. This paper describes a new method for Fenton treatment of complex wastewater without additional dosing of Fe(II) and H2O2, without iron-sludge accumulation, and with less consumption of pH regulators, using a novel bioelectrode system. Our new system includes a novel three-chamber microbial electrolysis unit and Fenton reaction unit, where Fenton reagents are generated by biotic and abiotic cathodes, while the bioanode simultaneously degrades biodegradable organics from the wastewater. The system's self-alkalinity buffering also waives the need for pH regulators. Dissolved organic carbon and 22 specific recalcitrant organics were removed by 99% and between 78 and 100%, respectively. The bioelectrode system generated 13 ± 3 mg/L dissolved Fe(II) and 5 ± 0.4 mg/L H2O2 for the Fenton reaction unit. The closed iron cycle avoided iron loss and iron sludge accumulation during operation. The pH regulator dosage and operating costs were just 9.7 and 1.4%, respectively, of what is required by classic Fenton. The low operating cost and reduction in chemical usage make it an efficient, sustainable alternative to the conventional treatment processes currently used for complex wastewater.

Activated Carbon Assisted Fenton-like Treatment of Wastewater Containing Acid Red G

The Fenton reaction as an effective advanced oxidation technology has been widely used in wastewater treatment for its stable effluent quality, simple operation, mild condition, and higher organic degradation with non-selectivity. However, the traditional Fenton reaction is limited by the sluggish regeneration of Fe2+, resulting in a slower reaction rate, and it is necessary to further increase the dosage of Fe2+, which will increase the production of iron sludge. Activated carbon (AC) has a strong adsorption property, and it cannot be ignored that it also can reduce Fe3+. In this study, the degradation of acid red G (ARG) by adding AC to the Fe3+/H2O2 system, the role of the reducing ability, and the reason why AC can reduce Fe3+ were studied. By adding three kinds of ACs, including coconut shell-activated carbon (CSPAC), wood-activated carbon (WPAC), and coal-activated carbon (CPAC), the ability of ACs to assist the Fe3+/H2O2 Fenton-like system to degrade ARG was clarified. Through the final treatment effect and the ability to reduce Fe3+, the type of AC with the best promotion effect was CSPAC. The different influence factors of particle size, the concentration of CSPAC, concentration of H2O2, concentration of Fe3+, and pH value were further observed. The best reaction conditions were determined as CSPAC powder with a particle size of 75 μm and dosage of 0.6 g/L, initial H2O2 concentration of 0.4 mmol/L, Fe3+ concentration of 0.1 mmol/L, and pH = 3. By reducing the adsorption effect of CSPAC, it was further observed that CSPAC could accelerate the early reaction rate of the degradation process of ARG by the Fe3+/H2O2 system. FT-IR and XPS confirmed that the C-O-H group on the surface of CSPAC could reduce Fe3+ to Fe2+. This study can improve the understanding and role of AC in the Fenton reaction, and further promote the application of the Fenton reaction in sewage treatment.

Preparation of magnetic N-doped iron sludge based biochar and its potential for persulfate activation and tetracycline degradation

The widespread use of tetracycline (TC) is a non-negligible cause of environmental pollution, such as antibiotic resistance genes (ARGs). Economical, green and efficient treatment methods are urgently needed. In this study, the magnetic N-doped iron sludge based biochar (N-BCFe) was prepared by a one-step pyrolysis method using the iron-containing sludge from groundwater treatment plants, sycamore leaves and urea as raw materials to active persulfate (PS) for TC degradation. The results show that under the optimal experimental conditions, the removal rate of 10 mg/L TC in the N-BCFe/PS system could reach 86.56% (120 min), accompanied by a 59.9% reduction in total organic carbon (TOC), and still 63.34% of TC was removed in the third recycle experiment. Combined with EPR, free radical quenching experiments and XPS spectra, it is confirmed that there are free radicals (SO4·−, ·OH) and non-radical (1O2) pathways in N-BCFe/PS system. And the former is related to Fe doping, while the latter is attributed to N doping (nitrogen-containing functional groups). Seventeen TC intermediates were identified by LC-MS technology, and three TC degradation pathways were established accordingly. Generally, this study illustrates the considerable catalytic potential of N-BCFe and provides valuable insights into the synthesis of catalysts prepared from iron-containing sludge as feedstock.

Efficient degradation of PFOA in water by persulfate-assisted and UV-activated electrocoagulation technique using Fe foam electrode

Perfluorooctanoic acid (PFOA), as a persistent organic pollutant, has been normally present in groundwater, surface water and seawater. It has been widely studied for its persistence, immunotoxicity and bioaccumulation. However, it is extremely difficult to be degraded due to high energy of the C-F bond. In this study, PFOA was removed from simulated and real industrial wastewater with Fe foam electrode by electrocoagulation (EC) technology. To achieve better degradation, persulfate (PS) and ultraviolet (UV) light were employed in the EC technology to realize the degradation of PFOA (PEC-PS). After 60 min treatment, the PEC system showed outstanding performance in that 87.5% of PFOA was degraded and 54.6% of PFOA was defluorinated under optimal conditions with smaller energy consumption and lesser iron sludge generation. Sulfate radical (SO4•−) was generated by the activation of ferrous ions under the synergy of UV light and electric energy. In this process, •OH and SO4•− and electron transfer are crucial in the reaction with PFOA and intermediates. The degradation mechanism of PFOA was summarized according to the quenching experimental results and the contribution of various reactive substances. The potential degradation pathways of PFOA were proposed according to the intermediates obtained through the HPLC-MS. It was found that PFOA showed a decomposition with the gradual losing of −CF2− units. The PEC-PS process of using Fe foam as electrode showed high-efficiency degradation and better recoverability, indicating the great potential of PEC-PS technique in efficient and thorough cleaning of PFOA in actual industrial wastewater.

Zero sludge discharge strategy for Fenton oxidation wastewater treatment technology: Biological regeneration and in-situ cyclic utilization - A feasibility study

Classical homogeneous Fenton oxidation wastewater treatment technology (abbreviated as Fenton process) shows advantages of simply process, high efficiency and low equipment requirements. However, the generated Fenton iron sludge (FIS) is a kind of hazardous solid waste and the treatment cost is high. The resource utilization of the FIS is of great significance for the cost reduction and further application of the Fenton process. In this study, we propose a zero sludge discharge strategy of FIS, biological regeneration and in-situ cyclic utilization. In this strategy, the FIS is firstly experienced an anaerobic biological digestion process to convert the Fe(Ⅲ) to Fe(Ⅱ) by iron reducing bacteria (IRB) using the residual organics as an electron donor; then the digested sludge is acidified to extract the Fe(Ⅱ) resource in the form of Fe2+ ion; finally, the Fe2+ ion is in-situly cyclic utilized for Fenton oxidation and the residual sludge containing abundance of IRB flows back to the anaerobic digestion tank. This strategy is verified by using both artificial and practical FIS. Besides temperature, it is found that the Fe/organics ratio, P (in the form of orthophosphate) level and solid content show a great effect on the conversion efficiency of the Fe(II) during anaerobic digestion of the iron sludge. The optimal pH for extraction of Fe(II) resource from the digested sludge is around 3.5, under which not only over 90% of the regenerated Fe(Ⅱ) could be extracted, but also the activity of the IRB in the residual sludge could be preserved. The extracted Fe2+ ions are cyclic utilized used for Fenton oxidation and show comparable performance to that of the commercial ferrous sulfate. The Fenton process coupling of the biological regeneration and in-situ cyclic utilization strategy could lead to an over 60% cost reduction and zero waste discharge. This study provides a sustainable strategy for the disposal of iron sludge and can promote the further popularization of the Fenton process.

Chromium Removal in the Presence of NOM during Fe(II) Reductive Precipitation for Drinking Water Treatment

Cr(VI) is toxic and carcinogenic, which fuels discussions on reducing existing standards for maximum Cr concentrations in drinking water. Fe(II) reductive precipitation is a common and economical method for achieving very low Cr(tot) concentrations (<5 µg/L). While Cr(VI) is reduced to Cr(III), Fe(II) is oxidized to Fe(III). The resulting Cr(III) and Fe(III) have low solubilities at neutral pH, precipitate as hydroxides, and can be removed by conventional media filtration. The presence of natural organic matter (NOM) in the raw water source can, depending on pH, concentration, type of NOM, and contact time, affect this process in various ways, from promoting Cr(VI) reduction, to re-reducing Fe(III), to forming stable complexes with Cr and Fe, thus, impairing chromium removal. The presented data showed that NOM, whether dominated by terrestrial humic acid, or of aquatic origin, could substantially impair chromium removal at neutral pH conditions. In particular, the ultimate removal of Cr(III) was affected. Soluble complexes and/or colloids of Cr(III), Fe(III) and NOM in the size range of 10 kDa–0.1 µm were formed, that could not be removed by conventional media filtration. Presence of iron sludge (>50 mg/L Fe(III)) mitigated the negative impacts of NOM on Cr(VI) reduction and Cr(III) removal. However, even 100 mg/L Fe(III), the highest applied sludge concentration, did not lead to a decrease in Fe(II) dosing requirements under the given conditions. A molar ratio higher than the given stochiometric ratio of [Fe(II)]:[Cr(VI)] = 3 was necessary for sufficient Cr(VI) removal.

Thiosulfate-assisted Fe2+/persulfate pretreatment effectively alleviating iron dose and enhancing biotransformation of waste activated sludge into high-value liquid products.

Ferrous-based acidogenic fermentation (AF) as a means to treat waste activated sludge (WAS) and produce short-chain fatty acids (SCFAs) has drawn increasing attention, but the massive amount of "iron sludge" that it produces not only significantly increases costs and difficulty of disposal but also poses risks to the environment and human health. This study explored a novel approach to not only reduce the iron dosage required by AF but also to improve its performance by introducing a thiosulfate (TS)-assisted Fe2+/persulfate (TAFP) pretreatment. Effects of the TAFP pretreatment on WAS disintegration and biodegradability, SCFA production, and microbial community structure with different persulfate-Fe2+-thiosulfate molar ratios at 4:4:0 (R1), 4:3:1 (R2), 4:2:2 (R3) and 4:1:3 (R4) were investigated. The results showed that the TAFP pretreatment by a remarkable margin promoted the disintegration of WAS as well as the biodegradability of the organics released, owing to the production of robust free radicals (SO4•- and •OH) triggered by the thiosulfate and Fe2+ cycles. 48-day AF tests further showed maximum SCFA production, ranging roughly between 1283 and 1395mg COD/L in the TAFP pretreated samples, much higher than Control (<120mg/L) and R1 (around 593mg COD/L). At the meantime, the Fe2+ dosage was reduced by 50% in R3 than that of R1. However, a prolonged lag phase of SCFA generation was observed between days 7 and 25, which was ascribable to the acidic conditions (pH<4.5) closely related to impaired metabolic activities as well as electron transfer efficiencies and limited activities of acidogenic enzymes (i.e., pyruvate-ferredoxin oxidoreductase). Despite such lag phase, the economic and environmental assessment of AF of TAFP-pretreated WAS had a higher net SCFA yield and less "iron sludge" than either without any pretreatment or with Fe2+/persulfate-only pretreatment.

Low strength wastewater anammox start-up and stable operation by inoculating sponge-iron sludge: Cooperation of biological iron and iron bacteria

The application of anaerobic ammonium oxidation (Anammox) technology in low-strength wastewater treatment still faces difficult in-situ start-ups and unstable operations. Sponge-iron sludge (R1) was used as a novel inoculum to provide a promising solution. Conventional activated sludge (R0) was used as the control. However, little is known about the feasibility and performance during the start-up and operation of Anammox combined with biological iron and iron bacteria in an iron sludge system. Anammox was successfully started both in R1 (87 days) and R0 (89 days) with a low-strength influent (with a nitrogen loading rate (NLR) of 43.64 ± 0.41 g N/(m3⋅d)). During long-term operation, the R0 nevertheless produced higher nitrates (9.7 ± 0.1 mg/L) than expected. In contrast, R1 presented no excess nitrate production (2.1 ± 0.06 mg/L). The total inorganic nitrogen (TIN) removal efficiency increased from 78.2 ± 7.1% in R0 to 86.1 ± 4.3% in R1. The iron sludge in R1 was divided equally into three parts and three different nitrogen-feeding methods were used over the 34 days of operation, as follows: first using a mixture of ammonium (27.15 ± 1.0 mg/L) and nitrite (32.7 ± 1.7 mg/L), then only ammonium (27.15 ± 1.0 mg/L) and lastly only nitrite (32.7 ± 1.7 mg/L) as the influent. R1 was a coupled system composed of Anammox, Feammox, and NOx−-dependent Fe(II) oxidation (NDFO). The contribution of Feammox and NDFO to TIN removal was 27.1 ± 1.2% and 31.9 ± 0.7%. However, Anammox was the primary nitrogen transformation pathway. X-ray diffraction (XRD) analysis shows that iron hydroxide (Fe(OH)3) and iron oxide hydroxide (FeOOH) were generated in R1. The produced Fe(OH)3 and FeOOH were capable of participating in Feammox and formed a Fe(II)/Fe(III) cycle which further removed nitrogen. Therefore, a highly stable and impressive nitrogen removal performance was demonstrated in the iron sludge Anammox system under the cooperation of biological iron and iron bacteria. The study considered the enrichment of norank_c_OM190, Desulfuromonas, and Thiobacillus and their contribution to the Anammox, Feammox, and NDFO processes, respectively. This study provides a new perspective for the start-up and stable operation of low-strength wastewater Anammox engineering applications.

Photocatalytic Activity of Zero-Valent Iron Nanoparticles Highly Dispersed on Porous Carbon Materials

During the traditional homogeneous Fenton reaction process for water treatment, the consumption rate constant of Fe2+ is much greater than its regeneration rate constant, which makes Fe2+ an almost stoichiometric loss and produces iron sludge waste. In this article, highly dispersed zero-valent Fe nanoparticles loaded on porous carbon materials (Fe-EMC) were synthesized by a one-step calcination method using Flammulina velutipes natural carbon source and Fe(NO3)3 as raw materials to solve the aforementioned problem. The as-prepared Fe-EMC materials are characterized by X-ray diffraction analysis, scanning electron microscopy, electron probe microanalyzer, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and N2 adsorption–desorption measurements. It exhibits excellent photocatalytic activity for the degradation of methylene blue (MB) dyes under a broad pH region. Under conditions of 0.3 g/L Fe-EMC, 0.2 M/L H2O2, pH 7.0–11.0, and 50 mg/L MB, 97.98% of the MB dyes in the solution were completely degraded within 1 h. It was attributed to the efficient regeneration cycle between Fe2+ and Fe3+ in the Fenton-like system with light irradiation, which can promote the generation of active oxygen species.

CuCo and sulfur doped carbon nitride composite as an effective Fenton-like catalyst in a wide pH range.

The heterogeneous Fenton-like reaction, as an advanced oxidation process, is widely recognized attributed to its recyclability, wide pH response range, easy solid-liquid separation, and non-production of iron sludge. Recently, the bimetallic catalysts have attracted intense attention due to their high catalytic performance and excellent stability over a wide pH range. In this article, CuCo/SCN bimetallic catalyst was prepared by pyrolysis method with sulfur doped carbon nitride (SCN) as the carrier. Under the conditions of pH = 7, catalyst dosage of 0.8 g/L, and concentration of H2O2 of 15 mM, 20 mg/L of methyl orange (MO) can be completely removed within 1 h. With the synergistic action between bimetals and sulfur doped carbon nitride, the CuCO/SCN involved Fenton-like system exhibited excellent catalytic degradation efficiency and strong stability for MO in neutral and weak alkaline conditions. The EPR characterization proved that OH and O2 − were the main active components. Furthermore, CuCo/SCN involved Fenton-like system has good adaptability. Bimetallic CuCo/SCN catalyst has great application potential in the degradation of environmental pollutants.

Coupling photocatalytic water oxidation on decahedron BiVO4 crystals with catalytic wet peroxide oxidation for removing organic pollutions in wastewater

Catalytic wet peroxide oxidation (CWPO) is widely used to remove organic pollutants in industrial wastewater, however, the remained challenges in circulating Fe3+/Fe2+ ions, inevitably forming iron sludge and massively consuming H2O2 limit its applications. Herein, the coupling of CWPO and photocatalytic water oxidation on decahedron BiVO4 crystals was reported to solve the above challenges. Using spatial photogenerated charge separation between different facets of BiVO4, efficiently circulating Fe3+ to Fe2+ was realized with a selectivity of ~100%, which greatly inhibits the formation of iron sludge. Moreover, in-situ formation of H2O2 through two-hole-involved photocatalytic water oxidation on BiVO4 replenishes the H2O2 consuming in the CWPO process. The coupling system exhibits a much higher total organic carbon (TOC) removal than the CWPO process in various pollutants, giving an improvement of rate constant to more than 10 times. This work provides a potential coupling route for advanced degradation of organic pollutants in wastewater treatment.

Magnetic biochar based on platanus leaves and iron sludge for persulfate activation and catalytic degradation of tetracycline

In this study, a novel magnetic biochar (BC-Fen), synthesized by one-step pyrolysis at 800 °C using platanus leaves and iron-containing sludge in different mass ratios, was used to activate persulfate (PS) for tetracycline (TC) degradation. Results show that BC-Fe1:2 exhibited excellent degradation performance (91.4%), much better than the raw biochar (BC) (77.1%). And electron paramagnetic resonance (EPR) techniques demonstrated radical (SO4·−, ·OH, O2·−) and non-radical (1O2) pathways in BC-Fe1:2/PS, S O4·− was further proved to be the main active species by quenching experiments. Liquid chromatography-mass spectrometry (LC-MS) was utilized to identify the intermediates of TC and three possible degradation pathways were proposed. Additionally, the BC-Fe1:2 was weakly affected by pH and the coexisting anions in wastewater, and could still remove 80% of TC in the third cycle experiment. This study provides a new idea for converting backwash iron-containing sludge from groundwater treatment plants into valuable catalysts, and provides new insights into the key role of low-cost magnetic biochar in PS activation.

Preparation of Fe3O4@C with water treatment residuals and its potential in the magnetic coagulation process

The resource utilization of water treatment residuals is a hot topic in current research. In this work, a kind of carbon-doped magnetic nanoparticle (Fe 3 O 4 @C) was prepared using backwashing iron mud from the groundwater treatment plant as raw material. The Fe 3 O 4 @C was used for the magnetic coagulation (MC) process to evaluate the superiority of the MC process via comparing it with the traditional coagulation (TC) process. Meanwhile, the Fe 3 O 4 was also used for reference in the MC process to assess the potential and feasibility of the Fe 3 O 4 @C. The synthesized Fe 3 O 4 @C was characterized by SEM, TEM , XRD , VSM , and XPS . Then, coagulation performances and flocs properties were evaluated in both TC and MC processes. In addition, the various roles of magnetic particles in the MC process were discussed. The results showed that the participation of Fe 3 O 4 @C increased the turbidity removal rate from 90.3% to 96.8%, the ultra-violet absorbance at 254 nm (UV 254 ) removal rate from 81.4% to 86.5%, the dissolved organic carbon (DOC) removal rate from 60.4% to 66.9%, and the residual Al content dropped from 2.54 mg/L to 2.07 mg/L. Especially, it is found the Fe 3 O 4 @C had the potential to remove more small molecular weight organic matter during the coagulation process. The flocs were loose and easily broken due to the formation of the Al species-nanoparticle cluster. Fe 3 O 4 @C involved MC process can achieve the same water treatment efficiency as Fe 3 O 4 involved MC process, or even better. Especially, the magnetic seed of Fe 3 O 4 @C based on waste utilization can not only reduce the cost, but also has a good adsorption on small molecular organics. This provides a new reference way for the utilization of iron mud and the development of magnetic seed in the MC process. • Carbon-doped magnetic nanoparticles (Fe 3 O 4 @C) were prepared as magnetic seeds. • Fe 3 O 4 @C could improve the removal of small molecular organics by adsorption. • Recycle utilization of iron sludge was realized.

A novel alginate/PVA hydrogel -supported Fe3O4 particles for efficient heterogeneous Fenton degradation of organic dyes

A major deficiency of the traditional Fenton technology in practical application is the production of a large amount of sludge. In this paper, we first synthesized a kind of magnetic alginate/PVA hydrogel microspheres by inverse emulsion method along with coprecipitation method. Through the cross-linking action of iron ions, sodium alginate and polyvinyl alcohol formed a stable interpenetrating network structure, in which the magnetic Fe3O4 nanoparticles were stably wrapped. After drying, a novel alginate/PVA hydrogel-supported magnetic Fe3O4 particles were obtained. FTIR,XRD，SEM and EDS were used to confirm the composition and structure of the composite material. Furthermore, the resultant alginate/PVA hydrogel-supported Fe3O4 particles perform excellent broad-spectrum catalytic activity for Fenton degradation of organic dyes. Especially, compared with the classical Fenton catalyst, the double-crosslinked structure in the alginate/PVA hydrogel endows the magnetic Fe3O4 particles with high stability and recyclability. After reuse for six times, the mass loss of the composite material is only 10 %, which can avoid the generation of iron sludge in Fenton-like reaction to the greatest extent.

Adsorption of As(V) by magnetic alginate-chitosan porous beads based on iron sludge

Magnetic alginate-chitosan porous beads based on iron (Fe) sludge (M-ACFBs) were prepared by waterworks iron sludge, sodium alginate, chitosan, and magnetic nanoparticles. The magnetic nanoparticles were also synthesized using waterworks iron sludge through the co-precipitation method. In addition, a double gel network of sodium alginate and chitosan was constructed to improve the pH stability of the beads. At the same time, iron sludge served as the functional body for As(V) adsorption. The beads have uniform bead size (∼2 mm), high specific surface area (115.4 m2/g), distinguished mesopores (5.7 nm in size), and strong saturation magnetization (∼15.0 emu/g). The ratio of magnetic nanoparticles to iron sludge was optimized, and the effects of pH, contact time, temperature, and coexisting ions on As(V) adsorption effect were studied. Also, the adsorption kinetics and adsorption isotherms are thoroughly discussed. The adsorption mechanism of M-ACFBs on As(V) is concluded as ligand exchange and electrostatic attraction, and the maximum adsorption capacity is as high as 14.2 ± 0.4 mg/g. It is found that magnetite (Fe3O4) can form a maghemite (γ-Fe2O3) layer in an oxygen-rich environment, and at the surface of this γ-Fe2O3 layer, Fe(II), O2, and As(V) undergo complex redox reactions, leading to the appearance of As(III), which leads to a challenge for the disposal of arsenic-loaded adsorbents. This study provides a reference pathway for the resource utilization of backwash iron sludge and As(V) removal from water.

Ascorbic acid enhanced the zero-valent iron/peroxymonosulfate oxidation: Simultaneous chelating and reducing

The activation of peroxymonosulfate (PMS) with zero-valent iron (Fe(0)) generates reactive oxygen species that can effectively degrade aromatic organic contaminants. However, the low conversion rate from Fe(III) to Fe(II) and the aggregation of iron sludge restrict its wide application. In this study, a water-soluble vitamin, ascorbic acid (H2A), was combined with the conventional Fe(0)/PMS system to enhance the degradation of benzoic acid (BA). The removal efficiency of BA in the H2A/Fe(0)/PMS system was improved by 59.4% compared with the conventional Fe(0)/PMS system under the optimal conditions of 0.03 g/L Fe(0), 0.4 mM PMS, 0.1 mM H2A and initial pH of 4.5. This result was mainly credited to the excellent reducing and chelating ability of H2A, which promoted the generation of reactive oxygen species by accelerating the reduction of Fe(III) and forming Fe(III)-ascorbate complexes. Alcohol quenching and electron spin resonance (ESR) experiments demonstrated that the main reactive oxygen species for the degradation of BA in the H2A/Fe(0)/PMS system were •OH and SO4∙-. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analyzed the surface chemistry and phase transformation of fresh and used Fe(0). Based on the above experimental analysis, the degradation mechanism of BA in the H2A/Fe/PMS system was systematically explored. Common water matrix including Cl-, HCO3-, NO3- and humic acid inhibited the degradation of BA, while SO42- had negligible impacts. In addition, the degradation performance of the H2A/Fe(0)/PMS system in natural water environment and its degradation of various pollutants were also investigated. In summary, this study provides new insights for enhanced removal of refractory organics by improving the conventional Fe(0)/PMS system.

Cost-effective Application of Fenton Oxidation Process to the Biologically Treated Leachate Containing Refractory Organics and Nitrite

This paper presents an economic application of Fenton oxidation process to biological treatment effluent containing high-strength nitrite and refractory organics. To cope with two major drawbacks in applying Fenton oxidation process to the nitrite-rich effluent simultaneously, i.e. expensive reagent and excessive sludge production, we examined two pretreatment methodologies prior to the application of Fenton process in the following way. First, residual nitrite in the biologically treated leachate (BTL) was removed with iron-catalyzed air oxidation (ICAO) as a cost-effective pretreatment. The time necessary for ICAO reaction is approx. 5 hr. Secondly, to reduce organic loads iron sludge was regenerated for an alternative coagulant. With the regenerated iron sludge, the dose of Fenton’s regent can be reduced by half to remove residual COD in the BTL.