On the mechanism of microbe inactivation by metallic iron

Application of iron and steel slags in mitigating greenhouse gas emissions: A review

The comprehensive consideration of climate warming and by-product management in the iron and steel industry, has a significant impact on the realization of environmental protection and green production. Blast furnace slag (BFS) and steel slag (SS), collectively called iron and steel slags, are the main by-products of steelmaking. The economical and efficient use of iron and steel slags to reduce greenhouse gas (GHG) emissions is an urgent problem to be solved. This paper reviewed the carbonization and waste heat recovery of iron and steel slags, and the utilization of iron and steel slags as soil amendments, discussed their application status and limitations in GHG reduction. Iron and steel slags are rich in CaO, which can be used as CO₂ adsorbents to achieve a maximum concentration of 0.4-0.5 kg CO₂/kg SS. Blast furnace molten slag contains a considerable amount of waste heat, and thermal methods can recover more than 60 % of the heat energy. Chemical methods can use waste heat in the reaction to generate gas fuel, and iron in slags can be used as a catalytic component to promote chemical reaction. Waste heat recovery saves fuel and reduces the CO₂ emissions caused by combustion. When iron and steel slags are used as soil amendments, the iron oxides, alkaline substances, and SiO₂ in iron and steel slags can affect the emission of CH₄, N₂O, and CO₂ from soil, microorganisms, and crops, and achieve a maximum reduction of more than 60 % of the overall GHG of paddy fields. Finally, This paper provided valuable suggestions for future GHG reduction studies of iron and steel slags in energy, industry, and agriculture.

Fe0-Supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage

Results from different research suggest that metallic iron (Fe0) materials enhance anaerobic digestion (AD) systems to remove organics (chemical oxygen demand (COD)), phosphorus and nitrogen from polluted water. However, the available results are difficult to compare because they are derived from different experimental conditions. This research characterises the effects of Fe0 type and dosage in AD systems to simultaneously remove COD and nutrients (orthophosphate (PO43−), ammonium (NH4+), and nitrate (NO3− Lab-scale reactors containing domestic sewage (DS) were fed with various Fe0 dosages (0 to 30 g/L). Batch AD experiments were operated at 37 ± 0.5 °C for 76 days; the initial pH value was 7.5. Scrap iron (SI) and steel wool (SW) were used as Fe0 sources. Results show that: (i) SW performed better than SI on COD and PO43− removal (ii) optimum dosage for the organics and nutrients removal was 10 g/L SI (iii) (NO3− + NH4+) was the least removed pollutant (iv) maximum observed COD, PO43− and NO3− + NH4+ removal efficiencies were 88.0%, 98.0% and 40.0% for 10 g/L SI, 88.2%, 99.9%, 25.1% for 10 g/L SW, and 68.9%, 7.3% and 0.7% for the reference system. Fe0-supported AD significantly removed the organics and nutrients from DS.

Mitigate nitrate contamination in potato tubers and increase nitrogen recovery by combining dicyandiamide, moringa oil and zeolite with nitrogen fertilizer

Potato is considered a nitrogen (N) intensive plant with a low N use efficiency (NUE). The current study introduced an excellent approach by combining dicyandiamide (DCD), moringa seed oil (MSO), or zeolite (ZE), with N fertilizer for maximizing potato tuber yields and NUE as well as minimizing tubers nitrate (NO₃^-) accumulation. The impact of these materials on soil N availability and gaseous emissions (NH₃, and N₂O) was investigated under incubation conditions. A 2-year field experiment were carried out with seven treatments [without N (control), N fertilizer (350 kg N-urea ha^-1 as a recommended dose; Urea_RD), 75% of N recommended dose with DCD (Urea_75%RD+DCD), Urea_75%RD with 2% MSO (Urea_75%RD+MSO_2%), Urea_75%RD with 4% MSO (Urea_75%RD+MSO_4%), Urea_75%RD with 0.5 Mg ZE ha^-1 (Urea_75%RD+ZE_R1), and Urea_75%RD with 1.0 Mg ZE ha^-1 (Urea _75%RD+ZE_R2)]. We also conducted a 40-days incubation trial with the same treatments; however, urea was added at the rate of 200 mg N kg^-1 soil for all treatments, excluding the control. The addition of DCD, MSO, and ZE with urea under incubation conditions delayed the nitrification process, thereby causing a rise in NH₄⁺-N content and a decrease in NO₃^--N content. Ammonia-oxidizing bacteria (AOB) was inhibited (p ≤ 0.01) in treatments Urea+DCD, Urea+MSO_4%, and Urea+ZE_R2. The highest NUE indexes were recorded in treatment Urea_75%RD+DCD. The highest NO₃^- accumulation (567 mg NO₃^- kg^-1) in potato tubers was recorded in treatment Urea_RD. Whilest, the lowest NO₃^- content (81 mg NO₃^- kg^-1) was in treatment Urea_75%RD+DCD. The lowest cumulative N₂O emissions and highest cumulative NH₃ volatilization were observed in the treatment Urea+DCD under incubation conditions. Our findings demonstrated that N fertilizer rate could be reduced by 25%, while the tuber yields increased with an acceptable limit of NO₃^- content, resulting in economical, agronomical, and environmental benefits.

Industrial wastes: Fly ash, steel slag and phosphogypsum- potential candidates to mitigate greenhouse gas emissions from paddy fields

Waste management and global warming are the two challenging issues of the present global scenario. Increased human population has set the platform for rapid industrialization and modern agriculture. The industries such as energy, steel, and fertilizers play a significant role in improving the social, and economic status of human beings. The industrial production of energy (that involves combustion of coal), production of steel items and diammonium ammonium fertilizer generate a huge amount of wastes such as fly ash (FA), steel slag (SS) and phosphogypsum (PG), respectively. Inappropriate dumping of any kind of waste poses a threat to the environment, therefore, scientific management of waste is required to reduce associated environmental risks. These wastes i.e. SS, FA, and PG being rich sources of oxides of calcium (CaO), silicon (SiO₂), iron (FeO), and aluminum (Al₂O₃), etc. may affect the release of greenhouse gases from the soil. The information associated with the application of FA, SS, and PG onto the paddy fields and their impacts on methane and nitrous oxide emissions are highly fragmented and scarce. The present review extensively and critically explores the available information with respect to the effective utilization of FA, SS, and PG in paddy cultivation, their potential to mitigate greenhouse gases emission and their associated mechanisms. The fine grid assessment of these waste management provides new insight into the next level research and future policy options for industries and farmers.

Effects of steel slag and biochar amendments on CO2, CH4, and N2O flux, and rice productivity in a subtropical Chinese paddy field

Steel slag, a by-product of the steel industry, contains high amounts of active iron oxide and silica which can act as an oxidizing agent in agricultural soils. Biochar is a rich source of carbon, and the combined application of biochar and steel slag is assumed to have positive impacts on soil properties as well as plant growth, which are yet to be validated scientifically. We conducted a field experiment for two rice paddies (early and late paddy) to determine the individual and combined effects of steel slag and biochar amendments on CO₂, CH₄, and N₂O emission, and rice productivity in a subtropical paddy field of China. The amendments did not significantly affect rice yield. It was observed that CO₂ was the main greenhouse gas emitted from all treatments of both paddies. Steel slag decreased the cumulative CO₂ flux in the late paddy. Biochar as well as steel slag + biochar treatment decreased the cumulative CO₂ flux in the late paddy and for the complete year (early and late paddy), while steel slag + biochar treatment also decreased the cumulative CH₄ flux in the early paddy. The biochar, and steel slag + biochar amendments decreased the global warming potential (GWP). Interestingly, the cumulative annual GWP was lower for the biochar (55,422 kg CO₂-eq ha^-1), and steel slag + biochar (53,965 kg CO₂-eq ha^-1) treatments than the control (68,962 kg CO₂-eq ha^-1). Total GWP per unit yield was lower for the combined application of steel slag + biochar (8951 kg CO₂-eq Mg^-1 yield) compared to the control (12,805 kg CO₂-eq Mg^-1 yield). This study suggested that the combined application of steel slag and biochar could be an effective long-term strategy to reduce greenhouse gases emission from paddies without any detrimental effect on the yield.

Rapid Aerobic Inactivation and Facile Removal of Escherichia coli with Amorphous Zero-Valent Iron Microspheres: Indispensable Roles of Reactive Oxygen Species and Iron Corrosion Products

Zero valent iron (ZVI) is recently regarded as a promising alternative for water disinfection, but still suffers from low efficiency. Herein we demonstrate that amorphous zerovalent iron microspheres (A-mZVI) exhibit both higher inactivation rate and physical removal efficiency for the disinfection of Escherichia coli than conventional crystalline nanoscale ZVI (C-nZVI) under aerobic condition. The enhanced E. coli inactivation performance of A-mZVI was mainly attributed to more reactive oxygen species (ROSs), especially free •OH, generated by the accelerated iron dissolution and molecular oxygen activation in bulk solution. In contrast, C-nZVI preferred to produce surface bound •OH, and its bactericidal ability was thus hampered by the limited physical contact between C-nZVI and E. coli. More importantly, hydrolysis of dissolved iron released from A-mZVI produced plenty of loose FeOOH to wrap E. coli, increasing the dysfunction of E. coli membrane. Meanwhile, this hydrolysis process lowered the stability of E. coli colloid and caused its rapid coagulation and sedimentation, favoring its physical removal. These findings clarify the indispensable roles of ROSs and iron corrosion products during the ZVI disinfection, and also provide a promising disinfection material for water treatment.

Bacterial indicator reduction in dairy manure using hybrid zero-valent iron (h-ZVI) system

Novel and efficient animal wastewater treatment technologies of bacteria reduction are needed for preventing disease outbreak in animal herds and safeguarding environmental health. Zero-valent iron (ZVI) has been used to treat bacteria contaminated water for the past decades, but its passivation issue has been a major challenge. In this study, batch tests were performed to evaluate the effect of a hybrid zero-valent iron (h-ZVI) or a mixed ZVI/Fe₃O₄ media system on reduction of Escherichia coli (E. coli) levels. The h-ZVI media was created through a wet chemical process that uses nitrate to oxidize ZVI in the presence of externally added Fe²⁺ (aq.). Transforming ZVI into a h-ZVI system could overcome the passivation of ZVI and increase the reactivity of the media. The results demonstrated that E. coli cells in the bulk phase were removed rapidly by h-ZVI media. Majority of E. coli was attached (or adsorbed) to the surface of h-ZVI media within a few minutes, which suggested that adsorption was the dominant mechanism for bacterial removal in the initial phase. This adsorption was confirmed by fluorescence microscopy with CTC-DAPI double staining and transmission electron microscopy (TEM). Increasing contact time steadily inactivated E. coli; all cells were inactivated after 120 min of contact. The TEM results indicated that h-ZVI inactivated E. coli by causing direct damage on bacterial cell membrane. The results of this study strongly suggest that h-ZVI treatment can be used in water treatment industry where bacterial contamination is concerned.

White Teeth and Healthy Skeletons for All: The Path to Universal Fluoride-Free Drinking Water in Tanzania

Fluorosis has been prevalent in the great East African Rift Valley (EARV) since before this region was given a name. In the Tanganyika days, Germans reported elevated fluoride concentrations in natural waters. In the 1930s, the clear relationship between high fluoride level and mottling of teeth was established. Since then, the global research community has engaged in the battle to provide fluoride-free drinking water, and the battle is not yet won for low-income communities. An applicable concept for fluoride-free drinking water in the EARV was recently presented, using the Kilimanjaro as a rainwater harvesting park. The Kilimanjaro concept implies that rainwater is harvested, stored on the Kilimanjaro mountains, gravity-transported to the point of use, eventually blended with natural water and treated for distribution. This article provides a roadmap for the implementation of the Kilimanjaro concept in Tanzania. Specifically, the current paper addresses the following: (i) presents updated nationwide information on fluoride contaminated areas, (ii) discusses the quality and quantity of rainwater, and current rainwater harvesting practices in Tanzania, (iii) highlights how low-cost water filters based on Fe0/biochar can be integrating into rainwater harvesting (RWH) systems to provide clean drinking water, and (iv) discusses the need for strict regulation of RWH practices to optimize water collection and storage, while simplifying the water treatment chain, and recommends strict analytical monitoring of water quality and public education to sustain public health in the EARV. In summary, it is demonstrated that, by combining rainwater harvesting and low-cots water treatment methods, the Kilimanjaro concept has the potential to provide clean drinking water, and overcome fluorosis on a long-term basis. However, a detailed design process is required to determine: (i) institutional roles, and community contributions and participation, (ii) optimal location and sizing of conveyance and storage facilities to avoid excessive pumping costs, and (iii) project funding mechanisms, including prospects for government subsidy. By drawing attention to the Kilimanjaro concept, the article calls for African engineers and scientists to take the lead in translating this concept into reality for the benefit of public health, while simultaneously increasing their self-confidence to address other developmental challenges pervasive in Africa.

Investigating the suitability of Fe0 packed-beds for water defluoridation

A commercial granular metallic iron (Fe⁰) specimen was used to evaluate the suitability of Fe⁰ materials for removing aqueous fluoride (F^-) (water defluoridation). Experiments were performed to characterize the defluoridation potential of the tested Fe⁰ as influenced by the presence of chloride (Cl^-) and bicarbonate (HCO₃^-) ions using tap water (H₂O) as operational reference system. Duplicate column studies were conducted for 120 days (4 months) using an initial F^- concentration of 22.5 mg L^-1, columns flow rates were about 17 mL h^-1. Each column contained a reactive layer (11 cm) made up of 100 g of Fe⁰ in a 1:1 volumetric Fe⁰:sand mixture. The reactive layer was sandwiched between two layers of the same sand. A pure sand column was used as control system. After the F^- removal experiments, the columns were flushed by methylene blue (MB) and Orange II for 21 days. Removal studies revealed (i) no F^- removal in the control system, (ii) no F^- significant removal on the Cl^- system, (iii) limited F^- removal in the HCO₃^- system, and (iv) the best F^- removal efficiency in tap water (H₂O). Dye flushing studies confirmed the ion-selective nature of the Fe⁰/H₂O system and demonstrated the relatively low efficiency of the same for F^- removal. The overall results challenge the prevailing perception that water defluoridation using granular Fe⁰ is not possible and suggest that effective water defluoridation in Fe⁰ packed-beds is pure a site-specific design issue.

Arsenic removal by discontinuous ZVI two steps system for drinking water production at household scale

Different countries in Europe still suffer of elevated arsenic (As) concentration in groundwaters used for human consumption. In the case of households not connected to the distribution system, decentralized water supply systems, such as Point of Use (POU) and Point of Entry (POE), offer a direct benefit for the consumers. Field scale ex-situ treatment systems based on metallic iron (ZVI) are already available for the production of reduced volumes of drinking water in remote areas (village scale). To address drinking water needs at larger scale, we designed a pilot unit able to produce an elevated daily volume of water for human consumption. We tested the long-term As removal efficiency of a two steps ZVI treatment unit for the production of 400 L/day clean water based on the combination of ZVI corrosion process with sedimentation and retention of freshly formed Fe precipitates. The system treated 100 μg/L As(V)-contaminated oxic groundwater in a discontinuous operation mode at a flow rate of 1 L/min for 31 days. Final removal was 77-96% and the most performing step was aeration/sedimentation (A/S) tank with a 60-94% efficiency. Arsenic in the outflow slightly exceeded the drinking water limit of 10 μg/L only after 6000 L treated and Fe concentration was always below 0.2 mg/L. Under proposed operating conditions ZVI passivation readily occurred and, as a consequence, Fe production sharply decreased. Arsenic mobility attached to particulate was 13-60% after ZVI column and 37-100% after A/S tank. Uniform amorphous cluster of Fe nanoparticles (100 nm) formed during aeration drove As removal process with an adsorption capacity corresponding to 20.5 mg_As/g_Fe. Research studies often focus only on chemico-physical aspects disregarding the importance of biological processes that may co-occur and interfere with ZVI corrosion, As removal and safe water production. We explored the microbial transport dynamics by flow cytometry, proved as a suitable tool to monitor the fate of both single cells and bioactive particles along the treatment train of the pilot unit. A net release of bioactive particles, representing on average 26.5% of flow cytometric events, was promoted by the ZVI filter, with densities 10 times higher than those found in the inflow. In conclusion, the proposed system was efficient to treat large daily volumes of As contaminated groundwater. However, filter design and operating conditions should be carefully adapted to specific situation, since several key factors affect As removal efficiency. An effort in the optimization of ZVI filter design should be made to reduce fast observed ZVI passivation and low As adsorption capacity of the whole filter. More attention to biomass retention and bioactive particles travelling within the unit should be given in order to elucidate bacteria influences on As removal efficiency and related sanitary risks on long term basis.

Aluminum electrocoagulation as pretreatment during microfiltration of surface water containing NOM: A review of fouling, NOM, DBP, and virus control

Electrocoagulation (EC) is the intentional corrosion of sacrificial anodes (typically aluminum or iron) by passing electricity to release metal-ion coagulant species and destabilize a wide range of suspended, dissolved, and macromolecular contaminants. It can be integrated ahead of microfiltration (MF) to effectively control turbidity, microorganisms, and disinfection by-products (DBPs) and simultaneously maintain a high MF specific flux. This manuscript summarizes the current knowledge on MF pretreatment by aluminum EC particularly focusing on mechanisms of (i) electrocoagulant dosing, (ii) (bio)colloid destabilization, (iii) fouling reductions, and (iv) enhanced removal of viruses, natural organic matter (NOM), and DBP precursors. Electrolysis efficiently removes hydrophobic NOM, viruses, and siliceous foulants. Aluminum effectively electrocoagulates viruses by physically encapsulating them in flocs, neutralizing their surface charge and reducing electrostatic repulsion, and increasing hydrophobic interactions between any sorbed NOM and free viruses. New results included herein demonstrate that EC achieves DBP control by removing NOM, reducing chlorine-reactivity of remaining NOM, and inducing a slight shift toward more brominated trihalomethanes and haloacetic acids. EC reduces MF fouling by forming large flocs that tend to deposit on the membrane surface, i.e. decrease pore penetration and forming more permeable cakes and by reducing foulant mass in case of significant floc-flotation.

Treatment of reverse osmosis (RO) concentrate by the combined Fe/Cu/air and Fenton process (1stFe/Cu/air-Fenton-2ndFe/Cu/air)

To decompose or transform the toxic and refractory reverse osmosis (RO) concentrate and improve the biodegradability, 1stFe/Cu/air-Fenton-2ndFe/Cu/air were developed to treat RO concentrate obtained from an amino acid production plant in northern China. First, their operating conditions were optimized thoroughly. Furthermore, 5 control experiments were setup to confirm the superiority of 1stFe/Cu/air-Fenton-2ndFe/Cu/air and synergistic reaction between Fe/Cu/air and Fenton. The results suggest that the developed method could obtain high COD removal (65.1%) and BOD5/COD ratio (0.26) due to the synergistic reaction between Fe/Cu/air and Fenton. Under the optimal conditions, the influent and effluent of 1stFe/Cu/air-Fenton-2ndFe/Cu/air and 5 control experiments were analyzed by using UV, FTIR, EEM and LC, which confirm the superiority of 1stFe/Cu/air-Fenton-2ndFe/Cu/air. Therefore, the developed method in this study is a promising process for treatment of RO concentrate.

Nanoscale Metallic Iron for Environmental Remediation: Prospects and Limitations

The amendment of the subsurface with nanoscale metallic iron particles (nano-Fe(0)) has been discussed in the literature as an efficient in situ technology for groundwater remediation. However, the introduction of this technology was controversial and its efficiency has never been univocally established. This unsatisfying situation has motivated this communication whose objective was a comprehensive discussion of the intrinsic reactivity of nano-Fe(0) based on the contemporary knowledge on the mechanism of contaminant removal by Fe(0) and a mathematical model. It is showed that due to limitations of the mass transfer of nano-Fe(0) to contaminants, available concepts cannot explain the success of nano-Fe(0) injection for in situ groundwater remediation. It is recommended to test the possibility of introducing nano-Fe(0) to initiate the formation of roll-fronts which propagation would induce the reductive transformation of both dissolved and adsorbed contaminants. Within a roll-front, Fe(II) from nano-Fe(0) is the reducing agent for contaminants. Fe(II) is recycled by biotic or abiotic Fe(III) reduction. While the roll-front concept could explain the success of already implemented reaction zones, more research is needed for a science-based recommendation of nano-Fe(0) for subsurface treatment by roll-fronts.

Inactivation and magnetic separation of bacteria from liquid suspensions using electrosprayed and nonelectrosprayed nZVI particles: observations and mechanisms

Here, nonelectrosprayed nanoscale zerovalent iron (NE-nZVI), electrosprayed nZVI (E-nZVI) and preoxidized nZVI (O-nZVI) particles were applied to inactivating Bacillus subtilis, Escherichia coli as well as bacteria in various wastewater samples. In addition, magnetic separation was applied to the mixture of 0.2 mL bacterial sample and 1.8 mL E-nZVI or NE-nZVI suspensions. Bacterial concentrations and optical density of the supernatants were analyzed using culturing, optical adsorption and qPCR tests. In general, for wastewater samples the inactivations were shown to range from 1-log to 3-log. PCR-DGGE analysis indicated that no gene mutation occurred when bacteria were treated with nZVI. Using magnetic separation, significant physical removals, revealed as a function of nZVI type (NE-,E- and O-nZVI) and bacterial concentration, up to 6-log were obtained. E-nZVI and NE-nZVI were shown to react differently with B. subtilis and E. coli, although exhibiting similar inactivation rates. qPCR tests detected higher amount of DNA in the supernatants from mixing E. coli with NE-nZVI, but less for E-nZVI. However, the opposite was observed with B. subtilis. Our data together with optical adsorption analysis suggested that the inactivation and magnetic separation mainly depend on Fe(0)/Fe(3)O(4) shell compositions, the type of bacteria (aerobic and anaerobic) and their concentrations.