Methacrylic acid in situ modified steel converter slag/natural rubber composites: Resourceful utilization of steelmaking solid wastes

As a by-product of the steelmaking industry, the large-volume production and accumulation of steel converter slag cause environmental issues such as land occupation and dust pollution. Since metal salts of unsaturated carboxylic acid can be used to reinforce rubber, this study explores the innovative application of in-situ modified steel slag, mainly comprising metal oxides, with methacrylic acid (MAA) as a rubber filler partially replacing carbon black. By etching the surface of steel slag particles with MAA, their surface roughness was increased, and the chemical bonding of metal methacrylate salt was introduced to enhance their interaction with the molecular chain of natural rubber (NR). The results showed that using the steel slag filler effectively shortened the vulcanization molding cycle of NR composites. The MAA in-situ modification effectively improved the interaction between steel slag and NR molecular chains. Meanwhile, the physical and mechanical properties, fatigue properties, and dynamic mechanical properties of the experimental group with MAA in-situ modified steel slag (MAA-in-situ-m-SS) were significantly enhanced compared with those of NR composites partially filled with unmodified slag. With the dosage of 7.5 phr or 10 phr, the above properties matched or even exceeded those of NR composites purely filled with carbon black. More importantly, partially replacing carbon black with modified steel slag reduced fossil fuel consumption and greenhouse gas emission from carbon black production. This study pioneered an effective path for the resourceful utilization of steel slag and the green development of the steelmaking and rubber industries.

Enhanced ammonia removal in tidal flow constructed wetland by incorporating steel slag: Performance, microbial community, and heavy metal release

Utilizing alkaline solid wastes, such as steel slag, as substrates in tidal flow constructed wetlands (TFCWs) can effectively neutralize the acidity generated by nitrification. However, the impacts of steel slag on microbial communities and the potential risk of heavy metal release remain poorly understood. To address these knowledge gaps, this study compared the performance and microbial community structure of TFCWs filled with a mixture of steel slag and zeolite (TFCW-S) to those filled with zeolite alone (TFCW-Z). TFCW-S exhibited a much higher NH4+-N removal efficiency (98.35 %) than TFCW-Z (55.26 %). Additionally, TFCW-S also achieved better TN and TP removal. The steel slag addition helped maintain the TFCW-S effluent pH at around 7.5, while the TFCW-Z effluent pH varied from 3.74 to 6.25. The nitrification and denitrification intensities in TFCW-S substrates were significantly higher than those in TFCW-Z, consistent with the observed removal performance. Moreover, steel slag did not cause excessive heavy metal release, as the effluent concentrations were below the standard limits. Microbial community analysis revealed that ammonia-oxidizing bacteria, ammonia-oxidizing archaea, and complete ammonia-oxidizing bacteria coexisted in both TFCWs, albeit with different compositions. Furthermore, the enrichment of heterotrophic nitrification-aerobic denitrification bacteria in TFCW-S likely contributed to the high NH4+-N removal. In summary, these findings demonstrate that the combined use of steel slag and zeolite in TFCWs creates favorable pH conditions for ammonia-oxidizing microorganisms, leading to efficient ammonia removal in an environmentally friendly manner.

Aqueous mineral carbonation of three different industrial steel slags: absorption capacities and product characterization

Heavy carbon industries produce solid side stream materials that contain inorganic chemicals like Ca, Na, or Mg, and other metals such as Fe or Al. These inorganic compounds usually react efficiently with CO2 to form stable carbonates. Therefore, using these side streams instead of virgin chemicals to capture CO2 is an appealing approach to reduce CO2 emissions. Herein, we performed an experimental study of the mineral carbonation potential of three industrial steel slags via aqueous, direct carbonation. To this end, we studied the absorption capacities, reaction yields, and physicochemical characteristics of the carbonated samples. The absorption capacities and the reaction yields were analyzed through experiments carried out in a reactor specifically designed to work without external stirring. As for the physicochemical characterization, we used solid-state Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscope (SEM). Using this reactor, the absorption capacities were between 5.8 and 35.3 g/L and reaction yields were in the range of 81-211 kg CO2/ton of slag. The physicochemical characterization of the solid products with solid FTIR, XRD and SEM indicated the presence of CaCO3. This suggests that there is potential to use the carbonated products in commercial applications.

Stability of soil slope in Almaty covered with steel slag under the effect of rainfall

The issue of rainfall-induced slope failure has attracted more attention from geotechnical engineers as a consequence of global warming. Current cumulative waste disposal has generated scientific interest in the utilization of waste materials in geotechnical design for climate change adaptation measures. Taking into consideration the effect of slope height and angle, steel slag-a waste product derived from the production of steel-was investigated as a slope cover against rainfall. To assess the stability of the slope and the infiltration of water into the soil, numerical analyses were conducted using both SEEP/W and SLOPE/W software in conjunction with rainfall conditions. Based on the findings, it can be concluded that increasing the slope's elevation and inclination will have an adverse effect on its safety factor. Steel slag can nevertheless be utilized for minimizing rainwater infiltration into the slope, as indicated by the pore-water pressure variations and graphs of the safety factor versus time. For a 20-m slope height, steel slag slopes have demonstrated a lower factor of safety difference in comparison to the initial slope without remediation. Regardless of slope angle and slope height, the safety factor reduces marginally during rainfall.

Mechanistic study on the promotion of Ca2+ leaching in steel slag through high-temperature solid waste modification

Indirect carbonation of steel slag is an effective method for CO2 storage, reducing emissions, and promoting cleaner production in the steel industry. However, challenges remain, such as low Ca2+ leaching rates and slag management complexities arising from variations in mineral compositions. To address this, a high-temperature modification process is proposed to alter the mineral composition and facilitate the synergistic utilization of calcium and iron. This study delves into the effects of various solid waste modifications on the leaching of Ca2+ and the total iron content within steel slag. Results show that high-basicity modified slag forms Ca2(Al, Fe)2O5, reducing calcium leaching. Low-alkalinity modified slag produces calcium-rich aluminum minerals and also reduces the leaching of Ca2+ ions. At a basicity of 2.5, coal gangue, fly ash, and blast slag achieve maximum Ca2+ leaching rates of 88.93%, 89.46%, and 90.17%, respectively, with corresponding total iron contents of 41.46%, 37.72%, and 35.29%. Upgraded coal gangue exhibits a 50.02% increase in calcium leaching and a 15.58% increase in total iron content compared to the original slag. This enhances CO2 fixation and iron resource utilization. Overall, the proposed indirect carbonation and iron enrichment modification offer a novel approach for the resource utilization and environmental stability of steel slag.

Sustainable assessment and synergism of ceramic powder and steel slag in iron ore tailings-based concrete

Solid waste management is a critical issue worldwide. Effectively utilizing these solid waste resources presents a viable solution. This study focuses on Iron ore tailings (IOTs), a solid waste generated during iron ore processing, which can be used as supplementary cementitious materials (SCMs) but have low reactivity, hindering their large-scale application in concrete production. To address this, ternary SCMs were prepared using ceramic powder (CP) and steel slag (SS) to enhance the performance of concrete incorporating IOTs. The study found that the synergistic effect of CP and SS significantly improved the compressive strength of concrete, with a notable increase of up to 21% compared to concrete with IOTs alone. Mercury intrusion porosimetry (MIP) and backscattering electron (BSE) analyses revealed that the ternary SCMs significantly optimized the characteristics of the interfacial transition zone (ITZ), which in turn enhanced the compressive properties of the concrete. This contributed to maintaining the structural integrity of the concrete, even amidst variations in the pore structure. Importantly, the incorporation of ternary SCMs led to a 23% reduction in carbon emissions, from 400.01 kg CO2/m3 to 307.48 kg CO2/m3, and elevated eco-strength efficiency from 0.1 to 0.14. The study highlights the role of multi-material synergy in developing composite SCMs systems, fostering the sustainable advancement of green building materials.

Utilizing different types of biomass materials to modify steel slag for the preparation of composite materials used in the adsorption and solidification of Pb in solutions and soil

This study utilized discarded steel slag (SS) as raw material and prepared modified steel slag materials (SS-SBC, SS-NBC, SS-BHA) through modification with biomass materials such as straw biochar (SBC), nutshell biochar (NBC), and biochemical humic acid (BHA). These materials were then applied for the removal of Pb from both solution and soil. The physical and chemical properties of the materials were analyzed using characterization techniques such as SEM, EDS, XRD, and BET. The specific surface area of the modified materials increased from the original 3.8584 m2/g to 34.7133 m2/g, 181.7329 m2/g, and 7.7384 m2/g, respectively. The study then explored the influence of different adsorption conditions on the adsorption capacity of Pb in solution, determining the optimal conditions as follows: initial concentration of 200 mg/L, adsorbent mass of 0.04 g, temperature of 15 °C, and pH = 2. To further investigate the adsorption process, kinetic and isotherm models were established. The results indicated that the adsorption process for all three materials followed a pseudo-second-order kinetic model and Freundlich isotherm model, suggesting a multi-layer chemical adsorption. Thermodynamic analysis revealed that the adsorption process was an exothermic spontaneous reaction. Soil cultivation experiments were conducted to explore the effects of different material addition amounts and cultivation times on the passivation of Pb-polluted soil. Analysis of heavy metal forms in the soil revealed that the addition of modified materials reduced the acid-extractable form of Pb in the soil and increased the residual form, which is beneficial for reducing the migration of Pb in the soil. FT-IR and XPS analyses were employed to study the functional groups, element composition, and valence states before and after adsorption passivation of Pb by the three materials. The results confirmed that the adsorption mechanisms of SS-SBC, SS-NBC, and SS-BHA mainly involved electrostatic adsorption, ion and ligand exchange, and surface precipitation. This study not only provides a new material for adsorbing and immobilizing heavy metals in soil and water but also offers a new approach for the resource utilization of steel slag waste.

Microwave deicing properties and carbon emissions assessment of asphalt mixtures containing steel slag towards resource conservation and waste reuse

A large amount of solid waste, such as steel slag (SS), is generated annually. At the same time, the shortage of road construction materials is becoming a concern. In this study, to recycle and reuse SS as a substitute for natural aggregates to achieve resource conservation and sustainable development of roads were conducted. First, the electromagnetic performance of SS was explored to evaluate its wave-absorbing properties. Next, the effect of different SS contents on heating properties, surface temperature, heating uniformity, and ice melting time (IMT) were investigated. Finally, the carbon emissions assessment (CEA) of conventional asphalt mixture (CAM) and steel slag asphalt mixture (SSAM) was compared. Results indicated that SS has ferromagnetic behavior and higher electromagnetic parameters, showing better wave-absorbing properties than limestone. There were three stages during microwave heating (MH): ice melting, moisture emitting, and stabilization. In addition, heating uniformity tends to be poor with the increase of SS, and samples with 100 % content of SS have the highest standard deviation of 21.04 °C and 20.77 °C after 270 s at -10 °C and - 20 °C. Samples containing 50 % SS have the best microwave deicing properties, which can reduce the IMT by 28.57 % to 46.18 % at different initial freezing temperatures and ice thickness compared to CAM. Furthermore, CEA revealed that CAM and SSAM's carbon emissions over road construction's life cycle are similar (around 27,000 kg) and originate mainly from the mixing and raw material extraction phases. However, SSAM leads to better environmental and economic benefits and provides an exemplary resource conservation and waste reuse solution.

Vanadium extraction from steel slag: Generation, recycling and management

The metal vanadium has superior physical and chemical properties and has a wide range of applications in many fields of modern industry. The increasing demand for vanadium worldwide has led to the need to guarantee sustainable vanadium production. The smelting process of vanadium and titanium magnetite produces vanadium-bearing steel slag, a key material for vanadium extraction. Herein, vanadium production, consumption, and steel slag properties are discussed. A detailed review of methods for extracting vanadium from vanadium-bearing steel slag is presented, including the most commonly used roasting and leaching method, and direct leaching, bioleaching and enhanced leaching methods are also described. Finally, the rules and regulations of steel slag management are introduced. In general, it is necessary to further develop environmentally friendly vanadium extraction methods and technologies from vanadium containing solid wastes. This study provides research directions for the technology of vanadium extraction from steel slag.

Manufacturing non-sintered ceramsite from dredged sediment, steel slag, and fly ash for lightweight aggregate: production and characterization

Green and low-carbon materialization for dredged sediment (DS) is limited due to its low pozzolanic activity. In this study, a novel DS-based non-sintered lightweight aggregate (LWA) is developed by steel slag (SS) and fly ash (FA) activation. Process optimization is performed by the response surfaces, and the basic properties and characterization of the optimal product are investigated. Results indicated that the optimized design ceramic aggregate (ODCA) was prepared as follows: raw pellets comprising of 59.2% DS, 5% SS, 35.8% FA, 5% MK, 5% H2O2, and 2‰ foam stabilizer were activated by alkali activator (1.5 weight ratio of 14 M NaOH to water glass) and then cured at 80 °C and 95% humidity for 24 h. The basic and environmental performances of ODCA were in accordance with standards, whose bulk density was as low as 665.8 kg/m3, the high cylinder compressive strength was 6.143 MPa, and leaching concentrations of heavy metals were controllable. The regulation mechanism of LWA performances could be summarized as follows. SS and FA additives played the role for the mechanical strength enhancement and passivation of heavy metals, which promoted the formation of sillimanite, chabazite, and C-S-H / C-S-A-H gels in ODCA. The bulk density of ODCA was greatly reduced by H2O2 addition, where ODCA had an open-pore structure with a median pore size of 4969.75 nm. Note that C-S-H/C-S-A-H were the key hydration products to give ODCA light density and high mechanical strength, simultaneously.

Properties and mechanisms of steel slag strengthening microbial cementation of cyanide tailings

The advantages of microbial induced carbonate precipitation (MICP) as bio-cementation technology for tailings-solidification are under extensive investigation. In order to improve performance of bio-cementation, many strengthening materials were applied to the bio-cementation of tailings. Steel slag (SS) is a kind of industrial solid waste, its chemical composition and mineral composition are similar to cement, and it has a certain application prospect as an auxiliary cementing material. In this study, the properties and mechanism of SS strengthening MICP cementation of cyanide tailings (CT) were investigated. The results showed that Sporosarcina pasteurii growth is not inhibited by SS, and Sporosarcina pasteurii can promote the hydration reaction of SS, providing a suitable alkaline environment and Ca2+, promoting the production of more CaCO3 in the MICP process. When 200 mL of CT leachate was added 1.4 g SS (200-400 mesh), the adsorption of Cu, Pb, Zn, Cd, total cyanide (T-CN), and free cyanide (F-CN) reached 48.05%, 44.28%, 36.25%, 16.67%, 79.05%, and 67.20%, respectively. The maximum unconfined compressive strength（UCS） of the cemented body (with 5%, 150 mesh SS) was 1.97 MPa, which was 3.396 times as high as that without SS. The cemented body with the addition of SS (5%, 150 mesh) contained more carbonate bound Cu (2.75%), Pb (4.89%), Zn (5.37%), and Cd (5.75%), and less exchangeable Cu (3.65%), Pb (6.85%), Zn (2.27%), and Cd (4.42%) than that without SS. In summary, the addition of SS improved the UCS of cemented bodies and the stability of heavy metals and cyanide, reduced the environmental risks existing in the process of CT storage. Meanwhile, it also provides new ideas for resource utilization of industrial solid waste SS and improvement of mine filling materials.

Effect of two different crystal forms of alumina on hydration properties and mechanical properties of steel slag-cement composite cementitious materials

In the present work, two common nano-alumina (NA) with different crystal forms (α-NA and γ-NA) are used to research the effects of steel slag-cement composite cementitious materials, which include the hydration properties and mechanical properties. The results show that the NA can enhance the strength of steel slag-cement composite cementitious materials, especially the early strength. Meanwhile, when the addition amount of γ-NA was 1%, the maximum compressive strength and flexural strength at 28 d were 35.43 and 5.21 MPa, respectively; when the addition amount of α-NA was 3%, the maximum compressive strength and flexural strength at 28 d were 36.27 and 4.89 MPa, respectively. In addition, according to the analysis of X-ray diffractometer and differential thermal analysis, it was concluded that the effects of the two types of alumina on the strength were mainly pozzolanic effect and filling effect. The pozzolanic effect of γ-NA was significantly stronger than that of α-NA. However, the large surface area of γ-NA affected the dispersion of the particles and the filling effect. According to scanning electron microscope analysis, compared with α-NA, γ-NA had significantly more hydration products and tighter adhesion. In conclusion, the addition of NA not only improved the properties but further realized the value-added utilization of steel slag.

Environmental, economic and engineering performances of aqueous carbonated steel slag powders as alternative material in cement pastes: Influence of particle size

The low reactivity and volume expansion issue of steel slag limits its application as alternative to cement. Studies demonstrated that aqueous carbonation (AC) can enhance the cementitious properties of finely sized steel slag as a cementitious supplementary material (SCM). However, the impact of particle size on the CO2 uptake capacity and its association of performance of carbonated steel slag remains unexplored. This study aims to optimize the grinding levels by examining the fineness of the steel slag used as SCM to reduce the high-energy consumption while maintaining the CO2 sequestration and properties of SCM. The results show that reducing the size of steel slag is favorable for CO2 sequestration (particle size 22.4-112.6 μm corresponds to sequestration of ∼88.5-37.9 kg CO2/t steel slag) and improve the leaching of Mg ions for mineralization. The life cycle assessment shows that the global warming potential of AC of steel slag is ∼96.2-24.9 kg CO2-eq/t steel slag, which can offset the carbon emissions due to further grinding. The 28-day compressive strength of the cement pastes blended with finer carbonated steel slag was also relatively higher due to the formation of mono-carboaluminates and stabilization of ettringite in facilitating the bond strength between the carbonated steel slag particle and the cement paste matrix. According to 3E (engineering, environmental and economic) triangle model, 22.4 μm steel slag powder showed the best comprehensive performance, including an increased revenue of 40.8 CNY/ton steel slag.

Carbonation of steel slag at low CO2 concentrations: Novel biochar cold-bonded steel slag artificial aggregates

Carbonation technology resolves the volume expansion of steel slag by combining CO2 with f-CaO, but the previous stringent carbonation conditions (99%vol) significantly limit the application prospect of steel slag. To achieve the carbonation of steel slag at lower CO2 concentrations, a novel cold-bonded artificial aggregates (CASSAs) based on steel slag and biochar is produced in this paper. The carbon capture capacities of CASSAs with different biochar contents (5 wt%, 10 wt%, and 15 wt%) are investigated in a low-CO2 concentration environment (10.79 % vol) and natural environment using the porosity and CO2 adsorption capacity of biochar. The changes in the performance of CASSAs before and after carbonation are investigated at different curing ages (7 d and 28 d). The results reveal that biochar increases the pores of the CASSAs. At 7 d, B15 achieves complete carbonation at low concentrations and can uptake 6.5 wt% of CO2. CO2 adsorption capacity by biochar in the natural environment facilitates the diffusion of CO2 in CASSAs. Regarding mechanical properties, the addition of biochar makes B15 at 7 d half as strong as B0, but B15 exhibits long-term strength development. B15 at 7 d has a strength of 8.49 MPa after carbonation, which is almost the same as B0. In addition, B15 achieves a net CO2 emission of -39.9 kg/ton. This study combines biochar with CASSAs to provide a potential method to carbonate steel slag at low CO2 concentrations. A new methodology was also used to quantitatively assess the ability of biochar CASSAs to solidify CO2 under low concentration conditions and natural environments from a macroscopic perspective. Biochar CASSAs have great potential to realize resource utilization and carbon capture from steel slag.

Application of common industrial solid waste in water treatment: a review

Industrial solid waste has a wide range of impacts, and it is directly or indirectly related to land, atmosphere, water, and other resources. Industrial solid waste has a large amount of production, complex and diverse components and contains a variety of harmful substances. However, as industrial by-products, it also has a lot of available value. Industrial solid waste has been continuously studied in water treatment due to its special composition and porous and loose structure. It is known that there are few reviews of various industrial solid wastes in the field of wastewater treatment, and most of them only discuss single industrial solid waste. This paper aims to sort out the different studies on various solid wastes such as fly ash, red mud, wastewater sludge, blast furnace slag and steel slag in dyeing, heavy metal, and phosphorus-containing wastewater. Based on the modification of industrial solid waste and the preparation of composite materials, adsorbents, coagulants, catalysts, filtration membranes, geological polymers, and other materials with high adsorption properties for pollutants in wastewater were formed; the prospect and development of these materials in the field of wastewater were discussed, which provides some ideas for the mutual balance of environment and society. Meanwhile, some limitations of solid waste applications for wastewater treatment have been put forward, such as a lack of further researches about environment-friendly modification methods, application costs, the heavy metal leaching, and toxicity assessment of industrial solid waste.

Co-disposal of municipal solid waste incineration bottom ash (MSWIBA) and steel slag (SS) to improve the geopolymer materials properties

In previous studies, municipal solid waste incineration bottom ash (MSWIBA) exhibited low compressive strength when made into geopolymer materials due to the lack of active Ca. The introduction of steel slag (SS) not only supplements MSWIBA with active Ca, but also enables further treatment of SS, an underutilized solid waste. In this study, mechanical properties, XRD, TGA, FTIR and MIP are the means to evaluate this binary geopolymer. The heavy metal leaching concentration of this geopolymer was used as a basis for assessing its environmental impact. The results show that the introduction of SS helps to improve the compressive strength of geopolymers. The introduction of SS supplements the active Ca and promotes the production of C-(A)-S-H gels. Increasing the alkali doping on this basis contributes to the dissolution of active substances in MSWIBA and SS and promotes the generation of silica-aluminate gels, which likewise contributes to the development of compressive strength of geopolymers. The activation of MSWIBA by alkali can be used as an aluminum removal process, which can reduce the volume of harmful pores in the geopolymer. The solidification efficiency of heavy metals after the introduction of SS can be>90%.

Enhanced electrokinetic remediation by magnetic induction for the treatment of co-contaminated soil

The electroplating industry site is an important reservoir of per- and poly-fluoroalkyl substances (PFASs) and heavy metals. In this work, a novel electrokinetic in-situ chemical oxidation system was established to restore an actual soil co-contaminated with high concentrations of heavy metals (Cr, Cu, Zn and Ni) and PFASs. Potassium persulfate (PS, K₂S₂O₈) and industrial waste steel slag were used as the oxidant and activator, respectively. The steel slag was evenly added in the soil, while PS was dosed in the cathode chamber. Citric acid fermentation broth produced by Aspergillus niger was added in the anode chamber to act as the metal chelator. A periodic alternating magnetic field was employed to enhance the catalytic performance of steel slag for PS. After 15-day treatment, 86.7% of PFASs and 87.2% of heavy metals were removed without PFASs accumulation in the electrolyte, with a defluorination percentage of 79.2%. The remediated soil had no phytotoxicity for wheat seed growth based on 7-day cultivation results. The quality of remediated soil could reach the national Class II criteria for residential use. Electron paramagnetic resonance spectroscopy analysis demonstrated that SO₄^•- and ^•OH were the major oxidative radicals responsible for PFASs degradation. Adding steel slag in the soil performed better than that in the cathode chamber based on pollutant removal and alleviating soil acidification. Magnetic induction could enhance PS activation by promote the corrosion of steel slag and thermal activation, thus increasing electrical current and electroosmotic flow, enhancing the transport of citric acid and PS, significantly improving the removal efficiency of heavy metals and PFASs.

The synergistic hydration mechanism and environmental safety of multiple solid wastes in red mud-based cementitious materials

Red mud (RM) is a solid waste material with high alkalinity and low cementing activity component. The low activity of RM makes it difficult to prepare high-performance cementitious materials from RM alone. Five groups of RM-based cementitious samples were prepared by adding steel slag (SS), grade 42.5 ordinary Portland cement (OPC), blast furnace slag cement (BFSC), flue gas desulfurization gypsum (FGDG), and fly ash (FA). The effects of different solid waste additives on the hydration mechanisms, mechanical properties, and environmental safety of RM-based cementitious materials were discussed and analyzed. The results showed that the samples prepared from different solid waste materials and RM formed similar hydration products, and the main products were C-S-H, tobermorite, and Ca(OH)₂. The mechanical properties of the samples met the single flexural strength criterion (≥ 3.0 MPa) for first-grade pavement brick in the Industry Standard of Building Materials of the People's Republic of China-Concrete Pavement Brick. The alkali substances in the samples existed stably, and the leaching concentrations of the heavy metals reached class III of the surface water environmental quality standards. The radioactivity level was in the unrestricted range for main building materials and decorative materials. The results manifest that RM-based cementitious materials have the characteristics of environmentally friendly materials and possess the potential to partially or fully replace traditional cement in the development of engineering and construction applications and it provides innovative guidance for combined utilization of multi-solid waste materials and RM resources.

CO2 sequestration and CaCO3 recovery with steel slag by a novel two-step leaching and carbonation method

The steel smelting process produces extensive CO₂ and Ca-containing steel slag (SS). Meanwhile, the low value utilization of steel slag results in the loss of Ca resources. CO₂ sequestration utilizing SS can reduce carbon emissions while achieving Ca circulation. However, conventional SS carbon sequestration methods suffer from slow reaction rates, finite Ca usage efficiency, and difficulty separating the CaCO₃ product from SS. Herein, an innovative two-step leaching (TSL) and carbonation method was presented based on the variations in leaching efficiency of activated Ca under different conditions, aiming at efficient leaching, carbon sequestration, and high-value reuse of SS. This method employed two NH₄Cl solutions in sequence for two leaching operations on SS, allowing the Ca leaching rate to be effectively increased. According to the findings, TSL could increase the activated Ca leaching rate by 26.9 % and achieve 223.15 kg CO₂/t SS sequestration compared to the conventional one-step leaching (CSL) method. If part of the CaCO₃ is recovered as a slagging agent, about 34.1 % of the exogenous Ca introduction could be saved. In addition, the CO₂ sequestration of TSL did not significantly decrease after 8 cycles. This work proposes a strategy that has the potential for recycling SS and reducing carbon emissions.

Quantitative analysis and phase assemblage of basic oxygen furnace slag hydration

Basic oxygen furnace (BOF) slag from steelmaking could be applied as a binder in building materials, reducing the CO₂ footprint and solid waste, which is relevant for industrial waste management and circular economy. However, its use is mostly restricted because its hydraulic activity is poorly understood. The BOF slag was hydrated in this study, and its reaction products were systematically characterized using XRD, QXRD, and SEM/EDX-based phase mapping. Internal consistency checks of the data were performed between the analytical techniques. The results revealed that the composition of the amorphous hydration products could be identified and quantified, and the main hydration products were hydrogarnets and C-S-H gel. An extended milling process significantly improved the reactivity, and all the major slag phases, including wüstite, participated in the reaction. Brownmillerite formed hydrogarnets during the first 7 days of hydration. The new hydration products contributed to the immobilization of vanadium and chromium. Particle size played an important role in the amount of C₂S reacting, the composition of the hydrogarnets and C-S-H gel, their proportions, and the immobilization capacity. Based on the findings, an overall hydration reaction was developed.

Effect mechanism of steel slag on CO2 capture in hydraulic lime

Steel slag (SS) inhibits the early hydration of cement, limiting its application in cement-based materials. In this study, SS was used to prepare hydraulic lime (HL), and the effect of SS on CO₂ capture in HL was investigated. SS inhibited the carbonation of HL in the early stages but promoted carbonation in the later stages. Adding more than 10% SS inhibited the formation of hydration products, and the reduction of hydration products inhibited the carbonation product content, increased the porosity of the hydration mortar, promoted the later stage carbonation rate, and reduced the compressive strength. The carbonation area and captured CO₂ content of the mortars in SS-HL increased exponentially with an increasing carbonation curing age. With an increasing SS content, the carbonation area and the degree of CO₂ capture decreased then increased after 1 day and after 3 days of accelerated carbonation curing, respectively.

Study on hydration mechanism and environmental safety of thermal activated red mud-based cementitious materials

Red mud (RM) cementitious materials were prepared with the thermally, thermoalkali- or thermocalcium-activated RM, steel slag (SS), and other additives. The effects of different thermal RM activation methods on the cementitious material hydration mechanisms, mechanical properties, and environmental risks were discussed and analyzed. The results showed that the hydration products of different thermally activated RM samples were similar with the main products being C-S-H, tobermorite, and Ca(OH)₂. Ca(OH)₂ was mainly present in thermally activated RM samples, and the tobermorite was mainly produced by samples prepared with thermoalkali- and the thermocalcium-activated RM. The mechanical properties of the samples prepared by thermally and thermocalcium-activated RM had early-strength properties, while the thermoalkali-activated RM samples were similar to the late-strength type of cement properties. The average flexural strength of thermally and the thermocalcium-activated RM samples at 14 days were 3.75 MPa and 3.87 MPa respectively, whereas, the 1000 °C thermoalkali-activated RM samples only at 28 days was 3.26 MPa; the above data could reach the single flexural strength (3.0 MPa) of the first-grade pavement blocks of the building materials industry standard of the People's Republic of China-concrete pavement blocks (JC/T446-2000). The optimal preactivated temperature for different thermally activated RM was different; the optimal preactivated temperature for both thermally and thermocalcium-activated RM was 900 °C, and the flexural strength was 4.46 MPa and 4.35 MPa, respectively. However, the optimal preactivated temperature of thermoalkali activated RM at 1000 °C. The 900 °C thermally activated RM samples had better solidified effects for heavy metal elements and alkali substances. 600~800℃ thermoalkali activated RM samples had better solidified effects for heavy metal elements. Different temperatures of thermocalcium-activated RM samples showed different solidified effects on different heavy metal elements, which may be due to the influence of thermocalcium activation temperature on the structural changes of the hydration products of the cementitious samples. In this study, three thermal RM activation methods were proposed, and the co-hydration mechanism and environmental risk study of different thermally activated RM and SS were further elucidated. This not only provides an effective method for the pretreatment and safe utilization of RM, but also facilitates the synergistic resource treatment of solid waste and further promotes the research process of replacing part of traditional cement with solid waste.

Combining organizational and product life cycle perspective to explore the environmental benefits of steel slag recovery practices

Sustainability in steel production is considered a global challenge which needs to be faced with coordinated actions. The aim of this study is to assess the environmental improvements of a steel mill in a circular economy perspective, through the Organizational Life Cycle Assessment (O-LCA) and the Product Life Cycle Assessment (P-LCA) methodologies. This study explores to what extent the improvements and the efforts to recover the steel slag can be detected using an organization perspective and making a comparison with the more traditional product perspective. The results obtained show that the case in which the steel slag is recovered has lower impacts than the case in which it is landfilled through both O-LCA and P-LCA applications and that the percentage variations are similar for 8 categories out of 10 demonstrating that for our case study, O-LCA and P-LCA can detect the efforts to recover slag similarly. Two categories, namely ADP-minerals&metals and EP-freshwater, are affected by the greater amount of metal and mineral raw materials needed if the slag is not treated and by the steel slag landfill disposal more significantly. What the results tell us is that the variations obtained for this study in the P-LCA application are greater than those obtained in O-LCA application, due to two methodological aspects, namely the application of allocation procedures and the choice of the system boundaries. Finally, it emerges that O-LCA methodology can detect environmental improvements of circularity practices, but the reduction of the impacts is less clear than P-LCA application. What is transferable is that O-LCA and P-LCA methodologies are not interchangeable to quantify the environmental benefits and address the efforts to improve a process in terms of circularity.

Suppression of arsenic leaching from excavated soil and the contribution of soluble and insoluble components in steel slag on arsenic immobilization

A huge amount of soil is excavated by tunnel and road construction projects in urban, coastal, and mountainous regions. These projects enable the effective use of underground spaces, and generally, the excavated soil is expected to be reused after treatment, which is required due to the potential release of geogenic arsenic from the soil. The present study investigated the level of water-soluble arsenic and arsenic phases in excavated soil in order to identify how arsenic is immobilized by soluble calcium and insoluble components in steel slag. The soluble calcium was found to suppress the level of water-soluble arsenic as well as arsenic in fraction 1 (nonspecifically bound) identified by sequential extraction from the soil but increased the level of fraction 2: specifically bound arsenic. The insoluble component did not suppress the level of water-soluble arsenic, but decreased and increased the arsenic levels in fractions 2 and 3 (amorphous iron/aluminum oxide bound), respectively. A column percolation test demonstrated that the arsenic that was inhibited from leaching by the addition of steel slag was the fractions 1 and 2 arsenic. The amounts of arsenic released in the serial batch leaching test were comparable with levels leached regardless of the addition of steel slag. These results indicate that both soluble calcium and insoluble components of steel slag have different roles in suppressing arsenic leaching from excavated soil. Based on these results, it is suggested that steel slag could be utilized to suppress arsenic release, thus enabling the reuse of excavated soil.

Mechanical and durability assessments of steel slag-seashell powder-based geopolymer concrete

Globally, an increasing carbon footprint has had a negative effect on the ecosystem and all living things. One of the sources that produces these footprints is the cement manufacturing process. Therefore, it is crucial to produce a cement substitute to reduce these footprints. The production of a geopolymer binder (GPB) is one of these possibilities. In this study, sodium silicate (Na₂SiO₃) was used as an activator in the production of geopolymer concrete (GPC) together with steel slag and oyster seashell as precursors. The materials of the concrete were prepared, cured, and tested. Workability, mechanical, durability and characterization test were conducted on the GPC. The results showed that adding a seashell increased the slump value. The optimum GPC compressive strength on a 100 × 100 × 100 mm³ cube for 3, 7, 14, 28, and 56 curing days was obtained with 10% seashell, while seashell replacement exceeded 10% declined in strength. Portland cement concrete achieved better mechanical strength when compared to steel slag seashell powder geopolymer concrete. However, steel slag seashell powder-based geopolymer gained better thermal properties than Portland cement concrete at 20% seashell replacement.

Converting industrial waste into a value-added cement material through ambient pressure carbonation

Converting industrial wastes into value-added building products in an environmental management strategy is a challenging yet vital component of the industrial process. Steel slag (SS), an industrial waste by-product from the steel-making process, is typically disposed of in landfill which consumes land resources and pollutes the environment. This paper explores the possibility of a closed-loop system to convert steel slag into a cement material through carbonation activation, thereby significantly reducing the amount of steel slag waste sent to landfills across Canada. The production of this cementing material can occur next to the steel mill, utilizing steel slag and carbon dioxide collected on-site to fabricate carbon-negative products. To save energy and allow production to be feasible on an industrial scale, ambient pressure (AP) carbonation is developed to reduce carbon emissions while improving their performance. High pressure (HP) carbonation curing and normal hydration (NH) references were also implemented at the same time to justify the application of AP carbonation in reducing CO₂ emission. The results of this study found AP carbonation-activated SS compacts have comparable CO₂ uptake (about 7.5 tons CO₂/100 tons slag) and mechanically compressive strength values as those subjected to HP carbonation, suggesting that AP could be used to replace HP in carbonation curing to ensure a lower energy input. Additionally, AP seemed to possess as effective carbonation as HP. The studies investigated by multiple techniques including X-ray diffractometer (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopic analysis, and scanning electron microscopy (SEM) aim to identify the microstructure development of carbonated SS paste to assess carbonation results. Developed with life cycle assessment (LCA), environmental impact evaluation shows that AP presents a smaller global warming potential (GWP) value than HP. The comparable CO₂ sequestration, satisfactory engineering properties, enhanced microstructure and lesser environmental impact in AP carbonation confirm the feasibility of replacing high pressure with extremely low pressure to cure concrete products. The use of AP carbonation for cement material created using steel slag reduces carbon emissions, energy usage, and natural resource consumption.

Effect of steel slag on ammonia removal and ammonia-oxidizing microorganisms in zeolite-based tidal flow constructed wetlands

The ammonia removal performance of tidal flow constructed wetlands (TFCWs) requires to be improved under high hydraulic loading rates (HLRs). The pH decrease caused by nitrification may adversely affect the NH₄⁺-N removal and ammonia-oxidizing microorganisms (AOMs) of TFCWs. Herein, TFCWs with zeolite (TFCW_Z) and a mixture of zeolite and steel slag (TFCW_S) were built to investigate the influence of steel slag on NH₄⁺-N removal and AOMs. Both TFCWs were operated under short flooding/drying (F/D) cycles and high HLRs (3.13 and 4.69 m³/(m² d)). The results revealed that a neutral effluent pH (6.98-7.82) was achieved in TFCW_S owing to the CaO dissolution of steel slag. The NH₄⁺-N removal efficiencies in TFCW_S (91.2 ± 5.1%) were much higher than those in TFCW_Z (73.2 ± 7.1%). Total nitrogen (TN) removal was poor in both TFCWs mainly due to the low influent COD/TN. Phosphorus removal in TFCW_S was unsatisfactory because of the short hydraulic retention time. The addition of steel slag stimulated the flourishing AOMs, including Nitrosomonas (ammonia-oxidizing bacteria, AOB), Candidatus_Nitrocosmicus (ammonia-oxidizing archaea, AOA), and comammox Nitrospira, which may be responsible for the better ammonia removal performance in TFCW_S. PICRUSt2 showed that steel slag also enriched the relative abundance of functional genes involved in nitrification (amoCAB, hao, and nxrAB) but inhibited genes related to denitrification (nirK, norB, and nosZ). Quantitative polymerase chain reaction (qPCR) revealed that complete AOB (CAOB) and AOB contributed more to the amoA genes in TFCW_S and TFCW_Z, respectively. Therefore, this study revealed that the dominant AOMs could be significantly changed in zeolite-based TFCW by adding steel slag to regulate the pH in situ, resulting in a more efficient NH₄⁺-N removal performance.

Solidification/stabilization of soil heavy metals by alkaline industrial wastes: A critical review

Solidification/stabilization technology is one of the most desirable technologies for the remediation of heavy metal contaminated soils due to its convenience and effectiveness. The annual production of alkaline industrial wastes in China is in the hundreds of millions of tons. Alkaline industrial wastes have the potential to replace conventional stabilizers because of their cost effectiveness and performance in stabilizing heavy metals in soils. This paper systematically summarizes the use of four alkaline industrial wastes (soda residue, steel slag, carbide slag, and red mud) for the solidification/stabilization of heavy metal contaminated soils and provides a comprehensive analysis of the three mechanisms of action (hydration, precipitation, and adsorption) and factors that influence the process. In addition, the environmental risks associated with the use of alkaline industrial wastes are highlighted. We found that soda residues, steel slag and carbide slag are appropriate for solidification/stabilization of Pb, Cd, Zn and Cu, while red mud is a potential passivation agent for the stabilization of As in soils. However, implementation of remediation methods using alkaline industrial wastes has been limited because the long-term effectiveness, synergistic effects, and usage in soils containing multiple heavy metals have not been thoroughly studied. This review provides the latest knowledge on the mechanisms, risks, and challenges of using alkaline industrial wastes for solidification/stabilization of heavy metal contaminated soils.

Exploring the Potential for Steel Slags Valorisation in an Industrial Symbiosis Perspective at Meso-scale Level

A greater reuse of steel slags would bring considerable benefits both from an environmental and economic point of view. The development of tools and strategies to monitor at different scales resources and waste flows would allow for better resource planning and a more sustainable management on territory. The aim of this study is to investigate and analyse the supply chain that deals with the management of steel slags at meso-level, in order to investigate the state of implementation of industrial symbiosis (IS), its potential and its improvement. A Mass Flow Analysis (MFA) has been implemented, through big data analysis coming from the integration of regional and provincial databases with a careful data processing from questionnaires. This integrated methodology has proved to be a valid tool to monitor the recovery and reuse, the implementation of industrial symbiosis and to plan improvement actions. This paper reports a representation of the current situation regarding the production, recovery and reuse of these materials in production processes for which they are suitable, with a view to their full exploitation, following the principles of circular economy and an analysis of the mutual exchange that occur among steelmaking plants and other business partners in a network of industrial companies. The results showed that most of the steel slags managed at meso-level (Province of Brescia, Italy) is still unfortunately destined for landfill with low percentage of them classified as by-product highlighting as the IS is not adequately applied. Of the slag destined for treatments and recovery processes, almost all of them are Electric Arc Furnace slag, which are mainly reused for hydraulically bound base layers and road sub-bases (about 85% of the total recovered) and as aggregates for the production of cement and bituminous mixes (about 15% of the total recovered). Results shows as further effort should be made in term of policies and strategies to incentivize IS and to increase the recovery.

Graphical abstract

Effects of iron-based substrate on coupling of nitrification, aerobic denitrification and Fe(II) autotrophic denitrification in tidal flow constructed wetlands

The aerobic properties of nitrification and the anaerobic properties of denitrification in constructed wetlands are difficult to reconcile. In this study, two constructed wetlands were constructed with pyrite and steel slag in combination with zeolite, and their respective nitrification and denitrification capacities were evaluated under different tidal strategies. The steel slag wetland achieved 70.89 % and 46.04 % removal rates of NH₄⁺-N and total nitrogen (TN), and the carbon consumption of denitrification was 1.51 mg BOD/mgN, which was better than pyrite wetland. Microbial analysis showed that Fe(II) autotrophic denitrification and aerobic denitrification occurred in both wetlands, and they were coupled with nitrification to achieve simultaneous removal of NH₄⁺-N and TN. Microbial co-occurrence network and k-core decomposition analysis indicated that the core genus of steel slag wetlands was nitrifying bacteria. This study provides new insights into the application of tidal flow wetlands to treat rural sewage.

Removal of p-Nitrophenol from simulated sewage using steel slag: Capability and mechanism

The steel slag was investigated for the removal of p-nitrophenol (4-NP) from simulated sewage by batch adsorption and fixed-bed column absorption experiments. The results showed that the maximum adsorption capacity was 109.66 mg/g at 298 K, pH of 7, initial concentration 100 mg/L, and dose 0.8 g/L. The adsorption process fitted the Langmuir isothermal adsorption model and followed pseudo-second-order kinetic models, the activation energy of adsorption (Ea) was 10.78 kJ/mol, which indicated that the adsorption was single-molecule layer physical adsorption. The regeneration efficiency was still maintained at 84.20% after five adsorption-desorption cycles. The column adsorption experiments showed that the adsorption capacity of the Thomas model reached 13.69 mg/g and the semi-penetrating time of the Yoon-Nelson model was 205 min at 298 K. Fe₃O₄ was identified as the main adsorption site by adsorption energy calculation, XRD and XPS analysis. The FT-IR, Zeta potential, and ionic strength analysis indicated that the adsorption mechanism was hydrogen bonding interaction and electrostatic interaction. This work proved that steel slag could be utilized as a potential adsorbent for phenol-containing wastewater treatment.

Recycling of crushed concrete and steel slag in drainage structures of geotechnical works and road pavements

A crushed concrete aggregate, processed from construction and demolition waste and a siderurgical aggregate, processed from electric arc furnace steel slag, were selected based on their very high availability worldwide and known technical feasibility to be used in construction works. Given the association of their presence to the possibility of reducing the drainage capacity of unbound granular layers of road pavements and drainage structures which they may be associated with, there are studies and regulations that do not recommend their use. The causes that are at the origin of restrictions are mainly the possibility of formation of tufa and recementation phenomena. This behaviour has also hampered their recycling in drainage structures of geotechnical works. Therefore, it was considered that it would be relevant to investigate the drainage capacity of those recycled aggregates, using a leachate produced in a municipal solid waste landfill and tap water. To reference their behaviour, two natural aggregates, a basalt and a limestone, were also studied under identical test conditions. The results obtained showed no reduction in the drainage capacity of the recycled aggregates, similarly to what was observed with the natural aggregates. The possibility of building drainage structures with the tested aggregates is verified.

Saturated hydraulic conductivity of compacted steel slag-bentonite mixtures--A potential hydraulic barrier material of landfill cover

The feasibility of using steel slag and bentonite mixtures to construct the hydraulic barrier of a landfill cover was explored in the present study. Fine-grained steel slag (SS; particle diameter < 1 mm) and sodium-activated calcium bentonite (SACB) were used to prepare compacted specimens, and the saturated hydraulic conductivity (k_s) was measured using a flexible-wall permeameter. Influential factors including SACB content (BC), SS gradation, water-washing treatment of SS and compaction water content (ω_comp) were investigated. The hydraulic conductivity results were interpreted in microscopic scale through mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM). It was found that when BC was below 10%, the k_s value of the specimens prepared with well graded SS was about one order of magnitude lower than that of the specimens prepared with poorly graded SS. This was due to less macropores caused by better SS gradation. Yet, the effects of SS gradation on k_s diminished as BC further increased to 15%, suggesting the dominant role of BC on k_s at high BC. Water-washing treatment of SS helped reduce k_s significantly to 1.2 × 10^-10 m/s at BC of 10%, owing to less multivalent cations and hence lower osmotic swelling reduction caused by cations. Controlling ω_comp 1-2% wetter than the optimum water content (ω_opt) also helped reduce k_s significantly, owing to the reduction of macropores. Accordingly, it is suggested to use well-graded SS mixed with 10% SACB and then compact at ω_comp slightly wetter than ω_opt to the degree of compaction greater than 90% in engineering practice.

Steel slag as a potential adsorbent for efficient removal of Fe(II) from simulated acid mine drainage: adsorption performance and mechanism

Acid mine drainage is an extraordinarily acidic and highly heavy metal ion-contaminated leachate, seriously threatening the environment. In this work, an industrial solid waste of steel slag is the adsorbent to remediate the simulated acid mine drainage containing a large amount of Fe(II) ions. Due to the excellent physicochemical properties and structures, steel slag exhibited remarkable Fe(II) removal performance. Its maximum removal efficiency was up to 100%. The initial pH, the dosage and particle size of steel slag, and initial concentration of heavy metal ions on Fe(II) removal efficiency were determined. The pseudo-second-order model and Freundlich isotherm model well described the adsorption behavior of steel slag, implying that the adsorption of Fe(II) by steel slag was mainly multilayer chemisorption. The thermodynamic study demonstrated that the adsorption process was endothermic and spontaneous; the enthalpy change was calculated to equal 91.21 kJ/mol. Mechanism study showed that the entire removal process of Fe(II) by steel slag was completed by electrostatic adsorption, chemical precipitation, and surface complexation in cooperation, and the chemical precipitation was the dominant mechanism. Meaningfully, this study provides a valuable strategy and path for engineering applications of AMD remediation by steel slag, which is prospective as an ideal candidate for Fe(II) ions elimination, inspiring the future development of "Treating the wastes with wastes."

A review on the utilization of waste material in asphalt pavements

Recycling of waste and disposal has become a vital environmental issue that creates serious concern worldwide. The use of waste material in pavement structure is one of the essential initiative for the future towards sustainable environment. This study imparts a review on waste materials such as plastic waste, crumb rubber, glass fibre, steel slag, crushed concrete and Low Density Polyethylene (LDPE) and their use in asphalt pavements. The waste materials act as modifiers and have the capability to upgrade the performance of pavement and provide green technology with eco-friendly environment. Utilization of waste material as an asphalt binder  enhanced the engineering properties of asphalt pavements. It may be regarded as a smart strategy for sustainable development as it is cost-effective, economical, efficiency and productivity. Moreover, it approached to minimize the pollution. Further, many researchers have investigated the outcomes of asphalt pavement with waste and observed that it achieved the properties and performance of asphalt mixtures while reducing pavement damage, failure and deformation.

Environmental benefit assessment of steel slag utilization and carbonation: A systematic review

The rapid increase in steel slag generation globally highlights the urgent need to manage the disposal or utilization processes. In addition to conventional landfill disposal, researchers have successfully reused steel slag in the construction, chemical, and agricultural fields. With the large portions of alkaline silicate mineral content, steel slag can also be used as a suitable material for carbon capture to mitigate global warming. This article comprehensively reviews the environmental performance of steel slag utilization, especially emphasizing quantitative evaluation using life cycle assessment. This paper first illustrates the production processes, properties, and applications of steel slag, and then summarizes the key findings of the environmental benefits for steel slag utilization using life cycle assessment from the reviewed literature. This paper also identifies the limitations of quantifying the environmental benefits using life cycle assessment. The results indicate steel slag is largely utilized in pavement concrete and/or block as a substitution for natural aggregates. The associated environmental benefits are mostly attributed to the avoidance of the large amount of cement utilized. The environmental benefits for the substitution of traditional energy-intensive material and carbonation treatment are further discussed in detail. Due to the presence of heavy metals, the potential risks to human and ecological health caused by the manufacturing process and usage stage are examined. Finally, the current challenges and global social implications for steel slag valorization are summarized.

Sulfur-zinc modified kaolin/steel slag: A particle electrode that efficiently degrades norfloxacin in a neutral/alkaline environment

In this work, sulfur and zinc were used to modify the steel slag/kaolin particle electrodes. Sulfur-zinc modified kaolin/steel slag particle electrodes (S-Zn-KSPEs) was successfully prepared. In a wide pH range (pH 3-10), S-Zn-KSPEs could efficiently degrade norfloxacin at low voltage (4 V) within 90 min. The removal rate of NOR by S-Zn-KSPEs was about 100% in acidic environment, more than 90% in neutral environment, and more than 80% in alkaline environment. And S-Zn-KSPEs could also efficiently degrade methylene blue, diuron, levofloxacin and other refractory pollutants under neutral conditions. S-Zn-KSPEs showed good stability and recyclability, and could maintain high catalytic activity after 8 cycles in a neutral or alkaline environment. The possible degradation mechanism and the degradation pathway of norfloxacin are proposed. In addition, S-Zn-KSPEs also showed a higher treatment effect in the treatment of actual surface water bodies. And S-Zn-KSPEs had a strong acid-base buffering capacity, which could avoid some pretreatment measures of wastewater in practical applications.

Characteristics of the immobilization process of arsenic depending on the size fraction released from excavated rock/sediment after the addition of immobilization materials

Chemical immobilization is an effective technique to suppress the release of arsenic from naturally arsenic-containing excavated rock/sediment. For designing the chemical immobilization technique, it is important to understand that the immobilization of arsenic depends on the sizes of ionic arsenic and arsenic retained on the colloids and suspended particles that are released from the excavated rock/sediment. Tests on the size fractionation of the arsenic released and the subsequent immobilization were conducted. The total amount of the size fraction of arsenic released from six excavated rock/sediment ranged from 0.16 to 0.75 mg kg^-1. The distributions of size fraction of arsenic released were categorized into three types: the dominant fraction was suspended particle fraction (SP-F) and ionic fraction (I-F), and a compatible amount of SP-F and I-F was included. Steel slag, calcium oxide, and ferrihydrite, which can effectively and stably immobilize ionic arsenic with different mechanisms, decreased the total amounts of the size fraction of arsenic released at 28%-84%, 59%-83%, and 57%-84%, respectively. Ferrihydrite and calcium oxide greatly reduced the I-F and the small and large colloid fractions. The steel slag was effective in reducing the SP-F at >86 %. In most arsenic fractions, the immobilized arsenic was not re-released at <7 %. This study provides the first experimental evidence of the variation in the released arsenic size depending on the excavated rock/sediment. In addition, the size fraction of the arsenic that could be immobilized depended on the immobilizing material. Thus, it is suggested that the combined application of immobilization materials would present a useful approach for immobilizing various released arsenic phases and preventing immobilized arsenic from re-release.

Natural pyrite improved steel slag towards environmentally sustainable chromium reclamation from hexavalent chromium-containing wastewater

Currently, varied processes adopted to remove hexavalent chromium from aqueous solution have been realized to cause secondary pollution. As such, this study explored a green method for aqueous hexavalent chromium (Cr(Ⅵ)) reclamation by waste steel slag (SS) enhanced by natural pyrite (NP). Compared with the sole SS or NP, more efficient Cr(Ⅵ) removal was achieved by NP-SS at an initial pH value ranging from 1 to 8, resulting in a final pH value of 7-8. Cr(Ⅵ) in the solution could be initially reduced to Cr(III) by Fe²⁺ provided by NP, which was then bound with the OH^- in the solution and the supersaturated calcium silicate hydrate on the surface of SS. In addition, the stearic acid anions existing on the surface of SS could promote the adsorption of Cr(III) to form chromium stearate. The used adsorbent could be potentially used for chromium smelting. Overall, this study provides a feasible and environmental sustainable solution to chromium reclamation from hexavalent chromium-containing wastewater.

Application of Bacillus mucilaginosus in the carbonation of steel slag

The stacking of steel slag has detrimental effects mainly for the waste of resources and the pollution of environment. In this study, a novel method based on microbially induced calcium precipitation (MICP) was proposed by utilizing a type of microorganism named Bacillus mucilaginosus, which could secrete carbonic anhydrase (CA) through the metabolism process, accelerating the hydration of carbon dioxide (CO₂) and thus facilitating the formation of carbonate ions (CO₃^2-). First, comparing the biologically deposited calcium carbonate with the chemically deposited one, it was found that the crystallinity and crystal size of the biological deposition was lower, leading to its cementitious properties. Under the condition of 1 wt. (weight) % dosage, the carbonation degree increased from 66.34 to 86.25% and the compressive strength improved greatly from 7.4 to 11.2 MPa as well. The weight gain rate of biologically carbonated specimens was also twice as much as the directly carbonated ones. This work strongly demonstrated that biological carbonation technology could not only improve the CO₂ sequestration potential of steel slag but also enhance the mechanical properties and durability of steel slag products. KEY POINTS: • Bacillus mucilaginosus could resuscitate and proliferate in the steel slag environment. • B. mucilaginosus secreted carbon anhydrase, which could accelerate the hydration of CO₂ and facilitate the precipitation of calcium carbonate. • Biologically carbonated steel slag had greater mechanical performance than directly carbonated one.

Production, characterisation, utilisation, and beneficial soil application of steel slag: A review

Slags are a co-product produced by the steel manufacturing industry and have mainly been utilised for aggregates in concreting and road construction. The increased utilisation of slag can increase economic growth and sustainability for future generations by creating a closed-loop system, circular economy within the metallurgical industries. Slags can be used as a soil amendment, and slag characteristics may reduce leachate potential of heavy metals, reduce greenhouse gas emissions, as well as contain essential nutrients required for agricultural use and environmental remediation. This review aims to examine various slag generation processes in steel plants, their physicochemical characteristics in relation to beneficial utilisation as a soil amendment, and environmental implications and risk assessment of their utilisation in agricultural soils. In relation to enhancing recycling of these resources, current and emerging techniques to separate iron and phosphorus slag compositions are also outlined in this review. Although there are no known immediate direct threats posed by slag on human health, the associated risks include potential heavy metal contamination, leachate contamination, and bioaccumulation of heavy metals in plants, thereby reaching the food chain. Further research in this area is required to assess the long-term effects of slag in agricultural soils on animal and human health.

Leaching of Metals from Steel Slag and Their Ecological Effects on a Marine Ecosystem: Validating Field Data with Mesocosm Observations

Steel slag is being used worldwide for a variety of applications, among which is underwater dyke reinforcement. In the present study the leaching and bioaccumulation of 18 inorganic compounds from basic oxygen furnace (BOF) steel slag were monitored in marine experimental ecosystems (mesocosms) for 12 wk. Triplicate mesocosms were installed at 2 refreshment rates, one reflecting the situation in the Oosterschelde estuary where BOF steel slag was applied and the other at a 35 times lower rate. Vanadium in both water and biota turned out to be the best tracer for the presence of BOF steel slag in the mesocosms. The mesocosm data helped to interpret the results of a 4-yr field sampling program in the Oosterschelde estuary where no elevated levels of vanadium in water or biota were found near locations where steel slag was applied. Also, no ecological impact could be established in the field, which was in line with the observations in the mesocosms. The present study shows the added value of a tailor-made mesocosm study for realistic risk assessment and provides support for applying this tool as a basis for designing efficient field monitoring programs. Environ Toxicol Chem 2021;40:2499-2509. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The application of steel slag in a multistage pond constructed wetland to purify low-phosphorus polluted river water

To investigate the effect of a constructed wetland (CW) with steel slag as the filler on water contaminated by low phosphorus levels, a multistage pond CW system was designed in this study. Low-phosphorus polluted river water was used as the research object. This study explored the effects of using steel slag as a CW filler on phosphorus removal and the total phosphorus (TP) purification effect of the wetland system. The results showed that the TP removal rates in the ecological pond, oxidation pond, surface flow wetlands and submerged plant pond were 5.17%, 8.02%, 21.56%, and 16.31%, respectively. Intermittent increases in phosphorus concentration were observed in the reactors and were caused by the decay of plant tissues, which released pollutants. Because steel slag was added to the filler, the TP concentrations in the effluent of the first- and second-level horizontal subsurface CWs increased by 151.13% and 16.29%, respectively, compared to the influent concentration. The 20th to 40th days of the test run was a period of rapid phosphorus release of the system. The use of steel slag has a potential risk of phosphorus release when applied in CWs used to purify low-phosphorus contaminated water bodies.

Estimating the variability of steel slag properties and their influence in phosphorus removal ability

Steel slag has been proven to be an effective phosphorus (P) removal media, and a potential aid to mitigate point and nonpoint P pollution in freshwater systems. However, the behavior of steel slag as a P sorption material (PSM) is often oversimplified through the generalization of its chemical and physical properties, preventing proper design of P removal structures. In this work, we tested eighteen steel slag samples from different batches, production processes, and steel-making plants, for the purpose of relating slag origin and chemical and physical properties to P removal ability, under two different flow regimes. Slag samples were also coated with aluminum (Al) and tested for P removal. Characterization included elemental composition, particle density, buffer capacity, and P removal ability. There was great variability in the evaluated properties across slag sources and origin, compelling the individual characterization of steel slag samples, since their intrinsic characteristics were key variables in determining their potential P removal capacity. Specifically, electrical conductivity (EC), bulk density, particle density and magnesium (Mg) content could explain around 70% of the variability of P removal by uncoated steel slags. Increasing residence time (RT) always increased P removal for uncoated slags. Steel slags showed a high variability in their P removal ability, but such variability was considerably decreased by coating the slags with Al. Additionally, the Al-coating process significantly improved P removal performance under more rapid flows (lower RT).

Study on mathematical model of hydration expansion of steel slag-cement composite cementitious material

This paper studies the expansion laws of steel slag-cement composite cementitious material, and establishes the mathematical model of hydration expansion of steel slag-cement composite cementitious material. Xinyu converter steel slag was mixed into the cement to prepare 25 × 25 × 280 mm steel slag-cement pastes. After boiled and autoclaved tests, the volume changes before and after the steel slag-cement pastes were measured to verify the accuracy of the established mathematical model. The experimental results show that when the content of steel slag is lower than 30%, the hydration expansion of Xinyu converter steel slag conforms to the established mathematical model. When the content of steel slag is 40%, the measured expansion ratio is 10.4% larger than the calculated expansion ratio of the mathematical model; when the content is 50%, due to a large number of internal micro-cracks, the measured expansion ratio is 19.5% larger than the calculated expansion ratio of the mathematical model. When the steel slag content reaches 60%, the steel slag-cement pastes warps seriously, and the mathematical model is no longer applicable to this situation. The mathematical model can accurately predict the expansion ratio when the steel slag content is less than 30%. Therefore, it can be concluded that when the steel slag content is less than 30%, there are only a few cracks in the steel slag-cement pastes, which is of good stability. So the results show that the maximum content of steel slag to prepared the steel slag-cement pastes is 30%.

Hazardous characteristics and variation in internal structure by hydrodynamic damage of BOF slag-based thin asphalt overlay

For the sustainable development of society, recycling of solid waste has received considerable attention worldwide. In this research, steel slag was used to replace natural aggregate in the thin asphalt overlay, and the hazardous characteristics and internal microstructure of this overlay were explored. The resistance to hydrodynamic damage of the overlay containing steel slag was also evaluated and compared with that of the traditional overlay. The results indicate that steel slag has potential leaching risk, which can lead to environmental hazards in long-term leaching processes. However, the recycling of steel slag in thin asphalt overlay inhibits the release of toxic heavy metals due to the encapsulation effect, thereby reducing the leaching concerns. Steel slag can significantly reinforce the skeleton structure and enhance the ability of the asphalt overlay to bear the load. The superior skeleton stability and moisture resistance of the steel slag asphalt overlay were observed after hydrodynamic treatment compared with overlays made of natural aggregate. The variations in the volumetric parameters and connectivity in the steel slag asphalt overlay are significantly less than those in conventional overlay after hydrodynamic treatment. This indicates that the volumetric characteristics of steel slag asphalt overlays are less affected by hydrodynamic pressure.

Immobilization persistence of Cu, Cr, Pb, Zn ions by the addition of steel slag in acidic contaminated mine soil

Adding steel slag to the acidic contaminated mine soil can immobilize heavy metal ions, but immobilization persistence of the metal ions needs to be determined. In this study, dynamic column simulation experiments were set up to compare the immobilization persistence of Cu, Cr, Pb and Zn ions in original soil and with the addition of slag, lime or fly ash to the soil during a simulated 36-month of acid rain leaching. After adding slag and lime, the pH, organic matter content and cation exchange capacity of soil were significantly increased. Compared with the original soil, additions of slag and lime to the soil were able to persistently immobilize the metal ions, whereas fly ash additions had little effect. During simulation, the metal ion concentrations in the slag group leaching solution were essentially consistent with Standard IV for groundwater. The metal ions were immobilized to form instable hydroxides and stable fractions following adding slag to soil. The hydroxide could rerelease metal ions by acid rain leaching, part of which were re-immobilized into stable fractions by entering slag lattice and complexing with soil organic matter. Therefore, adding slag to soil can persistently immobilize metal ions for heavy metal-contaminated acidic mine soil.

Degradation of norfloxacin wastewater using kaolin/steel slag particle electrodes: Performance, mechanism and pathway

In this work, kaolin/steel slag particle electrodes (KSPEs) were synthesized using a calcination method, and they were used to degrade norfloxacin (NOR) wastewater in three-dimensional (3D) reactor. Characterization methods used by KSPEs included SEM, XRF, XRD and BET. The effects of cell voltage, initial pH, KSPEs dosage and initial NOR concentration on NOR degradation were studied in the optimization experiment of operating parameters. The NOR degradation rate and COD removal rate can reach 96.02% and 93.45% under the optimal parameters within 30 min, and energy consumption is 0.99 kWh m^-3. As a result, KSPEs shows excellent catalytic performance and cycling, and still has high electrocatalytic activity after 10 cycles. Finally, the degradation mechanism and degradation pathways of KSPEs to treat NOR are proposed.

Environmental performance and functional analysis of chip seals with recycled basic oxygen furnace slag as aggregate

Resource utilization of industrial waste is a significant global challenge. Steel slag, a typical industrial by-product in the steel-making process, pollutes the environment and causes ecological deterioration. In this study, steel slag was recycled in chip seals as the aggregate, and the functional and environmental performance of the chip seal with recycled steel slag was determined. Economic costs were also discussed and compared with conventional surface layers. The results indicated that recycling steel slag as the aggregate in chip seal has a lower pollution risk and higher environmental benefits compared with those used for landfilling and dumping. Steel slag can significantly increase the heating and de-icing efficiencies of chip seal compared with basalt, particularly for microwave heating. The self-bonded function represented by the durability of aggregate retention can be enhanced by steel slag. The cost of the chip seal containing steel slag and steel fiber is only increased by 0.14 USD/m² than that of ordinary chip seal, indicating a remarkable economic efficiency of chip seal with de-icing and self-bonded functions.

Iron reduction and diopside-based glass ceramic preparation based on mineral carbonation of steel slag

In this article, a new process for treating steel slag and CO₂ simultaneously and preparing calcium carbonate, metallic iron, and glass ceramics without wastewater or gas production is proposed. The reduction of iron and preparation of diopside glass ceramics are studied in this paper, and the results show that the carbon thermal reduction product of the original slag does not reach its melting point, and the slag and iron are well separated in the samples containing the leached steel slag and added silica. Part of the parent glass is converted into a glass ceramic after heat treatment, and the crystalline phases of samples are melilite, diopside, and partial melilite, and diopside and anorthite, respectively. The crystallization activation energy of the best sample in this article is E = 660.664 kJ/mol. The Avrami indices calculated at different heating rates are all less than 3, which indicates that the crystallization mode of the glass involves surface crystallization. This finding is consistent with the results for the prepared glass ceramics.