Reduction of steel slag leachate pH via humidification using water and aqueous reagents

Enhanced activation of peroxymonosulfate for advanced oxidation processes using solid waste: A novel and easy implement high-value utilization process of slag

A simple, efficient and low energy-consuming process available to generate resultful radicals from PMS for organic pollutants removal had been employed in this study. Slag had been used as the activator for organic pollutants degradation under slag/PMS advanced oxidation process. In this work, effects of slag with or without pretreatment on pollutant removal were studied and radical species generated by slag were measured. Calcination pretreatment is one efficient method to enhance the degradation efficiency significantly. Due to Fe3O4 and Fe2O3 became the dominant phases after calcination, it was about 8.6-flods increasing after comparing the pollutant removal efficiency for different slag/PMS system with calcination pretreatment or not. Organic pollutant neither degraded in PMS system at 25 °C nor being absorbed by slag system for 60 min. On the contrary, up to 90% pollutant concentration reduction achieved in the slag/PMS process. During this process, both •OH and SO4•- had been detected once slag and PMS interaction in wastewater. Through the free radicals quenching tests,•OH should be the key free radical in this advanced oxidation process for the organic pollutant removal under this alkaline condition. In general, organic degradation rate was determined by the slag dosage, and the maximum degradation efficiency was mainly controlled by the PMS usage. This work is expected to broaden the high-value reutilization way for industrial solid waste.

Simultaneous nitrate and phosphorus removal in novel steel slag biofilters: Optimization and mechanism study

The simultaneous nitrate (NO3--N) and phosphorus (P) removal systems are considered to be an effective wastewater treatment technology. However, so far, there are few studies on system optimization to improve NO3--N and P removal. In this study, nine simultaneous NO3--N and P removal biofilters (SNPBs) were constructed to treat simulated wastewater. In order to optimize the NO3--N and P removal, different material loading positions were set: (1) red soil, steel slag, and rice straw (RSR), (2) steel slag, red soil, and rice straw (SRR), and (3) red soil, rice straw, and steel slag (RRS). Results showed that the above three treatments had mean removal efficiencies of 58%-91% for NO3--N and 55%-81% for TP, with the best N and P removal occurring in the SRR. The TN mass balance indicated that microbial removal was responsible for 78.2% of the influent TN in the SRR biofilter. The key microorganisms were Enterobacter, Klebsiella, Pseudomonas, Diaphorobacter, and unclassified_f_Enterobacteriaceae, which accounted for 61.9% of the total microorganisms. The main P-removal mechanism was the formation of Al-P, Fe-P, and Ca-P in red soil or steel slag layer. In addition, the decrease of SRR effluent pH from 11.86 in 1-7 days to 7.75 in 8-50 days indicated that red soil and rice straw had a synergistic effect on water pH reduction. These results suggest that a reasonable combination of steel slag with red soil and rice straw not only simultaneously removes NO3--N and P but also additionally solves the problem of high pH caused by 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.

Removal of CO2 from Biogas during Mineral Carbonation with Waste Materials

Biogas represents a source of renewable energy that could provide a replacement for fossil fuels to meet the increasing demand for energy. The upgrading of biogas through the removal of CO₂ to a content of 95-97% of CH₄ is necessary to increase its calorific value. This review focuses on biogas upgrading technologies using wastes or residues that enable the performing of mineral carbonation. In this research, we analyzed a natural biogas or synthetic one with a content of about (40-50%) of carbon dioxide. The chemical absorption is also briefly described in this study, due to its being the first step in innovative absorption and regeneration processes using mineral carbonization. Wastes with high calcium contents, i.e., ashes, steel-making slags, and stabilized wastewater anaerobic sludge, were considered for direct carbonization, taking into account the leaching of particles from carbonated wastes/residues. Moreover, the different types of reactors used for mineral carbonation have been described. The presented technological solutions are easy to use and economical, and some of them also take into account the regeneration of reagents. However, in the context of their direct use in biogas plants, it is necessary to consider the availability of wastes and residues.

Landfill leachate treatment by immediate one-step lime precipitation, carbonation, and phytoremediation fine-tuning

The high pollutant load of sanitary landfill leachates poses a huge challenge in the search for efficient and environment friendly solutions for their treatment. The objective of this work was to study an integrated solution of environmentally friendly technologies - immediate one-step lime precipitation (IOSLP), carbonation (CB), and phytoremediation (Phyt) - to treat a sanitary landfill leachate. In the leachate sample treatment by IOSLP, the influence of CaO concentration (18.2-33.3 g_CaO L^-1) and stirring time (2-60 min) on the sludge sedimentability and pollutant removal was studied. Organic load and ammonia nitrogen (AN) removal increases with CaO added, as well as sludge volume. Stirring time has a small influence on organic load and AN removal, presenting a minimum for sludge volume. Thus, the best operational conditions were chosen as 27.6 g_CaO L^-1, and 40-min stirring time, with 64% chemical oxygen demand (COD) removal. Sludge humidity was 2.1%, making dewatering needless. IOSLP supernatant was submitted to CB by atmospheric CO₂, and 100% removals in AN and hardness were attained. Effluents from IOSLP and IOSLP + CB were utilized in Phyt tests, with Vetiver (Chrysopogon zizanioides (L.) Roberty). The best COD removal (37%) during Phyt was attained for the samples treated by IOSLP + CB.

The utilization of alkaline wastes in passive carbon capture and sequestration: Promises, challenges and environmental aspects

Alkaline wastes have been the focus of many studies as they act as CO₂ sinks and have the potential to offset emissions from mining and steelmaking industries. Passive carbonation of alkaline wastes mimics natural silicate weathering and provides a promising alternative pathway for CO₂ capture and storage as carbonates, requiring marginal human intervention when compared to ex-situ carbonation. This review summarizes the extant research that has investigated the passive carbonation of alkaline wastes, namely ironmaking and steelmaking slag, mine tailings and demolition wastes, over the past two decades. Here we report different factors that affect passive carbonation to address challenges that this process faces and to identify possible solutions. We identify avenues for future research such as investigating how passive carbonation affects the surrounding environment through interaction with the biosphere and the hydrosphere. Future research should also consider economic analyses to provide investors with an in-depth understanding of passive carbonation techniques. Based on the reviewed materials, we conclude that passive carbonation can be an important contributor to climate change mitigation strategies, and its potential can be intensified by applying simple waste management practices.

Research status and future challenge for CO2 sequestration by mineral carbonation strategy using iron and steel slag

Mineral carbonation can simultaneously realize the effective treatment of CO₂ and iron and steel slag; thus, it is of great significance for the low carbon and sustainable development of iron and steel industry. In this article, the researches of mineral carbonation process using iron and steel slag as feedstock are reviewed, and the carbonation reaction mechanism and the parameters affecting the reaction rate and carbonation degree are analyzed. Furthermore, the effect of different enforcement approaches, such as ultrasonic enhancement, mixed calcination, microbial enhancement, and cyclic coprocessing on mineral carbonation reaction, is introduced. The additional effects of mineral carbonation, such as solving the problem of poor volume stability of steel slag, weakening the leaching of heavy metal ions, and reducing the pH of the leachate, are also illustrated. Moreover, issues related to mineral carbonation technology that should be emphasized upon soon, such as the production of valuable products, use of industrial wastewater, aqueous phase recycling use, multiparameter coupling analysis, and research on the properties of carbonation residues, are also discussed, which contribute some perspectives to the future development of mineral carbonation of iron and steel slag.

Biomethane production from anaerobic co-digestion and steel-making slag: A new waste-to-resource pathway

A proof of concept of using steel-making slag to upgrade biogas to biomethane is demonstrated in this study. Biogas is generated from the anaerobic co-digestion of sewage sludge and beverage waste. The CO₂ capture capacity of an alkaline liquor derived from the release of calcium from the steel-making slag is comparable to that of the commercial adsorbent monoethanolamine. Although only 5% of Ca in the steel-making slag was released to the alkaline liquor, 1 ton of steel-making slag could be capable of upgrading 10 m³ of biogas to over 90% methane content. The results also show that pH can be used as a surrogate parameter to monitor and control biogas upgrading. Further research to improve the release of calcium is essential for the acceleration of the weathering process of steel-making slag for subsequent construction applications.

Effect of steel slag in recycling waste activated sludge to produce anaerobic granular sludge

The amount of waste activated sludge (WAS) has grown dramatically in China. WAS is considered as a problematic and hazardous waste, which should be disposed in a safe and sustainable manner. In order to recycle WAS to an anaerobic granular sludge (AnGS) process for anaerobic digestion, Fe powder and steel slags (rusty and clean slags) were used to enhance the granulation process. The results demonstrated that both rusty and clean slags encouraged the development of granular sludge. Adding 10 g/L clean slags could increase AnGS granulation rate by 37%. In the presence of clean slags, extracellular polymeric substances (EPS) concentration in granules increased noticeably to 715 mg/g mixed liquor suspended solids (MLSS). High throughput sequencing analysis exhibited more diversity and higher abundance of functional microbial communities in the batch bottle with 10 g/L clean slags. This study suggested that adding clean slags at 10 g/L dosage was a sustainable and effective method for the sludge granulation.

Applying steel slag leachate as a reagent substantially enhances pH reduction efficiency for humidification treatment

A cost-effective, easy-to-implement, and sustainable approach is needed to mitigate the production of alkaline leachate from steel slags that are reused or disposed in the environment. To address this issue, a humidification treatment process, which is operated by wetting a stack of steel slag using aqueous reagents and letting atmospheric CO₂ to be passively diffused into the slag pores to induce slag carbonation reaction, was previously developed. In this study, we demonstrate that the leachate of raw steel slag can be recycled and used as a humidification reagent to substantially enhance the treatment efficiency as well as to enable operating the process with neither synthetic chemical consumption nor wastewater discharge. In a 24-h study, a 0.61-unit reduction in slag pH is achieved using a raw slag leachate as a reagent, which is substantially greater than a 0.28-unit reduction using deionized water. The net amount of CaCO₃ produced during an extended humidification duration of 4 weeks is increased by 2.7-fold when the leachate is used instead of deionized water. A series of systematically designed experiments demonstrates that the pH (11.0) and ionic strength (0.0048) are the two major characteristics of the raw slag leachate that contribute to the enhanced efficiency of humidification treatment. With further demonstration at larger scales in follow-up studies, the novel humidification process that utilizes the leachate generated on-site as a reagent is expected to be a feasible alternative for alkali waste treatment prior to its reuse or disposal.

Mechanism for alkaline leachate reduction through calcium carbonate precipitation on basic oxygen furnace slag by different carbonate sources: Application of NaHCO3 and CO2 gas

Carbonate treatment was tested as a means to mitigate the generation of alkaline leachate from basic oxygen furnace (BOF) slag. BOF slag was treated with 0.1, 0.5, and 1.0 M concentrations of NaHCO₃ solution for 48 h at a liquid/solid ratio of 5 L/kg. At 1.0 M NaHCO₃, the pH of the leachate decreased from 12.0 to 11.3 because less free CaO was dissolved from the treated slag. Approximately 1.59 mg-Ca²⁺/g-slag of free CaO was dissolved from the untreated BOF slag while only 0.06 mg-Ca²⁺/g-slag was liberated from the treated slag. When the data from X-ray photoelectron spectroscopy and thermogravimetric analysis were taken together, formation of CaCO₃ precipitates on the surface of the treated BOF slag was evident. Surface precipitation of CaCO₃ was more pronounced when CO₂ gas was used as an alternative carbonate source. Carbon dioxide treatment further decreased the leachate pH to 8.3, probably because it liberated more Ca²⁺ from BOF slag during the treatment than 1.0 M NaHCO₃ solution due to the pH difference (pH 6.6 and 9.6, respectively), in turn generating more CaCO₃ precipitates. Scanning electron microscopy analysis revealed that more CaCO₃ was precipitated on the CO₂ gas-treated slag surface than on the NaHCO₃-treated slag. This study identifies the leachate pH reduction-mechanism and the effect of carbonate source which are expected to contribute to the environmentally safe management of BOF slags.