Green remediation of contaminated sediment by stabilization/solidification with industrial by-products and CO2 utilization

Resource utilization of drinking water treatment aluminum sludge in green cementing materials: Hydration characteristics and hydration kinetics

As a byproduct of water treatment, drinking water treatment aluminum sludge (DWTAS) has challenges related to imperfect treatment and disposal, which has caused potential harm to human health and the environment. In this paper, heat treatment DWTAS as a supplement cementitious material was used to prepare a green cementing material. The results show that the 800°C is considered as the optimum heat treatment temperature for DWTAS. DWTAS-800°C is fully activated after thermal decomposition to form incompletely crystallized highly active γ-Al2O3 and active SiO2. The addition of DWTAS promoted the formation of ettringite and C-(A)-S-H gel, which could make up for the low early compressive strength of cementing materials to a certain extent. When cured for 90 days, the compressive strength of the mortar with 30% DWTAS-800°C reached 44.86 MPa. The dynamic process was well simulated by Krstulović-Dabić hydration kinetics model. This study provided a methodology for the fabrication of environmentally friendly and cost-effective compound cementitious materials and proposed a "waste-to-resource" strategy for the sustainable management of typical solid wastes.

Management of dredged marine sediments in Southern France: main keys to large-scale beneficial re-use

Fifty million cubic meters of marine sediments are dredged each year in France in order to maintain harbor activities and sustain the economy of littoral territories. Because of anthropogenic activities in and around harbors, sediments can contain significant amounts of chemical and organic pollutants whose behavior during dredging must be addressed in order to avoid releasing risks for humans and the environment. French regulations come to govern the management of dredged sediments, considering them "safe" and possible to be dumped at sea or "contaminated" and needed to be treated on land as waste. In recent years, new constraints have been pushed toward the management of land. This management is, however, challenging as few channels are proposed to reuse marine sediments, and elimination appears to be economically and environmentally unsustainable. This study provides an overview of the technical and regulatory aspects related to dredged marine sediment management in France and aims to identify and discuss the limits of their valorization. Dredged sediments are mainly composed of particles with heterogeneous grain size, some being known for many applications such as building materials and growing media. However, several reasons have been put forward to explain why these particles are not reused when extracted from dredged sediments. Several technical, socio-economic, and regulatory obstacles explain the low demand for dredged sediments. This demand can be stimulated by government incentives and a good regulatory framework. National regulations could help streamline their reuse by removing their "waste" status and creating a regulated market for dredged sediment.

Investigation of the acicular aragonite growth behavior in AOD stainless steel slag during slurry-phase carbonation

This study presents a novel method for producing acicular aragonite using argon oxygen decarburization (AOD) slag while controlling the reaction temperature, reaction time, stirring speed, and the magnesium-to‑calcium stoichiometric ratio. This approach provides steel plants with an opportunity to decrease their CO2 emissions and promote efficient resource utilization and CO2 storage through the production of high-quality value-added products. The experimental results showed that reaction temperature was the most significant factor affecting the carbonation efficiency of AOD slag, followed by reaction time, stirring speed, CO2 partial pressure, and the liquid-to-solid ratio (L/S). The study also found that elevated temperature and prolonged reaction duration favored the preferential precipitation of aragonite. Additionally, raising the temperature and the magnesium-to‑calcium stoichiometric ratio was shown to enhance the formation of aragonite, affecting its crystal growth orientation and dimensions. The optimal combination of reaction parameters for the preparation of acicular aragonite was found to be the reaction time of 8 h, the magnesium-to‑calcium stoichiometric ratio of 0.8, the reaction temperature of 120 °C, and the stirring speed of 200 r·min-1. Under these conditions, the resulting acicular aragonite exhibited excellent overall uniformity, a large aspect ratio, and a smooth crystal surface, with a content of 91.49 %, a single crystal length ranging from 9.86 to 32.6 μm, and a diameter ranging from 0.63 to 2.15 μm. This study provides valuable insights into the efficient production of acicular aragonite from steel slag while reducing CO2 emissions and promoting the sustainable use of resources.

Synthesis and application of waste-based layered double hydroxide: A review

The synthesis of layered double hydroxide (LDH) from industrial wastes is a sustainable approach to aid circular economy and hazardous material disposal. In this review, the researches on the synthesis and application of waste-based LDH from 2010 to 2023 are summarized and discussed. At present, there are mainly four types of waste-based LDH produced from red mud, slag, fly ash and wastewater, with co-precipitation being the most typical synthesis method. Red mud is used as the trivalent metal source supplemented by chemical reagents or other types of waste as divalent metal source to produce red mud-based LDH. Slag can act as the sole metal source providing both divalent and trivalent metal sources for slag-based LDH. Fly ash was used either as the trivalent metal source or both divalent and trivalent metal sources to produce fly ash-based LDH. Wastewater-based LDH was typically synthesized by in-situ co-precipitation method to achieve the self-purification of wastewater. The impurities in waste-based LDH can act as a two-edged weapon. It may either hinder or promote the performance of waste-based LDH. The challenge in the synthesis of waste-based LDH lies in the efficient extraction of available metals. The future research prospects for waste-based LDH are suggested.

Stabilization/solidification of Mn-contaminated clay slurry by using CaO-GGBS: Effects of anions

Clay sediment is removed by dredging, resulting in the disposal of enormous waste sediment clay slurries that consumes land space, as well as risks the human health and the environment. Manganese (Mn) is often identified in clay slurries. Quicklime (CaO)-activated ground granulated blast-furnace slag (GGBS) can be used to stabilize/solidify (S/S) contaminated soils; nevertheless, few studies have been published on the S/S of Mn-contaminated clay slurries using CaO-GGBS. Moreover, the anions contained in clay slurries may affect the S/S efficiency of CaO-GGBS in treating Mn-contaminated clay slurries, but this effect has hardly been investigated. Therefore, this study investigated the S/S efficiency of CaO-GGBS in treating MnSO₄-bearing and Mn(NO₃)₂-bearing clay slurries. The effect of anions (i.e. SO₄^2- and NO₃^-) on the strength, leachability, mineralogy, and microstructure of Mn-contaminated clay slurries treated with CaO-GGBS was explored. Results showed that CaO-GGBS could improve the strength of both Mn-contaminated slurries to meet the strength requirement for landfill waste outlined by United States Environmental Protection Agency (USEPA). The Mn leachabilities of both Mn-contaminated slurries were decreased to be less than the Euro limit for drinking water after cured for 56 days. The MnSO₄-bearing slurry generally produced higher UCS while lower Mn leachability than Mn(NO₃)₂-bearing slurry at the same CaO-GGBS addition. CSH and Mn(OH)₂ were formed, thereby enhancing strength and reducing leachability of Mn. Ettringite in CaO-GGBS-treated MnSO₄-bearing slurry, which was formed by the supply of SO₄^2- from MnSO₄, further contributed to the strength enhancement and the decrease of Mn leachability. Ettringite was the factor leading to the difference in strength and leaching properties between MnSO₄-bearing and Mn(NO₃)₂-bearing clay slurries. Hence, anions contained in Mn-contaminated slurries significantly affected the strength and the Mn leachability, and need to be identified before CaO-GGBS was used to treat such slurries.

In-situ release and sequestration of CO2 in cement composites using LTA zeolites

This paper investigates the impact of in-situ release and sequestration of CO₂ on the compressive strength, volume of permeable voids, phase change, hydration reaction, and micro-morphology of cement mortars. Two Linde Type A (LTA) zeolites with micro-pore dimensions of 5 Å and 4 Å (i.e., LTA 5A and 4A zeolites) were employed as CO₂ carriers herein. The incorporation of 312 wt% plain LTA 5A and 4A zeolites increases the 1-day compressive strength of the mortars. However, the use of plain LTA 5A zeolite shows marginal contributions to the 7 and 28-day compressive strengths of the mortars, whilst using plain LTA 4A zeolite even deteriorates their 7 and 28-day compressive strengths. The micro-structural analyses reveal that the addition of LTA zeolites promotes the cement hydration and improves the mean chain length (MCL) of calcium aluminosilicate hydrates (C-A-S-H). Nevertheless, this introduces numerous weak points or even a porous structure to the cement matrix. In contrast, in-situ release of CO₂ via LTA zeolites significantly enhances the compressive strengths of the mortars at various ages, as this can further facilitate the hydration evolution and improve the MCL of C-A-S-H. Moreover, in-situ release of CO₂ brings an incremental content of calcium carbonates. The calcium carbonate contents in the specimens containing 12 wt% LTA 5A and 4A zeolites are increased by 5.3 wt% and 4.8 wt%, respectively. This leads to homogenous distributions of calcite with a grain size of 150600 nm. Thus, LTA 5A zeolite outperforms LTA 4A zeolite with regard to CO₂ uptake and the corresponding mechanical properties. This work presents in initial exploration into the application of porous pozzolanic materials in conjunction of CO₂ in cement-based materials.

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.

Recent Progress on Ex Situ Remediation Technology and Resource Utilization for Heavy Metal Contaminated Sediment

Sediment is an important part of aquatic systems, which plays a vital role in transporting and storing metals. Due to its abundance, persistence, and environmental toxicity, heavy metal pollution has always been one of the hot spots in the world. In this article, the state-of-art ex situ remediation technology for metal-contaminated sediments is elaborated, including sediment washing, electrokinetic remediation (EKR), chemical extraction, biological treatment, as well as encapsulating pollutants by adding some stabilized/solidified materials. Furthermore, the progress of sustainable resource utilization methods, such as ecosystem restoration, construction materials (e.g., materials fill materials, partition blocks, and paving blocks), and agriculture use are reviewed in detail. Finally, the pros and cons of each technique are summarized. This information will provide the scientific basis for selecting the appropriate remediation technology in a particular scenario.

Does Current Knowledge Give a Variety of Possibilities for the Stabilization/Solidification of Soil Contaminated with Heavy Metals?-A Review

Stabilization/solidification of contaminated soil is a process that allows simultaneous strengthening of the soil structure, disposal of contamination and recycling of industrial waste, implemented as substitutes for Portland cement or additives to improve the properties of the final product obtained. Extremely intensive development of studies pertaining to the S/S process prompted the authors to systematize the binders used and the corresponding methods of binding the contamination, and to perform an analysis of the effectiveness expressed in geomechanical properties and leachability. The study pays close attention to the types of additives and binders of waste origin, as well as the ecological and economic benefits of their use. The methods of preparing and caring for the specimens were reviewed, in addition to the methods of testing the effectiveness of the S/S process, including the influence of aging factors on long-term properties. The results of the analyses carried out are presented in the form of diagrams and charts, facilitating individual evaluation of the various solutions for the stabilization/solidification of soils contaminated with heavy metals.

Recycling coral waste into eco-friendly UHPC: Mechanical strength, microstructure, and environmental benefits

On islands far away from the mainland, the raw materials for concrete production are often more difficult to obtain. Converting the coral waste generated during the island construction process into a marine ultra-high performance concrete (UHPC) mixture is an eco-friendly strategy. Coral powder (CP) is used to partially replace cement and silica fume (SF), and its mechanical strength, microstructure and environmental benefits are evaluated. Results show that using a small amount of CP (5%) to replace cement can improve the mechanical properties of UHPC, but the strength of UHPC decreases with the further increase of CP content. From the perspective of nanoindentation test, an appropriate amount of CP refines the pore structure of the UHPC matrix and increases the content of C-S-H, especially the proportion of high-density C-S-H. When 15% of SF is replaced by CP (SF15), the strength of UHPC decreases due to the decrease of C-S-H phase and the deterioration of microstructure. In terms of the width of the interface transition zone, the width of the C5 sample (CP replace 5% cement) is decreased by 16.7% compared with the control group, while the width of the SF15 group is increased by 38.9%. Compared with conventional UHPC, CP-based UHPC has lower carbon emission and non-renewable energy consumption, which effectively utilizes waste and promotes sustainability.

Combined Treatment of Cr(VI)-Contaminated Soils by Reduction, Adsorption, and Solidification

Remediation of Cr(VI)-contaminated soil usually includes reducing Cr(VI) to Cr(III) with sub-sequent solidification. In this paper, a treatment technique that combines reduction, adsorption, and solidification was proposed. By introducing an adsorbent into the reduction process, the remediation effectiveness was improved and the amount of reducing and solidified agent was decreased. Synthetic precipitation leaching procedure (SPLP), unconfined compressive strength (UCS) test, and scanning electron microscope (SEM) analysis were carried out to evaluate the remediation effect under different agent combinations and different agent-adding procedures. The results of SPLP showed that the reduction/adsorption/solidification treatment significantly reduced the leachability of Cr. UCS increased with increasing dosage of cement and CaS5, and decreased with an increasing dosage of vermiculite. The best agent dosage was CaS5 of 2 times molar stoichiometric ratio of Cr(VI), 15% of vermiculite, and 20% of cement. Orthogonal test showed that for soil with low Cr(VI) content, CaS5 dosage was the most important factor that affected the leachability of Cr. Cement and vermiculite have greater impact in limiting the leachability of Cr when Cr(VI) content in soil increased.

An overview of operations and processes for circular management of dredged sediments

Dredging is an essential technique to maintain proper water depths in ports and bays. Many dredged sediments are considered as toxic waste due to their significant amounts of metals and other pollutants. In consequence, they need to be treated to reduce this toxicity and avoid pollutant resuspensions. Physical operations and chemical, thermal and biological processes have been conventionally used to this aim, but the traditional linear sediment approach is often unsustainable and economically and environmentally demanding. Considering the increasing people's awareness in environmental issues, more efficient dredged sediment management schemes are required. Some authors are making significant efforts to improve circularity in sediment management processes by taking advantage of the mineral composition of sediments to obtain products for the building and road construction sectors, therefore decreasing the need of raw materials while reducing the amounts of sediments wasted to landfills. However, information related to the characteristics of these products, their mechanical behaviour and their functionality is still scarce, being sediment-based by-products developed mainly at low Technological Readiness Level (TRL), showing low global impact in the market. To implement circular economy in the dredged sediment sector, some technical and socio-political barriers must be still overcome. To this aim, further research and technological applications must be developed, with the support of decision makers and stakeholders. This review aims at giving an overview of the circular trends applied to toxic dredged sediment management, pointing at current opportunities, barriers and constraints that hinder its wide development.

Sewage sludge ash-incorporated stabilisation/solidification for recycling and remediation of marine sediments

Finding suitable disposal sites for dredged marine sediments and incinerated sewage sludge ash (ISSA) is a challenge. Stabilisation/solidification (S/S) has become an increasingly popular remediation technology. This study sheds light on the possible beneficial use of ISSA together with traditional binders to stabilise/solidify marine sediments. The performance of the binders on S/S of sediment 1 (clean) and sediment 2 (contaminated) was also compared. The results showed that the use of ISSA as part of the binder was effective in promoting the strength of the sediment with a high initial moisture content due to ISSA porous and high water absorption characteristics. The sediments treated with 10% cement and 20% ISSA attained the highest strength. Also, cement hydration as well as pozzolanic reactions between ISSA and Ca(OH)₂ made contributions to the strength development. This was supported by the microstructural analysis, in particular the porosity results. In terms of environmental impacts, two leaching tests (toxicity characteristic leaching procedure and synthetic precipitation leaching procedure) found that all the S/S treated sediment by 10% lime and 20% ISSA resulted in the lowest leachate concentrations under the on-site reuse scenario or under simulative acidic rainfall conditions. Therefore, recycling waste ISSA with lime can be used as an appealing binder to replace cement to stabilise/solidify dredged marine sediments for producing fill materials.

Recent advances in PCB removal from historically contaminated environmental matrices

Despite being drastically restricted in the 1970s, polychlorinated biphenyls (PCBs) still belong among the most hazardous contaminants. The chemical stability and dielectric properties of PCBs made them suitable for a number of applications, which then lead to their ubiquitous presence in the environment. PCBs are highly bioaccumulative and persistent, and their teratogenic, carcinogenic, and endocrine-disrupting features have been widely reported in the literature. This review discusses recent advances in different techniques and approaches to remediate historically contaminated matrices, which are one of the most problematic in regard to decontamination feasibility and efficiency. The current knowledge published in the literature shows that PCBs are not sufficiently removed from the environment by natural processes, and thus, the suitability of some approaches (e.g., natural attenuation) is limited. Physicochemical processes are still the most effective; however, their extensive use is constrained by their high cost and often their destructiveness toward the matrices. Despite their limited reliability, biological methods and their application in combinations with other techniques could be promising. The literature reviewed in this paper documents that a combination of techniques differing in their principles should be a future research direction. Other aspects discussed in this work include the incompleteness of some studies. More attention should be given to the evaluation of toxicity during these processes, particularly in terms of monitoring different modes of toxic action. In addition, decomposition mechanisms and products need to be sufficiently clarified before combined, tailor-made approaches can be employed.

Remediation of soils and sediments polluted with polycyclic aromatic hydrocarbons: To immobilize, mobilize, or degrade?

Polycyclic aromatic hydrocarbons (PAHs) are generated due to incomplete burning of organic substances. Use of fossil fuels is the primary anthropogenic cause of PAHs emission in natural settings. Although several PAH compounds exist in the natural environmental setting, only 16 of these compounds are considered priority pollutants. PAHs imposes several health impacts on humans and other living organisms due to their carcinogenic, mutagenic, or teratogenic properties. The specific characteristics of PAHs, such as their high hydrophobicity and low water solubility, influence their active adsorption onto soils and sediments, affecting their bioavailability and subsequent degradation. Therefore, this review first discusses various sources of PAHs, including source identification techniques, bioavailability, and interactions of PAHs with soils and sediments. Then this review addresses the remediation technologies adopted so far of PAHs in soils and sediments using immobilization techniques (capping, stabilization, dredging, and excavation), mobilization techniques (thermal desorption, washing, electrokinetics, and surfactant assisted), and biological degradation techniques. The pros and cons of each technology are discussed. A detailed systematic compilation of eco-friendly approaches used to degrade PAHs, such as phytoremediation, microbial remediation, and emerging hybrid or integrated technologies are reviewed along with case studies and provided prospects for future research.

Sustainable ex-situ remediation of contaminated sediment: A review

Routine waterway dredging activities generate huge volumes of dredged sediment. The remediation of dredged contaminated sediment is a worldwide challenge. Novel and sustainable ex-situ remediation technologies for contaminated sediment have been developed and adopted in recent years. In this review paper, the state-of-art ex-situ treatment technologies and resource utilisation methods for contaminated sediment were critically reviewed. By applying different techniques, sediment could been successfully transformed into sustainable construction materials, such as ceramsite, supplementary cementitious materials, fill materials, paving blocks, partition blocks, ready-mixed concrete, and foamed concrete. We highlighted that proper remediation technologies should be cleverly selected and designed according to the physical and chemical characteristics of sediment, without neglecting important aspects, such as cost, safety, environmental impacts, readiness level of the technology and social acceptability. The combination of different assessment methods (e.g., environmental impact assessment, cost-benefit analysis, multi-criteria decision analysis and life cycle assessment) should be employed to comprehensively evaluate the feasibility of different sustainable remediation technologies. We call on the scientific community in a multidisciplinary fashion to evaluate the sustainability of various remediation technologies for contaminated sediment.

Hydration Activity and Carbonation Characteristics of Dicalcium Silicate in Steel Slag: A Review

Dicalcium silicate is one of the main mineral phases of steel slag. Ascribed to the characteristics of hydration and carbonation, the application of slag in cement production and carbon dioxide sequestration has been confirmed as feasible. In the current study, the precipitation process of the dicalcium silicate phase in steel slag was discussed. Meanwhile, the study put emphasis on the influence of different crystal forms of dicalcium silicate on the hydration activity and carbonation characteristics of steel slag. It indicates that most of the dicalcium silicate phase in steel slag is the γ phase with the weakest hydration activity. The hydration activity of γ-C2S is improved to a certain extent by means of mechanical, high temperature, and chemical activation. However, the carbonation activity of γ-C2S is about two times higher than that of β-C2S. Direct and indirect carbonation can effectively capture carbon dioxide. This paper also summarizes the research status of the application of steel slag in cement production and carbon dioxide sequestration. Further development of the potential of dicalcium silicate hydration activity and simplifying the carbonation process are important focuses for the future.

Sustainable stabilization/solidification of arsenic-containing soil by blast slag and cement blends

Arsenic (As) is a naturally occurring trace element that may pose a threat to human health and the ecosystem, while effective remediation and sustainable reuse of As-containing soil is a challenge. This study investigated the geoenvironmental characteristics of a geogenic As-rich soil, and green binders (ground granulated blast slag (GGBS) and cement blends) were employed for the stabilization/solidification (S/S) of the soil under field-relevant conditions. Results indicate that the use of 10% binder could effectively immobilize As and chemical stabilization/physical encapsulation jointly determined the leaching characteristics of the S/S soils. The geogenic As could be effectively immobilized at the pH range of 5.5-6.5. The increasing use of GGBS enhanced the strength of the 28-d cured S/S soils because of long-term pozzolanic reaction, but also slightly improved the As leachability. Besides, the moisture content of the contaminated soils should be suitably adjusted to allow for desirable compaction of S/S soils, which resulted in high compressive strength and low of As leachability. Results show that soil moisture content of 20% was the most appropriate, which resulted in the highest strength and relatively lower As leaching. In summary, this study presents a sustainable S/S binder for recycling As-contaminated soil by using a combination of cement and GGBS.

Environmental cost of energy consumption and economic growth: can China shift some burden through financial development? An asymmetric analysis

The link between CO2 emission and its possible determinant, namely financial development, foreign direct investment, economic growth, and energy consumption, gives a significant environmental research place. This study attempts to determine the nonlinear link between CO2 emissions and its determinants in China for the period 1985-2019. Based on the theoretic links and foundation, we develop an asymmetric autoregressive distributed lag (ARDL) model using annual data in this study. The findings of the nonlinear ARDL suggest that domestic credit from the banks (DCB) has a negative effect on environmental pollution in the short and long run, while domestic credit to the private sector (DCP) has an insignificant negative role in environmental pollution in China. The results show that positive change in DCB causes a decrease in CO2 emissions, while negative change increases it in the short and long term. The coefficient for a positive shock in DCP is a negative, and negative shock in DCP has a significant positive impact on environmental pollution in the short and long run. This means that nonlinear ARDL financial development effects on environmental pollution have deviated from the linear ARDL estimates. Empirical findings also support the presence of asymmetric long-run cointegration among the indicators. The findings also show that energy consumption and real GDP have a significant positive impact on CO2 emissions in the long-run, while foreign direct investment (FDI) improves the environment. Based on the empirical findings, the study endorses the suitable channelization of the finance market towards clean energy projects without compromising economic growth.

Ex situ remediation of sediment from Serbia using a combination of electrokinetic and stabilization/solidification with accelerated carbonation treatments

The application of three simple and cost-effective technologies for ex situ remediation of the sediment of Begej River in Serbia is presented in this paper. In the first step, conventional electrokinetic treatment (EK) was carried out to reduce the amount of contaminated sediment and enhance the accumulation of metals. Subsequently, stabilization/solidification (S/S) treatment was applied to the remaining portion of polluted sediment to immobilize the accumulated metals. At the same time, the influence of accelerated carbonation on the effectiveness of the treatment was evaluated. The immobilizing agents used in this study included bio ash produced by combustion of wheat and soy straw mixture and bio ash derived from molasses incineration. After the treatments, the risk assessment was performed by using the sequential extraction procedure (SEP) and TCLP and DIN 3841-4 S4 leaching tests. The results obtained after the EK treatment revealed a reduction in the amount of polluted sediment to a half. Leaching tests and SEP performed on S/S mixtures after a 28-day maturation period indicated that accelerated carbonation decreased the mobility of critical metals, especially in wheat and soy straw mixtures. Moreover, based on the leaching tests, all prepared mixtures were categorized as non-hazardous and safe for disposal according to the relevant Serbian regulations. The newly developed method that combines EK and S/S treatments with the addition of accelerated carbonation produced reduced volumes of stabilized sediment which is safe for disposal.

Enhancement of cement-based solidification/stabilization of a lead-contaminated smectite clay

The cement-based solidification/stabilization is commonly used to remediate heavy-metal-contaminated clayey soils. The major problem associated with this method is heavy-metal precipitation, which retards cement hydration. The objectives of this paper are to study the influence of pH-dependent lead solubility patterns on the solidification/stabilization of contaminated smectite and to overcome the problems associated with cement hydration in this process through NaOH treatment. A series of physicochemical experiments were performed on untreated and NaOH-enhanced samples. Contaminated smectite with 5-100 cmol/kg-soil of lead nitrate was solidified/stabilized by 10-50% cement. This research demonstrates that solidification/stabilization is a pH-dependent phenomenon. Enhancement increases the pH of contaminated soil in which lead components transfer to a soluble form. Hereafter, as the results of XRD reveal, a decrease in lead precipitation on cement components is observed. Consequently, a noticeable increase in CSH formation is detected. The capsulation of lead ions by CSH improves the setting-time and unconfined compressive strength of solidified/stabilized samples. Furthermore, the TCLP results show a significant reduction in samples' lead-leaching abilities. Therefore, enhancement has changed the governing retention phenomena from precipitation/stabilization in lead carbonate form to mainly capsulation/solidification by CSH. Moreover, the results show a noticeable reduction in the required cement content.

Recycling drinking water treatment sludge into eco-concrete blocks with CO2 curing: Durability and leachability

Drinking water treatment sludge (DWTS) can be recycled into low-strength concrete blocks for construction use. The sodium sulfate resistance and leaching behaviours of the DWTS-derived blocks are investigated in this study. The experimental results show that the addition of DWTS degrades the sodium sulfate resistance of the concrete blocks, however CO₂ curing compensates for such property, especially in the case of blocks incorporating 30% DWTS. The improvement can be attributed to the formation of crystalline CaCO₃ during CO₂ curing for microstructure refinement evidenced by X-ray Computed Tomography and Scanning Electron Microscopy. Leaching analyses show that Cu and Al concentrations increased with increasing DWTS content, and CO₂ curing adversely increased the leachability of metals due to the decrease of pH, especially at early leaching stage. Nevertheless, the total leaching concentrations of Cu and Al after 60-day test is far below the prescribed limitations, regardless of samples subject to air curing or CO₂ curing. In summary, sludge-derived blocks exposed to CO₂ curing are safe and behave well in aggressive environments. Therefore, this study showcases a green technology that successfully recycling DWTS into value-added and durable concrete blocks with low environmental impacts.

Evaluating the environmental impact of contaminated sediment column stabilized by deep cement mixing

Deep cement mixing (DCM) method is a widely used geotechnical technique for increasing ground stabilization before construction works. However, the environmental influence of stabilized ground on the surrounding area remains a concern. A physical model experiment of DCM-treated sediment column was conducted to investigate both geotechnical and environmental effects on the surrounding sediment. The DCM column contained the cement-stabilized contaminated sediment and surrounded by uncontaminated sediment. The physical behaviour, including settlement, pore water pressure, and total pressure were measured under different loadings. Simultaneously, the migration of the major ions into seawater, and leaching of potentially toxic elements into the surrounding sediment were evaluated. The results revealed that the leaching of major ions from the DCM column followed the dissipation of excess pore water and migrated to the seawater above the sediment surface. Nevertheless, the leaching behaviour of potentially toxic elements into the surrounding sediment and variation of pH value after the DCM treatment were within an acceptable level. Therefore, the contaminated marine sediment could be effectively stabilized and solidified by in-situ remediation with minimal secondary pollution to the surrounding environment.

Evaluating the environmental impacts of stabilization and solidification technologies for managing hazardous wastes through life cycle assessment: A case study of Hong Kong

Proper management of hazardous materials arouses widespread environmental concerns due to its enormous ecological and health impacts. The development of green stabilization/solidification (S/S) technology for resourceful utilization of hazardous materials, as well as the immobilization of potentially toxic elements is of great scientific interests. Cement-based S/S is often considered a low-cost and highly efficient technology, but the environmental sustainability of a broad spectrum of S/S technologies has yet to be evaluated. Therefore, this study assessed the environmental sustainability of S/S technologies for managing two common types of hazardous wastes, i.e., contaminated marine sediment and municipal solid waste incineration fly ash (MIFA) by using life cycle assessment (LCA). A total of 17 scenarios under three strategies for sediment and two strategies for MIFA S/S technologies were comprehensively evaluated. The LCA results identified the most preferable S/S technology in each strategy. In particular, Scenario 1 (mixture of sediment with a small percentage of ordinary Portland cement and incinerated sewage sludge ash) of Strategy 1 (use as fill materials) would be the preferred option, as it reduces about 54% and 70% global warming potential compared to those of Scenarios 2 and 3, respectively. This is the first initiative for evaluating the environmental impacts of a wide range of recently developed S/S technologies using green/alternative binders for diverting hazardous wastes from disposal. The results can serve as a decision support for the practical application of the environmentally friendly S/S technology for sustainable remediation.

Closed-circulating CO2 sequestration process evaluation utilizing wastes in steelmaking plant

The wastes network system exploration in metallurgical process imposes of great significance for advancing green circular economy in steel plant. This paper originally proposes a closed-circulating CO₂ sequestering process for wastes appreciation and harmless disposal, and the effect of two circulation strategy, i.e. Slag circulation strategy and cold-rolling waste water(CRW) circulation strategy, on the CO₂ uptake efficiency, carbonation degree and desalination rate were systemically discussed. Then, their kinetics are analyzed by model and molecular simulation in detail, respectively. In addition, the energy consumption and the cost are simulated for comprehensively evaluating its superiority. The experimental and molecular simulation results all show that the peak values for both strategies could be achieved when circulation times is in the range of three to five. CRW circulation strategy has a better CO₂ uptake efficiency than slag circulation strategy, the CO₂ uptake efficiency is about 487kgCO₂/tslag and corresponding desalination rate is 48.9%, when CRW is circulated for five times at 60 °C and 20 L/g for 90 min. Adopting CRW circulation strategy, the CO₂ sequestration efficiency is averagely doubled comparing to previous results. 129%-183% energy consumption and 35.6% cost would be reduced, which represents that the proposed routine is economical to step forward to industrial application.

Dredged marine sediments stabilized/solidified with cement and GGBS: Factors affecting mechanical behaviour and leachability

Management strategies for the safe disposal of contaminated dredged marine sediment constitute a global-scale environmental issue. The stabilization/solidification method was investigated as a sustainable approach to the recycling of the sediment as a construction material. A systematic study of the factors affecting the mechanical performance and contaminant release was performed. The physico-chemical variables selected to assess the potential re-use of the sediment treated with Ordinary Portland cement (OPC) and Ground Granulated Blast Furnace Slag (GGBS) in an aquatic environment were: curing duration (7, 28, 56 and 98 days), curing temperature (5, 20 and 40 °C) and ambient (leachate) pH (1, 4, 7 and 10). Unconfined compressive strength (UCS) tests were conducted and extended-duration tank leaching tests were used to characterize the long-term leaching of Al, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba, Pb. The results showed that S/S methods provide excellent immobilization of metals in marine sediment at a pH range of 4 to 10. Immobilization efficiencies of >99.9% for Mn, Fe, Zn, As, Ba, Pb and >97.8% for Al, Cu and Zn are reported over 100 days. GGBS replacement is an effective way to further improve sediment properties by enhancing strength, mitigating sediment alkalization and offering a better immobilization capacity for Fe, Ni and Zn. The release of metals (Al, Mn, Cu, As, Ba and Pb) was strongly associated with a coupling effect of the physico-chemical factors, with metal-specific responses to curing temperature, curing duration and pH. Mn mobility showed a dramatic sensitivity to ambient pH while Ba was less pH-dependent. Al release is related to strength and leached out by dissolution in all situations considered. Considering that dredged marine sediments may contain multiple metal contaminants which exhibit individual responses to remediation, treatment with GGBS may be considered a potentially suitable management option.

Selecting the best stabilization/solidification method for the treatment of oil-contaminated soils using simple and applied best-worst multi-criteria decision-making method

Oil-contaminated soils resulted from drilling activities can cause significant damages to the environment, especially for living organisms. Treatment and management of these soils are the necessity for environmental protection. The present study investigates the field study of seven oil-contaminated soils treated by different stabilization/solidification (S/S) methods, and the selection of the best treated site and treatment method. In this study, first, the ratios of consumed binders to the contaminated soils (w/w) and the treatment times for each unit of treated soils were evaluated. The ratios of consumed binders to the contaminated soils were between 6 and 10% and the treatment times for each unit of treated soils were between 4.1 and 18.5 min/m³. Physicochemical characteristics of treated soils were also determined. Although S/S methods didn't change the water content of treated soils, they increased the porosity of soils. Unexpectedly, the cement-based S/S methods didn't increase the pH of the treated soils. The highest and the lowest leaching of petroleum hydrocarbons was belonging to S/S using diatomaceous earth (DE) and the combination of Portland cement, sodium silicate and DE (CS-DE), respectively. The best acid neutralization capacity was obtained for soils treated using the combination of Portland cement and sodium silicate (CS). Based on the best-worst multi-criteria decision-making method (BWM-MCDM), the soils treated using CS-DE was select as the best. The BWM-MCDM can be used as an effective tool for the selection of the best alternative in all areas of environmental decontamination.

The rainfall effect onto solidification and stabilization of heavy metal-polluted sediments

Rainfall makes impacts on the process of solidification/stabilization (S/S) and the long-term safety of solidified matrix. In this study, the effect of rainfall on solidification/stabilization process was investigated by the rainfall test. The unconfined compressive strength (UCS) and toxicity characteristic leaching procedure (TCLP) were adopted to characterize the properties of S/S sediments before and after the rainfall test. The samples cured for 28 days were selected for semi-dynamic leaching tests with a simulated acidic leachant prepared at pH of 2.0, 4.0 and 7.0. The effectiveness of S/S treatment was evaluated by diffusion coefficient (D_e ) and leachability index (LX). The results indicated that UCS decreased at maximum deterioration rate of 34.23% after 7 days of curing, along with the minimum rate of 7.98% after 28 days by rainfall, with greater than 14 days referred. The rainfall had little effect on the leaching characteristics of heavy metals during the curing process. However, the simulated acid rain made significant impacts on the leaching behaviours of the heavy metals in the S/S materials. All the values of cumulative fraction of leached heavy metals were less than 2.0%, exhibition of good stabilization of cement. Furthermore, the calculated diffusion coefficient (D_e ) for Cu was 1.28 × 10¹ cm² s^-1, indicating its low mobility of heavy metal ions in S/S sediments. Furthermore, the calculated diffusion coefficients (D_i ) for Cd, Cu and Pb were 7.44 × 10^-11, 8.18 × 10^-12 and 7.85 × 10^-12 cm² s^-1, respectively, indicating their relatively low mobility of heavy metal in S/S sediments.

Geochemical features and potential environmental implications of heavy metals in mining-impacted sediments, south China

The present study was initiated to investigate the geochemical features and associated pollution risks of selected heavy metals in sediments near an active copper sulfide mines, south China. These results indicated that legacy contamination in sediments were mainly Cd (11.9 mg/kg), Cu (0.106%), Pb (0.189%), Zn (0.0958%), and As (0.158%). Furthermore, the geochemical variability of most elements, ranging from 5.66% for K₂O to 24.99% for Cd, was relatively lower. On the spatial scale, the variation patterns of multi-elements did not show a decreasing trend. The multivariate statistical analysis revealed that the significant enrichment of the studied elements was mostly related to the geochemical background and anthropogenic sources. Besides that, the stable climate might have positive influences on the leachability patterns of heavy metals in sediment profiles. According to the results of the potential ecological risk index (PERI), Cd, Cu, Pb, and As were identified as the riskiest elements due to their rather higher contribution ratios to pollution risk. In response to continuous exposure risks, the significant enrichment of these mining-derived elements should be preferentially concerned. Finally, some reasonable action is proposed for aquatic environment protection. Graphical abstract.

Pennisetum sinese: A Potential Phytoremediation Plant for Chromium Deletion from Soil

Chromium is one of the major pollutants in water and soil. Thus, it is urgent to develop a new method for chromium removal from the environment. Phytoremediation is a promising approach for heavy metal pollution recovery. As a perennial giant grass with a fast growth rate, Pennisetum sinese has been widely used as livestock feed, mushroom culture medium and biomass energy raw material. Interestingly, we have found a high adsorption capacity of P. sinese for chromium. P. sinese was treated with different concentrations of chromium for 15 days. Results showed that P. sinese plantlets grew well under low concentrations (less than 500 μM) of chromium (VI). The plantlet growth was inhibited when treated with high concentrations of chromium (more than 1000 μM). Up to 150.99 and 979.03 mg·kg−1 DW of chromium accumulated in the aerial part and root, respectively, under a treatment of 2000 μM Cr. The bioaccumulation factor (BCF) of P. sinese varied from 10.87 to 17.56, and reached a maximum value at the concentration of 500 μM. The results indicated that P. sinese showed strong tolerance and high accumulation capability under Cr stress. Therefore, the chromium removal potential of P. sinese has a great application prospect in phytoremediation.

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.

Microwave irradiation assisted sodium hexametaphosphate modification on the alkali-activated blast furnace slag for enhancing immobilization of strontium

An inadvertent leakage of ⁹⁰Sr into the environment can induce an easy accumulation in biosphere and cause a continuous radiation to the surrounding ecosystem. In this study, sodium hexametaphosphate (Na₆O₁₈P₆) was employed to modify the blast furnace slags (BFS) to enhance the chemical stabilization of Sr²⁺ ions in the BFS-based cementitious materials. Microwave irradiation (MW) was used to further increase the binder activity of BFS samples and strengthened the mechanical strengths and durability of BFS-based blocks. A combination of experimental factors including the mass ratio of Na₆O₁₈P₆ to BFS-Sr_0.1 of 15%, the ratio of solid to liquid of 1:4 mg/L, the output power of 650 W, and the activation time of 3 min was most conductive to achieving an optimal microwave-irradiation process. Four extraction solutions were sorted by their leaching abilities following as MgSO₄ solution > H₂SO₄ solution > CH₃OOH solution > deionized (DI) water based on their leaching results. Compared with microwave irradiation, an addition of Na₆O₁₈P₆ to BFS samples obtained a better compressive strength for BFS-based blocks. However, a microwave-irradiation treatment was more effective in improving the resistances of blocks to gamma irradiation and thermal-thaw changes. Exposing to gamma irradiation over 6 months and enduring to thermal-thaw tests over 15 cycles, the microwave-treated blocks only lost 3.29% and 2.23% of leaching removal efficiencies in deionized water, respectively. Microwave irradiation increased the mechanical strengths of BFS-based blocks and inhibited leaching of Sr²⁺ ions from matrices mainly by strengthening hydration reactions and Sr²⁺ encapsulation.

Soil amendments for immobilization of potentially toxic elements in contaminated soils: A critical review

Soil contamination by potentially toxic elements (PTEs) has led to adverse environmental impacts. In this review, we discussed remediation of PTEs contaminated soils through immobilization techniques using different soil amendments with respect to type of element, soil, and amendment, immobilization efficiency, underlying mechanisms, and field applicability. Soil amendments such as manure, compost, biochar, clay minerals, phosphate compounds, coal fly ash, and liming materials are widely used as immobilizing agents for PTEs. Among these soil amendments, biochar has attracted increased interest over the past few years because of its promising surface properties. Integrated application of appropriate amendments is also recommended to maximize their use efficiency. These amendments can reduce PTE bioavailability in soils through diverse mechanisms such as precipitation, complexation, redox reactions, ion exchange, and electrostatic interaction. However, soil properties such as soil pH, and clay, sesquioxides and organic matter content, and processes, such as sorption/desorption and redox processes, are the key factors governing the amendments' efficacy for PTEs immobilization in soils. Selecting proper immobilizing agents can yield cost-effective remediation techniques and fulfill green and sustainable remediation principles. Furthermore, long-term stability of immobilized PTE compounds and the environmental impacts and cost effectiveness of the amendments should be considered before application.

Potentially toxic elements in solid waste streams: Fate and management approaches

Solid wastes containing potentially toxic elements (PTEs) are widely generated around the globe. Critical concerns have been raised over their impacts on human health and the environment, especially for the exposure to PTEs during the transfer and disposal of the wastes. It is important to devise highly-efficient and cost-effective treatment technologies for the removal or immobilisation of PTEs in solid wastes. However, there is an inadequate overview of the global flow of PTEs-contaminated solid wastes in terms of geographical distribution patterns, which is vital information for decision making in sustainable waste management. Moreover, in view of the scarcity of resources and the call for a circular economy, there is a pressing need to recover materials (e.g., precious metals and rare earth elements) from waste streams and this is a more sustainable and environmentally friendly practice compared with ore mining. Therefore, this article aims to give a thorough overview to the global flow of PTEs and the recovery of waste materials. This review first summarises PTEs content in various types of solid wastes; then, toxic metal(loid)s, radioactive elements, and rare earth elements are critically reviewed, with respect to their patterns of transport transformation and risks in the changing environment. Different treatments for the management of these contaminated solid wastes are discussed. Based on an improved understanding of the dynamics of metal(loid) fates and a review of existing management options, new scientific insights are provided for future research in the development of high-performance and sustainable treatment technologies for PTEs in solid wastes.

Long-term application of stabilization/solidification technique on highly contaminated sediments with environment risk assessment

After dredging of contaminated sediment, additional remediation technique is required before its final disposal. For this purpose, this research was based on the long-term stabilization/solidification (S/S) process of highly contaminated sediment (dominantly by heavy metals) from a European environmental hot spot, the Great Bačka Canal. Due to optimisation of remediation techniques, this sediment is treated with selected immobilization agents: kaolinite, quicklime and Portland cement. The use of pseudo-total metal content (selected priority substances: Cr, Ni, Cu, Cd, Zn, Pb and As) in untreated sediment, determined that sediment urgently requires remediation. Short-term (after 7 and 28 days) and long-term (after 7 years) monitoring were done in order to estimate the concentrations of metals and effect on biota from S/S mixtures during this processes. The environmental risk assessment encompassed the application of several appropriate analytical methods: the pseudo-total metal content, the German standard leaching test - DIN 3841-4 S⁴ and Toxicity Characteristic Leaching Procedure - TCLP test leaching tests and sequential extraction procedure (BCR) on S/S mixtures, testing the aging process and toxicity effects. After simulating real environmental conditions using all tests in all three mixtures, metals do not exceed the prescribed limit values and as such S/S mixtures are classified as non-hazardous waste. Sequential extraction procedure showed that the highest percentage of metals are in the residual phase, bound to silicates and crystalline structure. After 7 years of S/S mixture aging, kaolinite showed the highest binding capacity that was reflected in the content of metals in the residual phase (34.8% of Ni to 77.6% of Cr). DIN and TCLP leaching tests confirmed that the exchangeable phase has a minor effect on the environment. Accordingly, this remediation technology could be well applied for final disposal of this and similar extremely contaminated sediment dominantly with inorganic pollutants.

Fabrication and environmental applications of multifunctional mixed metal-biochar composites (MMBC) from red mud and lignin wastes

This study fabricated a new and multifunctional mixed metal-biochar composites (MMBC) using the mixture of two abundant industrial wastes, red mud (RM) and lignin, via pyrolysis under N₂ atmosphere, and its ability to treat wastewater containing various contaminants was comprehensively evaluated. A porous structure (BET surface area = 100.8 m² g^-1) was created and metallic Fe was formed in the MMBC owing to reduction of Fe oxides present in RM by lignin decomposition products during pyrolysis at 700 °C, which was closely associated with the transformation of liquid to gaseous pyrogenic products. The potential application of the MMBC was investigated for the removal of heavy metals (Pb(II) and Ni(II)), oxyanions (As(V) and Cr(VI)), dye (methylene blue), and pharmaceutical/personal care products (para-nitrophenol and pCBA). The aluminosilicate mineral, metallic Fe, and porous carbon matrix derived from the incorporation of RM and lignin contributed to the multifunctionality (i.e., adsorption, chemical reduction, and catalytic reaction) of the MMBC. Thus, engineered biochar composites synthesized from selected industrial wastes can be a potential candidate for environmental applications.

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

A slag humidification process that aims to reduce the leachate pH of steel slag via carbonation was accomplished by simply wetting the slag and exposing it to the atmosphere for passive diffusion of atmospheric CO₂. The optimization parameters of the process were studied. Results showed that by wetting the slag using various aqueous solutions (deionized water, NaCl solution and NaOH solution), such that its moisture content nearly reaches its water holding capacity, a significant reduction in leachate pH could be achieved. Pretreatment of the slag using 1 M NaOH and subsequent humidification using deionized water showed the best efficiency of 1.1 pH unit reduction among the tested conditions. Slag pretreatment could substantially enhance the carbonation degree on the slag surface, leading to conservation of the treatment effectiveness up to three times the leachate replenishment using deionized water. The benefit of the alkaline treatment to promote slag carbonation could also be achieved using a low to moderate (0.005-0.1 M) NaOH solutions for humidification without the pretreatment step. A 72.5% increase in the treatment efficiency could be achieved via a humidification treatment using 0.005 M NaOH solution compared to that using deionized water. This study shows the promise of humidification treatment as a low-cost, easily implemented, and environmentally friendly slag pH neutralization process that can be applied in the field for slag treatment prior to its use or disposal in the environment.

Green remediation of As and Pb contaminated soil using cement-free clay-based stabilization/solidification

Stabilization/solidification (S/S) is a low-cost and high-efficiency remediation method for contaminated soils, however, conventional cement-based S/S method has environmental constraints and sustainability concerns. This study proposes a low-carbon, cement-free, clay-based approach for simultaneous S/S of As and Pb in the contaminated soil, and accordingly elucidates the chemical interactions between alkali-activated clay binders and potentially toxic elements. Quantitative X-ray diffraction and ²⁷Al nuclear magnetic resonance analyses indicated that the addition of lime effectively activated the hydration of kaolinite clay, and the presence of limestone further enhanced the polymerization of hydrates. X-ray photoelectron spectroscopy showed that approximately 19% of As^[III] was oxidized to As^[V] in the alkali-activated clay system, which reduced toxicity and facilitated immobilization of As. During the cement-free S/S process, As and Pb consumed Ca(OH)₂ and precipitated as Ca₃(AsO₄)₂·4H₂O and Pb₃(NO₃)(OH)₅, respectively, accounting for the low leachability of As (7.0%) and Pb (5.4%). However, the reduced amount of Ca(OH)₂ decreased the degree of hydration of clay minerals, and the pH buffering capacity of the contaminated soil hindered the pH increase. Sufficient dosage of lime was required for ensuring satisfactory solidification and contaminant immobilization of the clay-based S/S products. The leachability of As and Pb in high-Ca S/S treated soil samples was reduced by 96.2% and 98.8%, respectively. This is the first study developing a green and cement-free S/S of As- and Pb-contaminated soil using clay minerals as an environmentally compatible binding material.

Calcium-Based Sustainable Chemical Technologies for Total Carbon Recycling

Calcium carbide, a stable solid compound composed of two atoms of carbon and one of calcium, has proven its effectiveness in chemical synthesis, due to the safety and convenience of handling the C≡C acetylenic units. The areas of CaC₂ application are very diverse, and the development of calcium-mediated approaches resolves several important challenges. This Review aims to discuss the laboratory chemistry of calcium carbide, and to go beyond its frontiers to organic synthesis, life sciences, materials and construction, carbon dioxide capturing, alloy manufacturing, and agriculture. The recyclability of calcium carbide and the availability of large-scale industrial production facilities, as well as the future possibility of fossil-resource-independent manufacturing, position this compound as a key chemical platform for sustainable development. Easy regeneration and reuse of the carbide highlight calcium-based sustainable chemical technologies as promising instruments for total carbon recycling.

Fe-rich biomass derived char for microwave-assisted methane reforming with carbon dioxide

Microwave-assisted methane reforming with carbon dioxide was dealt with in this work, using a Fe-rich biomass-derived char by one-step preparation. The main factors on the reforming reaction and stability of this catalyst were evaluated, together with a series of characterization on the produced gas and the used char. The char obtained from biomass pyrolysis with Fe₂O₃ addition of 10% exhibited the best performance on dry reforming reaction. A target CH₄ conversion of 95% over this char was realized at 800 °C. Moreover, H₂/CO ratio achieved with this char was prone to approach the stoichiometric value. Compared to CO₂ conversion, CH₄ conversion was more promoted with the increase of CO₂/CH₄ ratio. The variation of CO₂/CH₄ ratio also leaded to a noticeable changes on H₂/CO ratio. More importantly, the selected char presented a satisfied stability, evidenced by the total decrease of 4.8% for CH₄ conversion and 3.1% for CO₂ conversion in the test of 160 min. This was contributed to a depressed in-situ carbon consumption and a moderate deterioration of porous structure. Gaseous products obtained with the appropriate char in a long run had a syngas content of 88.79% and H₂/CO ratio of 0.92 on average.

Novel synergy of Si-rich minerals and reactive MgO for stabilisation/solidification of contaminated sediment

Disposal of significant amounts of dredged contaminated sediment poses an economic and environmental problem worldwide. Transforming contaminated sediment into value-added construction materials using low-carbon MgO cement is a sustainable option; however, the weak mechanical strength and unreliable water-solubility of MgO cement restrict its practical engineering applications. This study elucidates the potential role of industrial Si-rich minerals in the performance enhancement of MgO-based products via the promotion of magnesium silicate hydrate (M-S-H) gel formation. Quantitative X-ray diffraction and ²⁹Si nuclear magnetic resonance analyses indicated that compositions and crystallinities of the Si-rich minerals significantly influence the formation and polymerisation of the M-S-H gel. Pulverised fly ash was found to be a promising Si-rich mineral for generating polymeric M-S-H gel, whereas incinerated sewage sludge ash samples demonstrated a low degree of polymerisation, and the use of glass powder samples gave a low yield of M-S-H. The formation of M-S-H gel enhanced the compressive strength and water resistance (strength retention after water immersion). Further experiments demonstrated that Si-modified MgO cement can transform dredged sediment into fill materials with satisfactory mechanical properties and contaminant immobilisation. Therefore, the synergy between reactive MgO and Si-rich industrial waste is a novel option for sustainable remediation and environmental applications.

State of the art and future challenges for polycyclic aromatic hydrocarbons is sediments: sources, fate, bioavailability and remediation techniques

Polycyclic aromatic hydrocarbons (PAHs) are amongst the most abundant contaminants found in the aquatic environment. Due to their toxicity and carcinogenicity, their sources, fate, behaviour, and cleanup techniques have been widely investigated in the last several decades. When entering the sediment-water system, PAH fate is determined by particular PAH and sediment physico-chemical properties. Most of the PAHs will be associated with fine-grained, organic-rich, sediment material. This makes sediment an ultimate sink for these pollutants. This association results in sediment contamination, and in this manner, sediments represent a permanent source of water pollution from which benthic organisms may accumulate toxic compounds, predominantly in lipid-rich tissues. A tendency for biomagnification can result in critical body burdens in higher trophic species. In recent years, researchers have developed numerous methods for measuring bioavailable fractions (chemical methods, non-exhaustive extraction, and biomimetic methods), as valuable tools in a risk-based approach for remediation or management of contaminated sites. Contaminated sediments pose challenging cleanup and management problems, as conventional environmental dredging techniques are invasive, expensive, and sometimes ineffective or hard to apply to large and diverse sediment sites. Recent studies have shown that a combination of strategies including in situ approaches is likely to provide the most effective long-term solution for dealing with contaminated sediments. Such in situ approaches include, but are not limited to: bioaugmentation, biostimulation, phytoremediation, electrokinetic remediation, surfactant addition and application of different sorbent amendments (carbon-rich such as activated carbon and biochar) that can reduce exposure and limit the redistribution of contaminants in the environment.

Transforming waterworks sludge into controlled low-strength material: Bench-scale optimization and field test validation

In order to reduce landfill disposal of waterworks sludge (alum sludge) and incinerated sewage sludge ash (ISSA), this study proposed an innovative approach for upcycling them into value-added controlled low-strength materials (CLSM). Waterworks sludge interfered with cement hydration reaction and delayed the stiffening time of CLSM (>10 h) due to its high organics content (5%). The addition of triethanolamine (TEA) with a dosage of 0.1 wt% of binder effectively shortened the stiffening time to 4.5 h. The lab-scale results suggested that the optimal CLSM design (6% cement, 14% ISSA, 8% sludge, and 72% recycled fine aggregate at a water/binder ratio of 1.2) complied with the standard requirement of flowability (>200 mm), stiffening time (<5 h), and compressive strength (0.3-2.1 MPa). The pilot-scale field tests further confirmed that the sludge-incorporated CLSM achieved a high flowability (220 mm), short stiffening time (4 h), low compressive strength (1.38 MPa), and easy re-excavatability after 3 months. This study demonstrated that waterworks sludge can be potentially transformed into sustainable construction materials for urban development.