Low Carbon Stabilization and Solidification of Hazardous Wastes

By Lei Wang

Low Carbon Stabilization and Solidification of Hazardous Wastes details sustainable and low-carbon treatments for addressing environmental pollution problems, critically reviewing low-carbon stabilization/solidification technologies. This book presents the latest state-of-the-art knowledge of low-carbon stabilization/solidification technologies to provide cost-effective sustainable solutions for real-life environmental problems related to hazardous wastes including contaminated sediments. As stabilization/solidification is one of the most widely used waste remediation methods for its versatility, fast implementation and final treatment of hazardous waste treatment, it is imperative that those working in this field follow the most recent developments.

Low Carbon Stabilization and Solidification of Hazardous Wastes is a necessary read for academics, postgraduates, researchers and engineers in the field of environmental science and engineering, waste management, and soil science, who need to keep up to date with the most recent advances in low-carbon technologies. This audience will develop a better understanding of these low-carbon mechanisms and advanced characterization technologies, fostering the future development of low-carbon technologies and the actualization of green and sustainable remediation.

• Focuses on stabilization/solidification for environmental remediation, as one of the most widely used environmental remediation technologies in field-scale applications
• Details the most advanced and up-to-date low-carbon sustainable technologies necessary to guide future research and sustainable development
• Provides comprehensive coverage of low-carbon solutions for treating a variety of hazardous wastes as well as contaminated soil and sediment

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 24, 2021
• ISBN: 9780128242520

Book preview

Chapter 1

Overview of hazardous waste treatment and stabilization/solidification technology

Xinni Xiong¹, Yuying Zhang¹, Lei Wang² and Daniel C.W. Tsang³,    ¹Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China,    ²Institute of Construction Materials, Technische Universität Dresden, Dresden, Germany,    ³Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong

Abstract

Hazardous waste management is one of the most significant sectors in utility services. In view of the growing amount of waste and the shortage of energy and resources, the conventional waste treatment approaches could hardly fulfill the need of sustainable development, and would cause various concerns to the environment, ecological systems, and public health. The current environmental challenges highlight the demands to achieve sustainable development goals (SDG) by means of state-of-the-art technologies to evolve from a linear economy to a circular model. This chapter provides an overview on the current situations and emerging approaches on hazardous waste treatment and remediation. The concept and cutting-edge technologies of sustainable waste management for achieving SDG are discussed. Current waste treatment including landfilling, waste-to-energy technique, hydrothermal treatment, washing and extraction, phytoremediation, bioremediation, sintering and melting, and cement rotary kilns coprocessing, and advanced oxidation and photocatalysis are discussed. As a cost-efficient and promising technology, stabilization/solidification is demonstrated in terms of efficiently treating soil/sediment, industrial waste, and radioactive waste. The work summarizes the state-of-art advances and frontiers on green chemistry, sustainable development, and the circular economy.

Keywords

Waste management; sustainable treatment; hazardous waste; green remediation; stabilization/solidification; state-of-the-art technologies

1.1 Introduction

The progress of urbanization and technological developments have driven the steady increase of the amount of waste across the world. Improper disposal of waste would cause severe crisis to public health as well as the environment, such as pollution to air, soil, and groundwater. The economic, environmental, and social challenges presented from growing urbanization are becoming a great concern for scientists. In particular, the uncontrolled hazardous waste, which contains toxic organic compounds or heavy metals, are likely to spread and accumulate through the food chain web, and cause threats to human health.

Waste streams are generally divided into municipal solid waste (MSW), construction and demolition, commercial and industrial waste, agricultural and forestry wastes, and mining and quarrying wastes, although standards and definitions usually vary from country to country (Wilson, 2015). According to the New South Wales Environment Protection Authority (EPA), the classification of waste is judged by a series of steps, and general solid waste includes putrescible feature (e.g., animal waste, food waste, manure) and nonputrescible ones (e.g., building and demolition wastes, garden waste, virgin excavated natural material, wood waste) (NSW EPA, 2014). Besides, restricted solid waste and hazardous waste are determined by chemical assessment and evaluations like specific contaminant concentration and/or toxicity characteristics leaching procedure. It was estimated that 2.01 billion tons of MSW are generated annually, which is expected to increase to 3.40 billion tons by 2050 (The World Bank, 2018). The global annual solid waste may range from 2.5 to 4 billion metric tons, as estimated by a wide spectrum of nongovernmental organizations (NGOs) and international research agencies (Un-Habitat, 2010). As shown in Fig. 1.1A, the regional amount of waste production shows a positive trend with the level of economic development, and the growing trend in the coming decades poses a great burden to the world, which indicates the appeal of efficient and effective waste disposal approaches. The conventional waste treatment approaches include landfilling, incineration, and compositing. Landfill constitutes the great proportion (36.6%) of the global current waste disposal approaches (Fig. 1.1bB), among which approximately 8% is treated in sanitary landfills accompanied with gas collection systems to recycle renewable energy (The World Bank, 2018). Particularly, low-income countries are more tending to adopt open dumping than middle- and high-income countries, which are suffering from the problems of inefficiency, secondary pollution, greenhouse gas emission, and inadequate spaces. On the other hand, the shortage of food and energy in the developing areas also arouses great concerns. Nowadays, scientists and stakeholders are attempting to gradually convert the current waste management systems from the linear economy mode to resource management corresponding to the circular economy (Wilson et al., 2015). Sustainable waste treatment is of great importance to achieve a circular economy and reach a closed-loop in diverse industries.

Figure 1.1 (A) Projected global waste generation, and (B) percentage of world waste treatment approaches. Reproduced from The World Bank, 2018, <https://datatopics.worldbank.org/what-a-waste/trends_in_solid_waste_management.html>.

Therefore this chapter provides an overview of the current situations and emerging approaches for worldwide waste management. The concept and cutting-edge technologies of sustainable waste management for achieving SDG will be discussed. Stabilization/solidification (S/S) as a typical cost-efficient and promising method for sustainable waste management is discussed. The chapter will elucidate the principles, parameters, and evaluate the performance to present future prospects for research directions. The work summarizes the state-of-art advances and frontiers of green chemistry, sustainable development, and the circular economy.

1.2 Sustainable waste management

Waste management is a multidisciplinary topic that addresses issues including society, economy, and the environment, for example, climate change, human health, food and resource security, as well as sustainable production and consumption. Waste management involves the processes of waste collection, transportation, processing, as well as waste recycling or disposal. Sustainable waste management systems include advanced management strategies to minimize environmental challenges and protect resources (Demirbas, 2011). It was widely recognized that the 3R (reduce, reuse, and recycle) principle of waste management encourages the general public to reflect on the entire life cycles of products and services, and explore solutions to preserve more natural resources for our future generations. It arouses the design for resource-saving and long-life products. Waste reduction by minimizing the amount of waste at the source is the top priority, while recycling by using waste as raw materials for other purposes is another promising alternative.

As initiated by the United Nations (UN), a sum of 17 SDG were proposed in 2015 as part of the 2030 Agenda, which aroused the world to take action to eliminate poverty, protect the globe, and improve living qualities and prospects. The framework of 17 SDGs including energy, climate change, and sustainable consumption and production exposes emerging trade-offs in terms of infrastructure decision making, such as investments in sustainable infrastructure. The proper and sustainable waste management would lead to the effective realization of several human rights. The diversity and potential consequences of soil and groundwater pollution impacts, as well as the limited capacity in developing countries for addressing such impacts, render the establishment of the advanced waste management system as a key cross-cutting issue for achieving SDGs (Dermatas, 2017). Adshead et al. (2019) developed metrics for evaluating the performances of infrastructure-linked targets and combined them with a systems model that could quantify the infrastructure demands in the future so as to assess the investments and policies to meet such demands. They found that strategies of cross-sectoral infrastructure investments and policies with regard to energy, water, wastewater, and solid waste sectors were able to mobilize SDG targets by means of quantitative indicators to reveal their interdependent nature and address uncertainties. It is imperative to implement and promote sustainable services through key policy tools such as direct regulation, economic incentives, as well as social instruments (Rodić and Wilson, 2017). For example, a legal framework involving both public and private sectors, financial supports for the services, and collaborative projects with civil societies such as green groups and media are potential ways to resolve the waste management issues.

In addition, the integration of private services (e.g., small-scale enterprises) into the official system with the collaboration of NGO or community-based organization (CBO) would bring about socioeconomic benefits and contribute to ecological sustainability, and enhance public health services (Baud et al., 2001). Such alliances would be more preferred in developing areas, where waste trade and recycling facilitates could contribute to a cleaner environment with reduced waste and enhanced recycling, an improved financial situation, and more employment. Another case study in Brazil also supported the approach of integrated public–private partnerships (PPPs) and a deliberative mode in order to realize sustainable solid waste management (Kruljac, 2012). The waste management collecting systems have been evolving through the history. For example, a Waste Purchase program was carried out in 1989 supplying basic waste collection to the designated area, when the citizens used to purchase the surplus food and exchange waste for locally grown food from the farmers. Similarly, the Green Exchange program was initiated in 1991, stimulating regularized form of such scenario and encouraging waste recycling among low-income neighborhoods. These programs have accelerated the formation of PPPs involving the economic sector as a facilitative approach, which is mediated by NGOs or other CBOs for sustainable waste management (Kruljac, 2012). The significance of integrated solid waste management was highlighted in a work on the Indian MSW management system, which compared different scenarios such as composting, anaerobic digestion, incineration, and landfilling (Pujara et al., 2019). Life cycle assessment (LCA) and waste-to-energy (WTE) practices are critical tools to realize environment and economic sustainability as well as to minimize the adverse impacts associated with MSW. To achieve the SDG goals of sustainable consumption and production, practical measures can be implemented, including public education, improvement of manufacture and business process, integration of supply chain, waste redistribution, and recovery. Meanwhile, researchers are expected to advance the technologies on standardized data collection and update concept definitions, examining the awareness of the need concept in waste redistribution, as well as exploring the behavior of consumers and properties of logistics networks (Lemaire and Limbourg, 2019).

It is encouraged to realize synergies among the SDG targets through circular economy practices. For example, the circular economy is referring to practices which could lead to abundant benefits such as cost reduction and increase in employment, innovation, productivity, and resource efficiency in both developed and developing areas, and these actions are frequently taken in the fields on ecodesign, material reuse, refurbishment, remanufacturing, repair, product sharing, and industrial symbiosis (Schroeder et al., 2019). In this regard, SDG7, SDG 8, and SDG 12 (i.e., Affordable and Clean Energy, Decent Work and Economic Growth, Responsible Consumption and Production) are all applicable in promoting the circular economy. Besides, other SDGs such as Clean Water and Sanitation (SDG 6) and Life on Land (SDG 15) are also commonly involved in the progress of sustainable development and circular economy. Businesses with global supply chains are actively engaged to enlarge capacity for scaling-up circular economy practices. A study investigated a multiobjective model on a hazardous waste management system at the large scale so as to resolve safety and economic issues for decision makers and facility developers, which highlighted that the estimation of costs should be an essential provision (Yilmaz et al., 2017). Therefore building models on circular economy business could facilitate the maximization of resources and efficiency on energy utilization, reusing or recycling waste materials to move from nonrenewable to renewable resources.

1.3 Overview of current waste treatment technologies

Hazardous materials are waste materials that have substantial or potential harm or threats to public health or the environment. Hazardous waste refers to the waste that contains risky properties such as toxicity, corrosivity, flammability, and reactivity, which would bring about potential dangerous outcomes to the environment, ecological system, and human health (Yilmaz et al., 2017; Xiong et al., 2019b; Zhang et al., 2021). Hazardous waste may originate from industrial manufacture involving some materials with potentially toxic elements (PTE) or radioactive properties, which further result in processed wastes in different forms as well as contaminated soils/sediment. The identification of hazardous waste should comply with specific standards or regulations. Hazardous waste management should require strict controls in regard to handling, transportation, processing, and disposal. Improper disposal of hazardous waste would cause severe environmental problems such as water/soil/air pollution and climate change. For example, open dumping or direct burning would result in the release of toxic elements and organic compounds circulating through the food web as well as emissions of greenhouse gas into the atmosphere (Wilson, 2015). Leachate from hazardous waste would potentially cause severe contamination to the soil, surface water, and groundwater with continuous discharge of potentially toxic elements. Uncollected waste tends to block the drains to provoke floods and even spread infectious disease threatening public health, such as gastrointestinal and respiratory infections. Lots of evidence in the literature has proved the association between hazardous waste and various health problems including cancers in liver, bladder, breast, and testis, non-Hodgkin lymphoma, asthma, congenital anomalies overall, and anomalies of the neural tube, urogenital, connective, and musculoskeletal systems (Fazzo et al., 2017). Therefore special hazardous materials require specific treatments before disposal or even further use.

1.3.1 Landfill

Landfill is one of the most popular options for waste disposal, with a relatively low recycling rate due to the operating cost. However, landfill requires increasingly higher costs for land use and management in the long term, and it is also likely to result in severe pollution. Sanitary landfill is practiced as an improved form of landfill in developed countries; it is built with some architectural upgrades and aims to realize the optimal utilization of wastes with minimal contamination to the environment (Das et al., 2019). In recent years, limited land use has gradually become a concern to accommodate the huge amount of waste generated each day. Besides, landfills, known as waste mountains, are dangerous areas to some extent, since they are attractive for scavengers, trash collectors, and informal workers (Fatimah et al., 2020). The further treatment of landfill leachate, which is regarded as an important hazardous waste with a high concentration of heavy metals, inorganic salts, and ammoniacal nitrogen and organochloro compounds (Gautam et al., 2019), becomes a significant concern. In recent years, advanced oxidation processes (AOP) have been frequently employed for treating hazardous landfill leachate. Besides, electrocoagulation is regarded as a cleaner option. Biological treatments involving endogenous denitritation and Anammox processes can be used for nitrogen removal for mature leachate (Miao et al., 2019). Moreover, with time increasing, more problems may emerge, such as the generation, accumulation, and release of microplastic in the landfill systems. It was found in a study that 99.36% of microplastics in the landfill leachate came from the fragmentation of the buried plastic waste (He et al., 2019), which is likely to pose a severe threat to the groundwater and soil.

1.3.2 Waste-to-energy technique

WTE is a growing technique to efficiently recycle organic waste for energy production, which has been put into use in many areas and countries. Most WTE facilities utilize the incineration technology to burn the waste and collect low-pressure steam for electricity production, district heating, cooling, and other industrial uses (Bourtsalas et al., 2019). After incineration treatment, the volume of MSW waste can be significantly reduced by approximately 85%–90%, the mass by 60%–90%, and the organic matter by approximately 100% (Leckner, 2015). However, municipal solid waste incineration (MSWI) bottom ash and fly ash are inevitably generated along with the incineration processes. The MSWI bottom ash has been widely used as secondary raw materials in the construction area considering its low concentrations of contaminants (Yin et al., 2018), whereas the MSWI fly ash is classified as hazardous waste in numerous countries due to its high content of PTE (lead, cadmium, mercury, molybdenum, nickel, selenium, etc.) and other contaminants (dioxins, furans, sulfate, chloride, acids, etc.). Therefore toxic MSWI FA requires prudent and adequate treatment. Under the guidance of the European Union (EU) waste hierarchy, WTE technology is able to build synergies with energy and climate policy (D’Adamo and Rosa, 2016). Both MSW disposal and greenhouse gas emission could be relieved by this advanced approach, contributing to considerable profits. A study showed that using a WTE plant as an alternative to landfill could result in € 25.4 profits per 1000 tons of waste and 370 kg prevention of CO2eq per ton of waste (Cucchiella et al., 2017).

1.3.3 Hydrothermal treatment

Hydrothermal treatment is a novel technology for the treatment of organic wastes. Biomass such as food waste, agricultural waste, and forestry residues are mainly composed of cellulose, hemicellulose, and lignin. Although, waste biomass is commonly disposed of via the abovementioned conventional treatment methods (e.g., landfilling and incineration), a more environment-friendly and sustainable treatment approach by converting food waste into value-added chemicals has emerged in recent years (Yu and Tsang, 2017), which employs biomass valorization technologies and obtains various resources and energy, including acids, sugars, biofuel, biogas, and biochar ((Xiong et al., 2017); Xiong et al., 2019a). To achieve sustainable development and resource recovery, a number of physical, chemical, and biological methods for biomass valorization have been emerging in the past few decades. For example, biochemical conversions could convert the soluble sugars and nutrients in waste biomass into hydrolysates by means of chemical catalyzed or enzymatic hydrolysis, combined with microbial fermentation to produce target value-added chemicals of some commercial value. Besides, the thermochemical conversion is an attractive option, by breaking down the glycosidic bonds to release the units and going through further conversions and isomerization like dehydration to generate chemicals. These state-of-the-art practices could achieve waste reduction and recycle useful resources by converting waste biomass into consumer, specialty, commodity, and niche chemicals (Xiong et al., 2019a). In the future, more research is required for parameter optimization and exploration of the underlying mechanisms so as to improve the technologies to be more efficient and sustainable. It is also suggested to perform LCA or technoeconomic evaluations with effective tools on the current strategies on green production (Mak et al., 2019, 2020). Besides, integrated biorefinery for comprehensive utilization of biomass for producing a wide range of valuable products shows great prospects. Moreover, pilot trials should be conducted for the existing lab-scale research and scaled up for commercial manufacturing in the industries to achieve a sustainable biorefinery as well as circular economy (Xiong et al., 2019a).

1.3.4 Washing and extraction

Washing and extraction involve selectively removing compounds from waste using a solvent and then extracting them for separation. This method has been widely used in the remediation of PTE contaminated soil and has provided practical and constant choices for realizing clean soil (Hasegawa et al., 2019; Yin et al., 2021). In recent years, the washing and extraction approach has also been applied in hazardous waste treatment (Bogush et al., 2019; Gao et al., 2008; Yang et al., 2017). It is extremely effective for washing soluble salts such as NaCl and KCl from hazardous waste and eliminates the potential to cause salinization of the environment. Also, some soluble PTE could be removed by the washing process and could be recovered after extraction and concentration (Yang et al., 2017). Water washing of MSW incineration air pollution control residue that was considered as hazardous waste worldwide has been widely investigated and applied in laboratory-scale and full-scale applications. Bogush et al. (2019) found that water washing could substantially remove 23% dry mass of soluble salts from air pollution control residue, significantly reduce concentrations of the associated major elements, and increase concentrations of insoluble matrix elements and potential pollutants.

Improving the selectivity of extraction solutions (Zhang et al., 2019), incorporation of inorganic and organic amendments (Guo et al., 2019), and applying supercritical fluids in the oxidation–extraction process (Cao et al., 2017; Liu et al., 2016) could further enhance the efficacy of washing and extraction process for hazardous waste treatment. However, the generation of wastewater after washing and extraction treatment is a problem. Meanwhile, the processes of extraction and recovery are complex compared with other treatment methods, that is, cement-based treatment and chelating agent treatment (Spence and Shi, 2004; Fuentes et al., 2018; Chen et al., 2021).

1.3.5 Phytoremediation

Phytoremediation, a natural, solar-driven and environment-friendly method, especially in combination with other methods, such as biological, physical, and chemical methods, has recently aroused great interest and led to applications in hazardous materials treatment (Kumar Yadav et al., 2018). Phytoremediation is a good approach to harvest PTE from contaminated soils, mine tailings, and hazardous waste (Maletic et al., 2019; Sarwar et al., 2017). It has also been proven as an efficacious, sustainable, and economical strategy (Sarwar et al., 2017). Numerous organic and inorganic compounds are amenable during the phytoremediation process, which makes it a significant strategy for the treatment of hazardous materials (Wang et al., 2017). Metal sequestration, compartmentalization in individual cell organelles, as well as exclusion and inactivation by exudation of organic ligands are three main PTE of the control strategies involved in phytoremediation (Choppala et al., 2014). Meanwhile, phytoremediation is most applicable when costly pollution remediation approaches and processes are not available (Bhandari, 2018).

By using native plants, phytoremediation can serve a dual purpose of site remediation and ecological restoration. However, hyperaccumulators applied in phytoremediation are usually slow-growing and produce relatively less harvestable aboveground biomass under natural conditions (Wei et al., 2020). Therefore some transgenic plants with high biomass production, more metal accumulation ability, strong metal toxicity tolerance, and adaptability to multiple environments would enhance the efficacy of phytoremediation (Sarwar et al., 2017). Meanwhile, a combination of phytoremediation with other approaches, such as chemical-assisted phytoextraction and microbial-assisted phytoremediation, might also be beneficial for hazardous waste remediation.

1.3.6 Bioremediation

Bioremediation was developed as a cost-efficient, environment-friendly, and sustainable option for hazardous waste treatment (Azubuike et al., 2016; Chaturvedi et al., 2021). It involves applying some bacteria and other microorganisms to extract or degrade inorganic and organic contaminants. Some types of microorganisms are capable of releasing enzymes that can tackle harmful pollutants from contaminated sites. Bioremediation can either be conducted ex situ or in situ, depending on cost, site features, types and concentration of contaminants, and so on. The most significant step to successful bioremediation is site characterization, which establishes the most suitable and efficient bioremediation techniques (ex situ or in situ) (Vikrant et al., 2018). The ex situ bioremediation approaches need additional expense, which is attributed to excavation and transportation, which makes them more expensive. Nonetheless, bioremediation has been proven efficient and reliable in the remediation of many contaminated sites due to its ecofriendly characteristics. Microbial and plant-assisted bioremediation as well as other types of combination approaches would further enhance the efficacy of bioremediation (Ojuederie and Babalola, 2017).

1.3.7 Sintering and melting

Sintering and melting are thermal treatment methods that could be options for hazardous waste treatment and they are extremely efficient for the treatment of volatilizable toxic substances. Sintering is the process of fusing substrates together into one solid material by adjusting both pressure and heat, while the melting process combines substrates by heating them until they liquefy and combine as one material. In recent years, sintering was applied in the recycling and disposal of MSW incineration fly ash that is regarded as a hazardous waste in many countries. By using a high-temperature sintering process, the daily disposal capacity of MSW incineration fly ash was 100 t/day in an industrial scale facility, which has offered a new way to alleviate the environmental problem caused by storage and landfill of such hazardous waste (Peng et al., 2020). Meanwhile, the sintering and melting process also provided a new way for recycling hazardous mining wastes and coal fly ash into lightweight aggregates and foamed ceramics, respectively (Park et al., 2018).

1.3.8 Cement rotary kilns coprocessing

Cement rotary kilns coprocessing of wastes as alternative raw materials in the cement industry is a good example of industrial symbiosis for improved material resource efficiency (Bogush et al., 2020). Some hazardous wastes that are rich in silicon, calcium, aluminum, iron, and sulfur are suggested as substitutes for part of raw materials in cement kilns (Nidheesh and Kumar, 2019; Verbinnen et al., 2017; Yin et al., 2018). Cement rotary kilns coprocessing of hazardous wastes containing lower carbon-related components in cement kilns would contribute to the decrease of CO2 emissions compared to limestone (Bie et al., 2016). This would promote the process of realizing a carbon-negative society. Some PTE (e.g., Pb, Zn, and Cu) and most organic toxic substances in the hazardous waste can be chemically immobilized and/or completely destroyed, because of the high temperature (over 1450°C) and alkaline atmosphere during the coprocessing procedure (Huber et al., 2016; Peng et al., 2020). However, a good understanding of the influences of PTE from hazardous waste on the cement manufacturing process, cement quality, and the environment is necessary.

1.3.9 Advanced oxidation and photocatalysis

AOP and photocatalysis processes have drawn more interests in the field of water treatment due to their high removal efficacy in degrading and mineralizing organic pollutants from water (Yang et al., 2020). Meanwhile, AOPs and photocatalysis processes are environmentally friendly compared with other treatment processes, such as physical and chemical approaches, because AOPs and photocatalysis processes would not release substances of damaging residues or transform organic contaminants from one phase to another (Sharma and Feng, 2019). To be specific, a variety of organic contaminants would be degraded or mineralized into intermediate products or CO2 and H2O during the AOPs process and photocatalysis processes. TiO2 photocatalysis is a powerful AOP, which can oxidize virtually all organic contaminants by reactive oxygen species and produces no harmful end products at the same time (Athanasekou et al., 2018). Therefore AOPs and photocatalysis processes have the potential for application in the treatment of hazardous waste or wastewater produced by the treatment of hazardous wastes (Nomura et al., 2020).

1.3.10 Other treatments

Other hazardous wastes treatments, such as cement-based solidification/stabilization and resource recovery, have also stimulated interest from researchers (Cecchi et al., 2020; Gomes et al., 2020; Wang et al., 2019c). At the same time, cotreatments of different wastes have been encouraged in recent years. Cotreatment or codisposal of hazardous waste and other industrial wastes with a reasonable combination can have a synergistic effect, which would aid in formulating value-added waste-based products and even help to foster a circular economy and zero-waste cities

1.4 Sustainable stabilization/solidification

S/S is the most widely used treatment method employing cementitious materials or other stabilizers to decrease the leachability of hazardous materials by means of both physical and chemical approaches, which have been commonly adopted for hazardous waste control in recent years. The S/S materials stabilize the hazardous waste by chemical reactions, and meanwhile immobilizes some specific components by solidifying them, encapsulating, destroying, sorbing, or other pathways (Dassekpo et al., 2018). Ordinary Portland Cement (OPC) is one of the most popular binders used for S/S. By means of cement-based S/S technology, toxic elements could be immobilized and settled in the cement hydration products in hydroxide or complex forms through reactions with cement. Meanwhile, the alkaline environment for the solidified body could prevent the infiltration of toxic elements. Moreover, the hydration products, such as calcium silicate hydrate, AFt-(ettringite), and AFm-(monosulfate), are insoluble and beneficial for toxic elements immobilization (Fan et al., 2018).

The United States Environmental Protection Agency (USEPA) noted S/S to be the optimum demonstrated available technology for treating various hazardous wastes, which accounts for around 40% of the profitable market for hazardous waste treatment (Wang et al., 2015, 2018a). Stabilization can largely save the cost of waste management compared with other technologies. The cost of operating stabilization at onsite remediation consumed $40–$100 per ton of treated waste, among which approximately 40%–50% accounts for the reagents used (Shi and Spence, 2004). Several crucial factors should be taken into consideration to realize the economical operation system, including the waste amount, S/S formulations, and selection of site location as well as treatment scenarios. Additionally, there is emerging research investigating the utilization of industrial byproducts and/or waste as raw materials for the synthesis of reactive binders for S/S with desirable durability, strength, and economic advantages.

1.4.1 Stabilization/solidification for contaminated soil and sediment

Soil/sediment contamination is an increasingly great concern, as the inorganic and organic toxic pollutants may transfer via food chain, thus threatening the sustainability of agroecosystems, food security, and human health. Soil/sediment remediation technologies mainly include immobilization or extraction. Immobilization aims to stabilize the metals by reducing the leachability in the soil matrix and converting the toxic elements to less soluble or bioavailable forms so as to decrease the toxicity and their hazards to the environment. Extraction, on the other hand, draws the PTE from the contaminated medium in order to decrease the metal concentrations and the volume of the contaminated soil/sediment (Tajudin et al., 2016). Compared with conventional soil remediation technologies, such as bioremediation, soil/sediment washing, and thermal deposition, S/S shows the advantageous properties like relatively low cost and convenient operation with outcomes of comprehensive strength, as well as superior resistance toward biodegradation (Du et al., 2010). Therefore S/S technology is currently receiving more and more attention in the scientific community. It effectively immobilizes the contaminants in soil/sediment by transferring them into a less hazardous form and encapsulating them with the formation of a durable matrix.

Supplementary cementitious materials (SCMs), such as fly ash, calcined clays, silica fume, and natural pozzolans, have drawn much research interest in the past decades due to reduction of environmental impacts from the cement industries. An innovative limestone calcined clay cement (LC³) which was fabricated by substituting 50% cement clinker with two SCMs (i.e., limestone and calcined clay) for S/S of Zn-contaminated soil was evaluated in a study (Reddy et al., 2019). The formation of precipitates including Ca6Al2(SO4)3 (OH)12·26H2O (ettringite), and 3CaO·SiO2 (Alite) could explain Zn immobilization, which retained the leachable Zn concentrations below the relevant regulatory limit of hazardous waste. Another work on a cement-based S/S treatment was carried out on a Cr-contaminated soil, indicating the processes of hydration reaction related to alkaline conditions, improved up to 10% the residual fraction of Cr for stabilization (Senneca et al., 2020). Solidification resulted from metal incorporation in the hydration products, greatly contributing to the effectiveness of the process, while the formation of ettringite was postponed due to the internal microcracking of the aged specimens, and it might reduce the efficiency, as the release of toxic elements is mainly controlled by the diffusion mechanism.

Recently, a low-carbon and cement-free approach for treating As- and Pb-contaminated soil via S/S was studied by employing environmentally compatible clay minerals as binding material, which showed that leachability was reduced by 96.2% and 98.8%, respectively. The coaddition of calcined clay and limestone was proved to present a synergistic effect on improving hydrates polymerization and the properties of S/S products, where lime could activate the calcined clay to form calcium alumina hydrates as well as calcium silicate hydrates (C-S-H), while the presence of limestone enhanced the conversion of metastable hydroxyl-rich AFm into a stable carbonate-rich form (Wang et al., 2019a). In addition, waste valorization can be achieved by designing a sustainable soil or sediment remediation method by means of S/S. For example, a study prepared the red mud-incorporated S/S binder and showed excellent efficiency for As immobilization (99.9%) from sediment (Wang et al., 2019b). The S/S binder facilitated Fe-As complexation and presented superior compatibility with As, promoting both in situ and ex situ S/S of As-contaminated sediment. Utilization of calcium-rich industrial by-products [i.e., calcium carbide residue and ground granulated blast furnace slag (GGBS)] for augmenting the efficacy of CO2 curing proved effective for immobilizing contaminants in the sediments (Wang et al., 2018b). Further application such as ecopaving blocks of the S/S products showed a green solution to converting contaminated sediment into renewable and value-added materials. As for stabilizing persistent, toxic, and bioaccumulative contaminants, different carbon-based additives at 2% concentration were evaluated on aged poly- and perfluoroalkyl substances (PFASs)-contaminated soil, revealing that the efficiency of S/S relied on PFAS perfluorocarbon chain length as well as functional groups. Powdered activated carbon and Rembind were reported to present up to 99.9% stabilization efficiency on longer-chained PFASs, for example, perfluorooctane sulfonate (Sörengård et al., 2019). Nevertheless, optimization of S/S binder composition should be assessed depending on the specific cases of contaminated soil/sediment due to the complex processes of binding soil to aggregates.

The performance of S/S treatment on toxic elements-contaminated soils can be assessed by a wide range of approaches including durability test, expansion/shrinkage test, unconfined compressive strength test, hydraulic conductivity test, setting time test, and chemical leaching test (Du et al., 2010). A researched treated and recycled clay waste by S/S, found that addition of NaOH and Na2SiO3 solutions as well as low-calcium class F fly ash showed an enhanced efficiency (Dassekpo et al., 2018). The experimental and analysis results demonstrated the occurrence of geopolymerization reaction among clay waste, fly ash, and added chemicals, reaching the optimum compressive strength with fly ash ratio of 30%.

In summary, S/S remediation by means of abundant binders can effectively treated a wide range of hazardous waste while obtaining value-added materials. An OPC-based S/S method incorporated with other potential additives such as incinerated fly ash, lime, and GGBS were frequently reported in recent research and presented satisfactory performances as well as shortening the remediation cost. However, cement-based S/S is reported to account for massive CO2 emissions since OPC production results in a high carbon footprint, ranging from 0.66 to 0.82 t CO2 per ton of OPC, and it also suffers from the problems of decalcification, degradation, and potential leaching (Wang et al., 2019a). Although cement presents a high immobilization efficiency for metallic elements, it is still insufficient for cement to simultaneously immobilize both metallic and metalloid elements. Therefore searching for low-carbon and cost-efficient cementitious alternatives is of great significance in the scientific community. Besides, recycling renewable materials from agricultural waste (e.g., rice hush ash) and industrial waste (e.g., incinerated sewage sludge ash) would also be beneficial for developing soil/sediment technologies, and promoting sustainable and green remediation (Tajudin et al., 2016).

1.4.2 Stabilization/solidification for industrial waste

Industrial wastes, such as metallurgical and wastewater treatment sludge, fly and bottom ashes from coal/lignite-fired power plant, steel industry byproducts, and pharmaceutical wastes, have high chances of containing toxic elements. The industrial waste can also be immobilized into ceramics and glass-ceramic materials via S/S technologies. Inertization of PTE of the industrial solid waste into sustainable and green products can be an effective and beneficial option for industrial waste treatment (Karayannis et al., 2017). Industrial byproducts, such as fly and bottom ashes from the lignite power station and fluidized bed combustion ash, can be recycled and utilized as raw materials for ceramics fabrication due to their abundant oxygen content. It was reported that clay showed an affinity toward solid wastes with oxides, favoring the manufacture of ceramics/glass–ceramics with relatively low lixiviation levels and reduction of environmental impacts. Nevertheless, incorporation of water-soluble salts into ceramic materials containing additives, such as ammonium lignosulfonate, barium carbonate, and calcarenite, is likely to lead to efflorescence phenomena, which might affect the quality of the products (Karayannis et al., 2017).

MSWI fly ash, is massively produced as a byproduct from waste incineration, which consists of a considerable amount of metals/metalloids. As a group of hazardous waste, MSWI fly ash contains large quantities of toxic elements, such as Hg, Pb, Cd, Cr, Cu, Zn, dioxin, etc. Treatment of MSWI fly ash for air pollution control includes the following three dominant solutions, that is, separation (e.g., advanced separation and chemical extraction), S/S, and thermal processes. Particularly, S/S processes (e.g., chemical stabilization, accelerated carbonation, and cement solidification) can transform hazardous components into less toxic forms (Phua et al., 2019). Among the currently available treatment technologies for treating incinerator fly ash (IFA), S/S technology is regarded as an effective and efficient way for treating hazardous materials. Pretreatment such as water washing was sometimes conducted on MSWI fly ash, aiming to remove chlorides. For example, a recent study showed that MSWI fly ash with washing pretreatment and addition of 4% ferrous sulfate was applied as an additive for improving the strength of cement-stabilized soil, achieving 67% and 89% reduction of chromium and lead leaching, respectively (Liang et al., 2020). The leaching toxic elements in cement-stabilized soil were effectively inhibited, resulting from the physical encapsulation of hydration products as well as the stabilization of chemical agents. Cement solidification technology is capable of preventing environmental pollution resulting from MSWI fly ash, which is beneficial for the volume/weight reduction, acceleration of process, and recycling of heat energy (Bie et al., 2016). It was reported that the cement quantity, the leachate pH, and the vibrating time for leaching are the dominant factors that influence heavy metal leaching (Bie et al., 2016). MSWI fly ash can be stabilized through the reactions with the metastable vitrified amorphous slag (VAS). By using the metastable state of VAS for S/S of IFA, the phases of heavy metals, as well as Cl arose by reacting with silicate or aluminosilicate matrices with VAS and/or the incorporation into the newly formed crystals, and the potential ecological risk was sharply decreased (Zhang et al., 2016). Type C and Type F fly ash are commonly used for S/S. Type C fly ash is more expensive with remarkable cementitious properties and can be used independently for S/S, while the latter one is a pozzolanic material and should be mixed with other material containing lime to obtain good mechanical properties (Shi and Spence, 2004).

The S/S processes can be achieved by physical and chemical approaches including encapsulation, fixation, and adsorption with waste materials, for example, OPC. However, production of OPC suffers from high carbon emission as well as the low durability resulting from compatibility between cement and soil/clay. Therefore the exploration of low-carbon alternatives is attracting more and more attention among scientists. SCMs derived from industrial byproducts can improve the physicochemical properties of cement-based products by additionally forming C-S-H gel. SCMs (i.e., silica fume) and green stabilizer [i.e., potassium dihydrogen phosphate (KDP) and wood waste-derived biochar] were studied for S/S of IFA in a low-carbon and high-efficient way (Chen et al., 2019). It was found that the incorporation of SCMs (20wt.% of binder) favored the formation of cement hydrates, thus decreasing Pb leachability by 36.3%. KDP and biochar served as stabilizers, and contributed to immobilizing toxic elements as well. Recently, greener cement binders consisting of sustainable and renewable materials have been explored in S/S. Magnesia (MgO)-based cements, self-healing cementitious material, could meet these demands and be resistant to acidic erosion. By means of incorporation of microcapsules, such materials could self-repair the cracks, and hence present excellent durability and resilience in the cement–soil systems (Shen et al., 2019). Cement binders involving waste-derived additives corresponding to the circular economy concept is increasingly popular. Slags are often used for the formation of insoluble (C−S−H) gels in the processes of cement hydration, inhibiting the increase of pH as well as improving metal immobilization and strength.

Cr-bearing electroplating sludge is largely generated from electroplating industries. The complex and low-crystallinity mixture with high contents of highly water-soluble and mobile hexavalent chromium (Cr(VI)) would pose a severe threat to the environment and ecological systems, for example, by causing respiratory cancers. S/S technologies could transform the immobilized contaminants into a less soluble form followed by encapsulation in the durable matrix. Recently, a study investigated the treatment of Cr-bearing electroplating sludge via S/S technologies involving physical encapsulation, chemical bonding, as well as absorption by an alkali-activated slag binder, which was produced from blast furnace slag (BFS) and alkali (Chen et al., 2020). The hydrated products of the binders with low-crystallinity contained dense calcium silicate hydrate gels, which could effectively immobilize Cr(VI) in the structure. Particularly, the leachable Cr(VI) and total Cr were both found to decrease with higher liquid–solid ratio, water glass dosage, water glass modulus ratio, and curing time. BFS is an economic option, and it was reported to cost around 90% of OPC in addition to its other superior properties for reduction as well as sulfide precipitation (Shi and Spence, 2004). Ornamental rock solid waste could serve as a useful material for S/S of galvanic solid waste, as its fine particles contain hydroxyl groups with sorptive capability as well as electronegative density (Barreto et al., 2020). It was found that toxic substances including Zn, Cr, Mn, Fe, and petroleum hydrocarbons from galvanic solid waste could be effectively immobilized more than 95% with 0.06 kg OPC per kilogram of waste. Therefore S/S technology can assure good treatment and management on a wide range of hazardous industrial wastes, and meanwhile reduce the consumption of resources and energy.

Plasma vitrification is a technology with various advantages for stabilizing MSWI fly ash, which is a one-step process to simultaneously achieve the destruction of organic composition, heavy metal fixation, as well as volume reduction. The processes with high energy density as well as high temperature could shorten the reaction time and produce slag with improved physicochemical properties for construction material. A LCA and sensitivity analysis proved the effectiveness of thermal plasma vitrification as a green approach for IFA sustainable treatment by comparing with some other scenarios (Pei et al., 2020). This technology can greatly reduce the environmental impact such as toxicity and bioavailability threatening the ecological systems as well as human health.

S/S is an advanced technology that retains toxic contaminants from hazardous waste in the microstructure and safely uses them as building materials, which corresponds with the principles of green chemistry and sustainable development. Besides, waste conversion into favorable raw materials for other industrial sectors is a promising opportunity to achieve symbiosis, ample coordination, and a circular economy, which is actively advocated by some environmental policies (Karayannis et al., 2017). Nevertheless, some scientific questions are expected for further exploration. For example, the mechanisms of heavy metal immobilization might vary constantly with external environmental conditions and structures of matrix materials, while the heavy metal hydrolyzes could cause pH change and cement hydration. Besides, future prospect also requires more studies on long-term security as well as stability of the materials after S/S (Fan et al., 2018). The effect of variance of the environment such as acid rainfall on shear strength of S/S treated soil would be a concern, and the reduction of pH value would tend to release more heavy metals. Precautionary measures are necessary to guarantee the long-term stability of S/S-treated soil (Du et al., 2010). Additionally, it is expected to further explore the effect of characteristics and content of the toxic elements on the S/S-treated soils. The long-term performances of S/S could be investigated by in-the-field monitoring, to provide refined and timely monitoring data by technological innovations (Shen et al., 2019). Therefore interdisciplinary breakthroughs in big data and sensor technology are believed to show great prospects in the future. Advanced microscopic, spectroscopic, and mineralogical analyses might be beneficial for the demonstration of S/S alteration pathways and determining factors.

1.4.3 Stabilization/solidification for industrial waste radioactive waste

Treatment of radioactive waste is one of the crucial problems to be solved around the world. To solve this problem, various technologies and knowledge have been developed in recent decades. Cement-based treatment was the most traditional approach for S/S of radioactive waste and Portland cement was commonly used due to its simple operation procedure and low cost. In recent years, SCM, alternative calcium-based cements, magnesium-based cements, alkali-activated materials, and calcium sulfoaluminate (CSA)-based cement were also suggested to tackle radioactive waste due to their advantages over the Portland cement. For example, the chemistry of CSA-based cements allows for advantages in comparison to traditional formulations, including better chemical compatibility, lower porosity, lower alkalinity, lower viscosity, lower bleed, and higher early strength. Meanwhile, the expansive CSA-based formulations can also help in sealing underground disposal facilities. The lower alkalinity of the Portland alternative cement would promote the precipitation of many radionuclides as hydroxides as well as a lower corrosion potential of encapsulated metals such as aluminum. Novel cement with the capacity of self-desiccation would cause lower internal humidity and thus inhibit further corrosion, which would ensure the long-term durability of S/S matrix. However, novel cement formulations may still need to be tailored for specific wastes to obtain good S/S efficacy.

Glass-based waste forms have been widely applied for S/S of radioactive waste. The process where radioactive wastes are immobilized into glass waste forms is often regarded as vitrification. By the vitrification process the hazardous waste constituents are immobilized into glass forms that hinder the release of radioactive elements to the environment. Several vitrification techniques have been developed and some mature techniques, such as the Joule-heated ceramic melter, hot crucible induction-heated melter, and cold crucible induction melter have been commercially applied. Borosilicate glasses with good processability, chemical durability, thermal ability, and radiation tolerance are compositionally flexible to accommodate a wide range of waste constituents, which make them be the chief option for S/S of high-level radioactive waste vitrification. However, some challenging elements in the radioactive wastes are not compatible with borosilicate glasses, thus promoting the need for the development of novel glasses (i.e., phosphate glasses). Crystalline ceramic waste forms for S/S of radioactive wastes have been developed for a long time, where a multimineral phase titanate ceramic material namely Synthetic rock or Synroc becomes impregnated to immobilize high-level radioactive waste (Gregg et al., 2020). Accordingly, ceramic waste forms are expected to play an important role, as they enable a high incorporation (or waste loading) of radioactive elements within the ceramic structure whilst demonstrating excellent durability.

The geochemistry and geochemical materials have been well designed and applied for the treatment of radioactive wastes (Koťátková et al., 2017; Peterson et al., 2018). In terms of radioactive treatment techniques, the selection and evaluation of potential geochemistry and geochemical immobilization materials (cementitious materials, ceramics, glass, bitumen, etc.) for the treatment of radioactive waste after S/S are significant for meeting the target disposal life cycle. Meanwhile, the classification of radioactive wastes would facilitate understanding and simplify the management of the multiple elements of a diverse system (Hosan, 2017).

For the efficient and specific treatment of radioactive waste, the investigation of the interactions between the radioactive elements and immobilization matrices are necessary. Spectroscopic techniques, developing geostatistical models and computer simulations of radionuclide transportation, as well as applying density functional theory and molecular dynamics would be the future directions for potentially strengthening the accurate prediction of radionuclides migration

1.5 Conclusion and prospects

Waste is another form of resources, and the urgent need for developing sustainable hazardous waste treatment and management is growing in view of the fast urbanization and energy crisis. The UN SDG outline a framework for scientists and stakeholders to strive for state-of-the-art technologies as well as policies. The development on smart technology that employs mathematic modeling provides a top to bottom approach to design and optimize the waste management system, accompanied with LCA tools. In recent decades, technologies on waste valorization and WTE have made great progress for the recycling of organic waste. Current waste treatment, including landfilling, WTE technique, hydrothermal treatment, washing and extraction, phytoremediation, bioremediation, sintering and melting, cement rotary kilns coprocessing, and advanced oxidation and photocatalysis suffer from the disadvantages like high cost, long-term leachability of the toxic elements, secondary pollutions, and limited efficiency. On the other hand, S/S is an advanced technology that retains toxic pollutants into their microstructure for the safe use of hazardous waste management; it has shown great efficacy in successfully treating soil/sediment, industrial waste, and radioactive waste. Further investigations on high compatibility, long-term stability, and interdisciplinary breakthroughs are expected to improve the technology.

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