Solidification and multi-cytotoxicity evaluation of thermally treated MSWI fly ash
Graphical abstract
Introduction
The growing generation of municipal solid waste (MSW) has become a serious environmental issue around the world (Zhang et al., 2019). Generally, the management of MSW includes a series of technologies for waste reduction, recycling, decontamination, and reuse (Guerrero et al., 2013; Wang et al., 2018). In recent years, the incineration technology has been widely adopted in the waste treatment industry, resulting in the noticeable reduction in the waste weight (∼70 %), volume (∼90 %) and organic pollutants (∼100 %). Also, it achieves safer disinfection in comparison to other traditional methods and enables the recovery from waste heat to available energy (Huber et al., 2018b; Shan et al., 2012; Wang et al., 2018). However, the incineration plants will produce concomitant solid residues, which are often divided into two types: bottom ash (85–95 % by volume) and fly ash (5–15 % by volume). MSWI fly ash is on the list of hazardous wastes in most countries because of its high content of toxic heavy metals, which have potential perniciousness to plants, animals, and human beings ultimately (Asthana et al., 2010; Kang et al., 2020; Liu et al., 2018). Therefore, to avoid the environmental risks caused by MSWI fly ash, it is imperative to solidify and stabilize the leachable heavy metals in MSWI fly ash through appropriate treatments prior to recycling and final disposal.
The management approaches of MSWI fly ash include separation, thermal treatment, and stabilization/solidification (Mu et al., 2018). Although the novel innovated technique is zero fly ash incineration system, it has not been popularized in industries due to the introduction of additional materials and processes for pretreatment, or reliance on other technologies. Now thermal treatment is deemed as an efficient method owing to its high cost-effectiveness and operational feasibility. More importantly, its inhibitory effect on the leachability of heavy metals is noticeable (Huber et al., 2018a; Nowak et al., 2012). During high-temperature heating, the processes of mineralogical phase transformation, melting, vitrification, and recondensation occur in the matrix particles (Lindberg et al., 2015; Sobiecka and Szymanski, 2014). These chemical and physical changes strive to agglomerate and densify the matrix granulars, thereby forming a continuous vitreous phase on the surface to immobilize the heavy metals.
The existing ecotoxicity assessments of fly ash are mainly based on the toxicity characteristic leaching procedure (TCLP), which is a representative technique recommended by the U.S. Environmental Protection Agency to detect the leachability and mobility of heavy metals in the solid waste (Halim et al., 2003). In China, the TCLP test is also an evaluation template approved by hazardous waste identification for extraction toxicity (China, 2007a). Nevertheless, the static TCLP is unable to completely predict the long-term leaching risk and biosecurity of fly ash in various and complicated leaching environments. On the other hand, the harmful components contained in fly ash will eventually generate biological magnification in wildlife and human along the food chains as well as the water circulation system (Alimba et al., 2016; Garg and Mishra, 2010). Previous literature has revealed that, after the solidification treatment, the safety of the products met the regulations according to the local or other international relevant test techniques, whereas they still behaved non-negligible biological toxicity (Phoungthong et al., 2016; Wang et al., 2015). Accordingly, it is more systematic and comprehensive to appraise the environmental safety of the fly ash by combining TCLP with other evaluation methodologies, for instance, the chemical sequential extraction procedure (Weibel et al., 2018), the seed germination experiment (Ribé et al., 2014), the plant growth test (Pandey et al., 2009), and the bacterial inhibition test (Gong et al., 2017b). Although these toxicity assessment methods can detect the ecotoxicity and biotoxicity of MSWI fly ash or its stabilized products to some extent, research on whether the fly ash will have potential biotoxic risks towards animals and humans is still insufficient.
Multi-cytotoxicity evaluation is a novel method to assess the biological toxicity of solid waste. TCLP extracts were co-incubated with Vero, BHK-21, and PK-15 cells to indicate the toxic effects of incineration residues on the digestive system of animals (Huang et al., 2008). Besides, simulated landfill leachates were reported to induce genotoxicity and DNA damage in three human cell lines (Alimba et al., 2016). These reports support the feasibility of multi-cytotoxicity assessment, which not only increases the knowledge on the specific cytotoxicity and tolerance of various cell lines from different biological tissues and organs but also provides a novel toxicity evaluation platform for the solid waste.
Herein, in the present work, we utilized the thermal treatment to detoxify MSWI fly ash and investigated the effects of heating temperature and residence time on the leaching behaviors of five heavy metals by the TCLP test and the chemical sequential extraction procedure. The inhibitory mechanisms of thermal processing on the leaching toxicity of heavy metals were discussed. The phytotoxicity of leachates from the original and treated fly ash on two leafy vegetables (var. ramosa Hort. and Brassica chinensis L.) was also investigated. Additionally, in vitro assays were carried out to determine the cytotoxicity and cellular tolerance of leachate samples on multiple cells including mouse embryo osteoblast precursor cells (MC3T3-E1), human osteosarcoma cells (MG63), murine mammary carcinoma cells (4T1), mouse fibroblast cells (L929), and human umbilical vein endothelial cells (HUVECs).
Section snippets
Sampling of MSWI fly ash
The MSWI fly ash used in the study was sampled from a local waste-to-energy plant, which had a MSW disposal capacity of 1800 tons/day. The incineration furnace was equipped with an air pollution control (APC) system consisted of the semidry lime scrubber and the fabric filter. Thus, the samples had been treated by the desulfurization and dechlorination processes of the lime slurry injection before sampling. Afterwards, the raw fly ash (RFA) was desiccated at 105 ℃ for 24 h in a drying cabinet,
TCLP leaching concentration of heavy metals
The leaching concentrations of Cd, Cr, Cu, Pb, and Zn in RFA and treated samples are presented in Fig. 1. Among the five heavy metals in RFA, apparently, Pb (8.489 mg/L) had the highest leachability under the simulated acidic leaching conditions, followed by Zn (0.9561 mg/L), Cr (0.1101 mg/L), Cu (0.0524 mg/L), and Cd (0.0115 mg/L). Moreover, only the concentration of leachable Pb exceeded the limitation (5 mg/L) specified in the identification standards for hazardous wastes (China, 2007a). As
Conclusions
Overall, we found that the morphology and mineral phase of MSWI fly ash changed obviously during thermal treatment, resulting in the significant inhibition of leachability and mobility of heavy metals. And yet for all that, the plant germination tests showed that the leachate derived from treated fly ash still exhibited non-negligible phytotoxicity with increasing the cultivation time. Furthermore, the multi-cytotoxicity assays demonstrated that the cytotoxic effects of leachate from treated
Authors distribution
Conceived and designed the experiments: Yong Chen, Yi Deng, Weizhong Yang. Performed the experiments: Yong Chen, Li Xu. Analyzed the data: Yong Chen, Yi Deng, Weizhong Yang. Contributed reagents/materials/analysis tools: Yi Deng, Weizhong Yang. Wrote the paper: Yong Chen, Yi Deng, Weizhong Yang.
Declaration of Competing Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This work is jointly funded by the National Natural Science Foundation of China (81801848, 81961160736), Sichuan Science and Technology Program (2018HH0140, 2017FZ0046, 2018JZ0026, 2019YJ0554), Chengdu International Science and Technology Cooperation Foundation (2017-GH02-00025-HZ), State Key Laboratory of Polymer Materials Engineering (Grant No.: sklpme2019-2-05), Sichuan University Postdoctoral Interdisciplinary Innovation Foundation, as well as Hong Kong Scholar Program. The authors would
References (51)
- et al.
Chemical characterization of simulated landfill soil leachates from Nigeria and India and their cytotoxicity and DNA damage inductions on three human cell lines
Chemosphere
(2016) - et al.
Genotoxicity monitoring of industrial wastes using plant bioassays and management through vermitechnology: a review
Agric. Nat. Resour.
(2017) - et al.
Sustained proliferation in cancer: mechanisms and novel therapeutic targets
Semin. Cancer Biol.
(2015) - et al.
Solid waste management challenges for cities in developing countries
Waste Manage.
(2013) - et al.
Evaluating the applicability of a modified toxicity characteristic leaching procedure (TCLP) for the classification of cementitious wastes containing lead and cadmium
J. Hazard. Mater.
(2003) - et al.
Cytotoxicity of municipal solid waste incinerator ash wastes toward mammalian kidney cell lines
Chemosphere
(2008) - et al.
Combined disc pelletisation and thermal treatment of MSWI fly ash
Waste Manage.
(2018) - et al.
Comparative life cycle assessment of MSWI fly ash treatment and disposal
Waste Manage.
(2018) - et al.
Heavy-metal reduction and solidification in municipal solid waste incineration (MSWI) fly ash using water, NaOH, KOH, and NH4OH in combination with CO2 uptake procedure
Chem. Eng. J.
(2020) - et al.
Thermal treatment of solid residues from WtE units: a review
Waste Manage.
(2015)
Intermediate-calcium based cementitious materials prepared by MSWI fly ash and other solid wastes: hydration characteristics and heavy metals solidification behavior
J. Hazard. Mater.
Detoxification of solid waste containing Cr(VI) with phosphate by thermal treatment
Chem. Eng. J.
Influence of ignition of waste fishbone on enhancing heavy metal stabilization in municipal solid waste incineration (MSWI) fly ash
J. Clean. Prod.
Cytotoxic and genotoxic potential of Cr(VI), Cr(III)-nitrate and Cr(III)-EDTA complex in human hepatoma (HepG2) cells
Chemosphere
Heavy metal removal from MSW fly ash by means of chlorination and thermal treatment: influence of the chloride type
Chem. Eng. J.
Heavy metal removal from municipal solid waste fly ash by chlorination and thermal treatment
J. Hazard. Mater.
Application of fly ash on the growth performance and translocation of toxic heavy metals within Cajanus cajan L.: implication for safe utilization of fly ash for agricultural production
J. Hazard. Mater.
Variation of the phytotoxicity of municipal solid waste incinerator bottom ash on wheat (Triticum aestivum L.) seed germination with leaching conditions
Chemosphere
The influence of pH on the leaching behaviour of inorganic components from municipal solid waste APC residues
Waste Manage.
Assessment of mobility and bioavailability of contaminants in MSW incineration ash with aquatic and terrestrial bioassays
Waste Manage.
Carcinogenic and non-carcinogenic evaluations of heavy metals in protein foods from southwestern Nigeria
J. Food Anal.
Can washing-pretreatment eliminate the health risk of municipal solid waste incineration fly ash reuse?
Ecotox. Environ. Saf.
Waste-to-energy, municipal solid waste treatment, and best available technology: Comprehensive evaluation by an interval-valued fuzzy multi-criteria decision making method
J. Clean. Prod.
Extraction of heavy metals from MSWI fly ash using hydrochloric acid and sodium chloride solution
Waste Manage.
Thermal treatment of the fly ash from municipal solid waste incinerator with rotary kiln
J. Hazard. Mater.
Cited by (30)
Carbothermal treatment of municipal solid waste incineration fly ash: Purification and valuable elements extraction
2024, Separation and Purification TechnologyPreparation and mix proportion optimization of red mud-fly ash-based cementitious material synergistic activated by carbide slag and MSWIFA
2024, Construction and Building MaterialsA comprehensive review of toxicity of coal fly ash and its leachate in the ecosystem
2024, Ecotoxicology and Environmental SafetySynthesis of cordierite using municipal solid waste incineration fly ash as one additive for enhanced catalytic oxidation of volatile organic compounds
2024, Science of the Total Environment