Solidification and multi-cytotoxicity evaluation of thermally treated MSWI fly ash

Highlights

• MSWI fly ash with lower leaching toxicity is not equivalent to biosecurity.

• Multicellular assays are a novel approach to determine the toxicity of solid wastes.

• Normal cells possess higher cellular tolerance to the leachate than cancer cells.

Abstract

Municipal solid waste incineration (MSWI) fly ash produced in waste-to-energy plants possesses a serious threat to human health. Although the traditional methods including toxicity characteristic leaching procedure and sequential extraction approach can partially evaluate the reduction of heavy metals leaching from thermally treated MSWI fly ash, the potential threat towards organisms is frequently ignored in previous literature. Considering this, herein we systematically assess the cytotoxicity of heat-treated samples using multiple cells from different biological tissues/organs for the first time. The results indicate that the leachability and transferability of heavy metals are declined after treatment. The biological assays demonstrate that the leachates from the treated residues induce lower phytotoxicity and cytotoxicity compared with the original samples. Moreover, according to the cellular responses of multiple cells to the leachates, normal cells (MC3T3-E1, HUVEC, and L929) are more tolerant to the leachates than cancerous cells (4T1, MG63), and the skin fibroblasts (L929), which often interact with the external circumstance, have the best cellular tolerance. This work provides a novel platform to determine the potential biosecurity of MSWI fly ash-derived products towards organisms, when they are served as secondary building materials in the constructional industry that may be contact with animals and human beings.

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).