Research articleImmobilization of trace heavy metals in the electrokinetics-processed municipal solid waste incineration fly ashes and its characterizations and mechanisms
Graphical abstract
Introduction
The increasing combustion of the municipal solid waste (MSW) has caused the mass production of MSWI fly ash which contains some toxically organic pollutants (e.g., dioxins) and heavy metal elements (HM) and is commonly considered to be difficult to handle (Li et al., 2016). Various techniques have been studied to remove or solidify the contaminants in the particles of fly ashes, including the metal isolation, toxicity reduction, solidification/stabilization (S/S), and phytoremediation etc. (Quina et al., 2018; Silva et al., 2017; Viader et al., 2017; Wang et al., 2015). However, the issues including the high cost (i.e., in regard to the consumption of the chemical reagents, pollutant leaching), landfilling expansion, and low processing efficiencies have been observed with the utilization of the above remediation methods for the MSWI fly ashes (Bontempi, 2017; De Boom and Degrez, 2015; Kozakova et al., 2013; Weidemann et al., 2016). To overcome these problems, the technology of the electrokinetic remediation (EKR) being broadly applied in situ or ex-situ, especially for the solid wastes with the characteristic of the low permeability, is conducted and researched in the detoxification of the MSWI fly ashes (Huang et al., 2015, 2017; Lopez-Vizcaino et al., 2017). In the electrokinetics process, a pair of electrodes are settled at the ends of the stacking sample to impose a suitable range of voltages over the contaminated solids (Ding et al., 2017; Fu et al., 2017). With a low direct electrical (DC) current traversing the stacking area, the HM ions in the solid matrix were released and subsequently migrated to a specifically narrower region through the interactions of the electrolysis (i.e., electrochemical decomposition of the water), electromigration, electro-osmosis, and electrophoresis happening in the sampling areas (Chowdhury et al., 2017; Guto, 2017). Correspondingly, the volume of waste needing impermeable dump is obviously declined. However, a moiety with contaminated abundance is left required to be handled with more attention (Huang et al., 2015; Li et al., 2016).
The remnant moiety of the electrokinetics-processed MSWI fly ashes is a hazardous material, even more toxic than the raw MSWI fly ash, required to be cautiously treated before the landfilling disposal (Mu et al., 2018; Yang et al., 2018). The technique of solidification/stabilization (S/S) technology can efficiently immobilize the HM ions in the cementitious matrices through the coupled functions of physical encapsulation and chemical fixation and reduce the leaching toxicities of the pollutants (Billen et al., 2015; Yakubu et al., 2018). MSWI fly ash is assigned to the system of CaO-SiO2-Al2O3-SO3-Cl based on the mineral compositions, sufficient in the phases of CaO, SO3, and Cl, and inadequate in SiO2 and Al2O3 (Berber et al., 2017; Min et al., 2017; Silva et al., 2017; Yang et al., 2018). The solid wastes with richer SiO2 and Al2O3 commonly are chosen to complement the source materials to enhance the formation of the calcium silicate hydrate (i.e., C-S-H), geopolymer of three-dimensional aluminosilicate structure (i.e., C-A-S-H), ettringite (i.e., AFt), and monosulphate (i.e., AFm) through the hydration reactions and the polycondensation of silica and alumina precursors (Gong et al., 2017; Zhou et al., 2017). Overall, the leachable HM elements can be significantly reduced via immobilization with cement- and geopolymer-based materials. However, the obvious amplification of the volume for the solidified body compared with the equal mass of MSWI fly ashes in quality due to the addition of source materials and binders during the curing process has severely squeezed the availability of the landfilling capacity in the cities and elevated the conducting costs of the transportation (Karagiannidis et al., 2013; Tang et al., 2016; Wang et al., 2015; Wei et al., 2011).
As described above, to effectively dispose the remnant moiety of the electrokinetic-processed MSWI fly ashes (denoted R-MSWI fly ashes) using S/S technology as well as avoid the apparent expansion of the solidified contaminated wastes, the electrochemically treated samples in a specific region adjacent to the cathode compartment was excavated and prepared for the research of the alkali-activated S/S without the mixture of the other source materials. The physiochemical characteristics of the MSWI fly ashes were different before and after the EKR experiments, mainly pertaining to the changes of the pore size distribution, elemental compositions, and mineral constituents (Kirkelund et al., 2015; Li et al., 2016; Peppicelli et al., 2018). Thus, the optimum combination of the curing parameters based on three constant factors for the immobilization of the detected HM elements including the mass ratios of alkali activators to MSWI fly ashes (denoted A-M mass ratio) and the curing temperatures was preferentially determined using the raw MSWI fly ashes as a control object before the curing exploration of the R-MSWI fly ashes. The two most common indicators, the compressive strengths and toxicity leaching were measured to determine the effectiveness of the immobilization of HMs in the samples and the potential applications of the cementitious matrices. X-ray diffraction, X-ray fluorescence, thermos-gravimeter, scanning electron microscopy, FTIR spectroscopy were comprehensively used to further elucidate the forming mechanisms of the solidified bodies based on the two different precursors (i.e., MSWI and R-MWSI fly ashes) from the integrated point of view. The study not only effectively solves the contamination of the MSWI fly ashes by a closed-loop technique, also demonstrates the high potentiality of the method for treating other hazardous solid wastes.
Section snippets
Experimental materials and preparation
The samples of MSWI and R-MSWI fly ashes were used in this study as the main source materials to prepare the solidified specimens. The MSWI fly ash samples were captured from a bag-filter in Tongxing Waste Power Generation Plant, Chongqing, China. The sampling particles were dried in a thermostatic heater at 60 °C for 4 h (h) and then sifted through a 200-mesh sieve. The remnant specimen was collected in the region of the sample chamber near the cathode compartment after the EKR experiments and
Influence of the proposing time
The time setting commonly affects the dissolution and migration of the inorganic pollutants in the sampling chamber during the EK process. The high removals of HMs should be guaranteed to meet the strict disposal requirement in the process of EKR tests. While ensuring an appropriate testing duration was essentially important for the whole experiments. Therefore, the proposing time in the electrokinetics should be reasonably set before the curing process. As shown in Fig. 1, the mean leaching
Conclusions
This study has comprehensively explored the chemical characterizations of surfaces and colloids of the electrokinetics-treated MWSI fly ashes and corresponding solidified materials for the first time. A closed-loop concept is proposed to ultimately solve the contamination of the trace heavy metals in MSWI fly ashes. The variables including the A-F ratios and curing temperatures both have a significant effect on the S/S performance of the MSWI-based matrices. The A-F ratio of 30% g·g−1 and
Declaration of interests
The authors have no competing interests to declare.
Acknowledgements
The research is not funded by any organization.
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