Removal of heavy metals in medical waste incineration fly ash by Na₂EDTA combined with zero-valent iron and recycle of Na₂EDTA: Acolumnar experiment study

ABSTRACT

In this study, an effective circulating system was developed to remove heavy metals in medical waste incineration (MWI) fly ash. MWI fly ash (MWIFA)-column experiments were performed to remove Cu, Pb, Zn, Cd, and Ni from MWIFA using EDTA disodium (Na₂EDTA). Iron-column experiments were conducted to study the removal effect of zero-valent iron on the five heavy metals from washing wastewater. Toxicity Characteristic Leaching Procedure (TCLP) test method was employed to evaluate heavy metals toxicity of MWIFA residues generated after 0–0.2 mol/L Na₂EDTA solution treated. After being washed by 0.2 mol/L Na₂EDTA solution, TCLP leaching values of Cu, Pb, Zn, Cd, and Ni were the lowest and satisfied the standard (GB 5085.3–2007), and the leaching values were 58.4 ± 2.0 mg/L, 2.81 ± 0.14 mg/L, 64.3 ± 4.0 mg/L, 0.156 ± 0.005 mg/L, 0.381 ± 0.006 mg/L. Concentrations of Cu, Pb, Zn, Cd, and Ni in iron-column effluent were reduced by 99.7%, 91.6%, 91.6%, 75.4%, and 75.7%, respectively. Na₂EDTA was recovered and recycled to the removal of heavy metals from MWIFA. Comparing new Na₂EDTA solution with recycled Na₂EDTA solution, recycled Na₂EDTA and water could be reutilized to dispose MWIFA. The removal efficiencies of Cu, Pb, Zn, Cd, and Ni by recycled 0.2 mol/L Na₂EDTA solution were 67.1%, 68.8%, 63.2%, 73.9%, 50.7%, respectively, the removal efficiencies using recovered Na₂EDTA decreased by 2.6%, 3.9%, 3.3%, 4.2%, and 1.6%, respectively. Successive batch experiments were also conducted to evaluate industrialization potential and reusable times for recycled Na₂EDTA. After four recirculation cycles, extraction efficiencies of Pb and Cd (removal efficiency at different cycles divided by removal efficiency of new Na₂EDTA) declined toward 80%. Results from this research indicated that this circulating system possessed industrialization potential.

Implications: An effective circulating system was developed to remove heavy metals in MWI fly ash (MWIFA). Integration of Na₂EDTA with Fe⁰ promoted the removal of heavy metals from MWIFA. Na₂EDTA, NaCl and water were stepwise extracted from iron-column effluent, respectively. Recovered Na₂EDTA can still effectively remove heavy metals from MWIFA. Results from this research indicated that this circulating system possessed industrialization potential.

Introduction

Incineration of dangerous medical waste has significant advantage for both volume and mass reduction of waste as well as decomposition of organic compounds (Kim et al. 2019). It can thermally eliminate all pathogens (in the temperature rang of 1,100–1,200°C) (Papamarkou et al. 2018; Shaaban 2007), and at the same time, it may decrease the waste weight more than 70 wt% (90%, in terms of their original volume) (Alvim-Ferraz and Afonso 2005; Kougemitrou et al. 2011; Papamarkou et al. 2018; Sabiha et al. 2008; Tsakalou et al. 2018; Zhao et al. 2009) It is suitable for all kinds of medical wastes except for specific types such as radioactive or batteries (Tsakalou et al. 2018). One of the main drawbacks of the incineration process is that the significant amounts of Medical Waste Incineration (MWI) Bottom Ash (BA) and Fly Ash (FA), usually enriched with toxic substances, are generated (Tsakalou et al. 2018). Because of containing highly toxic organic pollutants (dioxins, furans, and polycyclic aromatic hydrocarbons), leachable alkali chlorides, and a lot of soluble heavy metals such as Cu, Pb, Zn, As, Cd, and so on, Medical Waste Incineration Fly Ash (MWIFA) is classified as hazardous wastes (Liu et al. 2018; Xie and Zhu 2013). Leaching concentrations for many kinds of soluble heavy metals in MWIFA exceed the maximum admissible values for hazardous waste landfill, MWIFA yet cannot fulfill the limited value for deposit in hazardous waste landfill without a prior treatment (Kim et al. 2019; Liu et al. 2018; Xie and Zhu 2013). It is known that metals in MWIFA are not biodegradable, can be easily precipitated into the soil and eventually leached into underground water if not immobilized or removed in time (Wang et al. 2012), The dispersion of heavy metals from ash into the air, soil, and water leads to significant environmental pollution (Cozea et al. 2018; Necsulescu et al. 2008;), which may indirectly cause harm to human beings and the environment. When heavy metals in MWIFA enter environment after long-term leaching, the environment and human health will be seriously threatened. Heavy metals are nonrenewable resources, which should be recycled for sustainable waste management (Deliyanni et al. 2017). Developing new recovery technologies for heavy-metal recycling and residual ash disposal is essential (Liu et al. 2019). In recent years, many scholars in the world have conducted many investigations related to heavy metals in MWIFA, especially focused on removal of heavy metals from FA (Meer and Nazir 2018). Acid leaching pretreatment can facilitate the separation and recovery of heavy metals from MWIFA together with other post-treatment, such as thermal treatment, melting, solvent extraction, sulfidation precipitation, flotation, and immobilization treatment (Liu et al. 2017b; Mangialardi 2004; Yang et al. 2014). Many kinds of lixiviants have been applied in the acid leaching pretreatment process: organic acids, such as tartaric acid, acetic acid, oxalic acid, and citric acid; inorganic acids, such as HNO₃, HCl, H₂SO₄; chelating reagents, such as Diethylenetriaminepentaacetic acid (DTPA), Ethylenediaminetetraacetic acid (EDTA), Ethylenediaminedisuccinic acid (EDDS), and Nitrilotriacetic acid (NTA), citric acid (CA), and alkaline lixiviants, such as sodium hydroxides and ammonium salts (Liu et al. 2018; Sumalatha, Naveen, and Malik 2019). After acid leaching, soluble heavy metals such as Zn and Pb are almost completely transferred into liquid phase from solid phase, which is more easily removed and recovered in a subsequent disposal process (Liu et al. 2019). In previous literatures, comprehensive comparison of extensive lixiviants utilized for removal of heavy metals from three kinds of FA (Municipal solid waste incineration fly ash, MWIFA, and coal FA) are conducted by studying in detail their reaction conditions, metals leached, and percentage extraction achieved (Liu et al. 2018; Meer and Nazir 2018); among those agents, chelating reagents, which can form stable complexes with almost all the heavy metals over a broad pH range (Bilgin and Tulun 2016; Sumalatha, Naveen, and Malik 2019), present better leaching ability (Liu et al. 2018). Because of the toxicity and potential carcinogen of DTA and DTPA (Neilson, Artiola, and Maier 2003), EDTA has become an important chelating agent that can detoxify many heavy metals, presenting the following advantages: a low degree of biodegradability in groundwater and in soil, and a high degree of complex formation with respect to heavy metals (Cheikh et al. 2010). EDTA disodium (Na₂EDTA) is the most utilized kind of EDTA chemicals due to the high solubility. In previous studies, researchers have widely utilized EDTA to extract heavy metals from sludge (Cheikh et al. 2010; Gitipour et al. 2016), cathode ray tube waste (Pant, Singh, and Upreti 2014), contaminated soil (Bilgin and Tulun 2016; Voglar and Lestan 2014), hazardous fine fraction at an old glass dump (Jani and Hogland 2018), and municipal solid waste incineration fly ash (MSWIFA) (Fedje et al. 2010; Hong et al. 2000; Hong, Tokunaga, and Kajiuchi 2000; Janoš et al. 2002). EDTA is effective and useful for extraction of heavy metals from MSWIFA, because EDTA could form stable water-soluble complexes with various metal ions (Hong, Tokunaga, and Kajiuchi 2000); however, few literature studies have been reported about treating heavy metals in MWIFA with EDTA disodium (Na₂EDTA). MSWIFA and MWIFA are all classified as FA; nevertheless, MWIFA contains large amounts of chlorines and carbons, which makes the constituents of MWIFA more complex than MSWIFA, and thus, treatment of MWIFA is also more difficult (Liu et al. 2018). Descripted in a latest research literature, an experimental investigation was conducted to reduce heavy metal concentration from MWIFA using ethylene diaminetetra acetic acid disodium (Na₂EDTA) as a chelating reagent, the heavy metals removal results showed that Na₂EDTA was effective to reduce heavy metals concentration (Ababneh et al. 2020). Based on the above-mentioned research results, we can get the significant information that EDTA is an effective and efficient chelating reagent which can be applied to recover heavy metals from MWIFA. Chelating reagents present better leaching ability, chelated metals are highly stable under wide range of pH conditions, and the removal of chelated metals using most existing technologies is difficult (Strawn and Baker 2009; Zhu et al. 2019); however, they are more harmful to the environment (Liu et al. 2018). Treatment of heavy metal-complex has become a critical issue in environmental protection; the main problems faced in the treatment of chelated heavy metals are how to effectively eliminate toxic heavy metals and how to safely dispose of chelating agents (Zhu et al. 2019).

In a latest review regarding removal of chelated heavy metals from aqueous solution, different methods to remove chelated heavy metals from wastewater, such as electrolysis, membrane separation, replacement-coprecipitation, adsorption, titanium oxide (TiO₂) photocatalysis, and fenton-like oxidation, are discussed in detail for the first time, along with their detailed mechanisms (Zhu et al. 2019).

In recent years, the use of ZVI for the treatment of toxic contaminants in groundwater and wastewater has received wide attention, and encouraging treatment efficiencies have been documented (Fu, Dionysiou, and Liu 2014). As a readily available, inexpensive, nontoxic, abundant, cheap, and moderately strong reducing agent, ZVI has been regarded as a potential candidate in the field of contaminated groundwater remediation and wastewater treatment (Fu, Dionysiou, and Liu 2014; Guan et al. 2015b).

ZVI is a reactive metal with standard redox potential (E⁰ = −0.44 V) and effective reductant when reacting with oxidized contaminants (Fu, Dionysiou, and Liu 2014). Removal mechanism of contaminants by ZVI concerns the directional transfer of electrons from ZVI to the contaminants, which transforms the latter into nontoxic or less toxic species (Fu, Dionysiou, and Liu 2014). Removal efficiencies of EDTA-chelated heavy-metal ions mainly depend on the reactivity of ZVI, easy surface passivation, and particle aggregation, which significantly limit the effectiveness and practical application of NZVI (Lv et al. 2019). In addition, it can have an adverse effect on the ecosystem by releasing NZVI into the environment. In order to solve these shortcomings, this study uses irregular spiral iron scraps. Although the specific surface area of the iron filings is not as large as NZVI, and the reaction activity is not as high as NZVI, it overcomes some of its disadvantages. It is cheap, easy to obtain, and large in quantity, which greatly improve the practicality of iron filings.

In this study, column leaching experiments were conducted to understand the removal efficiencies of heavy metals from MWIFA using Na₂EDTA. After being treated, the toxicity analysis of MWIFA residues was conducted by TCLP test to assess risk of MWIFA residues. Iron-column with ZVI was utilized to recover heavy metals from heavy metal-EDTA complexes in waste Na₂EDTA solution after MWIFA-column experiments. Na₂EDTA in the iron column effluent was recycled to treat MWIFA, and the effectiveness of recovered Na₂EDTA was also assessed.

Materials and methods

Materials

MWIFA used in this experiment was obtained from a medical waste incineration plant in Zhengzhou city, Henan province, China. A random method was applied, sample-collecting period lasted one month, and approximately 5 kg fresh MWI sample was withdrawn from storage location in the incineration plant; the subsequent complete mixture of MWIFA was conducted to ensure the representativeness and homogeneity of sample. Before experiment, all raw materials are dried in the oven at 105°C up to constant weight. MWIFA samples were mixed evenly and sealed in a dry sealed container after being sieved by a 2 mm screen.

Total heavy metals analysis

A homogenized dry sample of 0.100 g was weighed and then put in a Teflon digestion tank, digested via a microwave digestion unit for 2 h at 200°C after adding 10 mL HNO₃, 6 mL HF, and 2 mL HClO₄. After digestion, the sample was then placed on a hot plate at 140°C to heat to nearly 1 mL. After cooled to room temperature, the solution was diluted to a volume of 50 mL with deionized distilled water after being filtered through 0.45 µm cellulose acetate filter membrane. Concentrations of Cu, Pb, Zn, Cd, and Ni in the digests were then determined by inductively coupled plasma atomic emission spectrometry (ICP-MS).

Column experiments

Schematic diagram of continuous column experiments was shown in Figure 1. Column experiments were performed at a plexiglass cylinder. Column was successively packed with quartz sand (2 mm thick), MWIFA (30.00 g), fine quartz sand (above MWIFA, 2 cm below the water outlet) and thick quartz sand (2 cm thick, to the top of the MWIFA column) from water inlet to water outlet. Prepared 500 mL different concentrations of Na₂EDTA (0, 0.02, 0.05, 0.08, 0.1, 0.2 mol/L), and the Na₂EDTA solutions were introduced into MWIFA columns by the peristaltic pump with a controlled flow rate of 2 mL/min, respectively. The pH values of collected effluent were measured by pH meter, and the total solution was sampled at 0, 5, 10, 20, 30, 45, 60, 75, 90, 105, 120, and150 min. Then each obtained sample was measured by ICP-MS after being acidified.

Figure 1. Schematic diagram of column experiments.

According to Rathor, Chopra, and Adhikari (2017), the removal efficiencies of heavy metals from MWIFA were calculated using Equation (1)(1) $R=\frac{C-C_0}{C}\times 100\mathrm{\%}$(1) :

(1) $R=\frac{C-C_0}{C}\times 100\mathrm{\%}$(1)

where R is the removal efficiency, and C and C₀ (mg/kg) are the initial and final concentration of the heavy metal in MWIFA.

The 90 g iron filings were put into a homogenous solution of ethanol and water with a volume ratio of 1:1 after being treated with a weak acid, and then heated to 50°C in a water bath for 2 h and then dried with nitrogen. Collected solution obtained after MWIFA-column using different concentrations of Na₂EDTA as extracting solution was passed through a column containing treated iron filings (iron-column) by using a peristaltic pump, and the effluent was tapped with a beaker. And at 0, 5, 10, 20, 30, 45, 60, 75, 90, 105, 120, 150 min, the experiment was performed according to the procedure of the previous step. Two parallel experiments were performed during the experiment to ensure the effectiveness of the experiment.

Toxicity characteristic leaching procedure (TCLP)

A toxicity characteristic leaching procedure (TCLP) test was conducted in accordance with the US Environmental Protection Agency (USEPA) Method 1311 to evaluate the leaching toxicity of solid wastes (Lu, Hsu, and Lin 2019). The extraction fluid (solution # 1 pH 4.93 ± 0.05 and solution # 2 pH 2.64 ± 0.05) of the sample was determined by a preliminary experimental method. The solid-to-liquid ratio was fixed at 1:20. The extraction was performed using capped polypropylene (PP) bottles loaded on a rotary tumbler at 30 ± 2 rpm for 18 h ± 2 h. After extraction, the mixture was centrifuged by high-speed centrifuge and filtrated through 0.45 µm cellulose acetate filter membrane prior to metal measurement. Leachate was acidified before being analyzed by ICP-MS. All TCLP experiments were carried out in duplicate to ensure the validity of the experiment.

Recovery of Na₂EDTA

NaOH solution (1.0 mol/L) was added dropwise to the solution obtained after the column experiment until the pH exceeded 13.0; static settlement of 5 h was utilized to ensure a complete precipitation (solid substance Ⅰ). The precipitation (solid substance Ⅰ) was separated via centrifugation at 3,000 rpm and dried at 60°C. The pH of the homogeneous mixture of all the collected solution (solution Ⅰ) after solid-liquid separation was adjusted to less than 1 for formation of white precipitate with HCl solution (12.0 mol/L). Static settlement of 5 h was utilized to ensure a complete precipitation (solid substance Ⅱ) in the same way. Solid substance Ⅱ was separated via centrifugation at 3,000 rpm and dried at 60°C. Homogeneous mixture of all the collected solution (solution Ⅱ) after solid-liquid separation would be utilized in the subsequent experiment. Precipitation Ⅱ was dissolved in NaOH solution (1.0 mol/L) to form a new solution (solution III). White solid (noted as solid substance III) was obtained after the evaporation of new solution (solution III) in 80°C water bath. Solution Ⅱ was adjusted to pH = 7 using 1 mol/L HCl then distilled to collect the resulting white solid (noted as solid substance Ⅳ) and water. Three kinds of solid substance (Ⅱ, III, Ⅳ) were detected by X-ray powder diffraction (XRD), and the concentration of heavy metals in the final solution was determined by ICP-MS. All the above experiments were performed duplicate for data analysis to ensure the validity of the experiment.

Assessment of recycled Na₂EDTA

Successive batch experiments were also conducted to evaluate industrialization potential and reusable times recycled Na₂EDTA. Removal efficiencies of heavy metals from MWIFA at different recirculation cycles using recycled Na₂EDTA (new Na₂EDTA was defined as cycle 0) were assessed using Equation (2)(2) $P_i=\frac{R_i}{R_0}\times 100\mathrm{\%}$(2) :

(2) $P_i=\frac{R_i}{R_0}\times 100\mathrm{\%}$(2)

where P_i (P is abbreviation for percentage) is defined as the extraction efficiency at different recirculation cycles using recycled Na₂EDTA (new Na₂EDTA is defined as cycle 0), R is the removal efficiency, and R₀ and R_i(%) are initial removal efficiency using new Na₂EDTA and removal efficiency of the heavy metals from MWIFA at different recirculation cycles using recycled Na₂EDTA (new Na₂EDTA was defined as cycle 0). The i is the serial number of recirculation cycles(0, 1, 2, 3, 4 in this experiment).

Characterizations and measurements

All solutions were filtered through 0.45 µm cellulose acetate filter membrane and diluted with 3% HNO₃ (wt%). Cu, Pb, Zn, Cd, and Ni in solution were quantitatively analyzed by the ICP-MS analysis. Mineralogical properties were conducted by X-ray powder diffraction (UltimaIV Series, Rigaku, Japan) using CuKα radiation and identified from the JCPDS-ICDD databases. The voltage and current with 40 kV and 40 mA are adopted during the measurement, respectively. The measurement was carried out in the 2θ range of 10–50° with the speed of 10° per minute, XRD scans run at 0.02° per step.

Results and discussion

Total concentration of heavy metals

Table 1 showed the concentration of five heavy metals in MWIFA sample and soil background values in China. Concentrations of Cu, Pb, Zn, Cd, and Ni in MWIFA sample were 6067.2 ± 422.3 mg/kg, 2219.7 ± 20.6 mg/kg, 24,252.5 ± 416.4 mg/kg, 60.3 ± 12.1 mg/kg, and 227.1 ± 17.8 mg/kg, respectively. Generally, heavy metals concentrations in MSWFA sample far exceeded the corresponding soil background values in China, which made them a potentially hazardous source of environmental pollution. Moreover, Cu, Pb, Zn, Cd, and Ni had been proved to possess high toxicity to the ecosystem, and MWIFA is classified as hazardous waste (Liu et al. 2018; Xie and Zhu 2013).

Table 1. Heavy metal concentrations in the fly ash sample and soil background value in China (mg/kg dry weight).

Heavy Metals	Cu	Pb	Zn	Cd	Ni
Fly ash sample	6,067.2 ± 422.3	2,219.7 ± 20.6	24,252.5 ± 416.4	60.3 ± 12.1	227.1 ± 17.8
Soil background value	400	500	500	1	200

Heavy metals removal and leaching behavior of MWIFA residue

Batch experiments were conducted to elucidate the effects of Na₂EDTA concentration on removal efficiencies of Cu, Pb, Zn, Cd, and Ni from MWIFA at different concentrations of Na₂EDTA solution. The results of this experiment were shown in Figure 2; we can conclude that Na₂EDTA was also effective in solubilizing Cu, Pb, Zn, Cd, and Ni from MWIFA due to the high stability constants of Na₂EDTA with heavy metals, and this conclusion was similar with the results descripted in previous literature (Hong, Tokunaga, and Kajiuchi 2000). In our research, removal efficiency of heavy metals increased with simultaneous increase of Na₂EDTA concentration. When Na₂EDTA concentration increased from 0.00 to 0.20 mol/L, removal efficiencies of Cu, Pb, Zn, Cd, and Ni increased from 0.5% to 69.7%, 1.9% to 72.7%, 1.0% to 66.5%, 5.8% to 78.1%, and 2.9% to 52.3%, respectively. The removal efficiencies of Cu, Pb, Zn, Cd, and Ni in our research were close to the results in the previous study (Hong, Tokunaga, and Kajiuchi 2000).

Figure 2. Effect of Na₂EDTA concentration on removal efficiency of Cu, Pb, Zn, Cd, and Ni.

US EPA TCLP test was a frequently-used method to evaluate the leaching toxicity of heavy metals in solid wastes. Figure 3 shows the leaching concentrations of Cu, Pb, Zn, Cd, and Ni from MWIFA sample and MWIFA residue obtained after heavy metals extraction using Na₂EDTA by TCLP. EPA TCLP regulatory standards were applied for Pb and Cd; however, there were no standard limits for Zn, Cu, and Ni in EPA TCLP regulatory standards, and thus, Chinese regulatory levels of national standard GB 5085.3–2007 were employed as supplementary reference standard in the study. As shown in Figure 3, although the TCLP leaching amounts of Cu, Pb, Zn, Cd, and Ni was smaller than the regulatory limits when the concentration of Na₂EDTA was 0.1 mol/L, TCLP leaching amount of Zn was 85.8 ± 3.0 mg/L, which was close to 100 mg/L of its regulatory limit value. Therefore, the concentration of Na₂EDTA further increased. After washed by 0.2 mol/L Na₂EDTA solution, TCLP leaching values of Cu, Pb, Zn, Cd, and Ni were the lowest and satisfied the standard (GB 5085.3–2007, Identification standards for hazardous wastes–-Identification for extraction toxicity), and the leaching values were 58.4 ± 2.0 mg/L, 2.81 ± 0.14 mg/L, 64.3 ± 4.0 mg/L, 0.156 ± 0.005 mg/L, 0.381 ± 0.006 mg/L. Combining removal efficiency of heavy metal with TCLP leaching, 0.2 mol/L was chosen as the optimal concentration for removal of heavy metals, also the suitable concentration to realize the TCLP leaching amounts of Cu, Pb, Zn, Cd, and Ni all far less than the standard limits.

Figure 3. Leaching concentrations of heavy metal from MWIFA sample and MWIFA residues obtain after heavy metals extraction using different concentration of Na₂EDTA (0,0.02,0.05,0.08,0.1,0.2 mg/L) using USEPA Method 1311 (the red horizontal line means corresponding standard limits).

Heavy metal removal from leachate with zero-valent iron (ZVI)

EDTA-chelated M^Ⅱ (M refers to divalent metal) could be removed by ZVI, EDTA-chelated Cu^II was taken for example to elucidate the removal mechanism. In theory, the EDTA-chelated Cu^II was first replaced by Fe^III ions, which was originated from the oxidation of Fe⁰ (Liu et al. 2017a). EDTA-chelated Cu^II could not be reduced by Fe⁰ during Fe⁰ corrosion (Ku and Chen 1992); the primary product, Fe^Ⅱ, may stay at the interface, or may be transported away from the surface of Fe⁰, where it was subjected to homogeneous oxidation to Fe^III by dissolved oxygen (DO) (Guan et al. 2015a). The pH values of MWIFA-column effluent (also the influent of iron-column) are shown in Figure 4; acid environment of the solution facilitated the formation of Fe^Ⅱfrom Fe⁰. The stability constant of Fe^III-EDTA (log K = 25.2) was remarkably higher than Cu^II-EDTA (log K = 18.8) (Liu et al. 2017a). Then, the released free Cu^II ions were further removed through multiple pathways, including precipitation, adsorption onto the surface of ZVI corrosion products, and reduction by ZVI (Karabelli et al. 2008; Jiang et al. 2015). Some reaction of the above process are described as Equations (3)(3) $2\mathrm{F}{\mathrm{e}}^0+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}+4\mathrm{O}{\mathrm{H}}^{-}$(3) –(7(7) $\mathrm{C}{\mathrm{u}}^{\mathrm{I}\mathrm{I}}+\mathrm{F}{\mathrm{e}}^0\to \mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}+\mathrm{C}{\mathrm{u}}^0$(7) ) (Guan et al. 2015a; Ju and Hu 2011; Jiang et al. 2010):

(3) $2\mathrm{F}{\mathrm{e}}^0+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}+4\mathrm{O}{\mathrm{H}}^{-}$(3)

(4) $4\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}+4\mathrm{O}{\mathrm{H}}^{-}$(4)

(5) $\mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}+\mathrm{C}{\mathrm{u}}^{\mathrm{I}\mathrm{I}}-\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}\to \mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}\mathrm{I}}-\mathrm{E}\mathrm{D}\mathrm{T}\mathrm{A}+\mathrm{C}{\mathrm{u}}^{\mathrm{I}\mathrm{I}}$(5)

(6) $\mathrm{C}{\mathrm{u}}^{\mathrm{I}\mathrm{I}}+2\mathrm{O}{\mathrm{H}}^{-}\to \mathrm{C}\mathrm{u}{\left(\mathrm{O}\mathrm{H}\right)}_2\downarrow$(6)

(7) $\mathrm{C}{\mathrm{u}}^{\mathrm{I}\mathrm{I}}+\mathrm{F}{\mathrm{e}}^0\to \mathrm{F}{\mathrm{e}}^{\mathrm{I}\mathrm{I}}+\mathrm{C}{\mathrm{u}}^0$(7)

Figure 4. The pH values of MWIFA-column effluent at different sample time.

Stability constant of Fe^III-EDTA (logK = 25.2) (Liu et al. 2017a; Zhu et al. 2019) was also significantly higher than Pb^II-EDTA (log K = 17.9) (Neilson, Artiola, and Maier 2003), Zn^II-EDTA (log K = 16.4) (Neilson, Artiola, and Maier 2003), Cd^II-EDTA (log K = 16.4) (Neilson, Artiola, and Maier 2003), and Ni^II-EDTA (log K = 18.61) (Kornev et al. 2017), so Fe^III could replace the heavy metals in these five heavy metal-EDTA complexes, which achieved the decomplexation effect of ZVI on heavy metal-EDTA complexes.

ZVI in the iron-column first produced Fe^II ions, Fe^II ions were easily oxidized to Fe^III ions. In the above process, OH⁻ will be generated. The production of OH⁻ increased the pH of iron-column effluent. Then, the released free Cu^II, Pb^II, Zn^II, Cd^II, and Ni^II ions reacted with the OH⁻ generated continuously in the above process to form hydroxide the pH value of the solution declined, Fe(OH)₂ and Fe(OH)₃ colloids produced during the reaction. Otherwise, the surface of the ZVI corrosion production could also adsorb Cu, Pb, Zn, Cd, and Ni particles. These could decrease concentrations of Cu, Pb, Zn, Cd, and Ni in the waste liquid, which was also consistent with Figure 5. Changes of Cu, Pb, Zn, and Ni in iron-column influent and effluent are shown in Figure 5. The concentrations of Cu, Pb, Zn, Cd, and Ni in iron-column influent and effluent were 242 ± 18 mg/L, 82.3 ± 8.0 mg/L, 765 ± 16 mg/L, 1.24 ± 0.03 mg/L, 2.16 ± 0.09 mg/L, and 0.785 ± 0.040 mg/L, 6.93 ± 0.10 mg/L, 64.6 ± 6.0 mg/L, 0.305 ± 0.030 mg/L, and 0.524 ± 0.005 mg/L, respectively. The concentrations of the five heavy metals decreased by 99.7%, 91.6%, 91.6%, 75.4%, and 75.7%, respectively.

Figure 5. Concentration variation of Cu, Pb, Zn, Cd, and Ni in iron-column influent and effluent.

Recovery of Na₂EDTA and reuse of the recovered Na₂EDTA solution

XRD was employed for qualitative analysis of solid substanceⅡ, III, and Ⅳ using EDTA, Na₂EDTA and NaCl as reference. One aim of the experiment was to recover EDTA, Na₂EDTA, and NaCl; solid substance Ⅱ, III, and Ⅳ were noted as recycled EDTA, recycled Na₂EDTA, and recycled NaCl, respectively. XRD patterns of recycled EDTA, recycled Na₂EDTA, new EDTA, and Na₂EDTA are shown in Figure 6. Although there were some miscellaneous peaks in recycled EDTA and recycled Na₂EDTA, they were basically consistent with the EDTA and Na₂EDTA peaks. Combination of the abovementioned two results proved that EDTA and Na₂EDTA were the main products of the recovery. As shown in Figure 7, XRD pattern of solid substance Ⅳ exactly matched XRD pattern of the new NaCl. Therefore, NaCl was considered to be the main component of solid substance Ⅳ.

Figure 6. XRD patterns of recycled EDTA, Na₂EDTA, and new EDTA, Na₂EDTA.

Figure 7. XRD patterns of recycled NaCl and new NaCl.

Recycled Na₂EDTA was used to remove Cu, Pb, Zn, Cd, and Ni from the original MWIFA; the results are shown in Figure 8. The removal rates of Cu, Pb, Zn, Cd, and Ni were 67.1%, 68.8%, 63.2%, 73.9%, and 50.7%, respectively. Compared with the removal of Cu, Pb, Zn, Cd, and Ni with the new Na₂EDTA, the removal rates using recovered Na₂EDTA decreased by 2.6%, 3.9%, 3.3%, 4.2%, and 1.6%, respectively. Recycled Na₂EDTA also possessed the high removal efficiency of heavy metal from MWIFA. This circulating system possessed industrialization potential for MWIFA disposal to simultaneously realize removal of heavy metals, cyclic utilization of Na₂EDTA, and extraction of NaCl. The concentrations of Cu, Pb, Zn, Cd, and Ni in the final recovered water, discharge standard of pollutants for municipal wastewater treatment plant (GB18918-2002) and class V water standard of surface water environmental quality standard (GB3838-2002) were shown in Table 2. The concentrations of Cu, Pb, Zn, Cd, and Ni in the recovered water were 0.166 mg/L, 0 mg/L, 0.431 mg/L, 0.00521 mg/L, and 0.0165 mg/L, respectively, which were lower than those limits in the two standards.

Table 2. Comparison of heavy metal concentrations in recycled water and two standards.

Heavy metals (mg/L)	Cu	Pb	Zn	Cd	Ni
Recycled water	0.166	0	0.431	0.00521	0.0165
S1^a	0.5	0.1	1	0.01	0.05
S2^b	1	0.1	1	0.01	0.05

^aDischarge standard of pollutants for municipal wastewater treatment plant (GB18918-2002).

^bClass V water standard of surface water environmental quality standard (GB3838-2002).

Figure 8. Removal efficiencies of Cu, Pb, Zn, Cd, and Ni from MWIFA using new Na₂EDTA and recycled Na₂EDTA solution.

Based on the results noted above, conclusions were drawn that Na₂EDTA could be recycled and reutilized for recovery of heavy metals from MWIFA. Besides, reusable times for the recycled Na₂EDTA would be significant information to assess the industrialization potential. Furthermore, successive batch experiments were also conducted for that evaluation, and the results are shown in Figure 9. Extraction efficiency by recycled Na₂EDTA decreased gradually with simultaneous increase of Na₂EDTA cycles. After 4 recirculation cycles, extraction efficiencies of Pb and Cd (removal efficiency at different cycles divided by removal efficiency of new Na₂EDTA) declined toward 80%, more recirculation cycles more extraction efficiencies attenuation, increased cycles would decrease the extraction efficiency below 80% (especially Pb and Cd).

Figure 9. Extraction efficiency at different recirculation cycles using recycled Na₂EDTA (new Na₂EDTA is defined as cycle 0).

Conclusion

Na₂EDTA was used to remove Cu, Pb, Zn, Cd, and Ni from MWIFA and possessed obvious removal effect. TCLP leaching values of the five heavy metals in MWIFA residue after washing with 0.2 mol/L Na₂EDTA solution all lowered standard values, TCLP leaching values of Cu, Pb, Zn, Cd, and Ni were the lowest and satisfied the standard, and the leaching values were 58.4 ± 2.0 mg/L, 2.81 ± 0.14 mg/L, 64.3 ± 4.0 mg/L, 0.156 ± 0.005 mg/L, 0.381 ± 0.006 mg/L, removal efficiencies of Cu, Pb, Zn, Cd, and Ni were 69.7%, 72.7%, 66.5%, 78.1%, and 52.3%, respectively.

Economical and practical ZVI (iron filings) was used for decomplexing the complexes. Concentrations of Cu, Pb, Zn, Cd, and Ni in the iron-column effluent (relative to the iron-column influent) were reduced by 99.7%, 91.6%, 91.6%, 75.4%, and 75.7%, respectively.

In order to reduce the consumption of Na₂EDTA, the possibility of recycling Na₂EDTA from iron-column effluent was realized. In the process of regulating pH and evaporation, EDTA, NaCl, and water can be recovered from iron-column effluent. Recycled water reached the discharge standard of pollutants for municipal wastewater treatment plant (GB18918-2002) and the class V water standard of surface water environmental quality standard (GB3838-2002).

Recycled Na₂EDTA possessed the high removal efficiency of heavy metal from MWIFA compared with the removal of Cu, Pb, Zn, Cd, and Ni with the new Na₂EDTA, removal efficiencies using recovered Na₂EDTA decreased by 2.6%, 3.9%, 3.3%, 4.2%, and 1.6%, respectively.

Successive batch experiments were also conducted to evaluate industrialization potential and reusable times for the recycled Na₂EDTA. After four recirculation cycles, extraction efficiencies of Pb and Cd (removal efficiency at different cycles divided by removal efficiency of new Na₂EDTA) declined toward 80%, more recirculation cycles, more extraction efficiencies attenuation.

Acknowledgments

The authors are grateful for the administrative support from Wei-Min Liu and the technical support from Fei-Hua Yang. The authors are all grateful for the special support from the graduate student of Dr. Chun-Feng Wang.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was financially supported by National Key Research and Development Program of China [2018YFC1903602] and National Key Research and Development Program of China [2017YFC0703206].