Evaluating the Concentration and Leachability of Heavy Metals in Electric Arc Furnace Dust: Implications for Environmental Management

Iron, comprising approximately 5% of the Earth's crust, is primarily extracted via the carbon reduction method [1], [2]. Among iron alloys, steel reigns prominent, produced through three principal methods: traditional high furnace, direct iron ore reduction, and via the melting of sponge iron and steel scrap using an electric arc furnace (EAF) [1], [2]. Since 1996, crude steel production has surged worldwide, with nearly 1.7 billion tons being generated in 2017, and 30% of this amount produced via the EAF method [3].

However, each ton of steel produced using EAFs concurrently generates approximately 15-30 kg of electric arc furnace dust (EAFD) [4]. This by-product is characterized by the presence of heavy metals such as Cd, Pb, Cu, Mg, Cr, and Fe, leading to its classification as hazardous waste under the European waste and EPA lists [4], [5]. A survey of the literature indicates that about 3.7 million tons of EAFD are produced annually as a steel industry byproduct worldwide [6]. Consequently, an understanding of the physical and chemical characteristics of EAFD is paramount for its processing, recycling, and safe disposal.

Sustainable development hinges on the efficient use, reuse, and recycling of resources. This approach reduces waste disposal, lowers greenhouse gas emissions, conserves raw materials, and optimizes energy and water management. Nevertheless, EAFD, due to its toxic composition, particle size, high volume and weight, and daily production rate, presents serious environmental issues when left exposed in open environments. It contributes to soil, groundwater, and air pollution [7], [8]. Therefore, the concentration of heavy metals in EAFD and the environmental issues related to its disposal necessitate investigations into its recycling and reuse. The recovery of iron concentrates and zinc for use in steel or cast-iron production, or for commercial purposes, and the transformation of EAFD into non-hazardous waste are significant considerations in EAFD processing [9], [10].

The environmental fate of pollutants is often determined by the aqueous solubility, a fundamental molecular characteristic. The solubility of compounds, hydroxides, and salts with weak anions is directly influenced by pH. The pH of a washing solution can alter the quantity of pollutants released into the environment [11]. pH-dependent leaching tests, which are more practical and relevant than merely determining the total concentration, are commonly used to ascertain the environmental risks of waste materials and study the mobility of cations and anions [12].

Within the context of Esfarayen-Iran Steel factory, approximately 3,000 tons of dust are produced annually by EAFs. This dust is stored under inadequate conditions, exacerbating the risk of environmental harm as the dust is exposed to natural elements such as rain, potentially leading to the leaching of a substantial proportion of heavy metals. The present study aims to quantify the heavy metals in dust samples from this site and identify the most leachable toxic elements in response to water exposure.

The study was conducted within a geographical area demarcated by Latitude $40^{\circ} 36^{\prime \prime}-17^{\circ} 37^{\prime}$ and Longitude $57^{\circ} 56^{\prime \prime}-58^{\circ} 7^{\prime \prime}$. Sampling of Electric Arc Furnace Dust (EAFD) at the Esfarayen Steel Factory was performed at three randomly selected locations, designated A, B, and C. At each location, five samples were collected and composited, yielding a total weight of 3 kg per location. Newly produced EAFD was categorized as sample A, slightly aged EAFD as sample B, and the oldest EAFD as sample C.

Samples were subjected to laboratory analysis wherein they were initially dried in an oven for 24 hours at 110℃. A 5 g portion of each sample was subsequently digested using 50 ml of Aqua regia (comprising a 3:1 ratio of 33% HCl and 65% $\mathrm{HNO}_3$) in a 100 ml Erlenmeyer flask [13]. To ensure complete digestion, samples were heated at 90℃ for 90 minutes, followed by a heating period at 120℃. After cooling, the samples were filtered and diluted with distilled water in a 100 ml volumetric flask, then stored at 4℃ until the determination stage. The concentrations of the metals under study were quantified using a flame atomic absorption spectrometer (FAAS - Perkin Elmer AAnalyst 240). Dust chloride content was measured via the argentometric titration method [14], [15].

To evaluate the quantity of metals leachable from the dust, 20 g samples were transferred to Erlenmeyer flasks, and 50 ml of distilled water with varying pH levels (1, 3, 5, 7, 11) was added, alongside diluted solutions of HCl, $\mathrm{HNO}_3$, and $\mathrm{H}_2 \mathrm{SO}_4$ (1, 3M) [4].

The pH and electrical conductivity (EC) of the prepared samples in solution (comprising a 1:2.5 ratio of dust to distilled water) were measured using a multimeter [16].

Table 1 presents the concentration of heavy metals within Electric Arc Furnace Dust (EAFD) samples. Iron (Fe) concentrations are found to consistently exceed those of other elements in all dust samples. Comparisons with previous studies suggest that the zinc content in the current study is less than those reported in prior investigations, potentially attributable to the primary substance and furnace charging compounds [17], [18], [19], [20], [21]. High concentrations of chloride present in EAFD are noted to hinder the value of recycled zinc oxide due to the adverse effect of chlorine on zinc production, as well as its potential to release persistent environmental pollutants such as dioxin ($\mathrm{C}_4 \mathrm{H}_2 \mathrm{O}_2$) [22]. Table 1 illustrates a high chlorine content that could impede the zinc extraction process, indicating that the commercial recovery of zinc from the investigated EAFD may not be economically feasible.

The chemical composition of EAFD is revealed to be complex and varied, influenced by factors including the type of scrap used, the type of steel produced, working conditions, and the process types [23]. Minor alterations in the ratio of charged materials in a melting process can change the chemical composition of EAFD, therefore heavy metal amounts in different samples could reflect the source of raw materials. The fact that Fe comprises approximately 37.6% of EAFD's chemical composition suggests that it could be recovered as raw material from dust. The significant quantity of Fe in EAFD supports the potential for waste reuse in various applications, including concrete production and use as a magnetic adsorbent [20].

$\mathrm{pH}$ levels significantly influence the accumulation and mobility of heavy metals in soil or solid waste. Metals are generally more cationic and are attracted by the negative adsorbent, while in alkaline soils, metals tend to sediment and demonstrate less mobility. Therefore, the risk associated with heavy metals in acidic samples is greater than in alkaline samples [24], [25]. High electrical conductivity also suggests the presence of cationic and absorbable metallic elements. As presented in Table 1, the physicochemical properties analysis of dust reveals higher pH values in newly deposited samples. Over time, dust samples stored at the depot center decrease in alkalinity and electrical conductivity. This supports the idea that dust storage in unroofed areas leads to the removal of alkali and metal ions from the samples [23].

Prior studies have suggested that the ability to wash heavy metals from EAFD decreases as pH increases, however, the leaching of lead (Pb) and chromium (Cr) increases with pH increment [23]. Our results on heavy metal leaching deviate slightly from these previous studies. Heavy metals such as cadmium (Cd) and Pb are recognized as toxic, unnecessary for life, and persistent pollutants that cause harm across various levels of the food chain [26]. Figure 1 shows the leaching of dusts with water at different pH levels, with the highest amounts of leached zinc (Zn) and magnesium (Mg) observed at a pH of 7. An increase in alkalinity dramatically reduces the amount of Zn and Mg [20].

Figure 2 illustrates the highest dissolution of Cd and Pb at a pH of 5. This suggests that during rainfall, the potential for leaching and the influence of Cd and Pb from dust to soil and groundwater is elevated. The amounts of cobalt (Co), copper (Cu), and nickel (Ni) remain roughly constant in acid and neutral environments and are less dependent on water pH, whereas Fe shows a significant rise in an alkaline environment. Manganese (Mn) tends to leach only at pH = 3, with no change in leaching observed with increasing pH [23].

Comparing the total amount of heavy metals with the amount of leached metals in Figure 3, it is found that Co and Cd metal ions demonstrate the highest leaching rates compared to other studied heavy metals. The leaching percentages of metals at pH = 5 were determined to be in the order $\mathrm{Co}>\mathrm{Cd}>\mathrm{Zn}>\mathrm{Pb}>\mathrm{Cu}>\mathrm{Mg}>\mathrm{Fe}$, Ni, Mn. EAFD samples collected from an unroofed outdoor location at the Esfarayen Steel factory were directly exposed to rain and wind. Given the high concentrations of toxic metals such as Cd and Pb in these samples, there exists the potential for long-term soil and groundwater pollution caused by agents such as rainfall, displacement, and emission by wind [23], [26].

Figure 4 demonstrates that the percentage of Zn leaching increases with rising concentrations of hydrochloric acid (HCl) and sulfuric acid ($\mathrm{H}_2 \mathrm{SO}_4$). However, changes in nitric acid ($\mathrm{HNO}_3$) concentration from 1 to 3 M does not notably influence metal leaching. Both Pb and Ni exhibit the lowest reactivity and leaching levels in both concentrations of the utilized acids. Conversely, the percentage of copper (Cu) leaching increases at a concentration of 3 molar $\mathrm{H}_2 \mathrm{SO}_4$. Less than 10% of iron (Fe) was found in both concentrations, and the percentage of Co and Mn increased with a shift in concentration from 1 molar to 3 molar acids [20], [23].

The findings of this study on Electric Arc Furnace Dust (EAFD) highlight the viability and cost-effectiveness of recovering iron concentrates from the waste material. However, the low quantity of zinc (Zn) in EAFD, coupled with the presence of chloride ions, renders its recovery economically unfeasible. The leaching behavior of metals at different pH levels indicates that Zn and magnesium (Mg) exhibit the highest leaching potential when rainwater pH is neutral. Conversely, cadmium (Cd), lead (Pb), and manganese (Mn) metals are more prone to leaching in acidic environments. The leaching of iron (Fe), cobalt (Co), copper (Cu), and nickel (Ni) remains relatively constant in both acidic and alkaline conditions. Notably, at a pH of 5, the leaching order of heavy metals was observed as $\mathrm{Co}>\mathrm{Cd}>\mathrm{Pb}>\mathrm{Zn}>\mathrm{Cu}>\mathrm{Mg}>\mathrm{Ni}>\mathrm{Mn}>\mathrm{Fe}$.

Furthermore, the washing of EAFD using 1 molar and 3 molar concentrations of hydrochloric acid (HCl), nitric acid ($\mathrm{H}_2 \mathrm{SO}_4$), and sulfuric acid ($\mathrm{H}_2 \mathrm{SO}_4$) demonstrated that $\mathrm{H}_2 \mathrm{SO}_4$ had a more pronounced effect on metal leaching compared to $\mathrm{HNO}_3$ and HCl. Increasing the concentration of these acids from 1 to 3 molar resulted in higher percentages of metal washing. However, the concentration of $\mathrm{H}_2 \mathrm{SO}_4$ inversely affected the leaching of Cd. Pb and Ni exhibited the lowest leaching levels among the studied metals.

The physicochemical properties analysis of dust from the steel factory's arc furnace highlights the importance of recycling, reuse, and proper management and maintenance of this waste. Implementing measures such as avoiding wind distribution and reducing leaching through rainfall can be achieved by applying a layer of oil mulch to the surface of collected EAFD, enhancing adhesion and reducing permeability.

Overall, this study provides valuable insights for reducing waste disposal, preserving raw materials, managing energy and water resources, and mitigating soil and groundwater pollution associated with EAFD.