Sample Pretreatment

The collected slag samples were dried, passed through a 0.075 mm sieve, and then bagged and labeled for use. The methods of total heavy metal digestion and effective leaching of heavy metals from slag samples were based on references [11] and [14]. The morphology of heavy metals in the samples, including exchangeable state (F1), carbonate bound state (F2), iron (manganese) oxide bound state (F3), organic bound state (F4), and residue state (F5), were determined using the Tessier five-step extraction method [15] (see Table 1).

Analytical Instruments and Reagents

The Shimazu X-ray diffractometer (XRD-6000) was used to analyze the mineral composition of the slag, while thermal field emission scanning electron microscopy (Nova Nano SEM 450) was used to determine the microstructure and perform an energy spectrum analysis on the slag. The total amount of heavy metals was determined using the American Perkinelmer inductively coupled plasma emission spectrometer (PE Avio550 Max), and the form of heavy metals was determined using the American Perkinelmer inductively coupled plasma mass spectrometer (PE NexION300X). The chemical reagents used in pretreatment and testing the power plant’s bottom slag samples are all of a high-purity grade.

Potential Ecological Risk Index

In 1980, Swedish scientist Hakanson proposed a potential ecological hazard index (RI) evaluation method based on the characteristics of heavy metals and their environmental behavior [16]. The calculation formula is as follows:

C_rⁱ is the single pollution coefficient of a heavy metal, C_sⁱ is the measured content of a heavy metal element in mg/kg, and C_nⁱ calculates the required parameter ratio in mg/kg for this element.

E_rⁱ is the potential ecological risk coefficient of a heavy metal, and T_rⁱ is the toxicity corresponding coefficient of a heavy metal.

RI is the comprehensive potential ecological risk index of multiple heavy metals at a certain point.

The E_rⁱ and RI values were used to divide the potential ecological harm levels into different levels [17]. The standard treatment values of the toxic response coefficients of the pollutants [18] were Pb = 5, Ni = 5, Cu = 5, Cr = 2, Cd = 30, and As = 10. The hazard levels were divided into different degrees based on the E_rⁱ and RI values. Table 2 shows the relationship between E_rⁱ and RI and the degree of pollution.

Risk Assessment Coding

The RAC method systematically evaluates potential risks associated with heavy metals by analyzing their morphological characteristics. It was proposed by Perin et al. [19], and the calculation formula is shown as follows:

where FRAC is the percentage (%) of the exchangeable state content and the carbonate binding state content in the sum of the five forms, and CF1 is the exchangeable state content in mg/kg; CF2 is the content of carbonate binding state in mg/kg; CF3 is the content of iron (manganese) oxide binding state in mg/kg; CF4 is the content of organic binding state in mg/kg; and CF5 is residue content in mg/kg. To quantitatively evaluate environmental risks, the RAC method divides risks into five risk levels: no risk (FRAC<1%), low risk (1%≤FRAC <10%), medium risk (10%≤FRAC<30%), high risk (30%≤FRAC < 50%), and very high-risk (FRAC≥50%).

Data Analysis and Quality Control

Excel 2016 and SPSS 24.0 analyzed and statistically processed the total amount of heavy metals, various form content, comprehensive potential ecological risk index, and RAC value, while the Origin 2021 software drew the graphics. The standard reference material (GBW (E) 0300013) for analyzing heavy metal content in cement clinker was used for quality control during the test.

Morphology Observation and Energy Spectrum Analysis

As shown in Fig. 1, the slag particles exhibit a black or gray color and mainly consist of large-sized, massive particles. They possess an irregular and multi-angular surface topography that appears rough with obvious porous features after sintering. Microscopically, the surface is smooth but contains tiny holes. The particle size of the slag is generally distributed in 0.02−2 mm and is classified as sand.

In Fig. 1, particles of different sizes were selected for point scanning, and the energy spectra of the corresponding points are shown in Fig. 2. The elemental composition and mass fraction of each detected point are shown in Table 3. The slag mainly contained five elements, O, Ca, C, Si, and Al, of which the Ca concentration accounted for 36.27% of the chemistry. The slag particles were mainly calcium-based compounds.

Mineral Composition Characteristics

The minerals in the slag are mainly mullite (M) and quartz (Q), and a small amount of kaolinite (K), pyrite (P), gypsum (G), anatase (An), potassium feldspar (O), hematite (H), and other mineral phases (Fig. 3). Jade6.5 software and PowderX software analysis simulation calculations were used to perform a phase analysis. The contents were: mullite 33.0%, quartz 11.7%, kaolinite 9.8%, pyrite 6.9%, gypsum 7.4%, anatase 6.4%, potassium feldspar 19.6%, and hematite 5.2%.

Fig. 1.

Scanning electron microscopy (SEM) image of furnace bottom slag.

Nutrient Characteristics

Based on the nutrient classification standard of the second National Soil Survey [20], the pH value of the furnace slag in the power plant was 8.68, which is alkaline. The available potassium content of the slag was 268.16 mg/kg, which is in the grade I abundance level. The organic matter content was 35.11 g/kg, which is in the rich level of grade II. The available phosphorus content was 5.70 mg/kg, which was in the deficient level of Ⅳ. Lastly, the nitrogen alkali-hydrolyzed content was 26.51 mg/kg, classified in the extremely deficient class Ⅵ.

Total Amount and Available State of Heavy Metals

The average total concentration of the six heavy metal elements in the slag is lower than the screening value of the soil pollution risk of agricultural land (basic items), and the average concentrations of the effective state are much lower than the lower screening value of soil pollution risk of agricultural land (Table 4).

Compared with the background concentration of the Ningxia soil, only the Pb in the slag was lower than the background concentration. The average total concentrations of the other heavy metals exceeded the background value of the Ningxia soil. The total concentration of As exceeded the standard 5.36 times, Cd exceeded the standard 4 times, Cu exceeded the standard 2.13 times, Cr exceeded the standard 1.93 times, and Ni exceeded the standard 1.16 times. The average concentration of the six kinds of heavy metals was lower than the soil environmental background value in Ningxia.

Fig. 2.

Energy spectrum analysis of furnace bottom slag.

The Occurrence Form of Heavy Metals

The Pb, Cr, Ni, Cd, and As mainly exist in the residual state (F5), and the residual state content accounts for 99.21%, 95.75%, 71.84%, 98.56%, and 74.22%, respectively (Fig. 4). Cu mainly occurs in the residual (F5) and organically combined (F4) states, and its concentration accounts for 47.61% and 32.66%, respectively. Ni and As mainly exist in residual and exchangeable states, where exchangeable state content accounts for 28.05% and 25.49%, respectively. Ni and As elements in the exchangeable states indicate that As and Ni are enriched on the slag particles and can easily migrate into the environment.

Fig. 3.

X-Ray diffraction (XRD) pattern of furnace bottom slag.

Fig. 4.

Occurrence forms of six heavy metals in furnace bottom slag.

Potential Ecological Risk Index

The total amounts of E_rⁱ and RI of the six heavy metals in the slag were calculated using the Ningxia soil background value as a reference ratio. The evaluation results based on the total amount show that the E_rⁱ index of the potential ecological risk of six heavy metal elements in the slag is in the order of Cd>As>Cu>Ni>Cr >Pb (Table 5). The E_rⁱ index of Cd is 118.64, a grade III ecological hazard, and the E_rⁱ index of the As element is 53.56, a grade II ecological hazard (Table 5). The E_rⁱ of other elements was lower than 40, and the pollution degree was at the slight ecological hazard level. Cd was the most important ecological risk contributing factor, at a rate of 60.45%. The comprehensive potential ecological risk index RI of the six heavy metals in the slag is 196.27, and the RI index is less than 300, which indicates that the comprehensive potential ecological risk index RI value of the total six heavy metals in the slag is at a Ⅱ level and a medium ecological hazard level.

Risk Assessment Coding

The F_RAC values of As and Ni were 25.58% and 28.15%, respectively, with moderate risk, indicating that under the influence of a certain external environment, As and Ni elements in slag will migrate and adapt to the environment (Fig. 5). The F_RAC values of Cd, Cr, and Cu were between 1% and 10%, with a low-risk effect. The F_RAC value of Pb was less than 1%, which is a risk-free grade. The ecological risk degree of the six heavy metals in the slag was Ni>As>Cu>Cr>Cd>Pb.

Fig. 5.

Risk Assessment (RAC) evaluation index of six heavy metals in furnace bottom slag.

Combined with the results of the potential ecological risk index method and the risk evaluation coding method, the Cd, Ni, and As in the slag are the main potential polluting elements. In particular, Ni and As in the exchangeable states in the slag accounted for 28.05% and 25.49%, respectively, indicating these two elements’ large potential environmental migration. If large-scale resource utilization of slag is carried out, there may be a risk of Cd, Ni, and As, which can cause environmental pollution, endangering the ecology. Although slag has a good microstructure and a high content of available potassium and organic matter, it is not recommended to be used as a soil and water conditioner. However, it can be applied to road-based paving, the production of construction materials, ceramsite firing, and cement filling. Alkaline materials can effectively solidify heavy metals, passivate heavy metal activity, improve their stability, and reduce their environmental pollution risk [23-25]. In addition, leaching agents (such as chelating agents and organic acids) can also be used to pretreat the slag, and the leaching can reduce the dissolution of Cd, Ni, and As elements in the slag to achieve harmless utilization [26].