Mineralogical and geochemical characteristics of barium slag and ecological risks associated with heavy metals

2.2.1 Chemical composition and mineralogical characteristics analysis

The particle size distribution of the BS of different particle grades was analysed using a Mastersizer 2000 (Malvern Panalytical) laser particle size analyser. The primary chemical components were analysed using Thermo Scientific ARL Perform X-ray fluorescence spectrometry (XRF, melt sheet method, 50 kV, 50 mA), and the mineral composition was analysed using Bruker D8 advance X-ray diffraction (XRD, Cu target, 40 kV, 40 mA, scanning rate: 10°/min). In addition, the micromorphology and microelement distribution were analysed using a German ZEISS Gemini 300 (0.02–30 kV, 10×, 1,000,000×) scanning electron microscope and a Smart Ed spectrometer.

2.2.2 Geochemical analysis

• Heavy-metal content

Referring to ‘Determination of Metal Elements in Solid Waste by Inductively Coupled Plasma Mass Spectrometry’ (HJ 766-2015) (MEPC, 2015), the pre-treated BS samples of each grade were weighed at 0.2 g (accurate to 0.0001 g). Here, the microwave digestion method was employed, and the samples were dissolved using hydrochloric acid (1.19 g/mL), nitric acid (1.42 g/mL), hydrofluoric acid (1.49 g/mL) and hydrogen peroxide (30 %). After removing the excess acid from the digestion solution, it was dissolved with deionised water and then filtered. The filtrate was then diluted to a volume of 50 mL, and the supernatant was analysed using inductively coupled plasma mass spectrometry (ICP-MS) on a Prekin EImer ICP-MS NexlON 1000G instrument. The concentrations of heavy-metal ions (ions of Cu, Zn, Pb, Cr, Cd, Ni, Fe, Mn, As and Ba) in the solution were measured and used to calculate the heavy-metal content of the BS as follows:

• Identification of leaching toxicity

The BS leaching solution was prepared according to ‘Solid Waste leaching Toxicity Leaching Method of sulphuric acid and nitric acid Method’ (HJ/T299-2007) (EPAC, 2007b), and BS leaching toxicity was identified according to ‘Hazardous Waste Identification Standard Leaching Toxicity Identification’ (GB5085.3-2007) (EPAC, 2007a). Here, 10.0 g of pre-treated BS samples of each grade were placed into an extraction bottle, and the leaching solution with pH = 3.20 ± 0.05 was added at a liquid–solid ratio of 10:1 (mL/g). The leaching solution was then oscillated at 30 ± 2 r/min under 23 °C ± 2°C for 18 ± 2h and then filtered through a 0.45-μm filter membrane. The concentrations of heavy-metal ions (ions of Cu, Zn, Pb, Cr, Cd, Ni, As and Ba) in the filtrate were determined through ICP-MS.

• Bioavailable test and calculation

The concentrations of bioavailable heavy metals in the BS were determined. Using a mixed solution of 0.005 mol/L DTPA + 0.1 mol/L TEA + 0.01 mol/L CaCl₂·2H₂O as the extraction solution, the solution pH was controlled at 7.3, and 2.0 g pre-treated samples of each BS grade was mixed with the extraction solution at a liquid–solid ratio of 10:1 (mL/g). After oscillating the mixture at 200 r/min under 25 °C + 2°C for 2 h on an oscillating device, the supernatant was filtered using a 0.45-μm filter membrane (He et al., 2020; Kaniki and Tumba, 2019; Lewis et al., 2021; Lindsay and Norvell, 1978), and the concentrations of heavy-metal ions (ions of Cu, Zn, Pb, Cr, Cd, Ni, Fe, Mn, As and Ba) in the filtrate were determined through ICP-MS. This represented the concentration of bioavailable heavy metals (denoted as C_bioavailable).

The bioavailable fraction of heavy metals (BAM1) was calculated as follows:

• Bioaccessible test and calculation

With reference to the unified bioaccessibility model (Wragg et al., 2011), the gastrointestinal digestion characteristics of Chinese people, the concentration of heavy metals in the BS dissolved in the gastrointestinal system was measured by simulating gastric and intestinal juices as the extraction solution. This represented the bioaccessible concentration of heavy metals (denoted as C_{bioaccessible}).

The simulated gastric and intestinal juices were prepared as follows. To prepare the simulated gastric juice, 16.3 mL 37 % hydrochloric acid was mixed with 800 mL deionised water and then 10.0 g pepsin was added to the mixture. The volume was fixed to 1 L. The simulated intestinal liquid was prepared by mixing 6.8 g KH₂PO₄ with 10.0 g pancreatic enzyme, adjusting the pH to 6.8 ± 0.05, diluting with deionised water and setting the volume to 1 L (Lei et al., 2020; Li et al., 2017; Rui et al., 2019).

The tests for the gastric-phase (GP) concentration of the bioaccessible heavy metals were performed as follows: 10.0 g of pre-treated BS samples of each grade were mixed with 225 mL of the simulated gastric juice in a 1-L HDPE extraction bottle, and the pH was adjusted to 1.1 ± 0.05 with a 37 % HCl or 1-mol/L NaOH solution. The samples were then oscillated at 30 r/min under 25 °C on an oscillating device for 1 h. After centrifugation at 4000 r/min for 20 min, the concentrations of heavy-metal ions (ions of Cu, Zn, Pb, Cr, Cd, Ni, Fe, Mn, As and Ba) in the supernatant were measured using ICP-MS. This represented the GP bioaccessible concentration of heavy metals (Lei et al., 2020; Rui et al., 2019).

The gastrointestinal-phase (GIP) bioaccessible concentration of heavy metals was determined as follows: 600 mL of the simulated intestinal fluid was added to the GP fluid and the pH was adjusted to 6.3 ± 0.05. Samples were taken at 0.5, 1, 2, 4, 6, 8, 10 and 12 h at 30 ± 2 r/min under 25 °C. After centrifuging at 4000 r/min for 20 min, the concentrations of heavy-metal ions (ions of Cu, Zn, Pb, Cr, Cd, Ni, Fe, Mn, As and Ba) in the supernatant were measured through ICP-MS. This represented the GIP bioaccessible concentration of heavy metals (Lei et al., 2020; Rui et al., 2019).

The bioaccessible fraction of heavy metals (BAM2) was calculated as follows:

• Test of heavy-metal form

The content of four geochemical fractions of heavy metals (ions of Cu, Zn, Pb, Cr, Cd, Ni, Fe, Mn, As and Ba) in the BS samples of each grain grade was determined through BCR sequential extraction (Li and Yuejie, 2020; Liu et al., 2018; Rauret et al., 1999; Zhang et al., 2008), which includes weak-acid soluble fraction (B1), reducible fraction (B2), oxidisable fraction (B3) and residual fraction (B4). BCR sequential extraction procedure is shown in Table 1.

2.3.1 Bioaccessibility assessment

Bioaccessibility was graded into the following four classes according to the BAM2 value (Xu et al., 2021): low (1–15 %; LR), moderate (15–30 %; MR), high (30–50 %; HR) and very high (>50 %; VHR) risks.

2.3.2 Risk assessment code

The migration risk was evaluated using a risk assessment code (RAC), which is calculated as follows (Chen et al., 2023; Du et al., 2020):

3.1.1 Composition and particle size distribution of BS

The mass fraction of each BS grade after screening is presented in Table S1, and the particle size distribution curve of each BS grade is shown in Fig. 1.

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Fig. 1

Particle size distribution of the BS.

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The mass proportion of BS1 was 52.55 %. The difference in the mass proportion of BS composition among BS2, BS3 and BS5 was not significant, with values of 13.52 %, 13.10 % and 11.50 %, respectively. The mass proportion of BS4 was only 9.33 %.

The particle size distribution curves of the BS of different particle grades exhibited different shapes, and the overall distribution was negatively skewed. The particle size of BS2 (Fig. 1(a)) exhibited a unimodal distribution, with a median particle size (MPS) of 810.40 μm and a maximum frequency diameter (MFD) of 789.76 μm (accounting for 10.33 %, which was greater than the mass diameter proportions of other BS samples). The particle sizes of BS3 and BS4 (Fig. 1(b) and (c), respectively) exhibited bimodal distributions, with MPS values of 309.30 and 108.10 μm, respectively, and MFD values of 339.92 and 133.75 μm, respectively, corresponding to 6.96 % and 7.23 %, respectively. BS5 (Fig. 1(d)) exhibited a three-peak distribution, and the peak height gradually decreased with decreasing particle size. Here, the peak height of the fine grain decreased most obviously, with an MPS of 20.47 μm and MFD of 39.78 μm, and the proportion of MFD was 3.58 %. Moreover, Fig. 1 also showed that the screened BS had a nominal particle size range, whereas its actual particle size distribution deviated from this range. This discrepancy may be attributed to the sifting perforation of a minute quantity of BS particles.

In summary, the proportion of larger particles (≥850 μm) and smaller particles (<850 μm) in BS was close. The particle size distribution of BS with smaller particles displayed a negative skewness, while the shape of its distribution curve varied greatly. This particle size distribution model revealed the heterogeneity of thermodynamic and kinetic behavior during the carbothermal reduction and leaching process of barite, as well as indicated poor granularity sorting in BS.

3.1.2 Chemical composition analysis

The chemical compositions of the BS samples of different particle grades are shown in Table 2, and the overall proportion of the chemical components in the BS of different particle grades is shown in Table S2.

The chemical composition of BS was primarily SiO₂, BaO and SO₃, accounting for a total of 76.15 %, followed by CaO, Al₂O₃ and Fe₂O₃, accounting for 5.19 %, 4.51 % and 3.25 %, respectively. In addition, K₂O, MgO, Na₂O, TiO₂, V₂O₅, MnO, ZnO, CuO, Cr₂O₃ and NiO were detected. The chemical composition of the BS samples of different particle grades was the same but at varying proportions. Here, three general cases were identified. First, the proportion of the chemical composition increased with decreasing particle size, e.g. BaO, SO₃, Fe₂O₃, MnO and ZnO. Second, the proportion of the chemical components decreased with decreasing particle size, e.g. SiO₂, Al₂O₃, TiO₂, K₂O and P₂O₅. Finally, the proportion of the chemical composition remained relatively constant, e.g. CaO, MgO and Na₂O.

Table 2 also shows that the proportion of heavy-metal oxides, e.g. BaO, Fe₂O₃, MnO and ZnO, in the BS of each grain grade increased with the decreasing grain grade. Cr₂O₃ was detected in BS1 and BS2; CuO was detected in BS1, BS4 and BS5 (content changes negligibly) and NiO was only detected in BS2. However, CdO, PbO and As₂O₃ were not detected in the BS of any grade.

The mass proportion of BS1 in the BS was 52.55 % (Table S1); the overall mass proportions of each chemical component in BS1 were >40 %. This finding indicated that the main occurrence of each chemical component was in the BS with a particle size of 850 μm and greater. The proportions of BaO, Fe₂O₃, ZnO and MnO in BS1 were 48.69 %, 50.74 %, 50.41 % and 51.07 % respectively, and the smallest proportions of BaO, Fe₂O₃, ZnO and MnO were found in BS4, i.e. only 11.09 %, 9.49 %, 10.27 % and 8.42 %, respectively.

In summary, the chemical composition of BS with different particle sizes was essentially identical, with SiO₂, BaO, and SO₃ being the predominant chemical components. The content of SiO₂ decreased as particle size decreased while the content of BaO and SO₃ increased. Additionally, larger particles had a higher proportion of SiO₂ and a relatively small amount of Al₂O₃, TiO₂, and P₂O₅ compared to smaller particles. CaO, Fe₂O₃, MgO, Na₂O, K₂O and other metal oxides with minor content showed a random distribution within each particle size range. A significant positive correlation was observed between BaO and SO₃, suggesting a shared source for the two elements. Moreover, their concentrations increased with the increase of BS particle size.

3.1.3 Mineral composition analysis

The spectrogram of MBS (ungraded BS) shown in Fig. 2 demonstrates that the BS primarily contained quartz phase (SiO₂) and barium-containing mineral phases in the form of BaSiO₃, BaSO₄, BaAl₂Si₂O₈, BaS₂O₃H₂O, etc. The elements in barite ore (e.g. Si, Al and Fe) primarily occurred in quartz, clay, sericite, and hematite that coexisted or were associated with them. These elements in the associated minerals formed barium-containing mineral phases, e.g. BaSiO₃, BaAl₂O₄ and BaAl₂Si₂O₈, in carbothermal reduction (Zhang et al., 2019). However, the low content of the heavy metals and the iron-containing phase were not detected in the XRD test of the BS.

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Fig. 2

XRD contrast patterns of BS with different particle sizes. (a) XRD pattern of MBS and graded BS. (b)–(d) Amplification of the XRD patterns in 2θ.

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Combined with the chemical composition test results for the BS (Table 2) and the XRD pattern analysis (Fig. 2), the XRD patterns of the BS at each particle size exhibited relative shifts in the characteristic diffraction peaks of SiO₂ (BS1: 26.62°, BS2: 26.69°, BS3: 26.71°, BS4: 26.69° and BS5: 26.69°). The phenomenon of weakening (2θ = 20.84° and 26.66°) or even disappearance (2θ = 20.84°) of diffraction peaks was observed, and the smaller the particle size, the more pronounced this weakening and disappearance. The shift in the characteristic peak may have been caused by the displacement crystal transformation of the quartz phase in carbothermal reduction (Zhang et al., 2019), and the weakening and disappearance of the diffraction peak may have been caused by the solid-phase reaction between SiO₂ and Ba, Al and Fe in the barite ore to form barium-containing silicate or silico-aluminate minerals. Smaller mineral particles exhibited higher surface activity, which facilitated solid-phase reactions; thus, smaller particle sizes led to more clear weakening and disappearance of the diffraction peaks.

As shown in Fig. 2, the characteristic peaks corresponding to the barium-containing mineral phases, e.g. BaSiO₃, BaAl₂Si₂O₈ and BaS₂O₃H₂O, became stronger with increased peak sites as the grain size decreased, indicating that the reduction in the mineral particle size promoted formation of the barium-containing phases and contributed to the formation of other phases, thereby complicating the phase composition of the BS. Notably, no characteristic diffraction peaks corresponding to compound phases containing other heavy metals (Mn, Zn, Cu, Cr, Ni and As) or the water-soluble BaS compounds were observed in the XRD pattern. This could be due to the low content or poor crystallisation performance related to these minerals or they may have been encapsulated within other mineral phases.

3.1.4 Morphological structure and element distribution analysis

The BSs of different particle sizes were further characterised using SEM-EDS (Fig. 3 and Figs S1–S5). Clearly, the mineral morphological and structural characteristics of the BS of different grain grades considerably differed. The particles exhibited irregular shapes and diverse surface structures. Coarse particles with distinct outlines were observed in the BS1 and BS2 grade samples, and fine particles were dispersed and attached to the surface. With increasing particle size, the fine particles gradually increased, more fine particles or particle aggregates adhered to the surface of the larger particles (and even formed a fine particle attachment layer) and the particle boundary gradually blurred. In addition, the BS particles in the BS4 and BS5 samples exhibited more irregular fine particle aggregates, and the distribution of the small and medium particle aggregates in BS5 was more uniform. Furthermore, the EDS all-element scan revealed that various elements in the BS of each particle grade were mixed with each other, indicating a complex particle composition. In addition, with decreasing particle size, the mixing phenomenon became more pronounced and the particle composition increased.

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Fig. 3

Representative SEM-EDS images (C, O, Al, Si, S, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba and Pb) of the BSs.

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Fig. 3

Representative SEM-EDS images (C, O, Al, Si, S, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba and Pb) of the BSs.

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Fig. 3

Representative SEM-EDS images (C, O, Al, Si, S, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba and Pb) of the BSs.

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Fig. 3

Representative SEM-EDS images (C, O, Al, Si, S, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba and Pb) of the BSs.

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Fig. 3

Representative SEM-EDS images (C, O, Al, Si, S, Ca, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba and Pb) of the BSs.

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EDS analysis showed significant variations in the contents of various elements in the BS. For example, the levels of C, Si, Ba and O were notably high, followed by those of S, Ca and Fe. In contrast, the levels of Al, Pb, Cd, As, Zn, Cu, Ni, Mn and Cr were lower (or undetected). In addition, clear correlations were observed in EDS single-element scanning between the following elements: Si and O; Ba and S; Ba S and O; Ca, S and O; Fe and O and Al, Si and O. Based on the XRD diffraction pattern in Fig. 2 and the XRF analysis data given in Table 2, the slag was inferred to contain quartz, aluminosilicate, hematite, sericite, BaSO₄ and BaS.

The high C content was possibly caused by incomplete combustion of coal fuel during carbothermal reduction (Yang et al., 2021). The presence of BaS may be attributed to incomplete leaching in the water or reverse reaction in hydrolysis, resulting in amorphous precipitation. In addition, BaSO₄ originated from unreacted barite residues (Agarwal, 2019; Zhang et al., 2019). The C and Si contents reduced with a decrease in BS size, dropping from 60.24 % and 3.87 % in BS1 to 32.71 % and 2.73 % in BS5, respectively. Conversely, the Ba, S, Fe and Ca contents gradually increased with decreasing particle size, dropping from 10.17 %, 1.4 %, 1.01 % and 0.93 % in BS1 to 30.64 %, 4.10 %, 4.05 % and 3.60 % in BS5, respectively. In addition, the O content remained constant at approximately 20 % in all analysed samples. A correlation analysis between Ba and S was conducted using EDS single-element scan data. According to this analysis, the BaS content increased with decreasing particle size.

In summary, the main minerals phases in BS were silicon oxide and barium sulfate, characterized by large particle sizes, well-defined outlines, and clear boundaries. Additionally, its surface was coated with irregular flocculent particles primarily composed of BaS, aluminosilicate, sericite, hematite, and various barium salt phases.

3.3.1 Leaching toxicity test results

The leaching toxicity test results of barium residue for each grade are presented in Table 4. Cd, Pb and Cu were undetected in the leaching solution of the BS with different grain sizes. The concentration of Cr in the leaching solution of BS1 was 3.76 μg/L; however, no Cr was detected in the other fractions. The concentrations of As in BS4 and BS5 were 1.22 and 1.89 μg/L, respectively; however, As was not detected in the other fractions. Zn and Ni were detected in the leaching solution of the BS with mean values of 227.4 and 5.198 μg/L, respectively. Notably, the most significant leaching toxic substance in the BS samples was Ba, and the extracted Ba contained both water-soluble barium sulphide and acid-soluble barium salts. The concentration range of Ba in the leaching solution for each particle grade was 590–699 mg/L, which exceeded the limit of 100 mg/L mentioned in the ‘Hazardous Waste Identification Standard for Leaching Toxicity Identification’ (HJ5085.3-2007). Thus, the BS was classified as hazardous waste. In addition, there was no significant difference in the concentration of barium ions in the leaching solution among different grain grades. Therefore, the risk properties of BS were concluded to remain unchanged with varying grain size. The leaching concentration of other heavy metals in each particle grade did not exceed the specifications in HJ5085.3-2007, and this concentration did not significantly vary with the different particle grades. In addition, there was little correlation between the concentration of the leaching solution and the particle grade of BS.

3.3.2 Bioavailability analysis

The analysis results for heavy-metal bioavailability in the BS of each grain grade are presented in Fig. 4.

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Fig. 4

Bioavailable fractions of heavy metals in BS.

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There were no significant differences in the bioavailability of the same heavy metals among the different grain grades. The bioavailability of Cu, Pb and Cr was relatively low (0–0.02 %, 0.02–0.26 % and 0.02–0.11 %, respectively), while that of Cd (1.06–3.09 %), Fe (1.36–3.58 %) and As (1.94–4.54 %) was slightly higher. Zn, Ni, Mn and Ba showed the highest bioavailability, reaching 15.56–21.58 %, 30.11–47.42 %, 16.31–28.20 % and 19.44–21.82 %, respectively. The potential migration abilities of Zn, Ni, Mn and Ba in BS in plants were clearly stronger than those of other heavy metals, indicating that Zn, Ni, Mn and Ba had strong enrichment ability in plants and could be absorbed by plants in large quantities. Thus, crops should not be planted around BS landfills. However, considering the high bioavailability of heavy metals, e.g. Zn, Ni, Mn and Ba, in BS for plants, phytoremediation can be considered a remediation and treatment technology for BS storage yards to reduce the environmental risks caused by the migration and diffusion of heavy metals, such as Zn, Ni, Mn and Ba, into the environment surrounding BS landfills(Carvalho et al., 2019; Han et al., 2020; Lu et al., 2019; Ribeiro et al., 2018; Viana et al., 2021a; Viana et al., 2021b).

The average Ba content in BS reached 20.86 %, which far exceeded the contents of Zn, Ni, Mn and other heavy metals. However, there was no significant difference in the bioavailability between Ba and these three heavy metals. It was indicated that the content characteristics of heavy metal bioavailability components in BS were not only closely related to their total amount, but also affected by the physical and chemical properties of the environment. BS occurred in various forms, and the dissolution and migration properties of different barium-containing compounds were related to the acidity or alkalinity of the solution system. Under the tested pH value of the extraction solution tested for bioavailability (∼7.3), it was nearly impossible for both acid-soluble and -insoluble barium compounds to dissolve and migrate. Thus, the bioavailability of Ba in the BS did not differ significantly from that of Zn, Ni and Mn. The bioavailability of heavy metals in BS was related to its total amount and was affected by its form of occurrence. Notably, the pH conditions of the surrounding environment may change the solubility and migration of Ba in BS, which must be considered.

3.3.3 Bioaccessibility analysis

The bioaccessibility of heavy metals in the BS obtained through PBET in vitro simulation experiments is shown in Figs. 5 and 6.

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Fig. 5

Bioaccessible fractions of heavy metals in the gastric phase.

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Fig. 6

Bioaccessible fractions of heavy metals in the gastrointestinal phase.

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There was no significant difference in the bioaccessibility of the same heavy metal in the GP among the BSs of different grain grades. The bioaccessibility of Zn, Ni, Mn and Ba was 6.86–6.89 %, 11.45–11.50 %, 8.33–8.37 % and 7.49–7.52 %, respectively. In addition, the bioaccessibility of Cd, Fe and As was approximately constant in BSs of different grain grades at approximately 0.77 %, 1.36 % and 1.41 %, respectively. The bioaccessibilities of Pb and Cr were <0.05 %, and Cu was essentially insoluble in the GP. The mean bioaccessibility of heavy metals in the GP of BS was in the order of Ni > Mn > Ba > Zn > As > Fe > Cd > Pb > Cr > Cu. Clearly, different heavy metals, such as Ni, Mn, Ba and Zn, were relatively easily absorbed by the human body through the stomach, and the possibility of Pb, Cr and Cu being absorbed was very low. The protease in the gastric digestive environment was concluded to have a significant impact on the solubility of heavy metals. Here, the heavy metals formed metalloproteins with proteins, which helped the metal ions maintain their solubility. In addition, the acidic environment of the gastric fluid was conducive to improving the activity of protease, thereby promoting the formation and aggregation of GP metalloproteins to a certain extent as well as enhancing their dissolution and diffusion in the GP digestive fluid. No significant difference was observed in the bioaccessibility of metals in the GP among the five BS grades. The BAM2 value for all heavy metals in the GP was <15 %. Based on the risk classification evaluation of the BAM2 value, it was identified that the GP bioaccessibility of heavy metals in the BS posed low risk.

Fig. 6 showed that the GIP bioaccessibility of the same heavy metal in the BS at the GIP stage substantially changed over time. Herein, the gastrointestinal bioaccessibility of As was the most prominent, being higher than that of other heavy metals, and increase became more clear over time. Additionally, Zn, Ni, Mn, and Ba were also easily absorbed by the human GIP, and their GIP bioaccessibility was generally high and sustained over time with order of As > Ba > Ni > Zn > Mn. The bioaccessibility of Pb, Cr, and Cd exhibited a continuous increase over time, while that of Cu and Fe gradually declined until Fe reached zero. This might be due to the increase in pH from 1.1 to 7.3 during digestion in the stomach and small intestine, thereby resulting in the precipitation of Fe, Cu, and other heavy metals dissolved in the gastric juice in the small intestine and being re-deposited. Simultaneously, the pH environment of the intestinal fluid was not conducive to the formation of metalloproteinases. The metals adsorbed on the surface of pepsin only based on their charge were easily hydrolysed and precipitated after entering the intestinal phase, which reduced the GIP bioaccessibility.

These results revealed that in the GP and GIP stages, the bioaccessibility of Ni, Mn, Ba and Zn in the BS was relatively high; however, despite its high total content, the bioaccessibility of Fe was relatively low and even decreased to zero with the decreasing particle size. In addition, the bioaccessibility of As, with a low total content, continued to increase from the GP to the intestinal phase. The proportion of heavy metals in the BS that could be absorbed after entering the human digestive system varied, suggesting that multiple factors contribute to difference in the bioaccessibility of heavy metals in barium residue. Notably, the bioaccessibility of heavy metals in the BS was <100 %, indicating incomplete absorption during digestion and potentially overestimating negative health risks when assessing population health based on the full values of heavy metals(Xu et al., 2021).

3.3.4 Potential migration analysis

The heavy-metal pollution of BS was related to the total amount of heavy metals and was affected by the activity, solubility and curing degree of each element. The occurrence forms of heavy metals in the BS for each grain grade obtained using the BCR sequential extraction method are presented in Fig. 7. Mn, Ba, Fe and Ni primarily occurred in the form of a weak-acid soluble fraction in the BS with average proportions of 78.08 %, 64.75 %, 50.13 % and 43.78 %, respectively, followed by the reducible form with average proportions of 13.26 %, 23.53 %, 29.14 % and 24.24 %, respectively. The average occurrence proportions of the oxidisable forms of Mn, Ba, Fe and Ni were 3.45 %, 3.31 %, 12.25 % and 12.14 %, respectively, and those of the residual forms were 5.21 %, 5.41 %, 8.47 % and 19.84 %, respectively. These results indicated that heavy metals such as Mn, Ba, Fe and Ni in BS were extremely easy to release and could migrate and pollute the surrounding ecological environment under weak acid conditions. In addition, Cu and Cr primarily occurred in residual form, with average proportions of 53.40 % and 52.34 %, respectively. The average proportions of the oxidisable forms of Cu and Cr were 39.84 % and 18.65 %, respectively, and those of the reducible forms of Cu and Cr were 6.75 % and 24.83 %, respectively. Notably, 0 % and 4.17 % of Cu and Cr occurred in the weak-acid soluble form, indicating that Cu and Cr in BS primarily exist as insoluble salts that are difficult to release and migrate, thereby making them relatively stable in the natural environment with negligible pollution risk to the surrounding environment. In addition, Zn, Pb, Cd and As primarily occurred in the reducible form with average proportions of 61.95 %, 58.19 %, 75.16 % and 45.86 %, respectively, as well as in the oxidisable form with average proportions of 8.38 %, 10.37 %, 10.33 % and 36.56 %, respectively. The average proportions of the residual form of Zn, Pb, Cd and As were 12.30 %, 31.34 %, 14.51 % and 17.57 %, respectively, and those of the weak-acid soluble form of Zn, Pb, Cd and As were 17.38 %, 0.02 %, 0 % and 0 %, respectively. These findings indicated that Zn, Pb, Cd and As in BS were released, transported and inter-transformed through precipitation or chelation with organic matter in reducible or oxidisable environment, which posed pollution risks.

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Fig. 7

Chemical fraction distribution of heavy metals in BS.

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The RAC value of heavy metals in the BS is presented in Fig. 8. The risks associated with heavy metals such as Zn, Ni, Fe, Mn and Ba were high. The RAC values of Mn and Ba in each grade BS were >50 %, which represented a very high ecological risk. In addition, Ni and Fe were classified as very high ecological risks in BS1, BS2, BS3 and BS4 but were classified as no ecological risks and high ecological risks in BS5, respectively. Cr posed a low ecological risk in BS1, BS2, BS3 and BS4; however, Cr was found to pose no ecological risk in BS5. Cu, Cd and As had no ecological risk in the BS and did not pose migration risk; thus, attention should be paid to the ecological risks that may arise from the migration of Mn, Ba, Ni and Fe in BS during storage.

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Fig. 8

Calculated RAC values of the studied heavy metals in BS.

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