Health and Environmental Risk Assessment of Utilization Products of Aluminum–Chromium Slag

Abstract

Aluminum–chromium slag (ACS), a by-product of aluminothermic reduction, which is used to produce metallic chromium and its alloys, contains toxic, carcinogenic hexavalent chromium (Cr(VI)). Therefore, improper ACS utilization may severely harm human health and the environment. This study analyzed the Cr(VI) contents, leaching characteristics, and surface concentrations in ACS and four industrially utilized products derived from it (fused alumina for refractories, ferrochromium, aluminum–chromium bricks, and high-chromium bricks). A risk assessment framework was established to evaluate their human health and environmental risks. Results showed 111 mg/kg Cr(VI) in the ACS, with its leaching concentration (7.8 mg/L) exceeding China’s hazardous waste standard. The Cr(VI) contents in the products were low (from <2 mg/kg to 16 mg/kg), and their maximum leaching concentration was below the detection limit (<0.004 mg/L). Furthermore, the four products were found to have acceptable levels of human health risk (<10⁻⁵ carcinogenic risk and <1 noncarcinogenic hazard quotient) under two risk assessment methods (particle-contact- and surface-contact-based methods). Additionally, the predicted concentration of leached Cr(VI) in groundwater (0.008 mg/L) was below the drinking water standard (0.05 mg/L). Cr(VI) limit standards for the products were then proposed based on the risk assessment (≤31 mg/kg content, ≤0.189 mg/m² surface concentration, and ≤0.259 mg/L leaching concentration). Overall, these results may provide a reference for the safe utilization and risk management of ACS and other solid wastes.

1. Introduction

Aluminum–chromium slag (ACS) is a by-product of metal chromium, ferrochrome, and other alloys produced by aluminothermic reduction. Approximately 50–60 million tons of ACS are produced annually in China. With Al₂O₃ (85–90% wt) and Cr₂O₃ (5–10% wt) as its main components, ACS is a high-quality raw material in refractories [1]. For example, chrome–corundum bricks made from ACS are used in Ausmelt furnaces, zinc volatile kilns, and copper converters [2]. ACS is also a raw material of magnesium–aluminum–chromium spinel bricks, which are employed in ferrous metallurgy [3]. Moreover, chromium and aluminum in ACS are separated through melting carbonization to produce raw materials for fused alumina and carbonized chromium [4]. However, ACS is a hazardous solid waste, containing a certain amount of toxic, carcinogenic hexavalent chromium (Cr(VI)) [5]. ACS accumulation and landfills can severely pollute soil and groundwater. ACS utilization products can also have Cr(VI) if they are not managed properly, harming industrial workers and the environment. However, limited studies have focused on the characteristics and risks of Cr(VI) in ACS utilization products.

The potential environmental risks of utilization products generated from solid wastes should be evaluated through strict safety assessments [6]. Leaching tests are widely employed to simulate the pollutant infiltrations into groundwater during land disposal, particularly the toxicity characteristic leaching procedure and synthetic precipitation leaching procedure (SPLP) [7]. The leaching environmental assessment framework was developed to provide a flexible basis for leaching assessment in multiple scenarios for waste treatment, disposal, use, and remediation [8,9]. The chemical forms of metals in waste or waste utilization products can be evaluated by sequential extraction leaching tests, which can precisely identify the mobility of heavy metals rather than their total content [10,11]. The risk assessment code, potential risk index method, and synthesis toxicity index use calculated ratios and risk levels to evaluate the environmental risk of the utilization products of municipal solid waste incineration fly ash, coal-based solid waste, and slags [12,13,14]. However, these risk levels are relative and qualitative, as well as lack of specific exposure scenarios. Researchers also use health risk assessment models to address the carcinogenic and noncarcinogenic risks of recycled products that can have potential human contact [15,16]. The risk of groundwater contamination of solid wastes utilized as road or pave materials has been analyzed using fate, transport, and dilution models [17,18]. A universal risk assessment model has yet to be developed for the utilization products of solid wastes, especially for products used as construction materials or replacement for industrial raw materials. However, large quantities of industrial slags are utilized in the manufacture of construction materials and refractories materials [19,20]. A case-specific risk assessment framework should be established to address their environmental concern by identifying exposure scenarios and exposure receptors, and selecting appropriate assessment models based on product life cycles.

In this study, ACS-derived fused alumina products for refractory were collected to determine their residual Cr(VI) contents. A systematic framework of risk assessment was established to evaluate the associated human health and environmental risk levels. Based on the findings, a set of control criteria for Cr(VI) was proposed. The results may provide a reference for the management of the utilization of ACS and other solid wastes.

2. Materials and Methods

2.1. Materials

Samples were collected from a production plant of high-temperature materials in Liaoning Province, China. This plant mixes ACS with petroleum coke and smelts the mixture at 2000 °C to produce fused alumina for refractories (FAR), generating ferrochromium (FC) as a by-product. FAR granules are mixed with auxiliary materials in specific proportions, including phosphoric acid, aluminum dihydrogen phosphate, chromium oxide, zirconium oxide and ACS, to produce alumina–chromium bricks (ACB) and high-chromium bricks (HCB). The plant utilizes 20,000 t/a of ACS and produces approximately 30,000 t/a of various refractory materials.

Three ACS samples and six samples of each of the four products were collected in the plant’s warehouse. Each sample weighed over 2 kg. Pictures of the samples are presented in Figure 1. Except for the wipe sampling test, the samples were pulverized and sieved (100 mesh) before analysis.

2.2. Sample Analysis

Heavy metal content analysis. The Cd, Ni, Pb, Cu, Zn, and total Cr (Cr(T)) contents in the ACS were determined via inductively coupled plasma–mass spectrometry (Agilent 7900, Agilent, Santa Clara, CA, USA) following the HJ 766-2015 standard [21]. The Hg and As contents in the ACS were determined using atomic fluorescence spectroscopy (Beijing Haiguang AFS-8530, Beijing Haiguang Instrument Co., Ltd., Beijing, China) following the HJ 702-2014 standard [22]. The Cr(VI) contents in the ACS and the four products were determined through flame atomic absorption spectroscopy (Agilent 280FS AA, Agilent, Santa Clara, CA, USA) following the HJ 687-2014 standard [23].

Toxicity leaching test. A toxicity leaching test was performed on the ACS samples. A leaching solution was created according to the HJ/T 299-2007 method [24]. The Cr(VI) concentration in the leaching solution was determined using ultraviolet–visible spectroscopy (Puxi TU-1810, Beijing Purkinje General Instrument Co., Ltd., Beijing, China) following the GB/T 15555.4-1995 standard [1,25], and the concentrations of other heavy metals were determined using the HJ 766-2015 and HJ 687-2014 methods.

Maximum availability leaching test. This test was conducted on all the product samples in two stages. The eluate pH values were 7 and 3.2 in the first and second stages, respectively, with the liquid-to-solid (L/S) ratio being 50 L/kg in each stage. Approximately 10 g of the solid samples was mixed with 500 mL of water using a magnetic stirrer for 2 h in the first stage and for 7 h in the second stage. A sulfuric acid–nitric acid solution was employed to adjust the pH of the leaching system. Eluates from the two stages were filtered through a membrane filter, combined, and brought to a final volume of 2 L before analysis. The Cr(VI) and Cr(T) concentrations in the solution were determined according to the GB/T 15555.4-1995 and HJ 766-2015 methods, respectively. The maximum extractable content (MEC) of Cr(VI) or Cr(T) was calculated as follows:

$$C_{\mathrm{m}\mathrm{e}\mathrm{c}}={\displaystyle \frac{C_{\mathrm{a}}\times {\mathrm{L}\mathrm{S}}_1}{m_{\mathrm{s}}}}$$

(1)

where $C_{\mathrm{m}\mathrm{e}\mathrm{c}}$ is the MEC (mg/kg), $C_{\mathrm{a}}$ is the Cr(VI) or Cr(T) concentration in the final solution (mg/L), ${\mathrm{L}\mathrm{S}}_1$ is the volume of the final solution (L), and m_s is the weight of the solid sample used in the leaching test (kg).

Wipe sampling. Wipe sampling was conducted on the ACB and HCB samples according to the US Environmental Protection Agency (EPA) and ASTM methods [26,27]. Samples were collected by wiping the whole area of one brick twice using a 7.5 × 7.5 cm² gauze sheet. A maximum availability leaching test was performed on the gauze sheet after wipe sampling, and the surface concentration of Cr(VI) was calculated as follows:

$$C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}={\displaystyle \frac{C_{\mathrm{a}}\times {\mathrm{L}\mathrm{S}}_1}{{\mathrm{A}}_{\mathrm{b}\mathrm{r}}}}$$

(2)

where $C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}$ is the surface concentration of Cr(VI) in a refractory brick (mg/m²) and ${\mathrm{A}}_{\mathrm{b}\mathrm{r}}$ is the brick area (m²).

2.3. Health Risk Assessment

As shown in Figure 2, two methods were employed to investigate the human health risk of the four products. Method I addressed the risk of workers who may contact particles released by the products. The associated equations were adopted from the Technical Guidelines for Risk Assessment of Soil Contamination of Land for Construction (HJ 25.3-2019) [28,29], which is similar to the method used by the US EPA to create its Regional Screening Levels tables [30,31]. The considered exposure pathways were oral ingestion, dermal contact, and inhalation.

The carcinogenic and noncarcinogenic risks were calculated as follows:

$$\mathrm{C}{\mathrm{R}}_{\mathrm{o}1}={\mathrm{S}\mathrm{F}}_{\mathrm{o}}\times C_{\mathrm{t}}\times {\displaystyle \frac{\mathrm{O}{\mathrm{I}\mathrm{R}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {{\mathrm{A}\mathrm{B}\mathrm{S}}_{\mathrm{o}}\times \mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}}}\times {\displaystyle \frac{\mathrm{C}\mathrm{F}}{\mathrm{A}\mathrm{T}\mathrm{c}\mathrm{a}}}$$

(3)

$${\mathrm{C}\mathrm{R}}_{\mathrm{d}1}={\mathrm{S}\mathrm{F}}_{\mathrm{d}}\times C_{\mathrm{t}}\times {\displaystyle \frac{{\mathrm{A}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{S}\mathrm{A}}_{\mathrm{d}}\times {\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}\times \mathrm{E}\mathrm{v}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}}}\times {\displaystyle \frac{\mathrm{C}\mathrm{F}\times {\mathrm{A}\mathrm{B}\mathrm{S}}_{\mathrm{d}}}{\mathrm{A}\mathrm{T}\mathrm{c}\mathrm{a}}}$$

(4)

$$\mathrm{C}{\mathrm{R}}_{\mathrm{i}1}={\mathrm{S}\mathrm{F}}_{\mathrm{i}}\times C_{\mathrm{t}}\times {\displaystyle \frac{{\mathrm{I}\mathrm{R}\mathrm{A}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{F}\mathrm{I}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}\times \mathrm{P}\mathrm{M}\times \mathrm{P}\mathrm{I}\mathrm{A}\mathrm{F}\times \mathrm{f}\mathrm{s}\mathrm{p}\mathrm{i}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}}}\times {\displaystyle \frac{1}{\mathrm{A}\mathrm{T}\mathrm{c}\mathrm{a}}}$$

(5)

$${\mathrm{C}\mathrm{R}}_1={\mathrm{C}\mathrm{R}}_{\mathrm{o}1}+{\mathrm{C}\mathrm{R}}_{\mathrm{d}1}+\mathrm{C}{\mathrm{R}}_{\mathrm{i}1}$$

(6)

$${\mathrm{H}\mathrm{I}}_{\mathrm{o}1}=C_{\mathrm{t}}\times {\displaystyle \frac{{\mathrm{O}\mathrm{I}\mathrm{R}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {{\mathrm{A}\mathrm{B}\mathrm{S}}_{\mathrm{o}}\times \mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}}}\times {\displaystyle \frac{1}{\mathrm{A}\mathrm{T}\mathrm{n}\mathrm{c}}}\times {\displaystyle \frac{1}{{\mathrm{R}\mathrm{f}\mathrm{D}}_{\mathrm{o}}}}$$

(7)

$${\mathrm{H}\mathrm{I}}_{\mathrm{d}1}=C_{\mathrm{t}}\times {\displaystyle \frac{{\mathrm{A}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{S}\mathrm{A}}_{\mathrm{d}}\times {\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}\times \mathrm{E}\mathrm{v}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}}}\times {\displaystyle \frac{{\mathrm{A}\mathrm{B}\mathrm{S}}_{\mathrm{d}}}{\mathrm{A}\mathrm{T}\mathrm{n}\mathrm{c}}}\times {\displaystyle \frac{1}{{\mathrm{R}\mathrm{f}\mathrm{D}}_{\mathrm{d}}}}$$

(8)

$${\mathrm{H}\mathrm{I}}_{\mathrm{i}1}=C_{\mathrm{t}}\times {\displaystyle \frac{{\mathrm{I}\mathrm{R}\mathrm{A}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{F}\mathrm{I}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}\times \mathrm{P}\mathrm{M}\times \mathrm{P}\mathrm{I}\mathrm{A}\mathrm{F}\times \mathrm{f}\mathrm{s}\mathrm{p}\mathrm{i}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}}}\times {\displaystyle \frac{1}{\mathrm{A}\mathrm{T}\mathrm{n}\mathrm{c}}}\times {\displaystyle \frac{1}{{\mathrm{R}\mathrm{f}\mathrm{D}}_{\mathrm{i}}}}$$

(9)

$${\mathrm{H}\mathrm{I}}_1={\mathrm{H}\mathrm{I}}_{\mathrm{o}1}+{\mathrm{H}\mathrm{I}}_{\mathrm{d}1}+{\mathrm{H}\mathrm{I}}_{\mathrm{i}1}$$

(10)

where $\mathrm{C}{\mathrm{R}}_{\mathrm{o}1}$, ${\mathrm{C}\mathrm{R}}_{\mathrm{d}1}$, $\mathrm{C}{\mathrm{R}}_{\mathrm{i}1}$, and ${\mathrm{C}\mathrm{R}}_1$ are the carcinogenic risks of oral ingestion, dermal contact, and inhalation and the total carcinogenic risk under method I, respectively; ${\mathrm{H}\mathrm{I}}_{\mathrm{o}1}$, ${\mathrm{H}\mathrm{I}}_{\mathrm{d}1}$, ${\mathrm{H}\mathrm{I}}_{\mathrm{i}1}$, and ${\mathrm{H}\mathrm{I}}_1$ are the noncarcinogenic risks (hazard quotient, HQ) of oral ingestion, dermal contact, and inhalation and the total noncarcinogenic risk under method I, respectively; and $C_{\mathrm{t}}$ is the product’s total Cr(VI) content (mg/kg). The toxicity parameters are presented in

Table 1. Toxicity parameters of Cr(VI).

Parameter	Unit	Value
SFo	(mg/kg/d)−1	0.5
SFd	(mg/kg/d)−1	20
SFi	(mg/kg/d)−1	51.1
RfDo	mg/kg/d	0.003
RfDd	mg/kg/d	7.5 × 10−5
RfDi	mg/kg/d	2.35 × 10−5
ABSd	dimensionless	0

SF_o, SF_d, and SF_i are the carcinogenic slope factors for the exposure pathways of oral ingestion, dermal contact, and inhalation, respectively. RfD_o, RfD_d, and RfD_i are the reference doses for oral ingestion, dermal contact, and inhalation, respectively. ABS_d is the absorption factor for dermal contact.

[28,31], the other parameters are listed in

Table 2. Parameters for risk assessment method I.

Parameter	Definition and Unit	Value
EDa	Exposure duration of adults, a	25
EFa	Exposure frequency of adults, d/a	250
BWa	Body weight of adults, kg	61.8
IRAa	Daily air inhalation rate of adults, m3/d	14.5
OIRa	Oral ingestion rate of dusts of adults, mg/d	50
Ev	Contact frequency with surface, d−1	12
AFa	Adherence rate of soil on skin for adults, mg/m2	0.2
SAd	Dermal contact area, cm2	800
PM	Content of inhalable particulates in air, mg/m3	0.425
PIAF	Retention fraction of inhaled particulates in body, dimensionless	0.75
fspi	Workshop airborne particulate fraction, dimensionless	0.8
EFIa	Exposure frequency of workers to particles in a workshop, d/a	84
ABSo	Absorption factor of oral ingestion, dimensionless	1
CF	Convention factor, kg/mg	10−6
ATca	Time for carcinogenic effect, d	27,740
ATnc	Time for non-carcinogenic effect, d	9125

[28,32], with the values detailed in the Supplementary Materials.

Method II addressed the risk of workers who may contact the surfaces of the two refractory bricks. It involved equations from the studies by May et al. [33] and Cao et al. [34], which are as follows:

$${\mathrm{C}\mathrm{R}}_{\mathrm{o}2}={\mathrm{S}\mathrm{F}}_{\mathrm{o}}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}\times {\mathrm{S}\mathrm{A}}_{\mathrm{g}}\times {\mathrm{F}}_{\mathrm{g}}\times \mathrm{E}\mathrm{v}\times {\mathrm{F}\mathrm{T}}_{\mathrm{s}\mathrm{s}}\times {\mathrm{F}\mathrm{T}}_{\mathrm{s}\mathrm{m}}\times \mathrm{H}\mathrm{T}\mathrm{M}\mathrm{E}\times {\displaystyle \frac{{\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}\times {\mathrm{A}\mathrm{T}}_{\mathrm{c}\mathrm{a}}}}$$

(11)

$${\mathrm{C}\mathrm{R}}_{\mathrm{d}2}={\mathrm{S}\mathrm{F}}_{\mathrm{d}}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}\times {\mathrm{S}\mathrm{A}}_{\mathrm{d}}\times {\mathrm{F}}_{\mathrm{d}}\times \mathrm{E}\mathrm{v}\times {\mathrm{F}\mathrm{T}}_{\mathrm{s}\mathrm{s}}\times {\mathrm{A}\mathrm{B}\mathrm{S}}_{\mathrm{d}}\times {\displaystyle \frac{{\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}\times {\mathrm{A}\mathrm{T}}_{\mathrm{c}\mathrm{a}}}}$$

(12)

$${\mathrm{C}\mathrm{R}}_{\mathrm{i}2}={\mathrm{S}\mathrm{F}}_{\mathrm{i}}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}\times {\mathrm{I}\mathrm{R}\mathrm{A}}_{\mathrm{a}}\times \mathrm{K}\times {\displaystyle \frac{{\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}\times {\mathrm{A}\mathrm{T}}_{\mathrm{c}\mathrm{a}}}}$$

(13)

$${\mathrm{C}\mathrm{R}}_2={\mathrm{C}\mathrm{R}}_{\mathrm{o}2}+{\mathrm{C}\mathrm{R}}_{\mathrm{d}2}+\mathrm{C}{\mathrm{R}}_{\mathrm{i}2}$$

(14)

$${\mathrm{H}\mathrm{I}}_{\mathrm{o}2}={\mathrm{S}\mathrm{A}}_{\mathrm{g}}\times {\mathrm{F}}_{\mathrm{g}}\times \mathrm{E}\mathrm{v}\times {\mathrm{F}\mathrm{T}}_{\mathrm{s}\mathrm{s}}\times {\mathrm{F}\mathrm{T}}_{\mathrm{s}\mathrm{m}}\times \mathrm{H}\mathrm{T}\mathrm{M}\mathrm{E}\times {\displaystyle \frac{{\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}\times {\mathrm{A}\mathrm{T}}_{\mathrm{n}\mathrm{c}}}}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}\times {\displaystyle \frac{1}{{\mathrm{R}\mathrm{f}\mathrm{D}}_{\mathrm{o}}}}$$

(15)

$${\mathrm{H}\mathrm{I}}_{\mathrm{d}2}={\mathrm{S}\mathrm{A}}_{\mathrm{d}}\times {\mathrm{F}}_{\mathrm{d}}\times \mathrm{E}\mathrm{v}\times {\mathrm{F}\mathrm{T}}_{\mathrm{s}\mathrm{s}}\times {\mathrm{A}\mathrm{B}\mathrm{S}}_{\mathrm{d}}\times {\displaystyle \frac{{\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}\times {\mathrm{A}\mathrm{T}}_{\mathrm{n}\mathrm{c}}}}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}\times {\displaystyle \frac{1}{{\mathrm{R}\mathrm{f}\mathrm{D}}_{\mathrm{d}}}}$$

(16)

$${\mathrm{H}\mathrm{I}}_{\mathrm{i}2}={\mathrm{I}\mathrm{R}\mathrm{A}}_{\mathrm{a}}\times \mathrm{K}\times {\displaystyle \frac{{\mathrm{E}\mathrm{F}}_{\mathrm{a}}\times {\mathrm{E}\mathrm{D}}_{\mathrm{a}}}{{\mathrm{B}\mathrm{W}}_{\mathrm{a}}\times {\mathrm{A}\mathrm{T}}_{\mathrm{n}\mathrm{c}}}}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}\times {\displaystyle \frac{1}{{\mathrm{R}\mathrm{f}\mathrm{D}}_{\mathrm{i}}}}$$

(17)

$${\mathrm{H}\mathrm{I}}_2={\mathrm{H}\mathrm{I}}_{\mathrm{o}2}+{\mathrm{H}\mathrm{I}}_{\mathrm{d}2}+{\mathrm{H}\mathrm{I}}_{\mathrm{i}2}$$

(18)

where $\mathrm{C}{\mathrm{R}}_{\mathrm{o}2}$, ${\mathrm{C}\mathrm{R}}_{\mathrm{d}2}$, $\mathrm{C}{\mathrm{R}}_{\mathrm{i}2}$, and ${\mathrm{C}\mathrm{R}}_2$ are the carcinogenic risks of oral ingestion, dermal contact, and inhalation and the total carcinogenic risk under method II, respectively; ${\mathrm{H}\mathrm{I}}_{\mathrm{o}2}$, ${\mathrm{H}\mathrm{I}}_{\mathrm{d}2}$, ${\mathrm{H}\mathrm{I}}_{\mathrm{i}2}$, and ${\mathrm{H}\mathrm{I}}_2$ are the HQs of oral ingestion, dermal contact, and inhalation and the total noncarcinogenic risk under method II, respectively; and $C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}$ is the Cr(VI) concentration on the brick surfaces (mg/m²). The other parameters are listed in

Table 3. Parameters and values used in risk assessment method II.

Parameter	Definition and Unit	Value
SAd	Dermal contact area, m2	0.08
Fd	Fraction of available dermal area that contacts the surface, dimensionless	0.25
SAg	Dermal surface available for ingestion, m2	0.28
Fg	Fraction of available dermal area that contacts mouth, dimensionless	0.1
FTss	Fraction of dust transferred from surface to skin, dimensionless	0.1
FTsm	Fraction of dust transferred from skin to mouth, dimensionless	0.3
HTME	Hand to mouth events, dimensionless	3
K	Resuspension factor, m−1	5 × 10−8

, with the values detailed in the Supplementary Materials.

The risk-based Cr(VI) limits for the products were calculated as follows:

$${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{j}}=\min \left({\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{c}\mathrm{a},\mathrm{j}}{,\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{n}\mathrm{c},\mathrm{j}}\right) (\mathrm{j} = 1,2 \mathrm{f}\mathrm{o}\mathrm{r} \mathrm{M}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{d} \mathrm{I} \mathrm{o}\mathrm{r} \mathrm{I}\mathrm{I})$$

(19)

$${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{c}\mathrm{a},1}={\displaystyle \frac{\mathrm{A}\mathrm{C}\mathrm{R}\times C_{\mathrm{t}}}{{\mathrm{C}\mathrm{R}}_1}}{\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{c}\mathrm{a},2}={\displaystyle \frac{\mathrm{A}\mathrm{C}\mathrm{R}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}}{{\mathrm{C}\mathrm{R}}_2}}$$

(20)

$${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{n}\mathrm{c},1}={\displaystyle \frac{\mathrm{A}\mathrm{H}\mathrm{Q}\times C_{\mathrm{t}}}{{\mathrm{H}\mathrm{I}}_1}}{\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{n}\mathrm{c},2}={\displaystyle \frac{\mathrm{A}\mathrm{H}\mathrm{Q}\times C_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}}{{\mathrm{H}\mathrm{I}}_2}}$$

(21)

where ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},1}$ and ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},2}$ are the risk-based Cr(VI) limits obtained using method I (mg/kg) and method II (mg/m²), respectively; ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{c}\mathrm{a},1}$ and ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{c}\mathrm{a},2}$ are the carcinogenic-risk-based Cr(VI) limits obtained using method I and II, respectively; ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{n}\mathrm{c},1}$ and ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{n}\mathrm{c},2}$ are the noncarcinogenic-risk-based Cr(VI) limits calculated using method I and II, respectively; $\mathrm{A}\mathrm{C}\mathrm{R}$ is the acceptable carcinogenic risk level (10⁻⁵); and $\mathrm{A}\mathrm{H}\mathrm{Q}$ is the acceptable noncarcinogenic risk level (1).

2.4. Environmental Risk Assessment

The environmental risk of ACS utilization products arises from their contamination of groundwater when they are abandoned and disposed of in open dumps (Figure 2). The exposure concentration of Cr(VI) in groundwater was calculated and compared with the acceptable level: 0.05 mg/L [35]. The equations are as follows:

$$C_{\mathrm{g}\mathrm{w}}={\displaystyle \frac{C_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}}}{\mathrm{D}\mathrm{A}\mathrm{F}}}$$

(22)

$$C_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}}={\displaystyle \frac{C_{\mathrm{a}}\times {\mathrm{L}\mathrm{S}}_1}{{\mathrm{L}\mathrm{S}}_2}}$$

(23)

where $C_{\mathrm{g}\mathrm{w}}$ is the exposure concentration of Cr(VI) in groundwater (mg/L), $C_{\mathrm{l}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{h}}$ is the Cr(VI) concentration (mg/L) in a leaching system with an L/S ratio of 10 L/kg [36], and ${\mathrm{L}\mathrm{S}}_2$ is the volume (0.1 L) of the solution in a leaching system with an L/S ratio of 10 L/kg. $\mathrm{D}\mathrm{A}\mathrm{F}$ is the dilution-attenuation factor (dimensionless) [37]; it was set to 5.17, according to HJ 25.3-2019 [28], and the value is detailed in the Supplementary Materials.

2.5. Quality Control and Data Analysis

Detection analysis was performed in a commercially qualified laboratory. The data were analyzed using Microsoft Office Excel 2019, and data below the detection limit (DL) were replaced by DL/2 for calculation. Pearson correlation analysis was conducted using Origin 2021.

3. Results and Discussion

3.1. Heavy Metals in ACS and Utilization Products

The total contents and leaching concentrations of different heavy metals in the ACS are presented in Figure 3. The average Cr(T) content in the ACS samples was 4.12%, equivalent to 6.03% of Cr₂O₃. The Cr(VI) content was 111 mg/kg, which exceeded the risk intervention level for soil contamination in industrial sites in the GB 36600-2018 standard [38]. The Cr(VI) leaching concentration (obtained via HJ/T 299-2007, which is similar to the US EPA SPLP) was 7.8 mg/L, exceeding China’s hazardous waste identification standard of 5 mg/L for leaching toxicity [39]. These results were consistent with previously reported ones [1,5], indicating that ACS should be managed as a hazardous waste. The other heavy metals in the ACS—As, Hg, Cd, Pb, Ni, Cu, and Zn—were all in low levels. Their contents were much lower than the screening levels for the soil of residential land in GB 36600-2018, and their leaching concentrations were much lower than the standards for drinking water. Thus, Cr(VI) was the most concerning contaminant in the ACS.

The total contents and MECs of Cr(VI) and Cr(T) in the four ACS-derived products are presented in Figure 4. The Cr(VI) contents in the FAR and FC were below the DL (<1 mg/kg), indicating that the Cr(VI) in the ACS was effectively reduced by petroleum coal during smelting. The Cr(VI) contents in the ACB and HCB were 8.3 ± 1.0 and 14.5 ± 1.8 mg/kg (mean ± standard deviation), respectively. ACB and HCB were primarily produced using FAR, Cr₂O₃, and other ingredients; however, an amount of untreated ACS was used as well. The results indicated that the usage of ACS in the production of ACB and HCB should be controlled to avoid high Cr(VI) contents in the products.

The Cr(VI) MECs of the four products were all below DL (<0.8 mg/kg); 0.4 mg/kg is used in Figure 4. Although Cr(VI) was detected in the ACB and HCB, Cr(VI) was solidified/stabilized during brick production. Except in the case of the FAR, whose Cr(T) MEC was below the DL (0.4 mg/kg), the Cr(T) MECs were above the DL (0.6–3.4 mg/kg) and were less than 0.01% of the total Cr(T) contents. Yang et al. [40] established a set of limits of heavy metal MECs for cement products produced from solid wastes, which can be used to evaluate the environmental impacts of construction materials made from solid wastes. In their study, the Cr(T) MEC limits were 51 and 22 mg/kg for water supply pipeline and pavement scenarios, respectively, which were both higher than the MECs of the four analyzed products. Furthermore, the Cr(VI) MEC limits proposed by Yang et al. were 0.43 and 0.18 mg/kg, respectively, which were lower than the DL of the employed leaching method.

Surface concentrations of Cr(VI) and Cr(T) of ACB and HCB are shown in Figure 5. The surface concentrations of Cr(VI) in ACB and HCB were 0.039 ± 0.002 and 0.051 ± 0.007 mg/m² (mean ± standard deviation), respectively. Surface concentrations and contents of Cr(VI) of the two bricks showed a significant positive correlation (R² = 0.44, p < 0.05). Although wipe sampling was proposed a long time ago [41], only a few reports have been published on this method; they focus on polycyclic aromatic hydrocarbons and polychlorinated biphenyls [41,42]. The current study is among the limited works on Cr(VI) surface wiping results obtained from field-collected samples.

3.2. Human Health Risk of ACSUtilization Products

Equations (3)–(10) were employed to calculate the carcinogenic and noncarcinogenic risks of workers who may be exposed to ACS-derived products via particle contact. The Cr(VI) content results were used in the calculation process, and DL/2 was used for nondetectable values. The results are shown in

Table 4. Human health risk levels of ACS utilization products calculated using method I.

Products	Ct (mg/kg)	Carcinogenic Risk				Non-Carcinogenic Risk
		CRo1	CRd1	CRi1	CR1	HIo1	HId1	HIi1	HI1
FAR	1 *	9.11 × 10−8	0	2.32 × 10−7	3.23 × 10−7	1.85 × 10−4	0	5.87 × 10−4	7.72 × 10−4
FC	1 *	9.11 × 10−8	0	2.32 × 10−7	3.23 × 10−7	1.85 × 10−4	0	5.87 × 10−4	7.72 × 10−4
ACB	7 10 8.3 1.0	6.38 × 10−7 9.11 × 10−7 7.59 × 10−7 9.41 × 10−8	0 0 0 0	1.62 × 10−6 2.32 × 10−6 1.93 × 10−6 2.40 × 10−7	2.26 × 10−6 3.23 × 10−6 2.69 × 10−6 3.34 × 10−7	1.30 × 10−3 1.85 × 10−3 1.54 × 10−3 1.91 × 10−4	0 0 0 0	4.11 × 10−3 5.87 × 10−3 4.89 × 10−3 6.06 × 10−4	5.40 × 10−3 7.72 × 10−3 6.43 × 10−3 7.97 × 10−4
HCB	12 16 14.5 1.8	1.10 × 10−6 1.46 × 10−6 1.32 × 10−6 1.61 × 10−7	0 0 0 0	2.78 × 10−6 3.71 × 10−6 3.36 × 10−6 4.08 × 10−7	3.87 × 10−6 5.16 × 10−6 4.68 × 10−6 5.68 × 10−7	2.22 × 10−3 2.96 × 10−3 2.68 × 10−3 3.26 × 10−4	0 0 0 0	7.04 × 10−3 9.39 × 10−3 8.51 × 10−3 1.03 × 10−3	9.26 × 10−3 1.24 × 10−2 1.12 × 10−2 1.36 × 10−3

The asterisk (*) indicates that DL/2 was used. For numbers for ACB and HCB are the minimum, maximum, mean, and standard deviation.

. ${\mathrm{C}\mathrm{R}}_1$ and ${\mathrm{H}\mathrm{I}}_1$ varied from 3.23 × 10⁻⁷ to 5.16 × 10⁻⁶ and from 7.72 × 10⁻⁴ to 1.23 × 10⁻², respectively. The HCB showed the highest risk level, and the FAR and FC showed the lowest. The human health risk levels of the ACB and HCB calculated using the surface Cr(VI) concentrations from method II are shown in

Table 5. Human health risk levels of refractory bricks produced from ACS calculated using method II.

Products	Csurf (mg/m2)	Carcinogenic Risk				Non-Carcinogenic Risk
		CRo2	CRd2	CRi2	CR2	HIo2	HId2	HIi2	HI2
ACB	0.035	1.85 × 10−6	0	4.54 × 10−9	1.86 × 10−6	3.76 × 10−3	0	1.15 × 10−5	3.77 × 10−3
	0.041	2.17 × 10−6	0	5.32 × 10−9	2.18 × 10−6	4.40 × 10−3	0	1.35 × 10−5	4.41 × 10−3
	0.039	2.04 × 10−6	0	5.00 × 10−9	2.04 × 10−6	4.13 × 10−3	0	1.27 × 10−5	4.14 × 10−3
	0.002	1.10 × 10−7	0	2.69 × 10−10	1.10 × 10−7	2.23 × 10−4	0	6.83 × 10−7	2.23 × 10−4
HCB	0.042	2.22 × 10−6	0	5.45 × 10−9	2.23 × 10−6	4.50 × 10−3	0	1.38 × 10−5	4.52 × 10−3
	0.061	3.23 × 10−6	0	7.92 × 10−9	3.24 × 10−6	6.54 × 10−3	0	2.01 × 10−5	6.56 × 10−3
	0.051	2.72 × 10−6	0	6.66 × 10−9	2.72 × 10−6	5.50 × 10−3	0	1.69 × 10−5	5.52 × 10−3
	0.007	3.92 × 10−7	0	9.60 × 10−10	3.92 × 10−7	7.93 × 10−4	0	2.44 × 10−6	7.95 × 10−4

For numbers for ACB and HCB are the minimum, maximum, mean, and standard deviation.

. The ${\mathrm{C}\mathrm{R}}_2$ values varied from 1.86 × 10⁻⁶ to 2.18 × 10⁻⁶ and from 2.23 × 10⁻⁶ to 3.24 × 10⁻⁶, while the ${\mathrm{H}\mathrm{I}}_2$ values varied from 3.77 × 10⁻³ to 4.41 × 10⁻³ and from 4.52 × 10⁻³ to 6.56 × 10⁻³ for the ACB and HCB, respectively. All ${\mathrm{C}\mathrm{R}}_1$, ${\mathrm{C}\mathrm{R}}_2$, ${\mathrm{H}\mathrm{I}}_1$, and ${\mathrm{H}\mathrm{I}}_2$ values were below the acceptable risk levels (10⁻⁵ and 1), indicating that the four products have acceptable human health risks under both risk assessment methods.

For noncarcinogenic risk, an HQ value of 1 is widely adopted as the acceptable threshold in numerous studies [16]. Regarding carcinogenic risk, risks below 10⁻⁶ are considered negligible, and carcinogens between 10⁻⁴ and 10⁻⁶ are deemed acceptable [30]. In this study, a risk level of 10⁻⁵ was selected, which is higher than the recommended value (10⁻⁶) in HJ 25.3-2019 [28], yet lower than the threshold commonly applied in occupational health settings (10⁻⁴) [43].

The carcinogenic effect of Cr(VI) was more dominant than the noncarcinogenic effect. The contributions of the exposure pathways of oral ingestion, dermal contact, and inhalation to the carcinogenic risk were 28.2%, 0%, and 71.8%, respectively, under method I and 99.8%, 0%, and 0.2%, respectively, under method II. Having ABS_d = 0, Cr(VI) posed no risk effect through the dermal contact pathway. However, hand-to-mouth movements could facilitate oral ingestion through dermal contact.

Sensitivity analysis was performed to assess the impact of model parameters on health risk characterization. Sensitivity analysis is usually conducted on the parameters of risk assessment models by calculating sensitivity rates (SRs) [44]. The SRs of the two risk assessment methods are presented in Figure 6. ATca, EFIa, fspi, PIAF, PM, and EDa were the key parameters of method I; in other words, the absolute values of their SRs exceeded 50%. Meanwhile, ATca, BWa, EDa, EFa, HTME, FTsm, FTss, Fg, SAg, and Ev were the key parameters of method II. Therefore, the particle matter in workshops and hand-to-mouth movements should be reduced to minimize human health risks for workers.

3.3. Environmental Risk of ACS Utilization Products

Contaminants may undergo a complex process involving degradation, attenuation, and dilution from their release from solid wastes or contaminated soil to their mixing with groundwater [17]. The use of leaching concentrations obtained from conservative leaching tests and the DAF [30] is a simple preliminary screening method for evaluating environmental risks. Equations (22) and (23) were employed to investigate the environmental risk of the products under the assumption that they were disposed of in an informal landfill and Cr(VI) would be transported into the associated groundwater. The Cr(VI) leaching concentrations in the four products in the maximum availability leaching test were all under the DL (0.004 mg/L). A value of 0.002 mg/L was used in Equation (23), and the ${\mathrm{C}}_{\mathrm{g}\mathrm{w}}$ of the products was 0.008 mg/L, which was below the drinking water standard (0.05 mg/L). According to the same equations, the ${\mathrm{C}}_{\mathrm{g}\mathrm{w}}$ of the ACS was 1.51 mg/L, indicating that Cr(VI) conversion into Cr(III) is essential for safe ACS utilization.

3.4. Derivation of Limits for Cr(VI) in Products

Considering that the efficiency of Cr(VI) reduction may vary during manufacturing and raw ACS may be used to produce ACBs and HCBs, limits should be established for Cr(VI) in products. According to Equations (19)–(21), ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{c}\mathrm{a},1}$ and ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{n}\mathrm{c},1}$ were 31 and 1300 mg/kg, respectively, and ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{c}\mathrm{a},2}$ and ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},\mathrm{n}\mathrm{c},2}$ were 0.189 and 9.3 mg/m², respectively; therefore, ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},1}$ and ${\mathrm{L}\mathrm{L}}_{\mathrm{C}\mathrm{r},2}$ were 31 mg/kg and 0.189 mg/m², respectively. The Cr(VI) leaching concentration limit was 0.259 mg/L, which could be simply calculated by multiplying the drinking water standard (0.05 mg/L) with the DAF (5.17). The Cr(VI) limits for ACS products, as derived through risk assessment methods, are summarized in

Table 6. Cr(VI) concentration limits derived using risk assessment models.

Type of Cr(VI) Limit for Utilization Products	Value
Limit of Cr(VI) content in product (mg/kg)	31
Limit of Cr(VI) concentration on surface of refractory brick (mg/m2)	0.189
Limit of Cr(VI) leaching concentration in product (mg/L)	0.259

. These values can serve as a reference for controlling Cr(VI) levels in ACS-derived products and aid in assessing the potential health risks to workers who handle similar materials.

The study did not account for potential variations in Cr(VI) content within the products under extreme service conditions—such as high temperature and high corrosivity—that may occur during their operational lifespan. Further investigation will be valuable to evaluate changes in Cr(VI) levels in ACS-derived products under typical service conditions. More comprehensive data would then allow for the establishment of robust Cr(VI) concentration limits.

Risk assessment and the screening levels based on its results provide a powerful tool for environmental management, thereby promoting the sustainable utilization of solid waste. ACS-derived refractory materials are typically used either as raw materials for other refractory products, or as linings for industrial furnaces and kilns. The primary exposure scenarios of concern include occupational risks for workers handling these products and environmental risks following their end of use. Several studies have evaluated the environmental risks of solid waste-derived products through various leaching tests. For example, Gao et al. [45] and Wang et al. [46] investigated the leaching behavior of bricks manufactured with municipal solid waste incineration fly ash, comparing the leaching concentrations to municipal landfill entrance limits, which are obviously less stringent than groundwater quality standards. This assessment strategy may not be directly applicable to other types of solid waste-derived products. Other studies have employed health risk assessment model to evaluate the carcinogenic risks and noncarcinogenic HQs of such products [16,47]. However, these studies often applied default models and parameters designed for contaminated soils without necessary modifications. In contrast, this study conducted a comprehensive risk assessment of the ACS-derived products by adapting the models for health risk assessment of contaminated site and surface contact with industrial equipment, as well as models for evaluating groundwater contamination potential. Furthermore, the assessment parameters were customized to more accurately represent real exposure scenarios, thereby enhancing the applicability of the assessment results.

The findings of this study also suggest the measures to mitigate the environmental risks of the ACS-derived products. As indicated in 3.1, Cr(VI) present in ACB and HCB was most likely introduced through the use of untreated ACS. Therefore, reducing or eliminating the use of raw ACS during ACB and HCB production could decrease Cr(VI) levels in the final products. Additionally, personal protective equipment such as gloves and masks can help minimize the hand-to-mouth contact and inhalation exposure, which represent critical pathways for occupational health risks related to ACB-derived products.

4. Conclusions

Herein, we analyzed the heavy metal contents in ACS and products derived from it (refractory materials). A risk assessment framework was established to evaluate the human health and environmental risk levels of the ACS-derived products.

The Cr(VI) content in the ACS was 111 mg/kg, with leaching concentration (7.8 mg/L) exceeding China’s hazardous waste standard, but the proportions of other heavy metals were low. The Cr(VI) contents of the utilized products were significantly low (from <2 to 16 mg/kg), with the corresponding maximum leaching concentration being below the DL (<0.004 mg/L). The Cr(VI) surface concentrations of the ACB and HCB were 0.039 ± 0.002 and 0.051 ± 0.007 mg/m² (mean ± standard deviation), respectively. The human health risk levels of the four products under the two risk assessment methods were acceptable. The carcinogenic effect of Cr(VI) was more dominant than the noncarcinogenic effect. Oral ingestion and inhalation were the exposure pathways contributing health risks. The sensitive parameters of the risk assessment models were identified by calculating SRs.

Environmental risk assessments showed that the predicted concentration of leached Cr(VI) in groundwater (0.008 mg/L) was below the drinking water standard (0.05 mg/L). Then, Cr(VI) limits (≤31 mg/kg content, ≤0.189 mg/m² surface concentration, and ≤0.259 mg/L leaching concentration) for the products were proposed based on the performed risk assessment. Overall, these results provide a scientific basis for the safe utilization and risk management of ACS and its derived products as well as a reference for the managing other solid wastes.