Evaluation of Heavy Metal Contamination and Associated Human Health Risk in Soils around a Battery Industrial Zone in Henan Province, Central China

Abstract

This research investigated the contamination characteristics, sources, and health risks of five metals in soils from two villages named DK and SXC, downstream from a battery industry hub in Xinxiang city, Henan Province, China. The average concentrations of Cd, Pb, Ni, Cu, and Zn in DK were 5.93, 41.31, 71.40, 62.20, and 115.83 mg/kg, respectively, and in SXC were 2.04, 30.41, 41.22, 36.18, and 96.04 mg/kg, respectively. The single factor pollution index (P_i) revealed a consistent descending order of Cd > Cu > Zn > Ni > Pb in DK and SXC. The geo-accumulation index (I_geo) indicated that the Cd pollution in DK was extreme, and in SXC was at a heavy to extreme level. The potential ecological risk index (PERI) indicated that Cd presented a significantly high ecological risk while it was low for other metals. Principal component analysis classified them into the anthropogenic origin of Cd and common mixed origin of others. The elevated levels and pollution load of heavy metals with closer proximity to the battery factory imply that the factory is a probable source of contamination. Overall, the health risks posed by heavy metals were more pronounced for local children compared to adults, with Cd being the primary contributor to both pollution and health risks. This investigation provides a crucial basis for the heavy metal pollution management and related risk prevention in areas affected by electronic waste irrigation.

1. Introduction

Soil contamination by heavy metals is a global issue of significant concern due to heavy metal bioaccumulation, toxicity, persistence, and the potential risks that they pose to food security, human health, and ecosystems [1,2,3,4]. Soils are commonly perceived as persistent repositories for heavy metals, with those in suburban areas being particularly susceptible to sewage irrigation, industrial operations, vehicular emissions, and the disposal of municipal waste [5,6,7]. There have been numerous reports on soil heavy metal contamination in city suburbs or in the vicinity of industrial areas [1,5,8,9,10,11,12]. Yang et al. (2022) [5] reported that sewage irrigation led to the significant contamination of vineyard soil in the suburbs of Kaifeng city, with particularly high levels of heavy metals, notably cadmium (Cd) and zinc (Zn).

The pollution status, source identification, and risk assessment of heavy metals in contaminated soils have been comprehensively investigated [7,8,9,13,14]. Various assessment methods, including the pollution index (P_i), geo-accumulation index (I_geo), and potential ecological risk index (PERI), are extensively employed to evaluate the contamination levels of heavy metals in soil. Xiao et al. (2015) [7] observed that surface soil samples from the steel industrial area in Anshan city, Liaoning, China had moderate to high levels of Cd, Zn, copper (Cu), and lead (Pb) pollution, resulting in significant ecological risks, but were practically unpolluted by nickel (Ni). The primary contributors to the presence of these metals were identified as industrial emissions and vehicular activities. Soil contamination by heavy metals has become a critical global issue of concern that requires urgent resolution due to its implications for human health [1,5,8,15].

Heavy metals present in agricultural soils can be transferred to humans via ingestion, dermal absorption, inhalation [9,13,16], or through the food chain [5], either directly or indirectly, which eventually results in substantial negative impacts on human health. Per the carcinogen classification by the World Health Organization’s International Agency for Research on Cancer (IARC), Cd is classified as carcinogenic to humans, while Pb is considered possibly carcinogenic, potentially leading to a spectrum of pathological alterations across various organs [15]. Cd predominantly accumulates in the kidneys and liver [17], and chronic exposure to Cd can result in bone demineralization, damage to the intestines, as well as cardiovascular and reproductive issues [18,19]. Elevated levels of Pb in the bloodstream can result in impaired psychomotor development, characterized by a reduction in intelligence quotient, particularly in children [17]. Other heavy metals, such as Zn, can cause gastrointestinal disorders, sleep disorders, as well as the damage of biochemical processes [17,20]. The potential exposure health risk assessment is an invaluable tool for the thorough evaluation of the detrimental impacts of heavy metals across various environmental media, facilitating the formulation of environmentally sustainable policies [9].

Xinxiang city holds the distinction of being the principal hub for the battery industry and a leading center for new energy battery and material manufacturing in central China, earning it the reputation as a city synonymous with the battery industry. Over the past 50 or 60 years, more than 200 battery companies have gathered here. The production and processing activities of battery materials, such as nickel powder, cadmium plate as electrode material, and electrolytic waste containing cadmium, have for years discharged large amounts of industrial wastes into the environment, resulting in the soils in the vicinity of the battery enterprise cluster in Xinxiang city being impacted to varying extents by the accumulation of heavy metals. Under the pressure of environmental protection, local battery production has also been fully transformed from highly polluting nickel–cadmium batteries to more environmentally friendly lithium batteries. The city also houses various types of highly polluting industries including featured equipment manufacturing, coal chemical industry, automobile and parts, etc. The soil environment and nearby residents in the vicinity of the battery industry cluster may be facing serious threats from heavy metal contamination. Nevertheless, investigations into the characteristics of soil heavy metal pollution and its associated health risks, particularly for children, in agricultural regions at different proximities to the battery industry cluster, are scarcely documented. Consequently, soil samples were obtained from two villages situated at varying distances from the battery industrial area. The objectives of this study were: (1) to investigate the accumulation and contamination levels of soil heavy metals (Cd, Pb, Ni, Cu, and Zn); (2) to ascertain the potential sources of these heavy metals through multivariate analysis; and (3) to evaluate the potential ecological and health risks of these heavy metals to local residents.

2. Materials and Methods

2.1. Study Area and Soil Sampling

The research area is situated in the northern suburbs of Xinxiang city, northern Henan Province, China, as depicted in Figure 1. This locality is part of the Haihe River Basin and lies within the Warm Temperate Zone, characterized by a continental monsoon climate. The region experiences an average annual temperature of 14.0 °C and receives an average annual rainfall of 573.4 mm. The soil under investigation was identified as Fluvisol, according to the International Union of Soil Sciences Working Group WRB (2015) [21], originating from alluvial deposits left by flooding from the Yellow River.

Xinxiang city is recognized as the epicenter of China’s battery industry, hosting a significant congregation of battery manufacturing entities. Historically, effluents from these battery facilities were directly discharged into the river system, precipitating severe contamination of both water and sediment with heavy metals. Such practices in the past facilitated the infiltration or introduction of heavy metals into adjacent agricultural lands, as documented by Wang et al. (2021a,b) [22,23] and Li et al. (2022) [24]. Part of the traditional farmland in the vicinity of the battery industry cluster was gradually converted into orchards and woodlands in recent years. The present investigation concentrated on two villages, DK and SXC, located downstream from the battery industrial zone. The proximities of the central areas of DK and SXC to the battery facility are approximately 5 and 18 km, respectively.

In this study, a cumulative total of 26 topsoil samples were gathered, comprising 17 from woodland soils in DK and 19 from farmland soils in SXC, utilizing a core drill. Each sample represented a composite derived from five sub-samples collected via the quincunx sampling method. For each sampling location, approximately 1.0 kg of soil samples (0–20 cm depth) was collected and subsequently stored in polyethylene bags, which were then appropriately labeled and transported to the laboratory for analysis. Upon arrival, all soil samples underwent air-drying at room temperature, followed by pulverization and sieving through a 2 mm sieve and then a 0.149 mm sieve for the determination of soil pH and heavy metal content.

2.2. Chemical Analysis and Quality Control

The soil pH was determined in a soil-to-water mixture (1:2.5, w/v) using a pH meter (PHS-3E, Shanghai, China). The digestion of soil samples was conducted in triplicate utilizing a concentrated mixture of acids (HNO₃, HF, and HClO₄) on a hotplate, adhering to the method outlined by Lu (2000) [25]. The quantification of Cu, Ni, Zn, Pb, and Cd within the digested solutions was performed using flame atomic absorption spectroscopy (PinAAcle 900T, Shelton, CT, USA), and each digestion solution was measured three times. To verify the reliability and accuracy of the experimental outcomes, a national standard soil sample [26] was integrated within the digestion and analysis stages, yielding recovery rates of between 88% and 113%.

2.3. Pollution Assessment Methods

The evaluation of soil heavy metal contamination levels was conducted using several indices, including the single factor pollution index (P_i), Nemerow integrated pollution index (P_N), geo-accumulation index (I_geo), and potential ecological risk index (PERI), with each assessment method detailed in the Supplementary Material (SI). The classifications for P_i and P_N, along with the categorization of potential ecological risk levels, follow the guidelines in Zang et al. (2017) [14], Yang et al. (2022) [5], and Xie et al. (2022) [15], as outlined in Table S1 [5,14,15,27,28,29,30,31,32,33].

2.4. Health Risk Assessment

The assessment of the carcinogenic and non-carcinogenic risks to human health from soil heavy metals was conducted following the guidelines provided by the USEPA (2011) [34]. As classified by the IARC of the WHO, Cd is categorized as a class 1 carcinogen, and Pb as a class 2B carcinogen, both of which may cause cancer in human beings. The average daily doses (ADDs, mg·kg⁻¹·day⁻¹) of a pollutant through oral ingestion, dermal absorption, and inhalation were determined by employing Equations (1)–(3):

$$AD{D}_{ingest}=\frac{C_{soil}\times IngR\times EF\times ED}{BW\times AT}\times CF$$

(1)

$$AD{D}_{dermal}=\frac{C_{soil}\times SA\times AF\times ABS\times EF\times ED}{BW\times AT}\times CF$$

(2)

$$AD{D}_{inhale}=\frac{C_{soil}\times InhR\times EF\times ED}{PEF\times BW\times AT}$$

(3)

where C_soil is the concentration of the target contaminant in soil (mg·kg⁻¹), and the details of exposure parameters and their values are presented in Table S2 [5,13,15,16,34,35,36,37].

The calculation of the non-carcinogenic ((4)–(7)) and carcinogenic ((8)–(11)) risk assessment is listed below:

$$H{Q}_{ing}=\frac{AD{D}_{ing}}{Rf{D}_o}$$

(4)

$$H{Q}_{der}=\frac{AD{D}_{der}}{Rf{D}_o\times GIABS}$$

(5)

$$H{Q}_{inh}=\frac{AD{D}_{inh}}{Rf{C}_i}$$

(6)

$$HI={\displaystyle \sum HQ}$$

(7)

$$C{R}_{ing}=AD{D}_{ing}\times S{F}_o$$

(8)

$$C{R}_{der}=AD{D}_{der}\times \frac{S{F}_o}{GIABS}$$

(9)

$$C{R}_{inh}=AD{D}_{inh}\times IUR$$

(10)

$$CR={\displaystyle \sum C{R}_i}$$

(11)

Table S3 [5,13,15,35,38,39] presents the reference doses and slope factors for soil heavy metals, detailing their values through ingestion, dermal contact, and inhalation pathways.

2.5. Statistical Analysis

Statistical analysis was conducted utilizing SPSS software (version 16.0; SPSS Inc., Chicago, IL, USA) and Microsoft Excel 2010. The Kolmogorov–Smirnov (K-S) test, employed through SPSS 16.0, assessed the normality of the data set (p < 0.05). Furthermore, principal component analysis (PCA) was utilized to explore the associations among metals. In the PCA, varimax rotation with Kaiser normalization was applied to enhance the interpretability. Additionally, the geographic distribution of sampling sites was illustrated using ArcGIS 10.0 software.

3. Results

3.1. Soil Heavy Metal Concentrations

Table 1. Descriptive statistics of heavy metal concentrations and selected soil properties in soils.

	Parameter	Cd a	Pb a	Ni a	Cu a	Zn a	pH
DK (n = 17)	Average	5.93	41.31	71.40	62.20	115.83	7.94
	Minimum	3.83	36.44	63.29	43.67	106.26	7.59
	Maximum	8.09	47.31	82.12	77.73	135.09	8.58
	S.D. b	1.42	4.00	6.56	14.62	10.03	0.32
	C.V. c	24.00	9.68	9.19	23.51	8.66	4.02
SXC (n = 19)	Average	2.04	30.41	41.22	36.18	96.04	8.01
	Minimum	1.60	24.61	25.69	19.73	56.80	7.53
	Maximum	2.73	34.11	59.10	52.05	127.90	8.45
	S.D. b	0.31	2.77	9.15	9.14	21.45	0.25
	C.V. c	15.24	9.11	22.19	25.25	22.33	3.14
Reference standard	Background values d	0.063	19.7	28.0	21.4	65.1
	Soil risk screening values e	0.6	170	190	100	300

^a mg kg⁻¹; ^b Standard deviation; ^c Coefficients of variation (%); ^d [31]; ^e [40].

illustrates the descriptive statistical analysis of heavy metal concentrations and pH values within the agricultural soils from DK and SXC. The soil pH ranged from 7.59 to 8.58 and 7.53 to 8.45, with average values of 7.94 and 8.01 in DK and SXC, respectively, which showed that the soils in this study area were weakly alkaline. In DK, the concentrations of Cd, Pb, Ni, Cu, and Zn varied from 3.83 to 8.09, 36.44 to 47.31, 63.29 to 82.12, 43.67 to 77.73, and 106.26 to 135.09 mg·kg⁻¹, with average concentrations recorded at 5.93, 41.31, 71.40, 62.20, and 115.83 mg·kg⁻¹, respectively. Conversely, the concentration ranges for Cd, Pb, Ni, Cu, and Zn in the SXC soil samples were observed as 1.60–2.73, 24.61–34.11, 25.69–59.10, 19.73–52.05, and 56.80–127.90 mg·kg⁻¹, with their respective average concentrations being 2.04, 30.41, 41.22, 36.18, and 96.04 mg·kg⁻¹, respectively.

The levels of Pb, Ni, Cu, and Zn in the soil samples from both the DK and SXC villages were found to be within the risk screening values set by the National Soil Environmental Quality Standard of China [40] for farmland soils, as outlined by the Ministry of Ecology and Environment of China and the State Administration for Market Regulation (2018) [40]. In contrast, the concentrations of Cd at all tested sites in DK and SXC surpassed the stipulated risk screening value, indicating that the battery manufacturing plant may have played a significant role in the contamination of soil with Cd.

3.2. Pollution Assessment of Soil Metals

In this investigation, the assessment of heavy metal contamination in soil was conducted using the pollution index (P_i and P_N), geo-accumulation index (I_geo), and potential ecological risk index (PERI), with the findings presented in

Table 2. The P_i, I_geo, and $E_r^i$ of soil heavy metals in DK and SXC.

Sample Sites	Elements	Pi			Igeo			${\mathit{E}}_{\mathit{r}}^{\mathit{i}}$
		Range	Average	S.D.	Range	Average	S.D.	Range	Average	S.D.
DK (n = 17)	Cd	6.385–13.477	9.877	2.194	5.341–6.419	5.934	0.331	1824.38–3850.51	2821.99	626.83
	Pb	0.214–0.278	0.243	0.022	0.302–0.679	0.478	0.128	9.249–12.008	10.485	0.939
	Ni	0.333–0.432	0.376	0.032	0.591–0.967	0.760	0.121	11.301–14.665	12.750	1.085
	Cu	0.437–0.777	0.622	0.135	0.444–1.276	0.918	0.330	10.202–18.162	14.532	3.163
	Zn	0.354–0.450	0.386	0.031	0.122–0.468	0.242	0.111	1.632–2.075	1.779	0.143
SXC (n = 19)	Cd	2.667–4.554	3.401	0.504	4.082–4.854	4.418	0.205	761.90–1301.27	971.77	144.010
	Pb	0.145–0.201	0.179	0.016	−0.264–0.207	0.036	0.132	6.246–8.659	7.719	0.685
	Ni	0.135–0.311	0.217	0.047	−0.709–0.493	−0.062	0.320	4.587–10.553	7.361	1.590
	Cu	0.197–0.521	0.362	0.089	−0.702–0.697	0.123	0.391	4.610–12.161	8.454	2.078
	Zn	0.189–0.427	0.320	0.070	−0.781–0.390	−0.063	0.347	0.873–1.965	1.475	0.321

. The P_i for each metal, calculated in accordance with the soil risk screening values for soil contamination on agricultural land in China (GB 15618-2018, pH > 7.5), showed a significant variation among the metals. In general, the order of P_i values for the metals across soil samples from both DK and SXC was Cd > Cu > Zn > Ni > Pb, indicating a greater contamination by Cd. The average P_i values for these metals were higher in the soils from DK than in those from SXC, pointing to a more severe accumulation of heavy metals in DK. For Cu, Zn, Ni, and Pb, the P_i values were all below 1.0 in both the DK and SXC soils, classifying these metals at the unpolluted level. However, the P_i values for Cd were notably higher, ranging from 6.39 to 13.48 in DK and 2.67 to 4.55 in SXC, with average values of 9.88 and 3.40, respectively.

Nemerow integrated pollution index (P_N) analysis demonstrated that the P_N values for the soil samples varied between 4.65 and 9.76 for DK and 1.94 and 3.32 for SXC, with average values of 7.17 for DK and 2.49 for SXC, respectively. This categorization placed all the DK soil samples within the heavily polluted range. In contrast, for SXC, the distribution of soil samples across the pollution categories was as follows: 5.26% slightly polluted, 84.21% moderately polluted, and 10.53% heavily polluted.

Table 2 presents the calculated values of the I_geo for heavy metals in the soil samples from DK and SXC. According to these results, the average I_geo values for all metals in DK followed a descending order of Cd > Cu > Ni > Pb > Zn, whereas for SXC, the order was Cd > Cu > Pb > Ni > Zn. In DK, the I_geo range for Cd (5.34–6.42) indicated a status of extreme pollution. For Zn, Ni, and Pb, the I_geo values suggested a spectrum from unpolluted to moderate pollution, and Cu exhibited values within the moderately (57.14%) or unpolluted to moderately (42.86%) polluted spectrum. Conversely, in SXC, the I_geo value for Cd fell within the heavily to extremely polluted range, while the values for Cu, Zn, Ni, and Pb were all below 1.0, indicating that these metals did not surpass the unpolluted to moderately polluted threshold.

In this analysis, the potential ecological risk coefficients ($E_r^i$) for the five metals in DK followed a sequence of Cd > Cu > Ni > Pb > Zn, whereas in SXC, the sequence was Cd > Cu > Pb > Ni > Zn. Across all the sampling locations within the study region, the ecological risk index for Cd was categorized as high ($E_r^i$) > 760), underscoring the significant danger that Cd presents to both human health and ecosystems. Table 2 reveals that the potential ecological risk coefficients for Cu, Zn, Ni, and Pb were all below 30, signifying a low level of ecological risk.

3.3. Source Analysis of Hazardous Elements in Soil

Through this analysis, two principal components with eigenvalues exceeding 1 were identified (refer to

Table 3. Total variance explained and the component matrixes for heavy metals in soil.

Total Variance Explained
Component	Initial Eigenvalues			Extraction Sums of Squared Loadings a			Rotation Sums of Squared Loadings b
	Total	% of Variance	Cumulative (%)	Total	% of Variance	Cumulative (%)	Total	% of Variance	Cumulative (%)
1	4.068	81.361	81.361	4.068	81.361	81.361	2.658	53.152	53.152
2	0.643	12.867	94.229	0.643	12.867	94.229	2.054	41.077	94.229
3	0.188	3.767	97.995
4	0.060	1.205	99.200
5	0.040	0.800	100.000
Component matrix					Rotated component matrix
Element	PC1		PC2		Element	PC1		PC2
Cd	0.783		0.605		Cd	0.213		0.967
Pb	0.970		0.045		Pb	0.715		0.657
Zn	0.836		−0.511		Zn	0.969		0.145
Cu	0.950		−0.117		Cu	0.803		0.520
Ni	0.955		0.021		Ni	0.719		0.629

^a Extraction method: principal component analysis. ^b Rotation method: varimax with Kaiser normalization.

and Figure 2), cumulatively accounting for 94.23% of the overall variance. The rotation of the component matrix revealed that the first principal component (PC1), which accounted for the largest proportion of variance at 53.15%, exhibited strong correlations with Pb, Cu, Ni, and Zn. Conversely, the second principal component (PC2) was responsible for 41.08% of the variance and was exclusively associated with Cd.

3.4. Health Risk Assessment

The assessment of health risks associated with long-term exposure to potentially toxic heavy metals in soil for both adults and children is detailed in Figure 3 and Figure 4. For adults and children in DK, the non-carcinogenic risk indices (HI) spanned 0.287–0.526 and 1.835–3.318, respectively, with average HI values calculated at 0.410 for adults and 2.600 for children. Meanwhile, in SXC, the HI values for adults and children ranged from 0.134 to 0.210 and 0.865 to 1.343, respectively, with average values of 0.169 for adults and 1.088 for children. The calculated total carcinogenic risk (CR) for adults and children in DK ranged from 2.55 × 10⁻⁵ to 5.36 × 10⁻⁵ and 4.64 × 10⁻⁵ to 9.75 × 10⁻⁵, respectively. In SXC, these values ranged from 1.07 × 10⁻⁵ to 1.82 × 10⁻⁵ for adults and from 1.95 × 10⁻⁵ to 3.31 × 10⁻⁵ for children.

4. Discussion

The highest concentrations for these five heavy metals were all detected in the samples from DK, which is situated nearer to the battery manufacturing site, indicating a more pronounced accumulation of heavy metals in DK compared to SXC. The variation coefficient (CV) for these metals in DK soils demonstrated a descending sequence of Cd > Cu > Pb > Ni > Zn (Table 1). The significant CVs for Cd and Cu suggest considerable variability across different locations and highlight the substantial human-induced disturbances, as supported by the findings of Zang et al. (2017) [14] and Yang et al. (2022) [5]. As detailed in Table 1, the levels of all heavy metals in the DK soil samples exceeded their respective background values, as established by the Soil Survey Office of Henan Province (2004) [31]. Furthermore, the percentages of SXC soil samples exceeding the background concentrations for heavy metals were as follows: 100% for Cd, 100% for Pb, approximately 89.47% for both Ni and Cu, and 84.21% for Zn. This pattern underscores a more pronounced accumulation of Cd and Pb compared to Ni, Cu, and Zn. This result is consistent with previous studies in this area [23,24].

The single factor pollution index (P_i) established Cd as the sole metal with an average P_i exceeding 1.0, rendering all of the DK soil samples highly polluted (100%), whereas the SXC samples were classified as moderately (21.05%) or highly (78.95%) polluted by Cd. These outcomes unequivocally identify Cd as the predominant contaminant metal within the study locale. Also, the P_N values for soil samples further substantiate the presence of moderate to high levels of heavy metal pollution in numerous soils across the studied region. The average I_geo values for these metals were consistently higher in DK than in SXC, indicating a more significant accumulation of heavy metals in DK soils. These findings align with prior analyses, illustrating that the soils in DK are either highly or extremely contaminated with Cd, whereas SXC exhibited a relatively lesser degree of Cd pollution, remaining at or above the moderately polluted classification. Potential ecological risk quantifies the vulnerability of diverse biological communities to toxic agents, highlighting the potential ecological threats posed by hazardous substances [7]. By integrating the individual metal ecological risk indices ($E_r^i$) with the overall PERI (Table 2), the soils were identified as carrying a very high potential ecological risk primarily due to Cd, while presenting low ecological risks for Cu, Zn, Ni, and Pb. Consequently, the overall ecological risk from heavy metals at all the sampling sites was deemed very high, predominantly attributed to Cd contamination.

Principal component analysis (PCA) has been recognized as a potent methodology for identifying the sources of hazardous elements within soil environments [4]. While Cd was solely associated with PC2, it has been identified as being derived predominantly from anthropogenic sources. Extensive research has documented that the presence of Cd in soil is largely a result of industrial actions, such as sewage irrigation and atmospheric deposition (both dry and wet), as well as agricultural practices, including the use of phosphorus fertilizers and livestock manures [1,4,7]. The significant coefficient of variation observed for soil Cd levels in DK underscores the extensive impact of human activities on soil Cd. The contamination of soils within the studied region is likely attributable to the prolonged discharge of pollutants from battery manufacturing facilities and the irrigation practices utilizing electronic wastewater [23]. Meanwhile, large amounts of phosphate fertilizers might be applied in this traditional agricultural field. An elevated level of Cd pollution load as the distance from the battery factories decreases suggests that the battery factory likely contributed to soil Cd contamination. Pb, Cu, Ni, and Zn all have high loads on PC1 (Table 3), which might be considered as a mixed origin. Numerous investigations have established that Ni primarily originates from natural geological processes, including rock weathering and pedogenic processes [4,8,41]. Additionally, a substantial body of research indicates that Ni finds extensive application in various industrial activities, notably in the production of rechargeable batteries and electroplating operations [41,42]. The high value of C.V., especially in SXC, confirms that soil Ni is seriously affected by human disturbance, as does the existence of many nickel–cadmium battery factories.

As essential trace elements for animals, Cu and Zn can improve disease resistance, promote growth and development, and further improve meat quality. Hence, Cu and Zn are commonly utilized as dietary supplements in feed to bolster immunological functions and facilitate growth promotion. However, Cu and Zn in feed additives are absorbed so inefficiently that almost 90% of them are excreted by animals, which eventually produces various Cu- and Zn-enriched livestock manures [4]. The use of diverse fertilizers, including metal-enriched manures, commercial fertilizers, and sewage sludge, in addition to copper-based pesticides, can elevate the concentration of the associated heavy metals in agricultural soils, particularly Cu and Zn. The aforementioned result of a higher level of Cu and Zn as the distance from the battery factories decreases suggests that the battery factory likely contributed to soil Cu and Zn accumulation. Moreover, various soluble and insoluble Zn salts in the parent material of soils indicate their lithogenic source [4]. This indicates that Cu and Zn have a heterogeneous origin.

Much research has shown that Pb is an anthropogenic element in surface soils [4,7,8,43]. Pb originating from industrial emissions, incomplete coal combustion, and Pb aerosols has the capacity for long-range transport, in contrast to vehicle exhaust, which has limited dispersal capabilities [44]. The aforementioned result that a higher level of metals in the topsoil from DK than that from SXC suggests that industrial emissions and the improper disposal of wastes can lead to heavy metal pollution around the battery industrial area. Also, the moderate loads of Pb, Cu, and Ni in PC2 (Table 3, Figure 2) indicate the contribution from anthropogenic sources. In this study, Cd, Pb, Cu, and Ni were identified to share anthropogenic origins associated with the battery industrial sector, which is involved in the production of batteries and related materials.

The peak HI values for adults in both DK and SXC were significantly below 1, suggesting that the likelihood of noncancerous health implications for adults falls into a low or negligible category [5,7]. It was observed that the health risk metrics for the exposure to soil heavy metals in DK were uniformly higher than those in SXC, aligning with the comparative concentrations of heavy metals. Both the maximum and average HI values for the children in DK and SXC exceeded 1 but remained below 10, indicating the presence of a non-carcinogenic risk to the children in these areas. Yang et al. (2022) [5] showed a significant contamination of vineyard soil in the suburbs of Kaifeng city due to sewage irrigation, especially Cd and Zn, but there was no non-carcinogenic risk for adults and children. Figure 3 illustrates that among these three routes of exposure to soil metals, dermal contact posed the greatest non-carcinogenic risk, succeeded by ingestion and inhalation. Moreover, Cd was identified as the predominant contributor, accounting for more than 79.23% (in DK) and 65.17% (in SXC) of the total non-carcinogenic risk from soil metal exposure, with the contribution of these five heavy metals to the total non-carcinogenic risk ranked as follows: Cd > Pb > Ni > Cu > Zn.

Carcinogenic risk analysis determined that the highest CR for children was attributed to oral intake, implicating ingestion as a significant pathway for potential health risks (Figure 4). Conversely, dermal contact was identified as the primary pathway contributing to CR for adults. The total CR was within the range of 10⁻⁶ to 10⁻⁴ (as depicted in Figure 4), signifying an acceptable level of risk for adults and children in both the DK and SXC areas [15,35]. This suggests a low probability of cancer for the inhabitants, with a notably lower risk for adults compared to children. Consequently, the results reveal a negligible non-carcinogenic risk and low CR for adults, alongside low non-carcinogenic and carcinogenic risks for children exposed chronically to the soils of these woodland and farmland regions.

5. Conclusions

This study investigated the concentrations, pollution characteristics, and associated health risks of heavy metals in the surface soils from two villages, DK and SXC, located downstream from a cluster of battery industries in Xinxiang city, Henan Province, China. Soils within the electronic sewage irrigation areas of DK and SXC exhibited overall elevated metal concentrations, with Cd levels at all the sampled locations surpassing the established risk screening threshold. The P_i of heavy metals demonstrated a consistent order of Cd > Cu > Zn > Ni > Pb in both villages. The soil samples from DK were universally categorized within the heavily polluted range, whereas SXC exhibited a varied distribution across the slightly polluted, moderately polluted, and heavily polluted classifications. Based on the I_geo, the DK soils were determined to be highly or extremely contaminated by Cd, whereas SXC presented a relatively lesser degree of contamination, maintaining levels above the moderately polluted classification. The potential ecological risk index (PERI) indicated a low risk for Cu, Zn, Ni, and Pb, yet the overall ecological risk across all sampling sites was deemed very high, predominantly due to Cd contamination. PCA suggested that Cu, Zn, Ni, and Pb originated from mixed sources, whereas Cd was primarily attributed to anthropogenic activities. The disparity in Cd pollution between DK and SXC implicated the battery manufacturing activities as a significant source of soil Cd contamination. Children face higher health risks than adults, with a nonnegligible non-carcinogenic risk to the children in both villages, manifesting that children are at a slight risk from heavy metals. The principal route of non-carcinogenic exposure was identified as dermal contact, with ingestion and inhalation also contributing. CR assessment indicated an acceptable level of risk in both DK and SXC, with ingestion and dermal contact being the major pathways for children and adults, respectively. This work underscores the necessity for heightened scrutiny and can serve as a reference for the specific safety distance between battery industrial zones and agricultural areas.