Introduction

The pedosphere is an essential sphere within the Earth’s critical zone, and there exists a synergistic relationship between soil quality and the regional ecological environment [1, 2]. The soil environment, as the most active geochemical system for element migration and material cycling in nature, is highly susceptible to various pollution sources [3-6]. It is generally believed that the enrichment and accumulation of heavy metal elements in the soil are the combined result of anthropogenic activities and the release from the weathering of parent rocks [7-9]. In industrial areas, agricultural regions, and urban residential areas, human activities predominantly contribute to heavy metal pollution [10-13]. However, in underdeveloped regions and remote mountainous areas, the accumulation of heavy metal elements in the soil is primarily influenced by regional geological background factors [14-18].

The southwestern region of China is the largest karst distribution area in the world [19] and also an area with geochemical anomalies of multiple metal elements [20- 22]. The regional geochemical survey conducted in China during the 1980s at a scale of 1:200,000 indicated that the concentrations of various heavy metal elements in soil and sediments in the southwestern region of China were higher than the background values of soil elements in China [23, 24]. Numerous studies have confirmed that the weathering of carbonate rocks results in significant enrichment of heavy metal elements in soils. This is con-sidered the main reason for the widespread exceedance of soil heavy metal element concentrations in the karst regions of southwestern China [25-27]. In previous studies, researchers have conducted extensive work on spatial distribution characteristics [28-30], environmental risk assessment [31-33], pollution source analysis [34-36], and other aspects of heavy metal elements in karst regions. However, there is no definitive conclusion regarding the exact causes of excessive enrichment of soil heavy metal element concentrations. Current research generally suggests that the weathering of carbonate rocks into soil involves significant dissolution of carbonate minerals, leading to the enrichment of trace metal elements in the residual insoluble residues [37, 38]. If that is the case, under conditions without external input of heavy metals, the concentration of heavy metal elements in both carbonate rock bedrock and soil should exhibit a positive correlation. However, the extensive leaching of carbonate minerals often results in the loss of elemental geochemical information between the carbonate rock parent material and the soil [39], making it difficult to discuss the enrichment mechanism of soil heavy metal elements from the perspective of parent rocks. Additionally, despite being weathering products derived from the same parent rock (carbonate rock), variations in mineral composition can result in different degrees of geochemical behavior of heavy metal elements during the weathering process [40]. Existing studies have confirmed that the overlying soil in karst areas is a product of in situ weathering residues of acid-insoluble constituents in the underlying bedrock [41, 42]. Therefore, it is reasonable and feasible to utilize the acid-insoluble fraction of carbonate rocks as a ‘bridge’ to understand the elemental geochemical information between the rock-to-soil weathering sequence.

This study focuses on the enrichment mechanisms of heavy metal elements in weathered soils derived from carbonate rocks. By comparing the geochemical behavior of heavy metal elements under natural weathering conditions between carbonate rocks and clastic rock profiles in the study area and conducting acid-insoluble fraction extraction experiments on carbonate rocks, we further investigate the role of acid-insoluble fraction in influencing the mineralogical occurrence and migration extent of heavy metal elements. This study can not only deepen our understanding of the geochemical behavior of heavy metal elements in karst regions but also provide a scientific reference for regional soil heavy metal pollution prevention and control.

Materials and methods

Sampling, Preparation, and Testing Methods

Sample Collection

The main objective of this study is to investigate the geochemical behavior of heavy metal elements under natural weathering conditions. Therefore, it is crucial to deter-mine whether the collected samples can objectively reflect the natural background values of these elements. To avoid excessive human impact on the content of heavy metal elements in soils, the selected profile locations for this study were all situated in the southern region of Guizhou Province (Fig. 1). In comparison to other regions within the province, this area has a relatively lower population density and fewer industrial and mining enterprises. Moreover, each profile is located in areas far from human activities (> 500m) without any traces of accumulated profiles.

This study collected rock and soil samples from the underlying bedrock and overlying weathered soils of different stratigraphic formations (Cambrian, Carboniferous, and Permian) in the study area. A total of 20 rock samples (15 carbonate rocks and 5 clastic rocks) and 92 soil samples (60 carbonate rocks and 32 clastic rocks) were collected. During fieldwork, fresh rock frag-ments were collected as much as possible for the bedrock samples, while soil samples were obtained by trenching from bottom to top. The collected samples were sealed in self-sealing bags for preservation and transported to the laboratory for further processing. Rock samples were washed with an ethanol solution to remove surface im-purities, while soil samples were carefully cleaned to eliminate plant roots and gravel mixed within. After the samples were air-dried naturally, they were ground in an agate mortar to pass through a 200-mesh sieve and then stored in bags for further use.

Microscopic Rock Identification

This study selected representative carbonate rock samples, which were cut into quadrangular, thin sections using a cutting machine. After washing, the sections were wiped clean with a silk cloth and then air-dried. A suit-able amount of neutral gum solution was dropped onto the sections to create thin slides placed on glass slides. The positioning of the glass slides was adjusted using stainless steel tweezers to ensure a thin, uniform, secure, and bubble-free adhesive layer, facilitating microscopic observation.

Preparation of Acid-Insoluble Residue

In this study, an appropriate amount of carbonate rock samples were taken and subjected to crushing. The crushed samples were then placed into a 1L beaker, followed by a gradual addition of 1M HCl for reaction. Once the reaction inside the beaker ceased, the supernatant liquid was removed using a siphoning pipette, and additional HCl was added. This process was repeated until no further reaction occurred (indicating the endpoint of the reaction). Subse-quently, the solid residue (insoluble residue) in the beaker was thoroughly rinsed with ultrapure water. After confirm-ing the neutrality of the supernatant liquid using pH test paper, the insoluble residue in the beaker was transferred, dried, weighed, and stored in a bag for further use. This study prepared a total of 15 samples of insoluble residues from car bonate rocks using the aforementioned method.

Trace Element Testing of Samples

The rock, soil, and insoluble residue samples were weighed using an analytical balance, with 50mg placed into PTFE digestion vials. Then, 1mL of concentrated HNO₃ and 0.6 to 0.8mL of HF were added. These vials were inserted into the corresponding steel containers and digested for a minimum of 15 hours in a thermostatic incubator. After cooling down, approximately 2mL of 3% di-lute nitric acid was added, and the digestion continued for another 5 hours in the thermostatic incubator. Once cooled, the undisturbed solution from the digestion vials was carefully transferred to polyethylene terephthalate (PET) cen-trifuge tubes and diluted to a final volume of 10mL with ultrapure water. After thorough mixing, the mixture was centrifuged and left undisturbed for 2 to 3 hours before conducting testing. The trace element analysis and testing work was conducted at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences. The analytical instrument used for this purpose was the Plasma Quant MS Elite Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The relative standard deviation (RSD) of the replicate testing for trace element analysis of all samples was found to be less than 10%, indicating excellent repeatability.

Fig. 1.

Location of the study area and distribution of sampling points.

Results

Characteristics of Heavy Metal Element Content (mg/kg)

This study aimed to investigate the variation characteristics of heavy metal element content during the weathering process of different lithological bedrocks and the differences in elemental geochemical behaviors. The heavy metal element content in weathered profiles of carbonate rocks and clastic rocks in the study area was tested. The content of heavy metal elements in carbonate rocks, clastic rock bedrocks, and their weathered soils in the study area is presented in Table 1. The average content of Cr, Ni, Cu, Zn, Cd, and Pb in carbonate rocks is 21.61, 36.72, 3.21, 15.81, 0.22, and 3.8 mg/kg, respectively. In clastic rock bedrock, the content of Cr, Ni, Cu, Zn, Cd, and Pb is 89.51, 33.69, 27.85, 72.7, 0.139, and 31.41 mg/kg, respectively. Except for Cd, the average heavy metal element content in clastic rock bedrocks is higher than that in carbonate rocks. In weathered soils derived from carbonate rocks, the average content of Cr, Ni, Cu, Zn, Cd, and Pb is 137, 51.2, 29.6, 169, 3.95, and 57.7 mg/kg, respectively. The average content of Cr, Ni, Cu, Zn, Cd, and Pb in weathered soils derived from clastic rocks is 66.7, 23.4, 23.5, 70.2, 0.128, and 32.7 mg/kg, respectively. The heavy metal element content in weathered soils derived from carbonate rocks is higher than that in weathered soils derived from clastic rocks.

The enrichment factor (EF) is commonly used to indicate the degree of enrichment of elements in soil compared to their content in the bedrock. The enrichment factors (EF) of Cr, Ni, Cu, Zn, Cd, and Pb in weathered soils derived from carbonate rocks are 6.34, 1.39, 9.22, 10.69, 17.95, and 15.18, respectively, indicating varying degrees of enrichment of heavy metal elements in the soil compared to the bedrock. In contrast, the EF values for Cr, Ni, Cu, Zn, Cd, and Pb in weathered soils derived from clastic rocks fluctuate around 1, with values of 0.74, 0.69, 0.84, 0.97, 0.92, and 1.04, respectively. This suggests that there is no significant enrichment of heavy metal elements in weathered soils derived from clastic rocks compared to the bedrock (Table 1). Overall, there are significant differences in the geochemical behavior of heavy metal elements during the weathering process of carbonate rocks and clastic rocks. Weathered soils derived from carbonate rocks exhibit a substantial increase in the content of heavy metal elements compared to the parent rock, showing a strong enrichment effect. In contrast, weathered soils derived from clastic rocks mostly inherit the geochemical information of the parent rock, resulting in relatively homogeneous soil heavy metal element content (Fig. 2).

Fig. 2.

Characteristics of heavy metal element content (mg/kg). a) bedrock, b) Weathered soil).

Discussion

The Control Effect of Weathering Patterns

Soil is the weathering product of parent rock, and they both have a natural material inheritance [47]. From a material structure perspective, the differences in lithology are essentially differences in mineral composition. The variations in mineral composition among different lithologies can undoubtedly affect the geochemical behavior of elements during the weathering-to-soil process [48]. Clastic rocks are primarily composed of silicate minerals (feldspar, quartz, and clay minerals) and tend to exhibit strong resistance to weathering, presenting an isotropic weathering pattern. Carbonate rocks are mainly composed of carbonate minerals (calcite, dolomite), and during the weathering-to-soil process, the rock-forming elements such as Ca and Mg are extensively leached by weathering fluids, resulting in significant loss of their mass and volume, thus reflecting a non-isotropic weathering pattern.

Fig. 3.

Characteristics of heavy metal content in weathering process of carbonate rock (mg/kg). a) Carbonate, b) Acid-insoluble Substances, C) Weathered Soil.

Table 3.

Characteristics of heavy metals content during weathering of carbonate rock(mg/kg).

Stratum	Samples		Cr	Ni	Cu	Zn	Cd	Pb
Cambrian	Rock N=5	Max	82.9	50.7	9.50	107.00	0.29	35.20
		Min	8.88	32.1	2.47	6.55	0.02	1.71
		Mean	40.1	39.6	4.92	33.56	0.08	9.99
	Acid non-soluble substance N=5	Max	241	37.9	31.2	195	0.349	72
		Min	65.1	11.3	9.99	11.3	0.072	8.62
		Mean	113	22.54	18.698	82.32	0.188	34.004
	Soil N=21	Max	120	64.80	38.30	480.00	1.37	244.00
		Min	70.2	18.80	14.20	46.00	0.11	22.90
		Mean	94.9	39.60	28.11	163.92	0.53	75.72
	EF soli/rock		2.37	1.00	5.71	4.88	6.63	7.58
Carboniferous	Rock N=5	Max	13.0	39.00	4.88	18.00	0.36	0.83
		Min	3.71	25.90	1.62	3.11	0.06	0.22
		Mean	9.04	33.08	2.71	9.66	0.24	0.50
	Acid insoluble substance N=5	Max	2138	1605	104.29	1151	55.6	183.59
		Min	139	46.2	31.25	82.5	0.33	10.55
		Mean	768	452.7	60.02	649	13.41	54.42
	Soil N=21	Max	212	86.50	53.80	346.00	16.10	68.90
		Min	19.9	8.65	10.20	23.40	0.28	11.90
		Mean	128	48.43	27.91	169.85	4.16	46.76
	EF soli/rock		14.16	1.46	10.30	17.58	17.33	93.52
Permian	Rock N=5	Max	24.9	41.00	2.58	5.68	0.84	1.14
		Min	10.3	35.50	1.71	2.53	0.08	0.60
		Mean	15.8	37.50	1.99	4.21	0.34	0.92
	Acid insoluble substance N=5	Max	1151	266	120	698	10.96	42.01
		Min	95.5	35.1	14.75	31.5	0.35	7.55
		Mean	579	191	63.49	320	3.93	28.71
	Soil N=18	Max	355	113	49.6	278	17.90	70.10
		Min	50.0	14.65	7.35	36.35	0.50	11.30
		Mean	194	65.93	33.22	173.69	7.13	49.28
	EF soli/rock		12.28	1.76	16.69	41.26	20.97	53.57

This study calculated the correlation between the enrichment factors and mass transfer coefficients of heavy metal elements in weathered soils derived from clastic rocks and carbonate rocks (Fig. 4). The results indicate a significant and strong positive correlation between the enrichment factors and mass transfer coefficients of Cr, Ni, Cu, Zn, Cd, and Pb in weathered soils derived from clastic rocks, with R-squared values of 0.9335, 0.9977, 0.9139, 0.8337, 0.5642, and 0.9826, respectively. This suggests that under the constraint of isotropic weathering patterns, the accumulation of heavy metal elements in weathered soils derived from clastic rocks is primarily controlled by the extent of element migration. In other words, without the addition of exogenous heavy metal elements, the soil tends to inherit the heavy metal element content from its parent rock. In contrast, for weathered soils derived from carbonate rocks, there is no linear relationship between the enrichment factors and mass transfer coefficients of Cr, Ni, Cu, Zn, Cd, and Pb. Moreover, significant enrichment occurs even when a substantial portion of the heavy metal elements have been leached (Fig. 4). The reason behind this phenomenon is closely related to the distinct weathering pattern exhibited by carbonate rocks.

Fig. 4.

Correlation between Enrichment Factors and Mass Transfer Coefficients.

The weathering-to-soil process of carbonate rocks can generally be divided into two stages. During the ear-ly stage of weathering, carbonates dissolve, resulting in a significant loss of Ca and Mg ions. In the later stage, weathering mainly involves the deep weathering of silicate minerals [42, 49]. Intense leaching occurs during the weathering-to-soil process of carbonate rocks, leading to the profound natural evolution of elements and substances. In this study, due to the differences in lithology, carbonate rocks and clastic rocks display distinct geochemical behaviors of heavy metal elements during weathering-to-soil processes. The enrichment factors of heavy metal elements in weathered soils derived from clastic rocks fluctuate around a baseline value of 1. In contrast, the enrichment factors of heavy metal elements in weathered soils derived from carbonate rocks can reach values above ten, with Pb being enriched 78 times compared to the parent rock. This means that even in carbonate rocks with relatively depleted heavy metal content [50], significant enrichment of various trace elements, including heavy metals, occurs during the weathering-to-soil process. Therefore, the accumulation of heavy metal elements during the weathering-to-soil process of carbonate rocks can be understood as the result of the absolute mass loss of elements and the relative enrichment of their concentrations. The unique non-isotropic weathering pattern of carbonate rocks plays an important role in this phenomenon.

The Influence of Acid-Insoluble Residues in Carbonate Rocks

The previous discussion demonstrated that the unique weathering pattern of carbonate rocks leads to significant enrichment of heavy metal element content. However, despite this finding, there are considerable differences in the geochemical behavior of heavy metal elements during the weathering-to-soil process of carbonate rocks from different geological epochs. The leaching and enrichment levels of heavy metal elements in weathered soils derived from Carboniferous and Permian carbonate rocks are notably higher than those from the Cambrian epoch (Fig. 4). This study attempts to clarify these differences from the perspective of acid-insoluble residues in carbonate rocks.

In the natural weathering process of rocks, the fate of elements is often closely related to the minerals in which they are present [51]. Therefore, to understand the geochemical behavior of heavy metal elements during the weathering of carbonate rocks, it is important to first clarify the occurrence characteristics of these elements in different mineral phases. Carbonate rocks are primarily composed of easily soluble carbonate minerals and insoluble silicate minerals. The former is the main rock-forming mineral of carbonate rocks, which is extensively leached by weathering fluids during the weathering-to-soil process. On the other hand, the latter remains a residue during the weathering-to-soil process and represents the major soil-forming material of carbonate rocks. Based on the mineral composition characteristics of carbonate rocks, this study divides carbonate rocks into two end-members: The Acid-Soluble Mineral Phase (ASMP) and the Acid-Insoluble Mineral Phase (AIMP). The acid-soluble mineral phase represents carbonate minerals, while the acid-insoluble mineral phase partially represents the characteristics of residual dissolution products. Therefore, to unravel the geochemical behavior of heavy metal elements during the weathering process of carbonate rocks, understanding the occurrence and behavior of these elements in the acid-soluble and acid-insoluble mineral phases is crucial. Based on the results of acid-insoluble residue extraction tests (Table 3) and heavy metal element content analysis in acid-insoluble residues (Table 4), this study calculated the mass fraction of heavy metal elements in different mineral phases of carbonate rocks. The calculations indicate that heavy metal elements are distributed among various mineral phases of carbonate rocks, but overall, they tend to occur more in the acid-soluble mineral phase (Fig. 5). Furthermore, there exists a distribution pattern where higher acid-insoluble residue content is associated with a greater tendency for heavy metal elements to occur in the acid-insoluble mineral phase. For instance, in Profile Є5, where the acid-insoluble residue content is 38.52%, heavy metal elements are predominantly present in the acid-insoluble mineral phase. Conversely, in Profile Є3, with an acid-insoluble residue content of 1.86%, heavy metal elements are mostly found in the acid-soluble mineral phase. It can be said that the acid-insoluble residue content controls the mass distribution of heavy metal elements in different mineral phases of carbonate rocks, and this distribution characteristic determines the fate of heavy metal elements during the weathering-to-soil process to some extent. The lower acid-insoluble residue content in Carboniferous and Permian carbonate rocks results in less residual soil-forming material during their weathering-to-soil process. As a consequence, the degree of weathering experienced by these rocks is more intense, which explains why the leaching of heavy metal elements in the weathered soils derived from Carboniferous and Permian carbonate rocks is higher compared to the Cambrian period.

Previous studies have provided mineralogical and geochemical evidence to demonstrate that the overlying soil in karst areas is the in-situ weathering residue of acid-insoluble minerals from the underlying bedrock [42,49]. In this study, we introduced acid-insoluble minerals as a ‘bridge’ connecting the elemental geochemical information between carbonate bedrock and soil. The process of carbonate rock weathering into the soil was divided into two stages: 1. Bedrock-dissolved residue (acid-insoluble minerals) and 2. Dissolved residue-soil. Enrichment factors for heavy metal elements were calculated separately for these two stages (Table 4) in our research. The calculation results indicated that heavy metal elements generally exhibited enrichment characteristics during the first stage, with coal and Permian strata showing significantly higher enrichment factors compared to the Cambrian strata. This reflects the concentration enrichment of heavy metal elements due to the extensive leaching of carbonate minerals during the weathering stage, from bedrock to dissolved residue. However, the enrichment factors of heavy metal elements noticeably decreased in the second stage, even displaying depletion characteristics. This suggests that during the evolution of dissolved residue into soil under the influence of weathering and leaching processes, heavy metal elements partially migrated out of the system. The content of heavy metal elements during the process of carbonate rock weathering into the soil demonstrated an initial significant increase followed by a slight decrease, implying that acid-insoluble minerals greatly influence the distribution characteristics of heavy metal elements in soil. To verify this hypoth-esis, this study conducted a correlation analysis between the content of heavy metal elements in acid-insoluble minerals from carbonate rocks and weathered soil. The results indicated variations in the correlation between acid-insoluble minerals from different geological eras and the content of heavy metal elements in weathered soil. Specifically, the correlation was relatively strong for the Cambrian and Permian strata, while it was weaker for the Carboniferous strata (Fig. 6). The weak correlation between acid-insoluble minerals from the Carboniferous strata and the content of heavy metal elements in weathered soil can be attributed to their lower acid-insoluble mineral content (ranging from 0.12% to 3.71%, with a mean value of 0.95%). The poor correlation between acid-insoluble minerals and the Ni content in weathered soil for the Cambrian, Carboniferous, and Permian strata also supports this notion, as Ni is predominantly associated with acid-soluble mineral phases, which are prone to intense leaching during the process of weathering into the soil (Fig. 5). Additionally, the weak correlation observed between acid-insoluble minerals from the Carboniferous strata and the content of Cu, Zn, and Pb in weathered soil suggests a similar control factor affecting these chemical-ly akin chalcophile elements [52] during the process of weathering into the soil.

Fig. 5.

Mass percentage of heavy metals in different mineral phases (%).

The correlation between the content of heavy metal elements in acid-insoluble minerals from carbonate rocks and weathered soil does not exhibit a simple linear relationship across different geological eras. This indicates that the geochemical behavior of heavy metal elements during the process of carbonate rock weathering into the soil is influenced by multiple factors, making it a com-plex natural evolutionary process. However, overall, the concentration of heavy metal elements in acid-insoluble minerals from carbonate rocks partially controls the characteristics of heavy metal element content in weathered soil. The Carboniferous and Permian strata exhibit significantly higher concentrations of heavy metal elements in their acid-insoluble minerals compared to the Cambrian strata. Consequently, the corresponding weathered soil inherits this distribution characteristic of the heavy metal element content. This constitutes an important reason for the variations in the enrichment levels of heavy metal elements in weathered soil derived from carbonate rocks of different geological periods.

Fig. 6.

Correlation between the content of heavy metal elements (mg/kg).

Conclusions

This study investigates the enrichment mechanisms of heavy metal elements in weathered carbonate rock soils under natural background conditions. The findings provide the following insights: