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Article

Unravelling the Effect of Low-Molecular-Weight Dissolved Organic Matter on Antimony Enrichment in Groundwater of the Xikuangshan Sb Mining Area, China

by
Tongchun Qin
1,
Zijian Li
2,
Qianqian Sun
2 and
Chunming Hao
1,2,*
1
Nantong Institute of Technology, Yongxing Street 211, Nantong 226002, China
2
North China Institute of Science and Technology, Sanhe 065201, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(8), 1206; https://doi.org/10.3390/w17081206
Submission received: 25 February 2025 / Revised: 6 April 2025 / Accepted: 9 April 2025 / Published: 17 April 2025
(This article belongs to the Section Water Quality and Contamination)
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Versions Notes

Abstract

:
The effect of low-molecular-weight dissolved organic matter (LDOM) on antimony enrichment in groundwater remains unclear. In this study, the spectroscopic and molecular characteristics of high- and low-Sb groundwater are compared using optical spectrophotometry, ultrafiltration, and Fourier transform ion cyclotron resonance mass spectrometry. The results demonstrated that although the mean DOM concentration in LDOM groundwater (3.98 mg/L) accounted for only 69.22% of the mean DOM concentration, the proportion of Sb(V) within the total Sb varied between 80.29% and 99.56%. LDOM was characterized by higher biological and fluorescence index values, a greater H/C ratio, and reduced double-bond equivalent values compared with high-molecular-weight dissolved organic matter. High abundances of LDOM can enhance the primary enrichment of Sb(V) within the total Sb concentration via competitive adsorption and, as energy and electron acceptors for microbial communities facilitate Sb(III), oxidation within groundwater systems. This study provides new perspectives on understanding how DOM influences the migration and speciation transformation of Sb in groundwater environments.

1. Introduction

As a priority heavy metal, products containing Sb are widely used in flame retardants, batteries, and the automotive tire and alloy industries, consequently leading to significant Sb contamination [1]. Increasing evidence has shown that the Sb content in soils, sediments, and water has increased significantly in recent decades globally, particularly near mining, smelting, fossil-fuel combustion, and ammunition sites [2,3]. Recent surveys have revealed that the maximum detected concentration of Sb in the groundwater of the Xikuangshan Sb mine is 18.402 mg/L [4,5], which exceeds the Chinese standards for drinking water quality (0.005 mg/L) [6] and unpolluted water body limits (0.001 mg/L) by 3680 and 18,402 times, respectively [7]. Oxidation of Sb2S3 is a major natural source of Sb pollution in groundwater [8,9,10]. During this process, Sb(III) and Sb(V) are the main species detected, with Sb(V) and Sb(III) dominating in oxic and anoxic environments, respectively [11,12]. The toxicity of Sb species is comparable to that of arsenic, with Sb(III) posing a greater risk than Sb(V). However, owing to its weaker adsorption affinity for Fe/Mn hydroxides and higher solubility, Sb(V) generally exhibits greater mobility than Sb(III) in the environment [13,14,15]. Arsenic generally occurs as As(III) and As(V) compounds in groundwater. Therefore, analyzing the processes by which Sb(III) is oxidized to Sb(V) is essential for a comparison of the biogeochemical behaviors of As and Sb in groundwater environments.
Dissolved organic matter (DOM), which contains an abundance of functional groups, such as carboxyl, hydroxyl, carbonyl, and amide groups, is crucial for Sb mobilization and transformation [5,11,16]. Moreover, DOM can bind Sb to form Sb-Fe-DOM or Sb-DOM complexes [17,18,19], compete for adsorption with Sb on mineral surfaces [20,21], act as an electron acceptor [14,22], and induce Sb(III) oxidation under sunlight [14,23,24,25], thereby increasing Sb content in groundwater. The molecular weight of DOM serves as a fundamental characteristic, with a substantial impact on its bioavailability [11,26,27]. Previous studies have shown that DOM with different molecular weights exhibit distinct fluorescence properties and molecular structures, which have an impact on Sb migration in groundwater [11,14]. DOM with a high molecular weight (HDOM, >100 kDa) show a greater adsorption capacity for Fe minerals, which promote the formation of Sb-Fe-DOM or Sb-DOM complexes by binding with Sb species [25,26]. Furthermore, microorganisms can typically degrade low-molecular-weight DOM (LDOM, <1 kDa), which is considered bioactive and easily utilized [28,29]. Consequently, the degradation process of microorganisms can accelerate the oxidation of Sb(III) to Sb(V) [24,30], and the competitive adsorption of Sb(V) in an alkaline oxic environment with high concentrations of HCO3− [5,24,31]. Therefore, understanding the functions and effects of LDOM is essential to manage environmental Sb contamination.
Although the effects of the spectroscopic and molecular signatures of DOM on Sb migration in Xikuangshan Sb mining groundwater have been confirmed in our previous research [5,25], the effect of LDOM on Sb(V) enrichment and Sb(III) oxidation is not extensively understood. Therefore, considering the North Mine of the Xikuangshan Sb mining area, China, as the study site, this study aims to (1) compare the spectroscopic properties and molecular characteristics of LDOM between high- and low-Sb groundwater and (2) identify the roles of LDOM in the migration of Sb in shallow groundwater systems through ultrafiltration. The findings of this study provide new perspectives on Sb enrichment in groundwater.

2. Materials and Methods

2.1. Study Area

The North Mine of Xikuangshan Sb mining area is located 13 km north of the city of Lengshuijiang in Hunan province, China, between 111°30′00″–111°31′22″ E and 27°46′00″–27°48′00″ N. It covers approximately 0.50 km2 in a mountainous region with a mean annual precipitation of approximately 1382 mm and mean annual evaporation of approximately 903 mm [4,5,9]. The study area is impacted by an F75 fault to the west and a lamprophyre vein to the east (Figure 1). The Magunao Aquifer (D3s2 groundwater) and Shetianqiao Aquifer (D3x4 groundwater) constitute the two primary aquifers under investigation, exhibiting limited hydraulic connectivity with each other. Large residential areas, grasslands, vegetable plantations, and abandoned mining residual slags cover the weathered D3x4 water layer.
The D3x4 groundwater, an extensively distributed and exposed groundwater system, is 258 m thick and serves as the main source of drinking water for residents in the study area. Moreover, the D3x4 groundwater mainly consists of micrite, sandy limestone, mudstone, and shale, exhibiting an average hydraulic conductivity of 0.0092 m/d. The main water supply for the D3x4 groundwater originates from precipitation, whereas discharge generally occurs through seasonal springs and mine drainage. Despite the general groundwater flow moving from northeast to southwest, the flow direction of the D3x4 groundwater in the North Mine of the Xikuangshan Sb mining area is from southeast to northwest [32,33]. Additional details regarding the study area have been presented by Hao et al. [4,5,9,25].

2.2. Groundwater Sampling and Analyses

A total of 80 groundwater samples were systematically collected from the D3x4 groundwater between September and October 2024, as shown in Figure 1. Twenty groundwater samples were used to analyze the Sb concentration, species, and HCO3− concentration; twenty groundwater samples were used to assess DOM concentration and conduct Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS, Billerica, MA, USA) analyses; and 40 groundwater samples were used to perform fluorescence spectroscopy on DOM with varying molecular weights. Before collecting the samples, each of the 500 mL sampling bottles were rinsed 2–3 times sequentially with distilled water and D3x4 groundwater samples. To determine the levels and species of Sb, the D3x4 groundwater samples were initially passed through a 0.45 μm glass fiber membrane and subsequently treated with equal parts dilution of nitric acid with a 1:1 (v/v) dilution, along with 5% (v/v) 0.25 M ethylenediaminetetraacetic acid. They were then covered with aluminum foil to ensure a pH below 2.0 and avoid photochemical reactions [11,12]. Samples for DOM and FT-ICR MS detection were acidified with H2SO4 to a pH below 2 through 0.22 μm membrane filters in the field [25,29,34]. LDOM D3x4 groundwater (LDOM groundwater) samples and high-molecular-weight DOM D3x4 groundwater (HDOM groundwater) samples were processed using a polymer-enhanced ultrafiltration system (CFUS, Sartorius Vivaflow® 200, Gottingen, Germany) equipped with 1 kDa and 0.45 μm millipore filters by applying pressure with N2 gas to facilitate the filtration. The pH, oxidation redox potential (ORP), and total-dissolved-solids content of all D3x4 groundwater samples were measured in the field using a portable pH/ORP meter (HANNA H18424, Padua, Italy) and portable conductivity meter (HANNA H1833, Padua, Italy), respectively.
Sb concentrations and species were determined using HPLC-HG-AFS instrumentation (Qingdao), achieving a relative standard deviation within ±5% and analysis precision of 0.001 mg/L. Sb species concentration in HDOM and LDOM groundwater were listed in Table S1. The levels of bicarbonate ions (HCO3−) were measured using acid-base titration, achieving a measurement accuracy of 1.0 mg/L. The LDOM and HDOM samples were examined using a total organic carbon analyzer (TOC-5000, Kyoto, Japan) to determine the DOC concentrations with an analytical precision of 0.01 mg/L. The UV-visible absorbance, as well as fluorescence-spectroscopy measurements and molecular signatures of LDOM and HDOM, were obtained using a UV-visible spectrophotometer (Dayton, NJ, USA) and SolariX XR-15T FTMS (Billerica, MA, USA), respectively.

2.3. EEM-PARAFAC and FT-ICR MS Analysis

The EEM data were obtained at an excitation range of 250–450 nm with increments of 5 nm and an emission range of 220–520 nm with increments of 2 nm. Scanning was performed at a speed of 500 nm/min. The PARAFAC model can be utilized to examine extensive datasets containing hundreds or thousands of EEMs, thereby facilitating the identification of LDOM and HDOM components. The average fluorescence intensities were obtained from three repeated measurements, in which the PARAFAC-modeled EEM was subtracted from the measured EEM. This process ensured that the maximum deviation did not exceed 10% when compared with the original measured EEM intensities [29,35,36]. The appropriate number of components for the models was determined and verified through independent residual analysis and split-half analysis after outlier identification was performed. The fluorescence intensities and relative abundances of six distinct organic components (C1, C2, C3, C4, C5, and C6) within the LDOM and HDOM were satisfactorily modeled in Table 1.
Finally, the humification index (HIX), which indicates the extent of DOM humification and its sources; the biological index (BIX), serving as a marker for autochthonous biological origins; the specific UV absorbance at 254 nm (SUVA254), reflecting the aromatic character of DOM; and the fluorescence index (FI), used to differentiate between terrestrial and microbial DOM origins, were calculated in detail based on previous descriptions by Hao et al. [5], Liu et al. [37], and Jia et al. [11].
Molecular formulas were primarily determined based on the elemental compositions of 1H(1–120), 16O(1–50), 12C(1–60),14N(0–5), and 32S(0–3). These calculations were conducted for mass peaks exhibiting a signal-to-noise ratio (S/N) of 5 or higher, and the “nitrogen rule” was simultaneously applied in the process [38,39]. The double-bond equivalent (DBE), indicating the degree of unsaturation in DOM molecules, modified aromaticity index (AImod), reflecting the aromaticity of DOM molecules and nominal oxidation state of carbon were determined as described by Zark et al., Zhang et al., and Xu et al. [40,41,42].
Using the H/C and O/C ratios, the LDOM and HDOM constituents were classified into seven distinct groups within a van Krevelen diagram [25,29,34], as shown in Table 2. Among these compounds, those with a higher H/C ratio (H/C > 0.50) have previously been classified as bioreactive DOM [25,27], whereas those with a higher O/C ratio (O/C > 0.50) exhibit greater humic DOM characteristics [39,43].

3. Results

3.1. Geochemical Characterization of HDOM and LDOM Groundwater

The geochemical data of the HDOM and LDOM groundwater samples are listed in Table S1. The Sb concentrations in the HDOM and LDOM groundwater samples varied between 0.001 and 20.800 mg/L (average of 3.641 mg/L) and 0.001 and 20.600 mg/L (average of 3.519 mg/L), respectively. Moreover, Sb(V) accounted for 82.74–99.58% of the total Sb in HDOM groundwater samples (mean value of 3.203 mg/L), whereas in LDOM groundwater samples, the percentage of Sb(V) in the total Sb ranged from 80.29% to 99.56% (average of 3.095 mg/L). These minor differences between HDOM and LDOM groundwater suggest that Sb primarily exists in a dissolved form in LDOM groundwater, with Sb(V) being the predominant species [11]. Based on the natural background values and statistical criteria [4,5,9], the D3x4 groundwater samples were categorized into two groups: those with low antimony concentrations (<0.015 mg/L, namely low-Sb groundwater) and those with high antimony concentrations (≥0.015 mg/L, namely high-Sb groundwater).
The average DOM concentration in LDOM groundwater (3.98 mg/L) was noticeably lower than that of the HDOM groundwater (5.75 mg/L) (Table S1), indicating that the LDOM groundwater contained a lower amount of DOM. However, the concentrations of Sb and DOM exhibited a positive correlation (R2 = 0.56) in the original DOM groundwater (Figure 2a), suggesting that DOM significantly influenced the mobilization and transport of Sb in these groundwater types. The concentration of Sb exhibited a comparable range in both the high- and low-Sb groundwater samples in both HDOM and LDOW (Figure 2b), further confirming that the presence of LDOM may enhance the Sb concentration in D3X4 groundwater [5,11].

3.2. Fluorescent Characteristics of DOM in HDOM and LDOM Groundwater

Fluorescent characteristic indicators such as BIX, HIX, FI, and SUVA254 of the HDOM and LDOM groundwater are shown in Figure 3a and Table S1. Compared with the HDOM groundwater samples, BIX slightly increased, whereas HIX and SUVA254 obviously decreased in the LDOM groundwater samples, indicating a stronger biological source and less humic character in the LDOM [27,29]. Moreover, the FI value slightly increased from a mean value of 1.64 (terrestrially and microbially derived zones) in HDOM groundwater to a mean value of 1.78 (microbially derived zones) in LDOM groundwater [44,45,46,47,48], further demonstrating a larger contribution from microbial sources in LDOM [44,49,50]. Notably, low-Sb groundwater had similar HIX, SUVA254, FI, and BIX values to those of high-Sb groundwater, suggesting that the fluorescence characteristics of LDOM were dominant in both high- and low-Sb groundwater.
Six different fluorescent components (C1, C2, C3, C4, C5, and C6) were successfully identified in all high- and low-Sb groundwater samples using the PARAFAC models. C1, C3, C5, and C6 had similar excitation and emission wavelengths and were aligned with humic-like substances. In contrast, C2 and C4 were categorized as protein-like components [5,51,52]. In contrast to HDOM, the relative proportions of C1, C3, C5, and C6 in LDOM groundwater samples decreased noticeably, whereas a significant increase was observed in the relative proportions of C2 and C4 (Figure 3b). These findings suggest that protein-like compounds from microbial sources were predominantly observed in the LDOM groundwater samples.
This observation provides further evidence that protein-like components were dominant in high-Sb groundwater DOM, whereas humic-like components were predominantly present in low-Sb groundwater DOM (Figure 3c). This indicates that protein-like components were more favorable for Sb concentration enrichment in groundwater [5,29].

3.3. Molecular Characteristics of DOM in HDOM and LDOM Groundwater

A total of 3632–6278 molecular formulas were detected in groundwater LDOM, whereas 3041–5338 formulas were observed in the groundwater HDOM (Table S2). The groundwater LDOM, characterized by relatively higher molecular abundances, exhibited greater complexity than HDOM. This increased complexity may be attributed to a wider variety of DOM molecules resulting from microbial decomposition processes [29,34,35,43]. In general, the H/C ratios were comparatively elevated, and DBE and AImod values were reduced compared to HDOM (Figure 4b). This suggests that the groundwater LDOM contains a greater abundance of newly saturated and less aromatic compounds [25,53,54], which aligns with the higher BIX and lower HIX and SUVA254 values observed. Compared to HDOM, groundwater LDOM had similar levels of abundance for lipids, proteins/aliphatics, unsaturated hydrocarbons, carbohydrates, lignin-like compounds, tannin-like compounds, and condensed aromatics in the van Krevelen diagram (Figure 4a), indicating that groundwater LDOM can reflect the predominant molecular signatures of the initial DOM in groundwater [38,55].
Based on the correlations with Sb concentrations in the LDOM groundwater, the optical properties, PARAFAC components, and molecular characteristics of the DOM were divided into two categories, as shown in Figure 5a. The HIX, SUVA254, C1, C3, C5, C6, and O/C ratios were positively correlated, indicating the presence of more stable and aromatic DOM components. In contrast, the second group, which included BIX, FI, C2, C4, H/C ratio, DBE, and AImod, exhibited negative correlations, reflecting more reactive DOM components. Generally, the high-Sb groundwater DOM exhibited higher HIX, SUVA254 values, and O/C ratios in Figure 5a, while exhibiting lower BIX values and H/C ratios compared to low-Sb groundwater DOM. This suggests that the high-Sb groundwater DOM was more unsaturated, richer in humic substances, and less susceptible to biodegradation. These findings are consistent with those of earlier studies by Hao et al. (2022) and Jia et al. (2023) [5,11], further confirming that the increased prevalence of recalcitrant organic compounds in high-Sb groundwater likely contributes to Sb enrichment.
In the van Krevelen diagram (Figure 5b), the Sb mean concentration, BIX, FI, and protein-like components (C2 and C4) clustered within the range characterized by protein/aliphatic (highly unsaturated) compounds, exhibiting H/C ratios greater than 1.2. Conversely, parameters such as HIX and SUVA254 aligned with the intervals of lignin- and tannin-like (humic and less biodegradable) compounds, marked by O/C ratios exceeding 1.2, as illustrated in Figure 5b. The strong correlation between Sb concentration and optical indices, PARAFAC components, and molecular signatures in groundwater LDOM highlights that biodegradable protein-like components play a crucial role in Sb mobility, which cannot be ignored in high-Sb groundwater.
As shown in Figure 6, for both HDOM and LDOM groundwater, the lignin- and tannin-like molecules exhibited strong positive correlations with Sb concentrations. These molecules were typically characterized by their aromatic properties and resistance to degradation [38,39,56]. These findings suggest a positive association between Sb levels and the presence of aromatic and recalcitrant DOM compounds in both HDOM and LDOM groundwater. The number of lignin- and tannin-like substances positively correlated with the Sb concentration and was lower in LDOM groundwater than in HDOM groundwater, further suggesting that groundwater LDOM exhibits fewer aromatic structures and higher bioreactivity [25,34,35]. Previous research has shown that humic-like components with a higher HIX are linked to DOM molecules characterized by lower H/C ratios, higher DBE values, and elevated AImod indices. In contrast, protein-like components with higher BIX values are associated with higher H/C ratios and lower DBE and AImod values [25,27,29,43].

4. Discussion

Based on the aforementioned findings, LDOM groundwater exhibited higher BIX and FI values, along with lower HIX and SUVA254 values in its fluorescent characteristics. Additionally, it demonstrated an elevated H/C ratio and comparatively reduced DBE and AImod values in its molecular signature. LDOM is considered to be easily degraded by microorganisms, whereas HDOM is considered a resistant substance that hinders microbial degradation [25,39,57]. Moreover, Sb existed mainly as total Sb, and 82.74% of the samples exceeded the natural background value (0.015 mg/L) and belonged to high-Sb groundwater in the LDOM groundwater. This indicates that bioreactive LDOM with a higher BIX value and H/C ratio can facilitate Sb enrichment in groundwater.

4.1. Competitive Adsorption Effect

A moderate positive correlation existed between DOM and HCO3− in the LDOM groundwater (R2 = 0.71), whereas a weak positive correlation was observed in the HDOM groundwater (R2 = 0.13) (Figure 7a), indicating that the presence of HCO3− in the LDOM groundwater was mainly due to the microbial degradation of LDOM [51,58]. Under anaerobic conditions, microorganisms can biodegrade DOM to obtain electrons or energy, thereby supporting microbial respiration and leading to CO2 production [59]. This process subsequently enhances the dissolution of carbonate minerals, thereby increasing the HCO3− concentration in the LDOM groundwater [51,60,61]. The concentration of Sb(V) increased with increasing HCO3− levels (R2 = 0.79), whereas Sb(III) enrichment in the LDOM groundwater samples was negligible (R2 = 0.18) (Figure 7b). This suggests that competitive adsorption effects reduce Sb(V) removal from Fe/Mn hydroxide surfaces, whereas Sb(III) removal is minimally influenced by HCO3− [28,31,51]. Sb(III) and Sb(V) can form complexes with goethite, which are observed as monodentate or bidentate inner-sphere configurations via infrared spectroscopy or X-ray absorption fine-structure analysis [28,31]. Previous research has demonstrated that the surfaces of Fe/Mn hydroxides have a greater binding preference for Sb(III) than for Sb(V), attributed to the larger atomic radius of Sb(III) (0.076 nm) compared to Fe(III) (0.064 nm) [2,8,62,63]. Therefore, the increase in HCO3− led to Sb(V) being readily displaced from the surfaces of Fe/Mn hydroxides into groundwater enrichment, owing to intense competition for adsorption sites [8,64].
LDOM can also compete with Sb(V) for binding sites on Fe/Mn hydroxides [13,65,66], resulting in LDOM groundwater primarily existing as Sb(V) species. Owing to the greater number of functional groups in oxygen-rich DOM, LDOM was preferentially adsorbed onto Fe/Mn hydroxides and experienced stronger competition with Sb(V) for binding site [67,68,69,70]. The elevated protein-like components in LDOM intensified competition with Sb(V) [68]. The protein-like components in LDOM exhibited a moderate (R2 = 0.43) positive correlation with the Sb concentration in LDOM groundwater (Figure 8b), whereas no obvious correlations were observed between humic-like components and Sb concentration. This indicates that these easily degradable compounds in LDOM, characterized by higher BIX values, lower HIX and SUVA254 values, and greater proportions of C2 and C4, may contribute to increasing Sb concentrations in the LDOM groundwater through competitive adsorption. The fluorescence intensities of the protein-like components initially decreased and then decreased more gradually as the Sb concentration increased. In contrast, only slight quenching effects were observed for the humic-like components (Figure 8a). These findings further indicate that protein-like components served as more effective complexing agents for Sb than humic-like components in the LDOM groundwater [52,71,72,73].
The pH level may have a substantial impact on the development of O-, OH, and OH2+ surface functional groups within the LDOM [74]. In an acidic environment, Sb may attach to LDOM through electrostatic forces or by creating inner-sphere complexes. Conversely, in alkaline settings, LDOM promotes Sb mobility by generating soluble aqueous complexes [73,74]. Furthermore, under near-neutral conditions, the complexing of Sb(V) with LDOM was less pronounced than that of Sb(III), facilitating the mobilization of Sb(V). Hence, the weakly alkaline environment in D3x4 water (Table S1) can cause competitive adsorption between LDOM and Sb(V) on Fe/Mn hydroxide sites, promoting Sb(V) enrichment in high-Sb groundwater [72,75].

4.2. Microorganisms Sb(III) Oxidative Dissolution

Previous research demonstrated that HDOM contains a greater amount of humic substances with quinone functional groups than LDOM, facilitating higher levels of microbial Fe reduction in natural settings. According to Li (2022), the addition of HDOM notably enhanced Fe(III) reduction during incubation compared to samples in which HDOM was not included. Typically, HDOM consists of a higher proportion of aromatic and hydrophobic structures, whereas the LDOM remaining in the solution is more biologically accessible to microorganisms [76]. Humic-like components in HDOM, which have high molecular weights, aromaticity, HIX, and O/C ratios, exhibited higher electron-acceptor capacities than protein-like components [48,77]; however, lower concentrations of protein-like LDOM substances similarly revealed a significant influence of microbially derived DOM on Sb(III) oxidation. For example, 1,4-benzoquinone, which had a low DBE value, could efficiently oxidize Sb(III) under alkaline groundwater conditions, leading to the creation of hydroquinone by transferring electrons between O2 and labile DOM [28]. Furthermore, the co-oxidation of Sb(III) and Fe(II) facilitates the generation of Fe(IV) and Sb(V) through an outer-sphere electron-acceptor mechanism involving bioreactive LDOM (such as EDTA and oxalate) [22,23,30].
The Sb(III) oxidative reactions were highly dependent on pH and ORP, affecting the generation of reactive oxygen species in LDOM groundwater. As shown in Table S1, LDOM may facilitate Sb(III) oxidation by affecting the generation of semiquinone radicals and other reactive species in higher pH value and alkaline conditions [68,78,79]. The transformation of Sb(III) in LDOM groundwater is influenced by both biological and non-biological oxidation processes, leading to a progressive reduction in Sb(III) levels and corresponding increase in Sb(V) [68,70,80]. The concentration of Sb(V) was positively correlated with the combined fluorescence intensity of C2 and C4, whereas the oxidation ratio of Sb(III)/Sb progressively decreased, as illustrated in Figure 9. This phenomenon proved that Sb(III) oxidation was stronger in the presence of protein-like DOM substances. At the same time, as shown in Table S1, the positive ORP value indicated an oxidizing environment, which facilitated Sb(III) oxidation in LDOM groundwater. Previous studies had proved that Sb(III) can be rapidly oxidized to Sb(V) in oxic groundwater conditions in the absence of high concentrations of LDOM [8,24].
Furthermore, bioreactive LDOM can provide increased energy and electron yields for microbial respiration, thereby enhancing the microbially driven release of Sb [10,12]. Although different metabolic pathways are involved in Sb(III) oxidation, two bacterial strains, Ensifer sp. NLS4 and Shewanella sp. NLS1, which are capable of oxidizing Sb(III) to generate Sb(V), have been identified in previous studies under conditions of limited nutrients [81]. In addition, Thiobacillus and Rhizobium spp. within the tailing microbiomes have been recognized as key populations capable of oxidizing both As(III) and Sb(III). However, the oxidation rate of Sb(III) is significantly higher than that of As(III) [12]. At mining locations, Sb-sulfide minerals, LDOM (ligand-like protein components of DOM), and tailings serve as attachment surfaces and energy sources for aerobic microorganisms under increasing-pH conditions, ultimately leading to complete Sb(III) oxidation [16,82]. Microorganisms may play a role in the oxidation of Sb(III) through a metabolic pathway similar to that of As(III) (e.g., arsenite oxidase Aio, comprising aioA and anoA) during the degradation of LDOM [12]. Notably, the initial bacterial community structure and diversity in Sb-polluted soil were weakened and diminished by Sb(III) and Sb(V), whereas Sb(III) decreased the variety of bacteria and abundance of particular bacterial species more strongly than Sb(V) [10].

5. Conclusions

Insights derived from the analysis of the spectral and molecular characteristics of DOM with varying molecular weights revealed that LDOM exerts a significant influence on antimony mobility. Sb exists mainly as dissolved Sb in LDOM groundwater, with Sb(V) being the dominant species. Moreover, LDOM, which is characterized by higher BIX and FI values in its fluorescent properties, a higher H/C ratio, and relatively lower DBE and AImod values in its molecular structure, demonstrated greater bioreactive activity than HDOM.
The presence of HCO3−, resulting from the biodegradation of LDOM, enhanced Sb(V) enrichment in LDOM groundwater, which was attributed to the stronger competitive adsorption of Sb(V) compared with Sb(III) at Fe/Mn hydroxide sites. In addition, protein-like DOM substances, which function as energy and electron acceptors, can enhance Sb(III) oxidation during the co-oxidation of Sb(III) and Fe(II), Sb(III) photooxidation, and microbially driven processes.
Therefore, our findings provide a novel perspective on how the molecular weights of DOM influence Sb(V) enrichment and Sb(III) oxidation in high-Sb groundwater and solve issues (e.g., tailing and rebound) related to the future remediation of Sb-polluted groundwater.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w17081206/s1, Table S1: Descriptive statistics of geochemistry data, DOM, HIX, SUVA254, BIX, and FI in HDOM and LDOM groundwater; Table S2: Statistics of molecular signatures of DOM in HDOM and LDOM groundwater.

Author Contributions

Methodology, Q.S.; Investigation, Z.L.; Writing—original draft, T.Q.; Project administration, C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Natural Science Foundation of Hebei Province (D2024508003; D2021508004), the Fundamental Research Funds for the Nantong Institute of Technology (Grant No. 2023XK(B)05), and the projects from “Jiangsu Marine Structure Service Performance Improvement Engineering Research Center” and “Nantong Building Structure Key Laboratory (CP12015005)”.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the study area and sampling sites.
Figure 1. Location of the study area and sampling sites.
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Figure 2. Plots of (a) the association between DOM and Sb concentration and (b) boxplots illustrating the Sb concentration differences in groundwater with high and low Sb levels, categorized by HDOM and LDOM.
Figure 2. Plots of (a) the association between DOM and Sb concentration and (b) boxplots illustrating the Sb concentration differences in groundwater with high and low Sb levels, categorized by HDOM and LDOM.
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Figure 3. Box and whisker plots of (a) fluorescent characteristics index; (b) percentages of C1, C2, C3, C4, C5, and C6; (c) percentages of the combined proportions of C1, C3, C5, and C6 and the combined proportions of C2 and C4 in HDOM and LDOM groundwater.
Figure 3. Box and whisker plots of (a) fluorescent characteristics index; (b) percentages of C1, C2, C3, C4, C5, and C6; (c) percentages of the combined proportions of C1, C3, C5, and C6 and the combined proportions of C2 and C4 in HDOM and LDOM groundwater.
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Figure 4. Graph of (a) van Krevelen diagram of DOM compositions for 20 groundwater samples. HDOM groundwater samples were assigned a blue color, while LDOM groundwater samples were designated with a green color. The uncovered points indicate the differences in molecular signatures between HDOM and LDOM groundwater. Graph of (b) molecular signatures indices in HDOM and LDOM groundwater.
Figure 4. Graph of (a) van Krevelen diagram of DOM compositions for 20 groundwater samples. HDOM groundwater samples were assigned a blue color, while LDOM groundwater samples were designated with a green color. The uncovered points indicate the differences in molecular signatures between HDOM and LDOM groundwater. Graph of (b) molecular signatures indices in HDOM and LDOM groundwater.
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Figure 5. Plots of (a) correlation analysis of molecular compositions indices, optical parameters, and Sb concentrations in groundwater and (b) average characteristics of organic molecules associated with BIX, HIX, FI, and SUVA254, (C1, C2, C3, C4, C5, and C6), and Sb concentrations in van Krevelen diagram.
Figure 5. Plots of (a) correlation analysis of molecular compositions indices, optical parameters, and Sb concentrations in groundwater and (b) average characteristics of organic molecules associated with BIX, HIX, FI, and SUVA254, (C1, C2, C3, C4, C5, and C6), and Sb concentrations in van Krevelen diagram.
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Figure 6. Notable relationships between molecular characteristics and Sb from Spearman’s analysis (r ≥ 0.5, p < 0.01) for HDOM (b) and LDOM (a) groundwater samples. The blue to orange colors in the color bar denote the correlation coefficients (r), increasing from −1.0 to 1.0. The number of molecules correlated with Sb concentration is obviously more in HDOM groundwater than in LDOM groundwater.
Figure 6. Notable relationships between molecular characteristics and Sb from Spearman’s analysis (r ≥ 0.5, p < 0.01) for HDOM (b) and LDOM (a) groundwater samples. The blue to orange colors in the color bar denote the correlation coefficients (r), increasing from −1.0 to 1.0. The number of molecules correlated with Sb concentration is obviously more in HDOM groundwater than in LDOM groundwater.
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Figure 7. Correlation analysis of (a) DOM concentration and (b) Sb(V) and Sb(III) concentration versus Sb concentration versus HCO3− concentration in LDOM groundwater.
Figure 7. Correlation analysis of (a) DOM concentration and (b) Sb(V) and Sb(III) concentration versus Sb concentration versus HCO3− concentration in LDOM groundwater.
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Figure 8. Correlation analyses of (a) fluorescent intensity and (b) percentage of the combined proportions of C2 and C4 compared to Sb concentration in LDOM groundwater.
Figure 8. Correlation analyses of (a) fluorescent intensity and (b) percentage of the combined proportions of C2 and C4 compared to Sb concentration in LDOM groundwater.
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Figure 9. Plots depicting the Sb(V) concentration and the oxidation ratio of Sb(III)/Sb versus fluorescence intensity of the combined proportions of C2 and C4 in LDOM groundwater.
Figure 9. Plots depicting the Sb(V) concentration and the oxidation ratio of Sb(III)/Sb versus fluorescence intensity of the combined proportions of C2 and C4 in LDOM groundwater.
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Table 1. Six distinct organic components (C1, C2, C3, C4, C5, and C6) within the LDOM and HDOM groundwater.
Table 1. Six distinct organic components (C1, C2, C3, C4, C5, and C6) within the LDOM and HDOM groundwater.
TypeC1C2C3C4C5C6
%
HDOM groundwater (n = 20)
Max30.7445.4627.3542.5721.6535.90
Min3.7212.892.220.921.040.51
Mean22.3422.8517.3620.4610.846.14
SD7.358.215.4813.425.537.14
LDOM groundwater (n = 20)
Max30.4047.1922.1068.8215.077.41
Min2.3015.655.153.871.530.16
Mean16.8629.5513.7128.287.713.87
SD8.749.315.1717.274.412.24
Table 2. Seven distinct groups in DOM constituents were categorized into the van Krevelen diagram.
Table 2. Seven distinct groups in DOM constituents were categorized into the van Krevelen diagram.
CategoriesH/C RatioO/C Ratio
lipids1.50–2.200–0.30
protein/Aliphatic compounds1.50–2.200.30–0.67
unsaturated hydrocarbons0.70–1.500–0.10
carbohydrates1.50–2.500.67–1.20
lignin-like substances0.70–1.500.10–0.67
tannins-like materials0.50–1.500.67–1.20
condensed aromatics0.20–0.700–0.67
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Qin, T.; Li, Z.; Sun, Q.; Hao, C. Unravelling the Effect of Low-Molecular-Weight Dissolved Organic Matter on Antimony Enrichment in Groundwater of the Xikuangshan Sb Mining Area, China. Water 2025, 17, 1206. https://doi.org/10.3390/w17081206

AMA Style

Qin T, Li Z, Sun Q, Hao C. Unravelling the Effect of Low-Molecular-Weight Dissolved Organic Matter on Antimony Enrichment in Groundwater of the Xikuangshan Sb Mining Area, China. Water. 2025; 17(8):1206. https://doi.org/10.3390/w17081206

Chicago/Turabian Style

Qin, Tongchun, Zijian Li, Qianqian Sun, and Chunming Hao. 2025. "Unravelling the Effect of Low-Molecular-Weight Dissolved Organic Matter on Antimony Enrichment in Groundwater of the Xikuangshan Sb Mining Area, China" Water 17, no. 8: 1206. https://doi.org/10.3390/w17081206

APA Style

Qin, T., Li, Z., Sun, Q., & Hao, C. (2025). Unravelling the Effect of Low-Molecular-Weight Dissolved Organic Matter on Antimony Enrichment in Groundwater of the Xikuangshan Sb Mining Area, China. Water, 17(8), 1206. https://doi.org/10.3390/w17081206

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