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Article

Development and Evaluation of Selenium-Enriched Compound Fertilizers for Remediation of Mercury-Contaminated Agricultural Soil

by
Yuxin Li
1,
Guangpeng Pei
2,
Yanda Zhang
3,
Shuyun Guan
2,
Yingzhong Lv
1,
Zhuo Li
1 and
Hua Li
4,*
1
Pomology Institute, Shanxi Agricultural University/Shanxi Key Laboratory of Germplasm Improvement and Utilization in Pomology, Taiyuan 030031, China
2
College of Resources and Environment, Shanxi Agricultural University/Key Laboratory of Sustainable Dryland Agriculture of Shanxi Province, Jinzhong 030801, China
3
College of Forestry, Shanxi Agricultural University, Jinzhong 030801, China
4
School of Environment Science and Resources, Shanxi University, Taiyuan 030006, China
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(8), 1842; https://doi.org/10.3390/agronomy15081842
Submission received: 22 June 2025 / Revised: 14 July 2025 / Accepted: 29 July 2025 / Published: 30 July 2025
(This article belongs to the Section Innovative Cropping Systems)
Browse Figures
Versions Notes

Abstract

Agricultural soil contaminated with mercury (Hg) poses a serious threat to ecosystems and human health. Although adding an appropriate amount of selenium (Se) can reduce the toxicity and mobility of Hg in soil, Se alone is prone to leaching into groundwater through soil runoff. Therefore, Se-enriched compound fertilizers were developed, and their remediation effect on Hg-contaminated agricultural soil was determined. The Se-enriched compound fertilizers were prepared by combining an organic fertilizer (vinegar residue, biochar, and potassium humate), inorganic fertilizer (urea, KH2PO4, ZnSO4, and Na2SeO3), and a binder (attapulgite and bentonite). A material proportioning experiment showed that the optimal granulation rate, organic matter content, and compressive strength were achieved when using 15% attapulgite (Formulation 1) and 10% bentonite (Formulation 2). An analysis of Se-enriched compound fertilizer particles showed that the two Se-enriched compound fertilizers complied with the standard for organic–inorganic compound fertilizers (China GB 18877-2002). Compared with the control, Formulation 1 and Formulation 2 significantly reduced the Hg content in bulk and rhizosphere soil following diethylenetriaminepentaacetic acid (DTPA) extraction by 40.1–47.3% and 53.8–56.0%, respectively. They also significantly reduced the Hg content in maize seedling roots and shoots by 26.4–29.0% and 57.3–58.7%, respectively, effectively limiting Hg uptake, transport, and enrichment. Under the Formulation 1 and Formulation 2 treatments, the total and DTPA-extractable Se contents in soil and maize seedlings were significantly increased. This study demonstrated that Se-enriched compound fertilizer effectively remediates Hg-contaminated agricultural soil and can promote the uptake of Se by maize. The results of this study are expected to positively contribute to the sustainable development of the agro-ecological environment.

1. Introduction

In recent decades, rapid industrialization has led to soil pollution of increasing severity, with heavy metal pollution becoming a global environmental problem [1,2]. Soils are important reservoirs of mercury (Hg) in the environment, with atmospheric deposition being the main source of Hg enrichment in surface soils [3]. The Hg content in surface soils ranges from 3.8 to 618.2 μg kg−1 with an average of 74.0 μg kg−1 across the globe, and 41.2% of examined sites exceed the reported soil Hg background worldwide (60.0 μg kg−1) [3]. In China, a 1.6% exceedance rate for safe Hg levels was identified in soil samples, with a regular downward trend from the southeast to the northwest [4,5]. In the 1980s and 1990s, family workshops were the main mode of production in small-scale gold mines, with amalgamation being the key technology in the extraction process. However, Hg was released during this process, with an emission factor of 15 g Hg g−1 Au. The excessive accumulation of Hg in agricultural soil through atmospheric deposition not only damages the soil environment and affects crop yields but also contaminates the food chain and ultimately endangers human health [1,2,6].
A variety of physical, chemical, and biological remediation techniques are used to remediate Hg-contaminated soils, including thermal desorption, electrokinetic remediation, soil washing, and phytoextraction [4,7]. However, these technologies have several serious drawbacks and limited applicability for agricultural restoration [8]. For example, thermal desorption consumes a lot of energy and destroys the ecological structure of the soil; electrokinetic remediation has a poor effect on the removal of Hg in heterogeneous soil; soil washing is prone to secondary pollution and the loss of essential elements; and the phytoextraction process requires a long duration, with the enriched plants requiring further treatment [4,9,10]. Chemical stabilization has emerged as a promising in situ technique, aiming not at total Hg removal but reducing mobility and availability through chemical immobilization. This technique has the relative advantages of lower ecological disturbance, operational convenience, and cost-effectiveness compared with some alternatives [7,11]. Therefore, cost-effective and environmentally stable Hg-contaminated soil remediation technology would not only effectively alleviate the current situation in China—in which there is a conflict between humans and land—but could also meet the future agricultural development needs of remediation while still supporting production.
Selenium (Se) is an essential element for animals and humans and it is also a beneficial element for plants [12]. Unfortunately, due to its uneven distribution in agricultural soils, Se deficiency has become a global health problem. About 0.5–1 billion people in more than 40 countries have an insufficient Se intake [13,14]. To solve this problem, many countries have increased the Se content in crops by adding exogenous Se to the soil, with excellent results [15,16,17]. Furthermore, Se can antagonize Hg and reduce its toxicity and mobility by forming stable Hg–Se complexes in soil or rhizosphere regions and activating the antioxidant systems in plants [18,19,20,21]. Thus, the application of exogenous Se is an effective measure for treating Hg-contaminated soil and the safe production of crops [20]. In soil, the bioavailability of different Se species follows the order: SeO42− > organic Se > SeO32− > Se0 > Se2− [22]. The soil Se speciation varies with soil pH and redox conditions, primarily existing as water-soluble SeO32− and SeO42− [22]. In well-aerated alkaline soils, SeO42− predominates as the major form [23]. However, its high bioavailability and mobility facilitate leaching into groundwater via soil runoff [23]. In contrast, the bioavailability of SeO32− is reduced in soils with high levels of Fe/Al oxides, clay minerals, and organic matter (OM) because of its strong affinity for forming stable complexes with these constituents [14,24]. Moreover, Tran et al. [25] found that in alkaline soil, SeO32− was more effective in reducing the Hg content in pak choi than SeO42−. However, following its application, Se will readily leach into groundwater with soil runoff, reducing its utilization rate and polluting the water environment [26]. Therefore, there is an urgent need to determine safe and effective Se application measures.
A Se-enriched compound fertilizer that integrates nutrient-release properties with Hg-contaminated soil remediation would reduce the ecological risks of Hg while increasing Se uptake by crops. Certain clay minerals (e.g., attapulgite and bentonite) are excellent fertilizer carriers and binders due to their unique layered structure and ion exchange properties [27,28]. Organic materials, such as vinegar residue, biochar, and potassium humate, not only improve soil physiochemical properties but may also affect the morphological transformation of heavy metals through complexation and ion exchange [29,30,31]. Previous studies of Se–Hg antagonism have mainly focused on the application of Se alone, with only a few investigating the synergistic effect of Se sources and various functional materials in the remediation of Hg-contaminated agricultural soil.
To address this knowledge gap, we hypothesized that Se-enriched compound fertilizers would simultaneously enhance Hg immobilization in soil and reduce its bioavailability while also promoting Se uptake by crops. This study developed Se-enriched compound fertilizers using Na2SeO3 as the Se source; clay minerals (attapulgite and bentonite) as the binders; and vinegar residue, biochar, and potassium humate as the organic materials, subsequently adding inorganic fertilizers: urea, KH2PO4, and ZnSO4. The optimal formulation was screened through the pH value, granulation rate, OM content, and compressive strength. By collecting in situ Hg-contaminated agricultural soil and conducting a maize pot experiment, the remediation effect of the Se-enriched compound fertilizer was evaluated. This study aimed to (1) develop and optimize Se-enriched fertilizers using attapulgite/bentonite binders, and (2) validate their efficacy in reducing Hg mobility/plant uptake and improving Se bioavailability in Hg-contaminated agricultural systems.

2. Materials and Methods

2.1. Experimental Materials

2.1.1. Soil Sampling, Preparation, and Analysis

Mercury-contaminated soils were collected from the topsoil (0~20 cm) of farmland in Xinzhou City, Shanxi Province, China (located at 39°14′ N, 113°30′ E), during the 2021 fallow season using a five-point sampling method. These soils were then mixed and transported to the laboratory. Maize is a long-term crop in this area. There are also many small mines in this region, especially gold mines. Due to the historical problems of gold mining, smelting, and transportation, the Hg content in the agricultural soil of the study area was 5.38 mg kg−1, 1.58-fold higher than the risk screening value for soil contamination in agricultural land defined by the Chinese government (China GB 15618-2018) [32]. The excessive Hg in the soil has had a negative impact on soil ecology and both crop yield and quality. Soil samples were air-dried, pretreated (removal of the plant residues and stones), and ground through a 2 mm sieve for further analysis and experimentation.
The physicochemical properties of the experimental soil are provided in Table 1. The mechanical composition of the soil was determined using the hydrometer method (China NY/T 1121.3-2006) [33]. Soil pH was measured using a pH meter (ST3100, Ohaus Instrument Co., Ltd., Shanghai, China) in a suspension with a soil–water ratio of 1.0:2.5 w/v [34]. Soil electric conductivity was determined using a conductivity meter (Rex DDSJ-308A, Shanghai INESA Scientific Instrument Co., Ltd., Shanghai, China) (HJ 802-2016) [35]. Soil OM content was determined using the external heating potassium dichromate volumetric method [36]. Soil cation exchange capacity was measured using the hexamminecobalt trichloride solution–spectrophotometric method (China HJ 889-2017) [37]. The total Hg content in the soil was determined following sample digestion using HNO3-HCl, followed by liquid chromatography–atomic fluorescence spectrometry analysis (LC-AFS 6500, Beijing Haiguang Instruments Co., Ltd., Beijing, China) according to China GB/T 22105.1-2008 [38]. The total Se content in the soil was determined following sample digestion using HNO3-HClO4, followed by analysis using the LC-AFS 6500 according to China NY/T 1104-2006 [39].

2.1.2. Plant Material

The maize (Zea mays L.) cultivar ‘Luyu 1611’ was used in this study, with an approximately 126-day growth cycle. This hybrid cultivar exhibits resistance to northern leaf blight (Setosphaeria turcica), head smut (Sporisorium reilianum), and gray leaf spot (Cercospora zeae-maydis), coupled with lodging resistance and tolerance to high-density planting. It was a commonly used cultivar in the experimental regions. Pretreated with commercial protectants (fungicides and insecticides), the seeds were sown in the experimental soil.

2.1.3. Selenium-Enriched Compound Fertilizer Materials

Vinegar residue, biochar, and potassium humate were used as organic fertilizers. The inorganic fertilizers were KH2PO4, ZnSO4, and Na2SeO3. The selected binders were attapulgite and bentonite (sodium base). The vinegar residue was pyrolyzed at 700 °C for 2 h in an N2 atmosphere to obtain biochar. The organic fertilizer raw material was crushed and passed through a 0.3 mm sieve for further analysis and experimentation.
The properties of the Se-enriched compound fertilizer materials are shown in Table 2. The pH of the raw materials was determined using a pH meter (ST3100), and the total Hg and Se contents were determined following sample digestion using HNO3-HF-HClO4, with measurements made using a LC-AFS 6500 (China GB 18877-2002) [40]. The total N, P2O5, and K2O contents were determined according to China GB/T 17767.1-2008 [41], China GB/T 8573-2017 [42], and China GB/T 17767.3-2008 [43], respectively. The total nutrient content was the combined value of N + P2O5 + K2O.

2.2. Development of the Se-Enriched Compound Fertilizer

2.2.1. Granulation Method

The raw materials of the Se-enriched compound fertilizer were mixed in the following proportions: urea, 22.51%; KH2PO4, 9.59%; ZnSO4, 3.30%; Na2SeO3, 0.44%; and potassium humate, 40.00%. The binder doses were set to 10%, 15%, and 20%, and the vinegar residue and biochar doses were changed accordingly (Table 3).
The extrusion granulation method (JZL-80 laboratory extrusion granulator, Changzhou Yongchang Granulation and Drying Equipment Co., Ltd., Changzhou, China) was used for granulation. The particles were then dried (FG-1.0 laboratory boiling dryer, Changzhou Yongchang Granulation and Drying Equipment Co., Ltd., Changzhou, China) to obtain Se-enriched compound fertilizer. After drying, the particles were poured into a 1 mm sieve for screening. Particles ≥1 mm were used as the Se-enriched compound fertilizer.

2.2.2. Analysis of Particle Properties

The pH of the particles was measured with a pH meter (ST3100) (material–water = 1:5). The granulation rate was the proportion of ≥1 mm particle mass to the total particle mass. The compressive strength of the particles was measured using a WQYC-10c compressive strength tester (Hebi Metallurgical Machinery Equipment Co., Ltd., Hebi, China). The moisture content of the particles was measured using the vacuum oven method (China GB/T 8576-2010) [44]. The OM content of the particles was measures using the potassium dichromate volumetric method (China GB 18877-2002) [40]. The analytical methods used for particle total N, P2O5, K2O, Hg, and Se content were the same as those used for the organic fertilizers, as described in Section 2.1.3. The chloride ion content of the particles was measured using the ammonium thiocyanate titration method (China GB 15063-2001) [45]. Arsenic (As), cadmium (Cd), lead (Pb), and chromium (Cr) were determined following sample digestion via HNO3-HF-HClO4, with measurements using an atomic absorption spectrophotometer (AA 140/240, Varian Spectrum, Palo Alto, USA) for Cd, Pb, and Cr and the LC-AFS 6500 for As.

2.3. Pot Experiment

2.3.1. Experimental Design

Based on the requirements for soil testing in Shanxi Province, Se toxicity thresholds in soil (3 mg kg−1), and the nutritional requirements of maize, the amount of Se-enriched compound fertilizer applied was set to 0.23 g kg−1 (Se application was about 1 mg kg−1). Correspondingly, the amounts of ordinary chemical fertilizer applied were N = 0.03 g kg−1, P2O5 = 0.01 g kg−1, and K2O = 0.02 g kg−1, using a completely randomized design. The treatments are shown in Table 4; each was replicated in 3 different pots (3 independent pots).
The Hg-contaminated agricultural soil was loaded into plastic pots with a diameter of 18.5 cm and a height of 21.5 cm. Field conditions were simulated by placing the pots in a greenhouse with controlled conditions. Three maize seeds were planted in each pot. Soil moisture was maintained at 15% gravimetric water content (equivalent to 60% field capacity). This was achieved via daily weighing and water replenishment to compensate for evapotranspiration losses. The seedlings were thinned to one per pot after germination.
Maize was harvested after 40 days of growth (seedling stage). The leaves and roots were separated, and the leaves were washed with distilled water. The rinsed roots were soaked in EDTA-Na2 (5 mM) for 20 min and then washed again with distilled water. The leaves and roots were then dried at 105 °C for 30 min and at 75 °C to a constant weight. The completely dried biomass was ground into a 0.15 mm fine powder to determine Hg and Se contents.
The bulk soil and rhizosphere soil were collected. The bulk soil was obtained from areas away from the plant roots after removing the top 1–2 cm of surface soil. The plant roots were removed, and loose soil was gently shaken off, leaving approximately 1 mm of soil firmly attached to the roots. The root–soil samples were placed in 25 mL centrifuge tubes containing sterile 0.86% NaCl solution and incubated in an ice bath for 30 min, with shaking every 5 min. The plant roots were then removed and centrifuged at 4000× g for 30 min at 4 °C. The sediment at the bottom of the tube was the rhizosphere soil sample. The mixed samples of bulk soil and rhizosphere soil were air-dried and sieved to determine the Hg and Se contents.

2.3.2. The Hg and Se Analysis

The analytical techniques used to determine total Hg and Se content in the soil are described in Section 2.1.1. The available Hg and Se contents were determined following extraction using diethylenetriaminepentaacetic acid (DTPA) and the LC-AFS 6500 instrument according to China GB/T 23739-2009 [46].
The Hg content in maize was determined following digestion with HNO3 and using the LC-AFS 6500 according to China GB 5009.17-2014 [47]. The Se content in maize was determined following digestion with HNO3-HClO4 and using the LC-AFS 6500 according to China GB 5009.93-2017 [48].
The bioconcentration factor (BCF) and transfer factor (TF) of Hg and Se in maize tissues were calculated as follows [49]:
BCF = The content of element in plant tissue ÷ The content of element in soil
TF = The content of element in aboveground tissues ÷ The content of element in roots

2.3.3. Quality Control

During the analysis of Hg and Se contents in the soil samples, the national reference material GBW07403 (GSS-3) [50] was used for quality control. During the analysis of Hg and Se contents in the plant samples, the national reference material GBW10012 (GSB-3) [51] was used for quality control and to calibrate analytical instruments. Prior to the analysis of each batch of samples, quality control was performed using reference materials to ensure the accuracy and reliability of the analysis of that batch. The comparison of measured values versus certified reference values is shown in Table 5.

2.4. Data Analysis

Data processing for Se-enriched compound fertilizer properties and Hg/Se contents in soils and plants was performed using SPSS 25.0. Experimental data are expressed as mean ± standard error. Initially, a one-way analysis with Tukey’s post hoc test was applied. When F-values indicated significance (p < 0.05), the least significant difference (LSD) test was employed for planned pairwise comparisons among different treatments at p < 0.05.

3. Results

3.1. Binder Dosage Effects on Fertilizer Performance

The pH value is an important indicator in determining the binder ratio of compound fertilizer. In this study, three binder ratios with different doses were used to screen the appropriate raw material ratio by pH value. As shown in Table 6, the pH of the mixed materials in the six test proportions was 6.28–6.46. This was within the standard range of organic–inorganic compound fertilizers (5.5–8.0), and therefore, the mixed material could be further granulated. The granulation rate, OM content, and compressive strength of the Se-enriched compound fertilizer are shown in Figure 1.
Figure 1 shows that the highest granulation rates for each Se-enriched compound fertilizer formulation were achieved in the A2 (98.37%) and B3 (98.85%) treatments, although they were not significantly different. The OM contents of the Se-enriched compound fertilizer decreased with the increased binder dose. Specifically, compared with B1, the OM contents of B3 significantly decreased by 19.77%. Furthermore, for the same binder dose, the OM content of the Se-enriched compound fertilizer with attapulgite was significantly higher (3.42–22.25%) than it was with bentonite. The A2 (23.00 N) and B1 (23.33 N) treatments had the highest compressive strengths, which were significantly higher than those of A1, A3, B2, and B3.
Considering the pH, granulation rate, OM content, and compressive strength of the Se-enriched compound fertilizer, the A2 and B1 treatments were selected as having the most appropriate proportions; these treatments were defined as Formulation 1 and Formulation 2, respectively.
The Se-enriched compound fertilizers in Formulations 1 and 2 were comprehensively evaluated, and the results are shown in Table 7. Formulations 1 and 2 were in line with the relevant standard for organic–inorganic compound fertilizers (China GB 18877-2002) [40]; therefore, they could be used in the Hg-contaminated soil remediation experiment in the next stage of the study.
Table 7. The evaluation indices and determination values of the compound fertilizer.
Table 7. The evaluation indices and determination values of the compound fertilizer.
IndicatorGB 18877-2002
Standard [40]		Formulation 1Formulation 2Agronomic Significance
N (%)None11.17 ± 0.1212.06 ± 0.06Meeting the nutrients requirement of plant
P2O5 (%)None4.91 ± 0.084.67 ± 0.08
K2O (%)None9.66 ± 0.559.37 ± 0.21
Total nutrients (N + P2O5 + K2O) (%)≥15.025.74 ± 0.5726.10 ± 0.23
Moisture content (%)≤10.02.5 ± 0.031.7 ± 0.01Affecting storage stability
Organic matter (%)≥2058.26 ± 0.3858.45 ± 1.29Improving soil structure
Particle size (1.00–4.75 mm) (%)≥7093 ± 0.4492 ± 0.37Affecting use
pH value5.5–8.06.3 ± 0.026.3 ± 0.02Reducing nutrient fixation
Chloride ion (%)≤3.00.04 ± 0.000.11 ± 0.01Avoiding salt stress on crops
Arsenic (%)≤0.0050<DL<DLBelow the ecological risk threshold
Cadmium (%)≤0.0010<DL<DL
Lead (%)≤0.0150<DL<DL
Chromium (%)≤0.05000.0003 ± 0.00000.0004 ± 0.0000
Mercury (%)≤0.0005<DL<DL
Selenium (%)None0.20 ± 0.010.20 ± 0.03Supplement selenium
Note: Formulation 1, 15% attapulgite. Formulation 2, 10% bentonite. <DL, below the detection limit.

3.2. Soil Available Hg Reduction and Se Increase Caused by Fertilizers

The total Hg, total Se, DTPA-Hg, and DTPA-Se contents in soil under the different treatments are shown in Figure 2. Though there were no significant differences in soil total Hg content among the different treatments. Compared with the CK, the DTPA-Hg content in bulk soil and rhizosphere soil did not change significantly after applying the chemical fertilizer, but it decreased significantly after applying Formulations 1 and 2. The DTPA-Hg content decreased by 40.1% (bulk soil, Formulation 1), 47.3% (bulk soil, Formulation 2), 56.0% (rhizosphere soil, Formulation 1), and 53.8% (rhizosphere soil, Formulation 2), respectively.
Compared with the CK, the total and DTPA-Se contents in the soil were almost unaffected by chemical fertilizers, but they significantly increased after applying Formulations 1 and 2. However, there was no significant difference in the total and DTPA-Se contents between Formulations 1 and 2. Specifically, compared with the CK, the total Se content in the soil increased by 0.64 mg kg−1 (bulk soil, Formulation 1), 0.67 mg kg−1 (bulk soil, Formulation 2), 0.62 mg kg−1 (rhizosphere soil, Formulation 1), and 0.67 mg kg−1 (rhizosphere soil, Formulation 2), respectively. In comparison with the CK, the DTPA-Se content increased significantly by 0.22 mg kg−1 (bulk soil, Formulation 1), 0.21 mg kg−1 (bulk soil, Formulation 2), 0.14 mg kg−1 (rhizosphere soil, Formulation 1), and 0.15 mg kg−1 (rhizosphere soil, Formulation 2), respectively.
Figure 2b,c show that the DTPA-Hg and DTPA-Se contents in the bulk soil were significantly higher than in the rhizosphere soil under the same treatment.

3.3. Maize Hg Uptake Restriction and Se Uptake Enhancement with Fertilizers

The effect of Se-enriched compound fertilizer on the uptake of Hg and Se in maize is shown in Figure 3. Compared with the CK, the chemical fertilizer had no significant effect on the uptake of Hg or Se by maize, while Formulations 1 and 2 significantly reduced the Hg content and significantly increased the Se content in maize tissue. The Hg content in roots was significantly reduced by 29.0% under the Formulation 1 treatment and 26.4% under the Formulation 2 treatment compared with the CK. The Hg content in the leaves was significantly reduced by 58.7% under the Formulation 1 treatment and 57.3% under the Formulation 2 treatment compared with the CK. The Se content in roots was significantly increased by 0.82 mg kg−1 under the Formulation 1 treatment and 0.90 mg kg−1 under the Formulation 2 treatment compared with the CK. The Se content in leaves was significantly increased by 0.15 mg kg−1 under the Formulation 1 treatment and 0.14 mg kg−1 under the Formulation 2 treatment as compared with the CK.
As shown in Figure 4, compared with the CK, the Hg-BCF in roots was significantly reduced by 28.7% after applying of Formulation 1, but there was no significant change among the other treatments. The applications of Formulations 1 and 2 significantly reduced the Hg-BCF in leaves by 580.8% and 567.4%, respectively, compared with the CK. Additionally, compared with the CK, the Se-BCF in roots was significantly increased by 173.7% under the Formulation 1 treatment and 177.7% under the Formulation 2 treatment.

4. Discussion

4.1. The Effect of Different Binder Dosages on the Properties of Se-Enriched Compound Fertilizer

Granulation is a key process in the production of granular organic–inorganic compound fertilizers, and binders are a critical factor influencing this process [52]. All formulations met the China GB 18877-2002 [40] pH standard for organic–inorganic fertilizers, and a bentonite dosage of 10% yielded a significantly lower pH than 15% or 20% bentonite. This was due to the increased bentonite content (pH 10.02) in the latter two mixtures, which led to a corresponding decrease in vinegar residue (pH 4.17), thereby gradually increasing their pH [53].
The granulation rate is a critical factor in the whole production process, influencing fertilizer production costs and energy consumption [54]. When the binder doses were 10–20%, the granulation rate of attapulgite reached 98.02%, and that of bentonite reached 97.97%. This indicated that the attapulgite and bentonite exhibited strong pelletizing performance. The nutrient content of a compound fertilizer is one of the key indicators of its effectiveness. With an increase in binder dosage, the OM content of Se-enriched compound fertilizer particles significantly decreased. This was related to the reduction in the vinegar residue dose caused by the increased binder dose, with the OM contents of both attapulgite (0.01%) and bentonite (0.02%) being significantly lower than that of vinegar residue (12.37%). Compressive strength has a significant influence on the transportation and storage of fertilizer and is one of the indicators used to evaluate its fluidity and transmission performance [55]. The compressive strength of the particles was largest with an attapulgite dose of 15% and a bentonite dose of 10%. With a further increase in the binder dose, the compressive strength of the particles decreased. This was because excessive binder absorption causes expansion, resulting in its adhesive properties being lower than its disintegration properties [55]. Therefore, considering the pH, granularity rate, OM content, and compressive strength of the Se-enriched compound fertilizer, an attapulgite dose of 15% (Formulation 1) and a bentonite dose of 10% (Formulation 2) were selected as the binder doses in the follow-up experiment.

4.2. Remediation Effectiveness of Se-Enriched Compound Fertilizer for Hg-Contaminated Soil

The Se-enriched compound fertilizer significantly increased the Se content in the soils. However, the soil Se contents (0.80–0.87 mg kg−1) with Formulations 1 and 2 were below the national standard (3.00 mg kg−1) (China DZ/T 0295-2016) [56]. Notably, the DTPA-Se content in the bulk soil was significantly higher than that in the rhizosphere soil. This was because, under the influence of root exudates, more stable Hg–Se complexes were formed in the rhizosphere region [57]. However, a portion of the available Se was absorbed by plants, reducing the DTPA-Se content in the rhizosphere soil. The Se concentration in soil is the main driving factor determining the Se concentration in plants [58], which can also be used to evaluate the stress resistance of plants [59]. As shown in Figure 3b, Se accumulation in maize roots increased significantly under Formulations 1 and 2, likely due to the increased soil Se concentrations and increased need for plants to mitigate Hg stress [60,61]. Li et al. [62] also found that the Se content of plant tissues increased significantly with the application of exogenous Se.
With the application of Formulations 1 and 2, the DTPA-Hg content was reduced, which was significantly higher than in the CK and chemical fertilizer treatment. Moreover, the reduced DTPA-Hg content in the rhizosphere soil was higher than in the bulk soil, indicating that the rhizosphere was the primary region for the formation of insoluble Hg–Se and its complexes [57]. The decrease in the DTPA-Hg content could be attributed to Se facilitating the transformation of soil Hg from the effective forms (exchangeable, carbonate-bound) into the stable forms (OM-bound, residue state) [63].
Under the Formulations 1 and 2, the Hg content of maize tissues was significantly decreased because of the reduced DTPA-Hg in rhizosphere soil (Figure 2b). The results of this study aligned with those of Tran et al. [25], Zhang et al. [57], Shanker et al. [64], and Mounicou et al. [65] regarding Hg–Se antagonism, in which the Hg–Se complexes in the rhizosphere and/or roots were a key factor limiting the bioavailability, uptake, transport, and accumulation of Hg. Two novel conclusions emerge from this study. First, unlike the standalone Se application, the Se-enriched compound fertilizer system enhanced Hg–Se complex stability through OM complexation, electrostatic adsorption, and ion exchange [66,67]. Second, the alkaline pH (7.7) in the study soil decreased Se availability, resulting in BCF and TF values in maize leaves with no significant changes (Figure 4e,f). Formulations 1 and 2 significantly reduced the Hg-BCF in both the roots and shoots of maize seedlings. Qian et al. [68] also found that exogenous Se could significantly reduce the BCF of pak choi tissues. Previous studies have confirmed that enrichment with a large number of Hg Se complexes in soil and rhizosphere regions (including rhizosphere soil and plant root surface) was one of the main factors inhibiting the migration of Hg to plants. While this study confirmed Hg–Se interactions through conventional analyses, advanced techniques reported in the literature have provided molecular-scale evidence for Hg–Se complex formation in similar remediation systems—including capillary reversed phase chromatography coupled with inductively coupled plasma mass spectrometry (capRPLC-ICPMS), X-ray absorption near-edge structure (XANES), synchrotron X-ray fluorescence spectroscopy (SR-XRF) [21], and transmission electron microscopy spectroscopy [69].
Organic matter is a critical variable in the adsorption of Hg due to the strong interaction between Hg and the thiol groups (-SH) and/or functional groups containing OM sulfur [70,71]. The elevated OM content (about 58%) obtained from potassium humate, vinegar residue, and biochar in Formulations 1 and 2 enhanced Hg immobilization through the role of OM in retaining Hg [67]. Moreover, ternary complexes formed between the Se and OM particles, decreasing the availability of soil Se [72,73]. Additionally, reducing DTPA-Hg in soil and Hg in maize could contribute to physical immobilization, such as the large specific surface area in the vinegar residue and biochar [30], which could adsorb Hg, Se, and their complexes.
In summary, the Se-enriched compound fertilizer developed in this study had a strong remediation effect on Hg-contaminated agricultural soil and could significantly reduce the risk of maize Hg exposure.

5. Conclusions

In this study, vinegar residue, biochar, and potassium humate were used as organic fertilizers, combined with inorganic nutrients (urea, KH2PO4, ZnSO4, and Na2SeO3) and different binder doses (attapulgite and bentonite) to develop Se-enriched organic–inorganic compound fertilizers. The optimal granulation rate, OM content, and compressive strength of Se-enriched compound fertilizer particles were achieved with an attapulgite dose of 15% and a bentonite dose of 10%. The two Se-enriched compound fertilizer formulations met the organic–inorganic compound fertilizer standard for use in China. Formulations 1 and 2 significantly reduced the DTPA-Hg content in bulk soil and rhizosphere soil, as well as the Hg content in maize seedling tissue. The total and DTPA-Se in bulk and rhizosphere soil and the Se content in plants significantly increased after applying the Se-enriched compound fertilizers. The results of this study demonstrate the effectiveness of applying Se remediation agents to Hg-contaminated farmlands. However, the Se contents of soil and crops should be closely monitored to ensure that the safe threshold is not exceeded and there is no toxic effect on the environment.

Author Contributions

Conceptualization, Y.L. (Yuxin Li) and H.L.; methodology, Y.L. (Yuxin Li) and G.P.; software, S.G. and Y.Z.; validation, Y.L. (Yuxin Li) and H.L.; formal analysis, Y.L. (Yuxin Li) and S.G.; investigation, Y.L. (Yuxin Li) and G.P.; resources, Y.L. (Yingzhong Lv) and H.L.; data curation, Y.L. (Yuxin Li) and G.P.; writing—original draft preparation, Y.L. (Yuxin Li); writing—review and editing, Y.L. (Yuxin Li) and H.L.; visualization, Y.Z. and Z.L.; project administration, Y.L. (Yingzhong Lv) and H.L.; funding acquisition, G.P. and Y.L. (Yingzhong Lv) All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Project of Shanxi Province Key Lab Construction, China (No. Z135050009017-1-4), the Fundamental Research Program of Shanxi Province, China (No. 202303021212103, No. 202303021221057), the Shanxi Key Laboratory of Germplasm Improvement and Utilization in Pomology (No. PILAB20241507), the Research Award Fund for Outstanding Doctor Working in Shanxi, China (No. SXBYKY2023019), and the Scientific Research Starting Project for the Doctor of Shanxi Agricultural University, China (No. 2023BQ14).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 15 01842 g001
Figure 1. Granulation rate, organic matter content, and compressive strength of Se-enriched compound fertilizer under different proportioning schemes. (a) The granulation rate of Se-enriched compound fertilizer; (b) the organic matter content of Se-enriched compound fertilizer; (c) the compressive strength of Se-enriched compound fertilizer. A1, 10% attapulgite. A2, 15% attapulgite. A3, 20% attapulgite. B1, 10% bentonite. B2, 15% bentonite. B3, 20% bentonite. Different letters represent significant differences between treatments at p < 0.05.
Figure 1. Granulation rate, organic matter content, and compressive strength of Se-enriched compound fertilizer under different proportioning schemes. (a) The granulation rate of Se-enriched compound fertilizer; (b) the organic matter content of Se-enriched compound fertilizer; (c) the compressive strength of Se-enriched compound fertilizer. A1, 10% attapulgite. A2, 15% attapulgite. A3, 20% attapulgite. B1, 10% bentonite. B2, 15% bentonite. B3, 20% bentonite. Different letters represent significant differences between treatments at p < 0.05.
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Agronomy 15 01842 g002
Figure 2. Variations in the total and DTPA-extractable Hg and Se contents in soil under different treatments. (a) The total Hg content in soil. (b) The DTPA-extractable Hg content in soil. (c) The total Se content in soil. (d) The DTPA-extractable Se content in soil. Different letters represent significant differences between treatments at p < 0.05.
Figure 2. Variations in the total and DTPA-extractable Hg and Se contents in soil under different treatments. (a) The total Hg content in soil. (b) The DTPA-extractable Hg content in soil. (c) The total Se content in soil. (d) The DTPA-extractable Se content in soil. Different letters represent significant differences between treatments at p < 0.05.
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Agronomy 15 01842 g003
Figure 3. Variations in the Hg and Se contents in maize under different treatments. (a) The Hg content in maize. (b) The Se content in maize. Different letters represent significant differences between treatments at p < 0.05.
Figure 3. Variations in the Hg and Se contents in maize under different treatments. (a) The Hg content in maize. (b) The Se content in maize. Different letters represent significant differences between treatments at p < 0.05.
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Figure 4. The bioconcentration factor (BCF) and transfer factor (TF) of Hg and Se in maize. (a) The BCF of Hg in maize roots. (b) The BCF of Hg in maize leaves. (c) The TF of Hg in maize leaves. (d) The BCF of Se in maize roots. (e) The BCF of Se in maize leaves. (f) The TF of Se in maize leaves. Different letters represent significant differences between treatments at p < 0.05.
Figure 4. The bioconcentration factor (BCF) and transfer factor (TF) of Hg and Se in maize. (a) The BCF of Hg in maize roots. (b) The BCF of Hg in maize leaves. (c) The TF of Hg in maize leaves. (d) The BCF of Se in maize roots. (e) The BCF of Se in maize leaves. (f) The TF of Se in maize leaves. Different letters represent significant differences between treatments at p < 0.05.
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Table 1. Basic physicochemical properties of the experimental soil.
Table 1. Basic physicochemical properties of the experimental soil.
Soil PropertiesValue
TextureLoam
Clay (%)5.14
Silt (%)41.78
Sand (%)53.08
pH7.70
Electrical conductivity (μs cm−1)269
Cation exchange capacity (mmol kg−1)15.16
Organic matter (g kg−1)19.41
Total Hg (mg kg−1)5.38
Total Se (mg kg−1)0.27
Table 2. Basic properties of the raw materials of the Se-enriched compound fertilizer.
Table 2. Basic properties of the raw materials of the Se-enriched compound fertilizer.
Basic PropertiesVinegar ResidueBiocharPotassium HumateAttapulgiteBentonite (Sodium-Based)
pH4.179.979.045.4210.02
Total Hg (%)00000
Total Se (%)00000
N (%)0.510.080.100.050.05
P2O5 (%)0.120.031.420.010.79
K2O (%)--10.14--
Total nutrients (%)0.630.111.520.060.84
Table 3. Proportioning scheme of the compound fertilizer.
Table 3. Proportioning scheme of the compound fertilizer.
LabelsUrea
(%)		KH2PO4
(%)		ZnSO4
(%)		Na2SeO3
(%)		Potassium Humate
(%)		Vinegar Residue
(%)		Biochar
(%)		Attapulgite
(%)		Bentonite
(%)
A122.519.593.30%0.4440.009.165.0010.000.00
A222.519.593.30%0.4440.004.165.0015.000.00
A322.519.593.30%0.4440.000.004.1620.000.00
B122.519.593.30%0.4440.009.165.000.0010.00
B222.519.593.30%0.4440.004.165.000.0015.00
B322.519.593.30%0.4440.000.004.160.0020.00
Table 4. The experimental treatment.
Table 4. The experimental treatment.
LabelsControlChemical FertilizerFormulation 1Formulation 2
TreatmentReceived no treatmentAdded Chemical fertilizerAdded Formulation 1 of compound fertilizerAdded Formulation 2 of compound fertilizer
Table 5. The comparison of measured values versus certified reference values.
Table 5. The comparison of measured values versus certified reference values.
GBW07403 (GSS-3)Measured ValuesGBW10012 (GSB-3)Measured Values
Se (mg kg−1)0.09 ± 0.020.08 ± 0.010.021 ± 0.0080.022 ± 0.004
Hg (mg kg−1)0.060 ± 0.0040.062 ± 0.0020.00160.0017 ± 0.001
Whether it met the standardsYes
Table 6. The pH of the Se-enriched compound fertilizer under different proportioning schemes.
Table 6. The pH of the Se-enriched compound fertilizer under different proportioning schemes.
LabelsA1A2A3B1B2B3
pH6.31 ± 0.03 b6.28 ± 0.02 b6.33 ± 0.02 b6.32 ± 0.02 b6.42 ± 0.01 a6.46 ± 0.01 a
Note: A1, 10% attapulgite. A2, 15% attapulgite. A3, 20% attapulgite. B1, 10% bentonite. B2, 15% bentonite. B3, 20% bentonite. Different letters represent significant differences between treatments at p < 0.05.
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MDPI and ACS Style

Li, Y.; Pei, G.; Zhang, Y.; Guan, S.; Lv, Y.; Li, Z.; Li, H. Development and Evaluation of Selenium-Enriched Compound Fertilizers for Remediation of Mercury-Contaminated Agricultural Soil. Agronomy 2025, 15, 1842. https://doi.org/10.3390/agronomy15081842

AMA Style

Li Y, Pei G, Zhang Y, Guan S, Lv Y, Li Z, Li H. Development and Evaluation of Selenium-Enriched Compound Fertilizers for Remediation of Mercury-Contaminated Agricultural Soil. Agronomy. 2025; 15(8):1842. https://doi.org/10.3390/agronomy15081842

Chicago/Turabian Style

Li, Yuxin, Guangpeng Pei, Yanda Zhang, Shuyun Guan, Yingzhong Lv, Zhuo Li, and Hua Li. 2025. "Development and Evaluation of Selenium-Enriched Compound Fertilizers for Remediation of Mercury-Contaminated Agricultural Soil" Agronomy 15, no. 8: 1842. https://doi.org/10.3390/agronomy15081842

APA Style

Li, Y., Pei, G., Zhang, Y., Guan, S., Lv, Y., Li, Z., & Li, H. (2025). Development and Evaluation of Selenium-Enriched Compound Fertilizers for Remediation of Mercury-Contaminated Agricultural Soil. Agronomy, 15(8), 1842. https://doi.org/10.3390/agronomy15081842

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