Combined Approaches for the Remediation of Cadmium- and Arsenic-Contaminated Soil: Phytoremediation and Stabilization Strategies

Abstract

The prolonged duration of phytoremediation poses a risk of heavy metal dispersal to the surrounding environment. This study investigated a combined remediation approach for cadmium (Cd)- and arsenic (As)-contaminated soil by integrating phytoremediation with stabilization techniques. Bidens pilosa was utilized as the phytoremediator, and steel slag, pyrolusite, and FeSO₄ were employed as stabilizing agents in the pot experiments. Key metrics such as soil moisture content, root length, plant height, and heavy metal concentrations in Bidens pilosa were measured to evaluate the remediation efficacy. Additionally, the bioavailability, leaching toxicity, and chemical forms of Cd and As, along with other soil properties, were analyzed. The results indicated that the optimal restoration effect was achieved by combining steel slag, pyrolusite, and FeSO₄ with stabilizers in a ratio of 2:1:10. Additionally, the optimal dosage of these materials was found to be 9% by weight. Mechanistic studies, including heavy metal speciation analysis, X-ray photoelectron spectroscopy (XPS), and microbial community diversity analysis, revealed that the stabilization effects were primarily due to the interactions of anionic and cationic ions, chelation by organic acids secreted by plant roots, and enhanced microbial activity. A cost–benefit analysis demonstrated the technical, economic, and commercial viability of the combined remediation approach.

1. Introduction

Heavy metal contamination, particularly by cadmium and arsenic, poses significant threats to environmental and human health due to their persistence and toxicity [1]. In China, approximately 10 million hectares of farmland, accounting for over 8% of the total arable land, were found to be contaminated with heavy metals, leading to an annual reduction of about 10 billion kilograms of grain [2]. Industrial sites and mining activities contribute significantly to this pollution, affecting over 2 million hectares of land, with an additional 33,000 to 47,000 hectares impacted annually [3].

The escalating issue of soil contamination with cadmium and arsenic, resulting from widespread mineral extraction, separation, smelting, and deep processing, poses a severe environmental threat [4]. In 2002, a survey by the Ministry of Agriculture of China found that 10.3% of rice samples exceeded the cadmium limit [5,6]. Recent studies have indicated that this issue persists. For instance, a study conducted in the Chengdu Plain of Sichuan revealed substantial cadmium accumulation in soils, with about 78% of grain samples exceeding the national food standard for cadmium levels [7]. This continued exposure poses serious health risks, including skeletal and kidney damage, hypertension, and cancer. The Itai-Itai disease in Japan, caused by cadmium-contaminated rice [8], exemplifies the severe health risks associated with cadmium exposure. Arsenic contamination has been found to be prevalent in groundwater and soil, mainly due to natural geological processes and human activities like pesticide application and industrial processes. Regions such as the Hetao Basin in Inner Mongolia have frequently reported groundwater arsenic levels exceeding the World Health Organization’s recommended limits, posing significant risks to local communities [9]. Chronic arsenic exposure can lead to skin lesions, cardiovascular diseases, and various cancers, including skin, lung, and bladder cancers. The high arsenic levels in the groundwater of the Qinling region has been found to exacerbate health risks for local residents [10].

Previous research has extensively explored the various physical, chemical, and biological remediation techniques for controlling heavy metal contamination in soils. Common physical and chemical remediation methods include excavation, electroremediation, chemical leaching, and stabilization [11]. Chemical stabilization using soil amendments—such as carbon-based, lime-based, sepiolite-based, bentonite-based, iron-based, clay-mineral-based, silicate-based, modified nanomaterials, and other synthetic agents—has been deemed to be effective for remediating contaminated farmland [12,13,14,15,16,17,18].

Phytoremediation, known for its in situ application, environmental friendliness, and cost-effectiveness, has been widely used for the remediation of heavy-metal-contaminated soils [19,20]. Different strategies have been adopted based on the level of contamination to maximize the effectiveness of phytoremediation. Phytoextraction, which utilizes hyperaccumulators to remove heavy metals from the soil, is suitable for mildly–moderately contaminated soils. Phytostabilization involves using plants to immobilize pollutants, thereby reducing their bioavailability and environmental hazards. Through root adsorption and soil stabilization, phytostabilization effectively reduces the bioavailability and mobility of heavy metals in the soil [21,22]. The use of commercially available metal-tolerant plants for phytostabilization has gained research interest. Bidens pilosa has been shown to have potential for the phytoremediation of heavy-metal-contaminated soils due to its ability to accumulate and tolerate various heavy metals, including cadmium, arsenic, lead, and zinc [23]. Studies have shown its effectiveness in stabilizing these metals in mining areas, reducing their phytotoxicity, and decreasing metal mobility in the rhizosphere. Thus, vegetation cover on contaminated soils could be restored, mitigating environmental risks [24]. Utilizing native plants for soil restoration was particularly effective in severely polluted mining areas, where environmental pressures selected for tolerance mechanisms. These plants have exhibited higher resistance through effective exclusion mechanisms as well as having a greater tolerance to harmful concentrations [25].

However, the extraction rate of a single plant is relatively slow, and biomass accumulation is often affected under high heavy metal stress. Generally, field experiments require over a year for an objective evaluation of phytoextraction performance, while phytoremediation projects based on phytoextraction take several years or even decades [26]. During the remediation process, plant roots produce substances such as small molecular organic acids under bacterial action, promoting heavy metal dissolution. When the dissolution rate of heavy metals exceeds the extraction rate by plants, the risk of heavy metals diffusing into the external environment remains high. Therefore, “assisted natural remediation” became a new trend in phytoremediation technology. This approach involves adding amendments to chelate, adsorb, precipitate, and chemically react with metal ions to accelerate the remediation process, thereby reducing the bioavailability of metals [27].

Thus, the additional application of soil amendments plays a crucial role in enhancing the phytostabilization of contaminated soils. For instance, amendments could be added to improve the physicochemical properties of the soil. Previous studies have often involved the addition of synthetic amino polycarboxylates (such as ethylenediaminetetraacetic acid (EDTA) and glutamic acid diacetate tetrasodium (GLDA)), natural amino polycarboxylates (such as (S,S)-ethylenediamine-N,N-disuccinic acid (EDDS) and nitrilotriacetic acid (NTA)), and natural small molecular organic acids (such as citric acid (CA) and tannic acid (TA)) to chelate metals, promoting their dissolution and uptake by plants [28]. For instance, the study by Hong Chen et al. [29] demonstrated that the addition of EDTA and HEDTA increased the Cd and Ni contents in sunflower stems and leaves from 34 mg/kg and 15 mg/kg to 115 mg/kg and 117 mg/kg, respectively. However, Kos et al. [30] found that adding EDTA and 10 mmol/kg [S,S]-EDDS to soil significantly enhanced plant lead uptake but also caused the leaching of most of the total lead that was initially present in the soil, which was not a safe phytoremediation approach. Additionally, most chelating agents were found to be toxic and difficult to degrade in the soil.

In recent years, using minerals or industrial materials that are rich in metal oxides for heavy metal soil remediation has become a popular research topic. For example, Zhang et al. [31] used zeolite, diatomaceous earth, and sodium bentonite as main remediation components, supplemented with monocalcium phosphate and fulvic acid, to study Cd remediation in mildly alkaline soil with wheat. Field trials showed that Cd concentrations in rhizosphere and non-rhizosphere soils decreased by 27.3–31.2% and 34.3–54.2%, respectively. Jain et al. [32] planted Jatropha, Gliricidia, and sunflower in contaminated soil mixed with fly ash, finding that Jatropha achieved the highest remediation efficiency and plant biomass in the 10% fly ash treatment group, with remediation efficiencies for aluminum, iron, manganese, and zinc increased by 187.79%, 150.36%, 118.67%, and 137.43%, respectively, compared to the control. The team of Lee [33] used lettuce with limestone and red mud to treat contaminated soil, discovering that Ca(NO₃)₂-extractable As, Cd, Pb, and Zn were reduced by 58%, 98%, 98%, and 99%, respectively, after remediation, significantly decreasing the total accumulation of heavy metals in lettuce shoots. Liu et al. [34] used carbide slag to remediate soil co-contaminated with arsenic and cadmium and planted brown rice. The results indicated that the addition of carbide slag promoted the formation of iron–manganese plaques on plant roots, thereby stabilizing both arsenic and cadmium simultaneously.

The added minerals primarily fixed heavy metals through complexation, ion exchange, and electrostatic adsorption. By converting unstable components into stable ones, the heavy metal content in the soil could be reduced [31]. Minerals and alkaline industrial wastes containing alkaline cations mainly increased soil pH, causing oxides of calcium, iron, and manganese to form insoluble complexes or adsorb heavy metals into their crystal structures, or reduce highly active and toxic high-valence metal ions to stable, low-toxicity, low-valence metal ions, thus reducing metal bioavailability [35].

From the above studies, it is evident that combining metal-oxide-containing minerals and industrial waste with plants significantly improved the soil remediation efficiency. Compared to synthetic soil amendments, metal-oxide-containing minerals and industrial waste offered greater economic benefits and environmental sustainability.

Bidens pilosa, a perennial herb of the Asteraceae family, was widely distributed in polluted areas of the Huilong Arsenic Smelting Plant. It adapted to harsh living conditions in contaminated mining environments. Utilizing Bidens pilosa as a replacement for traditional crops like rice was a feasible strategy to prevent heavy metals from entering the food chain, reducing associated health risks. Its medicinal value could offset economic losses. A study using Bidens pilosa and a series of amendments (economical materials: steel slag, pyrolusite, and FeSO₄ = 2:1:10) in pot experiments demonstrated its effectiveness in remediating arsenic-contaminated soils. At the post-remediation point, the effectiveness was evaluated by analyzing heavy metal forms, X-ray photoelectron spectroscopy (XPS), and microbial diversity to explore potential remediation mechanisms. The results aimed to provide theoretical and practical support for establishing effective remediation systems for multi-metal-contaminated farmlands.

2. Materials and Methods

2.1. Assessment of Contaminated Soil Characteristics and Pollution Levels

The test soil was sourced from the area surrounding of the Huilong Arsenic Smelting Plant in Zhongshan County, located in Guangxi, South China, and situated at 24.46° N, 111.28° E, with an elevation of 173 m. The sampling area was shown in Figure 1. Soil sampling was executed as per the methods described by Barbara [36], with 30 samples collected from the shallow layer (0–20 cm), mixed well, and stored in 14 cm × 20 cm sealed bags, each weighing 1 kg. After air-drying for 24 h, the soil samples were sifted through a sieve of 10 mesh (2 mm) and reserved for further use.

The primary physicochemical attributes of the soil included a pH value of 6.25, a moisture content of 12.28%, an organic matter content of 13.5 g/kg, an available nitrogen content of 663 mg/kg, a phosphorus content of 360 mg/kg, a potassium content of 64 mg/kg, and a cation exchange capacity of 6.37 meq/100 g.

The contamination assessment, detailed in

Table 1. The content and pollution characteristics of heavy metals in soil.

Items	Cd	As
Total heavy metal (mg/kg)	0.38 ± 0.04	76.14 ± 0.23
Bioavailable metal content (mg/kg)	0.25	0.13
Agricultural Soil Risk Screening Standard Value (mg/kg)	0.3	30
Agricultural Soil Risk Control Standard Value (mg/kg)	2	150
PI value	1.26	2.54
Pollution Level	Slight pollution	Moderate pollution

, focused on the site’s heavy metal levels. Cd and As were the primary concerns, exceeding the threshold for safe agricultural use. While Cd displayed mild pollution levels, As was significantly more concerning, showing severe pollution levels. The pollution index (PI) [37] was calculated using the following formula:

$$PI={\displaystyle \frac{Ci}{S\mathrm{i}}}$$

(1)

where C_i (mg/kg) denoted the metal concentration of the soil, while S_i (mg/kg) was the environmental quality standard for soil.

2.2. Sources and Composition of Soil Remediation Materials

In this experiment, industrial by-products, specifically steel slag and soft manganese ore (notably pyrolusite), were utilized as substrates in the soil remediation study. The steel slag, obtained from Guangxi Chongzuo Jinxin Manganese Industry Co., Ltd. (Chongzuo, China), primarily consists of 44.32% calcium oxide, supplemented by 18.12% iron (III) oxide and 12.12% silicon dioxide. Conversely, pyrolusite, sourced from Hunan Leiyang Xingfa Manganese Industry Co., Ltd. (Leiyang, China), was rich in manganese (II) oxide at 23.67%, along with substantial amounts of silicon dioxide at 44.00% and iron (III) oxide at 22.54%. These materials also contained significant levels of aluminum oxide, suggesting their potential efficacy in soil amendment. Additionally, the ferrous sulfate (AR, Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China) used in our study was supplied by Guangxi Hezhou Jianlian Biotechnology Co., Ltd. (Hezhou, China). We employed X-ray fluorescence analysis (XRF) to detect these key components critical for phytoremediation. The plants chosen for this remediation effort, which were sourced from contaminated areas, were well-adapted to environments enriched with the specified minerals, enhancing their suitability for this application.

2.3. Experimental Design of Potted Plant and Material–Plant Combined Remediation

The plants used in the preliminary screening were local plants collected from contaminated sites (including dogtooth root, small pomegranate, false odor grass, and white flower ghost needle grass). The final selection of plant seeds was purchased from Dukou Jiajia Seed Industry Company. Mature and plump seeds with basically the same size were selected and disinfected with 0.5% sodium hypochlorite solution (AR, Shanghai Macklin Biochemical Technology Co., Ltd.) for 10 min; then, they were rinsed with deionized water before sowing.

Potted plant experiment:

(1) Tolerance plant screening: Cynodon dactylon, Bidens Pilosa, and Boehmeria nivea were chosen as the remediation plants through a pre-pot experiment. A series of pots, each containing 400 g of soil, were used to sow pre-disinfected and pre-soaked plant seeds at a rate of 20 g per square meter. After being cultivated in a greenhouse under environmental conditions of a temperature range of 18–25 °C and a relative humidity of 65–75% for 60 days, plant samples were harvested to assess their physiological parameters and heavy metal concentrations. At 60 days, the plants were harvested, and root length, plant height, and water content data were recorded three times, with the average taken. The heavy metal content in plant tissues was measured. At the same time, soil samples were collected three times to detect the effective heavy metal content in the soil, and the average value was taken.

(2) Material–plant combined remediation experiment: Steel slag, pyrolusite, and ferrous sulfate were combined in various mass ratios (the different mass proportions among the three) to fabricate distinct composite materials. Specified quantities of these composites, ranging from 1% to 19% by soil mass, were homogeneously mixed with the soil. Each flowerpot was filled with 400 g of soil, which was subsequently blended with different proportions of the steel slag, soft manganese ore, and ferrous sulfate to create varied ratios of iron–manganese material, constituting 11% of the soil mass, and allowed to stabilize for a period of 14 days. Following this stabilization phase, 20 seeds of selected phytoremediation plants were sown in each container, integrating the material with the plants for comprehensive remediation. After a growth period of 60 days, soil samples were collected for analytical assessment. The optimal composition of the iron–manganese material was determined based on the reduction rate of bioavailable states of heavy metals in the soil, with higher reduction rates indicating superior remediation efficacy.

The optimal dosage of stabilizer materials was determined based on the research of the optimal formulation. Potted plants were treated with different dosages of 1%, 3%, 5%, 7%, 9%, 13%, 15%, 17%, and 19% (based on soil quality) to stabilize the above stability and plant seeding. Then, soil samples were collected for further analysis. The optimal dosage of the iron–manganese material was established based on maximizing the decrease in bioavailable heavy metal concentrations in the soil. After the growth cycle, the harvested plants were air-dried, and each plant was individually sealed for subsequent measurement of bioavailable heavy metal content. All experiments were conducted in an artificial climate incubator (25 °C with 75% humidity and full illumination).

2.4. Analysis Method of Soil and Plant Samples

(1)

Bioavailable Heavy Metal Content Analysis in Soil:

This study used NaHCO₃ as an extractant to extract arsenic from soil, and calculated the bioavailability of arsenic through ICP-MS (NEXION 2000, PerkinElmer, Waltham, MA, USA) analysis [38]. A 10 g of the sample, sieved through a 2 mm screen, were taken in a 250 mL conical flask. Then, 150 mL of 0.5 mol/L NaHCO₃ solution was added. The mixture was shaken at the magnetic stirrer (Cimarec+, Thermo Scientific, Waltham, MA, USA) with 25 °C and 200 r/min for 2 h and filtered through a 0.45 μm microporous membrane (MF-Millipore™ Filter membrane, pore size 0.45 µm) for subsequent ICP-MS (Nexlon 300×, Perkin Elmer, USA) analysis. We repeated each test three times and took the average.

Bioavailable Cd and Pb were extracted using a diethylene–triamine–penta-acetic acid (DTPA) solution (0.005 mol/L DTPA(AR, Shanghai Macklin Biochemical Technology Co., Ltd.), 0.01 mol/L CaCl₂ (AR, Shanghai Macklin Biochemical Technology Co., Ltd.), and 0.1 mol/L triethylamine (TEA, AR, Shanghai Macklin Biochemical Technology Co., Ltd.)) at a soil-to-water ratio of 1:5 (w/v), and the results were obtained via ICP analysis [39,40]. We repeated each test three times and took the average. The reduction rate (η) of the bioavailable heavy metal [41,42] in the soil after material remediation was calculated using the following formula:

$$\mathsf{\eta } \left(\%\right)={\displaystyle \frac{{\mathsf{\eta }}_{\mathrm{Before}}{-\mathsf{\eta }}_{\mathrm{After}}}{{\mathsf{\eta }}_{\mathrm{Before}}}}\times 100$$

(2)

where η_Before measures the bioavailable heavy metal concentrations in the soil before combined remediation, and η_After measures them after combined remediation.

(2)

Heavy Metal Speciation Distribution Analysis in Soil:

Arsenic speciation was determined using the Tessier method, and cadmium speciation was analyzed using the BCR sequential extraction method [43]. Repeated each test three times and taken the average.

(3)

Total Heavy Metal Content Analysis in Plants:

The test samples (0.04 g of the plant under investigation) were digested in a polytetrafluoroethylene (PTFE) crucible [44]. The sample was treated with a mixture of HCl and HNO₃ (AR, Shanghai Macklin Biochemical Technology Co., Ltd.) in a 3:1 volume ratio, heated until the volume reduced to approximately 3 mL, and then allowed to digest at low temperatures until clear. The solution was transferred into a pre-labeled container, and the crucible was washed with 1% nitric acid. The filtrate was obtained by filtering the wash solution through a filter membrane (0.22 μm) and was then used for ICP-MS analysis. We repeated each test three times and took the average.

(4)

Plant Extraction Indices:

The bioconcentration factor (BCF) and translocation factor (TF) [45] indicated the vegetal potential to absorb and distribute metals.

BCF was the comparative level of heavy metal concentrations in plant tissue ($C_{Plant}$) versus soil ($C_{Soil}$) [46]. The formula for calculating BCF is as follows:

$$\mathrm{B}\mathrm{C}\mathrm{F}={\displaystyle \frac{C_{Plant} }{C_{Soil}}}$$

(3)

TF measured the comparison of metal concentrations from the shoot ($C_{Plant shoot}$) versus the roots ($C_{Plant root}$) [47]. The formula for calculating TF is as follows:

$$\mathrm{T}\mathrm{F}={\displaystyle \frac{C_{Plant shoot}}{ C_{Plant root}}}$$

(4)

(5)

Statistical analysis

The data were subjected to one-way ANOVA using SPSS 29.0.2 software. At least three independent replicates of each sample and determination were tested, and mean values and respective standard deviations were calculated.

2.5. Analysis Methods for Combined Remediation Mechanisms

(1) In our study, chemical phenomena such as chemical migration during soil remediation were observed through X-ray photoelectron spectroscopy (XPS). Before testing, the obtained soil samples underwent pre-treatment involving three washes with deionized water, followed by filtration and drying within a vacuum oven maintained at 40 °C [48]. The pre-treated samples were then prepared according to the testing standards and subsequently analyzed using a Thermo Scientific Nexsa spectrometer.

(2) To thoroughly examine the microbial diversity within rhizosphere soil samples from plants involved in phytoremediation, a systematic sequence of procedures was meticulously followed. Initially, DNA was extracted from the samples and subsequently amplified using PCR techniques, as referenced in previous studies [49]. Following the amplification process, the PCR products were evaluated and quantified with SynGene’s Gene Tools Analysis Software, ensuring the amplified products met the necessary quality standards. Only the products that passed this quality check were then purified. At the post-purification stage, the libraries were prepared on the Illumina platform, and sequencing was conducted to delve into the genetic material. Finally, the base calling was executed on the acquired sequencing images, allowing for the generation of raw data reads. This methodical approach not only ensured the accuracy of the data but also provided a comprehensive analysis of the microbial communities.

3. Results and Discussion

3.1. Results of Potted Experiments on Remediation Plants

As shown in Figure 2, the contaminated soil of the Zhongshan Huilong Arsenic Smelting Plant in Guangxi was used as the planting soil for pot experiments. After 60 days of greenhouse cultivation, Cynodon dactylon, Bidens pilosa, and Boehmeria nivea exhibited robust growth and substantial biomass. The results of the greenhouse pot experiment demonstrated that these three test plants were equipped with good adaptability towards the contaminated soil, and they exhibited strong tolerance to As and Cd pollution. Samples of the three plant species and the soil after remediation were collected and subjected to analysis. The results are presented in

Table 2. The growth parameters of plants (p < 0.05).

Plants	Average Root Length (cm)	Average Plant Height (cm)	Moisture Content (%)
Cynodon dactylon	9.1 ± 0.1 c	9.6 ± 0.5 b	68.1 ± 0.8 c
Bidens pilosa	9.7 ± 0.5 b	8.5 ± 0.7 c	71.5 ± 0.6 b
Boehmeria nivea	11.7 ± 0.4 a	12.7 ± 0.4 a	75.5 ± 0.8 a

The table presented the one-way ANOVA results for plant height, root length, and water content for three plant species. Each set of data had a sample size of n = 3. The letters “a”, “b”, and “c” denote significant differences among the means, with the same letter indicating no significant difference. The significance level is indicated by p, with p < 0.05 denoting a significant difference.

,

Table 3. Belowground (BG)/aboveground (AG), As and Cd content (mg/kg), bioconcentration factor (BCF), and translocation factor (TF) of plants.

Heavy Metal	Cynodon dactylon				Bidens pilosa				Boehmeria nivea
	BG	AG	BCF	TF	BG	AG	BCF	TF	BG	AG	BCF	TF
As	73.20	16.80	0.43	0.23	63.35	15.95	0.33	0.25	7.40	4.50	0.07	0.61
Cd	0.09	0.03	0.13	0.33	0.15	0.25	0.62	1.67	0.07	0.01	0.08	0.14

and

Table 4. Reduction rate of bioavailable heavy metals in soil after plant remediation.

Plants	Reduction Rate of Bioavailable As (%)	Reduction Rate of Bioavailable Cd (%)
Cynodon dactylon	22.1	16.3
Bidens pilosa	33.8	35.4
Boehmeria nivea	29.4	24.7

.

In the study, as presented in Table 2, the adaptability of three plant species in polluted soil environments was assessed, revealing significant insights into their potential for phytoremediation. Cynodon dactylon exhibited robust growth, with an average root length of 9.1 cm and an average plant height of 9.6 cm. Both indicators of growth were favorable, suggesting healthy vegetative development under stress conditions. Additionally, the moisture content of Cynodon dactylon was recorded at 68.1%, which fell within the normal physiological range of 65%-80%, indicating efficient water uptake and retention capabilities.

Bidens pilosa also demonstrated commendable growth characteristics, with a root length of 9.7 cm and a plant height of 8.5 cm. Although shorter in stature compared to Cynodon dactylon, the slightly greater root length of Bidens pilosa might indicated a superior capacity for nutrient uptake in contaminated soils. Furthermore, the moisture content for Bidens pilosa was measured at 71.5%, which was well within the optimal range for maintaining metabolic activities in stressful environments.

Among the species studied, Boehmeria nivea showed the most impressive growth parameters, with an average root length of 11.7 cm and a plant height of 12.7 cm, surpassing the other two species. Additionally, Boehmeria nivea maintained a moisture content of 75.5%, suggesting it possesses enhanced adaptability traits, potentially enabling it to thrive in contaminated soils more effectively than the other species tested. This higher moisture content also supported its ability to efficiently manage water stress under adverse conditions.

Suitable plants used in the remediation project should exhibit strong tolerance to heavy metals in soil [50]. The bioconcentration factor was a crucial indicator for measuring the tolerance and accumulation capacity of a certain plant towards heavy metals [51]. Therefore, the bioconcentration factor was a critical factor in selecting remediation plants. According to Table 3, Cynodon dactylon, Bidens pilosa, and Boehmeria nivea had bioconcentration factors of 0.43, 0.33, and 0.07 for arsenic, and 0.13, 0.62, and 0.08 for Cd, respectively. The bioconcentration factors of Bidens pilosa for both As and Cd were greater than 0.3, indicating Bidens pilosa had the best overall remediation effect among the three plants. Additionally, only the translocation factor of Bidens pilosa for Cd was greater than 1, demonstrating its outstanding ability of Cd translocation; as a result, Bidens pilosa was found to be the best candidate for subsequent research.

For the pollution in contaminated soil, despite the fact that the total contents of As and Cd are important indicators, their biological effects should given more attention; these were mainly determined by the bioavailability of As and Cd. As observed in Table 4, to varying degrees, all three plants succeeded in reducing the bioavailability of As and Cd, and the reduction effectiveness ranked as Bidens pilosa > Cynodon dactylon > Boehmeria nivea. Bidens pilosa had the highest reduction rate in the bioavailability of As and Cd. Therefore, Bidens pilosa was chosen as the remediation plant for further research.

Understanding the physiological morphology and seed dispersal methods of Bidens pilosa contributed to our understanding of the application’s effectiveness in combined remediation. Therefore, a corresponding study was conducted. The seeds of Bidens pilosa were collected, air-dried, and stored at room temperature. Fully developed seeds were selected for observing their morphological characteristics, as shown in Figure 3.

As shown in Figure 3A, Bidens pilosa exhibited robust growth in the wild environment, with significant plant height and abundant biomass, making it highly suitable for greening the soil in the contaminated area. Additionally, this herbaceous plant could increase the content of organics, quick-acting nitrogen, and quick-acting potassium in the soil, thereby enhancing the fertility of the soil in the contaminated area [52,53], creating favorable conditions for crop cultivation after remediation.

As depicted in Figure 3B, Bidens pilosa had well-developed breeding organs, a large seed-carrying capacity, and well-formed seed morphology, showing the high development of its breeding organs. Moreover, the seeds in the breeding organs were easily dispersed and effectively carried by external forces such as wind and birds. This indicated that the breeding organs of Bidens pilosa demonstrated outstanding adaptability and competitiveness in ecology and reproductive biology.

Seed dispersal was a crucial aspect of plant life history, which significantly influenced the geographical distribution of plants. As shown in Figure 3C, Bidens pilosa seeds were relatively large, which meant they contained abundant stored nutrients and resulted in higher seedling survival rates and increased tolerance to adverse environmental conditions. The seeds had 2–3 spines on the top, with numerous small backward-pointing barbs, facilitating attachment to the skin and clothing of animals and humans for effective dissemination (as shown in Figure 3D). In conclusion, Bidens pilosa possessed advantages such as significant biomass, strong reproductive ability, and widespread seed dispersal, making it suitable for application in combined remediation.

3.2. Results of Material–Plant Combined Remediation Experiment

In a combined material–plant remediation experiment, Bidens pilosa was used alongside a formulated mixture of steel slag, pyrolusite, and FeSO₄, which were applied to contaminated soil. The composition was tested at a fixed 11% material usage, varying the mass ratios to determine the optimal combination. According to the findings presented in Figure 4, the most effective ratio for soil remediation was 2:1:10 for steel slag, pyrolusite, and ferrous sulfate, respectively [54].

Figure 4 illustrates how the adjustments in the mass ratios of steel slag (SS), manganese ore (Py), and ferrous sulfate (FS) influenced the bioavailability of arsenic (As) and cadmium (Cd). Figure 4A,B showed the impact of varying steel slag ratios. A lower ratio enhanced arsenic stabilization, increasing the bioavailability reduction from under 80% to over 90%. Conversely, reducing steel slag detrimentally affected cadmium stabilization, decreasing the bioavailability reduction from over 90% to under 80%.

In Figure 4C, ferrous sulfate was added to the binary system of steel slag and manganese ore, and the variation in the mass ratio of ferrous sulfate (in five different mass ratios) was investigated for the combined remediation of materials and plants. When the mass ratio of ferrous sulfate in the ternary system increased from 6 to 10 (in the five formulations), the overall reduction rate of arsenic bioavailability increased from below 80% to above 85%, thereby promoting the solidification and stabilization of arsenic in the soil. Conversely, a higher mass ratio of ferrous sulfate in the ternary system resulted in a poorer reduction effect on the bioavailability of Cd (Figure 4D). As the mass ratio of ferrous sulfate in the ternary system increased from 6 to 10, the overall reduction rate of Cd bioavailability after combined remediation decreased from above 88% to below 80%, hindering the solidification and stabilization of Cd in the soil. The results from Figure 4C,D indicated that an increased mass ratio of ferrous sulfate in the ternary system promoted the solidification and stabilization of As in the soil, while a decreased mass ratio hindered the solidification and stabilization of Cd.

The overall data from Figure 4 indicated that optimizing the ternary system’s mass ratios could concurrently stabilize As and Cd in the soil of the Zhongshan Hui Long arsenic smelter. Under the optimal formulation (2:1:10 ratio of steel slag, manganese ore, and ferrous sulfate), the remediation reduced the bioavailability of As by 90.2% and Cd by 89.3%.

From the experimental results above, the combined material–plant remediation potted experiments were further conducted, with the material dosage in the soil as a variable, to determine the optimal material dosage.

Figure 5 illustrates the dynamic relationship between material dosage and the reduction in bioavailability for arsenic (As) and cadmium (Cd) in contaminated soil. The graph demonstrates that the reduction rate of As bioavailability increased with higher material dosages up to 9%, beyond which it stabilized. Specifically, the bioavailability of As decreased significantly from 53.1% to 90.7% as the material dosage rose from 1% to 9%. Conversely, while the reduction rate for Cd initially rose from 60.5% to 89.1% within the same dosage range, it declined to 76.3% when the material dosage was increased further to 19%.

The trends observed could be attributed to the interaction between the remediation materials and the soil contaminants. The formation of stable complexes between the introduced iron ions and arsenic oxides reduced the mobility of arsenic, thereby enhancing its immobilization in the soil [55]. In contrast, the introduction of sulfate ions likely led to the formation of soluble cadmium sulfate complexes, increasing the mobility of Cd, especially at higher material dosages [56].

The optimization of material dosage in soil remediation was crucial for balancing cost efficiency and environmental impact. In our study, we determined that a 9% material dosage is optimal. This dosage effectively reduced the bioavailability of arsenic (As) and cadmium (Cd) in the contaminated soil from the Zhongshan Hui Long arsenic smelter in Guangxi, achieving reductions of 90.7% and 89.1%, respectively.

3.3. The Study of Combined Remediation Applied in Actual Field Site

In order to further verify the remediation effectiveness of the combined remediation scheme in actual field sites, an on-site material–plant combined remediation experiment was conducted in the contaminated soil based on the selected remediation plant and material formulation. The experimental results are presented in

Table 5. The reduction rates of As and Cd in soil at the remediation site over time.

Days	The Reduction Rate of Bioavailability for As (%)	The Reduction Rate of Bioavailability for Cd (%)
30 days	89.3	88.5
60 days	88.7	89.2
90 days	89.1	89.4
120 days	88.9	89.7

.

From Table 5, it can be observed that, when the remediation time was 30 days, the reduction rates of bioavailability for As and Cd in the site soil was as high as 88%, indicating that the combined remediation approach exhibited strong adaptability to the field environment. As the remediation time extended to 60 days, 90 days, and 120 days, the reduction rates of bioavailability for As and Cd in the site soil remained above 88%, demonstrating the stable and persistent remediation effect of the combined remediation approach.

3.4. Study on Combined Remediation Mechanism

To assess the impact of remediation materials on soil, leaching toxicity experiments were conducted on the remediation materials, and the results are shown in

Table 6. Leaching toxicity analysis (mg/L).

Items	Cd	As
Leaching concentrations of heavy metals	0.03 ± 0.01	0.02 ± 0.01
Wastewater discharge standards	0.1	0.5

.

Based on the data presented in Table 6, the leached concentrations of the eight heavy metals from the material were markedly lower than the upper limits specified in the Comprehensive Wastewater Discharge Standards (GB 8978-1996) [57]. Consequently, the remediation material demonstrated excellent safety and environmental compatibility.

To assess the impact of the remediation material on the physicochemical properties of the soil, measurements of soil pH value, redox potential, and conductivity were taken before and after remediation, and the results are detailed in

Table 7. Physicochemical properties of soil before and after remediation (p < 0.05).

	pH Value	Eh (mV)	EC (μS/cm)
The soil before remediation	6.72 ± 0.05 a	461±4.36 a	67.9 ± 1.15 b
The soil after remediation	6.59 ± 0.05 a	426±6.56 b	83.5 ± 2.17 a

The table displayed the significant differences in the physicochemical properties (pH, Eh, and EC) of the soil before and after treatment of the contaminated soil, based on the results of a one-way ANOVA. Each set of data had a sample size of n = 3. The letters “a” and “b” indicate significant differences among the means, with the same letter indicating no significant difference. The significance level is denoted by p, with p < 0.05 indicating a significant difference.

.

As shown in Table 7, the influence of ferrous sulfate on soil properties was evaluated. Before remediation, the soil had a pH of 6.72, a redox potential of 461 mV, and an electrical conductivity of 67.9 μS/cm. After applying ferrous sulfate, the pH decreased by 0.13 to 6.59, remaining within the acceptable range of ±0.2, which indicated minor acidification that was still within safe limits for biological activity.

The redox potential decreased by 35 mV to 426 mV, influenced by the reducing conditions induced by ferrous sulfate. However, this level was within the normal range for upland soils (200–750 mV), suggesting suitable conditions for most plant and microbial life.

Furthermore, the soil’s electrical conductivity increased by 15.6 μS/cm to 83.5 μS/cm, attributed to the higher ion concentration from the ferrous sulfate. Despite this increase, the conductivity stayed below 200 μS/cm, ensuring the soil did not reach saline–alkali conditions.

Overall, the remediation using ferrous sulfate was effective, adjusting soil properties within environmentally safe limits, and thus supporting its use in similar remediation efforts.

To gain a deeper understanding of the changes in heavy metal properties in the soil of the contaminated area after material–plant combined remediation, an analysis of the chemical forms of As and Cd in the soil before and after remediation was conducted. The results are shown in Figure 6.

Based on the observations from Figure 6A, during the joint remediation process, the percentage of highly reactive exchangeable arsenic decreased by 0.6 percentage points, while the percentages of bound iron–manganese and organically bound arsenic each decreased by 3.1 percentage points. Additionally, the percentage of less reactive residual arsenic significantly increased by 4.7 percentage points. According to the independent t-tests on different forms of arsenic, the equivalence test of variance showed no significant differences across the five forms (P1 > 0.05), which provided a basis for conducting equivalence tests on the means. The results indicate significant differences (P2 < 0.05) for all chemical forms except for exchangeable and water-soluble arsenic, confirming that the changes in all other chemical forms were statistically significant. This demonstrated that the remediation materials effectively transformed highly reactive arsenic into more stable residual forms, thereby promoting the immobilization of arsenic in the soil and reducing its bioavailability.

As illustrated in Figure 6B, during the remediation process, the percentage of highly reactive acid-extractable cadmium decreased by 3.2 percentage points, and the percentage of reducible forms decreased by 2 percentage points, while the percentage of oxidizable forms increased by 3.3 percentage points. Furthermore, the percentage of less reactive residual cadmium increased by 1.9 percentage points. The independent t-test results, including the test of variance equivalence which showed P1 > 0.05, allowed for mean equivalence testing, revealing significant changes in the metal forms of cadmium after remediation (P2 < 0.05). This indicates significant differences in the migration of cadmium forms before and after remediation. These results suggested that the remediation materials effectively transformed acid-extractable cadmium into more stable oxidizable forms and reducible forms into more stable residual forms, thereby enhancing the immobilization of cadmium in the soil and reducing its bioavailability.

In order to investigate the changes in heavy metals and other important elements in the soil before and after remediation, XPS spectra analysis was conducted on the corresponding soil samples. The results are shown in Figure 7 and Figure 8.

Figure 7A,B represented XPS full spectra of the soil before and after remediation, respectively. Upon comparing the two figures, it was observed that the peak intensities of Si2p at 103.11 eV and Al2p at 74.93 eV decreased, indicating the involvement of these elements in the stabilization process of heavy metals. The mechanism was that SiO₂ and Al₂O₃ participated in hydration reactions [15], as illustrated by reaction Equations (5) and (6):

SiO₂ + Ca(OH)₂ + nH₂O→CaO·SiO₂·(n + 1) H₂O

(5)

Al₂O₃ + Ca(OH)₂ + nH₂O→CaO·Al₂O₃·(n + 1) H₂O

(6)

The produced calcium aluminum hydrate and calcium silicate hydrate improved the solidification and stabilization ability of remediation material towards Cd through physical encapsulation and chemical reactions (involving Ca²⁺ undergoing ion exchange with Cd²⁺), thereby diminishing the bioavailability of Cd in the soil [58,59].

By comparing the fine spectra of each element before and after remediation, notable changes were observed in Cd, As, and Fe. The detailed XPS spectra results for Cd, As, and Fe are presented in Figure 8.

Figure 8A displays the XPS spectra for Cd 3d in the soil both before and after remediation. Post-remediation, the Cd 3d peak position shifted positively, although the peak area remained largely unchanged. This shift was linked to the introduction of organic acids and salts from plant root exudates into the soil, which formed stable organically bound complexes with Cd, reducing its bioavailability [60].

Figure 8B shows the As 3d XPS spectra. Before and after remediation, the binding energies for As (V) were observed at 45.67 eV and 45.58 eV, and for As (III) at 44.87 eV and 44.78 eV, respectively [61]. The post-remediation spectra indicated a decrease in As (III) binding energy and peak area, alongside an increase in both parameters for As(V). This suggested oxidation of As (III) to As(V) facilitated by soil oxidants like Fe (III) and Mn (IV), reducing As bioavailability [62].

Figure 8C illustrated the Fe 2p XPS spectra, where pre-remediation peaks at 707.78 eV and 720.43 eV, and post-remediation at 710.17 eV and 722.93 eV, were indicative of Fe (III) oxide. The peaks at 710.78 eV and 723.34 eV pre-remediation, and 713.08 eV and 725.54 eV post-remediation, corresponded to FeOOH [63]. The oxidation of As (III) to As(V) by Fe (III) resulted in less toxic As(V), which then underwent adsorption and co-precipitation with hydroxylated iron, thereby reducing As activity and increasing the binding energy of Fe (III) in the soil [64,65]

Fe₂O₃ + AsO₃³⁻ + 4H⁺→2Fe²⁺ + AsO₄³⁻ + 2H₂O

(7)

FeO(OH)(H₂O)_1+x + AsO₄³⁻→AsO₄³⁻·FeO(OH)(H₂O)_1+x

(8)

3.5. Study on the Microbial Community Diversity in Soil

The integration of material- and plant-based remediation strategies resulted in significant changes to soil microbial populations, which subsequently affected the speciation and bioavailability of heavy metals. This study aimed to explore the link between microbial community shifts and the bioavailability of arsenic and cadmium pre- and post-remediation.

Microbial DNA was extracted from soil samples and analyzed through high-throughput sequencing of the 16S V3V4 regions. The sequencing data were processed via QIIME, mothur, and DADA2 programs to ascertain the genus-level relative abundance of species, with the findings illustrated in Figure 9.

Notable shifts in the dominant bacterial genera were evident when comparing soil samples before and after remediation. Initially, predominant genera included S0134_terrestrial_group, A4b, Marinobacter, Ohtaekwangia, Lysobacter, Limnochordaceae, Paeniglutamicibacter, Iamia, and Limnobacter. Post-remediation, the prevalent genera shifted to JG30-KF-CM45, Devosia, SBR1031, Nocardioides, Pseudarthrobacter, Altererythrobacter, Pseudoxanthomonas, Sphingomonas, Paenarthrobacter, Sphingopyxis, Pseudomonas, and Streptomyces.

The analysis revealed that, initially, only three genera—Marinobacter, Limnobacter, and Iamia—showed considerable resistance to heavy metals [66,67,68]. Post-remediation, this resistance expanded to ten genera, including JG30-KF-CM45, Devosia, and others. These genera were known for their ability to mitigate heavy metal bioavailability through adsorption, chelation, or biotransformation processes [69,70,71,72,73]. Streptomyces and Pseudomonas were known to immobilize heavy metals, thereby reducing their environmental toxicity. Meanwhile, Devosia could transform metals into stable organic complexes, and genera, like Nocardioides and Pseudarthrobacter, accumulated heavy metals, decreasing their mobility and bioavailability. Further studies have indicated that genera such as Gemmatimonas and Pseudarthrobacter not only tolerate high levels of heavy metals but also enhance plant growth [74,75,76,77].

3.6. The Mechanism of Combined Remediation of Soil Heavy Metals by Material–Plant Collaboration

According to the experimental results and related mechanism studies of the combined remediation towards the contaminated soil of the Zhongshan Huilong arsenic smelting plant in Guangxi, a mechanism was proposed, as illustrated in Figure 10.

1. After combined remediation, the exchangeable and weakly bound forms of As in the soil were transformed into stable organically bound and residual forms, while the extractable and reducible forms of Cd in the soil were transformed into more stable reducible and residual forms [78].

2. The introduction of combined remediation materials increased the content of Fe(III) and Mn(IV) in the soil, oxidizing highly active As(III) to less active As(V), thereby reducing the bioavailability of As in the soil [79].

3. During the growth process of the remediation plants, organic acids released by plant roots formed complexes with Cd in the soil through chelation, increasing the stable organically bound form of Cd after remediation [80], and thereby reducing the bioavailability of Cd in the soil.

4. After combined remediation, the abundance of microbial genera with high tolerance to heavy metals increased in the soil. These genera reduced the bioavailability of As and Cd in the soil through adsorption, chelation, or biological enrichment [81]. Moreover, certain genera could also promote plant growth, facilitating vegetation restoration in the contaminated area and enhancing the effectiveness of combined remediation.

3.7. Cost–Benefit Analysis of Combined Remediation

As detailed in

Table 8. Cost-effectiveness and treatment effect analysis of the remediation.

Remediation Scheme	Remediation Cost ($/Ha.)	Remediation Effect	Reference
Single material remediation	4631.3	31.0% As, 44.3% Cd, and 96.8% Pb	[83]
Single material remediation	3909.1	85.4% As	[84]
Single material remediation	6460.4	30.2% As and 28.6% Pb	[85]
Single phytoremediation	5811.9	26.7% As, 11.4% Cd, and 23.3% Pb	[85]
Material–plant combined remediation	3100.9	90.7% As and 89.1% Cd	This work

, a cost–benefit analysis was performed to assess the economic feasibility of the combined remediation method used in this study. The analysis highlighted that using locally accumulated steel slag, which otherwise occupied land resources [82] and leveraging Bidens pilosa—a native plant with very low seed costs—significantly reduced the remediation expenses.

Further examination of Table 8 showed that the combined remediation approach not only performed better than most methods documented in existing literature regarding the stabilization of heavy metals but also benefited from the simplicity of material preparation and the low cost and availability of plant seeds.

4. Conclusions

In this study, the combined remediation of cadmium- and arsenic-contaminated soil by phytoremediation and stabilization was studied, and the main conclusions were as follows:

(1) With a focus on the characteristics of heavy metal pollution in the soil of the impact area of the Zhongshan Huilong arsenic smelting plant in Guangxi, research was conducted on the reduction in bioavailability of As and Cd in the soil. Local materials (steel slag, pyrolusite and FeSO₄) and remediation plants (Cynodon dactylon, Bidens pilosa, and Boehmeria nivea) were utilized. Steel slag, pyrolusite, and ferrous sulfate were identified as the optimal remediation materials, and Bidens pilosa was recognized as the best remediation plant. The optimal material ratio of 2:1:10 for steel slag, pyrolusite, and ferrous sulfate, with a dosage of 9% by weight of the contaminated soil, resulted in a 90.7% reduction in arsenic bioavailability and an 89.1% reduction in cadmium bioavailability in the soil. Field experiments at the Zhongshan Huilong arsenic smelting plant in Guangxi confirmed the stability and effectiveness of this remediation strategy, achieving a reduction of over 88.5% in bioavailability for both metals.

(2) Based on the experimental results and related mechanism studies of the combined remediation of soil heavy metals in the impact area of the Zhongshan Huilong arsenic smelting plant in Guangxi, this study proposed a mechanism for the combined remediation of soil heavy metals by the synergy of material and plant. The introduction of remediation materials into the soil increased the content of Fe(III) and Mn(IV), oxidizing highly active As(III) to less active As(V), thereby reducing the bioavailability of As in the soil. During the growth process of the remediation plants, organic acids released by plant roots formed complexes with Cd in the soil through chelation, increasing the stable, organically bound form of Cd after remediation, thereby reducing the bioavailability of Cd in the soil. After combined remediation, the abundance of microbial genera with high tolerance to heavy metals increased in the soil. These genera reduced the bioavailability of As and Cd in the soil through adsorption, chelation, or biological enrichment. Additionally, certain genera also promoted plant growth, facilitating vegetation restoration in the contaminated area and enhancing the effectiveness of combined remediation.

(3) Through a cost–benefit analysis of the remediation, the combined remediation technology in this study demonstrated favorable technical, economic, and commercial feasibility.