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Article

The Addition of Degradable Activators Enhances Sedum alfredii Phytoremediation Efficiency in Cd-Contaminated Soils

by
Honggang Li
1,
Ling Huang
2,
Zhiliang Chen
2,*,
Hang Wei
2,
Mengqiang Sun
2,
Xiaoqing Huang
1,
Haochao Li
1 and
Qianjun Liu
1
1
School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China
2
Guangdong Engineering Technology Research Center of Heavy Metal Pollution Control and Restoration in Farmland Soil, South China Institute of Environmental Sciences, Ministry of Ecology and Environment, Guangzhou 510275, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(7), 3207; https://doi.org/10.3390/su17073207
Submission received: 19 December 2024 / Revised: 26 March 2025 / Accepted: 1 April 2025 / Published: 3 April 2025
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Abstract

:
Soil cadmium (Cd) pollution is a critical environmental issue that requires urgent remediation. Sedum alfredii Hance, known for its high biomass, strong stress tolerance, and suitability for harvesting, serves as an excellent hyperaccumulator. This study used field experiments to investigate the enhancement of Cd phytoremediation in soil using three activators: citric acid (CA), malic acid (MA), and polyaspartic acid (PASP). The results showed that the biomass of Sedum alfredii was increased by 8.95–28.37% by the addition of these activators, significantly boosting its Cd accumulation efficiency, with an average removal rate increase of 12%. Among all activators, CA exhibited the most substantial enhancement effect, with enrichment coefficients of 36.26% and 11.56% for the aboveground parts and roots of Sedum alfredii, respectively, and a 21.15% increase in the Cd removal rate. Although PASP had a less pronounced effect on biomass and Cd uptake, with decreases of 15.25% and 35.34% in the aboveground parts and roots, respectively, it significantly impacted soil Cd speciation and increased the activation rate by 20%.

1. Introduction

Cadmium (Cd) can migrate into soil through surface runoff, irrigation, and sediment deposition, subsequently accumulating in crops and plants, and posing a threat to human health through the food chain [1]. Seeking economically effective methods for the remediation of cadmium-contaminated soil is urgent. Among the Cd-contaminated soil remediation technologies, phytoremediation is the most promising method for heavy metal pollution due to its low cost and environmental friendliness [2].
Phytoremediation is employed using hyperaccumulator plants to absorb heavy metals from the soil. Heavy metals are taken up from the soil, water, or solid sediments into the plant with the assistance of their root systems and associated microorganisms. Then they are transported to the aboveground parts before being harvest and achieve the goal of heavy metal removal [3]. Among these plants, the annual or perennial succulent herb Sedum alfredii Hance [4] stands out as an excellent Cd hyperaccumulator due to its high biomass, strong stress tolerance [5], and suitability for harvesting [6]. However, phytoremediation technology has the disadvantages of a long remediation and treatment cycle and low efficiency [7]. To address these limitations, phytoremediation enhancement techniques have been developed, including chemical enhancement [8], microbial enhancement measures [9], agronomic measures [10], and genetic modification engineering [11]. The bioavailability of soil contaminants is improved, or plant growth is effectively promoted, through chemical enhancement methods, significantly improving the performance of plants in soil remediation. It is one of the effective measures to overcome the inefficiency and time-consuming nature of phytoremediation [12]. Chemical enhancement remediation technology, through the addition of activators, is used to alter the form and bioavailability of heavy metals in the soil, thereby increasing the ability of plants to extract heavy metals from the soil.
Activators are broadly classified into three main categories based on their chemical nature: (i) synthetic aminopolycarboxylic acids (e.g., ethylenediaminetetraacetic acid [EDTA] and glutamic acid diacetic acid [GLDA], (ii) natural aminopolycarboxylic acids (e.g., ethylenediaminedisuccinic acid [EDDS] and nitrilotriacetic acid [NTA]), and (iii) natural small-molecule organic acids (e.g., citric acid [CA], malic acid [MA], and tartaric acid [TA]) [13]. Among these, synthetic aminopolycarboxylic acids such as EDTA and GLDA promote heavy metal dissolution in soil by forming stable metal complexes. This process enhances metal mobility, facilitating their uptake by plant roots and subsequent translocation to aboveground tissues. However, these synthetic compounds can also compromise plant root cell membrane permeability, suppress growth, and persist in soil due to their low biodegradability. Their accumulation may induce phytotoxicity, harm soil fauna and microorganisms, and contribute to secondary environmental pollution [14]. For instance, EDTA exposure has been shown to restrict plant growth, resulting in poor seed germination, leaf wilting, chlorosis, necrosis, and impaired transpiration [15,16,17].
Low-molecular-weight natural organic acid activators, such as citric acid (CA) and malic acid (MA), are harmless to plants in practical applications and are easily degradable [18]. Moreover, soil microbial activity can be enhanced, and plant growth can be promoted, by them. The mechanisms to increase plant heavy metal uptake include the following: 1. Lowering soil pH causes heavy metals that are in an insoluble state or fixed in soil minerals to be released [19]; 2. Hydrogen ions dissociated from acidic groups can be adsorbed onto the cation adsorption sites on the surfaces of soil particles and minerals, resulting in the displacement of metal ions [20]; 3. Some heavy metals can be formed into soil-chelating agent–heavy metal complexes with low-molecular-weight organic acids and be immobilized in the soil, reducing the migration of heavy metals [21]. Polyaspartic acid (PASP), a biodegradable surfactant with multiple carboxyl and hydroxyl groups [1], is synthesized from L-aspartic acid [22]. It can be effectively used to form complexes with Cd2+, replacing Cd in the soil, thereby reducing the toxicity of Cd to plants [23] and improving the extraction efficiency of heavy metals by Brassica napus L. and Solanum nigrum L. [24]. Additionally, the activity of antioxidant enzymes in plants can be increased by PASP, thereby enhancing their defense against heavy metal stress [25].
The enhancement of phytoremediation for heavy metal-contaminated soil using EDTA [26], PASP [27], CA [28], and MA [29] has been validated in pot experiments. However, pot experiments are conducted under controlled conditions, and it is impossible to fully simulate field application conditions such as climatic changes, soil background conditions like moisture, nutrients, and microbial environment, and the rhizosphere environment resulting from the interaction between plant roots and field soil [30]. This study selected three natural organic acid activators: citric acid (CA), malic acid (MA), and polyaspartic acid (PASP), and investigated the enhancement effects of these activators, both individually and in combination, on the remediation of soil Cd contamination by Sedum alfredii through field experiments.

2. Materials and Methods

2.1. Plant Material, Soil Properties, and Field Profile

The Sedum alfredii seedlings used in this study were obtained from the Southeast Sedum Breeding Base in Qingyuan, China. The breeding base is located in a subtropical monsoon climate zone, characterized by an average annual temperature of 20–25 °C and relative humidity of 70–85%. During the growing season (spring and summer), temperatures typically range from 22 to 30 °C, with an average daily light intensity of 800–1200 μmol/m2/s. The soil at the breeding base is well drained and rich in organic matter, and has a pH range of 6.0–7.0. Regular irrigation and balanced fertilization are applied to ensure optimal growth conditions. The location of the experimental field is shown in Figure 1; it is located in Sanshui District, Foshan City (latitude 23°06′ N, longitude 112°84′ E), which belongs to the subtropical monsoon humid climate zone. Agricultural soils in the area are contaminated by Cd from sewage irrigation, with an average pH of 7.23, an organic matter (SOM) content of about 21.04 g/kg, and a cation exchange capacity (CEC) of 26.6 cmol/kg. In the experimental area, the original natural background soil type is classified as Anthrosols according to the World Reference Base for Soil Resources (WRB). Two rounds of Sedum alfredii remediation experiments were conducted from November 2018 to 18 July 2020. The first round was conducted from 11 November 2018 to 2 April 2019, without the addition of activators. The planting of Sedum alfredii reduced the Cd concentration in the soil from 0.98 mg/kg to 0.89 mg/kg, with a removal rate of 9.18%. From 28 February 2020 to 18 July 2020, this study further conducted a natural activator-enhanced remediation of Cd-contaminated soil using Sedum alfredii.

2.2. Field Experiment

The experimental field was divided into 6 columns and 3 rows, with each plot measuring 8 m in length and 11 m in width. There are 18 plots in total with a 20 cm gap between plots. After ridge division, plowing, and mulching, 42 kg/acre of potassium humate was added and evenly plowed into the soil. The transplanting distance of Sedum alfredii was approximately 20 cm. One month after transplanting, activators were added. Six treatments were set up, including control check (CK), CA (42 kg/acre), MA (42 kg/acre), PASP (42 kg/acre), CA (21 kg/acre) + PASP (21 kg/acre), and MA (21 kg/acre) + PASP (21 kg/acre), with each treatment replicated three times.

2.3. Sample Collection, Preparation, and Analysis

Five spots of Southeastern Sedum were randomly collected from each test plot as a mixed sample, for a total of 18 soil samples and 18 plant samples.
The collected Sedum alfredii was thoroughly washed with tap water to remove impurities adhering to the entire plant, followed by at least three rinses with deionized water. The plants were then drained, and surface moisture was removed with filter paper. The plants were separated into aboveground and underground parts, and their fresh weight was recorded before being dried in an oven at 75 °C. The dried samples were then ground, sieved, and prepared for analysis.
The soil pH was measured using the electrode method. A soil-to-water ratio of 2.5:1 was oscillated for 5 min, allowed to stand for 30 min, and the supernatant was measured with a pH meter. The Cd content in Sedum alfredii was determined in accordance with the method specified in GB 5009.268-2016 [31], using inductively coupled plasma mass spectrometry (ICP-MS). The content of Cd in the soil was determined according to GB/T 17141-1997 [32], using graphite furnace atomic absorption spectrophotometry. The Tessier five-step extraction method was used to measure the speciation of Cd in the soil [33].

2.4. Statistical Analysis of Data

Mean and standard deviation of experimental data. One-way analysis of variance (ANOVA) was performed using SPSS (Statistics 26).
The removal rate of soil cadmium (Cd) was calculated using the following formula:
EXEF HM   = C 0 − C C d C 0 ∗ 100 %
where EXEFHM represents the removal efficiency of soil cadmium, %; C0 is the initial cadmium content in the soil and CCd is the cadmium content in the soil after remediation, mg/kg.
The ratio of Cd taken up by plants to Cd in the exchangeable and carbonate-bound morphology in the soil as an activated part of total Cd is called the activation rate.
The activation rate of Cd is calculated as follows:
η = C 0 − C C d + C e C 0 ∗ 100 %
where η represents the activation rate of soil cadmium, %; Ce represents the sum of the concentrations of exchangeable and carbonate-bound Cd in the soil after remediation, mg/kg.
The bioconcentration and transport of Cd by Southeast Sedum were estimated by the transport factor (TF) and bioconcentration factor (BCF).
The bioconcentration factor (BCF) is an index of the ability of plants to absorb Cd from the soil into their aboveground and root systems. It is calculated as follows:
BCF =   C a C s
where Ca (mg/kg) indicates the Cd concentration in the dry branches at harvest and Cs is the initial soil Cd concentration.
The translocation factor (TF) gives an indication of the plant’s ability to translocate metals from roots to aboveground parts. This parameter was calculated as follows:
TF =   C a C r
where Ca (mg/kg) and Cr (mg/kg) represent the Cd concentrations in the plant aboveground and in the roots, respectively. Generally, plants exhibiting a TF (Translocation Factor) greater than 1 indicate a strong capacity for heavy metal absorption and translocation, making them suitable for phytoextraction. Conversely, when the TF is less than 1, the plant’s ability to absorb and translocate heavy metals is relatively weak, with heavy metals primarily accumulating in the roots and less being transferred to the aboveground parts. Such plants are more appropriate for phytostabilization purposes.

3. Results and Discussion

3.1. Cadmium Removal

The removal of cadmium from soil through phytoremediation is influenced by factors such as the geographical location of the experiment, soil physicochemical properties, plant type, and pollution load. The removal rates of soil Cd are shown in Figure 2. The removal rate of Cd by the CK group was 4.82%. After activators were added, the soil Cd removal rates ranged between 9.27% and 25.97%, showing considerable variation, with an average removal rate increase of 12%. The highest Cd removal rate was achieved with the addition of CA, reaching 25.97%, which was 21.15% higher than that of the CK group. The second-highest removal rate was achieved with MA, which increased by 16.54%. The enhancement effect of the activators PASP + MA and PASP + CA on the remediation of Cd by Sedum alfredii was not significant.
The enhanced Cd removal efficiency of Sedum alfredii with CA diverges from previous pot-based studies (Table 1), a discrepancy attributable to three interrelated factors. The hyperaccumulation capacity of Sedum alfredii—a species evolutionarily adapted for metal sequestration—contrasts sharply with non-hyperaccumulators like Celosia argentea L. [34]. This intrinsic difference likely underpins the observed efficiency gap. Furthermore, the accumulation of Cd by plants generally decreases with an increase in soil heavy metal concentration [35]. Plant growth and biomass can be affected by excessive Cd through reduced mineral nutrient uptake and disrupted biochemical metabolic processes [36]. Once heavy metals enter the plant, cell division, photosynthesis, respiration, and mineral nutrient absorption can be adversely affected, leading to symptoms such as curled young leaves, stunted growth, and significantly reduced biomass [37]. For example, when the initial concentration was 20.09 mg/kg, the remediation effect of Sedum alfredii on Cd was not significant [38]. Finally, compared to field experiments, smaller volumes and fewer cultivated plants are involved in pot experiments, making the control of soil, fertilizer, and water conditions easier. This facilitates the regulation of soil temperature and moisture content, promoting Cd absorption by Sedum alfredii. The Cd content in the aboveground and root parts can reach several hundred mg/kg [38], leading to significant differences in Cd accumulation by individual Sedum alfredii plants between pot and field experiments [39]. Compared to CA and MA, weaker bonds with Cd are formed by PASP. When mixed with MA or CA, the rhizosphere environment of Sedum alfredii can be affected by PASP, resulting in an insignificant enhancement effect of the PASP + CA and PASP + MA combinations on the remediation by Sedum alfredii.

3.2. Activation of Cd in Soil

The forms of heavy metals in soil are important factors affecting their mobility and bioavailability. Heavy metals in soil are classified into five forms by the Tessier five-step extraction method: exchangeable, carbonate-bound, Fe-Mn oxide-bound, organically bound, and residual forms. Most plants can only absorb heavy metals that are in dissolved form in the soil solution, weakly bound to soil particles (exchangeable form), and partially in the carbonate form, while those bound to the soil solid phase are difficult for plants to absorb [44]. In this study, the exchangeable fraction, the carbonate-bound fraction of Cd in remediated soil, and the Cd extracted by Sedum alfredii are collectively defined as the available form of soil Cd. The Fe-Mn oxide-bound and organically bound forms, which can transform into available forms under certain environmental conditions [45], are defined as potentially available forms. Chemical activators promote the transformation of potentially available forms into available forms through processes such as acidification, complexation, precipitation, and redox reactions, thereby facilitating the phytoremediation of contaminated soil [46].
After enhanced remediation, the distribution of Cd forms in the soil is shown in Figure 3. In the soil, Cd is mainly found in available and residual forms. Compared to the control group, the exchangeable and carbonate-bound forms of Cd were increased to varying degrees after treatment with activators. Under dynamic equilibrium, other forms of heavy metals gradually decreased. As shown in Figure 2, the activation rate of Cd in the soil was increased by 19% and 20% under CA and PASP treatments. PASP had the most significant impact on the distribution of Cd speciation in the soil. The carboxyl groups of PASP can be coordinated with Cd to form metal chelates [47], thereby rapidly and significantly increasing the availability of metals in contaminated soil. Small-molecule organic acids like CA can introduce H+ into the soil, altering the bioavailability of heavy metals, and can also form soluble chelates with heavy metal ions in the soil, promoting the adsorption of metal ions to soil particles and enhancing their activity and mobility [48]. Compared to CA, MA has a lower metal chelation ability [49], but its activation effect on soil Cd was still notably observed.
Organic acids, in addition to promoting the transformation of Fe-Mn oxide-bound and organically bound forms into exchangeable forms, can also reduce Cd toxicity by enhancing the expression of enzymatic and non-enzymatic antioxidants and certain tolerance genes within plants. This helps maintain normal cell morphology [50]. As a result, the absorption of Cd ions and complexes by plants is enhanced, improving the phytoremediation capacity of plants for Cd-contaminated soil. This study found that both CA and MA not only increased the available Cd content in the soil but also promoted the growth of Sedum alfredii.

3.3. Absorption and Remediation Effect of Sedum Alfredii on Cd

As shown in Figure 4, the Cd content in the aboveground parts and roots of Sedum alfredii under different treatments ranged from 6.61 to 10.63 mg/kg and 8.36 to 14.43 mg/kg, respectively. The Cd concentration in the aboveground parts was increased by 36.32% and 11.6% by the CA treatment, followed by the MA treatment, which respectively increased it by 7.69% and 11%. The hydroxyl or carboxyl groups in CA and MA can form stable compounds with Cd, facilitating the absorption and accumulation of Cd by Sedum alfredii, further increased the Cd concentration in the stems and roots [51]. However, under PASP treatment, the Cd content in the aboveground parts and roots of Sedum alfredii decreased by 15.25% and 35.34%, respectively, which was the lowest among all activator combinations. PASP did not significantly affect the Cd absorption capacity of Sedum alfredii, showing a marked difference from the results of pot experiments reported in the literature [42]. The reason might be that PASP, being an alkaline activator, increases soil pH and reduces H+, which decreases the exchange of heavy metal cations adsorbed on the surface of soil particles with H+, thereby slowing down the desorption of Cd from the surface of soil particles.
The key to increasing the accumulation of heavy metals in plants and enhanced phytoremediation with activators is to promote the dissolution of heavy metals in the soil and their translocation from plant roots to shoots. The Bioconcentration Factor (BCF) represents the ability of plants to accumulate heavy metals, while the Transport Factor (TF) indicates the translocation of heavy metals from the soil into the plant. The BCF and TF for each treatment group are shown in Table 2. Except for PASP, the Cd bioconcentration factor of Sedum alfredii was increased by all activator combinations. The BCF for Cd in the shoots and roots ranged from 7.43 to 11.95 and 9.4 to 16.21, respectively. The physicochemical conditions of the soil are altered by CA, which, being a weak acid, when applied, transforms the chemical forms of Cd from less absorbable to more absorbable by plants, thereby enhancing Cd uptake [52]. CA increased the BCF of Cd in the shoots and roots of Sedum alfredii by 36.26% and 11.56%, reaching 11.95 and 16.21, respectively, the highest among all activator combinations.
The Transport Factor (TF) of plants is another crucial factor in phytoremediation [53]. Activators can increase the bioavailability of heavy metals and the permeability of root cell membranes, facilitating the entry of metal–chemical reagent complexes into the plant [53]. Compared to free metal ions, metal–chemical reagent complexes are more easily transported through the cortex to the xylem vessels because they do not bind to groups on the surface of root cortex cells, such as carboxyl groups or polysaccharides [54]. The TF values for each group showed no significant differences, ranging between 0.6 and 0.8, and were not significantly different from other field experiments [34]. This indicated that the translocation of Cd from the roots to the shoots of Sedum alfredii was not significantly enhanced by the activators used in this study.

3.4. Biomass of Sedum alfredii

The biomass of plants is a key factor in determining the efficiency of phytoremediation [55]. Under the same conditions, plants with greater biomass can accumulate more heavy metals within their tissues [56]. Biomass is influenced by a variety of complex factors, including soil heavy metal concentration, the type and concentration of chemical additives, climate change, and soil background conditions such as moisture, nutrients, and microbial environment. As shown in Figure 5, activators increased the biomass of Sedum alfredii, with individual dry weights ranging from 17.02 g to 20 g, representing an increase of 8.95% to 28.37%. The addition of CA resulted in the greatest increases in both shoot and root dry weights, with increases of 28.35% and 30.3%, respectively.
The biomass of Sedum alfredii in this study was found to be higher than that reported in pot experiments (Table 1). A larger growth space is provided by field planting, allowing plant roots to be freely extended and water and nutrients to be absorbed. In contrast, root growth and, consequently, biomass accumulation are limited by container size in pot-grown plants. Additionally, more natural sunlight is received by field-grown plants, enhancing photosynthesis and promoting growth and biomass accumulation. Another reason is that a relatively low level of Cd contamination was present in the soil in the study area, resulting in less stress on the plants. When soil heavy metal concentrations are below the critical level that affects plant growth, no significant impact on plant development is observed. However, when soil heavy metal concentrations reach the critical threshold, plant growth can be severely inhibited [52]. Excessive Cd can affect plant growth and biomass by reducing nutrient uptake and disrupting biochemical metabolic processes [36].
Most studies reported that the application of chemical reagents increased leaf necrosis and reduced plant biomass [57], while a few studies indicated that chemical reagent treatments did not cause biomass loss and might even improve plant growth [58]. In our study, plant shoot biomass, the amount of extracted Cd, and phytoextraction efficiency were significantly increased by the addition of CA relative to the control soil, suggesting that a beneficial effect on the growth of Sedum alfredii was provided by CA. This might be related to the structure of CA, which has three carboxyl groups, a special molecular structure, and charged properties that can participate in plant growth and development processes. Plant growth was promoted by CA by providing available phosphorus and iron compounds [59], decomposing humic substances, activating auxins, and mitigating stress damage to photosynthetic organs [60,61]. PASP was reported to enhance plant nutrient absorption and extend fertilizer utilization efficiency to improve plant nutrient levels [62]. It was also believed to be capable of promoting seedling stem growth, root activity, chlorophyll content, and plant biomass [63]. However, in this study, significant changes in the biomass of Sedum alfredii were not observed with the addition of PASP. This could be related to the active hydrothermal conditions in the study area, which promoted soil microbial activity and affected the effectiveness of PASP.
The biomass of phytoremediation plants is also related to the concentration of activators. If the concentration of activators is too high, it can adversely affect plant growth, leading to lower remediation efficiency [56]. For example, EDTA has been shown to reduce the aboveground biomass of Boehmeria nivea L. [64] and inhibit the growth of Lolium perenne L.

3.5. Correlation Analysis

A correlation analysis was conducted on soil pH, Sedum alfredii biomass, activation rate, Cd content in Sedum alfredii, and Cd removal rate. The correlation heatmap and linear relationship graph (Figure 6) showed that the removal rate was significantly positively correlated with both the activation rate and biomass (p < 0.05), with correlation coefficients of 0.6 and 0.84, respectively. This indicated that the available Cd and the biomass of Sedum alfredii were important factors in improving the phytoremediation efficiency using activators. The bioavailability of soil heavy metals was determined to be a crucial factor in the amount of heavy metals absorbed by plants. Plants with larger biomass were found to accumulate more heavy metals in their roots and stems, thereby increasing the soil Cd removal rate. A correlation analysis was conducted on soil pH, Sedum alfredii biomass, activation rate, Cd content in Sedum alfredii, and Cd removal rate. The correlation heatmap and linear relationship graph (Figure 6) showed that the removal rate was significantly positively correlated with both the activation rate and biomass (p < 0.05), with correlation coefficients of 0.6 and 0.84, respectively. This indicated that the available Cd and the biomass of Sedum alfredii were important factors in improving the phytoremediation efficiency using activators. The bioavailability of soil heavy metals was determined to be a crucial factor in the amount of heavy metals absorbed by plants [65]. Plants with larger biomass were found to accumulate more heavy metals in their roots and stems, thereby increasing the soil Cd removal rate.
The initial pH of the test soil was 7.63, and after applying different combinations of chemical reagents, the pH of the treated soils ranged from 7.40 to 7.70, showing no significant changes. It could be attributed to the soil used in the experiment, as it was alkaline with inherent buffering capacity. In addition, soil pH could be influenced not only by potassium fertilizers and organic acids but also by other environmental factors in the farmland, such as microorganisms, flora, and fauna. The pH had no significant impact on Cd removal and the growth of Sedum alfredii, with correlation coefficients of 0.25 and 0.15.

4. Conclusions

The available Cd content in the soil and the biomass of Sedum alfredii were increased by the addition of activators, raising the average Cd removal rate from 4.82% to 16.82%. The biomass of Sedum alfredii and its Cd removal rate were significantly increased by the application of organic acids CA and MA alone, with CA and MA enhancing the Cd removal rate to 25.98% and 21.36%, respectively, which was higher than previously reported in pot experiments. The remediation effect in Cd-contaminated soil by Sedum alfredii was greatly enhanced by CA and MA. PASP was found to be the most effective activator, increasing the available Cd by 14% without affecting the mobility and accumulation of Cd in the soil. Although the application of activators showed great potential in phytoremediation, it might increase the risk of heavy metals migrating into groundwater. Therefore, it is important to balance the remediation effect and preserving groundwater when choosing activators.
In view of the complexity and variability of the soil environment in the field, the field experiment involves uncontrollable environmental variables, rainfall fluctuations and extreme temperatures may affect the metabolic activity of Sedum alfredii and the degradation rate of the activator, and the pH-buffering capacity of the farmland soil may reduce the activation efficiency of the activator. At present, there are few studies on the comprehensive practical application of composite activators for enhanced phytoremediation, especially on the mechanism. In this paper, we made a preliminary discussion on the comprehensive effects of composite activators on the remediation of cadmium-contaminated farmland by Sedum alfredii, and the analysis of the mechanism is still to be further researched.

Author Contributions

Software, H.L. (Honggang Li), X.H. and H.L. (Haochao Li); Validation, Z.C.; Investigation, H.L. (Honggang Li), L.H., H.W. and M.S.; Data curation, H.L. (Honggang Li), L.H., H.W. and M.S.; Writing—original draft, H.L. (Honggang Li); Writing—review & editing, L.H., Z.C. and Q.L.; Project administration, Z.C.; Funding acquisition, Z.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported financially by the Natural Science Foundation of Guangdong Province (2024A1515011090); National Natural Science Foundation of China (42377262); Foshan Science and Technology Bureau (2220001018511); Guangxi Key Research and Development Program (CinnamonaceaeAB24010113); Special Funds for the Basic Research and Development Program in the Central Non-profit Research Institutes of China (PM-zx703-202305-205); Innovative Research Group Project of the National Natural Science Foundation of China (U22A20606).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the authors declare that data will be made available on request.

Conflicts of Interest

The authors declare no competing interests.

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Figure 1. Summary map. (a) China Map. (b) Guangdong area. (c) Foshan prefecture in Guangdong. (d) Overview of experimental field.
Figure 1. Summary map. (a) China Map. (b) Guangdong area. (c) Foshan prefecture in Guangdong. (d) Overview of experimental field.
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Figure 2. Removal and activation rates of soil Cd.
Figure 2. Removal and activation rates of soil Cd.
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Figure 3. Morphological distribution of Cd after treatment.
Figure 3. Morphological distribution of Cd after treatment.
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Figure 4. Cd concentration in the shoots and roots of Sedum alfredii.
Figure 4. Cd concentration in the shoots and roots of Sedum alfredii.
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Figure 5. Biomass of shoots and roots of Sedum alfredii.
Figure 5. Biomass of shoots and roots of Sedum alfredii.
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Figure 6. Correlation relationship diagram.
Figure 6. Correlation relationship diagram.
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Table 1. Cd accumulation in different plants in pot experiments.
Table 1. Cd accumulation in different plants in pot experiments.
Reference[24][38][40][41][39][24][42][43][34]
Cd
removal (%)		0.080.043.918.35.839.72%26.6813.0%14.63
Concentration
(mg/kg)		Root0.0715.64.238725.61.436140027.889.1
Shoot1.7212.621.720322.21.141.4787.861.6
Biomass
(g/plant)		Root0.680.741.470.180.231.15 t/hm20.21.69 t/hm20.18 kg/m2
Shoot5.80.8 g2.21.21.61.6
ActivatorCACACACACAPASPPASPCANone
Initial Cd (mg/kg)3.9120.090.8510.43.030.948.623.680.41
PlantsSedum plumbizincicolaNHE Sedum alfrediiSedum alfrediiSedum alfrediiSedum alfrediiPennisetum sinese R.Sedum alfrediiCelosia argentea L.Sedum alfredii
Table 2. Bioconcentration and translocation factors of Southeast Sedum after the activator addition.
Table 2. Bioconcentration and translocation factors of Southeast Sedum after the activator addition.
TreatmentBCF TF
ShootsRootsShoots
CK8.77 ± 2.814.53 ± 6.560.6 ± 0.04
CA11.95 ± 3.5716.21 ± 1.430.73 ± 0.14
MA9.44 ± 2.2716.21 ± 2.030.6 ± 0.18
PASP7.43 ± 0.799.4 ± 1.140.8 ± 0.14
CA + PASP9.2 ± 0.3917.23 ± 3.760.6 ± 0.08
MA + PASP9.14 ± 0.8715.44 ± 4.910.66 ± 0.03
Values shown are the mean ± SD (n = 3).
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Li, H.; Huang, L.; Chen, Z.; Wei, H.; Sun, M.; Huang, X.; Li, H.; Liu, Q. The Addition of Degradable Activators Enhances Sedum alfredii Phytoremediation Efficiency in Cd-Contaminated Soils. Sustainability 2025, 17, 3207. https://doi.org/10.3390/su17073207

AMA Style

Li H, Huang L, Chen Z, Wei H, Sun M, Huang X, Li H, Liu Q. The Addition of Degradable Activators Enhances Sedum alfredii Phytoremediation Efficiency in Cd-Contaminated Soils. Sustainability. 2025; 17(7):3207. https://doi.org/10.3390/su17073207

Chicago/Turabian Style

Li, Honggang, Ling Huang, Zhiliang Chen, Hang Wei, Mengqiang Sun, Xiaoqing Huang, Haochao Li, and Qianjun Liu. 2025. "The Addition of Degradable Activators Enhances Sedum alfredii Phytoremediation Efficiency in Cd-Contaminated Soils" Sustainability 17, no. 7: 3207. https://doi.org/10.3390/su17073207

APA Style

Li, H., Huang, L., Chen, Z., Wei, H., Sun, M., Huang, X., Li, H., & Liu, Q. (2025). The Addition of Degradable Activators Enhances Sedum alfredii Phytoremediation Efficiency in Cd-Contaminated Soils. Sustainability, 17(7), 3207. https://doi.org/10.3390/su17073207

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