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Article

Polyvinylpolypyrrolidone Immobilized Cu, Cd and Zn in Soils and Reduced Their Uptake by Oilseed Rape

by
Yiliu Wang
1,2,
Diedrich Steffens
3,
Yunsheng Jia
4,* and
Huoyan Wang
1,2,*
1
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 211135, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Department of Plant Nutrition, Justus-Liebig University Giessen, Heinrich-Buff Ring 26-32, 35392 Giessen, Germany
4
Nanjing Institute of Environmental Sciences, Ministry of Ecology and Environment, Nanjing 210042, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(10), 2258; https://doi.org/10.3390/agronomy15102258
Submission received: 28 August 2025 / Revised: 19 September 2025 / Accepted: 21 September 2025 / Published: 23 September 2025
(This article belongs to the Section Soil and Plant Nutrition)
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Abstract

Organic amendments application has been proposed as an efficient method for remediation of heavy metals-contaminated soils. This study evaluated the performance of the water-insoluble organic material polyvinylpolypyrrolidone on decontaminating water and soil polluted by heavy metals Cu, Cd and Zn via batch trials, soil incubation and pot experiments with oilseed rape. The adsorption process of Cu, Cd and Zn by polyvinylpolypyrrolidone included a rapid step which achieved 92%, 76% and 87% of adsorption capacities within 10 min, followed with a slow step before reaching equilibrium which varied from 4 to 24 h among the three heavy metals. The maximum adsorption capacities were 327, 330 and 186 mg g−1 for Cu, Cd and Zn, respectively. With application doses of polyvinylpolypyrrolidone ranging from 10 to 60 g kg−1, the DTPA-extracted Cu, Cd and Zn showed 59–96%, 27–93% and 13–83% reduction compared to no addition. Moreover, the uptake of Cu, Cd and Zn by oilseed rape were significantly inhibited with polyvinylpolypyrrolidone amendments, and the effects improved with the accrual of polyvinylpolypyrrolidone. Intriguingly, the application of polyvinylpolypyrrolidone showed insignificant influences on nutrients taken up by oilseed rape. Results of the present study indicated that polyvinylpolypyrrolidone is a promising organic amendment for heavy metal (Cu, Cd and Zn) stabilization in polluted water and soil.

1. Introduction

Heavy metals such as chromium (Cr), arsenic (As), cadmium (Cd), copper (Cu) and Zinc (Zn) accumulate in water and soil largely through industrial discharge, agricultural chemicals and improper waste disposal [1,2]. Due to their non-biodegradability, toxicity and persistence, heavy metals bioaccumulation leads to toxic effects on agricultural productivity, ecosystem stability and human health [3,4,5]. Chronic exposure to heavy metals can cause severe health issues, including organ damage and carcinogenic risks [6], emphasizing the urgent need for effective remediation strategies.
Different remediation strategies, such as electrokinetic remediation [7], bioremediation [8], chemical remediation [9], physical remediation and combined techniques have been proposed to decontaminate heavy metal-polluted soils [10,11]. Electrokinetic remediation combines the technique of electrodialysis with the electromigration of ions in the polluted soils. The efficiency of electrokinetic remediation largely depends on soil properties such as pH, soil organic carbon, soil pore structure, lime content and speciation of heavy metals [12,13]. Bioremediation such as phytoremediation and microorganism remediation have shown great potential as environment-friendly alternatives to the physical remediation methods which were environmentally destructive [14]. However, bioremediation is time-consuming and not always competent for soils contaminated with multiple or high levels of heavy metals that even super-accumulative plants could not survive. Chemical remediation is mainly via the application of inorganic or organic amendments to immobilize soil heavy metals. Inorganic amendments such as zeolite [15], hydroxyapatite [16], beringite [17], iron oxides [18], lime [19], bauxite residue and red mud [20] have been reported to reduce the mobility and availability of one or several heavy metals in soils. However, the efficiency of inorganic amendments on heavy metal stabilization were limited in neutral soils [21]. On the other hand, many organic amendments have been used to enhance the solubility of heavy metals in contaminated soils, such as EDTA (ethylenediaminetetraacetic acid) [22], EDDS (ethylenediaminedisuccinic acid) [23], organic acids [24], cyclodextrins [25], saponin [26] and other organic chelators [27]. For most organic amendments, not only heavy metals but also large quantities of soil nutrients were extracted [11], and some organic amendments may pollute soil by being adsorbed onto soil particles. Using water-insoluble adsorber to sequester heavy metals is also an efficient way for heavy metal remediation in water and/or soils. A synthetic polymer, called polyvinylpolypyrrolidone, showed promise due to its high binding capacity and resistance to degradation [28], yet its application in soil–crop systems remained underexplored [29]. We hypothesized that polyvinylpolypyrrolidone application could immobilize high levels of soil heavy metals and reduce plant uptake of heavy metals, owing to strong binding capacity of polyvinylpolypyrrolidone to heavy metals and its well stability.
The present study evaluated the efficacy of water-insoluble polyvinylpolypyrrolidone in remediating Cu, Cd and Zn in contaminated water and soil–crop systems. The objectives were to (1) quantify the heavy metal adsorption capacities of polyvinylpolypyrrolidone, (2) analyze its impact on crop growth and metal accumulation and (3) elucidate its potential as a sustainable alternative to conventional amendments.

2. Materials and Methods

2.1. Materials

The polyvinylpolypyrrolidone material of pharmaceutical grade was purchased from BASF (Ludwigshafen, Germany). The commercial product is a water-insoluble powder and its main component is cross-linked polyvinylpyrrolidone with a structural formula as shown in Figure S1. The morphology of the polyvinylpolypyrrolidone material was examined with SEM (Hitachi SU 3800, Tokyo, Japan) as shown in Figure S2, which indicated high porosity (>80%) and surface area (3.84 m2 g−1) of the powder polymer [30]. The particle size of polyvinylpyrrolidone is typically <10 μm based on the SEM image.

2.2. Batch Trials

To investigate the time dependent kinetics of heavy metal adsorption by polyvinylpolypyrrolidone, 0.5 mol L−1 CuCl2, CdCl2 and ZnCl2 solutions were mixed with polyvinylpolypyrrolidone at a ratio of 20:1 (mL g−1). After shaking for certain time intervals (10 min, 30 min, 1 h, 2 h, 4 h, 8 h, 12 h and 24 h) on a thermostatic oscillator at 120 rpm, the supernatant was obtained by centrifugation at 10,000 rpm for 10 min and stored at 4 °C for further analysis. To figure out the potential adsorption mechanisms of heavy metals by polyvinylpolypyrrolidone, the results of adsorption kinetic batch trials were simulated with four mathematical models, i.e., pseudo first- and second-order models, Elovich model and intra-particle model. To study the adsorption isotherm, eight concentrations (0, 0.001, 0.005, 0.01, 0.05, 0.1, 0.5 and 1 mol L−1) of CuCl2, CdCl2 or ZnCl2 solutions were shaken (120 rpm) with polyvinylpolypyrrolidone at a ratio of 20:1 (mL g−1) for 2 h, respectively. The supernatant was separated and stored as described above. The possible interferences between the studied heavy metal cations of adsorption by polyvinylpolypyrrolidone were also explored. There were seven treatments with various heavy metal solutions, i.e., (1) Cu, (2) Cd, (3) Zn, (4) mixed solutions with Cu and Cd, (5) mixed solutions with Cu and Zn, (6) mixed solutions with Cd and Zn, (7) mixed solutions with Cu, Cd and Zn. The concentrations of Cu, Cd and Zn in seven treatments were all 0.1 mol L−1. Polyvinylpolypyrrolidone was added to the above seven solutions at a ratio of 20:1 (mL g−1) and shaken for 2 h. The supernatant was obtained and stored as above. All batch trials were conducted in centrifuge tubes at room temperature (N = 3).

2.3. Soil Incubation Experiments

To test the effects of polyvinylpolypyrrolidone on soil heavy metals immobilization, three soils differing in heavy metal content were used, i.e., two heavy metal-contaminated soils, Georgien (pH 7.4 in 0.01 mol L−1 CaCl2, clay content 50.2%) and Kirchhain (pH 5.5, clay content 36.6%) and one uncontaminated soil Klein Linden (pH 6.3, clay content 22.7%). The unpolluted Klein Linden soil contained about 1 mg kg−1 DTPA extracted Cu and Zn, while 0.1 mg kg−1 Cd. The two polluted soils (Georgien and Kirchhain) contained 8–220 mg kg−1 Cu, 97–106 mg kg−1 Zn and 1.5–2.5 mg kg−1 Cd, respectively. The concentrations of DTPA-extracted Cu, Cd and Zn in the three soils were 1.1–221.6, 0.10–2.57 and 1.0–106.7 mg kg−1, respectively. Polyvinylpolypyrrolidone was added to soil at four levels: 0, 10, 20 and 60 g kg−1 soil (<2 mm). For the unpolluted Klein Linden soil, before addition of polyvinylpolypyrrolidone, 2, 4 and 8 mg kg−1 was added with CdCl2 solutions. Distilled water was added to maintain 60% water holding capacity. Soil samples were collected after 1-, 14-, 42- and 98-day(s) incubation, dried at 40 °C and then sieved to <2 mm for further analysis.

2.4. Pot Experiments

The two heavy metal-polluted soils, i.e., Georgien and Kirchhain, were used for pot experiments performed in a greenhouse without additional illumination at ambient temperature of 15–25 °C. Four doses (0, 10, 20 and 60 g kg−1 soil) of polyvinylpolypyrrolidone was mixed with above two soils thoroughly before putting in plastic pots, respectively. Three rape seeds were sown evenly at the soil surface for each pot. Nitrogen was applied as a dose of 4 g kg−1 with NH4NO3 solutions to each pot 3 weeks after being sown. All seedlings were harvested after 7 weeks. The plant samples were dried at 80 °C to constant weights and then ground and saved for further analysis.

2.5. Heavy Metal Determinations and Statistic Analyses

The contents of heavy metals in soil samples were extracted with DTPA solutions [30]. Heavy metal concentrations were determined by Atomic Absorption Spectroscopy (AAS, Perkin-Elmer 2380, Springfield, IL, USA) [31]. Heavy metal contents of plant samples were determined with AAS after acid digestion [32]. Data analyses and graph plotting were performed with RStudio software version 4.4.2 [33]. Assumptions of normality and homogeneity of variance were tested by Shapiro–Wilk and Levene’s test, respectively [34]. With verified assumptions, two-way mixed ANOVA was performed to compare means of treatment under various doses and incubation durations for each heavy metal of each soil type. One-way ANOVA was performed to compare means of various doses for each heavy metal of each soil type at a certain incubation time. If the assumptions of normality and homogeneity of variance were violated, the non-parametric Friedman’s ANOVA was performed [35]. The statistical significance was accepted with p < 0.05.

3. Results

3.1. Adsorption Kinetics and Isotherms

The adsorption of Cu, Cd and Zn by polyvinylpolypyrrolidone were similar and mainly contained two stages (Figure 1): (1) a rapid initial adsorption phase within 10 min which reached 92%, 76% and 87% of equilibrium adsorption capacities, respectively; and (2) a slow adsorption step before equilibrium. After 4 h, the polyvinylpolypyrrolidone contained the highest amounts of Cu (257.4 mg g−1). For Cd and Zn, the equilibrium adsorption capacities of polyvinylpolypyrrolidone were 351.8 (24 h) and 164.7 (8 h) mg g−1, respectively. Within 10 min, 37.4%, 23.7% and 22.0% of initial Cu, Cd and Zn were removed by polyvinylpolypyrrolidone. Hence, the adsorption rates increased in the order of Cu > Zn > Cd, while the equilibrium capacity increased as Cd > Cu > Zn. With 1 mol L−1 of heavy metals, the adsorption capacities of polyvinylpolypyrrolidone were 263.1 mg g−1 Cu, 171.3 mg g−1 Zn and 286.6 mg g−1 Cd, respectively. The removal efficiencies of Cu, Cd and Zn by polyvinylpolypyrrolidone decreased with raised heavy metal concentrations in solutions. With the initial concentrations of solution Cu, Cd and Zn increased from 0.001 to 1 mol L−1, the proportions of heavy metals removed by polyvinylpolypyrrolidone decreased from 100% to 20.7%, 100% to 13.1% and 100% to 12.7%, respectively.
The parameters and correlation coefficients (R2) of four kinetic models and two isotherm models are summarized in Table 1. For Cu and Zn, the R2 values decreased in the order of pseudo second order > pseudo first order > Elovich > intra-particle diffusion (Figures S3–S6); while for Cd, it was pseudo second order > Elovich > intra-particle diffusion > pseudo first order. With R2 values ≈ 1, the pseudo second-order model best fitted the adsorption kinetic data for all three heavy metals. Regarding adsorption isotherm models, both Langmuir and Freundlich models R2 values were >0.92 for all three heavy metals (Figures S7 and S8). Based on parameters of Langmuir simulation, the maximum adsorption capacities of Cu, Cd and Zn by polyvinylpolypyrrolidone were 327, 330 and 186 mg g−1, respectively. The separation factor (RL) of the adsorption was 0.166, 0.086 and 0.089 for Cu, Cd and Zn, respectively.

3.2. Affinities of Cu, Cd and Zn to Polyvinylpolypyrrolidone

The effects of heavy metal coexistence on their adsorption by polyvinylpolypyrrolidone was explored (Figure 2). Overall, the adsorption capacities (mg g−1) by polyvinylpolypyrrolidone decreased in the order of Cd > Cu > Zn no matter whether the three metals were presented solely or together in solutions. When mixed with Cu + Cd, Cu + Zn and Cu + Cd + Zn, the polyvinylpolypyrrolidone showed 22.6%, 1.83% and 23.2% decline of Cu adsorption compared to mixed with 0.1 mol L−1 Cu alone, respectively. For Cd, the Cu + Cd, Cd + Zn and Cu + Cd + Zn treatments received 22.6%, 31.1% and 36.3% less Cd absorption by polyvinylpolypyrrolidone than Cd treatment, respectively. Much reduction in adsorption capacities occurred for Zn when mixed with Cu and/or Cd, as there were 29.2%, 37.5% and 68.9% less Zn have been absorbed of Cu + Zn, Cd + Zn and Cu + Cd + Zn treatments than Zn treatment, respectively.

3.3. Effects of Polyvinylpolypyrrolidone on Soil Heavy Metal Immobilization

In general, the DTPA-extracted amounts of all three heavy metals showed clear decline trends with increased doses of polyvinylpolypyrrolidone (Figure 3). For DTPA-extracted Cu (DTPA-Cu), in the Klein Linden and Kirchhain soils, compared to no polyvinylpolypyrrolidone application treatment, addition of 10, 20 and 60 g polyvinylpolypyrrolidone kg−1 soil reduced DTPA-Cu by 59–73, 78–88 and 88–96%, respectively. In the highly Cu-polluted Georgien soil, DTPA-Cu was also sharply reduced by 45, 65 and 88%, respectively. During incubation, the DTPA-Cu showed fluctuations especially for the Georgien and Kirchhain soils. Compared to no polyvinylpolypyrrolidone, the DTPA-extracted Zn (DTPA-Zn) was significantly reduced by 13–45, 26–60 and 64–83% in three soils with polyvinylpolypyrrolidone application doses of 10, 20 and 60 g kg−1, respectively. One day after incubation, addition of polyvinylpolypyrrolidone reduced 15.3%, 12.3% and 36.5% more DTPA-Zn in Kirchhain soil than in Georgien soil, even though DTPA-Zn content in the two soils was similar. In the unpolluted Klein Linden soil, the DTPA-Zn showed consistent reduction among soil incubation for all doses of polyvinylpolypyrrolidone. In the unpolluted Klein Linden soil, polyvinylpolypyrrolidone application doses of 10, 20 and 60 g kg−1 soil decreased DTPA-extracted Cd (DTPA-Cd) by 27–45, 38–93 and 51–84%, respectively. In the two polluted soils incubated for ≤42 days, the concentrations of DTPA-Cd decreased with increased doses of polyvinylpolypyrrolidone; while after 98 days incubation, 10 or 20 g kg−1 doses of polyvinylpolypyrrolidone showed similar or even less DTPA-Cd than 60 g kg−1 doses of polyvinylpolypyrrolidone.
Without polyvinylpolypyrrolidone application, DTPA-Cu showed 48.4, 12.9 and 15.2% reduction in the Klein Linden, Georgien and Kirchhain soils after 14 weeks incubation, respectively. Accordingly, the DTPA-Cd and -Zn showed 43.1%, 12.9% and 12.1% and 41.0%, 21.0% and 12.3% decline in three soils after 98 days incubation, respectively.
To explore the effects of Cd content on polyvinylpolypyrrolidone immobilization of soil Cd, four doses of CdCl2 were applied (Figure 4). One day after incubation, only 59.2%, 65.6% and 64.8% of Cd that had been applied with 2, 4 and 8 mg kg−1 into Klein Linden soil could be extracted by DTPA without polyvinylpolypyrrolidone addition. When mixed with polyvinylpolypyrrolidone, the ratios became 48.3–50.4%, 45.6–61.2% and 41.3–59.6%, respectively.

3.4. Effects of Polyvinylpolypyrrolidone on Plant Uptake of Heavy Metals

After grown for 7 weeks, the fresh weight of oilseed rape showed insignificant differences between four doses of polyvinylpolypyrrolidone at the Kirchhain soil, while there was an accrual (non-significant) at the Georgien soil (Table 2). No significant differences were observed of Fe, K, Ca and Mg contents of oilseed rape plants between four application levels of polyvinylpolypyrrolidone at both soils.
For plants grown at the Georgien soil, the plant Cu, Cd and Zn contents decreased by 24.2–44.9%, 13.9–75.0% and 19.2–39.1% for 10, 20 and 60 g kg−1 doses of polyvinylpolypyrrolidone compared with no polyvinylpolypyrrolidone addition, respectively (Table 2). In the Kirchhain soil, the plant Cu, Cd and Zn contents were reduced by 24.2–44.4%, 65.0–81.7% and 69.2–79.5% for polyvinylpolypyrrolidone doses increased from 10 to 60 g kg−1 compared to the control treatment, respectively.

4. Discussion

4.1. Adsorption Characteristics of Heavy Metals by Polyvinylpolypyrrolidone

The adsorption of heavy metals (Cu, Cd and Zn) by polyvinylpolypyrrolidone can be described as a rapid adsorption step followed by a slow step restricted by the rate of the reaction between polyvinylpolypyrrolidone and heavy metals. Based on the correlation coefficients (R2), the pseudo second order simulated all three heavy metals quite well, which indicated that the adsorption of Cu, Cd and Zn by polyvinylpolypyrrolidone could be a chemical adsorption procedure. The pH of polyvinylpolypyrrolidone is about 8.6 which could enable negative potentials for organic ligands of polyvinylpolypyrrolidone. Polyvinylpolypyrrolidone could sorb heavy metals via dipolar bonds between metal cations and deprotonated organic groups of the adsorbent [29]. It was also reported that the adsorption of Zn by polyvinylpolypyrrolidone mainly controlled by chemisorption mechanism [29]. Moreover, polyvinylpolypyrrolidone could form complex with metals via its nitrogenous heterocyclic groups [29]. The hydroxyl and/or nitrogenous groups of polyvinylpolypyrrolidone could have different affinities to various metal cations. Similarly, Kaschl et al. 2002 reported that the complexation capacity with soil humic acids increased in the order of Cu > Cd ≥ Zn [36]. The chemisorption dominated adsorption mechanisms of Cu, Cd and Zn by polyvinylpolypyrrolidone could result in selective adsorption of various metals. Our results indicated adsorption rates in the order of Cu > Zn > Cd. The phenomenon of selective adsorption of Cu, Cd and Zn, and also for other metals by polyvinylpolypyrrolidone needs further attention, which would deepen our understanding of adsorption mechanisms and impact factors of various metals by polyvinylpolypyrrolidone.
Based on the results of Langmuir model simulation, the maximum adsorption capacity of heavy metals by polyvinylpolypyrrolidone were 330, 327 and 186 mg g−1 for Cd, Cu and Zn, respectively, which indicated that polyvinylpolypyrrolidone could be efficient in the removal of heavy metals. The separation factors (RL) for all three heavy metals were ranged between 0 and 1, which confirmed favorable adsorption of Cu, Cd and Zn by polyvinylpolypyrrolidone [37]. Previous study reported that the adsorption of Cu by the hybrid of polyvinylpolypyrrolidone and mesoporous silica was also well-fitted by Langmuir model, and the corresponding maximum adsorption capacity was 128 mg g−1 [28], which was only 39% of the present study. The maximum adsorption capacity of the hybrid of polyvinylpolypyrrolidone and mesoporous silica (128 mg g−1) is much lower than that of the present study (327 mg g−1). This could be due to shortened exposed surface of polyvinylpolypyrrolidone after mixing with mesoporous silica. When mixed with mesoporous silica, part of the surface of polyvinylpolypyrrolidone was attached to silica, which could decrease the adsorption sites of polyvinylpolypyrrolidone for Cu. The removal efficiencies of Cu by the hybrid of polyvinylpolypyrrolidone and mesoporous silica were times higher than that of the solely mesoporous silica [28], which meant that polyvinylpolypyrrolidone could be more effective than mesoporous silica in the adsorption of Cu. Our results also indicated that the effective removal of heavy metals could be inhibited by competed ions, especially for the removal of Zn by polyvinylpolypyrrolidone when Cu and/or Cd were coexisted.

4.2. Immobilization of Heavy Metals in Soils by Polyvinylpolypyrrolidone

The DTPA-extracted amounts of heavy metals were considerably decreased with increased doses of polyvinylpolypyrrolidone in both unpolluted and highly polluted soils (Figure 3). For example, compared with no polyvinylpolypyrrolidone application, DTPA-Cu, -Cd and -Zn declined by 59–96%, 27–93% and 13–83% after various amounts of polyvinylpolypyrrolidone were added, respectively. This result indicated that polyvinylpolypyrrolidone promoted soil heavy metals immobilization which left fewer heavy metals that could be extracted by DTPA. In contrary, little or no effect of polyvinylpolypyrrolidone on soil Cd immobilization was reported by Szewczuk-Karpisz et al. [38]. The authors indicated that short examination time (1 day) was not enough for optimal adsorption of soil Cd by polyvinylpolypyrrolidone. Indeed, our results suggested that from 1 to 98 day(s) after incubation, the DTPA-Cd showed 52.9, 45.0 and 65.3% reduction for Klein Linden, Georgien and Kirchhain soils, respectively. It meant that large amounts of DTPA-Cd became unextractable after 1 day. Above results also indicated that the adsorption of Cd by polyvinylpolypyrrolidone in solutions reached equilibrium at 24 h (Figure 1). In soils, the adsorption of Cd by polyvinylpolypyrrolidone powder could be much slower than in solutions due to limited contact between polyvinylpolypyrrolidone and soil DTPA-Cd. The DTPA-Cd kept decreasing from 1 to 98 day(s) especially for 10 and 20 g kg−1 polyvinylpolypyrrolidone application treatments, which indicated that more soil Cd may be passivated by polyvinylpolypyrrolidone addition with longer periods. Hanauer et al. [29] reported that after 12 months of polyvinylpolypyrrolidone (6%) addition, the soil total Cd content decreased from 0.242 mg kg−1 to 0.002 mg kg−1. Moreover, the immobilization impact of polyvinylpolypyrrolidone was the strongest among other amendments such as iron grit, zeolite and biochar irrespective of cropping or not [29].
Our results also showed that for Cu and Zn, the immobilization procedure was faster. The DTPA-Cu and -Zn became stable or even accrual after 14 days incubation, which indicated that 2 weeks incubation were adequate for the immobilization of soil Cu and Zn by polyvinylpolypyrrolidone. The results of adsorption kinetics also suggested that the adsorption of Cu and Zn by polyvinylpolypyrrolidone was faster than Cd in solutions, which could be due to different affinities of functional groups of polyvinylpolypyrrolidone to various metals as we discussed above. Intriguingly, the DTPA-extracted metal concentrations showed different fluctuation trends in various soils. In the Georgien soil with neutral pH (7.4), the DTPA-Cu showed more apparent arise trends compared to the Kirchhain soil with acidic pH (5.5). As discussed above, the adsorption of Cu by polyvinylpolypyrrolidone is mainly dominated by chemisorption, which is reversible. The acidic soil condition could induce re-release of Cu into soil solutions and lead to accrual of DTPA-Cu, which needs more evidence.
The immobilization effects may be sustained for several years, as Kaplan et al. [39] reported that the remediation effects of polyvinylpolypyrrolidone on soil Cu, Cd and Zn could be stable after five years. Notably, the above study evaluated only soils with neutral pH values. Our results suggested that the DTPA-Cu showed slight increase for all three polyvinylpolypyrrolidone application treatments in the Kirchhain soil (pH = 5.5), which could be due to re-release of reversibly chemisorbed Cu. Thus, the stability of the remediation effects of polyvinylpolypyrrolidone on soil Cu under acidic soil required verification by long-term trials.

4.3. Influences of Polyvinylpolypyrrolidone on Plant Growth and Plant Uptake of Heavy Metals

The application of polyvinylpolypyrrolidone significantly reduced oilseed rape plant uptake of Cu, Cd and Zn with the effects improved as lifted doses of polyvinylpolypyrrolidone at both soils (Table 2). The Cu content of oilseed rape plant grown in Cu uncontaminated soils could be 1.6 to 8.8 mg kg−1 [40]. Toxic effects on the growth of oilseed rape may occur with Cu content > 15 mg kg−1 in oilseed rape plants [40]. The Cu contents of plant in the Georgien soil were about 3 to 25 times higher than that grown in normal soils. Thus, the quite high uptake of Cu may inhibit the growth of oilseed rape plant, which was confirmed by the plant dry weights that were negatively correlated with plant Cu contents. Without polyvinylpolypyrrolidone addition at the Georgien soil, the growth of the upper leaves was stunted, and the leaf tissues between veins were discolored, while the symptoms of Cu toxicity were alleviated in treatments with polyvinylpolypyrrolidone application. Low soil Cu could stimulate oilseed rape growth, while high content of soil Cu would inhibit the growth of rape plant. As Yang et al. [41] reported, the fresh weight of rape raised until the soil received 100 mg kg−1 dose of Cu (added as CuSO4). In the present study, rape seeds were sowed right after polyvinylpolypyrrolidone application. Based on the results of soil incubation, the immobilization impact of polyvinylpolypyrrolidone on soil Cu improved after 2 weeks. Thus, the rape plant seedlings may exposure to less toxic Cu if rape seeds were sowed 2 weeks after application of polyvinylpolypyrrolidone.
The hindered growth of rape seedlings could be attributable to toxicity of Cd and Zn as well. As Baryla et al. [42] reported, the dry weight of leaves and stems of rape was decreased when soil Cd content exceeded 25 mg kg−1, with Cd contents > 50 mg kg−1 of both leaves and stems. Our results showed that the rape plant Cd contents grown at both the Georgien and Kirchhain soils were all less than 8.0 mg kg−1. This level of shoot Cd content may not inhibit the growth of oilseed rape as this crop plant has been found to have no symptoms of Cd toxicity with shoot Cd ranging from 32.4 to 44.8 mg kg−1 [43]. The low uptake of Cd by rape could be due to low available soil Cd after polyvinylpolypyrrolidone addition, since the DTPA-Cd was 2.6 and 1.6 mg kg−1 one day after polyvinylpolypyrrolidone application of the Georgien and Kirhhain soil, respectively. On the other hand, plants take up Cd by accidental entry via specific and non-specific transporters of plant essential elements such as Cu and Zn [36]. Thus, high soil Cu and Zn contents could inhibit the uptake of Cd by rape plants. As Shahid et al. [44] reported, the cooccurrence of Zn and Cd could reduce root uptake of Cd by up to 50%, which may be due to similar chemical behaviors of two heavy metals in the soil–plant system and then competition of transporters of essential elements.
Rape uptake of all three heavy metals irrespective of essential and non-essential were significantly decreased after application of polyvinylpolypyrrolidone, while the absorption of other nutrients was not affected (Table 2). In most cases, the reduction effects showed insignificant differences between doses of 10 and 20 g kg−1, and between 20 and 60 g kg−1, while they had significant differences among 10 and 60 g kg−1. Since Cu and Zn are essential micro-nutrients for crops, cautions should be taken when considering using polyvinylpolypyrrolidone to remediate multiple heavy metal-polluted soils such as those with high levels of Cd but low levels of Cu and Zn, as high doses of polyvinylpolypyrrolidone may lead to shortage of available Cu and Zn for crops. Our results indicated that the DTPA-Cu could be less than 1 mg kg−1 if more than 20 g kg−1 polyvinylpolypyrrolidone were added into soils with not very high available Cu, such as Kirchhain soil in the present study. Thus, we suggest polyvinylpolypyrrolidone application doses of 20 g kg−1 for the remediation of soil with high levels of Cu and Zn, such as Georgien soil. This application rate was also recommended by Hanauer et al. [29]. However, we recommend less than 10 g kg−1 for soils with relatively high concentrations of Cd but low levels of Cu.

5. Conclusions

Batch trials, soil incubation and pot experiments were conducted to evaluate the adsorption performance and immobilization effects of polyvinylpolypyrrolidone on Cu, Cd and Zn in water and soil, respectively. The maximum adsorption capacities were 327, 330 and 186 mg g−1 for Cu, Cd and Zn, and 92%, 76% and 87% of them achieved within 10 min, respectively. Kinetic modeling results indicated that the adsorption of Cu, Cd and Zn by polyvinylpolypyrrolidone could be chemisorption dominant processes. The application of polyvinylpolypyrrolidone significantly reduced the DTPA extracted soil Cu, Cd and Zn contents by 59–96%, 27–93% and 13–83%. Thus, oilseed rape-accumulated heavy metals were markedly inhibited with polyvinylpolypyrrolidone amendments, while there was no suppression of macro-nutrients taken up by rape plants. We suggest polyvinylpolypyrrolidone application doses of 20 g kg−1 for the remediation of soil with high levels of Cu and Zn like the Georgien soil, and less than 10 g kg−1 for soils with relatively high concentrations of Cd but low levels of available soil Cu and Zn to avoid shortage of their supply for plant growth. Our results also indicated selective adsorption rates in the order of Cu > Zn > Cd by polyvinylpolypyrrolidone. Further research is needed on the impact factors and long-term remediation effects on soil physicochemical and biological properties before the promising organic amendment polyvinylpolypyrrolidone could be the widely accepted soil heavy metal remediation choice.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/agronomy15102258/s1. Figure S1: The structural formula of polyvinylpolypyrrolidone; Figure S2: The morphology of polyvinylpolypyrrolidone; Figure S3: Pseudo first order model result of adsorption kinetics of Cu, Cd and Zn by polyvinylpolypyrrolidone; Figure S4: Pseudo second order model result of adsorption kinetics of Cu, Cd and Zn by polyvinylpolypyrrolidone; Figure S5: Elovich model result of adsorption kinetics of Cu, Cd and Zn by polyvinylpolypyrrolidone; Figure S6: Intra-particle model result of adsorption kinetics of Cu, Cd and Zn by polyvinylpolypyrrolidone; Figure S7: Langmuir model result of adsorption isotherms of Cu, Cd and Zn by polyvinylpolypyrrolidone; Figure S8: Freundlich model result of adsorption isotherms of Cu, Cd and Zn by polyvinylpolypyrrolidone.

Author Contributions

Y.W.: Data curation, Formal analysis, Visualization, Writing—original draft, Writing—review and editing. D.S.: Conceptualization, Methodology, Supervision, Funding acquisition. Y.J.: Visualization, Validation, Writing—review and editing, Funding acquisition. H.W.: Conceptualization, Methodology, Writing—original draft, Supervision, Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Jiangsu Province, grant number BK 20240276 and the Special Fund of Chinese Central Government for Basic Scientific Research Operations in Commonweal Research Institute, grant number GYZX 240411.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Acknowledgments

We gratefully acknowledge Chen and Zhou for their help in data processing and manuscript revision.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Adsorption of Cu, Cd and Zn by polyvinylpolypyrrolidone as affected by adsorption time. N = 3.
Figure 1. Adsorption of Cu, Cd and Zn by polyvinylpolypyrrolidone as affected by adsorption time. N = 3.
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Figure 2. Adsorption capacities of polyvinylpolypyrrolidone affected by companion heavy metal ions. The concentrations of Cu, Cd and Zn were all 0.1 mol L−1 for all treatments. N = 3.
Figure 2. Adsorption capacities of polyvinylpolypyrrolidone affected by companion heavy metal ions. The concentrations of Cu, Cd and Zn were all 0.1 mol L−1 for all treatments. N = 3.
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Figure 3. Concentrations of DTPA extracted heavy metals (A: Cu; B: Cd; C: Zn) in three soils after 1 to 98 day(s) application with various doses of polyvinylpolypyrrolidone. Significant differences between various application doses of polyvinylpolypyrrolidone were labeled with different lowercase letters. (p < 0.05, N = 3).
Figure 3. Concentrations of DTPA extracted heavy metals (A: Cu; B: Cd; C: Zn) in three soils after 1 to 98 day(s) application with various doses of polyvinylpolypyrrolidone. Significant differences between various application doses of polyvinylpolypyrrolidone were labeled with different lowercase letters. (p < 0.05, N = 3).
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Figure 4. Concentrations of DTPA extracted Cd in Klein Linden soil treated with CdCl2 after incubation with various doses of polyvinylpolypyrrolidone for 1 to 98 day(s). Significant differences between various application doses of polyvinylpolypyrrolidone were labeled with different lowercase letters. (p < 0.05, N = 3).
Figure 4. Concentrations of DTPA extracted Cd in Klein Linden soil treated with CdCl2 after incubation with various doses of polyvinylpolypyrrolidone for 1 to 98 day(s). Significant differences between various application doses of polyvinylpolypyrrolidone were labeled with different lowercase letters. (p < 0.05, N = 3).
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Table 1. Modeling parameters of adsorption kinetics and isotherms for Cu, Cd and Zn by polyvinylpolypyrrolidone.
Table 1. Modeling parameters of adsorption kinetics and isotherms for Cu, Cd and Zn by polyvinylpolypyrrolidone.
ModelsHeavy MetalsConstantsR2
Pseudo first orderCuqe = 0.256; k1 = 15.810.963
Cdqe = 0.315; k1 = 10.930.232
Znqe = 0.161; k1 = 13.260.832
Pseudo second orderCuqe = 0.256; k2 = 249.90.999
Cdqe = 0.346; k2 = −1.090.998
Znqe = 0.164; k2 = −2.640.999
ElovichCuα = 0.251; β = 0.0020.341
Cdα = 0.293; β = 0.0170.939
Znα = 0.156; β = 0.0030.631
Intra-particle diffusionCuC = 0.249; kp = 0.0020.072
CdC = 0.269; kp = 0.0180.931
ZnC = 0.153; kp = 0.0030.351
LangmuirCuqmax = 0.327; kL = 0.079; RL = 0.1660.995
Cdqmax = 0.330; kL = 0.094; RL = 0.0860.995
Znqmax = 0.186; kL = 0.156; RL = 0.0890.997
FreundlichCun = 2.11; kF = 0.0410.954
Cdn = 2.63; kF = 0.0540.927
Znn = 2.60; kF = 0.0370.936
Table 2. Plant growth (mean ± standard deviation) and content of metals in rape plant as affected by various application doses of polyvinylpolypyrrolidone in two polluted soils. Significant differences between various application doses of polyvinylpolypyrrolidone were labeled with different lowercase letters. (p < 0.05, N = 3).
Table 2. Plant growth (mean ± standard deviation) and content of metals in rape plant as affected by various application doses of polyvinylpolypyrrolidone in two polluted soils. Significant differences between various application doses of polyvinylpolypyrrolidone were labeled with different lowercase letters. (p < 0.05, N = 3).
SoilDoses
g kg−1		Dry Weight
g pot−1		CuCdZnFeKCaMg
mg kg−1%
Georgien00.28 ± 0.08 ab43.4 ± 4.4 a7.2 ± 0.8 a161 ± 26 a70 ± 15 a5.1 ± 1.0 a4.3 ± 1.0 a0.66 ± 0.07 a
100.36 ± 0.04 ab32.9 ± 1.5 b6.2 ± 0.3 ab130 ± 17 ab72 ± 3 a5.2 ± 0.3 a4.0 ± 1.0 a0.66 ± 0.03 a
200.44 ± 0.07 a28.5 ± 1.2 bc4.5 ± 2.3 b126 ± 15 b87 ± 16 a5.6 ± 0.4 a3.8 ± 0.8 a0.64 ± 0.05 a
600.45 ± 0.03 a23.9 ± 2.0 c1.8 ± 0.2 c98 ± 4 b80 ± 10 a4.8 ± 0.3 a4.0 ± 1.0 a0.68 ± 0.06 a
Kirchhain00.58 ± 0.06 a9.9 ± 0.9 a6.0 ± 1.1 a318 ± 45 a79 ± 7 b3.3 ± 0.2 a3.0 ± 0.6 a0.53 ± 0.04 a
100.52 ± 0.03 a7.5 ± 1.4 b2.1 ± 0.1 b98 ± 3 b110 ± 17 a3.1 ± 0.1 a3.1 ± 0.7 a0.48 ± 0.07 a
200.57 ± 0.09 a7.0 ± 1.2 b1.1 ± 0.4 b96 ± 25 b107 ± 10 a3.0 ± 0.2 a3.5 ± 0.8 a0.52 ± 0.02 a
600.53 ± 0.03 a5.5 ± 0.5 b1.6 ± 0.3 b65 ± 5 b109 ± 12 a3.1 ± 0.3 a3.1 ± 0.7 a0.47 ± 0.06 a
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Wang, Y.; Steffens, D.; Jia, Y.; Wang, H. Polyvinylpolypyrrolidone Immobilized Cu, Cd and Zn in Soils and Reduced Their Uptake by Oilseed Rape. Agronomy 2025, 15, 2258. https://doi.org/10.3390/agronomy15102258

AMA Style

Wang Y, Steffens D, Jia Y, Wang H. Polyvinylpolypyrrolidone Immobilized Cu, Cd and Zn in Soils and Reduced Their Uptake by Oilseed Rape. Agronomy. 2025; 15(10):2258. https://doi.org/10.3390/agronomy15102258

Chicago/Turabian Style

Wang, Yiliu, Diedrich Steffens, Yunsheng Jia, and Huoyan Wang. 2025. "Polyvinylpolypyrrolidone Immobilized Cu, Cd and Zn in Soils and Reduced Their Uptake by Oilseed Rape" Agronomy 15, no. 10: 2258. https://doi.org/10.3390/agronomy15102258

APA Style

Wang, Y., Steffens, D., Jia, Y., & Wang, H. (2025). Polyvinylpolypyrrolidone Immobilized Cu, Cd and Zn in Soils and Reduced Their Uptake by Oilseed Rape. Agronomy, 15(10), 2258. https://doi.org/10.3390/agronomy15102258

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