3.1. Solution composition over time
The elemental composition of the solutions after the extraction process are shown in Table 1b. The concentrations of metals measured in the solutions before plant introduction, i.e. t = 0 h, showed that, compared to deionized water, using 10 mM.kg− 1 tartaric acid extracted 18% more Cd, 11% more Zn and 7% more Pb, while using 3 mM.kg− 1 Na-EDTA extracted 88% more Cd and 15% more Zn. Except for Cu (for which the tartaric solution induced a higher extraction rate from the soil compared to the Na-EDTA solution (6%)), higher concentrations of metals were measured when Na-EDTA was used as the extractant, despite the Na-EDTA solution being less concentrated than the tartaric acid. The concentration of dissolved organic matter was higher in the Na-EDTA solution than the tartaric acid treatment and the deionized water. The solubilization of the nutrients (Ca, K, Mg, Na) by the different solutions was dependent on the extraction solution and the element. In more detail, calcium was measured in higher concentrations in the tartaric acid treatment and the Na-EDTA treatment compared to the deionized water, with 77% and 25% higher concentrations than in deionized water, respectively. Magnesium was found in similar concentrations in the treatments with tartaric acid and Na-EDTA, which had 60% higher concentrations of Mg compared to the deionized water treatment. The concentrations of potassium were highest in the Na-EDTA treatment, followed by tartaric acid and finally deionized water. Tartaric acid did not affect the concentrations of sodium while the solution of Na-EDTA had almost 6-times higher concentrations of Na relative to the deionized water solution. Moreover, when comparing the amount measured in the soil used for the extraction and the amount measured in the 10 L extracting solutions after extraction, it can be seen that tartaric acid extracted the highest proportion of Ca (22.5%), Mg (10.6%), Al (15.6%), Fe (5.3%), Pb (3.2%), and Zn (7.2%); while using Na-EDTA extracted the least Fe (3.7%) and Mn (0.5%) but the highest K (16.5%), Mg (10.6%), DOC (8.2%), Zn (7.5%) and Cd (7.4%). It can be noted that the use of Na-EDTA was particularly aggressive towards the soil and removed essential fertility elements, such as K, DOC and nitrogen, in the form of ammonium and nitrate.
3.2. Total metal concentrations
Over the 360 h of the experiment time course, concentrations of Cd in solution varied greatly, with the concentrations decreasing in the order Na-EDTA > Tartaric acid > Deionized water. For Pb, Cu and Zn, concentrations decreased from 0 to 24 h then slightly increased, with the highest concentrations measured in the Na-EDTA treatment (Fig. 1).
At the end of the experiment (t = 360 h), Cd was higher in the Na-EDTA solution (7-fold) than in the tartaric acid and the deionized water treatments. The concentrations of copper were similar in the Na-EDTA solution and the deionized water but lower in the tartaric acid. Concentrations of lead and zinc decreased in the order Na-EDTA > Deionized water > Tartaric acid. The comparison of the concentrations of metals between the initial (t = 0 h) and the final (t = 360 h) samplings showed that the growth of water lettuce in the deionized water removed 92%, 53%, 84% and 83% of Cd, Cu, Pb and Zn, respectively: In comparison, its growth in the tartaric acid solution removed 94%, 77%, 90% and 91% of Cd, Cu, Pb and Zn, respectively, and only 71%, 45%, 78% and 65% of Cd, Cu, Pb and Zn when plants grew in the Na-EDTA solution.
3.3. Metal speciation and DOM concentration in the solutions
The concentrations of dissolved organic matter were higher in the Na-EDTA solution than the tartaric acid treatment and the deionized water throughout the experiment time course (Table S1). At the end of the experiment (t = 360 h), compared to the deionized water, the concentration of DOM was on average 57% higher in the Na-EDTA and 30% lower in the tartaric acid treatment.
Using the chemical composition of the growing solutions, the Visual MINTEQ software was used to model the forms of the metals found in the solution. These forms were grouped into four categories: (i) free ions (Cd2+, Cu2+, Pb2+, Zn2+), (ii) the metals linked to fulvic and humic acids (FA/HA), (iii) the metals linked to LMWOA and (iv) the metals in “other” forms (Fig. 1).
Throughout the experiment time course, Cd was mainly found as free ion Cd2+. The increase in Cd concentration in the solution with tartaric acid and especially Na-EDTA was mainly attributed to an increase in free Cd ion and, to a lesser extent, the “other” fraction (composed mainly of CdCl+, CdSO4, CdNO3 and CdF+). In addition, Na-EDTA greatly increased the concentration of Cd associated mainly with acids. Over the 360 h of the experiment time course, concentrations of free Cd2+ ions tended to decrease especially in the deionized water and tartaric acid treatments, while the concentration of Cd associated with fulvic and humic acids increased with time in the Na-EDTA treatment. This implies that Cd consumption by plants was mostly through the uptake of free Cd2+ ions.
At the initial time (t = 0 h), Cu was mostly present as free Cu2+ ions, followed by Cu associated with fulvic and humic acids, Cu associated with LMWOA and the “other” Cu forms (composed mainly of CuSO4, CuOH+, CuNO3+, CuCl+ and CuF+), in the deionized water treatment. The use of tartaric acid did not affect the relative concentrations of Cu present as different species, while Na-EDTA decreased concentrations of free Cu2+ ions and increased the fraction associated with fulvic and humic acids. Over the experiment time course, the decrease in the total concentrations of Cu were primarily due to a decrease in the concentrations of free Cu2+ ions, reflecting an uptake of Cu by plants via free Cu ion assimilation.
Although the concentration decreased over time, Pb was mostly present as free Pb2+ ions, in all three treatments. Such a decrease in the concentrations of free Pb2+ ions is resultant of the uptake of the Pb2+ ions by the plants. The second most encountered form, “other” (i.e., PbSO4, PbCl+, PbOH+, PbNO3+, PbF+), also decreased with time, while the concentrations of Pb associated with fulvic and humic acids tended to increase. The main effect of the Na-EDTA treatment was the increase in the concentration of Pb associated with fulvic and humic acids.
Similar to the other elements, the main form of Zn in the solution was free Zn2+ ion, concentrations of which decreased over time in all three treatments, relative to plant uptake. The main effect of the treatment was a small increase in the concentration of Zn associated with humic and fulvic acids with Na-EDTA, which was more important with time.
3.4. Nutrient contents
Concentrations of nutrients concentrations tended to stay stable until hours t = 8 or 24, after which they decreased in all treatments. At the end of the experiment (t = 360 h), nutrient concentrations decreased in the order Na-EDTA > Tartaric acid > Deionized water. Compared to the concentrations measured in the solutions at the beginning of the experiment, after 360 h, plants consumed 86%, 74%, 97% and 96% Ca, Mg, K and Na, respectively in the control, 83%, 73%, 94% and 83% in the tartaric acid and 70%, 55%, 97% and 80% in the Na-EDTA treatment.
Regarding their ionic forms, the anion concentrations showed more variability through time (Table S1). At the end of the experiment, all anion concentrations decreased compared to t = 0, except for fluoride and sulphate. All treatments presented similar concentrations except for chloride (tartaric acid > Na-EDTA > deionized water) and sulphate (deionized water > Na-EDTA > tartaric acid).
Similar to the reported concentrations of anions, an important variability with time was seen in the case of organic acid (Table S1). On average, concentrations of lactate decreased in the order tartaric acid > Na-EDTA > deionized water; acetate decreased in the order Na-EDTA > deionized water > tartaric acid; formate in the order tartaric acid > deionized water > Na-EDTA; and malate and oxalate decreased in the order deionized water > tartaric acid > Na-EDTA.
This shows that there were higher concentrations of nutrients in the Na-EDTA and tartaric solutions than the deionized water, but the Na-EDTA treatment extracted the most elevated concentrations of Na which could induce salt stress in plants.
3.5. Metal accumulation in plants
The accumulation of Cd, Cu, Pb and Zn onto the roots and inside the leaves and the roots of Pistia stratiotes was monitored over the 360 hours of the experiment (Fig. 2). Most of the metals accumulated inside the roots, followed by an accumulation inside the leaves and onto the root surfaces.
The sorption of Cd onto root surfaces increased with time but generally decreased in the order Tartaric acid > Na-EDTA > Deionized water (Fig. 2). The highest levels of Cd adsorption to root surfaces were measured at 24 h for the control, 360 h for the solutions containing tartaric acid and at 72 h for the Na-EDTA. Copper sorption onto the roots showed greater variability over both time and treatments compared to Cd, although Cu sorption was generally higher with tartaric acid solution and Na-EDTA solution compared to deionized water and decreased with time, except at the final sampling. The highest concentrations were found at 2 h for the control, 360 h for the tartaric acid and 4 h for the Na-EDTA. Sorption of Pb onto the roots was low in all treatments at the beginning of the experiment and increased with time, with higher levels of sorption associated with the Na-EDTA and tartaric acid treatments compared to the deionized water treatment. The highest concentrations were measured at 360 h for the tartaric acid and Na-EDTA treatments and 72 h for the control. Finally, Zn sorption onto the roots increased with time and was higher when tartaric acid or Na-EDTA was used to obtain the growing solutions than deionized water solution. The concentration in the control did not evolve much while the highest concentrations in the tartaric acid and Na-EDTA treatments were measured at 360 h.
The different treatments used to obtain the growing solutions had little effects on the concentrations of Cd, Cu, Pb and Zn in the shoot tissues, while the concentrations in the root tissues were more affected (Fig. 2). Concentrations of Cd in plant tissues were elevated in the tartaric acid and Na-EDTA treatments relative to the deionized water treatment, while concentrations of Pb decreased with both treatments, Cu initially decreased then increased in the treatments with tartaric acid and Na-EDTA, and Zn was little affected by time or treatment. Except for Cu, for which concentrations in the roots decreased while concentrations in the shoots remained the same with time; the concentrations of Cd, Pb and Zn increased with time in both shoots and roots and for all three treatments. Maximum concentrations of Cd, Pb and Zn in shoot tissues were measured at 360 h in all treatments while maximum concentrations of Cu in shoots was measured at 360 h only in the case of Na-EDTA and at 24 h for the other two treatments. Maximum concentrations of Cd in root tissues were observed at 24 h for the control, 360 h for the tartaric acid and 168 h for the Na-EDTA treatment. Maximum concentrations of Cu in both plant tissues were measured in the initial phases of the experiment: 4 h for the control, 8 h for the tartaric acid and 2 h for the Na-EDTA. In the control, the maximum concentrations of Pb in root tissues occurred at 360 h while in the other two treatments it was earlier (72 h). Finally, in all cases, maximum concentrations of Zn in roots were measured at 360 h.
When looking at the repartition of the metals within the plants, it can be seen that the treatments “tartaric acid” and “Na-EDTA” had mainly an effect in the latter phases of the experiment (i.e,. after 24 h) (Fig. 2). More precisely, at the last sampling, the proportion of metals sorbed on the surface of the roots increased while the proportion in the leaves decreased, with a more pronounced effect observed in the case of tartaric acid.
The translocation factors were low in all treatments and for all elements at the beginning of the experiment (Table 2), except for Zn. They all increased over the time of the experiment but remained below 1. The treatment effect was mostly visible at the last sampling points, in which a higher TF was observed in the deionized water treatment relative to the other two treatments. The exception to this was Pb which showed the reverse. The translocation efficiency followed the mobility of the elements, rather than the essentiality, with higher TF values for Zn and Cd than Pb and Cu, regardless of the treatments. In accordance with TF, bioconcentration factors were higher in the roots than in the leaves (Table 2). Moreover, BCFs increased with time for all elements and both organs, except for Cu. On average, for both roots and leaves, the highest values of BCF were associated with the uptake of Cu, followed by Cd, Zn and Pb was the lowest.
Table 2
Translocation (a) and bioconcentration factors for leaves (b) and roots (c) in the different treatments and during the 360h experiment. Data are presented as mean ± sd (n = 3). DW = deionized water, TA = tartaric acid, EDTA = Na-EDTA.
Translocation factor | 1 | 2 | 4 | 8 | 24 | 72 | 168 | 360 |
Cadmium | DW | 0.039 ± 0.008 | 0.023 ± 0.005 | 0.022 ± 0.004 | 0.040 ± 0.004 | 0.067 ± 0.005 | 0.088 ± 0.004 | 0.204 ± 0.113 | 0.514 ± 0.230 |
TA | 0.021 ± 0.003 | 0.025 ± 0.001 | 0.018 ± 0.004 | 0.037 ± 0.023 | 0.051 ± 0.027 | 0.102 ± 0.006 | 0.244 ± 0.142 | 0.158 ± 0.004 |
EDTA | 0.057 ± 0.043 | 0.024 ± 0.003 | 0.024 ± 0.004 | 0.024 ± 0.006 | 0.029 ± 0.007 | 0.104 ± 0.034 | 0.147 ± 0.019 | 0.333 ± 0.140 |
Lead | DW | 0.099 ± 0.010 | 0.060 ± 0.007 | 0.086 ± 0.009 | 0.104 ± 0.005 | 0.059 ± 0.012 | 0.023 ± 0.001 | 0.031 ± 0.008 | 0.069 ± 0.004 |
TA | 0.064 ± 0.012 | 0.070 ± 0.011 | 0.071 ± 0.014 | 0.117 ± 0.088 | 0.054 ± 0.016 | 0.042 ± 0.002 | 0.093 ± 0.011 | 0.088 ± 0.000 |
EDTA | 0.062 ± 0.009 | 0.077 ± 0.008 | 0.089 ± 0.010 | 0.038 ± 0.008 | 0.038 ± 0.015 | 0.057 ± 0.002 | 0.071 ± 0.002 | 0.140 ± 0.039 |
Copper | DW | 0.037 ± 0.001 | 0.028 ± 0.005 | 0.033 ± 0.008 | 0.037 ± 0.001 | 0.050 ± 0.003 | 0.078 ± 0.002 | 0.080 ± 0.033 | 0.242 ± 0.050 |
TA | 0.037 ± 0.004 | 0.037 ± 0.002 | 0.039 ± 0.004 | 0.058 ± 0.027 | 0.075 ± 0.008 | 0.090 ± 0.001 | 0.078 ± 0.024 | 0.096 ± 0.006 |
EDTA | 0.037 ± 0.001 | 0.037 ± 0.004 | 0.058 ± 0.006 | 0.043 ± 0.001 | 0.035 ± 0.009 | 0.102 ± 0.008 | 0.075 ± 0.005 | 0.092 ± 0.003 |
Zinc | DW | 0.155 ± 0.015 | 0.110 ± 0.010 | 0.125 ± 0.006 | 0.156 ± 0.005 | 0.114 ± 0.005 | 0.073 ± 0.002 | 0.112 ± 0.034 | 0.297 ± 0.009 |
TA | 0.171 ± 0.023 | 0.122 ± 0.015 | 0.117 ± 0.023 | 0.202 ± 0.158 | 0.129 ± 0.023 | 0.120 ± 0.011 | 0.220 ± 0.056 | 0.196 ± 0.011 |
EDTA | 0.164 ± 0.025 | 0.165 ± 0.008 | 0.201 ± 0.005 | 0.123 ± 0.011 | 0.090 ± 0.035 | 0.178 ± 0.011 | 0.173 ± 0.003 | 0.273 ± 0.026 |
Bioaccumulation Factor: Leaves | 1 | 2 | 4 | 8 | 24 | 72 | 168 | 360 |
Cadmium | DW | 8.43 ± 1.97 | 5.10 ± 1.07 | 8.28 ± 1.75 | 14.58 ± 1.68 | 25.38 ± 2.44 | 23.18 ± 1.25 | 52.20 ± 26.50 | 134.26 ± 64.17 |
TA | 4.46 ± 0.79 | 5.37 ± 0.24 | 6.32 ± 1.81 | 14.41 ± 8.93 | 15.80 ± 9.02 | 26.07 ± 0.99 | 64.53 ± 34.88 | 94.01 ± 0.15 |
EDTA | 6.32 ± 4.31 | 3.77 ± 0.31 | 4.73 ± 0.26 | 4.47 ± 1.18 | 6.28 ± 1.75 | 21.87 ± 4.40 | 39.77 ± 5.03 | 70.58 ± 31.03 |
Lead | DW | 1.89 ± 0.27 | 1.97 ± 0.25 | 3.36 ± 0.33 | 6.72 ± 0.47 | 5.52 ± 0.95 | 3.58 ± 0.24 | 5.62 ± 1.66 | 14.72 ± 0.47 |
TA | 1.16 ± 0.02 | 2.04 ± 0.22 | 3.25 ± 0.76 | 6.16 ± 4.72 | 7.06 ± 2.79 | 6.35 ± 0.58 | 10.63 ± 0.65 | 11.28 ± 0.09 |
EDTA | 1.28 ± 0.19 | 1.57 ± 0.17 | 3.43 ± 0.18 | 2.82 ± 0.55 | 3.82 ± 1.58 | 7.78 ± 0.87 | 8.46 ± 0.39 | 17.94 ± 4.91 |
Copper | DW | 65.91 ± 0.34 | 54.23 ± 8.04 | 67.98 ± 16.94 | 73.23 ± 6.10 | 92.95 ± 5.82 | 83.65 ± 5.18 | 90.31 ± 23.18 | 81.36 ± 12.62 |
TA | 65.48 ± 0.30 | 56.06 ± 3.94 | 65.11 ± 7.71 | 106.78 ± 46.26 | 108.88 ± 16.44 | 76.14 ± 0.29 | 85.25 ± 26.01 | 80.50 ± 4.57 |
EDTA | 65.54 ± 1.19 | 87.99 ± 7.48 | 82.28 ± 6.21 | 80.48 ± 1.27 | 75.03 ± 24.10 | 84.56 ± 4.90 | 87.02 ± 3.24 | 117.25 ± 10.21 |
Zinc | DW | 10.98 ± 1.22 | 8.10 ± 0.61 | 11.19 ± 0.85 | 15.90 ± 0.88 | 15.77 ± 0.37 | 11.07 ± 0.29 | 17.82 ± 4.94 | 48.53 ± 3.61 |
TA | 10.24 ± 2.03 | 7.76 ± 1.29 | 11.68 ± 2.57 | 19.97 ± 14.93 | 17.75 ± 4.26 | 15.36 ± 1.50 | 26.47 ± 5.17 | 33.40 ± 2.76 |
EDTA | 8.28 ± 0.45 | 10.73 ± 0.94 | 12.06 ± 1.33 | 10.85 ± 1.03 | 9.87 ± 4.18 | 18.06 ± 0.29 | 20.56 ± 0.43 | 41.89 ± 4.73 |
Bioaccumulation Factor: Roots | 1 | 2 | 4 | 8 | 24 | 72 | 168 | 360 |
Cadmium | DW | 215.7 ± 7.5 | 222.0 ± 1.5 | 371.8 ± 4.4 | 367.2 ± 3.6 | 380.2 ± 6.2 | 262.2 ± 1.0 | 260.0 ± 14.5 | 259.2 ± 9.1 |
TA | 215.8 ± 7.8 | 214.7 ± 1.6 | 353.5 ± 12.5 | 387.6 ± 1.2 | 306.7 ± 16.9 | 255.7 ± 6.5 | 268.5 ± 13.1 | 595.8 ± 16.6 |
EDTA | 115.4 ± 12.0 | 157.7 ± 7.0 | 202.1 ± 24.8 | 185.1 ± 3.7 | 218.4 ± 6.9 | 215.1 ± 27.5 | 271.0 ± 1.0 | 210.9 ± 4.6 |
Lead | DW | 19.0 ± 0.7 | 32.8 ± 0.4 | 38.9 ± 0.0 | 64.6 ± 1.4 | 93.8 ± 2.3 | 155.6 ± 3.6 | 178.0 ± 7.8 | 212.4 ± 3.9 |
TA | 18.5 ± 3.7 | 29.2 ± 1.6 | 45.8 ± 1.7 | 52.1 ± 1.0 | 128.2 ± 13.6 | 152.1 ± 7.2 | 115.1 ± 7.3 | 127.7 ± 0.8 |
EDTA | 20.6 ± 0.0 | 20.4 ± 0.1 | 38.6 ± 2.4 | 74.1 ± 0.6 | 100.7 ± 3.1 | 136.1 ± 9.3 | 118.5 ± 8.1 | 128.2 ± 0.3 |
Copper | DW | 1803.6 ± 62.4 | 1950.0 ± 50.4 | 2068.2 ± 8.8 | 1955.8 ± 108.7 | 1862.6 ± 23.1 | 1078.4 ± 36.8 | 1161.1 ± 187.0 | 338.1 ± 17.9 |
TA | 1773.8 ± 195.6 | 1514.1 ± 38.7 | 1670.2 ± 14.1 | 1864.3 ± 72.1 | 1439.4 ± 73.9 | 848.9 ± 13.4 | 1091.5 ± 9.1 | 835.2 ± 1.5 |
EDTA | 1753.4 ± 94.9 | 2350.6 ± 27.0 | 1429.7 ± 33.9 | 1885.3 ± 63.0 | 2112.2 ± 137.0 | 836.1 ± 114.5 | 1165.0 ± 34.2 | 1273.7 ± 74.7 |
Zinc | DW | 71.0 ± 0.8 | 73.9 ± 1.0 | 89.7 ± 2.3 | 102.1 ± 2.3 | 138.3 ± 2.4 | 151.7 ± 0.3 | 159.3 ± 4.3 | 163.1 ± 7.0 |
TA | 59.6 ± 3.8 | 63.4 ± 2.9 | 99.8 ± 2.5 | 100.8 ± 4.7 | 136.9 ± 8.7 | 128.3 ± 0.7 | 121.2 ± 7.5 | 169.9 ± 4.9 |
EDTA | 50.8 ± 5.1 | 65.0 ± 2.4 | 59.9 ± 5.0 | 88.0 ± 0.5 | 109.4 ± 4.0 | 102.0 ± 7.9 | 118.7 ± 4.5 | 153.4 ± 2.7 |