Results for the solubilization of metals from biosolid- (BSL-) treated soils by simulated organic acid-based synthetic root exudates (OA mixtures) of differing composition and concentrations are presented. This study used two BSL-treated Romona soils and a BSL-free Romona soil control that were collected from experimental plots of a long-term BSL land application experiment. Results indicate that the solubility of metals in a BSL-treated soil with 0.01 and 0.1 M OA mixtures was significantly higher than that of 0.001 M concentrations. Differences in composition of OAs caused by BSL treatment and the length of growing periods did not affect the solubility of metals. There were no significant differences in organic composition and metals extracted for plants grown at 2, 4, 8, 12, and 16 weeks. The amount of metals extracted tended to decrease with the increase of the pH. Results of metal dissolution kinetics indicate two-stage metal dissolution. A rapid dissolution of metals occurred in the first 15 minutes. For Cd, Cu, Ni, and Zn, approximately 60–70% of the metals were released in the first 15 minutes while the initial releases for Cr and Pb were approximately 30% of the total. It was then followed by a slow but steady release of additional metals over 48 hours.
Organic acids (OAs) provide attractive options for extracting agents not only because they are biodegradable [
The extent of complexation depends on the characteristics of the OAs involved (number and proximity of carboxylic groups), their concentrations, types of metal, and the pH of the soil [
OAs may be adsorbed onto the hydroxyaluminum-montmorillonite (HyA-Mt) complex. Cambier and Sposito [
Early research by Eaton [
The pH of the rhizosphere is also important in determining metal and nutrient mobilization and uptake. It also affects microbial activity in the vicinity of the root. Root induced pH change in the rhizosphere is a known phenomenon [
The objectives of this study were the following: To use an OA mixture as a substitute for actual OAs in root exudates to solubilize metals in BSL-treated soils. To test the solubilization of metals in BSL-treated soils by the different concentrations of the OA mixture of corn (
In order to meet these objectives, the researchers needed to assess OA mixture-specific metal solubility and dissolution rate constants of BSL-treated soils.
Two BSL-treated Romona soils and the BSL-free Romona soil control from the field plots of a long-term BSL land application experiment were used [
Chemical properties of the soil used for the experiment.
Biosolid treatment |
pH |
Total concentration (mg kg−1) | |||||
---|---|---|---|---|---|---|---|
Cd | Cr | Cu | Ni | Pb | Zn | ||
Control | 7.7 | 0.5 | 37 | 105 | 24 | 25 | 95 |
135 Mg ha−1 | 6.9 | 11 | 243 | 188 | 88 | 120 | 559 |
1,080 Mg ha−1 | 6.1 | 26 | 596 | 478 | 215 | 396 | 1466 |
One gram of the soils was mixed with 10 mL of OA mixtures in 50 mL Teflon test tubes. The contents were shaken and allowed to equilibrate at 298°K using a rotary mixer, SA-12 Motor Speed Control (B & B Motor and Control Corp., Long Island City, NY), which rotated the capped test tubes head to tail at approximately 1 rpm for 48 hours. The speed of rotation was maintained constant in all treatments. One mL of chloroform was added to each test tube to control microbial activity and prevent decomposition of OA mixtures during equilibration. The pH and EC of the system in the beginning and at the end of the reaction period were monitored and attempts were made to keep the pH constant. Three OA mixture concentrations 0.001, 0.01, and 0.1 M in 13.5 mM Ca(NO3)2 along with a 13.5 mM Ca(NO3)2 blank were tested. Each treatment combination was replicated two times. After equilibration, the soil suspensions were centrifuged for 20 minutes at 8,000 rpm to separate the solution and solid phases. The solution phase was passed through a 0.45
All experiments were repeated. Between-group differences were determined by one-way analysis of variance (ANOVA), followed by Student-Newman-Keuls test using a probability level of
Organic acids in rhizosphere are difficult to collect because the volume produced is limited and the components of the root exudates are readily biodegradable. To evaluate the OAs’ ability in the rhizosphere to solubilize metals in the soils, a large amount of root exudates must be collected. Because of the difficulty in collecting and preserving root exudates, it is imperative that an OA-based synthetic root exudate (OA mixture) be formulated.
The OA mixture should contain the primary chemical components responsible for metal complex formation and should be in the concentration and pH ranges commonly observed in the rhizosphere. In addition, the OA mixtures should also be prepared under the same background chemical matrix of the soil solution. In this manner, the OA mixture would exhibit comparable ability of reacting with metals as the actual root exudates.
To simulate OAs in root exudates, an OA mixture of similar composition to the root exudate composition was formulated. A series of experiments were conducted to test the effect of OA compositions, the total concentration of metals, and the concentration of OAs in the dissolution of metals in the BSL-treated soils. The amount of metals extracted by the OA mixtures, representing compositions of OAs recovered from the rhizosphere of corn grown on BSL-treated medium and the standard (STD) sand medium [
Amounts of Cd extracted by organic acid (OA) mixtures of various compositions and concentrations
OA concentration | OA composition |
Cd concentration (mg kg−1) | |
---|---|---|---|
Biosolid | Standard | ||
0.1 M | 2nd | 2.11 |
1.86 |
4th | 2.36 |
2.58 | |
8th | 1.98 |
1.63 | |
12th | 1.79 |
1.60 | |
16th | 1.71 |
1.62 | |
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0.01 M | 2nd | 0.39 |
0.31 |
4th | 0.41 |
0.43 | |
8th | 0.44 |
0.35 | |
12th | 0.47 |
0.37 | |
16th | 0.51 |
0.43 | |
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0.001 M | 2nd | 0.29 |
0.20 |
4th | 0.32 |
0.24 | |
8th | 0.36 |
0.32 | |
12th | 0.29 |
0.23 | |
16th | 0.30 |
0.29 | |
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13.5 mM Ca(NO3)2 | Not applicable | 0.10 |
Although the OA compositions of the root exudates recovered from plants were grown for various lengths of time from different plant species, they did not vary significantly under our system [
Mole fraction of organic acids collected in root exudates of corn.
Organic acid | Molecular weight | Mole fraction ratio |
---|---|---|
Acetic | 60.05 | 0.287 |
Butyric | 88.11 | 0.209 |
Glutaric | 132.12 | 0.004 |
Lactic | 90.08 | 0.366 |
Maleic | 116.07 | 0.042 |
Oxalic | 90.04 | 0.043 |
Propionic | 74.08 | 0.010 |
Pyruvic | 88.06 | 0.0004 |
Succinic | 118.09 | 0.006 |
Tartaric | 150.09 | 0.032 |
Valeric | 102.13 | 0.001 |
Figure
Estimated solution concentrations (16-week average) of organic acid mixtures in root exudates of corn
Treatment | Estimated concentration (mM) | |||
---|---|---|---|---|
Standard (control) | Biosolid-treated | |||
Mean | SD | Mean | SD | |
Blank | 2.05 | 0.63 | 3.41 | 0.87 |
Planted | 5.23 | 1.21 | 12.9 | 2.04 |
Percentages of metal extracted (means ± SD where
Quantitative measurements of root exudates from plant roots indicate that OA concentrations as high as 50 mM were found within 1 mm from the root surface [
The rhizosphere typically extends 1–5 mm outward from the interface of the root and soil but the pH measurement is complicated. Instead of directly measuring the pH of the rhizosphere, it was deduced and estimated from data found in literature. Studies of the rhizosphere changes in pH along roots of crops and pH changes at different distances from roots are listed in Table
Reported pH ranges of rhizosphere.
Plant | Genotype | pH range | Reference |
---|---|---|---|
Barley | Bowman |
6.2–7.6 |
Gollany and Schumacher [ |
Dorirumugi | 4.8–7.1 | Youssef and Chino [ | |
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Corn | Pioneer-3737 |
5.2–7.6 |
Gollany and Schumacher [ |
— | 4.8–6.7 | Fischer et al. [ | |
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Clover | Trikkala | 6.2–7.1 | Hinsinger and Gilkes [ |
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Daisy fleabane | — | 5.1–6.3 | Zhang and Pang [ |
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Nectarine tree | Maxim | 5.3–8.2 | Tagliavini et al. [ |
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Oat | Hytest |
6.0–7.6 |
Gollany and Schumacher [ |
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Rape | — | 5.7–6.4 | Ruiz and Arvieu [ |
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Rye | Standard | 5.6–7.1 | Hinsinger and Gilkes [ |
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Sordan | S-757 |
6.0–7.6 |
Gollany and Schumacher [ |
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Sorghum | SC-33-8-9EY |
6.3–7.6 |
Gollany and Schumacher [ |
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Soybean | Hawkeye | 5.5–7.1 |
Römheld and Marschner [ |
PI-54169 | 5.8–7.6 | Gollany and Schumacher [ | |
Toyosuzu | 5.1–7.0 | Youssef and Chino [ | |
— | 4.7–7.1 | Riley and Barber [ |
Zhang and Pang [
It is apparent that pH at or near the root of living plants was altered by root exudation. It varied from pH = 4-5 at the soil-root interface and gradually varied to the level comparable to the pH of bulk soils over approximately 5 mm of distance. The pH = 4.8 was chosen as the pH for the synthetic root exudates based on the results of effects of pH on the dissolution of metals in the BSL-treated soils.
The pH of the rhizosphere may vary from 4.0 to 8.0 (Table
The amounts of metals extracted tended to decrease with the increase of the pH and, at the same pH level, the OA mixture typically extracted more metals than the 13.5 mM Ca(NO3)2 electrolyte solution (Figure
Effects of pH on the extraction of metals (four replicates) in biosolid-treated Romona soil (135 Mg ha−1) by 13.5 mM Ca(NO3)2 electrolyte solution and 0.01 M organic acid mixtures. The differences of metal concentrations among the pH values were tested by one-way ANOVA. Values followed by the same uppercase letter were not significantly different at
In general, the holding capacity of soils for metals increases with increasing pH. Exceptions are Cr and Mo, which are commonly more mobile under alkaline conditions. Accordingly, a decrease in plant uptake of Cu, Mn, and Zn was observed when soil pH was increased [
Metals in the soils were not readily extractable by the neutral electrolyte solution blank that contained 13.5 mM Ca(NO3)2 and, in general, less than 0.25% of any metals were solubilized (Figure
For Pb, the concentrations in the extracts of blank were below limits of quantification of AAS (<0.001 mg kg−1). For the control soil, the total metal concentrations and percent of metals extracted were considerably lower than those in BSL-treated soils.
In BSL-treated soils, the amount of metals extracted was proportional to the amount present in soils and the percentage of the total metals extracted from BSL-treated soils increased with the concentration of OA mixtures; however, the percentage of extraction did not appear to change significantly with the BSL loading when soils were extracted by the 0.1 M OA mixture. In general, Cd, Cu, Ni, and Zn were more readily extractable by the OA mixtures (Figure
The batch equilibrium method was used to study the kinetics of metal dissolution in BSL-treated soils. The experimental procedures were similar to those previously described in the metal solubility study, with the exception that samples were equilibrated for time periods ranging from 15 minutes to 48 hours. When the BSL-treated soils (1,080 Mg ha−1) were equilibrated with 0.1 M OA mixture, the amount of BSL-borne metals solubilized in the OA mixtures increased from 1.67 to 2.48, 1.46 to 5.04, 20.3 to 33.4, 16.1 to 24.1, 0.77 to 2.33, and 205 to 282 mg kg−1 for Cd, Cr, Cu, Ni, Pb, and Zn, respectively, when equilibration time increased from 15 minutes to 48 hours (Tables
Metals release kinetics of biosolid-treated Romona soil (1,080 Mg ha−1) in different concentrations of organic acid (OA) mixtures.
Element | OA concentration | Metal concentration (mg kg−1) | |||||||
---|---|---|---|---|---|---|---|---|---|
15 min | 30 min | 1 hr | 2 hrs | 4 hrs | 8 hrs | 24 hrs | 48 hrs | ||
Cd | 0.001 M | 0.31 | 0.32 | 0.33 | 0.35 | 0.37 | 0.40 | 0.46 | 0.47 |
0.01 M | 1.16 | 1.21 | 1.24 | 1.28 | 1.35 | 1.49 | 1.73 | 1.75 | |
0.1 M | 1.67 | 1.70 | 1.75 | 1.82 | 1.90 | 2.03 | 2.40 | 2.48 | |
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Cr | 0.001 M | 0.34 | 0.40 | 0.44 | 0.49 | 0.57 | 0.73 | 1.09 | 1.12 |
0.01 M | 0.85 | 0.95 | 1.03 | 1.16 | 1.39 | 1.67 | 2.49 | 2.57 | |
0.1 M | 1.46 | 1.78 | 1.91 | 2.17 | 2.53 | 3.17 | 4.79 | 5.04 | |
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Cu | 0.001 M | 2.41 | 2.48 | 2.63 | 2.78 | 2.88 | 3.14 | 3.81 | 3.89 |
0.01 M | 10.7 | 11.1 | 11.7 | 12.1 | 12.8 | 13.8 | 16.8 | 17.5 | |
0.1 M | 20.3 | 21.2 | 22.5 | 23.7 | 25.2 | 26.4 | 32.1 | 33.4 | |
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Ni | 0.001 M | 2.53 | 2.64 | 2.83 | 2.94 | 3.13 | 3.28 | 3.72 | 3.79 |
0.01 M | 9.80 | 10.2 | 10.9 | 11.4 | 12.1 | 12.5 | 14.5 | 14.8 | |
0.1 M | 16.1 | 16.8 | 18.0 | 18.8 | 19.5 | 20.8 | 23.3 | 24.1 | |
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Pb | 0.001 M | 0.13 | 0.14 | 0.15 | 0.18 | 0.22 | 0.29 | 0.42 | 0.46 |
0.01 M | 0.34 | 0.36 | 0.38 | 0.45 | 0.54 | 0.67 | 0.99 | 1.03 | |
0.1 M | 0.77 | 0.82 | 0.87 | 1.02 | 1.09 | 1.89 | 2.01 | 2.33 | |
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Zn | 0.001 M | 25 | 26 | 28 | 29 | 31 | 32 | 36 | 38 |
0.01 M | 104 | 108 | 111 | 115 | 123 | 129 | 145 | 148 | |
0.1 M | 205 | 211 | 220 | 227 | 235 | 245 | 275 | 282 |
Metals release kinetics of biosolid-treated Romona soil (135 Mg ha−1) in different concentrations of organic acid (OA) mixtures.
Element | OA concentration | Metal concentration (mg kg−1) | |||||||
---|---|---|---|---|---|---|---|---|---|
15 min | 30 min | 1 hr | 2 hrs | 4 hrs | 8 hrs | 24 hrs | 48 hrs | ||
Cd | 0.001 M | 0.055 | 0.059 | 0.063 | 0.066 | 0.068 | 0.073 | 0.089 | 0.091 |
0.01 M | 0.18 | 0.19 | 0.20 | 0.21 | 0.23 | 0.25 | 0.29 | 0.30 | |
0.1 M | 0.79 | 0.83 | 0.88 | 0.92 | 1.01 | 1.10 | 1.26 | 1.29 | |
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Cr | 0.001 M | 0.07 | 0.08 | 0.09 | 0.10 | 0.13 | 0.16 | 0.22 | 0.23 |
0.01 M | 0.23 | 0.27 | 0.30 | 0.34 | 0.38 | 0.46 | 0.69 | 0.71 | |
0.1 M | 0.91 | 1.07 | 1.13 | 1.23 | 1.47 | 1.80 | 2.63 | 2.68 | |
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Cu | 0.001 M | 0.86 | 0.88 | 0.92 | 0.97 | 1.05 | 1.15 | 1.38 | 1.42 |
0.01 M | 2.21 | 2.35 | 2.49 | 2.63 | 2.80 | 2.93 | 3.59 | 3.66 | |
0.1 M | 7.6 | 7.8 | 8.3 | 8.9 | 9.4 | 10.1 | 12.6 | 12.7 | |
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Ni | 0.001 M | 0.53 | 0.55 | 0.59 | 0.62 | 0.66 | 0.68 | 0.79 | 0.81 |
0.01 M | 1.57 | 1.67 | 1.80 | 1.93 | 2.01 | 2.13 | 2.52 | 2.57 | |
0.1 M | 6.8 | 7.3 | 7.8 | 8.4 | 8.9 | 9.5 | 11.0 | 11.3 | |
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Pb | 0.001 M | 0.021 | 0.023 | 0.025 | 0.030 | 0.035 | 0.044 | 0.063 | 0.065 |
0.01 M | 0.06 | 0.07 | 0.08 | 0.09 | 0.11 | 0.13 | 0.19 | 0.20 | |
0.1 M | 0.32 | 0.34 | 0.38 | 0.45 | 0.53 | 0.62 | 0.90 | 0.94 | |
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Zn | 0.001 M | 3.96 | 4.19 | 4.35 | 4.48 | 4.69 | 4.91 | 5.60 | 5.66 |
0.01 M | 21.4 | 22.6 | 23.3 | 24.8 | 25.7 | 27.1 | 30.7 | 31.0 | |
0.1 M | 100 | 103 | 108 | 114 | 117 | 121 | 134 | 137 |
Metals release kinetics of control Romona soil in different concentrations of organic acid (OA) mixtures.
Element | OA concentration | Metal concentration (mg kg−1) | |||||||
---|---|---|---|---|---|---|---|---|---|
15 min | 30 min | 1 hr | 2 hrs | 4 hrs | 8 hrs | 24 hrs | 48 hrs | ||
Cd | 0.001 M | 0.0013 | 0.0014 | 0.0015 | 0.0015 | 0.0016 | 0.0017 | 0.002 | 0.002 |
0.01 M | 0.0084 | 0.0088 | 0.0091 | 0.0097 | 0.0102 | 0.0115 | 0.0137 | 0.014 | |
0.1 M | 0.020 | 0.021 | 0.022 | 0.023 | 0.025 | 0.027 | 0.031 | 0.032 | |
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Cr | 0.001 M | 0.010 | 0.012 | 0.013 | 0.016 | 0.019 | 0.023 | 0.034 | 0.035 |
0.01 M | 0.015 | 0.018 | 0.020 | 0.023 | 0.028 | 0.035 | 0.052 | 0.053 | |
0.1 M | 0.11 | 0.13 | 0.14 | 0.16 | 0.20 | 0.23 | 0.35 | 0.36 | |
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Cu | 0.001 M | 0.1071 | 0.1139 | 0.1207 | 0.1275 | 0.1343 | 0.1433 | 0.170 | 0.177 |
0.01 M | 0.35 | 0.36 | 0.39 | 0.41 | 0.44 | 0.47 | 0.56 | 0.58 | |
0.1 M | 1.97 | 2.03 | 2.12 | 2.21 | 2.34 | 2.51 | 3.11 | 3.17 | |
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Ni | 0.001 M | 0.0603 | 0.063 | 0.0675 | 0.0702 | 0.0737 | 0.0773 | 0.0891 | 0.090 |
0.01 M | 0.39 | 0.42 | 0.47 | 0.48 | 0.51 | 0.53 | 0.63 | 0.64 | |
0.1 M | 1.07 | 1.12 | 1.18 | 1.24 | 1.33 | 1.42 | 1.64 | 1.67 | |
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Pb | 0.001 M | n.d. |
n.d. |
n.d. |
n.d. |
n.d. |
n.d. |
n.d. |
n.d. |
0.01 M | n.d. |
n.d. |
n.d. |
n.d. |
n.d. |
n.d. |
n.d. |
n.d. | |
0.1 M | 0.014 | 0.016 | 0.017 | 0.019 | 0.024 | 0.030 | 0.042 | 0.044 | |
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Zn | 0.001 M | 0.30 | 0.32 | 0.34 | 0.35 | 0.37 | 0.38 | 0.44 | 0.45 |
0.01 M | 1.95 | 2.06 | 2.17 | 2.26 | 2.34 | 2.44 | 2.77 | 2.79 | |
0.1 M | 5.3 | 5.6 | 6.0 | 6.2 | 6.5 | 6.8 | 7.8 | 7.9 |
The percentages of total BSL-borne metals extracted were 13.8, 1.63, 7.95, 14.1, 1.03, and 24.2% for Cd, Cr, Cu, Ni, Pb, and Zn, respectively. If OAs are responsible for converting metals in solid phases into plant available forms, the amount and rate of the metals’ solubilization would be indicative of the metals’ availability to plants.
The metal dissolution kinetics data was plotted as a fraction of the total dissolved metals (
Metals of control and biosolid-treated soils solubilized by organic acid (OA) mixtures in the first 15 minutes of equilibration.
Biosolid treatment |
Concentration of OA mixtures | Metals dissolved in 15 minutes | |||||
---|---|---|---|---|---|---|---|
(% of total dissolved) | |||||||
Cd | Cr | Cu | Ni | Pb | Zn | ||
Control | 0.001 M | 63 | 29 | 61 | 67 | n.d. |
67 |
0.01 M | 60 | 28 | 60 | 61 | n.d. |
70 | |
0.1 M | 63 | 31 | 62 | 64 | 32 | 67 | |
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135 Mg ha−1 | 0.001 M | 60 | 30 | 61 | 66 | 32 | 70 |
0.01 M | 60 | 32 | 60 | 61 | 30 | 69 | |
0.1 M | 62 | 34 | 60 | 60 | 34 | 73 | |
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1,080 Mg ha−1 | 0.001 M | 66 | 30 | 62 | 67 | 28 | 66 |
0.01 M | 66 | 33 | 61 | 66 | 33 | 70 | |
0.1 M | 67 | 29 | 61 | 67 | 33 | 73 |
Time-dependent Cd, Cr, Cu, Ni, Pb, and Zn dissolutions of the biosolid-treated Romona soil (1,080 Mg ha−1) in 0.1 M organic acid mixtures (
A variety of chemical reactions occur in soils and reactions often take place simultaneously. Reaction time may vary from millisecond scale for ion exchange reactions to days (or months or years) for sorption/desorption reactions to reach equilibrium [
Kinetics constant for metal dissolution reaction extracted by organic acid (OA) mixtures according to (
Biosolid treatment |
Metal extracted by OAs | Zero- and first-order kinetics constant | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Cd | Cr | Cu | Ni | Pb | Zn | ||||||||
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| ||
Control | 0.001 M | 0.01 | 0.11 |
0.04 | 0.09 | 0.43 | 0.09 | 0.24 | 0.12 | n.d. |
n.d. |
1.2 | 0.12 |
0.01 M | 0.03 | 0.10 |
0.06 | 0.09 | 1.40 | 0.09 | 1.56 | 0.14 | n.d. |
n.d. |
7.8 | 0.13 | |
0.1 M | 0.08 | 0.12 |
0.44 | 0.08 | 7.88 | 0.07 | 4.28 | 0.12 | 0.06 | 0.09 | 21.2 | 0.13 | |
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135 | 0.001 M | 0.22 | 0.09 |
0.28 | 0.09 | 3.44 | 0.09 | 2.12 | 0.12 | 0.84 | 0.09 | 15.8 | 0.11 |
0.01 M | 0.72 | 0.12 |
0.92 | 0.08 | 8.84 | 0.09 | 6.28 | 0.12 | 0.24 | 0.09 | 85.6 | 0.13 | |
0.1 M | 3.16 | 0.13 |
3.64 | 0.09 | 30.4 | 0.08 | 26.8 | 0.14 | 1.28 | 0.08 | 400 | 0.13 | |
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1,080 | 0.001 M | 1.24 | 0.10 |
1.36 | 0.09 | 9.64 | 0.08 | 10.1 | 0.14 | 0.52 | 0.08 | 100 | 0.12 |
0.01 M | 4.64 | 0.10 |
3.40 | 0.08 | 42.8 | 0.07 | 39.2 | 0.12 | 1.36 | 0.08 | 416 | 0.12 | |
0.1 M | 6.68 | 0.07 |
5.84 | 0.08 | 81.2 | 0.08 | 64.4 | 0.12 | 3.08 | 0.08 | 820 | 0.10 |
For the zero-order rapid metal release, the metal dissolution of the soil increased with the concentration of OA mixtures and the amount of metals dissolved by the OA mixture of a given concentration increased with the amount of BSL-borne metals in the soils. When the concentrations of OA mixtures varied from 0.001 to 0.1 M,
The total metals that were extractable by the OA mixtures were calculated as the sum of metal rapidly released by the zero-order reaction (
Amounts of metals that were extractable from the biosolid-treated Romona soil (1,080 Mg ha−1) by 0.1 M organic acid mixture.
Element | Rapid release (mg kg−1) | Slow release (mg kg−1) | Total release (mg kg−1) |
---|---|---|---|
Cd | 1.67 | 0.86 | 2.53 |
Cr | 1.46 | 3.74 | 5.20 |
Cu | 20.3 | 13.7 | 34.0 |
Ni | 16.1 | 8.60 | 24.7 |
Pb | 0.77 | 1.73 | 2.50 |
Zn | 205 | 80.0 | 285 |
Under this metal release model, plants grown on BSL-treated soils would be expected to absorb metals from the rapid release pool first. As the metal release followed a zero-order reaction, one would expect the plant uptake, and therefore tissue concentration, of metals to remain essentially the same until metals in this pool are exhausted, at which time the plant uptake of metals, and therefore the tissue concentration, is expected to decrease as metals in the slow release pool would be available to plants at a much slower rate.
The data presented in Tables
The slope† of metal extracted from the biosolid-treated Romona soil (1,080 Mg ha−1) by 0.1 M organic acid mixtures according to (
Element |
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Cd | 6.68 | 3.40 | 1.75 | 0.91 | 0.48 | 0.25 | 0.10 | 0.05 |
Cr | 5.84 | 3.56 | 1.91 | 1.09 | 0.63 | 0.40 | 0.20 | 0.11 |
Cu | 81.2 | 42.4 | 22.5 | 11.9 | 6.30 | 3.30 | 1.34 | 0.70 |
Ni | 64.4 | 33.6 | 18.0 | 9.40 | 4.88 | 2.60 | 0.97 | 0.50 |
Pb | 3.08 | 1.64 | 0.87 | 0.51 | 0.27 | 0.24 | 0.08 | 0.05 |
Zn | 820 | 422 | 220 | 113 | 58.8 | 30.6 | 11.5 | 5.88 |
First-order kinetics constant for metal dissolution reaction extracted by organic acid (OA) mixtures according to (
Biosolid treatment |
Metal extracted by OAs | First-order kinetics constant ( |
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---|---|---|---|---|---|---|---|
Cd | Cr | Cu | Ni | Pb | Zn | ||
Control | 0.001 M | 4.71 | 0.34 | 4.11 | 4.87 | n.d. |
5.00 |
0.01 M | 3.99 | 0.30 | 3.90 | 3.93 | n.d. |
5.42 | |
0.1 M | 4.30 | 0.37 | 4.49 | 4.46 | 0.36 | 4.92 | |
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135 Mg ha−1 | 0.001 M | 4.13 | 0.38 | 4.05 | 4.65 | 0.38 | 5.63 |
0.01 M | 3.88 | 0.51 | 4.03 | 3.95 | 0.40 | 5.33 | |
0.1 M | 3.96 | 0.48 | 3.81 | 3.79 | 0.50 | 6.06 | |
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1,080 Mg ha−1 | 0.001 M | 4.93 | 0.32 | 4.30 | 4.81 | 0.24 | 4.85 |
0.01 M | 5.16 | 0.40 | 4.41 | 4.81 | 0.32 | 5.75 | |
0.1 M | 6.25 | 0.37 | 4.88 | 5.53 | 0.43 | 6.80 |
Metals present in BSL-treated soils are more extractable by an OA mixture than indigenous metals of the soil. In BSL-treated soil, more than 90% of metals extracted may be attributed to BSL-borne metals. In general, the amount of metals extracted decreased with the increase of the pH, and at the same (4.8) pH level, the OA mixture extracted more metals than the 13.5 mM Ca(NO3)2 electrolyte solution. In general, Cd, Cu, Ni, and Zn were more readily extractable by the OA mixtures and readily absorbed by plants grown on BSL-treated soils than Cr and Pb. The amount of metals extracted was a function of concentration of OA mixtures. Higher concentrations of OA mixture resulted in greater extraction of metals from the BSL-treated soils. The percentages of total BSL-borne metals extracted were 13.8, 1.63, 7.95, 14.1, 1.03, and 24.2% for Cd, Cr, Cu, Ni, Pb, and Zn, respectively. If OAs were responsible for converting metals in solid phases into plant available forms, the amount and rate of metals’ solubilization would be indicative of metals’ availability to plants. A rapid dissolution of metals occurred in the first 15 minutes of mixture. For Cd, Cu, Ni, and Zn, approximately 60–70% of the metals were released. For Cr and Pb, the initial releases were approximately 30% of the total. The data of the metal dissolution kinetics in BSL-treated soils may fit either the two-site bicontinuum model in which significant amounts of the soluble metals were dissolved rapidly, following a zero-order dissolution kinetics and the remaining soluble metals released slowly over a long period of time, following a first-order dissolution kinetics, or first-order dissolution kinetics alone.
The authors declare that they have no competing interests.
The authors gratefully acknowledge Mr. D. Thomason, Mr. W. Smith, and Ms. N. J. Krage for technical assistance. This research was supported by the Water Environmental Research Foundation (WERF-97-REM-5) and California Baptist University’s Microgrant.