In the Laboratory
Competitive Sorption between Oxalate and Phosphate in Soil: An Environmental Chemistry Laboratory Using Ion Chromatography W Kang Xia*† and Gary Pierzynski Department of Agronomy, Kansas State University, Manhattan, KS 66506; *
[email protected] Soil chemistry, a key component of environmental chemistry, has been largely ignored in undergraduate chemistry curricula over the years. In recent years, several experiments on the analysis of organic and inorganic contaminants in soils, designed for environmental chemistry laboratory courses, have been published in this Journal (1–9). However, lab exercises that illustrate important chemical reactions in soils are lacking. Soil is essential in supporting life on earth. The sustainable management of this important natural resource requires an understanding and appreciation of chemical characteristics of soils and their effect on the bioavailability and fate of soil nutrients as well as contaminants. Experiments that include important soil-chemistry topics are valuable in fulfilling the increasing need of connecting abstract scientific concepts to real-world issues in chemistry curricula (10). Phosphorus is an essential nutrient for plants (11). Sorption of phosphate in soils, especially in highly weathered tropical soils, is one of the major reasons for phosphorus deficiency in plant production (12). Phosphate sorption is primarily attributed to ligand-exchange reactions between hydroxyl, –OH, or –OH2 groups exposed on the surfaces of soil minerals, and phosphate in soil solution (Figure 1).1 The degree of phosphate sorption in soil is significantly affected by many soil properties, such as mineralogy, pH, and the presence of simple organic acids. The dominant minerals in soils of temperate regions are layer silicate minerals. In contrast, oxide minerals, such as aluminum and iron oxides and hydroxides, are the major mineral components in tropical soils (13). These possess more surface hydroxyls (13) and, therefore, phosphate sorption is favored in tropical soils compared to soils in temperate regions. Soil solution pH has a profound effect on surface hydroxyls (Figure 2). Low pH causes surface hydroxyls to accept H+ to form –OH2 groups. Since the –OH2 ligand is easier to displace from surface bonding sites than hydroxyl, lowering soil solution pH increases phosphate sorption by facilitating ligand-exchange reactions (13). The excess hydroxyls in high pH soil solutions compete with surface hydroxyls for the H+, creating negative charges on the mineral surfaces, thereby reducing sorption of negatively charged phosphates due to electrostatic repulsion. In addition, solution pH also has a significant influence on species distribution of phosphate (Figure 3A). High pH increases the negative charges on phosphate, which contribute to the electrostatic repulsion. Solution pH affects the speciation of other weak acids such as oxalate in a similar manner (Figure 3B).
Figure 3 shows the effect of pH on species distribution of phosphate and oxalate. At pH 4.5, H2PO4᎑ is the predominate phosphate species. With increasing pH, the relative abundance of H2PO4᎑ decreases, while the relative abundance of HPO42᎑ increases and becomes the most dominant around pH 9.5. The pH values of most of the surface soils are in the range of 4.0–9.0. At this pH range, the predominate oxalate species is C2O42᎑. The decreased sorption of oxalate on oxide mineral surfaces with increasing solution pH is mainly due to increased negative charges on the mineral surfaces. Research has shown that simple organic acids released by microorganisms, root exudation, and decomposition of organic matter may influence phosphate sorption in soils (14– 16). The competitive effect of oxalate on phosphate sorption is one example. The stability constants for Al-oxalate and FeO HO
P
OH
_
O
O
+ OH2
OH
OH
Fe
Fe
Al
HO
P
OH2
O
OH
Fe
Fe
Al
OH
+
OH
−
2OH
−
Mineral surface Mineral surface
O
HO
_
P O
O _
HO
O P
+
OH2
OH2
OH
OH
Fe
Fe
Al
Fe
O
O
Fe
Al
+
Mineral surface Mineral surface
Figure 1. Two schematic models (top and bottom) for phosphate ligand-exchange reaction on oxide mineral surfaces.1 Low pH
High pH
OH −1/2
OH2 +1/2 M Mineral surface
H
+
+
M
+ OH −
Mineral surface
O -3/2 M
+
H2O
Mineral surface
†
Current address: Department of Crop and Soil Sciences, The University of Georgia, 3111 Miller Plant Sciences Building, Athens, GA 30602.
Figure 2. Schematic model illustrating the influence of solution pH on surface hydroxyls. M represents Fe or Al.
JChemEd.chem.wisc.edu • Vol. 80 No. 1 January 2003 • Journal of Chemical Education
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In the Laboratory
oxalate complexes are 106.1 and 107.6, respectively, and are 6.6 and 4.1 orders of magnitude greater than that for Alphosphate and Fe-phosphate complexes, respectively (17). Due to its higher stability constants for forming complexes with Al or Fe sites on mineral surfaces, oxalate can effectively compete with phosphate for surface binding sites and, therefore, significantly increase solution phosphate concentration by reducing phosphate sorption through ligand-exchange reactions on mineral surfaces (14–16). Other simple organic acids such as citrate, formate, and acetate can have similar effects on phosphate sorption in soils. In phosphorus-deficient soils, certain cereal plants and southern pines can exude large quantities of oxalate in the root rhizosphere (18, 19), a strategy used by some plants to increase phosphate bioavailability in soil. We have developed a laboratory experiment to introduce undergraduate or master-level students majoring in science to sorption and desorption reactions in soils. In this lab exercise, competitive sorption between oxalate and phosphate was evaluated for an aluminum oxide and a temperate soil at two different pH levels using a batch equilibrium technique. In addition, students were also exposed to the basic theory of ion chromatography (IC) and given hands-on experience in using IC for simultaneous detection of multiple anions in water samples.
A
Relative Abundance (%)
100
H 3PO4
H 2PO4
−
HPO4
2−
PO4
3−
80
60
Competitive Sorption of Oxalate and Phosphate Sediments from the previous phosphate sorption study were mixed with 25 mL of 0, 0.1, 0.3, 1.0, and 3.0 mM oxalate solution at the desired pH (pH 4.6 or 8.0). There were two replicates per treatment. Because only negligible amounts of phosphate remained in the sediment water, rinsing the sediment before oxalate addition was not necessary. The mixtures were shaken for 1 h and then centrifuged following the previously described procedures. Concentrations of phosphate and oxalate in the supernatants were analyzed by IC. The amount of oxalate sorbed was calculated based on the difference between the amount of oxalate initially added and that in the solution after the 1-h reaction period. The amount of phosphate desorbed was calculated based on phosphate concentration detected in the supernatant. Ion Chromatography Analysis
20
0
2
4
6
8
10
12
14
pH B H 2C 2O4 100
Relative Abundance (%)
The students in this lab were divided into four groups. Each group worked with a different treatment combination of pH level (pH 4.6 or 8.0) with either aluminum oxide or temperate soil, which was collected from Manhattan, KS. Ten samples were prepared by each group using the following procedures. The students mixed 25mL of 15.81 mM KH2PO4 solution at the desired pH with 0.3 g of aluminum oxide or soil in a 50-mL plastic centrifuge tube. Samples were shaken on an end-to-end shaker at low speed for 1 h. After shaking, the samples were centrifuged at 1000 rpm for 10 min. Supernatants were carefully poured out and analyzed for phosphate by IC. The remaining sediments were used for the competitive sorption study described in the following section. The amount of phosphate sorbed was calculated based on the difference between the amount of phosphate initially added and that in the solution after the 1-h reaction period.
40
0
HC 2O4
−
C 2O 4
2−
80
The IC eluent solutions and three working standard solutions were prepared by our lab assistant beforehand. Approximately 5 mL supernatant was filtered through a 0.2-µm Whatman IC filter into a 5-mL IC autosampler vial. Each vial was then fitted with a 0.45-m filter cap (Dionex) and placed in the autosampler. Procedures for starting up and for conditioning the IC system were demonstrated to the students. Each group of students created their own method, following the steps and parameters listed in the handout, before using the IC. Schedule Arrangement
60
40
20
0 0
2
4
6
8
10
12
14
pH Figure 3. Influence of solution pH on species distribution of (A) phosphate and (B) oxalate.
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Sorption of Phosphate
This is a 4-h laboratory experiment consisting of a 1-h lecture and a 3-h demonstration and lab exercise. Students were first asked to set up the phosphate sorption experiment at the beginning of the laboratory period. The 1-h lecture was then given while the samples for the phosphate sorption step were on the shaker. The 1-h lecture included discussion of the concepts of sorption and desorption reactions in soil, the chemistry of phosphate and oxalate, basic theory and operation of IC, and IC data interpretation. After the 1-h lecture, the oxalate sorption experiments were set up by the
Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu
In the Laboratory
students and the demonstration of IC operation and method development were performed during the second hour-long shaking period. Data collection time for each sample by IC was approximately 20 min. After all the samples were run, chromatograms for the standards and two samples were uploaded onto the Web site for this course. Each student was required to calculate the amounts of sorbed and desorbed phosphate and sorbed oxalate for the two samples. Data for other samples were calculated by the instructor and uploaded onto the Web. The students were asked to compile all the data and prepare a lab report including data interpretation and discussion. Hazards Proper attention and caution should be exercised when handling the centrifuge. Both the 1 M HCl and 1 M NaOH are corrosive. Gloves, safety glasses, and a lab coat are needed during the lab exercise. Results and Discussion The peak identification in each sample (Figure 4A) was performed based on the ion chromatogram for a standard mixture of oxalate and phosphate (Figure 4B). Concentrations of oxalate and phosphate in each supernatant solution
were calculated using their peak areas and the linear correlation between peak areas and the concentrations of the target compound in the standard solutions (Figure 5). Total phosphorus contents and water soluble phosphate levels in soils normally range from 50 to 1500 mg kg᎑1 and less than 0.01 to 1 mg L᎑1, respectively (20). The competition between oxalate and phosphate on mineral surfaces exists regardless of whether the ions are at high or low concentrations. Due to IC detection limits for phosphate and oxalate, concentrations higher than what is found in a normal soil were used in this lab exercise in order to better demonstrate the concentration changes due to the competitive sorption between the two anions. Figure 6 clearly illustrates the effect of pH on phosphate sorption. The amount of phosphate sorbed on the aluminum oxide surface at pH 4.6 was 10 times as much as that sorbed at pH 8.0. A similar trend was also found for the soil sample that has layer silicates as the major mineral component. Low pH favors phosphate sorption on mineral surfaces. At the same pH, significantly more phosphate was sorbed on aluminum oxide than on the soil because there are more phosphate binding sites on the aluminum oxide surface. More phosphate was released back into solution when the phosphate-loaded aluminum oxide and soil were exposed to increasing concentrations of oxalate at pH 4.6 (Figure 7). This suggests that oxalate has a stronger affinity for mineral A 0.4
1.2 1.0 0.8 0.6 0.4 0.2 0.0 3
4
5
6
7
8
Retention Time / min
Concentration 兾 (mmol兾L)
Conductivity / µS
A
0.3
phosphate 0.2
0.1
y = 0.0333x + 0.0168 R 2 = .9990
0.0
B
0
2
4
6
8
10
Peak Area
12
1
10
B
8
0.4
6 4 2 0 3
4
5
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7
8
Retention Time / min
Peak #
Component Retention Name Time / min Peak Area
1
Phosphate
4.72
9.45
2
Oxalate
5.96
17.27
Concentration 兾 (mmol兾L)
Conductivity / µS
2
0.3
oxalate 0.2
y = 0.0204x + 0.0126
0.1
R 2 = .9995 0.0 0
Figure 4. Ion chromatograms for (A) a supernatant solution sample (dilution factor, 2) and (B) a standard mixture containing 0.330 and 0.364 mM phosphate and oxalate, respectively. Retention time and peak area for each compound are listed in the table.
5
10
15
20
Peak Area Figure 5. Concentration vs peak area for phosphate and oxalate standards.
JChemEd.chem.wisc.edu • Vol. 80 No. 1 January 2003 • Journal of Chemical Education
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In the Laboratory
Phosphate sorbed 兾 (mmole兾g)
surface binding sites than phosphate. Oxalate sorption also increased as the aluminum oxide and the soil were exposed to increasing concentrations of oxalate (Figure 7). Similar to phosphate, more oxalate was sorbed on the aluminum oxide than on the soil sample. The same trend was observed at pH 8.0 (Figure 8). The effect of pH on oxalate sorption was similar to that on phosphate sorption (Figures 7 and 8). 0.20
0.168
0.15
Conclusion In summary, this lab exercise successfully exposed the students, most of whom were environmental sciences majors, to important chemistry concepts such as sorption and desorption, ligand-exchange reactions, stability constants, and mineral surface chemistry, all of which can be applied to the soil environment. The students found this experiment to be an effective means to gain experience in analytical techniques as well as to see the practical aspects of chemical analysis. Comparing to the colorimetric test on phosphate in water samples conducted several weeks prior to this exercise, the students were intrigued by the remarkable capability of IC to simultaneously detect multiple anions within minutes.
0.10
Acknowledgments 0.065
0.05
0.008
0.016
0.00 oxide pH=4.6
oxide pH=8.0
soil pH=4.6
soil pH=8.0
Figure 6. Phosphate sorption on an aluminum oxide and a Kansas soil.
We wish to acknowledge the assistance of Nishantha Fernando, a graduate student, for lab preparation. We sincerely appreciate feedback from the students in AGRON 615, Soil and Environmental Chemistry. The Department of Agronomy at Kansas State University generously provided laboratory space for this course. Support for this study was provided by the Kansas Agricultural Experiment Station (Contribution Number 02-81-J).
oxide
0.014 0.012 0.010 0.008 0.006
soil 0.004 0.002 0.000 0
1
2
3
4
Oxalate Concentration 兾 (mmole兾L)
Phosphate Desorbed 兾 (mmole兾g)
Phosphate Desorbed 兾 (mmole兾g)
0.016
0.014
oxide 0.012 0.010
soil
0.008 0.006 0.004 0.002 0.000 0
1
2
3
4
Oxalate Concentration 兾 (mmole兾L)
0.014 0.014
Oxalate Sorbed 兾 (mmole兾g)
Oxalate Sorbed 兾 (mmole兾g)
0.016
0.012 0.010 0.008 0.006
oxide 0.004 0.002
soil
0.012
oxide
0.010
soil
0.008 0.006 0.004 0.002
0.000 0
1
2
3
4
0.000 0
Oxalate Concentration 兾 (mmole兾L) Figure 7. Competitive sorption between phosphate (top) and oxalate (bottom) on an aluminum oxide and a Kansas soil at pH 4.6.
74
1
2
3
4
Oxalate Concentration 兾 (mmole兾L) Figure 8. Competitive sorption between phosphate (top) and oxalate (bottom) on an aluminum oxide and a Kansas soil at pH 8.0.
Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu
In the Laboratory W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Note 1. The charges on –OH2 and –OH are not indicated because the charges on –OH2 and –OH functional groups vary depending on which element in a mineral they are attached to. Based on Pauling’s Rules (13), if –OH2 and –OH functional groups are attached to elements (such as Al and Fe) with 6 valence bonds and +3 valence in a mineral, the charge on –OH2 and –OH are +1/2 and ᎑1/2, respectively. If –OH2 and –OH functional groups are attached to elements (such as Si) with 4 valence bonds and +4 valence in a mineral, the charge on –OH2 and –OH are +1 and 0, respectively.
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