Research Article

Removal of Cd and Pb in calcareous soils by using Na2EDTA recycling washing†

Shujuan Zhang1,2, Zhihui Yang1,2,*, Baolin Wu1,2, Yangyang Wang1,2, Ruiping Wu1,2, Yingping Liao1,2

1 School of Metallurgical Science & Engineering, Central South University, Changsha, China

2 Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, Changsha, China

Correspondence: Z. Yang, School of Metallurgical Science & Engineering, Central South University, Changsha,

410083, China

E-mail: yangzh@csu.edu.cn

Keywords: Heavy metal removal, Na2EDTA recycling, soil remediation, soil amelioration

Abbreviations: AAS, atomic absorption spectrophotometer; Me-EDTA, metal complexes with

ethylenediaminetetraacetic acid, DTPA, diethylenetriaminepentaacetic acid

†This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as an ‘Accepted Article’, doi: [10.1002/clen.201200634]. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Received: November 21, 2012 / Revised: April 10, 2013 / Accepted: April 17, 2013

Abstract

Soil-column experiments were conducted for removing of Cd and Pb from calcareous soils by using

ethylenediaminetetraacetic acid disodium (Na2EDTA). Up to 84.4% and 73.5% of diethylenetriaminepentaacetic acid

(DTPA)-extractable Cd and Pb were removed by using 0.04 mol/L Na2EDTA at 1:8 of soil to solution ratio (w/v). More

than 99% of Cd and 97% of Pb in leachate collected from washing procedure were precipitated at pH 10 by using Na2S

+ Ca(OH)2, while only 56.9% of Cd and 97.5% of Pb were removed with FeCl3 + NaOH. The recovered Na2EDTA

from leachate treatment did not lose much of its chelating capacity in the first washing process. However, in the second

and third batch washing process, the removal of DTPA-extractable Cd decreased by 26.2 and 20.2%, while the removal

of DTPA-extractable Pb decreased by 33.0 and 41.1%, respectively. Due to the high content of Na+ in the treated soil,

several chemical amendments were employed to remove Na+. (NH4)2SO4 was the most suitable amendment reagent and

the optimal concentration of (NH4)2SO4 was 0.08 mol/L at a soil-to-solution ratio of 1:3. The total cost for recycled

Na2EDTA washing decreased by 68.9% as compared with fresh Na2EDTA washing.

1 Introduction

Contamination of soils with heavy metals is a common problem throughout the world and various treatment

technologies were rapidly developed in the past decade [1, 2]. One of the most promising treatment methods is to

separate the metals from soil by using chelating agents [3-6]. Of the many prevalent chelating agents,

ethylenediaminetetraacetic acid disodium (Na2EDTA) has been proved to be the most effective due to its strong

chelating ability [7]. But the high cost and necessary removal from the spent fluid have hampered the full-scale

application in soil washing projects. These hindrances could be eliminated if Na2EDTA could be recovered

cost-effectively and reused [8]. Although several recycling methods have been demonstrated on a laboratory-scale�

there are still no practical and commercially available methods for recycling Na2EDTA in wastewater [9-11].

Previous researches have shown that there are at least three possible techniques to recover and regenerate Na2EDTA.

The first method involves substituting of the metal ions in EDTA complexes by zero-valent metals which can result in

precipitation of the metallic contaminants and liberate Na2EDTA [12]. The second method involves electrolytic

recovery of metals and Na2EDTA from Me-EDTA solution in a two-chamber cell separated with a cation exchange

membrane to prevent EDTA anodic oxidation [13]. The third method involves precipitating the liberated metals by

adding suitable reagents to destabilize the metal complexes [14]. A number of studies were performed to set up the

precipitation process of metals from complexes with EDTA. NaOH, Ca(OH)2, FeCl3, Fe(NO3)3, NaH2PO4, Na2HPO4

and Na2S were the most often used reagents. Lim et al. [15] developed a method to recycle Me-EDTA solution.

Chelated Cd, Pb and Ni were initially replaced by Fe3+ at pH 3, and the dissociated Cd, Pb and Ni were then precipitated

with PO43-. Finally, Fe(III)-EDTA complex were dissociated at a high pH and Fe(OH)3 precipitation was formed. Chang

et al. [16] recycled chelant from Cu-EDTA solution by substituting Cu with Fe(0) under acidic condition. Fe2+ ions in

the Fe(�)-EDTA complex were then precipitation as Fe(OH)3 at high pH after NaOH added, thus liberating Na2EDTA.

Zeng et al. [17] separated metals from EDTA with Na2S, resulting in 70.8% ~ 99.8% recovery of Cd, Pb, Zn and Cu

through precipitation in the form of metal sulfides. However, few papers have been reported about reuse of the recycled

Na2EDTA for several times.

The content of Na+ in the treated soil washed by recycled Na2EDTA was inevitably very high. The sodium soil would

have low permeability and poor drainage that can affect the growth and yield of most crops [18, 19]. Unfortunately,

little work has been done to ameliorate such soil. Cultivation of certain salt tolerant forage species on calcareous

sodium soil may be a low cost and environmentally acceptable strategy. However, the time-consuming and difficulty for

soil amelioration with high Na+ content greatly limited its application. Chemical agents followed by washing were

normally applied to remove Na+ from the colloid’s cation exchange sites [20]. Many laboratory studies have identified

the effect of gypsum on physical properties of the sodium soil [21, 22], but high gypsum applications might bring

deleterious impact on the sodium soil [23]. Thus, it is necessary to develop an eco-friendly non-toxic amelioration

technique to improve the permeability of the soil.

This study focuses on investigation of the feasibility of recycling Na2EDTA which used to remove heavy metals from

polluted soil. The objectives were: (1) to optimize the condition for heavy metal removal in soils by using Na2EDTA

washing; (2) to recover Na2EDTA from the leachate and reuse of the recovered Na2EDTA; (3) to select a suitable

amelioration technique to amend the washed soil by recycled Na2EDTA treatment and to optimize the dose of the

ameliorant.

2 Materials and methods

2.1 Soil sample

The moist soil sample of the surface soil (0-10 cm) of a Dystric Cambisol (FAO-UNESCO, 1994) was collected from a

contaminated field located in northwest China (104°35’E, 36°48’N). The Dystric Cambisol is a typical soil in

Northwestern China. The soil was heavily contaminated by Pb and Cd because of long-term irrigation of heavy-metal

containing wastewater.

The soil samples were air-dried at room temperature (20-30°C), ground to pass through a 2-mm sieve, then

homogenized and stored prior to experiment. The general chemical properties of the tested soil were: 1.40% organic

matter, 279.8 mmol kg-1 cation exchange capacity, 15.8 mg kg-1 total Cd, 274.0 mg kg-1 total Pb, 5.38 mg kg-1

diethylenetriaminepentaacetic acid (DTPA)-extractable Cd, 81.8 mg kg-1 DTPA-extractable Pb, 293.0 mg kg-1 water

soluble sodium, 201.9 mg kg-1 exchangeable sodium and pH (H2O) 7.31.

2.2 Soil remediation by using Na2EDTA washing

A batch of column experiments were conducted with different concentrations of Na2EDTA (0.01~0.12 mol/L) at a

soil-to-solution ratio of 1:10 to choose the optimal Na2EDTA concentration. Briefly, 300 g of contaminated soil were

placed in a Plexiglas column (locally fabricated) with 5 cm in diameter and 30 cm in height. A plastic mesh (D = 0.2

mm) was placed at the bottom of the column to retain the soil. Then, 3 L of Na2EDTA solution were passed through a

soil column by using a peristaltic pump. After washing procedure ceased, the moist soil was taken from the column,

air-dried at room temperature and then passed through a 2-mm sieve. Cd and Pb in soils were extracted with DTPA at

1:5 of a soil-to-solution ratio (w/v). The suspension was shaken for 2 h and the concentrations of Cd and Pb in

supernatant were determined.

Another batch of column experiment was conducted to choose the optimal soil to solution ratio. Soil-to-solution ratios

varied from 1:2 to 1:14. The washing process was conducted as it was mentioned above. All experiments were

performed in triplicate to ensure reproducibility of the results.

2.3 Removal of Cd and Pb in leachate

Two chemical methods of Na2S + Ca(OH)2 and FeCl3 + NaOH were chosen to liberate Cd and Pb from EDTA complex.

In order to minimize the volume of leachate, the soil was washed with the optimal concentration of Na2EDTA at

soil-to-solution ratio of 1:2. The leachate was collected to analyze the contents of Cd and Pb and then treated with Na2S

+ Ca(OH)2 and FeCl3 + NaOH, respectively. For Na2S + Ca(OH)2 treatment, the leachate was adjusted to pH values

from 8 to 13 with Ca(OH)2, added with Na2S at varied molar ratios (5:1~20:1) of Na2S to Cd and Pb, and then stirred for

2 h. Thereafter, 2% of polyacrylamide was added at a ratio of 6 mL/L (v/v) to enhance the flocculation. The suspended

liquid was filtered and supernatant was used to analyze the concentrations of Cd and Pb. For FeCl3 + NaOH treatment,

the leachate was adjusted to pH values from 8 to 13 with NaOH. The left procedure was similar to that of Na2S +

Ca(OH)2 treatment.

2.4 Soil remediation by washing with the recycled Na2EDTA

The removal of Cd and Pb in soils by using the recycled Na2EDTA was evaluated and a fresh Na2EDTA solution with

the same concentration was employed as control. In the first batch experiment, 300 g of contaminated soil were washed

with 600 mL of 0.04 mol/L of fresh Na2EDTA solution. The leachate was collected and recovered by Na2S + Ca(OH)2 at

pH 10. The recycled Na2EDTA solution was reused to wash the residual soil for additional three cycles. Prior to use, Cd

and Pb in leachate for each cycle were treated with Na2S + Ca(OH)2 as described above. In successive washing

procedures, the volume of the leachate was compensated to 600 mL with fresh 0.04 M Na2EDTA solution. After

washing procedure ceased, the moist soil was taken from the column, air-dried at room temperature and then passed

through a 2-mm sieve. Cd and Pb in soils were extracted with DTPA at 1:5 of soil-to-solution ratio (w/v). The

suspension was shaken for 2 h and the concentrations of Cd and Pb in supernatant were determined.

2.5 Amelioration of the sodium in soils

The residual soil treated with recycled Na2EDTA contained a quite high concentration of water soluble and

exchangeable sodium. The sodium soils possess poor physical properties and fertility problems that affect growth and

yield of most crops. Thus the amelioration of the Na2EDTA -treated soils is necessary. Six ameliorants were chosen in

the study: (1) gypsum (CaSO4 · 2 H2O), (2) ammonium sulfate ((NH4)2SO4), (3) monocalcium phosphate

(Ca(H2PO4)2 · H2O), (4) diammonium hydrogen phosphate ((NH4)2HPO4), (5) ammonium acetate (NH4AC), (6) H2O.

The concentration of the above six ameliorants was 0.04 mol/L at a soil-to-solution ratio of 1:3. The procedure was

described as follows: 100 g of soil was placed in a Plexiglas column with 4 cm in diameter and 10 cm in height. A

plastic mesh (D = 0.2 mm) was placed at the bottom of the column to retain the soil. Then 300 mL of ameliorant

solution added into the soil column by a peristaltic pump. After the washing procedure ceased, the moist soil was

air-dried and passed through a 2-mm sieve. Water soluble sodium and exchangeable sodium in soil were extracted and

determined with atomic absorption spectrophotometry (AAS) (Purkinje Genera, China).

2.6 Analysis method

The DTPA-extractable Cd and Pb were extracted by 0.005 mol/L DTPA + 0.1 mol/L triethanolamine + 0.01 mol/L

calcium chloride of soil/water 1:5 w/v. The concentrations of Cd and Pb in solution were determined with ASS. Soil pH

was measured by using soil/water 1:5 and a pH meter. The cation exchange capacity of soils was determined by the

ammonium acetate (NH4Ac)/sodium acetate (NaAc) method. Organic matter was analyzed by Walkley-black titration.

The water soluble sodium was extracted at 1:5 of soil-to-water ratio. Exchangeable sodium was extracted with 1 mol/L

ammonium acetate (NH4AC) buffer (pH 7.0) at a 1:5 ratio of soil to solution. Sodium concentration in solution was

determined with AAS.

3 Result and discussion

3.1 Effect of Na2EDTA concentration on the removal of DTPA-extractable Cd and Pb

Figure 1 shows the removal of DTPA-extractable Cd and Pb in soils by washing with different concentrations of

Na2EDTA and different ratios of soil/Na2EDTA solution (w/v). As shown in Fig. 1A, the removal of DTPA-extractable

Cd and Pb increased with increasing the concentration of Na2EDTA. The removal percentages of DTPA-extractable Cd

and Pb increased from 70.4 to 89.7% and from 44.2 to 79.0%, respectively, when Na2EDTA concentration increased

from 0.01 to 0.12 mol/L. However, the large amount removal of DTPA-extractable Pb occurred in the range of

Na2EDTA concentration from 0.01 to 0.04 mol/L since the removal percentages of DTPA-extractable Pb increased

substantially from 44.2 to 71.2%. Thereafter, little removal of DTPA-extractable Pb was observed in the Na2EDTA

concentration range of 0.06 to 0.12 mol/L. This result was not surprising since a much disproportional gain was also

observed by Palma et al. [24]. This phenomenon could be explained by that these fractions including exchangeable,

acid soluble, reducible and a part of the oxidizable fractions were almost completely extracted with low concentration

of Na2EDTA [25]. In this study, it was found that Cd was easier to be removed from soil than Pb when Na2EDTA was

used as washing agents. This result was in agreement with the result of Lafuente et al. [26]. They have reported that Pb

was more difficult to liberate than Cd. This phenomenon could be explained by that Pb has great affinity to the surface

of sorbates than Cd [27, 28].

In order to investigate the effect of ratios of soil to Na2EDTA solution on heavy metal removal, a series of washing tests

were conducted at varying the soil/solution ratios from 1:2 to 1:14 when the concentration of Na2EDTA maintained at

the 0.04 mol/L. As shown in Fig. 1B, the removal of DTPA-extractable Cd and Pb increased obviously over the range of

soil to solution ratios from 1:2 to 1:8 and appeared to plateau at the ratio near 1:8. Therefore, soil to solution ratio of 1:8

would be the best choice.

3.2 Removal of Cd and Pb in leachate

Regeneration of Na2EDTA is essential to reduce the costs in a reasonable level. In the experiment, two chemical

precipitation methods (Na2S + Ca(OH)2 and FeCl3 + NaOH)) were used to remove Cd and Pb in leachate derived from

0.04 mol/L of Na2EDTA washing at soil to solution ratio of 1:2. In the precipitation method of Na2S + Ca(OH)2,

Ca(OH)2 provided Ca2+ ions that competed with metals, replaced the chelated heavy metals and thus encouraged the

release of Cd and Pb from the Na2EDTA complex. While Na2S was used as an anionic precipitant that provided HS-/S2-

anions to compete with Na2EDTA for the precipitation of heavy metals as metal sulfides. However, the precipitation

method of FeCl3+NaOH was mainly based on the conditional constant of EDTA complexes with Fe3+, Pb2+ and Cd2+.

Fe-EDTA has the highest complex constant followed by Pb-EDTA and Cd-EDTA at highly acidic condition. Lim et al.

[15] reported that >90% of EDTA would form Fe-EDTA or Fe-HEDTA at pH 3.

Figure 2 shows the removal percentages of Cd and Pb in leachate with different doses of Na2S (the molar ratios of Na2S

to Cd and Pb were 5, 10 and 20) and varying pH values of 8, 9, 10, 11, 12 and 13. The result showed that Na2S was very

effective to remove Pb and Cd from leachate. When pH increased from 8~13 at 5:1 molar ratio of Na2S to Cd and Pb,

the removal percentages of Cd and Pb increased from 95.0 to 99.2% and 60.5 to 99.6%, respectively. However, a large

amount of Cd and Pb was removed at pH 10. A similar result was found by Hong et al. [29] who achieved almost

complete precipitation of Pb and Cu with Na2S (Na2S/Na2EDTA 1:1) at pH 10. In addition, Cd and Pb removal was

dependent on Na2S amount. For instance, higher Cd and Pb removal percentages (>99%) were found at 20:1 and 10:1

molar ratios of Na2S to Cd and Pb than at 5:1 molar ratio at pH < 10. However, the differences of Cd and Pb removal

among different molar ratios of Na2S to Cd and Pb were not significant at pH > 10. Therefore, 5:1 molar ratio of Na2S to

Cd and Pb achieved a satisfied removal of Cd and Pb in leachate at pH 10. The residual Cd and Pb in the leachate were

0.047 and 0.878 mg/L, respectively, which can meet the Integrated Wastewater Discharge Standard (GB8978-1996)

suggested by the Chinese Ministry of Environment.

The removal of Cd and Pb in leachate using FeCl3 + NaOH were also studied with different pH (8~13). As shown in Fig.

3, only 7.7 and 7.9% of Cd and Pb were removed by using FeCl3 + NaOH over the pH range from 8~10. However, the

removal percentages of Cd and Pb increased significantly with the increase of the pH value. The maximum removal of

Cd (56.9%) and Pb (97.5%) were obtained at pH 13. It was obvious to find that Pb removal percentage at pH 13 was

significant higher than Cd. Although a part of Cd in leachate can be removed by FeCl3, the removal rate was not

satisfactory even at high molar ratio of FeCl3 to heavy metals and high pH value (pH 13) (Fig. 3).

3.3 Reuse of the recovered Na2EDTA solution

The recovered Na2EDTA was used for removal of Cd and Pb in contaminated calcareous soils and the results is shown

in Fig. 4. The column washing was initially performed at concentration of 0.04 mol/L Na2EDTA and soil-to-solution

ratio of 1:2. The Na2EDTA was recovered from leachate by Na2S at pH 10. The recovered Na2EDTA was further used to

wash soil for three cycles. The removal percentages of DTPA-extractable Cd reached 80.1% over the first batch

washing process, which was similar to the removal efficiency of Cd (81.3%) by fresh Na2EDTA. However, the removal

of DTPA-extractable Cd decreased by 26.2 and 20.2% for the 2nd and 3rd batch washing process comparing to the fresh

Na2EDTA (Fig. 4A), and the removal of DTPA-extractable Pb decreased by 9.7, 33.0 and 41.1% in each batch washing

process (Fig. 4B), respectively. Obviously, the removal efficiency of Cd and Pb by the recycled Na2EDTA solution had

a slightly decrease after recycling several times. This could be attributed to Na2EDTA loss. For instance, in the first

washing cycle, 600 mL of 0.04 mol/L fresh Na2EDTA was used for washing 300 g soil and the mass of Na2EDTA was

8.93 g. After washing, 510 mL leachate were collected and the recovered concentration of Na2EDTA was 0.037 mol/L

in leachate. The mass of recycled Na2EDTA was 7.02 g. Thus the mass loss of Na2EDTA was 21.39% because of soil

adsorption. Therefore, when the recovered Na2EDTA was used for remove Cd and Pb in soils, the removal percentages

decreased since the loss of Na2EDTA (Fig. 4). Overall, the recycled Na2EDTA was still effective in heavy metal

extraction at least in the first batch washing process.

3.4 Amelioration of sodium soil

The improvement of the sodium accumulation by using ameliorants was mainly associated with the decrease in Na+

concentration. Calcium salt was the most common ameliorant which could provide a source of Ca2+ to replace excess

Na+ from the cation exchange sites and subsequently remove of Na+ by washing. In addition, ammonium salts can also

lower the soluble and exchangeable Na+ content [30]. The ammonium ion can increase the movement of water through

sodium soil and make the displacement of Na+ through hydrolysis become possible. Fig.5 compares the effects of

various amendments on the removal of water soluble sodium and exchangeable sodium. The concentration of water

soluble sodium and exchangeable sodium were significantly decreased by the amendments and the maximum removal

rate of water soluble sodium and exchangeable sodium reached up to 94.5 and 95.0%, respectively. The overall

efficiency in decreasing sodium of the soil was in the order: (NH4)2HPO4 > (NH4)2SO4 > CaSO4 · 2 H2O > NH4AC >

Ca(H2PO4)2 · H2O > H2O. It was obvious that ammonium salts were more efficient to decrease Na+ concentration than

calcium salt except for CaSO4 · 2 H2O. Considering the high price of (NH4)2HPO4, (NH4)2SO4 was more suitable for

amelioration of the sodium soil.

The effect of (NH4)2SO4 concentration on Na+ removal was evaluated (Fig. 6A). The result showed that the removal of

Na+ increased with increasing concentration of (NH4)2SO4 when (NH4)2SO4 concentration was <0.08 mol/L. The

removal efficiency of water soluble sodium was exceeded by exchangeable sodium. This phenomenon was mainly due

to the replacement of exchangeable Na+ by NH4+ from exchange sites and then changed into water soluble Na+ [31].

The percentages of Na+ removal did not vary significantly when (NH4)2SO4 concentration increased from 0.08 to 0.1

mol/L. Therefore, the optimal (NH4)2SO4 concentration was 0.08 mol/L.

The effect of solution to soil ratio was also evaluated at 0.08 mol/L of (NH4)2SO4 concentration. As shown in Fig. 6B,

increasing the solution to soil ratio was favorable for the removal of water soluble and exchangeable Na+. At 3:1 of

solution to soil ratio, there was a relative large removal amounts of water soluble sodium and exchangeable sodium.

The removal of water soluble sodium and exchangeable sodium were 92.3 and 97.1%, respectively.

3.5 The cost-benefit analysis

Na2EDTA is a costly chemical agent for removing heavy metals in soils. In our experiments, the cost for fresh

Na2EDTA washing amounted to $250.8/t soil. However, if Na2EDTA can be recycled, it can substantially decrease the

expenditure. The cost of recycled process included the added cost of Na2S, pH maintenance, and NH4+ salt amelioration.

When Na2EDTA was recycled and the soil was washed one cycle by fresh Na2EDTA solution and three cycles by the

recovered Na2EDTA solution, the amount of Na2EDTA consumption decreased by 71% (Table 1). The total expenditure

including Na2EDTA, Na2S, Ca(OH)2 and (NH4)2SO4 was $77.91/t soil. The cost for soil washing with recycled

Na2EDTA decreased by 68.9% compared with treatment without Na2EDTA recycling.

4 Conclusions

Na2EDTA can be used in the removal of Cd and Pb in Dystric Cambisol with a loamy texture and low content of organic

matter. The removal percentages of DTPA-extractable Cd and Pb in the soils reach up to 84.4 and 73.5%. In order to

reduce the consumption of Na2EDTA, the possibility of recycling Na2EDTA from leachate can be accomplished. Cd and

Pb in the leachate can be precipitated by Na2S + Ca(OH)2 and Na2EDTA can be released for recycling. The recovered

Na2EDTA is capable of being reused for soil remediation at least several times with slight loss of its chelating capacity.

Due to the high content of Na+ in the treated soil, it is essential to ameliorate such soil. The results of post-amelioration

soil suggest that (NH4)2SO4 is the most suitable ameliorant. The cost-benefit analysis indicates that the total cost for

recycled Na2EDTA washing decreased by 68.9% as compared with fresh Na2EDTA washing.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (51074191), National Science

Found for Distinguished Young Scholars of China (50925417) and National Public Welfare Research Project of Land

Resource (201211067-3) for financial support.

The authors have declared no conflict of interest.

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Table 1 Cost comparison of fresh Na2EDTA washing and the recycled Na2EDTA washing

Recovered Na2EDTA solution washing Agent

Fresh Na2EDTA

washing Na2EDTA Na2S Ca(OH)2 (NH4)2SO4

Mass of agent (kg/t soil) 104.23 29.77 5.72 3.7 31.71

Price ($/t agent) 2407.70 2407.70 288.9 64.20 136.40

Cost ($/t soil) 250.80 71.70 1.65 0.24 4.32

Total cost 250.8 77.91

Fig. 1 Effect of Na2EDTA concentrations on the removal of DTPA-extractable Cd and Pb

Fig. 2 Removal of Cd and Pb in leachate by adding different doses of Na2S (HM represents Cd and Pb)

Fig. 3 Removal of Cd and Pb in leachate by different doses of FeCl3 (HM represents Cd and Pb)

Fig. 4 Comparison of DTPA-extractable Cd and Pb removal between 0.04 mol/L of fresh EDTA and its recycled

solutions

Fig. 5 Effect of different ameliorant agents on the removal of water soluble and exchangeable sodium in soils

Fig. 6 Removal of water soluble and exchangeable sodium under different (NH4)2SO4 concentrations

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

Figure 6.