Fuel 88 (2009) 1167–1172

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Fuel

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Oxidation of FGD-CaSO3 and effect on soil chemical properties when appliedto the soil surface

Liming Chen a,*, Cliff Ramsier b, Jerry Bigham c, Brian Slater c, David Kost a, Yong Bok Lee c, Warren A. Dick a

a School of Environment and Natural Resources, The Ohio State University, The Ohio Agricultural Research and Development Center, Wooster, OH, USAb Ag Spectrum Company, Dewitt, IA, USAc School of Environment and Natural Resources, The Ohio State University, Columbus, OH, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 May 2007Received in revised form 14 July 2008Accepted 17 July 2008Available online 10 August 2008

Keywords:FGD-sulfiteFGD-sulfateSoil qualityGypsumCoal combustion products

0016-2361/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.fuel.2008.07.015

* Corresponding author. Tel.: +1 330 263 3655; faxE-mail address: chen.280@osu.edu (L. Chen).

Use of high-sulfur coal for power generation in the United States requires the removal of sulfur dioxide(SO2) produced during burning in order to meet clean air regulations. If SO2 is removed from the flue gasusing a wet scrubber without forced air oxidation, much of the S product created will be sulfite (SO2�

3 ).Plants take up S in the form of sulfate (SO2�

4 ). Sulfite may cause damage to plant roots, especially in acidsoils. For agricultural uses, it is thought that SO2�

3 in flue gas desulfurization (FGD) products must firstoxidize to SO2�

4 in soils before crops are planted. However, there is little information about the oxidationof SO2�

3 in FGD product to SO2�4 under field conditions. An FGD-CaSO3 was applied at rates of 0, 1.12, and

3.36 Mg ha�1 to the surface of an agricultural soil (Wooster silt loam, Oxyaquic Fragiudalf). The SO2�4 in

the surface soil (0–10 cm) was analyzed on days 3, 7, 17, 45, and 61. The distribution of SO2�4 and Ca in the

0–90 cm soil layer was also determined on day 61. Results indicated that SO2�3 in the FGD-CaSO3 rapidly

oxidized to SO2�4 on the field surface during the first week and much of the SO2�

4 and Ca moved downwardinto the 0–50 cm soil layer during the experimental period of two months. It is safe to grow plants in soiltreated with FGD-CaSO3 if the application is made at least three days to several weeks before planting.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The use of high-sulfur coal for electric power generation in theUnited States requires the removal of SO2 produced during burningvia some type of flue gas scrubbing technology in order to meetclean air regulations. Many FGD (flue gas desulfurization) technol-ogies including wet limestone, Mg-enhanced lime, and lime spraydrying are currently used in the power industry, and the wet lime-stone processes have predominated in the US [1]. Magnesium-en-hanced lime FGD and lime spray FGD technologies often produceSO2�

3 -containing FGD products [2]. If SO2 is removed from the fluegas using a wet scrubber without forced air oxidation, the FGDproduct created will contain as much as 30–87% of S as SO2�

3 [3].In the United States, the wet limestone without forced air processis still used, and a large amount of FGD-CaSO3 is produced.

Plants take up S mostly in the form of SO2�4 [4]. At low soil pH,

SO2�3 may cause damage to plant roots [5,6]. However, CaSO3 can

rapidly oxidize to CaSO4 in oxygenated environments [7]. The reac-tion rate is strongly affected by concentrations of O2 and dissolvedSO2�

3 , pH, temperature, and catalysts such as Co2+, Cu2+, and Mn2+

ll rights reserved.

: +1 330 263 3788.

[8–11]. Ritchey et al. [6] reported that CaSO3 was completely oxi-dized to CaSO4 in aqueous solutions in 5 weeks, and approximatelyone-third of CaSO3 was oxidized in a soil slurry in 8 days. Lee et al.[11] found that CaSO3 was completely oxidized in soil–FGD mix-tures in 21 days. Wendell and Ritchey [12] reported that whenFGD-CaSO3 product was applied to the soil surface, movement ofAl, Ca, and SO2�

4 in columns of an acid soil was significantlyincreased.

Gypsum (CaSO4 � 2H2O) is a quality source of both Ca and S forplant nutrition. Gypsum amendments can improve the physicaland chemical properties of soils by promoting soil aggregation,increasing water infiltration rates and movement through the soilprofile, and mitigating subsoil acidity and Al toxicity [13]. Whenused as a soil amendment, gypsum is often applied to the surfaceof fields without mixing. This is especially so when applied tono-tillage fields. FGD-CaSO3 could function similarly to gypsumwhen surface-applied if the CaSO3 is oxidized rapidly to CaSO4.However, there is little information on the oxidation of FGD-CaSO3

to CaSO4 and movement in cultivated soils under field conditions.The objectives of this study were to determine the oxidation rate ofFGD-CaSO3 applied onto the soil surface of an agricultural field andto evaluate the impact of FGD-CaSO3 application and oxidation onthe movement of plant nutrients and toxic elements in the soil.

Table 3Weekly precipitation and average temperature for the duration of the experiment in2007 and soil moisture on sampling dates at Wooster

Time interval(date)

Precipitation(cm)

Averagetemperature (�C)

Sampledate

Soil moisture(% by mass)

5/19–25 3.7 12.2 5/19 22.55/26–6/1 2.28 22.2 5/22 20.56/2–6/8 2.00 17.9 5/26 21.56/9–6/15 0 15.6 6/5 19.86/16–6/22 6.17 21.86/23–6/29 1.60 20.26/30–7/6 2.38 21.0 7/3 20.17/7–7/13 5.56 21.27/14–7/19 1.98 22.2 7/19 18.3

Total 25.7

1168 L. Chen et al. / Fuel 88 (2009) 1167–1172

2. Materials and methods

Field studies were conducted on an agricultural soil (Woostersilt loam, Oxyaquic Fragiudalf) located near Wooster, OH. Organicmatter, pH, and concentrations of selected elements extracted byMehlich-III extractant [14] from experimental field soil (0–20 cm)are presented in Table 1. The Wooster field had been in continuouscorn for three years from 2003 to 2006 under a conventional tillagemanagement system. FGD-CaSO3 was obtained from AmericanElectric Power Company, Conesville, OH, and FGD-gypsum was ob-tained from Cinergy Corporation, Cincinnati, OH (Table 2). In theseFGD materials, SO4–S was measured by ion chromatography (IC),and SO3–S was determined by subtraction of SO4–S from total Sthat was determined by dry combustion method. All other ele-ments were analyzed by inductively coupled plasma (ICP) emissionspectrometry. The chemical composition of any FGD product isinfluenced by the type of coal, desulfurization process, and sorbentused. It is also influenced by the location where fly ash is removedfrom the flue gas stream. The difference in the concentrations ofelements including Mg and B between the two study materials isdue to the coal sources used. FGD-CaSO3 was applied at rates of0, 1.12, and 3.36 Mg ha�1 to the soil surface, and FGD-gypsumwas applied at only a single rate of 1.12 Mg ha�1 as a positive com-parison. Rates used were normal field recommended rates requiredto improve soil physical and chemical properties such as water andair infiltration and subsoil acidity. These treatments were appliedto 1 � 1 m plots arranged in a randomized block with fourreplicates.

The field experiment was carried out from May 19 to July 19,2006. Weekly precipitation and average temperature at the exper-

Table 1Organic matter, pH, and concentrations of selected elements extracted by Mehlich-IIIextractant from experimental field soil (0–20 cm) at initiation of experiment

pH Organicmatter(%)

Al B Ca Cu Fe K Mg Mn P S Zn

mg kg�1

7.1 3.1 775 0.43 1440 1.5 195 111 311 94 50 51 6.7

Table 2Characteristics of the FGD-CaSO3 and FGD-gypsum

Parameter FGD-CaSO3 FGD-gypsum

Sulfur component (%)SO3–S 22.7 0SO4–S 2.0 18.7

Major elements (g kg�1)Ca 304 213S 247 187

Other elements (mg kg�1)Al 577 228As <1.3 <11B 165 5.8Ba 20.7 5.5Cd <0.05 <1.0Cr <0.19 <1.0Cu 1.69 <3.0Fe 654 222K 206 284Mg 3660 112Mn 20.5 1.3Mo <0.225 <3.0Ni 3.66 <3.0Pb <0.77 <5.0Se <2.3 <25.0Zn 7.1 4.8

imental site and soil moisture on the sampling days are presentedin Table 3. On days 3, 7, 17, 45, and 61 after treatments, five soilcores from depth of 0 to 10 cm were collected from each plotand combined to form one sample. Immediately after returningto the lab, a subsample of 20 g soil was put in a test tube, and30 mL double deionized water was added. After reciprocatingshaking for 1 h, the soil suspension was centrifuged, and the super-natant was filtered through a 0.45 lm membrane filter. The pH andelectrical conductivity of the extracts were measured using a pHmeter and a combination glass electrode. The elements Ca, Mg, K,Al, Fe, Mn, and Zn in the extracts were analyzed by inductively cou-pled plasma emission spectrometry, and SO4–S was determined byion chromatography. A subsample of 30 g soil was dried in a forcedair oven at 65 �C for four days for moisture determination. On day61 after treatment, two soil cores from each plot were collected toa depth of 90 cm using a hydraulic coring device and sub-sampledat 10-cm intervals. The sub-samples were then mixed to obtain asingle sample for each depth increment in each plot, and soils wereextracted with water and analyzed as described above.

3. Results and discussion

All treatments of FGD-CaSO3 and FGD-gypsum gradually de-creased in the surface (0–10 cm) soil pH during the first 17 days,and then pH gradually recovered after that time (Fig. 1). Wendelland Ritchey [12] and Lee et al. [11] also observed that soil andleachate pH decreased when FGD-CaSO3 or FGD-gypsum was sur-face-applied or mixed with soil under laboratory conditions. Lee etal. [11] found that the decrease of pH was approximately 0.5 unitswhen the initial soil pH values were 4.0 and 5.1, and there was lit-tle or no divergence in leachate pH with and without FGD-CaSO3

when the initial soil pH was 7.8. Wendell and Ritchey [12] reportedthat surface application of FGD-CaSO3 did not depress leachate pHas much as when incorporated into soil. The electrical conductivityin the soil extracts was significantly increased by FGD-CaSO3 orFGD-gypsum treatment (Fig. 1). The electrical conductivity trendedto decrease over time and was similar for the treatments of1.12 Mg ha�1 FGD-CaSO3 or FGD-gypsum. The solubility in wateris only 0.054 g L�1 for CaSO3 and 2.1 g L�1 for gypsum [11]. Thus,the similar electrical conductivity for FGD-CaSO3 or FGD-gypsumtreatment suggests that all the FGD-CaSO3 had been oxidized toSO2�

4 .The concentrations of water-soluble SO4–S, Ca, Mg, and K in the

soil surface layer (0–10 cm) were significantly increased by FGD-CaSO3 or FGD-gypsum treatment and gradually decreased overtime (Fig. 2). The concentrations of SO4–S, Ca, Mg, and K by treat-ment with 1.12 Mg ha�1 FGD-CaSO3 were similar to those by treat-ment with FGD-gypsum at all days measured. These resultsindicated that the SO2�

3 in the FGD-CaSO3 was rapidly oxidized inthe first three days after surface application. Wendell and Ritchey

6.5

6.6

6.7

6.8

6.9

7.0

7.1

7.2

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

0

200

400

600

800

1000

1200

ControlFGD-CaSO3 1.12 Mg Ha-1

FGD-CaSO3 3.36 Mg ha-1

FGD-gypsum 1.12 Mg ha-1

Time (days)

pHEC

(μS

cm-1

)

Fig. 1. Changes in pH and electrical conductivity (EC) in water extracts of soils (0–10 cm) over time after surface application of FGD-CaSO3 or FGD-gypsum.

L. Chen et al. / Fuel 88 (2009) 1167–1172 1169

[12] also found that S in leachate was mostly in the form of SO4–Swhen FGD-CaSO3 was surface-applied on soils and leached withdeionized water. Lee et al. [11] reported that oxidation of SO2�

3 insoil–FGD mixtures needed 21 days, but oxygen was probably

0

100

200

300

400

500

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

0

10

20

30

40

50

60

70

Time

Mg

(mg

kg-1

)SO

4- S (m

g kg

-1)

Fig. 2. Changes in water-soluble SO4–S, Ca, Mg, and K in the surface 10 cm so

limiting the rate of oxidation in the soil–FGD mixture. Even thoughFGD-CaSO3 and FGD-gypsum contained little or no Mg and K (Table2), water-soluble Mg and K were significantly increased by FGD-CaSO3 or FGD-gypsum treatment. This is because Ca ions havegreater affinity for exchange sites on soil particles than Mg and Kions. Application of FGD-CaSO3 and FGD-gypsum displaced andmobilized Mg and K.

Application of FGD-CaSO3 or FGD-gypsum significantly de-creased the concentrations of water-soluble Al, Fe, Mn, and Zn inthe 0–10 cm soil surface layer (Fig. 3). Wendell and Ritchey [12] re-ported when FGD-CaSO3 was incorporated into the soil, leachate Alwas increased and exchangeable Al in the soil was decreased. Thissuggests that the decrease of Al was attributed to leaching loss byCa replacement. Aluminum and Mn phytotoxicities are very com-mon in acid soils that can significantly reduce agricultural produc-tion. Toxic concentrations of soluble and exchangeable Al and Mninhibit root growth and nutrient uptake. Manganese toxicity hasoften been observed in Wooster soil. Application of FGD-CaSO3 orFGD-gypsum reduced Al and Mn in soil, thus mitigating Al andMn toxicities to crops. Iron and Zn are plant essential micronutri-ents. The decrease of these elements was induced by releasingfrom soil particles by cation exchange with Ca and moving down-ward with water [15,16]. Zinc especially may need to be replaced iftoo much is leached from soil.

FGD-CaSO3 contained 165 mg B kg�1 (Table 2). Boron is anessential element for plants and is the most widespread micronu-trient deficiency throughout the world. Boron concentrations in al-falfa are usually between 30 and 80 mg kg�1, and alfalfa is Bdeficient if the concentration at the top 15 cm of plant is less than30 mg kg�1 [17]. It is generally recommended to apply 1–3 kg B ha�1 to soil for optimal alfalfa growth. Alfalfa yield increasedwith the increase of applied B to 4.2 kg ha�1 on Coastal Plain soilsof East Texas [18]. Application of FGD-CaSO3 at 3.36 Mg ha�1

provided 0.55 kg B ha�1 to soil, and water-soluble B in soil wasincreased by the treatment (Table 4). These results indicated thatFGD-CaSO3 may provide B for crop growth. If acidic minedspoils are treated with very high rates of FGD, for example

0

50

100

150

200

250

300

350

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70

10

20

30

40

50

60

70ControlFGD-CaSO3 1.12 Mg ha-1

FGD-CaSO3 3.36 Mg ha-1

FGD-gypsum 1.12 Mg ha-1

(days)

Ca

(mg

kg-1

)K

(mg

kg-1)

il layer over time after surface application of FGD-CaSO3 or FGD-gypsum.

0

10

20

30

40

50controlFGD-CaSO3 1.12 Mg ha-1

FGD-CaSO3 3.36 Mg ha-1

FGD-gypsum 1.12 Mg ha-1

0.00

0.05

0.10

0.15

0.20

0.25

0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 700 10 20 30 40 50 60 70

0

5

10

15

20

25

30

35

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Time (days)

Mn

(mg

kg-1)

Zn (m

g kg

-1)

Fe (m

g kg

-1)

Al (

mg

kg-1)

Fig. 3. Changes in water-soluble Al, Fe, Mn, and Zn in the surface 10 cm soil layer over time after surface application of FGD-CaSO3 or FGD-gypsum.

Table 4Effect of FGD-CaSO3 or FGD-gypsum on the concentrations of water-soluble B in0–10 cm soil layer

Treatment Time after surface application

3 days 7 days 17 days 45 days 61 days

mg kg�1

Control 0.15 0.09 0.06 0.05 0.031.12 Mg ha�1 FGD-CaSO3 0.16 0.08 0.13 0.03 0.163.36 Mg ha�1 FGD-CaSO3 0.22 0.37 0.20 0.11 0.171.12 Mg ha�1 FGD-gypsum 0.05 0.12 0.03 0.04 0.05

4.5 5.0 5.5 6.0 6.5 7.0 7.50

20

40

60

80

100

ControlFGD-CaSO3 1.12 Mg ha-1

FGD-CaSO33.36 Mg ha-1

FGD-gypsum 1.12 Mg ha-1

0 200 400 600 800 10000

20

40

60

80

100

pH

Soil

dept

h (c

m)

Soil

dept

h (c

m)

EC (μS cm-1)

Fig. 4. Changes in pH and electrical conductivity (EC) in water extracts of soilcollected at different depths 61 days after surface application of FGD-CaSO3 or FGD-gypsum.

1170 L. Chen et al. / Fuel 88 (2009) 1167–1172

224–448 Mg ha�1 fly ash or FGD material mixed with fly ash, con-centrations of B in the soil can reach phytotoxic levels [19]. How-ever, plant B toxicity is not expected to occur if FGD-CaSO3 isapplied at normal rates for agricultural soil amendments.

At the end of the experiment (day 61), the treatment of FGD-gypsum slightly increased the pH in the 10–30 cm soil layers,while FGD-CaSO3 at the rate of 3.36 Mg ha�1 decreased the pH inthe 10–20 cm soil layer (Fig. 4). All treatments of FGD-CaSO3 andFGD-gypsum significantly increased the electrical conductivity inthe soil layers from 0–50 cm, and the increase was greatest forFGD-CaSO3 at 3.36 Mg ha�1.

On day 61, treatment of FGD-CaSO3 at 3.36 Mg ha�1 signifi-cantly increased water-soluble SO4–S, Ca and Mg in the soil layersof 0–90 cm (Fig 5). Application of FGD-CaSO3 or FGD-gypsum at1.12 Mg ha�1 increased water-soluble SO4–S, Ca, and Mg in the soillayers of 0–30 cm. The impact of 1.12 Mg ha�1 FGD-CaSO3 wassimilar to that of FGD-gypsum. Zaifnejad et al. [20] conducted acolumn experiment using FGD-gypsum and observed increases ofexchangeable Ca, Mg and S in all the soil layers of 0–90 cm afterthe column was leached for 39 days. Because CaSO3 is very slightlysoluble, the similar downward movement of SO4–S and Ca in thesoil treated with FGD-CaSO3 or FGD-gypsum suggests that theCaSO3 had been oxidized to CaSO4 in the soil surface.

Sixty-one days after treatments, concentrations of water-solu-ble Al were significantly decreased in the soil layers of 0–30 cm

by all treatments of FGD-CaSO3 or FGD-gypsum (Fig. 6). Concentra-tions of water-soluble Zn were decreased in the soil layers of

0 50 100

0 50 100

150 200 250 300 3500

20

40

60

80

100

ControlFGD-CaSO3 1.12 Mg ha-1

FGD-CaSO3 3.36 Mg ha-1

FGD-gypsum 1.12 Mg ha-1

150 200 250 3000

20

40

60

80

100

0 10 20 30 40 500

20

40

60

80

100

SO4-S (mg kg-1)

Ca (mg kg-1)

Mg (mg kg-1)

Soil

dept

h (c

m)

Soil

dept

h (c

m)

Soil

dept

h (c

m)

Fig. 5. Changes in water-soluble SO4–S, Ca, and Mg in the soil at different depths 61days after surface application of FGD-CaSO3 or FGD-gypsum.

0 10 15 20 25 30 350

20

40

60

80

100

ControlFGD-CaSO3 1.12 Mg ha-1

FGD-CaSO3 3.36 Mg ha-1

FGD-gypsum 1.12 Mg ha-1

Zn (mg kg-1)0.00 0.05 0.10 0.15 0.20 0.25 0.30

0

20

40

60

80

100

Al (mg kg-1)

Soil

dept

h (c

m)So

il de

pth

(cm)

5

Fig. 6. Changes in water-soluble Al and Zn in the soil at different depths 61 daysafter surface application of FGD-CaSO3 or FGD-gypsum.

L. Chen et al. / Fuel 88 (2009) 1167–1172 1171

0–90 cm. The impact of 1.12 Mg ha�1 FGD-CaSO3 was similar tothat of FGD-gypsum. Subsoil Al is a major factor limiting crop yieldin many areas of the world. Lime is approximately 200 times lesssoluble than gypsum and incorporating lime into topsoil has no ef-fects on subsoil acidity. Thus, surface application of gypsum hasbecome a recommended procedure for ameliorating subsoil Al tox-icity. Our results indicate that FGD-CaSO3, like gypsum, is a goodproduct for acid-subsoil amelioration.

4. Conclusion

Sulfite in FGD-CaSO3 was rapidly oxidized to SO2�4 within three

days when applied to the moist soil surface of a field in the spring.There were no differences in the concentrations of water-solubleSO4–S and Ca in the soil surface (0–10 cm) over time or at different

depths 61 days after 1.12 Mg ha�1 of FGD-CaSO3 or FGD-gypsumwas applied to the soil. Effects of FGD-CaSO3 on water-solubleMg, K, Al, Fe, Mn, and Zn in soils were similar to those of FGD-gyp-sum when applied at the rate of 1.12 Mg ha�1. The benefits of add-ing FGD-CaSO3 to the soil are similar to those of adding gypsum forsoil chemical properties. It is safe to grow plants in soil treatedwith FGD-CaSO3 if the application is made at least three days toseveral weeks before planting.

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