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Journal of Environmental Quality 30:1071-1080 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Waste Management

Effect of Flue Gas Desulfurization Residue on Plant Establishment and Soil and Leachate Quality

Tracy Punshona, Domy C. Adrianoa and John T. Weberb

a Savannah River Ecology Lab., Univ. of Georgia, Drawer E, Aiken, SC 29802
b Dep. of Soil Science, Univ. of Adelaide, PMB 1, Glen Osmond, South Australia, 5064

Corresponding author (punshon{at}srel.edu)

Received for publication December 8, 1999.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL APPROACH
 RESULTS
 DISCUSSION
 REFERENCES
 
Effects on soil quality and crop establishment after incorporation of flue gas desulfurization by-product (FGD) into soil as an amendment was assessed in a mesocosm study. Mesocosm units received applications equivalent to 0, 2.5, 5.0, 7.5, and 10% FGD residue [0, 25, 50, 75, and 100 tons acre-1]. Germination, biomass production, and elemental composition of corn (Zea mays L. var. Dekalb DK-683), soybean [Glycine max (L.) Merr. var. Haskell Pupa 94], radish (Raphanus sativus L. var. Sparkler), and cotton (Gossypius hirsutus L. var. Deltapine 51) were determined. The quality of leachates and soil were also determined periodically. Flue gas desulfurization residue did not affect germination and all application rates stimulated aboveground biomass. Plants grown in FGD-amended soil contained significantly elevated tissue concentrations of As, B, Se, and Mo. The FGD residue elevated surface soil pH from 5.5 (Control) to 8.1 (at 10% FGD). Leachate pH was unaffected by FGD, but salinity rose sharply with increasing application rates of FGD. Leachates contained higher concentrations of B, with small increases in Se and As. Flue gas desulfurization residue application caused an increase in total B, As, Mo, Se, and extractable Ca in the soil, but decreased Mn and Zn. Using FGD residues could have beneficial effects on crop establishment without detrimental effects on soil or leachate quality, at an optimum rate of approximately 2.5%. This material could alleviate surface acidity, and B and Mo deficiencies in plants.

Abbreviations: FGD, flue gas desulfurization residue • EC, electrical conductivity • ANOVA, analysis of variance • GLM, general linear model • SAR, sodium adsorption ratio


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL APPROACH
 RESULTS
 DISCUSSION
 REFERENCES
 
FLUE GAS DESULFURIZATION (FGD) by-products result from the removal of SOx from gaseous emissions during coal combustion. Power plants utilize several techniques to extract S from waste gases, classified as either wet or dry scrubbing processes (ACAA, 1998). In dry scrubber systems, which are the emphasis of the current study, a three-stage process first cools the gases, and then either a lime- or sodium-based extractant is sprayed into the system, followed by a final filtration step. For the lime-based reagent usually hydrated lime is used, and in the case of sodium, carbonate or bicarbonate is often used. Hydrated lime is one of the cheapest extractant, although SOx removal efficiency is higher using sodium. Waste gases react with the Ca component of the extractant, giving rise to various Ca–S products such as calcium sulfite (CaSO3 · 1/2H2O or hannebachite) and calcium sulfate (CaSO4 · 2H2O or gypsum) (Miller, 1995). The use of a hydrated lime extractant frequently results in a FGD by-product that contains relatively little gypsum.

Flue gas desulfurization residues are typically very variable by-products due to the variety of techniques and extractants used in their production. For instance, the characteristics of the parent coal, specifically the S content influences pH of the material. Using a dolomitic lime as the extractant often confers higher concentrations of Mg to the final by-product (Miller, 1995). The composition of soluble salts and trace elements is also influenced by the dewatering, stabilization, storage, and disposal stages. Flue gas desulfurization residues generally do not contain a great deal of trace elements, although this is increased when they are mixed with fly ash (stabilization). Trace elements from the parent coal condense on the surface of fly ash particles during combustion, generally adhering in higher concentrations on the smaller, more mobile particles. Comprehensive surveys of the elemental compositions of different FGD residues suggest that the gypsum content can vary between 20 and 80% (Miller, 1995). Dick et al. (2000) reviewed the mineralogy of a range of FGD products, as well as comparative studies on elemental composition.

Coal combustion by-products (CCBPs) consist mainly of fly ash, with 57187652 Mg (tonne) produced in 1997 compared with 22697611 Mg of FGD (ACAA, 1998). Re-use of the materials is generally low, and remains at approximately 33% for fly ash and 29% for FGD. Fly ash has a variety of applications in the construction industry, the majority being used for cement, concrete, and grout. Flue gas desulfurization residues are not pozzolanic and are therefore not as extensively used in construction unless they are mixed with another cementitious material (Hower et al., 1996). There are reports of FGD by-products being used in self-leveling floors, thin-layer applications for renovation, and in mining mortars (Schlieper et al., 1997). Mixing FGD residue with fly ash and lime produces an innocuous monolithic mass that can also be used as a roadbase (Smith and Colella, 1997). Research into increasing the use of FGD residues in construction indicates that calcium sulfite–rich FGD has a greater compressive strength and water-resistance than gypsiferous materials, which have a tendency to leach when in contact with water.

One re-use option is the application of FGD residues to degraded land, and may present a more cost-effective alternative to the use of commercial gypsum and lime, which are used as soil conditioners (Shainberg et al., 1989). The beneficial effects of FGD residue application to soil are similar to that of lime, which include an increase in surface pH (Shainberg et al., 1981) and alteration of water holding capacity as a result of the introduction of fine particles. The risks involved with FGD application are those associated with trace elements leaching from the material into the soil (Klein et al., 1975) combined with an increase in electrical conductivity (EC); these issues have yet to be clarified satisfactorily in field-scale experiments.

The variation in the mineralogical properties and elemental composition of FGD residue makes their effect on the soil impossible to predict reliably. Some parent coals may contain more potentially toxic trace elements than others, making their residues of more concern. Key elements of concern that tend to result from applying FGD residues to soil are B, Mo, As, and Se. Boron solubility from CCBPs has been cited most frequently as a cause of poor crop growth and germination in previous amendment studies (Aitken and Bell, 1985; Kukier and Sumner, 1996). Other workers have used grass and legume species with innate trace element tolerances in an attempt to manage this trace element transfer (Parker et al., 1991).

Should use of these by-products as a soil conditioner prove to be cost-effective and an environmentally safe alternative to landfilling, it will be necessary to categorize the materials into those suitable for either construction or agriculture. This would ideally influence the processes used by power plants to ensure they are appropriate for the chosen end-use. For example, higher aeration during desulfurization results in a residue that contains more gypsum, whereas insufficient oxygen favors production of calcium sulfite. Power plants may also ensure alternative appropriate disposal of their by-products by offering transportation–handling–placement systems through which the cost per ton of disposal can be minimized compared with landfilling (Sevim and Unal, 1998).

The objectives of this research were to evaluate the effects of FGD residue on (i) germination and early biomass of crop plants, (ii) trace element transfer into plant tissue, and (iii) soil and leachate quality. The study aimed to elucidate these effects in a large-scale mesocosm study using a soil representative of the area and locally obtained FGD material.


   EXPERIMENTAL APPROACH
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL APPROACH
RESULTS
 DISCUSSION
 REFERENCES
 
Establishment of the Mesocosm Study
Fresh flue gas desulfurization residue was obtained from a power plant in Cope, South Carolina. The by-product was air-dried and homogenized in a large-volume soil mixer; differential application rates of the residue were prepared by mixing material with a local Orangeburg series soil (fine-loamy, siliceous, thermic, Typic Paleudult). The soil had a pHwater (1:1 ratio) of 5.4 (±0.7), 0.5% organic C; 84.8% sand, 10.9% silt, and 4.2% clay. Mesocosm units consisted of galvanized iron cattle tanks, 2.4 m in diameter, and 60-cm depth, representing an area of approximately 4.67 m2 (equivalent to 0.0012 acres). Each tank was fitted with a drainage tap for leachate collection. The FGD–soil mixtures approximately equivalent to 0, 55.5, 111.1, 166.6, and 222.2 Mg FGD ha-1 (0, 25, 50, 75, and 100 tons FGD acre-1) were applied to the mesocosm tanks, above layers of gravel and clean soil to a depth of 15 cm, mimicking a topsoil application (Fig. 1). Mesocosm units were set up in a restricted access area at the Savannah River Ecology Laboratory, with four replicate tanks per treatment. Mesocosms were allowed to equilibrate for 4 mo after incorporation of FGD. Leachate samples were collected, and pH and electrical conductivity (EC) were measured periodically and after each rainfall. Subsequent leachate collections were carried out 9, 27, 43, 84 and 86, 96, and 121 d after set-up.



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  		Fig. 1. Mesocosm unit construction.

 
On 1 June 1998 mesocosm units were planted with four crop species: corn (Zea mays L. var. Dekalb DK-683), soybean (Glycine max L. var. Haskell Pupa 94), radish (Raphanus sativus L. var. Sparkler), and cotton (Gossypius hirsutus var. Deltapine 51) at rates of 20 plants of each species per unit. Due to the risk of bird damage, emergent seedlings were covered with wire-mesh cages until fully established. Germination began 6 d after planting, and rates of emergence were monitored for the proceeding 20 d. Crop plants were grown for 10 wk, with measurements of shoot height, commencing 16 d after planting, on a daily basis until harvest.

Flue Gas Desulfurization By-Product and Soil Characterization
The elemental composition of all of the experimental samples was determined using a Perkin Elmer PE Sciex Elan ICP-MS. The FGD used was a weathered material, originating from an Eastern anthracite coal, with approximately 2% S and 9% ash, considered a low-grade gypsum product. The FGD by-product came from a dry scrubber process, which used hydrated lime as the extractant, and the material was stabilized by combination with fly ash. Extractions were carried out to determine total and plant-available elemental composition (Table 1). pH and EC were determined using an Orion 250 A+ glass-bulk electrode, soaked in 10% HCl before use, using a deionized water/soil ratio of 1:1. Measurements were repeated three times and the arithmetic mean of the pH readings was determined. The material had a pH of 9.3, and an electrical conductivity of 3.56 dS m-1. Total elemental composition of FGD was determined by digesting 0.5-g samples of dried, ground material in a 1.5-mL aqua regia (1:3:1 HNO3/HCl/H2O) added to 5 mL concentrated HF (40% v/v) in a Teflon vessel (Sparks et al., 1996). Extractable metals were determined by shaking 5 g of dried sample with 50 mL distilled, deionized water overnight, afterwhich they were filtered (Whatman no. 41), and were confirmed using a double acid extraction technique (20 mL 0.05 M HCl + 0.5 M H2SO4 to 5 g dried material: shaken for 30 min, centrifuged, and filtered using Whatman no. 41 paper).


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  		Table 1. Elemental composition of flue-gas desulfurization residue (FGD) from a power plant in Cope, SC.{dagger}.

 
Determination of the total and extractable elemental concentration of FGD-amended soil samples was carried out at the end of the experiment following the harvest and removal of plant material; 0.25-g samples of dried, sieved FGD-amended soils were analyzed for total elemental concentration (using HF and H3BO3); extractable elemental composition was determined by extracting 0.5 g soil in 0.1 M HNO3 (Alloway, 1995) to avoid matrix interference due to excess Ca, encountered when Ca(NO3) extracts are analyzed by ICP-MS. Boron determination in soil samples was carried out using the hot water extraction technique.

Preparation of Plant Material
Plant material was oven-dried at 60°C until no further weight loss. Material was separated into leaves (including petioles) and roots and weighed to determine dry biomass. Due to the presence of a distinct lignified stem in corn plants, this was apportioned separately for analysis. The material was ground to a fine powder (1 mm stainless steel screen) and digested in 10 mL 5 M HNO3 by microwave (CEM Corp. MDS-2000) in pure Teflon PFA vessels.

Analysis of Mesocosm Leachates
Leachate samples were collected throughout the experiment, with measurements of pH and electrical conductivity taken immediately after collection. Leachate samples were then acidified and maintained in cold storage before analysis by ICP-MS.

Statistical Analyses
Analysis of variance (ANOVA), general linear models (GLMs), and Tukey's statistical test were used to determine significant differences within the data, using a probability level of P < 0.05 in all cases. In instances where ANOVA and Tukey's tests were too conservative, the nonparametric Kruskal-Wallis test was used to account for differential variability of data sets.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL APPROACH
 RESULTS
DISCUSSION
 REFERENCES
 
Flue Gas Desulfurization Mineralogy
Crystalline composition, determined by x-ray diffraction, showed the FGD residue to be predominantly composed of amorphous glassy material (46%), hannebachite (26.6% CaSO3 · 1/2H2O), mullite (17% Al2Si2O13), in addition to portlandite or hydrated lime [5.5% Ca(OH)2] and quartz (3% SiO2).

Effect on Crop Germination and Growth
Monitoring of germination rates in mesocosm units showed no significant inhibitory or stimulatory effect of FGD on early plant establishment. Biomass measurements (expressed as g dry matter plant-1) taken at the end of the 10-wk growing period showed a marked increase in shoot biomass as a result of FGD addition (Fig. 2). Statistical analysis of the data showed that shoot biomass was significantly increased by FGD amendment in corn (F4 = 18.98, P < 0.0001) and radish leaves (F4 = 6.31, P = 0.004) only.



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  		Fig. 2. Mean dry weight of corn, cotton, soybean, and radish in response to FGD amendment (means and SD of bulked samples where n = 4).

 
In the case of cotton and radish plants ANOVA tests, as applied elsewhere, were too conservative to separate the growth differences that clearly occurred (Fig. 2). Further analysis of this data (Kruskal-Wallace two-sided: with controls tested against grouped FGD treatments) showed a significant mean separation between control and FGD-treated plants (cotton P = 0.0017 and for radish P = 0.0077). Furthermore, by removing control plants and testing for differences in root growth between treated plants only, it emerged that there were no between-treatment differences for either cotton or radish, where P = 0.0655 and P = 0.3994 for cotton and radish, respectively. This indicated that the increase in root biomass was not proportional to the level of FGD amendment, although there was a general improvement.

Effect on Elemental Composition of Plant Tissue
Significant trends are summarized in Table 3 to indicate where concentrations were significantly (determined by one-way ANOVA) increased ({uparrow}), decreased ({downarrow}), or unaffected (–) by FGD amendment. This showed significant increases in As, Se, Mo, B, and to some extent Ca within tissues, and decreases in Mn, Na, and Zn. Nickel, Pb, Fe, and Cu concentrations were unaffected by FGD amendment.


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  		Table 3. Trends in the elemental composition of leaf and root tissues of corn, cotton, soybean, and radish grown on soil amended with FGD residue.{dagger}.

 
Control concentrations of As in leaves of soybean, corn, and cotton plants ranged from 0.1 to 0.5 mg kg-1. Radish plants contained relatively high background concentrations of As, with concentrations of 1.1 (±0.07) mg kg-1 detected in the shoots of the controls. Radish germinating in 10% FGD contained between 0.8 and 2.3 mg As kg-1, and root tissue concentrations rose from 0.1 to 0.6 to 0.7 to 2.6 mg As kg-1 at the highest FGD application rate. Soybean plants showed the greatest increases in As relative to background concentrations, with 10% FGD-treated plants containing approximately seven times the As concentration of controls (soybean leaf: 6.79 x background; roots: 7.05 x background) (Fig. 3). Sensitivity and toxicity to As varies widely between plant species, and the higher background As concentration observed in both radish and corn plants may be an indication of a comparatively greater tolerance to As. In general, legumes and rice (Oryza sativa L.) tend to be more susceptible to As toxicity. Data for innate As resistance was collated in Adriano (2001), with radish and corn plants designated as moderately tolerant and soybean as less tolerant, and the current comparative germination experiment supports these general trends. Radish and corn both contained more As than soybean and cotton, both in control and amended soils, and the overall increase in their As uptake as a result of amendment was lower than in the less resistant soybean. Critical tissue As concentrations vary widely between literature source and test species used, ranging from 1 mg kg-1 in soybeans to between 20 mg kg-1 in barley (Adriano, 2001). Symptoms indicative of As toxicity (wilting of new-cycle leaves, retardation of root growth, root discoloration, and necrosis of leaf tips) were not observed in any of the crops. Arsenic mobility may have been increased by a predominance of Ca in the treated soil as a result of FGD addition. Higher concentrations of Fe and Al (which predominate in fly ash) in the soil can cause less-soluble As species Fex and Alx(AsO4)x to predominate, whereas elevated Ca may have shifted the equilibrium in favor of Ca3(AsO4)2, which is relatively more mobile and available for plant uptake. In highly weathered southeastern U.S. soils, Fe and Al hydroxides and oxy-hydroxides would act as an As adsorber under normal conditions, although in situations where there is a pH increase (due to FGD addition) a decrease in overall positive charge on these oxides may have caused a release of As into the soil solution.



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  		Fig. 3. Concentrations of As in leaf and root tissues of corn, cotton, soybean, and radish as influenced by FGD amendment (means ± SD where n = 4).

 
Background Se concentrations in leaves varied within the range 0 to 1.9 mg kg-1. Radish grown in unamended soil contained particularly high concentrations of Se in their leaves (2.3 ± 0.03 mg kg-1) in contrast with the other crops. Plants grown in soil amended with 10% FGD contained 5.2 to 18.7 mg Se kg-1. Similarly, root tissue concentrations ranged from 0 to 1.3 to 0.43 to 16.3 mg kg-1 between 0 and 10% FGD. The greatest increase was observed in radish roots at the highest FGD application rate, reaching a total of 18.71 (±5.2) mg Se kg-1, which is equivalent to {approx}37 times the background Se levels for this species (Fig. 4).



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  		Fig. 4. Concentrations of Se in leaf and root tissues of corn, cotton, soybean, and radish as influenced by FGD amendment (means ± SD where n = 4).

 
Significant B accumulation occurred only in the leaf tissues of the crops tested. The greatest increases were seen in soybean and corn, approximately four times the background concentrations at 10% FGD (Fig. 5). Additionally, there was an increase in the concentration of Mo, most notably in soybean leaves. In this case, concentrations rose from 0.3 (±0.02) to 91 (±27) mg Mo kg-1 at 10% FGD (Adriano, 1980) (F4 = 10.00; P = 0.0001). Molybdenum concentration in other plant species remained <10 mg kg-1. The higher Mo concentration in soybeans may be the result of increased requirement by the symbiotic rhizobia associated with the roots of leguminous plants, and this may also be linked to the observed yield increases in this species. Molybdenum is an element frequently associated with CCBP addition and frequently occurs at elevated levels in plant material grown on CCBP amended soil, although Mo toxicity has not been observed in the field (Adriano et al., 1980). In this study, elevated concentrations of Mo were found in plant tissues, although there were no significant increases in either leachate or soil Mo. Molybdenum toxicity is generally thought to be expressed at lower concentrations in animal than in plant tissues; plants containing Mo levels >20 mg kg-1 may cause toxicity symptoms (molybdenosis) in grazing animals (Adriano, 2001); this can be generally considered an upper limit for determining safe levels of Mo accumulation in aboveground plant parts. Therefore, the levels of Mo observed in soybean leaves in this study may be considered excessive. High Mo concentrations within plant materials are usually associated with fly ash application (Elseewi and Page, 1984), possibly due to the FGD material applied in this study having a relatively high fly ash content and therefore considerably enriching the material with Mo (32.8 mg kg-1). Additionally, Mo increases are also a common result of the liming of acid soils (Dick et al., 2000).



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  		Fig. 5. Concentrations of B in leaf and root tissues of soybean and corn as influenced by FGD amendment (means ± SD, where n = 4).

 
There was also a reduction in tissue Mn in all crops, in both the leaves and roots equally. At the highest level of amendment, Mn concentrations were reduced to approximately half that of untreated plants. A reduction in Mn solubility and therefore in plant uptake has been observed as a result of excessive liming to reduce surface acidity. Labanauskas (1965) noted that soils with pH > 6.5 create more favorable conditions for Mn oxidation from Mn(II) to the less soluble oxidation state of Mn(IV). The critical concentration for Mn deficiency in most plant species are similar; ranging from between 10 and 20 mg Mn kg-1 in fully expanded leaves, below which net photosynthesis and chlorophyll content begin to rapidly decline (Ohki et al., 1981). All FGD application rates reduced the Mn content of plant tissues to the same degree (i.e., to approximately half that of untreated plants), and levels of Mn of all species approached the deficiency threshold. Increase in biomass also followed a similar pattern, with a distinct lack of proportionality between the rate of FGD and the biomass, indicating that Mn deficiency may be the limiting factors in biomass enhancement when FGD is added to the soil.

Effect of Flue Gas Desulfurization on Leachate Quality
pH and salinity data were analyzed using a repeated measures ANOVA with autoregressive error structure (SAS version 7: Proc. Mixed). Sampling dates were considered categorically rather than linearly due to the different intervals between sampling dates, and influential factors considered were treatment, time, and their interactions. The analysis supported observed trend differences between leachate pH and salinity, with no significant treatment effect for pH (F4 = 1.55; P = 0.1971), but a significant change over time (F1 = 16.3; p = 0.0001), with no interaction. Conversely for salinity, treatment, time, and interactions between them were all highly significant (P < 0.0001 in all cases). Salinity of the leachate water was dramatically increased by FGD incorporation, and appeared closely linked to the amount of FGD applied. Salinity reached a maximum of 3.385 (±0.6) dS m-1 at 10% FGD compared to a background of 0.05 to 0.4 dS m-1 (Fig. 6). Leachate salinity fell in the higher treatments and at the end of the monitoring period was approximately 2.2 dS m-1 in all treated units.



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  		Fig. 6. Electrical conductivity of sequential leachate samples taken from FGD mesocosms (means ± SD where n = 4).

 
The soluble salt concentrations of the leachate were also used to calculate the sodium adsorption ratio (SAR), a semiquantitative indicator of water quality and of the extent of sodicity of a substrate. Weathered southeastern soils (e.g., the Orangeburg series used here) are particularly sensitive to dispersion (Kaplan et al., 1996) because the levels of soluble salts are inherently low. Clay dispersion is thought to occur when the SAR >= 4. Sodium adsorption ratio values of mesocosm leachates collected 86, 98, and 121 d after the study was established showed a steady increase from a background value of 1.65 to 3.96 at 10% FGD. At the highest FGD application rate, SAR continued to rise over the three monitoring dates (3.65, 3.25, and 3.96, respectively), and was strongly correlated with electrical conductivity and the level of FGD application, although it did not rise above this threshold level.

Elemental composition of leachates was determined in samples taken 86, 98, and 121 d after establishment, during which germination experiments were in progress (Table 4). Significant differences in elemental composition in response to FGD amendment, time and the experimental replication were determined using the general linear model (GLM) multiple comparisons statistical test (P = 0.05) (Table 2). Boron concentration in leachates were increased by FGD amendment, and remained elevated over the duration of the study. The remaining elements—Na, Mg, K, and Ca—were all elevated by FGD and changed significantly with time.


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  		Table 4. Elemental composition of mesocosm leachate (means ± standard deviation where n = 4). Significant FGD-treatment effects were established using GLM statistical tests, and only elements influenced by FGD are shown. Different letters denote significant differences between treatments, as indicated by Tukey's statistical tests. P < 0.05 in all cases.

 

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  		Table 2. Total and extractable elemental composition of mesocosm soil immediately after crop harvest (means ± SD). Significantly influenced elements (determined by ANOVA analysis) only are shown. Means separations determined by Tukey's statistical analysis where P < 0.05.

 
The As concentrations of leachates from 10% FGD treatment were approximately 0.001 to 0.002 mg kg-1 higher after 98 d, with the background concentration remaining steadily at {approx}0.002 mg kg-1. Selenium in the leachate from treated mesocosms was elevated above background concentrations to a much greater extent. With a stable control response of <0.004 mg kg-1, mesocosm treated with 10% FGD residue yielded leachate which contained 0.0271 mg Se kg-1, although this concentration fell to 0.0174 mg kg-1 when monitored again after 121 days (Fig. 4).

Additionally, Ca concentration was elevated from a background level of between 13 and 26 mg kg-1, to between 379 and 634 mg kg-1, with 10% FGD (equivalent to approximately 30–50 x increase). The concentrations of other macronutrients also increased considerably with the addition of FGD, including Mg and K.

Effect on Soil Quality
Soil samples collected below the incorporation layer after crop harvest indicated a stable pH increase in FGD-treated units. Background pH remained at 5.5 and increased to 8.1 in the 7.5 and 10% treatments. Similarly, the salinity of the soil was also higher in FGD-treated units, elevated to between 2.9 and 3.3 dS m-1 from a background of 0.2 (±0.01) dS m-1.

Analysis of the total elemental concentration of amended soils showed an elevation of As, Be, B, Co, Ni, and Se. The most significant elevations were in As and to a lesser extent Se. As concentrations rose from a background level of 0.9 (±0.1) to 2.4 (±0.6) mg kg-1. Amendment of soils with FGD also increased the plant-available concentrations of As, B, Ca, K, Mg, Se, Sr, and Tl (Table 2). The most notable trends were the 10-fold increases in both As and Se, with the higher rates of FGD addition. Arsenic increased from 0.15 to 1.8 mg kg-1 and Se from 0.02 to 0.18 mg kg-1. Increase in the concentration of hot water–extractable B as a result of 10% FGD was approximately five times the background levels (Table 2). In addition, FGD addition caused significant increases in the availability of nutrients including Mg, K, and Ca.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL APPROACH
 RESULTS
 DISCUSSION
REFERENCES
 
Addition of FGD residue to weathered southeastern U.S. soils resulted in a significant increase in the aboveground biomass of corn, cotton, soybean, and radish plants. The greatest increases in biomass were seen in corn and radish plants. The material used contained appreciable amounts of fly ash (Punshon et al., 1999) and caused accumulation of high concentrations of B, As, and Se in plant tissues. Despite inherent variation in the composition of FGD materials, many published studies have emphasized the presence of elevated B levels in plant tissues, and attributed reduction in germination and biomass to B phytotoxicity. In this study, there was no observable reduction in germination, and only soybean and corn accumulated significant concentrations of B. Weathered or ponded FGD materials are generally considered to contain less B than fresh materials, and there is thought to be a close correlation between the B status of the material and uptake into plant tissues (Kukier and Sumner, 1996). Clark et al. (1999) tested 15 different types of CCBP, three of which were FGD residues containing predominantly CaSO3, and used corn to determine potential phytotoxic effects. They found that plants grown on soils amended with FGD residues contained shoot concentrations similar to those observed in this study: {approx}300 mg B kg-1 in soybean (Fig. 5) (as opposed to 358 mg kg-1 in corn). The similarity of this study to those of other workers (Kukier and Sumner, 1996; Stehouwer et al., 1996; Crews and Dick, 1998; Wright et al., 1998; Sloan et al., 1999) endorses the general trend that high levels of FGD application increase shoot B concentration. However, B accumulation characteristics were plant species-specific. By comparing the responses of a range of crop species, this study showed soybean plants take up more B than corn (230 ± 69 mg kg-1), cotton, or radish, with a corresponding suppression in biomass at FGD rates above 2.5% FGD. Corn accumulated less B (108 ± 19 mg kg-1), and had consistently higher shoot biomass. Although B toxicity thresholds vary from plant to plant, in general tissue concentrations >100 mg kg-1 are considered to be toxic (Adriano, 2001; Clark et al., 1999). Clark et al. (1999) also pointed out that the B toxicity threshold varies with the growth stage of the particular crop, with concentrations >25 mg kg-1 toxic to young corn plants and >100 mg kg-1 toxic to those nearer ear formation. Comparing different crop species allows a safe application rate to be determined, but also allows the selection of a crop that responds to FGD addition with increased biomass without accumulating toxic concentrations of B. This trend was also observed for As and Se. Corn plants accumulated the highest concentration of As (2.6 mg kg-1), although in all cases As uptake remained an order of magnitude below the published toxicity thresholds (20–100 mg As kg-1). Response to As enrichment in the substrate and subsequent uptake is a species-specific characteristic, and is thought to be moderately higher in crops such as corn and radish; the elevated As uptake of both of these species in this study supports this. Radish plants accumulated more Se than the other crops, with cotton taking up the least. Levels of Se accumulation of these crops stayed below {approx}20 mg kg-1, well below the average Se tissue levels for cultivated crop plants, grains, and native grass species (30 mg kg-1 under field conditions) (Adriano, 2001).

Another point of concern is the likelihood for Mo accumulation within plant tissue to levels that would be toxic to grazing animals. Amendment of Mo-deficient soils would be a valuable application venue for this material, although overapplication to Mo-sufficient soils may prevent grazing end uses, especially where leguminous plants are used.

Boron was the only element that is found to increase significantly in mesocosm leachate and which remained elevated throughout the sampling period. With a maximum leachate concentration of 172.8 (±55) µg L-1 (121 d after application of 10% FGD) the B concentration remains below the desirable upper limit for drinking water standards (1 mg L-1), and water intended for long-term irrigation of sensitive crops (0.75 mg L-1). Monitoring the physical and chemical characteristics of the soil–FGD mixture showed a long-term increase in pH and EC of mesocosm leachate. There was a significant increase in leachate salinity, there was a steady decrease during the monitoring period, and from this study it is reasonable to assume that after a sufficient equilibration period, salinity will return to near background levels. Postharvest soil samples showed that pH increases were strongly related to FGD application. This pH shift may prove beneficial where surface acidity is a problem or in situations where metal bioavailability is a concern. While mesocosm leachate did not contain significantly elevated concentrations of either As or Se, carryover effects of FGD addition remain in the form of increases in the metal loading of the soil. With repeated applications of FGD that contain particularly high As and Se concentrations, food chain transfer and human health problems may result. The As and Se added to the soil following incorporation of FGD are within the normal range of concentrations on a nationwide scale; in general total soil As varies between 0.2 and 40 mg kg-1, whereas in most cases total soil Se is between 0.2 and 2.0 mg kg-1; between 1 and 6 mg Se kg-1 is considered normal (Adriano, 2001).

The alteration in soil pH as a result of FGD addition alters the bioavailability of many important trace elements. Boron generally exists in the soil as neutral or negatively charged ions that have a tendency to leach freely through the soil profile, and where conditions are more alkaline B uptake is generally suppressed. Arsenic availability is strongly influenced by the addition of high concentrations of gypsum (CaSO4 · 2H2O), due to the competition between sulfate and arsenate ions for sorption sites. This may have been a factor in the high tissue As concentrations in this study. Similarly, changes in pH altered the availability of the plant-essential Mn, a frequently observed effect of excessive liming, with tissue Mn concentrations in all crop species approaching deficiency levels. Supplementary levels of Mo from the FGD material resulted in increased concentrations of this element within the shoot and root material of soybean, possibly due to the increased Mo requirement typical of legumes, although corresponding yield increases were not observed. It is suggested that to raise the beneficial effects of FGD by-product application, and to avoid problems with trace element toxicity, rates may not exceed 2.5%.


   ACKNOWLEDGMENTS
 
The authors thank Brad Reinhart and Dr. Brian Jackson for technical assistance. This research was supported by Financial Assistance Award no. DE0FC09-96SR18546 from the U.S. Department of Energy to the University of Georgia Research Foundation.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 EXPERIMENTAL APPROACH
 RESULTS
 DISCUSSION
 REFERENCES
 
This study was funded by the Electrical Power Research Institute.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL APPROACH
 RESULTS
 DISCUSSION
 REFERENCES
 


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