Abstract
Coal fly ash (FA) has a potential to be used as a soil amendment for growing plants. Toxicity of heavy metals present in FY, FA high salinity, and high pH of coal FA may potentially restrict or even prevent plant growth on the media with high concentration of FA. Sphagnum peat moss (SPM) shows a potential to ameliorate coal FA based plant media by improving the texture of such media, making media less harder, decreasing high pH of the media, and potentially binding heavy metals present in FA. Therefore, the aim of this study was to determine the effects of growth media containing differing concentration of FA and/or sphagnum peat moss (SPM) on growth of selected plants. The following plant species have been tested: barley (Hordeum vulgare), oats (Avena sativa), rye (Secale cereale), wheat (Triticum aestivum), Regreen; a hybrid between wheatgrass (Agropyron cristatum) and winter wheat (Triticum aestivum), Triticale; a hybrid between wheat (Triticum aestivum) and rye (Secale cereale),, and perennial ryegrass (Lolium multiflorum). The addition of SPM to FA based plant growth media expressed ameliorative role, allowing the growth of seedlings on such media. In addition, our results indicate that the transfer of heavy metals from coal FA to plants and possibly to a food chain either did not exist or was very low.
Keywords: coal ash, cereal crops, heavy metals, plant growth
INTRODUCTION
Coal fly ash (FA) is constantly generated as a by-product by coal or thermal power stations (Kumar et al. 2010). Fly ash belongs to the most challenging waste materials in the world, and it may contribute to degradation of arable land and contamination of air, water, soil and organisms in the zone of influence.
The potential for heavy metal contamination of groundwater in and around coal FA disposal sites is one of the main areas of concern regarding proper disposal of these by-products (Bilski et al., 1995). Besides the use of coal FA for construction purposes, soil amendments are the most extensively studied reutilization option (Openshaw et al. 1992, Kramer, 2007). In fact, several studies have shown that FA can be used as a soil amendment for growing plants (Bilski et al. 2011). Toxicity of heavy metals present in FA, FA high salinity and pH may potentially restrict or prevent plant growth on the media with high concentration of FA (Bilski et al. 1995). The pozzolanic nature of coal FA could cause plant growth media to harden in the presence of its high content, thereby making it difficult to support plant growths (Alva et al., 2000).
Sphagnum peat moss (SPM) shows a potential to ameliorate coal FA based plant media by improving the texture of such media, making media less harder, decreasing high pH , and potentially binding heavy metals present in FA (Sheng-Fu et al. 2005). This potential was a rationale for proceeding with our research.
Many herbaceous plants, primarily grasses, which exhibit rapid growth, are moderately resistant to environmental stress, and are therefore often used as cover crops in environmental restoration and remediation projects (Bilski and Foy, 1987, Koo et al. 2006). It constitutes the rationale for choosing rye, barley, oats, and regreen to be grown on coal ash based media in our experiments.
We hypothesized that growth media composed of FA, SPM and soil will support growth for selected plants, and that it will allow for amelioration of FA. Therefore, the aim of this experiment was to determine the effects of growth media containing differing concentration of FA, SPM and/or soil on growth of several plant species including rye, barley, oats, and regreen, and to determine concentration of heavy metals including Ca, Cu, Mn, Fe, Zn, Pb, Cr, B and Se in growing plants and growth media.
MATERIALS AND METHODS
Two selected coal FA, from semi-bituminous coal from Montana and from North Dakota lignite were used as a supplement to the growth media.
The following plant growth media were used:
Soil (Fargo clay) as a control
FA from semi-bituminous coal from Montana (NDSU FA1)
50% FA NDSU + 50% SPM (weight based)
30% FA NDSU + 30% SPM + 30% soil
FA from North Dakota lignite coal (VCSU FA2)
50% FA VCSU + 50% SPM
-
30% FA VCSU + 30% SPM + 30% soil
1NDSU FA – coal FA obtained from North Dakota State University power plant, Fargo, North Dakota, USA
2VCSU FA – coal FA obtained from Valley City State University power plant, in Valley City, North Dakota, USA
Four plant species have been tested: barley (Hordeum vulgare), oats (Avena sativa), rye (Secale cereale), and Regreen, a hybrid between wheatgrass (Agropyron cristatum) and winter wheat (Triticum aestivum).
The rationale for choosing listed above plant species was the diversity of its nutritional requirements combined with the fact that these species are resistant to unfavorable environmental conditions and these plants are popular cereal crops. Fibrous root system of cereal crops should contribute to their ability to establish growth on media with very fine particles, providing only marginal support for emerging seedlings.
On every Petri dish, 30 seeds were planted and covered with thin layer of growth media, and watered to approximate field capacity of growth media. Plants were grown for 21 days, harvested, dried, and weighed. The concentrations of Ca, Cu, Mn, Fe, Zn, Pb, Cr, B and Se in growth media was determined, and the concentrations of the same elements in young plants were analyzed after digestion of combined plant samples (one sample consisting of 3 replication of every treatment). Before the digestion plant samples were washed (to remove possible adhering FA particles), oven dried at 60° C to constant weight, and grounded to pass a 0.841 mm screen. Plant samples were wet-digested in a nitric-perchloric acid mixture prior to elemental analysis (Carlson and Adriano 1991)
Chemical analysis was performed using inductively coupled plasma (ICP) emission spectrophotometry (Bilski and Alva, 1995). For statistical evaluations, the result of plant dry weight and every element tested in the soil and plant tissue will be treated as separate experiments in performing analysis of variance. The data will be analyzed statistically using ANOVA and Statistical Analysis System (SAS, 2005).
RESULTS
Weight of several plants grown on different growth media is presented on Fig. 1–4.
Fig. 1.
Weight of barley.
a,b,cP<0.05 values with different superscripts differ.
Fig. 4.
Weight of regreen plants.
a,b,cP<0.05 values with different superscripts differ.
The weight of barley plants was similar for growth media composed of 100% soil or containing NDSU FA with or without soil or SPM. However, weight of barley was less (P<0.05) in growth media containing VCSU FA than in soil control. In addition, plants seeded on VCSU FA did not grow.
The weight of rye was similar for growth media composed of 100% soil or containing VCSU FA with SPM, and was greater (P<0.05) for growth media containing NDSU FA with SMP and soil than in soil control. However, rye plants seeded on 100% NDSU FA, NDSU FA with SMP, 100% VCSU FA or VCSU FA with SMP and soil did not grow.
The weight of jerry oats was similar for growth media composed of 100% soil or containing NDSU FA with SPM and soil, and was less (P<0.05) for growth media containing 100% NDSU FA, NDSU FA with SMP or VCSU FA with SPM than in soil control. However, jerry oats plants seeded on 100% VCSU FA or VCSU FA with SMP and soil did not grow.
The weight of regreen was similar for growth media composed of 100% soil or containing NDSU FA with SPM or NDSU FA with SPM and soil, and was less (P<0.05) for growth media containing 100% NDSU FA, VCSU FA with SMP or VCSU FA with SPM and soil than in soil control. However, regreen plants seeded on 100% VCSU FA did not grow.
Concentration of several elements in growth media and harvested plans is presented in Tables 1–8.
Table1.
The concentration of As in growth media and in plants (mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 4.5a | 4.7 | 0.0a | 0.67a | 4.8a |
| VCSU FA | 157.0b | ng | ng | ng | ng |
| NDSU FA | 1.6a | 0.0a | ng | 9.3b | 0.0b |
| 50% VCSU FA+50% SPM | 105.3c | 2.9b | 4.4b | 4.0c | 5.8a |
| 33% VCSU FA+33% SPM+33% soil | 23.8d | 5.8c | ng | ng | 2.9c |
| 50% NDSU FA+50% SPM | 3.1a | 2.2d | ng | 2.8c | 2.2c |
| 33% NDSUFA+33% SPM+33% soil | 3.3a | 2.0d | 0.6a | 2.0c | 2.0c |
P<0.05, Values with different superscripts differ within a column; ng - no germination.
Table 8.
The concentration of Tl in growth media and in plants (mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 0.2a | 24.3a | 23.2a | 23.7a | 24.2a |
| VCSU FA | 6.9b | ng | ng | ng | ng |
| NDSU FA | 9.5c | 24.6a | ng | 83.5b | 23.8a |
| 50%VCSU FA+ 50% SPM | 5.4d | 23.7a | 23.9a | 30.8c | 48.1b |
| 33% VCSU FA+33% SPM+33% soil | 4.5d | 22.8a | ng | ng | 69.5c |
| 50% NDSU FA + 50% SPM | 3.9d | 24.9a | ng | 22.9a | 23.2a |
| 33% NDSU FA+ 33% SPM+ | 1.1e | 22.8a | 22.8a | 24.2a | 24.1a |
P<0.05 Values with different superscripts differ within a column.
The greatest concentration of As was in VCSU FA alone, and growth media containing VCSU FA. The concentration of As in plant growing media ranged from 1.6 mg/kg in NDSU FA, through 4.6 mg/kg in the soil (our control treatment), to a 157 mg/kg in VCSU FA, and in plants from not detectable level to 5.8 mg/kg. Concentration of As in plants did not reflect level of As in growth media.
The highest concentration of B was in VCSU FA, and it was ~100-fold greater than in the soil and ~3 fold greater than in NDSU FA. The concentrations of B in plant growth media ranged from 22 to 2274 mg/kg. The concentration of B in plants in all media containing FA was greater than in soil control. Overall, oats seedlings expressed the tendency to accumulate the highest amounts of boron.
The highest concentration of Fe was in VCSU FA, ~2-fold greater than in the soil and ~1.5-fold greater than in NDSU FA. The concentrations of Fe in plant growth media ranged from 6,500 to 41,220 mg/kg. Overall, barley seedlings expressed the tendency to accumulate the highest amounts of boron.
The concentration of Mo was significantly higher in coal FA based media than in soil control. The concentrations of Mo in plant growth media ranged from 1.3 to 14.2 mg/kg. Greater accumulation of Mo was observed for seedlings grown on FA based media, as compared to soil control.
The concentration of Zn was greater in higher in soil control than in coal FA based media. Accumulation of Zn in seedlings was affected by media type. The concentrations of Zn in plant growth media ranged from 27.2 to 131.5 mg/kg, and it seems that it did not depend on Zn concentration in soil.
The concentration of Sr in all media consisting of coal FA was up to 124-fold greater than in soil control, and was greater in VCSU FA than in NDSU FA. The concentrations of Zn in plant growth media ranged from 48 to 282 mg/kg. The accumulation of Sr in plant tissues seems e not to reflect Sr concentration in growth media.
The concentration of Ti in all growth media with coal FA was greater than in soil control. The concentrations of Ti in plant growth media ranged from 7.1 to 35.4 mg/kg, All tested cereal crops appeared not to hyper accumulate Ti from growth media.
The concentration of Tl was greater in growth media based on coal FA than in soil control. The concentrations of Tl in plant growth media ranged from 22.8 to 69.5 mg/kg. The accumulation of Tl in plant tissues seems not to reflect Tl concentration in growth media.
DISCUSSION
The present study demonstrated that growth of selected cereal plants on media containing FA was suppressed. Inhibitory effects on plant growth were especially observed in growth media consisting of VCSU FA, or containing VCSU FA. Out of tested plants, rye growth was suppressed by all growth media containing FA. However, barley growth was not affected by addition of NDSU FA to growth media, but was suppressed by presence of VCSU FA in growth media. Addition of SPM with or without soil reversed inhibitory effects of NDSU FA on growth of rye and jerry oats, and VCSU FA on rye growth. It is with an agreement with findings of other research trials (Bilski et al. 2011, Kramer 2007, Chu 2008). These experiments demonstrated that plant response to FA in growth media depends not only on plant species, but also to a chemical composition of coal FA.
The concentration of As in plant growth media was up to 157 mg/kg. This As concentration level meets the standards for considering plant growth media as phytotoxic (Kabata-Pendias and Adriano, 1995). Such high concentration occurred only in VCSU FA, but As level in NDSU FA was 1.6 mg/kg lower than in soil control (4.5 mg/kg). Background concentrations of As in soil range from 1 to 40 mg/kg, with an average value of 5 mg/kg (Beyer & Cromartie, 1987), thus, the level of As in our soil control was very close to the average level determined for soil. In our experiment elevated levels of As concentration in growth media was not reflected by As concentration in plant tissues. The level of As in plant seedlings in our study reached 5.81 mg /kg, which is almost within the range of As concentration in plants grown in natural conditions (DeKoe 1994).
The concentration of B in all growth media containing coal FA reached 2274 mg/kg in VCSU FA and exceeded several-fold values present typically in the soil (usually up to 26- 33 mg/kg; ATSDR, 2007) Coal FA based media in our experiments contained significantly higher amounts of B than noted in literature (Mikkelsen and Camberato, 1995). Coal FA could be an effective source of B for plants (Bilski and Alva, 1995). The physiological B concentration is in the range of 5–30 mg/kg. However, the range between adequate supply and toxicity for B is quite narrow (Kabata-Pendias and Mukherjee, 2010). In our experiment, B concentration in plants reached 608 mg/kg which indicates toxic level. In addition, plants in our experiment expressed the most common symptom of excessive B accumulation, such as necrosis along leaf margins and at the growing points, reflecting the buildup of B following the transpiration stream (Mikkelsen and Camberato, 1995). Thus, in our experiment, seedlings limited growth or no growth of plants on media containing FA was due to B toxicity.
In our experiment, the concentration of Fe in growth media was 1.8%. The common range of Fe contents in soils is between 0.1 and 10% (Kabata-Pendias and Mukherjee 2010a), and Although the concentration of Fe in FA was higher than in soil control, it didn’t exceed 10% - the highest value naturally occurring in soils. It indicates, that the levels of Fe present in all our growth media and accumulated in plant tissues was not toxic.
In the present experiment, concentration of Mo in growth media containing FA was several-fold greater than in soil control. High concentration of Mo is generally associated with alkaline soils. Mo concentrations in agricultural soils can reach 50 mg/kg (Hornick et al., 1977). Soil control pH in our experiments were alkaline (pH 8.3–8.5), but Mo concentration in soil used in our study was not elevated. Despite this, plants grown on media with elevated concentration of Mo showed significantly elevated accumulation of Mo, as compared to plants grown on the soil. Concentrations of Mo in plant leaves typically range from 0.1 to 1.5 ppm on a dry matter basis, and in our studies plants grown on Mo rich media very significantly exceeded this concentration reaching 22.8 mg/kg in oats grown on media composed of 50% VCSU FA and 50% SPM. Concentrations of Mo in plants above 10 mg/kg might be toxic for animals fed with these plants (Kabata-Pendias and Mukherie, 2010).
Our results (Table 5) indicate that the concentration of Zn in coal FA based plant growth media was lower than in the soil control, but all values were within mentioned above acceptable limits. As a result of relatively low Zn concentration in growth media, Zn uptake by plant seedlings remained within acceptable limits (Adriano, 1986).Average Zn concentration in non-contaminated soils range from 40 to 90 mg/kg. Low levels are found in sandy soils (10–30 mg/kg), while high contents are found in clays (95 mg/kg; Adriano, 1986). Physiological l levels of Zn in most crops and pastures range from 10 mg/kg to 100 mg/kg (Mortvedt & Gilkes, 1993).
Table5.
The concentration of Zn in growth media and in plants ( mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 40.5a | 83.5a | 54.7a | 84.9a | 73.3a |
| VCSU FA | 32.9b | ng | ng | ng | ng |
| NDSU FA | 27.2c | 86.7a | ng | 76.3b | 73.7a |
| 50%VCSU FA+ 50% SPM | 36.8b | 82.7a | 41.4b | 84.5a | 93.7b |
| 33% VCSU FA+33% SPM+33% soil | 25.6c | 89a | ng | ng | 131.5c |
| 50% NDSU FA + 50% SPM | 16.7d | 71.7b | ng | 74.8b | 73.6a |
| 33% NDSU FA+ 33% SPM+ | 21.1c | 81.3a | 37.2b | 73.1b | 91.3b |
P<0.05 Values with different superscripts differ within a column; ng - no germination.
Strontium is found naturally in soil in amounts that vary over a wide range, but the typical concentration is 250 mg/kg. The disposal of coal FA, incinerator ash, and industrial wastes may increase the concentration of Sr in soil (ATSDR, 2004). The concentration of Sr in coal FA, on the other hand, may vary depending on FA source and be as low as < 1mg/kg and as high as 3,900 mg/kg (Mattigod et al., 1999). In our experiment, the concentration of Sr in the soil (33 mg/kg) was below mentioned above typical concentration level 250 mg/kg. Sr concentration in coal ash in our study (Table 6) was also within the typical for FA limits for NDSU FA but in VCSU FA exceeded a typical concentration for FA. The levels of plant accumulation of Sr reflected the higher concentration in FA based media, and for plants grown on media containing FA reached the level of 784 mg/kg. Despite such high accumulation of Sr in plants, it does not seem to create any toxic effects, because frequently Sr levels in forage crops reach even higher values than in our experiments (Kabata-Pendias and Adriano, 1995).
Table6.
The concentration of Sr in growth media and in plants ( mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 33a | 121a | 75a | 152a | 83a |
| VCSU FA | 4099b | ng | ng | ng | ng |
| NDSU FA | 2833c | 282b | ng | 784b | 477b |
| 50%VCSU FA+ 50% SPM | 3469d | 76c | 214b | 99c | 48c |
| 33% VCSU FA+33% SPM+33% soil | 842e | 68c | ng | ng | 781a |
| 50% NDSU FA + 50% SPM | 1673f | 149a | ng. | 224d | 147d |
| 33% NDSU FA+ 33% SPM+ | 805e | 126a | 159c | 186a | 173d |
P<0.05 Values with different superscripts differ within a column; ng - no germination
Heavy clay soils, like the soil used in our experiment (Fargo clay) contain more titanium (Ti) than sandy soils (Monier-Williams, 1950, Kabata-Pendias and Mukherjee, 2010), and the average concentration in soil appears to be below 5 g/kg. Our soil contained only a fraction of this amount, and also FA used in our experiment didn’t contain more than 5 g Ti per kg. The concentration of Ti in plants was also within physiological limits (Anke and Seifert, 2004).
Thallium content of soils is controlled by parent material as well as by contamination, and its concentration varies from 0.01 to 2.8 mg/kg Takeda et al., 2004), but it is enhanced in loamy and organic soils. Our results (Table 8) indicate a moderate Tl concentration of 0.2 mg Tl/kg, but the concentration of Tl in FA reached 9.5 mg/kg. The concentration of Tl in young plant tissue in plants grown on media with FA reached almost 70 mg Tl/kg, which is unusual and may cause further plant growth inhibition and a decrease of chlorophyll concentration in growing plants (Tyler, 2005, Kabata-Pendias and Mukherjee, 2010).
CONCLUSIONS
Our results demonstrated that the addition of SPM to coal FA and soil can provide beneficial conditions for the growth of selected plans, especially for barley and regreen. Thus, SPM shows potential as ameliorative factor for FA utilization as plant growth media. In addition, our results indicate that the transfer of heavy metals from coal FA to plants and possibly to a food chain either did not exist or was very low. However, additional in depth research should be undertaken to test possible toxic effects on further plant growth, and on animal health if these plants would be used as a feed in order to make final recommendations.
In our experiment, we intentionally utilized VCSU FA with very high concentration of heavy metals out of several coal FA tested. Although, we did not get any plant growth on VCSU FA, we were able to demonstrate, that using additives such as SPM and/or soil we might be able to utilize FA with high heavy metals concentration for purposes related to agriculture purposes. The long-term significance of our research is based on the contribution to the stabilization of coal FA piles by covering such piles with plants. It would help to avoid toxic leaching from coal FA piles and to prevent wind erosion and air contamination of fly ash dust.
Fig. 2.
Weight of rye.
a,bP<0.05 values with different superscripts differ.
Fig. 3.
Weight of oats plants.
a,b,cP<0.05 values with different superscripts differ.
Table2.
The concentration of B in growth media and in plants (mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 22a | 48a | 20a | 55a | 19a |
| VCSU FA | 2274b | ng | ng | ng | ng |
| NDSU FA | 903c | 280b | ng | 238b | 345b |
| 50%VCSU FA + 50% SPM | 1676d | 448c | ng | 399c | 363b |
| 33% VCSU FA+33% SPM+33% soil | 358e | 372d | ng | ng | 371b |
| 50% NDSU FA + 50% SPM | 364e | 474c | 483b | 591d | 464c |
| 33% NDSU FA+ 33% SPM+33% soil | 195f | 341d | 305c | 608e | 498d |
P<0.05 Values with different superscripts differ within a column; ng - no germination
Table3.
The concentration of Fe in growth media and in plants ( mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 18610a | 840a | 397a | 527a | 650a |
| VCSU FA | 41220b | ng | ng | ng | ng |
| NDSU FA | 6500c | 295b | ng | 334b | 199b |
| 50%VCSU FA+ 50% SPM | 25800d | 897a | 264 | 643c | 262c |
| 33% VCSU FA+33% SPM+33% soil | 16436a | 589c | ng | ng | 325d |
| 50% NDSU FA + 50% SPM | 10704a | 362d | ng | 214d | 308d |
| 33% NDSU FA+ 33% SPM+33% soil | 12696a | 360d | 203b | 234d | 276c |
P<0.05 Values with different superscripts differ within a column; ng - no germination
Table4.
The concentration of Mo in growth media and in plants (mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 0.52a | 2.9a | 1.3a | 4.2a | 1.36a |
| VCSU FA | 6.28b | ng | ng | ng | ng |
| NDSU FA | 3.4c | 4.4b | ng | 12.0b | 3.7b |
| 50%VCSU FA+ 50% SPM | 5.1b | 13.7c | 14.5b | 22.8c | 3.6b |
| 33% VCSU FA+33% SPM+33% soil | 1.6d | 10.7c | ng | ng | 2.5c |
| 50% NDSU FA + 50% SPM | 2.68c | 8.2d | ng | 12.1b | 14.2d |
| 33% NDSU FA+ 33% SPM+33% soil | 1.35d | 10.3c | 6.4c | 11.8b | 7.5e |
P<0.05 Values with different superscripts differ within a column; ng - no germination
Table 7.
The concentration of Ti in growth media and in plants (mg/kg−1).
| Media | Growth media |
Barley | Rye | Jerry Oats |
Regreen |
|---|---|---|---|---|---|
| Soil | 41a | 35.4a | 10.5a | 23.6a | 20.4a |
| VCSU FA | 1444b | ng | ng | ng | ng |
| NDSU FA | 1440b | 29.8b | ng | 44.1b | 22.9a |
| 50%VCSU FA+ 50% SPM | 965c | 26.1b | 18.1b | 23.2a | 7.1b |
| 33% VCSU FA+33% SPM+33% soil | 224d | 25.9b | ng | ng | 9.2b |
| 50% NDSU FA + 50% SPM | 770e | 23.2b | ng | 14.8c | 11.6b |
| 33% NDSU FA+ 33% SPM+ | 449f | 15.4c | 18.1b | 14.3c | 22.1a |
P<0.05 Values with different superscripts differ within a column.
Acknowledgments
Supported by North Dakota INBRE Grant Number P20 RR016741 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH).
The authors would like to thank Dr. Marinus Otte and Dr. Donna Jacob for performing chemical analysis of FA samples and plants, and Dr. Anna T. Grazul-Bilska for consultation and constructive comments; all from North Dakota State University, Fargo, ND.
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