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Impact of application of coal combustion products to soil on soil characteristics, concentration of elements in plant material and crop product safety

Colin Birch1,2, Doug George and Premawansa Dissanayake

1 The University of Queensland, Gatton Campus, Gatton, 4343
2
Present Address The University of Tasmania, Burnie Campus, Burnie, 7320

Abstract

Field and glasshouse experiments using coal combustion products (CCP) from Tarong and Millmerran power stations have been conducted since 2005 to assess effects of their use in land management on soil properties, uptake of nutrient and non-nutrient elements, crop safety and crop yield – this paper concentrates on the first three, drought having limited crop yield responses. CCP from the flue (‘fly ash') and furnace (bottom ash') have been applied at up to 80 t/ha to ferrosols and vertosols. In glasshouse studies, soils have been cropped repeatedly using chick pea, cowpea and peanut, while maize, sorghum and peanut have been grown in field trials. The impact of CCP application on soil physical and chemical properties was assessed. Elemental concentrations in vegetative and reproductive plant parts were determined by plant analysis. There has been very little or no evidence of increased uptake of elements that may cause adverse impacts on plants (eg B, Se) or plant products (eg. Cd), concentrations being well within prescribed limits. Decreases have also occurred. No evidence of increase in uptake of either nutrient or non-nutrient elements from year to year was found, and reduced availability of manganese in ferrosols, probably due to liming effects of the CCPs has occurred. Fly ash increased soil pH in vertosols (from 6.1 to 6.6) and ferrosols (5.2 to 5.4). The application of CCPs from the two power stations is discussed from environmental and product safety viewpoints.

Key Words

Coal ash, resource recovery, coal management, soil quality

Introduction

Increasing demand for electricity in Australia is predominantly being satisfied by burning of fossil fuels, principally coal. Coal fired power stations produce substantial quantities of coal combustion products (CCP), the non-combustible mineral components of coal. CCP contain heavy metals that can enter the environment, add to the environmental load of them and may be taken up by plants in quantities presenting a potential health risk (Adriano et al. 2002). CCP can be used in industrial applications e.g. manufacture of concrete but the proportion of production used is small (Kirk et al. 2003). Hence, new beneficial uses of CCP must be found in primary or secondary industries to reduce disposal in land fill. For example, CCP has been shown to have potential for restoration of degraded land when mixed with biosolids (Robinson et al. 2003). Land management in agriculture provides a potential opportunity to use CCP for soil amelioration provided benefits can be shown at an acceptable level of risk. Benefits could include improved water infiltration and storage and greater plant root proliferation, which should result in higher dry matter and grain yield. The concentration of heavy metals in plants must be assessed against established safety standards in USEPA (1995), NH&MRC (1999) and EnHealth (2001) over an extended period.

Metal and non-metal ions can move from CCP particles to soil by displacement from cation exchange sites on both fly ash (very fine CCP recovered from the gas stream) and bottom ash (CCP from the coal furnace), slow dissolution of the CCP and by diffusion from within the particles. Little quantitative assessment of the risk to environmental safety or accumulation of heavy metals in plants is evident from earlier research (Adriano et al. 2002). The principal objectives of this research are to assess the effects of application of fly ash and bottom ash from Tarong and Millmerran power stations on (i) soil physical and chemical properties and (ii) elemental composition of above ground plant material.

Materials and Methods

Glasshouse experiments design and cultural conditions

Glasshouse trials were conducted using a vertosol from Gatton and ferrosol, from Kingaroy, Queensland. Fly ash and bottom ash were mixed into the top 10 cm of the soil at 0, 10, 20, 40 and 80 t/ha, prior to planting of chickpea (Cicer arietinum) in 2005. Three replicates arranged in randomised complete blocks were used, with each soil considered a separate experiment. Two plants were grown in 25 cm pots, irrigated regularly and sufficient fertiliser applied to ensure non-limiting water and nutrient supply, In subsequent years, soils treated initially with 0, 20 and 80 t/ha of fly ash and bottom ash were replanted with cowpea (Vigna unguiculata) in 2006-07 and peanut (Arachis hypogaea) (in the ferrosol only) in 2007-08. One plant was harvested per pot in the mid vegetative stage of each crop, the other grown to maturity. At both sampling times, dry matter yield of whole plant tops was determined.

Field experiments design and cultural conditions

Field trials were established in commercial farms at several rainfed sites in southern Queensland (Table 1).

Table 1. Field trials using fly and bottom ash at Kumbia and Millmerran from 2005 to 2008.

Site

Soil

Years

Ash Source

Ash Type

Ash rates (t/ha)

Crops

Kumbia A

Ferrosol

2005-08

Tarong

Fly

0, 10.5, 21, 35, 70

Peanut-maize-peanut

       

Bottom

0, 5.5, 11, 22, 44

Peanut-maize-peanut

Kumbia B

Ferrosol

2005-08

Tarong

Fly

0, 10.5, 21, 35, 70

Maize-peanut-maize

       

Bottom

0, 5.5, 11, 22, 44

Maize-peanut-maize

Millmerran

Vertosol

2006-08

Millmeran

Fly

0, 5.5, 11, 22, 44

Sorghum - sorghum

       

Bottom

0, 5.5, 11, 22, 44

Sorghum - sorghum

Coal ash was applied by Gason spreader to 20 m x 6 m (Kumbia, bottom ash) or 20 m x 12 m (Millmerran, fly ash and bottom ash) plots arranged in randomised complete blocks and 50 m x 20 m demonstration plots (Kumbia, fly ash). The surface 10 cm of soils were assessed prior to ash application months by standard analytical procedures for pH, organic carbon, conductivity, and extractable nutrients and Al. Soil water holding capacity at pF2 and pF4.2 was measured by pressure plate and soil bulk density by standard laboratory procedures. Tests were repeated in each cropping season. Crops were planted using the cooperating farmers’ equipment, and fertilised according to farmers’ commercial practice. At both sampling times, dry matter yield of whole plant tops was determined. In field experiments, plants were harvested during the mid-vegetative stage, flowering (some crops), during grain fill (some crops) and at maturity, and separated into vegetative and reproductive parts as appropriate. Plants were analysed for elemental composition of whole plants during the vegetative stage and in reproductive plant parts and residue at maturity, using standard sample preparation and digestion procedures. Nutrient concentrations in plant digests were assessed using ICP at The University of Queensland Central Analytical Facility. Plant samples from the control and highest rates of ash application were analysed for concentration of Al, B, Cu, Fe, Mn, Zn, Cr, Co, Be, Ni, As, Se, Cd, Pb, Sb, Sr, Mo, Ag, Sn, Ba and V, all being present though mostly in low concentrations in CCP, plus Ca, P and K.

Results

Soil properties

There were small changes in extractable Al (reduced by bottom ash from 41.5 to 35.1 mg/kg) and B (increased from 8.4 to 11.1 mg/kg by fly ash) at Millmerran at the highest rate of CCP application. No other nutrients were affected significantly at this site. Both fly ash and bottom ash reduced extractable Mn from initially quite high values (1500 – 2000 mg/kg) by 30% and fly ash reduced extractable Ba from 286 to 223 mg/kg in the ferrosol. Only small changes in other soil chemical characteristics occurred, the most important being fly ash increasing soil pH in the vertosol at Millmerran from 6.1 to 6.6 and ferrosol at Kingaroy from 5.2 to 5.4 after one year of exposure to the highest application rate. There have been very small and mostly non-significant changes in soil bulk density while plant available water content increased significantly in ferrosol soil treated with fly ash.

Concentration of elements in plants

There were some significant effects of application of CCP in 2005 on concentration of nutrients in whole plants and plant parts (Table 2). In addition, there were few effects of bottom ash on concentration of elements in peanut in 2006-07 – small reduction in concentration of Al and small increase in concentration of Ca in fruits, but no effects on concentrations in tops. In addition to data in the table, there were substantial accumulations of Sr in peanut tops in both the control (161 000 μg/kg) and treated plants (174 000 μg/kg), but the concentration in kernels remained low (<3200 μg/kg) in the ferrosol. Similar trends occurred with Ba, the respective figures being 254 000 μg/kg, 294 000 μg/kg and near 3000 μg/kg.

Table 2. Significant effects of application of bottom ash and fly ash in 2005 to 2007 on concentration of nutrients in whole plants and plant parts (mg/kg). (+ or - sign indicates increase or decrease in concentration).

Site, year, soil, crop, plant part

Significant change - bottom ash

Concentration in Control

Concentration in highest bottom ash rate

Significant change - fly ash

Concentration in Control

Concentration in highest fly ash rate

Glasshouse, 2006, vertosol, cowpea, plant tops

Cu (+)

7.5

13.2

Fe (+)

88

107

Zn (+)

41

57

Mn (+)

53

75

Co(+)

0.39

0.84

Cr (+)

0.39

2.81

Cr (+)

1.64

3.30

Mo (+)

0.83

1.84

Ni (+)

0.56

1.60

Pb (-)

1.04

0.59

Cd (-)

0.11

0.03

     

Pb (-)

1.04

0.64

     

Glasshouse, 2006, ferrosol, cowpea, plant tops

Al (-)

64

44

Zn (+)

63

72

Fe (-)

105

85

Cr (-)

0.16

1.27

Co (-)

0.51

0.36

Ni (-)

1.72

0.93

Cr (+)

0.16

0.62

Cd (+)

0.03

0.07

Ni (-)

1.72

0.88

Mo (+)

1.37

2.61

Millmerran, 2006-07, sorghum, grain

Al (-)

13

3.0

Al (-)

13.3

3.3

Cr (-)

2.88

1.76

Cr (-)

2.88

2.01

Ba (+)

0.30

1.00

Cd (+)

0.03

0.08

Millmerran, 2006-07, sorghum, residue

Al (-)

437

177

Al (-)

437

215

Fe (-)

188

115

Fe (-)

188

116

Co (+)

0.97

1.09

As (+)

0.13

0.18

Ca (-)

3800

3500

Cd (+)

0.06

0.17

     

Ca (-)

3800

3100

Kumbia, 2006-07, maize, grain

Ni (+)

0.04

0.32

Mo (-)

0.25

0.13

Ca (+)

30

70

Ba (+)

1.00

1.58

Kumbia, 2006-07, maize, residue

Ni (+)

0.20

1.45

Al (-)

823

335

Ca (+)

3000

3800

Fe (-)

1260

477

     

Zn (+)

15

28

     

Ni (+)

0.70

4.74

     

Mo (-)

0.71

0.31

     

Ba (-)

70.9

46.6

Discussion

The generally small effects on soil physical properties may be related to the severe conditions (prolonged drought) under which field experiments were conducted, as other research indicates that beneficial effects occurred (Wearing et al. 2004). The general lack of effect of CCP on extractable concentrations of elements in soils indicates that changes were of no practical significance. For most nutrient and non-nutrient elements, changes in concentration in plant parts were small and non-significant, and even where increases in the concentrations of elements such as Cd occurred, they have not reached sufficiently high values in reproductive parts to be of concern from a health viewpoint The increases in concentrations of elements such as nickel, chromium (in vertosol), and cobalt (ferrosol) indicate that these elements are becoming available for uptake by plants following diffusion, solubilisation and exchange from CCP or solubilisation of soil sources following change in soil pH. However, the latter have been small, so CCP emerges as the main source.

Decreases in concentrations of Fe, Mn, Mo and increases in concentrations of some nutrients are also likely to be due to the alkalinising effect of CCP, especially fly ash, and consequent changes in solubility of nutrients. High concentrations of Sr and Ba in some crops and soils in control and CCP treatments must be related to background levels and soil chemistry, as application of CCP did not increase them in plant tops, and in ferrosol reduced Ba concentration occurred in maize tops. The results indicate that there are few, if any, adverse environmental or product quality risks under rainfed production conditions over the time frame of the experiments reported here. Continued studies on the experimental areas are required to quantify long term effects. There is also evidence of beneficial change to nutrient and non-nutrient elements in plants.

Acknowledgement

The financial support of Cement Australia for this research is gratefully acknowledged.

References

Adriano DC, Weber J, Bolan NS, Paramasivam S Koo B and Sajwan, K. S. (2002) Effects of high rates of coal fly ash on soil, turfgrass, and groundwater quality, Water, Air and Soil Pollution 139: 365-85.

EnHealth 2001. Guidelines for assessing human health risks from environmental hazards. EnHealth Council, Canberra.

Kirk DW, Jia CQ, Yan J and Torrenueva AL (2003) Wastewater Remediation Using Coal Ash J Mater Cycles Waste Management 5: 5-8.

NH&MRC (National Health and Medical Research Council) (1999). Toxicity assessment for carcinogenic soil contaminants NH&MRC Canberra.

Robinson TJ, Spark KM and Swift RS (2003) The potential for biosolids and coal fly ash mixtures for the revegetation of degraded lands, Faculty of Natural Resources, Agriculture and Veterinary Science, The University of Queensland, 2003.

USEPA (1995) Proposed Guidelines for Ecological Risk Assessment. EPA/630/R-95/002. US Environmental Protection Agency, Washington.

Wearing C, Birch CJ, and Nairn JD (2004) An Assessment of Tarong Bottom Ash for Use on Agricultural Soils, Developments in Chemical Engineering and Mineral Processing. 12: 1-14.

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