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Published online 6 July 2006
Published in J Environ Qual 35:1260-1268 (2006)
DOI: 10.2134/jeq2005.0229
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Plant and Environment Interactions

Root Penetration of Sealing Layers Made of Fly Ash and Sewage Sludge

Clara Neuschütz*, Eva Stoltz and Maria Greger

Department of Botany, Stockholm University, Lilla Frescativ 5, S-10691 Stockholm, Sweden

* Corresponding author (neuschuetz{at}botan.su.se)

Received for publication June 9, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Fly ash and sewage sludge are suggested materials for constructing sealing layers covering mine tailings impoundments. Little is known, however, of their effect on vegetation or resistance to root penetration. We investigate: (i) the ability of different plant species to grow in sealing layers comprising fly ash and sewage sludge, (ii) the impact on plant growth of freshly hardened fly ash compared to aged and leached ash, and (iii) the plant stress response to fly ashes of different properties. A 6-mo greenhouse study using birch (Betula pendula Roth.), Scots pine (Pinus sylvestris L.), Kentucky bluegrass (Poa pratensis L.), and willow (Salix viminalis L.) demonstrated that no roots could grow into a compacted layer consisting only of ash, while a 6:4, ash–sludge mixture admitted roots into the upper part and a 1:9, ash–sludge mixture was totally penetrated (to 15 cm in depth) by roots of willow and Scots pine. Freshly hardened ash prevented root growth more effectively than aged ash did, as was observed in tests using reed canarygrass (Phalaris arundinacea L.) and pea (Pisum sativum L.). Furthermore, extracts of highly alkaline ash were more toxic to pea in a 48-h toxicity test than less alkaline ash was. However, stress responses to diluted ash extracts of lower pH, measured as enzyme capacities in dwarf bean (Phaseolus vulgaris L.), were more related to the metal and ion contents. Root penetration of sealing layers is most effectively prevented if little sewage sludge is added, and if ash of high alkalinity is chosen.

Abbreviations: BA1-3, fly ash from combustion of bio fuel at three thermal power stations in Sweden • EC50, dilution of ash extract causing a 50% reduction in root growth compared to that of control plants • G-6-PDH, glucose-6-phosphate dehydrogenase • GlDH, glutamate dehydrogenase • ICDH, isocitrate dehydrogenase • MSWA, fly ash from a municipal solid waste-fed thermal power station in Sweden • POD, peroxidase • WA, fly ash from a wood construction waste-fed thermal power station in Sweden


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
FORMATION of acidic and metal-rich drainage water is a problem often related to the disposal of sulfidic mine tailings. Sulfuric acid and dissolved metals are produced when metal sulfides, such as pyrite (FeS2), in fine-grained mining waste react with oxygen and water (Rimstidt and Vaughan, 2003). To prevent this, mine tailings should be covered with a material that prevents penetration of air and water. A common technique is to use a compacted sealing layer covered with a protective cover, which protects the sealing layer from external stress, such as drought, frost, and erosion, and supports the establishment of plants (Clemensson-Lindell et al., 1992; Elander et al., 1998). An efficient vegetative layer helps stabilize the cover treatment, by further preventing erosion and decreasing the amount of percolating water (Tordoff et al., 2000). However, it has become obvious that growing plants can also influence sealing layer durability (Simon and Müller, 2004), by creating pores and cracks with their roots. It had been assumed that a compacted sealing layer would prevent root growth, since root elongation decreases rapidly as soil strength increases (Materechera et al., 1991; Clark et al., 1998). Nevertheless, roots have been found growing even in highly compacted sealing layers of various materials, in dry cover treatments of mine tailings impoundments (unpublished data). These results indicate that the ability of sealing layers to resist root penetration may decrease with time, and that there is a need for further studies of the interactions between roots and materials used in dry cover treatments.

At many disposal sites for mine waste, moraine has been used in constructing dry cover layers (Clemensson-Lindell et al., 1992). However, since the mine tailings impoundments often occupy large areas, there is a search for materials, such as industrial waste products, which do not require further exploitation of the environment. For instance, fly ash from thermal power stations has been proposed as a suitable material for use in sealing layers. Its high alkalinity could help neutralize the acidic drainage water, and its pozzolanic ability enables the ash to harden, forming a strong and firm material (Steenari et al., 1999). Another promising waste product suitable for use in protective covers is sewage sludge from waste water treatment plants. It serves as a nutritious substrate for plant establishment (Sopper, 1993), and also as a permeability-decreasing component of sealing layers (Mácsik et al., 2003), since fly ash alone may not have a sufficiently low hydraulic conductivity (Palmer et al., 2000). Sealing layers consisting solely of compacted sewage sludge may become almost impermeable to water (Scandiaconsult Sverige AB, 2001); however, they do not have the same strength or resistance to degrading organisms as fly ash does (Wang and Viraraghavan, 1997), and could possibly increase the risk of root penetration owing to their nutrient content. The best solution is thus believed to be a mixture of the two materials (Mácsik et al., 2003).

Before using fly ash and sewage sludge on a larger scale as components of sealing layers, more knowledge is needed of the effects of growing roots on these materials, and of how vegetation reacts to contact with these materials. The roots of most plants cannot penetrate compacted soils with a mechanical resistance higher than 2 MPa (Materechera et al., 1991), a strength that could easily be achieved by fly ashes with good hardening ability (Fernández-Jiménez and Palomo, 2003). Predicting the hardening ability of a given fly ash is unfortunately not simple, since the quality of fly ashes varies greatly (Reijnders, 2005). In terms of chemical composition, fly ashes generally contain most plant nutrients except N, and will therefore stimulate plant growth when small amounts are added to soils (Demeyer et al., 2001). However, when higher amounts are added toxic responses have been observed in plants, and various explanations have been suggested for these. Toxicity caused by heavy metals has been suggested, based on observations of the induction of phytochelatins and the accumulation of high metal levels in the plants (Kumar et al., 2002; Tripathi et al., 2004). Others have recorded high Na concentrations in plants growing in fly ash, and have referred to the toxic responses as salt effects (Giordano et al., 1983), while nutrient deficiency due to high alkalinity has also been discussed (Wong and Wong, 1989; Gupta et al., 2002). A high pH in soils not only leads to low nutrient availability, but may also have direct toxic effects on plants (Kopittke and Menzies, 2004), such as disintegration of root cells and decreased formation of root hairs (Tang et al., 1993).

In choosing material with which to construct sealing layers it is important to know what factors can most effectively prevent root penetration, and if weathering of the sealing layer by, for example, water, may decrease its ability to withstand root penetration. Furthermore, the effects of adding sewage sludge, to decrease the hydraulic conductivity, on root penetration have yet to be thoroughly investigated. If sludge addition leads to enhanced nutrient conditions, lowered alkalinity, and decreased soil strength, the risk of root penetration will increase. Mixtures of sewage sludge and fly ash are reported to have beneficial effects on plant growth, and have therefore been used as nutrient-rich plant substrates (Rautaray et al., 2003; Sajwan et al., 2003; Schumann and Sumner, 2004).

This study examines the ability of roots of different plant species to penetrate sealing layers containing fly ash, with or without added sewage sludge. Furthermore, we wanted to determine whether the prevention of root penetration is due to material compaction, high pH, or rather the release of toxic substances from the fly ash. Our hypothesis was that a high degree of compaction of the fly ash is the key factor inhibiting root growth, assisted by high alkalinity and high salt and heavy metal contents; consequently, the aging and leaching of hardened fly ash can decrease the inhibiting effect on root growth. Furthermore, we hypothesized that adding a higher amount of sewage sludge would lead to increased root penetration, due to improved nutrient levels and lower mechanical resistance.

To test the hypotheses the following were performed: Exp. I: a study of the penetration of the roots of different plant species through sealing layers comprising various combinations of fly ash and sewage sludge; Exp. II: an investigation of the effects of using fresh vs. aged, hardened fly ash on root penetration and plant growth in the covering soil; Exp. III: a 48-h toxicity test with pea plants using extracts of five fly ashes of different origins, to identify what factors in fly ash may have toxic effects on plants; and Exp. IV: dilutions of the two fly ashes differing the most in terms of chemical properties were finally chosen for use in examining physiological stress responses at earlier stages in dwarf bean.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Root Penetration of Sealing Layers of Ash–Sludge Mixtures—Experiment I
To study the growth of roots through sealing layers containing fly ash, with or without added sewage sludge, four plant species were grown in PVC containers consisting of eight compartments, the inner dimensions of which were 50 by 11 by 6 cm (Fig. 1). The front face of the container was transparent and removable, to enable observation of the root growth, but covered with black plastic to prevent the growth of algae. From bottom to top each compartment contained: sand, 5 cm; mine tailings, 5 cm; a sealing layer, 15 cm; and moraine, 10 cm.


Figure 1
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  		Fig. 1. Container used for the cultivation of plants. Each container consisted of eight compartments, and in each compartment one plant or 1.0 g of seeds was introduced.

 
The mine tailings consisted of weathered sulfidic tailings from the mining area at Kristineberg, Sweden (65°04' N, 18°44' E), where the moraine was also collected. The fly ash originated from the incineration of wood construction waste at the Högdalen thermal power station, Stockholm, Sweden, and the sewage sludge from the Henriksdals wastewater treatment plant, Stockholm, Sweden. Three different mixtures of ash and sludge were used to make the sealing layers. The ash/sludge mass ratios were 10:0, 6:4, and 1:9 (wet wt.). The water content of the ash was 7.5 g kg–1 and of the sludge 250 g kg–1. Water was added (200 g kg–1) to the treatment with fly ash only. The bulk densities of the different ash–sludge mixtures dried at 105°C for 48 h are shown in Table 1, as are the gravimetric water contents of the fresh samples.


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  		Table 1. Bulk density (dried samples) and gravimetric water content of ash–sludge mixtures (n = 3, ± SE).

 
Plants of the following species were transplanted into the top layer of moraine: birch (2-wk-old seedlings), Scots pine (1-yr-old plants), willow (clone 78183, 15-cm fresh cuttings without roots), and Kentucky bluegrass (cv. Amason). Four replicates of each species were used, with one plant per compartment for the trees and 1.0 g of seeds per compartment for the grass. The plants were grown under greenhouse conditions, and were subject to 18 h of illumination per day and a day/night temperature of 19°C/17°C. After 6 mo, the front faces of the containers were removed and 75-cm3 soil samples were taken, using a sharp steel spatula, from the bottom of the moraine layer, from the top and bottom of the sealing layer, and from the mine tailings. Plant roots and soil from the samples were separated from each other, and dried at 105°C for 24 h before the dry weight was calculated. Only roots growing inside the soil samples were analyzed, while those growing along the sides of the container were disregarded. The pH of the soil samples was measured using a Metrohm 744 pH-meter (Herisau, Switzerland), according to the Swedish Standard, SS-ISO 10390.

Influence on Root Growth by Fresh and Aged Fly Ash—Experiment II
To examine the impact of the mechanical strength of the soil on root penetration, vs. the impact of compounds dissolved from the ash, a test was performed in which plants were grown in soil on top of fly ash hardened to different degrees of compaction, using fresh as well as aged fly ash. The fly ash used came from the same source as was used in Exp. I. To obtain ash layers of different strengths, 100 g of ashes (dry wt.) were mixed with 30, 40, 50, 60, and 70 mL of water and poured in 200-mL square plastic pots. A higher water content results in a more porous material after the hardening process. The bulk densities and content of fly ash in these ash–water mixtures are shown in Table 2. Bulk densities were calculated for samples dried at 105°C for 48 h, both for mixtures 7 d after hardening and for mixtures after termination of the experiment.


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  		Table 2. Bulk densities (dry wt.) of ash–water mixtures before and after Exp. II (n = 3 to 4, ± SE).

 
Triplicates of the ash–water mixtures (Table 2) were prepared 7 d before the start of the experiment, to represent freshly cured fly ash. The aged and leached ash–water mixtures were prepared 10 mo in advance, stored in a dark and dry storage room, and thereafter, before the start of the experiment, washed 3 x 1 h, each time with 150 mL of distilled water. At the start of the experiment the ash–water mixtures were covered with 100 g (FW) of potting soil (P-jord, SW, Svalöv, Sweden) containing 0.8 m3 m–3 peat and 0.2 m3 m–3 clay. Two plant species were planted, in three replicates: reed canarygrass (cv. Palaton, 1 g of seeds per pot) and pea (cv. Faenomen, five seeds per pot). Pots containing only soil and plants were used as controls. The pots were placed in a climate chamber and were subject to 16 h of illumination per day for 18 d, at a photon flux density of 200 to 250 µmol m–2 s–1, from halogen lamps (Osram, Powerstar HQI-E, Munich, Germany). The temperature was kept at 20°C and the relative humidity at 65 to 70%. The plants were watered with 50 mL of distilled water every second day. At the end of the experiment the plants were harvested, the shoots and roots from the different layers in the pots were dried at 105°C for 24 h for the determination of dry weights, and pH of the soil and ash layers was measured (Metrohm 744 pH-meter, Herisau, Switzerland) according to Swedish Standard SS-ISO 10 390.

Toxicity of Fly Ash Extracts for Pea—Experiment III
A short-term toxicity test was performed using extracts of five different fly ashes to compare toxicity effects with the contents of substances likely to have an adverse impact on plants. A modified version of the EPA OPPTS 850.4230 toxicity test was used for this purpose. Young pea seedlings were chosen to represent a terrestrial plant; this species is suitable for use in measuring root length growth, since side roots are not formed until the seedlings are several days old. The seeds were germinated in moist vermiculite 48 h before the start of the experiment. The seedlings were mounted in 15-mL test tubes, one in each test tube, containing a dilution series of extracts of fly ashes from five different Swedish thermal power stations. Three ashes originated from the incineration of bio fuel, from the Hässelby (BA1), Munksund (BA2), and Skellefteå (BA3) power plants, and two from the incineration of wastes, from the Högdalen (wood construction waste, WA) and Umeå (municipal solid waste, MSWA) power plants. Furnace facilities used at the power plants were as follows: pulverized fuel furnace (BA1), circulating fluidized bed boiler (CFB) (BA2, BA3, and WA), and grate-fired boiler (MSWA). Characteristics of the ash extracts are shown in Table 3. The extractions used mixtures of distilled water and ash at a liquid/solid ratio of 5:1; they were performed in 500-mL dark glass bottles shaken for 24 h at 150 rpm and at room temperature, and were thereafter filtered through 0.45 µm syringe filters (Filtropur S, Sarstedt, Germany). Nine different concentrations, ranging from 0.00 m3 m–3 to 1.00 m3 m–3, were prepared for the five ashes, by mixing the ash extracts with distilled water, and poured into triplicates of 15-mL test tubes, resulting in 27 tubes per ash type. Root lengths were measured at the start and after 48 h, and used for the construction of dose–response curves from which the EC50 values could be read. The pH values of the dilutions were measured before and after the experiment. Conductivity (Schott Handylab, Multi 12, Mainz, Germany), pH (Metrohm 744 pH-meter, Herisau, Switzerland), carbonate alkalinity (Swedish Standard SIS 02 81 39), and the content of 22 elements were analyzed in the concentrated extractions. The analyses of elements were performed by Analytica AB, Sweden, using the methods presented in Table 3.


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  		Table 3. Characteristics of various fly ash extracts used in Exp. III and IV. The EC50 value represents the proportion of ash extract in an ash extract–water solution needed to produce 50% decreased root growth. A lower EC50 value indicates a higher toxicity.

 
To distinguish the impact of plant roots on solution pH from that of CO2 dissolved from the air, dilutions of the fly ash extract with the highest alkalinity (BA1) were prepared both with and without plants. Three dilutions, 0.01, 0.05, and 0.20 (m3 m–3) ash extract in distilled water, were poured into 15-mL test tubes; 48-h-old pea seedlings were mounted in half of the test tubes, and root lengths and pH values in the solutions were measured before and after the test, which lasted 48 h. Each treatment was set up in triplicates. To increase the impact of air on the solutions, two additional series were prepared, with and without plants, in which the solutions were aerated via plastic tubes (1 mm i.d.) connected to an aquarium pump.

Physiological Stress Response of Bean to Fly Ash Extracts—Experiment IV
To study the physiological responses of plants to ash extracts at lower concentrations, two fly ashes were chosen for their differences in alkalinity and metal content (BA1 and MSWA, Table 3). The toxicity test with pea plants (Exp. III) gives only a short-term measure of impact. It is probable that the pattern may be different when plants are exposed to more diluted extracts for a longer time. Furthermore, dilution lowers the pH, changing the conditions for many elements that can affect plants. The dwarf bean (cv. Limburgse vroege) was chosen, since it has served as a model plant in various biological test systems used for studying the impact of contaminants on plants (Van Assche and Clijsters, 1990). Another early phytotoxic response to pollutants, apart from reduced growth, is the induction or reduction of enzyme activities (e.g. Mazhoudi et al., 1997; Lagriffoul et al., 1998; Schützendübel and Polle, 2002). For example, the activities of malic enzyme (ME), isocitrate dehydrogenase (ICDH), glucose-6-phosphate dehydrogenase (G-6-PDH), glutamate dehydrogenase (GlDH), and peroxidase (POD) are all reported to increase at high metal accumulation levels in plants (Van Assche et al., 1988).

Seeds were germinated between two layers of moist rock wool at 22°C. After 4 d the seedlings were mounted on floating polystyrene plates and transferred to 1-L plastic pots, four seedlings per pot, containing the solutions described below. The plants were grown in climate chambers at day/night temperatures of 21°C/15°C on Days 1 to 5 and 21°C/21°C on Days 6 to 10, with a light intensity of 120 µmol m–2 s–1 for 14 h per day. The relative humidity was 65%, and the pH (PHM 82 Standard, Radiometer, Copenhagen, Denmark) was measured in the afternoon every day. The experimental setup included two dilutions of each fly ash (0.005 and 0.020 m3 m–3), plus a control. A low level of nutrients was added (0.1 m3 m–3 Hoagland solution) to all treatments at the start and on Day 6, when the solutions were changed. After 10 d of treatment the plants were harvested and weighed; shoot length was measured, as well as root weight and area of primary leaves.

For enzyme measurement, samples of primary leaves and roots (1 g FW) were immediately frozen in liquid N and thereafter stored at –70°C. The remaining parts of the plants were dried at 105°C for 24 h to enable dry weight measurement. The frozen primary leaves (1 g fresh wt.) were homogenized using an ultra mixer together with 5 mL of extraction buffer (pH 7.8) containing 0.1 M Tris-HCl, 0.001 M ethylenediaminetetraacetic acid (EDTA), and 0.001 M dithiotreitol. Polyvinylpyrrolidone was added (0.2 g) to absorb the poly phenols. After filtration through a nylon mesh and centrifugation for 10 min at 13 500 g and 4°C, the supernatant was kept on ice. The capacities of following enzymes were measured spectrophotometrically in the supernatant, according to Bergmeyer et al. (1983) and Mocquot et al. (1996): ME (E.C. 1.1.1.40), ICDH (E.C. 1.1.1.42), G-6-PDH (E.C. 1.1.1.49), GlDH (E.C. 1.4.1.2), and POD (1.11.1.7). Guaiacol was used as the substrate for the peroxidase reaction.

Statistical Analyses
Statistical calculations were performed using the software Statistica version 6.0 (StatSoft, 2001). Analyses of root growth and the pH differences between treatments in Exp. I, as well as the differences between plant responses in Exp. III and IV were performed using analysis of variance (ANOVA), or Kruskal-Wallis ANOVA where variances differed between treatments. Multiple comparisons of means were performed using the Tukey honestly significant differences (HSD) test at the 5% significance level. Differences between series of different ash mixtures in Exp. II were studied using analysis of covariance (ANCOVA).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The roots of none of the plants managed to grow into the sealing layer consisting of only fly ash (Table 4), while birch roots did not grow into the sealing layer in any of the treatments. With a sealing layer consisting of a 6:4 ash–sludge mixture, roots of willow and Kentucky bluegrass were found in the upper part of the layer, and with a sealing layer consisting of a 1:9 ash–sludge mixture, roots of willow and Scots pine grew to the full depth of the sealing layer. Thus, a lower content of fly ash in the sealing layer gave a lower resistance to root penetration. No roots were detected in the mine tailings in any of the treatments. Although Kentucky bluegrass had a high root biomass in the moraine layer, fewer roots were found in the sealing layer, than was the case with willow and Scots pine roots.


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  		Table 4. Root growth [g roots (g soil)–1] (dry wt.) in different layers and with different ash:sludge mixtures in the sealing layer. The volume of each sample was 75 cm3. "-" indicates that no roots were detected (n = 4, ±SE).

 
The ash/sludge ratio in the sealing layer also proved to influence the pH in the moraine layer. Increasing the proportion of fly ash raised the pH in both the sealing and moraine layers (P < 0.001, Kruskal-Wallis ANOVA), and in some treatments even in the mine tailings (Table 5). Increased pH in the moraine layer correlated with decreased root growth, which was significant (P < 0.05) for willow and Scots pine. Root growth into the sealing layer also seemed to affect pH in this layer, since the treatments with the highest root biomass in the sealing layers (ash–sludge mixtures with willow plants) also had the lowest pH values (Table 5).


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  		Table 5. pH in different layers and with different ash–sludge mixtures in the sealing layer. The letters a to c indicate significant differences (P < 0.05) between treatments within layers and species (n = 4, ±SE).

 
Plants growing in soil on freshly hardened ash layers had significantly lower shoot growth than did those grown on aged fly ash (Table 6). Storing the ash for 10 mo and then washing it resulted in a decreased pH in the ash layer and in the soil above; however, the latter effect was only significant (P < 0.05) in pots containing reed canarygrass. Almost no roots were able to grow into fresh fly ash, except those of pea seedlings planted in the mixture with lowest bulk density; roots of both plant species were found in the three mixtures of aged ash with the lowest bulk densities (Table 6). During the experiment the bulk densities decreased in the most porous mixtures (Table 2); however, this decrease was similar in both fresh and aged ash, so it could not explain the increased root penetration of the aged ash. Even shoot growth was negatively affected by increased bulk density, though this was only significant for the treatment using grass growing on aged ash. Surprisingly, soil pH significantly decreased with increasing ash concentration, and so could not have caused the decreased shoot growth.


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  		Table 6. Growth of roots into the ash layer and shoot growth (dry wt.) of pea and reed canarygrass growing in soil on ash layers consisting of fly ash–water mixtures of different bulk densities (a low content of fly ash represents a low bulk density, see Table 2). Control pots contain only soil (n = 3, ±SE).

 
Analysis of the extracts revealed that the five fly ashes differed in terms of metal content and other characteristics (Table 3). Generally, bio ashes had higher pH and alkalinity values than waste ashes did, while the waste ashes contained higher concentrations of heavy metals such as Cd and Cu; however, for concentrations of most other elements, such generalizations could not be made. Toxicity to pea seedlings was generally higher in extracts from bio ashes than from waste incineration ashes (EC50 values shown in Table 3), and seems to be connected to the degree of alkalinity (Fig. 2). Measurement of pH before and after the experiment revealed that in dilutions in which alkalinity values were calculated as being below 4 mmol HCO3– dm–3, the pH decreased from above 10 to below 7 (Fig. 3), possibly affected by living plant roots. The dilution level at which this effect occurred varied between the ashes, according to the alkalinity of the undiluted extracts.


Figure 2
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  		Fig. 2. Plot of alkalinity and EC50 of extracts from five different fly ashes. The two highly alkaline ash extracts affect plants at low concentrations, while those with lower alkalinity are less toxic (n = 3 for calculation of EC50, using one extraction of each fly ashes).

 

Figure 3
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  		Fig. 3. pH before and after pea cultivation in dilution series of two of the ash extracts used in the toxicity test. These are extracts of fly ashes with (a) high and (b) low alkalinity, originating from the incineration of bio fuel (BA1) and wood construction waste (WA), respectively. Seedlings growing in dilutions in which the pH decreased to below 7 all displayed growth similar to that of the controls (n = 3, ±SE).

 
The decreased pH observed in more highly diluted extract solutions was partly caused by the activity of the living plant roots. This was proven in the test in which pea seedlings were grown in extracts from the most alkaline ash (BA1), and pH before and after 48 h of cultivation was compared between tubes with and without plants. During the test, the pH decreased more in solutions with plants than in solutions without plants (Fig. 4). This effect was particularly evident in non-aerated test tubes. In the strongest solution consisting of 0.20 m3 m–3 fly ash extract the plants showed symptoms of toxicity, and the pH decreasing effect was also less obvious.


Figure 4
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  		Fig. 4. pH at start and after 48 h in three dilutions of extract of alkaline bio fly ash, with and without pea cultivation. Results from aerated solutions are shown in Fig. 4a, and from non-aerated solutions in Fig. 4b. The dilutions with the ash extract concentrations 0.01, 0.05, and 0.20 (m3 m–3) had following alkalinities: 1.1, 5.4, and 21.6 (mmol HCO3– dm–3), respectively (n = 3, ±SE).

 
The growth parameters indicated negative effects on dwarf beans cultivated in MSWA extracts, compared to results for controls and BA1-treated plants; however, these differences were only statistically significant for the weights and shoot lengths of plants cultivated in the strongest concentration of MSWA compared to controls (Table 7). Since the pH was unregulated, it was initially higher in BA1 extracts than in MSWA extracts and controls (Table 7). With time, however, pH decreased in all solutions.


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  		Table 7. Growth parameters of dwarf beans cultivated in diluted extracts (LS 5) of BA1 (bio fly ash) and MSWA (municipal solid waste fly ash), and pH of solutions at start and end of Exp. IV (n = 3, ±SE). Letters indicate significant (P < 0.05) differences between treatments.

 
In the primary leaves of dwarf beans, enzyme capacities were in most cases higher in plants treated with MSWA extracts than in the control plants or those treated with BA1 extracts (Table 8), indicating a stronger response to stress. In roots, however, few differences occurred (Table 8).


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  		Table 8. Capacities of malic enzyme (ME), isocitrate dehydrogenase (ICDH), glucose-6-phosphate dehydrogenase (G-6-PDH), and guaiacol peroxidase (GPOD) expressed as mU (g FW)–1 in primary leaves and roots of dwarf beans after 10 d of treatment. Capacity of glutamate dehydrogenase (GlDH) is not shown, since there were no significant differences. Control plants were cultivated in nutrient solution. The extracts were made from fly ash from the incineration of bio fuel (BA1) and municipal solid waste (MSWA) (n = 3, ±SE). Letters indicate significant differences (P < 0.05), a–c for leaves and x–z for roots.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
We examined how plant root growth would be affected by fly ash when this waste product is used in constructing sealing layers. We considered whether adding sewage sludge or aging and leaching the fly ash would increase the risk of roots penetrating the sealing layer, and what fly ash properties would have the greatest impact on plant roots. From our results we conclude that plant root growth is inhibited by an increasing proportion of fly ash in a sealing layer (Tables 4 and 6). We suggest that this is primarily due to the increased soil strength and penetration resistance of the material, as the bulk densities, in both Exp. I and II, increased with increasing fly ash content relative to the sewage sludge and water contents, respectively (Tables 1 and 2). The two main factors determining the penetration resistance of a soil are bulk density and water content (Vaz et al., 2001), and many studies have observed a clear correlation between soil strength and root growth (e.g. Materechera et al., 1991; Misra and Gibbons, 1996; Godefroid and Koedam, 2004). In our experiments the limiting bulk density for roots to be able to penetrate the ash mixtures appeared to be approximately 1.0 g cm–3 (Tables 1, 2, 4, and 6).

Another reason for the increased root penetration of sealing layers containing sewage sludge could be the effect of added N. Despite the highly variable content of elements in fly ashes, they do in general (and in wood fly ashes in particular) contain a beneficial mixture of all elements, except N, essential for plants (Demeyer et al., 2001). Sewage sludge on the other hand contains large amounts of N: for example, the average total N content in sludge from Swedish waste water treatment plants is 38 g kg–1 (dry wt.) (Statistika Centralbyran, 2002). A combination of fly ash and sewage sludge would thus produce a nutrient-rich substrate that could easily attract growing roots. Such mixtures have also been suggested as ameliorating additives to promote plant establishment (e.g. Rautaray et al., 2003; Sajwan et al., 2003). Therefore, the addition of sewage sludge to sealing layers will promote root growth rather than prevent it, unless the compaction is great enough. Higher ratios of sewage sludge in the sealing layers also displayed a tendency to stimulate root growth in the covering moraine layer (Table 4), which could be the effect of N released from the sludge, or the effect of lowered pH (Table 5) increasing the availability of nutrients from the sludge, ash, or moraine. The pH of moraine used on top of sealing layers consisting of only fly ash (pH 8.2–9.9) (Table 5) is high enough to create unfavorable conditions for plant growth, by decreasing the availability of nutrients (Marschner, 1995). Plants adapted to noncalcareous soils, such as Scots pine (Carlile and Brown, 1968), will likely have difficulties surviving in such conditions. In constructing a protective layer, the depth must be great enough to ensure that the underlying alkaline sealing layer has no negative effects on the plants; as well, the material used should be rich in nutrients, to decrease the incentive for plant roots to grow into the sealing layer, especially when plant species with high nutrient requirements, such as willow, are established (Nixon et al., 2001).

Though the high pH of fly ash obviously has a negative effect on plant root growth (Table 4), plants also have a strong ability to alter pH toward neutral values (Marschner, 1995). Even in highly alkaline environments plants may down regulate pH, as shown in experiments with limed mine tailings in which pH in the drainage water was decreased from 10 to approximately 7 by the cultivation of cotton grass (Eriophorum angustifolium Honckeny) (Stoltz and Greger, 2002). The ability of plants to decrease the pH is, on the other hand, strongly related to the soil's buffering capacity (Schaller, 1987), so to study the effect of high pH on plants, the soil alkalinity must also be known. In our experiment, pea roots managed to lower the pH from above 10 to below 7 as long as the solution's buffering capacity was no higher than approximately 2 to 4 mmol HCO3– dm–3 (Fig. 3). Dissolution of CO2 from the air also has a pH-decreasing effect (Neal et al., 1998), but as seen in Fig. 4, the cultivation of plants greatly enhanced that effect in the solutions.

Factors other than alkalinity have been shown to influence plant growth in ash-containing substrates (Degenhardt and Gimmler, 2000; Kumar et al., 2002). Freshly hardened fly ash proved to have a negative effect on plant growth compared to aged ash, although pH was only slightly higher in the soil over top of the fresh ash and never exceeded 7 (Table 6). The lower shoot growth of both pea seedlings and reed canarygrass in soil over top of fresh ash could instead be an effect of dissolved substances. In the toxicity test with pea seedlings, alkalinity appeared to be the main factor affecting plant growth. However, when the solutions were diluted and plants exposed for a longer time, other factors obviously became more important in terms of affecting plant growth. During the experiment, pH dropped to 5 to 6 in the MSWA solutions and to 7 to 8 in the BA1 solutions (Table 7), low enough to support plant growth and to make the impact of other elements observable. In contrast to the results of the toxicity test, in which seedlings were more sensitive towards BA1 extracts, dwarf beans cultivated in MSWA extracts showed a stronger stress response (Table 8) by increasing the activities of ME, ICDH, G-6-PDH, and GPOD, which by many authors have been connected to elevated stress levels (Van Assche et al., 1988; Schaaf et al., 1995; Schützendübel and Polle, 2002). The effects on both growth and enzyme activities were greater in leaves than in roots, which indicate the influence of an element easily transported to the shoot. Induction of POD has been observed in leaves of plants treated with Zn, which is to a great extent transported to the shoots, while treatment with Cu, which is accumulated in the roots, resulted in higher enzyme activities in roots (Cuypers et al., 2002).

The fly ash extracts all varied in metal concentration (Table 3), and since metals are dependent on pH for their dissolution and availability (Berrow and Burridge, 1991), varying pH and alkalinity will also determine the speciation and solubility of the metals. We suggest that the higher stress level of plants cultivated in MSWA solutions having lower pH values than the BA1 solutions (Table 8) is a result of the higher solubility of toxic elements in these solutions. The measured elements found in the MSWA extract at levels at least twice as high as those in the BA1 extract and that could be considered as having toxic effects were: B, Cd, Hg, Na, and Pb (Table 3). Toxic levels of accumulated metals (e.g. Tripathi et al., 2004), have been recorded in plants grown in fly ash, and accumulation of B has been singled out as a major reason for the phytotoxicity of fly ash from coal combustion (Aitken and Bell, 1985). Salt effects have, moreover, been suggested as an important factor causing toxic effects in plants growing in MSW fly ash (Giordano et al., 1983). To identify which of these factors were the most detrimental for plants further research is needed. The low root growth of Kentucky bluegrass into the sealing layers of ash–sludge mixtures in Exp. I (Table 4) may well be due to a salt effect, since this species has displayed a lower tolerance towards salinity, relative to other grass species (Torello and Symington, 1984; Alshammary et al., 2004).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Growth of plant roots is effectively prevented if a high proportion of fly ash is used in constructing sealing layers. Its pozzolanic property makes ash form a strong and impenetrable layer, with a high mechanical strength that inhibits root growth. Addition of sewage sludge decreases the bulk density and resistance of the sealing layer to root penetration. Furthermore, this study demonstrated that substances released from the fly ash may negatively impact plant roots, even when they are growing in the protective covering layer. This effect is particularly evident shortly after the sealing layer is constructed and the fly ash is fresh, and will diminish with time. A high pH further prevents root growth, especially when highly alkaline fly ashes are used. In fly ashes of lower alkalinity, released elements have a more pronounced effect on plants, since many of these elements are more soluble at low alkalinities. Apart from the pH effect, we suggest that the fly ashes used in this study had a phytotoxic effect due to high levels of Na, B, Cd, and Pb. The complex composition of leachate from fly ash makes it difficult, however, to pinpoint the exact reason for its phytotoxicity, especially since its various components may interact in synergistic or antagonistic ways.


   ACKNOWLEDGMENTS
 
This work was funded by Boliden Mineral AB, Skellefteå Kraft AB, Stockholm Vatten, Umeå Energi, VA-Forsk, and Värmeforsk. The authors want to express their appreciation to Prof. Jaco Vangroensveld at Limburg Universitaire, Diepenbeek, Belgium for guidance in performing the biological tests with dwarf beans, Patrik Dinnetz for advice in statistical calculations, and Prof. Lena Kautsky for valuable comments on the manuscript.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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E. Stoltz and M. Greger
Root penetration through sealing layers at mine deposit sites.
Waste Management Research, December 1, 2006; 24(6): 552 - 559.
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