Effect of Composts, Lime and Diammonium

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Effect of Composts, Lime and Diammonium Phosphate on the Phytoavailability of Heavy Metals in a Copper Mine... Article in Pedosphere · October 2009 DOI: 10.1016/S1002-0160(09)60158-2

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Pedosphere 19(): 1–, 2009 ISSN 1002-0160/CN 32-1315/P c 2009 Soil Science Society of China ° Published by Elsevier Limited and Science Press

Effect of Composts, Lime and Diammonium Phosphate on the Phytoavailability of Heavy Metals in a Copper Mine Tailing Soil∗1 M. J. KHAN1,∗2 and D. L. JONES2 1 Department 2 School

of Soil and Environmental Sciences, NWFP Agricultural University, Peshawar (Pakistan) of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd LL57 2UW (UK)

(Received , 200; revised , 200)

ABSTRACT Soil samples from a historic copper mine tailing site at the Parys Mountain, North Wales (UK) were amended with green waste compost (GC), GC+30% sewage sludge (GCS), lime and diammonium phosphate (DAP), to determine the effect of amendments on DTPA- and Ca(NO3 )2 -extractable metals in the mine tailing and on the phytoavailability of heavy metals by a lettuce (Lactuca sativa L.). Both compost were added at the rate of 10% by weight, lime was added as calcium carbonate equivalent (pH = 7) and DAP at a 2 300 mg kg−1 soil level. The experiment was arranged in randomised complete design with three replicates in pots under control environment. Addition of lime resulted in the largest reduction in metal extractability with DTPA and Ca(NO3 )2 and phytoavailability of Cu, Fe and Zn while DAP was effective in lowering Pb extractability and phytoavailability. With exception of Zn, all other metals extracted decreased with time after amendment applications. The distribution of heavy metals between and within the four procedures of potentially bioavailable sequential extraction (PBASE) procedure varied significantly (P < 0.001). Stronger relationships were noted between the metals extracted with PBASE SE1 and Cu, Pb (P < 0.01) and Fe (P < 0.001) in the lettuce. These results indicate that addition of lime is sufficient to restore the vegetative cover to a high metal mines wastes while DAP is good for stabilizing Pb, but its detrimental role on plant growth and the risk associated with presence of N in DAP (through N leaching) may restrict its chances for remediation of contaminated sites. Key Words:

amendments, heavy metals, lettuce, mine tailing, potentially bioavailable sequential extraction

Citation: Khan, M. J. and Jones, D. L. 2009. Effect of composts, lime and diammonium phosphate on the phytoavailability of heavy metals in a copper mine tailing soil. Pedosphere. 19(???): ???–???.

INTRODUCTION Mining and smelting activities have contaminated soil and water resources with heavy metals throughout the world. Geochemical weathering processes acting upon metallurgical wastes and byproducts initiate the process of transporting heavy metals from contaminated areas and redistributing them to surrounding soils, streams and groundwater (Fuge et al., 1993; Paulson, 1997). Besides mining and smelting activities, heavy metal pollution also stems from fertilizers, pesticides, biosolids and automobile emissions (Sparks, 2003). Soil remediation technologies based on the excavation, transport, and landfilling of metal contaminated soils are highly effective in lowering risk; however, they can also be expensive to implement. Phytostabalization also known as phytorestoration is a plant based remediation technique, providing hydraulic control, which suppresses the vertical migration of contaminants into groundwater, and immobilizing contaminants by chemical fixation with various soil amendments (Berti and Cunningham, 1997). This also uses plants to immobilize the contaminants in the soil and groundwater through absorption and accumulation by the roots or precipitation in the root zone of the ∗1 Project

supported by the Higher Education Commission (HEC) of Pakistan and NWFP Agricultural University, Peshawar in collaboration with CAZS Natural Resources, University of Wales Bangor, UK. ∗2 Corresponding author. E-mail: [email protected].

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M. J. KHAN AND D. L. JONES

plants. The latter could be achieved by in-situ chemical immobilization through the addition of chemical and organic amendments, which is less expensive and may provide a long-term remediation solution through the formation of low solubility and/or precipitation of minerals (McBride and Martinez, 2000; Basta and McGowen, 2004). Research on chemical immobilization of heavy metals has included alkaline and phosphate-based materials (Hooda and Alloway, 1996; Derome, 2000). Alkaline materials (CaCO3 ) reduce heavy metal solubility by increasing pH and concomitantly increasing the metals sorption by soil (Filius et al., 1998). Addition of phosphate materials has proven effective for immobilizing Pb. McGowen et al. (2001) found that highly soluble diammonium phosphate (DAP) was most effective for immobilizing Cd, Pb and Zn in soil. Few studies have investigated the use of municipal biosolids (sewage sludge), composts, manure and peat (Brown et al., 1996; Basta and Sloan, 1999; Li et al., 2000). Addition of organic materials buffers soil pH and reduces heavy metal uptake by plants. Organic matter with reactive groups such as hydroxyl, phenoxyl, and carboxyl can effectively control the adsorption and complexation of heavy metals in the soil (Alloway, 1995; McBride et al., 1997; Lee et al., 2004). There are several methods that estimate the risk of heavy metals in soil. The term bioavailability is used to describe the solubility and potentially availability of heavy metals for plant uptake (Hettiarachchi and Pierzynski, 2002). Several chemical laboratory methods are capable of estimating the bioavailability of heavy metals to plants and animals. Chemical fractionation methods involving sequential extractions are used to determine the levels of metals ranging from soluble to the residual trapped in minerals lattices (Lake et al., 1984; Ure, 1990; Basta and Gradwohl, 2000). Sometimes metal fractionation or sequential extractions are used to describe the metal behaviour under different soil or environmental conditions. However, they can not be entirely specific for a given fraction within the soil and an additional problem of redistribution of extracted metals to the soil constituents exists (Zwonitzer et al., 2003). The uses of traditional methods of diethylenetriamine pentaacetic acid (DTPA) extraction (Lindsay and Norvell, 1978) and NH4 NO3 extraction (Risser and Baker, 1990) are still very common in estimating heavy metal phytoavailability with a fairly high degree of reliability but are limited only to fertilizer formulation. To assess the potential food chain risks associated with heavy metal contamination, the bioassays using plant or animal models are useful for establishing subsequent relationships. Bioassay methods provide valuable contaminant bioavailability information but have the disadvantage of being lengthy. Brown et al. (2003) used DTPA and Ca(NO3 )2 to extract available and extractable metals, respectively, while in this article both of the methods were used to extracted bioavailable metals. The DTPA soil test (Lindsay and Norvell, 1978) is a traditional method used in most laboratories for determining metals under normal soil condition (pH ≈ 7) and does not appear suitable to gauge the bioavailable fraction particularly for Pb, while Basta and Sloan (1999) reported that Ca(NO3 )2 extractants accurately assessed the phyto- and bioavailable fraction of several metals. Therefore, both the extractants need to be compared with respect to their extraction capacity and correlation with plant uptake. The objectives of this study were: 1) to evaluate the effectiveness of organic, alkaline (lime) and soluble phosphate (DAP) amendments to reduce contaminant bioavailability to lettuce; 2) to compare DTPA and Ca(NO3 )2 extraction procedures and their correlation with plant uptake; and 3) to evaluate the effect of treatments on the potentially bioavailable sequential extraction (PBASE) fractionation of heavy metals. MATERIALS AND METHODS Soil and organic amendments Surface soil (< 20 cm depth) with elevated residual concentrations of Cu, Fe, Pb and Zn (Table I) was collected from the abandoned copper mine tailing at the Parys Mountain, North Wales of UK. Soil was air-dried and sieved (< 2 mm) for analysis of its physical and chemical properties, as follows of sand 729 g kg−1 , silt 115 kg−1 , clay 156 kg−1 , pH 3.35, electrical conductivity (EC) 0.16 dS m−1 , lime

PHYTOAVAILABILITY OF HEAVY METALS

3

requirement 16.8 t ha−1 , and total organic carbon (TOC) and nitrogen (TON) 5.7 and 0.19 g kg−1 , respectively. TABLE I Heavy metal content in the soil collected from the abandoned copper mine tailing at the Parys Mountain, North Wales of UK Heavy metal

Total

DTPA-extractable mg

Cu Cd Fe Pb Zn a) Not

1 905±102 5.76±0.05 4 229±48 7 692±39 1 036±13

7.53±0.2 NDa) 107±21 1 424±6 3.16±0.4

Ca(NO3 )2 -extractable

kg−1 5.03±0.9 ND 3.77±0.2 3 744±40 5.78±009

detected.

The organic amendments consisted of green waste compost (GC) and GC mixed with 30% (v/v) −1 treated sewage (GCS). For GC, the pH is 8.33, EC 1.06 dS m−1 , NO− , NH− 3 -N 6.68 mg kg 4 -N 15.73 −1 −1 −1 −1 mg kg , Olson-P 433 mg kg , TOC 201 g kg and TON 12.1 g kg . For GCS, the pH is 7.12, EC −1 −1 2.30 dS m−1 , NO− , NH− , Olson-P 720 mg kg−1 , TOC 220 g kg−1 3 -N 9.13 mg kg 4 -N 41.95 mg kg and TON 15.0 g kg−1 . The concentrations of heavy metals in GCS were low and thus not reported. Experimental design and process Four amendments, DAP, lime, GC and GCS, were incorporated into 2-kg soil in a 3-L plastic pot. The control consisted of 2-kg untreated soil. DAP was applied at the rate of 2 300 mg P kg−1 soil according to the earlier study of McGowen et al. (2001), who reported that 2 300 mg P kg−1 as DAP was most effective in immobilizing Pb and Zn. Agricultural grade ground lime of 70% was added as a lime requirement equivalent of achieving the target pH of 7. The GC and GCS amendments were added to the soil at a rate of 10% dry weight basis. This application rate is common when biosolids are used for restoration of contaminated soils (Sopper, 1993). All the soil amendment treatments were performed in triplicate. After addition of the amendments, deionized water was added to bring the soil to field capacity (33 kPa pressure and 250 g kg−1 water) and the soils were incubated at 25 ◦ C under controlled condition in the greenhouse for 90 days. Soil moisture was maintained and the soils were mixed thoroughly weekly. Thirty-gram soil sample was collected from each treatment pot for metal analysis at intervals of 30 d after setup and until 90 d. After 90-d incubation period, the soils were uncovered and placed into a 20-cm pot containing 1.5-kg soil over a 3-cm layer of vermiculite. Pre-germinated 7-day old seedlings of lettuce were transplanted into each pot in a completely randomised block design with three replicates. The pots were kept in the green house under controlled temperature for16 h daylight at 20 ◦ C and 8 h darkness at 18 ◦ C per day until the lettuce reached maturity (78 days). The plants were watered as needed with dilute nutrient solution (1 g L−1 ) of commercial plant fertilizer (N:P:K = 30:15:25). Lettuce was harvested 2 cm above the soil surface, washed with deionised water, dried at 70 ◦ C for 48 h and ground to pass 2-mm sieve. Finely ground samples of 0.5 g were wet digested in 10-mL concentrated nitric acid and kept overnight. The lettuce digests were evaporated to about 1 mL in a digestive block at 180 ◦ C and then diluted to 20 mL with distilled water (Zarcinas et al., 1987). The digested samples were filtered through Wahtman No. 40 filter paper and the filtrates were collected for analysis of the heavy metals. Soil samples were also collected after the plant harvest for metal sequential extraction procedure as well as DTPA extraction. Sample analysis Particle size of the tested soil was determined using the hydrometer method (Gee and Bauder, 1986). Moisture content was determined by oven-drying at 105 ◦ C for 24 h whilst pH was determined

4


in 1:1 (w/v) soil:water extract and EC in 1:1 (v/v) soil:water extract (Smith and Doran, 1996). Calcium carbonate equivalent was determined using Shoemaker-McLean-Pratt (SMP) single buffer method (Shoemaker et al., 1962). TOC and TON were determined using a CHN analyser (LECO CHN2000, Leco Corporation, St. Joseph, MI, USA). Total metal contents, after aqua regia (4:1 HCl:HNO3 , v/v) extraction, were estimated using flame atomic absorption spectrometry (AAS) with a deuterium background as appropriate (McGrath and Cunliffe, 1985). Available (plant available) and extractable (both plant available and soluble) metals in the soil before and after treatment application of 30, 60, 90 and 180 d were determined using DTPA and 0.5 mol L−1 Ca(NO3 )2 (Lindsay and Norvell, 1978; Basta and Gradwohl, 2000). The DTPA extraction was done with 0.005 mol L−1 DTPA solution, added to soil at 2:1 ratio with a 2-h shaking time and filtered through Whatman No. 42 filter paper, and the metal concentrations were measured with AAS. The effect of soil treatments on metal extractability was also evaluated by the PBASE method (Basta and Gradwohl, 2000) with a slight modification. In this method, 1 g soil was extracted firstly with 20 mL of 0.5 mol L−1 Ca (NO3 )2 (SE1). The sample was shaken for 16 h end-to-end on a reciprocal shaker and then centrifuged at 5 000 × g for 15 min. The supernatant was decanted, filtered through a 0.45-µm membrane filter (Millipore), acidified with 0.5 mL concentrated HCl and stored at 4 ◦ C until metal analysis by AAS. In the second extraction procedure of the PBASE procedure (SE2), 20 mL of 1 mol L−1 NaOAc solution adjusted to pH 5 was added to the solid residue of the previous procedure and shaken for 5 h. After extraction, the resulting supernatant was prepared for analysis as above. In the third procedure of the procedure (SE3), 20 mL of 0.1 mol L−1 Na2 EDTA solution adjusted to pH 7 was added to the residues in the tube and shaken for 6 h. The resultant supernatant was filtered but not acidified with HCl because acidification causes precipitation of EDTA salts. For the final procedure of the PBASE procedure (SE4), 20 mL of 4 mol L−1 HNO3 was added to the residue in the tube and the mixture was transferred into Pyrex glass tube, heated on block digester for 1 h at 80 ◦ C and shaken for 16 h. The extract was filtered through a 0.45-µm membrane filter (Millipore) before metal analysis. Statistical analysis All the data were subjected to two-way analysis of variance (ANOVA) using GenStat statistical package for window (Rothamsted Research of UK, version 8). Mean comparisons were made using least significant difference (LSD) test after it was determined that F -value was significant at 1% or 5% level of probability. General linear model was used for describing the relationship between the metals of DTPA extraction and PBASE extraction and lettuce metal uptake. RESULTS AND DISCUSSION Soil properties The copper mine tailing soil was a sandy loam with very low pH (pH = 3.35). The EC was low (0.16 dS m−1 ) with the total organic nitrogen content of 0.19 g kg−1 that may not sustain crop growth without fertilizers addition. Total Cu, Fe, Pb and Zn in the soil were well above the permissible soil limits for agricultural activities (Holmgren et al., 1993) indicating a highly contaminated soil (Table I), however, the level of Cd was not high enough to be phytotoxic. Both DTPA- and Ca(NO3 )2 -extractable heavy metals indicated that there were a large amount of potentially bioavailable Cu, Fe, Pb and Zn (Basta and Gradwohl, 2000; Basta and McGowen, 2004) while Cd was not detectable by either extraction method. Effect of the amendments and incubation interval on extractable heavy metals The additions of amendments significantly reduced the Cu contents extracted by DTPA and Ca(NO3 )2 , however, the magnitude of reduction was different. In case of DTPA extraction, the addition of lime


5

was more effective in reducing Cu, while DAP addition enhanced the phytoavailability of Cu with time (Fig. 1). The Cu extractability was reduced over time in the treatments of GC, GCS and lime including the control whereas a significantly higher Cu content was noted 180 d after the addition DAP amendment. This increase might be due to the acidification arising from the dissolution of DAP (Sposito, 1989; Basta and McGowen, 2004) and the initial lower organic matter content of the soil (Khattak and Khan, 1996). The Ca(NO3 )2 -extractable Cu was significantly lowered in the amended soil compared to the control, but there was no significant difference among the various treatments (Table II). Although in all case Ca(NO3 )2 -extracted Cu decrease slightly over time, there was no significant difference either (Table II). Moreover, the effect of DAP on the Cu content extracted by Ca(NO3 )2 was different from that of DTPA, which might be due to the strong chelating nature of DTPA matrix. The Cu extraction potential of DTPA was higher than that of Ca(NO3 )2 (Fig. 1).

Fig. 1 Diethylenetriamine pentaacetic acid (DTPA)-extractable Cu, Fe, Pb and Zn and Ca(NO3 )2 -extractable Cu, Fe, Pb and Zn in the copper mine tailing soils amended with green waste compost (GC), GC+30% sewage sludge (GCS), lime and diammonium phosphate (DAP). Vertical bars represent the standard errors of means (n = 3). TABLE II Analysis of variance for the heavy metals extracted by two methods in soils sampled at four different times and amended with different immobilizing treatments dfa)

Source of variation

Time Treatment Method Time × treatment Time × method Treatment × method Time × treatment × method a) Degree

of freedom;

b) Least

3 4 1 12 3 4 12

Cu

Fe

Pb

Zn

LSDb)

F

LSD

F

LSD

F

LSD

F

NSc) 1.18 0.72 NS NS 1.61 NS

NS < 0.001 < 0.001 NS NS < 0.001 NS

2.40 1.94 1.51 3.99 2.98 3.02 6.12

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001

28.98 32.46 20.56 62.50 38.19 45.15 88.77

< 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001

1.32 0.66 NS 1.64 1.37 0.94 NS

< 0.001 < 0.001 NS < 0.001 < 0.001 < 0.001 NS

significant difference;

c) Non

significant at 5% level of probability.

The effect of treatments on DTPA-extractable Fe was more pronounced and the order of Fe reduction due to the amendments was lime > GC > GCS > control > DAP. Similar to Cu content,

6


the addition of DAP increased the DTPA-Fe extractability (Fig. 1). The increased immobilization due to lime and organic amendments arises from adsorption, precipitation and complexation (Hooda and Alloway, 1996; Bolan and Duraisamy, 2003). The trend of Ca(NO3 )2 -extractable Fe was different from DTPA-extractable Fe. The Fe amount extracted during the first three month of incubation remained fairly constant and the effect of amendment was not visible compared to the control (Fig. 1). However, after plant growth, all the treatments significantly reduced Fe contents in the extracts, which may be due to chemical changes induced by plant growth (Hettiarachchi and Pierzynski, 2002). The reduction of Ca(NO3 )2 -extractable Fe due to the addition of amendment remained same compared to the control. The Fe contents in the extracts were significantly reduced with time after the amendment application as shown by both extraction methods. The extraction capacity of DTPA was several times larger than that of Ca(NO3 )2 , which may be due to the chelating nature of DTPA and the different pH of the matrices as well as the lower pH of the soil. The effect of amendment treatments, time of incubation and methods of extraction on Pb were all significant (P < 0.001) and so were their interactions. The addition of DAP significantly (P < 0.001) reduced the Pb content as shown in both methods of extraction (Fig. 1) while lime addition increased the DTPA-extractable Pb even more than control. But in case of Ca(NO3 )2 extraction, lime was the least effective stabilizing amendment. Brown et al. (2003) reported higher DTPA-Pb extraction (> 300-fold) compared to a Ca(NO3 )2 matrix. Basta and Gradwohl (2000) reported that Ca(NO3 )2 extracted only 0.2% of the total Pb. The results of the organic amendments were between the values noted for DAP and lime treatments. These results were consistent with those reported in other studies (Hettiarachchi et al., 1997; Mench et al., 1998; Hettiarachchi and Pierzynski, 1999; Chen et al., 2003; Basta and McGowen, 2004). The efficiency of DAP immobilization stems from the formation of anglesite (PbSO4 ) or lead phosphate that may control the Pb solubility in soils (McGowen et al., 2001). Based on geochemical computer speciation models, Basta and McGowen (2004) reported that the formation of lead hydroxypyromorphite [Pb5 (PO4 )3 OH] after application of DAP was the most probable solid phase controlling Pb solubility in many soils, which supported the conclusion that DAP would be a more effective treatment than lime and other organic amendments. However, the application of DAP must be carefully performed as high levels of DAP may be detrimental to crops due to P toxicity and soil acidification. For long-term stability of Pb, liming material may be used with DAP to offset potential soil acidification. Results of various organic and inorganic amendment treatments for Zn were similar to those for Cu and Fe but different from Pb (Fig. 1). Unlike Pb results, lime was the most effective for reducing the bioavailability of Zn by increasing its immobilization. These results suggest that the formation of smithsonite (ZnCO3 ), a Zn-containing solid phase, is controlling the Zn solubility rather than zinc pyromorphite [Zn5 (PO4 )3 OH] or hopeite [Zn3 (PO3 )2 ·4H2 O] (Nriagu, 1984). These results seem in apparent contradiction to the previous work of McGowen et al. (2001) and Basta and McGowen (2004), who reported that phosphate based immobilization was more effective. Wang and Zhang (2001) have shown that the combined application of phosphate and lime is more effective. However, in all these studies, the soil used was not acidic as it was in the present research. The beneficial effect due to lime may arise because it increases the soil pH. Lime is also less expensive than soluble phosphate fertilizers (DAP). Unlike Cu, Fe and Pb, the Zn content extracted by both method remained fairly constant during the first three months of incubation but significantly increased when the lettuce was grown in the soil, which may be explained by chemical changes induced by plant growth (Hettiarachchi and Pierzynski, 2002). The extraction capacity of the both extraction methods was comparable for the first three months. Chemical fractionation of heavy metals The distribution of heavy metals between and within the four extraction procedures by PBASE varied widely (Fig. 2). The mean values of distribution across the treatments for Cu, Fe, Pb and Zn in SE1 fraction were 1%, 2%, 3% and 17%, respectively, and the distribution in the first two extracts of


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PBASE (SE1+SE2) was 4%, 3%, 47% and 38%, respectively. The highest amounts of Cu and Fe (85% and 61%, respectively) were extracted by SE4. According to Basta et al. (2001), metal bioavailability is related to its extractability, so the overall relative bioavailability of the heavy metals in the fraction should be SE1 > SE2 > SE3 > SE4. The results of the present study suggested that metal solubility (i.e., SE1 extractable) in copper mine soils was Zn > Pb > Fe > Cu, which agrees with earlier studies (Elliot et al., 1986; Ma and Rao, 1997; Basta et al., 2001). The distribution of Cu and Zn within the four PBASE extraction fractions of our study agrees with the earlier work of Basta and Sloan (1999). All the amendments applied were equally effective in reducing Cu in SE1 fraction compared to the control while DAP was more effective (P < 0.001) in reducing soluble Fe and Pb (Basta et al., 2001). The addition of lime was more effective in reducing Zn solubility (Pierzynski and Schwab, 1993; Basta and Sloan, 1999). None of the amendments affected the very insoluble (occluded) PBASE fraction (SE4).

Fig. 2 Cu, Fe, Pb and Zn in the potentially bioavailable sequential extraction fractions, expressed as PBASE-extractable, in the copper mine tailing soils amended with green waste compost (GC), GC+30% sewage sludge (GCS), lime and diammonium phosphate (DAP) for 180 d.

Phytoavailability of heavy metals in amended soil Analysis of variance was carried out on the plant metal concentrations to test for significant differences between different treatments (Table III). Lettuce Cu contents in various treatment soils followed the trend of control > GC > GCS > DAP > lime (Fig. 3). The reduction magnitude of Cu in lettuce was 95%, 91%, 59% and 56% by lime, DAP, GCS and GC, respectively, compared to control. This indicates that lime and DAP performed considerably better than the compost treatments. Although liming is primarily aimed at ameliorating soil acidity, it is increasingly being accepted as an important management tool in reducing the toxicity of heavy metals (Brallier et al., 1996; Brown et al., 1997; Bolan et al., 2003). Organic amendments tend to enhance the phytoavailability of heavy metals, however, plant uptake is inversely related to pH (Logan and Chaney, 1983; Narwal et al., 1983). These results are also in agreement with those of Basta and Sloan (1999), who reported greater heavy metal accumulation by lettuce grown in soil treated with alkaline biosolids than in soil receiving agricultural limestone. TABLE III Analysis of variance for heavy metal concentrations in lettuce grown under different amendment treatments Heavy metal

F

Least significant difference

Standard error of the mean

Degree of freedom

Cu Fe Pb Zn

< 0.001 < 0.001 < 0.001 < 0.001

11.36 53.95 20.23 19.48

5.1 24.21 9.08 8.74

4 4 4 4

The Fe concentrations in lettuce varied from 415 to 112 mg kg−1 DW, a decrease of 73% due to application of GCS compared to control, however, there were no significant differences between the amendments (Fig. 3).

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Fig. 3 Phytoavailability of Cu, Fe, Pb and Zn to lettuce grown on copper mine tailing soils amended with green waste compost (GC), GC+30% sewage sludge (GCS), lime and diammonium phosphate (DAP). Vertical bars represent the standard errors of means (n = 3).

Lead concentrations were significantly reduced (P < 0.001) with the additions of different amendments and the reduction due to the application of DAP was 92% followed by GCS and GC, while lime with 37% reduction was least effective (Fig. 3). Additions of phosphate materials have proven extremely effective as a chemical immobilization treatment for Pb (Basta and McGowen, 2004). Ruby et al. (1994) indicated that adequate levels of soil phosphate were responsible for the formation of insoluble complexes and the reduction in potentially bioavailable Pb. Other research has shown that highly soluble phosphate source (i.e., DAP) enhances the potential for the formation of lead pyromorphite, which reduces the phytoavailability of Pb (Ma et al., 1993; Pierzynski and Schwab, 1993; Hettiarachchi et al., 1997; Ma and Rao, 1997; Khan and Jones, 2008). Moreover, these results are contrast with the work of Zwonitzer et al. (2003), who observed no influence of P addition on Pb concentration of sorghum tissue in contaminated soils. These contradictions may be due to the differences in the crop used. Although DAP was superior to all other treatments in reducing Pb bioavailability, it was problematic as it restricted the root growth. It caused poor structure due to soil dispersion and the plant growth remained stunted throughout the growth period. All the amendments significantly reduced the concentrations of Zn in lettuce compared with the control (Fig. 3). Liming decreased Zn contents from 150 to 35.7 mg kg−1 DW, a decrease of 76%, however, the differences between the treatments were not statistically significant. The general decrease of the metal concentrations in plant, resulting from an increase of soil pH due to the addition of lime, is in general agreement with the findings of Smith (1994), Hooda and Alloway (1996), Basta and Sloan (1999), Basta et al. (2001) and Usman et al. (2005). Relationship between metal extractability and metal phytoavailability The relationship between heavy metals extracted with the PBASE procedure and metal contents in lettuce is shown in Table IV. There were strong relationships (P < 0.001) between Cu, Fe and Pb extracted by SE1 and the corresponding phytoavailable metals. A weaker relation was established between Pb extracted with SE2 and lettuce uptake of Pb (P < 0.05). Summations of SE1 with other PBASE fractions did not improve the relationship found between SE1-extractable and lettuce uptake of metal contents of Cu, Fe and Pb. Moreover, Zn in lettuce was not correlated with SE1- and SE2-extractable Zn and was only weakly correlated with SE3-extractable Zn. These results contradicted the previous study of Basta and Gradwohl (2000) and the reasons for this may be due to the narrow range of metal concentrations in the soils of their study while in our study there was a wide range of metal contents in the soil and plant imposed by the additions of amendments. Furthermore, the traditional DTPA method of metal extraction did not show correlation at all for Zn or showed very weak correlation for Pb, which may not be surprising since acidic soil was used in this study and the DTPA test is primarily devised for soils of pH 7 or higher.


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TABLE IV Coefficient of correlation between heavy metals extracted with four extraction procedures of the potentially bioavailable sequential extraction (SE1–SE4) and diethylenetriamine pentaacetic acid (DTPA) and the corresponding metal contents in lettuce Extraction procedure

Cu

Fe

Pb

Zn

SE1 SE2 SE3 SE4 SE1–SE2 SE1–SE3 SE1–SE4 DTPA

0.69** 0.28 0.02 0.00 0.64** 0.58** 0.08 0.18

0.88*** 0.68** 0.02 0.52* 0.86*** 0.06 0.00 0.23

0.73** 0.46* 0.03 0.01 0.61** 0.45* 0.47* 0.53*

0.00 0.07 0.41* 0.04 0.15 0.07 0.12 0.05

*, **, ***Significant at P < 0.001, P < 0.01, and P < 0.05 levels, respectively.

CONCLUSIONS The application of organic and inorganic amendments significantly reduced both DTPA- and Ca(NO3 )2 extractable heavy metals. The addition of lime was superior in reducing Cu, Fe and Zn while DAP proved better for reducing Pb phytoavailability. The extraction capacity of DTPA was higher than Ca(NO3 )2 . The distribution of heavy metals within the PBASE fractions varied between metal species. The lowest amount of Cu, Fe and Pb were extracted with SE1 procedure while highest amounts were extracted by SE4 except for Pb, the highest amounts of which were accomplished by SE2 procedure. The effects of the amendments were pronounced on the SE1 and S2 fractions while SE4 fraction was not affected by the addition of amendments. Plant tissue concentrations of Cu, Fe and Zn was reduced significantly by the addition of lime while DAP was superior in reducing Pb in tissues. Strong relationships were found between heavy metals extracted with SE1 and metal phytoavailability for Fe (P < 0.001), Cu and Pb (P < 0.01) and a weaker relationship between Zn extracted with SE3 and lettuce Zn uptake. The summation of PBASE fractions did not improve the correlation between SE1-extractable Cu, Fe and Pb and the corresponding uptake of lettuce. ACKNOWLEDGEMENTS We are thankful to Dr. Leaon Clarke from School of Marine Sciences, University of Wales, Bangor, UK for facilitating my studies in the laboratory. We are also thankful to Dr. Phil Hollington from CAZS Natural Resources, University of Wales, Bangor, UK for his all out support in planning the experiments and correcting the manuscript. The help of Mr. Julian Bridges and Miss Louise Bastock from Greenhouse staff at Penyffridd, University of Wales, Bangor, UK in handling the experiment in greenhouse and providing conveyance facility between the greenhouse and department is duly acknowledged. REFERENCES Alloway, B. J. 1995. Heavy Metals in the Soil. Blackie Academic and Professional, Glasgow, UK. Basta, N. T. and Gradwohl, R. 2000. Estimation of Cd, Pb, and Zn bioavailability in smelter-contaminated soils by a sequential extraction procedure. J. Soil Contam. 9: 149–164. Basta, N. T., Gradwohl, R., Snethen, K. L. and Schroder, J. L. 2001. Chemical immobilization of lead, zinc, and cadmium in smelter-contaminated soils using biosolids and rock phosphate. J. Environ. Qual. 30: 1 222–1 230. Basta, N. T. and McGowen, S. L. 2004. Evaluation of chemical immobilization treatments for reducing heavy metal transport in a smelter-contaminated soil. Environ. Pollut. 127: 73–82. Basta, N. T. and Sloan, J. J. 1999. Application of alkaline biosolids to acid soils: Changes in solubility and bioavailability of heavy metals. J. Environ. Qual. 28: 633–638. Berti, W. R. and Cunningham, S. D. 1997. In-place inactivation of Pb in Pb-contaminated soils. Environ. Sci. Technol. 31: 1 359–1 364.

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