Abstract: The present research was carried out with eight specimens of sludge from Apex Weaving and Finishing Mills Ltd. Gazipur, Bangladesh, to determine the concentration of heavy metals (Pb, Cd, Zn, Cu, Cr, Fe and Mn) in the sludge samples and an assessment was made with the heavy metal content in agricultural soil. Atomic Absorption Spectrometry (AAS) method was employed for the analysis of Pb, Cd, Cr, Zn, Cu and UV-Spectrophotometric method was used for Fe and Mn, respectively. The mean concentration of lead (Pb), cadmium (Cd), chromium (Cr) was 79.13, 6.27, 4.35 mg kg-1 and for zinc (Zn), copper (Cu), iron (Fe) and manganese (Mn) it was 7.90, 1.34, 195.2 and 3.97 g kg-1, respectively. All the heavy metal concentrations except chromium in the sludge samples were higher than that of in agricultural soil. In addition, the study concluded that pre-treatment process for reducing the amount of heavy metal is mandatory before the sludge can be used as a soil conditioner or fertilizer in the agricultural soil.
INTRODUCTION
The textile industry, the flagship of Bangladesh-now earns more than 70% of the country`s total export income. A recent report also revealed that Bangladesh`s RMG (Ready Made Garments) exports to the world markets reached an all time high value of over USD 9.35 billion in the end of 2007 (BGMEA, 2008). This textile and dyeing industries now viewed as a major environmental threat in the industrial area of Bangladesh and they contribute huge amounts of sludge in wastewater treatment processes (Karim et al., 2006). Although characteristics of sludge depend on the wastewater treatment process and sludge stabilization methods, it contains substantial amounts of toxic heavy metals (Chen et al., 2005; Singh et al., 2004). Another recent investigation reported that elevated levels of heavy metals in vegetables are found from the areas having long term uses of treated or untreated wastewater (Sharma et al., 2006, 2007).
Heavy metals are very harmful because of their non-biodegradable nature, long biological half-lives and their potential to accumulate in different body parts (Manaham, 2005; Wilson and Pyatt, 2007). Excessive accumulation of heavy metals in agricultural soils through wastewater irrigation, may not only result in soil contamination, but also affect food quality and safety (Muchuweti et al., 2006). Some research also confirmed that heavy metals such as Cd, Pb, Cu, Zn and Ni have carcinogenic or toxic effects on human beings and environment (Trichopoulos, 2001; Turkdogan et al., 2002; Kocasoy and Sahin, 2007).
The management of sludge is becoming increasingly difficult due to the presence of heavy metals (Zorpas et al., 2008). It is now established that application of sludge into land can increase soil water-holding capacity, decrease soil bulk density, increase soil aeration and root penetrability and stimulate soil microorganism activity (Kvarnstrom et al., 2000). In addition, land utilization of sludge could represent a step forward to more sustainable farming practices and municipal waste management. Achieving this purpose it is pivotal to know the heavy metal content in textile sludge as without investigating toxic substances it is not feasible to use sludge as a soil conditioner or fertilizer in agricultural land.
MATERIALS AND METHODS
Sludge Sample Collection
The sludge samples were collected from Apex Weaving and Finishing
Mills Ltd. Gazipur, Bangladesh in mid-January, 2004. The collected samples
were stored into separate plastic container and stored at ambient temperature
prior to treatment. The sludge samples were homogenized by manual mixing,
air-dried for 24 h, disaggregated using a pestle and mortar made by porcelain
to pass through a 2 mm mesh sieve.
Digestion of Sample with Aqua-Regia
An aliquot of 0.200 g of powdered sludge of each sample was taken
in a silica crucible (150 cm3). Then 1M concentrated hydrochloric
acid (9 cm3) was added followed by 1 M concentrated nitric
acid (3 cm3). The content of the crucible was carefully heated
in sandbath nearly to dryness in fumehood. After cooling the crucible
at room temperature, deionized water was added to the sample and was filtered
through a filter paper (Whatman No. 42). The filtrate was collected in
a measuring flask and was preserved for the determination of Pb, Cd, Zn,
Cu, Cr, Fe and Mn. All regents used were Merck, Analytical Grade (AR)
including standard stock solutions of known concentrations of different
heavy metals.
Heavy Metal Analysis
Heavy metals analysis were carried out using AAS (Perkin Elmer AAnalyst
400). The AAS was calibrated for all the metals by running different concentrations
of standard solutions. Average values of three replicates were taken for each
determination. The detection limits for Pb, Cd, Zn, Cu and Cr were 3.0, 1.60,
1.60, 3.0 and 3.0 mg L-1, respectively. Iron determination was carried
out by hydroxylamine and with 1, 10 phenanthroline at pH 3.2-3.3. At pH between
2.9 and 3.5, rapid color was formed in the presence of an excess phenanthroline
and the reddish-orange iron (II) complex absorbs at 515 nm (Greenberg
et al., 1998). A quantitative estimation of Mn in the digested samples
was carried out using a UV Spectrophotometer (Sequoia-Tuner, Model-390) following
permanganate oxidation method at 522 nm. Because of its distinctive color and
stability, permanganate ion is preferably used as the determination form for
manganese.
RESULTS AND DISCUSSION
The present study found that the average concentration of lead in the sludge samples was 79.13 mg kg-1 although the content of lead in the eight sludge samples ranged from 73.61-86.02 mg kg-1 in Table 1. This result revealed that examined sludge samples contained relatively higher amount of Pb than that of agricultural soil. Threshold natural background of Pb in agricultural soil in China is ≤35 mg kg-1 (Wong et al., 2002). Kabata-Pendias and Pendias (2000) reported that the maximum content of Pb was 50 mg kg-1 in light soils used for cultivation.
Usually, sludge contains high concentration of cadmium, because wastewater discharged from mill contains cadmium from dyes. The average concentration of cadmium of the sludge samples was 6.27 mg kg-1 in Table 1. The permissible level of Cd in agricultural soils set by Dutch authorities is 1 to 5 mg kg-1 (Chen et al., 1997). Natural background level of Cd in agricultural soil in China is ≤0.20 mg kg-1 (Wong et al., 2002). The values of Cd found in the present investigation were higher than those of above critical value set by other researchers. In addition, some cadmium compounds are able to leach through soils to ground water. When cadmium compounds bind the sediments of rivers, they can be more easily bioaccumulated or re-dissolved when sediments are disturbed, such as during flooding. Therefore, the use of sludge as a soil conditioner or fertilizer in arable soils can cause severe pollution with Cd and the production of crops and vegetables may be at a risk.
| Table 1: | The concentrations of heavy metals in all sludge samples |
| Table 2: | A comparison of heavy metal content (mg kg-1) in soil from other regional study with present study |
| NA: Not Available, aSource: Awashthi (2000), bSource: Bowen (1966), cSEPA (1995) | |
The maximum Zn value in light soil used in cultivation in India given by Kabata-Pendias and Pendias (2000) was 100 mg kg-1. The threshold natural background value of Zn in crop soils and paddy soils in China is ≤100 mg kg-1. In Table 2, the concentration of Zn was found 7906.5 mg kg-1 in the sludge samples, which was higher than those of permissible levels given by different nations. Similarly, the Cu content in the sludge samples was 1347.7 mg kg-1 which was also extremely higher than that of in China (≤35 mg kg-1) and India (20-30 mg kg-1). Some well documented studies disclosed that heavy metals such as zinc (Zn) and copper (Cu) are the principal elements restricting the use of sludge for agricultural purposes (Su and Wong, 2003; Udom et al., 2004; Dai et al., 2007).
The average concentration of chromium (Cr) in these samples was 4.35 mg kg-1. The maximum content of Cr reported by Kabata-Pendias and Pendias (2000) in soil used in cultivation was 100 mg kg-1. Natural background of Cr in agricultural soils in China is ≤ 90 mg kg-1. The Cr content in soils obtained from the present study was lower than the permissible levels recommended by the above sources.
Long term exposure of iron from the sludge into soils may contaminate it and change the soil structure and thus make it harmful for cultivation. Kisku et al. (2000) found the concentrations of Fe in agricultural soils in India varying from 289.3-338.5 mg kg-1 dry weight. Samantaray et al. (2001) reported that the Fe content ranged from 1422.5-1593.0 mg kg-1 in agricultural soils near a metalliferous chromite mine spoil in India. The Fe contents found in the sludge samples were higher than that of the study by above two researchers. However, the safe limit for iron is not available in Indian and Chinese standard. The average amount of manganese (Mn) in the sludge samples was 3974.1 mg kg-1 which was very higher than the range of uncontaminated soil in India (Table 2).
CONCLUSION
The concentrations of Cd, Zn, Cu and Mn in the sludge samples were exceed the safe limit set by India and China whereas Pb and Cr found within the safe limit (Table 2). The present study failed to compare the results with Bangladesh standard as the Department of Environment; Government of Bangladesh has not yet established any standard for heavy metal content in agricultural soil. Therefore, it is very necessary to establish a safe or standard limit for the concentration of heavy metal in sludge that can be used as a fertilizer as well as soil conditioner.