Solubility of Trace Elements and Heavy Metals from Stabilized Sewage Sludge by Fly Ash
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- Hongling, Z., Lina, S. & Tieheng, S. Bull Environ Contam Toxicol (2009) 83: 752. doi:10.1007/s00128-009-9794-5
Stabilized sewage sludge (SS) by fly ash (FA) and alkaline mine tailing as artificial soil, to be applied on the ecological rehabilitation at mining junkyard, offers a potential viable utilization of the industrial by-product, as well as solves the shortage of soil resource in mine area. In this study, trace element and heavy metal soil solution concentrations arising from fly ash, sewage sludge, mine tailing, and artificial soil mixtures were investigated in a laboratory incubation. It was found that total Cd, Pb, and Zn contents in artificial soils were significantly lower than the control standards for pollutants in sludges from agricultural use (GB 4284-84). Soil solution Cd and Pb concentrations were obviously reduced by mixing sewage sludge with alkaline fly ash. Initial soil solution Cd, Pb, and Zn concentrations in artificial soils were 1.773–14.672, 4.05–24.95, and 133–608 μg L−1, respectively, and after 35-days incubation, soil solution Cd, Pb, and Zn concentrations gradually decreased and were approaching control levels by the end of the experiment, and finial soil solution were decreased to 0.037–0.365, 2.12–7.34, and 29–509 μg L−1, respectively.
KeywordsStabilized sewage sludgeFly ashMine tailingTrace elementsSolubility
In China, many abandoned mine lands need revegetation. In these abandoned mine lands revegetation is hindered by the lack of suitable topsoil (Wang and Cai 2006). The use of plowland soil for ecological remediation wastes both manpower and material resources, but also can not solve the soil resource shortage for ecological remediation (Shen et al. 2004).
Fly ash is a byproduct of coal fired power plants, and is composed of particulate matter collected from the flue gas stream. In China, 1.8 × 108 t fly ash (FA) from the generation of electricity was produced each year (Ben and An 2004). Land-filling is the traditional method of disposal of fly ash, however, the dual factors of increased cost and stricter legislation have prompted research into alternative methods of disposal or utilization of this waste material (Abbott et al. 2001; Kriesel et al. 1994). Over the last 25 years, numerous studies on the use of fly ash as a soil amendment have been performed (Adriano et al. 1980). Although benefits associated with the application of FA to soils have been reported, the poor acceptance of agronomic of FA is due to low organic C content, high salinity, and environmental concerns over potentially toxic elements (Carlson and Adriano 1993). Biosolids is a useful source of organic matter, a pool of a slow-release essential nutrients (nitrogen, phosphorus, sulphur and magnesium) and microorganisms. The mixing of an organic waster product such as with fly ash has been proposed to increase the macronutrient content of the resulting mixture while reducing odor and improving handling properties of the organic waste (Jackson and Miller 2000; Belmonte et al.2006). Field trials utilizing FA/biosolids mixtures as fertilizers for maize produced comparable yields to conventional fertilization techniques. The results of a pot study showed that the coal ash, reservoir sediments and sewage sludge mixed in proper proportions could greatly promote plant growing and increase production (Shen et al. 2004). However, heavy metals content associated with the land application of FA and SS should be investigated. Many researchers have noted an increased availability of trace elements in fly ash-amended soil (Tolle et al. 1983). Adriano et al. (1980) reported that Cadmium uptake in sudangrass resulting from sewage sludge application was reduced in the presence of fly ash, and Cd, Zn, Mn uptake in tall wheat grass were reduced when SS was mixed with fly ash.
Trace metal availability is mostly a function of solubility, soil extractions solution should provide a reasonable technique for assessing whether a trace element is available for plant uptake or leaching. Trace element availability from land application of single FA or sewage sludge is well documented (El-Mogazi et al. 1988; Carlson and Adriano 1993; Gibbs et al. 2006; Belmonte et al. 2006). However, a few attempts were made to investigate trace element availability from land application of FA/SS mixtures and the feasibility of the FA/SS mixtures without natural soil used in the ecological remediation of the dumping site for mullock in China. This study will provide valuable information and data for the application of these artificial soils to mining areas.
Materials and Methods
Basic physicochemical properties of the compositions
Available N (mg kg−1)
Olsen P (mg kg−1)
Available K (mg kg−1)
Cd (mg kg−1)
Pb (mg kg−1)
Zn (mg kg−1)
Coal fly ash
Artificial soils (A–E) of sewage sludge mixed with mine tailing and fly ash in weight proportions
One thousand grams of every dry artificial soil sample was placed in a plastic bucket. The details of treatments A, B, C, D, and E are listed in Table 2. Three replicates were designed. All treatments were wet to the moisture of 17% (W/W) by adding deionized water and re-wet to this moisture content throughout the incubation period, on the day prior to every soil solution sampling. The plastic buckets were open to the atmosphere (25°C), but were strictly controlled to avoid any leaching. By this approach, the authors minimized the effect of soil loss during sampling, and thus were better able to keep each treatment at an equivalent moisture content for the duration of the study.
The soil solution was periodically extracted (Days 1, 5, 7, 15, 22 and 35) from the incubated treatments, by centrifugation. One hundred grams of moist soil was sampled from each plastic bucket and then was put into a plastic bottle and 10 ml deionized water was added. Although the artificial soil was at 17% moisture content, it was still necessary to add a further 10 ml of deionized water to the soil sample prior to centrifugation, to extract a sufficient volume of the soil solution. Sufficient time was allowed for this added water to percolate into the soil, because the soil was still below field holding capacity and this was a rapid process. The artificial soil was centrifuged at 3,000 r/min for 25 min. After centrifugation, the soil solution in the bottle was filtered (0.22 μm). The solid cake collected on the filter paper was carefully returned to the corresponding bucket and remixed evenly with the bulk remaining soil.
Aliquots of the filtered soil solution were taken for measurement of the cations of Ca, Mg in the leachates by means of ion chromatography (IC-1010 China). Measurements of pH and Eh were made on a slurried (1:1 soil/H2O) sub-sample of the incubated soil. Cd, Pb, and Zn contents in artificial soil solutions were analyzed by means of atomic absorption spectrophotometry (AA-6300, Shimadzu, Japan).
SPSS 12.0 statistical package was employed for Statistical analysis. Analysis of variance was used to test for significant differences in trace element solubility in individual treatments and interactive effects of the mixed waste treatments.
Results and Discussion
In reactions (1)–(4), two hydrogen ions in the artificial soil were exchanged for one calcium or magnesium (Carlson and Adriano 1993). The increase in the bulk solution concentration of these two cations was closely linked to nitrification processes (nitrification releases H+ into soil solution) and the resultant pH changes in the artificial soil treatments (Zhang et al. 2007). The decrease in pH associated with nitrification increased exchangeable acidity, thus, the variable charge portion of the cation exchange capacity (CEC) of the soil was reduced resulting in increased bulk solution concentrations of Ca and Mg (Van Breemen et al. 1984; Jackson and Miller 2000).
Zinc is a micronutrient and is needed by plants in small quantities and use as catalyst in numerous biological processes. Excessive quantities of Zn can render biosolids hazardous to human health, plant, and animal life (McGrath and Cunliffe 1985; Gupta and Gupta 1998). In general, soils that contain greater than 300 mg kg−1 extractable Zn is considered to be phytotoxic to plants (Levy et al. 1999; Brallier et al. 1996). Previous study showed total Zn contents in artificial soil treatments were extremely lower than the Control Standard (GB 4284-84). The highest content (140.87 mg kg−1) was detected in D treatment (FA:SS = 1:2), while lowest total Zn content (32.26 mg kg−1) was in E treatment (FA:SS:MT = 2:1:1; Zhang et al. 2007). Brallier et al. (1996) and Levy et al. (1999) reported that soils contain >300 mg kg−1 extractable Zn is considered to be phytotoxic to plants. Thus, the extractable Zn in artificial soil treatments was much lower than threshold levels that may cause phytotoxicity.
Mixing fly ash with sewage sludge increased the solution Ca and Mg concentrations. Total Cd, Pb, and Zn contents in artificial soils were significantly lower than the Control standards for pollutants in sludges from agricultural use (GB 4284-84, 1985). The results obtained form incubation experiment suggested that alkaline fly ash stabilized soluble Cd and Pb from sewage sludge. Finial soil solution Cd, Pb, and Zn concentrations in artificial soils were 0.037–0.365, 2.12–7.34, and 29–509 μg L−1, respectively. This indicated that application of these artificial soils composed of fly ash and sewage sludge would not lead to Cd, Pb, and Zn contamination and could be used for ecological reconstruction in mining areas.
The project was supported by the Natural Science Foundation of Liaoning (No. 20062002), the National Basic Research Project (973) of China (No. 2004CB418506), and the High Technology Research and Development Program (863) of China (No. 2007AA06405).