Leaching Potential of Heavy Metals, Nitrogen, and Phosphate From Compost-Amended Media

By Xia, Y P; Stoffella, P J; He, Z L; Zhang, M K; Et al

Leaching potential of nutrients and heavy metals was evaluated from a peat-based medium (containing 70% peat, 20% perlite, and 10% vermiculite) amended with varying proportions (0%, 25%, 50%, 75%, or 100%) of compost (biosolids and yard waste, 1:1 by weight). The compost contained small amounts of Zn, Cu, Pb, and Cd. However, the leachate fractions of Zn, Cu, Pb, and Cd in compost accounted for only 0.19%, 0.23%, 0.05%, and 0.27%, respectively of the total concentrations. Except for Cu, the concentrations of Zn, Pb, and Cd were higher in the leachates of peat-based medium than the compost amended media. The concentrations of Cd and Pb in the first leachate of the peat-based medium exceeded the drinking water standards (USEPA 1989). However, the concentrations of Cd, Cr, Cu, and Pb in all the compost amended media were below the limit of the drinking water standards. The concentrations of total P and PO^sub 4^-P in leachates increased with increasing proportion of compost in the media. Concentration of NO^sub 3^-N in the first leachate was high and decreased in the subsequent leachings for all the compost amended media. These results suggested that the biosolids-yard waste compost may be a safe and acceptable replacement or partial replacement to peat-based medium without increased leachability of nutrients and heavy metals.

Introduction

Composts can be utilized as a complete or partial additive to commercial peat-based media in the flori-culture and nursery industry (Bugbee et al. 1991; Burger et al. 1997; Raymond et al. 1998; Wilson et al. 2001). For environmental safety, if compost is used in horticulture, it is necessary to evaluate the leachability of nutrients and heavy metals to surface water and groundwater.

Although the extractability of heavy metals from municipal solid wastes (MSW) compost and biosolids (sewage sludge) has been assessed, few studies have focused on leaching of heavy metals from biosolids-yard waste compost as a whole or incorporated into growth media. The objective of this study was to assess the leachable concentrations of heavy metals Cd, Cr, Cu, Mn, Pb, Zn, and the available N and P in peat-based media amended with various proportions of compost, and to identify factors that influence the leaching potential.

Materials and Methods

Column Experiments

A biosolids-yard waste (1:1, by weight) compost was obtained from the composting facility in Palm City, Florida. A peat-based medium (mixing peat, perlite and vermiculite at the ratio of 70:20:10) (The Scotts Co., Marysville, Ohio) was mixed with the biosolids-yard waste compost at 0%, 25%, 50%, 75%, or 100% compost ratios (by volume), respectively (Table 1). Media were saturated with deionized water and then packed into plexiglass tubes (30 cm long by 7.5 cm inner diameter). The bottom of each column was made of a plexiglass plate on which there were 20 holes with 2-mm-diameter of 2-mm. The media were packed to 15 cm high in the tubes, and filter papers were placed on both the bottom and surface of the column.

Deionized water (150 mL) was supplied to each column using a peristaltic pump, at a rate of 1.5 ml min^sup -1^. For one month, 10 leaching events were conducted every other day in a laboratory with temperature of 23~25C. The cross section area of column was 44.2 cm^sup 2^. The amount of water leached was equivalent to 330mm rainfall, or 25% of the mean annual rainfall in Fort Pierce, Florida. A randomly complete block experimental design was used with three replications of columns for each medium. Leachate effluents for each leaching were collected in 1000-mL containers below the soil columns, and stored in 250-mL polyethylene bottles for the analysis.

TABLE 1.

Chemical properties of peat-based media amended with a biosolids- yard waste compost.

Analytical Methods

Subsamples of the leachate effluent were analyzed for pH, electric conductivity (EC) using a pH/ion/conductivity meter (Accumet Model 50, Fisher Scientific, Norcross, Georgia). The remaining effluent was centrifuged at 5000 RCF for 15 min and filtered through a Whatman no. 42 filter paper prior to analysis. The concentrations of NO^sub 3^-N and PO^sub 4^-P in the leachates were measured by ion chromatography (IC DX 500; Dionex Corporation Sunnyvale, California). The concentrations of heavy metals were determined using an inductively coupled plasma atomic emission spectrometry (ICP-AES, J-Y Horiba Group, Edison, New Jersey). Subsamples of the compost amended media were ground to pass a 0.5- mm sieve and digested with a three-acid method (HNO^sub 3^: HClO^sub 4^: H^sub 2^SO^sub 4^=8:1:1, 10 mL) for determining total elemental concentrations using the ICP-AES. The concentrations of total C and N in the medium sample were measured using a CN Analyzer (Vario MAX CN, Elementar Analysensysteme GmbH, Germany). Available NO^sub 3^-N in the compost amended media was extracted by snaking a 4.0g sample (oven-dry basis) in 40 mL 1M KCl for 1 h, and NO^sub 3^-N concentration in the filtrate was analyzed with a Cadmium Reduction- Auto-analyzer (AA III, Bran Luebbe, Buffalo Grove, Illinois). All data were analyzed with the SAS models (SAS Institute 2000).

Results and Discussion

Leaching of Dissolved Salts

The EC values of leachates decreased rapidly with leaching sequence. High EC values only occurred in the first leachate samples from the compost amended media (Figure 1). The decrease in EC occurred in the first five leaching events for each column followed by a minimal change in the subsequent leaching events (Figure 1). The majority of dissolved salts in compost amended media were leached during the first three leaching events. After six leaching events, the EC values were below 1.0 ds m^sup -1^ for each of the compost-amended media, and below 0.3 ds m^sup -1^ for the peat- based medium.

FIGURE 1. Electrical conductivity (EC) in leachates from several compost amended media over a 30-d period (error bar indicates standard error).

Leaching of NO^sub 3^-N and PO^sub 4^-P

The concentrations of NO^sub 3^-N in the first leachates were as high as 1,996 and 1,022 mg L^sup -1^ in the 100% or 0% compost, respectively, and decreased approximately by 50% with each additional leaching for the first five leaching events (over 12 d) (Figure 2). Li et al. (1997) also observed peak concentrations of NO^sub 3^-N in leachates from biosolids and MSW. The peak concentration of NO^sub 3^-N in leachates was not consistent with their available NO^sub 3^-N (KCl extractable) in the media, since available NO^sub 3^-N was higher in the peat-based medium than the 100% compost medium (Xia et al. 2005). Higher peak concentration of NO^sub 3^-N in leachates from the 100% compost medium was probably due to its lower C/N ratio. The medium containing more compost with a lower C/N ratio was expected to have higher NO^sub 3^-N leachability due to mineralization of the compost.

There were different P concentration changes in leachates between peat-based medium and compostamended media (Figure 3). For the peat- based medium, the concentrations in leachates decreased rapidly with leaching sequence (Figure 4). PO^sub 4^-P in the leachates from peat- based medium decreased from the first to the tenth leachate. However, for compost amended media, the concentrations of total P and PO^sub 4^-P in leachates increased slightly from the second leaching event. Except for the first leaching event, in which P concentrations in the leachates were much higher in peat-based medium than the four compost amended media, the concentrations of total P and PO^sub 4^-P in leachates increased with increasing proportion of compost in the media (Figures 3 and 4) because of high total and available P in the compost (Table 1) (Zhang et al. 2004; Xia et al. 2005).

FIGURE 2. Concentrates of NO^sub 3^-N in leachates from several compost amended media over a 30-d period (error bar indicates standard error).

FIGURE 3. Concentrations of total dissolved P (TDP) in leachates from compost amended media over a 30-d period (error bar indicates standard error).

FIGURE 4. Concentrates of PO^sub 4^-P in leachates from several compost amended media over a 30-d period (error bar indicates standard error).

Leaching of Cd, Cu, Pb, and Zn

The concentrations of heavy metals in leachates generally decreased with increasing proportions of the compost in the medium (Table 2). Except for Cu, concentrations of total Zn, Pb, and Cd in the leachate of the compost were lower than that of peat-based medium, and decreased with increasing compost in the media (Figures 5a-d and 6). Similar results were also reported by Sawhney (1992 1994) in his leaching experiments of municipal solid waste (MSW) compost mixed with varying amounts of growth medium.

TABLE 2.

Concentrations of heavy metals in the first leachate from peat- based media amended with a biosolids-yard waste compost.

The concentrations of Cd and Pb in the leachates for all five treatments decreased with leaching progress (Figure 5a and Figure 5c). Initially, the concentrations of heavy metals decreased rapidly, and were below detection limit in the leachate of the 3th or 4th leaching events (6 or 9 days). For example, concentrations of Cd and Pb in the leachates from 100% compost medium decreased from 0.41 g kg^sup -1^ and 5.46 g kg^sup -1^ in the first leachate to below detection limits in the thirdand fourth leachate, respectively.

FIGURE 5. Concentrates of metals in leachates from several compost amended media over ten leaching events: a-Cd, b-Cu, c-Pb, and d-Zn (error bar indicates standard error).

TABLE 3.

Total heavy metals leached from peat-based media amended with a biosolids-yard waste compost in ten leaching events

High concentrations of Cd and Pb, 7.65 and 162.1 g L^sup -1^, respectively occurred in the first leachate for the peat-based medium (Table 2), and exceeded the drinking water standard of 5 g L^sup -1^ for Cd and 20 g L^sup -1^ for Pb (USEPA 1989). Studies have shown that if the media pH is near neutral, leaching of the heavy metals Cd, Cr, Cu, Mn, Ni, Pb, and Zn may be minimal (Bugbee et al. 1991; Tisdell and Breslin 1995). The decreased pH value of peat-based medium (Table 1) may explain the higher initial leaching of the heavy metals, even though the total amount and available concentrations of Cd, Cu and Zn in the peat-based medium were much lower than in the 100% compost medium (Xia et al. 2005). Available Pb in the peat-based medium was higher than any compost amended media, as indicated by its higher concentration in the first leachate (Table 2).

The Zn concentrations in leachates from all compost amended media decreased with leaching events (Figure 5d). In the first leachate, a high concentration of Zn was detected in peat-based medium, but the concentration was lower in the compost-amended media. Except for the 100% compost, concentrations of Zn for all the media decreased dramatically with leaching events, and were as low as 52~56 g kg^sup -1^ in the leachate of the tenth leaching event. The concentrations of Cu in peat-based medium were much lower than the compost medium, and decreased from 44.3 g kg^sup -1^ in the first leachate to less than 20.0 g kg^sup -1^ in the second leachate (Figure 5b). The concentrations of Cu in all the leachates of the five treatments were below its drinking water standards of 1300 g L^sup -1^ (Table 2, USEPA 1989).

The amounts of Pb, Zn, and Cd leached from the peat-based medium were high, but decreased with increasing proportions of the compost in the media (Table 3). The leachable percentages of Zn, Pb, and Cd in peat-based medium were 5.16%, 6.83%, or 3.78%, respectively, but the values decreased to 0.19, 0.05, and 0.27 percentage of total amount in the compost, respectively (Figure 6).

FIGURE 6. Comparison of the leachable percentages of four heavy metals from several composted amended media at 0, 25, 50, 75, or 100% (error bar indicates standard error).

Conclusions

The biosolids-yard waste compost contained a considerable amount of heavy metals, but the concentrations of heavy metals in the leachates were generally low and decreased with increasing leaching events. The concentrations of Cd and Pb in the first leachate of the peat-based medium exceeded the drinking water standard (USEPA 1989). The high leachable fractions of total Cd, Pb, and Zn from the peat- based medium may be associated with the acidic condition of the medium. However, the concentrations of Cd, Cr, Cu, and Pb in all of the compost amended media (25, 50, 75, or 100%) were below the limits of the drinking water standards. These results suggest that the biosolids-yard waste compost product may be safe and an acceptable amendment to the commercial peat-based medium without increasing leachability of heavy metals. The concentrations of NO3- N in the first leachates from compost amended media were extremely high and far exceeded the drinking water standard. High leachable P and PO4-P in the filtrates were associated with the high concentration of total and available P in the compost amended media.

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Y.P. Xia1, P.J. Stoffella2, Z.L. He2, M.K. Zhang3, D.V. Calvert2, X. E. Yang3 and S.B. Wilson2

1. Department of Horticulture, College of Agriculture and Biotechnology, Zhejiang University, Hangzhou, China

2. University of Florida, Institute of Food and Agricultural Sciences, Indian River Research & Education Center, Fort Pierce, Florida

3. Ministry of Education Key Laboratory of Environmental Remediation and Ecological Health, College of Natural Resource and Environmental Science, Zhejiang University, Hangzhou, China

Copyright J.G. Press Inc. Winter 2007

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