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Soil Chemistry

Release Behavior of Copper and Zinc from Sandy Soils

^a Univ. of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, FL 34945-3138
^b Ministry of Education Key Lab. of Environmental Remediation and Ecological Health, College of Natural Resource and Environmental Sciences, Zhejiang Univ., Huajiachi Campus, Hangzhou 310029, P.R. China

^* Corresponding author (zhe{at}ufl.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Copper (Cu) and zinc (Zn) transport after agricultural applications have accumulated in the sediments and are suspected to affect fish health in the Indian River Lagoon and St. Lucie Estuary, South Florida. Minimal information is available on the release of soil Cu and Zn to water and its relation to their concentrations and physicochemical forms in the soils. Sandy soil samples (n = 13) with a wide range of total Cu and Zn content were collected from forestland and commercial citrus groves in the Indian River area, Florida. The soils were subjected to column leaching and batch extractions to understand the release behavior of Cu and Zn as affected by soil-water contact time, soil/water ratio, pH, and electrolyte concentration and cations. Copper released in batch extractions that simulated long-term leaching was primarily from exchangeable and carbonate-bound fractions, whereas Zn was primarily from the carbonate-bound fraction. The Cu and Zn released from the column represented short-term leaching and were primarily from their exchangeable fractions. Leached Zn decreased linearly as the solution pH was raised from 3.0 to 9.0. Leached Cu was at a minimum at pH 5 to 7 and increased at higher or lower pH beyond that range. These results indicate that long-term saturated conditions after precipitation enhanced Cu and Zn release to water from the sandy soils. The released Cu decreased with increasing Ca concentration but increased with sodium (Na) concentration in the soil solution because of their differential effects on soil colloids, especially organic matter (flocculated by Ca²⁺ and dispersed by Na⁺) because organically bound Cu is the dominant fraction in the soils. Increasing Ca, K, Na, or NH₄⁺ concentration generally increased Zn release through cation exchange. These findings merit attention in the development of best management practices to reduce transport of heavy metals from land to water.

Abbreviations: ICP–AES, inductively coupled plasma atomic emission spectrometer • S, soil

	INTRODUCTION

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INCREASED anthropogenic inputs of heavy metals in agricultural soils from the application of municipal waste, manure, fungicides, and pesticides have caused considerable concern regarding their influence on water quality (Alloway, 1995; Moore et al., 1998). Soils contaminated with Cu and Zn can contribute to the enrichment of Cu and Zn in surface runoff, subsequently deteriorating the quality of receiving surface waters (Moore et al., 1998). Most of the soils in Florida under commercial citrus production are sandy. Repeated applications of Cu- and Zn-containing agricultural chemicals have resulted in Cu and Zn accumulation in the soils (Zhu and Alva, 1993). High Cu and Zn concentrations were also detected in the sediments of the St. Lucie Estuary, South Florida (Haunert, 1988; He et al., 2003b); these high concentrations are suspected to affect fish health in the Estuary and the Indian River Lagoon. Monitoring studies revealed increased Cu and Zn loads in surface runoff from citrus groves in the St. Lucie watershed, and the concentrations of Cu and Zn in surface runoff were correlated with extractable soil Cu and Zn (Zhang et al., 2003).

Copper and Zn loads in surface runoff are not always related to total Cu and Zn in the soils (He et al., 2005). Soil properties, metal characteristics, and environmental factors influence Cu and Zn concentrations and loads in surface runoff or leachate (Zhang et al., 2003; He et al., 2004). The movement of heavy metals in soils may occur in sandy, acid, low-organic-matter soils if subjected to heavy rainfall or irrigation (Dowdy and Volk, 1983). Copper and Zn have moderate mobility in slightly acid soils (Elliott et al., 1986; Hesterberg et al., 1993; Jorgensen, 1991). Copper and Zn loads in surface runoff from the St. Lucie watershed were related to water discharge and extractable Cu and Zn in the soils (Zhang et al., 2003; He et al., 2004). Similar relationships were observed with P loading in surface runoff (He et al., 2003a), implying that in addition to their availability, soil moisture, precipitation, and irrigation affect the transport of P and metals from soil to receiving waters.

Copper and Zn concentrations and physicochemical forms in surface soil directly influence the movement of Cu and Zn, especially in sandy soils (Cezary and Singh, 2001; Edwards et al., 1997; Moore et al., 1998; Scokart et al., 1983). Dissolved Cu and Zn concentrations in surface runoff were significantly correlated with soil Cu and Zn levels extracted with 0.01 M CaCl₂ (Zhang et al., 2003). Temporal and spatial variations occurred for surface runoff Cu and Zn concentrations in a specific site and among several monitoring sites (He et al., 2004). In addition to field management practices such as fertilization, liming, spraying, and irrigation, these variations can be also related to speciation of Cu and Zn in the soil and soil solution. Phytoavailable Cu and Zn were significantly correlated with fulvic complex Cu and exchangeable Zn (Krishnamurti and Naidu, 2002). The concentrations of heavy metals in soil solution are primarily controlled by sorption-desorption and dissolution-precipitation reactions at the soil particle–water interface, but different mechanisms are involved in the partitioning between solid and liquid phase in soil (Hayes and Traina, 1998). Up to 56% of Cu in soil solution was in the form of organic complexes, whereas >80% of Zn was in form of free ion (Saeki et al., 2002). Soil pH and organic matter were reported to account for approximately 70% of the variability in Cu partitioning and 80% of the variability in bioavailable Cu in 40 soils collected from the USA, Canada, and the UK (Impelliteri et al., 2003). For Zn, soil pH alone accounted for approximately 75% of the variability in Zn partitioning and 80% of the variability in bioavailable Zn.

Although studies have been conducted to understand adsorption of Cu or Zn to soil minerals (Atanassova, 1999; Cavallaro and McBride, 1984), minimal information is available on the release behavior of Cu and Zn in soil-water systems. The objectives of this study were to understand Cu and Zn release characteristics from sandy Spodosols to surface runoff in relation to their chemical association in the soil and to examine the effects of major soil solution properties that are involved in the release of these metals, including pH, soil/water ratio, contact time, electrolyte, and type of cations.

	MATERIALS AND METHODS

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Soil Analysis
Surface soil samples (0–5 cm) were randomly collected from St. Lucie County, FL. Ten samples were collected from several commercial citrus groves (called citrus soil) and three from adjacent noncultivated forestland (called forest soil) with a native population of sand pine (Pinus clausa vasey). Each soil was mapped as Wabasso (sandy, siliceous, hyperthermic, Alfic Alaquods). Soil samples were air-dried, and a subsample was passed through a 2-mm sieve for analysis. The remaining soil was used for column leaching and batch extraction studies. Selected properties of the soils are presented in Table 1.

Copper and Zn in the soils were fractionated into exchangeable, carbonate-bound, organically bound, oxide-bound, and residual fractions by a procedure modified from that of Amacher (1996). Soil samples (2.0 g) were sequentially extracted with 20 mL of 0.1 M Mg(NO₃)₂, 20 mL of 1 M NaOAc, 40 mL of 0.1 M Na₄P₂O₇, and 40 mL of 0.2 M ammonium oxalate + 0.2 M oxalic acid + 0.1 M ascorbic acid (pH 3) for separation of exchangeable, carbonate-bound, organically bound, and oxide-bound fractions, respectively (Table 2). After each extraction, the suspension was centrifuged at 7500 x g (relative centrifuge force) for 30 min, and the supernatant was passed through a Whatman # 42 filter paper. The soil residue was rinsed three times with 5 mL of ethanol and evaporated to dryness before next extraction. Steps 3, 4, 6, and 8 in the original method of Amacher (1996) were omitted because the soils had low organic C and oxide contents (Table 1). Total Cu and Zn contents in the soils were determined by digesting 1 g of soil with HNO₃–HF-HClO₄ (Reed and Martens, 1996). Copper and Zn concentrations in the extracts and digesters were determined using an inductively coupled plasma atomic emission spectrometer (ICP–AES) (Ultima; JY Horiba Inc., Edison, NJ). Residual Cu and Zn were calculated by subtracting the sum of exchangeable, carbonate-bound, organically bound, and oxide-bound from their total soil concentrations.

Column Leaching
Soil Cu and Zn leachability was determined using soil columns. Each column was prepared in the laboratory using a Plexiglas leaching column (10-cm long and 7.5-cm i.d.). The bottom of the column consisted of a Plexiglas plate containing several 5-mm-wide holes. The plate was covered with a nylon cloth to prevent loss of soil during the leaching process. Each 300-g soil sample was packed to form a column 5 cm high. Two disks of filter paper (Whatman # 42) were placed on the top of soil before leachings to prevent disturbance by applied water. Before setting up the leaching experiment, the soil columns were positioned upright in a plastic pan (14.2 cm inner diameter and 3.7 cm height), and deionized water was added in small amounts to the pan to moisten the column from the bottom upward until it reached field-holding capacity. The columns were placed in stands for 2 d at 24°C. About 110 mL (1 pore volume) of deionized water was applied to the top of each soil column at a rate of 2 mL min^–1 on a daily basis using a peristaltic pump and repeated for 10 d. This application rate did not allow any ponding on the top of the column. Leachates were collected in 1000-mL beakers below the soil columns and filtered through a 0.45-µm membrane filter for analysis. Copper and Zn concentrations in the filtrate were determined using the ICP–AES.

To study the effects of pH on Cu and Zn leaching, three soils (S1, S4, and S11) were leached using 110 mL of different pH leachants (deionized water adjusted to pH 3, 4, 5, 7, and 9 with dilute HNO₃ and NaOH). The previously described measurements, including the studies of fractionation of Cu and Zn, batch extraction, and column leaching, were conducted on triplicate samples.

Statistical Analysis
Measurements of extractable and leachate Cu and Zn were conducted on triplicate samples. A randomized complete-block experimental design was used. Each variable was subjected to ANOVA using the Statistical Analysis System (SAS version 8.2, SAS Institute, 2001) for each soil. Treatment (fraction) means were separated by Duncun's multiple range test (P < 0.05 level). An additional ANOVA was conducted for each fraction. An orthogonal contrast was partitioned from the main effect of soil type to determine difference between the two soil groups (citrus versus forest) for each fraction. Multiple regression analyses (stepwise procedure) (SAS Institute, 2001) was conducted to evaluate the relationships between water-extractable or leachable Cu or Zn and their fractions in the soils.

	RESULTS AND DISCUSSION

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Fractions of Copper and Zinc in the Soils
Total soil Cu and Zn ranged from 1.6 to 400.1 mg Cu kg^–1 and from 2.1 to 100.6 mg Zn kg^–1, respectively (Table 1). The Cu and Zn concentrations in the soils used for citrus production (citrus soils: 67.7–400.1 mg Cu kg^–1 and 24.4–100.6 mg Zn kg^–1) were 10 to 110 times higher than those soils originating from undisturbed forest (forest soils: 1.6–3.6 mg Cu kg^–1 and 2.1–4.0 mg Zn kg^–1). The concentrations and proportions of Cu and Zn in the five fractions varied among the soils (Table 3). For the forest soils, Cu was present mainly in organically bound and oxide-bound fractions. No carbonate-bound Cu was detected. The mean proportion of total Cu as various fractions decreased in the order of organically bound (60.1%) > oxide-bound (21.8%) > residual (15.5%) > exchangeable (2.7%). Zinc was present primarily in residual and exchangeable fractions in the forest soils (Table 3). The proportion of total Zn as exchangeable fraction was 23.2 to 44.1%. Mean percentages of total Zn as various fractions decreased in the order of residual (44.4%) > exchangeable (30.1%) > organically bound (16.6%) > oxide-bound (4.8%) > carbonate-bound (4.1%) (Table 3).

The concentrations of each Cu fraction in the 13 soils were correlated with total Cu (r = 0.75–0.97; P < 0.01). The concentrations of the carbonate, organic, oxides, and residual Zn fractions were each correlated with soil total Zn (r = 0.87–0.92; P < 0.01), but there was no correlation between exchangeable Zn and total Zn (r = –0.07). Soil pH was negatively correlated with exchangeable Cu and Zn (r = –0.76 and –0.93, respectively; P < 0.01) and positively correlated with the carbonate-bound fraction of Cu or Zn (r = 0.66 and 0.81, respectively; P < 0.01).

Effect of Contact Time
The amounts of Cu and Zn released increased with contact time and varied among the soils (Fig. 1 ). Copper and Zn released from the forest soils (S1–S3) during the extraction were minimal (<0.2 mg kg^–1), and did not significantly increase with contact times. The amounts of Cu and Zn released from the citrus soils (S4–S13) ranged from 0.5 to 4.0 mg kg^–1 and 1.2 to 8.0 mg kg^–1, respectively, which is significantly higher than those from the forest soils (P < 0.01). The highest amounts of Cu and Zn (8.5 and 4.0 mg kg^–1, respectively) were released from S4 soil, which agrees with its highest levels of Cu and Zn accumulation (Table 1).

View larger version (22K): [in this window] [in a new window]   Fig. 1. Changes of Cu and Zn released from the soils with extraction time. Error bars depict SE.

For most of the soils, the contact time to reach extraction equilibrium was about 24 to 48 h. The results suggest that prolonged waterlogging soil conditions after a rain event can enhance the release of Cu and Zn in the soils, thus increasing their transport potential.

Effects of Soil/Water Ratios
Soil samples were equilibrated with deionized water in various soil/water ratios for 24 h. The quantities of Cu and Zn released per unit of soil weight increased with increasing water content (Fig. 2 ), likely due to increased solubility of some Cu and Zn compounds in the more diluted extraction systems. The differences in Cu and Zn release among the various soil/water ratios were greater in the soils with high concentrations of Cu and Zn than those with low concentrations of Cu and Zn. These results suggest that total amounts of Cu or Zn released from the soil would increase though Cu or Zn concentrations in the surface runoff might decrease with increasing rainfall due to dilution. Similar results were also obtained with P loads in surface runoff, which is closely related to water discharge rate from the field (He et al., 2003a).

View larger version (21K): [in this window] [in a new window]   Fig. 2. The amount of Cu and Zn released from the soils as a function of the soil/water ratio. Error bars depict SE.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Cumulative amount of Cu and Zn released from the soils as a function of leaching events. Error bars depict SE.

Changes of Copper and Zinc Leached from Soils with pH
Leaching solution pH had a differential effect on the leachability of Cu and Zn (Fig. 4 ). The cumulative amount of Zn leached increased with decreasing pH for all the three soils (Fig. 5 ). Others have also reported increased Zn mobility as soil pH decreases (Chuan et al., 1996). However, the changes of Cu leaching with pH were different from that of Zn; leaching of Cu from the soils was at minimum at pH 5 to 7 and increased with an increase or a decrease in pH beyond that range (Fig. 5). The increased amounts of Cu leached at high pH were probably due to increased organic matter dissolution and subsequent increases in the release of organically bound Cu fraction. At low pH (<5.0), the solubility of soil Cu compounds increased, and the desorption of adsorbed Cu was enhanced due to the competition of H⁺ for adsorbing sites and increased positive charge on soil surface, resulting in greater Cu leaching potential from the soils.

View larger version (40K): [in this window] [in a new window]   Fig. 4. Effects of leaching solution pH on the amount of Cu and Zn leached from the soils. Error bars depict SE.

View larger version (25K): [in this window] [in a new window]   Fig. 5. Effects of solution pH on cumulative amounts of Cu and Zn leached from three soils.

View larger version (35K): [in this window] [in a new window]   Fig. 6. Effects of electrolyte type on the release of Cu and Zn from the soils. Error bars depict SE.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The leaching losses of Cu and Zn in the sandy soils are not directly related to their total contents in the soils but are affected by their chemical associations with other soil components, the pH and chemical composition of soil solution, soil moisture, and leaching duration. The release of Cu and Zn within a short time was primarily due to their exchangeable fractions, but carbonate-bound and exchangeable fractions contribute to the released Cu and Zn if the soils are subjected to a long-term leaching. Solution pH has a differential effect on the leaching of Cu and Zn. Leached Zn decreased as pH increased from 3.0 to 9.0, whereas leaching of Cu was at minimum at pH 5–7 and increased at higher or lower pH. The accumulative amounts of Cu and Zn released from the soils increased with increasing soil–water contact time and the water/soil ratio. Increasing Ca concentration in soil solution decreased Cu release, but the reverse was true if Na concentration in soil solution was increased. The release of Zn from the soils with a larger proportion of exchangeable Zn (>23% of the total Zn) was enhanced by raised concentrations of Ca, K, Na, or NH₄ in the soil solution. These findings indicate that the release of Cu and Zn in soil is affected by soil properties, environmental conditions, and management practices such as fertilization and irrigation. The results from this study merit attention in the development of best management practices to reduce the transport of heavy metals from land to water bodies.

Received for publication August 2, 2005.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by He, Z. L. Articles by Stoffella, P. J. Search for Related Content PubMed Articles by He, Z. L. Articles by Stoffella, P. J. Agricola Articles by He, Z. L. Articles by Stoffella, P. J. Related Collections Water Quality Heavy Metals Soil Chemistry