The leachability, bioaccessibility, and speciation of Cu in the sediment of channel catfish ponds

Abstract

There have been growing concerns about the environmental impact of Cu applied in the catfish pond aquaculture. In this paper, sediments taken from three commercial catfish ponds were studied for content, leachability, bioaccessibility, and speciation of sediment-bound Cu. Results showed that the Cu was concentrated in the top 10 cm of the sediments and the peak Cu concentrations ranged from the background level to about 200 mg/kg. Toxicity characteristic leaching procedure showed only 1–8% of sediment Cu was leachable while bioaccessible Cu, evaluated by physiological based extraction test, accounted for up to 40–85% of total Cu. Due to the high redox potential in the surface sediments, acid-volatile sulfide was not a significant binding phase. The sequential extraction results showed that the residual phase (forms in lattices of primary and secondary minerals) was the major Cu fraction in the first two pond sediments but carbonate-bound, Fe/Mn oxide-bound and organically bound Cu, as well as the residual fraction, seemed equally important in the third pond.

Introduction

The channel catfish (Ictalurus punctatus) has been farmed extensively in the central and southern USA for many decades. The water surface being used for catfish production in the United States has expanded from about 1000 hectares (ha) in the early 1960s (Boyd et al., 2000) to about 80,000 ha in 2001 (USDA, 2004). One of the noteworthy chemicals often encountered in aquaculture practice is copper sulfate, which has been the most commonly used algaecide for about a century in the USA to control the off-flavor problem (e.g. fishy taste) caused by blue-green algae in channel catfish ponds (Riemer and Toth, 1970). As a matter of fact, copper sulfate pentahydrate (CuSO₄ · 5H₂O) is currently the only algaecide approved by the USEPA for the use in catfish ponds (USEPA, 2003). Typically, copper is applied to the fishponds at about 1% (w/w) of the total alkalinity of the pond water, and applications are made at intervals of 2–10 days from early summer to early fall. Individual ponds typically receive about 50 kg/ha of CuSO₄ · 5H₂O or about 12.5 kg/ha Cu each year. Based on these numbers, in 2001, the 80,000 ha of channel catfish ponds in the USA (mainly in Alabama, Arkansas, Louisiana, and Mississippi) received a total of 4,000,000 kg of CuSO₄ · 5H₂O or 1,000,000 kg of Cu (USDA, 2004). According to Liu et al. (2006), nearly 100% of the applied Cu would transfer to the pond sediment. Although contaminated sediments in rivers, lakes and other surface water bodies have been fully studied for decades (Allen, 1995, Förstner and Wittmann, 1981, Locat et al., 2003, Vdovic et al., 2006), there have been few reports to evaluate the potential environmental problems associated with such a high amount of Cu accumulated in the sediments of channel catfish ponds.

Cu contamination in the catfish pond sediment might be different from that in other surface water sediments due to the following reasons:

• Catfish ponds receive a high load of copper as mentioned above. Most of it accumulates in the sediment.

• Pond sediment is frequently disturbed by aquaculture management activities. For example, in order to increase dissolved oxygen content in the water and reduce the accumulation of the toxic substances in the bottom, the pond sediment is often exposed to the air through pond water aeration, forced water circulation, and sediment drying and even tilling. Hence, pond sediment environments may differ from those in the lakes or reservoirs in terms of the anaerobic/aerobic conditions, which may affect the speciation and thus availability of Cu in the sediment.

• Periodic drainage and sediment removal result in the frequent output of Cu-laden sediment to the surrounding water bodies or farmland, which makes environmental evaluation of Cu-laden pond sediment necessary.

Although Boyd et al. (2000) have proposed several best management practices for catfish aquaculture to prevent the Cu-laden sediment from entering the environment, assessment of the leachability, bioavailability, and speciation of sediment-bound Cu were not included in the paper. Han et al. (2001) have researched the Cu speciation in the sediments of commercial catfish ponds using the sequential extraction procedures (SEP), indicating the Cu was primarily bound with the carbonates, organic matter and iron oxides. Nevertheless, in Han et al.'s paper, sediment samples were exposed to the air during sampling, transporting and treatment and the anaerobic condition, under which the sediments usually existed, was not taken into account at all. This might have resulted in an underestimate of the importance of metal-sulfide binding in the anaerobic horizons characteristic of most natural in-place sediment.

Di Toro et al. (2001) revealed that the toxicity of a heavy metal resulted from a competitive equilibrium in the solution among the heavy metal ion, other co-existing cations, organic and inorganic ligands, and the biotic ligand (the target organ) through the competitions between the heavy metal and the co-existing cations for the biotic ligand and between biotic ligand and other ligands for the heavy metal. Toxicity and mortality occur when the competing result, or the concentration of the metal bound to the biotic ligand, exceeds a threshold concentration (Di Toro et al., 2001). Therefore, if there are enough strong binding ligands such as sulfide and carbonate to quantitatively sequester the metal through forming precipitants, theoretically, there would be no or much less biotic ligand-available metal in the solution to exert the toxicity toward organisms. In this case, the metal is considered to be biologically unavailable and hence nontoxic. Thus, the availability of a heavy metal is controlled by strong metal-binding solid phases (Bull and Williamson, 2001). In freshwater sediment the major binding phases with Cu are carbonates, Fe/Mn oxides, organic matter, acid volatile sulfides (AVS) and clay minerals (Fang et al., 2005, Peng et al., 2004, Yu et al., 2001). Commonly used methods to assess the speciation of Cu in anaerobic sediment are sequential extraction procedures (Peltier et al., 2005, Sáenz et al., 2003, Yu et al., 2001) and acid-volatile sulfides (AVS)/simultaneously extracted metals (SEM) method (Ankley et al., 1993, Berry et al., 1996, Fang et al., 2005, Hansen et al., 1996, Yu et al., 2001).

Another important factor controlling the availability of the soil/sediment-bound Cu is the soil solution chemistry (Amonette, 2002), i.e., the properties of the environment in which the soil-bound chemical is present. For example, the availability of soil-bound Cu might be low in the natural water bodies such as lakes or rivers with a pH around 7. However, if this soil were to be swallowed accidentally by cattle or human beings, its availability would likely increase several times in the stomach where the solution pH is around 2. There are several methods available to evaluate the availability of soil/sediment bound metals under different solution environments such as the toxicity characteristic leaching procedure (TCLP) and physiological based extraction test (PBET). The former is often applied to evaluate the leachability of solid waste in the natural environment (especially in landfill) (Ghosh et al., 2004, USEPA, 1992) and the latter to evaluate the bioaccessibility of contaminates in animal or humans' stomach (Fendorf et al., 2004, Ruby et al., 1996, Yang et al., 2002).

Generally speaking, the major purpose of this paper is to evaluate the leachability, bioaccessibility, and speciation of Cu in the catfish pond sediment. The detailed objectives are as follows:

• to assess the role of sulfides in binding Cu in fishpond sediment using the AVS/SEM extraction method;

• to evaluate the leachability of the Cu-laden sediment using the TCLP method;

• to estimate the bioaccessibility of the Cu-laden sediment using the PBET method;

• to study the speciation of Cu in different sediments using the sequential extraction procedure (SEP); and

• to explore the relationship between Cu speciation and its environmental availability.