Fate, transport, and environmental availability of copper(II) applied in catfish aquaculture ponds and enhanced immobilization of soil-bound lead using a new class of stabilized iron phosphate nanoparticles
Copper sulfate has been the most commonly used algaecide for about a century in the U.S. to control the off-flavor problem caused by blue-green algae in channel catfish (Ictalurus punctatus) ponds. In 2001, the ~80,000 hectares of channel catfish ponds in the U.S. received a total dose of 4,000,000 kg of CuSO4•5H2O or 1,000,000 kg of Cu2+. However, no detailed studies have been available pertaining to the potential adverse impacts of the copper applied in catfish ponds on human and environmental health. A pilot-scale study and various field measurements at commercial ponds were conducted to investigate the environmental fate of copper applied as an algaecide in catfish ponds. In the pilot study, a total of 774 g Cu(II) were applied to an experimental catfish pond over a period of 16 summer weeks. Copper mass balance indicated that virtually all Cu(II) applied was retained in the sediment. Approximately 0.01% of the total Cu applied was taken up by fish and 0.1% remained in pond water. Data from three commercial fishponds of different ages (1-25 years) and with different sediment types (acidic, neutral and calcareous) supported the pilot-scale observation. Field monitoring of groundwater quality suggested that the copper leaching into the groundwater surrounding the ponds was insiginificant. Sediments taken from the three commercial catfish ponds were studied for content, leachability, bioaccessibility, and speciation of sediment-bound Cu(II). Results showed that copper was concentrated in the top 10 cm of the sediments. Leachability tests based on the toxicity characteristic leaching procedure (TCLP) showed ~1-8% of sediment-bound copper was leachable, while the bioaccessible copper, determined following a physiological based extraction test (PBET) procedure, accounted for up to ~40-80% of total Cu. Becasue of the high redox potential in the surface sediments, acid volatile sulfide was not a significant sink for copper. Tests following a sequential extraction method revealed that the residual phase copper (i.e. Cu bound in the lattices of primary and secondary minerals) was the major Cu fraction in the ponds with acidic and calcareous sediments but carbonate-bound, Fe/Mn oxide-bound and organically bound Cu, as well as the residual fraction, seemed equally important in the pond with neutral sediment. Effects of various soil fractions and soil compositions on the leachability and bioaccessibility of soil-bound Cu were investigated with three representative soils (calcareous, neutral, and acidic). The synthetic precipitation leaching procedure (SPLP) was used to assess the metal leachability, the PBET was used to assess the bioaccessibility, and a selective dissolution approach was applied to fractionate the soil fractions. Data showed that soil carbonates played an important role in Cu desorption from soil. The leachability of Cu bound in carbonate-rich soils was less than that in soils with lower carbonate content. However, the bioaccessibility of copper in carbonate-rich soils was greater than that for soils with low carbonate content. Leachability and bioaccessibility of Cu in different particle size fractions fractionated on were found to be correlated with the carbonate contents in each fraction. Results also showed Fe/Mn oxides, organic matter and clay minerals are responsible for Cu retention under acidic leaching conditions, and clay minerals consistently showed the strongest affinity for Cu. This study developed a new class of iron phosphate (vivianite) nanoparticles, prepared with sodium carboxymethyl cellulose (CMC) as a stabilizer, and tested the feasibility of applying the nanoparticles for in-situ immobilization of lead (Pb2+) in soils. TEM measurements indicated that the mean particle size was about 8.4±2.9 nm (standard deviation). Batch test results showed that the CMC-stabilized nanoparticles can effectively reduce the TCLP leachability and PBET-based bioaccessibility of Pb2+ in the 3 representative soils. When the soils were treated with the nanoparticles at a dosage ranging from 0.61 to 3.0 mg as PO4 3-/g-soil for 56 days, the TCLP leachability of Pb2+ was reduced by up to 95%, whereas the bioaccessibility of Pb2+ in the soils was reduced by 31~47%.