Article pubs.acs.org/est
Disinfection of Ballast Water with Iron Activated Persulfate Samyoung Ahn,†,‡ Tawnya D. Peterson,*,† Jason Righter,† Danielle M. Miles,† and Paul G. Tratnyek† †
Institute of Environmental Health, Division of Environmental and Biomolecular Systems, Oregon Health & Science University, 3181 SW Sam Jackson Park Road, Portland, Oregon 97239-3098, United States ‡ Department of Environmental Education, Sunchon National University, Sunchon, Jeonnam, 540-950, South Korea S Supporting Information *
ABSTRACT: The treatment of ballast water carried onboard ships is critical to reduce the spread of nonindigenous aquatic organisms that potentially include noxious and harmful taxa. We tested the efficacy of persulfate (peroxydisulfate, S2O82−, PS) activated with zerovalent iron (Fe0) as a chemical biocide against two taxa of marine phytoplankton grown in bench-scale, batch cultures: the diatom, Pseudonitzshia delicatissima and the green alga, Dunaliella tertiolecta. After testing a range of PS concentrations (0−4 mM activated PS) and exposure times (1−7 days), we determined that a dosage of 4 mM of activated PS was required to inactivate cells from both species, as indicated by reduced or halted growth and a reduction in photosynthetic performance. Longer exposure times were required to fully inactivate D. tertiolecta (7 days) compared to P. delicatissima (5 days). Under these conditions, no recovery was observed upon placing cells from the exposed cultures into fresh media lacking biocide. The results demonstrate that activated PS is an effective chemical biocide against species of marine phytoplankton. The lack of harmful byproducts produced during application makes PS an attractive alternative to other biocides currently in use for ballast water treatments and merits further testing at a larger scale.
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INTRODUCTION The transport of ballast water is important for the maintenance of stability of ships as they travel at sea. However, the discharge of large volumes of water during ballast water exchange poses a serious threat to marine ecosystems worldwide through the introduction of nonindigenous aquatic species,1−3 threatens human health through the potential transmission of pathogenic organisms,4 and results in significant negative economic impacts due to these threats.5 A notorious example of negative impacts arising from ballast water discharge is the introduction of the zebra mussel Dreissena polymorpha in North America. Since its discovery in Lake St. Clair (Michigan, U.S., and Ontario, Canada) in 1988, it has become established in >50% of waterways in the U.S., with costs on the order of billions of dollars for control.6 Another prominent issue related to ballast water discharge is the apparent global increase in the prevalence of harmful algal blooms (HABs).7−9 In Australia’s Derwent and Huon estuaries, for example, the establishment of a new woodchip mill in the early 1970s coincided with the introduction of the toxic alga Gymnodinium catenatum.10,11 By the late 1980s, toxin-producing dinoflagellates responsible for paralytic shellfish poisoning were present in the ports of Hobart (G. catenatum), Melbourne (A. catenella), and Adelaide (A. minutum).12 Later studies confirmed that the increase of HAB species at these sites was not due to more vigilant monitoring but resulted from a real increase in the presence of nonindigenous dinoflagellate species in Australian waters.13 There are sufficient examples of these and other emerging threats to warrant a globally integrated effort to reduce the potential risk of invasions. In recognition of © 2013 American Chemical Society
these problems, the United Nations International Maritime Organization (IMO) member states adopted an international law, the International Convention for the Control and Management of Ships’ Ballast Water and Sediments. The Ballast Water Convention (BWC) adopted in 2004 requires that ships develop ballast water management plans and control the discharges of invasive species (http://www.imo.org/). The ratification of the BWC has spurred efforts to develop and implement improved technologies to optimize ballast water treatment. Currently, there are more than 50 ballast water management systems available, 15 of which were deemed to have sufficiently detailed testing data available for adequate performance assessment.14 Five types of treatments are prominent: (i) deoxygenation and cavitation, (ii) filtration and chlorine dioxide, (iii) filtration and UV, (iv) filtration + UV + titanium dioxide, and (v) filtration and electro-chlorination. Recommended practices generally involve a filtration step followed by treatment (alone or in combination), which can include UV light, the use of biocides, deoxygenation, ozone, or electric pulse techniques.15 In particular, the use of oxidizing agents to treat ballast water is an attractive option due to their relative ease of application, their efficiency against a wide spectrum of organisms, and their effectiveness under a variety of conditions. Received: Revised: Accepted: Published: 11717
June 5, 2013 September 5, 2013 September 11, 2013 September 11, 2013 dx.doi.org/10.1021/es402508k | Environ. Sci. Technol. 2013, 47, 11717−11725
Environmental Science & Technology
Article
activation by Fe0 is eq 3 where Mn+ is Fe2+ either in solution or as a surface species. A more complete conceptual model requires consideration of the structure of the oxide film that mediates corrosion on Fe0 under such conditions. Initially, oxidization by strong oxidants like PS make the outer oxide surface layer more protective, but eventually this process is balanced with autoreduction of these oxides by the underlying Fe0. This results in a bilayeredstructured passive filmprotypically with an inner layer of Fe3O4 and an outer layer of Fe2O3that conveys Fe(II) sites to the surface where they can react with oxidants from the solution.34,35 The near-surface Fe(II) is likely to be mainly responsible for production of SO4 − (via eq 3)and the resulting oxidation of contaminantsrather than direct contact between PS and Fe0 or Fe(II) in solution. An analogous conceptual model probably applies to systems where Fe0 appears to activate H2O2, producing Fenton-like pathways of contaminant oxidation.36−39 While prior work on Fe0 activated PS (or H2O2) has demonstrated its potential advantages for remediation of contaminated groundwater and soil, other applications of this chemistry have not been explored. Here, we investigate an apparently novel application of PS: as a biocide for the disinfection of ballast water. We hypothesized that the partially passivated Fe0 that comprises the containment vessels for ballast water might provide gradual PS activation and that this system would disinfect ballast water by a combination of direct (oxidation by PS) and indirect (oxidation by SO4•−, HO•, etc.) processes. The proposed application seems consistent with the chemistry of typical ballast waters: e.g., relatively high concentrations of Cl− should favor sustained corrosion of Fe0 and therefore activation of PS, and the resulting addition of SO42− to the water would be inconsequential compared with background levels in seawater. However, the overall performance of such a system is hard to predict from its process components,17,40,41 so this study was designed to survey a range of potentially relevant operating conditions in controlled bench-scale, batch reactors.
The chemical treatment of choice needs to fulfill three criteria: (i) the chemical should not accelerate corrosion of the metal in the ballast water tank walls or cause deterioration of infrastructure; (ii) the chemical should be safe for release upon ballast water discharge; and (iii) the IMO must approve the use of a biologically “active ingredient” through international convention. The latter incorporates elements related to worker safety in handling the toxicant at sea in quantities necessary for effective ballast water treatment.15,16 For many chemical biocides, tests for toxicity have been carried out under conditions that are not relevant to ballast waters, which include, for example, the absence of light.17 Moreover, natural degradation processes for potentially toxic byproducts in the environment often involve bacteria, which themselves may be the targets of the biocide. To make the matter more complex, assessing toxicity under conditions found in the marine environment is often difficult due to variations in temperature, redox conditions, presence of other toxic compounds in the natural environment, as well as the potential toxicity of breakdown products.18 Finally, for microorganisms that do not swim, germinate, or generate visible colonies within a timely period, assessing the viability of the cell can be problematic. One resource that can aid in the identification and evaluation of alternative biocides for ballast water treatment is the extensive body of research that has been done on treatment processes used in purification of drinking water, treatment of wastewater, and remediation of groundwater. In all three of these applications, chemical oxidants have been used extensively. One chemical oxidant with a long history of use in chemical process applications, but that has only recently received widespread attention for use water treatment, is persulfate (peroxydisulfate, S2O82−, PS). In aqueous systems, PS can cause oxidation in three ways (i) by direct reaction of PS, (ii) through sulfate radical (SO4•−) that arises from breakdown of PS, or (iii) through other reactive oxygen species (esp. HO•) that form from radical reactions initiated by sulfate radical. Since the direct oxidation of most contaminants by PS is slow, optimization of water treatment technologies based on PS generally involves accelerating the formation of SO4•−, HO•, and other radicals (i.e., activation). Activation of PS can be achieved by a variety of reactions, including homolysis of the peroxide bond using heat, light, and microwave or ultrasound irradiation (eq 1);19−23 heterolysis of the peroxide bond by OH− (eq 2),20,24,25 or surface-bound hydroxyl moieties;26,27 and an oxidation−reduction process (analogous to the Fenton reaction) involving reduction of PS by low-valent metals (Mn+) such as aqueous Fe(II) and Ag+ (eq 3),19,28,29 ferrous iron oxides like Fe3O4,30 or zerovalent iron (Fe0).31−33
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EXPERIMENTAL SECTION Decomposition of Persulfate in Seawater. Materials and methods for these background experiments are described in the Supporting Information. Phytoplankton Cultures. Two marine algal cultures were used to test the efficacy of PS/Fe0 as a biocide: the diatom, Pseudonitzschia delicatissima, and the green alga, Dunaliella tertiolecta. P. delicatissima was isolated from Monterey Bay, California (by K. Hayashi and provided by R. Kudela, University of California, Santa Cruz), and D. tertiolecta was provided by J. Case (University of California, Santa Barbara). P. delicatissima was identified by scanning electron microscopy and nucleotide sequencing.42,43 Many species within the genus Pseudonitzschia (including P. delicatissima) are capable of producing the potent neurotoxin, domoic acid. Each isolate was grown in triplicate 60 mL culture flasks containing 30 mL f/2 medium44 buffered with 0.005 M Tris and without the addition of EDTA. All stock cultures were maintained in a controlled environmental chamber at 15 °C on a 12:12 h lightdark cycle under a light intensity of 80 μmol photons m−2 s−1 provided by “cool-white” broad-spectrum fluorescent lighting. Preparation of Chemical Biocide. Na2S2O8 was obtained under the Klozur brand from FMC Corp. (Philadelphia, PA). Concentrated stock solutions (100 mM) of persulfate were
Δ or hυ
S2 O82 − ⎯⎯⎯⎯⎯⎯→ 2SO•− 4
(1)
S2 O82 − + OH− → HSO4 − + SO52 −
(2)
S2 O82 − + Mn + → SO4•− + SO4 2 − + Mn + 1
(3)
Although Fe0 might be expected to rapidly reduce PS to sulfate without generating useful quantities of SO4•−, several studies have shown that Fe0 can activate PS to produce rapid and sustained degradation of organic contaminants.32,33 These studies concluded that the most likely mechanism for PS 11718
dx.doi.org/10.1021/es402508k | Environ. Sci. Technol. 2013, 47, 11717−11725
Environmental Science & Technology
Article
prepared in DI water and stored in the refrigerator at 4 °C. Tris buffer (0.005 M) was prepared by adding 0.0606 g of Tris base (Sigma-Aldrich) in 50 mL f/2 media. HCl (6 N, 42 μL) was added to adjust the solution pH to 8.27. Fe0 was obtained from Fluka (Milwaukee, WI; >99%,
