VIEWPOINT pubs.acs.org/est
Dissolved Organic Carbon a Practical Consideration in Application of Biotic Ligand Models in Chinese Waters Hao Chen,†,* Binghui Zheng,† Fengchang Wu,† and Kuen Benjamin Wu†,‡ †
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China ‡ HydroQual Inc., Mahwah, New Jersey 07430, United States
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he biotic ligand models (BLMs), which were originally developed upon the acute copper toxicity to fishes as a result of binding of the copper ions to the fish gills, has evolved in the last two decades to cover a range of aquatic organisms and heavy metal ions and has proven valid in a wide range of water chemistry variables.1 3 The BLMs explicitly consider that only the biotic ligand-bound metal speciation is responsible for the toxicity to the organisms, and recommend that the bioavailability of the metals be taken into consideration when making environmental criteria and risk assessment of metals in waters. The U.S. Environmental Protection Agency (EPA) has recommended the use of the BLMs in establishment of copper water quality criteria (WQC). The European Union (EU) has launched an European Union Water Framework Directive, and requires that all EU member states to ensure that all inlands and coastal waters achieve good water quality status by 2015, and in particular evaluation of chronic toxicity of metals (e.g., nickel) to organisms by means of BLMs has been suggested to be incorporated into WQC regulatory framework. The BLMs are able to calculate, based upon some measurable water chemistry variables, the toxic metal speciation bound by the biotic ligands in the organisms. One of the most important water chemistry variables affecting the quantity of bioavailable speciation for a given toxic metal is dissolved organic carbon (DOC), and its concentration is usually determined for 0.45-μm r 2011 American Chemical Society
filtered water samples on modern total organic carbon (TOC) analyzers which may employ high temperature (e.g., >800 °C) combustion or wet chemistry oxidation to “completely” oxidize the organic carbon. However, it is expensive to measure DOC on a regular basis in many laboratories, and DOC values have long been missing in the regular water quality monitoring in Chinese waters. Although Chinese government and scholars have realized the limitations of implementation of WQC only based upon total dissolved metal concentrations in the waters, implementation of the BLMs to WQC in Chinese waters still has a long way to go. The first step, among many possible to-do lists (e.g., selection of typical Chinese species sensitive to a given toxic metal in question), may be to include the DOC measurement as a necessary item in regular water quality monitoring in some regions where the measurements are affordable to the local environmental monitoring agency laboratories, and such practice may be spreading gradually to other regions after careful assessment of the action. In the meantime, a technical guideline of DOC measurements should be made to take into account the diversity of aquatic environments nationwide that may impose strong effects on choice of appropriate approaches for DOC measurements. For instance, given low DOC but relatively high total inorganic carbon (IC) in a specific region, nonpurgeable organic carbon (NPOC) mode is a better choice than the conventional mode of TC-TIC (i.e., subtraction of the total inorganic carbon from the total carbon) on a TOC analyzer that employs high temperature combustion methods. Application of inappropriate DOC methods would lead to up to several factors of errors in the DOC values. In contrast to DOC measurements, chemical oxygen demand (COD) has long been a mandatory item in Chinese water quality monitoring, and as such, a large body of data has been collected nationwide, and this situation will continue. An approach alternative to direct DOC measurement is to establish a regionspecific relationship between DOC and COD, and the DOC in the regions may therefore be estimated accordingly. Similar to establishing a relationship for COD vs DOC, it is also possible to establish a region-specific relationship for UV vis absorbance vs DOC, as UV vis absorbance measurements have been commonly practiced on simple benchtop instruments with enough precision in many laboratories nationwide in China. However, Received: October 25, 2011 Accepted: October 25, 2011 Published: November 03, 2011 9835
dx.doi.org/10.1021/es203690w | Environ. Sci. Technol. 2011, 45, 9835–9836
Environmental Science & Technology
VIEWPOINT
cautions must be taken to protect against utilization of a given region-specific relationship to other regions. Another important issue in application of the BLMs may be related to calibration of the BLMs against site-specific DOC compositions. Although the BLMs have been well calibrated in U.S. and EU, especially in a wide range covering ca. 80% water environment conditions in the latter, it is not certain whether the generic parameters (e.g., log K values for metal binding by natural organic matter, NOM) validated in U.S. and EU conditions may be applicable to Chinese waters nationwide, as anthropogenic inputs of non-NOM organic carbon into the polluted waters show dramatic temporal and spatial variation nationwide. As compared to the relative “cleaner” waters in EU and U.S., the composition of the non-NOM organic materials and their binding abilities to metals in question in Chinese waters impose a big challenge in application of the BLMs. The above-mentioned issues are not only limited to Chinese waters, but also relevant to waters in other developing countries with elevating environmental issues and problems. Adaptation of the BLMs to typical waters in these countries and regions therefore should take into consideration the specific conditions (e.g., economical and technical aspects).
’ AUTHOR INFORMATION Corresponding Author
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’ ACKNOWLEDGMENT This comment came from the fruitful discussion in The First International Workshop on Nickel Biotic Ligand Models in Chinese Waters in 2011. We are grateful to the Nickel Producers Environmental Research Association (NIPERA) for financial support. ’ REFERENCES (1) Playle, R.; Dixon, D.; Burnison, K. Copper and cadmium binding to fish gills: 331 Modification by dissolved organic carbon and synthetic ligands. Can. J. Fish. Aquat. Sci. 1993, 50 (12), 2667–2677. (2) Di Toro, D.; Allen, H.; Bergman, H.; Meyer, J.; Paquin, P.; Santore, R. Biotic ligand model of the acute toxicity of metals. 1. Technical basis. Environ. toxicol. chem. 2001, 20 (10), 2383–2396. (3) Wilkinson, K. J.; Slaveykova, V. I. Predicting the bioavailability of metals and metal complexes: Critical review of the biotic ligand model. Environ. Chem. 2005, 2 (1), 9–24.
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dx.doi.org/10.1021/es203690w |Environ. Sci. Technol. 2011, 45, 9835–9836
