Differentiating metal from ammonia toxicity in toxicity identification evaluations

摘要

Ammonia is a common nutrient and originates from sewage, industrial ana farm wastes, fertilisers, and from natural decomposition processes (Sarda and Burton 1995). In water ammonia exists as un-ionised ammonia (NH_3) and as the ammonia ion (NH4+). The toxicity of ammonia solutions to freshwater macroinvertebrates is primarily attributed to the NH3 (the un-ionised species), with the ammonium ion (ionised species) being relatively less toxic (Kendall 1986). High concentrations of ammonia often co-occur with high levels of heavy metals and organic pollutants. This can lead to confounding toxicity test results (Ankley et al. 1990; Ankley and Schubauer-Berigan 1995; Ferretti et al. 2000). Distinguishing ammonia toxicity from toxicity caused by metals or organic pollutants is therefore important in determining appropriate methods for ecological risk assessment. Toxicity Identification Evaluation (TIE) studies have indicated that ammonia is often partly responsible for the toxicity of sediment samples (Ankley et al. 1990; Schubauer-Berigan and Ankley 1991; Burkhard and Jenson 1993). Within standard TIE procedures, the graduated pH test and EDTA-chelation are treatments that have been proposed as methods to differentiate between ammonia and metal toxicity (Norberg-King et al. 1991). EDTA-chelation is used as an indicator of metal toxicity (Norberg-King et al. 1991; Hockett and Mount 1996). However, it is not clear from literature whether EDTA can also influence ammonia toxicity. Most positively chargedions will interact with EDTA to some extent, but especially divalent transition metals, including cadmium, copper, nickel, lead and zinc, have high affinities (Garvin 1964). The toxicity of ammonia is pH dependent because of the pH-dependent equilibriumbetween ionised and unionised ammonia. However, the toxicity of metals and organic acids is also dependent on the pH (Norberg-King et al. 1991; Schubauer Berigan et al. 1993). Therefore, toxicity changes after pH adjustments can not be interpreted as resulting exclusively from ammonia and changes in toxicity after addition of EDTA can not be exclusively related to metals. As alternatives, the equitoxic solution test and the zeolite test can be used (Mount et al. 1989). The equitoxic solution test is difficult to perform because pH adjustments and pH control within 0.1 pH unit is required. The zeolite test has the following disadvantages: both ammonia and metals are removed from porewater and most important, the material can leach toxic artefacts (Mount et al. 1989).