Chronic Toxicity of Iron to Aquatic Organisms Under Variable pH, Hardness, and Dissolved Organic Carbon Conditions.

A series of chronic toxicity tests were conducted exposing three aquatic species to iron (Fe) in laboratory freshwaters. The test organisms included the green algae Raphidocelis subcapitata, the cladoceran Ceriodaphnia dubia, and the fathead minnow Pimephales promelas. They were exposed to Fe (as iron (III) sulfate) in waters under varying pH (5.9 to 8.5), hardness (10.3 to 255 mg/L CaCO 3 ), and dissolved organic carbon (DOC) (0.3 to 10.9 mg/L) conditions. Measured total Fe was used for calculations of biological effect concentrations as dissolved Fe was only a fraction of nominal and did not consistently increase as total Fe increased. This was indicative of the high concentrations of Fe required to elicit a biological response and that Fe species that did not pass through a 0.20 or 0.45 µm filter (dissolved fraction) contributed to Fe toxicity. The concentrations frequently exceeded the solubility limits of Fe(III) under circumneutral pH conditions relevant to most natural surface waters. Chronic toxicity endpoints (10% effect concentrations; EC10s) ranged from 442 to 9,607 µg total Fe/L for R. subcapitata growth, from 383 to 15,947 µg total Fe/L for C. dubia reproduction, and from 192 to 58,308 µg total Fe/L for P. promelas growth. Toxicity to R. subcapitata was variably influenced by all three water quality parameters, but especially DOC. Toxicity to C. dubia was influenced by DOC, less so by hardness, but not by pH. Toxicity to P. promelas was variable, but greatest under low hardness, low pH, low DOC conditions. These data were used to develop an iron-specific, bioavailability-based multiple-linear regression model (MLR) as part of a companion publication. This article is protected by copyright. All rights reserved. Environ Toxicol Chem 2023;00:0-0. © 2023 SETAC.