Contact toxicity of metals in sewage sludge: evaluation of alternatives to sodium chloride in the microtox® assay

Abstract

The presence of chloride ions in the Microtox® test can cause problems when testing metal toxicity, both due to extraction of metals from solid samples and formation of chloro complexes of metals in the liquid phase. To investigate alternatives to NaCl in the Microtox test, the toxicity of Cu, Cd, Pb, and Zn to Photobacterium phosphoreum was tested in 28 osmotic surrogates for NaCl. It was found that Na+ must be present to keep the blank luminescence stable for 30 min. The results point to NaClO4 as the most satisfactory surrogate solution as it has an inert behaviour and does not form complexes with any metal of environmental interest. Raw, digested, and reference sewage sludges were tested in the osmotic surrogates. The EC50 values for sludges were lower in solutions of NaNO3, Na2SO4, and NaClO4, and higher in sucrose, mannitol, and KCl, compared to NaCl. NaClO4 can be recommended as an osmotic surrogate for sewage sludge testing. Another problem with the Microtox assay is the lack of pH control in the cuvette. Copper toxicity tests were carried out in Tris buffer and KH2PO4 at two different concentrations and at pH 7 and 8. The results show that 1 mM phosphate buffer, pH 7.0, can be recommended for solid samples. However, owing to formation of KClO4 a buffer containing potassium is not recommended in combination with NaClO4.

Lutetium Speciation and Toxicity in a Microbial Bioassay: Testing the Free-Ion Model for Lanthanides

“…Because of reported problems with pH control in this biotest system (25,32), we checked the pH at the end of the experiments by collecting the contents of three replicate wells (ca. 650 µL) in an Eppendorf vial with a pipet.…”

Metal Bioavailability and Toxicity in Sediments

“…Toxicity due to solids associated metals has been shown (Carlson-Ekvall & Morrison, 1992). Problems with using NaCl as an osmotic diluent have been studied, with some suggested replacements (Carlson-Ekvall & Morrison, 1995). This test has been available for several years, but has not been widely adopted, due to a number of uncertainties regarding data interpretation.…”

Discriminating the Ecotoxicity due to Metals and to Low pH in Acid Mine Drainage

“…Further-more, chloride ions can change the speciation of metals, thus altering the true toxicity of the e%uent (Kross and Cherryholmes, 1993;Carlson-Ekvall and Morrison, 1995).…”