Toxicity screening of metals with special reference to quantitative approach

A series of metals Cr(6+), Al(3+), Cd(2+), Pb(2+), Cu(2+), Zn(2+), and Hg(2+) were tested in three systems--Microtox, Motility Test, and Growth Zone Inhibition Test. Toxicity endpoint of each metal was variable from system to system. Of the three systems, Microtox was the most sensitive system. In this system, Hg(2+) reacted as the most toxic element having EC(50) value 0.08 mg/L while Cd(2+) and Cr(6+) were least toxic with their EC(50) values 19.4 and 21.0 mg/L respectively. MEC(90) value of Motility Test was always needed more concentrations of toxicant in comparison to other systems. In comparison to Microtox, ten times more concentration of Hg(2+) (1.4 mg/L) was required to find out its MEC(90) value. Growth Zone Inhibition Test was very simple method from handling point of view. The usual practice of evaluation of toxicity screening in this system is either qualitatively or semi-qualitatively. Hence a study was designed to establish a quantitative technique, Growth Inhibition Test, as an alternative to this test using the same sensor organism B. cereus, which allows determination of MAC as well as MIC. MIC for Hg(2+) was found to be 0.03 mg/L in Growth Inhibition Test while the same element was needed more concentration (1.0 mg/L) in the case of Growth Zone Inhibition test to produce halo. However, all these systems including Growth Inhibition Test showed Hg(2+) was the most toxic element.