Effects of Humic Acid and Water Hardness on Acute Toxicity and Accumulation of Cadmium in the Freshwater Fish (Puntius gonionotus Bleeker)

The effects of humic acid and water hardness on acute cadmium toxicity and accumulation were determined, using Puntius gonionotus Bleeker. The toxicities were expressed as the median lethal concentration (LC50) and the 96-h LC50 value of cadmium was 2.30 mg/L. Three different types of water hardness were used: soft, moderately-hard and hard, with total hardness concentrations of 55, 113, and 225 mg/L CaCO3, respectively. The acute cadmium toxicity and accumulation significantly decreased by increasing water hardness concentration. An increase in humic acid concentrations (0.5, 5.0, and 50 mg/L) also decreased cadmium toxicity and accumulation. Moreover, simultaneous increases in humic acid and water hardness concentrations also resulted in a decrease in cadmium toxicity. However, they were less effective in decreasing cadmium accumulation than the effect of water hardness or humic acid alone.