Effects of Humic Acids on the Toxicity and Accumulation of Zinc and Cadmium to Chironomid Larvae, Rheocricotopus spp.

Humic substances in aquatic environments can bind metal ions in their carboxylic groups, thereby altering the toxicity and availability of metal ions to aquatic organisms. This study investigated the effects of dissolved humic acids (HAs) on the toxicity and accumulation of waterborne zinc (Zn2+) and cadmium (Cd2+) ions in the larvae of Rheocricotopus species (Chironomidae, Diptera). Chironomid larvae were separately exposed to Zn2+ (0.1 and 1.0 mg/L) and Cd2+ (0.03 and 0.1 mg/L) in the presence of dissolved HAs at concentrations ranging from 0 to 50 mg/L. The metal species and the complexation capacity of dissolved HAs were predicted using the model WinHumicV for speciation modeling. The uptake and accumulation of free [Zn2+] and [Cd2+] in the larval bodies were determined using the ion-selective fluorescence probes; however, the total metal ions were determined through the acid digestion method. WinHunicV model estimated a progressive decrease in the availability of free [Zn2+] and [Cd2+] in the test solutions as the concentration of metal–humic acid complexes increased. The accumulated free [Zn2+] and [Cd2+] in chironomid larvae were found to be statistically significant as the concentrations of dissolved HAs increased (i.e., ≥ 10 mg/L) in both zinc and cadmium treatments. The total Cd2+ accumulation by chironomid larvae exposed to 0.1 mg/L test solution with dissolved HAs did not show notable change. Despite the decrease in free [Zn2+] and [Cd2+] activity due to dissolved HAs, the total accumulated Zn2+and Cd2+ in the larvae were higher than predicted on the basis of free metal ions activity. Thus, the acute exposure study revealed that the presence of dissolved HAs in aquatic environments reduced the toxicity and accumulation of metal ions in chironomid larvae.