The influence of humic acid on the biological effects of selected trace metals

The influence of humic acid (HA) on the biological effects of cadmium, hexavalent chromium, and two forms of trivalent chromium (chromic chloride and chrome lignosulphonate) was assessed using common freshwater invertebrates. The acute toxicity was determined using Daphnia pulex, while D. magna was used in bioaccumulation studies. Final concentrations of HA tested were 0, 0.5, 5, and 50 mg/l. Humic acid was observed to significantly increase, decrease, or not influence both the acute toxicity and bioaccumulation of cadmium depending on HA concentration and time point examined. With hexavalent chromium, HA had little influence on the biological effects. The acute toxicity was significantly decreased by 48 h with 5 and 50 mg/l HA, while bioaccumulation was significantly decreased by 96 h with 50 mg/l HA. No significant effects were observed at the remaining time points with either acute toxicity or bioaccumulation. While HA had little influence on the biological effects of hexavalent chromium, 50 mg/l HA significantly decreased acute toxicity and bioaccumulation at the majority of the time points examined with both trivalent chromium compounds. The remaining two HA concentrations (0.5 and 5 mg/l) either had no influence or significantly decreased the toxicity and uptake of the compounds.; Additionally, dialysis studies were used to determine the influence of HA on the bioavailability of the metals. As with the biological effects, HA had little influence on the bioavailability of hexavalent chromium, while having a greater influence on cadmium and chromic chloride. There was a HA concentration dependent pattern of binding (a maximal decrease in percent free metal followed by metal release from HA) observed with cadmium and chromic chloride. This metal release from HA may have resulted from an associated time dependent decrease in pH of the surrounding water. Humic acid had little influence on the bioavailability of chrome lignosulfonate.