Sorption of copper and norfloxacin onto humic acid: effects of pH, ionic strength, and foreign ions.

Copper (Cu) and norfloxacin (Nor) are frequently used as feed additives for animal growth promotion, which results in a great probability of Cu and Nor coexisting in animal excretion and in soils. Sorption of Cu and Nor on soil organic matter (SOM) can markedly affect their environmental fate. Thus, humic acid (HA), a major fraction of SOM, was chosen to investigate the cosorption behaviors of Cu and Nor on HA under different solution chemistry conditions (pHs, ionic strengths, and foreign ions). The addition of Nor decreased the maximum adsorption capacity (Q) of Cu and an increasing effect was observed with increasing Nor concentration. Meanwhile, the addition of Cu also markedly inhibited the sorption of Nor on HA. The Q of Cu increased with increasing pH from 3.0 to 5.0 whether Nor was present or not, but more addition of Nor led to less increment in Q of Cu at the same pH. The Q of Nor was observed at pH 4.0 without Cu, but that was found at pH 5.0 and 3.0 with the addition of 20 and 100 mg L Cu, respectively. The sorption of Cu on HA decreased with increasing ionic strength and followed an order of NaHPO > NaSO ≈ NaNO at pH 5.0 whether Nor was present or not. Additionally, the higher valence cation had a stronger inhibition effect on Cu sorption. The competition between Cu and Nor for sorption on HA under the same conditions indicated that the coexistence of Cu and Nor may enhance the feasibility of their mobility and environmental risk.