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 Cu2+ and Nor coexisting in animal excretion and in soils. Sorption of Cu2+ 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 Cu2+ and Nor on HA under different solution chemistry conditions (pHs, ionic strengths, and foreign ions). The addition of Nor decreased the maximum adsorption capacity (Qm) of Cu2+ and an increasing effect was observed with increasing Nor concentration. Meanwhile, the addition of Cu2+ also markedly inhibited the sorption of Nor on HA. The Qm of Cu2+ 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 Qm of Cu2+ at the same pH. The Qm of Nor was observed at pH 4.0 without Cu2+, but that was found at pH 5.0 and 3.0 with the addition of 20 and 100 mg L−1 Cu2+, respectively. The sorption of Cu2+ on HA decreased with increasing ionic strength and followed an order of NaH2PO4 > Na2SO4 ≈ NaNO3 at pH 5.0 whether Nor was present or not. Additionally, the higher valence cation had a stronger inhibition effect on Cu2+ sorption. The competition between Cu2+ and Nor for sorption on HA under the same conditions indicated that the coexistence of Cu2+ and Nor may enhance the feasibility of their mobility and environmental risk.