Background organic matter significantly influences the removal of emerging contaminants in natural water. In this work, the adsorption of ciprofloxacin (CPX) onto a series of magnetic multifunctional resins (GMA10-GMA90) in the presence and absence of humic acid (HA) was conducted to demonstrate the effect of HA. Both hydrophobic and ion exchange interactions contributed to CPX adsorption. Negative charge-assisted hydrogen bonds also participated in the adsorption process, resulting in the high adsorption amount of anionic CPX onto the negatively charged GMA30 under basic solutions. HA could impact CPX adsorption not only as a competitive adsorbate but also as an additional adsorbent. At pH 5.6, the additional adsorption sites provided by adsorbed HA molecules on the resins dominated and thus facilitated the adsorption process. While at pH 10, HA inhibited the adsorption of CPX by directly competing for ion exchange sites and coexisting with CPX in the solution. The ratio of the amount of CPX adsorbed by dissolved HA to that by the resin reached as high as 1.61 for GMA90. The adsorbed HA molecules onto the resins could provide additional adsorption sites for CPX as proven by the enhanced CPX adsorption in HA-preloading systems at pH 5.6.
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