Metal ion-oxytetracycline interactions on maize straw biochar pyrolyzed at different temperatures

The interaction between metal ions and antibiotics greatly influences their fate and transport in the environment, but knowledge on the interaction between metal ions and antibiotics to biochar is still limited. Sorption of oxytetracycline (OTC) and metal ions (Cu2+ or Cd2+) to biochars pyrolyzed at 300 °C (BC300) and 600 °C (BC600) in single or binary systems (OTC with metal ion) were investigated. The sorption and interaction mechanisms involved were also explored. The results showed that biochars can effectively adsorb OTC, Cu2+ and Cd2+. The Langmuir parameter qmax for OTC was 26.12 mmol kg−1 for BC600, and Freundlich parameter Kf for Cu2+ and Cd2+ were 92.40 mmol1−n Ln kg−1 and 83.42 mmol1−n Ln kg−1 for BC300, respectively. The pyrolytic temperature of the biochars and metallic species co-determines the adsorption interaction of both OTC and metal ions in binary systems. In the presence of Cu2+, sorption of OTC to BC300 was dramatically enhanced, yet to BC600 it was insignificantly affected. Inversely, in the presence of OTC, the sorption of Cu2+ to BC600 was improved, yet to BC300 it was not influenced. The interaction among OTC, Cu2+ and biochars, namely the bridging effect, led to the formation of ternary complexes that enhanced the adsorption of biochars. However, the interaction between OTC and Cd2+ was not found through the UV-vis spectrum scanning, and the sorption of OTC with Cd2+ to the biochars was independent.