Influences of pH and metal ions on the interactions of oxytetracycline onto nano-hydroxyapatite and their co-adsorption behavior in aqueous solution

Abstract

In this study, interaction of oxytetracycline (OTC) onto nano-hydroxyapatite (nHAP) was evaluated as affected by pH and metal ions. Results showed that the adsorption process of OTC was highly pH- and metallic species-dependent. The amount of sorbed OTC at four pH were in the order of pH 8.0 > pH 10.0 > pH 5.5 > pH 3.0 and reached equilibrium around 120 min, indicating adsorption affinity of four species to nHAP followed the order of OTC − > OTC 2− > OTC ± > OTC + . Adding metal ions greatly increased the distribution coefficient (K d ) of OTC between adsorbents and aqueous phases, following the order of Fe 3+ > Cu 2+ > Pb 2+ > Cd 2+ = Ca 2+ in the pH range of 3.0–10.0. Moreover, the co-adsorption behavior of OTC and heavy metals onto nHAP was also explored at pH 5.5 for the first time. OTC adsorption was significantly enhanced with the co-existence of 0.25 mmol/L Cu 2+ or Pb 2+ . Inversely, the presence of 0.25 mmol/L OTC slightly led to the improvement of Cu 2+ adsorption and depression of Pb 2+ adsorption, yet Pb 2+ adsorption was obviously promoted with the co-existence of 0.1 mmol/L OTC. Meanwhile, adsorption of OTC and Cd 2+ showed unapparent variation in single or binary systems. The bridging effect involving metal ions, O- and N- containing groups in OTC molecules, and [tbnd]CaOH or [tbnd]POH sites of nHAP resulted in the formation of ternary complexes which were responsible for the promotion of their adsorption, while dissolution-precipitation was another key mechanism with the co-existence of Pb 2+ .