Improving the adsorption capacity of graphene oxide. Effect of Ca2+ on tetracycline retention

Tetracyclines (TCs) constitute a group of antibiotics that are commonly used to treat bacterial diseases, in veterinary medicine and as an additive in animal feed. This broad application has led to their accumulation in food products and the environment because sewage treatment plants cannot completely remove them. Therefore, the aim of this study was to synthesize graphene oxide (GO) and evaluate its TC adsorption properties in aqueous media. The effects of pH (between 2.5 and 11) and Ca²⁺ concentration (between 0 and 1 M) were thoroughly investigated. Structural, textural, and electrokinetic properties of the prepared GO were determined by N₂ adsorption/desorption, XRD, TEM, UV–vis, FTIR, XPS, thermogravimetry and electrophoretic mobility measurements. TC adsorption on GO is an interplay between the two main roles played by Ca²⁺: competitor or bridging cation. At low pH, there is cation exchange, and Ca²⁺ behaves as a competitor of the positively charged TC species, decreasing adsorption as calcium concentration increases. At high, the formation of Ca bridges between the surface and TC (GO-Ca²⁺-TC) is favored, increasing the adsorption of the antibiotic by increasing calcium concentration. Different combinations of Ca²⁺ and pH effects are important to improve the use of GO either as a pH-dependent and reversible TC adsorbent for decontamination or as pH-independent adsorbent for TC quantification with electrochemical sensors.