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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. [ABSTRACT FROM AUTHOR]

The absorption of humic acids on iron oxide-coated silicon plates were shown to be dependent on pH, with a decrease in pH increasing humic acid absorption. Desorption was achieved either through a change in pH with no corresponding change in the concentration of the humic acid or upon dilution with no corresponding change in pH. Desorption using the change in pH was found to be reversible, with additional increases in pH accelerating the desorption process.

The isoelectric point (IEP) of theedge surface of a montmorillonitesample was determined by using electrophoretic mobility measurements.This parameter, which is fundamental for the understanding of thecharging behavior of clay mineral surfaces, was never measured sofar because of the presence of permanent negative charges within themontmorillonite structure, charges that mask the electrokinetic behaviorof the edges. The strategy was to block or neutralize the structuralcharges with two different cations, methylene blue (MB+) and tetraethylenepentaminecopper(II) ([Cu(tetren)]2+), so that the charging behavior of the particles becomes that ofthe edge surfaces. Adsorption isotherms of MB+and [Cu(tetren)]2+at different ionic strengths (NaCl) were performed to establishthe uptakes that neutralize the cation exchange capacity (CEC, 0.96meq g–1) of the sample. At high adsorptive concentrations,there was a superequivalent adsorption of MB+(adsorptionexceeding the CEC) and an equivalent adsorption of [Cu(tetren)]2+(adsorption reaching the CEC). In both cases, structuralcharges were neutralized at uptakes very close to the CEC. Zeta potential(ζ) vs pH data at different ionic strengths of montmorillonitewith adsorbed MB+allowed to estimate an upper limit ofthe edge’s IEP, 5.3 ± 0.2. The same kind of data obtainedwith adsorbed [Cu(tetren)]2+provided a lower limit ofthe IEP, 4.0 ± 0.2. These values are in agreement with previouslyinformed IEP and point of zero charge of pyrophyllite, which is structurallyanalogous to montmorillonite but carries no permanent charges. Theimportance of knowing the IEP of the edge surface of clay mineralsis discussed. This value characterizes the intrinsic reactivity ofedges, that is, the protonating capacity of edge groups in absenceof any electric field generated by structural charges. It also allowsus to correct relative edge charge vs pH curves obtained by potentiometrictitrations and to obtain the true edge charge vs pH curves at differentelectrolyte concentrations. [ABSTRACT FROM AUTHOR]