Eliminating waste with waste: Removal of doxycycline in water by goethite modified phosphogypsum

Doxycycline (DOX) contamination in water has posed a serious threat to mankind health and environmental safety. Herein, a novel goethite modified phosphogypsum (GPG) composite was developed by impregnating synthesized goethite on phosphogypsum (PG) surface. The results obtained by pH gradient experiments indicate that GPG has superior adsorption efficiency and exhibits stable removal of DOX over a wide pH range of 4–11. The pseudo-second-order kinetic model and Langmuir isotherm model can better describe the adsorption process of DOX on GPG, indicating that the adsorption of DOX is a process characterized by the presence of chemical adsorption and monolayer coverage. Without adjusting the pH value of DOX solution, the maximum adsorption capacities of PG, goethite and GPG for DOX were 20.79 mg/G, 172.12 mg/g and 722.91 mg/g, respectively, according to Langmuir equation. The negative values of the thermodynamic parameters ΔH and ΔG in the studied range indicate the exothermic and spontaneous characterization of the DOX adsorption process. GPG exhibits good environmental resistance and reusability stability to DOX in electrolyte ion interference, real water samples and cyclicity experiments. The mechanism analysis shows that the adsorption of DOX on GPG is mainly controlled by the interaction of hydrogen bond, complexation and hydrophobic interactions. This study develops a convenient technique to synthesize effective adsorbents that can be used for DOX removal from real wastewater and provides new perspective on resourceful utilization of industrial wastes.