Adsorptive removal of tetracycline by sustainable ceramsite substrate from bentonite/red mud/pine sawdust

In this study, a novel, sustainable and efficient ceramsite substrate of constructed wetlands (CWs) were prepared for tetracycline (TC) removal by employing bentonite (Ben) and red mud (Rm) as the main materials and pine sawdust (Ps) as the additive. The optimal parameters for Ben/Rm/Ps ceramsite preparation were obtained via orthogonal and one-factor experimental designs, and the optimal parameters were presented as follows: mass ratio of Ben: Rm: Ps = 4:1:0.9, preheating temperature = 240 °C, preheating time = 20 min, calcining temperature = 1150 °C, and calcining time = 14 min. The properties of Ben/Rm/Ps-op ceramsite (obtained at the optimal condition) were first analyzed, including XRD and SEM, and demonstrated a microporous structure with some crystal strength components. Neutral condition and higher temperature were indicated conducive to improve the TC removal efficiency, while coexisting ions (Na(+) or Ca(2+)) showed adverse effect for TC adsorption by Ben/Rm/Ps-op. In addition, adsorption kinetics and isotherm could be well described by the second-order kinetics and linear isothermal model, respectively, which suggested chemisorption and multilayer adsorption thickness increased infinitely. The theoretical maximum TC adsorption capacity of Ben/Rm/Ps-op at 20 °C reached up to 2.5602 mg/g. In addition, Ben/Rm/Ps-op could effectively remove TC as the CWs substrate under a dynamic flow condition. Further, Ben/Rm/Ps-op exhibited high reusability capability and stability for TC removal, and the adsorption amount still remained for 2.13 mg/g (C(0) = 80 mg/L) after three consecutive cycles.