Morphological crystal adsorbing tetracyclines and its interaction with magnesium ion in the process of struvite crystallization by using synthetic wastewater.

Struvite (MgNH4PO4·6H2O) crystallization is a promising method of phosphorus recovery from wastewater. As for digestive livestock wastewater, the extensive residues of antibiotics could induce struvite recovery to spread antibiotic resistance and thereafter pose ecological risks to the environment. In this study, struvite crystals with different morphologies were produced from synthetic swine wastewater, and tetracyclines (TCs) adsorbing capacities were investigated. The important factors, including the existence of Mg2+ ions and initial TCs concentration, were examined. The predominant adsorption between TCs and struvite crystals was electrostatic interaction, with the maximum capacity at doxycycline (DXC) 876.5 μg/Kg, oxytetracycline (OTC) 1946.7 μg/Kg and tetracycline (TC) 2376.2 μg/Kg, respectively. Well-faceted struvite crystallites possessed high adsorption capacities than those of dendritic crystallite, due to higher Mg intensities on the crystallite surface. The increment of phosphorus concentration could trigger the transformation of struvite morphology from needle to dendritic shapes with X-shape as an intermediate stage, which would reduce Mg density in specific crystallite facets and therefore limit TCs adsorption onto struvite crystals. The existence of Mg2+ ion would inhibit TCs deprotonation and thereafter improve TCs adsorption onto struvite crystals. Further investigation revealed that continuously elevating initial TCs concentration would promote the formation of 1:2 transferring to 1:1 TCs-Mg chelates, which would result in a fluctuation following a drastic augment of TCs adsorption capacity.