Novel synthesis of aluminum hydroxide gel-coated nano zero-valent iron and studies of its activity in flocculation-enhanced removal of tetracycline

A newly designed aluminum hydroxide gel-coated nanoscale zero-valent iron (AHG@NZVI) with enhanced activity and dispersibility of NZVI was successfully synthesized. The AHG@NZVI composite was synthesized via control of the surface AHG content. AHG@NZVI-1, AHG@NZVI-2 and AHG@NZVI-3 were prepared under centrifugal mixing speeds of 1000, 2000 and 4000 r/min, respectively. The activity of AHG@NZVI was evaluated by its tetracycline (TC) removal efficiency. The effects of AHG content, pH value, reaction temperature, and presence of competitive anions on TC removal were investigated. TC could be removed by both adsorption and chemical reduction on AHG@NZVI-2 (centrifugal speed 2000 r/min) in a short time with high removal efficiency (≥98.1%) at the optimal conditions. Such excellent performance can be attributed to a synergistic interaction between aluminum hydroxide gel and NZVI: (1) AHG could enhance the stability and dispersity of NZVI; (2) aluminum hydroxide gel could absorb a certain amount of TC and Fe²⁺/Fe³⁺, which facilitated the mass transfer of TC onto the NZVI surface, resulting in acceleration of the reduction rate of TC by the AHG@NZVI composite; and (3) AHG-Fe²⁺/Fe³⁺ could absorb a certain amount of TC by flocculation. The kinetics of TC removal by AHG@NZVI composite was found to follow a two-parameter pseudo-first-order model. The presence of competitive anions slightly inhibited the activity of AHG@NZVI systems for TC removal. Overall, this study provides a promising alternative material and environmental pollution management option for antibiotic wastewater treatment.