Enhanced reductive removal of ciprofloxacin in pharmaceutical wastewater using biogenic palladium nanoparticles by bubbling H2

Abstract

To treat waste with waste and efficiently remove the organic pollutant, waste palladiums(ii) were adsorbed and reduced on microorganism surface to catalyze the reductive removal of ciprofloxacin in pharmaceutical wastewater. By optimizing conditions such as pH and temperature, the amount of biogenic palladium adsorbed and reduced on E. coli reached 139.48 mg g^-1 (Pd/microorganisms). Moreover, most of the Pd(ii) was reduced to nanometer-sized Pd(0) as characterized by TEM and SEM with EDXA. Using the obtained biogenic palladium, the reductive removal of ciprofloxacin is up to 87.70% at 25 °C, 3.03 folds of that achieved in the absence of H₂. The results show that waste E. coli microorganisms can efficiently adsorb and remove waste Pd(ii) and produce Bio-Pd nanoparticle catalysts in the presence of H₂. This biogenic palladium presents high catalytic activity and great advantages in the reductive degradation of ciprofloxacin. Our method can also be applied to other waste metal ions to prepare the biogenic metals, facilitate their recovery and reuse in degrading organic pollutants in wastewater to achieve "treating waste using waste".

Fig. 1. Curve of the Pd²⁺ concentration versus adsorption time during direct adsorption.

Fig. 2. TEM images of E. coli (A) and palladium (metal)-loaded E. coli (B) cells surface. Scale bars correspond to 500 nm, and metal precipitates can be found inside the periplasmic space as arrows indicate.

Fig. 3. SEM images of untreated pure E. coli (A and B) and palladium nanoparticles-loaded E. coli cells surface (C and D).

Fig. 4. Elemental composition of the Bio-Pd nanoparticles.

Fig. 5. XPS spectra of the prepared Bio-Pd nanoparticles in the presence (A) and absence (B) of H₂.

Fig. 6. Removal of ciprofloxacin (5 mg L⁻¹, 20 mL) by Bio-Pd at different pH values, 30 mg catalyst, 25 °C without the presence of H₂.

Fig. 7. Removal of ciprofloxacin (5 mg L⁻¹, 20 mL) with different amounts of Bio-Pd, pH 3.2, 25 °C, without the presence of H₂.

Fig. 8. Removal of ciprofloxacin (5 mg L⁻¹, 20 mL) under different conditions in the presence of H₂ (A) and in the absence of H₂ (B) (conditions: pH 3.2; 30 mg catalyst; 25 °C; CIP + H₂, control experiment).

Scheme 1. Possible process of biosorption and reduction of Pd(ii) in the presence of H₂.

Scheme 2. Proposed pathway for CIP reductive degradation by Bio-Pd in the presence of H₂.