A novel Fe(III) dependent bioflocculant from Klebsiella oxytoca GS-4-08: culture conditions optimization and flocculation mechanism

Abstract

In this work, the effect of cultivation factors on the flocculation efficiency (FE) of bioflocculant P-GS408 from Klebsiella oxytoca was optimized by the response surface methodology. The most significant factor, i.e. culture time, was determined by gray relational analysis. A total of 240 mg of purified P-GS408 was prepared from 1 liter of culture solution under the optimal conditions. GC-MS analysis results indicated that the polysaccharide of P-GS408 mainly contains Rhamnose and Galactose, and the existence of abundant hydroxyl, carboxyl and amino groups was evidenced by FTIR and XPS analyses. With the aid of Fe³⁺, the FE of kaolin solution by P-GS408 could achieve 99.48% in ten minutes. Functional groups of polysaccharide were involved in the first adsorption step and the zeta potential of kaolin solution changed from -39.0 mV to 43.4 mV in the presence of Fe³⁺ and P-GS408. Three-dimensional excitation-emission (EEM) fluorescence spectra demonstrates that the trivalent Fe³⁺ and Al³⁺ can bind efficiently with P-GS408, while those univalent and divalent cations cannot. With the help of SEM images, FTIR, zeta potential and EEM spectra, we proposed the P-GS408 flocculation mechanism, which consists of coordination bond combination, charge neutrality, adsorption and bridging, and net catching.

Figure 1. 3D surface graphs (A,C,E) and contour plots (B,D,F) of flocculation efficiencies showing the effect of variables: (A,B) Urea- Sucrose; (C,D) Time- Sucrose; (E,F) Time- Urea.

Figure 2

(A) Gas chromatogram of alditol acetate derivatives from P-GS408 and (B) FTIR spectra of P-GS408 and flocs.

Figure 3. XPS spectra of P-GS408 (A) and high resolution 1 s XPS spectra of C, O, and N from P-GS408 are shown in (B–D), respectively.

Figure 4

Effects of (A) pH and (B) temperature on flocculation efficiency (FE). The final concentrations of Fe³⁺ and P-GS408 in test solution were 1.5 mM and 4 mg/L, respectively. (C) Effect of various cations on FE. (D) Effect of Fe³⁺ concentration on FE with P-GS408 concentration varying from 2 mg/L to 8 mg/L. The final concentrations of cations and P-GS408 were kept at 1.5 mM and 4 mg/L, respectively. All the tests were performed in duplicate, and the results were averaged.

Figure 5. EEM fluorescence spectra of P-GS408 with different metal cations: (A) without any metal; (B) K⁺; (C) Ca²⁺; (D) Mg²⁺; (E) Al³⁺; (F) Fe³⁺.

The final concentrations of P-GS408 and metals in analytical solution were 50 mg C/L and 1.5 mM, respectively, except the Fe³⁺ concentration was 1.0 mM.

Figure 6. EEM spectra of two main fluorescence components obtained from PARAFAC analysis.

Figure 7. Working model of kaolin particle-Fe³⁺-P-GS408 bioflocculant.