Tetracycline Removal by Hercynite-Biochar from the Co-Pyrolysis of Red Mud-Steel Slag-Sludge

Abstract

The sludge-derived biochar is considered an effective emerging contaminants adsorbent for wastewater treatment. In this paper, red mud and steel slag (RMSS) was used for improving sludge dewaterability and enhancing the sludge-derived biochar adsorption capacity. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and a scanning electron microscope (SEM) were employed to comprehensively characterize the mineral composition, functional group, and morphology of the adsorbent. RMSS was able to improve the sludge dewatering performance by providing a skeleton structure to promote drainage and Fe(III) to decrease the Zeta potential. The dosage of 20 mg/g RMSS was able to reduce the specific resistance to filtration (SRF) and the Zeta potential of sludge from 1.57 × 10¹³ m/kg and -19.56 mV to 0.79 × 10¹³ m/kg and -9.10 mV, respectively. The co-pyrolysis of RMSS and sludge (2:8) induced the formation of biochar containing FeAl₂O₄ (PS80). The PS80 exhibited a large surface area (46.40 m²/g) and high tetracycline (TC) removal capacity (98.87 mg/g) when combined with H₂O₂ (PS80-H₂O₂). The adsorption process of TC onto PS80 and PS80-H₂O₂ was well described by the pseudo-first-order and pseudo-second-order kinetic model, indicating physisorption and chemisorption behavior. The results indicated that co-pyrolysis of RMSS sludge PS80-H₂O₂ could enhance the biochar adsorption capacity of TC, attributable to the degradation by ·OH generated by the heterogeneous Fenton reaction of FeAl₂O₄ and H₂O₂, the release of adsorbed sites, and the improvement of the biochar pore structure. This study proposed a novel method for the use of RMSS to dewater sludge as well as to induce the formation of FeAl₂O₄ in biochar with effective TC removal by providing a Fe and Al source, achieving a waste-to-resource strategy for the integrated management of industrial solid waste and sewage sludge.

Keywords: FeAl2O4; adsorption; biochar; dewaterability; tetracycline.

Figure 1

The effects of the RMSS dosage on the conditioning of sludge: (a) SRF and Wc; (b) zeta potential; SEM images of (c) raw sludge and (d) the conditioned sludge by RMSS (20% mg/g DS).

Figure 2

Rietveld refinement of (a) PS0, (b) PS33, (c) PS60, (d) PS80, and (e) PS100.

Figure 2

Rietveld refinement of (a) PS0, (b) PS33, (c) PS60, (d) PS80, and (e) PS100.

Figure 3

The effect of (a) different sludge-derived biochar and (b) different dosages of PS80 and (c) different initial pH on TC removal; (d) XRD patterns and (e) mineral constituents of different sludge-derived biochar.

Figure 4

TC removal capacity by PS80 and PS0 with and without H₂O₂.

Figure 5

SEM of biochar (a) PS0 and (b) PS80.

Figure 6

FTIR spectra of the PS80, PS80+TC, and PS80+H₂O₂+TC samples.