Study on the efficiency and mechanism of iron-modified cyanobacteria biochar in removing tetracycline from water

Using cyanobacteria as raw material to prepare biochar, the adsorption efficiency of cyanobacteria biochar on tetracycline at different temperatures was investigated and the optimal preparation temperature of cyanobacteria biochar was screened. Cyanobacteria biochar modified with different Fe/C mass ratios was prepared by the liquid phase reduction method. The removing efficiency, influencing factors and removing mechanism of the iron-modified cyanobacteria biochar for tetracycline were studied. The results showed that the iron-modified cyanobacteria biochar had a high removing capacity for tetracycline at 700 ℃ and an iron-carbon mass ratio of 1∶1, and the removing rate reached 87.2% in 60 min, which was 1.2 times of that before modification. The adsorption type was in accordance with the pseudo-second-order kinetic equation (R²>0.99). The relationship between the iron-modified cyanobacteria biochar tetracycline removal performance and its structure was discussed by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was proved that the main removal effects of tetracycline were adsorption and chemical degradation, the zero-valent iron as the electron donor promoted the redox reaction, and the oxygen-containing functional groups as the bridge of electron transfer played an important role in the adsorption and degradation process. The test results of influencing factors showed that the influence degree of anions was in the order of SO₄ ²⁻ > Cl⁻, the influence degree of cations was Ca²⁺ > Na⁺, and the influence degree of organic fulvic acid (FA) was weak relative to ionic strength. The iron-modified cyanobacteria biochar had good removal efficiency on tetracycline antibiotics, which could provide a new idea for cyanobacteria resource utilization.