The concentration of tetracycline (TC) in the supernatant was determined by UV-VIS. Since pH value has important effect on the existing forms, stabilities and characteristic absorption peaks of TC, the characteristic absorption peaks could be affected by pH value. As shown in Figure S1, the peak at 357 nm was almost insensitive to pH (2-8). Therefore, the peak of 357 nm was selected for detection. This observation was in agreement with Tang et al. in the study of TC sorption on magnetic biochar. As shown in Figure S2 and S3, the absorbance values increased with the increase in concentration of TC, with linear regressions of A=-0.003+0.0311C(1-15 ppm, R=0.999), and A=1.17+0.015C(20-120ppm, R=0.997).