Adsorptive removal of tetracycline from water using Fe(III)-functionalized carbonized humic acid

Humic acid (HA) was carbonized at 300, 400 and 500 °C and then functionalized with 1 wt%–12 wt% Fe(III) respectively [CHA300/400/500-Fe(III)]. Adsorption of such Fe(III)-functionalized carbonized HA as adsorbents to aqueous tetracycline (TC: 25 mg·L−1) was studied. The adsorption equilibrium time for CHA400-Fe(III) to TC was 6 h faster and the adsorption removal efficiency (Re) was two times higher than that of HA/CHA. The adsorption Re of CHA400-Fe(III) loaded 10% iron [CHA400-(10%)Fe(III)] to TC could reach 99.8% at 8 h and still kept 80.6% after 8 cycles. The adsorption kinetics were well fitted to the pseudo-second-order equation and the adsorption isotherms could be well delineated via Langmuir equations(R2 > 0.99), indicating that the homogeneous chemical adsorption of TC occurred on the adsorbents. The main adsorption mechanisms of TC were complexation Fe(III) and hydrophobic distribution. Electropositive and electronegative repulsion between TC and CHA400-(10%)Fe(III) at lowly pH(2) and highly pH(8–10) respectively, leaded to the relatively low adsorption capacity and more notable influence of ion concentration. When the pH was between 4 and 8, TC mainly existed in neutral molecules (TCH2), so the influence of ion concentration was not obvious. The dynamic adsorption results showed that the CHA400-(10%)Fe(III) could continuously treat about 2.4 L TC(27 mg·L−1) wastewater with the effluent concentration as low as 0.068 mg·L−1. Our study suggested a broad application prospect of a new, effective, low-cost and environment-friendly adsorbent CHA400-(10%)Fe(III) for treatment of low-concentration TC polluted wastewater. [Display omitted] •HA and Fe(III)-functionalized carbonized HA were used to remove tetracycline.•CHA400-(10%)Fe(III) showed the highest removal efficiency (99.8%) to TC (25 mg·L−1).•CHA400-(10%)Fe(III) has a double strong adsorption force of iron coordination and CHA's hydrophobic distribution.•CHA400-(10%)Fe(III) can be easily regenerated by ethanol and maintain high reusability.•CHA400-(10%)Fe(III) is effective in treating low-concentration TC wastewater.