Can Cr(VI) and tetracycline be simultaneously removed by stabilized FeS nanoparticles via aeration at circumneutral conditions?

This work revealed that aqueous Cr(VI) and tetracycline (TC) could be removed concurrently by oxygenation of carboxymethyl cellulose stabilized FeS nanoparticles (CMC-FeS NPs) at oxic and circumneutral conditions for the first time. CMC-FeS NPs were prepared at a CMC-to-FeS mass ratio of 1/2 and were used in the form of suspensions. Experimental results reveal that 5.0 mg/L of Cr(VI) and 30.0 mg/L of TC can be completely removed from the solution within 60 min by 45 mg/L of CMC-FeS NPs. At circumneutral pH, FeS oxidation yielded hydroxyl radicals (OH) in both free and surface-bound forms. Surface complexation, rather than electrostatic attraction, facilitated the adsorption of neutral TC onto CMC-FeS NPs, whereas the electrostatic attraction probably dominated the adsorption of HCrO4−/CrO42−. Further, the preference of TC oxidation by OH over Cr(III), the strong reducing capacity of FeS and the shield effect of Fe(hydr)oxides towards Cr(III) oxidation help Cr(VI) reduction outcompete Cr(III) oxidation. Compared with the single pollutant system, the coexistence of Cr(VI) and TC exhibited a mutual aid effect. Our findings validate a fascinating strategy capable of simultaneously removing aqueous Cr(VI) and TC, and shed light on the purification of water polluted by bio-refractory antibiotics and reducible heavy metals.