Conversion of Fe-rich sludge to KFeS2 cluster: Spontaneous hydrolysis of KFeS2 for the effective adsorption of doxycycline

Abstract

Fe-rich sludge is a solid waste considerably generated in coagulation, Fenton, and catalytic processes for wastewater treatment, in which it is commonly disposed in landfills. However, a limited portion of sludge is recycled as polymeric ferric flocculant and iron red. Herein, the Fe-rich sludge was simulated by the hydrolysis of FeCl3 and converted to a new one-dimensional KFeS2 cluster via a one-step hydrothermal route with the addition of K2S and KOH. The results showed that in the hydrothermal process, KFeS2 cluster grew radially from 2 μm to 10 μm with the increase in KOH concentration from 2 M to 5 M. The new cluster showed a high adsorption capacity of 2933.6 mg/L for doxycycline, which is 14 times that of sludge and higher than that of hematite nanoparticles, commercial polymeric ferric flocculant and pyrite. The adsorption isotherm complied with the Langmuir model, and the adsorption kinetics fitted well with the pseudo-second-order model. During adsorption, KFeS2 cluster was completely hydrolysed to release Fe/S-bearing colloids with a considerable number of Fe-SH/Fe-OH groups for the coordination of the NH2 group of tetracycline-type antibiotics, e.g. doxycycline. However, an inefficient removal of quinoline and p-nitrophenol was observed. With the proposed method, the used alkaline solution was completely recycled in the next round of KFeS2 synthesis without the generation of secondary waste. Such green method has potential applications in the resource utilisation of Fe-rich sludge.