In situ conversion of goethite to erdite nanorods to improve the performance of doxycycline hydrochloride adsorption

Abstract Goethite is the major component of solid waste from surface de-rusting and zinc-electrolysis processes, and such waste is conventionally disposed of in landfills. A new conversion method of goethite to nanorod erdite was investigated in this study to develop an alternative route for reutilising goethite-bearing waste. The application performance of nanorod erdite was also determined. Results showed that raw goethite was interlaced with spicula. After hydrothermal treatment at 160 °C for 10 h, goethite was converted to hexahedral pyrite (named as E-0) without NaOH whilst to nanorod erdite (named as E-2) with corresponding diameter and length of 0.2 and 1–2 μm in the presence of 2 M NaOH. The formed erdite nanorod (E-4) grew longer from 1 to 2 μm to 2–5 μm when the NaOH concentration increased from 2 M to 4 M. NaOH is important for the dissolution of structural Fe on goethite as Fe(OH)4−. NaOH enabled the replacement of the OH− of Fe(OH)4− by HS−, followed by polymerisation and crystallisation of (FeS2)nn-, thus resulting in erdite production. Erdite was not observed when NaOH was replaced by KOH, Ca(OH)2 and ammonia liquid, respectively forming KFeS2, portlandite and pyrite. The erdite-bearing products were effective in doxycycline hydrochloride (DCH) adsorption. The adsorption data of DCH fitted well with Jovanovich and pseudo-second order models, and the maximum adsorption capacity of DCH was in the following order: goethite