Conversion of Fe-bearing minerals in sludge to nanorod erdite for real electroplating wastewater treatment: Comparative study between ferrihydrite, hematite, magnetite, and troilite

Ferrihydrite, hematite, magnetite and troilite are typical Fe-bearing minerals, and served as major components of Fe-rich sludges generated in water treatment, refining, steel processing and mining industry, respectively. To recycle such Fe-rich sludges to prepare erdite-bearing flocculant, the phase transformation of the four Fe-bearing minerals to nanorod erdite should be clarified. In the study, the four minerals were hydrothermal treated to prepare nanorod erdite separately by simply adding Na S and NaOH. Among the four minerals, ferrihydrite was weakly crystallized and easily converted to nanorod erdite with diameter of 200–500 nm and length of 2–5 μm, along with dotted distribution of hematite as byproduct. Only a small portion of hematite was involved in the formation of nanorod erdite with length of 2 μm, but the involvement of magnetite and troilite in erdite formation did not occur. The Fe dissolution from Fe-bearing minerals in alkaline solution was a key step for erdite crystallization, which apparently enhanced at high NaOH concentration. By increasing NaOH concentrations from 1 to 6 M, radial growth of nanorod erdite from both ferrihydrite and hematite steadily improved to 5–10 μm and 5 μm, separately. Fer-P, the product of ferrihydrite, contained plenty of nanorod erdite and exhibited remarkable removal efficiency of Cr, Cu and Zn from real electroplating wastewater. By adding 0.5 g Fer-P, the removal percentage of Cr, Cu and Zn were all more than 99.95%, which were higher than other conventional reagents such as polyaluminium chloride, polyferric sulfate, powder activated carbon, and diatomite. The total residual heavy metal concentrations were less than 0.1 mg/L, which met the standard of discharging electroplating-wastewater of China. Nanorod erdite was spontaneously hydrolyzed to generate Fe/S-bearing colloid, which contained abundant Fe–S–H groups for chelating heavy metals in comparison with ligands in electroplating wastewater, maintaining the residual heavy metals at low level. In summary, rather than magnetite/troilite-rich sludge, ferrihydrite/hematite-rich sludge can be served as desirable resources to prepare erdite-bearing flocculant for real electroplating wastewater treatment