Regeneration of metal-containing alkali-activated adsorbent granules from a field experiment

Alkali-activated materials have become an active research topic as adsorbents for wastewater treatment. However, their regeneration is studied less frequently. In the present study, granular alkali-activated adsorbents were prepared from metakaolin or blast furnace slag with an inclusion of commercial MgCO₃/MgO/Mg silicate-rich mineral adsorbent. The granules were used in a field experiment to treat effluent from a closed mine site containing 4.3 mg/L Ni, 1.3 mg/L Mn, 0.5 mg/L Fe, and 0.6 mg/L Zn. The granule regeneration was compared with 0.3 M NaOH, 0.3 M NaCl, 0.03–1.5 M HNO₃, 0.3 M CH₃COOH, and 0.05 M EDTA-2Na solutions. The best-performing granule type was based on blast furnace slag with the commercial Mg-rich adsorbent and it could be regenerated effectively with 0.3 M HNO₃. The adsorption performance of the granules improved upon repeated regeneration (cumulative adsorption amounts in the field experiment reaching up to 1.0 mg/g Ni, 0.3 mg/g Mn, 0.1 mg/g Fe, and 0.2 mg/g Zn per cycle) which was likely due to enhanced specific surface area (reaching up 160–190 m²/g while the initial values were 0.5–20 m²/g). The granules had a mass loss of 27 % and 9.5 % during the first and second regeneration cycle, respectively, which is likely the limiting factor in their continued reuse.