Adsorption properties of alkali-activated stone wool

Alkali-activated materials are actively studied as adsorbents for toxic metals, nutrients, and dyes from wastewater. They are produced using an environmentally friendly process from low-cost aluminosilicate precursors, and thus have significant potential for fostering the development of clean technology. One of the potential precursors is stone and glass wool, for which alkali activation has been successfully demonstrated in construction products. In this study, alkali-activated stone wool was assessed for adsorption for the first time. Stone wool manufactured without organic resin (SW) and another stone wool with organic resin being removed by heat treatment (SW-O) were alkali activated using NaOH, Na 2 CO 3 , NaAlO 2 , or Na silicate solution, and characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy, thermogravimetry–mass spectrometry, and potentiometric titrations to understand the surface properties affecting adsorption. Comparative adsorption experiments were conducted using methylene blue (MB), cobalt (Co(II)), arsenite (As(III)), and NH 4 + at 100 mg × L −1 concentration, 1 g/L adsorbent dose, 24 h contact time, and pH of 7. The raw stone wools demonstrated almost no adsorption, whereas the alkali-activated stone wools adsorbed up to 78, 42, 3, and 18 mg × g −1 of MB, Co(II), As(III), and NH 4 + , respectively. The samples prepared with the SW Na silicate demonstrated higher adsorption capacities for MB and Co(II). For As(III), an adsorption capacity was similar regardless of the mixture. The highest adsorption capacity for NH 4 + was achieved with SW-O NaOH. The increased adsorption capacities were related to the increase in the specific surface area, more negative zeta potential, formation of tobermorite-like structures, and an increase in the quantity of surface OH groups. The varying adsorption levels of specified contaminants are possibly governed by their speciation at pH 7, aqueous radii, and the adsorption mechanism. This study demonstrated that stone wool-based adsorbents have practically relevant potential for wastewater treatment.