Investigation of performance and mechanism of zinc removal from polluted water by concrete fines derived from aggregate recycling: From problematic byproducts to effective adsorbent.

Concrete fines are byproducts produced from aggregate recycling. Because of their properties, they cannot be directly recycled for use in concrete manufacturing, which is problematic to move the cement and concrete industry toward sustainable development goals and reduce its environmental impact. Taking advantage of concrete fines unique properties was regarded as a possible research direction. The hydrated cement fraction was expected to provide alkalinity to neutralize the acidic solutions, while calcium related compounds were expected to provide the function of heavy metals removal. In this research, concrete fines were used to remove Zn from acid mine drainage as an active treatment. The removal performance was comprehensively investigated. The maximum capacity of Zn-adsorption is 111.9 mg/g, and almost 100% Zn can be removed for an initial Zn concentration of 20 mg/L. The dominant reaction mechanism of Zn adsorption to concrete fines was determined to be ion-exchange reaction with surface complexation and precipitation. The Zn ions in solution can exchange with the Ca ions in calcium silicates and calcium silicate hydrates in concrete fines and replace the protons released by ionization of the silanol group for complexation, and thus Zn removal is not limited to an alkaline environment or high initial Zn concentration. The acidity was alkalized by hydration reaction, mainly consuming calcium hydroxide. Based on these mechanisms, concrete fines are effective adsorbent to remove Zn without the need for the synergistic reactions of other metals and for making the aqueous solution strongly alkaline, even in the strongly acidic environments and in effluents with high Zn concentration. Through evaluation and comparison of Zn adsorption capacity with other materials, concrete fines were regarded as promising alternative adsorbent for Zn removal.