Effect of amino acids on the stability of anionic pollutants in fly ash blended cement

Fly ash has been used as a cementitious material for solid wastes immobilization. However, fly ash typically contains the hazardous elements Se, Cr, As, and B, and the leachability of these anions is a concern for its reuse. In this study, five amino acids were selected as models of simplified natural organic matter (NOM) in the pedosphere to modify the dissolution tests of Se, Cr, As, and B in ground fly ash blended cement. At the initial pH of 4.7, hydrocalumite and ettringite were easily dissolved, so Ca salt precipitates played an important role in immobilizing oxyanions. However, the stability of Ca salts was weakened by the ligand-promoted dissolution mechanism. In particular, the formation of complexes of H2Asp and HGly with Ca2+ promoted the release of Se, Cr, and As. H2Cys, on the other hand, acted as a reducing reagent to suppress Se and Cr mobility. At the initial pH of 12.0, the stable formation of hydrocalumite and ettringite effectively immobilized the anionic species. Under this condition, aside from the complexation of amino acids with Ca2+, the ion exchange between deprotonated amino acids and oxyanions in hydrocalumite interlayer may also threaten the stability of undesirable anionic species. However, the adsorption of amino acids on the surface of ettringite and hydrocalumite may inhibit their dissolution, resulting in a decrease in Se, Cr, As, and B release. This work provides a reference for understanding the stability of undesirable anionic pollutants in fly ash blended cement when they are exposed to geochemical environments.