Adsorption behavior and solidification mechanism of Pb(II) on synthetic C-A-S-H gels with different Ca/Si and Al/Si ratios in high alkaline conditions

Calcium silicate aluminate hydrate (C-A-S-H) is the main hydration product and its porous adsorption property can effectively inhibit Pb(Ⅱ) leaching from alkaline pore solution of alkali-activated municipal solid waste incineration (MSWI) fly ash compacts. The study examines how different ratios of Ca/Si and Al/Si in C-(A)-S-H affect Pb(Ⅱ) adsorption behavior and mechanism in high alkaline conditions. The results showed that Al-doped C-A-S-H gels have increased specific surface area and cumulative pore volume, and slightly decreased average pore size. Pb(OH)3- adsorption by C-A-S-H was mainly chemisorption with spontaneity. Temperature, pH value, Al/Si and Ca/Si ratios of C-A-S-H affected Pb(Ⅱ) adsorption. Equilibrium adsorption capacity (Qe) initially increased then decreased with higher Al/Si for C-A-S-H with Ca/Si 0.60 but decreased with higher Al/Si for Ca/Si 1.20. Active Si-OH (Q1 or Q2b) and Al-OH (Q2(1Al)) in aluminosilicate tetrahedra chains dehydrate and condense with Pb(OH)3- to form Si-O-Pb or Al-O-Pb bonds. Higher interlayer water content in low Ca/Si and Al/Si C-A-S-H facilitated ion exchange during Pb(Ⅱ) adsorption. After adsorption, aluminosilicate tetrahedra chain length increased while interlayer water content decreased. Ca(Ⅱ) and Pb(Ⅱ) in C-A-S-H underwent partial ion exchange during the adsorption process. Simulation results indicate Pb(OH)3- was more likely to polycondensate with silica tetrahedral bridging oxygen in Si2AlO2(OH)8 to form Si-O-Pb than Al-O-Pb bonds. These findings provide theoretical support for designing dosages of alkali activators and doped aluminosilicate minerals to improve the solidification efficiency of Pb(Ⅱ) in alkali-activated MSWI fly ash compacts.