Effect of alkali species on synthesis of K-F zeolitic materials from paper sludge ash for soil amendment

Zeolitic material including zeolite K-F (KAlSiO4·1.5H2O) with soil amendment properties was synthesized at 90 °C from paper sludge ash in KOH–NaOH and KOH–LiOH mixtures. The total alkali concentration in each solution was maintained at 4 mol/L and the relative amounts of the cationic species (K+/Na+ and K+/Li+) were varied. Zeolite K-F crystal could be obtained in KOH–NaOH solutions with Na/(Na + K) ratios lower than 0.5 or KOH–LiOH solutions with Li/(Li + K) molar ratios lower than 0.25, while hydroxysodalite (Na6Al6Si6O24·8H2O) and katoite (Ca3Al2(SiO4)(OH)8) were formed at other cation molar ratios in each mixture. The observed concentrations of Si and Al in the solution during the reaction explain the synthesis of reaction products with soil amendment properties. The properties of a product intended for K+ release depend on formation of an amorphous gel and zeolite K-F, while that for Ca2+ release depends on formation of an amorphous gel and calcium hydrate minerals, such as katoite. The formation of zeolite K-F in a KOH–LiOH solution with Li/(Li + K) ratio = 0.25 is faster than that in KOH alone and the product with good properties, high cation exchange capacity and high released amount of Ca2+, for soil amendment could be obtained.

• Zeolite K-F and hydroxysodalite were formed in KOH–NaOH solutions.
• Zeolite K-F and calcium hydrate minerals, such as katoite, were formed in KOH–LiOH solutions.
• Properties for K+ release depend on formation of an amorphous gel and zeolite K-F.
• Properties for Ca2+ release depends on formation of an amorphous gel and calcium hydrate minerals.
• Zeolite K-F formation in KOH–LiOH solution with Li/(Li + K) = 0.25 is faster than that in KOH alone.