Simultaneous uptake of ammonium and phosphate ions by compounds prepared from paper sludge ash

Al-containing CaO-SiO₂-H₂O phases were prepared by hydrothermal treatment of mixtures of paper sludge ash (PSA) with various silica and calcia sources and their properties were determined with particular reference to the simultaneous uptake of ammonium and phosphate ions, which are implicated in the eutrophication of lakes and ponds. After examination of various silica and calcia sources, Ca(OH)₂ and SiO₂ sol were selected as the most appropriate starting materials. Dry milling was found to be superior to wet milling in avoiding contamination from the milling media during mixing. Nine samples with three different Ca/Si ratios and Al₂O₃ contents were prepared with various mass ratios of Ca(OH)₂, PSA and SiO₂. The chemical compositions of the hydrothermal products of these mixtures moved towards the tieline of CaSiO₃-PSA, with respect to the starting compositions. The major phase formed in all samples was poorly crystalline C-S-H(I), with hydroxysodalite also formed in the Al-containing mixtures. All the products showed a capacity for the simultaneous uptake of ammonium and phosphate ions. The saturated sorption capacities calculated from the Langmuir equation ranged from 0.9 to 2.4 mmol/g for the ammonium ion and from 3.3 to 5.2 mmol/g for the phosphate ion. Since the sorption capacities for both ions increased with increasing Ca contents of the product, substitution of Ca²⁺ for NH₄⁺ and the formation of calcium phosphate phases such as apatite and brushite by precipitation are thought to be the main sorption mechanisms.