Preparation of sorbents containing ettringite phase from concrete sludge and their performance in removing borate and fluoride ions from waste water

Concrete sludge is an industrial waste slurry containing hydrated cement, aggregates and water. Solid sorbents containing ettringite, Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O, were prepared from concrete sludge by adding various amounts of aluminum sulfate to enhance ettringite formation. Anion exchange performance of the sorbents was examined using model waste waters containing boron or fluoride ions. The removal behavior depended on the calcium/aluminum ratio and the heat treatment temperature after drying. For the same Ca/Al ratio, improved removal performance was observed for sorbents treated at higher temperatures. The highest removal capacity was found when the sorbent was prepared with a molar ratio of Ca/Al of 3.2 and heat treatment at 175°C. The final concentrations of boron and fluoride were 6.3mg-B/L, and less than 4mg-F/L for initial concentrations of 100mg-B/L and 300mg-F/L. Treatment of the sorbents at higher temperature dehydrated the ettringite phase to form metaettringite phase. The sorbents prepared in the present study can be used in a boron and fluoride removal process that meets the effluent standard in Japan.