Synthesis of an efficient adsorbent from titanium gypsum for phosphate removal: Effect of co-presence of waste sludge

Utilization and disposal of titanium gypsum (TiG) have led to growing public concern. In this study, we proposed a novel scheme that could transform TiG to efficient adsorbents for phosphate (P) recovery. For the first time, TiG was calcined either alone or in the co-presence of waste sludge at different temperatures. The products from calcination of TiG alone had low adsorption for P (below 10 mg/g), because TiG was hard to be decomposed in an inert atmosphere at the studied temperatures. However, co-presence of waste sludge could substantially improve P adsorption to 70–160 mg/g, regardless of whether the waste sludge was directly mixed with TiG and then co-calcined in one crucible, or just co-calcined with TiG in two separate crucibles in the same atmosphere. The excellent adsorption of TiG-derived adsorbents was due to chemical adsorption and surface precipitation of P by the active sites (e.g. CaO, Ca(OH)2) generated from decomposition of waste sludge and TiG. Decomposition of TiG could be substantially enhanced by organic gaseous byproducts and resultant carbon from calcination of waste sludge. The adsorbent powders were then pelletized using bentonite or methyl cellulose as binders, and the pelletization process showed no significant detrimental effect on the adsorption ability. The findings of this study not only provide a novel utilization process for gypsum and waste sludge, but also provide a series of adsorbents promising in removal and recovery of P.