Phosphate immobilization using an acidic type F fly ash

Batch equilibration experiments using a low calcium (similar to 1 wt.% as C aO), acidic (pH similar to 4.5) Type F fly ash demonstrated phosphate immob ilization on the order of 100% to 75% for 50 and 100 me; P/1 solutions, res pectively. A loosely compacted column of fly ash similarly removed 10 mg P/ 1 for over 85 pore volumes, While the interactions between phosphate and ca lcium-rich (Type C) ashes are relatively well understood, insight into the mechanisms of phosphate immobilization in Type F ash necessitated a review of the phosphate chemistry and interactions with acidic geomedia. Phosphate adsorption was subsequently modeled using a constant capacitance model app roach (CCM) excluding precipitation reactions. Our CCM predictions of total phosphate immobilization (20%) were substantially less than the results of the batch equilibration experiments and phosphate adsorption predicted by other researchers examining near pure natural and synthetic geomedia due to the compositional heterogeneity of the fly ash. Nevertheless, for the amor phous and crystalline phases studied, the immobilization of phosphate in th e Type F fly ash is attributed to the formation of insoluble aluminum and i ron phosphates at low to medium values of pH. (C) 2000 Elsevier Science B.V . All rights reserved.