Experimental design methodology applied to adsorption of metallic ions onto fly ash

The objective of this study was to define operating conditions which would conciliate a high removal of the five metallic cations (Cu2+, Ni2+, Zn2+, C d2+, Pb2+) and a low desorption of these metal ions from the contaminated s orbents. To achieve this goal the strategy relied on the use of experimenta l design methodology. The influence of four parameters (fly ash/lime mass r atio, type of fly ash/lime sorbent solution temperature, and sorbent concen tration) on the removal at pH = 5 and the stabilization of the five metalli c ions was studied. In the first step, the influence of three parameters on the removal of Cu2+ ions was studied (R-2 = fly ash/lime mass ratio, type of sorbent, temperature). It was found that the same set of parameter value s would produce both the highest removal and the lowest desorption for this cation: R-2 = 9g g(-1), sorbent B (made by mixing fly ash and lime in wate r, then drying this paste at 105 degreesC for 24 h), temperature of suspens ion equal to 60 degreesC. The formation of calcium silicate hydrate (CSH), resulting from the pozzolanic activity of fly ash, is assumed to be partial ly responsible for these mechanisms. In the second step, simplex methodolog y and Doehlert matrix were used to find the conditions in a 2D space (sorbe nt concentration, temperature of solution) that would give the highest remo val from a solution containing five metallic cations and the lowest desorpt ion of these five cations adsorbed on the contaminated sorbents. Then, the system response that had to be optimized was the total metallic ions concen tration (TMIC, mol L-1). A TMIC was measured both for adsorption and leachi ng experiments. These responses were modelized using a second-order polynom ial and the surface responses were plotted for adsorption and desorption re sults. A difference was observed between operating conditions reaching the highest adsorption from those that gave the lowest desorption. However, an adsorbent concentration around 122 g L-1 and a solution temperature of 66 d egreesC would lead simultaneously to a high adsorption and a low desorption . (C) 2001 Elsevier Science Ltd. All rights reserved.