254 J SCI IND RES VOL 67 MARCH 2008 Journal of Scientific & Industrial Research Vol. 67, March 2008, pp. 254-256 *Author for correspondence Tel: + 90 212 4737070/17643; Fax: + 90 212 4737180 E-mail: nbalkaya@istanbul.edu.tr Zinc adsorption kinetics by phosphogypsum — A preliminary study Nilgün Balkaya 1 * and Hasan Cesur 2 1 Istanbul University, Deptt of Environmental Engineering, 34320 Avcilar-Istanbul, Turkey 2 Ondokuz May1s University, Deptt of Chemistry, 55139 Kurupelit-Samsun, Turkey Received 18 June 2007; revised 13 December 2007; revised 17 December 2007 This study reports kinetics of zinc adsorption by phosphogypsum, which was conditioned with lime before batch adsorption. Pseudo-second-order equation was found more appropriate for Zn 2+ adsorption than pseudo-first-order equation. Keywords: Adsorption, Aqueous solution, Kinetic, Phosphogypsum, Zinc Introduction Adsorption of zinc from aqueous solutions by physical materials including papaya wood 1 , synthetic zeolite A 2 , powdered waste sludge 3 , rice husk ash 4 and activated carbon 5 has been studied. This study presents kinetics of zinc adsorption by lime-preconditioned phosphogypsum (PG) using pseudo-first-order (PFO), pseudo-second-order (PSO) and intraparticle diffusion (ID) models. Materials and Methods PG, obtained from TÜGSA^ Fertilizer Plant in Samsun, Turkey, was sieved (particle size, 65 mesh) and then preconditioned 6 with lime milk. Physical and chemical characteristics of raw and lime-preconditioned PG have been published 6-8 . Aqueous solutions (100 ml) containing Zn 2+ ions (50 mg/l) were taken in Erlenmeyer flasks (250 ml) containing lime-preconditioned PG (1 g). These flasks were agitated (200 rpm) at room temperature (Clifton NE5 28D Model Shaking Water Bath). Initial pH (9.5) of adsorbate solution was adjusted using NaOH and HNO 3 before contact of adsorbent with metal ion solution. At the end of predetermined time intervals, content of flasks were separated by centrifugation and supernatant was analyzed for Zn 2+ using atomic adsorption spectrophotometer (ATI UNICAM 929 Model Flame AAS). Amount (mg/g) of Zn 2+ adsorbed on lime-preconditioned PG was calculated as q e = (C o -C e )V/W ...(1) where C o and C e are initial and equilibrium concentrations of Zn 2+ ion in solution, respectively (mg/l), V is volume of solution (l) and W is weight of adsorbent (g). Each experiment was performed twice. Reproducibility of measurements was within 5%. Results and Discussion PFO and PSO kinetic models correlate solute uptake, important in predicting reactor volume 9 . Pseudo-First-Order (PFO) Kinetic For PFO sorption rate constant 10-12 , plot of log (q e -q t ) against time has been found not linear (Fig. 1). Correlation coefficient (r) shows that this model cannot be applied to predict adsorption kinetic model. Consequently, adsorption behaviour cannot be explained by PFO model. Pseudo-Second-Order (PSO) kinetic PSO rate constant (k 2 ) and q e were calculated from intercept and slope of the plots of t/qt versus t (Fig. 2), where q t is sorption capacity at time t. Calculated q e value agree very well with experimental data. This indicates that adsorption of Zn 2+ from aqueous solution on lime- preconditioned PG obeys PSO kinetic model 13 . Also,