In the present study, the removal of lead ions from aqueous solutions was studied using activated carbons prepared from silk cotton hulls by three different acid activation methods; they are named as concentrated (conc.) H 2 SO 4 (AC), mixture of conc. H 2 SO 4 + H 2 O 2 (AC1), and mixture of conc.H 2 SO 4 + NH 4 S 2 O 8 (AC2). The surface properties of the adsorbent materials were determined by the Fourier Transform infrared spectroscopy, thermogravimetric analysis, Brunauer-Emmette-Teller, scanning electron microscope, and energy dispersive X-ray analyses. From the thermogravimetric analysis result, the formation of carboxylic acid and lactones are present in the order AC2 > AC1 > AC. Brunauer-Emmette-Teller result shows that the activated carbons are macroporous in nature. Batch adsorption experiments were performed as a function of pH, adsorbent dose, contact time, initial metal ion concentration, and temperature. The experimental equilibrium adsorption data fits well for the Langmuir, Freundlich, and Tempkin equations. The maximum adsorption capacities were 40.00 mg g À1 for AC and 83.33 mg g À1 for AC1 and AC2, respectively. Adsorption kinetic data were modeled using the pseudo-first-order and pseudo-second-order, and intraparticle diffusion equations and its rate constants were evaluated. The adsorption process followed the pseudo-second-order reaction kinetics, suggesting that it is presumably chemisorption. The calculations of the thermodynamic parameters revealed that the adsorption process was endothermic, but it is feasible and spontaneous under natural conditions. The reversibility of the process has been studied in a qualitative manner, and it shows that the spent activated carbons can be effectively regenerated for further use easily. Figure 8. Kinetic fit for the adsorption of lead onto SCHACs at 30 C (a) pseudo-first order; (b) pseudo-second-order; and (c) intraparticle diffusion model. C. DULCY EVANGELIN, S. G. GUNASEKARAN AND M. DHARMENDIRAKUMARAsia-Pacific Journal of Chemical Engineering 198

Kinetic study of aqueous polymerization of ethyl acrylate (EA) was carried out at 30 °C in dilute nitric acid medium by employing ammonium ceric nitrate (ACN)–methyl cellosolve (MC) and ACN–ethyl cellosolve (EC) as redox initiator systems. The ceric ion consumption was found to be first order with respect to ceric ion concentration with both initiator systems. The formation of complexes between Ce(IV) and reducing agents (RA) was observed. The order with respect to Ce(IV), reducing agents and monomer was evaluated for aqueous polymerization of EA by Ce(IV)–MC and Ce(IV)–EC redox initiator systems. The overall activation energy, Eoverall, for aqueous polymerization of EA was evaluated in the temperature region of 27–40 °C with both initiator systems. A kinetic mechanism for aqueous polymerization of EA initiated by redox initiator systems is presented. © 2001 Society of Chemical Industry

The novel biosorbent silk cotton hull, an agrowaste material, has been successfully utilized for the removal of cadmium(II) from aqueous solutions. The adsorption of cadmium onto three kinds of activated biosorbent has been studied: modified by concentrated sulfuric acid alone (AC), a mixture of concentrated sulfuric acid and hydrogen peroxide (AC1), and a mixture of concentrated sulfuric acid and ammonium persulfate (AC2). The adsorption studies were carried out to optimize the process parameters such as pH, adsorbent dosage, contact time, and initial metal ion concentration. Maximum metal removal was observed at pH 7.0 with a contact time of 90 min at stirring speed of 200 rpm with an adsorbent dosage of 4.0 g L −1 . The sorption isotherms were studied using the Langmuir, Freundlich, and Tempkin isotherm models. The maximum adsorption capacities were 100.00, 142.86, and 142.87 mg g −1 for AC, AC1, and AC2, respectively. Accordingly, the surface modification of the activated carbons AC1 and AC2 enhanced cadmium removal greatly. The experiments demonstrated that the removal of metal ions followed the pseudosecond-order kinetic model. The sorption mechanism is discussed in terms of the activated surface properties. A relationship between the oxygen content and sorption was found in this novel material. Desorption experiments were carried out using hydrochloric acid with a view to generate the spent adsorbent and to recover the adsorbed metal ions.

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