Abstract
The present study assesses the ability of two low-cost adsorbents - timber sawdust (TS- OH) and its alkaline treated analog (TS-ONa) - to remove two basic dyes, namely, Methylene Blue and Methyl Green, from aqueous solutions. The presence of new functional groups on the surface of TS-ONa resulted in a dramatic increase of surface polarity and the density of sorption sites, thereby improving the sorption efficiency of the cationic dyes. The results obtained from the sorp- tion characteristics have revealed that the sorption process for TS-ONa was uniform and rapid. The adsorption of cationic dyes reached equilibrium within the first 10 min of contact time and the trea- ted material acts efficiently in a wide pH range of dye solutions. The extent of adsorption was mea- sured through equilibrium sorption isotherms and analyzed using the Langmuir model. The monolayer saturation capacities for Methylene Blue are 694.44 and 1928.31 mg/g and for Methyl Green are 892.86 and 1821.33 mg/g for TS-OH and TS-ONa, respectively. Therefore, the chem- ically treated sawdust proved two- to threefold higher adsorption capacities of these dyes than those of the untreated analog. The exothermic nature of adsorption is demonstrated by a decrease of adsorption capacity with increasing temperature, and the negative value of free energy change indi- cated the spontaneity of adsorption. Desorption experiments with 1 M aqueous NaCl put into evi- dence that cationic dyes were completely desorbed from the matrices and the reusability of the TS- ONa matrix after three repeated cycles led to just a slight attenuation in its performance. These results show that alkaline treatment of a low value by-product of the timber industry leads to a powerful and efficient low-cost adsorbent, which may be used for the treatment of colored waste- waters.