Adsorption of selenite ion from aqueous solution by solid pineapple waste

Abstract

In the pineapple canning industry, almost forty percent of pineapple portion is rejected during the production process. Most of the rejected portion is used as ruminant and for alcohol production. In the present study, the untreated solid pineapple waste (SPW) was used to adsorb selenite ion (Se(IV)) from aqueous solution. The Se(IV) loaded and unloaded SPW were characterized using proximate analysis, atomic absorption spectrometry, scanning electron microscopy, energydispersive X-ray spectroscopy, thermogravimetry analysis, differential scanning calorimetry and Fourier transform infrared spectroscopy. Various parameters that affect the adsorption such as initial pH, contact time, temperature, mass of adsorbent and concentration of adsorbate were investigated to determine the optimum adsorption conditions. Kinetics of the adsorption was investigated by pseudo firstorder and pseudo second-order models. Equilibrium studies were carried out by Langmuir, Freundlich and Temkin isotherms. Thermodynamic studies of the adsorption were performed using van’t Hoff equation. Desorption studies of the Se(IV) loaded SPW were carried out under various conditions such as initial pH, desorption time, ratio mass of loaded SPW to reaction volume and concentrations of humic acid and competing ions such as chloride, phosphate and sulphate ions. Under the optimum adsorption conditions the Se(IV) content of SPW could be increased by 100 folds from less than 10 µg/g to nearly 1000 µg/g. The adsorption optimum conditions were at pH 6, adsorption duration t = 60 min, initial concentration of Se(IV) aqueous solution C0 = 10 mg/L, SPW weight was 3 g in 50 mL of reaction volume and temperature was 80°C. The adsorption rate of Se(IV) by SPW under the optimum conditions fitted the pseudo second order with rate constant, k2 = 4.8539 mg/g min while the adsorption of Se(IV) by SPW best fitted Temkin isotherm (R2 = 0.9963), followed by Langmuir isotherm (R2 = 0.9830) and the Freundlich isotherm (R2 = 0.9702), which indicated the dominance of chemisorption. The van’t Hoff equation applied to this adsorption indicated that the adsorption process is endothermic, favourable and spontaneous. The adsorbed Se(IV) can be leached out gradually under the influences of pH, ratio of mass of loaded SPW to reaction volume and the presence of humic acid, phosphate, sulphate and chloride ions