Screening of Significant Parameters Affecting Zn (II) Adsorption by Chemically Treated Watermelon Rind

Abstract

In recent years, agricultural wastes and biomass have been extensively investigated as low cost adsorbents in heavy metal removal owing to the facts that they are relatively cheap and exhibit high adsorption capacities. Watermelon rind is a natural and rich source of the non-essential amino acid citrulline, containing abundant amino and carboxyl groups, which have been proven to have great capability of binding heavy metals in aqueous solutions. In the present study, various chemical solvents were utilized to pre-treat watermelon rind in enhancing its Zn2+ ions adsorption performance. Improvement in the physical and chemical properties of chemically treated watermelon rind was evidenced through SEM and FTIR characterization results. Drastic improvement in the adsorption efficiency was observed compared to non-treated watermelon rind that resulted that at only 52.4 % removal percentage. NaOH solution was found to be the best pre-treatment solvent compared to Ca(OH)2, H2SO4, and C6H8O7, with the highest Zn2+ removal percentage attained up to 90.2 % using NaOH treated watermelon rind. The Zn2+ ions adsorption process using NaOH treated watermelon rind was then further investigated using a statistical tool. Fractional factorial design (FFD) was applied to evaluate the effects of 7 process parameters, namely solution pH, adsorption temperature, biosorbent dosage, initial metal ions concentration, contact time, concentration of pre-treatment solvent and stirring rate. Contribution of every parameter in influencing the adsorption efficiency was evaluated and factors that significantly affected the adsoprtion were elucidated by employing experimental design and analysis of variance in FFD. The result of factorial design revealed that solution pH, adsorption temperature, biosorbent dosage, initial metal ions concentration and the interaction effect between biosorbent dosage and initial metal ions concentration imposed significant effect (P < 0.05) to the removal percentage of Zn2+ ions at the end of the adsorption process. Effects of these process factors on the adsorption efficiency were investigated and discussed in detail