Removal of selected metal ions from aqueous media by agricultural wastes: Kinetic and thermodynamic studies

Abstract

In the present research study, biosorption efficacy of agro-wastes (rice husk, bagasse, peanut husk, cotton sticks and wheat bran) for U, Zr and Sr removal from aqueous media was investigated. Rice husk, bagasse and peanut husk were selected as most efficient biosorbent for the removal of U, Zr and Sr ions respectively. These selected biomasses were subjected to different pre-treatments (Physical and chemical) and modifications (immobilization). Batch biosorption affecting parameters like pH, sorbent dose, initial metal ion concentration and temperature were optimized for native, pre-treated and immobilized biomasses to get maximum removal. Maximum biosorption capacity values were found at pH (4-5), (3-4) and (7-9) for U, Zr and Sr ions respectively for native, pre- treated and immobilized biomasses. The amount of metal ions sorbed (mg/g) decreased with increasing biosorbent dose and increased at higher initial metal ion concentration. Linear and non-linear regression forms of pseudo-first and second-order were studied and value of R2 and six non-linear regression error functions namely hybrid fractional error function (HYBRID), Marquardt’s percent standard deviation (MPSD), average relative error (ARE), sum of the errors squared (ERRSQ/SSE), sum of the absolute errors (EABS) and Chi-square test (χ2) were used to predict the most optimum kinetic model. Sorbent- sorbate reaction nature was estimated by fitting equilibrium data by non-linear and transformed linear forms of the Langmuir, Freundlich and Redlich-Peterson isotherms and most optimum isothermal model was optimized by comparing linear and non-linear R2 value and non-linear regression error functions. Calculated values of thermodynamic parameters i.e. ΔG ̊, ΔH ̊ and ΔS ̊ showed that studied processes are feasible and spontaneous. Response surface methodology using face-cantered central composite design was used to design experiments for biosorption of U(VI), Zr(IV) and Sr(II) ions onto biomasses. Significance of main, interaction and square effects of quadratic model was determined by ANOVA, F-test and p value. Adsorption/desorption studies showed that biosorbents can be reused successfully. Effect of interfering ions (cations & anions) on the removal efficiencies was studied. The column biosorption was also done and effect of bed height, flow rate and initial metal ion concentration was also studied by breakthrough curves and applying Bed Depth Service (BDST) and Thomas model. BET, SEM-EDX, TGA, XRD and FTIR analysis were carried out to characterize the biomasses. The whole study proved that selected agro-wastes have good removal potential for U(VI), Zr(IV) and Sr(II) ions containing wastewater.