Isotherm, kinetic and thermodynamic of arsenic adsorption from aqueous solution using Theobroma cacao pod-modified feldspar composite

Abstract

In this study modified feldspar composite (MFC) adsorbent based on feldspar and Theobroma cacao pods (TCP) was prepared using calcination method, characterized and tested for the removal As (III) from aqueous solution. The results showed that the cation exchange capacity of the modified feldspar composite (30.66 ± 0.21 meq/100 g) was 5 times higher than that of raw feldspar (6.42 ± 0.45 meq/100g). More so, the novel biohybrid material, MFC has a surface area of 53.60 ± 0.3 m2/g and particle size of 105.4 ± 0.18. Response surface methodology (RSM) via central composite design (CCD) was utilized in the optimization of the efficiency of As (III) ions uptake by the novel composite (MFC) in 20 experiment runs. The optimization results revealed that predicted adsorption percentages of (99.72%) for As (III) ions was close to the experimental results (99.98%) for the metal ions at the optimum conditions of adsorption parameters (pH 5; 0.5 g; 100 mg/L; 120 min and 3280K). The adequacy and validation of the model was justified with the good agreement between R2 values (0.9959) and adjusted R2 (0.9981) and higher F values (≥ 147) from the analysis of variance (ANOVA) results. Furthermore, the results from isotherm studies showed that As (III) ions adsorption onto the adsorbent best fitted the Langmuir isotherm model, hence chemisorption process. The results of the kinetic studies showed that the rate of uptake of As (III) ions onto MFC active sites followed pseudo second order kinetic model. The Intraparticle diffusion though not the only rate - controlling step, played an important role in the metal ions uptake by the adsorbent. The thermodynamic results revealed that As (III) adsorption onto MFC surface was feasible, spontaneous with negative ∆H values, suggestive of exothermic process. The MFC, owing to its abundance and other properties as its improved cation exchange capacity and eco-friendliness has a good potential as a highly efficient alternative adsorbent to commercial activated carbons in water treatment.