Walnut shell treated with citric acid and its application as biosorbent in the removal of Zn(II)

In this work, walnut shell (WS) (Carya illinoinensis) treated with citric acid was used as biosorbent to remove zinc ions from aqueous solution. The effect of citric acid concentration and solution pH on the adsorption equilibrium was investigated in batch experiments. Additionally, the overall adsorption rate of Zn ions on walnut shell treated with citric acid was investigated by applying a diffusion model based on superficial diffusion. The results evidenced that a maximum adsorption capacity towards Zn(II) ions is obtained by using a citric acid concentration 2 M (WS2). The modification of walnut shell increases 2.5 times its adsorption capacity to remove Zn(II) from aqueous solutions due to the incorporation of carboxylic groups reaching a maximum adsorption capacity of 27.86 mg/g. The pH effect showed that the adsorption capacity was drastically increased by raising the solution pH from 3 to 6. The adsorption kinetics of Zn(II) on WS2 were interpreted correctly by the surface diffusional model (SDM). The values of Ds varied from 2.25 to 5.25 × 10−9 cm2/s, and increased exponentially as the mass of Zn(II) adsorbed at equilibrium augmented.