Zirconium(IV)-Loaded Amino Functionalized Walnut Shell for Efficient Adsorption of Phosphate and 2,4-Dichlorophenoxyacetic Acid from Water

A cost-effective adsorbent (AWS@Zr) was synthesized from walnut shell using Zirconium and amino group modification for the uptake of 2,4-dichlorophenoxyacetic acid (2,4-D) and phosphate (PO₄³⁻). Characterization of the adsorbents revealed a significant difference in the physicochemical parameters of pristine and functionalized walnut shell. Langmuir model was observed to predict adsorption of 2,4-D, while Freundlich model best-fitted PO₄³⁻ adsorption with chemisorption being the principal underlying mechanism. The adsorption phenomena were pH dependent with Langmuir maximum capacity of 227.4 ± 5.4 mg g⁻¹ and 73.9 ± 3.2 mg g⁻¹ for 2,4-D and PO₄³⁻, respectively. Kinetic models were also used to analyze the experimental data, and remarkable determined coefficients favor the pseudo-second-order kinetic model for the batch systems. The column experiments were carried out as a function of adsorbates flow rate, initial feed of 2,4-D and PO₄³⁻ concentration, bed depth. The results indicated both Thomas and Clark models could predict uptake of 2,4-D and phosphate with Thomas maximum capacity as 195.5 ± 1.0 for 2,4-D and 87.4 ± 0.7 mg g⁻¹ for PO₄³⁻ at optimum flow rate of 10 mL min⁻¹ and bed depth of 6 cm. Moreover, the column isotherm studies revealed that the Langmuir model predicted the adsorption data of PO₄³⁻, and 2,4-D, which was consistent with batch adsorption of 2,4-D. The studied pollutants onto AWS@Zr are PO₄³⁻ > 2,4-D based on the β⁻¹ obtained from the column’s mass transfer analysis. Adsorption–desorption studies revealed the reusability potentials of AWS@Zr. Zr and amino in surface of AWS@Zr play major role during removal of 2,4-D and PO₄³⁻. There is potential for AWS@Zr to remove some anionic pollutants from solution.