Anion-π interactions aiding in the adsorption of anions in the solution phase, though challenging to quantify, have attracted a lot of attention in supramolecular chemistry. We present the design of a polymer adsorbent that quantifies the adsorption of arsenate ions experimentally by optimizing anion-π interactions in a purely aqueous system and use density functional theory to compare these results with theoretical data. Arsenate anions are removed from water by amine-functionalized polydivinylbenzene using the comonomer 1-vinyl-1,2,4-triazole, which was cross-linked with divinylbenzene via radical polymerization in a hydrothermal procedure. The amine-functionalized polydivinylbenzene successfully removed arsenate anions from water with a capacity of 46 mg g-1, a 70% increase compared to the nonfunctionalized polydivinylbenzene (27 mg g-1) capacity under the same conditions. Adsorption is best described by the Sips isotherm model with a correlation coefficient R2 factor of 0.99, indicating that adsorption sites are homogeneous, and adsorption occurred by forming a monolayer. Kinetic studies indicated that adsorption is second order in the amine-functionalized polydivinylbenzene. Computational studies using density functional theory showed that the 1-vinyl-1,2,4-triazole comonomer improved the thermodynamic stability of the anionic-π interactions of polydivinylbenzene with arsenate anions. Electrostatic interactions dominate the mechanism of adsorption in polydivinylbenzene compared to the anion-induced interactions that dominate adsorption in amine-functionalized polydivinylbenzene.