The surface charge of the adsorbent 2-L ferrihydrite was calculated by the partial charge model for varying pH conditions. The value of the surface charge was used to predict the ability of the adsorbent to chemically adsorb and desorb the adsorbate. The crystal structure of the 2-L ferrihydrite material was regarded as two models: an exclusively octahedral model and a combined model comprising 20% tetrahedra and 80% octahedra. The partial charge model was used to calculate the surface charge of the adsorbent under varying pH conditions. In the exclusively octahedral model, the surface charge reached the highest value of +0.060 under acidic conditions, -0.088 in the neutral state, and the lowest value of -0.347 under alkaline conditions. In the case of the combined model, δ(OH) had the highest value of +0.056 under acidic conditions, -0.087 in the neutral state, and the lowest value of -0.332 under alkaline conditions. As a result, we confirmed that the surface charge of the adsorbent could have a positive value even in an acidic environment. That is, the surface charge of the adsorbent could become positive or negative according to the pH of the solution. In a solution in which the pH is below 10, the adsorbent could adsorb the negative phosphate because the δ(OH) would be positive. In contrast, in a solution with pH>10, the adsorbent could desorb the negative phosphate because the δ(OH) would be negative.