Drinking water companies in the Netherlands are actively investigating routes to reduce arsenic (As) to <1 µg L−1 in drinking water. Co-precipitation of As with iron during groundwater treatment is a promising method. When As(III) is present in raw water, permanganate (MnO4) can be dosed to oxidize As(III) to As(V) in order to improve As removal efficiency. The dosages of MnO4 and Fe(III) to achieve <1 µg L−1 As in the treatment effluents depend on the composition of raw water. The coprecipitation of As(III) and As(V) with ferrihydrite under variable raw water composition and redox environments, controlled by oxygen (O2) or MnO4 is modeled in this study by the generalized double layer model, and redox equilibrium reactions. Results show that the pH of the treatment process is critical to determine the As removal efficiency. At pH = 8 the highest As removal is obtained, followed by pH = 6 while pH = 7 gives the least removal. HCO3, PO4 and H4SO4 hamper the adsorption of As(V). In future work, the model outcome will be assessed by experiments. Furthermore, the model will be extended with oxidation kinetics in case oxidation (by e.g. oxygen) occurs at a slower rate than the (mean) residence time of the water in the process.