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Ferrosorp® Plus (FSP) is a commercial adsorbent obtained from the sludges of drinking water treatment plants, composed of mainly amorphous ferric oxy(hydroxides) and calcite among others impurities. This product is indicated to remove arsenic along with phosphates in water treatments[1]. A novel application can be the in-situ remediation of arsenic polluted aquifers through injection of modified product of a smaller size, allowing higher reactivity and mobility through porous media, as already observed for phosphates[2]. To this end, the original granular product (G-FSP) was sonicated (S-FSP), reducing the size from 0.5-2 mm to 0.001-0.1 mm, and increasing the surface area from 218 m2/g to 233 m2/g, respectively. The only crystalline phase detected with x-ray diffraction (XRD) was calcite, which accounted for 8.3%, as determined by thermogravimetric analysis (TGA). Batch kinetics studies of arsenite and arsenate adsorption on these two materials were evaluated at initial pH 4 and pH 9, the two extremes of pH range application according to the manufacturer. Adsorption equilibrium was reached after ~48 h and the experimental results fitted to a pseudo-second order kinetics model. Adsorption isotherms were determined in batch studies at the equilibrium pH (around 9.5) and fitted to a Freundlich model. It was observed that, despite the slight increase on the surface area in the sonicated by-product, the amount of arsenic adsorbed on both material sizes were similar; the Freundlich constant for granular and sonicated FSP were, respectively, 10.0 and 12.8 mg(1‑1/n)·L(1/n)/g for As(III), and 5.1 and 5.3 mg(1-1/n)·L(1/n)/g for As(V). The adsorbed arsenic concentrations at equilibrium were simulated using the PHREEQC software and the Dzombak and Morel (1992) surface complexation model. Simulations were fitted to reproduce the experimental results and to elucidate the role of calcite and of the released bicarbonate anions on arsenic adsorption onto FSP[3]. These experiments provide evidence about the role of the material size and of the calcite on arsenic adsorption, which can be extended to other applications, like aquifers polluted by acid drainage. In this case, the presence of calcite is advantageous as it counteracts acidity while adsorbing arsenic. Acknowledgements This work received the financial support of the Torres Quevedo (2019) grant number PTQ2019-010503. [1] HeGo BIOTEC GmbH, 2015, accessed 09/01/2022, [2] Martí, V., Jubany, I., Ribas, D., Benito, J. A., & Ferrer, B. (2021). Improvement of Phosphate Adsorption Kinetics onto Ferric Hydroxide by Size Reduction. Water, 13(11), 1558 [3] Appelo, C. A. J., Van der Weiden, M. J. J., Tournassat, C., & Charlet, L. (2002). Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic. Environmental Science & Technology, 36(14), 3096-3103.