Synthetic adsorbents based on Fe oxy-hydroxides (FeOOH) prepared under a wide range of pH-values via intense oxidation conditions of FeSO4 as well as the As(III) and As(V) adsorption mechanism are investigated using X-ray absorption fine structure (XAFS) spectroscopies at the Fe- and As-K-edges. Synthesis in an alkaline environment promotes the face-connectivity of the Fe(O,OH)(6) chains at the expense of edge-and corner-sharing linkage, which is consistent with the lower surface charge density and in turn with the lower arsenic adsorption capacity. Microstructural changes are also detected after As(V) adsorption onto FeOOH synthesized at pH 5.5: the ratio of face-/edge-sharing sites increases from approximately 0.4-0.7 as a function of the As(V)-loading. This modification of the polymeric Fe(O,OH)(6) structure at higher As/Fe ratios is attributed to strong As(V) bidentate mononuclear (E-2) and binuclear (C-2) adsorption. In contrast, no alterations in the FeOOH microstructure were observed, possibly due to the weaker E-2 linkage of As(III). (C) 2015 Elsevier B.V. All rights reserved.