This report describes the speciation of arsenic oxyanion mixtures using a liquid chromatography coupled with inductively coupled plasma mass spectrometry (LC-ICPMS) technique. The proposed method allowed a fast, sensitive and environmentally friendly determination of arsenic inorganic species in water. The speciation was carried out during adsorption experiments using dried sludge as an adsorbent. The dried used sludge was generated as discarded material from a water plant after coagulation and sedimentation stages and was selected due to its chemical and mineralogical composition. The sludge composition includes high proportions of silica, aluminium and iron minerals, which are known to be responsible for the adsorption of arsenic species. The As speciation results demonstrated a low limit of detection (LOD) of 5.26 ppb for arsenite and 9.85 ppb for arsenate species. The arsenate species was removed faster compared to the arsenite species when both are present as mixtures. These findings confirm that electrostatic interactions between the sludge components and charged H <inf>2</inf>AsO<inf>4</inf>^-, arsenate predominant species at pH 6.0, result in a faster equilibrium than H<inf>3</inf>AsO<inf>3</inf> (arsenite, a predominantly neutral species at pH 6.0). The adsorption was fitted by Langmuir and Freundlich models; the second one being the one which better represented the adsorption process. The Freundlich parameter k, the relative uptake capacity of the adsorbent, was 0.221 (mg g^-1) (L mg^-1)ⁿ for arsenite and 0.101 (mg g^-1) (L mg^-1)ⁿ for arsenate. The data from competitive adsorption tests suggested that the removal mechanism for arsenic species is due to the formation of an outer sphere complex via electrostatic or van der Waals attractions. The results show the capability of the dried sludge as a green alternative to remove trace levels of both arsenic oxyanions from contaminated or wastewaters. Also, the developed speciation protocol allowed a better understanding of the adsorption mechanism of arsenic species by Fe-rich sludge. © 2013 The Royal Society of Chemistry.