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The article describes a method for preconcentration and speciation of Cr(III) and Cr(VI) in river waters. It is based on ultrasound-assisted magnetic solid phase extraction employing a nanocomposite prepared from magnetite nanoparticles, manganese oxide and alumina oxide, and then funct i o n a l i z e d w i t h [ 3 -( 2 -am i n oe t h y l a m i n o ) p r o p y l ] trimethoxysilane (AAPTMS). By taking advantage of the oxidative properties of MnO 2 , the Fe 3 O 4 @MnO 2 ,Al 2 O 3 nanocomposite was used for the preconcentration of total chromium. The AAPTMS-modified nanocomposite, in turn, is selective for Cr(VI). The concentration of Cr(III) can be calculated as the difference between total chromium and Cr(VI). The preconcentrated chromium species were quantified using ICP-OES. The method was optimized using response surface methodology. Under optimal conditions, the limit of detection and quantification are 20 and 50 pg ⋅ mL −1 , respectively. The intraday and interday precisions of the method, expressed in terms of relative standard deviation, are 2.6 and 4.9%, respectively. In addition, the effects of potentially interfering ions were examined. The method was successfully applied to the speciation of chromium in spiked river water and a certified reference material.

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