Acid mine drainage (AMD) is an important environmental problem globally and has caused serious human health problems for downstream residents. Iron secondary minerals, as an important sink of contaminants, are common in sediments and floodplain soils. Stable transition metal isotopes have emerged as new tools to examine the sources and mechanisms controlling the cycle of trace elements in rivers and soils. In this study, we investigated the sorption of different crystallization structure of iron (III) oxyhydroxides by combining Cd adsorption isotherms at acidic condition, to quantify the magnitude of Cd stable isotope fractionation during adsorption process and to constrain the molecular mechanism responsible for fractionation in this system. At pH=6, significant Cd adsorption to both minerals surface was observed while all the Cd existed in the dissolved phase at pH=4. The amount of Cd adsorbed on iron oxyhydroxides between low and high ionic strength was not obvious, and Cd adsorbed on mineral surface could drive an isotopic offset between solution and solids, with isotopically light Cd preferentially sorbed (δ^114/110Cd~NIST\ 3108~ from -0.49‰ to -0.13‰). Cd signatures was constant as a function of the fraction of total Cd sorbed, indicating a reversible equilibrium isotope effect. The small difference of ∆^114/110Cd~solution-solid~ between the two ionic strengths with same iron oxyhydroxide indicates similar mechanisms driving Cd isotope fractionation. The Cd isotope composition of different iron oxyhydroxides were slightly indistinguishable. Those shifts may result from the structure and crystallinity difference. At high ionic strength, the presence of SO₄^2- dramatically promoted Cd adsorption, suggesting the formation of Cd-SO₄^2- ternary surface complexes on goethite. In this circumstance, we observed that the δ^114/110Cd of solid phase increased from -0.39 ‰ to -0.13 ‰, and the light δ^114/110Cd of solid phase attenuated implied that the increased content of Cd was derived from heavy δ^114/110Cd of the solution. Our findings emphasize the importance of inorganic ligands complexation in metal isotope sorption studies. Cadmium stable isotopes may be a novel tool for determining geochemical attenuates processes in river and soil environments.