The objectives of this investigation were to improve our understanding of organic acid transport in porous media by focusing on a model system involving phthalic acid and goethite-coated sand (GCS). This was specifically made by first recalibrating a molecularly sound phthalate surface complexation model to GCS and then applying this model to describe breakthrough curves (BTC) in a GCS packed column. ATR-FTIR spectra of phthalic acid adsorbed at goethite surfaces at pH 3.0 and 6.0 and at loadings from 2.0 to 40.8 mu mol/m(2) confirmed the coexistence of metal-bonded (MB) and hydrogen-bonded (HB) complexes at low pH and the predominance of HB complexes at high pH. This concept was incorporated into a surface complexation model used to describe BTC at influent pH (pH(in)) values of 3.0, 6.0, and 7.8. The BTC revealed strongly pH-dependent behaviors. At pH(in) 3.0, the BTC revealed one front/plateau behavior while at pH(in) 6.0 two fronts/plateaus occurred. The existence of a second front/plateau led to an overestimation of the sorbed amount compared to that observed in the batch and caused a failure in the prediction of BTC. Additional column investigations suggested that surface loadings of nonspecifically adsorbed complexes could vary with pH and ionic strength and that the two-step breakthrough behavior may have emerged as a result of the formation of surface species of different natures than those during the first step, with the latter even serving as attachment sites corresponding to the second step. These findings call for refinements in current day modeling approaches used in reactive transport studies.