The organoarsenical p-arsanilic acid (p-AsA) is used in the U.S. poultry industry as a feed additive and its structure resembles one of the stable biodegradation products of Roxarsone (ROX) in anaerobic environments. With the implementation of recent EPA MCL of total arsenic in drinking water (10 ppb), thereareconcernsaboutthefate of organoarsenicals introduced to the environment through the application of arsenic-contaminated manure. We report herein, for the first time, the thermodynamics of p-AsA binding to Fe-(oxyhydr)oxides using ATR-FTIR. ATR-FTIR spectra were used to quantify surface coverage of p-AsA, p-AsA(ads), by analyzing the broadband assigned to v(As-O) at 837 cm(-1). Adsorption isotherms were measured in situ at 298 K and pH 7 in the concentration range 1 microM to 40 mM. Values of Keq were obtained from Langmuir model fits and they range from 1411 to 3228 M(-1). We also determined the maximum adsorption capacities of Fe-(oxyhydr)oxides to p-AsA, and they range from 1.9 x 10(13) to 2.6 x 10(13) molecules/cm2. Our results suggest that p-AsA is more mobile than methylated and inorganic forms of arsenic and that the transport of nanoparticles with p-AsA(ads) might play a role in its mobility in geochemical environments.