Replacement of one anion from goethite with another provides useful insight into the irreversible adsorption of the first added anion in a binary system. The objective of this study was to investigate the irreversible adsorption of dimethylarsinate (DMA), monomethylarsonate (MMA), arsenate, and phosphate onto goethite at pH 4 in phosphate and arsenic binary systems by adding two anions sequentially. The density of irreplaceable phosphate or arsenic on goethite decreases to a limit with an increase in the initial concentration of the other anion. This limit is the density of MMA, arsenate, and phosphate that irreversibly adsorbs onto goethite, which depends on the adsorption density of these species in the adsorption phase. The highest limit of phosphate that cannot be replaced with DMA, MMA, and arsenate is respectively 1.9, 0.5, 0.8 mu mol m(-2). The limit of irreplaceable DMA is zero, and the highest limit of irreplaceable MMA and arsenate is 0.9 and 1.1 mu mol m(-2), respectively. The results indicate that the irreversible adsorption of one specific anion in arsenic and phosphate binary systems is affected not only by the adsorption density of this anion before the addition of the other anion but also by the nature of the other. (c) 2007 Elsevier Inc. All rights reserved.