Adsorption of arsenic (V) on kaolinite and on kaolinite–humic acid complexes Role of humic acid nitrogen groups A. Saada a, * , D. Breeze a , C. Crouzet a , S. Cornu b , P. Baranger a a Bureau de Recherches G eologiques et Mini  eres (BRGM)––Environnement et Proc ed es, 3 Avenue Claude Guillemin, BP-6009, Orl eans Cedex 2 F-45060, France b SESCPF, INRA d’Orl eans, av. de la pomme de pin, BP-20619, Ardon, Olivet Cedex 45166, France Received 18 March 2002; received in revised form 5 February 2003; accepted 11 February 2003 Abstract In order to investigate the influence of organic matter on arsenic retention, we used batch experiments at pH 7 to determine the adsorption of As(V) on three different solids: a crude, purified, Ca-exchanged kaolinite and two kaolinites coated with humic acids (HAs) having different nitrogen contents. We first examined the adsorption of each HA onto kaolinite, and then used the HA–kaolinite complexes to study As(V) adsorption. The results clearly show an influence of the HA coating on As adsorption. For example, with low initial As concentrations the solid/liquid partition coef- ficient (R d ) for both HA complexes is greater than that for the crude kaolinite. We found that increasing the initial As concentrations decreased the R d values of the HA-coated kaolinites until finally they were the same as the crude ka- olinite R d values. This suggests that adsorption occurs first on the HA sites and then, once the HA sites are saturated, on the remaining kaolinite sites. We also noted that the more reactive HA–kaolinite complex was the one with the highest N/C ratio. Comparing the amount of amine groups in the HA–kaolinite complexes with the total amount of adsorbed As indicates that the HA amine groups, due to their positive charge at pH 7, play a key role in the adsorption of As onto organic matter. Ó 2003 Elsevier Science Ltd. All rights reserved. Keywords: Clay minerals; Organic matter; Humic acids; Sorption; Arsenate; Nitrogen groups 1. Introduction The physicochemical properties of arsenic have, for economic, health and environmental reasons, been mea- sured and quantified since the beginning of the 20th century. The hydrogeochemical behaviour of arsenic is, however, very complicated due both to the affinity of As to being complexed by ligands of natural waters, and to the involvement of As in the biosphere. Consequently, the natural environment can contain high concentra- tions of arsenic arising from factors such as mineral dissolution, geothermal activity, meteoric leaching of mine wastes, production of coal fly ash, and the use of pesticides. Many adsorbent substrates in the natural environ- ment are known to have a strong reaction with respect to the transfer of many pollutants, including arsenic. Among these substrates are clays and organic matter in particular. Here we decided to investigate the role of humic–clay complexes on the adsorption, and conse- quently the mobility, of arsenic. Chemosphere 51 (2003) 757–763 www.elsevier.com/locate/chemosphere * Corresponding author. Tel.: +33-2-3864-3411; fax: +33-2- 3864-3680. E-mail address: a.saada@brgm.fr (A. Saada). 0045-6535/03/$ - see front matter Ó 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0045-6535(03)00219-4