ORIGINAL PAPER Competitive adsorption of metal ions onto goethite–humic acid-modified kaolinite clay E. I. Unuabonah 1 • B. I. Olu-Owolabi 2 • K. O. Adebowale 2 Received: 29 December 2014 / Revised: 19 November 2015 / Accepted: 11 January 2016 / Published online: 4 February 2016 Ó Islamic Azad University (IAU) 2016 Abstract A binary mixture of humic acid and geothite was prepared and used to modify kaolinite to produce geothite–humic acid (GHA)-modified kaolinite adsorbent useful for the adsorption of Pb 2? , Cd 2? , Zn 2? , Ni 2? and Cu 2? from Single and Quinary (5) metal ion systems. The cation exchange capacity (CEC) and specific surface area of GHA-modified kaolinite clay adsorbent were found to be 40 meq/100 g and 13 m 2 /g, respectively, with the CEC being five times that of raw kaolinite clay (7.81 meq/ 100 g). The Langmuir–Freundlich equilibrium isotherm model gave better fit to experimental data as compared with other isotherm models. In Quinary metal ion system, the presence of Zn 2? and Cu 2? appears to have an antag- onistic effect on the adsorption of Pb 2? , Cd 2? and Ni 2? , while the presence of Pb 2? , Cd 2? and Ni 2? shows a syn- ergistic effect on the adsorption of Zn 2? and Cu 2? . The GHA-modified kaolinite showed strong preference for the adsorption of Pb 2? in both metal ion systems. Brouers– Weron–Sotolongo (BWS) kinetic model gave better fit to kinetic data compared with other kinetic models used. Data from BWS kinetic model indicate that adsorption of metal ions onto GHA-modified adsorbent in both metal ion sys- tems followed strictly, diffusion-controlled mechanism with adsorption reaction proceeding to 50 % equilibrium in \ 2 min in the Single metal ion system and \ 1 min in the Quinary metal ion system. Adsorption of metal ions onto GHA-modified kaolinite is fairly spontaneous and endothermic in nature in both metal ion systems although the rate of metal ion uptake and spontaneity of reaction are reduced in the Quinary metal ion system. Keywords Competitive adsorption  Kaolinite  Goethite  Humic acid  Diffusion-controlled transport  Kinetics Introduction In natural systems, the bioavailability of heavy metal ions is largely controlled by adsorption–desorption reactions at the particle interface (Backes et al. 1995). Natural systems are composed of a heterogeneous mixture of mineral and organic solid phases as well as lots of organic and inor- ganic solutes that compete with heavy metal ions for adsorption sites on solids. The toxicity or bioavailability of these toxic heavy metal ions depends on their activities in solution (Buffle 1985; McBride 1994), and their migration to the biosphere depends on their speciation in the soil and aquatic systems (Choppin 1992; McCarthy et al. 1998). Humic acid is from humic substances (HSs) found in soil organic matter. The interaction of HS (Saada et al. 2003; Wang and Xing 2005; Alekseeva and Zolotareva 2013) and goethite (Grossi and Sparks 1994; Jung et al. 1998; Lackovic et al. 2003; Gimenez et al. 2007) with clays has been well studied. Wang and Xing (2005) pointed out in their study that the aliphatic fraction of humic acid is preferentially adsorbed by clay minerals, while the aro- matic fractions are left in solution. There are also reports on the separate use of humic acid, goethite, humic acid- Electronic supplementary material The online version of this article (doi:10.1007/s13762-016-0938-y) contains supplementary material, which is available to authorized users. & B. I. Olu-Owolabi iromidayobamidele@yahoo.co.uk 1 Environmental and Chemical Processes Research Laboratory, Department of Chemical Sciences, Redeemer’s University, Ede PMB 230, Osun State, Nigeria 2 Department of Chemistry, Faculty of Sciences, University of Ibadan, Ibadan, Nigeria 123 Int. J. Environ. Sci. Technol. (2016) 13:1043–1054 DOI 10.1007/s13762-016-0938-y