Journal of Hazardous Materials 279 (2014) 569–578 Contents lists available at ScienceDirect Journal of Hazardous Materials j o ur nal ho me pa ge: www.elsevier.com/locate/jhazmat Experimental determination and modeling of arsenic complexation with humic and fulvic acids Hoda Fakour, Tsair-Fuh Lin ∗ Department of Environmental Engineering and Global Water Quality Research Center, National Cheng Kung University, Tainan City, Taiwan h i g h l i g h t s • A modeling approach for arsenic complexation with humic and fulvic acids was proposed. • Both arsenic species form organic complexes, with higher affinity for arsenate. • The two-site binding model successfully simulates the As complexation with both HA and FA. • Number of binding sites is proportional to HA concentration for both arsenic species. • The apparent stability constants are independent of the HA concentrations. a r t i c l e i n f o Article history: Received 1 April 2014 Received in revised form 17 July 2014 Accepted 18 July 2014 Available online 27 July 2014 Keywords: Arsenic Complexation Fulvic acid Humic acid Iron based adsorbent Ligand binding a b s t r a c t The complexation of humic acid (HA) and fulvic acid (FA) with arsenic (As) in water was studied. Exper- imental results indicate that arsenic may form complexes with HA and FA with a higher affinity for arsenate than for arsenite. With the presence of iron oxide based adsorbents, binding of arsenic to HA/FA in water was significantly suppressed, probably due to adsorption of As and HA/FA. A two-site ligand binding model, considering only strong and weak site types of binding affinity, was successfully devel- oped to describe the complexation of arsenic on the two natural organic fractions. The model showed that the numbers of weak sites were more than 10 times those of strong sites on both HA and FA for both arsenic species studied. The numbers of both types of binding sites were found to be proportional to the HA concentrations, while the apparent stability constants, defined for describing binding affinity between arsenic and the sites, are independent of the HA concentrations. To the best of our knowledge, this is the first study to characterize the impact of HA concentrations on the applicability of the ligand binding model, and to extrapolate the model to FA. The obtained results may give insights on the com- plexation of arsenic in HA/FA laden groundwater and on the selection of more effective adsorption-based treatment methods for natural waters. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Arsenic is a semi-metallic element widely distributed in the earth’s crust and introduced into ground water through the dissolution of minerals and ores. Due to its toxicity and wide occurrence, the element is today recognized as one of the most dangerous inorganic pollutants and threats to drinking water supplies [1]. Therefore, to protect human health, the World Health Organization (WHO) recommended the maximum concentration of 10 g/L arsenic in drinking water [2]. Arsenic mobility in the environment is controlled primarily by sorption onto metal-oxide surfaces. One potentially important factor influencing stability of arsenic could be the presences of other dissolved substances. Natural organic matter (NOM), which comprises a prevalent constituent in natural waters, is highly reactive with both metals and surfaces, and is thus an apparent nominee to influence arsenic mobility and bioavailability [3–6]. In natural water, NOM can be classified into two ∗ Corresponding author. Tel.: +886 6 2364455; fax: +886 6 2752790. E-mail addresses: tflin@mail.ncku.edu.tw, lintf10@gmail.com (T.-F. Lin). groups, humic and non-humic fractions. The former fraction is more hydrophobic in character and comprises humic and fulvic acids, while the latter is less hydrophobic and includes hydrophilic acids, proteins, amino acids, and carbohydrates [7]. Humic acid (HA) represents 50–90% of dissolved organic carbon in aquatic and terrestrial soil systems [8]. It may complex with both metals and metalloids, adsorb onto min- eral surfaces, participate in redox and photochemical reactions, and form coatings on mineral surfaces [9]. A number of studies have demonstrated strong correlations between organic carbon content and As distribution in water systems, suggesting NOM plays an important role in controlling As transport [4,10,11]. Fulvic and humic acids have also been reported to form stable complexes with mineral surfaces and effectively block arsenic from adsorption on iron oxide [12]. Moreover, it has been shown that NOM may form NOM–As complexes in aquatic environments, facilitating the release of As from natural and contaminated environments [3,4]. The formation of organically As complexes could be a result of either direct association of As with NOM, through ligand exchange with the functional groups such as amine in NOM, or inherent metal content of NOM samples [6]. Although studies regarding the interactions between As and NOM are available (e.g. [3,13–15]), the complexation of As with NOM is not well understood and is still difficult to be quantified. A few parameters have been proposed to quantitatively http://dx.doi.org/10.1016/j.jhazmat.2014.07.039 0304-3894/© 2014 Elsevier B.V. All rights reserved.