Adsorption of Humic Acid on Goethite:  Isotherms, Charge Adjustments, and Potential Profiles

Abstract

The adsorption of natural organic matter (NOM) on mineral (hydr)oxide plays an important role in the evaluation of the speciation of toxic metal ions in the environment. Because both NOM and mineral oxide have variable charges that adjust upon adsorption, a good understanding of proton binding is required before the binding of metal ions can be understood. In this study, the adsorption of purified Aldrich humic acid (PAHA) on goethite was examined as a function of the environmental conditions (pH, salt concentration, and free concentration of PAHA) together with the proton adsorption to PAHA, goethite, and their mixtures. The induced charges on both components were separated on the basis of the difference between the charge/pH curves of the mixture and those of the single components. The electrostatic potential profile across the adsorbed layer was obtained as a numerical solution of the Poisson-Boltzmann equation using the charge density of the adsorbed PAHA and the goethite surface. From the quantitative evaluation of the induced charge on both components, it is revealed that the degree of the charge adjustment is related to the electrostatic affinity between the PAHA segments and the goethite surface, the electrostatic repulsion between the PAHA segments, and the electrostatic shielding by salt ions. Considering the charge distribution of the adsorbed PAHA at the goethite surface, it is concluded that the change of the charge adjustment is sensitive to that of the conformation of the adsorbed PAHA. From the detailed inspection of the assumptions made and the comparison with the reported theoretical calculations, the obtained potential profiles are considered to broadly reflect the true potential profiles. Because a charge adjustment is not frequently considered in detail in relation to the NOM adsorption on metal (hydr)oxides, the obtained results can form the basis for the further development of modeling of the adsorption of NOM on (hydr)oxide surfaces.

Predictive model of cationic surfactant binding to humic substances

“…[5,6]. This is also the case for PAHA, Saito et al [45] used b PAHA = 0.63. For the other HS investigated by…”

“…For PAHA the data of Saito et al [45] can be used. When no reliable proton titration data are available X H,0 can be estimated from titrations of the HS with the cationic polyelectrolyte, polyDADMAC, till the iso-electric point (IEP) of the complex is reached [12,13].…”

“…These results are well suited to test the surfactant MC procedure using the adopted Donnan model. The parameters of the Donnan model are derived from the PAHA literature [22,45] and collected in Table 1 the IEP has been used also to determine the optimal position of the calibration line for the surfactant concentration [22]. Given these experimental uncertainties it may be concluded that the LFH-D model works well for the CPC-PAHA system.…”

Heteroaggregation of nanoparticles with biocolloids and geocolloids

“…Geocolloids widely exist not only in surface water, but also in groundwater and in soils [51,52]. Typical geocolloids include clay minerals, metal oxides, and hydroxides, all of which play an important role in the fate and transport of pollutants and nutrients in the natural environment [51][52][53]. Geocolloids can be either 'particulate' (flocculated in the solid phase) or 'colloidal' (in suspension, or dissolved, in the solution phase) in the environment [52].…”

Humic substance charge determination by titration with a flexible cationic polyelectrolyte

“…The charge density of HS depends on the pH and at the pH conditions in most natural waters HS are negatively charged (Milne et al, 1995;Ritchie and Perdue, 2003). The charge density of humic particles is a dominant factor for the interactions of HS with: (1) heavy metal ions (Milne et al, 2003;Koopal et al, 2005;Merdy et al, 2006), (2) oppositely charged polyelectrolytes (Bolto et al, 1999;Kam and Gregory, 2001;Hankins et al, 2006), (3) proteins (Tan et al, 2008(Tan et al, , 2009), (4) cationic surfactants Yee et al, 2006;Ishiguro et al, 2007) and (5) mineral surfaces Filius et al, 2000;Saito et al, 2004;Weng et al, 2005). Therefore, for a good understanding of these interactions reliable information on charge characteristics of HS is of vital importance.…”