Waste Disposal in Clay Formations: Influence of Humic Acid on the Migration of Heavy‐Metal Pollutants

Abstract

The deep geological disposal of high-level radioactive waste is assumed to be a way of providing adequate protection for humans and the environment. For the safe isolation of radioactive waste, a low permeability and high sorption capacity of the geological barrier is important. Clay minerals play a major role in different concepts for the disposal of high-level nuclear waste in deep geological formations. Geological clay formations are considered to be a host rock for waste repository. [1] Natural clays can contain natural organic matter, for example, humic substances. In most aquatic systems, components of natural organic matter such as humic acids can act as ligands for complexing metal ions and may affect the speciation and thus the mobility of the metal.[2] By forming soluble complexes and colloids with numerous toxic heavy metals, including radioA C H T U N G T R E N N U N G nuclides, humic acids can influence the migration behavior of these pollutants in hematite [3] or geological clay formations, as reported, for example, for bentonite, [4] kaolinite, silica, and montmorillonite.[5] Kaolinite is a significant component of many soils and fracture-filling materials of crystalline hard rock. These clay formations are part of the natural barrier protecting the environment from the possible impact of radionuclide contamination.[6]Herein we report the sorption behavior of the trivalent lanthanides europium and gadolinium (as homologues of the actinides americium and curium), under conditions close to those of nature, in a binary system consisting of these heavy metals and well-crystallized kaolinite (KGa-1b) as a model mineral, and in the ternary system metal/kaolinite/humic acid. The influence of the concentration of lanthanide ion, pH value, or concentrations of different competing metal cations (water hardness, represented by calcium and magnesium) on the sorption of the lanthanide ion onto kaolinite in the presence or absence of humic acid was investigated. After the sorption experiments, free metal ions in the supernatant solution were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Speciation of the lanthanide-humic acid complexes are reported elsewhere.[7]The ternary system described in Figure 1 is most suitable to study metal migration under natural conditions. Research on the three binary systems can be seen as preliminary tests to understand partial aspects of the complete system. With humic acid, there is the possibility that metal humates could adsorb on the kaolinite surface through hydrophobic interactions, thus increasing retention of the lanthanide ion. Negatively charged complexes could inhibit sorption of the lanthanide and accelerate migration through clay minerals. Sorption behavior is strongly dependent on the experimental conditions as tests performed in the ternary system clearly show. The experiments that were performed here are based on the system metal/kaolinite with humic acid as an additional influencing factor.With humic acid present, the sorption of the lant...

Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH, ionic strength, carbonate and humic acid

“…A layer consists of a single sheet of SiO 4 tetrahedra combined into hexagonal rings, with a superimposed dioctahedral gibbsite type sheet of edge sharing octahedra (Strunz and Nickel, 2001). Some of the important papers reporting on the sorption of radiotoxic contaminants onto kaolinite are: Aksoyoglu et al, 1991;Kohler et al, 1992;Niitsu et al, 1997;Samadfam et al, 2000;Sakuragi et al, 2002;Payne et al, 2004;Stumpf et al, 2004;Křepelová et al, 2006Křepelová et al, , 2008Amayri et al, 2008;Buda et al, 2008;Kautenburger andBeck, 2008 and. The sorption ability of the various metal ions onto kaolinite was found to be strongly dependent on their oxidation state, thereby showing moderate (Np(V)), high (Am(III), Cm(III)) or very high sorption (Th(IV), Pu(IV)).…”

Benefits of multidimensional fractionation for the study and characterization of natural organic matter

“…NOM is of environmental, agricultural, industrial, and biomedical importance. For example, NOM, is studied for its effect on limiting nutrients in open oceans [1], its own role as organic nutrient [2,3], carbon cycling/storage [4,5], formation of harmful disinfection byproducts in drinking-and waste-water [6,7], sorption/transport/bioavailability of metals and radioactive waste [8][9][10] , and aerosol composition [11]. Moreover, humics and "humic-mimetics" (synthetically made HULIS) are studied for their antiviral, antibacterial, and cancer-related properties.…”

Complexation of europium and uranium by humic acids analyzed by capillary electrophoresis‐inductively coupled plasma mass spectrometry

“…Additionally, the actinide uranium as the principal component of nuclear fuel was used. Metal migration or sorption in the geosphere can be affected by a wide variety of chemical reactions; with all kinds of natural organic matter (NOM), especially humic substances []. Humic material is a large and heterogeneous group of macromolecules of different molecular weight (500–250 000 Dalton) and charge density, with complex and difficult structure [].…”

“…stances [6,7]. Humic material is a large and heterogeneous group of macromolecules of different molecular weight (500-250 000 Dalton) and charge density, with complex and difficult structure [8].…”