Montmorillonite—the most popular mineral of the smectite group—has been recognized as a low-cost, easily available mineral sorbent of heavy metals and other organic and inorganic compounds that pollute water. The aim of this work was to determine the sorption mechanism and to identify the reaction products formed on the surface of montmorillonite and organo-montmorillonite after sorption of molybdates (Mo(VI)) and tungstates (W(VI)). Montmorillonites are often modified to generate a negative charge on the surface. The main objective of the study was to investigate and compare the features of Na-montmorillonite (Na-M), montmorillonite modified with dodecyl trimethyl ammonium bromide (DDTMA-M), and montmorillonite modified with didodecyl dimethyl ammonium bromide (DDDDMA-M) before and after sorption experiments. The material obtained after sorption was studied by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The XRD pattern showed the presence of a new crystallic phase in the sample that was observed under an SEM as an accumulation of crystals. The FTIR spectra showed bands related to Mo–O and W–O vibration (840 and 940 cm−1, respectively). The obtained results suggest that molybdenum(VI) and tungsten(VI) ions sorb onto the organo-montmorillonite in the form of alkylammonium molybdates and tungstates.

Among the various technologies tested for removing the anionic species resulting from arsenic contamination, sorption methods have received unflagging interest. Being potential sorbent materials, clay minerals modified by cationic surfactants are often examined for this purpose. Among the clay minerals tested, information regarding sorption properties of expanded vermiculite modified with surfactants is scarce. Therefore, the present study aims to prepare organo-vermiculites modified with hexadecyltrimethylammonium (HDTMA) and benzyldimethylhexadecylammonium (HDBA) at surfactant concentrations of 0.5, 1.0, and 2.0 cation exchange capacity. Modified sorbents were identified and characterized using the analytical methods that can determine phase composition and textural properties of the samples. The sorption of As(III) and As(V) as a function of initial pH value, initial concentration of As(III, V), and initial dosage of sorbent was investigated. The results show that HDTMA and HDBA affect the properties of raw vermiculite. For instance, increase in the concentration of surfactants is often accompanied by a change in interlayer space or textural properties of vermiculite. It was observed that tested organo-minerals adsorbed As(V) to a greater extent compared to As(III). Various analytical studies were carried out and the results revealed the successful synthesis of organo-vermiculite. Moreover, the study also showed that the structure of organo-vermiculite has a significant impact on the uptake of As(III) and As(V) anions.In the last decades, various technologies for removing As from contaminated water have been investigated, such as oxidation, coagulation, ion exchange, precipitation, membrane filtration, biological treatment, and sorption [9][10][11]. However, most of these methods have drawbacks, including high costs, deficient removal, generation of waste products, or high reagent and energy requirement. Among the methods mentioned earlier, adsorption seems to be the most effective method [1,12]. For removal by adsorption technique, activated carbon, metal-organic frameworks, zeolites, bog iron ores, water treatment residuals, and activated alumina have been proven to be promising adsorbents for arsenic removal [13][14][15][16][17].Clay minerals are low-cost and nontoxic sorbents. Raw clays are able to sorb cations owing to their negatively charged surface; however, they display low sorption efficiency for negatively charged contaminants such as arsenic [18]. Nevertheless, clays can be chemically modified by organic surfactants, which change the charge on the clay surface from negative to positive [19]. The organo-clays are hydrophobic materials that can effectively sorb anions and organic compounds. Modified kaolinite, montmorillonite, and illite are the most popular clay minerals that are used as sorbents [20][21][22]. However, only a few studies have focused on using organo-vermiculites to sorb anions [23,24].Vermiculite is a 2:1 phyllosilicate mineral having a sandwiched structure with two tetrahedral shee...

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Bog iron ores are known for their sorption properties regarding heavy metals. However, they have not been commonly used as sorbents of arsenic compounds. The aim of this study was to investigate As(III) and As(V) immobilization by bog iron. The tests included varying initial As concentrations (0.01-20 mM), and initial pH values (2-12), and also sorption experiments to evaluate the competition between both As(III) and As(V) and heavy metal cations. The results showed that As removal by bog iron ores depends on the oxidation state of As-the removal of As(V) is lower than the removal of As(III). Immobilization of As was the most effective at medium initial concentrations of As (0.25-1 mM) in a slightly acidic or neutral pH environment. Competitive sorption experiments revealed that the occurrence of several ions in the solution significantly affects the sorption effectiveness. The bonding strength of As with a bog iron ore surface was estimated on the basis of three-step desorption experiments. Desorption of As resulted in the extraction of less than 50% of adsorbed As(III) and As(V). This study shows that bog iron ores constitute an appropriate adsorption material for arsenic especially at concentration range 0.25-5 mM, pH 5-10 for As(III) and 0.25-0.5 mM, pH 2-5 for As(V). However, there are no simple correlations between mineralogy and sorption capacity.

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One of the most effective methods for the immobilization of toxic metals involves the use of minerals from the apatite supergroup. The formation of cadmium chlorapatite may lead to successful entrapping of cadmium; thus, it is important to examine the solubility constant to determine the stability of cadmium in the the apatite structure. Cadmium chlorapatite was synthetized and characterized by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy. The solubility constant (log) Ksp of cadmium chlorapatite was -65.58. The Gibbs free energy of formation of cadmium chlorapatite reached -3950.48 kJ mol−1. The solubility constant turned out to be low but was enough for cadmium chlorapatiteto be considered a very stable compound..