Effects of organic acids on release of some elements from sepiolite and palygorskite minerals in saline conditions

Clay minerals have significant impacts on all physical, chemical and biological properties of soils. Weathering of clay minerals in soil could be affected by several factors such as redox status, dehydration, hydrolysis, dissolution, chelation (bond a metal ion with an organic capsule) and release of aluminum hydroxide into juicy clays structure through exchange position. Limited information is available about the influence of organic acids on dissolution of fibrous clay minerals especially in saline conditions. This study was carried out to identify the influence of studied parameters on release of magnesium, silicon and aluminum from sepiolite and palygorskite minerals.
A laboratory experiment was set up with a completely randomized design and factorial arrangement in three replications. Treatments included two types of mineral (sepiolite and palygorskite in size of 25-53 μm), two organic acids (0.01 M citric and oxalic) and a control and four salinity levels (0, 6, 12 and 24 dS/m). The sepiolite and palygorskite minerals were taken from a mine in Fariman city (northeastern Iran) and Tolsa Co. (Spain), respectively. Then their elemental composition and X-ray diffraction patterns were measured using Bruker Pioneer S4 and Bruker D8 machines, respectively. The concentrations of released elements were also measured by GBC atomic absorption spectrometer.
The results obtained from determination of elemental composition of sepiolite and palygorskite minerals showed that sepiolite has higher concentration of MgO than palygorskite which is compatible with those reported in literature. The presence of minor concentration of CaO in elemental composition of the both minerals suggests that both minerals have minor amounts of carbonates as the accompanying phase. This was further supported by the results of XRD analysis of the samples. The results also illustrated that the amount of released elements is highly affected by the type of organic acids, salinity and type of mineral. Among the organic acids, the amount of Mg and Al released from sepiolite and palygorskite in citric acid treated samples were greater than those from oxalic acid samples. However, oxalic acid was found to be more effective for extraction of Si from the minerals. With increasing salinity, the release of all elements was increased, however; in presence of organic acids the amount of released elements was reduced comparing with non-saline conditions. Moreover, the results illustrated that the ratio of released Si/ (Al) in sepiolite samples was higher than palygroskite.
The results of this study suggest that sepiolite is less stable than palygorskite and is much more weathered under the conditions studied.