Investigation of Phosphate Retention in some Allophanic and Non-Allophanic Nano-Clays from Karaj Formation

Nanoclays, due to their high specific surface area (SSA) chemical and mechanical stabilities, and a variety of surface and structural properties are widely applied. Some of their applications are using them as nano composite polymers, heavy metal ions absorbents, catalysts, photochemical reaction fields, ceramics, paper fillings and coatings, sensors and biosensors. Nano clays and Clays are the most important components constructing soil ecosystems. The physical and chemical properties of soils are mainly depending on the type and amount their clay fraction pertaining to considerable nanoclays. Nano clays have been frequently used to eliminate environmental contaminants from soil and water. Nano clays have also an effective role in the phosphate sorption and desorption from soil solution. Phosphate retention is highly affected by the chemical bonds of the materials, cristalographic properties and pH. In clay size particles there are different structures of nano particles such as alominosilicates with nano ball and nano tube construction. Soils with andic properties have amorphous clay minerals such as allophone. Allophane has a diameter of 3 to 5 nano meter under a transmission electron microscope (TEM) and its atomic Si/Al ratio ranges between 0.5 and 1. Allophane shows variable charge characteristics and high selectivity for divalent cations, and is highly reactive with phosphate.
The objective of this research was to inspect the effect of soil components particularly clay and nanoclay on the sorption of phosphate. To achieve this goal, we studied the amount of phosphate sorption by the natural nanoclays. Samples with andic and vitric properties which were previously formed on volcanic ash in Karaj were chosen in 5 pedons as two Andic ( > 5 percent volcanic glass, > 25 percent P retention, pH NaF > 8.6 and Alo ½ Feo > 0.4) and non Andic soils.. After removal of organic materials, soluble salts, carbonates and iron oxides from the soil, clay fraction was prepared for X-ray diffraction analyses. The nanoclay fraction was extracted using the method described by Li and Hu (2003). The specific surface area were determined using EGME method. Different forms of extractable aluminum, including pyrophosphate (Alp) and ammonium oxalate (Alo) extractable forms, as well as silica extractable by ammonium oxalate (Sio) were measured. Routine chemical analyses for organic carbon (OC), cation exchange capacity (CEC) were determined by standard methods. Particle size distribution was determined by the hydrometer method (after ultrasound dispersion). Allophane percentage was calculated using the formula provided in the soils under study by Mizota and Van Reeuwijk (1989). Nano particles were inspected using scanning electron microscope (SEM).
The studied soils were classified as Entisols, Andisols and Inceptisols. The results showed that the bulk of soil mineralogy was consisted of combination of illitic, chloritic, smectite and hydroxy interlayer minerals. In addition to sesquioxides, the crystallization degree of soil minerals was also important in phosphate retention. Results of SEM studies of Andisols implied the existence of different types of aluminosilicate nano particles as nano ball (Allophane), nano tubes (imogolite) and smectitic minerals. Hollow spherical structure was proposed for allophane. According to the SEM results, nano particles extracted from non andic soils were dominated by layered silicates (probably montmorillonite). Among physical properties which are effective on phosphate retention, the shape, size and porosity of the particles can be mentioned, all of which have impacts on the specific surface area of the particles. Soils with higher amounts of Alp and Sio were comprised more nanoclay (25,8 g per kg) and higher phosphate retention (%55). Various mechanisms were suggested by soil scientists for phosphate sorption on allophane (Nanoclays). Some of are ligand interchange, silicate replacing by phosphate in high phosphate concentration, and replacing phosphate by weak silicon bonds. There was a positive and significant relation between Sio and Alo amounts (R2= 0,976). The ratio of Alp/Alo in andic soils increased by increasing organic material (at least 0,02 and at most 0,11).
Phosphate retention in the studied soils had a significant relation with Alo (R2 = 0,991).The more percentage of nanoclay showed higher phosphate retention thus the highest amount of phosphate retention was determined in the sample containing highest nanoclays. Nanoclay shows high performance in removal of phosphate from solutions. Little amounts of nanoclays can remove great amounts of phosphate from solution.