Silica removal from the sample was done following the proce- dure of Dobrowolski. After finding the weight percent of silica in the sample by the Chung method, 100 mg of soil sample was put in a crucible. 300 ml of concentrated hydrofluoric acid was added drop- wise onto the surface of the powder and left for 20 min. This is particularly helpful for obtaining a smaller particle size of soil and for partial destruction of the skeleton of soil components. 300 ml of 60% polytetrafluoroethylene (PTFE) slurry was added which nearly equals the same molar concentration was of quartz in all samples. Double distilled water (2.4 ml) was added to make the volume 3 ml. Making the volume to 3 ml fulfills both the conditions of protocol, that are percent concentration of PTFE slurry and HF (6% PTFE slurry and 4% HF), and the same molar concentration of PTFE as that of quartz. Then the mixture was shaken for 2 min with a vortex- type shaker to get the slurries mixed. Immediately after, the sample was placed in a furnace; the temperature was increased gradually upto 600 1C. As Dobrowolski indicated, after evaporation of water at 1001C, the next reaction, that of PTFE decomposition, started at 480 1C. This decomposition occurs in the temperature range 480–580 1C forming very active F radicals which react with SiO2, resulting in the formation of volatile SiF4. The reaction mechanism is as follows: PTFE-R-CF2-CF2-CF2-C2F4 þ , followed by SiO2(s)þC2F4(g)-SiF4(g)þ2CO(g)
However, there is no adequate literature present on the chemical reaction of PTFE with other clay minerals and so more research needs to be done to investigate the reactions of PTFE with other clay minerals and to approve the here used methodology of sample preparation.