10 mg of the adsorbent were added to 10 mL of the dye solution having a fixed concentration and pH. The mixture was shaken(250 rpm) at room temperature. To measure the quantity of dye adsorbed on the support as a function of time at regular intervals from 10 to 900 min, liquid was separated from SiO2 particles by filtration through a 0.45 m PTFE Millipore filters. The adsorption process was monitored with a Cary 3E UV–vis–spectrophotometer by determining the concentration of the colorant in the supernatant. For each dye, pH-specific standard calibration curves have been made at the maximum absorption wavelengths and use in its respective pH range. For example, at pH 7, the absorbance values were taken at 500 and 625 nm for congo red and brilliant green,respectively. At a given time t, the adsorbed quantity of dye (qt) is given by the relation