The dissociation constant (pKa) value of bentazon at 25 C is 3.28 The solution pH higher than 3.28 indicates dissociation of bentazon producing bentazon ion in the solution. On the other hand pHPZC of the SFHAC is 6.4 and at pH below this point the surface of the AC is positive charged and above the pHPZC surface will show negative charge. Thus, as pH increases the rate of dissociation increases, resulting in more negative () ions in the solution ultimately leading higher adsorption due to electrostatic attraction between bentazon ion and the positively charged surface of the SFHAC until pHPZC. Afterwards above the pHPZC, the amount of adsorption has been reduced drastically due to electrostatic repulsion between the bentazon ion and negatively charged surface of the SFHAC. FTIR analysis also suggests the presence of carboxylic acids or alcohol functional groups which also responsible for the presence of negative ions in the solution. As a result of electrostatic attraction between surface and the pesticide ions leads to the enhancement of adsorption of bentazon until pH 6.