The Equilibrium contact time is very important in order to determine the adsorption capacity of the adsorbents. Fig. 5 describes the effect of contact time and initial concentration on the removal of bentazon by SFHAC. The adsorption study was conducted at different predetermined contact time and initial concentrations. Six initial concentrations 25, 50, 100, 150, 200 and 250 mg/L were selected for this study while adsorbent dose remained constant at a temperature of 30 C. It was observed that bentazon uptake was very fast for the first 30 min and gradually slowed down until equilibrium was attained. After the 8 h equilibrium period, no remarkable adsorption was observed. Already, it has been reported that the prepared SFHAC has large mesoporous surface area (1211.57 m2/g) with free functionalized carbon active sites which might regulate the higher removal% rate at the beginning.
But after equilibrium was reached, theses active sites were fully covered by the herbicide compound and raised intra-specific competition among the bentazon molecules that retarded the adsorption process.