Contact time is one of the important parameters influencing the efficiency of the adsorption process. The kinetic study was
conducted for As(III) adsorption onto coconut fiber at adsorbent dose of 10 g/L and initial adsorbate concentration of 0.40 mg/L when room temperature was 26 1 C. The experiments were conducted by varying time between 1–10 h. It is evident from Fig. 6 that uptake increased with the lapse of time and gradually nearly approached to a constant value. The uptake of As(III) was higher upto 6 h of contact time, after which it slowed down and the equilibrium approached at contact time of 8 h. This might be due to the reduction of driving force after longer period of operation [19]. Initially the sorbent sites were vacant, hence uptake was increasing with lapse of time. Later number of sites decreased and ultimately
saturation of adsorbent sites occurred after longer period of
operation [17]. Sorption of As(III) at 8 h contact time was
0.0378 mg/g. Reported equilibrium contact times by various
researchers are 4 h for maize leaves [11], 20 h for polymeric
Al/Fe modified montmorillonite [23], 6 h for waste iron slag [25],
22 h for iron impregnated potato peels [26] and 16 h for Fe-RHMCM-
4 (DHT-10) synthesized from rice husk silica [27].