It is well understood that the adsorption capacity of organic reactant on photocatalyst is a key factor for the degradation rate in photocatalytic oxidation process. Generally, it is observed that the rate of decomposition of organics over the photocatalyst is more pronounced if large number of target molecules is adsorbed on the catalyst surface, which either depends on the acidic/basic nature of the surface of the catalyst or surface modifications through change in pH of the system [36]. Moreover, the pH of solution has great influence on the surface charge of the semiconductor photocatalyst, which has been studied extensively for variety of photocatalysts. The removal of phenol on ZnO–bentonite nanocomposite at five different pH conditions viz. 2, 4, 7, 10 and 12 under standard CSTR conditions are studied (Fig. 8). It is observed that under basic conditions, the amount of phenol degraded is considerably higher as compared to acidic medium. These observations can be ascribed to two phenomenon: (a) Under acidic condition, surface of the nanocomposite is positively charged at both nano ZnO and clay sites that leads to the protonation of active sites and therefore, affects the adsorption of phenol moiety. Moreover, the protonation of phenol occurring under this condition also hindered the adsorption of phenol there by affecting its removal (