This study is aimed at evaluating the efficiency of hydrochloric acid, modified red mud in removing diclofenac
(DCF) from aqueous solution. Red mud produced was treated with 2.25 M HCl, noted RMH. Batch experiments were used
to determine the effects of time of stirring/initial concentration, pH, adsorbent dose and temperature on the efficiency of
DCF adsorption on RMH. pH was the main parameter that controls DCF adsorption, with removal of 95 % at pH of 4 in 10
minutes. But the increase in temperature decreased DCF adsorption from 95 % at 298 K to 62 % at 333 K. Equilibrium was
attained in 10 minutes for all the parameters tested. Langmuir isotherm fitted the adsorption with maximum adsorption
capacity values of 500 and 222 mg/g at 1050 and 500 mg/L initial concentrations respectively. Adsorption was very
favourable with very low RL values (0.003-0.0003). Adsorption energies obtained from the Dubinin–Radushkevich
isotherm revealed that physical adsorption was the main mechanism of adsorption. Pseudo-second order kinetic model best
explained the adsorption with initial sorption rates increasing with increasing initial concentration. The removal process
was very spontaneous and exothermic. HCl treated red mud is very efficient in removing DCF from aqueous solution.