This work aims to study ammonium removal in an aqueous solution by adsorption on natural and modified Palygorskite clay to improve water quality. The palygorskite (Pal) is a fibrous natural clay and was modified by acid treatment using 4 N and 6 N HCl solutions to enhance its adsorption capacity toward ammonium. The Pal modified were designated as Pal-Ac4 and Pal-Ac6. The ammonium adsorption isotherms on Pal, Pal-Ac4, and Pal-Ac6 were evaluated, and the effects of modification, pH, and temperature on the adsorption capacity were analyzed. At an equilibrium concentration of 300 mg/L and T = 25 degrees C, the adsorption capacity of Pal, Pal-Ac4, and Pal-Ac6 was 2.8, 9.2, and 5.3 mg/g, respectively, and the optimal acid treatment was that for Pal-Ac4. The adsorption capacity of Pal-Ac4 increased, reducing the solution pH from 7 to 3, and the maximum adsorption capacity of Pal-Ac4 was 28.5 mg/g at pH 3.