Removal of dichromate-, molybdate-, and nitrate ions from wastewater using modified natural montmorillonite

Abstract

Natural montmorillonite (cationic adsorbent) was modified with sodium metasilicate to invert its adsorption capacity for the ability to adsorb anions from wastewater. The textural characteristics decreased several times due to interlayer space blocking with a surface area of 2.7 m2/g. The ζ-potential changed from negative to positive (10.4 mV). The adsorption of inorganic anions Cr2O72-, MoO42- and NO3– was 0.11; 0.049; 0.052 mmol/g. Equilibrium was achieved within 25–30 min with 80–85 % adsorption in the first 10–15 min. The adsorption kinetics of all anions correlated well with the pseudo-second-order model with R2 0.993–0.999 and was largely determined by the external diffusion of the adsorbent to the surface. Adsorption increased with an increase in mass and charge of the adsorbent anions. The adsorption isotherms were best approximated by the Redlich-Peterson model, with a value of i = 0.6–0.65 at 293 K and i = 0.8–0.95 at 313 K. The activation energy of the process was: Ea(Cr2O72-) = 0.63 kJ/mol; Ea(MoO42-) = 11.59 kJ/mol; Ea(NO3–) = 22.49 kJ/mol). The values of the energy and enthalpy of adsorption indicated that nitrate anions were adsorbed on the surface due to the physical adsorption (orientation component). Besides, the chromate and molybdenum anions were adsorbed due to the formation of weak hydrogen bonds with protonated aluminol and silanol groups of montmorillonite. Finally, the removal of these anions was up to 90 % at a natural water pH of 6.5. It means that the reported method may be prospective to remove Cr2O72-, MoO42-, and NO3– from water.