Enhanced removal of Cd2+ by nano zero-valent iron modified attapulgite from aqueous solution: Optimal design，characterization and adsorption mechanism

Abstract

Nano zero-valent iron (nZVI) modified attapulgite composite (nZVI@ATP) was prepared by liquid phase reduction method for Cd2+ removal from aqueous solution. A series of characterization methods confirmed that nZVI particles with a particle size of 50–100 nm were evenly distributes on the surface of attapulgite after pickling, which greatly overcomes the agglomeration of nZVI. Composite material had the optimum removal rate of Cd2+ when the ratio of iron to attapulgite was 1: 2. More than 98% of Cd2+ was removed from aqueous solution using nZVI@ATP at an initial condition of 100 mg/LCd2+ under the conditions of 1 g/L of nZVI@ATP, pH 5.0 and a temperature of 25 ℃, much higher than that of attapulgite. Kinetic studies revealed that Cd2+ adsorption process followed the pseudo-second-order model, and basically reached the adsorption equilibrium after 2 h, indicating that chemisorption was the dominant adsorption mechanism. Adsorption thermodynamics showed that the adsorption process was a spontaneous endothermic reaction, accompanied by the increase in entropy. The Freundlich adsorption isotherm model could well fit the adsorption process. The presence of Na+, Ca2+ and Mg2+ in the solution strongly inhibited the removal of Cd2+. The mechanism of Cd2+ removal by nZVI@ATP involved multiple processes, mainly including surface-complexation adsorption and precipitation/co-precipitation. The results exhibited that nZVI@ATP has excellent potential as an adsorbent for the removal of Cd2+ from the contaminated water.