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

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 degrees C, 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/coprecipitation. The results exhibited that nZVI@ATP has excellent potential as an adsorbent for the removal of Cd2+ from the contaminated water.