Selective adsorption of Pb²⁺, Ni²⁺ and Cr³⁺ by polyacrylic acid/attapulgite composite adsorbents

Attapulgite (ATP) is a hydrous layer-ribbon magnesium aluminum silicate mineral with a unique three-dimensional structure. Due to its natural abundance, extremely good mechanical/thermal stability, its one-dimensional nanoscale, and a large quantity of silanol groups on the surface, attapulgite is a very good candidate for low-cost materials as adsorbents. To increase adsorption capacity and selectivity, attapulgite must be treated with organo-functional groups to produce specific sites on the surface. Polyacrylic acid/attapulgite (PAA/ATP) was prepared by grafting PAA onto γ-methacryloxypropyl trimethoxy silane (MPS)-modified attapulgite surface via solution polymerization. PAA/ATP was characterized by thermogravimetry (TG) and X-ray photoelectron spectroscopy (XPS). The selective adsorption of ternary mixed heavy metal ions (Pb²⁺, Ni²⁺ and Cr³⁺) by polyacrylic acid/attapulgite composite adsorbents was evaluated in batch experiments by varying contact time and initial ion concentration. The kinetic data was fitted with the pseudo-second-order kinetic model, and the equilibrium data obtained was analyzed with the Langmuir and Freundlich isotherm equations. The kinetic data followed pseudo-second-order equation, indicating the chemical adsorption was the rate-limiting step process. The equilibrium adsorption data of Pb²⁺ could be described well with the Langmuir isotherm model. Obviously, PAA/ATP showed excellent adsorption selectivity for Pb²⁺ compared with Ni²⁺ and Cr³⁺ in terms of distribution coefficient and separation factor, which was also confirmed by XPS.