Constructing nanostructured silicates on diatomite for Pb(II) and Cd(II) removal

Crystallizing nanostructured silicates on diatomite is an alternative way to prepare promising low-cost adsorbents for heavy metal removal. Here we report a facile hydrothermal process to build (Zn₂SiO₄ or Mn₇O₈(SiO₄)) layer on diatomite with the assistance of ethanediamine which is essential for the construction of hierarchical silicate nanostructures via an energetically favored heterogeneous nucleation process. The prepared ZnDt (Zn₂SiO₄/diatomite) was selected as an example to study the absorption behaviors toward lead Pb(II) and cadmium Cd(II) ions. The results reveal that the adsorption capacities of ZnDt composite for Pb(II) and Cd(II) ions reach 443.0 mg/L and 29.0 mg/L, respectively. The adsorption kinetics and isotherm analysis show that the adsorption process is properly described by pseudo-second-order and Langmuir models. Process parameters (pH value, ion strength, adsorption time, etc.) effecting the adsorption capacity were optimized, and high adsorption capacity for Pb(II) in the presence of coexistent cations (Cd²⁺ and Na⁺) was observed in competitive adsorption process. Our structural analysis and Zn²⁺ extractability determinations show that surface layer of Zn₂SiO₄ nanosheets is hydrolyzed in Pb²⁺ and Cd²⁺ aqueous solution, resulting in the dominant chemisorption of Pb²⁺ and Cd²⁺ on external surface sites to generate Pb₃(OH)₂(CO₃)₂ and Cd(OH)₂, respectively. The current work may inspire the development of new types of 3D nanostructured diatomite-based adsorbents for remediation of wastewater.