Immobilization of toxic inorganic anions (Cr2O72-, MnO4- and Fe(CN)63-) in metakaolin based geopolymers: A preliminary study

Geopolymer (GP) consists of polymeric Al and Si tetrahedral network, with alkali metal cation (Na+ or K+) balancing the negative charge of Al tetrahedra. Although immobilization of heavy metal cations, that could balance the negatively charged AI tetrahedra in GP, was extensively studied, this is the first study that deals with immobilization of metal anions (dichromate, permanganate and hexacyanoferrate). GP samples were prepared from metakaolin (MK) as an aluminosilicate material and sodium silicate activator (Si/Al molar ratio 2) in the presence of the three anions. The efficiency of immobilization was studied in terms of % leaching of metal anions using atomic absorption spectrophotometry and in terms of leaching of total metal anions by conductivity measurements. The % leaching was less than 10% when 0.2% (w/w) of metal anion salts were incorporated in the GP. However, the % leaching increased from 10% to 20% with increasing loading of dichromate salt from 0.2% to 1.5% (w/w) which indicates that GP has limited capacity (limited number of pores) for immobilization of metal anions. When silica gel and alumina were employed as additives to MK (changing Si/Al ratio), the leaching of dichromate and total ions increased substantially and the surface of GP becomes more crystalline as indicated by SEM and EDS studies. The present work suggests that the mechanism of heavy metals immobilization by balancing the negatively charged Al tetrahedra of GP is not true.