Novel porous phosphoric acid-based geopolymer foams for adsorption of Pb (II), Cd(II) and Ni(II) mixtures: Behavior and mechanism

In this work, novel porous phosphoric acid activated geopolymer foams (abbreviated as PAG) with open cell structure are synthesized from a simple and effective process. Commercial metakaolin (MK) is used as raw material, hydrogen peroxide solution and Triton X-100 act as pore former and foam-stabilizer agents, respec-tively. The resultant PAG samples possess macropores around several millimeters and micro-cracks on geo-polymer matrix. Different analytical instruments including optical microscope, X-ray diffraction, scanning electron microscopy equipped with energy-dispersive spectroscopy, Fourier transform infrared spectrometer, Brunauer-Emmett-Teller surface area measurement, etc. are applied to analyze the structural evolution. The obtained PAG is tested for heavy metals removal in unitary (Pb, Cd, Ni), binary (Pb-Cd, Pb-Ni, Cd-Ni) and ternary (Pb-Cd-Ni) metal solutions. The effect of pH on the simultaneous removal performance of Pb2+, Cd2+ and Ni2+ by PAG is studied. The maximum removal efficiency of Pb2+, Cd2+ and Ni2+ is achieved at pH = 7, and the selectivity sequences of the three metals adsorbed in ternary (Pb-Cd-Ni) metal solution is Pb2+ (11.99 mg/ g)> Ni2+ (6.16 mg/g)> Cd2+(2.81 mg/g). The adsorption mechanisms are heavy metals exchanged with H+ in geopolymer matrix and surface complexation with the abundant -OH groups (Si-OH, P-OH, Al-OH) on PAG. The selectivity sequence can be ascribed to the difference of the hydrated heavy metals' radii. The heavy metals adsorption capacities of acid activated geopolymer foams are higher than that by alkali activated geopolymer monoliths reported in literature, suggesting its high potential application in wastewater treatment.