Thorough understanding of the kinetics and mechanism of heavy metal adsorption onto a pyrophyllite mine waste based geopolymer

• Published in: Journal of molecular liquids 2018-08, Vol.263, p.428-441
• Main Authors: Panda, Laxmipriya, Rath, Swagat S., Rao, Danda Srinivas, Nayak, Binod B., Das, Bisweswar, Misra, Pramila K.
• Format: Article
• Language: English
• ISSN: 0167-7322; 1873-3166
• DOI: 10.1016/j.molliq.2018.05.016

The present work explored the effectiveness of a geopolymer, synthesized from pyrophyllite mine waste, as an adsorbent for heavy metals such as Co, Cd, Ni, and Pb from aqueous solution. The chemical analysis of mine waste sample revealed the presence of silica (49.38%) and alumina (24.95%) besides Ca and Mg-bearing phases indicating the sample to be a prospective source for the geopolymer formation. The X-ray diffraction studies of the waste sample showed the presence of pyrophyllite, quartz, muscovite with a little amount of anorthite and albite. The presence of these phases was also confirmed by the scanning electron microscopy-energy dispersive X-ray spectroscopy and electron probe microanalysis. The synthesized geopolymerized sample was found to be highly amorphous in nature and did not show any major peaks that were found in the original feed sample. The batch adsorption studies supported the enhanced adsorption capacity of the geopolymer in comparison to the original waste sample. The maximum removal in the range 98–99% of the heavy metals could be accomplished by the geopolymer at pH 7.8, temperature 343 K, and initial metal concentration of 10 ppm. The adsorption data were found to fit pertinently to Langmuir model, Temkin model and pseudosecond order rate kinetics. The mechanism of adsorption was discussed with schematic models. The swellings developed in the pyrophyllite sample during geopolymerisation rupture to give rise to a highly porous geopolymer that efficiently adsorbs heavy metals upto approximately 98-99% from water. [Display omitted] •A potent geopolymer from pyrophyllite mine waste was synthesized for heavy metals adsorption.•Maximum adsorption of 98.97% Co, 97.53% Pb, 98.87% Ni and 98.65% Cd occurred at optimized conditions.•The adsorption data fitted to Langmuir and Temkin models satisfactorily.•The positive change in entropy of adsorption was due to release of solvated water molecules to the surroundings.•The adsorption of metal ions followed pseudosecond order kinetics.