Contaminant retention affected by controlled and uncontrolled pH for fly ash-bentonite composites used as landfill liner material

Abstract

Contaminant retention characteristics of clay-based composites as affected by pH needs to be quantified for its design, functionality, and performance as a landfill liner material. The efficacy of four fly ash (FA) in bentonite-fly ash (B-FA) mixes was investigated for its lead (Pb2+) retention characteristics under controlled pH (pH = 5) and realistic uncontrolled pH conditions anticipated in the landfill liners. B-FA mixes with 30%, 50%, and 70% FA amendment, bentonite-sand (B–S) mixes, and parent materials were tested by batch sorption tests. Under uncontrolled pH conditions, the retention capacity of all parent materials and mixes was higher than the controlled pH condition due to significant precipitation reaction and complexation associated with pH. Beyond 1000 mg.L-1 initial concentration, B-FA mixes constituting FA with inherent pH > 10.5 were the most efficacious adsorbents. Under both pH conditions, the metal retention capacity of B-FA mixes was higher than that of conventional B-sand mixes. All results were analytically substantiated by detailed FESEM micrographs, XRD, and EDX spectra of samples before and after sorption. The Freundlich and Langmuir nonlinear retention model parameters indicate that the uncontrolled pH condition results in nonlinear sorption, the higher energy of sorption, and stronger interactive bonds between Pb2+ ions and sorbent.