Preparation of a surface modified fly ash-based geopolymer for removal of an anionic dye: Parameters and adsorption mechanism

Geopolymers have been recently studied as environmentally friendly and low-cost adsorbents especially for the removal of cationic species in wastewater treatment mainly because of their negative surface charge at spontaneous pH conditions. Although there are very few recent studies conducted with different geopolymer composites on anionic dyes, high cost, difficulty of the composite preparation and most importantly the necessity of very low pH values limit their usage. Hence, in this study, a simple and low-cost surface modification with CTAB was applied to a previously prepared fly ash-based geopolymer (GEO) for the removal of anionic Acid Blue 185 (AB185) without the need of strongly acidic conditions. Within this scope, the effects of CTAB dosage (1-5% by weight of GEO), adsorbent dosage (0.5-3.0 g L-1) and initial dye concentration (10-50 mg L-1) were studied as a function of retention time (5-300 min). For 40 min, the removal efficiency of AB185 substantially increased from 0.29 up to 79.36% for the respective GEO and its modified product with 4% CTAB (MGEO4). The efficiency increased with the adsorbent (MGEO4) dosage of up to 2.0 g L-1 at which 89.20% was obtained for 300 min. However, a little decrease was observed down to 81.10% for 3.0 g L-1. The efficiency values of 98.19 and 89.20% were obtained for the initial AB185 concentrations of 10 and 50 mg L-1, respectively. The LangmuirHinshelwood kinetic model is highly correlated with the experimental results. The high adsorption capacity attained in a very short time suggests that the main mechanism is based on physical adsorption via the electrostatic attraction between MGEO4 and AB185. Overall results have indicated that the CTAB-modified fly ash-based geopolymer can be effectively used for the adsorption of AB185.