Facile Synthesis and Characterization of Metakaolin/carbonate Waste-Based Geopolymer for Cr(VI) Remediation: Experimental and Theoretical Studies

The current study reports the fabrication of a geopolymer adsorbent through the activation of metakaolin (MK) and dolomite waste (DW) mixtures by sodium silicate (7.5 wt% of Na2O). Various percentages of MK (i.e., 5, 10, and 15 wt%) were employed, and the geopolymer with 15 wt% MK exhibited the highest compressive strength (44.58 MPa) as compared to the values of 27.18 MPa and 33.97 MPa achieved by the geopolymer with 5.0 wt% MK and 10.0 wt% MK, respectively. This remarkable geopolymer, known as DW/MK-G, was subsequently utilized to remove Cr(VI) from solutions within a temperature range of 25 to 55 degrees C. The as-synthesized DW/MK-G offered a high mechanical performance and active functional groups due to the formation of a well-developed calcium silicate hydrate phase. The experimental data were appropriately fitted to the Langmuir classical model with maximum adsorption capacities ranging from 50.44 mg/g to 42.49 mg/g. The captured Cr(VI) ions by the DW/MK-G binder followed the concept of the statistical monolayer model. The removed Cr(VI) number per site (n) extended from 1.05 to 1.31 producing a vertical geometry with a multi-ionic mechanism. Cr(VI) removal was an exothermic process as evidenced by the decrease in the adsorption sites density (N-M) from 44.08 to 23.25 mg/g and uptake capacity (Q(sat)) from 46.32 to 30.22 mg/g with improving temperature. The Cr(VI) adsorption onto DW/MK-G was governed by the physical interactions (i.e., Delta E < 30 kJ/mol). The attained thermodynamic functions clarified that the interaction between Cr(VI) and DW/MK-G active sites was spontaneous and exothermic.