Facile synthesis and characterization of metakaolin/carbonate waste-based geopolymer for Cr(VI) remediation: Experimental and theoretical studies

[Display omitted] •Dolomite waste/metakaolin-based geopolymer was employed as a low-cost adsorbent for Cr(VI) removal at different temperatures.•The developed geopolymer with 15 wt% MK offered the highest compressive strength.•Statistical physics models were utilized to understand the Cr(VI) adsorption mechanism.•The Cr(VI) adsorption capacities extended from 42.49 to 50.44 mg/g.•Thermodynamic parameters explained the spontaneous nature of Cr(VI) adsorption. 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 °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 (NM) from 44.08 to 23.25 mg/g and uptake capacity (Qsat) 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., ΔE