New trials of geopolymer silicate bricks based on natural Tunisian sands and their adsorptive properties

This study aims to reduce the impact of the cationic dye methylene blue (MB) by removing it from aqueous solutions using geopolymer silicate bricks made from local Tunisian silica sands combined with metakaolin. The chemical–mineralogical characterization (XRD, XRF) of the raw silica sand,collected from the Jebel Zemlet el Beidha region of the south of Tunisia, shows it to be mainly formed of quartz and potassim-based feldspars, with smaller levels of calcite and kaolinite.Between 10–30 wt% metakaolin was added to these silica sands (88.9% SiO2),and the geopolymerisation process was activated using a mixture of sodium silicate and NaOH solutions, valorizing this plentiful natural resource. The inorganic polymer silicate bricks prepared from these natural Tunisian silica sands had suitable properties, with a compressive strength ofup to 27 MPa,up to 51% water adsorption, and with a specific surface area measured at 24 m2/g.It was shown with adsorption tests that these silicate bricks are very effective in adsorbing methylene blue (MB) at relatively low concentrations in aqueous media. The adsorption of MB was rapid, efficient, and with an optimal discolouration rate of 73.17% for only 25 mg/L of MK30 in a 10-ppm MB solution after 4 h, and nearly 2/3 of this value was obtained after only 5 min. The MB-adsorption behavior of the silicate bricks was described satisfactorily using the Langmuir isotherm, and from this model a maximum retention rate of around 77.34 mg/g of MB for thegeopolymer silicate bricks was determined.