Tuning adsorption, structure and compressive strength of sepiolite- and metakaolin-based alkali activated monoliths for methylene blue removal from waste water

Adsorption is considered as the most beneficial waste water treatment method when compared to other physical, chemical, and biological treatment methods due to its economic feasibility, environmentally friendliness, and easy implementation. Alkali activated materials (AAMs) are promising materials as adsorbents for removing organic molecules from waste water owing to their low cost and availability, easy and environmentally-friendly synthesis, high adsorption capacity, chemical stability, regeneration/reusability capability, and high mechanical strength. Unfortunately, powder-formed adsorbents are mostly focused in literature although their practical utilization is limited due to problems in recovery, regeneration and mechanical performance. In this regard, a family of sepiolite (S)- and metakaolin (M)-based alkali activated monoliths (S-Monolith and M-Monolith) with varying porosities were synthesized and tested for methylene blue (MB) adsorption comparatively for the first time. Hydrogen peroxide (H2O2) is used as a foaming agent to adjust the porosity with varying concentrations in the ranges of 0–8 wt% and 0–1 wt%, for S-Monoliths and M-Monoliths, respectively. As porosity of S-Monoliths increases from 13% to 40%, MB uptake capacity rises from 6.6 to 10.3 mg g−1 while corresponding compressive strength values decrease gradually from 37 MPa to 6.5 MPa. On the other hand, as the porosity of M-Monoliths varies from 31.9% to 63.5%, corresponding MB uptake capacity increases from 4.3 to 7.8 mg s−1 whereas compressive strength values decrease from 28 MPa to 2.1 MPa. Both pseudo-first-order and pseudo-second-order adsorption kinetics show good fit to the experimental data, indicating that both physical and chemical adsorption processes govern the adsorption process. Adsorption isotherm model studies showed that Langmuir isotherm provides a better fit indicating that chemisorption controls the adsorption mechanism. Regeneration experiments conducted on samples with the highest porosities show that S- and M-Monoliths can be regenerated up to four cycles without loss in their integrity. Results show that the adsorption performance of AAM monoliths synthesized from different raw materials can be tuned by changing the porosity levels and these monoliths offer mechanically strong and sustainable options as adsorbents for industrial applications.