Green cylindrical mesoporous adsorbent based on alkali-activated phosphorous slag: synthesis, dye removal, and RSM modeling

Abstract

In this work, activated phosphorous slag based cylindrical adsorbent was synthesized for removal of basic violet (BV) and malachite green oxalate (MGO) dyes from aqueous solution. For this purpose, the adsorbent was prepared using a mixture of silica-fume and phosphorous slag as raw precursor and different concentration of NaOH as alkaline solution under a specified curing condition. Response surface methodology has been applied to determine the optimal adsorbent composition and experimental conditions (pH, initial concentration, adsorbent dosage, and contact time). The obtained results showed that the phosphorous slag combined with 30 wt% of silica-fume and 15 mol/L of NaOH was the optimal synthesized adsorbent with an acceptable strength of 6.25 MPa. The maximum adsorption capacity was measured as 46.58 mg/g for BV and 46.36 mg/g for MGO in the optimum condition. Specific surface area for the produced shaped adsorbent is 56.59 m2/g with average pore diameter of 20 nm which was classified as a mesoporous material. It was found from adsorption/desorption test that the overall adsorption capacity decreased about 11% after seven-cycle that result in a reliable reuse of the shaped adsorbent. The resulted high removal efficiency after several desorption cycles not only could be due to the porosity of the mesoporous adsorbent but also could be due to the presence of Si–O–Si bonds in its molecular-structure that result in higher adsorption efficiency. It was also found that silica-fume could play as a structure modifying agent that affect the molecular-structure of the adsorbent by formation of more Si–O–Si bonds due to increase in Si/Al molar ratio in the adsorbent composition.