Upcycling of Precipitated Silica from Bamboo Alkaline Black Liquor into a Mesoporous Silica-Based Adsorbent for Dye Removal

Dye wastewater pollution has become a critical environmental issue, while conventional silica adsorbents are often limited by high production costs and the use of chemical silicon sources. In this study, a black-liquor-derived silica-based adsorbent was prepared from bamboo alkaline black liquor through calcium hydroxide precipitation, acid leaching, washing, and calcination. The obtained product was systematically characterized by ICP-OES, XRD, SEM, and BET analysis. Phase analysis showed that the product was mainly composed of amorphous SiO2, together with a small amount of residual CaSiO3. The product was semi-quantitatively estimated to contain approximately 64.6 wt.% total SiO2 and 14.7 wt.% total CaSiO3, and the estimated SiO2 yield was about 4.6 wt.% based on dry black liquor solids. The adsorbent exhibited a mesoporous structure with a specific surface area of 60.04 m2/g, a pore volume of 0.23 cm3/g, and an average pore diameter of 7.5 nm. The effects of adsorbent dosage, initial methylene blue concentration, solution pH, and temperature on adsorption performance were investigated. Under the selected conditions of 0.5 g adsorbent, 50 mg/L methylene blue, pH 7, and 20 °C, the removal efficiency reached 82.94%. Adsorption isotherm analysis showed that the Langmuir equation provided the best fit among the tested models over the studied concentration range, and the linear Langmuir fit gave an apparent capacity parameter of 840.3 mg/g. However, this fitted value should not be interpreted as a physically realizable ideal monolayer uptake. This work provides a feasible route for converting silicon-containing bamboo alkaline black liquor into a mesoporous silica-based adsorbent for dye removal.