Fast, efficient and clean adsorption of bisphenol-A using renewable mesoporous silica nanoparticles from sugarcane waste ash

Abstract

Even with all the biological problems associated with bisphenol-A (BPA), this chemical is still being widely used, especially in thermal paper receipts. In this study, renewable mesoporous silica nanoparticles (MSN), obtained from sugarcane ash, functionalized with hexadecyltrimethylammonium (CTAB) were applied as an adsorbent in the removal of BPA from the aqueous solution. The versatility of this material and its BPA adsorption capacity were tested at different pH values, being practically constant at pH between 4 and 9, with a slight increase in pH 10 and a greater increase in pH 11. The removal time evaluation indicates a very fast adsorption process, removing almost 90% of BPA in the first 20 min of contact. The kinetic model indicates a monolayer formation of BPA molecules on the MSN-CTAB surface. The maximum adsorption capacity (Q _max) was 155.78 mg g^-1, one of the highest found in literature, and the highest for material from a renewable source.

Fig. 1. Aqueous dissociation scheme of BPA with pK_a values.

Fig. 2. Effect of the initial pH on the adsorption capacity of BPA. Conditions: 25 °C, initial concentration 100 mg L⁻¹, contact time 2 h and adsorbent mass 1.0 g L⁻¹ (see Fig. S2 and Table S1†).

Fig. 3. Scheme illustrating the organization of BPA on the surface of MSN-CTAB at different pH values.

Fig. 4. Percentage of BPA removal at 25 °C, at a contact time ranging from 0 to 180 min (initial concentration of 80.9 mg L⁻¹).

Fig. 5. (a) Pseudo first and second-order models kinetics plot for the removal of BPA by MSN-CTAB. Conditions: 25 °C, initial concentration 80.9 mg L⁻¹ and adsorbent mass 1.0 g L⁻¹ (more details in Table S3†). (b) FTIR-ATR spectra of bisphenol-A, MSN-CTAB and MSN-CTAB + bisphenol-A. And TG and DTG curves of (c) MSN-CTAB, (d) BPA and (e) MSN-CTAB + BPA. The measures were performed under an oxygen atmosphere (more details in Fig. S6 and Table S5†).

Fig. 6. Adsorption Langmuir and Freundlich isotherms for BPA adsorbed by silica nanoparticles. Conditions: 25 °C, contact time 1 h and adsorbent mass 1.0 g L⁻¹ (see Fig. S7, Table S6 and S7†).