Utilization of Biomass Fly Ash for Improving Quality of Organic Dye-Contaminated Water

Development of innovative methodologies to convert biomass ash into useful materials is essential to sustain the growing use of biomass for energy production. Herein, a simple chemical modification approach is employed to functionalize biomass fly ash (BFA) with 3-aminopropyltriethoxy silane (APTES) to develop an inexpensive and efficient adsorbent for water remediation. The amine-functionalized BFA (BFA–APTES) was fully characterized by employing a range of characterization techniques. Adsorption behavior of BFA–APTES was evaluated against two anionic dyes, namely, alizarin red S (ARS) and bromothymol blue (BTB). In the course of experimental data analysis, the computation tools of data fitting for linear and nonlinear form of Langmuir, Freundlich, and the modified Langmuir–Freundlich adsorption isotherms were used with the aid of Matlab R2019b. In order to highlight the misuse of linearization of adsorption models, the sum of the squares of residues (SSE) values obtained from nonlinear models are compared with R² values obtained from the linear regression. The accuracy of the data fitting was checked by the use of SSE as an error function instead of the coefficient of determination, R². The dye adsorption capacity of BFA–APTES was also compared with the nonfunctionalized BFA. The maximum adsorption capacities of BFA–APTES for ARS and BTB dye molecules were calculated to be around 13.42 and 15.44 mg/g, respectively. This value is approximately 2–3 times higher than the pristine BFA. A reasonable agreement between the calculated and experimental values of q_e obtained from the nonlinear form of kinetic models verified the importance of using equations in their original form. The experimentally calculated thermodynamic parameters including molar standard Gibbs free energy (Δ_adG_m⁰) and molar standard enthalpy change (Δ_adH_m⁰) reflected that the process of adsorption of dye molecules on the BFA–APTES adsorbent was spontaneous and exothermic in nature. Moreover, the used BFA–APTES adsorbent could be regenerated and reused for several cycles with significant dye adsorption capacity. The remediation capability of the BFA–APTES adsorbent against ARS dye was also demonstrated by packing a small column filled with the BFA–APTES adsorbent and passing a solution of ARS through it. Overall, we provide a simple and scalable route to convert BFA into an efficient adsorbent for water remediation applications.