Comparative study on efficiency of surface enhanced coal fly ash and raw coal fly ash for the removal of hazardous dyes in wastewater: optimization through response surface methodology

Abstract

Crystal violet (CV) dye, because of its non-biodegradability and harmful effects, poses a significant challenge for wastewater treatment. This study addresses the efficiency of easily accessible coal fly ash (CFA)-based adsorbents such as raw coal fly ash (RCFA) and surface enhanced coal fly ash (SECFA), in removing CV dye from waste effluents. Various analytical techniques such as FTIR, XRD, SEM, TEM, BET, zeta sizer and zeta potential were employed for the characterization of the adsorbents and dye-loaded samples. BET revealed that RCFA possesses a surface area of 19.370 m² g^-1 and SECFA of 27.391 m² g^-1, exhibiting pore volumes of 0.1365 cm³ g^-1 and 0.1919 cm³ g^-1 respectively. Zeta-sizer and potential analysis showed the static charges of RCFA as -27.3 mV and SECFA as -28.2 mV, with average particle sizes of 346.6 and 315.3 nm, respectively. Langmuir and Freundlich adsorption isotherms were also employed for adsorption studies. Employing central composite design (CCD) of response surface methodology (RSM), the maximum CV removal was 81.52% for RCFA and 97.52% for SECFA, providing one minute contact time, 0.0125 g adsorbent dose and 10 ppm dye concentration. From the thermodynamic studies, all the negative values of ΔG° showed that all the adsorption processes of both adsorbents were spontaneous in nature.

Fig. 1. FTIR Spectra of the adsorbents RCFA (a) and SECFA (c) and their respective dye-loaded samples RCFAL (b) and SECFAL (d).

Fig. 2. X-ray diffractograms of RCFA (a) and SECFA (b).

Fig. 3. SEM images of RCFA and SECFA where (a) and (c) represent unloaded samples while (b) and (d) represent dye-loaded samples respectively.

Fig. 4. TEM images of RCFA (a), SECFA (b) and fringes of SECFA (c).

Fig. 5. Plots indicating proximity between predicted and actual values for both adsorbents RCFA (a) and SECFA (b).

Fig. 6. 3D surfaces of RCFA and SECFA plotted at minimum adsorbent dosages and contact time with varying dye concentrations of 10 ppm (a) and (b), 100 ppm (c) and (d) and 200 ppm (e) and (f).

Fig. 7. 3D surfaces of RCFA and SECFA plotted at maximum dye concentration and minimum contact time with varying the adsorbents dosages of 0.0125 g (a) and (b), 0.025 g (c) and (d) and 0.05 g (e) and (f).

Fig. 8. 3D surfaces of RCFA and SECFA plotted at minimum adsorbent dosages and maximum dye concentrations with varying contact times of 1 minute (a) and (b), 5 minutes (c) and (d) and 10 minutes (e) and (f).

Fig. 9. (a) and (b) Show the Langmuir linear model fitting of RCFA (at 10 minutes and 0.05 g adsorbent) and SECFA (at 1 minute and 0.025 g adsorbent) respectively. While (c) and (d) show the Freundlich linear model fitting of RCFA (at 10 minutes and 0.05 g adsorbent) and SECFA (at 1 minute and 0.0125 g adsorbent) respectively.

Fig. 10. Regeneration studies of the adsorbents for CV dye removal.