Textile dye removal using diatomite nanocomposites: a metagenomic study in photosynthetic microalgae-assisted microbial fuel cells

Abstract

In this study, Coomassie brilliant blue (CBB), brilliant green (BG), and rhodamine (Rh) dyes were used to simulate dye-rich wastewater. Adsorption and degradation of these dyes (2 μM, 10 μM, and 30 μM) on diatomite (DE) were evaluated under light (L) and dark (D) conditions. The adsorption of dye-DE composites followed pseudo-second-order kinetics at all concentrations and conditions had R ² > 0.99, thus showing a good fit. The calculated equilibrium adsorption amount q _e,(cal) was coherent with the value of experimental q _e,(exp). The poorest adsorption and photocatalysis occurred at 30 μM, prompting the functionalization of dyes with TiO₂ and Fe₃O₄ nanoparticles (NP(s)). The highest dye degradation efficiencies (DG_eff) for 30 μM dyes were 86.79% (CBB-DE-Fe₃O₄, 72 h), 96.10% (BG-DE-TiO₂, 52 h), and 81.74% (Rh-DE-TiO₂, 48 h), with Rh-DE-TiO₂ showing the fastest degradation. Functionalized DE-dye (30 μM) nanocomposites were further tested in a photosynthetic microalgae-assisted microbial fuel cell with dye-simulated wastewater at the anode (PMA-MFC-1 with CBB-DE-Fe₃O₄, PMA-MFC-2 with BG-DE-TiO₂ and PMA-MFC-3 with Rh-DE-TiO₂) and Asterarcys sp. GA4 microalgae at the cathode. In dark anode chambers, PMA-MFC-3 achieved the highest DG_eff value of Rh dye as 88.23% and a polarization density of 30.06 mW m^-2, outperforming PMA-MFC-2 with BG dye and PMA-MFC-1 with CBB dye. The molecular identifier analysis of microbes in wastewater at the anode showed the dominance of Sphingobacteria and Proteobacteria in PMA-MFC-3 (Rh-DE-TiO₂) and COD removal of 61.36%, highlighting its potential for efficient dye degradation and bioelectricity generation. Furthermore, PMA-MFC-3 simultaneously demonstrated a superior microalgal lipid yield of 3.42 μg g^-1 and an algal growth of 8.19 μg g^-1 at the cathode.

Fig. 1. Pseudo-second-order kinetics for dyes: (A) CBB 2 μM, light and dark conditions; (B) CBB 10 μM, light and dark conditions; (C) CBB 30 μM, light and dark conditions; (D) BG 2 μM, light and dark conditions; (E) BG 10 μM, light and dark conditions; (F) BG 30 μM, light and dark conditions; (G) Rh 2 μM, light and dark conditions; (H) Rh 10 μM, light and dark conditions; and (I) Rh 30 μM, light and dark conditions.

Fig. 2. Time-dependent (A) concentration of the control CBB sample without DE under light condition; (B) control CBB sample without DE under dark condition; (C) DG_eff of control CBB without DE under light and dark conditions; (D) CBB with DE under light condition; (E) concentration of CBB with DE under dark condition; (F) DG_eff of CBB and DE under light and dark conditions; (G) concentration of CBB (30 μM) with DE and nanoparticles (TiO₂ and Fe₃O₄) and (H) DG_eff of the highest CBB dye concentration in the presence of nanoparticles (TiO₂ and Fe₃O₄).

Fig. 3. Time-dependent (A) concentration of the control BG sample without DE under light condition; (B) control BG sample without DE under dark condition; (C) DG_eff of control BG without DE under light and dark conditions; (D) BG with DE under light condition; (E) concentration of BG with DE under dark condition; (F) DG_eff of BG and DE under light and dark conditions; (G) concentration of BG (30 μM) with DE and nanoparticles (TiO₂ and Fe₃O₄) and (H) DG_eff of the highest BG dye concentration in the presence of nanoparticles (TiO₂ and Fe₃O₄).

Fig. 4. Time-dependent (A) concentration of the control Rh sample without DE under light condition; (B) control Rh sample without DE under dark condition; (C) DG_eff of control Rh without DE under light and dark conditions; (D) Rh with DE under light condition; (E) concentration of Rh with DE under dark condition; (F) DG_eff of Rh and DE under light and dark conditions; (G) concentration of Rh (30 μM) with DE and nanoparticles (TiO₂ and Fe₃O₄) and (H) DG_eff of the highest Rh dye concentration in the presence of nanoparticles (TiO₂ and Fe₃O₄).

Fig. 5. Comparison of the polarization curve for PMA-MFC1-3 enriched with diatomite: (A) PMA-MFC 1 (diatomite–Coomassie brilliant blue–Fe₃O₄); (B) PMA-MFC 2 (diatomite–brilliant green–TiO₂) and (C) PMA-MFC 3 (diatomite–rhodamine–TiO₂).

Fig. 6. Heat map of metagenomics data generated on the (A) initial and (B) final days at the anode section of PMA-MFC reactors 1–3 containing diatomite–Coomassie brilliant blue, brilliant green and rhodamine and nanoparticles on the initial and final days, respectively.

Fig. 7. Microalgae (Asterarcys sp.GA4) at the cathode for 40 days and measuring its (A) cell density, (B) biomass and (C) lipid yield on different days at the cathode of PMA-MFC 1–3.