Hybrid Ni@ZnO@ZnS-Microalgae for Circular Economy: A Smart Route to the Efficient Integration of Solar Photocatalytic Water Decontamination and Bioethanol Production

Abstract

Water remediation and development of carbon-neutral fuels are a priority for the evermore industrialized society. The answer to these challenges should be simple, sustainable, and inexpensive. Thus, biomimetic-inspired circular and holistic processes combing water remediation and biofuel production can be an appealing concept to deal with these global issues. A simple circular approach using helical Spirulina platensis microalgae as biotemplates to synthesize Ni@ZnO@ZnS photocatalysts for efficient solar water decontamination and bioethanol production during the recycling process is presented. Under solar irradiation, the Ni@ZnO@ZnS-Spirulina photocatalyst exhibits enhanced activity (mineralization efficiency >99%) with minimal photocorrosion and excellent reusability. At the end of its effective lifetime for water remediation, the microalgae skeleton (mainly glycogen and glucose) of the photocatalyst is recycled to directly produce bioethanol by simultaneous saccharification and fermentation process. An outstanding ethanol yield of 0.4 L kg^-1, which is similar to the highest yield obtained from oxygenic photosynthetic microorganisms, is obtained. Thus, the entire process allows effective solar photocatalytic water remediation and bioethanol production at room temperature using simple and easily scalable procedures that simultaneously fixes carbon dioxide, thereby constituting a zero-carbon-emission circular process.

Keywords: bioethanol production; biomimetics; biotemplating; photocatalysis; water decontamination.

Scheme 1

Schematic illustration of the circular process using microalgae (Spirulina plantensis) for water mineralization and bioethanol production.

Figure 1

a) Geometric parameters of Spirulina: wire diameter (d), helix diameter (D), and length (L). b) Schematic representation of the biotemplating process. Field emission‐scanning electron microscope micrographs and EDX mapping of c) fixed Spirulina platensis, d) Ni‐Spirulina, e) Ni@ZnO‐Spirulina, and f) Ni@ZnO@ZnS‐Spirulina.

Figure 2

a) UV–vis diffuse reflectance absorption spectra, b) photoluminescence (PL) spectra, and c) transient photocurrent response under UV‐filtered irradiation (λ > 400 nm) of Ni@ZnO‐ and Ni@ZnO@ZnS‐Spirulina.

Figure 3

a) Photodegradation and b) mineralization efficiency of MB (10 ppm in algae culture medium) under artificial and natural UV‐filtered sunlight irradiation of the Ni@ZnO‐ and Ni@ZnO@ZnS‐Spirulina photocatalysts. c) MB photodegradation efficiency of the photocatalysts during 25 consecutive recycling cycles under artificial UV‐filtered sunlight irradiation. d) Time‐dependent dissolution of Zn(II) from the photocatalysts under artificial UV‐filtered sunlight irradiation. Photocatalyst dosage = 0.5 mg mL⁻¹ and temperature = 25 ± 0.2 °C. The lines in (a) and (d) are guides to the eye.

Figure 4

Ethanol production from the glycogen and glucose from fresh Spirulina (open circles) and Spirulina recycled from the Ni@ZnO@ZnS‐Spirulina photocatalysts used for water remediation (closed circles) in a) the absence of enzymes and b) the presence of 1.5 U L⁻¹ α‐glucosidase and 3.5 U L⁻¹ α‐amylase. The yeast concentration in each case is ≈1.55 × 10⁷ yeast mL⁻¹ Ethanol Red. The data points and error bars are the mean values from three separate cultures and their corresponding standard deviation, respectively. The lines are guides to the eye.