Full Paper www.advancedscience.com Hybrid Ni@ZnO@ZnS-Microalgae for Circular Economy: A Smart Route to the Efficient Integration of Solar Photocatalytic Water Decontamination and Bioethanol Production Albert Serrà,* Raül Artal, Jaume García-Amorós, Borja Sepúlveda, Elvira Gómez, Josep Nogués, and Laetitia Philippe 1. Introduction Water remediation and development of carbon-neutral fuels are a priority for the evermore industrialized society. The answer to these challenges Unsustainable societal development should be simple, sustainable, and inexpensive. Thus, biomimetic-inspired and rapid global industrialization have caused a high stress on water and energy circular and holistic processes combing water remediation and biofuel resources, particularly drinking water and production can be an appealing concept to deal with these global issues. carbon fuels. Consequently, efficient water A simple circular approach using helical Spirulina platensis microalgae as decontamination and replacement of cur- biotemplates to synthesize Ni@ZnO@ZnS photocatalysts for efficient solar rent dense energy carriers with remov- water decontamination and bioethanol production during the recycling able alternatives are extremely important in mitigating the highly undesirable con- process is presented. Under solar irradiation, the Ni@ZnO@ZnS-Spirulina sequences of unsustainable growth on photocatalyst exhibits enhanced activity (mineralization efficiency >99%) health, economy, and enviçronment.[1] In with minimal photocorrosion and excellent reusability. At the end of its this context, the development of circular effective lifetime for water remediation, the microalgae skeleton (mainly and holistic processes mimicking the glycogen and glucose) of the photocatalyst is recycled to directly produce biological life cycle, where every end is a bioethanol by simultaneous saccharification and fermentation process. An new beginning, is crucial to achieving a more sustainable future.[2] outstanding ethanol yield of 0.4 L kg−1, which is similar to the highest yield The design and implementation of obtained from oxygenic photosynthetic microorganisms, is obtained. Thus, competitive circular processes must the entire process allows effective solar photocatalytic water remediation and integrate the biomimetic rationale to bioethanol production at room temperature using simple and easily scalable satisfy four essential criteria: simplicity, procedures that simultaneously fixes carbon dioxide, thereby constituting a efficiency, cost-effectiveness, and material availability. Sunlight, as the main engine zero-carbon-emission circular process. of most of the biological process, offers Dr. A. Serrà, Dr. L. Philippe Dr. J. García-Amorós, Prof. E. Gómez Empa Institute of Nanoscience and Nanotechnology (IN2UB) Swiss Federal Laboratories for Materials Science and Technology Universitat de Barcelona Laboratory for Mechanics of Materials and Nanostructures E-08028 Barcelona, Catalonia, Spain Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland Dr. J. García-Amorós E-mail: albert.serraramos@empa.ch Grup de Materials Orgànics R. Artal, Prof. E. Gómez Departament de Química Inorgànica i Orgànica Grup d’Electrodeposició de Capes Primes i Nanoestructures (GE-CPN) Universitat de Barcelona Departament de Ciència de Materials i Química Física Martí i Franquès, 1, E-08028 Barcelona, Catalonia, Spain Universitat de Barcelona Dr. B. Sepúlveda, Prof. J. Nogués Martí i Franquès, 1, E-08028 Barcelona, Catalonia, Spain Catalan Institute of Nanoscience and Nanotechnology (ICN2) CSIC and BIST The ORCID identification number(s) for the author(s) of this article Campus UAB, Bellaterra, E-08193 Barcelona, Spain can be found under https://doi.org/10.1002/advs.201902447. Prof. J. Nogués © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, ICREA Weinheim. This is an open access article under the terms of the Creative Pg. Lluís Companys 23, E-08010 Barcelona, Spain Commons Attribution License, which permits use, distribution and repro- duction in any medium, provided the original work is properly cited. DOI: 10.1002/advs.201902447 Adv. Sci. 2020, 7, 1902447 1902447 (1 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com the key advantages of being a virtually inexhaustible and clean viewed as one of the best alternatives because of its similarity source of energy. Consequently, diverse processes for the con- to petrol. However, the existing technology is unsustainable for version of solar radiation to chemical fuels, electricity, or heat the large-scale production of bioethanol. Nevertheless, photo­ have generated enormous interest as a solution to the issue synthetic microorganisms such as microalgae can produce of global energy deficiency and environmental pollution.[3] large amounts of biomass by consuming only sunlight, carbon In particular, ongoing energy research focuses on the use of dioxide, and some nutrients, serving as a high-carbon source. new materials to improve the photocatalytic efficiency in the In this context, microalgae, specially cyanobacteria, are one of visible-light range and development of artificial photosynthesis the most promising options for bioethanol (third-generation prototypes and strategies to obtain carbon-neutral fuels. In biofuel) production owing to their characteristics such as high addition, solar photocatalysis has been intensively studied for biomass productivity, high capacity to fixate carbon dioxide, water decontamination (i.e., pollutant mineralization).[3] and possibility of growth in seawater (salt tolerance).[6] Cur- Semiconductors based on metal oxides (e.g., ZnO, TiO2) rent efforts are concentrated on enhancing bioethanol yield and sulfides, such as CdS, have demonstrated great potential by developing diverse strategies to promote the extraction of as photocatalysts for different processes. However, their per- intracellular carbohydrates. Interestingly, the latter, mainly formance remains suboptimal because of i) limited visible- glycogen, can be easily extracted from some cyanobacteria, light photocatalytic activity; ii) insufficient efficiency to trap such as Spirulina platensis, without complex pretreatments due and harvest sunlight; iii) limited photocorrosion resistance, to their extremely brittle membrane; thus, these cyanobacteria and/or iv) high potential ecotoxicological effects on aquatic are excellent candidates for bioethanol production.[6] biota.[4] Hence, different approaches (e.g., doping, complex In the present study, we developed a novel hybrid structure architectures, or heterojunctions) are being pursued to miti- consisting of Spirulina plantensis microalgae plated by Ni@ gate their shortcomings. In particular, current research on ZnO@ZnS shells for a high-yield circular process merging photocatalyst synthesis also focuses on the use of bioinspired water decontamination and biofuel production. To close the and biomimetic architectures to improve light trapping and cycle, the clean water and the CO2 produced in this diverse pro- harvesting capabilities. Nevertheless, the combined overall per- cess are used to cultivate new algae. formance of current photocatalysts in the visible region of the The hybrid Ni@ZnO@ZnS-Spirulina has enabled, for the electromagnetic spectrum remains exceedingly low for com- first time, the integration of i) highly efficient biomimetic mercial exploitation. Therefore, the development of efficient, solar photocatalysis for water decontamination, ii) effective green, and sustainable photocatalysts that fulfill all the exigent bioethanol production by the Spirulina recycled at the end of performance requirements is still necessary.[4,5] the photocatalyst’s lifetime, and iii) fixation of carbon dioxide The current pursuit of sustainable solutions to energy and (Scheme 1). The Ni@ZnO@ZnS-Spirulina photocatalysts environmental issues is not only limited to the development exhibited excellent solar photocatalytic performance for the of efficient photocatalysts. Today, researchers also devote mineralization of methylene blue (MB) with minimal photocor- significant effort on the replacement of fossil fuels with rosion activity. Additionally, the recycled photocatalyst reached a bioenergy, especially carbon-neutral biofuels.[6] Bioethanol is conversion efficiency of ≈96% for bioethanol production (0.4 L Scheme 1. Schematic illustration of the circular process using microalgae (Spirulina plantensis) for water mineralization and bioethanol production. Adv. Sci. 2020, 7, 1902447 1902447 (2 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com of bioethanol per kg of dry photocatalyst) after saccharification (Figure 1d). Note that this Ni layer serves two purposes: i) to and fermentation. Therefore, an outstanding circular process provide the structure with magnetic functionality that enables for integrated CO2 fixation, water decontamination, and bioeth- magnetic collection of the hybrid microalgae (Figure S2, Sup- anol production has been demonstrated. porting Information) and ii) to allow the deposition of the ZnO semiconductor layer. As can be seen in Figure 1d, Ni is suc- cessfully deposited only on the Spirulina microhelix surface, 2. Results and Discussion which is consequently rough, microporous, and magnetic. The estimated thickness of the Ni shell is ≈ 1.5 µm. The Ni- 2.1. Preparation of the Hybrid Ni@ZnO@ZnS-Microalgae Spirulina is then completely covered by a thin ZnO layer that Photocatalyst fills the Ni micropores, as shown in Figure 1e. Finally, a nano- metric ZnS shell is successfully formed by partial sulfidation The photocatalytically active microalgae hybrid is composed of of the ZnO layer (Figure 1f), resulting in a Ni@ZnO@ZnS- Spirulina plantensis coated with an onion-like Ni@ZnO@ZnS Spirulina photocatalyst with an onion-like core@shell@shell shell. The Spirulina acts as the structural support of the magnetic structure. Sulfidation enhances the surface roughness, which and photocatalytic layers and as biomass reservoir for efficient is desirable to increase the effective surface area of a photocata- bioethanol production. Note that the Ni layer in the structure has a lyst. The average geometrical and structural parameters of the dual role, i) providing mechanical strength to the fragile Spirulina hybrid Spirulina are summarized in Table 1. In particular, the and serving as a suitable surface for the growth of ZnO and Brunauer–Emmett–Teller (BET) surface area of Ni@ZnO@ ii) given its magnetic properties, it enables the magnetic manipu- ZnS-Spirulina is 79.1 m2 g−1, which is 1.12 times higher than lation of the photocatalyst to transport it to the reaction site and that of the Ni@ZnO-Spirulina, suggesting that the sulfida- to easily recover it after its lifetime. Finally, the ZnO/ZnS hetero- tion, indeed, increases the surface roughness. However, it is structure coating constitutes a highly efficient and stable photo- 0.79 times lower than that of the Ni-Spirulina because of the catalytic layer for water decontamination by solar photocatalysis. micropore filling (Figure S3, Supporting Information). Notably, This multifunctional microstructure can be fabricated following the specific surface area accessible to pollutants and, more a simple, cost-effective, and scalable process, as follows (see importantly, light, plays an essential role in the photocatalytic Experimental Section in the Supporting Information for details): degradation efficiency by providing more active sites.[4a,5c] The successful growth of Ni, ZnO, and ZnS is confirmed by energy-dispersive X-ray (EDX) elemental mappings 2.1.1. Optimum Spirulina Cultivation (Figure 1), X-ray diffraction (Figure S4, Supporting Informa- tion), and X-ray photoelectron spectroscopy spectra (Figure S5, Prior to biotemplating, the microalgae were cultivated for Supporting Information). In particular, the EDX mapping of 5 days in a nitrate-deficient (4 × 10−3 m NaNO3) algal culture each Spirulina structure demonstrates that the biotemplating medium to increase the glycogen content, and consequently the process is consistent, that is, the growth is uniform and homo- bioethanol production yield, in the subsequent simultaneous geneous across the entire surface of microalgae, leading to a saccharification and fermentation.[6e,f,7] Under these conditions, multilayered core@shell@shell microstructure. the glycogen content (67.4 ± 2.1% w/w of dry the microalgae weight) is ≈2.5 times higher than that of the microalgae cultivated in conventional microalgae media (30 × 10−3 m 2.2. Basic Optoelectronic Characterization NaNO3), whereas the glucose content is not influenced by the nitrate ion concentration (Table S1, Supporting Information). The optoelectronic properties of Ni@ZnO- and Ni@ZnO@ In addition, the biochemical composition remains virtually ZnS-Spirulina were investigated by UV–vis diffuse reflectance, constant even after two weeks of fixation. photoluminescence spectroscopies, and photoelectrochemical tests (Figure 2). As can be seen in Figure 2a, the Ni@ZnO- Spirulina exhibits the characteristic spectrum of ZnO, with a 2.1.2. Biotemplating Process fundamental sharp absorption edge rising at ≈390 nm and a very small absorption intensity in the visible domain. However, Biotemplating (Figure 1; Scheme S1, Supporting Information) after sulfidation (Ni@ZnO@ZnS-Spirulina), the absorption involves three steps: i) Ni electroless plating, ii) ZnO chemical band is displaced to the visible domain, which should lead to deposition, and iii) ZnS sulfidation (see Methods for details).[8] a stronger photoresponse in this region. In fact, the bandgaps The process is initiated by the fixation of Spirulina plantensis by of the Ni@ZnO- and Ni@ZnO@ZnS-Spirulina structures, glutaraldehyde to maintain the initial shape and prevent micro- calculated using the Tauc relation (Figure S6, Supporting Infor- algae fragmentation. Prior to the Ni electroless deposition, Pd mation), are 3.21 ± 0.09 and 2.85 ± 0.09 eV, respectively.[4b,9] nanoparticles are adsorbed on microalgae surface to act as a The Ni@ZnO-Spirulina exhibits an intense and broadband nuclei catalyst to promote the Ni deposition. The Pd-covered photoluminescence spectrum centered at 630 nm, which is Spirulina biotemplate (Figure 1c) has the common compact indicative of the polycrystalline structure of the ZnO layer with helix structure of Spirulina plantensis var lonar consisting of multiple trapped electronic states due to the crystallographic a microhelix ≈3.9 µm in wire diameter, ≈23 µm in helix dia­ defects. Interestingly, sulfidation substantially reduces the meter, and ≈65 µm in length (8–12 turns). Subsequently, the luminescence intensity. This can be attributed to the passiva- surface of the Spirulina is completely covered with a Ni layer tion of some of the crystallographic defects by the mechanical Adv. Sci. 2020, 7, 1902447 1902447 (3 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com 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. stress generated by the incorporation of S atoms in the lattice the current increased rapidly under UV-filtered irradiation and during the sulfidation.[4b,9] recovered quickly when the light was turned off. Addition- As shown in Figure 2c, reversible, uniform, and stable ally, the photocurrent enhancement observed for Ni@ZnO@ photocurrent responses were observed for Ni@ZnO-, and Ni@ ZnS-Spirulina (about 6.4 times higher than that of Ni@ZnO- ZnO@ZnS-Spirulina; during ten on/off cycles (30 s/20 s), Spirulina) indicates an improved separation efficiency of the Table 1. Average feature sizes, BET surface areas, and bandgap energies of the different Spirulina structures. Spirulina Wire diameter (d) [µm] Helix diameter (D) [µm] Length (L) [µm] BET surface area [m2 g−1] Bandgap energy [eV] Ni 4.5–7.1 25–29 60–70 118.1 – Ni@ZnO 4.8–7.3 25–30 60–70 70.6 3.21 ± 0.09 Ni@ZnO@ZnS 4.8–7.4 25–30 60–70 79.1 2.85 ± 0.06 Adv. Sci. 2020, 7, 1902447 1902447 (4 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com photogenerated electrons and holes, attributed to the synergetic interaction between ZnO and ZnS. This confirms that ZnO@ ZnS core@shell photocatalysts have improved photocatalytic properties over pure ZnO photocatalysts. These optoelectric properties of the Ni@ZnO@ZnS- Spirulina already suggest its high potential for enhanced UV-filtered photocatalytic response. 2.3. Water Decontamination: Photocatalytic Mineralization of Persistent Organic Pollutants (POPs) The photocatalytic mineralization of POPs is governed by the generation of reactive oxygen species (ROS), mainly hydroxyl radicals (•OH), on the photocatalyst surface. However, oxygen superoxide ions (O2−) and singlet oxygen (1O2) can also play an important role; this role drastically depends on the pH. Consequently, prior to evaluating the photomineralization efficiency of ZnO-based Spirulina photocatalysts, the forma- tion of ROS species (Figure S7, Supporting Information) under UV-filtered simulated sunlight (>400 nm, light inten- sity of 680 ± 10 lx) at 25 °C was determined by using chem- ical selective radical quenchers.[4,10] Specifically, the kinetics and concentration of hydroxyl radicals was investigated using a fluorescein salt as the selective radical quencher, assuming zero order kinetics and equimolar reaction stoichiometry between hydroxyl radicals and fluorescein molecules. As can be seen in Figure S7a,b in the Supporting Information, Ni@ ZnO@ZnS-Spirulina exhibited ≈30 times higher production of hydroxyl radicals (0.48 × 10−6 m min−1) than did Ni@ZnO- Spirulina (0.016 × 10−6 m min−1), which suggests that Ni@ ZnO@ZnS-Spirulina has high potential in mineralizing POPs. In addition, the formation of oxygen superoxide ions was deter- mined by monitoring the reaction between 2,3-bis(2-methoxy- 4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt (XTT) and oxygen superoxide ions (Figure S7c, Supporting Information) by means of conventional time-resolved UV–vis spectroscopy. The formation of singlet oxygen was also deter- mined by monitoring the reaction of the anthracene platform of the SOSG reagent with singlet oxygen. In these experiments, Ni@ZnO@ZnS-Spirulina presented a relatively low activity in producing oxygen superoxide ions and singlet oxygen com- pared to Ni@ZnO-Spirulina. The photolytic formation of ROS from water was negligible. Consequently, a high photominer- alization efficiency of POPs is expected for Ni@ZnO@ZnS- Spirulina under UV-filtered simulated sunlight due to the high production of hydroxyl radicals. The water decontamination analysis was focused on Ni@ ZnO@ZnS-Spirulina because its optoelectronic characteristics and ROS generation are more adequate for solar photocatalysis than those of Ni@ZnO-Spirulina, as was recently demonstrated for different ZnO and ZnO@ZnS morphologies.[4b,9b] However, note that, for comparison, the main photocatalytic properties of the Ni@ZnO-Spirulina are given in Figure 3 and Figures S8–S10 in the Supporting Information. The degradation of MB—10 ppm of MB in an algae culture Figure 2. a) UV–vis diffuse reflectance absorption spectra, medium—was used as a model to investigate the photominer- b) photoluminescence (PL) spectra, and c) transient photocurrent response under UV-filtered irradiation (λ > 400 nm) of Ni@ZnO- and alization activity of the Ni@ZnO@ZnS-Spirulina under both Ni@ZnO@ZnS-Spirulina. artificial (light intensity of 680 ± 10 lx) and natural (average Adv. Sci. 2020, 7, 1902447 1902447 (5 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com 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. light intensity 1500 ± 300 lx) UV-filtered (>400 nm) sunlight time (t) (Figure S10, Supporting Information) allows the conditions. Both illumination conditions induce negligible calculation of the apparent kinetic constant and consequently, photolysis (i.e., light induced degradation without catalyst; the mass-normalized kinetic constant (knor), which can be Figure S11, Supporting Information). used to compare the photocatalytic activity of the Ni@ZnO@ As can be seen in Figure 3a, the MB adsorption on the ZnS-Spirulina photocatalysts to the state-of-the-art ZnO-based surface of Ni@ZnO@ZnS-Spirulina after 90 min under dark photocatalysts. conditions is 4.5%, indicating the good affinity of MB to ZnS. In The knor values of Ni@ZnO@ZnS-Spirulina (8.0 and general, both the large surface area and the adsorption affinity 7.4 min−1 g−1 under artificial and natural UV-filtered sunlight, promote an increase in the density of active photo­catalytic sites, respectively) are comparable to or better than those of bench- thereby leading to enhanced photocatalytic activity. mark materials reported in the literature (Table S2, Supporting Under UV-filtered sunlight irradiation, the absorption Information).[4b,9b,11] However, the photodegradation of persis- intensity of MB decreased continuously with irradiation time, tent organic pollutants can lead to the formation of substances indicating the photooxidation of the MB (Figure 3a; Figure S8, that are equally or more toxic. Consequently, determining the Supporting Information). A colorless solution is achieved after mineralization efficiency (complete degradation of organic 210 min of irradiation. The Ni@ZnO@ZnS-Spirulina shows pollutants to gaseous CO2 and inorganic ions) is an essential very high photocatalytic activity, with 88.9% and 86.5% of MB requirement before establishing Ni@ZnO@ZnS-Spirulina as molecules degraded after 180 min under artificial and natural an efficient and competitive photocatalyst for water decontami- sunlight irradiation, respectively (Table 2). nation by solar photocatalysis. In the case of Ni@ZnO@ZnS- It is well known that according to the Langmuir– Spirulina under artificial UV-filtered sunlight irradiation, the Hinshelwood model, the photocatalytic degradation of MB total organic carbon (TOC) of the solution decreases from 10 to at low concentrations follows pseudo first-order kinetics <0.05 ppm (detection limit of the TOC analyzer) after 210 min with respect to MB concentration. Therefore, the plot of the of irradiation, indicating that most of the organic molecules are logarithm of absorbance [− ln(At/A0)] as a function of reaction mineralized (Figure 3b). Adv. Sci. 2020, 7, 1902447 1902447 (6 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com Table 2. Photocatalytic performances of the Ni@ZnO@ZnS-Spirulina under artificial and natural UV-filtered sunlight irradiation (λ > 400 nm). Photo- catalyst dosage = 0.5 mg mL−1 and temperature = 25 ± 0.2 °C. Spirulina Artificial (light intensity of 680 ± 10 lx) Natural (average light intensity 1500 ± 300 lx) Irradiation time = 180 min Irradiation Irradiation time = 180 min Irradiation time = 210 min time = 210 min MB degradation knor [min−1 g−1] Mineralization efficiency MB degradation knor [min−1 g−1] Mineralization efficiency efficiency [%] [%] efficiency [%] [%] Ni@ZnO@ZnS 88.9 ± 1.4 8.0 >99 86.5 ± 1.3 7.4 96.9 ± 1.9 It is well known that hydroxyl radicals are the main actors 48 h under irradiation, thus exhibiting high anti-photocorrosion in the photomineralization of POPs due to their high oxida- properties promoted by the ZnS shell. Therefore, owing to its tion power. Nonetheless, the effects of hydroxyl radicals, super- excellent photocatalytic efficacy, photocorrosion resistance, oxide ions, and photogenerated holes were visualized through reusability, and recyclability the Ni@ZnO@ZnS-Spirulina trapping experiments performed during the photocatalytic is an excellent and ecofriendly photocatalyst for water reme- degradation of MB. Singlet oxygen was not considered, as its diation with minimal adverse ecotoxicological effects. In formation in these conditions is negligible under these experi- contrast, the Ni@ZnO-Spirulina showed much lower photo- mental conditions. Under UV-filtered simulated sunlight, catalytic water decontamination efficiency and much faster the degradation efficiency of the Ni@ZnO@ZnS-Spirulina degradation due to the photocorrosion effects (Figure 3; (88.9 ± 1.4%) decreased to ≈6%, ≈82%, and ≈48% in the pres- Figures S8 and S9, Supporting Information), thus highlighting ence of scavengers for hydroxyl radicals, superoxide ions, and the importance of the ZnS shell. photogenerated holes, respectively (Figure S12, Supporting Information). Therefore, as expected, hydroxyl radicals and photogenerated holes govern the mechanism of MB photo­ 2.4. Bioethanol Production degradation under UV-filtered simulated sunlight when Ni@ZnO@ZnS-Spirulina is used as a photocatalyst. Once the lifetime of the photocatalysts is reached, the struc- To investigate the reusability of the Ni@ZnO@ZnS- tures can be recycled to produce biofuel. The inorganic shells Spirulina, the photocatalysts were collected, washed, and dried of the hybrid Ni@ZnO@ZnS-Spirulina photocatalyst can be after each photocatalytic reaction cycle (180 min) and reused easily removed and the biomass of the Spirulina can be effi- for 25 consecutive cycles under artificial UV-filtered sunlight ciently converted to bioethanol in a simple and inexpensive irradiation. As can be seen in Figure 3c, the MB photodegra- simultaneous saccharification and fermentation process.[6e,12] dation efficiency remains virtually constant during the first Importantly, to show the innocuousness of the inorganic seven successive cycles, which denotes an excellent stability coating process, the ethanol production by the recycled and of Spirulina photocatalysts. However, after this, the efficiency fresh Spirulina was compared. decreases slowly after each cycle, possibly as a consequence The Ni@ZnO@ZnS-Spirulina photocatalysts were collected of weak photocorrosion (see below). After 25 cycles, the MB after their application for water decontamination and dried. photodegradation efficiency is only 10% lower than that at the The remaining Ni, ZnO, and ZnS were dissolved using an first cycle. After 25 reusability cycles, the BET surface area of acidic solution. Note that Ni(II) and Zn(II) can be easily recov- the Ni@ZnO@ZnS-Spirulina photocatalysts remained virtu- ered and separated by factorial crystallization and consequently, ally constant (77.4 m2 g−1). In addition, FE-SEM morphological reutilized for the synthesis of new Spirulina photocatalysts.[13] analyses (Figure S13, Supporting Information) showed that Subsequently, the microalgae were lysed by using a laboratory the photocatalyst surface and the microalgal skeleton were not homogenizer to extract the glucose and glycogen using 8 g of affected during these 25 reusability cycles. Therefore, Ni@ dried Spirulina in 200 mL of yeast extract peptone medium ZnO@ZnS-Spirulina exhibited a relatively high architectural (Formedium). The mixture was filtered, and the supernatant, stability without photocatalyst fragmentation. The reduction in which contained ≈26.9 g L−1 of glycogen and ≈0.7 g L−1 of the MB photodegradation (10%) efficiency over the course of glucose, was used to produce bioethanol. 25 reusability cycles can mainly be ascribed to a loss of photo- The bioethanol production involved a novel simultaneous catalyst during its magnetic collection and cleaning after each saccharification/fermentation process, where α-glucosidase cycle (or, less probably, to poisoning of the photocatalyst by the and α-amylase were combined with a commercial yeast. First, formation of nonmineralized carbonaceous species during MB the optimal concentration of enzymes and yeast were deter- photodegradation). mined by using solutions containing 26 g L−1 of glycogen or Additionally, the Zn(II) release was determined after 168 h 14 g L−1 of glucose to efficiently transform glycogen to glucose (1 week) of artificial UV-filtered sunlight irradiation to evaluate (saccharification) and glucose to ethanol (microbial fermenta- the photocorrosion activity and durability of the Spirulina-based tion). The best saccharification and fermentation results were photocatalysts. As shown in Figure 3d, the Zn(II) release from obtained using 1.5 U L−1 α-glucosidase, 3.5 U L−1 α-amylase, the Ni@ZnO@ZnS-Spirulina, and therefore the introduction of and an initial yeast concentration of 1.55 × 107 yeast mL−1. a new inorganic pollutant to the aqueous solution, is rather low. To show the importance of performing the saccharification The dissolution is less than 5% of the initial ZnO@ZnS after and fermentation simultaneously, the bioethanol production Adv. Sci. 2020, 7, 1902447 1902447 (7 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com 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. from recycled and fresh Spirulina was conducted in the pres- releasing only CO2 gas as residue. By using this gas to close ence or absence of enzymes. The direct fermentation without the circle, i.e., eliminating the produced residues, a green enzymes using ≈1.55 × 107 yeast mL−1 of Ethanol Red lead to circular strategy can be offered for water decontamination and only 0.48 ± 0.07 and 0.45 ± 0.10 g L−1 of ethanol using fresh bioethanol production. Consequently, as the last stage to close and recycled Spirulina, respectively, after 30 h of fermentation the circular process and start a new cycle, new microalgae are (Figure 4a). In contrast, simultaneous saccharification and fer- cultivated by using the i) clean water and CO2 gas obtained mentation (Figure 4b) offered an excellent way to obtain bioeth- from photocatalytic water decontamination and ii) CO2 gas gen- anol. Namely, 11.8 ± 0.3 and 11.9 ± 0.3 g L−1 of ethanol were erated during saccharification and fermentation (Scheme 1). obtained using fresh and recycled Spirulina, respectively, after The photosynthesis simultaneously fixes carbon into micro- the same time period. The drastic enhancement of the produc- algae and yields vital products such as oxygen and glycogen in a tion yield is due to the efficient conversion of glycogen to glu- single solar-driven process, converting Ni@ZnO/ZnS-Spirulina cose in the presence of enzymes. The conversion efficiencies in a zero-carbon-emission route for sunlight photocatalytic were 94.6 ± 2.5% and 94.4 ± 1.5% after 30 h of reaction at 31 ± water decontamination and bioethanol production. 0.3 °C in the presence of α-glucosidase and α-amylase, respec- tively, resulting in glucose concentrations of 25.7 ± 0.7 and 25.7 ± 0.4 g L−1 using fresh and recycled Spirulina (Figure S14, 3. Conclusions Supporting Information), respectively. The high bioethanol yield of the simultaneous saccharifica- Summarizing, a holistic circular process, mimicking the life tion/fermentation process is a clear evidence that the combina- cycle, using a hybrid Ni@ZnO@ZnS-Spirulina plantensis var tion of enzymes and yeast is essential in obtaining bioethanol lonar microalgae, which integrates CO2 fixation, water remedia- from the glycogen-rich microalgae by microbial fermentation. tion, and bioethanol production with minimal ecotoxicological Interestingly, the ethanol yield—based on the complete con- effect, has been successfully demonstrated. sumption of the microalgae glycogen and glucose—was ≈89% The Ni@ZnO@ZnS-Spirulina microalgae exhibited an of the theoretical yield. The ethanol yield obtained in this work excellent photocatalytic performance in the mineralization of (≈0.4 L kg−1 of dried biomass) is comparable to the highest persistent organic pollutants under sunlight irradiation, with levels achieved using carbohydrate feedstocks (Table S3, Sup- excellent durability. The solar photocatalytic efficacy was com- porting Information).[14] It is important to emphasize that the parable to or better than those of benchmark materials reported yield of recycled Spirulina was virtually identical to that of fresh in the literature. Importantly, the clean water and the carbon Spirulina, indicating that metallization and water decontamina- dioxide produced during mineralization could be exploited for tion processes did not damage the glycogen and glucose of the the cultivation of new microalgae within the circular process. microalgae. The recycling of Ni@ZnO@ZnS-Spirulina provided metal ions that can be recovered and reused to synthesize new photo- catalysts, while the microalgae skeleton was later exploited for 2.5. Closing the Cycle—CO2 Fixation bioethanol production, thereby minimizing the residues of the circular process. We showed that simultaneous saccharification ZnO-based hybrid photocatalysts based on microalgae can be and fermentation of the biomass allows for the direct produc- used for the efficient mineralization of persistent organic pol- tion of bioethanol without pretreatments, presenting yields that lutants and then recycled to directly produce bioethanol by are comparable to the highest levels achieved using carbohy- a simultaneous saccharification and fermentation process, drate feedstocks. Adv. Sci. 2020, 7, 1902447 1902447 (8 of 9) © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advancedscience.com Thus, the magnetic and photocatalytic plated microalgae B. Dam, V. Di Palma, T. Edvinsson, K. Fujii, H. Gardeniers, hybrids offer a green circular strategy to integrate CO2 fixation, H. Geerlings, S. M. Hashemi, S. Haussener, F. Houle, J. Huskens, water remediation, and bioethanol production, which can be B. D. James, K. Konrad, A. Kudo, P. P. Kunturu, D. Lohse, B. Mei, easily scaled for industrial applications. E. L. Miller, G. F. Moore, J. Muller, K. L. Orchard, T. E. Rosser, F. H. Saadi, J. W. Schüttauf, B. Seger, S. W. Sheehan, W. A. Smith, J. Spurgeon, M. H. Tang, R. Van De Krol, P. C. K. Vesborg, P. Westerik, Energy Environ. Sci. 2018, 11, 2768; d) S. Sorcar, Supporting Information J. Thompson, Y. Hwang, Y. H. Park, T. Majima, C. A. Grimes, J. R. Durrant, S.-I. In, Energy Environ. Sci. 2018, 11, 3183. Supporting Information is available from the Wiley Online Library or [4] a) Y. K. Mishra, R. Adelung, Mater. 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