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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 92 RSC Advances 1 Carbon Dioxide Bio-fixation and Wastewater Treatment via Algae 2 Photochemical Synthesis for Biofuels Production 3 Yafei Shen 4 Department of Environmental Science and Technology, Interdisciplinary Graduate School of Science 5 and Engineering, Tokyo Institute of Technology, G5-8, 4259 Nagatsuta, Midori-ku, Yokohama, 6 226-8502, Japan 7 8 *Corresponding Author 9 Email address: [email protected]; [email protected] 10 Tel: +81-45-924-5507; Fax: +81-45-924-5518 11 12 13 ABSTRACT Manuscript 14 We are faced with the problem of energy/carbon dioxide (CO 2) in the coming decades. 15 Microalgae has been considered as one of the most promising biomass feedstocks for 16 biofuels production. Meanwhile, the productivity of these photosynthetic microorganisms 17 in converting CO 2 into carbon-rich lipids, only a step or two away from biodiesel, greatly 18 exceed that of agricultural crops, without competing for arable land. Worldwide, research 19 and demonstration programs are being carried out to develop the technologies needed to 20 expand algal lipid production from a craft to a major industrial process. This paper 21 narrates the recent advances on microalgae used for biofuels (e.g., biohydrogen, biodiesel Accepted 22 and bioethanol) production, including their cultivation, harvesting, and processing. The 23 various aspects associated with the design of microalgae production units are described as 24 well, providing an overview of the current state of development of algae cultivation 25 systems (photobioreactors and open ponds). Algal cultivation systems integrated with the 26 algae-based biorefineries could yield a diversity of bioresources, such as biodiesel, green 27 gasoline, bio-jet fuel, isolated proteins, food starches, textiles, organic fertilizers), which 28 mitigate the costs of biofuels production. Utilizing the energy, nutrients and CO 2 held 29 within residual waste materials to provide all necessary inputs except for sunlight, the 30 algae cultivation becomes a closed-loop engineered ecosystem. Consequently, developing Advances 31 this biotechnology is a tangible step towards a waste-free sustainable society. 32 33 Keywords: photosynthesis; algae; biodydrogen; biodiesel; hydrothermal processing RSC 34 35 36 37 38 1 RSC Advances Page 2 of 92 39 1. Introduction 40 Natural photosynthesis is the process, by which sunlight is captured and converted into the 41 energy of chemical bonds of organic molecules that are the building blocks in all living 42 organisms, oil, gas and coal. These fossil fuels, the products of photosynthetic activity 43 millions of years ago, could provide the energy to power our technologies, heat our homes 44 and produce the wide range of chemicals and materials that support our life. As a 45 consequence of ever-growing utilization of fossil fuels, we are faced with a severe problem 46 of increasing levels of CO 2 and other greenhouse gases in the atmosphere with implications 47 for global climate change. 48 49 Photosynthesis as a successful energy generation and storage systems is derived from a 50 fact that the raw materials and power needed for biomass synthesis are available in almost 51 unlimited amounts; sunlight, water and CO 2. The core process of photosynthesis is the 52 water splitting by sunlight into oxygen and hydrogen equivalents. The oxygen is released Manuscript 53 into the atmosphere, where it is available for living organisms to breathe and for burning 54 fuels to drive our technologies. The hydrogen equivalents are used to reduce CO 2 to sugars 55 and other types of organic molecules. When fossil fuels, biomass and other biofuels are 56 burned to release energy, we are simply combining the ‘hydrogen’ stored in these organic 57 molecules with atmospheric oxygen to form water. Similarly, energy is also released from 58 the organic molecules constituting our food, when they are metabolized within our bodies 59 by the respiration process. Thus, in the biological world, photosynthesis brings about the 60 splitting of water into oxygen and hydrogen, whereas respiration is the reverse, combining Accepted 61 oxygen and hydrogen in a carefully controlled and highly efficient way so as to create the 62 metabolic energy. From an energetic view, the synthesis of organic molecules implies a 63 way of storing hydrogen and storing solar energy in the form of chemical bonds [1,2]. 64 65 This article comprehensively reviews the current progresses on green biofuels production 66 from algae, mainly consisting of four parts. The first part states the energy utilization along 67 with the CO 2 problem within the coming decades, and discusses the contributions that can Advances 68 be made from photosynthetic biofuels based on the successful principles of photosynthesis. 69 The global energy situation, CO 2 and solar energy capture, and photosynthetic biofuels are 70 presented as well. In particular, it emphasizes the potential of exploiting the vast amounts 71 of solar energy available to produce biofuels via algae photosynthetic reaction combining RSC 72 the advanced technologies. The second part describes the current barriers and challenges of 73 biofuels production from algal biomass, including the new technologies for cultivation, 74 harvesting and processing. The third part discusses the production of main biofuels (i.e., 75 biohydrogen, biodiesel and bioethanol) from algal biomass. In addition, the integration of 76 biodiesel and bioethanol production in the biorefinery approaches have been presented to 77 search for a better understanding of microalgae biofuel production and path forward for 78 research and commercialization. Ultimately, the integrated algal systems for wastewater 79 treatment and bioremediation to capture carbon (C), nitrogen (N) and phosphorus (P) from 2 Page 3 of 92 RSC Advances 80 specialty industrial, municipal and agriculture wastes are introduced. To bring more profits, 81 the value added biofuels and chemicals can be developed by the sustainable and applicable 82 ways. 83 84 1.1. Global Energy Consumption and Demands 85 Currently the global annual energy consumption rate is about in the region of 16.3 TW [3], 86 with the USA and the extended EU each representing about 40% of this. In the future, this 87 global value will rise due to industrialization in underdeveloped and developing countries 88 coupled with the increase of world population. Based on the current projections, the global 89 annual energy consumption rate will reach 20 TW or even more by 2030, doubled by 2050 90 and tripled by the end of the century [4-6]. About 85% of the total global energy consumed at 91 present comes from burning fossil fuels with the proportion approaching 90% for the 92 developed countries. Oil, gas and coal contribute approximately equally to this demand. 93 The remaining sources of energy are hydroelectric, nuclear, biomass and renewable, such 94 as solar, wind, tide and wave. At present, the utilization of biomass plays a dominated role Manuscript 95 in the underdeveloped regions such as Africa, where woody biomass and other organic 96 matters are used as fuels. 97 98 The low level contribution of non-fossil fuels to present-day global energy demand reflects 99 the readily available resources of oil, gas and coal. Even when oil reserves become limiting, 100 there will remain large reservoirs of gas (including from shale) and, particularly, coal to [7] 101 exploit . Therefore, in the global arena, the problem for the immediate future is not a Accepted 102 limitation of fossil fuel reserves but the consequences of its combustion. If the total fossil 103 fuel reserve is burnt, the CO 2 level would rise to values equivalent to those that existed on 104 our planet long before humankind evolved [8]. Despite of this consideration, it is certain that 105 fossil fuels will continue to be a major source of energy for some years to come but it is 106 vital that they should be used in such a way as to minimize CO 2 release into the atmosphere. [9] 107 Technologies for CO 2 sequestration have been developed . Hand in hand with this, there 108 is an improvement in the efficiency of energy use and supplementation whenever possible 109 from non-fossil fuel sources. Against this background, we must also strive to develop new Advances 110 technologies based on principles that have yet to be revealed from basic studies and in 111 particular those that focus on using the enormous amount of energy available to us as solar 112 radiation [10]. The sun provides solar energy to our planet on an annual basis at a rate of 113 1×10 5 TW. Therefore, the energy from 1 h of sunlight is equivalent to all the energy RSC 114 humankind currently uses in a year. We do have existing technologies to capture sunlight 115 and produce electricity and the efficiency and robustness of these photovoltaic systems is 116 improving daily [11-13].