Citrus processing wastes: Environmental impacts, recent advances, and future perspectives in total valorization

The recent advances and future perspectives in the complete valorization of citrus processing waste (CPW), a by-product of citrus processing industries, are presented in this review paper. First, the importance of valorization of CPW to develop a bio-economy and to reduce its negative environmental impacts is assessed. A brief survey of applications of native/modified CPW for nanoparticle, bio-sorbent, and biofertilizer production is presented. As the core part of the valorization scheme and regarding the environmental aspects, the perspectives for the application of CPW are via green extraction techniques, e.g., microwave- and ultrasound-assisted extractions, and biochemical processes. Furthermore, green extraction and biochemical techniques result in processes' intensification toward integrated biorefinery models. The superiority of green extraction techniques over traditional techniques, challenges for implementation, and the valuable extracts obtained by these methods as well as a summary of their analytical techniques are discussed. The challenges of bioconversion of CPW to biofuels and fermentative products and strategies to overcome them are later presented. Finally, a literature review on using the concept of green chemistry for the integrated biorefinery of CPW and its engineering challenges is presented and a biorefinery scheme is proposed accordingly.