Review Insights for the Valorization of Biomass from Portuguese Invasive Acacia spp. in a Biorefinery Perspective Ricardo Correia 1 , José Carlos Quintela 2, Maria Paula Duarte 1 and Margarida Gonçalves 1,3,* 1 Mechanical Engineering and Resources Sustainability Centre, Department of Sciences and Technology of Biomass, Faculty of Sciences and Technology, New University of Lisbon, 1099-085 Caparica, Portugal; [email protected] (R.C.); [email protected] (M.P.D.) 2 Natac Biotech, C/Electrónica 7, 28923 Alcorcón, Madrid, Spain; [email protected] 3 VALORIZA-Research Center for Endogenous Resource Valorization, Polytechnic Institute of Portalegre, 7300-555 Portalegre, Portugal * Correspondence: [email protected] Received: 25 November 2020; Accepted: 10 December 2020; Published: 16 December 2020 Abstract: Acacia spp. are widespread all over the Portuguese territory, representing a threat to local biodiversity and to the productivity of the forest sector. The measures adopted in some countries for their eradication or to control their propagation are expensive, have been considered unfeasible from practical and economical perspectives, and have generated large amounts of residue that must be valorized in a sustainable way. This review brings together information on the valorization of bark, wood, leaves, flowers, pods, seeds, roots, and exudates from Acacia spp., through the production of high-value bioactive extracts (e.g., antioxidant, antimicrobial, anti-inflammatory, antidiabetic, antiviral, anthelmintic, or pesticidal agents, suitable to be explored by pharmaceutical, nutraceutical, cosmetics, and food and feed industries), its incorporation in innovative materials (e.g., polymers and composites, nanomaterials, low-cost adsorbents), as well as through the application of advanced thermochemical processes (e.g., flash pyrolysis) and pre-treatments to decompose biomass in its structural components, regarding the production of biofuels along with valuable chemicals derived from cellulose, hemicellulose, and lignin. The knowledge of this research is important to encourage an efficient and sustainable valorization of Acacia spp. within a biorefinery concept, which can bring a significant economic return from the valorization of these residues, simultaneously contributing to forest cleaning and management, to reduce the risk of fires, and to improve the social-economic development of rural areas. Keywords: Acacia spp.; invasive species; biorefinery; bioactive extracts; antioxidant; antimicrobial; anti-inflammatory; biofuels; nanomaterials; adsorbents 1. Introduction The genus Acacia comprises a large group of more than 1350 species, widely distributed throughout tropical and warm temperate areas of the world [1]. Most of the species are native to Australia but are spread all over the world because of their wide variety of uses and economic importance such as for ornamental purposes, for sand and dune stabilization, as a fuel through the production of woodfuel and charcoal, as an important source of fodder, tannins for the leather industry, gums, and essences for perfumes [2–4]. In the Portuguese territory its introduction was mainly for stabilization of dunes and soil [5], as well as for ornamental purposes [6]. However, its worldwide cultivation has led to its establishment and invasion in many regions of the world, with negative impacts on biodiversity and on ecosystem properties and functions [7]. Forests 2020, 11, 1342; doi:10.3390/f11121342 www.mdpi.com/journal/forests Forests 2020, 11, 1342 2 of 42 Forests 2020, 11, x FOR PEER REVIEW 2 of 43 Acacia SomeSome key key features features contributing contributing to to the the success success of of Acacia spp.spp. as as invaders invaders include include their their phenotypic phenotypic plasplasticity,ticity, which which confers confers them them with with the the ability ability to to adapt adapt to to changing changing environments; environments; their their large large seed seed productionproduction andand accumulationaccumulation of of massive massive seed seed banks banks for long for periods;long periods; their high their capacity high forcapacity vegetative for vegetativereproduction reproduction from rhizomes from afterrhizomes disturbances, after disturbances, such as fires such and as cuttings; fires and and cuttings; their allelopathic and their allelopathicproperties, inhibitingproperties, the inhibiting growth ofthe neighboring growth of neighboring native species native [8–10 species]. [8–10]. AcaciaAcacia spp.spp. are are considered considered invasive invasive in in the the Portuguese Portuguese territory territory [11] [11,], where where the the occupied occupied area area almostalmost doubled doubled from from 1995 1995 to to 2010, 2010, passing passing from from 27 270101 ha ha to to 5351 5351 ha, ha, according according to to the the last last national national forestforest inventory inventory [12] [12].. Amongst Amongst invasive invasive AcaciaAcacia sppspp.,., A.A. cyclops cyclops AA.. Cunn. Cunn. e exx G. G. Don Don fil. fil.,, A.A. dealbata dealbata LinkLink,, A.A. longifolia longifolia (Andre(Andrews)ws) Willd. Willd.,, A.A. melanoxylon melanoxylon R.R. Br. Br.,, A.A. saligna saligna (Labill.)(Labill.) H. H. L. L. Wendl. Wendl.,, A.A. retinodes retinodes Schlecht.Schlecht.,, A.A. karrookarrooHayne, HayneA., mearnsiiA. mearnsiiDe Wild. De andWild.A. pycnanthaand A. pycnanthaBentham areBentham the most are representative. the most representative.Figure1 shows Figure examples 1 shows of invaded examples areas of by invadedAcacia areasspp. inby the Acacia Portuguese spp. in the territory. Portuguese territory. Figure 1. Examples of invaded areas by Acacia spp. in the Portuguese territory (elaborated from Figure 1. Examples of invaded areas by Acacia spp. in the Portuguese territory (elaborated from images images taken from Plantas Invasoras em Portugal (http://invasoras.pt), accessed on 1 November taken from Plantas Invasoras em Portugal (http://invasoras.pt), accessed on 1 November 2020). 2020). The threat to protected areas, and the negative impacts on the ecosystems and on the productivity of theThe forest threat sector to protected have motivated areas, expensiveand the negative removal impacts initiatives on by the local ecosystems authorities and in orderon the to productivityminimize its of proliferation the forest sector [13], have generating motivated high expensive amounts removal of biomass initiatives which by is usuallylocal authorities burned forin orderenergy to productionminimize its or proliferation landfilled. The [13] valorization, generating ofhigh some amountsAcacia ofspp. biomass for energy which may is usually have a burned positive forcontribution energy production to the economic or landfilled. sustainability The valorization of the involved of some operations Acacia spp. [14], for but en thisergy option may have is often a positivelimited contribution due to collection to the and economic transportation sustainability costs. of However,the involved finding operations high-value [14], but applications this option foris oftenthese limited species due can contributeto collection to and overcome transportation these costs costs. and However, make the processfinding economicallyhigh-value applications sustainable. forFurthermore, these species given can the contribute magnitude ofto theovercome invasion, these the radicular costs and system make of thesethe process species, economically the dispersion sustainable.of seeds in Furthermore, the natural environments, given the magnitude and the of years the invasion, necessary the before radicular soil nutrientssystem of andthese processes species, thereturn dispersion to similar of seeds pre-invasion in the natural levels, environments, their elimination and the by removalyears necessary has been before considered soil nutrients unfeasible and processesfrom practical return and to economicalsimilar pre perspectives-invasion levels, [15], their and haselimination the potential by removal of by itself has to been cause considered significant unfeasibleenvironmental from impacts,practical namelyand economical the destabilization perspectives of large[15], and volumes has the of soil.potential of by itself to cause significantAcacia environmentalspp. fractions, impacts, namely bark, namely wood, the leaves,destabilization flowers, of pods, large seeds volumes or roots, of soil. are rich sources of bioactiveAcacia secondary spp. fractions, metabolites namely (e.g., bark, amines wood, and leaves, alkaloids, flowers, cyanogenic pods, seeds glycosides, or roots cyclitols,, are rich fatty sources acids ofand bioactive seed oils, secondary gums, non-proteinmetabolites (e.g. amino, amines acids, and terpenes, alkaloids, tannins cyanogenic and other glycosides, flavonoids cyclitols, and simple fatty acidsphenolics) and se [4ed,16 ,oils,17] that gums, have non been-protein used inamino traditional acids, medicineterpenes, for tannins a wide and range other of ailments,flavonoids such and as simplediabetes, phenolics) worminfection, [4,16,17] dysmenorrhea, that have been eczema, used malaria, in traditional gout, jaundice, medicine abdominalpain, for a wide rangekidney of ailments, problems , such as diabetes, worm infection, dysmenorrhea, eczema, malaria, gout, jaundice, abdominal pain, kidney problems, constipation, leprosy, piles, pneumonia, rheumatism, fever and cancer [18]. These Forests 2020, 11, 1342 3 of 42 constipation, leprosy, piles, pneumonia, rheumatism, fever and cancer [18]. These secondary