**1. Introduction**

The concept of the circular economy—a system where waste generation is minimized by reintroducing residues and by-products into the production cycle—can be applied, to a large extent, to production processes that use natural resources. This is one of the bases that support the so-called bioeconomy, the need for the integral valorization of natural resources. In Europe, agriculture constitutes approximately 63% of the total biomass supply; forestry, about 36%; and fisheries, less than 1% [1]. It is therefore essential to focus on the recovery of waste generated by the agricultural sector in order to guide the economic strategy towards a circular economy and bioeconomy.

Spain is the leading country in olive and olive oil production with an average annual output of 9.8 million tons of olives, more than five times that of the second largest producer, Italy, with 1.8 million tons per year [2]. Spain represents 47% of worldwide olive production and 72% of European production. As consequence of this production, after harvest, a large number of different types of lignocellulosic materials are generated (pruning, leaves, stones, pomace, etc.), which generally have no industrial application and must be discarded. It is estimated that for the production of one kg of fruit, more than 0.8 kg of waste is generated, meaning more than 7.5 million tons of olive harvest waste per year, waste that could be valorized in Spain. Olive tree pruning biomass (OTPB), in common with any lignocellulosic material, mainly consists of cellulose, lignin and hemicellulose; and other non-structural minority compounds such as pigments, proteins, ashes, etc. This biomass can be fractionated into its various components by means of biorefinery processes. This fractionation of the OTPB into its lignocellulosic components has been widely studied by the scientific community, testing its application as a source of sugar [3], substrate for ethanol production [4], lignin [5], energy [6], building materials [7] and cellulose fibers for paper and cardboard production [8].

One of the most interesting avenues for the valorization of the agricultural residues is the production of nanocellulose as an alternative to wood sources [9]. Nanocellulose presents unique properties such as a high surface area, unique optical properties, lightweightness, stiffness and a high strength, in addition to its inherent properties in common with cellulose (renewable, biodegradable and sustainable) [10]. These properties allow the possibility of using this nanomaterial in many industrial sectors, expecting to reach a global turnover around 10,000 M€ in 2020 [11]. The wide range of applications of nanocellulose-based materials include the paper and cardboard industry [12], electronic devices [13], energy [14], cosmetics [15], composites [16], wastewater treatment [17], catalysts [18], construction [19], drug carriers [20] and biomedicine [21]. The use of agricultural residues, such as OTPB, as a source for the local, renewable and sustainable production of nanocellulose will allow countries with insufficient forest resources to produce these high value-added products.

Cellulose nanofibers (CNFs), also known as nanofibrillated cellulose, are one of the existing types of nanocellulose (along with cellulose nanocrystals and bacterial cellulose). CNFs are long (several microns), flexible (presenting both types of cellulose region, crystalline and amorphous), nanometric (1–100 nm in width) and are extracted from cellulosic fibers by mechanical methods [22]. The mechanical treatment aims at the isolation of the cellulose nanofibers by the delamination of the fibers. Several mechanical treatments have been studied, including high pressure homogenization [23], twin-screw extrusion [24], micro-fluidization [25] and ultrafine-friction grinding [26], the most commonly used. One of the great disadvantages of these treatments is the large number of passes that the fibers have to undergo and the long time required to produce delamination. Therefore, to facilitate and increase the effectiveness of the treatment, fibers are subjected to a previous process, known as pretreatment. Likewise, there are many pretreatments, but the most widely used and most effective are mechanical pretreatment [27], enzymatical pretreatment [28], TEMPO-mediated oxidation [29] and surface functionalization [30]. To study the effectiveness of the different treatments, it is crucial to determine the chemical composition of the source, to optimize the process of fiber obtention and to adequately characterize the final product.

In this work, olive tree pruning biomass has been valorized as a lignocellulosic source for the obtention of cellulose nanofibers as a high value-added product. The suitability of the chemical composition of the raw material and the fiber in cellulose nanofiber production has been studied. In order to study the effect of lignin on the effectiveness of nanofibrillation and its properties, the cellulose fiber was subjected to a bleaching process. Both types of fiber, bleached and unbleached, were subjected to two independent pretreatments, mechanical pretreatment and TEMPO-mediated oxidation, followed by high pressure homogenization treatment. The cellulose nanofibers obtained were widely characterized in terms of their chemical composition, morphology, thermal stability and crystallinity.

### **2. Materials and Methods**
