In Europe, the Water Framework Directive (WFD) of 2000 established guidelines for the protection of surface water, underground water and coastal water. The main objective was to maintain and/or to restore water quality in Europe. In France, the WFD has been incorporated into French legislation by several laws, the latest of which dates from July 27, 2015 and deals with chemical substances [1,2,3]. This regulation lists two main categories of unwanted substances: dangerous priority substances (DPS) considered persistent, highly toxic or causing bioaccumulation and priority substances (PS) presenting a significant risk for the environment. A total of 70 substances are concerned, of which 8 are metals, namely Hg and Cd as DPS; Pb and Ni as PS; and As, Cr, Cu and Zn. The surface treatment industry is a particular contributor to the release of these metals [2], and among the various industrial activities, this sector is still considered to be one of the largest consumers and polluters of water [1,2,3]. In addition, these eight metals are also the subject of particular attention insofar as they account for the calculation of the so-called metox index, an indicator used to quantify certain forms of toxic pollution and to calculate the taxes that certain high-risk industries must then pay to the French Water Authority. The World Health Organization (WHO) and other environmental protection agencies (US EPA) have also adopted guidelines on the release of metals and their presence in water compartments.

Currently, Europe is asking industrials to innovate by reducing and/or eliminating metals present in their discharge waters at trace levels. In theory, there are methods available including membrane filtration, evaporation, liquid-liquid extraction, adsorption and ion exchange [4,5,6,7,8,9]. For example, the surface treatment industries can upgrade their standard installations used to treat their wastewater before release by using the series of (sand) filtration + adsorption onto activated carbons + ion-exchange to tend toward zero pollution [6,8]. This technology is recognized for its effectiveness, but its widespread use is very limited due to its high cost. It is also extremely difficult, while being economically viable, to remove substances that are heterogeneous and variable in nature and that occur at very low concentrations in discharge waters. In fact, economic considerations must be examined because the industrial sector in Europe is characterized by a large proportion of small and medium-size enterprises that are not very enthusiastic about investing in additional equipment with high operating and maintenance costs. In recent years, many attempts have been made to find inexpensive alternative materials as biosorbents that are both effective and acceptable for industrial use from both a technological (simplicity) and economic point of view. Numerous reviews on this topic can be consulted [10,11,12,13,14,15,16,17,18,19].

A recent class of materials proposed for environmental applications such as the removal of pollutants from solutions is based on the use of plants such as hemp and flax. These two plants are annual, high-yielding crops grown for their fibers and seeds [20,21,22]. Hemp and flax are interesting raw, eco-friendly, lignocellulosic materials because of their ease of production (rapid growth, no pesticides), low cost, renewable character, particular chemical composition of their fibers (mainly cellulose, hemicelluloses and lignin), particular structure (multicellular fibers of fibrilar structure), physical and mechanical properties as well as their versatility. They are usable in the form of powder, fragments (shives), fibers and oils since the entire plant (seeds and plant stem) is recoverable [20]. Like other lignocellulosic fibers or plant fibers such as jute, ramie, sisal, kenaf and bamboo, hemp and flax contain a high content of cellulose and comprise three main constituents (cellulose, hemicelluloses, and lignin) and other minor components. Within the framework of a circular and ecological economy, products based on natural fibers of vegetal origin such as hemp and flax are attracting great interest for their numerous applications in various industrial fields such as textiles, the paper industry, the construction sector, composites (automotive applications, plastic, packaging), agrochemistry, mulch and animal bedding [20,21,22,23,24,25,26]. However, in the field of wastewater treatment, there are no real applications yet, but there is a growing interest in the academic field [25,26].

Indeed, hemp and flax in fiber form have been proposed for metallic ion removal from aqueous synthetic solutions [23,25,26,27,28,29,30,31,32,33,34,35]. Kostić’s group studied the capacity of hemp for removing Zn, Pb and Cd ions from aqueous monometallic solutions and indicated strong bonding of metal ions to functional groups such as the carboxylic, carbonyl and hydroxyl groups present in the hemp structure (cellulose, hemicelluloses, lignin and extractives) [27,32,35]. In a series of papers on the ability of hemp to act as a non-conventional, low-cost and effective adsorbent, Păduraru and co-workers also reported similar conclusions for other metals such as Cu, Cr, Ag and Co [28,29,30]. Balintova et al. [31] studied the removal of copper ions by hemp and reported biosorption capacities between 3.91 and 4.45 mg/g, which were similar to those of the conventional commercial materials used in the treatment of metal contaminated water. Other studies have reported similar findings and conclusions [23,24,33,34]. Flax has also been proposed for metal removal [36,37,38,39,40,41]. For example, Abbar et al. [36,37] showed that 1 g of flax was capable of adsorbing 8.4 mg of Zn, 9.9 mg of Cu and 10.7 mg of Pb. The fibers were brought into contact with the metals present in water at a pH between 4 and 7 (depending on the element to be complexed) for 1 h under agitation. The authors reported that the chemical groups present in the structure of polysaccharides were responsible for biosorption, similarly to the case of hemp fibers. Melia et al. [38] also showed that flax byproducts were interesting as unconventional materials for adsorbing Cd: For metal concentrations between 1.1 and 21.5 mg/L, flax was capable of removing more than 90% of Cd in less than 10 min for the lowest investigated concentrations. The maximum capacity reported by the authors was 3.36 mg/g. Abutaleb et al. [40] recently reported a maximum capacity of 40.9 mg/g for U adsorption onto flax fibers. All these results were obtained using hemp and flax in the form of fibers.

Recently, our group proposed for the first time the use of hemp-based biosorbents in a felt form to treat monometallic and polymetallic aqueous solutions [42,43,44,45,46]. Results of biosorption in batch mode showed that hemp-based felts could be used as efficient biosorbents for the removal of metals present in synthetic solutions [42,43] or in real effluents [44,45,46]. Pursuing our interest in such plant-based biosorbents, in this study, we compared the biosorption performance of two lignocellulosic-based felts (Figure 1) made of hemp and flax fibers and a smaller amount of synthetic fibers toward the removal of 7 metals, namely Al(III), Cd(II), Co(II), Cu(II), Mn(II), Ni(II) and Zn(II). These metals were selected based on their common presence in discharge waters from the French metal industry. Moreover, Cd, Cu, Ni and Zn are also on the list of priority pollutants defined by the French Water Agency [2]. Herein, studies concerning the effects of metal concentration, adsorbent dose, contact time and pH were evaluated using the batch method. To our knowledge, this is the first report on the comparison of metal removal from solutions by hemp- and flax-based materials in felt form, which might be an interesting option to apply in industrial scenarios due to its easy operation as a filter.