## 1 Introduction  
Recently, the materials science community witnessed a sky-rocketing number of research papers on biochar. A search on Web of Science with the keyword \"biochar\" returned the following results: 2 papers in 2001, 221 in 2011, 1413 in 2016, 4333 in 2020 and 3238 in 2021 (accessed on 12 August 2021), which means that over 5000 papers should be published on the topic by the end of 2021. This attests to the vitality of this branch of materials science. The rationale for the rush in research on biochar lies in the availability of biomass worldwide in different forms, such as agrowastes, i.e., peels [1], leaves [2], palm fromds [3] and seeds [4]. The production of biochar depends on various parameters: the initial composition of the biomass [5] and the pyrolysis parameters [6]. Biochar production falls within the energy sector as it yields bio-oil and biogas in addition to the biochar itself. Biochar is employed in agriculture for soil remediation and as an adsorbent in environmental applications, e.g., the removal of organic and inorganic pollutants. In the domain of materials science, biochar was found to be a unique source for making graphene [7] but raised hope, if valorized, for its use as a support of nanocatalysts [8]. In the latter case, biochar is usually produced then post-treated for the immobilization of metal ions, which is followed by in situ reduction, resulting in immobilized nanocatalysts [9]. For other purposes, metal ions are loaded on the biochar (or other carbon allotrope) and calcined to provide immobilized metal oxide nanocatalysts [10]. In order to reduce the steps needed to create biochar-immobilized nanocatalysts, we reasoned that the initial biomass could be loaded with metal ions and the modified biomass powder could be pyrolyzed in view of obtaining, in one step, nanocatalysts-coated biochar. This approach is attractive and has seldom been reported; it involves the design of rice husk biochar-immobilized copper metallic nanocatalysts for the catalytic cracking of biomass primary tar [11]. As an alternative method, hydrothermal carbonization may prepare hydroxar-immobilized copper nanoparticles in a single step, but this requires a long process time and additional NaBH\({}_{4}\) to reduce the metal ions [12]. Carbothermal production is another process that is mostly employed for recycling used batteries [13], although it has recently also been extended to designing carbon-loaded catalysts [14]. Olive stones are also widely valorized agrowastes, e.g., as biofil fillers for polymer composites [15; 16]. In 2017, olive stone powders were used as biosourced cellulosic supports of silver and gold nanoparticles, and the final olive pit-supported nanocatalysts facilitated the degradation of para-nitrophenol [17]. Herein, we take the advantage of this widely available agrowaste from Tunisia (a country in the top five producers of olive oil by volume [18]) to make nanocatalyst-decorated biochar. Instead of the noble metals, silver and gold, we instead targeted copper-nickel bimetallic nanocatalysts. These supported bimetallic nanoparticles served as electrocatalysts for the reduction of nitrates in wastewater [19] and as heterogeneous catalysts for the reduction of dyes [20]. Herein, we bridge the gap between the use of olive pit powder particles as a support of noble nanocatalysts and the unique, single-step making of biochar-immobilized low-cost bimetallic copper-nickel nanocatalyst. The final copper-nickel-decorated olive pit biochar hybrid was characterized in terms of its morphology, elemental and chemical composition, crystalline structure, thermal stability and response to a magnetic field. It was then tested as a potential heterogeneous catalyst of the discoloration of Methyl Orange aqueous solution.