## 1 Introduction  
Anthropogenic inputs of heavy metals into the environment pose a serious threat to humanity and ecosystems worldwide due to their toxicity, persistence, and bioaccumulation [1; 2]. Heavy metals, which are some of the most serious pollutants, derive from both industrial sources and domestic discharge, affecting environmental quality and human health [3]. As humans feed at the highest trophic level, and as heavy metals are biomagnified along the food web, humans are more likely to suffer serious health complications [3]. The main sources of heavy metal pollution in water and soils are industrial wastes, agricultural pesticides and fertilizers, and domestic wastewater [2; 3; 4; 5]. Among the heavy metals, copper is a relevant environmental pollutant released from diverse anthropogenic sources, such as industrial and mining activities, combustion processes, the production and use of phosphate-based fertilizers, and domestic wastewater discharge. There are also some important natural copper sources, such as plant decomposition, volcanic activity, and wildfires, which increase the copper concentration in different abiotic matrices [3]. Copper uptake in humans primarily occurs from the ingestion of polluted food and water, mostly in the form of Cu\({}^{2+}\). It is mainly absorbed by the small intestine and, to a lesser extent, by the stomach, where it may cause gastrointestinal diseases [5]. An excess of copper is toxic due to its displacing other metallic cofactors [5]. Different technologies have been developed to control and remove excess amounts of heavy metals from wastewater, such as chemical precipitation, electro-coagulation, solvent extraction, bioremediation, recovery by evaporation, membrane filtration, and adsorption by activated carbon or biomaterials [3]. Among these, adsorption methods are widely used as they offer many advantages due to the wide variety of adsorbent materials available [6]. In recent years, considerable attention has been paid to the use of biochar as an adsorbent for heavy metal removal, as it is less expensive, more efficient, and more environmentally friendly than alternative materials [7]. Biochar is a solid material composed of a carbon matrix with a highly porous structure, which, depending on the production conditions, could result in an extensive surface area, acting as a surface sorbent [8]. In addition to its low cost, biochar has other productive advantages, such as high availability and mechanical stability, and so could play an important role in the environmental control and removal of pollutants [8; 9]. Biochar is produced by pyrolysis, a process consisting of the thermal decomposition of organic biomass under limited oxygen conditions at temperatures ranging from 350 to 900 \({}^{\circ}\)C [7; 10]. The relevant operation parameters of pyrolysis are the temperature and residence time, as it is these which directly influence the physical and chemical properties of the final product [7; 8]. The properties of biochar also depend on the nature of the raw materials used [11; 12]. For example, seaweed biomass is considered a promising raw material for biochar production due to its wide distribution, rapid growth, and high efficiency for carbon dioxide (CO\({}_{2}\)) fixation [13]. The cultivation of algae allows atmospheric carbon dioxide to be sequestered, reducing greenhouse effects [14]. However, characterizations of the physical and chemical properties of seaweed biochar remain scarce even though these are crucial for determining its potential application as a bioadsorbent in aqueous solutions. The scarce available information indicates that the physicochemical properties of seaweed biochar include high concentrations of various trace elements, such as phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca), as well as the presence of key functional groups (e.g., carboxyl, hydroxyl, and phenolic groups) [15; 16]. These characteristics could make seaweed biochar a potential alternative for Cu\({}^{2+}\) adsorption in waste or drinking waters.