Sustainable biochar derived from apple wastes and enhanced with metal oxides green synthesized for water purification by adsorption

The preoccupations regarding the preparation of eco-friendly materials for environmental decontamination and waste recycling have increased. Biochars are sustainable materials that can be produced through pyrolysis of cost-effective biomass sources (e.g., agricultural wastes), with low cost and physicochemical stability, suitable for environmental applications. The study was focused on the preparation and characterization of the biochar obtained from apple residues, activated (A-ac) and functionalized with green synthesized metal oxides (Fe3O4 and NiO), for preliminary testing of pollutants (paracetamol, tartrazine, dibutyl phthalate, and bis(2-ethylhexyl) phthalate) removal from water. The green synthesis of the metal oxide was done in the presence of Urtica dioica L. (nettle) extract (ext) that was previously characterized for total polyphenolic content and antioxidant capacity. The novel materials prepared (A-ac-Fe3O4-ext, A-ac-NiO-ext, and A-ac-Fe3O4-NiO-ext) were analyzed for the determination of their morphological, compositional, and structural properties, and the determination of the specific surface area and porosity parameters. The analysis showed the presence of carbon, iron, oxygen, and nickel in the developed materials. XRD patterns of both A-ac-Fe₃O₄-ext and A-ac-Fe₃O₄-NiO-ext confirm the presence of Fe₃O₄ (magnetite), while NiO was not detected crystallographically but was confirmed by EDX, likely due to its dispersed state. The surface area for A-ac-NiO-ext was 681.6 m2/g, being the highest of the investigated samples. The increase in the IG/ID ratio observed by Raman spectroscopy upon NiO addition suggests enhanced structural ordering and interaction with biochar. Preliminary adsorption tests demonstrated that A-ac-Fe3O4-ext exhibited the highest adsorption efficiency for paracetamol (57.54%), tartrazine (37.48%), and dibutyl phthalate (74.98%). A-ac-NiO-ext showed moderate adsorption for paracetamol (46.36%) and high efficiency for tartrazine (64.36%), while A-ac-Fe₃O₄-NiO-ext exhibited the lowest performance, with only 23.23% and 18.8% removal, respectively. This work highlights the potential of sustainable biochar derived from apple wastes and enhancement with metal oxides green synthesized for environmental applications, particularly in the removal of pollutants from waters.