Desenvolvimento de hidrocarvões funcionalizados com óxido de grafeno para a remediação de águas contaminadas com nitrato

Abstract

Nitrate contamination in groundwater and surface water, caused by excessive fertilizer use, improper waste disposal, and other sources, is a serious environmental problem. Nitrate concentrations above 10 mg.L⁻¹ can form toxic compounds, such as nitrite and nitrosamines, posing health risks. Traditional removal methods are limited, leading to the search for alternatives, such as new materials for adsorption processes. Hydrochar, a porous carbon-rich material, is widely used for pollutant adsorption, but its efficiency for anion removal, such as nitrate, is limited due to its negative surface charge at certain pH levels. Graphene oxide, with its superior properties and active functional groups, emerges as an enhancer. Combining it with hydrochar results in a composite where hydrochar acts as a lignocellulosic support matrix. Thus, this work aimed to develop graphene oxide-functionalized hydrochar composites derived from sugarcane biomass to improve nitrate adsorption capacity in contaminated water. The synthesis of the materials was carried out from sugarcane straw using thermal and chemical processes to ensure a porous and functional structure. For the synthesis of Graphene Oxides, two routes were employed: via iron oxide (OG-Fe₃O₄) and using ferrocene (OG-Fe(C₅H₅)₂). Furthermore, four synthesis routes were adopted for the composites, including the hydrothermal route ((HC/OG-Fe₃O₄ I) and (HC/OG-Fe(C₅H₅)₂ I)) and the heterojunction route ((HC/OG-Fe₃O₄ II) and (HC/OG-Fe(C₅H₅)₂ II)). After synthesis, the materials were characterized morphologically and functionally. The effectiveness of OG-Fe(C₅H₅)₂ composites for nitrate removal was confirmed. Structural characterization by XRD showed the graphene oxide pattern with an interplanar distance of 0.70 nm around 11°, while FTIR identified oxygenated functional groups such as -OH and -COOH, essential for adsorption. With a point of zero charge at pH 5.6, hydrochar exhibited a positive charge in acidic pH, favoring electrostatic interactions with nitrate. SEM analysis revealed cavities and porosities, while thermal stability was enhanced by the high carbon content, and the adsorption kinetics followed the pseudo-second-order model, with rapid adsorption between 15 to 40 minutes. At pH 4, the composites removed up to 72% of nitrate, while at neutral pH, they maintained 67%, indicating potential for industrial applications without acidic adjustments. The study also detailed the synergy between electrostatic interactions and hydrogen bonds, aiding in nitrate ion capture. The robust performance at neutral pH, combined with rapid adsorption kinetics, positions the HC/OG-Fe(C₅H₅)₂ composite as a practical and efficient alternative for water treatment, x potentially reducing costs and simplifying processes without compromising nitrate removal efficiency.