Produção de biochar a partir de sabugo de milho: caracterização, adsorção de nitrogênio amoniacal e Scale-up da pirólise em reator de tambor rotativo

Abstract

Biochar has been extensively investigated as an adsorbent material for environmental applications. Although its use in agriculture has traditionally been associated with soil conditioning and carbon sequestration, biochar also exhibits high potential for wastewater treatment. In this context, the production of biochar from agricultural residues stands out as a sustainable alternative, enabling the valorization of these materials and the development of low-cost adsorbents, while providing additional environmental benefits such as CO₂ emission reduction and promotion of the circular economy. In this research project, biochar was produced by pyrolysis of corn cob, an abundant agricultural residue. Initially, the raw material was conditioned and characterized through thermogravimetric analysis (TGA), moisture and ash content determination, elemental composition, extractives content, lignin, and holocellulose analyses. Biochar production was conducted using a mixed 2 × 3 factorial experimental design, varying the carrier gas (N2 and autopyrolysis), particle size, and temperature, aiming to obtain a biochar with high cation exchange capacity and suitable properties for ionic adsorption. After the experimental design, segregation of corn cob composition during particle size separation was observed, as well as differences in adsorption capacities. This behavior motivated the production of powdered biochar under the best-defined atmospheric and temperature conditions, followed by post-pyrolysis acid treatment with phosphoric acid (H3PO4), resulting in the biochars denoted as Bp and BpH. Subsequently, physicochemical and functional characterizations were performed, including pH, point of zero charge (pHpzc), water retention capacity (WRC), electrical conductivity, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and total reflection X-ray fluorescence (TXRF), in addition to planned complementary analyses such as N2 physisorption, toxicity, and leaching tests. Characterization results indicated that acid treatment significantly altered the biochar surface, leading to reductions in pH, ash content, and conductivity, as well as modifications in surface functional groups. The pHpzc obtained for BpH was close to 3, providing a favorable condition for ammonium nitrogen (NH4+ < pKa = 8.8 < NH₃) adsorption. Considering the application in ammonium nitrogen removal, the BpH biochar (acid-treated) showed favorable kinetics, with the pseudo-first-order (PFO) model fitting the data, suggesting an overall kinetic controlled by external mass transfer and progressive site occupation, and an experimental ammonium nitrogen adsorption capacity (q) of ~53 mg g-1 in just 180 min. Equilibrium data showed sigmoidal behavior, and the Sips model provided the best fit, with an estimated maximum capacity of 64.07 mg g-1. In water from a fish farming depuration tank, an ammonium nitrogen adsorption capacity of 1.07 mg g-1 was obtained, similar to that obtained with a synthetic solution of similar concentration, in addition to reductions in concentrations of elements such as Na, P, K, and Ca, and inorganic carbon q = 3.90 mg g-1 and total carbon q = 2.73 mg g-1. In scale-up, corn cob pyrolysis in a rotary drum reactor resulted in a biochar yield of 24.86% and a pyrolytic liquid yield of 20.54% with acidic characteristics. The results demonstrated the viability of integrating corn cob valorization, biochar production, and its application in ammonium nitrogen removal, although operational adjustments and complementary evaluations are necessary for its continuous application.