Sorção de fósforo por biocarvão de resíduo de tabaco dopado com cátions e eficiência como fertilizante

Abstract

Biochars are carbonaceous materials with numerous energy, environmental and agricultural applications. Biochar is the product of the pyrolysis of organic residues subjected to oxygenlimited conditions and high temperatures. In the pyrolytic process, there is formation, mainly, of carboxylic and phenolic groups on the surface of biochars, conditioning a certain anionic character to the material, making it difficult to adsorb anions, such as phosphates. However, it is possible to change the functional groups and, consequently, the surface charge profile of the material through doping, a process that consists of adding cationic metals to the raw material to be carbonized, which will promote the formation of new surface functional groups capable of to adsorb anions. Some studies have shown a high potential for recovery of PO43- with the use of biochar submitted to metal doping treatments and reuse in agriculture. However, few works bring more in-depth information about such groups formed with different cations in biochars. There is also a great lack of information for the full understanding of the interactions between these new groups with anions, such as phosphates, under different conditions. Thus, the objective of this study was to know the capacity, mechanisms and structures in the sorption of P by biochars doped with acid (Al3+ and Fe3+) and basic (Ca2+ and Mg2+) cations, in addition to a subsequent evaluation of the agronomic performance of the resulting products as suppliers of P. Biochars were synthesized from tobacco residue at five pyrolysis temperatures (350, 400, 450, 500 and 550 ºC), with four doping cations (Al3+, Fe3+, Ca2+ and Mg2+) and absence of cations (control), making up a 5 x 5 factorial scheme, distributed in a completely randomized design, with three replications. With the selected biochars, the physical-chemical characterization was carried out using scanning electron microscopy coupled with dispersive X-ray spectroscopy, X-ray diffraction and infrared spectroscopy with Fourier transform. For P sorption studies, kinetic models and adsorption isotherms were adjusted and, subsequently, the use of extractors to evaluate desorption. Furthermore, biochar modified with Al enriched with P (BAl-P) and modified with Mg enriched with P (BMg-P) were used as phosphate fertilizers in the successive cultivation of corn-soybean, comparing them with the reference source triple superphosphate (ST), whose experiment consisted of three sources (BAl-P, BMg-P and ST), two doses of the nutrient (50 and 200 mg dm-3 P), plus a control treatment (without addition of P), distributed in a randomized block design, with four replications. In this experiment, shoot dry mass, P content and content, P recovery rate and P availability were evaluted after each crop. Metal doping increased the sorption capacity of P in biochar from the crystalline alteration and increase in surface functional groups, which acted as sorption sites. BMg-P was the treatment that showed the highest P sorption capacity and the highest agronomic efficiency in the successive corn-soybean cultivation, comparable to triple superphosphate, but with the characteristic of slow release of P.