Tratamento de biochar produzido a partir da pirólise de sabugo de milho para aplicação como substrato no cultivo hidropônico da alface (Lactuca sativa L.)

Abstract

One way to reuse residual corn biomass, especially from the cob, abundant in the western region of Paraná, is by using it as raw material in the pyrolysis process to form biochar. Biochar is a porous-structured product with numerous applications and benefits, such as providing nutrients to plants, and is widely used in agriculture. This study aims to explore the potential of corn cob biochar, with and without treatment, as a substrate in hydroponic lettuce (Lactuca sativa L.) cultivation. Before conducting the pyrolysis, the residual corn cob raw material was characterized, and the moisture content was found to be 8,327±0,274%, ash content was 1,463±0,006%, and extractives were 6,400±0,347%. The corn cob pyrolysis tests followed a factorial experimental design 23+PC (central point), in which different pyrolysis conditions were studied, including temperatures of 441, 500, and 559 °C, residence times in the reactor of 0,5, 1,0, and 1,5h, and biomass particle sizes of 2, 3, and 4 mm. Mathematical models were developed to explain the experimental responses, and the global desirability tool was applied, identifying that test 3 (temperature of 441 °C, time of 1,5h, and biomass size of 2 mm) was the most suitable for maximizing the selected responses of yield and cation exchange capacity (CEC). Under these pyrolysis conditions, the yield for the biochar was 27,306%, and the CEC was 36,757 cmol kg-1. Biochar produced under the conditions of test 3 was treated with phosphoric acid, followed by neutralization with magnesium hydroxide (designated as B2), and treated with phosphoric acid + magnesium hydroxide + sodium selenite (designated as B3), with B1 being the untreated biochar. The phosphorus (P) concentration increased from the acid treatment of 2,491±0,101 mg L-1 to 25,093±0,488 mg L-1. After treatment with magnesium hydroxide, the magnesium concentration obtained was 10,736±0,184 mg L-1, an element that was not detected in B1. After treatment with selenium, the concentration obtained was 13,360±0,205 mg L-1. A solubility test was conducted to verify the concentration of this element transferred to water, as the goal was to evaluate the potential of the treated biochar as a substrate in hydroponic cultivation, and excessive selenium concentrations can be toxic to plants. The selenium concentration obtained in the water after the solubility test was 0,725±0,015 mg L-1, appropriate for application in hydroponic cultivation. The B1 and B3 biochars were used as substrates in the hydroponic cultivation of lettuce at a biochar:perlite ratio of 1:1 (v/v) for B3 and 1:1 (v/v), as well as 0.5:1 (v/v) for B1. The highest growth parameters were obtained when applying the treated biochar, showing that the treatment was effective. After 4 weeks of cultivation, the average number of leaves reached 7,31±0,20, the average aerial part height was 11,25±0,34 cm, and the average leaf width was 4,81±0,10 cm. Based on the results obtained, it can be concluded that the biochars produced in the experimental trials were of good quality and can be applied in hydroponic lettuce cultivation, especially the trial 3 treated with P, Mg, and Se.