Complexation and Precipitation Mechanisms During Heavy Metal Adsorption on a Variety of Biochars

MEDINA, R.; WANNAZ, E.D.; SALAZAR M.J.; BLANCO A.; BARBERO MEDINA G.P.; PARDO S. D.

Montevideo

Congreso; SETAC Latin America 15th Biennial Meeting; 2023

Society of Environmental Toxicology and Chemistry

Since the industrial revolution, pollutant emissions have increased exponentially, generating significant environmental problems in nature. Nowadays, there are several methods for air, water and soil remediation. Biochar (BC) is a material obtained by pyrolysis of organic matter, rich in recalcitrant carbon, with a high cation exchange capacity and a large surface area. BC has great potential for the remediation of soils and water contaminated with heavy metals (HMs) because it immobilizes them by adsorption on its surface. The adsorption mechanisms vary from one BC to another, and may be different for each HM.In this work, two adsorption mechanisms (complexation and precipitation) of HMs (Cd, Cu, Pb and Zn) were studied on BCs produced at 3 pyrolysis temperatures (300, 400 and 500°C) from soybean straw, peanut husks and brewer´s malt. Each of the BCs was loaded with each of the HMs separately by batch adsorption assays with aqueous solutions. To study the complexation mechanism, FTIR spectra of BCs with and without HMs were compared. To study the precipitation mechanism, X-ray diffraction technique was used.The results obtained by FTIR show that the richness of functional groups decreases with increasingproduction temperature. That is why the mechanism of complexation with functional groups was more important in BCs produced at lower temperatures. The functional groups with the highest participation in the adsorption of HMs were C=O, C-O, C=C, COOH and COO-. Soybean straw, peanut husks, and their respective biochars showed participation of the complexation mechanism for all 4 HMs; malt BCs did not show this mechanism although un-pyrolyzed malt did.The results obtained by X-Ray diffraction showed that the precipitation mechanism of HMs is influenced by both the raw material used and the pyrolysis temperature. In the soybean BC produced at 400 °C only Pb precipitation occurred, while in the one produced at 500 °C Cd and Cu were also precipitated. In the peanut shell BC no precipitation phenomenon was observed. In the malt BC, precipitation was observed only for Pb, being more noticeable with increasing production temperature.In conclusion, soybean BCs must be recommended for soil and water remediation considering that they presented the highest adsorption efficiency for the 4 HMs and showed a greater participation of the complexation and precipitation adsorption mechanisms, both important for immobilization estability.