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

ROCIO MEDINA; WANNAZ EDUARDO DANIEL; MARIA JULIETA SALAZAR; BLANCO ANDRES; BARBERO GONZALO PEDRO; PARDO SHEILA DANIELA

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 significantenvironmental 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 highcation exchange capacity and a large surface area. BC has great potential for the remediation of soils andwater contaminated with heavy metals (HMs) because it immobilizes them by adsorption on its surface. Theadsorption 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) werestudied on BCs produced at 3 pyrolysis temperatures (300, 400 and 500°C) from soybean straw, peanuthusks and brewer´s malt. Each of the BCs was loaded with each of the HMs separately by batch adsorptionassays with aqueous solutions. To study the complexation mechanism, FTIR spectra of BCs with andwithout 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 moreimportant in BCs produced at lower temperatures. The functional groups with the highest participation in theadsorption of HMs were C=O, C-O, C=C, COOH and COO-. Soybean straw, peanut husks, and theirrespective biochars showed participation of the complexation mechanism for all 4 HMs; malt BCs did notshow this mechanism although un-pyrolyzed malt did.The results obtained by X-Ray diffraction showed that the precipitation mechanism of HMs is influenced byboth the raw material used and the pyrolysis temperature. In the soybean BC produced at 400 °C only Pbprecipitation occurred, while in the one produced at 500 °C Cd and Cu were also precipitated. In the peanutshell BC no precipitation phenomenon was observed. In the malt BC, precipitation was observed only forPb, being more noticeable with increasing production temperature.In conclusion, soybean BCs must be recommended for soil and water remediation considering that theypresented the highest adsorption efficiency for the 4 HMs and showed a greater participation of thecomplexation and precipitation adsorption mechanisms, both important for immobilization stability.